Introduction to Environmental Mineralogy 9811977917, 9789811977916

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Anhuai Lu Yan Li Changqiu Wang Hongrui Ding

Introduction to Environmental Mineralogy

Introduction to Environmental Mineralogy

Anhuai Lu • Yan Li • Changqiu Wang • Hongrui Ding

Introduction to Environmental Mineralogy

123

Anhuai Lu School of Earth and Space Sciences Peking University Beijing, China

Yan Li School of Earth and Space Sciences Peking University Beijing, China

Changqiu Wang School of Earth and Space Sciences Peking University Beijing, China

Hongrui Ding School of Earth and Space Sciences Peking University Beijing, China

ISBN 978-981-19-7791-6 ISBN 978-981-19-7792-3 https://doi.org/10.1007/978-981-19-7792-3

(eBook)

Jointly published with Science Press The print edition is not for sale in China (Mainland). Customers from China (Mainland) please order the print book from: Science Press. ISBN of the Co-Publisher’s edition: 978-703-04-3729-7 © Science Press and Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are reserved by the Publishers, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publishers, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publishers nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publishers remain neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Foreword

Human’s recognition and use of minerals have been confined to resource exploration for centuries since the dawn of mineralogy. Over recent decades, natural resources have been over-exploited and utilized, leading to growing environmental destruction. Earth science, as a basic discipline that hosts the two key topics of resources and environment, takes on the high responsibility of developing resources and protecting the environment. Therefore, exploring the environmental properties of natural minerals is imperative. It is the mineralogical environmental properties together with the resource attributes that construct the inherent nature of minerals. I reviewed the history of human usage of minerals at the opening ceremony of the First National Symposium on Environmental Mineralogy held by the Department of Geology of Peking University in May 2001 and proposed that the concept of minerals should be updated, as the environmental properties of minerals, i.e., environmental mineralogy, would be the emerging theme. Thirteen years later, I once again suggested that we should break through the traditional perception of minerals and encouraged vigorous development of intersection research between modern mineralogy and other disciplines at the National Symposium on Mineralogy Development organized by School of Earth and Space Sciences of Peking University in November 2014. Nowadays, the strengthening of the new interdisciplinary study of environmental mineralogy is a well illustration of the breakthrough to the traditional conception of minerals. I still remember clearly that the authors’ research on the environmental mineralogy began with the study of iron sulfide minerals for the treatment of heavy metal contaminants, and it is remarkable to see such a wealth of research findings today. The authors published a Chinese version of their monograph in 2015, which systematically addressed five major mineralogical environmental properties and applications: minerals recording environmental changes, minerals affecting environmental quality, minerals reflecting environmental evaluation, minerals treating environmental pollution, and minerals involving in bioaction. This monograph in English summarizes the main research achievements of the authors in the last 20 years, highlighting a third form of energy in nature in addition to solar photon and valence electron, the photoelectron energy of natural semiconducting minerals. The indirect harnessing of solar energy by other organic or inorganic materials on the earth’s surface occurs through the conversion of sunlight by semiconducting minerals. This work has further explored how mineral photoelectric energy contributed to the origin and evolution of early earth life and the growth and metabolic activities of microorganisms, proposing a new insight into the existence of “photoelectric” microorganisms in nature. Pathological mineralization in human body is also discussed in this book, and research on characteristics of mineralization in some major disease lesions was experimentally carried out. Moreover, the first infrared emission spectral characterization of minerals was investigated. These innovative research results have enriched the repository of modern mineralogy. Substances in nature are mainly composed of inorganic minerals and organisms. How can mineralogy keep pace with the development of biology? The emergence and prosperity of environmental mineralogy has opened the door to the future of modern mineralogy. The study of environmental mineralogy has greatly expanded the research objects and content of v

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traditional mineralogy, which has achieved considerable development in theory and broad prospects in application. This is also a tremendous progress of mineralogy since the conceptual breakthrough of minerals. The environmental mineralogy has promoted the traditional mineralogy into a new research stage, reaching new heights and achieving new goals. I firmly believe that environmental mineralogy plays a fundamental disciplinary role in systematic earth sciences, which is comparable to the role of resource mineralogy in traditional geology. The evolution of mineralogy from a traditional “minor subject” to a modern “major discipline” is just around the corner. In this regard, I am delighted to write the foreword to the work of Anhuai Lu and others. Beijing, China June 2022

Danian Ye Member of the Chinese Academy of Sciences

Preface

Mineralogy is an ancient natural discipline that, together with biology, constitutes the most fundamental natural material science. More than 5800 minerals have been found so far in nature. The initial understanding and exploration of nature by human beings included the recognition and use of minerals. For centuries after the emergence of mineralogy, minerals as resources have been valued and utilized, which has underpinned the importance of resource mineralogy as a fundamental subject in resource geology. It is believed that with the growing demand for natural resources and the consequent in-depth understanding of them, the supporting role of resource mineralogy as a basic discipline of resource geology will continue to be strengthened. Currently, the historical concept of minerals in the natural inorganic world is undergoing new changes. In the past, minerals have been considered as natural simple substances or compounds formed under various geological processes. The formation of minerals is confined to the lithosphere, and minerals are essentially regarded as the products of geological processes. Nowadays it is essential to include inorganic solids formed during interactions that are not purely geological between the lithosphere with hydrosphere, atmosphere, and biosphere in the category of natural minerals. This essentially involves not only the matter cycle among the atmosphere, hydrosphere, biosphere, and lithosphere in the earth’s surface but also reflects how materials circulation on the terrestrial surface has been strongly disturbed by anthropogenic activities since industrialization. The Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) clearly stated in 1997 that new materials formed purely from existing rocks and minerals exposed to the atmosphere and subjected to terrestrial surface processes can be considered as minerals. Evidently, minerals are natural simple substances or compounds formed during various natural processes, which include both geological processes and natural interactions of multiple spheres of the earth’s surface. The research domain of modern mineralogy is not limited to the lithosphere as in traditional mineralogical studies, but is more concerned with the basic mineralogical sciences involved in the process of lithospheric being influenced by the hydrosphere, atmosphere, and biosphere. This has greatly expanded the scope of modern mineralogical research, leading to the creation of environmental minerology, an entirely new field that reflects interactions from the lithosphere to the hydrosphere, atmosphere, biosphere, and even the soil circle. Environmental mineralogy, which emphasizes theoretical and applied research on the environmental properties of minerals, has also emerged at a time when the earth, the home of human beings, is facing serious threats of environmental contamination and ecological devastation. As an emerging discipline born in the early 1990s, environmental mineralogy focuses on the interaction between minerals and the various layers on the earth’s surface, as well as minerals reflecting nature evolution, preventing ecological damage, evaluating environmental quality, purifying pollution, and participating in biological processes. At present, the minerals with environmental response and their formation processes among the products of the interaction between lithosphere and the hydrosphere are becoming the main research objects of environmental mineralogy. And the main research target of environmental mineralogy is the vii

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eco-physiological responses of the formation, development, and changes of inorganic minerals during the interaction of multiple layers in the earth’s critical zone. In 2015, our monograph of nearly one million words, Introduction to Environmental Properties of Mineralogy in Chinese, was published by Science Press. The whole book was organized into three parts. Part I systematically introduced the main mineralogical environmental properties, the natural self-purification functions of minerals in the inorganic world, the environmental influences of synergistic mineral-microbial interactions, and the eco-physiological effects of biomineralization. Part II focused on the environmental properties of several typical minerals, such as pyrite, pyrrhotite, and sphalerite in the sulfide group, rutile and cryptomelane in the oxide group, silicate minerals like chrysotile and orthoclase, and sulfate minerals like jarosite in the oxygenate group. In this part, the synergistic effects of semiconducting minerals and microorganisms are also elaborated, and the characteristics of pathological minerals in human cardiovascular and several tumor lesions are initially discussed. In the last part, “mineral method” was introduced as the fourth category of pollution prevention methods, including the application of minerals in treating inorganic pollutants, degrading organic contaminants, purifying soot pollution, evaluating soil quality, preventing and controlling wastes, and disposing tailings. This monograph in English summarizes the main results of our research in the last 20 years and is divided into 10 chapters. Chapter 1 provides a brief overview of mineralogical environmental properties. Chapters 2–5 focus on the environmental properties of manganese oxides by comparing them with iron sulfide minerals, from their distribution on the ground, pore characteristics, and redox properties to their interaction with microorganisms. Chapters 6 –8 highlight the photoreduction and photooxidation properties of semiconducting minerals and how their photoelectron energy is utilized by microbial growth and metabolism. In Chap. 9, the environmental effects of biomineralization and the pathological manifestations of mineralization in human lesions are presented. The last chapter features the infrared emission spectra of minerals. The main findings of our research are to discover the mechanism of solar energy conversion by ferromanganese “mineral coatings” on the terrestrial surface and the natural photocatalytic system composed of suspended semiconducting Fe, Mn, and Ti oxides in the euphotic zone of the sea. Thus, based on the two known fundamental energy forms of solar photons and elemental valence electrons in nature, we proposed a third energy form of mineral photoelectrons on terrestrial and oceanic surfaces, opening up a whole new field of mineral photoelectron energy and rationalizing a novel theory of terrestrial origin of life. We first proposed that microorganisms can obtain mineral photoelectron energy for growth and metabolism, which functioned as a leading international hotspot research in this field. The environmental properties of mineral valence electron and mineral photoelectron redox and their interaction with microorganisms were mainly highlighted. This book was mainly completed by Anhuai Lu, Yan Li, Changqiu Wang, and Hongrui Ding at School of Earth and Space Sciences, Peking University. Graduate students participating in the work include Xiang Gao, Ning Li, Yanjun Guo, Juan Liu, Ruiping Li, Ming Lv, Rui Liu, Dongjun Zhao, Lijuan Wang, Xishen Zheng, Yan Li, Ruochen Yang, Zemin Luo, Chenzi Fan, Hongrui Ding, Xin Wang, Cuiping Zeng, Yun Zhu, Huiqin Zhang, Ping Yu, Yidong Yin, Yunhua Yan, Yi Liu, Fanlu Meng, Haoran Wang, Yan Li, Meiqian Zhu, Xinge Yang, Chao Quan, Hongyan Zuo, Cong Ding, Hanlin Zhang, Xiao Wang, Manyi Sun, Yuxiong Xiao, Yuan Li, Ying Liu, Guiping Ren, Xiaoming Xu, Fefei Liu, Yuwei Liu, Yanzhang Li, Ying Zhu, Xiang Ji, Yan Zhang, Jia Liu, etc. Shan Qin, Ruixia Hao, Yong Lai, Jianying Liu, Fang Mei, Bo Zhang, Kang Li, Chongqing Yang, and other teachers have participated in parts of the studies. Yuan Sun, Jia Liu, Xiao Ge, Yanzhang Li, Xiang Ji, Tianci Hua, Yuanlong Zhang, Ying Zhu, Yan Zhang, Huan Ye, Haoning Jia, Ziyi Zhuang, Junqi Wu, and other graduate students assisted in the completion of partial chapters and drawings. The research was funded by two projects of the National Key Basic Research and Development Program of China (973 Program) in the major scientific frontier areas. This work was also supported by the National Key Research and Development Program of China, the

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National Natural Science Foundation of China, Key International Collaborative Program, and several general programs. Our research was also supported by the laboratory building projects of Peking University in the national “211 Project” and “985 Project”. The accomplishment of this monograph also benefited from the development of academic organization, academic exchange, and discipline construction of environmental mineralogy in China. The Geological Society of China approved the establishment of the Environmental Mineralogy Branch in the Mineralogy Committee in 1999, and the Chinese Society for Mineralogy, Petrology and Geochemistry approved the establishment of the Environmental Mineralogy Committee in 2004. In 2003, Peking University established a Ph.D. program in Geology (Materials and Environmental Mineralogy) independently. In addition to publishing research results in domestic and international academic journals, we have also published research collections in Acta Petrologica et Mineralogica (1999-2019), Bulletin of Mineralogy, Petrology and Geochemistry (2006), Acta Geologica Sinica (2006), Elements (2012), and Geomicrobiology Journal (2012). We would like to express our sincere gratitude to the above-mentioned organizations and individuals. Due to the limitation of the author’s ability, deficiencies and shortcomings are inevitable in this book, and we sincerely welcome comments and criticisms from readers. Beijing, China July 2022

Anhuai Lu President of International Mineralogical Association (IMA)

Contents

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Environmental Property of Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Research Category of Environmental Property of Minerals . . . . . . . . . 1.1.1 Minerals Record Environmental Changes . . . . . . . . . . . . . . . 1.1.2 Minerals Affect Environmental Quality . . . . . . . . . . . . . . . . . 1.1.3 Minerals Reflect Environmental Evaluation . . . . . . . . . . . . . . 1.1.4 Minerals Control Environmental Pollution . . . . . . . . . . . . . . . 1.1.5 Minerals Participate in Biological Function . . . . . . . . . . . . . . 1.2 Natural Self-purification Function of Inorganic Mineral . . . . . . . . . . . 1.2.1 Surface Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Channel Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Structure Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 Ion Exchange Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Redox Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.6 Precipitation/Dissolution Effect of Mineral . . . . . . . . . . . . . . 1.2.7 Crystallization Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . 1.2.8 Hydration Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.9 Thermal Effect of Mineral . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Environmental Effects of the Synergism Between Minerals and Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Mineral Electron Energy Form . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Mineral Photoelectrons Promote the Origin and Evolution of Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Mineral Photoelectrons Promote the Growth and Metabolism of Photoelectrophic Microorganisms . . . . . . . . . . . . . . . . . . . 1.3.4 Microbial Photoelectrophic Nutrition Mode . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental Effects of Tunnel Structure Minerals . . . . . . . . . . . . 2.1 Octahedral Tunnel Effects of Cryptomelane . . . . . . . . . . . . . . . . 2.1.1 Channel Structure of Manganese Oxide . . . . . . . . . . . . . 2.1.2 Channel Effect of Natural Cryptomelane . . . . . . . . . . . . 2.1.3 Remarks on the Reactivity of Nanomineral Aggregates . 2.2 Channel Structure Effects of Potassium Feldspar Tetrahedron . . . 2.2.1 Channel Structure Characteristics of Potassium Feldspar 2.2.2 Ion Exchange Effect of Potassium Feldspar Channels . . . 2.3 Tubular-Texture Effects of Fibrous Serpentine . . . . . . . . . . . . . . 2.3.1 Crystal Structure of Fibrous Serpentine . . . . . . . . . . . . . 2.3.2 The Active Group of Fibrous Serpentine . . . . . . . . . . . . 2.3.3 The Active Behavior of Fibrous Serpentine . . . . . . . . . . 2.3.4 The Nanotube of Clinochrysotile . . . . . . . . . . . . . . . . . 2.3.5 Nano-fibriform Silica from Natural Chrysotile . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Photoactivity of Mn Oxides on Earth’s Surface . . . . . . . . . . . . . . . . . . 3.1 Nature Manganese Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Vast Distribution of Mn Oxides on Modern Earth . . . . . . . 3.1.2 Widespread Mn Coatings on Earth’s Surface . . . . . . . . . . . 3.1.3 Photoelectric Behavior of Mn (Oxyhydr)oxide . . . . . . . . . . 3.2 Electronic Structure of Natural Semiconducting Mn Oxides . . . . . . 3.2.1 Effect of Mn (or O) Vacancies . . . . . . . . . . . . . . . . . . . . . 3.2.2 Effect of Metal Cations . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Photocatalytic Self-reduction of Natural Mn Oxides . . . . . . . . . . . . 3.3.1 Photocatalytic Oxidation of Water by Mn4CaOx . . . . . . . . 3.3.2 Photocatalytic Self-reduction of Natural Mn Oxides . . . . . . 3.4 Environmental Functions of Mn Oxides Controlled by Mn Redox Cycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Reductive Dissolution of Mn Oxides Mediated by Organic Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Oxidative Formation of Mn Oxides and Heavy Metal Sorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Interaction Between Fe & Mn-Bearing Minerals and Microbes . . . . . . . . . . . 5.1 Reduction of Goethite by Cronobacter Sakazakii . . . . . . . . . . . . . . . . . . . 5.1.1 Total Protein and Fe(II) Concentration Changes . . . . . . . . . . . . . 5.1.2 Morphology of the Strain and Minerals . . . . . . . . . . . . . . . . . . . . 5.1.3 Coordination Structure and Fe Oxidation State of the Products . . . . 5.2 Reduction of Birnessite by a Novel Dietzia Strain . . . . . . . . . . . . . . . . . . 5.2.1 Anaerobic Reduction of Birnessite by 45-1b . . . . . . . . . . . . . . . . 5.2.2 Aerobic Reduction of Birnessite by 45-1b . . . . . . . . . . . . . . . . . . 5.2.3 Effect of AQDS on Reduction of Birnessite . . . . . . . . . . . . . . . . 5.2.4 Mineral Characterization of Bioreduced Samples . . . . . . . . . . . . . 5.3 Coupled Anaerobic and Aerobic Microbial Processes for Mn-Carbonate Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Birnessite Bioreduction by 45-1b Under Aerobic and Anaerobic Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Effect of Oxygen on Birnessite Bioreduction and Rhodochrosite Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 A Conceptual Model and Geologic Significances of Mn(II) Carbonate Precipitation at Anaerobic Sub-Interfaces in the Aerobic Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Redox Activity of Iron Sulfide and Mn Oxide . . . . . . . . . . . . . . . . . . . . . 4.1 Removal of Cr(VI) and Cr(III) from Aqueous Solutions and Industrial Wastewaters by Natural Pyrrhotite . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Characteristics of Pyrrhotite and Wastewater . . . . . . . . . . . . . 4.1.2 Effectiveness in Cr(VI) Removal . . . . . . . . . . . . . . . . . . . . . 4.1.3 Solid Phases After Cr(VI) Removal . . . . . . . . . . . . . . . . . . . 4.1.4 Process of Cr(VI) Removal . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Potential Industrial Application . . . . . . . . . . . . . . . . . . . . . . . 4.2 Reactivity of Mn Oxide Cryptomelane . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Occurrence and Characterization of Cryptomelane . . . . . . . . . 4.2.2 Oxidation of Phenols by Mn Oxide . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Photocatalytic Reduction Effects of Sphalerite and Sulfur . . . . . . . . . . 6.1 Mineralogical Characteristics of Natural Sphalerite . . . . . . . . . . . . . 6.1.1 Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2 Crystal Chemical Characteristics . . . . . . . . . . . . . . . . . . . . 6.1.3 Surface Charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Semiconducting Characteristics of Natural Sphalerite . . . . . . . . . . . 6.2.1 Optical Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Conduction and Valence Band Potentials . . . . . . . . . . . . . 6.3 Photocatalytic Activities of Natural Sphalerite . . . . . . . . . . . . . . . . 6.3.1 Photoreduction of Pollutants as Well as Carbon Dioxide by Sphalerite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Highly Efficient ZnO/ZnFe2O4 Photocatalyst from Thermal Treatment of Sphalerite . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Photoreduction of Inorganic Carbon(+IV) by Elemental Sulfur . . . . 6.4.1 Geochemistry of Tengchong Terrestrial Hot Spring with Abundant S0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Photoreduction of Carbonate to Produce HCOOH in the Presence of S0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.3 The Photoactivity of S0 Under UV Light . . . . . . . . . . . . . 6.4.4 Adsorption of Carbonate Molecules and Formation of Formate on S0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.5 Reaction Mechanisms Based on the Semiconducting Properties of S0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.6 Reaction Mechanisms Based on Broken Bonds Reacting with Adsorbed Molecules . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.7 Implications for Photoreactive S0 in Prebiotic Terrestrial Hydrothermal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Photocatalytic Oxidation Effects of Rutile . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Mineralogical Characteristics of Natural Rutile . . . . . . . . . . . . . . . . . . 7.1.1 Occurrence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2 Crystal Chemical Characteristics . . . . . . . . . . . . . . . . . . . . . . 7.1.3 Surface Charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Semiconducting Characteristics of Natural Rutile . . . . . . . . . . . . . . . . 7.2.1 Optical Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Electronic Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Conduction and Valence Band Potentials . . . . . . . . . . . . . . . 7.3 Photocatalytic Activities of Natural Rutile . . . . . . . . . . . . . . . . . . . . . 7.3.1 Photocatalytic Oxidation of Methyl Orange by Natural Rutile Under Visible Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Enhanced Visible-Light Response of Natural Rutile by Thermal Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 Explanations and Prospectives of Rutile Photocatalysis on Both Earth and Mars . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Interactions Between Semiconducting Minerals and Microbes . . . . . . . . . . . . 171 8.1 Interactions Between Semiconducting Minerals and Bacteria Under Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 8.1.1 Synergistic Pathway Between Semiconducting Minerals and Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

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8.1.2

Semiconducting Minerals Stimulate Growth of Non-phototrophic Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.3 Synergism Between Microorganisms and Semiconducting Minerals in Environmental Remediation . . . . . . . . . . . . . . . . . 8.2 Regulation and Influence of Mineral-Microorganism Electron Transfer on Microbial Community . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Semiconducting Minerals Regulate Extracellular Electron Transfer and Microbial Community Composition . . . . . . . . . . . 8.2.2 Photoelectron Energy of Semiconducting Minerals Affects Microbial Community and Function . . . . . . . . . . . . . . . . . . . . 8.3 Regulation and Influence of Mineral-Microorganism Electron Transfer on Microbial Strains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Extracellular Electron Transfer to Minerals Through External Circuit and Synergistically Enhanced by Semiconducting Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Extracellular Electron Transfer to Minerals Directly with Promotion from Semiconducting Minerals . . . . . . . . . . . . . . . . 8.3.3 Photoelectron Energy Utilized by Microbes to Accelerate Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Environmental Effects and Application of Pollutant Treatment . . . . . . . 8.4.1 Light Fuel Cell Tech for Pollution Treatment by Semiconducting Minerals Cooperating with Extracellular Electron Transform . . 8.4.2 SSC Enhanced LFC System for Wastewater Treatment . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Human Pathological Mineral Features . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Mineralization Characteristics of Psammoma Body Mineralization in Meningioma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.1 Morphology and Composition of Psammoma Body Mineralization in Meningioma . . . . . . . . . . . . . . . . . . . . . . 9.1.2 Characterization of Morphology, Chemical Composition and Microstructure of Separated PBs . . . . . . . . . . . . . . . . . 9.1.3 Discussion on the Formation Mechanism of Calcification . . . 9.2 Characteristics of Cardiovascular Mineralization . . . . . . . . . . . . . . . . 9.2.1 Cardiovascular System Mineralization . . . . . . . . . . . . . . . . . 9.2.2 Mineralogical Characterization of Calcification in Cardiovascular Aortic Atherosclerotic Plaque . . . . . . . . . . . 9.3 Characteristics of Psammoma Bodies in Ovarian Tumors . . . . . . . . . 9.3.1 Morphology and Distribution of Psammoma Bodies in Ovarian Tumors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.2 The Mineral Composition and Fine Structure of Psammoma Bodies in Ovarian Tumors . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Carbonate and Cation Substitution in Hydroxyapatite in Breast Cancer Micro-Calcifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Mineral Phase and Crystal Structure . . . . . . . . . . . . . . . . . . 9.4.2 Carbonate Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.3 Cation Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.4 Diagnostic Significance and Implications . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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10 Infrared Effect of Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 10.1 The Theory of Infrared Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 10.2 Thermal Emission Spectra of Carbonate Minerals . . . . . . . . . . . . . . . . . . 238

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10.2.1 The Characteristics of the Natural Carbonate Minerals . . . . . . 10.2.2 Infrared Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . 10.2.3 Infrared Emission Spectroscopy . . . . . . . . . . . . . . . . . . . . . . 10.2.4 The Effect of Crystal Chemistry on Characteristic Vibrations . 10.2.5 Infrared Radiation Properties of Minerals . . . . . . . . . . . . . . . 10.3 The Middle and Far-Infrared Spectroscopy Characteristics of Calcite, Dolomite and Magnesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Mineral Characteristics and Infrared Absorption Spectroscopy 10.3.2 Mid-Infrared Thermal Emission Spectroscopy . . . . . . . . . . . . 10.3.3 Mass of Metal Atoms Affects the Spectral Vibration Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.4 Effect of Antisymmetric Stretching Vibration of C–O Bond on the Emissivity of Carbonate Minerals . . . . . . . . . . . . . . . . 10.3.5 Influence of Crystal Structure on the Radiation Characteristics of Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Thermal Emission Spectra of Silicate Minerals . . . . . . . . . . . . . . . . . . 10.4.1 Infrared Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.2 Comparison of Absorption and Emission Bands of Silicate Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.3 Effect of Vibrating SiO4 Tetrahedron on Infrared Radiation Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.4 Geologic Implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Environmental Property of Minerals

1.1

Research Category of Environmental Property of Minerals

Resources and environment are the two main themes of contemporary earth sciences. And as a basic discipline of geoscience, the development of mineralogy should focus on the two themes. The understanding and utilization of the environmental properties of minerals is a further development of the knowledge and use of minerals as resources. Environmental mineralogy has also emerged at a time when the planet, on which human beings rely for survival, is facing serious threats of ecological destruction and environmental pollution. Research on the environmental property of minerals mainly focuses on how minerals record environmental changes, how minerals impact environmental quality, how minerals reflect environmental evaluation, how minerals control pollution, and how minerals participate in bioaction. The following aspects are highlighted in the study of environmental minerology: ① minerals as an information carrier reflecting environmental changes on different time and space scales; ② the negative impact of minerals on human health and the environment, and ways to prevent and control these effects; ③ the performance of minerals in controlling pollution and remediating environment; ④ the ability of minerals to load pollutants, and the mechanism and methods of minerals to evaluate environmental quality; and ⑤ the interaction between minerals and organisms at cell levels and the corresponding microscopic mechanism.

1.1.1 Minerals Record Environmental Changes Natural minerals are the product of the natural evolution of the earth. The environmental changes on the earth have left imprints during the occurrence, development, change and extinction of minerals on different spatial and temporal scales. Therefore, minerals containing abundant information that can reflect environmental changes became the © Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_1

informative carrier of environmental evolution. Generally, we can find this kind of environmental information through external micromorphology, internal microstructure, physical and chemical properties, spectral characteristics, and genetic occurrence of minerals. Glaciers and loess formed since the Quaternary period are often targeted in global change studies. The characteristics of heavy mineral particles in glaciers and primary and secondary minerals in loess documented abundant information on the formation and evolution of glaciers and loess, respectively (Ehrmann and Polozek 1999; He et al. 1997; Orgeira et al. 1998). In-depth study of these information carriers will be beneficial to reveal the characteristics and evolution patterns of global environmental changes on large temporal and spatial scales. As relatively independent natural complexs, lakes are the interface between lithosphere, atmosphere, hydrosphere and biosphere. Lacustrine deposits are continuous and contain abundant environmental information, providing a robust document for high-resolution environmental evolution research. At present, the commonly used environmental indicators in the lacustrine deposits are not only organisms such as pollens, diatoms, and ostracods but also inorganic minerals such as sedimentary minerals, carbonate content, oxygen and carbon isotope of authentic carbonate, the contents and proportion of trace elements and magnetic parameters. A detailed study of the mineralogical characteristics of stalactites and stalagmites produced in karst areas, (Davitaya et al. 1998; Genty and Deflandre 1998) like the study of the annual rings of trees, can precisely reveal the evolutionary patterns in terms of paleoclimate and paleoenvironment on a smaller time scale. To understand the changes of global climate and environment in the next few decades to 100 years, it is urgent to acquire an insight into the climate and environment change patterns on the 10–100 year scale, as well as the frequency and mechanisms of extreme climate events. Thus, it is critical to obtain continuous high resolution (year to season) natural records with precise chronology. Natural materials with annual and seasonal cycles that 1

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record periodic climate and environmental changes are referred to as natural clocks, such as tree rings, corals, muds and ice cores. Besides, the cave stalagmites have been discovered relatively recently as a geological natural clock. Although the ice cores are thick and continuous, they are only found in polar and alpine regions. Countable annual sequences of coral reefs generally reach hundreds of years, and their distribution is limited to the tropical oceans. The applicable tree rings are limited in the temperate semi-arid areas. It is difficult to find tree ring samples with ages over 1000 years under pure natural conditions. The ripple mud of Mar Lake spans a long time and can be used to track seasonal cycles, but its distribution is limited to the volcanic lake areas. Cave stalagmites, on the other hand, widely distributed from north to south in China, with a period from modern times to tens of thousands of years ago. It is a rare geological clock that can record short-scale climate and environment changes with high resolution. Stalactites and stalagmites are mainly composed of carbonate minerals including calcite and aragonite. Numerous domestic and international research have shown that the rhythmic variation of stalagmite micro-growth layers from micron to millimeter thick can reflect short-scale, highfrequency climatic vibrations. Changes in stalagmite luminescence intensity are important records of paleoenvironmental changes, and surface soil organic material carried from dripping water is the main cause of luminescence in the microlayer. In Broecker et al. (1960) used the 14C method to identify some fast-growing stalagmite layers in temperate climates as annual rings. In Baker et al. (1993) used the thermal ionization mass spectrometry-uranium dating to confirm that the luminescent micro-growth layers in the stalagmite was growing annually. Since then, several stalagmites with annually growth layers have been reported: a type of annual layer consists of luminous and nonluminous calcite generally formed under temperate climate conditions; an annual layer constituted by interbedded white loose and dark tight calcium carbonate depositions which is mainly formed in areas with obvious seasonal rainfall and temperature changes; the interbedded annual layers of calcite and aragonite that are mainly formed in the dolomitic rock zone. The stalagmites with micro-layer characteristics found in China so far mainly consist of radial fibrous calcite crystals. The calcite crystal bundles are perpendicular to the microlayer surface, which is mainly the interface of exogenous dark organic matter that comes from the overlying soil of the cave.

1.1.2 Minerals Affect Environmental Quality Weathering of rocks can lead directly to the destruction and decomposition of minerals, usually for the loss of some

1

Environmental Property of Minerals

active components. In particular, minerals that are unstable under surface conditions tend to undergo chemical and biological weathering. The toxic and harmful cations and anions derived from these weathering products have a great impact on the surrounding environment, affecting the local soil and water quality, and often causing local human health and ecological environment issues. To find the internal mechanisms and prevention methods of minerals damaging human health and ecological environment has become another major environmental property of minerals. Generally, minerals formed spontaneously in nature are originally able to exist stably in nature. That is, natural minerals are well compatible with ecological environment, which is one of the necessary conditions for mineral formation. However, people have taken as many methods as possible to find various mineral resources in order to maximize the mineral resources utilization. As a result, the minerals below the surface were passively moved to the surface, bringing an increase in the enthalpy of the minerals, a decrease in the temperature and pressure as well as an enhancement of oxidation in the medium minerals located, leading to a significant decline of their stability. This inevitably leads to the destruction and decomposition of minerals, and the resultant heavy metals and anionic substances directly impact the quality of surface waters and soils, with some metallic minerals, especially for those metallic minerals containing variable valence elements. A typical example of this is the acid mine drainage pollution from mines which seriously damaged ecological environment. The acid mine drainage mainly comes from the decomposition of metal sulfides. It is an environmental protection issue being explored to effectively prevent the oxidative decomposition of sulfide minerals under surface conditions (Gray 1997). Some radioactive natural minerals have also been brought directly to the surface by human mining activities, and their unreasonable use has brought extremely serious negative impacts on human health and even survival. Some minerals are considered as chemical barrier to dispose the nuclear waste properly (Pushkareva 1998). Nuclear waste is a special kind of pollutant with enormous hazards, and with massive exploration and utilization of nuclear energy, research on the safe disposal of nuclear waste has become an urgent task. Storage and landfills are currently common ways to dispose of nuclear waste, and the key technical issue is how to effectively prevent leakage of nuclear waste. Another type of mineral-induced harm to human comes from the processing and utilization of mineral resources, such as the generation of mineral dust (Ross et al. 1993). Asbestos-like amphibole is the main pathogenic factor of pulmonary disease among workers in asbestos mines and processing plants. These fibrous minerals can be found in the lung of those miners (Nayeb et al. 1998). During the

1.1 Research Category of Environmental Property of Minerals

utilization of coal, a large amount of SO2 and CO2 released from the thermal decomposition of minerals, severely impacting human health and atmospheric quality, and even forming acid rain. Minerals in the lithosphere are affected by the atmosphere, mainly by chemical changes such as oxidation and carbonation that occur with the direct participation of O2 and CO2 in the atmosphere or dissolved slightly in waters. Oxidation is a common process that has a great influence on minerals containing variable valence elements, such as pyrite transforming to limonite, rhodochrosite to psilomelane or cryptomelane. Carbonation of minerals is mainly due to the formation of HCO3− after the dissolution of CO2 in the atmosphere, leading to the increase of the medium acidity and dissolution of micro-soluble minerals, which had drawn little attention in the past. The study of the weathering of these minerals in the atmosphere is of great importance for the conservation of cultural relics. For example, it is essential to prevent the oxidation of paintings colored by sulfide mineral pigments, and for stone sculptures, to effectively prevent the acid dissolution of calcite by atmospheric carbonation. In summary, the destruction and decomposition of minerals caused by human living and production activities have brought negative impact on human health and the environment. It is possible to reduce or even avoid these impacts by studying the environmental properties of minerals, discovering the mechanism of mineral decomposition, and taking measures to prevent the destruction and decomposition of minerals.

1.1.3 Minerals Reflect Environmental Evaluation Natural environment quality evaluation mainly refers to the evaluation on the environmental conditions of natural landscape such as atmosphere, water, and soil. Inorganic minerals in soil, atmosphere and waters usually occur as simple substances or compounds, which are closely related to pollutants in the nature, especially inorganic pollutants. The capacity of minerals to carry pollutants partly affects the existent state, change process, migration ability and harm of pollutants, which directly affects the evaluation of environmental quality and ecological conditions. The research on the capacity of minerals to load pollutants mainly focused on objects such as mineral dust floating in the air, sediments in water and mineral particles in soil. The direct detection of pollutants in the air, water and soil is necessary to effectively investigate and evaluate the regional environment conditions. However, the in-depth investigation of minerals are instrumental in the analysis of the formation mechanism, damage extent and controlling measures of the contaminant. When evaluating soil

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environmental capacity, it is necessary to identify the mineral components in soil, such as clay minerals, iron oxides, manganese oxides, aluminum oxides and hydroxides. Soil environmental capacity refers to the self-purification ability of pollutants, which is inseparable from the adsorption, desorption, fixation, and release of heavy metals by soil minerals. The purification capacity of specific minerals in soil reflects the purification capacity and accommodation capacity of soil itself. In fact, the content of toxic and harmful elements in soil is not the only indicator to directly determine the quality of the soil environment. The other key issue is to reveal how pollutants exist in soil minerals and the balance between pollutants and various inorganic minerals. This helps to identify the relationship between contaminants and specific minerals at the mineral level, and eventually suggest mechanisms for environmental equilibrium between contaminants and minerals in soils, thus improving the self-purification ability of the soil and protecting food chain from pollution. Some heavy metals in the soil at high concentrations are not necessarily harmful and do not affect the food chain, such as mercury in the form of cinnabar (HgS). On the contrary, those heavy metals in lower concentrations are not necessarily harmless, such as Cd in the form of ions, which may be absorbed by crops and affect the food chain. The application of minerals impacting environmental quality to the field of soil heavy metal pollution evaluation may provide new insights into the prevention and control of soil heavy metal contamination. To understand the characteristics, distribution and formation mechanisms of atmospheric mineral particles is necessary for evaluating the atmospheric environmental quality, including the dust pollution conditions. Atmospheric particulates can generally be divided into two categories: primary and secondary particulates. Primary particles include local soot, resuspended dust, exotic dust and windblown dust. Soot particles are mainly generated by coal combustion including incompletely burned carbon particles and fully burned sulfates. Windblown and resuspended dust are mostly made up of natural minerals. The new inorganic solid substances in the atmosphere are minerals formed by natural processes. In-depth and systematic research on the morphology, particle size, chemical composition, crystal structure, physicochemical properties and formation mechanism of atmospheric mineral particles as well as their relationships with organic pollutants will undoubtedly provide scientific basis and technical support for atmospheric environmental quality evaluation, especially for the prevention of the increasingly severe haze pollution. Most of the current environmental quality evaluation methods are based on the detection of pollutant content and then the determination of quantitative evaluation criteria. Mineralogical methods have not been given enough attention to the evaluation of environmental quality.

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1.1.4 Minerals Control Environmental Pollution It is necessary to recognize and utilize the environmental properties of natural minerals in order to prevent pollution and remediate the environment. It is also beneficial to develop the basic environmental properties of minerals, including surface effect, pore effect, structure effect, ion exchange effect, oxidation effect, dissolution and precipitation effect, crystallization effect, hydration effect, thermal effect, semiconducting effect, nano effect of minerals and mineral-biological composite effect. We have found that natural inorganic minerals with self-purifying function are effective in preventing pollution, which is comparable to biological methods. Based on this, a new method to treat pollution using inorganic natural minerals is proposed. This mineral method is created on the basis of making full use of the laws of nature, reflecting the characteristics of natural self-purification, and perfecting the natural self-purification system and principle constructed by inorganic minerals and organisms together. The mineral method has developed new theories and technologies of pollution control and environment remediation, becoming the fourth pollution control method after physical, chemical and biological methods. The mineral method provides theoretical guidance and technical support for the prevention of inorganic and organic pollutants. The study of accelerated purification processes with human intervention is also of great practical importance for severely polluted local environments. The water shortage in eastern China is mainly associated with concentrated population, rapidly developing economy, and water pollution. Sewage treatment and reuse are the main methods to solve the water shortage problem. Groundwater pollution is becoming more and more serious, resulting in poor drinking water quality with high levels of fluoride, arsenic, phosphorus, nitrogen, nitrate, and even organic pollutants and trace heavy metals in the water. The widely distributed and highly reserved brackish water in arid and semi-arid areas has also not been effectively managed and reasonably utilized. The treatment and improvement projects of regional surface water and groundwater are not supported by general pollution control technologies. Therefore, a low-cost geological method—natural self-purification —is necessary to achieve large-scale pollution control. The development of low-cost and efficient technologies for treating wastewater has become a urgent national need. The ability of natural minerals to purify various pollutants is the basis of the geological method. Hence, only by fully understanding the mechanism and capacity of pollutant purification from the perspective of mineralogy can we achieve effective treatment of pollutants in the water. Technically, it is possible to design correspondingly

1

Environmental Property of Minerals

effective ways to deal with any pollutant. Lowering budgets and balancing environmental protection and economic development are considered key to pollution control technologies. In particular, many small-scale enterprises in developing countries with low productivity are more interested in developing low-cost and dissolvable technologies. Nowadays, national and international pollution control methods are mainly classified into physical, chemical and biological methods, while most of these technologies are complicated in equipment and high in cost, making it difficult to be widely applied in pollution control. As for the mineral method, some of the natural minerals used are often from mining waste. Treating waste with waste is beneficial to both pollution control and waste recycling, which is of great significance to zero-emission and zero-waste principles.

1.1.5 Minerals Participate in Biological Function Interactions between minerals and organisms are widespread on the earth. In particular, the mechanism of interaction between mineral crystals and biological cells at the nanoscale is a crossover research topic between the inorganic and organic worlds. In all spheres of the earth, biological action involves in the occurrence, development and transformation of minerals, and minerals also participate in the growth, development and evolution of organisms. Animals, plants, and even singlecelled creatures commonly contain inorganic minerals. The originally distinct organisms and minerals closely linked and eventually integrated to some extent blurring the microscopic boundary between inorganic and organic processes. Therefore, research on the interaction between minerals and biological systems will broaden the scope of mineralogy. The study of mineral-organism connections not only provides scientific basis for tracing life processes in the earth system, but also provides technical support for protecting the ecology. Natural minerals are the products of natural processes in different spheres of the earth. Microbial activity influences the occurrence, development, and transformation of minerals. Moreover, micro minerals can be directly deposited on the cell wall of bacteria or dispersed throughout biofilms. This unique natural process leaves its mark on the micro morphology and microstructure of minerals, making these minerals a medium for recording life information on the earth. Such studies are becoming one of the hottest topics in the international earth sciences exploring whether there are signs of life on Mars (Friedmann et al. 2001). A study on the reaction of iron minerals induced by an environmental bacterial complex (Sherriff et al. 1998) showed that the

1.2 Natural Self-purification Function of Inorganic Mineral

bacterial complexs capable of producing biofilms can control Eh and pH in aqueous medium, thus impacting the dissolution, deposition and transformation of iron minerals such as iron oxides and hydroxides. This research focused on how organisms influence the formation and transformation of minerals, tracing life process and revealing the mechanisms on how the mineral evolutions are dominated by microorganisms. Moreover, they identified the micro morphology and microstructure characteristics of minerals, providing scientific basis and method to trace the life process on earth. Extreme environments in western China are usually defined as deserts and Gobi covering more than 1.6 million km2, or 1/6 of China’s land area. These desert and Gobi areas are featured by strong wind, windblown dust, deficient water and soil, barren grass, and widespread salinization. The ground surface has become rounded and polished by wind erosion, without any growth conditions for trees, shrubs and herbs, and the interaction between inorganic minerals and organic organisms is seriously imbalanced. The exposed minerals are mainly feldspar, quartz and various gravels with better weathering resistance. Due to the lack of hydrated minerals, clay and other minerals with good adsorption ability, organisms are especially deficient and plants are basically undeveloped. Microorganisms live in extremely harsh environments with limited mineral and biological interactions in the surface system, so the processes of material and energy exchange are extremely inadequate in the west. The ultimate improvement of the material and energy exchange process in each sphere is aimed at adjusting the imbalance of mineral-biological interactions in the surface system, increasing the adsorption capacity of minerals, improving plant growth conditions and gradually increasing the soil development degree. Iron oxides and hydroxides can effectively adhere to the surface of quartz and other particles to form iron oxide film. In fact, the research on the removal of cation and anion pollutants by iron oxide and hydroxide films has always been a hot issue in the world (Korshin et al. 1997; Ryan et al. 1999; Khaodhiar et al. 2000; Thirunavukkarasu et al. 2001). The coated iron oxides and hydroxides on mineral particles with high specific surface area, variable valence and high charge density tend to adsorb substances with opposite charge. The key problem to be solved is how to ensure the fertilizer attached to the coated film is effectively absorbed by the plants. Once the first crop of vegetation matures, it greatly facilitates the growth of the second and even the next crop, eventually creating a soil layer that truly improves the ecological environment. Therefore, it is necessary to analyze the promotion of ecological construction by surface minerals and microorganisms under extreme environment in western regions. We can focus on improving the surface adsorption and charging properties of feldspar and quartz, enhancing the

5

adhesion of surface minerals to prevent fertilizer loss, cultivating drought and alkali tolerant plant species to adapt to the arid and saline environment, and cultivating matching microbial communities to improve the absorption capacity of plant roots to nutrients attached to the mineral surfaces. In order to ultimately improve the environmental quality and build a balanced relationship between soil, plants and hydrological microclimate system under extreme conditions, a new ecological method is proposed based on a balanced mineral-organism relationship in the desert with poor fertility and strong permeability. The objectives are to develop mineral methods for water and fertilizer conservation, to provide biological techniques for selecting plant species and promoting plant growth, to develop new methods for sand fixation under extreme environments, and ultimately to provide theoretical basis and techniques support for ecological construction under extreme environments in western China.

1.2

Natural Self-purification Function of Inorganic Mineral

The environmental property of mineral treating pollution lies mainly on its self-purification function. The purification function of biological communities has been studied and widely applied in pollution control for a long time. In the last decade, we have proposed the natural self-purifying effects of inorganic minerals for pollution control that is comparable to biological methods, and have developed a new mineralogical method for pollution control and environment restoration. Natural self-purification is a potential capacity endowed by nature to enable the long-term interdependence of humans and the earth, yet this function can only be effective under certain conditions. In fact, the self-purifying effects of nature have been well known and applied in pollution control and environment recovery, such as developing and utilizing the purifying functions of organisms including microorganisms and aquatic plants. However, the purification effect of natural minerals among the inorganic world has not received adequate attention. That is to say, human beings have utilized only part of the self-purifying function ascribed by nature, which consists of organisms and inorganic minerals. In addition, too much effort has been devoted to developing non-natural pollution control methods, which are complicated and expensive, and may even result in inevitable secondary contamination. Nowadays, the concept of environmental friendliness has been implemented in every field, and we are looking for more green industries that have potentials to develop pollution control technologies. The full use of natural self-purification, including organic and inorganic methods, reflects the principle of environmental

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friendliness in the field of pollution control and environment protection. In this section, the natural self-purification function of inorganic minerals, as a fundamental environmental property of minerals, is systematically described. The theory and method of using natural minerals for effective treatment of solid, liquid and gaseous pollutants are also presented.

1.2.1 Surface Effect of Mineral The surface of minerals is usually the interface between minerals and gases, solids and liquids, and even between two solid minerals. The surface effect of mineral is closely related to the surface properties of minerals. The polar surface has a strong adsorption effect, and chemical reactions often occur within a few nanometers in thickness on the surface. The chemical properties of mineral surface are associated with the chemical composition, atomic structure, and micromorphology of the surface.

1.2.1.1 Surface Chemical Composition In general, the chemical composition of mineral surface rarely represents that of the mineral since the mineral surface will rapidly be oxidized and even carbonized once exposed to air (Hochella 1995). In aqueous medium, however, most natural mineral surfaces are hydroxylated, protonated, charged and produce Lewis acid sites or Bronsted acid sites, while salt and sulfide minerals also have salt and sulfur groups. These complex surface products are often regarded as “indefinite” substances, with a different chemical composition compared to the overall phase and can passivate the surface chemical properties. Generally, the first and second monolayer on the surface have important reactivity. The surface adsorption is essentially the direct interaction of atoms and molecules in the medium with the atoms in the outermost layer of the surface (Hoffmann 1988). However, the “indefinite” substances widely distribute on the mineral surface and influence the surface properties. Therefore, these surface substances must be dissolved before the internal chemical composition of the minerals can be exposed. Changes in the chemical composition of mineral surfaces are also associated with the adsorption of other substances and desorption of some regional components of the mineral surface. These adsorption and desorption also affect atoms at the top layer of the mineral surface and the atoms at a certain depth, depending on the degree of surface solid diffusion (Hochella 1995). 1.2.1.2 Surface Crystal Structure Like the chemical composition of mineral surface, the structure of mineral surface is often inconsistent with its bulk crystal structure, which significantly affects the mineral

1

Environmental Property of Minerals

surface effect. The defects on mineral crystal plane are developed, especially the point defects caused by the absence of cations or anions. This vacancy-type point defect has a strong adsorption effect, which is very favorable for ions with opposite charge. The mineral crushing process will cause many split planes on the minerals, which are often concave or convex. The atoms on the split planes are prone to reconstruction and relaxation, that is, after crystal is ruptured by external force and form split planes, numerous electrically unsaturated polar surfaces are generated. The unsaturated state of polar surface promotes adjustment effects, such as slight changes in atomic bond length or bond angle. The relaxation can lead to the expansion of mineral cells, that is, the mineral cell constant changes after crushing, and the surface expansion enhances surface activity. The heat treatment of minerals can also promote the expansion of mineral cells, and the quenching treatment of minerals can lead to shrinkage of mineral cells. These can be reflected in the change of mineral surface structure.

1.2.1.3 Surface Micromorphology The surface of natural minerals is often rough with complex micromorphology. Even on the nearly flat cleavage plane of minerals, there will often be steps of 2–3 nm (Eggleston and Hochella 1992), constituting a more complex micromorphological feature. The micromorphology of mineral surface mainly includes platforms, steps, twists, vacancies and adsorbed atoms or molecules. The height of the common mineral surface steps frequently can range from single to multi-layered atomic layers. The atoms at the bottom of the steps or vacancy defects have higher coordination ability, while those on the platform have lower coordination ability. The atoms at the top edge of the step and at the top of the external corner of the twist have smaller coordination numbers, while the isolated adsorbed atoms at all platform positions have the lowest coordination numbers (Hochella 1995). Most researches under vacuum or non-vacuum conditions suggested that there are larger number of suspended bonds on the low-coordination surface, thus having the best chemical activity (Christmann and Ertl 1976). The structure on the mineral surface is often reorganized due to the suspended bonds. The “excess” charge on the cleavage planes with structural reorganization can be automatically compensated, while most growth planes and cleavage planes cannot be automatically compensated and thus form polar surfaces. Polarity is determined by the number of suspended bond charges on the mineral surface, affecting the reactivity of mineral surface. The mineral crystal plane is the crystal growth plane, which can be divided into F plane, S plane and K plane according to the sequence of surface polarity. The F plane, also known as the flat plane, grows slowly according to the layer growth mechanism. The F plane expands continuously with the

1.2 Natural Self-purification Function of Inorganic Mineral

growth of the crystal and appears on the final crystal. The S plane, also known as the step plane, is composed of the steps of the adjacent F plane growth layers. It grows faster than the F plane, and belongs to the nonsignificant crystal planes with small expansion. The K surface is rough and fastestgrowing, and tends to disappear during crystal growth. Mineral split plane is a new surface formed by sudden bond breaking through external force. In order to reduce the system energy, the surface atoms will move to a new stable equilibrium position by reconstruction and relaxation, and the coordination and charge distribution near the atom will change. There are often many suspended bonds on the split plane of mineral crystals, which enhances the surface reaction activity. Obviously, the micro-morphology characteristics of mineral surface largely affect its surface activity. In general, the activity of mineral split plane is greater than that of mineral cleavage plane and crystal plane. Of course, the mineral crystal plane has different crystallization direction and different surface activity.

1.2.1.4 Surface Charge The ideal mineral crystal surface is electrically neutral because the number of anions and cations on the crystal surface is balanced. However, there is unequal mass isomorphism in the actual mineral crystal structure, which can directly lead to the charge imbalance on the mineral surface. In addition, the change of chemical composition on the mineral surface, especially hydroxylation and protonation in the aqueous medium, makes the charged property of mineral surface change. That is to say, the surface chemical composition, surface crystal structure and surface micromorphology of minerals can be comprehensively reflected in the surface electrification of minerals. The pH value of the medium where the mineral is located is a key factor affecting the surface electrification of the mineral. In general, the zero charge point (pHzpc) represents the charged characteristics of mineral surfaces. When the mineral surface is electrically neutral, the pH value of the medium is the zero charge point of the mineral. Zero charge point of minerals can be measured by ZPC determinator or conventional titration. When the pH value is lower than pHzpc, the mineral surface has positive charge and can effectively adsorb anions; when the pH value is higher than pHzpc, the mineral surface is negatively charged and can effectively adsorb cations. Obviously, knowing the zero charge point properties of minerals can help us understand the adsorption properties of mineral surfaces and effectively utilize the adsorption effect of mineral surfaces. 1.2.1.5 Type of Adsorption The key to the mineral surface effect is the adsorption ability of the mineral surface. The adsorption of mineral surface can theoretically be divided into five basic types: non-bonding

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electrostatic adsorption, bonding monodentate adsorption and bidentate adsorption, precipitation adsorption, coprecipitation adsorption and solid solution diffusion adsorption (Hochella and White 1990). Taking the adsorption cation on the mineral surface as an example, electrostatic adsorption refers to the surface adsorption of a single cation with negative charge and hydrogen bond; monodentate adsorption and bidentate adsorption refer to the formation of ionic bond between a cation and one or two anions on the mineral surface, respectively; precipitation adsorption refers to the multi-layer adsorption with complete bonding between multiple same cations and mineral surface; coprecipitation adsorption refers to the multi-layer adsorption with multiple cations and mineral surface. The surface is fully bonded by multilayer adsorption, at least one of which is the same as that of the mineral composition; the diffusion adsorption of solid solution refers to the adsorption cation entering the crystal by diffusion, occupying the cation position in the lattice. The adsorption strength is different among the five adsorption types, which can be divided into non-bonding adsorption with lower adsorption strength and bonding adsorption with higher adsorption strength. In the bonding adsorption, the fixation degree of mineral surface on adsorbate is high, and the adsorbate is not easy to desorption from the mineral surface; in the absence of bonding adsorption, the adsorbate can be desorption from the mineral surface, and the adsorption and desorption reach a balance. It is important to distinguish different types of adsorptions in practical application. For example, bonding adsorption can effectively remove heavy metals in soil pollution control. In water pollution control, especially when recovering valuable metals, non-bonding adsorption is expected, because valuable metals can be removed. Therefore, it is often necessary to choose whether to use adsorption or desorption, and it is important to judge the type of bonding adsorption and non-bonding adsorption in application. A simple method is to add NaNO3 with different concentration as interfering agent in the mineral adsorption experiment system. The adsorption effect does not change with NaNO3 concentrations. There are more innovative researches on mineral surface adsorption with improved experimental technology and theoretical basis. The study of mineral surface adsorption promotes the development of mineral surface science, making it one of the most challenging research fields in earth science and environmental science. The interaction between minerals and their surrounding environment occurs at their interface, where within the scope of several layers of atoms. Therefore, the understanding of the basic process occurring on the mineral surface at atomic scale can also promote the deep understanding of the interaction in the global material circulation. It is possible to understand and predict global processes through direct observation and simulation of reaction mechanism at atomic scale.

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1.2.2 Channel Effect of Mineral Channel effect of minerals includes the molecular sieve effect, ion sieve effect and ion exchange effect in the structural pores of minerals. Mineral channels refer to the connected spaces composed of coordination polyhedrons in the crystals, which are different from cracks and pores generated when crystals are broken or stacked. The channel mineral with the largest channel size in nature is cacoxenite, and the channel size along the [0001] direction is 1.42 nm (Moore and Shen 1983). The pegmatite formed bulk ferrisicklerite in the early stage, fine-grained ferrisicklerite in the later stage, irregular shaped sicklerite by weathering, and finally formed colloidal cacoxenite in appropriate geochemical environments. The single crystal of cacoxenite is acicular, and the aggregates are radial, fibrous, shell-like or spherical. Cacoxenite is the weathering product of phosphate minerals in pegmatite, which can be prepared by hydrothermal synthesis under normal temperature and pressure. According to the pore size, porous materials are divided into three categories: microporous materials (pore size < 2.0 nm), mesoporous materials (pore size between 2.0 nm and 50 nm) and macroporous materials (pore size > 50 nm). The channel size of minerals is often less than 1.0 nm, belonging in the category of microporous materials. Zeolites used as molecular sieves are typical channel minerals. The channel size of most zeolites is 0.23– 0.52 nm, while only octahedral zeolite has a larger channel size of 0.74 nm. The channels of most natural minerals are called ultra-micro channels with a size of about 0.3 nm. In the past, attention was paid to microporous molecular sieves above 0.3 nm, while the ultra-micro channels were neglected. In general, minerals with alkali and alkaline earth metals such as K, Na, Ca and Ba in the crystal chemical formula tend to have ultra-micro channels. Since inorganic ions are less than 0.3 nm in diameter, minerals in nature with ultra-micro channels less than 0.3 nm in size are supposed to have ion sieve effect. Natural zeolite crystals with typical cage structure have one-dimensional, two-dimensional, and three-dimensional channels. Different zeolites are distinguished by the shape and size of the cage and the channels. The Na and Ca in the channels are in weak contact with the Si–Al skeleton, which can be easily replaced by other cations without breaking the lattice. This characteristic is favorable for removing toxic or harmful substances such as radioactive elements, heavy metal ions, ammonia, nitrogen and phosphate in wastewater. When the water in zeolite is removed during heating, the residual charge in the zeolite pores can adsorb external polar gas molecules, such as NH3, CO2, H2S and SO2. Only molecules with smaller diameter can enter the channel and

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Environmental Property of Minerals

be adsorbed, while those with larger diameter are excluded from the pore, so zeolite has molecular sieve effect. The channel size of natural cryptomelane is 0.46 nm, which is close to that of zeolite. The Mn–O octahedron is the basic structure of cryptomelane, with channels filled by alkali metals. This type of channels also has good molecule sieve effect, ion sieve effect and ion exchange effect like the well-known channels constructed by Si–O and Al–O tetrahedrons. The currently synthesized cryptomelane and todorokite were both constructed by [MnO6] octahedral chains and have channel effects, which are collectively called oxide octahedral molecular sieve (OMS). For example, the cryptomelane with 2  2 channels is called OMS-2, and the todorokite with 3  3 channels and a channel size of 0.69 nm are called OMS-1 (Vicat et al. 1986; Yin et al. 1994; Luo et al. 2000). The channel effect of cryptomelane is that ions or molecules larger than 0.46 nm in diameter are excluded from the channel, while the smaller can enter, suggesting the ion sieve or molecular sieve effect of cryptomelane. Channeling effects can be found in most minerals, including feldspar. The [SiO4] tetrahedron in feldspar can form channel structure by bridging in three-dimensional space. The channel of K-feldspar is 0.55 nm in length and 0.39 nm in width, which allow at least H2O molecules to enter and pass. The sericitization occurred in the middle stage of hydrothermal alteration of K-feldspar crystals (the reaction formula is 3K[AlSi3O8] + H2O = KAl2[AlSi3O10] (OH)2 + 6SiO2 + K2O). The small molecule water and healthy trace elements can form mineral water through feldspar mineral channels in fractured granite rocks. The micropores of feldspar are excellent molecular sieves, preventing the passage of bacteria (0.5–5.0 lm in diameter) and viruses (0.2–25.0 lm in diameter). The internationally accepted method of separating highly radioactive nuclear waste is landfilling, of which stopping the migration of radionuclides is the key. Tuff and granite were selected by most countries as the surrounding rock for recycling radioactive waste. Since water molecules and alkali metal ions can enter the feldspar channels, the more reactive radionuclides are certainly more accessible to the channels in comparison. Feldspar in tuff and granite serve natural barriers for having good porous structure, which absorb nuclides and effectively block their migration. The interlayers in clay minerals also have channel structures. The ion exchanges and thermal expansions occur in the montmorillonite and vermiculite layers, respectively. The channels parallel to fibers in sepiolite and palygorskite account for more than 1/2 of the fiber volume. Technically, the chrysotile and halloysite with special tubular structure and the diatomite with porous structure are not channel minerals. However, they have filtering effects on pathogenic

1.2 Natural Self-purification Function of Inorganic Mineral

bacteria, protozoa, and worms with large individual size. Even the organic pollutants, ammonia, nitrogen and oil in water can be removed by the filtering effect of these minerals. The pores between natural mineral particles also function as filters. Minerals as filtering materials should have strong mechanical strength (so as not to be broken due to the friction between particles in the washing process), chemical stability (so as not to cause water quality deterioration due to dissolution in the filtering process), large specific surface area, spherical appearance, rough surface, angularity, and certain particle size gradation. Till now, the widely used mineral filters include refined anthracite, refined quartz sand, bauxite ceramsite, and magnetite and pyrolusite. In the filtration process, the filtration materials mainly carries suspended solids and flocs in the water to achieve purification. Combined with surface adsorption, composite mineral adsorption and filtration materials can be prepared, such as a new adsorption and filtration material (Edwards and Benjamin 1989) prepared by fixing iron hydroxide on the surface of ordinary quartz sand, which not only functions as an ordinary filtration material, but also can effectively adsorb and remove heavy metals.

1.2.3 Structure Effect of Mineral The physical and chemical properties of minerals are determined by crystal structure and chemical composition. Although the minor change of crystal structure does not change the mineral species, it may induce great changes in mineral properties. Like the chemical composition, the atomic structure and electronic properties on the mineral surface are usually quite different from those of its interior. Theoretically, the surface structure of some metals can be inferred from their crystal structure. However, the surface characteristics are complex and are often reconstructed in order to achieve the lowest energy. In fact, the exposed mineral surface in unsaturated state will spontaneously adjust its structure. The surface will adsorb materials in the absence of any adsorbate on it, while it will readjust its structure in different ways after adsorbing molecules. The reconstruction degree of different crystal planes is also different (Hoffmann 1988). The atoms on the surface sometimes involve several layers below the surface, and their positions in the structure are different from those in the equilibrium state. These structural differences can be weak or significant. The surface atomic structure, especially the low symmetry structure, may relax after simple rupture on the exposed surface. In most cases, the relaxation is perpendicular to the surface, and the atomic distance between the first layer and the second layer can be reduced by 15%. By eliminating the free-swinging bonds, these surface layers

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will expand and even exceed the original state based on the surface adsorption. Another problem that is often neglected is the structural effect of adsorbate on mineral surface. Usually, the atoms on the mineral surface closest to the adsorbate will shift in space in order to better match the adsorbate structure. This situation often occurs when there is a strong interaction between the adsorbate and surface, that is, when the adsorbate and the surface have strong chemical activity and strong bond formation (Hochella 1995). The defects and dislocations in internal mineral structure significantly affect the characteristics of minerals, and often increase the activity of mineral surface. Therefore, the study of mineral structure modification has become an important pathway to improve the activity of minerals. Surface structural disorder is defined as the disorder of perfect order on the surface, which affects a variety of surface kinetic and thermodynamic processes, surface electronic properties and surface chemical effects. The kinetic and thermodynamic processes include crystal growth, transfer, phase transition and defect formation. Electronic properties can be affected by point defects or extended defects. Surface chemical reactions are affected by defect positions with different energies and kinetic factors. Research on atomic-level details of surface structure can reveal problems such as the relationship between charge transfer, energy level shift, local electronic structure, and chemical reactivity. In turn, the local electronic properties of adsorbed molecules also have a certain impact on the surface structure of minerals (Hoffmann 1988). The chemical composition of minerals changes during oxidation processes, which also leads to structural defects. For example, compared with hexagonal pyrrhotite, monoclinic pyrrhotite has higher removal efficiency of Cr(VI), indicating that monoclinic pyrrhotite has stronger reactivity. This is related to the structural defects caused by deficiency of Fe in monoclinic pyrrhotite (Fe1−xS), because this defect is the active site of chemical reaction. Theoretically, there is no Fe vacancy in hexagonal pyrrhotite (FeS), and the crystal structure is relatively complete, which partly reduces its chemical reaction activity. The removal efficiency of Cr(VI) by hexagonal pyrrhotite has increased after exposed in the atmosphere for a long time, and it may be related to the oxidation of the surface of hexagonal pyrrhotite. Due to the formation of magnetite on the surface and facture of hexagonal pyrrhotite (Ribbe 1974), Fe vacancies occur near the surface of hexagonal pyrrhotite: 3FeS + 2xO2 = 3Fe1 −xS + xFe3O4. The chemical activities at the surface and fracture of hexagonal pyrrhotite with structural defects are also improved. There are various kinds of slight changes in the crystal structure of minerals, including the development of structural defects on the mineral surface, the adjustment and reconstruction of the unsaturated state of mineral polar

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surface, the relaxation effect of minerals with the enhancement of surface activity, the expansion and contraction of the unit cell, such as crushing or heating resulting in volume increase, quenching leading to reduction, etc., and the thermal expansion of layered structure minerals such as vermiculite (Lu et al. 2003). These structural changes directly lead to obvious changes in the physical and chemical features of minerals. The structural transformation from hexagonal pyrrhotite with weak activity to monoclinic pyrrhotite with strong activity, polymorphic transformation from rutile, anatase to brookite with increasingly unstable properties, informing that the significant changes of mineral structure can lead to the change of mineral species, and eventually enhance their activity and reactivity.

1.2.4 Ion Exchange Effect of Mineral Ion exchange is common in natural minerals. Exchanged ions often have similar properties, valence, and radius. Ions exchanged actively with minerals generally have stronger binding energy than those exchanged passively. Some active alkali and alkaline earth metals in minerals, such as K, Na, Ca ions, are easy to exchange with transition metal ions. Most heavy metals and anionic pollutants exist in the form of cations and anions or anionic clusters. These cations and anions in environmental medium can be exchanged with similar cations or anions in minerals and fixed, leading to good ion exchange effects in minerals. Mineral ion exchange mainly occurs at different crystal structures, such as the surface of ionic lattice minerals, pore structure of mineral channels and interlayer of layered structure minerals.

1.2.4.1 Ion Exchange on the Surface of Ionic Lattice Mineral Calcite and aragonite, two typical ionic lattice minerals are polymorph of CaCO3, and the Ca2+ on its surface can be equivalently exchanged with cations such as Mn2+, Zn2+, Pb2+, Cd2+, Hg2+ in aqueous medium. Among them, Pb2+ has strong reactivity with calcite and aragonite, while Mn2+ and Cd2+ only have strong reactivity with aragonite, and no reactivity with calcite. They are fixed on the surface of carbonate minerals, such as lead carbonate, manganese carbonate, and cadmium carbonate (Abe et al. 1991). Apatite can exchange Ca2+ on its surface lattice under normal temperature and pressure with cations in solution, such as Pb2+, Cd2+, Hg2+, Zn2+, Mn2+, and the sequence of removal is Pb2 + > Cd2+  Zn2+ > Mn2+ > Hg2+ (Abe et al. 1991). Natural apatite mainly refers to fluorapatite and chlorapatite, and apatite in animal bones are hydroxyapatite, including carbon hydroxyapatite containing CO32−. Hydroxyapatite with poor stability has better ion exchange performance than fluorapatite and chlorapatite, which are more stable. Obviously, the

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Environmental Property of Minerals

removal effect of calcium carbonate minerals and apatite on heavy metal pollutants is mainly manifested as cation exchange effect of surface lattice, which is related with the good quality of groundwater in carbonate area. CO32− in carbonate also has good anion exchange function. For example, when calcium carbonate is used to neutralize waste sulfuric acid, SO42−can completely exchange CO32− in calcite to form gypsum (CaSO4
2H2O). Volume is more than doubled during the transformation from calcite to gypsum. CrO42− can also partially exchange CO32− in calcite to form Ca(CO3, CrO4) (Tang et al. 2007).

1.2.4.2 Ion Exchange in Channel Mineral The charge compensation from alkali metal or alkaline earth metal ions is often required when there is unequal isomorphism substitution in the structural polyhedrons of channel minerals. The cations compensating negative charges have a relatively large coordination number and are often located in the channels of lattice frame of mineral polyhedron. The binding force between the cation and lattice frame is weak, and it is easy to exchange with other ions, so it has cation exchange properties. It is well known that ion exchange of Na and Ca in zeolite channels can remove pollutants and has been widely applied. Zeolite has molecular sieve effect due to its fixed narrow channels. The change of cation type is one of the modifications of natural zeolite to improve ion exchange performance. Natural zeolites (e.g., clinoptilolite and mordenite) have high ion exchange selectivity for some cations, and the sequence of exchange selection is related to the hydration radius of ions. Ions with small hydration radius easily enter the zeolite framework for ion exchange, with strong exchange capacity. When Mn4+ is partially replaced by Mn3+ in cryptomelane, the negative charge will appear in the structural unit, requiring alkali metal cations like K+ to be arranged in the channels to balance the electricity valence. And the maximum exchange capacity is related to the number of octa-coordination Mn3+ in the crystal. If the number of cations that compensate the negative charge exceeds the equivalent of Mn3+, anions such as OH− will appear in the structure to compensate the excessive positive charge. These anions also have considerable mobility and anion exchange properties. 1.2.4.3 Ion Exchange in Layered Mineral Due to the substitution of Al3+ for Si4+ and the unequal substitution between divalent and trivalent cations in the octahedral layer, numerous changes take place in the structural unit layer in silicate minerals. The interlayer of most clay minerals contains K+, Na+, Ca2+, Ba2+ and other alkali and alkaline earth metal ions, and ion exchange is relatively active. The dispersion degree of clays in solution affects the kinetics of ion exchange and is closely related to the degree of isomorphism. For example, the isomorphism of

1.2 Natural Self-purification Function of Inorganic Mineral

montmorillonite can strengthen the connection between structural unit layers and make it difficult to disperse. However, after modification by immersing in electrolyte solution, such as converting Ca2+ based montmorillonite into Na+ based montmorillonite, the interlayer binding force of montmorillonite decreases. Thus, the interlayers become hydrophilic and easy to disperse and expand, which makes cations easy to diffuse into the interlayer, further greatly improving the ion exchange rate. When Al3+ substitutes for Si4+ in the tetrahedral layer of illite, the K+ compensating for the negative charge in the interlayer leads to the strengthening of the contact force of structural unit layer and the restriction of its ion exchange effect. Kaolinite is the simplest clay mineral with 1:1 type structure. The isomorphism of cations is unusual, but the OH− group is distributed at the edge of the layer, which can promote anion exchange. Due to the contact force between layers is very weak, the kaolinite is easy to disperse in water, thus determining the high ion exchange rate. Natural clay minerals are hydrophilic and have good purification function for inorganic pollutants. Moreover, by using organic modifiers to replace the exchangeable inorganic cations inside, the oleophilic and hydrophobic clay minerals can be synthesized to improve the purification capacity of hydrophobic organic pollutants.

1.2.5 Redox Effect of Mineral Oxidation and reduction reactions always occur together in nature. A redox reaction occurs only when there is a certain redox potential difference. It is essentially the process of electron transfer that affect some fundamental kinetic processes. Microscopically, the redox reaction is manifested as changes in the valence of elements, and macroscopically, they govern the evolution of the earth’s matter and the environment change. Minerals containing variable valence elements have redox effect. Minerals with higher valence elements are oxidizing and act as oxidants, while minerals with lower valence elements serve as reductants. S, Fe and Mn are common variable valence elements in nature. Minerals composed of variable valence elements are often some of the more unstable metallic minerals. Natural sulfide minerals are characterized by containing S2 − or S22−. When S2− or S22− loses electrons and are oxidized to 0 2+ 4+ S , S , S or S6+ in sulfide minerals, some high valent cations or organic compounds can be effectively reduced by them. Iron sulfide minerals, such as pyrrhotite (FeS) and pyrite (FeS2), can effectively reduce Cr6+ to Cr3+ and achieve the harmless treatment of heavy metal pollutant Cr6+. This is due to the multiple and large redox potential differences between the iron sulfide minerals and Cr6+, such as S/S2−–Cr6+/Cr3+, S/S22−–Cr6+/Cr3+, and Fe3+/Fe2+–Cr6+/Cr3+. Therefore, under

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certain conditions, S2−, S22− and Fe2+ can be effectively oxidized and Cr6+ can be effectively reduced. Natural manganese oxides contain mainly Mn4+ and a small amount of Mn3+. The oxidizing properties of Mn4+ and Mn3+ in manganese oxide minerals is not as good as that of Mn7+ in potassium permanganate, but they also possess considerable oxidizing ability and are ideal natural oxidants. The strong oxidizing ability of natural cryptomelane (KxMn8 4+ and Mn3+. −xO16) is related to its variable valence ions, Mn Cryptomelane can completely oxidize organic pollutants, phenol, to CO2 and H2O, and the oxidative decolorization rate of more than 10 kinds of organic printing and dyeing wastewater common in industry can reach more than 99% in a short time. It can be used to develop a new method for low-cost treatment of high-concentration, strongly polluting and intractable industrial organic wastewater containing unit phenol and polyols (Wei 2005).

1.2.6 Precipitation/Dissolution Effect of Mineral The dissolution effect refers to the dispersion and re-complexation of solutes in solvents, including the dispersion of solute molecules and ions as well as the recombination or complexation between solvent and solute molecules. The empirical theory of similar miscibility shows that the more similar the structure is, the easier the matter dissolves. According to the solubility product theory, the larger the Ksp value is, the easier it is to be dissolved; the smaller the Ksp value is, the harder it is to be dissolved. For example, NaCl crystal is soluble, while sulfide minerals are insoluble. As for insoluble compounds, the concentration of anions or cations decreases due to some chemical reactions. For example, the insoluble compound will continue to dissolve when there are redox reactions or reactions with the formation of less insoluble matter. In fact, “absolutely insoluble matter” does not exist. At room temperature, the solubility product (Ksp) of pyrrhotite (FeS) is 1.59  10–19, and that of nickel sulfide (NiS) is 1.07  10–21, which are both insoluble compounds. The reaction, FeS + Ni = NiS + Fe, can occur according to the relationship of solubility product of FeS and NiS. This is because in the FeS dissolution equilibrium, S ions tend to combine with Ni ions to form NiS crystals with smaller Ksp, which makes the insoluble FeS dissolution equilibrium change and thus dissolution and precipitation transformation occurs. Of course, this kind of dissolution is largely affected by the concentration of Ni ions, which can be called slight dissolution. Similarly, FeS can completely react with Co, Zn, Pb, Cu, Cd and Hg to form CoS, ZnS, PbS, CuS, CdS and HgS crystals with smaller Ksp. The slightly soluble metal sulfide minerals are often unstable in nature, and they contain variable valence

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elements, which are easy to be oxidized and decomposed, showing certain solubility in some aqueous conditions. The natural iron sulfides have positive effects in treating toxic wastewater containing Cr6+, Pb2+, Cd2+, Hg2+. The effect is determined by the slight dissolution of minerals (Fe2+, S2−, S22−) under certain conditions, and is the reflection of redox reaction (S/S2− and Cr6+/Cr3+, S/S22− and Cr6+/Cr3+, Fe3+/ Fe2+ and Cr6+/Cr3+) and precipitation transformation (S2− and Pb2+, S2− and Cd2+, S2− and Hg2+, S2− and Cr3+). The insoluble compounds such as PbS, CdS and HgS can be formed in the treated products and can be recovered. As for mineral crystals, when the crystal faces with different reticular density at different crystal orientations dissolve, the crystal faces with larger reticular density dissolve first, which is contrary to the crystal growth. The site with defects of mineral crystals is prone to dissolution, because the dislocation center releases energy with bonds breaking and dissolution. Minerals dissolve at the edge of crystal in unsaturated solution. These basic principles are also applicable to the slight dissolution of minerals. Mineral dissolution is also a process of constant consumption. When treating heavy metal pollutants using the mineral dissolution effect, there is no saturation limitation as the mineral adsorption process.

1.2.7 Crystallization Effect of Mineral The cations and anions in some ionic lattice minerals are toxic and harmful. In the process of mineral formation, especially during crystallization in solution, these cations and anions can be fixed to achieve pollution purification. For example, minerals containing Hg and Cr, Hg4HgCrO6 (Roberts et al. 1991) and Hg2Hg3CrO5S2 (Roberts et al. 1993), which are formed in the metal pit dump, can directly fix Hg and Cr. Industrial wastewater containing fluorine can be effectively treated by the crystallization of fluorite (CaF2). When treating wastewater containing Cr(VI) with natural iron sulfides, the excess S2− can react with Cr(III), the reduction product of Cr(VI), to form sulfochromite (Cr2S3). This is a new one-step process for chromium removal by reduction and precipitation. The formation of colloidal minerals containing nitrogen and phosphorus which are nodular, grapefruit, crusty, membranous and bell shaped aggregates on the surface, such as KMg[PO4]⋅6H2O, NH4Mg[PO4]⋅6H2O, Ca5[PO4]3(F,Cl), CaAl3(OH)6[HPO4] [PO4] and CaFe4[PO4,SO4]2(OH)8⋅nH2O, can provide new ideas for developing treatment methods for nitrogen and phosphorus wastewater. Adding magnesium salt to the polluted water can promote the crystallization of struvite (NH4Mg[PO4]⋅6H2O), which is an effective method for treating industrial wastewater rich in nitrogen and phosphorus and even poultry manure pollutants. K-type struvite

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Environmental Property of Minerals

(KMg[PO4]⋅6H2O) can be formed from denitrified cow dung. If magnesium sulfate is added, the volatile ammonia can be recycled to form (NH4)2[SO4]⋅Mg[SO4]⋅6H2O (Vriend 2001). Metal sulfide minerals with variable valence elements are often extremely unstable in nature, and they often cause acid mine drainage (AMD) pollution. The key to prevent AMD pollution is to effectively prevent these metal sulfide minerals from oxidization and decomposition. Since Fe3+, SO42− and H2O are main chemical components of AMD, spraying KOH solution on the waste rock piles of metal mines can form jarosite KFe3[SO4]2(OH)6 at normal temperature and pressure and within a large concentration range. Jarosite can maintain the characteristics of colloidal minerals under certain humidity, which can serve as the isolation layer of waste rock heap in polymetallic mines to isolate the atmosphere, to prevent the oxidation and decomposition of metal sulfide minerals in the AMD. According to the definition that minerals are formed in natural processes, solid ice obviously belongs to the category of minerals. Using the formation process of ice, water can be skillfully separated from wastewater, leaving heavy metal ions pollutants, which is the reverse process of separating heavy metal ions from water in normal water treatment. More importantly, the density of solid ice is smaller than that of liquid water, so the ice can float on water rather than settling underwater. It is this wonderful property of ice that aquatic organisms in freshwaters can be protected from freezing instead of being crushed by solid ice when winter comes in cold regions. What wonderful ecological protection function of inorganic mineral crystallization in nature!

1.2.8 Hydration Effect of Mineral The water in minerals includes structure water, crystal water, interlayer water, zeolite water and adsorbed water. The structure water usually exists in the form of OH−, occupying a fixed position in the mineral lattice. It has a specific content ratio of the chemical composition, with a high escape temperature (about 600–1000 °C). Since the radius of OH− is 0.133 nm which is larger than cations, the escape of OH− can lead to complete destruction and recombination of the crystal structure. Crystal water often occurs as neutral water molecules in oxysalt minerals containing large radius complex anions. Sometimes it forms hydrated cations of larger radius coordinated with cations of smaller radius. Crystal water also has a fixed position in the lattice, and its quantity is in simple proportion to the mineral composition. The escape temperature is usually 100–200 °C and does not exceed 600 °C. The loss of crystal water also causes the destruction and recombination of structure. Interlayer water, zeolite water and adsorbed water, which exist as neutral

1.2 Natural Self-purification Function of Inorganic Mineral

water molecules, are quite different. Their positions and quantities are greatly affected by ambient temperature and humidity and are often not fixed. They can escape at about 100 °C, and the escape temperature of zeolite water does not exceed 400 °C. Minerals that lose interlayer water, zeolite water and adsorbed water can reabsorb water, which means that these water molecules in minerals can exit or enter without causing changes in the structure. When minerals in the lithosphere are affected by hydrosphere, they are directly manifested as mineral hydration, sometimes as hydrolysis. Hydrolysis leads to the formation of minerals containing structure water. Numerous clay minerals are the products of hydrolysis of aluminosilicate minerals, accompanied by the precipitation of hydroxyl, which has a great influence on the environmental pH. Hydration leads to the formation of minerals containing crystal water, interlayer water, zeolite water and adsorbed water. The hydration of crystal water is often accompanied with the increase of mineral volume. For example, the anhydrite can expand by 30% in volume when hydration occurs to form gypsum. The water swelling of clay minerals such as montmorillonite has a significant influence on engineering foundation. The minerals with higher crystal water content in the inorganic world are ettringite (Ca6Al2[SO4]3(OH)12⋅24H2O) and challantite (Fe2+12Fe3 + 2[SO4]18O2⋅63H2O). Minerals containing interlayer water, zeolite water and adsorbed water have comparable effect with plants on regulating environmental water function, which are the best inorganic humidity-controlling and temperature-regulating substances in nature. The self-control mineral materials with function of absorbing and releasing water, also known as intelligent humidity control material or self-discipline humidity control materials. They can adjust their own water content according to the environmental humidity changes. Absorbed water in minerals can not only change atmospheric humidity but also affect the soil moisture. Hydration effect of minerals has great significance in the field of ecological construction.

1.2.9 Thermal Effect of Mineral The thermal effect of minerals mainly includes thermal stability and thermal instability, which are characterized by thermal expansion and thermal decomposition, respectively. The thermal effect plays an important role in coal desulfurization and dust removal projects, such as the thermal expansion of vermiculite and the thermal decomposition of calcite, which have unique purification functions for the prevention and control of coal dust-type air pollution. Dust pollutants mainly refer to SO2 and incompletely burned C particles. Calcite as sulfur fixation agent is often added in the process of pulverized coal forming, so that the

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SO2 produced during coal combustion react with CaO, the thermal decomposition product of calcite, to form CaSO4 fixed in the slag to reduce SO2 emission. In fact, due to the local reducing atmosphere with high concentration of C and CO within the burning coal, CaSO4 is easy to be reduced and decomposed, releasing SO2 again and affecting the sulfur fixation rate. Obviously, reducing the concentration of C and CO can ensure the sulfur fixation rate, which also means improving the conversion rate of C and CO to CO2, reflecting the full combustion of coal and reduction of C dust. The thermal expansion of natural vermiculite can enhance the oxidation atmosphere during coal combustion, preventing the decomposition of CaSO4 and increasing the sulfur fixation rate. The layered structure of vermiculite gives it a new thermal effect of dehydration and expansion at high temperature, and the resulting vapor pressure can make the interlayer region expand by 30–40 times. Adding a small amount of vermiculite can form a loose structure inside the briquette, which becomes a channel for oxygen input during combustion, thus reducing the excessive C and CO concentration inside the briquette, and effectively preventing the reductive decomposition of the formed CaSO4. This process can greatly improve the conversion rate of C and CO to CO2 and the combustion rate and sulfur fixation rate of coal, and reduce the C ash formed due to the incomplete combustion of coal, and significantly improve the sulfur fixation and dust removal level of the briquette (Lu et al. 2003). The coal production and consumption in China ranks first in the world, and coal pollution has become the main source of pollution in the atmosphere. Among them, SO2 and carbon dust pollutions are particularly serious. The civilian stoves are wildly and densely used in winter, causing a large amount of insufficient coal combustion. These coal combustions induce a large amount of carbon dust which is difficult for unified management and centralized treatment. In addition, the low-altitude emission of flue gas has a great impact on the ground concentration of SO2 and carbon dust, which are often diffused in the human’s respiratory zone, posing a great threat to human health. The development and utilization of mineral thermal effect has broad prospects in the prevention and control of dust-type pollutants generated during briquette combustion.

1.2.9.1 Photocatalytic Effect of Mineral Most oxide and sulfide minerals belong to semiconductor minerals, and their semiconductor effect is mainly manifested as photocatalytic oxidation or reduction. Semiconducting minerals generate photoelectron-hole pairs when exposed to light radiation. The separated electrons or holes can often be used under certain conditions, with photoelectrons having a reducing effect and holes having an oxidizing effect. In semiconducting redox system, the dissolved oxygen and water react with photoelectrons and holes in

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heterogeneous photocatalysis, generating free radical ⋅OH with high activity, which can oxidize and degrade a variety of organic pollutants, including those difficult to be degraded by organisms. The band gap of most metal oxide minerals is larger than 1.5 eV, and the wavelength range for photoelectron generation is 249–777 nm, mainly visible light with a small portion of ultraviolet light. The band gap of most metal sulfide minerals is less than 1.5 eV, and the maximum wavelength of photoelectron generation exceeds 921 nm, mainly infrared light and a small portion of visible light with the wavelength range of 345–740 nm. Obviously, semiconducting minerals with photocatalysis in the inorganic world can produce photoelectrons and holes under visible light. The generated photoelectrons can be used to reduce high-valent toxic inorganic pollutants, while the holes can oxidize toxic organic pollutants which are difficult to degrade. The impurities, lattice defects, heating characteristics and ultrafine grinding effect of natural minerals contribute to increasing the range of photo-responsiveness and enhancing the photocatalytic activity. For example, Ti in natural vanadium-bearing rutile is partially replaced by impurities such as V, Fe, Cu and Zn, causing lattice distortion and defects, and thus exhibiting good photocatalytic properties. The results suggest that the absorption in visible light range of rutile treated by grinding, heating, and quenching is greatly increased, and the degradation efficiency of halogenated hydrocarbons is significantly improved. Semiconducting photocatalysis of natural metal oxide and sulfide minerals play a positive role in the interaction of multiple spheres on the earth surface. This role is not only closely linked to the evolution of the inorganic materials, but may also be related to the early origin of life on the earth. For example, reduction reactions of CO2, H+ and N2 to HCOO−, H2, and NH4+ respectively can take place under the catalysis of semiconducting minerals. Additionally, reducing CO2 to formic acid is considered as the first step in the non-biological pathway for synthesis of organic molecules (Schoonen et al. 1998). In the past, it is believed that everything comes from sunlight (“everything” mentioned here mainly refers to organisms in the organic world). At present, however, it is fully reasonable to consider that the visible light catalysis also plays a key role in the occurrence, development and transformation of inorganic minerals, especially metal oxide and sulfide minerals. Systematic research on the photocatalysis of inorganic minerals plays an irreplaceable role in revealing the life process and environmental evolution of the earth. At present, the prevention and control of organic pollutants in the earth surface system, especially persistent organic pollutants that cannot be degraded by themselves, is becoming more and more urgent. The degradation of organic matter is essentially a process of oxidation or reduction decomposition, and the photocatalytic

1

Environmental Property of Minerals

oxidation and reduction of inorganic minerals is crucial. Therefore, it is necessary to fully understand and utilize the photocatalytic properties of metal oxides and sulfide semiconducting minerals in the earth surface system.

1.2.9.2 Nano Effect of Mineral The nano effect of materials refers to the special surface effect, small size effect and macroscopic quantum tunneling effect when the particle size reaches the nanometer scale, thus performing many physical and chemical properties different from that of macro materials. The nano effect of minerals is determined by its size. The mineralogical characteristics of nano crystals are different from those of general macro crystals. Natural nano minerals are mainly cryptocrystalline and colloidal minerals, and typical representative minerals are limonite, bauxite and psilomelane. The aggregates are stalactitic, reniform, botryoidal, oolitic, nodular, crustaceous and gelatinous. In the past, due to the limited analytical techniques, it is difficult to separate and identify these nanoscale colloidal mixtures. Only the main components can be determined by thermal analysis method, while the associated components with content less than 10% cannot be measured. The research on determination of chemical composition of colloidal minerals is extremely imperfect due to their complexity and strong adsorption properties. Hence, tracing and utilizing the composition of colloidal minerals are extremely difficult. With improved mineralogical observation methods, the understanding of these minerals is deepening. For example, the single crystal of nano minerals developed in cryptocrystalline aggregates can be observed by high resolution transmission electron microscopy. The nano minerals are often elongated needle-like, with cross-section diameter of nanometer scale and length up to micron level, such as goethite contained in limonite. It is worth noting that the nest-like aggregates composed of nano cryptomelane crystals in psilomelane develop channels with a size of 4–7 nm. Small water molecules, heavy metal complexes, and even small organic molecules can freely shuttle through these nanopores, and react with nano cryptomelane. That is, the nano effect of natural minerals does not necessarily require the crushing of minerals. In fact, the nano-pores in natural cryptocrystalline mineral aggregates contribute to the nano effect of minerals. Most minerals in human body are nano minerals, such as hydroxyapatite in bones and teeth, kidney stones, dental calculus and other pathological minerals. The formation mechanism of minerals in human body, especially pathological minerals, and their physiological and pathological effects remain to be further studied. Some studies suggest that some pathological mineralization may be related to nanobacteria in the blood (Nayeb et al. 1998). The chemical composition and morphological characteristics of mineralized nanobacteria are similar to mineral particles in calcified

1.3 Environmental Effects of the Synergism Between Minerals …

tissue cells and kidney stones. The asbestos-like amphibole is a major cause of lung disease among asbestos miners and processors, and these nanoscale fibrous minerals can be found in the lung of miners (Heaney and Yates 1998). Microorganisms can often directly form nano-minerals. For example, bacteria can concentrate rare metals including polluting toxic heavy metals on their surface and form tiny nano mineral unit cells. The property that bacteria transform trace metals to tiny mineral cells can be used to remove heavy metal pollutants from the environment (Beveridge 1998; Lovley 1998). Nano-minerals can also be obtained by modification of natural minerals, such as chrysotile, which is a onedimensional open nanotubular mineral. There is adsorbed water in chrysotile nanotubes and the mass fraction of water in nanotubes with an inner diameter of 2–3 nm is relatively high, accounting for about 1.24–1.43%. Therefore, the chrysotile can be used to prepare nano tubes or reactors, siphons and templates for nanowires. The outer layer of chrysotile tubes is an octahedron layer of magnesium hydroxide, and the surface is hydrophilic and polar, which can be dissolved in concentrated hydrochloric acid. The residual silica tetrahedron after acid etching present a fibrous pseudomorphosis, and the chrysotile becomes a nano silica fiber. Hydrophobic silica nanofibers can be prepared by grafting organic matter to realize the transformation from hydrophilic to hydrophobic.

1.2.9.3 Composite Effect of Mineral and Organism In the interaction of multiple spheres in the critical zone on the earth, the biological actions involve in the occurrence, development and transformation of minerals, and the minerals also participate in the growth and evolution of organisms, which allows the integration of two originally distinct domains in nature. In the complex natural purification system, the inorganic purification process involving organisms and the organic purification process involving minerals are ubiquitous. The release, migration, and fixation of heavy metals in the environment as well as the migration, transformation and degradation of organic matters are all significantly affected by the interaction of minerals and organisms. Minerals and organisms construct a complete natural self-purification system with composite effects in nature. The combined prevention and control methods of heavy metal pollutants by minerals and microorganisms are mainly presented in the following ways. The first path is that the microbial cells transform heavy metal ions from the environment to mineral crystals to fix them. For example, Enterobacter cloacae and Ochrobactrum anthropi were found to have strong reducibility for Cr(VI) and to promote the mineralization of Cr(III) (Cheng et al. 2012). Some fungi are highly tolerant to Pb(II) and can adsorb Pb(II) efficiently,

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especially converting Pb(II) into lead-bearing minerals. The second pathway involves birnessite with a layered structure that has good properties for the adsorption and immobilization of heavy metals. In the ternary system of microorganisms, Mn(II) and fixed heavy metals, microorganisms oxidize Mn(II) to form birnessite, and at the same time, they can adsorb and fix heavy metals through co-precipitation. The number of fixed heavy metals is significantly higher than that of using birnessite alone. For example, MnB1 bacteria-Mn(II)-Cu2+ system has this composition effect (Zhang 2014). When treating complex and highly concentrated pollutants, in order to target the coexistence of BOD and COD, the mineral methods are often used after the microbial treatment; or treatment with minerals followed by microbial methods. The effect of both methods is better than that of treatment with microorganisms or minerals alone, which fully shows the advantage of the mineral and biological composite effect on pollutant treatment engineering. In summary, the self-purification function of natural inorganic minerals is very abundant. It is conceived that with more in-depth research work, more self-purification functions of minerals will be found and utilized. The inorganic mineralogical methods of using natural minerals to control pollution and protect the environment are comparable to the well-known organic biological methods. These two methods together construct the natural self-purification system in nature, and play a crucial role in pollution control and environmental restoration based on their respective characteristics and advantages in all spheres with human-earth interaction. At present, a new round of research upsurge on mineralogical theories and applications which began with the study of mineral environmental properties has been rising in the world. It is the main research trend in the future to further explore the purification function of inorganic minerals, to develop the interaction theory between minerals and biological systems, to develop the application technology using natural minerals in pollution control and environmental restoration, to broaden the field of inorganic minerals in pollution purification, and to propose, summarize and refine the new basic theory and application methods of natural self-purification function of inorganic minerals, so as to develop and improve the natural self-purification system in nature constructed by inorganic minerals and organisms.

1.3

Environmental Effects of the Synergism Between Minerals and Microorganisms

The interaction of lithosphere, atmosphere, hydrosphere, and biosphere in the key zone on different temporal and spatial scales largely controls the evolution of the lithosphere, the liquid–gas cycle and the biological evolution. Sunlight

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directly or indirectly participates in this process. In the past, more attention has been paid to the influence of sunlight on the diurnal temperature, the physical weathering of minerals and rocks, the global circulation of water and gas, and the biological photosynthesis. Inspired by the extensive use of solar energy via the development and application of semiconductor photoelectric conversion materials, the research on the mechanism of solar energy conversion by natural semiconductor minerals has made more progress, especially the conversion of solar energy to chemical or biological energy by semiconductor minerals. Inspired by the extensive use of solar energy through the development and application of semiconducting photoelectric conversion materials, rich results have been achieved in the study of the mechanism of natural semiconducting minerals converting solar energy, especially to chemical or biological energy. The synergy between minerals and microorganisms mainly refers to the interaction between semiconducting minerals and non-photosynthetic microorganisms, which can result in mineral formation, decomposition and transformation. The essence of this process is that non-photosynthetic microorganisms can use sunlight energy mediated by semiconducting minerals (Lu et al. 2012).

1.3.1 Mineral Electron Energy Form 1.3.1.1 Element Valence Electron Energy Valence electrons in minerals generally come from the outermost or sub-outer layers of atoms, which are also bonding electrons. When conditions such as pH, Eh change, the original chemical bonds in the crystal lattice of minerals might be unstable. The atoms in the crystal lattice will effectively collide with the atoms from the medium after getting energy, followed by original chemical bonds overcoming the thermodynamic barrier and breaking, eventually recombining into new chemical bonds that are most stable under current conditions. From the perspective of energy transfer, the mineral valence electrons originally bound in the crystal lattice will transfer along the redox potential gradient influenced by the thermodynamics limitation and kinetics related to the temperature, pressure, catalyst, solvent, light and other reaction conditions. The mineral valence electrons finally exist as the newly formed chemical bonds in the form of chemical energy, accompanied by the activation, break and recombination of the original chemical bonds. Therefore, the valence electron energy of minerals is essentially a kind of chemical energy that exists inside the crystal lattice, which relates to the type of bonding atoms, chemical bonds, crystal structure and other factors. This process reflects the redox activity of elements in minerals during geochemical cycling.

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Environmental Property of Minerals

Notably, electrons from minerals transfer in two different ways in the mineral-solution system. For the first pathway, dissolution of the mineral surface occurs by hydrogen bonds in solution, and the anion and cation electrostatic interactions or electron pairs established by atomic valence electrons are disrupted. The anion then breaks away from the lattice into solution and effectively collides with ions or molecules from the medium, resulting in the formation of new bonds by transferring, deflecting or sharing valence electrons. The second pathway, molecules or ions from the medium collide with the mineral surface at active sites. Their electrons are shared with the metal atoms’ outer orbital electrons of the minerals to form covalent bonds, or they directly provide a pair of electrons to form coordination bonds with the empty orbitals of metal atoms. Then the electron transfer occurs with the variable-valence elements in the mineral lattice for adsorption and redox reactions, resulting in the destruction of temporary covalent or ligand bonds. The products are desorbed off the mineral surface into solution, allowing the active sites to be continuously regenerated for a new round of adsorption-redox reactions.

1.3.1.2 Semiconductor Conduction Band Electron Energy According to the energy band theory of solids, when atoms or ions are closely packed to form minerals, the outer valence electrons are delocalized. All valence electrons are shared by the atoms of the entire mineral lattice and can move throughout the solid, so it is called shared electron. The shared electrons on the molecular orbitals of semiconducting minerals are different from elemental valence electrons during their production and transformation. Each electric shell in an isolated atom corresponds to a defined energy. Semiconducting minerals are formed by a series of atoms arranged periodically. As these atoms approach each other, the electronic shells overlap, so that the electrons are no longer completely confined to one atom, but share a common motion on similar energy shells of adjacent atoms. This results in a linear recombination of atomic orbitals into molecular orbitals after stacking on top of each other, with a constant number of orbitals and a splitting of energy levels. The lower energy bonding orbitals form the valence bands and the higher energy antibonding orbitals form the conduction bands. For semiconductors, valence electrons occupy lowenergy bonding orbitals, while high-energy antibonding orbitals are mostly empty. After gaining energy, the electrons in the bonding orbital can jump to the anti-bonding orbital and become free electrons. In other words, electrons in the valence band are excited to the conduction band and become free electrons of relatively higher energy, leaving holes in the valence band and forming a reduced electron

1.3 Environmental Effects of the Synergism Between Minerals …

and oxidized hole pair. It should be noted that since electrons are in an unstable excited state in the antibonding orbital, a dynamic electron transfer channel needs to be established to effectively hinder the electron–hole recombination. Therefore, only when the electron donor or electron source captures holes, the electrons in the conduction band can be effectively transferred to the electron acceptor, so as to achieve the conversion of external excitation energy to chemical energy, and ultimately increase the electron energy of the donor and break the energy barrier of the reaction. Photoelectrons are the most typical form of electron energy in the conduction band of semiconducting minerals. When the incident photon energy is higher than the band gap of the semiconducting mineral, the valence band electrons of the semiconducting minerals will be excited to the conduction band to form a pair of reductive photoelectron and oxidative hole, which then triggers redox reactions to release and convert energy. Semiconducting minerals on the earth’s surface under sunlight carry the energy conversion functions. For example, iron and manganese oxides such as birnessite, goethite and hematite possess stable, sensitive, and long-lasting photon-photoelectron conversion capabilities (Lu et al. 2012). In general, semiconducting mineral photoelectrons also include hot electrons produced by minerals gaining thermal energy, but the two are produced in different ways. In crystals, the atoms are periodically arranged on the lattice and the interactions between the atoms make them always continuously do thermal vibrations around the equilibrium position, thus forming lattice waves. According to quantum theory, the energy of lattice waves is also quantized with phonons as the smallest energy unit. When heated, atomic vibrations are more intense at the hot end of semiconducting minerals due to the increase in temperature. The electrons or holes then interact with phonons, which causes the electrons in the atomic shell to absorb quantized energy and escape from the inner layer to form carriers. Various semiconducting Fe–Cu sulfides such as pyrite, chalcopyrite and bornite in nature have been proven to have excellent thermoelectric response properties (Herzig and Hannington 1995; Xu and Schoonen 2000; Uhlig et al. 2014; Qiu et al. 2014; Ang et al. 2015). When these minerals are in a temperature field, the carrier energy and concentration at their high temperature end is higher than that at the low temperature end, and carrier diffusion occurs. An electric field is then formed inside the mineral, which converts the thermal energy to produce free hot electrons. From the perspective of energy, light consists of quantized photons that emit energy in the form of electromagnetic waves. Light can cause photoexcitation of semiconductors, and eventually be converted to electrical energy. Heat, on the other hand, is an indicator for the degree of particle random motion, which travels as

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elastic waves in crystals through quantized phonons. Heat can cause thermal excitation of semiconducting crystals and be converted into electrical energy. It should be noted that the conduction band electron energy is closely related to and different from the valence electron energy of semiconductors. Photoelectrons and hot electrons are the basic particles that carry energy in the process of photoexcitation and thermal excitation. They all participate in redox reactions as intermediate excited state electrons, which are eventually converted to stable valence electrons and store the light or heat absorbed by semiconductors in the form of chemical energy. It can be seen that the conduction band electron energy of semiconductors is generated by the excitation of external energy. Although it is not directly involved in the reactions like valence electrons, it has a significant catalytic effect on the electron donor’s valence electrons to break through the thermodynamic barrier. Among them, the generation, transfer and energy of hot electrons are significantly different from photoelectrons, reflecting the diversity of conduction band electron energy of semiconductors. Both elemental valence electron energy and conduction band electron energy of semiconducting minerals have been demonstrated to promote the growth and metabolism of microorganisms. In the complex and changeable natural environment, the transfer of mineral valence electron energy is often accompanied by changes of mineral redox state. Some minerals containing variable valence elements widely distributed in the lithosphere, such as iron oxides, manganese oxides and sulfides, are active in the interaction with microorganisms within the earth's multiple spheres. And they often affect the growth and metabolism of microorganisms through extracellular electron transfer, promoting geochemical cycles driven by minerals co-operating with microorganisms (Lovley et al. 1987; Lovley and Phillips 1988; Myers and Nealson 1988; Jiao et al. 2005; Garcia et al. 2007; Shelobolina et al. 2012; Li et al. 2019). In the process, minerals act as media to convert energy not available to microorganisms, such as light and heat, to photoelectrons or hot electrons to be used by microorganisms. For example, minerals such as rutile (TiO2), sphalerite (ZnS) and greenockite (CdS) can absorb photon energy larger than their own forbidden bands to generate high-energy photoelectrons. Thus, they can not only capture and convert light energy, but also support the growth and metabolism of traditional non-photosynthetic microorganisms (Lu et al. 2012, 2019; Kornienko et al. 2016). Therefore, different mineral electronic energy represent different forms of energy obtained by microorganisms from the inorganic environment, reflecting the different roles that minerals play in coordinating with microorganisms and promoting their growth and metabolism.

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1.3.2 Mineral Photoelectrons Promote the Origin and Evolution of Life Geological research results have confirmed that the earliest life appeared on the earth at least 3.5 billion years ago (Nisbet 1987; Schidlowski 1988). Before the origin of life, enough organic matter must be accumulated on the earth to provide the material basis. Natural semiconducting sulfides such as sphalerite on the early earth were commonly found in pools formed around hot springs (Mulkidjanian et al. 2012). Experiments have shown that the sunlight catalysis of sphalerite can synthesize a variety of organic substances necessary for the origin of life (Zhang et al. 2007). Sphalerite photocatalysis also produced pyruvate and a-ketoglutarate, which are important intermediates in the reductive tricarboxylic acid cycle (r-TCA cycle) with the function of fixing carbon dioxide in early life on the earth (Guzman and Martin 2009). The photoelectrons produced by sphalerite photocatalysis can further participate in the cycle of reductive tricarboxylic acid. The photocatalytic properties of alabandite could facilitate the conversion of carbon dioxide to organic matter, which may also provide a material basis for the origin of early life (Urey 1962). As mentioned above, when a semiconductor mineral is excited by sunlight photons, the electrons in its valence band gain energy to jump to the conduction band, forming free electrons-photoelectrons, while positively charged photogenerated holes are formed in the valence band. In fact, the early reductive and weakly acidic earth surface was widely distributed with reductive inorganic substances that are capable of trapping the oxidative photo-generated holes produced in the photocatalysis of semiconducting minerals, thus generating the reductive photoelectrons (Hayase and Tsubota 1983; Yanagida et al. 1990; Peral and Mills 1993; Bems et al. 1999; Smirnoff and Wheeler 2000; Vaughan 2006). Photogenerated holes have strong oxidizing properties. The photogenerated hole potential of sphalerite can reach + 2.64 V, and that of rutile up to + 3.15 V (Schoonen et al. 1998). The reductive photoelectrons of semiconducting minerals can reduce carbon dioxide to organic substances, providing a material basis for the origin of life. The oxidative photogenic holes can be captured by the reductive substances in the medium, thus avoiding the destruction of the original cells. Mulkidjanian et al. (2012) confirmed that the natural metal sulfides rich in hot springs, such as sphalerite and alabandite, can form a protective sheath to protect primitive cells from damage by ultraviolet radiation. During the origin of life on earth, life appeared as primitive cells and could not have developed elaborate and complex photosynthetic systems. So, how did life activities use sunlight energy? Our research results had provided a new idea for this question. It was the natural semiconducting

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Environmental Property of Minerals

minerals that could absorb ultraviolet, protect early primitive life cells, and generate photoelectrons with higher energy. For example, the redox potential of photoelectrons from sphalerite is − 1.04 V, and the that of pyrite is − 1.19 V (Xu and Schoonen 2000). Undoubtedly, these mineral photoelectrons could be the initial source of energy for the metabolic activities of primitive cells in early life. The extremely complex natural system composed of organisms, inorganic minerals and sunlight, which was linked by the photocatalysis of natural semiconducting minerals, throughout the origin and evolution of life on the earth. It was the natural semiconducting minerals that photoelectrically synthesized the early organic matter, providing the material basis for the origin and evolution of early life. The photocatalysis of semiconducting minerals prevented damage from ultraviolet radiation to primitive cells. It is clear that natural semiconducting minerals had a significant impact on the origin and evolution of early life on the earth in terms of synthesizing materials, providing energy and protecting cells, and they are playing an important role in the interaction of multiple spheres in the critical zone on the earth’s surface.

1.3.3 Mineral Photoelectrons Promote the Growth and Metabolism of Photoelectrophic Microorganisms For a long time, it has been believed that microbes mainly use solar energy and chemical energy, and therefore the microbes on the earth are classified into two basic nutrient modes, phototrophy and chemotrophy. According to classical theory, phototrophic microorganisms can obtain light energy through photosynthetic pigments in their cells, while chemotrophic microorganisms can only obtain chemical energy in the form of elemental valence electrons from the oxidation process of organic or inorganic matter due to the lack of photosynthetic pigments, instead of directly converting and using solar energy. The relationship between mineral photoelectrons and microorganisms has only been discovered in recent years (Lu et al. 2012). What has been reported so far is the photocatalytic bactericidal effect of semiconductors. The reactive oxygen radicals (ROS) generated by photogenetic holes oxidized and decomposed microbial cell walls and kill bacteria (Malato et al. 2009; Dalrymple et al. 2010). However, the lifespan of ROS is very short, and only microorganisms directly adsorbed on the surface of semiconductors were damaged. Instead of damaging microorganisms, photoelectrons can significantly enhance the growth and metabolism, biochemical activity and substrate utilization of some microorganisms compared to photogenetic holes (Lu et al. 2012). This suggests that

1.3 Environmental Effects of the Synergism Between Minerals …

photoelectrons can be a source of energy required for the growth and metabolism of some microorganisms. By constructing a two-chamber photoelectrochemical system of semiconductor mineral anode and microbial cathode, the researchers found that the photogenerated electrons of semiconducting minerals can spontaneously flow driven by the potential difference between the cathode and anode, and transfer to the cathode through an external circuit to promote the growth and metabolism of microorganisms in the cathode chamber (Lu et al. 2012). Under simulated sunlight, the photoelectrons of semiconducting metal oxides represented by rutile (TiO2), metal sulfides represented by sphalerite (ZnS), and surface soil metal oxides represented by goethite (FeOOH) can be used by the chemoautotrophic microorganism Acidithiobacillus ferrooxidans (A. ferrooxidans) In the light-rutile-A. ferrooxidans two-chamber system (Fig. 1.1a), the cell concentration of A. ferrooxidans was significantly increased with a shorter lag period and an increment in logarithmic and stable growth phase compared to the breakout control experiment without light and without external electrons. The same results were observed in the light-sphalerite-A. ferrooxidans and lightgoethite-A. ferrooxidans systems. The Fe2+/Fe3+ redox pair is used as an electron mediator in this pathway. The bacteria continuously oxidize Fe2+ in the cathode chamber to produce Fe3+, which can be reduced to microbially available Fe2+ by photoelectrons generating from the semiconducting minerals, thus enabling the indirect use of light energy by A. ferrooxidans. The conduction band photoelectron potentials of natural rutile and sphalerite at pH 7.0 are − 0.36 V and − 1.58 V, respectively (vs. NHE) (Schoonen et al. 1998), while the standard electrode potential of Fe2+/Fe3+ is + 0.77 V (vs. NHE) (Rawlings 2005). Therefore, thermodynamically, the photoelectrons of natural semiconducting minerals can completely reduce Fe3+ to Fe2+. The efficiency of this non-photosynthetic microorganism to utilize extracellular photoelectrons is mainly regulated by both the wavelength (photon energy) and the light intensity (number of photons) of the irradiated light received by semiconducting minerals. The longer the light wavelength, the lower the photon energy, the lower the conversion efficiency, the lower the concentration of microorganisms when they reach the stable phase. In contrast, there is an optimal light intensity, that is, at 8 mW/cm2, the system achieves the highest photon-electron conversion efficiency and the highest concentration of microorganisms in the stable phase (Lu et al. 2012). The light-to-bioenergy conversion efficiency of the rutile-A. ferrooxidans system was calculated to be 0.13– 0.18‰, while that of the sphalerite-A. ferrooxidans system was 0.25–1.9‰ (Lu et al. 2012), which are obviously much lower than the 10‰ efficiency of photosynthetic energy conversion in plants (Blankenship et al. 2011). In the dual-chamber system, light energy needs to go through a

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certain conversion process before it can be used by microorganisms, and the photoelectron energy is continuously lost and reduced during the long-range electron transfer. Therefore, the conversion efficiency of light energy-bioenergy in this system is much lower than that in long-evolved and perfected photosynthesis. The phenomenon that mineral photoelectrons promote the “photoelectric” growth and metabolism of chemotrophic microorganisms is not an isolated case. Liu et al. (2015) formed a circuit of TiO2 and Si nanowires as light trapping units and compounded them with the non-photosynthetic bacterium Sporomusa ovata (S. ovata). The TiO2 is used as a photoanode to generate photoelectron-hole pairs, the holes are captured by water to produce O2, and a large number of bacteria are attached to the Si nanowire surface by pure culture. The results show that S. ovata could accept photoelectrons from the anode and fix CO2 to produce acetic acid (Fig. 1.1b). In addition to the autotrophic microorganisms, the heterotrophic Alcaligenes species can also obtain the photoelectron energy in a combined system of CdS and NAD, where the bacterial growth was supported by the regenerated NADH from the photoreduction of NAD by CdS photoelectrons (Shumilin et al. 1992). Further results show that the chemoheterotrophic Alcaligenes faecalis (A. faecalis) of the genus Alcaligenes, can obtain a significant growth promoting effect in the presence of mineral photoelectrons (Lu et al. 2012). In the dual-chamber system with semiconducting mineral photocatalysis, the adhesion density of A. faecalis on the cathode electrode reached 3  106 CFU/cm2, which was significantly higher than the control group without mineral photocatalysis at 2  103 CFU/cm2. This result indicates that the photoelectrons produced by mineral photocatalysis under sunlight can also be utilized by chemotrophic microorganisms and promote their growth.

1.3.4 Microbial Photoelectrophic Nutrition Mode Natural semiconducting minerals promote the growth and metabolism of non-photosynthetic microorganisms through photocatalysis, revealing a new way for non-photosynthetic microorganisms to utilize solar energy that may be widespread in nature. As we all know, natural semiconducting minerals such as metal oxides and sulfides are widely distributed on the earth’s surface (Vaughan 2006), and autotrophic and heterotrophic microorganisms and their microbial communities are also widely distributed. Reducing substances, such as ascorbic acid, humic acid and reducing inorganic substances are also widely present in nature (Hayase and Tsubota 1983; Smirnoff and Wheeler 2000; Vaughan 2006). These reducing substances can capture

20

1

Environmental Property of Minerals

Fig. 1.1 Schematic diagram of the dual-chamber research system. a Rutile-A. ferrooxidans system (Lu et al. 2012); b TiO2-S. ovata system (Liu et al. 2015)

oxidizing photoholes from semiconducting minerals that are excited by light (Yanagida et al. 1990; Peral and Mills 1993; Bems et al. 1999) so as to separate reducing photoelectrons. Photoelectrons can be transferred to microorganisms through a variety of extracellular electron transfer mechanisms, and are used by microorganisms for growth and metabolism. Obviously, recent research results on the interaction between semiconducting minerals and non-photosynthetic microorganisms pose new challenges to the traditional theory of microbial energy metabolism. Microorganisms can be reclassified into three main categories based on their

metabolic pathway: phototrophy, chemotrophy and photoelectrophy (Table 1.1). The photoelectrophy is a new way of energy utilization in which semiconducting minerals mediate the direct or indirect use of photoelectron energy by non-photosynthetic microorganisms, expanding the understanding of traditional microbial energy utilization pathways. Undoubtedly, this form of energy nutrition embodies the role of natural semiconducting minerals on the surface in synthesizing substances, providing energy and protecting cells. It may have had an important impact on the energy use of life in the early

1.3 Environmental Effects of the Synergism Between Minerals …

21

Table 1.1 Types of microbial energy metabolism Energy source

Solar photon

Elemental valence electron

Mineral photoelectron

Metabolic pathway

Phototrophy

Chemotrophy

Photoelectrophy

Type of nutrition

Photoheterotrophy

Photoautotrophy

Carbon source

Organics

CO2

Cognitive level

Known

Chemoheterotrophy

Chemoautotrophy

Organics

CO2

Known

earth and in extreme environments, and is playing an important role in the process of multi-sphere interaction at the earth’s surface. Sakimoto et al. reported in 2016 that photoelectrons promote the growth of microorganisms in the system of direct contact between semiconducting minerals and microorganisms. They used the non-photosynthetic microorganism Moorella thermoacetica (M. thermoacetica) to reduce the sulfhydryl group of cysteine to S2− and combined it with Cd2+ to produce CdS directly attached to the bacterial surface. Under light, the electron transfer process can be divided into two stages (Fig. 1.2a). In the early stage, due to the low level of hydrogenase expression, photoelectrons tend to rely on electron mediators such as ferric oxide reductase, cytochromes, riboflavin, and quinones on the cell membrane to pass into the cell. However, due to electron– hole recombination, the charge transfer efficiency of this pathway is low and insufficient to generate the energy required for the first step of the reduction in the WoodLjungdahl pathway. Therefore, as the expression of hydrogenase increases in the later stage, photoelectrons are gradually transferred into the cell through the more efficient hydrogenase pathway to facilitate the carbon fixation (Kornienko et al. 2016; Sakimoto et al. 2016). Escherichia coli (E.coli) after genetic recombination can also form a bacterial-CdS complex system through a similar mechanism, and can even produce H2 under aerobic conditions with light irradiation (Fig. 1.2b) (Wei et al. 2018). CdS can also promote the autotrophic denitrification of Thiobacillus denitrificans (T. denitrificans) under light (Fig. 1.2c) (Chen et al. 2019). In addition to CdS, the metal oxide TiO2 promotes both the hydrogen production process of genetically recombinant Escherichia coli (E. coli) (Fig. 1.2d) (Honda et al. 2016) and the denitrification of microorganisms by reacting with biofilms containing denitrifying bacteria under light. In the above mentioned system of direct contact between semiconducting minerals and microorganisms, although photoelectrons have not been proven to directly interact with outer membrane proteins for electron transfer, some membrane

Photoelectric heterotrophy

Photoelectric autotrophy

Organics

CO2

Lu et al. (2012)

proteins and humic acids can act as electron mediators and participate in photoelectrons in denitrification-valence electron transfer process (Fig. 1.2e) (Zhu et al. 2018). Hematite (Fe2O3) has also been shown to form a conductive network with Shewanella, and the conduction band photoelectrons produced by light can achieve electron transfer with the microbial outer membrane protein c-Cyts and generate significant photocurrent (Nakamura et al. 2009). Obviously, the photoelectrons generated by the semiconducting minerals deposited on the surface of the microbial cells under illumination still need to rely on a certain transmembrane electron transfer mechanism to transmit the photoelectrons to the microorganisms for use. The transmembrane energy loss of photoelectrons will have a negative impact on microbial energy utilization and conversion efficiency. Therefore, the Au nanoclusters with better biocompatibility were successfully implanted into M. thermoacetica cells (Fig. 1.2f), and the photoelectrons generated in the cells effectively overcome the transmembrane energy loss, thus exhibiting better performance than CdS-microbes. Au nanoclusters were also found to bind more stably to cell membranes than CdS, generating photoelectrons with a longer lifetime, which further ensures the transfer efficiency of photoelectrons (Zhang et al. 2018). The photoelectrophy is a new pathway of nonphotosynthetic microorganisms using photoelectron energy directly or indirectly mediated by semiconducting minerals to transfer solar energy, which expands our understanding of traditional microbial energy utilization. According to the basic energy forms in nature, previous studies have shown that the outermost or sub-outer valence electrons of the variable elements and solar photons are two different energy forms that can be utilized by microorganisms. In the process of extracellular electron transfer between semiconductor minerals and microorganisms, photoelectrons as a third natural basic energy form have been discovered to stimulate microbial growth and metabolism. The pathways that microorganisms utilize mineral photoelectron energy may have a profound impact on microbial life activities, as well as elements and energy cycles in nature.

22

Fig. 1.2 The compound interaction system of semiconducting minerals-photoelectrons-microbes. a Light- CdS-M. thermoacetica acetic acid production system electronic pathway (Sakimoto et al. 2016); b light-CdS-E. coli hydrogen production system (Wei et al. 2018); c light-CdS-T. denitrificans composite system electron transport

1

Environmental Property of Minerals

path (Chen et al. 2019); d light-TiO2-MV+/MV2+-E. coli hydrogen production system (Honda et al. 2016); e light-TiO2-biofilm denitrification system (Zhu et al. 2018); f light-nano Au-M. thermoacetica acetic acid production system (Zhang et al. 2018)

References

23

Fig. 1.2 (continued)

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24 Guzman MI, Martin ST (2009) Prbiotic meabolism: production by mineral photo electrochemistry of a-ketocarboxylic acids in the reductive tricarboxylic acid cycle. Astrobiology 9(9):831–842 Hayase K, Tsubota H (1983) Sedimentary humic acid and fulvic acid as surface active substances. Geochim Cosmochim Acta 47(5):947– 952 He X, Tang K, Lei X (1997) Heavy mineral record of the holocene environment on the loess plateau in China and its pedogenetic significance. Catena 29(3–4):323–332 Heaney PJ, Yates DM (1998) Adsorption of toxic metal cations to aqueous polymeric silica. In: 17th international mineralogical association conference, Toronto, Canada Herzig PM, Hannington MD (1995) Polymetallic massive sulfides at the modern seafloor—a review. Ore Geol Rev 10(2):95–115 Hochella MF (1995) Mineral surfaces: their characterization and their chemical, physical and reactive nature. Mineral Surf 5:17–60 Hochella MF, White AF (1990) Mineral-water interface geochemistry. Rev Mineral 23(1–16):309–364 Hoffmann R (1988) Solids and surfaces: a chemist’s view of bonding in extended structure. VCH Publishers, Canada Honda Y, Hagiwara H, Ida S et al (2016) Application to photocatalytic H2 production of a whole-cell reaction by recombinant, Escherichia coli, cells expressing [FeFe]-hydrogenase and maturases genes. Angew Chem Int Ed 55:8045–8048 Jiao Y, Kappler A, Croal LR et al (2005) Isolation and characterization of a genetically tractable photoautotrophic Fe(II)-oxidizing bacterium, Rhodopseudomonas palustris strain TIE-1. Appl Environ Microbiol 71(8):4487–4496 Khaodhiar S, Azizian MF, Osathaphan K et al (2000) Copper, chromium, and arsenic adsorption and equilibrium modeling in an iron-oxide-coated sand, background electrolyte system. Water Air Soil Pollut 119(1–4):105–120 Kornienko N, Sakimoto KK, Herlihy DM et al (2016) Spectroscopic elucidation of energy transfer in hybrid inorganic-biological organisms for solar-to-chemical production. Proc Natl Acad Sci USA 113 (42):11750–11755 Korshin GV, Benjamin MM, Sletten RS (1997) Adsorption of natural organic matter (NOM) on iron oxide: effects on NOM composition and formation of organo-halide compounds during chlorination. Water Res 31(7):1643–1650 Li Y, Wang X, Li YZ et al (2019) Coupled anaerobic and aerobic microbial processes for Mn-carbonate precipitation: a realistic model of inorganic carbon pool formation. Geochim Cosmochim Acta 256:49–65 Liu C, Gallagher JJ, Sakimoto KK et al (2015) Nanowire-bacteria hybrids for unassisted solar carbon dioxide fixation to value-added chemicals. Nano Lett 15(5):3634–3639 Lovley DR (1998) Geomicrobiology: interactions between microbes and minerals. Science 280(5360):54–55 Lovley DR, Phillips EJ (1988) Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl Environ Microbiol 54(6):1472–1480 Lovley DR, Stolz JF, Nord GL et al (1987) Anaerobic production of magnetite by a dissimilatory iron-reducing microorganism. Nature 330(6145):252–254 Lu AH, Zhao DG, Li JH et al (2003) Application of vermiculite and limestone to desulphurization and to the removal of dust during briquette combustion. Mineral Mag 67(6):1243–1251 Lu AH, Li Y, Jin S et al (2012) Growth of non-phototrophic microorganisms using solar energy through mineral photocatalysis. Nat Commun 3:768–775 Lu AH, Li Y, Ding HR et al (2019) Photoelectric conversion on Earth’s surface via widespread Fe- and Mn-mineral coatings. Proc Natl Acad Sci USA 116(20):9741–9746

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Luo J, Zhang QH, Huang AM et al (2000) Total oxidation of volatile organic compounds with hydrophobic cryptomelane-type octahedral molecular sieves. Microporous Mesoporous Mater 35–36 (99):209–217 Malato S, Fernandez-Ibanez P, Maldonado MI et al (2009) Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal Today 147(1):1–59 Moore PB, Shen J (1983) An X-ray structural study of cacoxenite, a mineral phosphate. Nature 306(5941):356–358 Mulkidjanian AY, Bychkov AY, Dibrova V et al (2012) Origin of first cells at terrestrial, anoxic geothermal fields. Proc Natl Acad Sci USA 109(14):E821–E830 Myers CR, Nealson KH (1988) Bacterial manganese reduction and growth with manganese oxide as the sole electron acceptor. Science 240:1319–1321 Nakamura R, Kai F, Okamoto A et al (2009) Self-constructed electrically conductive bacterial networks. Angew Chem 121 (3):516–519 Nayeb ZA, Case B, Vali H (1998) Lung fiber burden of two groups of Quebec asbestos miners and millers. In: 17th international mineralogical association conference, Toronto, Canada Nisbet EG (1987) The young earth: an introduction to Archaean geology. Cambridge University Press, Cambridge Orgeira MJ, Walther AM, Vasquez CA et al (1998) Mineral magnetic record of paleoclimate variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina). J S Am Earth Sci 11(6):561–570 Peral J, Mills A (1993) Factors affecting the kinetics of methyl orange reduction photosensitized by colloidal CdS. J Photochem Photobiol, A 73(1):47–52 Pushkareva RA (1998) Tritium adsorbtion in the montmorillonite. In: 17th international mineralogical association conference, Toronto, Canada Qiu P, Zhang T, Qiu Y et al (2014) Sulfide bornite thermoelectric material: a natural mineral with ultralow thermal conductivity. Energy Environ Sci 7(12):4000–4006 Rawlings DE (2005) Characteristics and adaptability of iron-and sulfur-oxidizing microorganisms used for the recovery of metals from minerals and their concentrates. Microb Cell Fact 4(1):1–15 Ribbe PH (1974) Sulfide mineralogy. Rev Mineral 1:284–350 Roberts AC, Bonardi M, Erd RC et al (1991) Wattersite, Hg4HgCrO6, a new mineral from the Clear Creek Claim, San Benito County, California. Mineral Rec 22:269–272 Roberts AC, Szymanski JT, Erd RC et al (1993) Deanesmithite, Hg2Hg3CrO5S2, a new mineral species from the Clear Creek Claim, San Benito County, California. Can Mineral 37(4):787–793 Ross MP, Nolan RP, Langer AM et al (1993) Health effects of mineral dusts other than asbestos. Mineral Geochem 28:361–407 Ryan JN, Elimelech M, Ard RA et al (1999) Bacteriophage PRD1 and silica colloid transport and recovery in an iron oxide-coated sand aquifer. Environ Sci Technol 33(1):63–73 Sakimoto KK, Wong AB, Yang P (2016) Self-photosensitization of nonphotosynthetic bacteria for solar-to-chemical production. Science 351(6268):74–77 Schidlowski MA (1988) 3800-million-yeat isotopic record of life from carbon in sedimentary rocks. Nature 333(6171):311–318 Schoonen MAA, Xu Y, Strongin DR (1998) An introduction to geocatalysis. J Geochem Explor 62(1–3):313–318 Shelobolina ES, Xu H, Konishi H et al (2012) Microbial lithotrophic oxidation of structural Fe(II) in biotite. Appl Environ Microbiol 78 (16):5746–5752 Sherriff BL, Brown DA, Sawicki JA (1998) Iron mineral reactions mediated by an environmental bacterial consortium. In: 17th international mineralogical association conference, Toronto, Canada

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25 Wei W, Sun P, Li Z et al (2018) A surface-display biohybrid approach to light-driven hydrogen production in air. Sci Adv 4(2):eaap9253 Xu Y, Schoonen MA (2000) The absolute energy position of conduction and valence bands of selected semiconducting minerals. Am Miner 85(34):541–556 Yanagida S, Yoshiya M, Shiragami T et al (1990) Semiconductor photocatalysis I. Quantitative photoreduction of aliphatic ketones toalcohols using defect-free ZnS quantum crysstallites. J Phys Chem 94(7):3104–3111 Yin YG, Xu WQ, Deguzman R et al (1994) Studies of stability and reactivity of synthetic cryptomelane-like manganese oxide octahedral molecular sieves. Inorg Chem 33(19):4384–4389 Zhang HQ (2014) Interaction between microorganisms and birnessite and its method of immobilizing heavy metals. Theses and Dissertations of Peking University, Beijing Zhang X, Ellery S, Friend C et al (2007) Photodriven reduction and oxidation reactions on colloidal semiconductor particles: implicaitons for prebiotic synthesis. J Photochem Photobiol, A 185 (2):301–311 Zhang H, Liu H, Tian Z et al (2018) Bacteria photosensitized by intracellular gold nanoclusters for solar fuel production. Nat Nanotechnol 13(10):900–905 Zhu N, Wu Y, Tang J et al (2018) A new concept of promoting nitrate reduction in surface waters: simultaneous supplement of denitrifiers, electron donor pool, and electron mediators. Environ Sci Technol 52:8617–8626

2

Environmental Effects of Tunnel Structure Minerals

2.1

Octahedral Tunnel Effects of Cryptomelane

There are more than 30 known manganese oxide/hydroxide minerals distributed in a wide variety of geological settings (Post 1999). Mn presents in most manganese oxide/hydroxide minerals in three different oxidation states: + 2, + 3, and + 4, giving rise to a range of multi- and mix-valent phases. The basic building block for most manganese oxides atomic structures is the [MnO6] octahedron, which can be assembled by sharing edges and/or corners to form a family of layer and tunnel structures. Typically, the manganese oxides are constructed of single, double, or triple chains of edge-sharing [MnO6] octahedra, and the chains share corners with each other to produce frameworks with tunnels (Post 1999). Cryptomelane (KxMn8−xO16, 0.2 < x < 1.0) is a typical tunnel structure manganese oxide belong to hollandite group, and is reported to have monoclinic I2/m and tetragonal I4/m polymorphs, with cell dimensions of a = 9.956 Å, b = 2.8705 Å, c = 9.706 Å, b = 90.95° (Post et al. 1982) and a = 9.866 Å, b = 9.866 Å, c = 2.872 Å, a = b = c = 90° (Vicat et al. 1986), respectively. It has a well-defined 2
2 channel structure and a channel size of 0.46 nm 0.46 nm constructed from edge-shared double [MnO6] octahedral chains (Vicat et al. 1986; Pasero 2005; Feng et al. 1999). The channels are partially filled with K+ and sometimes water molecules, which balance the charge of mixed Mn2+, Mn3+, and Mn4+ (Feng et al. 1999; Post and Burnham 1986). K+ in the channels can be ion-exchanged with other cations with appropriate sizes (Cu2+, Co2+, Ni2+, etc.) (Malinger et al. 2006). The special open Channel structure and variable valence of Mn give cryptomelane unique physicochemical properties such as cation-exchange capability, strong oxidation ability, and reversible multi-electron redox capacity (Malinger et al. 2006; Makwana et al. 2002; Thackeray 1997). Indeed, cryptomelane has been

© Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_2

recommended as a heterogeneous redox catalyst, chemical sensor, adsorbent, and battery material for environmental and industrial use (Malinger et al. 2006; Makwana et al. 2002; Thackeray 1997). In natural environments, cryptomelane is the major manganese oxide in the supergene oxidation zones of Mn-bearing crusts and manganese deposits. The supergene oxidized manganese ore generally occurs as cryptocrystalline aggregates with noticeable conchoidal, botryoidal, reniform, and stalactitic textures, which are characteristic assembles of colloidal minerals. Since colloidal minerals are in nano size and with a very poor crystalline quality, until the mid-twentieth century, cryptomelane had been identified by mineralogists and named as a new mineral species. Cryptomelane was first distinguished from psilomelane and named as “true psilomelane” in 1932 (Ramsdell 1932), but was known with little structure and composition information due to the limitation of measurement techniques at that time. Richmond and Fleischer characterized the distinct mineral species by X-ray diffraction and chemical analysis and then nominated the name “cryptomelane” in 1942 (Richmond and Fleischer 1942). In the 1990s, cryptomelane became the preferred mineral for isoto- pic dating due to its high content of K and widespread existence in the weathered crust (Hautmann and Lippolt 2000). But due to the complex mineralogy and typical occurrence of fine-grained, poorly crystalline aggregates, it is extremely challenging to conduct characterization on natural cryptomelane. Perhaps, for this reason, there are relatively few studies concerning it. Here we do mineralogical characterization on natural cryptomelane with the purpose to show it is not only an important Mn metal resource but also highly chemically active in the environmental geochemistry. Natural cryptomelane and its synthetic analogues could be used as cost-effective environmental materials in catalytic, cation-exchange, and redox reactions.

27

28

2.1.1 Channel Structure of Manganese Oxide Channel structure means that atoms, ions, or their structural units share angular tops and edges in minerals with ring and frame structures, forming holes or channels with one or more holes or channels extending in a certain direction. There are always exchangeable water molecules or other ions in the pore, and the diameter, length, and dimension of the channel vary with the composition, structure, and bond of the mineral. Zeolite is a typical mineral with a tunnel structure. Its molecular sieve, ion sieve, and ion exchange effect are not only used in catalysis, adsorption, separation, etc. but also used in new fields such as microlaser, nonlinear optical materials, and nanodevices. It is proposed that all structures with open channels and holes, containing zeolite water and exchangeable cations can be attributed to the category of the molecular sieve generally called. In this sense, the molecular sieve is a large family, which includes other types of molecular sieves, such as pillared clay and other layered silicate molecular sieves, oxide molecular sieves (such as cryptomelane, todorokite), in addition to the zeolite family, and molecular sieve with zeolite structure. Most natural minerals are in the range of microporous molecular sieves (pore size  2 nm). So far, the largest pore size found in nature is cacoxenite (pore size is 1.4 nm), the structure is composed of [PO4] tetrahedron, ferrite octahedron, and aluminum oxygen trigonal bipyramid (5 times coordination), which is a kind of large pore structure with various cation coordination, and its pore size is close to the level of mesoporous molecular sieve. Most manganese oxides and hydroxides have tunnel structures, and their crystal structure is similar to natural zeolite. The octahedron of manganese oxide minerals is equivalent to the tetrahedron of [SiO4] in zeolite, and the octahedron of [MnO6] is of single, double, and three chain structure. The four chains are surrounded by rectangular or square hollow one-dimensional channels in a near orthogonal mode, and the channel can be occupied by larger metal ions and water molecules. Some oxide minerals have many varieties, and their differences are mainly manifested in the connection mode of cation octahedron and the type of pore cation. The common manganese oxide minerals with tunnel structure mainly include todorokite, cryptomelane, hollandite, coronadite, psilomelane, pyrolusite, etc.

2.1.2 Channel Effect of Natural Cryptomelane Channel effect is essentially molecular sieve effect or ion sieve effect. It refers to a kind of adsorbent or membrane material with uniform micropores and pore size equivalent to that of ordinary molecules or ions. According to its effective

2

Environmental Effects of Tunnel Structure Minerals

size, it can be used to screen fluid molecules or ions of different sizes. This effect is called molecular sieve or ion sieve effect. Cryptomelane is a kind of mineral with tunnel structure, which is called Mn oxide octahedral molecular sieve. Compared with the zeolite, the [MnO6] octahedron of cryptomelane has a uniform one-dimensional framework with a channel size of 0.46 nm, which is close to that of zeolite (the channel size of most zeolite ranges from 0.23 to 0.52 nm, only the octahedron has a large channel size of 0.74 nm). Therefore, the [MnO6] octahedron of cryptomelane also has the properties of zeolite like molecular sieve or ion sieve and it shows good channel effect. Compared with limonite with larger channels, the higher structural stability of cryptomelane is due to its smaller channels and the close packing of [MnO6] accumulation in the structure. The channel effect of cryptomelane shows that the ions or molecules larger than the channel diameter of 0.46 nm are excluded from the channel, while those smaller than the channel diameter can enter the channel, which means that cryptomelane has the function of ion sieve or molecular sieve. As far as zeolite is concerned, it can also show ion sieve properties for some ions in solution and can be divided into complete ion sieve effect and partial ion sieve effect. The complete ion sieve effect means that the exchange ion is completely blocked out of the zeolite structure because it is larger than the channel size of the material, which makes the exchange reaction impossible. Some zeolites with dense structure show complete ion sieve effect on inorganic cations due to their small channel size, while those with open structure show complete ion sieve effect only in the exchange of some organic large ions. Part of the ion sieve effect is that the exchange ion is partially blocked and the exchange reaction cannot be carried out completely. Part of the ion sieve effect is mainly due to the existence of different exchange sites in zeolite, while some exchangeable cations are not easily exchanged. Therefore, the properties of ion sieves are mainly related to the structural characteristics of zeolite itself, the properties of exchange ions, and exchange conditions. Among them, the influence of the exchange ion itself on the properties of zeolite ion sieves is mainly manifested in the size of ion itself or hydrated ion (Zhang et al. 1986). The influence factors of the channel effect can be summarized as follows: ① the molecules and ions larger than 0.46 nm in channel diameter are excluded from the channel, thus showing complete molecular sieve or ion sieve effect; ② the K+ and Na+ ions in the channel of cryptomelane are located at (0,0,0) position and (0,0.5,0.5) position respectively, In theory, it is easy to be exchanged, which may not cause part of the ion sieve effect of cryptomelane, but the actual situation needs to be further studied.

2.1 Octahedral Tunnel Effects of Cryptomelane

In a word, the channel effect and ion exchange are complementary to each other in the channel materials (cryptomelane and zeolite materials). Therefore, when discussing one of the environmental properties, the other is bound to be involved. That is to say, as OMS-2 material, its channel effect and ion exchange environmental properties are reflected at the same time.

2.1.2.1 Ion Exchange of Cryptomelane In the crystal structure of cryptomelane, the good channel and K+ in the channel constructed by the octahedron of [MnO6] are similar to those in natural zeolite (Zhang et al. 1986). In the natural structure of cryptomelane, there are lower than tetravalent manganese ions in the basic skeleton unit of octahedron, which makes the structure appear redundant negative charge. Therefore, a valence cation K+ appears in the structure channel of the cryptomelane to compensate for these redundant negative charges and make the structure in a stable state. K+ in the channel of cryptomelane is similar to Na+, Ca2+ in the zeolite channel, and it is weak to combine with skeleton, so it can participate in ion exchange without destroying the crystal structure of cryptomelane. The research on zeolite molecular sieve has been mature. The structure and ion exchange properties of the cryptomelane are similar to zeolite. Therefore, the factors that affect the ion exchange of zeolite may also be the influencing factors of the ion exchange of the cryptomelane. For example, the capacity of ion exchange also depends on the charge of [MnO6] octahedron anion skeleton; it also has ion exchange selectivity. The ion selected depends on the size of the channel, the properties of the exchange ion (such as size, valence, hydration state), etc., namely, K+ and a small amount of Na+ and Ca2+ in the channel can only be exchanged with metal ions with similar ionic radius, charge number, electronegativity and degree of hydration in solution, and the radius of exchanged ions or hydrated ions must be smaller than the channel size of cryptomelane. In addition, the medium conditions of exchange reaction, such as the concentration of electrolyte, also affect the ion exchange. In the process of ion exchange, the ion exchange dynamics, that is, the exchange speed and the influence of exchange conditions on the exchange rate should also be considered. The cation exchange between minerals with ion exchange properties (such as zeolite and the cryptomelane) and the aqueous solution is a kind of complex reaction between solid and liquid. The exchange process is as follows (Zhang et al. 1986): (1) The cations in the solution diffuse through the surface of cryptomelane to the channels of cryptomelane; (2) The cation diffused into cryptomelane via exchange with the cation in the pore of cryptomelane; (3) The ions exchanged from cryptomelane diffuse into the solution.

29

In the above process, the diffusion of ions in the cryptomelane structure is the key to realizing ion exchange. Because the diffusion process is very slow, the whole ion exchange speed depends on the diffusion speed of ions, that is, the ion exchange speed is restricted by the diffusion speed of ions in the crystal structure. To sum up, the influencing factors of ion exchange performance are: ① the charge of [MnO6] octahedral anion skeleton affects the ion exchange capacity of cryptomelane; ② Only metal ions with similar radius, charge number, electronegativity, and degree of hydration of K+ and a small amount of Na+ and Ca2+ ions in the channel can be exchanged with each other. At the same time, the radius of the exchanged ions must be less than 0.46 nm of the channel diameter of cryptomelane to enter the channel; ③ The rate of ion exchange is controlled by the rate of ion diffusion. Therefore, the ion exchange is not an independent process but is closely related to the channel effect (molecular sieve or ion sieve effect). Cryptomelane has strong ion exchange performance because of its large channel structure and has high selectivity for metal ions (radius is about 0.14 nm) whose effective radius is close to its channel size. It was found that the adsorbed heavy metal ions exchanged with cryptomelane and entered into the channels (Tsuji and Komarneni 1993). This selective cation exchange property makes cryptomelane a useful ion sieve material for chromatographic separation, selective adsorption of heavy metals (O’Reilly and Hochella 2003), and removal of radionuclides. Under the control of ionic hydration radius and channel size, the adsorption of Li+, Na+, K+, Rb+, Cs+ is reversible ion exchange adsorption, which has high selective adsorption capacity for K+, Rb+, and can also selectively adsorb heavy metal Pb2+ into the channel of cryptomelane (Tsuji and Tanaka 2001).

2.1.2.2 Ion Exchange of Heavy Metals by Natural Cryptomelane The channel effect of cryptomelane applied to the treatment of heavy metal pollution shows good environmental properties and has good removal ability for Cd2+, Hg2+, Pb2+, and other heavy metal ions. Here, the ion exchange effect (Cd2+ as an example) shown by the channel effect of natural cryptomelane is described in detail. 1. Mechanism The partition coefficient (Kd) is often used to analyze the ion exchange process and the adsorption model is established (Bradbury and Baeyens 1999). Several different expressions that are often used in the study of selective ion exchange are selectivity plot, Vanselow equation, Langmuir curve, and power exchange function. One of the most widely used is the selection curve

30

2

Environmental Effects of Tunnel Structure Minerals

method. Assuming that there is only H+–Cd2+ binary ion exchange in the solution, the ion exchange reaction can be written as follows: Cd2 þ þ 2Hsþ $ 2H þ þ Cd2s þ

ð2:1Þ

Among them, Cds2+ and Hs+ are ions in cryptomelane (solid phase), and Cd2+ and H+ are ions in solution. The selection coefficient of ion exchange can be obtained from the following equation: K¼

½Cd2s þ ½H þ 2

½Hsþ 2 ½Cd2 þ 

ð2:2Þ

s According to Eqs. 2.1 and 2.2 (i.e., Kd ¼ ½½Cd Cd ), the results are as

K ¼ Kd ð

½Hs  2 Þ ½H

ð2:3Þ

When the concentration of Cd2+ in the solution and exchanger is far less than that of H+, i.e., [Cds2+]  [Hs+], while [Cd2+]  [H+], K and [Hs+] are constants ([Hs+]  Q, i.e., ion exchange capacity), then Eq. 2.3 can be rewritten as follows (Eq. 2.4):
 lg Kd ¼ lg KQ2  2lg½H þ  ð2:4Þ It can be seen that the plot of lg[H+] with lgKd should get a straight line with a slope of − 2. 2. Experimental Results In a series of 100 mL triangular flasks, 800 mg cryptomelane powder with a particle size of 120–160 mesh was added, and CdCl2 solution with a volume of 25 mL and Cd2+ concentration of 11.24 mg/L and HNO3 solution with a certain concentration were a added. The experimental temperature was 25 °C, and the supernatant was centrifuged after 72 h of continuous oscillation in a water bath oscillator with a rotating speed of 170 r/min. According to the calculation, the adsorption capacity of Cd2+ adsorbed by natural cryptomelane and Kd are obtained (Table 2.1), and the lgKd versus lg[H+] is plotted (Fig. 2.1).

Table 2.1 Adsorption capacity of Cd2+ under different concentrations of HNO3

[H+]/(mol/L) Concentration/(mg/L) Adsorption capacity/(mg/g)

Fig. 2.1 Logarithmic relationship between Kd and [HNO3] in Cd2+ adsorption by natural cryptomelane

As can be seen from Fig. 2.1, the relationship between lgKd and lg[H+] is not completely linear. pH from 3
10−3 to 1
10−1, Kd decreased linearly, but the slope was − 0.6968, which was quite different from the ideal exchange reaction (slope was − 2) of H+/Cd2+ binary ion exchange; on the contrary, the Kd increased when pH = 1.5
10−1. It shows that the ion exchange of Cd2+ by natural cryptomelane is not only the selective exchange of H+–Cd2+ binary system but also the exchange of mixed ions. In other words, Cd2+ not only exchanges with H+ but also with K+ and other channel cations. With the increase of the concentration of HNO3 solution, the Kd of Cd2+ adsorbed by natural potash manganese ore decreased, that is, the adsorption capacity decreased. The above experiments can not exactly give the corresponding relationship between the adsorbed Cd2+ and the released H+ or K+, but it shows that Cd2+ enters the cryptomelane by ion exchange with the ions (K+ and H+) in the channel or occupying the vacancy in the channel. After adsorption of Cd2+, the XRD pattern of cryptomelane is the same as that before the reaction, and no new phase appears. The results show that Cd2+ can replace K+ and enter into the channels of natural cryptomelane, and the structure of cryptomelane remains unchanged. That is to say, K+ in the channels of natural cryptomelane can be replaced by Cd2+, which indicates that natural cryptomelane has good ion exchange property, and the exchanged ions can enter into the channels.

3
10−2 5.27

5
10−2 6.83

8
10−2 7.16

1
10−1 7.99

1.5
10−1 6.47

0.24

0.18

0.16

0.13

0.19

Adsorption rate/%

53.08

39.21

36.33

28.95

42.43

Kd/(mL/g)

45.26

25.80

22.82

16.30

29.48

2.2 Channel Structure Effects of Potassium Feldspar Tetrahedron

2.1.3 Remarks on the Reactivity of Nanomineral Aggregates Mn oxides are highly chemically active in soils, rocks, and natural waters. Specifically, we have found that the manganese oxide cryptomelane (KxMn8−xO16 where x ranges from 0.2 to 1.0) with densely packed, nanoaggregates can display highly efficient catalytic activity in aqueous solutions. Under SEM, the cryptomelane nanoaggregates are observed as assembled with nanocrystals in nanofibers, nanoneedle, and nanorod form and with the diameter ranging from 10 to 80 nm. It is well-known that naturally occurring nanominerals provide unimaginably large surface areas for earth-influencing interfacial reactions and the exchange of electrons, elements, and energy. In the broadest context, this exchange provides an important pathway by which the hydrosphere, atmosphere, and solid earth communicate with one another. However, it is often thought that such nanomineral aggregates will not show unusual properties associated with well-dispersed nanominerals. That is, these aggregates might be assumed to predominately exhibit the properties of bulk minerals rather than their constituent nanocrystals. Moreover, it may be unlikely for natural conditions to ever promote the dispersion of these aggregates, and if for some reason the nanocrystals are dispersed, to preserve them in a nanoscale dispersion. Previous studies suggest that mesoporosity and permeability, defined as channels and channels in the dimensional range of 2–50 nm, can exist in natural nanocrystalline aggregates (Hochella and Banfield 1995; Hochella 2002; Hochella et al. 2008; Banfield and Zhang 2001), perhaps allowing reactive contributions from the surfaces of nanominerals in aggregates. The present study directly demonstrated this phenomenon for a manganese oxide cryptomelane that all natural samples have slit-shaped mesopores. The mesopores give aggregates exceptionally small-scale porosity and permeability and provide an abundance of reactive sites on internal nanomineral surfaces, driving the hydrogen peroxide breakdown reaction (H2O2 H2O + 1/2O2) at a rapid rate. Importantly, we have also demonstrated that natural cryptomelane aggregates with different mesopore size distributions show different rates of H2O2 decomposition reaction. Nanomineral aggregates of Mn oxides are widespread near and on the earth’s surface, which most likely plays an important role, yet to date still a largely undetermined one, in the cycling of earth materials, environmental behavior, and remediation efforts (Hochella et al. 2005; Banfield and Zhang 2001). Geoscientists, as well as environmental and materials scientists, will play an important role in developing a much better understanding of the nano-effect in natural nanoscaled minerals. Finally, such mineral aggregates,

31

including what we have seen with cryptomelane in this study, have great potential for use as ion exchange, heavy metal immobilization, organic oxidation, and catalytic materials.

2.2

Channel Structure Effects of Potassium Feldspar Tetrahedron

2.2.1 Channel Structure Characteristics of Potassium Feldspar The feldspar group belongs to the framework of aluminosilicate minerals, with the general chemical formula M [T4O8]. Where T is a trivalent or tetravalent cation with a small radius (0.02–0.07 nm), such as Al, Si, and a small amount of B, Fe, P, Ti, and Ge; M is the alkali metal and alkaline earth metal cations with a large radius (0.09– 0.15 nm), such as Na+, K+, Ca2+, and Ba2+, and a small amount of Li+, Rb+, Cs2+, Sr2+, and NH4+. Most of the naturally occurring feldspars are included in the three-component series of K[AlSi3O8]–Na[AlSi3O8]–Ca [Al2Si2O8], which is equivalent to a combination of three-terminal molecules of potassium feldspar (Or), albite (Ab), and anorthite (An). Therefore, the general formula of the chemical composition of feldspar minerals can be expressed as OrxAbyAn1−(x+y) or KxNayCa1−(x+y)[Al2−(x+y)Si2 +(x+y)O8]. Where 0  (x + y)  1, x is the mole fraction of potassium feldspar (Or); y is the mole fraction of albite (Ab); 1 − (x + y) is the mole fraction of anorthite (An) (Ribbe 1983). When the ammonia-containing hot spring acts on potassium albite, ammonium feldspar (NH4[AlSi3O8], Buddingtonite) can be formed. The minerals of the feldspar group include two series: the sub-potassium albite and the sub-plagioclase. Potassium endmembers of potassium albite are commonly referred to as potassium feldspar (K[AlSi3O8]), including sanidine (monoclinic), orthoclase (monoclinic), and microcline (triclinic). Sanidine has the highest crystallization temperature, followed by orthoclase and microclinte (Ribbe 1983).

2.2.1.1 Chemical Composition The theoretical chemical composition of orthoclase is K2O 16.9%, Al2O3 18.4%, and SiO2 64.7%, while naturally produced orthoclase always contains more or less Ab, usually up to 20%, and sometimes even up to 50%. It can also contain a small amount of An, which increases with the Ab contents. The chemical composition analysis of the feldspar in the 300 Ma granite rock mass collected from Barkol, Eastern Junggar, Xinjiang using Electron Probe Microanalyzer was shown in Table 2.2 (the data in the table are the average

32

2

Environmental Effects of Tunnel Structure Minerals

Table 2.2 Chemical composition analysis of alkaline feldspar samples (wt%) Sample

SiO2

Al2O3

Na2O

K2O

CaO

MgO

FeO

MnO

BaO

Or

Ab

An

Colorless

55.97

27.53

6.19

10.03

0.01

0.06

0.05

0.02

0.01

51.6

48.3

0.1

Pink

54.67

28.33

6.43

9.26

0.61

0.06

0.25

0.05

0.02

47.7

49.6

2.7

Zuo et al. (2022)

values of multiple points). The chemical composition is uniform, and the formulas of colorless and pink feldspar are Or51.6Ab48.3An0.1 and Or47.7Ab49.6An2.7 respectively, suggesting orthoclase.

2.2.1.2 Crystal Structure The space group of monoclinic sanidine is C2/m, which has the highest degree of symmetry and the simplest structure in the feldspar group. Because of its relatively simple structure, it is also the most thoroughly researched feldspar species so far. Therefore, the crystal and channel structure characteristics of feldspar are described here by taking sanidine as an example. The most important structural unit in feldspar crystals is a 4-ring composed of [TO4] tetrahedrons. There are two types of 4-ring, one is the ð201Þ 4-ring near the perpendicular to the axis a, and the other is the (010) 4-ring perpendicular to the axis b. Both of them are composed of two non-equivalent [TO4] Tetrahedron (T1 and T2) compositions. Oxygen is located at the corner apex of a nearly regular tetrahedron centered on Al and Si. All oxygen is shared by two T atoms to form a framework structure (Ribbe 1983). Machatschki (1928) first recognized the three-dimensional framework of [AlO4] and [SiO4] tetrahedrons in feldspar. Taylor (1933) and Taylor et al. (1934) measured the characteristics of the framework of feldspar and found that the key structural unit of feldspar is a four-membered ring composed of TO4 tetrahedrons. When it shares an angular vertex with a similar four-membered ring, a crank-shaft-type double chain parallel to the axis a is formed. The reveal of the crank-shaft structure is an important achievement in the history of feldspar structure research. To better understand the crystal structure characteristics of feldspar, the mineral structure of feldspar can be observed from different directions. The projection along the axis a direction shows that a 4-ring is composed of two pairs of non-equivalent T1 and T2 tetrahedrons, where the corners of a T1-T2 pair are upward (U). The other diagonal is top-down (D), and the four O2− (at 2.1 and 6.3 Å) in the middle are common oxygen. Along the axis a direction, the U tetrahedron and D tetrahedron are always connected in a broken line with a common angle. This direction forms a hollow structure. However, the structural channels in this direction are too small to let other ions in.

The projection along the axis c direction (Fig. 2.2) shows that the double crankshaft chain in the axis b direction is connected through the adjacent T2 corner OA2 oxygen atoms, thereby creating a crankshaft chain layer. The OA2 atom is located on the (010) symmetry plane (or pseudo-symmetry plane), and the OA1 oxygen atom is located on the secondary axis or pseudo-secondary axis. In short, the T2 tetrahedron only serves to connect the crankshaft within the layers, while the T1 tetrahedron is the connection between the layers. Some people describe the ideal projection of the feldspar grid on the (001) plane as a “dogface” projection. The feldspar structure forms a channel structure in the [001] direction, and the channels are occupied by K+, Na+, Ca2+, and Ba2+ plasma. Figure 2.2 shows the characteristics of the tetrahedral chains along the [110] and [1–10] directions [TO4] in the feldspar grid. They form an open tetrahedral chain. The internal space is relatively large and is occupied by large metal ions K+, Na+, Ca2+, Ba2 + , etc. The four Ts in [TO4] forming the 4-ring in the triclinic feldspar structure are not equivalent, presented as T1(o), T1(m), T2(o), and T2(m) respectively.

2.2.1.3 Microstructure Characteristics Figure 2.3 shows the TEM lattice fringes of the potash feldspar sample (collected from the Balikun granite in East Junggar, Xinjiang in Table 2.3), which are regular intersecting diamond patterns. The cross-web pattern is uniform throughout the image, suggesting the integrity of the structure. These stripes are made up of two sets of nearly orthogonal patterns with regular changes in contrast. The main spot in the center of the corresponding diffraction pattern is bright with small diffraction spots distributed around it in a pseudo-hexagonal shape, showing the monoclinic symmetry. The lattice fringe width is 0.59 nm and 0.65 nm respectively, corresponding to the (110) and ð111Þ crystal planes. 2.2.1.4 Channel Structure Characteristics

1. Channel Aperture Feldspar, like zeolite, belongs to the framework silicate minerals, in which the silicon-oxygen tetrahedrons are connected to form a framework. The tetrahedral layer in the zeolite and calcium zeolite framework is similar to

2.2 Channel Structure Effects of Potassium Feldspar Tetrahedron

33

Fig. 2.2 The ideal projection of feldspar structure along axis c on the plane (001) (Lu et al. 2006)

smaller and the channel structure cannot be formed. Therefore, the development of the channel structure in feldspar minerals is related to the mineral composition and symmetry. Generally, the higher the symmetry, the larger the aperture of the channel and the more obvious the channel effect.

Fig. 2.3 Lattice image and diffraction pattern of feldspar in Balikun granite body, East Junggar, Xinjiang (Zuo et al. 2022)

the feldspar minerals, and they are all composed of tetrahedral rings through shared corner oxygen. The silicon-oxygen tetrahedrons are connected to form a crank-shaft-type chain, which extends along the axis a direction; the chain and the chain are connected to form a three-dimensional framework through bridge oxygen. The crystal structure of feldspar minerals is similar, and all the corners of the [TO4] tetrahedron are connected in a frame shape. In low-symmetric feldspar minerals, such as plagioclase, due to the deformation of the framework, the symmetry is reduced, so that the lattice channels become

2. Channel Ion Types The diameter of the filled ions in the feldspar channels directly affects the size. When there are cations K+ and Ba2+ with larger radius in the channels, they can support the TO4 tetrahedral framework and form large and regular coordination polyhedrons, and the crystals appear as monoclinic symmetry. If smaller Na+ and Ca2+ cations are filled in the channels, these ions are smaller than the channels of the framework, causing the framework to collapse, and the coordination polyhedron becomes irregular to form a triclinic system (Smith 1974). Among the feldspar minerals, the FD of buddingtonite is the lowest (Table 2.3), and the structure porosity is the largest. Table 2.3 lists the structural characteristics of different feldspars, showing the cation radius of the six-coordinate channels. The types of channel ions in the feldspar structure vary greatly, from Li+ with a radius of 0.72 Å to Cs+ with a radius of 1.70 Å, indicating that the feldspar channel structure can be filled with cations of different radii.

2.2.1.5 Channel Structural Phase Transition Under certain pressure conditions, the channel structure of feldspar can be transformed into a new channel structure

34 Table 2.3 Structural characteristics of aluminosilicates of composition M[AlSi3O8] and M[Al2Si2O8]

2

Environmental Effects of Tunnel Structure Minerals

Composition

Name

r/Å

Vox/Å3

FD

Li[AlSi3O8]

Virgilite

0.74

20.7

25.6

Na[AlSi3O8]

Low-albite High-albite

1.02

20.8 20.8

25.6 23.9

K[AlSi3O8]

Low-microcline Sanidine

1.38

22.5 22.5

22.2 21.8

Rb[AlSi3O8]

–

1.49

23.3

21.7

Cs[AlSi3O8]

–

1.70

n.d

n.d

NH4[AlSi3O8]⋅nH2O

Buddingtonite

1.43

n.d

21.6

Mg[Al2Si2O8]

–

0.72

* 21.1

n.d

Ca[Al2Si2O8]

Anorthite

1.00

20.9

n.d

Sr[Al2Si2O8]

Slawsonite

1.16

21.7

n.d

Ba[Al2Si2O8]

Paracelsian

1.36

23.3

n.d

Ba[Al2Si2O8]

Celsian

1.36

23.0

n.d

Ba[Al2Si2O8]

Hexacelsian

1.36

23.6

n.d

Notes The table is modified according to Liebau (1985). r means the cationic radius when the coordination number is 6; Vox means the volume of per oxygen atom; FD means the skeleton density (Baur and Joswig 1996); n.d. means not measured

phase. Ringwood et al. (1967) succeeded in the transformation from diopside to hollandite structure under the conditions of 900 °C and 12 GPa. The hollandite type KAlSi3O8 is the only stable potassium-containing silicate mineral under the conditions of the lower mantle known so far. It belongs to the tetragonal crystal system. Al and Si atoms are six coordinated, a = 0.938 nm, c = 0.274 nm (Smith 1974). According to the high-temperature and high-pressure experiment, with the increase in pressure, albite can become a hollandite-type structure at 10–28 GPa and 1000 °C, and the volume will be reduced by 5.3%. Although the degree of symmetry of feldspar minerals is relatively low, the structure can accommodate large-radius rubidium, potassium, and sodium ions, which is closely related to the existence of large polyhedral voids in the structure. In the basanite-type high-pressure polytype feldspar structure, silicon (aluminum) forms [(Si,Al)O6] octahedrons. The octahedrons are connected with a common edge to form a double-strand parallel to the axis c, and the double strands are connected by corners to form a frame-like structure and form a large channel parallel to the axis c (Chen et al. 1995). Large radius alkaline ions Na+, Ca2+, K+, etc. are distributed in these large channels. The discovery of feldspar high-pressure polytype in nature indicates that alkaline elements exist in the form of feldspar high-pressure polytype in the deep earth, especially in the lower upper mantle and transition zone. It has become one of the main carrier minerals of large radius cations such as Na+, K+, Rb+, Sr2+, and Ba2+ in the deep earth.

2.2.2 Ion Exchange Effect of Potassium Feldspar Channels Feldspar has not been classified as a channel structure mineral for a long time. Because the ion exchange effect of feldspar is not as good as that of large channel zeolite minerals, and it belongs to the category of microchannels. There are various trace elements in feldspar, such as rubidium, strontium, barium, lead, etc. (Heier 1962; White et al. 2003), which exist in the structural channels of feldspar.

2.2.2.1 Exchange of Feldspar Channel Ions and Na+ Ions in High-Temperature Melt At 900 °C and one bar pressure, the plagioclase is transformed to low albite in the NaCl melt and then reacts with the KCl melt to become plagioclase (Orville 1963). Using this method can form albite and potash feldspar end-member components. Muller (1988) used molten NaCl melt to perform ion exchange with diatomite (KAlSi3O8) to form albite. Then it is treated with acid to produce hydrogen feldspar, which is further mixed with lithium nitrate melt to form lithium feldspar. Viswanathan (1972) mixed the feldspar with molten KCl and heated it for a short time at 810 °C. The Na+ in the feldspar could be replaced by K+ to form potassium feldspar. Conversely, when potassium feldspar is mixed with molten NaCl, Na+ ions can also replace K+ to generate albite. Chou and Wollast (1985, 1989) believed that alkaline feldspar can undergo repetitive ion exchange. Based on predecessors, we carried out ion-exchange experiments of feldspar and NaCl melt with different

2.2 Channel Structure Effects of Potassium Feldspar Tetrahedron

35

concentrations and different reaction times. After 99.9% purity, NaCl is ground in an agate mortar and mixed with potassium feldspar powder with a particle size of 60 lm. The mixing ratios are 2 g feldspar and 20 g NaCl and 2 g feldspar and 40 g NaCl respectively. Stir the NaCl and potassium feldspar powder evenly and put them into the crucible, and carry out the ion exchange experiment at the temperature of 810 °C After the reaction, the product is repeatedly washed with deionized water to remove unreacted NaCl and then dried at low temperature. The experimental results showed that the Na2O content in the original potassium feldspar before ion exchange was 3.35%, and the Na2O content increased significantly after the exchange (Table 2.4). After adding 20 g of NaCl for 12 h, the Na2O content in potassium feldspar powder reached 9.05%, and 11.9% after 72 h of reaction; after reacting with 40 g NaCl for 12 h, the Na2O content of potassium feldspar reached 9.96%, and after 72 h of reaction, it reached 15.9%. Potassium feldspar and NaCl undergo an ion exchange reaction. After reacting with 20 g NaCl for 30 h, the increase rate of Na2O content in feldspar became relatively slow, and the Na2O content only increased by 0.2% from 30 to 36 h. From 36 to 72 h, the Na2O content in feldspar only increased by 1.3%. This means that the reaction has reached a relatively balanced state. For an ion-exchange reaction with 40 g NaCl, the ion exchange effect is better than that with 20 g NaCl. This shows that the degree of feldspar ion-exchange reaction has a certain relationship with the concentration of the exchanger. When 40 g NaCl reacted with 2 g feldspar for 12 h, the content of Na2O in feldspar increased by 6.5%, which was greater than the ion exchange capacity after 24 h of reaction with 20 g NaCl sample, indicating that the reaction speed was faster. After 30 h, the reaction speed suddenly increased, but after 36 h, the reaction speed slowed down. Finally, the Na2O content of feldspar reached 15.9% after 72 h. From the two sets of ion exchange data, the alkaline ions in the feldspar channels have good chemical activity, and the exchange reaction between the alkaline ions has an obvious effect. After the ion exchange reaction of 2 g feldspar and 40 g NaCl, the Na2O concentration change test showed (Table 2.4) that the Na2O content in the reaction product increased with the increase of the exchange reaction time, and after 72 h, the equilibrium point was still not reached. It shows that the ion exchange reaction between Na+ and feldspar channel ions can continue.

Table 2.4 The change of Na2O content in ion-exchange reaction between feldspar and NaCl molten salt over time XRF data

As seen from the research results of molten salt statistical theory. Usually, molten salt can be regarded as a liquid composed of ions, the chemical bonds between ions are regarded as pure ionic bonds, and the halides of Na, Ca, Sr, and Ba are regarded as ionic melts. At higher temperatures, cations can obtain sufficiently high energy, which increases the diffusion rate of ions and makes the exchange reaction easier (Duan and Qiao 1990). The ion exchange experiment of feldspar shows that when there are different types of ions, feldspar with a high degree of symmetry is prone to ion exchange because the diameter of the channels of the feldspar with a high degree of symmetry is larger. However, when K+ is used to replace Na+, the ability of feldspar to release Na+ from its crystal is weak (Demir et al. 2001). The difference in ion radius can explain this phenomenon. The larger radius of K+ (1.38 Å) is more difficult to ion exchange with feldspar than the smaller radius of Na+ (1.02 Å), that is, it is difficult for K+ to enter the albite crystal lattice and undergo ion exchange reaction with Na+ in it. The ion exchange between feldspar and NaCl melt is a multiphase reaction between solid and melt. The exchange process includes: (1) The cations in the melt diffuse into the channel structure through the surface of the feldspar; (2) The cations diffused into the feldspar channels exchange reaction with channel ions; (3) The cations exchanged from the feldspar channels diffuse into the melt. The diffusion of ions in the feldspar structure is the key to achieving ion exchange. The ion exchange rate is controlled by the structural characteristics of the mineral. People have noticed the ion exchange performance of feldspar minerals for a long time (O’Nell 1967; Smith 1974), but due to the slow exchange rate, the research on it is limited to the study of mineralization theory, such as the migration and diffusion of elements. The ion exchange reaction of feldspar in the NaCl molten salt system can be expressed by the following equation (Eq. 2.5): 0 1 KA K½ðAl; SiÞ4 O8  þ NaCl , ½Na@ Al; Si A O8  þ KCl ð2:5Þ KB 4 In the formula, K+ and Na+ are exchangeable cations. The apparent distribution coefficient is defined as Eq. 2.6: K¼

Activity of Na ion in the feldspar phase The activity of Na ion in flux

ð2:6Þ

NaCl quality/g

20 g

The reaction time/h

12

24

30

36

72

12

24

30

36

72

Na2O content/%

9.05

9.33

10.4

10.6

11.9

9.96

11.5

12.7

14.7

15.9

Liu et al. (2006)

40 g

36

2

The symbol KA (A = adsorption) indicates the distribution coefficient of the cation Na+ from the salt phase into the feldspar phase; KD (D = desorption) indicates the distribution coefficient of the cation K+ from the feldspar phase into the salt phase. If KA = KD, the exchange reaches equilibrium, otherwise it does not reach equilibrium, indicating a hysteresis effect. According to the theory of ion exchange reaction, the kinetics of ion exchange reaction is controlled by two independent processes, the mineral-molten salt interface exchange and the diffusion of cations in the mineral. Partition coefficient K The number of Na ions exchanged per gram of feldspar ¼ The number of Na ions per gram of NaCl

ð2:7Þ The difference between the ion exchange partition coefficient and 1 is a measure of the hysteresis effect (Eq. 2.7). The closer this value is to 1, the greater the hysteresis effect of ion exchange. The apparent distribution coefficient of feldspar minerals is between 0.108 and 0.237 (Table 2.5), and there is a certain hysteresis from the numerical point of view. The ion exchange value of feldspar and NaCl molten salt increases with the increase of the reaction time. After 30 h of reaction, the change in the amount of ion exchange was significantly smaller, and the reaction was close to equilibrium (Fig. 2.4). The hysteresis effect becomes smaller with the extension of the reaction time, and the ion exchange reaction becomes more thorough. The ion exchange properties of channel minerals are mainly related to the structural characteristics of the mineral itself, the nature of the exchange cations, and the exchange conditions. Due to the small channel size of the feldspar, the exchange ions cannot exhibit a complete ion sieve effect, which is common property and characteristic of some microporous channel structure minerals (Zhang et al. 1986). Feldspar has a relatively dense framework, which can accelerate the ion exchange reaction of the channels under heating conditions. This is because the increase in

Table 2.5 Distribution coefficient of feldspar and NaCl molten salt ion-exchange (Liu et al. 2006)

Environmental Effects of Tunnel Structure Minerals

temperature can increase the activation energy of ions and accelerate the diffusion rate of ions. In the process of ion exchange between feldspar and molten salt, if the diameter of the cation is slightly larger than the free channel diameter of the mineral channel, the exchange can still occur due to the vibration of the feldspar oxygen atom ring. Therefore, the channel effect is also subject to certain conditions. In the process of ion exchange of channel minerals, the temperature often affects the ion exchange capacity. Some cations are large in diameter and cannot enter the mineral channels under normal circumstances. The high temperature increases the flexibility of the mineral crystal lattice and enlarges the channel to make ions enter it more easily. The ion exchange reaction of feldspar is mainly a partial ion sieve effect, which is an incomplete exchange caused by steric effects. Because of the single channel structure in the feldspar, the ions in the melt enter the feldspar channels and occupy the passage of ions before the exchange reaction is completed. This blocks the path of exchangeable ions that are not substituted inside, resulting in an incomplete ion-exchange reaction. In feldspar minerals, due to the difference in bond strength between ions of different valences and the [SiO4] framework, the mobility of high-valence cations is small, while the mobility of monovalent alkali metal ions is relatively large. The feldspar ion exchange capacity calculated by Pelte et al. (2000) is 3.9 meq/g, which is higher than 2.62 meq/g of mordenite, 3.86 meq/g of erionite, 3.45 meq/g of heulandite, and 2.64 meq/g of clinoptilolite. Therefore, feldspar has a certain ion exchange capacity and can be used as an ion exchanger. Feldspar can also exchange reactions with the ions in the aqueous solution to change the mineral composition and form feldspar containing “different impurity elements”, but the reaction speed is relatively slow. The speed of ion exchange reaction in the salt melt is much faster than that at room temperature. The increase in temperature accelerates the progress of the chemical reaction and facilitates the quantitative control of the reaction (Duan and Qiao 1990). Therefore, the high-temperature ion-exchange reaction can

Na2O percentage

Exchanged Na+ number

Ion partition coefficient KA/KD

20 g NaCl

40 g NaCl

20 g NaCl

40 g NaCl

20 g NaCl

40 g NaCl

0

3.35

3.35

0

0

0

0

12

9.05

9.96

24

9.33

Ion exchange time/h

1.11E+21

1.28E+20

0.108

0.125

11.5

1.16E+21

1.58E+21

0.113

0.154

30

10.4

12.7

1.37E+21

1.82E+21

0.133

0.176

36

10.6

14.7

1.41E+21

2.21E+21

0.137

0.214

72

11.9

15.9

1.66E+21

2.44E+21

0.161

0.237

2.2 Channel Structure Effects of Potassium Feldspar Tetrahedron

37

Fig. 2.4 Ion exchange partition coefficient of feldspar and NaCl. a 2 g feldspar and 40 g NaCl; b 20 g NaCl (Liu et al. 2006)

be used as an important method to study the effect of mineral microchannels.

2.2.2.2 Fixing Pb in the Feldspar Channels of Medium Temperature Powder Potash feldspar is one of the rock-forming minerals with higher lead content in magmatic rocks. Wedepohl (1974) studied the Pb content in 638 potash feldspar samples in granite, reaching n
10−6–300
10−6. In alkaline rocks with a feldspar content of 50–70%, the Pb content in feldspar accounts for about 70–95% of the total Pb content of the whole rock, indicating that potash feldspar is mineral-rich in trace lead. This is because feldspar and lead feldspar have similar structures and can form a complete solid solution series with potash feldspar, so Pb can be preferentially present in potash feldspar (Doe and Tilling 1967). The ion radius of Pb2+ is 0.119 nm, which is close to K+ (0.138 nm) and Na+ (0.102 nm). Pb2+ can exchange K+ and Na+ plasmas in the feldspar channels and enter the feldspar channels to form lead feldspar. In nature, lead feldspar generally does not exist in the form of a separate phase, and Pb is often present in various types of feldspar, especially potash feldspar, as a trace component (Zhang et al. 2004). Fouque and Levy once mixed the oxides of the corresponding components in a ceramic crucible and melted them at a high temperature to synthesize triclinic lead feldspar (Pb[Al2Si2O8]) with a density of 4.093 g/cm3 (Scheel 1971). Sorrell (1962) reacted kaolinite and halloysite with Sr, Ba, and Pb sulfates to synthesize a triclinic lead feldspar phase material with a content of about 10%. When Scheel (1971) synthesized spinel, a light yellow transparent long strip crystal appeared on the bottom of the ceramic cover, which was a solid solution of Pb[Al2Si2O8]-K[AlSi3O8], and its crystal structure and lattice constant showed monoclinic Crystalline lead

feldspar. Farquhar et al. (1997) used streak feldspar and Pb (NO3)2 solution to mix. The results show that Pb2+ and feldspar have an obvious ion exchange reaction, 80% of the K+ in feldspar will undergo ion exchange, and the ion exchange reaction speed will increase under acidic conditions. The potassium feldspar and Pb(NO3)2 were mixed and stirred, and then heated at 380 °C for reaction. XRD analysis showed that the d values of the five-strong peaks of the reaction product were 6.54, 3.42, 3.32, 3.27, and 2.57 Å (Table 2.6). It is very similar to Scheel’s synthetic lead feldspar with 5 strong peaks whose d values are 6.53, 3.45, 3.32, 3.27, and 2.56 Å. Although there is a certain difference in the intensity of the diffraction peaks, several characteristic peaks of the lead feldspar in the exchange product are obvious, which undoubtedly indicates that a new Pb-feldspar phase is formed. As for the difference in the intensity of the diffraction peak, it may be affected by the original mineral potassium feldspar. The diffraction peak of lead feldspar interferes with the diffraction peak of the original phase, which changes the intensity of the diffraction peak. Theoretical issues such as the position of Pb in the feldspar structure have been unresolved. Based on previous work and the results of the above ion-exchange experiments, from the analysis of the structural characteristics of feldspar, it is believed that the existence of Pb in the lead feldspar is related to the channel structure of the feldspar. The ionic radius of Pb2+ is close to that of K+ and Na+. Under certain temperature conditions, Pb2+ can completely exchange K+, Na+, etc. in the feldspar channels and enter the feldspar channel structure to form lead feldspar. Taking into account the channel structure characteristics of feldspar and the ion exchange performance, Pb2+ replaces K+, Na+, Ca2+, etc. in the feldspar channel structure, forming a new phase. The ion exchange reaction formula is shown as Eq. 2.8:

38

2

Table 2.6 XRD characteristics of lead feldspar, synthetic lead feldspar, and orthoclase

Environmental Effects of Tunnel Structure Minerals

Lead feldspar Pb [Al2Si2O8] (in this book)

Lead feldspar Pb[Al2Si2O8] (Scheel 1971)

Potash feldspar (K, Na)[AlSi3O8] (in this book)

d/Å

I/I0

d/Å

I/I0

hkl

d/Å

I/I0

6.54

72

6.53

100

110

6.50

6.1

4.60

20

4.60

20

021

4.58

1.0

3.80

47

3.80

35

111

3.78

6.0

3.61

19

3.61

20

131

3.56

30

3.42

47

3.45

72

112

3.48

10.7

3.32

98

3.31

50

202

3.35

29.2

3.27

100

3.26

50

002

3.25

100

2.98

81

3.00

25

131

2.96

1.7

2.57

19

2.56

60

331

2.56

3.9

Liu et al. (2006)




 K; Na; Ca1=2 ½AlSi3 O8  þ Pb2 þ
 ! Pb½Al2 Si2 O8  þ K þ ; Na þ ; Ca2 þ

ð2:8Þ

XPS was used to determine the bonding characteristics of feldspar and Pb2+ before and after the reaction (Table 2.7), and the binding energy error was ± 0.1 eV. The results (Fig. 2.5) showed that the binding energy of Pb 4f7/2 in the reaction product was 136.81–138.44 eV. The binding energies of PbO, Pb(OH)2, Pb(NO3)2, and PbSiO3 are 137.2, 137.3, 138.3, and 138.45 eV, respectively (Farquhar et al. 1997). The binding energy of Pb in feldspar has a similar binding energy to other Pb-containing compounds, indicating that they all have a strong binding ability to Pb. Lead compounds are dominated by ionic bonds, with strong bonds. This indicates that the lead in feldspar exists in the form of Pb2+, occupying the positions of channel ions such as Ca2+, K+, and Na+ in the channel structure.

Fig. 2.5 The XPS spectrum of the ion exchange reaction product of feldspar and Pb(NO3)2 and the partial enlargement of the binding energy of Pb 4f (Liu et al. 2006)

2.2.2.3 Fixing Cd in Feldspar Channels in Solution at Room Temperature Prepare 7 Cd2+ solutions with a concentration of 25 mg/L with CdCl2, mix with 325 mesh potassium feldspar powder

at pH 5 and room temperature, and set the reaction time to 12, 24, 48, 72, 96, 120, and 720 h. After the reaction, the supernatant was poured out, the concentration was tested,

Table 2.7 Binding energy and content of Pb2+ and channel ion XPS in feldspar minerals

Spectral line

Sample

Binding energy/eV

Content/ %

Sample

Binding energy/eV

Content/ %

Na 1s

1#

1072.01

1.186

2#

1071.90

1.267

Ca 2p3

348.10

0.219

347.90

0.166

K 2p3

293.07

2.72

293.11

1.829

1091.93

0.165

1071.98

0.582

0

0

0

0

Na 1s

1#Pb-feld

Ca 2p3

2#Pb-feld

K 2p3

293.06

0.696

293.02

1.625

Pb 4f7

137.03

0.932

136.81

0.501

Pb 4f7

138.44

7.453

138.09

4.599

Liu et al. (2006)

2.2 Channel Structure Effects of Potassium Feldspar Tetrahedron

Fig. 2.6 The removal rate of Cd2+ by feldspar at different times (Liu et al. 2006)

and the Cd2+ removal rate was calculated (Fig. 2.6). After a 120-h reaction, the removal rate was 96.25%; after a 720-h reaction, the removal rate reached 97.94%. After 120 and 720 h, the Cd2+ concentration in the solution did not change significantly, reflecting that the Cd2+ concentration in the solution reached equilibrium. Therefore, 120 h can be considered as the equilibrium time for feldspar to remove Cd2+. The potassium feldspar powder after 720 h of reaction was filtered out, rinsed repeatedly with deionized water, dried in an oven, and then subjected to an XRD test (Fig. 2.7). After the reaction, two new characteristic diffraction peaks with d values of 4.70 and 2.78 Å appeared in the product, and the original three diffraction peaks of 6.40, 3.17, and 2.97 Å were enhanced. Compared with the cadmium silicate XRD spectra numbered 31-0217 in the JCPDS standard card, the above five diffraction peaks are very similar or even the same as the corresponding five characteristic diffraction peaks of the cadmium-containing silicate phase. And it is consistent with the XRD diffraction

Fig. 2.7 XRD diagram of potassium feldspar sample and its reaction product with CdCl2 solution (Liu et al. 2006)

39

peak characteristics of cadmium aluminosilicate synthesized in the melt with a chemical composition of Cd[Al2Si2O8]. It fully shows that Cd2+ in the solution reacts with potassium feldspar and enters the potassium feldspar to form the cadmium feldspar phase, that is, the ion exchange reaction between Cd2+ and potassium feldspar channel ions. The XPS test of the reaction product of potassium feldspar and CdCl2 solution showed that the average binding energy of Cd 3d5 of the product was 406.6 eV, while the binding energy of CdCl2 was 405.5 eV (Fig. 2.8). This indicates that Cd2+ may enter the channels of potassium feldspar, and the chemical bonding environment of Cd2+ in potassium feldspar is different from that of CdCl2. The binding energy of Cd2+ in the potash feldspar structure is larger than that in CdCl2 (Seyama and Soma 1984), indicating that potash feldspar has a stronger binding capacity for Cd2+. The result of XPS analysis of the reaction product also showed that the K+, Na+, and Ca2+ contents in potassium feldspar have changed significantly (Table 2.8), decreasing by 0.929%, 0.269%, and 0.219% respectively. Ca2+ is no longer detectable on the surface of the product, and 0.584% of Cd2+ appears. This is also the result of a partial exchange reaction between potassium feldspar channel ions and Cd2+. Although XPS shows the near-surface information of minerals, it also reflects the changes in the composition of the near-surface channels of feldspar minerals from one side. This is consistent with the appearance of Cd-containing silicate in the XRD of the reaction product. In microporous mineral materials, due to the small diameter of the channels of some minerals, the binding of channel ions is stronger. Under normal temperature conditions, the ion exchange of the channels mainly occurs near the surface of the mineral, and cannot occur in the deep layer

Fig. 2.8 XPS full spectrum of the reaction products of potassium feldspar with CdCl2 solution and local amplification of Cd 3d binding energy (Liu et al. 2006)

40 Table 2.8 Content analysis of potassium feldspar channel ions and Cd2+ by XPS method

2 Sample

Oxide

Before reaction

K2O

9.29

K 2p3

2.720

Na2O

3.35

Na 1s

1.186

CaO

1.45

After the reaction

wB/%

Environmental Effects of Tunnel Structure Minerals

Spectral line

Content/%

Change after reaction and before reaction

Ca 2p3

0.219

K 2p3

1.791

K 2p3

− 0.929

Na 1s

0.917

Na 1s

− 0.269

Ca 2p3

0.000

Ca 2p3

− 0.219

Cd 3d5

0.584

Cd 3d5

+ 0.584

Liu et al. (2006)

of the mineral. The content of channel ions K+, Na+, and Ca2 + on the surface of feldspar decreases, while the content of Cd2+ increases, indicating that Cd+ occupies the cation positions in the channels, and not only surface adsorption occurs. The above experimental results at least show that under low-temperature conditions, some channel ions such as Na, K, or Ca in feldspar undergo ion exchange reactions with Cd ions, that is, feldspar has a certain channel ion exchange effect under low-temperature conditions.

2.2.2.4 The Feldspar Channels Block the Migration of Nuclide Radioactive nuclear waste is a kind of extremely harmful solid waste. It takes tens of thousands of years or even longer for the long-lived high-radioactive nuclear waste to be harmless. For example, the half-lives of radionuclides 238 U, 129I, 99Tc, 239Pu, 59Ni, and 94Nb are all over ten thousand years, and the half-life of 238U is as long as 4.47 billion years (Witherspoon 1996). So for the radioactive waste, especially high-level radioactive waste from the nuclear power plant isolated from the biosphere and providing reliable permanent disposal methods, not only developed countries but also developing countries are extremely concerned but have yet to officially build a country disposal repository, both at the site primary and evaluation stages, such as underground laboratory research and natural analogy (Wang et al. 2006a, b). In the disposal of radioactive nuclear waste, blocking the migration of radionuclides is the key problem, and the geological body is the host of the geological disposal repository of high-level nuclear waste and the last barrier to blocking the migration of radionuclides to the environment. Therefore, it is very important to select a geological body with good stability for the disposal of high-level nuclear waste (Chen 2000). At present, granite is preliminarily selected as the surrounding rock of the disposal reservoir with natural barrier function according to the geological conditions in most countries, including Argentina, Bulgaria, Canada, China, Japan, Czech

Republic, Finland, France, India, South Africa, Spain, Sweden, Switzerland, Ukraine (Witherspoon 1996). It should be noted that in many standards for site selection, the channel structure of the surrounding rock and the characteristics of mineral surface fractures, and the effective migration and diffusion rate of nuclides in the rock medium are designated as key indicators. In addition, the rock is required to have sufficiently high homogeneity and continuity in physical and chemical properties and sufficiently low permeability (IAEA 1994). Special studies have been conducted on how granite prevents the migration of nuclides (Eriksen and Locklund 1989; Ticknor and Cho 1990). We believe that these evaluation indicators only stay on the macroscopic level of whether there are fractures in the rock mass and whether there are fissures in the rock, but do not give a sufficient understanding of the microscopic details of the channel characteristics of the internal structure of the minerals that make up the rock. In recent years, in-depth analysis of the variability of artificial barriers and their blocking effects on radionuclides from the mineralogical level has received increasing attention (Curtis and Morris 2013; Cotter-Howells et al. 2000). It needs to be emphasized that in addition to radiation effects, high-level radioactive waste heats up due to radionuclide decay, which can make the surrounding rock temperature up to 200 °C (Witherspoon 1996). The thermal disturbance also promotes changes in the properties of the surrounding rock, which will greatly increase the rate of nuclide migration in the surrounding rock. Feldspar minerals are the main constituent minerals of granite, and the K-feldspar content in tuff is close to 50%. Therefore, the structural state of feldspar minerals largely determines the degree of natural barrier between granite and tuff. The atomic radius of typical high-radioactive nuclides is smaller than the K, Na, and Ca existing in the channels of feldspar minerals (Table 2.9). It is reasonable to believe that since alkali metal ions can enter the channels of feldspar minerals, radionuclides of the same size and more active are undoubtedly easier to enter the channels of feldspar minerals.

2.3 Tubular-Texture Effects of Fibrous Serpentine Table 2.9 Atomic radii of typical high radionuclides and size of fillings in feldspar channels

41

Atomic species

238

129

99

59

94

232

79

Radius/nm

0.152

0.133

0.136

0.124

0.146

0.180

0.140

Atomic species

210

107

93

Ca

Na

K

H2O

Radius/nm

0.141

0.137

0.160

0.197

0.190

0.235

0.138

U Po

I Pd

Tc Zr

Ni

Nb

Th

Se

Liu et al. (2006)

It is precisely the well-developed channel structure of feldspar minerals in granite and tuff that allows nuclide to enter the channel, and it is possible to block the migration of nuclide and become a natural barrier.

2.3

Tubular-Texture Effects of Fibrous Serpentine

Fibrous serpentine, also known as chrysotile, is a traditional industrial mineral. It has many excellent properties, such as heat resistance, heat insulation, low thermal conductivity, high resistance, high tensile strength, and good flexibility. In addition, it has been widely used in traditional industries. However, fibrous serpentine is detrimental to humans. Long-term unprotected exposure to the fibrous dust probably causes diseases such as asbestosis and mesothelioma. Even so, fibrous serpentine itself has many incomparable excellent properties, so there are not plenty of ideal artificial substitutes. Therefore, how to seek advantages and avoid disadvantages so that chrysotile can not only avoid harm to the environment but also make the best use of it is a problem worthy of in-depth discussion.

2.3.1 Crystal Structure of Fibrous Serpentine The ideal chemical formula of fibrous serpentine is Mg6[Si4O10](OH)8, which belongs to 1:1 trioctahedral layered silicate minerals, and the structural unit layer is formed by the combination of silica tetrahedron (T) and magnesite hydroxide octahedron (O) at a ratio of 1:1. The silica tetrahedron slices are connected into a [Si2O5]n hexagonal network, and the reactive oxygen species are all located on one side of the tetrahedron slices. Reactive oxygen species and hydroxyl groups act together as anions to form Mg–O2(OH)4 octahedral interlinked “brucite” plates (Fig. 2.9a). In the serpentine structure, the tetrahedral sheets are basically connected by covalent bonds, the octahedral sheets are mainly connected by ionic bonds, and the tetrahedral sheets and the octahedral sheets are also connected by ionic bonds. Each structural unit layer is connected to adjacent unit layers by weak molecular bonds and hydrogen bonds. Therefore, the fibrous crystal of fibrous serpentine has a strong chemical

bond chain is dominated by a covalent bond and ionic bond along the fibrous tube direction, while the molecular bond chain is dominated by a weak bond chain in the vertical tube direction. The axial lengths (axis a and b) of tetrahedral and octahedral plates are not the same along the direction of the structural layer. The Mg (OH) plate is 5.4 nm
0.93 nm and the SiO (O) tetrahedral plate is 5.0 nm
0.87 nm, which leads to the mismatch between them. According to previous studies, this mismatch can be adjusted in three ways: ① In octahedral sheets, Al3+ and Fe3+ with a smaller radius substitute for Mg2+ with a larger radius; in tetrahedral tablets, Al3+ and Fe3+ with a larger radius substitute for Si4+ with a smaller radius. ② Make the octahedral or tetrahedral sheet deformation. ③ Structural unit layer crimping with tetrahedral sheets inside and octahedral sheets outside (Fig. 2.9b). All three modes can exist simultaneously in a single serpentine mineral. Fibber serpentine is mainly adjusted in the third way to achieve mutual adaptation of structural units, thus forming a tubular structure and fibrous morphology. The elongation direction of most fibrous serpents is parallel to the axis a and the curl direction is the axis b. Curling can be expressed in different ways, such as tube type, spiral type, and scroll type (Zhu et al. 1986). Most fibrous tubes are hollow and a few are filled with amorphous materials (Jiang 1987). Electron-microscope study of serpentine shows that the outer diameter of its fibrous tubes was generally 11–85 nm, and most of them are in the range of 20–50 nm (Zhu et al. 1986; Jiang et al. 1981; Jiang 1987). The inner diameter is 2–25 nm, and most of them are less than 10 nm, belonging to natural one-dimensional nanotube minerals (Teng 1998).

2.3.2 The Active Group of Fibrous Serpentine The special structure of fibrous serpentine determines that it has high surface activity. Its activity mainly comes from several aspects: one is the end face of the fibrous ends, the inner and outer surface of the fibrous tube, and the unsaturated bond at the surface defect; second, the high surface energy caused by the large specific surface area (up to 100 m2/g) of their nanocrystals (Wicks and Whittaker 1975); the third is the additional internal and surface energy caused

42

2

Environmental Effects of Tunnel Structure Minerals

Fig. 2.9 The ideal crystal structure of chrysotile. a (100) plane projection; b curved plane (Wang et al. 2005)

by lattice bending due to the unique crimp structure. The unsaturated bond, especially the oxygen-containing unpaired electrons, suspended silicon and the hydroxyl surface on the fibrous surface are the most active. By analyzing the chemical bonds and the properties of chemical elements that constitute the active groups of fibrous serpentine, we can conclude that the active groups of fibrous serpentine can be divided into five categories based on the chemical composition and crystal structure of fibrous serpentine: unsaturated Si–O–Si, O–Si–O bonds, hydroxyl, Mg-containing bonds, and hydrogen bonds; Mg-containing bonds include Si–O–Mg, Mg–O–Mg, OH–Mg–OH, Mg– OH–Mg, and OH–Mg–O. 1. Unsaturated O–Si–O Bond This bond exists in the tetrahedral Si–O layer of fibrous serpentine and is a covalent bond. According to the valence bond theory and hybridized orbital theory, when the silicon-oxygen tetrahedron is formed, 1s orbital and 3p orbital in the outer orbit of silicon atom are recombined to form the sp3 hybridized orbital with equal energy. Four equivalent sp3 hybrid orbitals and four oxygen p orbitals form four equivalent r bonds, thereby forming [SiO4]4−. The theoretical value of each O–Si–O bond angle is 109.5°. The average Si–O distance is 0.162 nm, and the average O–O distance is 0.264 nm (Wang et al. 1984). There are two kinds of oxygen on the unsaturated O–Si–O bond. When oxygen is the bridged oxygen, after the break of the covalent bond between oxygen and silicon, oxygen will have an uncoupled electron. If it is the active oxygen, after the break of the ionic bond between oxygen and magnesium, the oxygen will become O−. The silicon-atoms in the bond are large, so the substituents are difficult to shield the silicon atoms, and the reagents are easy to attack the silicon-oxygen bond. In addition, the silicon atom is polarized and electrically positive, and the silicon-oxygen bond has a

large polarity, the electronegativity difference is 1.6, and the ionization degree is 35% (Luo and Jiang 1999). When this bond is broken, two types of free oxygen may be produced: O*− and O (* denotes an uneven electron), which are highly oxidizing and react easily with surrounding substances. Oxygen is chemically reactive and can form oxides with almost all elements except light noble gases. The mutual polarization of metal ions and oxygen is strong, and it is easy to form high oxidation state oxides of covalent bond types, such as V2O5, CrO3, MoO3, and Mn2O7. Almost all of the O*− or O separated from the unsaturated O–Si–O bond of serpentines can undergo REDOX reactions with surrounding substances, such as the peroxidation of unsaturated fatty acids. Thus, the unsaturated O–Si–O bond is a highly reactive group. 2. O–Si–O Bond This bond exists together with the O–Si–O bond in the tetrahedral silica-oxygen layer of fibrous serpentine as a covalent bond, in which two p orbitals of oxygen atom are combined with two equivalent sp3 hybrid orbitals of Si respectively to form two equivalent r bonds. The Si– O–Si bond angle is 141°, and the average Si–Si distance is 0.305 nm. The unsaturated Si–O–Si bond has strong chemical properties and special bonding properties. The unsaturated Si–O–Si bond is a suspension bond. The lack of adjacent atoms around the silicon atoms on the surface of the cross sections and surface defects at both ends of the fiberglass tube forms a number of suspension bonds. The bond has only one unsaturated electron, which can either give out an electron as a donor or accept an electron as an acceptor. That is, it is easy to combine with other atoms and stabilize, so it has high chemical activity (Wu 1991). A single silicon atom on a silicon oxide tetrahedron plate

2.3 Tubular-Texture Effects of Fibrous Serpentine Table 2.10 Suspended bonds on the surface of the tetrahedral silica-oxygen layer of fibrous serpentine

43

Direction

Silicon atoms on the surface

Density of surface silicon atomic/nm2

Number of broken bonds/nm2

Relative density of broken bonds

Parallel (001)

3

9.32

27.96

1.00

Parallel (100)

1 or 2

2.58

2.58 or 5.16

0.09 or 0.18

Parallel (010)

1

5.38

5.38

0.19

Wang et al. (2005)

may produce 1–3 suspension bonds (Table 2.10). This kind of suspension bond makes the mineral have very high activity, which is shown in its high adsorption to heavy metal ions and fluorine ions and its toxicity and carcinogenicity. This bond, like the O–Si–O bond, is sensitive to reagents and is prone to fracture. When the silicon atoms react with external substances, 3d orbitals can be used to improve their valence states, such as the interaction with compounds such as halides, acids, and bases. The chemical reaction formulas are shown as Eqs. 2.9–2.11:  Si  O  Si  þ MXn ! Si  O  MXn1 þ  Si  X ð2:9Þ  Si  O  Si  þ HA ! Si  A þ H2 O

ð2:10Þ

 Si  O  Si  þ M  OH ! Si  O  M þ  Si  OH ð2:11Þ where X is A halogen element, A is an anion, and M is A metal ion. These reactions can generate organosilicate bonds (Si–Cl, Si–OH, etc.) (Wu 1991). Due to the distribution of hydroxyl base on the outer surface of fibrous serpentine, it is easier to form Si–OH bond and increase the surface activity of fibrous serpentine. Thus, the unsaturated O–Si–O bond can not only adsorb heavy metal ions and positive complex ion groups in sewage but also adsorb anions (such as fluoride ions) and anion groups, making them fixed on the mineral surface. 3. Hydroxyl Hydroxyl exists in the crystal structure of serpentine magnesite octahedron, at the bottom of the film, that is, on the side connected to the silicon-oxygen tetrahedron sheet, there is one hydroxyl group in the octahedron; On the other side, the octahedron has three hydroxyl groups. Thus, the outer surface of fibrous serpentine is composed of a layer of –OH, which is electrically neutral. The electronegativity difference between hydrogen and oxygen is 1.35, less than 1.7, and the O–H bond is polar covalent. Hydroxyl groups have strong chemical activity,

and hydrogen atoms in –OH tend to bond covalently with more electronegative X atoms (such as F, O, and N atoms). The –OH is ready to combine with protons in solution, making the surface of fibrous serpentine alkaline. It is this layer of hydroxyl groups that gives fibrous serpentine a strong resistance to alkalinity, but very poor resistance to acid. The –OH is also easy to combine with metal ions in the solution, in sewage treatment, in precipitation reaction, or in the form of ionic bonds to fix it on the mineral surface. The –OH can also undergo a replacement reaction with anions, such as being replaced by fluorine ions to form fluorine fibrous serpentine (Luo and Jiang 1999). Hydroxyl groups are negative inducing groups that attract electrons more effectively than hydrogen atoms. This inducing effect is ubiquitous in organic molecules, such as binding to carcinogenic polycyclic aromatic hydrocarbon (PAH) molecules (Wan 2002). 4. Mg-Containing Bond The Mg–O bond exists between tetrahedral and octahedral sheets in the crystal structure of serpentines, while the Mg–OH bond is in the octahedral sheet, both of which are ionic bonds. Mg–O–Mg, OH–Mg–OH, Mg– OH–Mg, and OH–Mg–O bonds are all ionic bonds inside the octahedron and between the octahedral sheet and the tetrahedral sheet, except for the ionic bonds and covalent bonds in Si–O–Mg bonds. The electronegativity difference between Mg and O atoms is 2.3, and electron transfer occurs when the atoms are close to each other under certain conditions. They combine by electrostatic attraction to form an ionic bond. Due to the non-saturation and directionality of ionic bonds, an ion can combine with several different sign ions in any direction at the same time under certain external conditions as long as space permits. Therefore, Mg2+ can form a hexacoordination structure with oxygen, and the radius of Mg ion is 0.072 nm, similar to that of Ni2+, Fe2+, Sc2+, CO2+, Ga3+, Mn2+, Cu2+ and Cr3+ (Wu 1991; Peng et al. 2000). So Mg2+ can be replaced by these ions. In addition, Mg2+ and halogen can also form a hexacoordination structure. The electrostatic interaction between Mg2+ and O2− is relatively strong. Even

44

so, under certain conditions, such as an acidic environment, when the ionic bond of Mg–O bond and Mg–OH bond is broken, Mg2+ breaks out and connects with other anions or anion groups, or replaces oxygen or hydroxyl with halogen to form a coordinated octahedron with magnesium and fix it on the chrysotile. Therefore, the magnesium-containing bonds also have strong activity. 5. Hydrogen bond The hydrogen and oxygen in the chrysotile hydroxyl group are covalently bonded. The hydrogen atoms are distributed on the outer layer of the hydroxyl group, that is, the outer surface of the fiber. The hydrogen nucleus is almost exposed, and this exposed hydrogen nucleus is very small and has no inner electrons. It is not easy to be repelled by the electron cloud of other atoms, and it is easy to attract a unique pair of the electron cloud in another oxygen atom to form a hydrogen bond. This oxygen atom is the bridging oxygen in the silicon-oxygen tetrahedral layer. Therefore, hydrogen bonds mainly exist between the basic unit structure layers of chrysotile. In addition to oxygen atoms, this exposed hydrogen nucleus can also attract other atoms and atomic groups with greater electronegativity, such as F, N, and Cl, which exhibit certain chemical activity. The strength of the hydrogen bond and the intermolecular force have the same order of magnitude, but if there is a hydrogen bond between molecules, it will greatly strengthen the binding force between molecules. There is a wide range of substances that can form hydrogen bonds. HF, H2O, NH3, inorganic oxygen acids, carboxylic acids, alcohols, amines, and life-related proteins all have hydrogen bonds. The hydrogen on the hydroxyl layer of the outer wall of the chrysotile fiber tube has a certain effect on the properties of these substances.

2.3.3 The Active Behavior of Fibrous Serpentine The active group is the material base of its chemical and biological activity. Different active groups adsorb different heavy metal ions differently. The adsorption of anions (groups), the adsorption and catalytic decomposition of organic compounds are mainly achieved through the action of four active groups: –OH, hydrogen bond, unsaturated Si– O–Si, and magnesium-containing bond. All the active groups except the hydrogen bond can make fibrous serpentine have high biological activity, that is, biotoxicity and carcinogenicity, but the hydrogen bond of fibrous serpentine cannot be ignored for the adsorption of high electronegative anions (groups) and the influence on some hydrogen bond organic matter. Oxygen with uncoupled electrons, silicon

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Environmental Effects of Tunnel Structure Minerals

with dangling bonds, and hydroxyl groups on the surface of the outer column of the fibrous tube are the most active. Experiments of serpentine adsorbing heavy metal ions have achieved good results (Yang 1997; Guo and Yuan 2000). This adsorption is achieved by the hydroxyl and unsaturated Si–O–Si bonds of serpentine. On the one hand, the oxygen exposed by the broken Si–O–Si bond can bind with heavy metal ions, such as Cd2+, Cu2+, Pb2+, and Ni2+, so as to make them fixed. On the other hand, under certain conditions, the hydroxyl group on the surface of serpentine precipitates into the aqueous solution, making the solution alkaline. Under alkaline conditions, the above heavy metal ions and hydroxyl groups readily form insoluble hydroxides and precipitate, thus achieving the purification of water. In addition, heavy metal ions with similar Mg2+ radius and type in serpentine, such as Ni2+, Fe2+, Co2+, and Cr3+, may also be fixed by serpentine through substitution. The factors influencing the adsorption of heavy metal ions by serpentine mainly include serpentine particle size, dosage, pH value of the medium, etc. In general, the finer the particle size of serpentine is, the higher the adsorption rate is. This is obviously due to finer particle size, larger specific surface area, and more active groups on the surface. The influence of the amount of serpentine on the adsorption effect requires considering the concentration of heavy metal ions. There is usually an optimal amount, and the increase of the amount on top of the optimal amount has little effect on the improvement of adsorption. In terms of the influence of medium pH value on adsorption, usually under neutral or slightly alkaline conditions, the effect of serpentine adsorption of heavy metals is better, but also needs to specific analysis of different metal ions, such as the removal rate of serpentine on Cu2+, Fe3+ is almost independent of pH; the removal of Ni2+ and Cd2+ is the best when pH = 6–8. This may be due to the small solubility product of Cu2+ and Fe3+, and the pH of the hydroxide precipitates are 4.67 and 1.87, respectively, so that Cu(OH)2, and Fe(OH)3 precipitates can be formed in acidic solution, thus removing them from the water. However, the solubility product of other hydroxides is much higher than that of Pb(OH)2, Ni(OH)2, and Cd(OH)2, which can be completely precipitated and removed only when the amount of –OH in the solution is increased under the condition of medium and weak base. The adsorption of anions (groups) by serpentine is mainly achieved through the action of four active groups (–OH, hydrogen bond, unsaturated Si–O–Si, and magnesiumcontaining bond). Under certain conditions, the hydroxyl groups on the surface of serpentine can be replaced by halogens (e.g., F and Cl) or oxygen-containing anions, and fluoride ions, etc., are fixed on the mineral in the form of ionic bonds. The hydrogen bond layer distributed on the surface of the outer column of fibrous serpentine has high activity. The exposed hydrogen nuclei adsorb more

2.3 Tubular-Texture Effects of Fibrous Serpentine

electronegative atoms and groups such as O, F, N, and Cl. Serpentine as a fluorine removal agent is used to remove fluorine from water (Oppara et al. 1990; Jinadasa et al. 1991; Fu et al. 2002) to achieve the purpose of water fluoride reduction through the substitution of hydroxyl on the surface of serpentine by fluorine, or the adsorption of exposed hydrogen to fluorine. The fibrous serpentine can also adsorb similar anion (group) pollutants (such as Cl−) and other anion groups containing arsenic (such as H2AsO4−, VO3−, H2Cr2O7− and MnO4−) in water. In the adsorption and catalytic decomposition of organic matter, the high surface activity of fibrous serpentine may cause organic pollutants to be adsorbed and immobilized. The hydroxyl groups on the serpentine surface enter the aqueous solution to make the solution alkaline, and some organic pollutants can accelerate decomposition and hydrolysis under alkaline conditions, reduce toxicity, and even decompose into non-toxic substances. According to the characteristics of dangling bonds, silicon atoms can not only gain electrons but also lose electrons. As long as conditions permit, not only the characteristic bonds and basic bonds of organosilicon compounds: Si–OH, Si–Cl, Si–H, and Si–C bonds can be generated, but also organosilicon compounds can be formed with various substituents.

2.3.4 The Nanotube of Clinochrysotile Clinochrysotile is a natural one-dimensional nano-filament mineral, with cylindrical, spiral, multi-spiral, and cone-shaped tubular structures (Jiang et al. 1981; Jiang 1987), belonging to the natural one-dimensional hollow opening nanotube mineral. It has a large specific surface area and a completely regular pipe. Generally, the inner diameter of the tube is within the range of 2–6 nm, and water molecules and most ions can come and go freely. Therefore, there may be a small amount of adsorbed water in the nanotubes, that is, pipeline water. Pipeline water in serpentine nanotubes affects their physical and chemical properties (Li et al. 1982). It has been reported that heating can increase the relative strength of chrysotile asbestos (chrysotile serpentine), reaching the maximum value at about 300 °C, with an average increase of about 12%. The reason for the increase in strength is that heating causes the loss of adsorbed water in chrysotile fibers (Jiang 1987), that is, the tensile strength of chrysotile phase is greatly affected by pipe water. Water is a polar liquid with a high dielectric constant. When a small amount of adsorbed water is mixed with the mineral, the dielectric constant will change greatly (Chen et al. 1995). The pipeline water of serpentinous nanotubes is adsorbed water, which will increase the dielectric constant and decrease the resistivity. In the chemical reaction, the

45

pipe water in the serpentinous nanotubes is an effective accelerant and carrier, which can load or dissolve ions and other substances, and promote the mixing and movement of the reactant components. Hydroxylation or hydration of various ions can form hydroxylated ions or hydration ions and accelerate the reaction. Obviously, pipeline water is conducive to the adsorption of heavy metal ions and anions (or anion groups) such as Cl−. The inner wall of the clinochrysotile tube is a hexagonal mesh made of inert oxygen of silicon-oxygen tetrahedron. There are three active centers for the interaction between the nanotubes and the pipe water: the cation in the nanotubes, the holes in the inert oxygen hexagonal mesh of the silicon-oxygen tetrahedron, and the oxygen atoms on the substrate. The size of the holes on the inert oxygen hexagonal mesh of the tetrahedron is less than 0.27, and the water molecules will not completely enter the holes but protrude slightly above the oxygen surface. The oxygen in the water molecules points to the outside lattice of the tetrahedron, and its protons can form hydrogen bonds with the oxygen atoms opposite to the crystal holes. The distance between the hole centers on the inert oxygen hexagonal mesh of the tetrahedral silica-oxygen tetrahedron is 0.15–0.52 nm, and the shortest distance between water molecules is 0.26 nm (Wang and Xiong 2002). The water molecules embedded in the holes cannot form hydrogen bonds, but cover the inner wall of the nanotubes. Isomorphous substitution gives the lattice excess negative charge, and base oxygen lone pair electrons are easy to deformation in the nearby protons, under the influence of can form hydrogen bonds with covalent (Wang and Xiong 2002). Namely, clinochrysotile nanotubes can wall near the surface of the water molecules and surface oxygen atoms to form hydrogen bonds with covalent. In addition, the electrostatic interaction between cations and local charges of the water dipole causes water molecules to gather around ions in the form of coordination to form hydrated ions (Wang and Xiong 2002), that is, cations in nanotubes can also exist in the form of hydrated ions.

2.3.5 Nano-fibriform Silica from Natural Chrysotile The unique crystal structure of fibrous serpentine results in more –OH exposed on the outer surface of the fiber. The – OH is likely to interact with H+ ionized from HCl in solution, resulting in Mg2+ and other cations coordinated with – OH exposed to the outer surface with the dissociation of – OH and become unstable, easy to enter the solution. The amount of acid erosion increases with the increase of H+ concentration (Feng et al. 2000). When the serpentines reacted with acid, MgO was leached out in large quantities,

46

and the residual solid product was mainly SiO2 (OH)n. When the magnesium loss rate is greater than 90%, the residual material is filtered, rinsed, and dried to form white, loose material. Due to its high purity and unique nano-fibriform structure, it is called nano-fibriform silica (Wang et al. 2006a, b). Nano-fibriform silica, also known as light silicon dioxide or hydrated silicon dioxide. Its chemical formula is SiO2 nH2O. It shows white powder or granular texture, and it is light in weight. After absorbing moisture in the air, it becomes aggregated fine particles. Its surface area and dispersing ability are large, the mechanical strength and tear resistance are high, and the content of silica is 80–95% (Wang 1994). The preparation of nano-fibriform can be divided into three categories: dry pyrolysis, wet precipitation, and other methods. Dry pyrolysis includes gas phase pyrolysis and arc pyrolysis. The chemical composition, physical structure, and physico-chemical properties of nano-fibriform silica are closely related to the advanced manufacturing methods, equipment, and processes. The application fields of nano-fibriform silica with different quality are also different (Zhang and Zhou 1995; Yang and Sun 1999). Nano-fibriform silica has good electrical insulation and dispersion performance due to acid, alkali, and hightemperature resistance. Filler reinforcing agent is widely used in rubber, plastics, printing ink thickener, paint additives, synthetic and silicone grease thickener, tanneries, smoothing agent, pesticides dispersant, papermaking filler, synthetic resin, polyester resin, elastic polyurethane, additive adiabatic insulation filler, electrical, and electronic industry, and daily chemical raw materials. It is also used as an opening agent for polypropylene and non-toxic polyvinyl chloride plastic films, as well as an anticaking agent, and carrier for food, pesticide, and medicine (Zhan and Xu 1993; Xu et al. 2003). With regard to the basic structure of nano-fibriform silica, there are some differences in the basic structure of SiO2 particles between the traditional silica (fumed silica and precipitated silica) and nano-fibriform silica (Fig. 2.10). The fumed silica (Fig. 2.10a) is synthesized in the gas phase under high temperature. Its Si–O particles are characterized by a three-dimension structure and low hygroscopic capability due to the close arrangement of its molecules. The precipitated silica is synthesized in water medium with a low synthetical rate. Its Si–O particles have a mixed structure with two and three dimensions and a relatively high hygroscopic capability due to the loose arrangement of its molecules and the capillaries inside the particles (Fig. 2.10 b). Fig. 2.10c, d show the arrangement of the Si–O particles of the nano-fibriform silica in the vertical and parallel directions. The silicon-oxygen tetrahedral fragments from chrysotile exhibit a hexagonal network structure. So, the

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Environmental Effects of Tunnel Structure Minerals

silicon-oxygen tetrahedrons inside the Si–O particles are arranged in short-range order. Although the nano-fibriform silica is produced in water medium, some characteristics of its Si–O particles are similar to those of fumed silica, such as three-dimension structure and close molecule arrangement. The thickness of the Si–O particles of the nano-fibriform silica, which is the thickness of one layer silicon-oxygen tetrahedron, is smaller than 1 nm, and its structure is closer than that of the precipitated silica, indicating that the hygroscopic degree of the nano-fibriform silica is between those of the fumed silica and precipitated silica. The structure, size, and shape of Si–O particles, the aggregate, and the secondary aggregate of nano-fibriform silica are different from those of traditional silica. The Si–O particles of the traditional silica are rigid, solid, super fine, and spherical. The average diameters of the SiO2 particles of the fumed silica and the precipitated silica are 5–20 nm and 25–60 nm, respectively (Li 1998). Numerous Si–O particles are randomly combined to form a branch catenary aggregate. The shape of the aggregate is irregular. The secondary aggregate consists of numerous aggregates connected easily by hydrogen and van der Waals forces. The characteristics of the aggregate and secondary aggregate of SiO2 particles of the nano-fibriform silica are shown in Fig. 2.11. The aggregate consists of a large number of SiO2 particles arranged in orientation or semi-orientation and has a structure of hexagonal network of silicon-oxygen tetrahedral fragments from chrysotile (Fig. 2.11a, b). One secondary aggregate, which is made up of many aggregates also arranged in orientation or semi-orientation, is one fiber of the nano-fibriform silica. The secondary aggregates partially preserve the nano-fiber- and nanotube- structure of chrysotile (Fig. 2.11c). The secondary aggregate is very firm and difficult to disjoin by foreign forces. The secondary aggregate of the traditional silica is not so firm, and it is easy to be disjoined or massed by foreign forces. The average diameters of the aggregates of both fumed silica and precipitated silica are over 1 lm (Zheng et al. 1997; Li 1998; Wu et al. 2003; Xu et al. 2003). And the diameter of the nano-fibriform silica is less than 50 nm. The secondary aggregate of the nano-fibriform silica is more stable and more difficult to reunite than the traditional silica. The TEM images of the nano-fibriform silica (Fig. 2.12) show that such silica is nano-fibriform and porous. Dissolution of brucite layer and collapse of SiO tetrahedral sheets result in the formation of nano-fibriform silica. A single fiber of silica, which has many pores with an aperture of less than 6.5 nm on the rough surface (Fig. 2.12c1), is shorter and thinner than that of chrysotile (Wang et al. 2005). Its diameter is about 20–30 nm with a high aspect ratio. A single fiber is on the scale of micron or nanometer in length. The fibers with many ruptures in the vertical direction of the fibrous axis are straight (Fig. 2.12b3) in form. The layers or

2.3 Tubular-Texture Effects of Fibrous Serpentine

47

Fig. 2.10 Basic structure of SiO2 particles of different silicas. a Fumed silica; b precipitated silica (Yang and Sun 1999); c viewed in a parallel direction of hexagonal network; d viewed in a vertical direction of nano-fibriform network silica (Wang 2006)

Fig. 2.11 Characteristics of aggregate and secondary aggregate of SiO2 particles of nano-fibriform silica (Wang et al. 2005). a Aggregate of Si–O particles arranged along the direction of cylinda; b aggregate of Si–O particles arranged along the direction of section; c secondary aggregate (Wang 2006)

lines of the aggregates, arranged along the direction of the fibrous axis (Fig. 2.12a1, c1 and d1), form the nanofibers and the integrated (Fig. 2.12b2) or misshapen (Fig. 2.12b1) nanotubes. Obviously, the nano-fibriform silica partly preserves the nanofiber’s or nanotube’s structure of chrysotile. Figure 2.13 shows the pore structure of the nano-fibriform silica. Define two types of pores: ① pores in fibers and ② cumulate pores. The pores in fibers consist of the pores among SiO2 particles, among aggregates, remnants of nanotubes, and capillary tubes, which are formed by the dissolution of brucite octahedral sheets and collapse of Si–O tetrahedral sheets. The cumulate pores are voids among neighboring fibers. Because of such numerous pores, the silica has a higher hygroscopic degree and higher pore bulk compared with traditional silica.

Figure 2.14 shows the nitrogen adsorption and desorption isotherms of the nano-fibriform silica. The amount of adsorbed gas increases tardily with the increase of P/P0 in the lower P/P0 range. This is due to monolayer-multilayer molecule adsorption (Gregg and Sing 1982). The hysteretic loop is observed at the range of 0.4 < P/P0 < 1.0, which is associated with capillary condensation taking place in mesopores. The loop on the high P/P0 side (close to 1.0) is related to large pores (voids among neighboring fibers). The nitrogen adsorption isotherms of nano-fibriform silica belong to a typical Type IV proposed by the International Union of Pure and Applied Chemistry (IUPAC) (Temuujin et al. 2003). In Fig. 2.14a, the hysteretic loop resulting from capillary condensation can be observed. Its occurrence proves that the aggregate of nano-fibriform silica is a

48

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Environmental Effects of Tunnel Structure Minerals

Fig. 2.12 TEM of nano-fibriform silica. a1, c1 and d1 Layers or lines of aggregates arranged along the direction of the fibrous axis; b1 misshapen nanotubes; b2 integrated nanotubes; b3 rupture in the vertical direction of the fibrous axis (Wang 2006)

Fig. 2.13 Pore structure and type of nano-fibriform silica. a Pores in fibers; b cumulate pores (Wang 2006)

mesopore (2–50 nm in width) material. In general, the BET surface area (SBET) and MLD have a positive correlation (Le Van Mao et al. 1989). While the MLD value is 92.73% the SBET absorptive capacity and pore bulk of nano-fibriform silica is estimated to be 378 m2/g, 330 cm3/g, and 0.51 cm3/ g, respectively (Wang et al. 2005). The SBET of nano-fibriform silica is higher than that of the spf silica (232 m2/g) (Seledets et al. 2003), due to the Occurrence of

numerous pores less than 6.5 nm in the fibers of nano-fibriform silica (Fig. 2.12). The SBET of the fumed silica and the precipitated silica are 200–400 m2/g and 150– 300 m2/g, respectively (Li 1998; Seledets et al. 2003). The higher special surface area, a higher absorptive capacity, and a larger pore bulk imply more active groups on the surface of nano-fibriform silica than compared to traditional silica. Figures 2.14b, c show the pore size distribution of

References

49

References

Fig. 2.14 a N2 adsorption isotherms at 300 °C; b and c pore size distribution of nano-fibriform silica (Wang 2006)

nano-fibriform silica with desorption temperatures of 25 and 300 °C. The diameters of pores in nano-fibriform silica are all less than 6.5 nm, mostly 2.1 and 3.8 nm as indicated by the statistical result. The pore size distribution curves display that the differential bulk of 2.1–2.9 and 4.3–6.5 nm pores at desorption temperature of 25 °C is larger than that at 300 ° C, while the differential bulk of 2.9–4.3 nm is reverse. At 300 °C, all adsorbed water and a small amount of hydroxy in the nano-fibriform silica are released, resulting in an increase of 2.9–4.3 nm pores and a decrease of 2.1–2.9 and 4.3– 6.5 nm pores. In conclusion, Si–O particles of nano-fibriform silica are silicon-oxygen tetrahedral fragments from chrysotile with a thickness of less than 1 nm. They have a three-dimension structure of the hexagonal network and a close arrangement of molecules. The structure of the silicon-oxygen tetrahedrons in Si–O particles is in short-range order. One secondary aggregate, which is one fiber of the nano-fibriform silica, consists of a lot of SiO2 particles and aggregates arranged in orientation or semi-orientation. The secondary aggregates partly preserve nanofiber’s and nanotube’s structure of chrysotile and are very firm and difficult to disjoin by foreign forces. There are many pores in the fibers of nano-fibriform silica, whose diameters are all less than 6.5 nm, mostly 2.1 and 3.8 nm, according to the statistical result. There are two types of pores in the nano-fibriform silica: ① pores in fibers and ② cumulate pores (voids among neighboring fibers). The pores in the fibers consist of pores among SiO2 particles, aggregates, remnant nanotubes, and capillary tubes, which are formed by the dissolution of brucite octahedral sheets and collapse of Si–O tetrahedral sheets. Because there are numerous pores, this silica has a higher hygroscopic degree and higher pore bulk than traditional silica. These characteristics indicate that the nano-fibriform silica may be a high-quality filler and catalyzer carrier.

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Ribbe PH (1983) Feldspar mineralogy. In: Mineralogy (2nd edn), pp 24–73 Richmond WE, Fleischer M (1942) Cryptomelane, a new name for the commonest of the “psilomelane” minerals. Am Mineral: J Earth Planet Mater 27(9):607–610 Ringwood AE, Reid AF, Wadsley AD (1967) High-pressure KAlSi3O8, an aluminosilicate with sixfold coordination. Acta Crystallogr 23 (6):1093–1095 Scheel HJ (1971) Lead feldspar. Z Kristallogr—Cryst Mater 133 (1):264–272 Seledets O, Skubiszewska-Zieba J, Leboda R et al (2003) Surface properties of fumed silica/pyrocarbon prepared by pyrolysis of methylene chloride. Mater Chem Phys 82:199–205 Seyama H, Soma M (1984) X-ray photoelectron spectroscopic study of montmorillonite containing exchangeable divalent cations. J Chem Soc, Faraday Trans 1 80(1):237–248 Smith JV (1974) Feldspar minerals. I. Crystal structure and physical properties. Springer, Heidelberg Sorrell CA (1962) Solid state formation of barium, strontium and lead feldspars in clay-sulfate mixtures. Am Miner 47:291–309 Taylor WH (1933) The structure of sanidine and other feldspar. Z Kristallogr—Cryst Mater 85(5–6):425–445 Taylor WH, Darbyshire JA, Strunz H (1934) An X-ray investigation of the felspars. Z Kristallogr—Cryst Mater 87(1–6):464–498 Temuujin J, Okada K, MacKenzie KJ (2003) Preparation of porous silica from vermiculite by selective leaching. Appl Clay Sci 22 (4):187–195 Teng RH (1998) The intrinsic properties, criteria and research trends of nano-materials. J Iron Steel Res 10(2):61–65 (in Chinese with English abstract) Thackeray MM (1997) Manganese oxides for lithium batteries. Prog Solid State Chem 25(1–2):1–71 Ticknor K, Cho Y (1990) Interaction of iodide and iodate with granitic fracture-filling minerals. J Radioanal Nucl Chem 140(1):75–90 Tsuji M, Komarneni S (1993) Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure. J Mater Res 8(3):611–616 Tsuji M, Tanaka Y (2001) Spectroscopic study of a cryptomelane-type manganic acid exchanged by divalent transition-metal cations. J Mater Res 16(1):108–114 Vicat J, Fanchon E, Strobel P, Tran Qui D (1986) The structure of K1.33Mn8O16 and cation ordering in hollandite-type structures. Acta Crystallogr Sect B: Struct Sci 42(2):162–167 Viswanathan K (1972) Kationenaustausch an plagioklasen. Contr Mineral Petrol 37(4):277–290 Wan P (2002) Chrysotile industry and its structure adjustment and development in China. China Non-Metallic Miner Ind 29(5):8–12 (in Chinese) Wang ZG (1994) Manufacture of white carbon black. Inorg Chem Ind 3:22–24 (in Chinese) Wang PQ, Xiong HQ (2002) The study for quantitative analysis of water absorbed on clays and their hydration mechanism. Petroleum Industry Press, Beijing Wang P, Pan YL, Weng LB (1984) Systematic mineralogy. Geology Press, Beijing (in Chinese) Wang LJ, Lu AH, Wang CQ et al (2005) Study on the character of water in the nanotube of clinochrysotile. J Mineral Petrol 25(1):25–29 Wang J, Chen WM, Su R et al (2006a) Geological disposal of high-level radioactive waste and its key scientific issues. Chin J Rock Mechan Eng 25(4):801–812 (in Chinese with English abstract) Wang LJ, Lu AH, Wang CQ et al (2006b) Nano-fibriform production of silica from natural chrysotile. J Colloid Interface Sci 295(2):436– 439

References Wedepohl KH (1974) Handbook of geochemistry. Springer, Heidelberg and New York, pp 103–145 (II(4):82-C-1-82-O-1) White JC, Holt GS, Parker HF et al (2003) Trace elements partitioning between alkali Feldspar and Peralkalic quartz trachyte to Rhyolite magma; Part I: systematics of trace elements partitioning. Am Mineral 88(23):316–329 Wicks FJ, Whittaker EJW (1975) A reappraisal of the structures of the serpentine minerals. Can Mineral 13(3):227–243 Witherspoon PA (1996) Geological problems in radioactive waste isolation: second worldwide review. Lawrence Berkeley National Lab, University of California, USA, California Wu QH (1991) Surface chemistry and heterogeneous catalysis. Chemical Industry Press, Beijing (in Chinese) Wu W, Chen JF, Shao L et al (2003) Surface status and formation of ultrafine siliucon dioxide after polymer grafting modification. J Beijing Univ Chem Technol 30(2):1–4 (in Chinese with English abstract) Xu GZ, Bai JF, Li SS et al (2003) Effect of stirring intensity on the structure of white carbon. Inorg Chem Ind 35(1):31–32 Yang ZK (1997) Study on the treatment of wastewater containing copper by serpentine. Environ Sci Technol 77(2):17–19 (in Chinese)

51 Yang HK, Sun YJ (1999) Surface modification of silica by gas phase method. Silicone Mater 13(5):15–18 (in Chinise) Zhan GZ, Xu SM (1993) Production of white carbon black from non-metallic ore. Non-Metallic Mines 16(6):6–18 (in Chinese) Zhang XJ, Zhou CL (1995) Present situation of white carbon black production at home and abroad. Inorg Chem Ind 6:20–22 (in Chinese) Zhang QC, Yang HR, Han C (1986) Ion Exchange performance and application of natural zeolite. Science Press, Beijing (in Chinese) Zhang Q, Liu ZH, Qiu YZ et al (2004) Lead in potassium feldspar and contribution to mineralization. Geol Prospect 40(1):45–49 (in Chinese with English abstract) Zheng DM, Zhan XL, Liu XH, Yan J (1997) The summarization of superior quality processing about silica. J Nanchang Univ (Eng Technol) 19(2):93–95 (in Chinese with English abstract) Zhu ZZ, Fan LM, Liang WX et al (1986) A study of several kinds of asbestos minerals in China. Mineral Petrol 4:1–181 (in Chinese with English abstract) Zuo HY, Liu R, Lu AH (2022) The behavior of water in orthoclase crystal and its implications for feldspar alteration. Curr Comput-Aided Drug Des 12(8):1042

3

Photoactivity of Mn Oxides on Earth’s Surface

Mn oxides have long been ubiquitously spread on earth, from the Mn deposits in Precambrian strata (Roy 1997) to the Mn nodules spread over the sea floor (Bauman 1976; Jiang et al. 2007), and the Mn-enriched rock-soil surface coatings (Bauman 1976; Dorn 1991; Jiang et al. 2007; Lu et al. 2019; McKeown and Post 2001; Nash and McLaren 2011; Post 1999; Roy 1997; Xu et al. 2018). Mn oxides are diverse in both phase and structure and comprise mixed Mn (IV/III/II) valence states with layered, tunneled, or spineltype structures (Johnson et al. 2016; Post 1999). In Nature, Mn oxides are widespread in coatings on the upper surface of rocks and soils (Lu et al. 2019), where the dominant phase in the coatings is birnessite—a layered mineral with Mn (or O) vacancy and adsorbed cations (Lu et al. 2019; McKeown and Post 2001; Post 1999; Villalobos et al. 2006, 2003; Webb et al. 2005; Xu et al. 2019). As a semiconducting mineral, birnessite shows outstanding photocatalytic reactivity (Ling et al. 2017; Lu et al. 2019; Sherman 2005) and as revealed by theoretical calculations, the electronic structure of birnessite is sensitively affected by Mn (or O) vacancy abundance as well as the metal sorption sites (Kwon et al. 2008, 2009; Li et al. 2019a, b). These effects will further impact the photocatalytic performance of birnessite in various photoreactions, mainly during birnessite selfreduction (Li et al. 2019a, b) or reduction by organics (Sunda et al. 1983; Sunda and Huntsman 1990; Waite et al. 1988) as well as water photocatalytic oxidation (Marafatto et al. 2015; Sherman 2005). Notably, the oxygen evolution complex (OEC) in biological photosystem II (PSII) as Mn4CaO5 is structurally similar to birnessite (Hocking et al. 2011; Robinson et al. 2013; Yang et al. 2015), which draws an interesting parallel of the key functioning components in water oxidation and oxygen evolution between the organic and inorganic world. As a counterpart to Mn reduction, the oxidation of Mn(II) to form natural Mn oxides has been

© Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_3

proposed through catalytic oxidation by fungi and bacteria (Northup et al. 2010), or light-driven photooxidation by co-present semiconducting minerals (Madden and Hochella 2005; Xu et al. 2018, 2019). More recently, the photostimulated anaerobic formation of Mn oxides has been observed through the photooxidation of Mn(II) carbonate minerals (Liu et al. 2020; Lyons et al. 2020) or the biomineralization mediated by anoxygenic photosynthetic microorganisms (Daye et al. 2019; Lyons et al. 2020). These studies have implications on the light-dependent Mn(II) oxidation, and Mn oxides emergence, prior to the Great Oxidation Event (GOE) on earth, especially in oceanic euphotic zones. During the oxidative formation of birnessite, sorption sites of heavy metals and fine structure of birnessite will constantly interact with each other, which ultimately determines the biogeochemical cycle of Mn along with heavy metals. This chapter looks back on the mineral evolution processes of Mn oxides throughout geological history and the vast occurrence of Mn oxides in various modern natural settings. Notably, Mn coatings widespread on the earth’s surface as well as its remarkable performance of photon-to-electron conversion under solar irradiation are highlighted. Further, the effect of Mn (or O) vacancies and metal cation sorption on the electronic structure and photocatalytic activity of Mn oxides as revealed by density function theory (DFT) are summarised. Mn oxides actively participate in the photocatalytic redox cycling on the earth’s surface, involving both the photooxidation of Mn(II) and the photoreductive dissolution of Mn oxides. During these processes, heavy metal sorption/desorption, organic oxidation and even water splitting, and oxygen evolution can occur. This study provides novel insights into the photogeochemical cycling of natural Mn oxides as well as its environmental implications.

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3.1

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Nature Manganese Oxides

3.1.1 Vast Distribution of Mn Oxides on Modern Earth Mn oxides are widespread in modern natural settings, especially in both oceanic and terrestrial environments (Lu et al. 2019; McKeown and Post 2001; Nash and McLaren 2011; Post 1999). They are the predominant components of Mn nodules that develop across the ocean floor and on the bottom of fresh-water lakes (Bauman 1976; Jiang et al. 2007). These Mn oxides are generally birnessite, todorokite, and vernadite (Glasby 1977; Manceau et al. 2014). The formation of Mn-enriched nodules is thought to arise from upward diffusion of Mn through the underlying reducing sediments and further accumulated by the catalytic oxidation of Mn(II) adsorbed on preliminarily formed fine-grained MnO2 (Lei and Bostrom 1995). In addition, some Mn-oxidizing bacteria (such as the Pseudomonas putida and Bacillus species) have been found in marine nodules, which are considered responsible for the biogenic origins of Mn oxides (Crerar and Barnes 1974; Tebo et al. 2004; Villalobos et al. 2006; Webb et al. 2005). In terrestrial settings, Mn is the second most abundant heavy metal element in the earth’s crust, of which Mn(IV/III) are the most prevalent forms (Tebo et al. 2004). Especially in terrestrial weathering environments, the most representative Mn deposits are rock varnish, which typically attaches to the uppermost surface of rocks from arid to semiarid regions such as in the Gobi Desert (McKeown and Post 2001; Nash and McLaren 2011; Xu et al. 2018). The Mn enrichment of rock varnish has been reported to be * 50–200 times of that in the upper crust (Dorn 1991; Goldsmith et al. 2014; Thiagarajan and Lee 2004; Xu et al. 2019), and the dominant Mn oxides phases are birnessite, todorokite and romanechite (Garvie et al. 2008; Lu et al. 2019; McKeown and Post 2001; Xu et al. 2018). Several formation mechanisms for rock varnish have been proposed. Goldsmith et al. (2014), Thiagarajan and Lee (2004) proposed direct aqueous atmospheric deposition as one method, while Northup et al. (2010) suggested that biogenic Mn oxidation by fungi and bacteria will also induce Mn nucleation and precipitation. Recently, Xu et al. (2018, 2019) put forward a new theory that light-enhanced photocatalytic oxidation of Mn by the semiconducting minerals such as Fe oxides co-present in rock varnish is responsible for its formation.

3.1.2 Widespread Mn Coatings on Earth’s Surface Widespread occurrences of rocks have thin coatings of Fe and Mn (oxyhydr)oxides on their surfaces (Fig. 3.1) (Garvie

Photoactivity of Mn Oxides on Earth’s Surface

et al. 2008; Perry and Adams 1978; Sarmast et al. 2017). Such rocks often make distinctive landscapes in deserts (Fig. 3.1a), including the expansive Gobi region of China and Mongolia. Other examples include dark gray coatings on karst terrains (Fig. 3.1d) and reddish-yellowish or dark brown coatings covering soil particles (Fig. 3.1g). Rock varnish generally appears as a very dark (often black) coating that is well developed on bare rock surfaces in terrestrial arid and semiarid regions (Fig. 3.1a) (Dorn 1991; Goldsmith et al. 2014). Mn-rich coatings are generally found only on upper rock surfaces that are exposed to solar light, while the undersides of rocks are reddish to yellowish (the two lower right images in Fig. 3.2a). EDS mapping of Mn and Fe (Fig. 3.2b) indicates that these two elements are enriched in the mineral coatings but not in the bulk rock. The Mn concentrations in rock varnish surfaces are often enriched by two orders of magnitude relative to the whole rock or soil particle and also reflect the average crustal abundance of approximately 0.13 wt% (Taylor and McLennan 1985). In contrast, the undersides of the same rocks mainly consist of Fe (oxyhydr) oxides (i.e., hematite) with no Mn-rich minerals present (Fig. 3.2). Rocks with a dark Mn-rich varnish are also generally found on the sunlit side of landforms, and Mn-poor rocks commonly occur on the shaded side (Nealson 2015; Schindler and Dorn 2017). In addition, the growth rate of Mn-rich coatings was found to be much faster in high-UV flux or photic environments (Koschinsky and Heine 2017; Krinsley et al. 2009). Notably, the distribution of dark Mn-rich rock coatings overlaps the regions with strong solar irradiation. This pattern implies a close relationship between the formation of Mn oxides and solar light. Photocurrent-time curves are used to characterize the photovoltaic response of the samples, which provide information on the response time of photoswitching performance and photostability for semiconductor electrodes (Cheng et al. 2014; Cho et al. 2013). The photoelectrochemical measurements were carried out in a conventional three-electrode system consisting of the mineral electrode as the working electrode, a Pt sheet as the auxiliary electrode, and a SCE (saturated calomel electrode, 0.244 V vs. normal hydrogen electrode at 25 °C) reference electrode. 0.1 M Na2SO4 aqueous solution was used as the electrolyte. A 300 W Xe lamp (Trustech Co., China) was used as the excitation light source. The light was irradiated onto the mineral electrodes from the back face, i.e., through the quartz window, electrolyte, and the FTO substrate. The incident light intensity was adjusted to 120 mW/cm2. The photocurrent-time response of the mineral electrodes was determined by chronoamperometry with a potentiostat (CHI 660C, Shanghai Chenhua Instrument Co. Ltd, China), under a constant pre-pulsed potential of 1 V (vs. SCE), and using a hand-operated shutter to obtain light and dark response curves.

3.1 Nature Manganese Oxides

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Fig. 3.1 a Representative landscape of the Gobi Desert in northwest China with optical micrograph (lower left) showing mineral coatings and hand specimens (lower right) showing the color contrast between the surface and underside of rocks; b backscattered electron (BSE) image of a rock thin section collected from the Gobi Desert and the corresponding elemental mapping of Mn, Fe, and Si by energydispersive X-ray (EDX) spectroscopy; c high-resolution transmission electron microscopy (HRTEM) image of a Mn-rich coating from Gobi rock. The EDX spectrum (upper left inset) corresponds to a spot in an Mn-rich region; d representative landscape of karst in southwest China with an optical micrograph (lower left) and hand specimens (lower right) showing the color contrast between the surface and inside of

rocks; e normalized Mn K-edge X-ray absorption near-edge structure (XANES) spectrum of a karst Mn coating and the least-squares fits using linear combination fitting (LCF) of the spectra of d-MnIVO2 and (Ca0.999, MnII0.001)CO3; f fourier transform (FT) magnitude and imaginary part derived from Mn K-edge extended X-ray absorption fine structure (EXAFS) spectra of a karst Mn coating, synthetic d-MnO2 (layer structure) and b-MnO2 (tunnel structure); g representative landscape of the red soil region in southern China with optical micrographs (lower left in the polarized optical microscope and lower right in stereoscope); h BSE of a soil particle thin section collected from the red soil region and corresponding elemental mapping of Fe, Si (Lu et al. 2019)

The HRTEM image (Fig. 3.2c) of Mn oxide on a Gobi rock surface exhibits a lattice fringe spacing of 0.72 nm, corresponding to the {001} plane of birnessite. Furthermore, the Raman spectra of the rock varnish have sharp peaks from the Mn–O stretching vibration modes of birnessite at 591 cm−1 and 643 cm−1 (Fig. 3.3). Tunneled Mn oxides are not identified even though they typically have better crystallinity. In powders scratched from karst mineral coatings,

the Mn content is less than 0.1 wt%, making it difficult to detect Mn species by XRD, HRTEM, or Raman spectroscopy. Instead, X-ray absorption spectroscopy was used for these samples. The normalized Mn K-edge XANES spectrum of the karst Mn-rich coating displays a maximum absorption peak at 6561.5 eV (Fig. 3.2e), followed by a broad shoulder feature at * 6574.5 eV (indicated by arrows); these absorbance features closely match those of

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Photoactivity of Mn Oxides on Earth’s Surface

Fig. 3.2 Composition and structural features of the Gobi underside rock. a Back-scattered image and energy dispersive X-ray detector data (inset); b raman spectrum of the regions on the underside of rocks from the Gobi region. The EDX result shows that Fe instead of Mn is the dominant element in the underside of the rock sample. The Raman

spectrum shows diagnostic peaks of hematite, i.e., Fe–O symmetrical bending vibration modes at 292, 409, and 605 cm−1, and Fe–O symmetrical stretching vibration mode at 226 cm−1. All samples tested indicate that the Mn-rich coatings are poorly developed on the underside of the Gobi rocks (Lu et al. 2019)

Fig. 3.3 a Raman spectrum of rock varnish on quartzite; b Raman spectrum of rock varnish on feldspathic quartz sandstone sample. Three types of Fe/Mn oxide minerals, i.e., hematite (291 and 407 cm−1),

goethite (217, 279, and 393 cm−1) and birnessite (486, 591, and 635– 643 cm−1) can be identified in the Raman spectra (Lu et al. 2019)

d-MnO2, suggesting that the Mn local structure in karst Mn-containing coatings could predominantly be of the MnO2 type. The least-squares linear-combination fitting of the XANES spectrum gives 73 ± 1% d-MnIVO2 and 27 ± 1% (Ca0.999, MnII0.001)CO3, implying an average Mn oxidation state of * 3.46. In the Fourier transform (FT) magnitude of the EXAFS spectra (the top figure in Fig. 3.1f), the second main peak at R + dR * 2.5 Å can be attributed to Mn–Mn coordination, where adjacent MnO6 octahedra share edges with each other (McKeown and Post 2001; Villalobos et al. 2006; Webb et al. 2005) and form a

layer structure (Post 1999). The peak at R + dR * 3.0 Å could arise from the Mn–Mn coordination of MnO6 octahedra sharing corners, which occurs in tunnel-structure Mn oxides (McKeown and Post 2001; Post 1999; Webb et al. 2005). The spectrum of the karst Mn-rich coating resembles layer-structure d-MnO2 more than tunnel-structure b-MnO2, especially at * 3.0 Å; furthermore, the FT imaginary part is more similar to that of layer-structure d-MnO2, particularly at * 2.7 − 4.0 Å (Fig. 3.1f). These results suggest that the Mn oxides in this karst coating sample have mainly a layer structure.

3.1 Nature Manganese Oxides

3.1.3 Photoelectric Behavior of Mn (Oxyhydr) oxide Many decades of cutting-edge science have provided a deep understanding of biological light-harvesting capabilities that convert solar to chemical energy for the formation of biomolecules necessary for life. Such intricate purely organic processes are exhibited on this planet by chlorophyll- or rhodopsin-based phototrophic bacteria (Beja et al. 2000; Des Marais 2000; Sauer and Yachandra 2002). Although laboratory arrangements for extracellular phototrophy with the aid of semiconducting minerals have succeeded in stimulating the growth of the iron-oxidizing bacterium Acidithiobacillus ferrooxidans (A. ferrooxidans) (Lu et al. 2012) and the acetogens Sporomusa ovata (S. ovata) (Liu et al. 2015) and Moorella thermoacetica (M. thermoacetica) (Sakimoto et al. 2016a, 2016b), no evidence has yet emerged for the widespread existence of a nonbiological component of a light-harvesting system that could be an inherent component of any portion of life on this planet. Finding a natural inorganic system that might deliver solar energy to non-phototrophic organisms in nature would imply a role, analogous to photosystem II (PS II) or rhodopsin, for novel modes of extracellular phototrophic metabolisms. Such a system would also provide a distinctive driving force for redox (bio)geochemistry on earth’s surfaces. Layered Mn oxides have been reported as efficient catalysts in photochemical systems (Sakai et al. 2005). Likewise, in situ photoelectric mapping measurements (Figs. 3.4a–d) indicate that desert varnish samples are stable and sensitive photoelectric systems. A photocurrent is detectable only in the region of the Fe- or Mn-rich mineral coating (Figs. 3.4b and 3.5). When illuminating a spot in this area with a selected bias and laser power, the photocurrent is elevated in the ON state and immediately descends to the baseline in the OFF state (Fig. 3.4c), performing with high sensitivity and stability just like a natural photoelectric device. Notably, the photocurrent exhibits a close linear relationship with bias and laser power (Figs. 3.4d and 3.6), demonstrating a constant photon-to-electron conversion efficiency. At a fixed bias of 0.1 V and 442 nm laser excitation (a wavelength in the violet range with a photon energy of 2.8 eV, able to promote an electron from the top of the valence band to the bottom of the conduction band of semiconducting minerals in mineral coatings), the estimated photoresponsivity is 2
10−6 A/W, and the effective quantum efficiency (EQE) is 5.8
10−6 for the Fe-rich coating, while there is 1.7-fold enhancement for the Mn-rich coating. Similarly, red soil and powder separated from the outermost surface of karst rocks yield photocurrents that are sensitive to irradiation (Fig. 3.4e). In contrast to the rock substrates and non-semiconducting silicate minerals (quartz and feldspar),

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which do not produce photocurrents (Figs. 3.4b and 3.7), the mineral coatings behave as natural photoelectric generators and show long-term stability (Fig. 3.7). The capability of photo-to-electric transduction in the desert varnish coatings may be ascribed to naturally occurring minerals, which are identified as mainly birnessite (triclinic and hexagonal types), hematite, and goethite (Fig. 3.3). The bandgaps (Eg), as determined by oxygen Kedge absorption and emission spectra (Fig. 3.8), are 2.57, 2.10, 1.82 and 1.77 eV for goethite, hematite, hexagonal and triclinic birnessite, respectively, indicating that the coatings are all visible light-responding semiconducting minerals. A comparison of the redox potentials of H2O and humic acid (HA) (Struyk and Sposito 2001), one of the most common compounds in natural organic matter (NOM), with the valence band edges of the four minerals (Fig. 3.8b) demonstrates the potential for H2O and HA oxidation by photoexcited holes, allowing for the release of photoelectrons. In addition, many other compounds with reducing potentials, including dissolved transition metal ions, as well as a vast store of other inorganic and organic substances, are widespread in natural environments due to inexhaustible production by the decomposition of biomass or dissolution of minerals (Becking et al. 1960; Dorn and Oberlander 1981; Edwards et al. 2004). These compounds can all serve as possible electron donors to scavenge positively charged photoholes generated by the photocatalysis of Fe- and Mn-mineral coatings and then facilitate the outflow of photoelectrons (Fan et al. 2014; Lan et al. 2017; Sherman 2005; Sunda et al. 1983). According to in situ photoelectricity tests of Mn-rich coatings, a photocurrent density of approximately 358 nA/cm2 is generated when a varnish sample is irradiated by a simulated solar source with an average light intensity of 100 mW/cm2 and a small bias of 0.1 V is applied. In total, an estimated 2.23
1016 photoelectrons can be produced per second from a 1 m2 rock varnish-covered land surface under the current experimental conditions. Certainly, the amount of naturally produced photoelectrons is not uniform in varnish-covered areas, due to the uneven exposure to sunlight, availability of different electron donors, and distribution of mineral coatings. However, considering the vast rock varnish-covered land surface of 3.567
1013 m2, the global-scale production of photoelectrons emitted from the mineral coatings is not negligible. These photoelectrons could be a major source of extracellular electrical energy to fuel photoelectrotrophic bacteria (Lu et al. 2012, 2013). Our findings substantiate the implications of laboratory experiments on extracellular phototrophy, which have succeeded in stimulating the bacterial growth of iron-oxidizing bacterium A. ferrooxidans (Lu et al. 2012) and the acetogens S. ovate (Liu et al. 2015) and M. thermoacetica (Sakimoto et al.

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Photoactivity of Mn Oxides on Earth’s Surface

Fig. 3.4 Photoelectric measurement results of mineral coatings. a Schematic diagram of in situ photoelectronic measurement on a rock varnish sample; b photocurrent and EDX micro mapping of Fe-rich and Mn-rich varnish samples (the mapping region corresponds to the area marked by the black dashed line in a; c photocurrent-time curves collected from varnish samples and recorded at a selected bias or light

intensity. d good linear relationships between photocurrents collected from varnish samples and light intensity; e Photocurrent-time behavior of electrodes fabricated by mineral coating powders from red soil and karst rock. ON and OFF represent the state of the light source (Lu et al. 2019)

2016a, b) with the aid of semiconducting photocatalytic minerals. We conclude that the solar light response and photocurrent production of widespread semiconducting mineral coatings are capable of playing important roles in biogeochemical processes on the earth’s surface (Fig. 3.9). It should be noted that Mn is especially concentrated in the “coating” layer. The valence band potential of the main Mn-bearing mineral in coatings, for example, birnessite, is more positive than the redox potential of H2O/O2 (Fig. 3.8), suggesting that the photocatalytic oxidation of water to oxygen may occur in the topmost layer of the geosphere (Sauer and Yachandra 2002). In addition, the photoelectrons from the conduction band of birnessite, similar to other semiconductors, can provide a driving force for the reduction of elements such as iron and manganese (Matsunaga et al. 1995; Sherman 2005; Sunda et al. 1983), regenerate NADPH from NADP+ (Guo et al. 2018), and provide a new form of energy for bacterial metabolisms (Lu et al. 2013;

Sakimoto et al. 2016a). The inexhaustible electron donors (e.g., water, organics, and reduced ions) in nature can complete those electron transport chains and bring about continuous redox chemistry, in which the semiconducting mineral coatings act as the vital bridge. Overall, the newly discovered properties of these coatings broaden the pathways for utilizing solar energy from the well-known organic world to the mineral semiconductor world.

3.2

Electronic Structure of Natural Semiconducting Mn Oxides

Theoretical calculations have been widely applied to analyse the electronic structure of birnessite (Kwon et al. 2009; Ruetschi 1984). According to these results, the valence band (VB) and conduction band (CB) of birnessite mainly consist of O(2p) and Mn(3d) orbitals, respectively, whose electronic structure can be interpreted by crystal field theory

3.2 Electronic Structure of Natural Semiconducting Mn Oxides

Fig. 3.5 The EDX elemental distribution features of Fe-rich and Mn-rich desert varnish samples, respectively. a EDX elemental distribution patterns of a Fe-rich desert vanish thin section; b EDX composition result and pattern of randomly selected spot 1 in panel a; c EDX composition result and pattern of randomly selected spot 2 in panel a; d EDX elemental distribution patterns of a Mn-rich desert

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vanish thin section; e EDX composition result and pattern of randomly selected Spot 3 in panel d; f EDX composition result and pattern of randomly selected Spot 4 in panel d. The EDX results show that the content of Mn is as low as the background value in the coating of a Fe-rich varnish sample, and the content of Fe is extremely low in the coating of a Mn-rich varnish sample (Lu et al. 2019)

eg (e.g., dx2 y2 and dz2 ) symmetry (Kwon et al. 2009; Sherman 2005). Considering that 3d electron configuration of Mn(IV) ions is t32g e0g , , the valence band maximum (VBM) of birnessite is occupied by t2g triplet, and its conduction band minimum (CBM) is dominated by eg doublet (Xu and Schoonen 2000). Mn (or O) vacancies and introduced transition metal cations change the band structure of birnessite and will be discussed in detail below.

3.2.1 Effect of Mn (or O) Vacancies

Fig. 3.6 Close linear relationships between in situ photocurrents and bias at fixed laser power. a Result of Fe-rich desert varnish sample, R2 = 0.9988; b result of Mn-rich desert varnish sample, R2 = 0.9955 (Lu et al. 2019)

(Fig. 3.10). Due to the repulsion between O ions and the 3d orbitals of Mn in the [MnO6] octahedron, Mn 3d orbitals would be split into t2g (e.g., dxy, dyz, and dzx) symmetry and

Mn (or O) vacancies in birnessite can affect the electronic structure and reduce the bandgap (Kwon et al. 2008, 2009). For Mn vacancies, the surrounding O around the Mn vacancy is usually associated with four H atoms to balance the charge. By calculating the energy, the structure is stable when four extra H atoms lie out of the ab plane in proximity to each other (Kwon et al. 2009; Ruetschi 1984). Attributed to the Mn-3d and O-2p around the Mn-vacancy site, the bandgap decreases due to some newly-introduced states

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Fig. 3.7 Current curves versus time of Fe and Mn (oxyhydr)oxide “mineral coatings” as well as other minerals in the rock substrate. Photocurrent-time curves of Fe and Mn (oxyhydr)oxide mineral coatings are fixed to graphite electrodes in supporting electrolyte of 0.1 M Na2SO4 under simulated solar light irradiation (Xenon light source) and with an applied external bias of 1.0 V (vs. SCE). In this system, the electron donors are H2O in the anodic chamber, while the electron acceptors are oxygen in the cathodic chamber. Similar to non-semiconducting minerals such as quartz and feldspar, the remnants apart from Fe/Mn oxides from red soil show indistinctive photoresponse to the light with the current values approaching the baseline value (Lu et al. 2019)

Fig. 3.8 The bandgap structure and redox potential of some semiconducting minerals in mineral coatings. a Oxygen K-edge absorption (thin lines) and emission spectra (bold lines) of Fe (oxyhydr)oxides and Mn oxides in the desert varnish coatings. The bandgap (Eg) of each mineral is determined by calculating the energy difference between the top of the valence band and the bottom of the conduction band given by the inflection points on the adsorption spectra and at half-peak height on the emission spectra; b band edge

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Photoactivity of Mn Oxides on Earth’s Surface

emerging above its VBM. Further, when increasing the Mn vacancies from 3.3 to 12.5%, the bandgap of birnessite shows a reducing trend (as shown in Fig. 3.4). The bandgap of non-vacancy birnessite is indirect, while the bandgap of birnessite with Mn vacancy is direct at C (Figs. 3.11a, b). There is also a difference in the CBM and VBM partial charge of non-vacancy and Mn-vacancy birnessite (Figs. 3.11c, d). In non-vacancy birnessite, most of the electron and hole states are overlapping, while they are well-separated in birnessite with Mn vacancies (Kwon et al. 2009). The electronic transitions from VB to CB upon illumination would leave holes at VBM and excited electrons at CBM. The separation between the VBM and the CBM would promote photoinduced charge carries efficiently. Oxygen vacancies, as another common defect in birnessite, have been proposed to have dual roles in the alteration of electronic structure. They can generate an extra electron in each [MnO6] octahedron and convert Mn(IV) ðt32g e0g Þ into

unstable high-spin Mn(III) ðt32g e1g Þ. To maintain a low-energy state in the strong Jahn–Teller effect, the extra electron must occupy another split Mn(III) d-orbital, i.e., e1g ðdz2 Þ within the bandgap (Fig. 3.12) (Lucht and Mendoza-Cortes 2015; Peng et al. 2017). Such a configuration can remarkably decrease the bandgap of O-defected birnessite compared with the

positions of Fe (oxyhydr)oxides and Mn oxides with respect to the normal hydrogen electrode (NHE, V) and the absolute vacuum scale (AVS, eV). The upper rectangles represent the bottoms of the conduction bands, and the lower rectangles represent the tops of the valence bands (pH = 7). The dashed line indicates the redox potential of humic acid (HA) (Struyk and Sposito 2001) and H2O/O2 redox couples (Lu et al. 2019)

3.2 Electronic Structure of Natural Semiconducting Mn Oxides

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Fig. 3.9 Schematic model showing the topmost layer of earth’s semiconducting mineral coating together with PS II for harvesting and transforming solar energy. This coating lies parallel to the core, mantle, and crust, but clearly represents a vanishingly small amount of mass compared to these major earth components/layers. Nevertheless, the average abundances of Mn in the core, mantle, and crust have low values, as indicated in this figure, while Mn is extraordinarily

concentrated in the ultrathin layer of mineral coatings. These coatings, together with the PS II protein complex in oxygenic photosynthetic organisms, are believed to be responsible for harvesting and transforming solar energy in the geosphere and biosphere, respectively, which broadens the pathways for utilizing solar energy from the well-known organic world to the newly discovered mineral semiconductor world (Lu et al. 2019)

non-vacancy one. On the other hand, O vacancy also commonly exists in other metal oxides and works as active sites in reactions (Cao et al. 2019; Wang et al. 2017). In Nature, multiple surfaces may be exposed and therefore perform different photoactivity. Theoretical calculations have compared the function of O vacancies exposed on the (001) and (100) crystal face of birnessite. The formation energy of O vacancies is 1.45 eV for the (001) face and 1.43 eV for (100) face, indicating the latter is more available than the former. The DOS in Fig. 3.13 also shows O vacancies reduce the bandgap and the (100) crystal face exhibits half-metallicity characteristics with near-zero bandgap. These results suggest that O vacancies can effectively change the surface state, which is consistent with the experimental studies (Yang et al. 2018).

coordinate in two kinds of sites. One is cations incorporating into birnessite sheet (INC), resulting in surrounding ions distorting, and the other is cations forming triple-cornersharing (TCS) in the interlayer of birnessite, resulting in [MnO6] layers with either increased interlayer spacing or expanded average bonding distance between Mn and O atoms. The electronic structure of birnessite will be changed in both cases. Moreover, the electronic structure of transition metal cations will affect the coordination trend (i.e., in TCS and INC model). For example, through simulating the INC and TCS model of Co, Ni, Cu, and Zn cations in birnessite, the energy difference between forming INC and TCS species is − 214, − 23, + 4, and + 34 kJ/mol, respectively. For INC models, the increasing content of cations, exerts greater stress on the surrounding ions by Jahn–Teller distortion through the interaction between the 3d orbit of metal cations and ligands O. Qualitatively, with the electronic number increasing of Co, Ni, Cu, and Zn, the unoccupied 3d orbital reduces and thus decreases the Jahn–Teller distortion. In Zn ions, due to 3d orbits fully occupied, strong interaction would not happen and the INC site for Zn is unstable in birnessite (Kwon et al. 2013). The DOS of Zn in the INC site also shows a weak interaction because the 3d electronic is mainly located in a deep valence band without spin exchange.

3.2.2 Effect of Metal Cations Cation exchange capacity (CEC) is a significant property of layered minerals, which adsorb cations reversibly in order to compensate for the negative charge of layers (Mukhopadhyay 2011). Transition metal cations are commonly observed in natural birnessite because of their similar crystal field environments with Mn ions. Generally, cations would

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Photoactivity of Mn Oxides on Earth’s Surface

Fig. 3.11 a MnO2 band structure of non-vacancy and b Mn-vacancy, views of charge distributions of valence band maximum (VBM) hole states [orange (or light grey)] and c conduction band minimum (CBM) electron states [blue (or grey)] in non-vacancy and d Mn-vacancy (Kwon et al. 2008)

Fig. 3.10 The density of states (DOS) of a non-vacancy birnessite with its O-2p and Mn-3d contribution to the total DOS. Total DOS of birnessite with vacancies (filled areas) in b 4
4
1 supercell and c 2
2
1 supercell, with the corresponding total DOS of non-vacancy birnessite (Kwon et al. 2009)

In order to understand the influence of heavy metal incorporating pathway on the band structure and the photoassisted structural transformation of biogenic birnessite, it is thus critical to take one as an example. Among various heavy metal ions, Cu2+ is one of the most common heavy metal pollutants on the earth’s surface (Qin et al. 2017). Cu2 + fixation mechanism on synthetic hexagonal birnessite has been extensively investigated but derived different models: it

is once considered to sorb on layer vacancy sites as CuO6 or CuO4 complex (Kwon et al. 2013; Sherman and Peacock 2010), or along edge sites as dimers or polynuclear clusters (Peña et al. 2015; Qin et al. 2017); Cu2+ is also reported to enter into Mn vacancies (Qin et al. 2017; Sherman and Peacock 2010), especially at high pH values (Kwon et al. 2013; Sherman and Peacock 2010). The experimental conditions of Cu2+ sorption process in those studies seem to be similar, yet Cu2+ sorption sites revealed by XAS are quite diversified. Among various physical, chemical, and biological factors, the illumination condition is hypothesized to result in the discrepancy in Cu2+ sorption mechanism, which, however, could easily be neglected. Due to the semiconducting property, light can promote photocatalytic self-reduction and dissolution of birnessite (Frierdich et al. 2011; Kwon et al. 2009; Sherman 2005; Sunda et al. 1983),

3.2 Electronic Structure of Natural Semiconducting Mn Oxides

Fig. 3.12 The Jahn–Teller effect. For Mn3+ to the right, the Jahn– Teller effect causes elongation of the Mn–O bond compared to Mn4+ to the left. Because of the loss of degeneracy, the orbitals for the Mn3+ are

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offset to lower the energy of the partially filled orbitals (Lucht and Mendoza-Cortes 2015)

Fig. 3.13 a Top view of pure birnessite-type MnO2; b corresponding density of states (DOS); c (001) MnO2 O vacancy configurations; d corresponding DOS; e (100) MnO2 O vacancy configurations; f corresponding DOS (Yang et al. 2018)

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and thus cause a recrystallization process, which will influence the metal sorption pathway and sorption sites. Up till now, the mechanism of light on structural transformation of Cu2+-binding birnessite has received little attention.

3.3

Photocatalytic Self-reduction of Natural Mn Oxides

3.3.1 Photocatalytic Oxidation of Water by Mn4CaOx Water oxidation is one of the most important reactions in Nature (Hocking et al. 2011). The origin of water oxidation can be traced back to the late Archean, in which cyanobacteria served as the earliest important producers of O2 by photosynthesis and may produce a trace amount of oxygen accumulating in some local environments (Lyons et al. 2020). Interestingly, studies have shown that Mn oxide, a byproduct of photosynthesis, already existed before the emergence of cyanobacteria (Johnson et al. 2013). That is, the photocatalytic oxygen production by layered Mn oxides is possibly the embryonic form of photosynthesis, due to the similar fine structures of oxygen evolution complex (OEC) and Ca-birnessite (Fig. 3.14). In PSII, the photocatalytic oxygen production reaction occurs when Mn clusters, Mn4CaO5, are converted into the birnessite-like structure (Hocking et al. 2011). The Mn clusters have a cubic alkane structure, which is composed of highly distorted Mn4Ca-oxo and overhanging Mn4CaO5. During the process of photosynthesis (i.e., S-state cycle), the structure of Mn clusters and the oxidation state of Mn are changed (Fig. 3.15). Birnessite is a layered mineral with a repeated single-layer of MnO2. Each MnO2 layer is composed of [MnO6] octahedra with shared edges, and the interlayer between MnO2 layers contains alkaline cations and water molecules (Robinson Fig. 3.14 Structure comparison of the active centers in the PSII; b Ca-birnessite (Yang et al. 2015)

Photoactivity of Mn Oxides on Earth’s Surface

et al. 2013). The photocatalytic decomposition of water by birnessite (Fig. 3.16) is similar to the S-state cycle of PSII described above, whose processes both consist of the first oxidation of Mn(III), the formation of O–O bonds and the final reduction of Mn(IV) (Yang et al. 2015). There are two opinions on the evolution of the Mn4CaO5 cluster. The first is that the oxygen evolution complex (OEC) evolved from Mn-bearing proteins, whereby the transfer of electrons by Mn-bearing catalase may be the intermediate step of aerobic photosynthesis (Blankenship and Hartman 1998). However, there is no obvious similarity between the PSII core protein and Mn-bearing catalase, and there is no evidence that H2O2 was abundant in the Archean ocean. Olson (1970) proposed a hypothesis that Mn(III)porphyrin cytochrome could replace Fe-bearing cytochromes in the evolution of OEC and pointed out a series of nitrogen-containing compounds such as NO and NO2− as transition electron donors of intermediate redox potential. However, from today’s perspective of OEC, this may be not correct because Mn is not bound in cytochrome but exists in Mn4CaO5 clusters and whether the transition state of nitrogen compounds existed at that time is still questionable (Sauer and Yachandra 2002). The other viewpoint is that OEC is derived from the integration of Mn oxides because OEC is similar to some layered Mn oxides (e.g., birnessite) in the short-range structure (Russell et al. 2008; Sauer and Yachandra 2002). However, there is no integration process of Mn oxides in the formation of OEC (Johnson et al. 2013; Tamura and Cheniae 1987). If that occurred, the OEC-like function of natural Mn oxides could make a pioneering contribution to atmospheric oxygen before the GOE of the primitive earth. Moreover, such a model for abiotic oxygen production has been probably working on Mars, where Mn oxides have always been abundant and exposed to the sun (as described for the earth above) (Lanza et al. 2016).

3.3 Photocatalytic Self-reduction of Natural Mn Oxides

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Fig. 3.15 Scheme of the water oxidation on the surface of Ca-birnessite. The purple and blue atoms represent Mn(IV) and Mn(III), respectively. The yellow proton represents the proton to remove in the next step (Yang et al. 2015)

3.3.2 Photocatalytic Self-reduction of Natural Mn Oxides In aquatic environments, the vertical profile of Mn in a multitude of oceanic and lacustrine water columns shows a gradual decrease in the concentration of aqueous Mn with depth (Davison 1993; Klinkhammer and Bender 1980; Landing and Bruland 1987; Statham et al. 1998; Sunda and Huntsman 1990). Theoretically, aqueous Mn should be oxidized to Mn(III/IV) oxides by superoxide radicals generated by sunlight irradiation or organisms in the photic zone, resulting in a lower concentration. Contrary to these thermodynamic estimates, aqueous Mn predominates in the photic zone (Morgan 2005; Sunda et al. 1983; Tebo et al. 2004; Zhang et al. 2018). This phenomenon is related to the photocatalytic self-reduction of natural Mn oxides, which can be caused either by a ligand to metal charge transfer process involving organics or by the reduction of photoelectrons (Sunda et al. 1983; Sunda and Huntsman 1990, 1994). Narsito et al. (2008) showed that the charge transfer process involves three steps: ① adsorbing humic acid on MnO2 surface to form an intermediate complex; ② one

delocalised p electron is excited and transferred; ③ one electron transfers from an oxygen center to the manganese center of the intermediate complex, causing the reductive dissociation of Mn(IV) ! Mn(II) (Fig. 3.17a). Mn oxides have a suitable band gap of less than 3.1 eV and can absorb and convert photons to produce photoelectron-hole pairs under the excitation of visible light (Sherman 2005). The photoelectrons/holes can change the valence states and existing forms of elements. It means the sunlight irradiation can induce photoelectrons in the Mn 3d orbitals in the conduction band and holes in O 2p orbitals in the valence band. If the electrochemical potential of the valence band is above one half-reaction then the hole in the valence band could have oxidation potential (Pinaud et al. 2011; Sherman 2005). If the conduction-band potential is more negative than one half-reaction then the electron excited into the conduction band would cause a reduction. Sherman (2005) calculates that the conduction band for pyrolusite and birnessite is lower than the redox potential 1.23 V (SHE, Standard Hydrogen Electrode) of MnO2/Mn (II); that is, they could theoretically absorb sunlight to generate photoelectrons for self-photoreduction. Hence, a

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Fig. 3.16 The Kok cycle of S-state transitions in photosynthetic water oxidation (Sauer and Yachandra 2002)

photoelectron could be excited to the conduction band to reduce Mn(IV) to Mn(III) under certain sunlight irradiation (Luther 2005; Sakai et al. 2005). The valence band is mainly occupied by O 2p and eg of Mn 3d orbits. One electron in the O 2p orbital can transfer to the empty eg orbital of Mn(IV) under sunlight irradiation to form intermediate Mn(III) (Luther 2005; Sakai et al. 2005). Mn(III) is in a tetragonally distorted rather than in octahedral geometry, resulting in higher kinetic lability (ligand exchange) and reactivity (Marafatto et al. 2015; Chan et al. 2018). The eg of Mn(III) therefore is prone to accept the

Fig. 3.17 a Excitation of the humic acids aromatic system in the intermediate HA-MnO2 complex through p-p* transition, and reductive dissociation of exited intermediate of HA-MnO2 complex, producing water soluble Mn(II) of MnO22− (Narsito et al. 2008); b proposed model for the evolution of metal redox chemistry during d-MnO2 photoreduction (Marafatto et al. 2015)

3

Photoactivity of Mn Oxides on Earth’s Surface

photoelectron and thus the Mn(III) is further reduced to Mn (II) (Luther 2005). Hence, the photocatalytic self-reduction of Mn oxides is a two-step one-electron transfer process. The conduction band states are localized Mn 3d orbitals which contribute to electron transfer, consequently, promoting the reduction of Mn(IV) to Mn(III). Marafatto et al. (2015) have proved the two-step one-electron transfer process by light-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS), which involves: layered Mn(IV) oxide obtained photoelectron to be reduced to layered Mn(III) oxide; Jahn–Teller distorted Mn(III) migrated into the interlayer and then the interlayer staking increased (Fig. 3.17b). In addition, the formation of valence band holes accompanying the photocatalytic self-reduction and the generation of Mn(III) can oxidise water or dissolved organic matter (Chan et al. 2018). In the Raman spectra of oxygen evolution reaction (OER), Mn(III) induced local strain on the oxide sublattice is also observed, which further proves the one-electron transfer mechanism (Chan et al. 2018).

3.4

Environmental Functions of Mn Oxides Controlled by Mn Redox Cycling

3.4.1 Reductive Dissolution of Mn Oxides Mediated by Organic Matter In soil and sediment, Mn oxides are capable of reacting with surrounding organic matter such as citrate, oxalate, humic and fulvic acids (Flynn and Catalano 2019; Stone 1987; Wang et al. 2018; Wang and Stone 2006a, b). During redox

3.4 Environmental Functions of Mn Oxides Controlled …

reactions, Mn(IV/III) will be reduced to low-valent Mn (III/II), some of which can detach from the structure and release off as soluble Mn(II). The mechanisms for the redox reactions between Mn oxides and organics are intrinsically attributed to the adsorption, complexation, and reduction processes (Wang and Stone 2006b). Adsorption refers to the formation of surface complexes, Mn(IV/III)-organic anions, which are essential to the reactions on the (hydr)oxide surfaces. Complexation induces the release of surface-located Mn(III) and the formation of Mn(III)-organic complexes in solution, which cause the ligand-assisted dissolution. Reduction process involves the conversion of Mn(IV) and Mn(III) to dissolved Mn(II), i.e., reductive dissolution. The reduction efficiency of Mn oxides is closely affected by pH conditions. At more acidic pH, Mn reduction will proceed more completely and rapidly, giving rise to both the accumulation of structural Mn(III/II) and the massive dissolution of Mn oxides into Mn(II) (Flynn and Catalano 2019; Stone and Morgan 1984; Waite et al. 1988; Wang et al. 2018; Wang and Stone 2006b). The H+-enhanced Mn reduction could be rationalised by the H+-consuming property of the birnessite–organics reaction (Wang et al. 2018). Also, the protonation of the birnessite surface is beneficial for the surface adsorption of organic anions, thus facilitating further reactions (Stone and Morgan 1984). In addition, Mn (II) desorption from edge sites into solutions will be promoted under acidic conditions as well (Wang and Stone 2006a). The dissolved Mn(II) and the structural Mn(III/II) could further modify the fine structures of Mn oxides, or even promote the inter-transformation among different mineral phases (Hinkle et al. 2016; Lefkowitz et al. 2013). Especially for birnessite, the vacancy abundance, layer stacking, in-plane crystallinity within the phyllomanganate sheets, and local coordination structure centering on the Mn atoms will all be likely to alter (Flynn and Catalano 2019; Wang et al. 2018). Notably, these structural responses after the redox reaction with organics are also liable to be affected by pH conditions. At the weakly acid environment of pH 4–6, Mn (III/II) tend to migrate into the inter-layer space and adsorb above vacancies (Flynn and Catalano 2019; Wang et al. 2018), which will strengthen the layer stacking (Flynn and Catalano 2019; Marafatto et al. 2015); on the other hand, some of the Mn(III) also appears to locate at the layer edge sites, which, together with the abundant presence of dissolved Mn(II) in acidic solutions, might help induce the re-ordering of layer sheets towards triclinic symmetry (Flynn and Catalano 2019; Hinkle et al. 2016). Under neutral to alkaline solutions, the generated Mn(III) is more inclined to be incorporated into vacant sites and reside in the layer (Wang et al. 2018). Further, with the effect of Mn(II), additional mineral phases such as manganite, feitknechtite, and hausmannite may possibly form (Lefkowitz et al. 2013;

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Wang et al. 2018; Zhang et al. 2018). In addition, Mn oxide minerals like birnessite in the soil can be used as electron acceptors for microorganisms (such as the Dietzia species), accompanied by the oxidation of organic matter and fixation as carbonate minerals [e.g., rhodochrosite (Li et al. 2019a, b)]. It thus can be summarised that the coupling effect of birnessite and microorganisms in the soil system under sunlight can convert atmospheric CO2 into organics effectively, and further into carbonate minerals assisting with carbon sequestration.

3.4.2 Oxidative Formation of Mn Oxides and Heavy Metal Sorption Until recently, the formation mechanism of natural Mn oxides is still controversial and mainly divided into biogenic and abiotic oxidation. Naturally occurring Mn-oxidising microorganisms (bacteria and fungi) are responsible for the biogenic catalytic oxidation of Mn(II) into Mn(III/IV) oxides (Tebo et al. 2004; Villalobos et al. 2006; Webb et al. 2005). The key to the catalytic process should be attributed to the properly-modified pH and redox conditions of the local aqueous environment by microorganisms, or the release of metabolic products to oxidise Mn(II) chemically (Hullo et al. 2001; Richardson et al. 1988; Tebo et al. 2004). By contrast, Mn(II) is stable as redox-neutral at pH values up to 8.5 and hence the abiotic homogeneous oxidation of Mn(II) is very slow and requires years for completion (Kim et al. 2011; Tebo et al. 2005). However, some researchers have observed the rapid photooxidative generation of Mn oxides by coexisting natural semiconducting iron (hydr)oxides, such as hematite and goethite (Madden and Hochella 2005; Xu et al. 2019). Also, the photostimulated production of reactive oxygen species (ROS) in natural Mn coatings, like 1O2 and OH with strong oxidising capability, can contribute to Mn oxide formation (Xu et al. 2019). During the oxidative formation of layered Mn oxides, i.e., birnessite, some heavy metal ions are likely to be adsorbed or incorporated into the structure, such as Cu2+, Pb2+, Zn2+, and Ni2+ (Liu et al. 2018; Qin et al. 2017; Wang et al. 2012; Zhu et al. 2010). As revealed in these studies, the sorption sites and binding structures of the metal ions can be varied: most commonly, they are inclined to migrate into interlayer space and bind above the vacancy, forming a triple-cornersharing (TCS) complex, and rarely as part of the layered sheet as INC species, in addition, the metal ions can be adsorbed at layer edges to form multinuclear species by double corner-sharing or edge-sharing complexing (DCS or DES). The sorption pathways of heavy metals largely affect their preferential binding sites. When the metal cations are simultaneously co-precipitated with birnessite, they will

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predominantly adsorb above the layer vacancies; when the metal cations are post-adsorbed to birnessite, they tend to be incorporated into the vacancies or bind at layer edges (Li et al. 2019a, b). On the other hand, the metal sorption mechanism is sensitively impacted by the fine structure of birnessite. A large interlayer space will allow the layer accommodation of metal cations to bind at vacancies as TCS and INC species (Liu et al. 2018; Wang et al. 2012; Zhu et al. 2010), and these species will potentially induce the structural transformation toward tunneled Mn oxides (Manceau et al. 2014). In other cases when the birnessite is structurally accumulated with Mn(III), the metal ions will be repelled to the edge sites (Li et al. 2019a, b; Liu et al. 2018) The correspondingly formed DES and DCS complexes are structurally unsteady and readily released off from the layer, especially under extreme pH conditions (Qin et al. 2017).

3.5

Concluding Remarks

Mn oxides widely distributed among various natural settings on earth are diversified in mineral phase, crystal structure, and Mn oxidation state. The most common Mn oxides on earth’s surface occur in rock/soil surface Mn coatings, which are predominantly present as birnessite that has a layered structure and can contain interlayer metal cations. Natural birnessite displays a marked photoelectric response to visible light and acts as the semiconducting material. The electronic structure is sensitive to fine structures such as Mn/O vacancies and adsorbed metal ions. With excellent photocatalytic property, birnessite actively participates in the photoredox reactions on earth, during which birnessite itself is reduced while organics and even water are oxidised. Notably, the water oxidation and oxygen evolution complex in the biological photosynthesis system, Mn4CaO5, is featured by a birnessite-type structure. The oxidative formation of birnessite occurs mainly through the bio-catalysis effect by microorganisms such as Pseudomonas putida and Bacillus species; in other cases, the photooxidation mediated by coexisting semiconducting minerals or ROS gives additional pathways. Mn oxides act as heavy metal scavengers in the environment, while also acting to degrade organics as well as oxidize water. Together they are all involved in the biogeochemical cycle driven by Mn photocatalytic reduction and oxidation in Nature.

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70 Ruetschi P (1984) Cation-vacancy model for MnO2. J Electrochem Soc 131:2737–2744 Russell MJ, Allen JF, James MWE (2008) Inorganic complexes enabled the onset of life and oxygenic photosynthesis. Springer, Netherlands, Berlin Sakai N, Ebina Y, Takada K et al (2005) Photocurrent generation from semiconducting manganese oxide nanosheets in response to visible light. J Phys Chem B 109:9651–9655 Sakimoto KK, Wong AB, Yang PD (2016a) Self-photosensitization of nonphotosynthetic bacteria for solar-to-chemical production. Science 351:74–77 Sakimoto KK, Zhang SJ, Yang PD (2016b) Cysteine-cystine photoregeneration for oxygenic photosynthesis of acetic acid from CO2 by a tandem inorganic-biological hybrid system. Nano Lett 16:5883– 5887 Sarmast M, Farpoor MH, Boroujeni IE (2017) Soil and desert varnish development as indicators of landform evolution in central Iranian deserts. Catena 149:98–109 Sauer K, Yachandra VK (2002) A possible evolutionary origin for the Mn4 cluster of the photosynthetic water oxidation complex from natural MnO2 precipitates in the early ocean. Proc Natl Acad Sci USA 99:8631–8636 Schindler M, Dorn RI (2017) Coatings on rocks and minerals: the interface between the lithosphere and the biosphere, hydrosphere, and atmosphere. Elements 13:155–158 Sherman DM (2005) Electronic structures of iron(III) and manganese (IV) (hydr)oxide minerals: thermodynamics of photochemical reductive dissolution in aquatic environments. Geochim Cosmochim Acta 69:3249–3255 Sherman DM, Peacock CL (2010) Surface complexation of Cu on birnessite (d-MnO2): controls on Cu in the deep ocean. Geochim Cosmochim Acta 74:6721–6730 Statham PJ, Yeats PA, Landing WM (1998) Manganese in the eastern Atlantic Ocean: processes influencing deep and surface water distributions. Mar Chem 61:55–68 Stone AT (1987) Reductive dissolution of manganese(III/IV) oxides by substituted phenols. Environ Sci Technol 21:979–988 Stone AT, Morgan JJ (1984) Reduction and dissolution of manganese (III) and manganese(IV) oxides by organics. 1. Reaction with hydroquinone. Environ Sci Technol 18:450–456 Struyk Z, Sposito G (2001) Redox properties of standard humic acids. Geoderma 102:329–346 Sunda WG, Huntsman SA (1990) Diel cycles in microbial manganese oxidation and manganese redox speciation in coastal waters of the bahama-islands. Limnol Oceanogr 35:325–338 Sunda WG, Huntsman SA (1994) Photoreduction of manganese oxides in seawater. Mar Chem 46:133–152 Sunda W, Huntsman S, Harvey G (1983) Photoreduction of manganese oxides in seawater and its geochemical and biological implications. Nature 301:234–236 Tamura N, Cheniae G (1987) Photoactivation of the water-oxidizing complex in Photosystem II membranes depleted of Mn and extrinsic proteins. I. Biochemical and kinetic characterization. BBABioenerg 890:179–194 Taylor SR, McLennan SM (1985) The continental crust: its composition and evolution. Blackwell Scientific Publications, Oxford

3

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Tebo BM, Bargar JR, Clement BG et al (2004) Biogenic manganese oxides: properties and mechanisms of formation. Annu Rev Earth Planet Sci 32:287–328 Tebo BM, Johnson HA, McCarthy JK et al (2005) Geomicrobiology of manganese(II) oxidation. Trends Microbiol 13:421–428 Thiagarajan N, Lee CTA (2004) Trace-element evidence for the origin of desert varnish by direct aqueous atmospheric deposition. Earth Planet Sci Lett 224:131–141 Villalobos M, Toner B, Bargar J et al (2003) Characterization of the manganese oxide produced by Pseudomonas putida strain MnB1. Geochim Cosmochim Acta 67:2649–2662 Villalobos M, Lanson B, Manceau A et al (2006) Structural model for the biogenic Mn oxide produced by Pseudomonas putida. Am Sci 91:489–502 Waite TD, Wrigley IC, Szymczak R (1988) Photoassisted dissolution of a colloidal manganese oxide in the presence of fulvic acid. Environ Sci Technol 22:778–785 Wang Y, Stone AT (2006a) The citric acid–MnIII, IVO2 (birnessite) reaction. electron transfer, complex formation, and autocatalytic feedback. Geochim Cosmochim Acta 70:4463–4476 Wang Y, Stone AT (2006b) Reaction of MnIII, IV (hydr) oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds. Geochim Cosmochim Acta 70:4477–4490 Wang Y, Feng XH, Villalobos M et al (2012) Sorption behavior of heavy metals on birnessite: relationship with its Mn average oxidation state and implications for types of sorption sites. Chem Geol 292:25–34 Wang JL, Li JE, Jiang CJ et al (2017) The effect of manganese vacancy in birnessite-type MnO2 on room-temperature oxidation of formaldehyde in air. Appl Catal B 204:147–155 Wang Q, Yang P, Zhu M (2018) Structural transformation of birnessite by fulvic acid under anoxic conditions. Environ Sci Technol 52:1844–1853 Webb SM, Tebo BM, Bargat JR (2005) Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp strain SG-1. Am Miner 90:1342–1357 Xu Y, Schoonen MAA (2000) The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am Miner 85:543–556 Xu XM, Ding HR, Li Y et al (2018) Mineralogical characteristics of Mn coatings from different weathering environments in China: clues on their formation. Mineral Petrol 112:671–683 Xu XM, Li Y, Li YZ et al (2019) Characteristics of desert varnish from nanometer to micrometer scale: a photo-oxidation model on its formation. Chem Geol 522:55–70 Yang J, An H, Zhou X et al (2015) Water oxidation mechanism on alkaline-earth-cation containing birnessite-like manganese oxides. J Phys Chem C 119:18487–18494 Yang WJ, Zhu YF, You F et al (2018) Insights into the surface-defect dependence of molecular oxygen activation over birnessite-type MnO2. Appl Catal B 233:184–193 Zhang T, Liu L, Tan W et al (2018) Photochemical formation and transformation of birnessite: effects of cations on micromorphology and crystal structure. Environ Sci Technol 52:6864–6871 Zhu MQ, Ginder-Vogel M, Sparks DL (2010) Ni(II) Sorption on biogenic Mn-Oxides with varying Mn octahedral layer structure. Environ Sci Technol 44:4472–4478

4

Redox Activity of Iron Sulfide and Mn Oxide

Pyrrhotite is a common iron sulfide mineral in nature, which can be divided into hexagonal and monoclinic structure. Hexagonal pyrrhotite is also called meteorite pyrite, and its chemical composition is FeS. The chemical composition of monoclinic pyrrhotite is Fe1−xS, in which the Fe is not 1, because part of Fe2+ is replaced by Fe3+, in order to maintain charge balance, the Fe2+ position in the structure appears partial vacancy, resulting in the formation of absent structure or structural defects. Fe2+ and S2− in the lattice can both provide electrons, which makes pyrrhotite a natural reducing agent. Manganese oxides are ubiquitous in soils and sediments and are some of the most redox reactive mineral constituents in environments. Cryptomelane is known to be a manganese oxide octahedral molecular sieve for oxidation. Cryptomelane (KxMn8−xO16, 0.2 < x < 1.0) is a typical tunnel structure manganese oxide has a well-defined 2
2 tunnel structure and a tunnel size of 0.46 nm
0.46 nm constructed from edge-shared double MnO6 octahedral chains (Vicat et al. 1986; Bac et al. 1993; Pasero 2005). The tunnels are partially filled with K+ and water molecules, which balance the charge of mixed Mn2+, Mn3+, and Mn4+ (Feng et al. 1995; Post 1999). And the manganese element with variable valence is the main reason for the redox activity of manganese oxides.

4.1

Removal of Cr(VI) and Cr(III) from Aqueous Solutions and Industrial Wastewaters by Natural Pyrrhotite

Chromium, because of its widespread industrial use, is a common heavy metal contaminant in industrial regions throughout the world. Many Cr(VI) species are known carcinogens. In the U.S., chromium is, after lead, the second most important inorganic contaminant in groundwater. The safe disposal of large quantities of Cr(VI)-bearing toxic

© Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_4

industrial wastewaters is also one of the major environmental problems that China faces. Environmental remediation of Cr(VI)-contaminated sites or industrial wastewaters usually involves the reduction of Cr(VI) to the less mobile and less toxic Cr(III) species (Sedlak and Chan 1997). Reduction of Cr(VI) to Cr(III) took place if a strong chemical reductant (e.g., sodium sulfite) was added and Cr(III) was then removed through precipitation (e.g., Cr(OH)3) by adding sodium hydroxide or lime. This procedure usually required sophisticated equipment and expensive chemical reagents. It would therefore result in high operational costs and large quantities of toxic byproducts. Partial immobilization of aqueous Cr(VI) could be realized directly, without reduction to Cr(III), by the coprecipitation of the CrO42− ions into Barite, where CrO42− was substituted for SO42− in the crystal structure (Maria et al. 1997; Fernández-González et al. 1999). Treating industrial Cr(VI) wastes using natural materials such as iron sulfide minerals has shown some great potential and attracted the attention of the international scientific community (Zouboulis et al. 1995; Erdem and Tümen 1996; Pratt 1996; Blowes et al. 1997; Aide and Cummings 1997; Wittbrodt and Palmer 1997; Maurizio et al. 1998; Demoisson et al. 2005; Kim et al. 2002). Pyrite, in particular, has been proven to be effective in removing dissolved Cr(VI) from solutions (Zouboulis et al. 1995). In this chapter, we show another simple, effective, and economical alternative for treating Cr(VI)-bearing wastewaters using another natural iron sulfide mineral, pyrrhotite.

4.1.1 Characteristics of Pyrrhotite and Wastewater Pyrrhotite used in this study was obtained from the Dongshengmiao sulfide mineral deposit in Inner Mongolia, Northern China. Hand-picked samples were crushed, washed

71

72

with tap water, dried at room temperature, and then sieved. Pyrrhotite crystals were selected using a magnetic mineral separator. It was identified as monocline pyrrhotite with a0 = 1.1892 nm, b0 = 0.6897 nm, c0 = 2.2742 nm, b = 90° 11′, and a super-crystal structure of 2A2B4C by X-ray diffraction. Its chemical composition was 60 wt% for Fe and 39% for S by electron-probe analysis. This gave a crystal formula of Fe0.88S. The nonstoichiometric Fe to S ratio indicated vacancies at the Fe site in the crystal structure. The sum of the heavy metal (i.e., Mn, Cu, Pb, Zn, Cd, Co, and Ni) content of this mineral was less than 0.1 mmol/g, as determined by the flame atomic absorption spectrophotometric (AAS) analysis of the mineral after wet chemical digestion. Microscopic observation showed that the selected pyrrhotite particles fell into a restricted grain size range. Its reactive surface area was, therefore, proportional to its mass. Solutions with different Cr (VI) concentrations were prepared from reagent grade K2Cr2O7 and distilled water. Their pH values were adjusted to the predetermined values by adding small quantities of concentrated H2SO4 or NaOH solution and confirmed by pH measurements. Two Cr(VI)-bearing industrial wastewater samples were collected from two electroplating plants, one in Beijing and the other in the Fujian Province, Southeast China. The wastewater from the Beijing plant had a Cr(VI) concentration of 600 µmol/L and a pH of 6.86, while that from the Fujian plant had a Cr(VI) concentration of 3400 µmol/L and a pH between 2 and 3.

4

Redox Activity of Iron Sulfide and Mn Oxide

added to 50.0 mL of an experimental solution that contained 190 µmol/L Cr(VI). The minimum quantity of pyrrhotite required for the effective removal of Cr(VI) was proportional to the Cr(VI) concentration and the volume of wastewater to be treated (i.e., the total quantity of Cr(VI) to be removed). If 80–120 mesh of pyrrhotite was used to treat 1 L of wastewater, the relationship between the quantity of the solid (G, g) and the Cr(VI) concentration (C, µmol/L) was G = 0.212C (Fig. 4.3a), while for a wastewater with an initial Cr(VI) concentration of 250 µmol/L the minimum solid-to-solution ratio was 55 g/L (Fig. 4.3b). To effectively remove every µmol of Cr(VI) from the solution, therefore, a minimum of 0.22 g or 2.7 mmol of 80–120 mesh pyrrhotite was required. Three grams of 80–120 mesh pyrrhotite were used and then reused in six consecutive experimental runs under the following conditions: an initial Cr(VI) concentration of 190 µmol/L, 50.0 mL of solution, and 20 min reaction time (Table 4.3). The effectiveness of Cr(VI) removal was maintained in all the runs. Table 4.4 provided the results of seven parallel Cr(VI) removal experiments in wastewater collected from the Beijing electroplate plant. The wastewater had a pH of 6.86 and a Cr(VI) concentration of 600 µmol/L. Although the final pH was much higher than those observed from our previous experiments conducted in artificial Cr(VI)-bearing solutions, Cr(VI) was effectively removed (i.e., removal efficiency > 98%). Experiments conducted in wastewater from the Fujian plant yielded similar results.

4.1.2 Effectiveness in Cr(VI) Removal 4.1.3 Solid Phases After Cr(VI) Removal Table 4.1 presents the results that were obtained in solutions with different initial pH values. More than 90% of Cr(VI) was effectively removed within 2.5 h if the two results from the runs with the initial pH of 11.5 and 12.1 were disregarded. The pH became steady within 20 min of the experimental runs and fell into a narrow range of 3.5–4.5, with the exception of experiments conducted at extreme initial pH conditions (i.e., pH) 1.08 and 12.1) (Fig. 4.1). Figure 4.2 showed the minimum time required for reaching maximum Cr(VI) removal. With the exception of the initial pH of 7.63 experiments, maximum Cr(VI) removal was reached within 40 min, when 3.0 g of 80–120 mesh pyrrhotite was used to treat 50.0 mL of 190 µmol/L Cr (VI) solution. The effects of the quantity and the grain size of pyrrhotite on Cr(VI) removal were summarized in Table 4.2. The quantity of the Cr(VI) removal was related positively to the amount of pyrrhotite used and negatively to its grain size. Under our experimental conditions, Cr(VI) was effectively removed if more than 3.0 g of 80–120 mesh pyrrhotite was

At the end of the experiments, the liquid settled into two distinctive layers: a clear supernatant layer and a cloudy yellowish lower layer. The concentrations of Cr(VI) and total Cr, Cu, Pb, Zn, Cd, and Fe in both layers were determined. They were much higher in the lower layer than in the supernatant. In the supernatant, the Cr(VI) concentration was at 0.77 µmol/L, which was below the Cr(VI) industrial wastewater discharge standard in China. Its total Cr concentration, however, was at 920 µmol/L, which exceeded the discharge standard. In the traditional methods, this problem was resolved by adding Ca(OH)2 or NaOH to initiate the precipitation of Cr(OH)3. In this research, the content of total Cr in the supernatant liquid was effectively reduced to a level that was below the discharge standard simply by adding an excessive quantity of pyrrhotite. Desirable results of the contents of Cu, Pb, Zn, Cd, and Fe in the supernatant were also obtained when the reaction was extended to 10 h. In addition, when the reaction time was extended from 1 to 10 h, the content of Cr(VI) was

4.1 Removal of Cr(VI) and Cr(III) from Aqueous Solutions …

73

Table 4.1 Summary of the effects of initial solution pH and reaction time on the efficiency of Cr(VI) removal Reaction time = 2.5 h

Reaction time = 18 h

Initial pH

1.22

3.35

5.32

6.24

7.24

8.74

9.89

11.5

Initial Cr(VI) concentration/(µmol/L)

190

190

190

190

190

190

190

190

pH after reaction

1.00

3.56

3.89

4.08

5.48

5.30

5.73

10.0

Cr(VI) concentration after reaction/(µmol/L)

7.5

6.3

6.9

4.4

4.4

15.1

8.2

104

Cr(VI) removal efficiency after reaction/%

96.1

96.7

96.4

97.7

97.7

92.2

95.8

45.8

Initial pH

1.08

3.40

5.32

6.85

7.80

8.61

9.66

12.1

Initial Cr(VI) concentration/(µmol/L)

190

190

190

190

190

190

190

190

pH after reaction

1.14

3.61

3.84

3.77

4.23

3.98

4.47

8.95

Cr(VI) concentration after reaction/(µmol/L)

9.4

1.3

1.9

1.3

0.6

0.6

0.6

76

Cr(VI) removal efficiency after reaction/%

95.1

99.4

99.0

99.4

99.7

99.7

99.7

60.5

Lu et al. (2006)

Fig. 4.1 pH fell into a narrow range of 3.5–4.5 after 20 min of Cr(VI) removal reactions, with two exceptions observed at extreme initial pH conditions (i.e., pH 1.08 and 12.1) (Lu et al. 2006)

decreased from 5.8 to 1.2 µmol/L, which was much lower than the discharge standard of industrial wastewater (i.e., 1.5 mg/L or 29 µmol/L) and slightly above the standard of potable water (i.e., 0.05 mg/L or 0.95 µmol/L). It was apparent that Cr(VI) in the solution was effectively removed, resulting in yellowish precipitates. To determine the chemical compositions, the oxidation states, and the binding states of the solid products, two parallel experiments were conducted to generate relatively large quantities of solid precipitates by adding 5.0 g of 140– 160 mesh (96–107 µm) pyrrhotite to 100.0 mL of 3850 µmol/L Cr(VI) solutions with the initial pH of 1.87 and 11.6, respectively, and then letting the mixtures react for 40 h. The final pH values of 2.56 and 9.18 were obtained, and 95.7 and 44.6% of Cr(VI) were removed, respectively.

Results of XPS spectra of the precipitates, which were collected from the cloudy yellowish lower layer, were given in Fig. 4.4. The following Cr-bearing phases were positively identified using published standard Cr 2p3/2 binding energy: Cr2S3 (574.80 eV) and Cr2O3/CrOOH mixture (577.55 eV in Fig. 4.4b or 577.30 eV in Fig. 4.4c). Chromium sulfides are rare in nature. Brezinaite (Cr3S4) has been found in meteorites (Bunch and Fuchs 1969). When the mixtures of Cr and S of different proportions were heated to 1000 C and then cooled to room temperature, a series of chromium sulfides, such as R-Cr, CrS, Cr7S8, Cr5S6, Cr3S4, Cr2S3, etc., would be crystallized. To the best of our knowledge, this was the first time that a Cr2S3 phase was identified in an aqueous system. Further studies are needed to obtain the mineralogical and chemical properties of this phase.

74

4

Redox Activity of Iron Sulfide and Mn Oxide

Fig. 4.2 Reaction time required to reach maximum Cr(VI) removal in solutions of different initial solution pH values (Lu et al. 2006)

Table 4.2 Effects of the quantity and grain size of clino-pyrrhotite on Cr(VI) removal Grain size/mesh

40–80

80–120

80–120

80–120

80–120

80–120

80–120

120–160

Mineral quantity/g

3

1

2

3

4

8

16

3

Initial pH

7.51

7.89

7.06

7.38

7.78

7.23

7.35

7.34

Final pH

5.89

6.40

5.63

4.77

4.31

3.13

3.26

3.98

Initial Cr(VI) concentration/(µmol/L)

190

190

190

190

190

190

190

190

Final Cr(VI) concentration/(µmol/L)

67

120

65

24

5.0

3.8

1.3

5.0

Cr(VI) removal efficiency/%

65.4

38.9

66.3

87.6

97.4

98.0

99.4

97.4

Lu et al. (2006)

4.1.4 Process of Cr(VI) Removal

FeSðsÞ þ 2H þ ! Fe2 þ þ H2 SðaqÞ

ð4:2Þ

The experimental data were fitted to the Langmuir isotherm
ð4:1Þ G ¼ 4:58Ceq = 1:20 þ Ceq

Reduction of Cr(VI) to Cr(III) and precipitation of Cr(III) in acid solutions

where Ceq (µmol/L) is the equilibrium solution concentration of Cr(VI), and G (µmol/g) is the corresponding quantity of Cr(VI) adsorbed onto the surface of a unit mass of pyrrhotite. The fact that our experimental data fitted well to the Langmuir isotherm suggested that the adsorption of Cr2O72− or CrO42− ions onto the surface of pyrrhotite were an important step in Cr(VI) removal from solution. Reduction of Cr(VI) to Cr(III) and the consequent Cr(III) precipitation probably happened at the solution-mineral interface. This process could be expressed by the following reactions. Dissolution of pyrrhotite

ð4:3Þ

þ 4Cr2 O2 7 þ 15H2 SðaqÞ þ 2H ¼ 4Cr2 S3 ðsÞ þ 3SO2 4 þ 16H2 O 2 Cr2 O2 7 þ 4H2 SðaqÞ ¼ Cr2 S3 ðsÞ þ SO3 þ 4H2 O

ð4:4Þ

2þ Cr2 O2 þ 8H þ ¼ Cr2 O3 ðsÞ þ 6Fe3 þ þ 4H2 O 7 þ 6Fe

ð4:5Þ Reduction of Cr(VI) to Cr(III) and precipitation of Cr(III) in basic solutions 2þ CrO2 þ 4OH þ 4H2 O 4 þ 3Fe ¼ CrðOHÞ3 ðsÞ þ 3FeðOHÞ3 ðsÞ

ð4:6Þ

4.1 Removal of Cr(VI) and Cr(III) from Aqueous Solutions …

75

Fig. 4.3 a Linear relationship between the minimum quantity of clino-pyrrhotite required (G, g) to effectively treat 1 L of Cr(VI)bearing solutions and the Cr(VI) concentration (C, µmol/L) of the Cr(VI)-bearing solutions; b linear relationship between the minimum quantity of clino-pyrrhotite required (G, g) to effectively treat a Cr(VI)-bearing solution (Cr(VI) 250 µmol/L) and the volume of the solution (V, mL) (Lu et al. 2006)

Table 4.3 Results of Cr(VI) removal using recycled clino-pyrrhotite

Cycle

1

2

3

4

5

6

Initial pH

1.04

1.04

1.01

1.03

3.19

5.83

Initial Cr(VI) concentration/(µmol/L)

190

190

190

190

190

190

Final pH

1.08

1.08

1.04

1.01

1.98

3.82

Final Cr(VI) concentration/(µmol/L)

1.3

6.3

< 0.0019

< 0.0019

1.9

< 0.0019

Cr(VI) removal efficiency/%

99.4

96.8

100

100

99.0

100

Lu et al. (2006)

76

4

Redox Activity of Iron Sulfide and Mn Oxide

Table 4.4 Results of Cr(VI) removal of wastewater from the Beijing electroplate plant Exp. #

1

Final pH

6.84

Final Cr(VI) concentration/(µmol/L)

0.038

Cr(VI) removal efficiency/%

100

2

3 7.68 5.7

5.76 0.028

99.1

100

4 5.80 6.9 98.9

5

6

7

6.28

6.83

6.91

< 0.0019

< 0.0019

< 0.0019

100

100

100

Lu et al. (2006)

Fig. 4.4 a XPS scan of clino-pyrrhotite after the reaction; b XPS analysis of Cr-bearing phases precipitated at the clino-pyrrhotite surface in acidic solution; c XPS analysis of Cr-bearing phases at the clino-pyrrhotite surface in basic solution (Lu et al. 2006) 2þ 4CrO2 þ 6HS2 þ OH þ 10H2 O 4 þ 9Fe ¼ 2Cr2 S3 ðsÞ þ 9FeðOHÞ3 ðsÞ

ð4:7Þ

As the solubility of clino-pyrrhotite was extremely low, the previous reactions occurred most likely at or near the solution-mineral interface. Similar reactions were reported during acid mine drainage formation (Pratt 1996). The nonstoichiometry (i.e., Fe0.88S) of pyrrhotite was attributed to the existence of vacancies of the Fe sites, rather

than an excess of sulfur ions, in the mineral crystal structure. Charge balance was maintained by the existence of Fe3+ ions in the crystal structure (i.e., (Fe2+, Fe3+)S), as proposed by Vaughan and Craig. Hydrogen sulfide and a hydroxyl radical interacted with pyrrhotite at the vacant Fe sites on the surface (Knipe et al. 1995). Such types of interactions were cited as the classical Lewis base/acid-type reactions, with HS− and OH− acting as a base by donating a pair of electrons to a vacant Fe site (Knipe et al. 1995).

4.2 Reactivity of Mn Oxide Cryptomelane

It was most likely that the vacant Fe sites in pyrrhotite acted as surface reactive sites for Cr(VI) reduction. Similar to HS− and OH−, Cr(VI), in the form of Cr2O72− or CrO42−, was attracted to the vacant Fe sites of pyrrhotite. The Cr2O72 − or CrO42− ion had sufficient electron affinity to remove the trapped electrons from the vacant Fe sites, and consequently, its Cr(VI) was reduced to Cr(III).

4.1.5 Potential Industrial Application This laboratory bench-scale experimental study showed that natural pyrrhotite minerals could be used in the effective removal of Cr(VI) from aqueous solutions and industrial wastewaters. It would have great potential in the industrial-scale treatment of Cr(VI) bearing toxic wastewaters. The acidic condition that was generally associated with most industrial wastewater would in fact favor the removal of Cr(VI) by pyrrhotite. In an acidic medium, the proton was a reactant in both pyrrhotite dissolution and Cr-(VI) reduction reactions (e.g., see reactions 1, 2, and 4). Proton consumption would result in a reduction of the acidity of the waste solution. If an excessive amount of pyrrhotite was added to the acidic waste solution, the acid in the waste would be partially neutralized, and the waste solution could be discharged directly without the addition of any basic chemical reagent. Sodium sulfite (Na2SO3) used in the traditional methods for treating Cr(VI) wastewaters was in fact produced from natural pyrrhotite (FeS) or pyrite (FeS2). While one molecule of Na2SO3 provided only two electrons for the reduction of Cr(VI) to Cr(III), where SO32− was oxidized to SO42−, one molecule of pyrrhotite (FeS) would provide eight electrons for the Cr(VI) to Cr(III) reduction, as S2− was oxidized to SO42−. Pyrrhotite was therefore 4 times as effective as sodium sulfite. In addition, the ferrous iron in pyrrhotite would also contribute one electron for the Cr(VI) reduction when it was oxidized to ferric iron. From an industrial standpoint, it implied that our method would have the potential of being much more cost-efficient than the traditional methods. The grain size and the quantity of pyrrhotite were two of the most important factors in the removal of Cr(VI). In general, a small grain size and a large quantity of pyrrhotite and a weak acidic media favored the removal. Optimal Cr (VI) removal was found with a pyrrhotite grain size of 80– 120 mesh or 145 ± 28 µm, and the pH values were between 1 and 10. Under such conditions, the minimum quantity of pyrrhotite required for the effective removal of Cr(VI) was proportional to the Cr(VI) concentration and the volume of the wastewater, or in other words, to the total quantity of Cr (VI) to be treated. Our laboratory bench-scale experimental

77

results indicated that only 220 g or 2.7 mol of pyrrhotite with a grain size of 145 ± 28 µm was needed to remove effectively 1 mmol of Cr(VI) within 1 h from wastewater that had a pH between 1 and 10. From an industrial wastewater treatment perspective, one cubic meter of wastewater that contained  1 mmol/L of Cr(VI) would be effectively treated within an hour if 220 kg of 145 ± 28 µm pyrrhotite was added to the wastewater. Our method offered several important advantages over the traditional two-step methods (i.e., reduction of Cr(VI) to Cr(III) in an acidic medium Cr(III) precipitation in a basic medium). The treatment process was a simple addition of pyrrhotite to the wastewater. Both Cr(VI) reduction and consequent Cr(III) precipitation occurred when pyrrhotite was mixed with the Cr(VI)-bearing wastewater. No special types of equipment, costly chemicals, or complex operations were required. Chromium was precipitated out as solids and removed completely from the liquid phase. The treated wastewater was clean enough to be discharged directly into the natural environment. Furthermore, the solid residue of pyrrhotite could be reused after a simple rinse with water. As a result, the quantity of the solid waste generated in this method was much smaller than that produced by any of the previously reported treatment methods. The problem of the production of large quantities of solid toxic wastes that were usually associated with other methods was avoided. In addition, the total Cr content in the resultant solid wastes might be high enough for economic Cr recovery. In particular, our method offered two additional important advantages over the method proposed by Zouboulis et al. (1995) using fine grain size pyrite: that a much coarser mineral grain size was used in our method (i.e., 145 ± 28 µm vs 12 µm) and that pyrrhotite could be effectively reused in the treatment scheme, which, from an industrial standpoint, would further reduce the operational difficulty and cost.

4.2

Reactivity of Mn Oxide Cryptomelane

4.2.1 Occurrence and Characterization of Cryptomelane 4.2.1.1 Occurrence Natural cryptomelane mainly occurs in the supergene oxidation zones of manganese deposits and lateritic weathered profiles (Ostwald 1992; Vasconcelos et al. 1994; Linder 2001), where it is formed by authigenic precipitation (Vasconcelos et al. 1994). Meanwhile, cryptomelane has long been known to originate from a layered (birnessite or vernadite) precursor (Taylor et al. 1964; Glasby and Hodgson 1971). Some cryptomelane may occur in weathered ultramafic rocks (Llorca and Monchoux 1991), volcanic ash, marble (Nimfopoulos 1991), and old weathered shell (Li et al. 2002). In

78

China, manganese oxide ore is widely distributed at depressed places along the edge of the Jiangnan Old Terrane, the 23° N Zone in the south of China, which is 1500 km in length from east to west and 220 km in width from south to north as well as 50–60 m in depth (Lu et al. 2003). This type of manganese deposit is in favor of the formation of supergene manganese oxide ore due to the humidity climate in South China. Known as one of the largest manganese deposits in China, the Xialei manganese deposit is located in the south of the Youjiang fold belt, south China fold system, Guangxi Province. In this area, weathering of thick manganese carbonate sequences has resulted in massive manganese oxides layers, 10–30 m thick in some places. Cryptomelane is the most common manganese mineral in the eastern and central parts of the mining district, which is 9 km in length from east to west and 2.5 km in width from south to north. Xialei cryptomelane commonly occurs in reniform form (Fig. 4.5 a). While in the cracks and pores of manganese ore, cryptomelane may develop in veined form (Fig. 4.5b), and is leaden with strong metallic luster on the fresh surface, hard and brittle. The orebody in this type is very rich in Mn oxides as cryptomelane, psilomelane, pyrolusite, and nsutite, and with a very rare fraction of gangue minerals. The samples used for this study are collected from the central part of the mining district NC1 (reniform), and the eastern part of the mining district NC2 (veined).

4.2.1.2 Characterization of Natural Cryptomelane Under an optical microscope, Xialei cryptomelane samples show concentric circular, enteroliths, pisolitic and veined microstructures (Fig. 4.6). The morphologies of natural cryptomelane are more complicated under a scanning electron microscope (SEM). Figure 4.7 shows a great variety of morphologies as minute, but densely packed and ill-defined crystals (Fig. 4.7a, e), acicular and blade-like crystals (Fig. 4.7b, c), wavy, fibrous crystals (Fig. 4.7d, f). Importantly, all of the images show crystal dimensions and features fell down into the nano-range.

Fig. 4.5 Xialei cryptomelane ore. a In reniform form; b in veined form (Feng et al. 2015)

4

Redox Activity of Iron Sulfide and Mn Oxide

The XRD patterns of NC1 and NC2 (Fig. 4.8) show characteristic peaks of cryptomelane (JCPDS: 34-0168). While several impurity peaks of nsutite and quartz appeared in the XRD pattern of NC2. The chemical compositions of several randomly selected cryptomelane particles are characterized by electron microprobe analyses (EMPA) and listed in Table 4.5. In veined samples, the MnO2 content is over 90%, K2O is about 3%. While in reniform samples, the MnO2 content is about 90%, K2O more than 3%. Most samples contain Al2O3, SiO2, and Fe2O3, and part of the samples contain trace elements of Ni, Ba, Sr, and Mg. The corresponding chemical formulas of natural cryptomelane particles are calculated as shown in Table 4.6. Lattice fringes paralleled the crystal direction under a high-resolution transmission electron microscope (TEM). Generally, the lattice fringes of (101) plane have a spacing of about 0.7 nm (Fig. 4.9a). While, those with bigger spacing (about 0.9 nm) were also observed (Fig. 4.9b), possibly due to the intergrowth of tunnels with different sizes. Lattice defects are easily found in natural cryptomelane samples (Fig. 4.9c), which are probably caused by cation-exchange. In general, the (101) planes are homogeneous and continuous (Fig. 4.9a, b). Also, there are some lattice bending and lattice distortion (Fig. 4.9b, c). Lattice defects often develop in the transition zone of two different planes. For instance, in the transition zone boundary of (101) planes and (301) planes, where the lattice fringes of (101) planes (d = 0.7 nm) are broader than those of (301) planes (d = 0.3 nm), lattice fringes bend, narrow down and even disappear (Fig. 4.9c).

4.2.2 Oxidation of Phenols by Mn Oxide 4.2.2.1 Natural Cryptomelane Previous studies have shown that manganese oxides could oxidize phenolic compounds including phenol and substituted phenols (Stone 1987; Abecassis et al. 2005; Yang et al.

4.2 Reactivity of Mn Oxide Cryptomelane

79

Fig. 4.6 Optical microscope images of cryptomelane showing. a Veined structure; b concentric circular structure; c enterolithic structure; d pisolitic structure. I, II, III represent three periods of mineralization; cry is the abbreviation of cryptomelane (Feng et al. 2015)

2006). In our study, natural cryptomelane samples (NC2) with the manganese oxidation state (AOS) of 3.56 are used for reaction with phenolic wastewater. Two kinds of phenolic wastewater are collected from coal gas plants and gasification plants, respectively. The concentration of total phenols is measured by the bromide method commonly used in factories for water quality determination (EPA 320.1). An excessive bromide agent would react with the existing phenols and then the iodometric method (EPA 345.1) is used to measure excessive bromide agent to help calculate the total phenols. CODCr is measured by fast digestion spectrophotometric method (HJ/T 399-2007) using Model HATO CTL-12 COD rapid determination instrument. It has been established that pH has a considerable influence on the performance of phenol removal by manganese oxides (Stone 1987). The effect of pH on phenols removal rate is shown in Fig. 4.10. It is obvious that the efficiency of

phenol removal increases evidently in more acidic aqueous suspensions. After 3 h reaction, the removal efficiency was above 60% at pH 1–5, and at pH 1 the removal efficiency reached almost 100%. Solution pH determines the protonation reactions and formation of surface complexes between phenols and manganese oxide (Stone 1987). Moreover, pH is of great importance in changing the thermodynamic redox potentials of manganese oxides and phenols (Ukrainczyk 1992). At lower pH, cryptomelane has higher redox potential and is easier to take phenols degradation. Figure 4.11 displays phenols removal as a function of liquid temperature at 15, 20, 30, and 40 °C, respectively. It is found that high temperature is in favor of phenols removal. The removal rate of total phenols reaches 96.2% at 40 °C and 69.8% at 15 °C. The temperature may influence mineral surface chemistry properties such as intrinsic acidity constant and surface charge (Alvarez-Merino et al. 2008; Halter 1999). Some studies indicate that the pHPZC of metal

80

4

Redox Activity of Iron Sulfide and Mn Oxide

Fig. 4.7 Scanning electron microscope images of cryptomelane in samples a–d NC1 and e, f NC2 (Feng et al. 2015)

oxides/hydroxides generally decreases with temperature increases (Alvarez-Merino et al. 2008). Even a slight temperature change could also have a strong influence on the stability of the surface complex (Halter 1999). However, there are no available data on the effect of temperature on the surface charge and pHPZC of manganese oxides so far. The role of temperature on phenol removal behavior by manganese oxide is uncertain, which is based on surface

complexion reactions. More comprehensive studies would be done on the variations in surface charge as a function of temperature. Based on the above experiments, we choose the optimal experimental conditions (cryptomelane concentration of 200 g/L, the temperature of 40 °C, pH value of 3.0 and reaction time of 3 h) and conduct the reaction with five kinds of phenolic wastewater. The degradation efficiency of total

4.2 Reactivity of Mn Oxide Cryptomelane

81

Fig. 4.8 XRD patterns of natural cryptomelane (Feng et al. 2015)

Table 4.5 Electron microprobe analyses of cryptomelane in Xialei manganese deposit (wt%) Sample

MnO2

K2O

Na2O

CaO

BaO

SrO

Fe2O3

Al2O3

SiO2

MgO

NiO

Total

NC1-1

90.58

3.9

0.04

0.14

0.00

0.05

0.11

3.98

0.39

0.00

0.00

99.19

NC1-2

90.59

3.84

0.04

0.16

0.00

0.04

0.00

3.84

0.37

0.00

0.00

98.88

NC1-3

93.17

4.37

0.07

0.10

0.00

0.00

0.13

2.17

0.29

0.00

0.00

NC1-4

89.59

3.89

0.10

0.26

0.10

0.02

0.09

4.33

0.45

0.00

0.00

NC2-1

96.02

2.72

0.11

0.15

0.00

0.00

0.11

0.02

0.05

0.02

0.01

99.21

NC2-2

97.72

2.79

0.17

0.22

0.00

0.00

0.04

0.09

0.06

0.00

0.04

101.13

NC2-3

97.71

2.88

0.17

0.17

0.00

0.00

0.18

0.03

0.05

0.00

0.03

101.22

NC2-4

94.18

2.96

0.20

0.15

2.00

0.00

0.07

1.46

0.12

0.05

0.21

101.40

NC2-5

94.9

2.52

0.13

0.18

1.81

0.00

0.25

0.50

0.09

0.06

0.15

100.59

100.3 98.83

Feng et al. (2015) Notes NC1-1, 2, 3, 4 represent four different mineral particles randomly selected from sample NC1; NC2-1, 2, 3, 4, 5 represent five different mineral particles randomly selected from sample NC2

phenols, CODCr is illustrated in Table 4.7. Removal rates of total phenols reach 84.6–97.4% after suspension reaction. In the process, protons are consumed and Mn2+ is leaching in the solution with oxidation of phenols. Take phenol for example:

 MnðIVÞ  O þ C6 H5 OH þ H þ ! Mn2 þ þ C6 H4 O2 þ H2 O ð4:8Þ After removal of Mn ions in solutions by alkali precipitation method, CODCr removal rates reach close to and even higher than 50%. If air bubbles are introduced into the

82

4

Redox Activity of Iron Sulfide and Mn Oxide

Table 4.6 Chemical formula of Xialei cryptomelane Sample

Chemical formula

NC1-1

(K0.59Ca0.02Na0.01)0.62(Mn7.37Al0.55Si0.05Fe0.01)7.98O16

NC1-2

(K0.58Ca0.02Na0.01)0.61(Mn7.4Al0.53Si0.04)7.97O16

NC1-3

(K0.65Na0.02Ca0.01)0.68(Mn7.56Al0.3Si0.03Fe0.01)7.9O16

NC1-4

(K0.59Ca0.03Na0.02)0.64(Mn7.32Al0.60Si0.05Fe0.01)7.98O16

NC2-1

(K0.41Na0.03Ca0.02)0.46(Mn7.86Si0.01)7.87O16

NC2-2

(K0.41Na0.04Ca0.03)0.48(Mn7.85Al0.01Si0.01)7.87O16

NC2-3

(K0.43Na0.04Ca0.02)0.49(Mn7.85Fe0.02Si0.01)7.88O16

NC2-4

(K0.44Ba0.09Na0.05Ca0.02)0.6(Mn7.64Al0.2Ni0.02Fe0.01Si0.01Mg0.01)7.89O16

NC2-5

(K0.38Ba0.08Na0.03Ca0.02)0.51(Mn7.75Al0.07Fe0.02Si0.01Mg0.01Ni0.01)7.85O16

Feng et al. (2015)

Fig. 4.9 HRTEM images of cryptomelane showing. a Lattice fringes with the d-spacing of 0.7 nm, corresponding to (101) plane (inset is the result of selected area electron diffraction); b lattice fringes with the Fig. 4.10 The effect of pH on removal of phenols with original phenols of 4592 mg/L, cryptomelane concentration of 200 g/L, liquid temperature of 20 °C and reaction time of 3 h (Feng et al. 2015)

d-spacing of 0.9 nm, lattice defects are indicated by arrows; c the transition zone of (101) and (301) plane (Feng et al. 2015)

4.2 Reactivity of Mn Oxide Cryptomelane

83

Fig. 4.11 The effect of liquid temperature on the removal of phenols with original phenols of 4592 mg/L, cryptomelane concentration of 200 g/L, pH value of 2.0 and reaction time of 2 h (Feng et al. 2015)

Table 4.7 Indexes of original and treated phenolic wastewater by natural cryptomelane NC2

Water quality index

Wastewater from coal gas plant Before extraction

After extraction

Total phenols before treatment/(mg/L)

4592

1040

Total phenols after treatment/(mg/L)

320

CODcr before treatment/(mg/L)

Wastewater from plastic auxiliary plant

Wastewater from gasification plant Before extraction

After extraction

5376

4440

1180

160

140

148

147

6604

2036

18,473

3275

2975

Removal rate of total phenols/%

93.0

84.6

97.4

96.7

87.5

Removal rate of CODcr/%

49.3

57.4

63.1

70.8

48.3

Feng et al. (2015)

precipitation process of dissolved Mn(II), they can be collected in the form of manganese oxide and recycling used. Since the concentrations of organic compounds have been considerably reduced, the remaining compounds with relatively low concentrations facilitate further biological treatment. Thus, this method will be designed as a feasible pretreatment process for industrial wastewater.

4.2.2.2 Synthesized Mn Oxide It has been established that the pH has a considerable influence on the performance of phenol removal by manganese oxides (Stone 1987). The index of acid amount was

used instead of initial pH value because the initial reaction rate was too rapid to measure the initial pH value. The sulfuric acid dosage versus the rate of phenolic removal is shown in Fig. 4.12. It was obvious that the efficiency of phenol removal increased evidently in more acidic aqueous suspensions. After 2 h reaction, the concentration of total phenols was reduced to 82 mg/L and the removal efficiency was 92% when 0.16 mol/L sulfuric acid was added in (the initial pH value was roughly detected at about 2.3). It was much better than 729 mg/L at neutral conditions without the addition of acid. However, when sulfuric acid dosage exceeded 0.1 mol/L, the concentration of total phenols did

84

4

Redox Activity of Iron Sulfide and Mn Oxide

Fig. 4.12 The effect of sulfuric acid dosage on removal of phenols by the mixture of manganite and hausmannite and pH value of solution after reaction (Fan et al. 2010)

not reduce obviously and remained at about 80–100 mg/L. Consequently, to cut down the treatment cost, it was proposed that a sulfuric acid dosage of 0.075–0.1 mol/L would be suitable and the effluent pH was at 4.7 at these conditions. There were close ties between effluent pH and initial sulfuric acid dosage. Increased pH explained an amount of Mn2+ dissolved with phenols present in the suspensions. However, when acid dosage increased continuously even to 0.16 mol/L, effluent pH did not steadily decline but stabilized in the range of 4–5 at manganese oxide dosage of 40 g/L. This phenomenon was probably ascribed to surface acid–base characteristics and high adsorption capability of manganese oxide (Liu and Tang 2002). The initial rate of phenol oxidation significantly depends on pH, soluble manganese, oxide: phenol ratio and redox potentials of manganese oxides (Ukrainczyk 1992). Figure 4.13 represents the relationship between the percentage of phenol removal and reaction time. The residual phenols decreased remarkably to 174 mg/L in just 10 min corresponding to 82% removal rate. After half of an hour, the rate of phenol removals slowed down quite markedly. All the phenols would be almost removed from water body and pH 5 till reacting for 6 h. The change trend of solution pH was in conformity with that of phenol removal rate. It illustrated that proton was involved into the process of oxidation of phenols by manganese oxides. However, it was observed although 84% phenols had been removed, pH was still at 2.8 after 20 min reaction time. Except by phenol oxidation, at pH < 3.5 manganese oxides would dissolve by oxidation of water as well (Ukrainczyk 1992).

Figure 4.14 displays the phenol removal as a function of liquid temperature at 25, 40, 55 and 70 °C. It was found in our experiments that the effect of temperature on removal performance was limited. The concentration of total phenols in the wastewater was reduced from 969 to 99 mg/L at 70 ° C which was close to 114 mg/L at 25 °C after 2 h reaction. By contrast the variation scope of pH at higher temperature during the same time reaction was larger than that at low temperature. Temperature might influence mineral surface chemistry properties such as intrinsic acidity constant and surface charge (Ulrich and Stone 1989; Alvarez-Merino et al. 2008; Halter 1999). The few investigations into metal oxides/hydroxides indicate that pHPZC generally decreases with higher temperature (Alvarez-Merino et al. 2008). Small temperature changes could also have a strong influence on the stability of surface complex (Halter 1999). However, so far there are no available data on the effect of temperature on the surface charge and pHPZC of manganese oxides. The role of temperature on phenol removal behavior by manganese oxide is uncertain, which is based on surface complexion reactions. More comprehensive study would be done on the variations in surface charge as a function of temperature. Besides, in consideration of industrial process and easily volatile problems of phenols, it is appropriate to set the reaction temperature at room temperature or actual temperature of influents. A series of experiments were performed with five grades of manganese oxide particle size. In Fig. 4.15, it revealed that small particles resulted in a high removal efficiency of phenols. When the grain size was at > 198 µm, 74–107 µm

4.2 Reactivity of Mn Oxide Cryptomelane

85

Fig. 4.13 The effect of reaction time on removal of phenols by the mixture of magnetite and hausmannite and pH value of solution after reaction (Fan et al. 2010)

Fig. 4.14 The effect of temperature on removal of phenols by the mixture of manganite and hausmannite and pH value of solution after reaction (Fan et al. 2010)

and < 58 µm, 775 mg/L, 814 mg/L and 873 mg/L total phenols were removed, respectively. The solution pH increased from 3.2 to 4.6 with decreasing particle size. As is known, the specific surface area is inversely related with the manganese oxide particle size. The surface adsorption process is necessary for further chemical reaction of phenols on manganese oxide particle interface to occur (Liu and Tang 2000). Larger surface area provides more adsorption sites which assist phenols and proton be quickly adsorbed onto

the surface of manganese oxides to form complexes, and then accelerate the electron transfer rate. However, fine particles cause trouble in engineering for instance requiring long time for precipitation or more difficult to be separated from solution. So the optimal grain sizes would be controlled in the range of 75–150 µm. The effect of manganese oxide dosage on total phenol removal was analyzed in two ways: (1) adding the same amount of sulfuric acid (0.1 mol/L) at different initial pH or

86

4

Redox Activity of Iron Sulfide and Mn Oxide

Fig. 4.15 The effect of grain size on removal of phenols by the mixture of manganite and hausmannite and pH value of solution after reaction (Fan et al. 2010)

Fig. 4.16 The effect of oxide particle concentration on removal of phenols by the mixture of manganite and hausmannite and pH value of solution after reaction (*means adding the same amount of 0.1 mol/L sulfuric acid at different initial pH; #means adjusting the initial pH 3–3.5 with injections of different quantity of acid) (Fan et al. 2010)

(2) adjusting the initial pH 3–3.5 with injections of different quantity of acid. High affinity for protons of manganese oxide surface indicates that the dosage of acid should be regulated according to the oxide concentration, so as to keep the initial pH value as consistent as possible. As shown in Fig. 4.16, the removal of total phenols is related positively to the amount of manganese oxide particle as a result of closely

initial pH and changing trend in way (2). On the contrary, when sufficient protons are supplied in way (1), a high removal efficiency of 86% achieves at particle concentration of 10 g/L. There is no obvious enhancement with further increase of manganese oxide. These results show that manganese oxide oxidation process can effectively degrade most of phenols in gasification plant wastewater, but it is

4.2 Reactivity of Mn Oxide Cryptomelane

important to coordinate with the dosages of acid in order to guarantee good removal efficiency. Based on the above experimental results, the most important factors which affect the removal efficiency of total phenols are solution pH and manganese oxide concentration. The reactions of oxidation degradation of phenols attribute to the interfacial behavior on the manganese oxide particle (Stone and Morgan 1984). Solution pH determines the protonation reactions and formation of surface complexes between phenols and manganese oxide (Stone 1987). Moreover, pH is of great importance in changing the thermodynamic redox potentials of manganese oxides and phenols (Ukrainczyk 1992). For manganite and hausmannite used in our experiments, the redox potentials of them at two Mn2+ activities as a function of pH are compared in Fig. 4.17 (Hernández et al. 2003). It is easy for manganite and hausmannite to degrade phenol when pH < 4.5 and pH < 6 respectively at low Mn2+ activity of 10−6 M and this reaction is stronger at a more acidic environment. However, low pH may also facilitate the release of reduction product of Mn(II) or other oxidized organic substrate from manganese oxide surface into solution (Stone 1987) which results in abundance of Mn2+ activity. Released Mn(II) competes for the manganese oxide surface sorption sites (Liu and Tang 2000; Lee et al. 2002) as well as reduces the reduction potentials of manganese oxides. With the diffusion of Mn2+ products away from the surface, manganite and hausmannite can only oxidize phenol at pH < 2 and pH < 3.5 respectively when Mn2+ activity increases to 5∙10−2 M. Suspension pH rises quickly with released Mn(II) which slows and finally prevents the reaction. It is also suggested that there is a delicate relationship among phenol removal efficiency, manganese oxide and acid

Fig. 4.17 Electrochemical peak potential (vs. std. hydrogen electrode) of phenol and reduction potential of manganite (Man.) and hausmannite (Haus.) at 25 °C at 10−6 M and 5
10−2 M Mn2+ activity, respectively, as a function of pH (Fan et al. 2010)

87

dosage. On one hand, abundant manganese oxide particles are needed to provide enough sorption sites for phenols, which results in high initial rates of phenol oxidation; on the other hand, too many protonated surface sites will increase solution pH, which is unfavorable to the reaction. In addition, the compositions and structures of manganese oxides have effects on removal efficiency. The influence of four kinds of manganese oxides on the degradation efficiency of total phenols, CODCr and TOC was studied in Table 4.8 with the mixture of manganite and hausmannite, cryptomelane, K-birnessite and MnO2 (AR) respectively. Except MnO2 (AR), almost 90% of total phenols and 40% of TOC could be degraded by other three kinds of manganese oxides after suspension reaction. The final pH also remains low at 1.39 in MnO2 (AR)’s suspension because of its slow reaction rate. All of the mixture of manganite and hausmannite, cryptomelane and K-birnessite are similarly prepared with MnSO4 and alkali by air oxidation. Well developed nano-scale crystals and variable valence of Mn content (Mn3+/Mn4+) give them good performance. Although pure MnO2 in the form of pyrolusite has the similar crystal structure as manganite. It is considered as “inactive” for nonreactive nature of single Mn(IV). It is believed much more difficult to gain an electron by Mn4+ which requires a change in spin state than by Mn3+ (Ukrainczyk 1992). As in Table 4.8, reduction product of Mn(II) from oxidation of phenols by manganese oxides makes contributions to CODCr to a certain degree. After removal of Mn ions in solutions by alkali precipitation method, CODCr decreased rapidly to half its original concentration. In the meantime, about 600–700 mg/L TOC diminished again. Mn(OH)2 colloid formed from precipitation between the soluble Mn2+ and OH− has amorphous particles and high adsorption capacity. Part of organic pollutants were absorbed on its surface and removed from wastewater by filtration process. If air bubbles are introduced into precipitation process of dissolved Mn (II), it can be collected in the form of manganese oxide and recycling used. The constitutions of wastewater generate change and the concentration of contaminants decrease noticeably after treatment by manganese oxide method. In UV scan shown in Fig. 4.18, it is found that the contaminants which have strong absorption peaks between 200 and 350 nm are well removed by manganese oxide treatment. It is clear that the most of conjugated systems such as aromatic or heteroaromatic molecules are eliminated by this process. The residual absorbency shows some organic contaminants cannot be removed. The intensities of these peaks of the effluents treated by the mixture of manganite and hausmannite, cryptomelane and K-birnessite are much lower than that by MnO2 (AR). At the same time in GC–MS analysis, phenol which is extremely predominant in quantity in initial

88

4

Redox Activity of Iron Sulfide and Mn Oxide

Table 4.8 Indexes of original and treated phenolic wastewater by four kinds of manganese oxides Sample

Index Phenols/ (mg/L)

pH

Dissolved Mn/(mg/L)

TOC/(mg/L) Before removal of Mn

COD/ (mg/L) After removal of Mn

Before removal of Mn

After removal of Mn

969

7.25

–

2295

–

5825

–

Treated by manganite and hausmannite

99

4.08

2298

1547

809

6592

3020

Treated by cryptomelane

45

3.12

2570

1487

972

5653

2859

Treated by K-birnessite

99

4.25

2839

1528

969

6146

2870

Treated by MnO2 (AR)

846

1.39

151

2405

2200

6313

3996

Original wastewater

Fan et al. (2010)

Fig. 4.18 The UV scan results of original and treated phenolic wastewater by the mixture of manganite and hausmannite, cryptomelane, K-birnessite and MnO2 (AR) (Fan et al. 2010)

wastewater and 1,2-benzenediol are not detected in the effluents (Fig. 4.19a, c). Simultaneously, small peak of intermediate product of p-benzoquinone in phenol oxidation reaction is observed (Stone 1987) (Fig. 4.19b). Although other organic pollutants still remain, the intensities of abundance peaks decrease markedly and the average removal rate achieve about 40–50%. Since the concentrations of organic compounds have been considerably reduced, the remaining compounds are much easier to be utilized by microorganism for further biological treatment, which results in increased BOD5/CODCr of 0.36.

Manganese oxides were investigated for pretreatment of high concentration phenolic wastewater. Solution pH and manganese oxide dosage are considered as the primary factors. The increase of acidity and manganese oxide concentration can improve the total phenol removal efficiency evidently. It has been proved to provide much better performance in removal of total phenols, TOC and CODCr than MnO2 (AR), but similar activity with cryptomelane and K-birnessite. After treatment, phenol which constitutes an extremely high proportion in the raw wastewater and 1,2-benzenediol cannot be detected out. Manganese oxide

References

89

References

Fig. 4.19 The comparisons of a phenol, b p-benzoquinone, c 1,2-benzenediol in original and treated phenolic wastewaters by the mixture of manganite and hausmannite, cryptomelane, K-birnessite and MnO2 (AR) (Fan et al. 2010)

approach eliminates most of organic compounds and increases the biodegradability of the effluent. This method also offers several important advantages that no special equipment, costly chemicals and complex operations are required. Therefore, it will be designed as a feasible pretreatment process for industrial wastewater.

Abecassis WM, Jothiramalingam R, Landau MV et al (2005) Cerium incorporated ordered manganese oxide OMS-2 materials: improved catalysts for wet oxidation of phenol compounds. Appl Catal B-Environ 59(1–2):91–98 Aide MT, Cummings MF (1997) The influence of pH and phosphorus on the adsorption of chromium (VI) on boehmite. Soil Sci 162 (8):599–603 Alvarez-Merino MA, Fontecha-Camara MA, Lopez-Ramon MV et al (2008) Temperature dependence of the point of zero charge of oxidized and non-oxidized activated carbons. Carbon 46(5):778–787 Bac A, Sjc A, Dlb A et al (1993) Fracture-lining manganese oxide minerals in silicic tuff, Yucca Mountain, Nevada, U.S.A. Chem Geol 107(1–2):47–69 Blowes DW, Ptacek CJ, Jambor JL (1997) In-situ remediation of Cr (VI)-contaminated groundwater using permeable reactive walls: laboratory studies. Environ Sci Technol 31(12):3348–3357 Bunch TE, Fuchs LH (1969) A new mineral: brezinaite, Cr3S4, and the Tucson meteorite. Am Miner 54(11):1509–1518 Demoisson F, Mullet M, Humbert B (2005) Pyrite oxidation by hexavalent chromium: investigation of the chemical processes by monitoring of aqueous metal species. Environ Sci Technol 39 (22):8747 Erdem M, Tümen F (1996) Cr(VI) reduction in aqueous solutions by using pyrite. Turk J Eng Environ Sci 20(6):363–369 Fan C, Lu A, Li Y et al (2010) Pretreatment of actual high-strength phenolic wastewater by manganese oxide method. Chem Eng J 160 (1):20–26 Feng Q, Kanoh H, Miyai Y et al (1995) Alkali metal ions insertion/extraction reactions with hollandite-type manganese oxide in the aqueous phase. Chem Mater 7(1):148–153 Feng X, Li W, Zhu M et al (2015) Advances in understanding reactivity of manganese oxides with arsenic and chromium in environmental systems. [ACS symposium series] Advances in the environmental biogeochemistry of manganese oxides bk-2015-1197:1-27 Fernández-González A, Martn-Daz R, Prieto M (1999) Crystallisation of Ba(SO4, CrO4) solid solutions from aqueous solutions. J Cryst Growth 200(1):227–235 Glasby GP, Hodgson GW (1971) The distribution of organic pigments in marine manganese nodules from the Northwest Indian Ocean. Geochim Cosmochim Acta 35(8):845–851 Halter WE (1999) Surface acidity constants of a-Al2O3 between 25 and 70°c. Geochim Cosmochim Acta 63(s 19–20):3077–3085 Hernández L, Hernández P, Velasco V (2003) Carbon felt electrode design: application to phenol electrochemical determination by direct oxidation. Anal Bioanal Chem 377(2):262–266 Kim J, Jung PK, Chon M (2002) Reduction of hexavalent chromium by pyrite-rich andesite in different anionic solutions. Environ Geol 42:642–648 Knipe SW, Mycroft JR, Pratt AR et al (1995) X-ray photoelectron spectroscopic study of water adsorption on iron sulphide minerals. Geochim Cosmochim Acta 59(6):1079–1090 Lee HS, Park SJ, Tai IY (2002) Wastewater treatment in a hybrid biological reactor using powdered minerals: effects of organic loading rates on COD removal and nitrification. Process Biochem 38(1):81–88 Li J, Vasconcelos PM, Zhang J (2002) Behavior of argon gas release from manganese oxide minerals as revealed by 40Ar/39Ar laser incremental heating analysis. Chin Sci Bull 47(18):1502–1510 Linder P (2001) The adsorption characteristics of d-manganese dioxide: a collection of diffuse double layer constants for the adsorption of H+, Cu2+, Ni2+, Zn2+, Cd2+ and Pb2+. Appl Geochem 16(9– 10):1067–1082

90 Liu R, Tang H (2000) Oxidative decolorization of direct light red F3B dye at natural manganese mineral surface. Water Res 34(16):4029– 4035 Liu R, Tang H (2002) Surface acid–base characteristics of natural manganese mineral particles. Colloids Surf, A 197(1–3):47–54 Llorca S, Monchoux P (1991) Supergene cobalt minerals from New Caledonia. Can Mineral 29(1):149–161 Lu A, Gao X, Qin S, Wang C (2003) Cryptomelane(KxMn8−xO16): natural active octahedral molecular sieve(OMS-2). Chin Sci Bull 48 (9):920–923 Lu A, Zhong S, Chen J et al (2006) Removal of Cr(VI) and Cr(III) from aqueous solutions and industrial wastewaters by natural clino-pyrrhotite. Environ Sci Technol 40(9):3064–3069 Maria LP et al (1997) Differential redox and sorption of Cr (III/VI) on natural silicate and oxide minerals: EXAFS and XANES results. Geochim Cosmochim Acta 61(16):3399–3412 Maurizio P et al (1998) The reduction of chromium (VI) by iron (II) in aqueous solutions. Geochim Cosmochim Acta 62(9):1509–1519 Nimfopoulos MK (1991) Mineralogical and textural evolution of the economic manganese mineralisation in Western Rhodope Massif, N. Greece. Mineral Mag Ostwald J (1992) Genesis and paragenesis of the tetravalent manganese oxides of the Australian continent. Econ Geol Bull Soc Econ Geol 87(5):1237–1252 Pasero M (2005) A short outline of the tunnel oxides. Rev Mineral Geochem 57(1):291–305 Post JE (1999) Manganese oxide minerals: crystal structures and economic and environmental significance. Proc Natl Acad Sci USA 96(7):3447–3454 Pratt AR (1996) The low temperature surface geochemistry and kinetics of pyrrhotite weathering: influences on acid mine drainage (amd). https://ir.lib.uwo.ca/digitizedtheses/2624

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Redox Activity of Iron Sulfide and Mn Oxide

Sedlak DL, Chan PG (1997) Reduction of hexavalent chromium by ferrous iron. Geochim Cosmochim Acta 61(11):2185–2192 Stone AT (1987) Reductive dissolution of manganese(III/IV) oxides by substituted phenols. Environ Sci Technol 21(10):979–988 Stone AT, Morgan JJ (1984) Reduction and dissolution of manganese (III) and manganese(IV) oxides by organics. 1. Reaction with hydroquinone. Environ Sci Technol 18(6):450–456 Taylor R, Mckenzie R, Norrish K (1964) The mineralogy and chemistry of manganese in some Australian soils. Aust J Soil Res 2(2):235–248 Ukrainczyk L (1992) Oxidation of phenol in acidic aqueous suspensions of manganese oxides. Clays Clay Miner 40(2):157–166 Ulrich HJ, Stone AT (1989) The oxidation of chlorophenols adsorbed to manganese oxide surfaces. Environ Sci Technol 23 (4):421–428 Vasconcelos PM, Renne PR, Brimhall GH et al (1994) Direct dating of weathering phenomena by 40Ar/39Ar and K-Ar analysis of supergene K-Mn oxides. Geochim Cosmochim Acta 58(6):1635– 1665 Vicat J, Fanchon E, Strobel P et al (1986) The structure of K1.33Mn8O16 and cation ordering in hollandite-type structures. Acta Crystallogr 42(2):162–167 Wittbrodt PR, Palmer CD (1997) Reduction of Cr(VI) by soil humic substances. Blackwell Publishing Ltd 48(1):151–162 Yang C, Yu Q, Zhang L et al (2006) Solvent extraction process development and on-site trial-plant for phenol removal from industrial coal-gasification wastewater. Chem Eng J 117(2):179– 185 Zouboulis AI, Kydros KA, Matis KA (1995) Removal of hexavalent chromium anions from solutions by pyrite fines. Water Res 29 (7):1755–1760

5

Interaction Between Fe & Mn-Bearing Minerals and Microbes

Minerals and microbes have coevolved throughout the earth’s history. They interact at the microscopic scale, but their effects are manifested macroscopically. Minerals support microbial growth by providing essential nutrients, and microbial activity alters mineral solubility and the oxidation state of certain constituent elements. Microbially mediated dissolution, precipitation, and transformation of minerals are either directly controlled by microorganisms or induced by biochemical reactions that usually take place outside the cell. All these reactions alter metal mobility, leading to the release or sequestration of heavy metals and radionuclides. Microbes and minerals interact at all time and spatial scales. Redox reaction is an important interacting mode, where electrons are transferred from microorganisms to multivalent metal ions related to minerals, and vice versa. The type of compounds or elements that donate or accept electrons is determined by the species of microorganisms and the environmental conditions. Redox-active minerals, such as those that contain iron (Fe(II) and/or Fe(III)) and manganese (Mn(III) or Mn(IV)), are abundant in soil, aquatic, and subsurface sediments. They support microbial growth in at least four different ways: as electron sinks for heterotrophy-based respiration, as energy sources for autotrophic growth, by enabling cell-to-cell electron transfer, and as electron storage materials (Fig. 5.1). As the microbial cell envelope is neither physically permeable to minerals nor electrically conductive, microorganisms have evolved strategies to exchange electrons with extracellular minerals. In the absence of molecular oxygen (O2) and other respiratory terminal electron acceptors, dissimilatory metal-reducing bacteria (DMRB), such as Geobacter metallireducens GS-15 (Lovley et al. 1987; Lovley and Phillips 1988) and Shewanella oneidensis MR-1 (Myers and Nealson 1988; Lovley et al. 1989), oxidize organic matter or hydrogen (H2) and then transfer the released electrons to minerals that contain Fe(III) or Mn(III) or Mn(IV) for respiration (Fig. 5.1a). By contrast, metal-oxidizing microorganisms, including Rhodopseudomonas palustris TIE-1 and © Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_5

Sideroxydans lithotrophicus ES-1, use structural and soluble metal ions as electron and/ or energy sources to reduce O2, carbon dioxide (CO2) and nitrate (NO3−) for growth (Jiao et al. 2005; Emerson and Moyer 1997; Shelobolina et al. 2012; Bose et al. 2014) (Fig. 5.1b). Furthermore, semiconducting minerals, including hematite (a-Fe2O3) and magnetite (Fe(II)Fe(III)2O4), can function as conductors to transfer electrons between different microbial species (Kato et al. 2012) (Fig. 5.1c). For example, hematite and magnetite facilitate the transfer of electrons that are released from the oxidation of acetate by Geobacter sulfurreducens PCA to Thiobacillus denitrificans, which uses the received electrons to reduce NO3− to nitrite (NO2−). Finally, minerals such as magnetite and clay minerals that contain Fe(II) and Fe(III), act as electron-storage materials, or “batteries”, that receive electrons from electron-releasing microorganisms (for example, G. sulfurreducens PCA and S. oneidensis MR-1) when no other terminal electron acceptors are available, then donate the received electrons to other microorganisms (for example, R. palustris TIE-1 and Pseudogulbenkiania sp. strain 2002) when conditions change (Byrne et al. 2015; Zhao et al. 2015) (Fig. 5.1d). Therefore, the electrical interplay between microorganisms and minerals links the redox transformation of metal ions in minerals to the oxidation of organic carbon compounds and fixation of CO2 to organic compounds through photosynthesis and NO3− reduction.

5.1

Reduction of Goethite by Cronobacter Sakazakii

Fe occurs in many oxide and hydroxide minerals. Interaction between DMRB and Fe-bearing minerals is significant in Fe geochemical cycle (Ouyang et al. 2013; Dong 2010). Numerous studies have been done on the bioreduction of Fe-bearing minerals by DMRB. However, Cronobacter sakazakii had not been applied in mineral-bacteria 91

92

5 Interaction Between Fe & Mn-Bearing Minerals and Microbes

Fig. 5.1 Electrical interplay between microorganisms and minerals. a Microorganisms use minerals that contain metal ions as terminal electron acceptors for respiration. b electron and/or energy sources for

growth. c electrical conductors that facilitate electron transfer between microbial cells of the same and different species. d electron-storage materials, or batteries, to support microbial metabolism (Shi et al. 2016)

interaction research work so far, and crystal structure changes of highly crystalized minerals under DMRB reduction were less reported. The reduction ability of Cronobacter sakazakii (QF) on goethite was investigated in this section, which would enrich the knowledge of DMRB.

protein concentrations were distributed in a relatively concentrated area. Moreover, aqueous Fe(II) in the supernatant of undissolved samples was under the detection limit. This implied that no aqueous Fe(II) was released into the solution. Most Fe(II) had formed new mineral phases or been adsorbed on bacteria or minerals.

5.1.1 Total Protein and Fe(II) Concentration Changes

5.1.2 Morphology of the Strain and Minerals

The total protein and Fe(II) concentrations of the suspension at fixed time intervals are shown in Fig. 5.2. The protein and Fe(II) concentrations of the control group were kept at a relatively stable and low level throughout the experimental period. For the treatment group, the protein and Fe(II) concentration were significantly higher than the control group, and there were three periods through the cultivation. In the proliferation period, from the beginning to the 4th day, QF reduced the goethite rapidly and the Fe(II) was accumulated; in the stable growth period, from the 4th day to the 8th day, QF went on reducing the goethite to a peak point of 47.5 mg/L; in the decline period, the yield of Fe(II) slowed down; when the activities of QF ultimately stopped, Fe(II) was no longer accumulated and ultimately kept stable at the amount of 49.7 mg/L. Within the logarithmic and stationary phases of QF, Fe(II) concentration and total protein concentration had an approximately positive correlation (Fig. 5.3). Nevertheless, when QF activities almost stopped, Fe(II) concentration showed little variation. Fe(II) and total

After anaerobic incubation for 20 days, the interaction between goethite and bacteria came to a steady state. Scanning electron microscope (SEM) was applied to observe the bacteria and possible phase transformation of goethite during bioreduction. Goethite of the abiotic control group was acerose, non-directive, 0.3 − 1.5 lm in length, and 0.1 − 0.3 lm in width (Fig. 5.4a). Meanwhile, the bacteria in the experimental group used sodium acetate in the culture medium for its growth and metabolism and generated abundant metabolites which aggregated with goethite (Fig. 5.4b). Moreover, hexagonal-shaped crystallites (Fig. 5.4c) ranging in diameter from 0.5 to 1.5 lm were observed, which were similar in morphology to green rust minerals as described by Fredrickson et al. (Fredrickson et al. 1998; Jorand et al. 2013). The energy dispersive spectrometer (EDS) analyses of the hexagonal crystals revealed oxygen (O), iron (Fe), carbon (C), phosphorus (P), chlorine (Cl), and sodium (Na) (Fig. 5.3d), which was dramatically different from the composition of goethite.

5.1 Reduction of Goethite by Cronobacter Sakazakii

93

Fig. 5.2 Temporal evolution of total protein concentration and Fe (II) concentration of the suspension of experimental and control group under anaerobic culture (Wang et al. 2017)

Fig. 5.3 Relationship between total protein concentration and of Fe(II) concentration (Wang et al. 2017)

To confirm the new minerals observed in SEM, synchrotron radiation X-ray diffraction (SR-XRD) was applied simultaneously. The SR-XRD pattern for minerals of sterile controls showed all the peaks distributed at 0.5 nm and smaller d-spacing, which was in agreement with the synthetic goethite (Fig. 5.5). This indicated there were no

obvious changes in mineral constitutions and crystal structures in the control group. However, the SR-XRD Fe(II) and Fe(III) were formed in the solution when Fe(II) in goethite was reduced. Then, aqueous Fe(II) and Fe(III) precipitated with phosphate (PO43−), carbonate (CO32−), pattern of the bioreduction products revealed a series of new peaks with

94

5 Interaction Between Fe & Mn-Bearing Minerals and Microbes

Fig. 5.4 SEM of the minerals and bacteria in the samples after anaerobic cultivation for 20 days. a SEM of abiotic control goethite. b SEM of QF and bioreduced goethite after anaerobic cultivation.

c SEM of hexagonal crystals after microbial reduction under anaerobic condition. d composition of hexagonal crystal determined by EDS (Wang et al. 2017)

bigger d-spacing: 0.48, 0.603, 0.613, 0.684, 0.77, and 1.14 nm, along with the goethite peaks (Fig. 5.4). The mineral phases relating to these new peaks were mainly fougerite with layer structure (Trolard et al. 2007; Génin and Ruby 2004). For fougerite GR1 with one plane interlayer, the d-spacing (0.603, 0.613, 0.684, and 0.77 nm) varied with the interlayer anions, and 1.14 nm d-spacing was attributed to fougerite GR2 with two planes interlayer structure. These new mineral phases possibly resulted from the partial crystal

structure destruction and phase transition of goethite after bioreduction by QF. Firstly, aqueous sulfate (SO42−), or chloridion (Cl−) to form fougerite. In previous studies on this similar system, vivianite, siderite, and other Fe(II)bearing minerals could be found (Dong 2010). However, because of the differences in bacteria species and experimental conditions, these minerals were not found in this study.

5.2 Reduction of Birnessite by a Novel Dietzia Strain

Fig. 5.5 Mineral phase analysis of solid products by SR-XRD after anaerobic incubation for 20 days (Wang et al. 2017)

5.1.3 Coordination Structure and Fe Oxidation State of the Products Based on the SEM and SR-XRD results, X-ray absorption near edge structure (XANES) was applied to analyze the coordination environment and oxidation state of Fe atoms in the products. The normalized Fe K-edge XANES spectra of bacteria and goethite interact products of 0, 10, and 20 days were shown in Fig. 5.6. There were no significant differences among these spectra. XANES spectroscopy can provide direct structural information for the neighboring atoms around Fe. Therefore, the coordination environment for Fe didn’t change dramatically. For goethite, Fe atoms were surrounded by six O atoms in an octahedral coordination structure. After bioreduction, Fe atoms were still mainly surrounded by oxygen, forming Fe–O or Fe-OH octahedral coordination structure. Although the peak intensity of the main absorption peaks changed slightly, their positions shifted towards the lower energy at about 1 eV. The main edge position of the 0-day sample (barely reduced) was 7132.0 eV, shifted from about 0.8 eV to 7131.2 eV after 10 days of bioreduction, and then moved about 1 eV to 7131.0 eV through 20 days of bioreduction (Fig. 5.6a). The main absorption peaks attributed to the 1 s ! 4p transition, and the peaks shifted to higher energy with increasing electron transition energy and Fe oxidation (Sutton et al. 1993; Bajt et al. 1994). For Fe–O coordination octahedral structure in this experiment system, the higher energy of the main absorption was attributed to the higher Fe oxidation state. In these spectra, main peaks gradually moved towards the lower energy, indicating the Fe oxidation state of products decreased under the bioreduction of QF, and was negatively related to the growth of the bacteria.

95

The pre-edge feature, located 7111 − 7117 eV before the main K-edge crest of Fe (Fig. 5.6b), was related to the 1 s 3d transition. The energy position, splitting, and intensity distribution of the pre-edge feature varied systematically with spin state, oxidation state, and coordination structure (Westre et al. 1997). In our case of Fe–O, the pre-edge centroid position and total integrated intensity were closely associated with the Fe oxide state. Higher centroid position and bigger integrated intensity suggested higher oxidation state of Fe (Berry et al. 2003). The pre-edge characteristics of the examples were shown in Table 5.1, and two components contributed to the pre-edge of all the samples. Centroid position and total integrated intensity gradually decreased along with the bioreduction progress. These changes implied QF bacteria reduced goethite and changed the Fe oxidation state in the experimental process, which was in agreement with the main peak analysis. Taking the previous discussion on the main edge and pre-edge into account, the coordination environment of Fe didn’t change significantly, but Fe oxidation state decreased under the bioreduction of QF. As discussed above, Cronobacter sakazakii could utilize Fe(III) in goethite as an electron acceptor for its metabolism under anaerobic environment, and goethite is transformed to fougerite. During the bioreduction process, the metabolic activity of Cronobacter sakazakii played a dominant role.

5.2

Reduction of Birnessite by a Novel Dietzia Strain

Mn is the 10th most abundant element in the earth’s crust and second only to Fe as the transition metal with alternating redox states (Turekian and Wedepohl 1961; Post 1999). Microbially influenced transformations of Mn which have been previously reported to take place in soils, sediments, mine tailings, and marine environments, also play an important role in driving the geochemical cycling of Mn (Lovley et al. 2006; Jones et al. 2011). Recent research carried out the dissimilatory Mn(IV) reduction under anoxic conditions. In the absence of O2, some Mn-reducing organisms may use Mn-oxides as electron acceptors. While some laboratory studies observed that the presence of oxygen did not inhibit microbial Mn reduction due to the existence of a Mn- reductase system whose activity was inducible by Mn(II) and unaffected by O2 (Myers and Nealson 1988; Lovley and Phillips 1988). Besides the types of microbial species and O2 level, electron shuttles, such as humic acid and quinone-containing compounds, also have great influences on microbial Mn(IV) reduction rates (Lovley et al. 1996; Ruebush et al. 2006). Lovley proved that the addition of humic substances or anthraquinone-2,6- disulfonate (AQDS) greatly stimulated the reduction activity of Geobacter metallireducens.

96

5 Interaction Between Fe & Mn-Bearing Minerals and Microbes

Fig. 5.6 The normalized XANES spectra of products with Ifeffit. a The main absorption peaks shifted from 7132.003 eV to 7131.201 eV, and then to 7131.003 eV. b the pre-edge peaks of XANES spectra can be decomposed into multiple peaks (Wang et al. 2017)

Table 5.1 Pre-edge characteristics of the minerals

Sample

Component position/eV

Nor-malized height

Area

Centroid/eV

Total area

R2

Goethite + Bacteria 0d

7113.284

0.0123

0.0161

7113.957

0.0549

0.9997

7114.631

0.0195

0.0307

Goethite + Bacteria 10 d

7113.282

0.0124

0.0158

7113.939

0.0483

0.9997

7114.596

0.0203

0.0325

Goethite + Bacteria 20 d

7113.259

0.0133

0.0173

7113.879

0.0468

0.9998

7114.498

0.0376

0.0376

A fermentative facultative anaerobe, Dietzia strain DQ12-45-1b (45-1b), which was isolated from a microaerobic condition, was investigated for the reduction of a most common Mn(IV) oxide, birnessite. Given previously reported observations, microbial Mn(IV) reduction by 45-1b was further studied by examining possible constraints of cell densities, O2, and electron shuttles (AQDS) on Mn(IV) reduction rates as well as the resulting Mn-bearing mineral products.

5.2.1 Anaerobic Reduction of Birnessite by 45-1b The results of birnessite reduction by 45-1b were shown in Fig. 5.7 and summarized in Table 5.2. As observed in Fig. 5.7a, significant amounts of Mn2+ were produced in

bacterial treatments, which was considerably higher than the concentrations of Mn2+ in sterile control and killed control. In the initial 14 days, the amounts of Mn2+ increased with time, and 6%, 21%, and 42% of Mn(IV) was reduced with the initial cell concentration of 6.2  108, 2.5  109, and 1.0  1010 cells/mL, respectively. By contrast, the chemical reduction extent of Mn(IV) by acetate was below 5%. Since changes in Mn reduction rate could be roughly estimated from the Mn2+ release rate, three stages of Mn reduction process could be approximately obtained from Fig. 5.7a. The first stage is the initial 2 days when Mn2+ concentration increases but with a relatively lower rate than the following several days. Mn2+ release rates with initial cell concentrations of 6.2  108 cells/mL and 2.5  109 cells/mL during 2–14 days and that with 1.0  1010 cells/mL during 2–10 days were calculated to be 12.2, 48.0, and 114.9 lM/day, respectively, showing a positive

5.2 Reduction of Birnessite by a Novel Dietzia Strain Table 5.2 Anaerobic reduction of birnessite under different cell concentrations

97

Cell concentration/ (cells/mL)

AOS(Total Mn)

AOS(Insoluble Mn)

(Mn2+/Mn (Total))/%

Reduction extent/%

6.2  108

3.29

3.44

10

33

2.5  109

2.90

3.35

33

53

1.0  10

2.13

2.22

39

93

0

3.83

3.91

4

5

10

Notes All samples were biotreated for 28 days

correlation with the inoculated cell density. However, Mn2+ release rates after 14 days decreased to 9.1 and 26.5 lM/day in the treatments with two lower cell concentrations, indicating the bacterial activity associated with Mn bioreduction went down. Even a slight decrease in Mn2+ concentration after 14 days was observed with the highest inoculation concentration of 1.0  1010 cells/ mL, suggesting the bioreduction possibly stopped. Besides, tiny amounts of particles were observed in the medium after 14 days. All these evidences indicated the bioreduction of Mn with the highest cell concentration of 1.0  1010 cells/mL was close to completion on the 14th day. Consistently, the average oxidation state (AOS) of the residuals was measured to be 2.22 (Table 5.2) and found to be approaching 2. The slight decrease of Mn2+ concentration after 14 days, would be explained by adsorption of Mn(II) onto the cell or residual mineral surface, or else by forming Mn(II)/Mn(III) minerals (Fischer et al. 2008; Bratina et al. 1998; Burdige and Nealson 1985). So, in bacterial treatment with cell concentration of 1.0  1010 cells/mL, almost all Mn(IV) in birnessite was reduced to Mn(II) after 14 days and part of produced Mn(II) was present in an insoluble state. There was a positive relationship between the Mn reduction rate and the cell concentration, that the final reduction extent of birnessite was 33%, 53% and 93% (Fig. 5.7; Table 5.2), corresponding to the cell concentration of 6.2  108, 2.5  109, and 1.0  1010 cells/mL, respectively.  3MnO2 þ CH3 COO þ H2 O ! 3Mn2 þ þ 2HCO 3 þ 3OH

ð5:1Þ However, the anaerobic reduction of birnessite by 45-1b was found to be unaccompanied by bacterial growth. In the presence of birnessite as the sole electron acceptor, the concentrations of Mn2+ continuously increased in all three bacterial treatments during the stable bioreduction stage (day 2– 14), while the bacterial protein concentration kept stable (Fig. 5.7b). Particularly, the Mn2+ release rates were found to be in good proportion to the protein concentrations (Fig. 5.8). Therefore, we can confirm the anaerobic reduction of birnessite by 45-1b was not a direct biological process linking Mn reduction with bacterial growth. Considering the results of killed control and the positive correlation between the Mn2

+

release rates and protein concentrations, we speculate the bioreduction process may be an enzymatic reaction, which needs further demonstration.

5.2.2 Aerobic Reduction of Birnessite by 45-1b Under aerobic conditions, Mn2+ concentration of sterile treatment gradually increased over the experiment (Fig. 5.9 a) due to acetate reduction. In treatments with the initial inoculation cell concentration of 6.2  108, 2.5  109, and 1.0  1010 cells/mL, Mn2+ concentration in the initial 2 days sharply increased to 33.0, 98.7, and 180.0 lM, respectively. The initial Mn2+ release rates also showed a positive correlation with the inoculated cell density. Meanwhile, the pH drastically went up from 6.9 to 8.3 for the two higher cell concentrations even in the presence of HEPES buffer, which leads to the quick precipitation, re-adsorption of Mn(II) on the mineral surface, or re- oxidation of Mn(II) as reflected by the abrupt drop of Mn2+ concentration to zero in the following days. As expected, some white precipitations were observed in the suspensions with two higher cell concentrations of 2.5  109 and 1.0  1010 cells/mL, indicating the formation of new minerals related to Mn reduction. In the medium with the lowest cell concentration of 6.2  108 cells/ mL, both the maximum Mn2+ generation and the most drastic change in pH were recorded later than in the equivalent treatments with higher cell concentration (approximately at the 6th day). And after the 6th day, the Mn2+ concentration did not undergo a sudden change to zero but gradually decreased. No visible precipitates were observed in this treatment. Bacterial growth under the aerobic conditions was indicated by a time-course increase in protein and pH (Fig. 5.9 b). When grown in pure culture with O2 as the terminal electron acceptor, the 45-1b strains were found to be able to increase the pH value from neutral to alkaline scales (data not shown). Therefore, the more rapid increase in both soluble Mn2+ and pH values in bacterial treatments than those in sterile control, as well as the obvious positive correlation between the inoculated cell concentration and Mn reduction extent indicated that the aerobic Mn reduction was correlated

98

5 Interaction Between Fe & Mn-Bearing Minerals and Microbes

Fig. 5.7 a Aqueous Mn2+ concentrations. b protein concentrations (solid symbol)/pH values (hollow symbol) during anaerobic birnessite reduction with different initial cell concentrations (Zhang et al. 2015)

initial cell concentration of 6.2  108 cells/mL was 3.92 (Table 5.3), the same as the original birnessite. Although there were some Mn2+ continuously released in the initial 6 days of 6.2  108 cells/mL treatment, the Mn2+ concentration gradually decreased in the following days (Fig. 5.9a). Comparing Mn bioreduction in the presence and absence of O2, it could be found that Mn reduction extents under aerobic conditions were much lower than those under anaerobic conditions, although stain DQ12-45-1b grew more vigorous under aerobic conditions. These findings suggested O2 interfered with birnessite reduction, not only as an alternative electron acceptor to compete with Mn(IV) reduction but also as an oxidizer leading to reoxidation of Mn(II) in alkaline pH. Fig. 5.8 A positive relationship between protein and Mn2+ release rate (abscissa values were the mean values of proteins in 2 − 14 days) (Zhang et al. 2015)

with the bacterial growth. In a separate experimental batch, which was to examine the relationship between acetate consumption and cell concentration, we observed the depletion of acetate at around the 6th day. So, along with the depletion of the carbon source, the bacterial growth stagnated, and the protein concentration did not increase anymore (Fig. 5.9b). The pH went stable and was finally maintained at 9.6 − 9.7 (Fig. 5.9b). After 24 days, the AOS of insoluble Mn in the system with initial cell concentrations of 2.5  109 and 1.0  1010 cells/mL were 3.44 and 3.21, corresponding to 25% and 37% of Mn reduction, respectively (Table 5.3). The actual Mn reduction extent should be higher than the experimental valuaes, because under alkaline condition, re-oxidation of Mn(II) by O2 is feasible (Fendorf et al. 1993; Yang and Wang 2002). In comparison, the AOS of insolubles with an

5.2.3 Effect of AQDS on Reduction of Birnessite Addition of the humic acid analog AQDS generally enhanced Mn2+ release rates under anaerobic conditions (Fig. 5.10a). The Mn2+ release rate with AQDS was observed to be approximately one time higher than that without AQDS in the initial 2 − 14 days and more than 60% higher in 14 − 28 days (Table 5.4). Consistently, the AOS of the insolubles decreased from the initial 3.92 to 2.57 with AQDS and to 3.35 without AQDS. Under aerobic conditions, a slight enhancement of Mn2+ release rates was observed in the presence of AQDS in the first 2 days (Fig. 5.10b). However, unlike in anaerobic experiments, the addition of AQDS did not enhance the Mn reduction extent, and the AOS of the insolubles was 3.80 with AQDS and 3.44 without AQDS (Table 5.4). It is believed that AQDS could enhance the rate and extent of microbial metal reduction by shuttling electrons from bacteria to mineral surfaces and thus eliminating the requirement for the direct contact of bacteria with electron acceptors

5.2 Reduction of Birnessite by a Novel Dietzia Strain

99

Table 5.3 Aerobic reduction of birnessite under different cell concentrations (Mn2+/Mn(Total))/%

Reduction extent/%

Cell concentration /(cells/mL)

AOS(Total Mn)

AOS(Insoluble Mn)

6.2  10

3.91

3.92

substrate microflora > soil microflora (red > green > black). These results showed a good electron transfer process between mineral electrodes and varnish-cultured microflora under light irradiation, indicating that the EET between semiconducting minerals and electroactive bacterial communities may have occurred on the varnish under sunlight in a natural environment.

8.2 Regulation and Influence of Mineral-Microorganism …

177

Fig. 8.4 a Current densities between soil mineral electrode and four kinds of microflora solution mediums (bacterial communities from varnish, substrate, soil, and blank 1/10 LB) under light and dark

conditions; b current densities between substrate rock electrode and the four kinds of microflora solution; c current densities between varnish mineral electrode and different mediums (Ren et al. 2019)

8.2.1.4 EET Possibly Occurred on Varnish Under Natural Light Conditions Our research demonstrated that both electroactive microorganisms and semiconducting Fe/Mn oxides gathered on varnish. When these minerals are exposed to sunlight, the photoelectron holes could be produced and the EET process should exist on varnish for a long geological history. Fe/Mn oxides are abundant on the earth’s surface and serve as the most common natural electron acceptors for EET (Shi et al. 2016). However, under sunlight irradiation in daytime, semiconducting minerals should be stimulated, generating photoelectron-holes, and a rapid electron transfer process should occur on the varnish as shown in Fig. 8.5. Owing to the semiconducting properties, photoexcited holes combined with electrons from microorganisms. Therefore, Fe/Mn oxide no longer acted as an electron acceptor but took part in the EET process, delivering them to surrounding environments. At night or during the absence of sunlight, the oxidation of organic matter is realized due to microbial metabolism, in which Fe/Mn oxides serve as electron acceptors (Weber et al. 2006; Shi et al. 2012). With the cooperation of semiconducting minerals and sunlight, a local field effect is generated, resulting in more efficiently electron transfer process, and this phenomenon affects the structures of bacterial communities and facilitate electroactive microorganism accumulating on varnish over time. Notably, both electroactive genera and Fe/Mn semiconducting minerals are positively correlated with the EET process under the light. Nearly all karst stones have partial dark and light color sections under the same condition. Under sunlight, both microorganisms and minerals displayed close relationship

with the extracellular electron transfer process, which enhanced the understanding of microorganism-mineral interactions in natural karst environments. The stone samples were collected from four sites in karst areas of southeastern Yunnan Province of China, including Stone Forest Scenic (24°40′ N, 104°80′ E) in Shilin County (SL) located in Kunming City, Qingshui HETANG (QS), Puzhehei Scenic (PZ) Area and Shede Village (SD) (24°80′ N, 104°70′ E) in Qiubei County located in Wenshan Zhuang and Miao Autonomous Prefecture.

8.2.1.5 Composition of Bacterial Communities at Phylum and Class Levels The relative abundances of the top 10 abundant phyla identified from the dark and light samples were presented. Proteobacteria, Cyanobacteria, Bacteroidetes, Acidobacteria, Actinobacteria, Gemmatimonadetes, and Chloroflexi were the dominating phyla (Fig. 8.6a). Cyanobacteria (18–56%), Proteobacteria (11–29%), and Acidobacteria (6–18%) were among the highest phyla in the dark samples, whereas Proteobacteria (41–58%), Cyanobacteria (6–38%), and Bacteroidetes (5–9%), in the light samples. Additionally, the percentage of Acidobacteria was significantly different between the dark and light samples (approximately 18% and 1%, respectively). In addition, the average quantities of Actinobacteria and Chloroflexi in the dark samples were fivefold of those in the light samples. Furthermore, the top 10 class analysis results showed that Alphaproteobacteria was the most dominant in all the samples, followed by Oscillatoriophycideae, and Cytophagia (Fig. 8.6b). Most of the OTUs under the phylum Proteobacteria were observed in the light samples. In the dark

178

8 Interactions Between Semiconducting Minerals and Microbes

Fig. 8.5 Schematic diagram of possible EET process between microorganisms and Fe/Mn semiconducting minerals occurred on varnish at day and night times (Ren et al. 2019)

contribute to the results of PCA analysis, we calculated the PC1 values of different microorganisms. Nineteen electroactive microorganisms appeared (Table 8.4) and contributed significantly to the PCA result.

8.2.1.6 Mineral Composition Difference The bioactivity of most microbes is closely related to the surrounding environment. The inorganic mineral composition should be considered as a factor that can influence bacterial community structure in the local environment. The main minerals of dark and light samples were carbonate, clay minerals, quartz and so on. However, iron and manganese oxides were detected by Raman spectra in dark samples, but hardly to be detected in light samples. Moreover, the mineral phase was investigated that iron and manganese were more abundant in dark samples than the light samples through the Raman spectrum.

Fig. 8.6 a Taxonomy classifications of OTUs at phylum; b class levels for the dark and light samples (only top 10 enriched class categories are shown in the figure) (Ren et al. 2018a, b, c)

samples, Oscillatoriophycideae was the most dominant class, followed by Alphaproteobacteria. The taxa of Solibacteres class revealed a remarkable different diversity, and nearly no Solibacteres was observed in the light samples. Nevertheless, their average percentage was 6% in dark section samples. Furthermore, Chloracidobacteria and Nostocophycideae were observed in high proportions in the dark samples, whereas Gemmatimonadetes was abundant in the light samples. Cytophagia showed a consistent distribution in all samples. To understand the major microorganisms that

8.2.1.7 Electrochemical Characterizations of Electrodes With a higher concentration of Fe/Mn, the dark samples should have different behavior under sunlight. In order to confirm this, photocurrent-time response to light was determined by amperometric I-t curves under a constant potential of 1.0 V. The positive photocurrents corresponded to a photooxidation process, which indicated that the dark sample electrode, as a photoanode, was an n-type semiconductor (Hsu et al. 2012; Ohko et al. 1997; Sakai et al. 2005). All photocurrents immediately increased when the light was turned on, and rapidly went back to baseline when the light was off (Fig. 8.7). Moreover, the average photocurrent density of dark section samples was 6.2 mA/cm2, while it was only 1.4 mA/cm2 for light section samples. It is obvious that the dark sample had a remarkable response to solar light. The average photocurrent density of treated samples in which iron and manganese oxides were removed was

8.2 Regulation and Influence of Mineral-Microorganism … Table 8.4 List of all electroactive genera in alphabetical order and the references for electroactivity

179

Number

Genrea

PC1 (%)

Reference

1

Acaryochloris

22.994

Shevela et al. (2006)

2

Actinoplanes

26.482

Solina (2014)

3

Afifella

24.465

Ramasamy (2016)

4

Azospira

32.975

Sasaki et al. (2016)

5

Bdellovibrio

14.669

Barkeloo et al. (2010)

6

Candidatus Solibacter

23.913

Louro and Díaz-Moreno (2016)

7

Chroococcidiopsis

24.363

Ma et al. (2016)

8

Fimbriimonas

38.440

Feng et al. (2016)

9

Gemmata

29.139

Pisciotta et al. (2012)

10

Hymenobacter

20.607

Koch and Harnisch (2016)

11

Mesorhizobium

27.796

Sukkasem et al. (2008)

12

Nostoc

33.218

Jaki et al. (2000)

13

Phormidium

27.873

Häder (1981)

14

Pontibacter

22.172

Sasaki et al. (2016)

15

Rhodoplanes

33.812

Ringelberg et al. (2011)

16

Rubricoccus

28.551

Jeuken (2017)

17

Sphingomonas

23.194

Ding et al. (2015)

18

Spirosoma

33.539

Ng et al. (2016)

19

Sporocytophaga

25.835

Zhang et al. (2013)

Fig. 8.7 Photocurrent-time behaviors of different sample electrodes in the supporting electrolyte of 0.1 M Na2SO4 (Ren et al. 2018a, b, c)

1.7 mA/cm2. Iron and manganese oxides were proved to be the electrically active minerals, which could be activated by sunlight. Therefore, both microorganisms and minerals perform stronger activities for EET and facilitate the electroactive microorganisms gathering with sunlight. Fe/Mn elements are closely related to the metabolic processes of microorganisms, especially iron- and manganese-oxidizing bacteria. The formation Fe/Mn nodules may determine the microbial community in the nodules

and may result in differences between the distributions of the bacterial taxonomic groups of the soil and those of the nodules (He et al. 2008). Cyanobacteria produce manganese oxide deposits to facilitate resistance to high solar radiation (Parchert et al. 2012). Therefore, iron and manganese oxides are important factors influencing the distribution of bacteria. However, despite the remarkable advances in EET in the past few years, the characteristic of these electrically active minerals, which can be activated by visible or UV light, were often disregarded. Iron and manganese oxides protect microorganisms from UV radiation (Friedmann 1982; Hughes and Lawley 2003) and participate in the microbial EET process under sunlight. Electrically active minerals and sunlight produce a local field effect, which affects the structures of bacterial communities and facilitates the clustering of electroactive microorganisms over time. From an ecological perspective, electroactive microorganisms perform EET to cope with the limitations in habitat, such as the availability of dissolved electron donors or acceptors by utilizing solid ones, including humic acid (Klüpfel et al. 2014). Notably, our research demonstrated that electroactive microorganisms and iron and manganese oxides both gathered on the dark samples on karst area stone. The genera observed were all positively correlated with EET and may have included a considerable amount of electroactive microorganisms. The ferromanganese “mineral membrane” widely developed on rock varnish and at Karst area stones has

180

8 Interactions Between Semiconducting Minerals and Microbes

demonstrated excellent photocatalytic properties under sunlight, participating in a series of complex redox reactions on the earth’s surface. Surprisingly, the ferromanganese “mineral membrane” not only spreads on the land surface but also reaches out to the marine euphotic zones. Based on the most active and pivotal hydrosphere on the earth, the interplay between adjacent semiconducting minerals and electrochemically active microorganisms (EAMs) was scientifically researched in the marine euphotic zone of the Yellow Sea, China. Fe and Ti were found in suspended minerals by ICP-MS and EDS and SR-XRD and Raman indicated that semiconducting minerals of iron and titanium metal oxide in situ such as anatase, rutile, brookite, and goethite, notably, retained prominent photoresponse property and distinct photoelectric catalytic potential under visible light. Photocurrent density of the mineral electrode was increased by 24.98% better than dark current density. Compared with the photocurrent density of the blank electrode FTO (1.89 mA/cm2), the photocurrent density of the mineral electrode (13.56 mA/cm2) was about seven times under the voltage of 1.2 V (Fig. 8.8). 16S rRNA gene systematically emerged that electrochemically active microorganisms (EAMs), such as Pseudomonas and Paraclostridium, were pivotal in the solution and preponderant on the electrode respectively after a period of cultivation. The total relative abundance of Pseudomonas and Paraclostridium of these two representative bacteria gathering adequately on the electrode exceeded 90%, which was speculated to be associated with extracellular electron transport as electrochemically active microorganisms (Fig. 8.9). EAMs in the microbial community were more preferentially concentrated around electrodes and the microbial community structure changed according to environmental changes, which testified the electrochemically

active microorganisms with considerably directional selectivity and adaptability, as well as with the significance of directivity. Whereupon a dual-chamber reactor was established to probe into the mechanism of redox reaction and electron transfer process between microorganisms and suspended minerals. Importantly, compared with a graphite electrode, the maximum power density of the reaction was 2.78 times with the increase of suspended minerals as the electron acceptor under dark conditions, as well as 236.53% promotion with visible light (Fig. 8.10). Experiments of this dual-chambered constituted by the suspended minerals cathode and microorganic community anode indicated EAMs in situ of marine have the ability of extracellular electron transfer and suspended semiconducting

Fig. 8.8 Current density-time curves under the voltage of 1.2 V (Liu et al. 2021a, b)

Fig. 8.10 Polarization curves and power density curves in light and dark. Blue line: voltage; red line: power density (Liu et al. 2021a, b)

Fig. 8.9 Microbial species of original marine sample, solution and electrode at the genus level (Liu et al. 2021a, b)

8.2 Regulation and Influence of Mineral-Microorganism …

minerals in the euphotic zone can actively participate in and accelerate the process of EET. Pseudomonas, Vibrio, and Paraclostridium all belong to electroactive bacteria, which were the dominant bacteria widely distributed in the cultured solution and on the electrode respectively, continuously transmitting electrons in an anode chamber to increase the overall current. The semiconducting minerals in the cathode chamber demonstrated decent photoelectric response performance under the light. As dissolved oxygen was the terminal electron acceptor, minerals principally performed as an intermediate electron transfer process to favor the cathodic oxygen reduction. In the meanwhile, visible light stimulated the photoresponse of the semiconducting minerals, promoting more extracellular electrons from the anode and enhancing the metabolism of microorganisms. Accordingly, when these minerals are exposed widely to sunlight, photoelectron holes may be generated to promote the photoresponse, and the EET process should occur in the marine euphotic zone to sunlight. Overall, the results indicated that there was a distinct EET between the semiconducting minerals and the electroactive bacterial communities, stating that extracellular electron transfer between the suspended minerals and microbial communities was enhanced by mineral photoresponse under sunlight in the euphotic zone, which promoted the viability and metabolism of microorganisms, associated closely with the energy flow and element cycle.

8.2.2 Photoelectron Energy of Semiconducting Minerals Affects Microbial Community and Function Non-phototrophic organisms such as heterotrophs and chemoautotrophs are excluded from the realm of the solar light-dependent metabolism, owing to their lack of photosensitive cellular compounds. Nonetheless, this deficiency does not necessarily exclude the possibility that these organisms can directly or indirectly derive energy from sunlight through inorganic mediators such as semiconducting minerals. Similar to the important role of metals and metal-containing minerals in the evolution and metabolism of phototrophic life cycles (Edwards 1996; Wächtershäuser 2000; Tributsch et al. 2003; Russell and Martin 2004; Mulkidjanian 2009), minerals may be important in delivering solar energy to non-phototrophic organisms. Common semiconducting minerals, such as rutile (TiO2), sphalerite (ZnS) and goethite (FeOOH), are solar-responsive photocatalysts in nature (Lu et al. 2007; Li et al. 2008; Xu and Schoonen 2000). When the incident light energy is greater than the bandgap between the valence band and the conduction band of these minerals, photoelectron-hole pairs can be generated and redox reactions are triggered with the

181

release of energy. This form of energy, although indirectly, may be harvested by non-phototrophic microorganisms. The evidence is provided in favor of a novel pathway, in which solar energy is converted to chemical energy by photocatalysis of semiconducting minerals to support and stimulate the growth of non-phototrophic microorganisms. Under light irradiation, photocatalysis of semiconducting minerals produced photoelectrons that supported the growth of representative non-phototrophic microorganisms including heterotrophic Alcaligenes faecalis (A. faecalis) and chemoautotrophic and acidophilic Acidithiobacillus ferrooxidans (detailed in Sect. 8.3.3). Bacterial growth pattern was dependent on light wavelength and intensity, and closely matched the light absorption spectra of the minerals. The calculated photon-to-biomass conversion efficiency suggests that the light-induced and mineral-mediated pathway is probably less efficient than photosynthesis. Photoelectrons from the semiconducting minerals in the P-chamber also supported the growth of heterotrophic bacterium, A. faecalis, one of the most prevalent species of bacteria in natural soils (Gamble et al. 1977). In the presence of a simulated sun light source (Xe-lamp), photoelectrons from rutile in the P-chamber flew to the C-chamber, generating a higher current than that in the dark (Fig. 8.11). A higher current in the light experiment significantly stimulated the growth of A. faecalis on the graphite electrode of the C-chamber, as revealed by the environmental scanning electron microscope (ESEM) images (Fig. 8.12). Compared to the sparse distribution of a few short rod-shaped bacteria on the electrode surface in the dark, the light promoted the attachment of numerous rod-shaped bacteria to the electrode surface to form a visible biofilm (Fig. 8.12a). Quantitative analyses indicated that colony-forming units (CFU) in the presence of light (3
106 CFU cm−2 of the electrode area) were three orders of magnitude higher than that in the dark (2
103 CFU/cm2), where the initial cell concentration in the C-chamber was 8.3
107 mL−1 of the soil extract. These observations suggest that A. faecalis responded to photoelectrons and its growth was substantially stimulated by photoelectrons. A. faecalis has versatile functions including degradation of polyaromatic hydrocarbons (Peng et al. 2008) and plastics (Shah et al. 2008), arsenite oxidation (Stolz et al. 2006), heterotrophic nitrification-aerobic denitrification to remove ammonium in wastewater (Joo et al. 2005), siderophore production, and promotion of plant growth in the soil rhizosphere (Sayyed et al. 2010). In the presence of the semiconducting mineral CdS and light, NAD can be photoreduced to NADH via the direct electron transport from CdS to the enzyme transport chain of an Alcaligenes species (Shumilin et al. 1992). The reducing power of NADH has been observed to support the growth of heterotrophic bacteria such as Escherichia coli (Kim et al. 2009).

182

8 Interactions Between Semiconducting Minerals and Microbes

Although the exact biochemical pathway of the photocatalyzed microbial growth is yet to be determined, it is likely that a similar mechanism may be responsible for the progressive enrichment of A. faecalis in the soil sample as a result of the photocatalysis by semiconducting minerals. Our data actually provided such evidence that the growth of non-phototrophic microorganisms could be stimulated by solar energy through the photocatalysis of semiconducting minerals. Photoelectrons generated by mineral photocatalysis are transferred across a relatively long distance through a conducting matrix to sustain the growth of non-phototrophic microorganisms. Non-phototrophic heterotrophs exist in abundance in both ancient and modern environments (Rothschild and Mancinelli

2001; Dong et al. 2007). On early anoxic earth, metal sulfide photocatalysis might have provided an energy-yielding pathway to support microbial growth. Once the atmosphere became oxygenated owing to the emergence of photosynthesis, mineral photocatalysis by Fe and Mn oxides could have supported various metabolic pathways such as sulfate reduction, nitrate reduction and iron oxidation. Our data demonstrate that the aerobic denitrifying (A. faecalis) bacteria can grow on the energy released from the photocatalysis of sulfides and oxides. In the modern environment, nonphototrophic microorganisms have been found in a number of extreme environments ranging from the driest desert (Dong and Yu 2007) to the most acidic environments (Nordstrom and Alpers 1999; Baker and Banfield 2003). In these environments, organic matter is scarce and is generally produced by a subset of microorganisms such as chemolithotrophs (Nordstrom and Alpers 1999). The abundant existence of semiconducting minerals such as metal sulfides and oxides in the same environments (Nordstrom and Alpers 1999; Baker and Banfield 2003) can produce photoelectrons through photocatalysis, which might have indirectly or directly fuelled non-phototrophic microorganisms, such as A. faecalis. The photoelectrochemical characteristic of semiconducting minerals plays an imperative role in supporting the growth of many electroactive microorganisms. Photoelectrons with appropriate energy could stimulate the diversity and maintain the community structure of bacteria and archaea while lower or higher electric potential caused lower diversity and more divergent microbial assemblages. The abundance of genes related to the electron transport chain was significantly higher in photoelectron- regulated setups than that in open circuit ones, indicating that the extracellular photoelectrons experienced the transmembrane transition and entered the bacterial electron transport chain coupling cell anabolism and catabolism. This part of research proposed the potential impact of photoelectrons produced by

Fig. 8.12 Environmental scanning electron micrograph images of A. faecalis cells on the cathode surface after 20 days of growth. a In the light (continuous wavelength 380–760 nm and intensity of * 13.5

mW/cm2) and b in the dark. The semiconducting mineral in the P-chamber is rutile, and the bacterium in the C-chamber is A. faecalis in soil extract. Scale bar, 5 µm (Lu et al. 2012b)

Fig. 8.11 Variations of electric current with time in the light and in the dark. The semiconducting mineral in the P-chamber is rutile, and the bacterium in the C-chamber is A. faecalis in soil extract. Light was provided with wavelength of 380–760 nm and an intensity of * 13.5 mW/cm2. The presence of electric current under the dark condition is inferred to be due to naturally present redox couples in soil extract (Lu et al. 2012b)

8.3 Regulation and Influence of Mineral-Microorganism Electron …

semiconducting minerals on marine microorganisms and the putative electron flow under sunlight in natural environments. In the photic zone of the marine, through absorbing the solar energy, electrons in the valence band of semiconducting minerals transit to the conduction band that has a higher energy level. This transition process provided a photoelectron in the conduction band and a photogenerated hole in the valence band. These photoelectrons could act as the electron donor for biotic metabolism and produce the origin organic matters on the earth (Lu et al. 2012a, b, 2019), providing a substantial basis for the origin and evolution of marine life. The microbial anabolism, catabolism, and reproduction can be supported by extracellular phototrophy with the aid of semiconducting minerals, providing a new form of energy for bacterial metabolisms. An electrochemical system with graphite electrodes is introduced to simulate the photoelectrons triggered by polar energy from semiconducting minerals in the marine photic zone, at the western Yellow Sea, China. The potential between anode and cathode remained relatively stable during the whole experimental period (Fig. 8.13). The relationship between the relative abundance of dominant phyla, alpha-diversity, and applied potentials helps us better understand the effects of simulated photoelectrons on specific microbes and the interactions of the overall microbial community (Fig. 8.14d–f). No significant correlation was identified for the potential and relative abundance of dominant phyla, highlighting that they were not linearly correlated as optimal conditions normally occurred in the intermediate state (Liu et al. 2018; Sheng et al. 2016), like the medium potential condition in current systems. Spearman’s pairwise correlation between the relative abundance of Proteobacteria and Bacteroidetes exhibited a negative

183

correlation (p < 0.05), indicating that the two phyla might contain species could use photoelectrons for metabolisms and exhibited a competitive relation during the photoelectron intervention process. The negative correlation between potential and richness/diversity index of the bacterial community demonstrated that photoelectrons selected specific species of microbes. The Euryarchaeota had a positive correlation with potential, which was opposite to other phyla, demonstrating a different electron energy preference and electron capture and transport way from other phyla. It also indicated that Euryarchaeota might contain species with extracellular electron capture and transition ability. Accordingly, it could be induced that not only bacterial but also archaeal community in photic zone had response to the simulated electrons. The principal coordinate analysis (PCoA) was conducted to study how microbial beta-diversity (composition) varied under different applied potentials (Fig. 8.15a). Within the single group using the same original sample for incubation, the position between the point of OC and M, which represented the original sample and sample under medium potential, was closer than that of OC and L or H, demonstrating that with a middle potential of − 0.30 V the simulated photoelectrons best supported the microbial reproduction and metabolism. This was confirmed by the Bray–Curtis dissimilarity between open circuit settings and low/medium/high potential settings (Fig. 8.15b), as the medium one harbored the lowest microbial dissimilarity away from the original sample. Similarly, the previous study agreed that the marine semiconducting metal (oxide) particles had a higher impact on the biomass and potential succession of bacteria than phylogenetic composition (Fabrega et al. 2011).

8.3

Regulation and Influence of Mineral-Microorganism Electron Transfer on Microbial Strains

8.3.1 Extracellular Electron Transfer to Minerals Through External Circuit and Synergistically Enhanced by Semiconducting Minerals

Fig. 8.13 The current of each setting during the experiment (Liu et al. 2020)

It explored photocatalysis on the cathodic side of an MFC by using a semiconductor mineral of rutile as an alternative cathodic catalyst. The objective was to integrate a semiconductor material as the cathodic catalyst into the conventional MFC design, offering the additional function of photoelectrochemical reactions. This mechanism may enhance the electron transfer to the terminal electron acceptors in an MFC, increasing its potential of power generation. A naturally existing semiconductor mineral,

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8 Interactions Between Semiconducting Minerals and Microbes

Fig. 8.14 Spearman’s pairwise correlation between applied potential, alpha-diversity, and abundant microorganisms (Liu et al. 2020)

“impure” rutile (containing trace metals), was used in this part. In addition, mechanisms of the “solar-assisted” electron transfer by a natural rutile-coated cathode and its potential applications were discussed. Power generation of the MFC equipped with a rutilecoated cathode was examined in two phases. During the initial phase of 100 + h incubation, the system was incubated in the dark to establish a stable baseline MFC operation. During the second phase, the MFC was incubated sequentially in the dark and light. When the MFC was switched from dark to light conditions, an instantaneous increase of voltage from 337 to 384 mV was observed. We attribute this voltage

increase to the photocatalysis of rutile catalyst, which enhanced the electron transfer from the cathode to oxygen. To verify this observation, the lamp was turned off, and the voltage declined instantaneously to the pre-illumination level. The lamp was turned on again, a repeatable instantaneous increase of the voltage occurred, and the voltage stabilized at approximately 383 mV (Fig. 8.16), similar to what was previously observed. These results indicated that an increase of power output was ascribed to the photocatalytic reaction by the rutile-coated cathode. This was further investigated by the current–voltage (I-V) characteristics recorded in a standard three-electrode

8.3 Regulation and Influence of Mineral-Microorganism Electron …

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Fig. 8.15 a The red rectangle indicates genus enriched with applied potentials; principal coordinate analysis (PcoA) of bacterial community; b Bray– Curtis dissimilarity between each applied potential group and open circuit group (Liu et al. 2020)

setup. As shown in Fig. 8.17, both curves in dark and under visible light illumination showed cathodic current. The observed cathodic current in the dark was attributed to the baseline cathodic reduction of oxygen. The cathodic photocurrent was much higher than the current in the dark, suggesting a higher kinetic of electron transfer resulting from the additional photocatalytic reaction. The photogenerated electrons catalyzed by the rutile-coated cathode reacted at its interface with oxygen in the air-saturated catholyte, and the photogenerated holes were removed via the back contact and resulted in a cathodic photocurrent. Experiments were conducted to evaluate the performance of the rutile-coated cathode. From the polarization curves (Fig. 8.18), the internal resistance of the MFC equipped with a rutile-coated cathode in the light and dark was determined to be 65 and 85 X, respectively. Correspondingly, the maximum power densities obtained from the MFC with and without visible light irradiation were 12.03 and 7.64 W/m3, respectively. In comparison, the maximum power density for the MFC equipped with a graphite cathode was 5.73 W/m3.

Fig. 8.16 Voltage generation over time in an MFC containing rutile-coated cathode under visible light and in the dark (Lu et al. 2010)

Fig. 8.17 Current versus potential curves for the rutile-coated electrode in 0.5 M Na2SO4 electrolyte in a standard three-electrode setup (Lu et al. 2010)

The results suggest that the catalytic efficiency of rutile-cathode can be substantially enhanced when irradiated by light, although it is also catalytically active in the dark. In an MFC, the cathode is responsible for transferring electrons generated from the anode to the terminal electron acceptor, thus closing the electrical circuit. In the absence of light, rutile catalysis is presumed similar to other catalyst such as Pt; however, the light sensitivity of this group of semiconductor material indicates a unique function that may be leveraged in MFC applications. Under light illumination, photoinduced electron–hole pairs are generated at the surface of the rutile-coated cathode. The photogenerated electrons (ecb−) can react with the terminal electron acceptors (such as O2) at the cathode-electrolyte interface, thus leaving the photogenerated holes (hvb+), which diffuse to the back contact and serve as electron acceptors for electrons transferred from the anode. Therefore, this light-sensitive system converts incident photons into electron–hole pairs and provides an electrical gradient that triggers the separation of ecb− and hvb+ (Lu et al. 2007). The whole process could be summarized in Fig. 8.19.

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Fig. 8.18 Comparison of polarization and power density curves in an MFC containing different cathodes (Lu et al. 2010)

Fig. 8.19 Hypothesized shift of electron potentials along the transport chain in the MFC containing rutile-coated cathode: (1) loss due to electrolyte resistance; (2) loss due to electron transfer through the cellular respiratory chain; (3) loss at the anode; (4) loss due to the external resistance; (5) loss at the cathode; (6) increase due to the transformation of electrons to photoelectrons under light irradiation; (7) loss due to reduction of the electron acceptor (Lu et al. 2010)

From the MFC perspective, enhancing the cathodic reaction rate will cause higher power output. The irradiated semiconductor-coated cathode improves the reaction kinetics at the cathode by incorporating a photocatalytical pathway, as indicated by the lower overpotential. The total overpotential (ηtot) of the cell can be determined by estimating the difference between open circuit voltage (EOCV) and measured voltage (V) in a polarization state (Lee et al. 2008), because the voltage of an MFC in a polarization state follows.

8 Interactions Between Semiconducting Minerals and Microbes

In an MFC, the microbially mediated oxidation of substrates and the resultant electron transfer in the anode chamber is a bioelectrochemical process. The acceptance of electrons from the external circuit by semiconductor minerals and the subsequent fill of the photogenerated holes is a photoelectrochemical process. Our study on using a semiconductor-coated cathode in MFCs successfully coupled those two electrochemical processes into a single MFC design. In such an MFC, the energy potential of the electrons in the cathode was elevated after absorbing light energy by the rutile-coated cathode (indicated by the higher voltage output as shown in Fig. 8.16). In the cathode chamber of an MFC with a semiconductor-coated cathode, the electrons transferred to the cathode via external circuit are recharged by switching to photoelectrons through the semiconductor photocatalysis, leading to an energy increase of electrons. The photogenerated electrons are known to possess high reductive activity, which would enable a reductive degradation of a broader range of constituents of higher redox potentials (e.g., O2, chlorinated ethenes, hexavalent chromium, nitrate, uranium, and Fe3+). Therefore, MFCs containing a rutile-cathode or other semiconductor-coated cathodes may offer an effective way for reductive degradation in the cathode chamber as well as oxidative degradation in the anode chamber for contaminants of interests. Microbial fuel cell (MFC) is a well-known bioelectrochemical device mainly used for power generation and contaminant remediation (Chaudhuri and Lovley 2003; Lovley 2006; Du et al. 2007; Oh and Logan 2005). The efficiency of an MFC is influenced by many factors, such as electricigens, electrode material, electron donor and accepter species, equipment configuration, etc. (Oh et al. 2004; Rabaey and Verstraete 2005; Logan et al. 2006). Among those electron transfer processes, the anodic microbial electron transfer is critical to the whole MFC performance, principally because all electrons are produced from the oxidation of anodic “fuels” by microorganisms. In this part, the photocatalysis of semiconducting minerals was extended to silicon solar cells (SSC). SSC is a stable, low cost and commercialized power generation device, which can transform solar energy into electric energy. We optimized the MFC-SSC system by taking into consideration of the compatibility of SSC and MFC. In order to explore the electron transfer mechanisms, the performances of a sole MFC, a sterilized MFC-SSC and an MFC-SSC were compared in terms of power output, potential variation, anodic substrate oxidation efficiency, and electron transfer efficiency. The polarization and power density curves of MFC, sterilized MFC-SSC and MFC-SSC were shown in (Fig. 8.20). The open circuit voltage (OCV) of MFC-SSC was 1208 mV, which nearly double the value in sole MFC

8.3 Regulation and Influence of Mineral-Microorganism Electron …

Fig. 8.20 Polarization and power density curves of MFC, MFC-SSC and sterilized MFC-SSC (Ding et al. 2014)

(665 mV). The system resistance of MFC-SSC (120 X) was a little higher than that of MFC (99 X), which was due to SSC itself having an internal resistance of 20 X under the light. The maximum power density of MFC-SSC was 19 W/m3, much higher than that of MFC (7.5 W/m3), indicating a great improvement of electron transfer efficiency by incorporation of SSC into MFC. It should be noticed that the OCV of MFC-SSC closely approximated the sum of MFC (665 mV) and SSC (600 mV), and its internal resistance nearly equaled the sum of MFC (99 X) and SSC (20 X) under the light. In order to clarify whether the MFC-SSC is the simple summation of MFC and SSC, we did a control experiment with a sterilized anode MFC and an SSC. The OCV of sterilized MFC-SSC was about 580 mV, similar to SSC itself. And its internal resistance was as large as 900 X, much higher than that of MFC or MFC-SSC. Consistently, the maximum power density was only 0.9 W/m3, much lower than that of MFC or MFC-SSC. It was very clear that both the power output and the internal resistance of MFC-SSC didn’t equal the sum of those in MFC and sterilized MFC-SSC. The effect of electrooxidation of SSC was much weaker than the bio-oxidation of MFC. So, it is sure that MFC-SSC was not a simple summation of MFC and SSC. Obviously, the SSC in sterilized MFC-SSC couldn’t make an effective electron transfer, and only it cooperated with the anodic microorganisms could realize a rapid electron transfer. Notably, the polarization and power density curves of MFC-SSC sharply dropped when the current reached the limiting current of SSC at about 3.4 mA. That value was lower than the maximum current of MFC (about 6.5 mA), which indicated the incorporation of SSC did not bring an overload on the bio-anode of MFC. The limiting current of SSC (3.4 mA) was higher than the output current (3.1 mA) of MFC when it reached its maximum power output. So,

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MFC was considered to be able to function under its most efficient condition when it was united with SSC to output a maximum power density. The anode potential correlated with the electrode reactions and reflected the electron balance states between the anode and the external circuit. When a cell run in the open circuit, electrons would accumulate at the anode, resulting in a low anodic potential. When the current increases, the anodic potential shifts to a more positive value and then established a new balance. As shown in Fig. 8.21, the anodic potential of MFC gradually increased from − 457 to − 357 mV versu SCE from open circuit to nearly short circuit. When the current exceeded 5 mA, it was difficult to record the exact values of anodic potential because it was much more difficult for the electrode potential to get steady under a higher current. That also suggested it was more and more difficult for anodic microorganisms to “pump” enough electrons from the oxidation of substrate to establish a balance state, and the anodic reaction equilibrium might be broken near this current value. So, for anodic microorganisms, there should be a limiting current in the cell, which was principally the upper limit of microbial capability. It was observed that the limiting current of MFC-SSC was about 3.4 mA (Fig. 8.21), which was also the limiting value of SSC. Below the limiting current value, the anodic potentials of MFC-SSC overlapped those of MFC, indicating the thermodynamics of the anodic reactions were unaffected by the SSC. When the MFC-SSC reached the maximum current of 3.4 mA, any adjustment on the circuit couldn’t bring on the continue increase of the current. Simultaneously, the anodic potential kept stable around − 415 mV. In fact, the anodic potential of MFC-SSC slightly fluctuated during the whole experimental process, which concentrated at − 440 ± 20 mV versus SCE (with a relative deviation

Fig. 8.21 Electrode potentials of microbial anode in MFC and MFC-SSC (Ding et al. 2014)

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8 Interactions Between Semiconducting Minerals and Microbes

about ± 4.5%) and was used acetate as the electron donor and inoculated mixed strains at the anode (Cheng et al. 2006). The comparison of anodic potentials in MFC and MFC-SSC indicated the addition of SSC didn’t affect the thermodynamics of anodic reactions, and it only improve the kinetics of microbial reactions. According to the current and electric quantity data, the current of the MFC-SSC maintained near 1.0 mA, while it was only 0.4–0.5 mA in the MFC. Electric quantity had an approximate linear accumulation with a rate of 0.037 mmol/h in MFC-SSC, as compared with 0.018 mmol/h in MFC. For comparison, Table 8.5 summarized the total quantities of transferring electrons, the quantities of oxidized organic substrate, as well as the microbial oxidation efficiency (MOE) and coulomb efficiency. The chemical oxygen demand (COD) of the anodic medium in MFC decreased from 1641 to 1421 mg/L by 13.4% within 24 h, while it significantly decreased from 1593 to 842 mg/L by 47.1% in MFC-SSC. The MOE significantly increased from 0.0571 mmol/h in MFC to 0.195 mmol/h in MFC-SSC, indicating a great promotion of anodic oxidation efficiency. All those results supported that the anodic oxidation rate of MFC-SSC was significantly improved, so to generate more electrons per unit time to be transferred in the circuit and ultimately realize an enhanced engineering of the system. The MFC-SSC was demonstrated as a cooperating system of MFC and SSC with much higher electron transfer efficiency than either sole MFC or SSC. In fact, the SSC itself didn’t offer electrons to the whole MFC-SSC system, and it only elevated the electrons’ energy. All electrons flowing in the circuit originated from the anodic microorganisms. So, the improved power output was the cooperating effects of MFC and SSC, which two behaved with different working mechanisms in the integrated MFC-SSC system. The bio-oxidation on the anode of MFC supplied electrons, and then SSC converted light energy to electric energy to form a build-in electric field and drive the electrons transfer.

8.3.2 Extracellular Electron Transfer to Minerals Directly with Promotion from Semiconducting Minerals The interaction between semiconducting minerals and microorganisms, such relationship is a promising way to

Table 8.5 Quantity of anodic organic fuel oxidation and transferring electrons, calculated MOE and coulomb efficiency

System

increase the efficiency of bioelectrochemical systems. Herein, the enhancement of electron transfer between birnessite, hematite, anatase photoanodes and Pseudomonas aeruginosa PAO1 under visible light was investigated. The performance of the light-birnessite-PAO1 system was analyzed to investigate electron transfer between semiconducting birnessite and P. aeruginosa PAO1 under visible light irradiation. First, in situ LSV was conducted in an abiotic control of LBNS medium with a potential window of 0.4–1.0 V. Upon light illumination, the abiotic control can generate a photocurrent that increased from 5.21 lA/cm2 at 0.5 V to 99.23 lA/cm2 at 1.0 V (Fig. 8.22). The photocurrent output of the light-birnessite-PAO1 system reached 279.57 lA/cm2 at 0.9 V, this output was 322% higher than that in the abiotic control. To gain further insights, we collected live P. aeruginosa PAO1 culture, killed them by autoclave sterilization, injected them back into the system, and measured the LSV again. Upon light illumination, the dead culture (control) produced a photocurrent that increased from 43.27 lA/cm2 at 0.5 V to 196.01 µA/cm2 at 1.0 V. The obtained value was substantially lower than that in the live cell culture. The dark current in the dead cell culture remained higher than that in the abiotic control. This discrepancy was possibly due to the conductivity difference between the dead cell culture and medium and several conductive compounds that may be present in the system. Therefore, an enhanced electron transfer can occur between birnessite and P. aeruginosa PAO1 under light irradiation, and the enhanced current output should be attributed to the photoenhanced electrochemical interaction between birnessite and P. aeruginosa PAO1. Multi-condition I-t measurements were compared to explore bioelectron transfer in the light-birnessite-PAO1 system. First, the repeatability of electron transfer was investigated. We compared the performances of the stable run of the light-birnessite-PAO1 system (marked as “Stable System”) and the abiotic control (marked as “Bir + LBNS”). Second, we analyzed the electron contribution percentages of the biofilm and redox species in the culture. We collected the birnessite electrode from the “Stable System” and placed a new sterile birnessite (marked as “Bir (new) System”). Finally, we placed the removed birnessite from the “Stable System” into a new reactor with sterile LBNS (marked as “Bir (with PAO1) + LBNS”). Figure 8.23 shows the results of reduplicative 500 s dark/light illumination cycles. Under

Acetate oxidation quantity/mmol

System electron quantity/mmol

MOE /mmol/h

Coulomb efficiency/%

MFC

1.37

0.438

0.0571

3.99

MFC-SSC

4.68

0.892

0.195

2.38

Relative percentage (%)

341.6

203.6

341.6%

–

8.3 Regulation and Influence of Mineral-Microorganism Electron …

Fig. 8.22 In situ LSV curves of birnessite in live P. aeruginosa PAO1 culture (red), autoclaved dead P. aeruginosa PAO1 (blue), and LBNS medium (dark) under dark or light exposure at a scan rate of 10 mV/s. A dotted line indicates the value of photocurrent density at 0.8 V (Ren et al. 2018a)

light illumination, the “Stable System” generated an increased average photocurrent from 25.1 ± 1.2 lA/cm2 to 40.1 ± 1.5 µA/cm2, whereas photo/dark currents totaled 13.1 ± 0.2 and 9.3 ± 0.2 µA/cm2, respectively, in the abiotic control. This result demonstrates that the enhanced current was due to the contribution of P. aeruginosa PAO1. The photocurrents remained stable, indicating that birnessite can continuously and repeatedly promote the bioelectrons pumped out from P. aeruginosa PAO1 under the light condition. Subsequently, we analyzed the performance of the “Bir (new) System.” As shown in Fig. 8.23, the average photocurrent in the “Bir (new) System” reached 32.8 ± 0.3 lA/cm2 in the first 100 min, which was lower than that in the “Stable System” (approximately 7 lA/cm2). Given that the birnessite electrode was replaced by a new one, the P. aeruginosa PAO1 biofilm was removed at the same time, thereby influencing balance in the system. Therefore, the current in the “Bir (new) System” was reduced. The photocurrent reverted to 40 lA/cm2 in 300 min, suggesting that a new balance was established. In the “Bir (new) System”, the P. aeruginosa PAO1 cell culture originated from the “Stable System” and the soluble conductive biological compounds were widely distributed in the system. The bioelectrons can still transfer out indirectly, realizing a photocurrent of 32.8 ± 0.3 lA/cm2. Then, the P. aeruginosa PAO1 cells gradually combined with birnessite, and the current steadily increased over time. Hence, the enhanced photocurrent was mainly due to the additional redox species in the cell culture. For the “Bir (with PAO1) + LBNS,” the biofilm-coated birnessite generated an average photocurrent of 15.1 ± 0.4 lA/cm2 in the first 50 min. This photocurrent was

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Fig. 8.23 I-t curves of the light-birnessite-PAO1 system under different culture conditions of on/off light cycles (“Stable System”: a stable light-birnessite-PAO1 system; “Bir (new) System”: the birnessite electrode was removed from the “Stable System” and replaced with a new birnessite; “Bir (with PAO1) + LBNS”: the removed birnessite from the “Stable System” was placed into a new reactor with sterile LBNS; “Bir + LBNS”: abiotic control) (Ren et al. 2018a)

considerably lower than that in the “Stable System” (nearly 25 lA/cm2). This notable reduction suggests that a considerable number of bioelectrons were originated from the cell culture in the light-birnessite-PAO1 system. The bioelectrons from the biofilm accounted for a small fraction of the total photocurrent. Given the new LBNS, the cells grew vigorously, and the photocurrent increased gradually. P. aeruginosa PAO1 cells were attached on the surface of birnessite, but they needed additional time to biosynthesize electron shuttles before achieving a high current output similar to that in the “Bir (new) System”. Thus, we concluded that the enhanced photocurrent output resulted from birnessite anode harvesting of bioelectrons from P. aeruginosa PAO1 under light energy. Bioelectrons were attributed primarily to the additional redox species associated with P. aeruginosa PAO1 and secondarily to the biofilm on birnessite. To further understand the electroconductibility of P. aeruginosa PAO1 biofilm, we washed the birnessite electrode, which was removed from the “Stable System”, and then its electroconductibility was measured by conductive cAFM. Adhesion results (Fig. 8.24a) showed a higher force in the portions with P. aeruginosa PAO1 cells and extracellular polymeric substances on the birnessite surface. The distribution areas are consistent with topography results. Figure 8.24b presents the current image of the biofilm on birnessite. Given that the recorded currents originated from the Z direction, resistance for cell + birnessite was higher than that for birnessite. Therefore, less conductance was apparent in areas covered by cells. Nonetheless,

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electroconductibility was readily observed in all areas with or without cells. These results indicate that electrons were able to transfer between birnessite and P. aeruginosa PAO1 at the cellular scale even though the top surface was covered with cells. Therefore, bioelectrons can possibly transfer from both the P. aeruginosa PAO1 biofilm and the redox species in the solution. Based on the I-t results (Fig. 8.23), it’s speculated that additional redox species associated with P. aeruginosa PAO1 appeared in the system. In situ cyclic voltammetry (CV) was utilized to demonstrate the existence of soluble bioelectron shuttles in the system. As shown in Fig. 8.25a, a pair of redox peaks were identified, and the positions of oxidation and reduction peaks were located at − 90 and − 290 mV versus Ag/AgCl, respectively. The peak current ratio ipa/ipc = 1(ipa and ipc denote the anodic and cathodic peak currents, respectively) indicated a reversible reaction. These results suggest that electron shuttles were present in the system, and an indirect electron transfer occurred. P. aeruginosa produces phenazines as soluble redox mediators (Wang et al. 2008; Bodelón et al. 2016). Different phenazine compounds (such as pyocyanin, phenazine-1carboxylate, and 1-hydroxyphenazine) feature different redox potentials. The peaks in CV results were closer to the value of pyocyanin (Bosire et al. 2016). Therefore, electron transfer may be evident between birnessite and P. aeruginosa PAO1 (under light irradiation) through the redox-active pyocyanin. To identify the soluble redox mediators that played key roles during electron transfer, we isolated redox species from the live cell culture by using CHCl3 and HCl (Fig. 8.25b inset picture) and demonstrated them by SERS analysis. As shown in Fig. 8.25b, the peaks at 409, 492, 520, 547, 635, 810, 1074, 1181, 1356, 1513, and 1608 cm−1 matched well with the signals of pyocyanin (Bodelón et al. 2016). Therefore, the soluble pyocyanin biosynthesized by P. aeruginosa PAO1 was involved in promoting electron transfer between semiconducting birnessite and P. aeruginosa PAO1 under light irradiation.

Fig. 8.24 AFM images of birnessite electrode with cell. a Adhesion information; b tunneling AFM (TUNA) current (Ren et al. 2018a)

8 Interactions Between Semiconducting Minerals and Microbes

Figure 8.26 shows the proposed schematic mechanism of enhanced EET between semiconducting birnessite and P. aeruginosa PAO1. During the metabolic processes of P. aeruginosa PAO1, bioelectrons were produced and transferred to the extracellular conductive components. Soluble bioelectron shuttles were synthesized and widely distributed in the cell culture. Under light irradiation, the semiconducting properties of birnessite can be activated, thereby generating photoelectrons and holes. Photoelectrons deviated from the Mn 3d conduction band and transferred to the external circuit. The photoexcited holes in the O 2p and Mn 3d valence bands appeared at the same time. These holes featured more positive potentials and were easily combined with bioelectrons. Therefore, rapid bioelectron transfer occurred on the surface of birnessite. Electrons were also continuously pumped out with the contribution of bioelectrons originating from both the biosynthesized extracellular shuttles (such as pyocyanin) and P. aeruginosa PAO1 cells in the biofilm. Thus, birnessite no longer acted as an electron acceptor but reacted with bioelectrons and delivered them to the external circuit, thereby realizing an enhanced electron transfer between semiconducting birnessite and P. aeruginosa PAO1. Both mineral and biological processes evolved over the course of a long geological history and this process will expand the knowledge on mineral-microbe interactions and elucidate how minerals influence microbial species. Another appropriate example is the interaction between a hematite photoelectrode and Pseudomonas aeruginosa PAO1 under visible light irradiation. The performance of the hematite photoelectrode in the light-hematite-PAO1 system with or without wild-type PAO1 was studied. The results of the amperometric I-t curves measurements (Fig. 8.27) showed that the system produced negligible current without PAO1 in dark conditions. When PAO1 was present, the average current generated was 3.0 ± 0.1 µA/cm2 and 4.1 ± 0.1 µA/cm2 at the FTO and hematite electrode surface. The higher value of current in “Hem + PAO1” might be due to the specific surface area of the hematite being much larger than the FTO

8.3 Regulation and Influence of Mineral-Microorganism Electron …

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Fig. 8.25 a CV comparison between live cell culture and fresh LBNS; b SERS spectra of pyocyanin isolated from light-birnessite-PAO1 system (Inset picture: Liquid–liquid extraction of pyocyanin in live cell culture by CHCl3 and hydrochloric acid) (Ren et al. 2018a)

Fig. 8.26 Schematic diagram of enhanced electron transfer between birnessite and P. aeruginosa PAO1 driven by visible light irradiation (Ren et al. 2018a)

Fig. 8.27 Amperometric I-t curves of the light-hematite-PAO1 system with different conditions under light on/off cycles (Ren et al. 2017)

electrode. In the dark, electron transport was thought to occur in the conduction band Fe 3d through the hematite electrode, while at this time the charges in the valence band

O 2p were not free-flowing unless excited by light or heat (Sherman 2005). With light illumination, the abiotic control generated a measurable average photocurrent of 7.5 ± 0.2 µA/cm2 in the LBNS electrolyte. The average photocurrent output for the light-hematite-PAO1 system reached 18.1 ± 0.2 µA/cm2 under light illumination, which was almost 2.4 times higher than that of the value of the abiotic control under the same conditions. Moreover, it is worthwhile mentioning that this was only 0.6 µA/cm2 higher than the dark current in the “FTO + PAO1” experimental group, which suggests that the light had a negligible effect on PAO1. Therefore, the significant enhanced current output in “Hem + PAO1” could be ascribed to the transfer of bioelectrons from PAO1 to the hematite anode and subsequently the external circuit. When the hematite electrode was irradiated by visible light, photoexcited holes prepared at the valence band of the hematite could combine with bioelectrons and more electrons flowed into the external circuit. In order to thoroughly investigate the electron transfer process between hematite and PAO1 belonging to a direct or an indirect route, the electrode in system was replaced by a

192

new hematite which had no bacteria on the surface. As shown in Fig. 8.28 (marked as line “New Hem + PAO1”), the value of the current density was nearly invariable in dark conditions. However, the value of the average photocurrent decreased by 1.8 µA/cm2 approximately, which could be attributed to the contribution of direct electron transfer by PAO1 and may be realized by extracellular conductive components (such as pili). Subsequently, the bio-photoanode was observed by light microscopy (Leica Dmi8, Leica Microsystems, Wetzlar, Germany), and some bacterial cells were distributed on the hematite surface. The ESEM image of the electrode showed that these cells had a rod-shaped morphology and dispersed randomly (Fig. 8.29). Therefore, some cells stayed on the hematite surface and direct electron transfers occurred in the light-hematite-PAO1 system, however this only accounted for a small proportion. In order to demonstrate that the indirect electron transfer route played a significant role in the system, the live cell culture was taken out and new LBNS injected into the device. The I-t curves with the new LBNS medium were examined (marker as “Hem + LBNS”). As shown in Fig. 8.28, the photocurrent density decreased from 19.5 ± 0.5 µA/cm2 to 7.6 ± 0.3 µA/cm2, which comes to a similar level as the abiotic control. Taking the above results into account, we could conclude that the indirect transfer route of live cell culture may have played a critical role in enhancing electron transfer. Furthermore, the live cell culture was centrifuged at 10,000 r/min for 5 min, then after removing the supernatant and resuspending the cell pellet into the device containing the new LBNS and new hematite electrode, the current density in “Hem + LBNS + Cells” was only 7.9 ± 0.2 µA/cm2 with a negligible current growth of less than 0.3 µA/cm2 compared with the “Hem + LBNS”.

Fig. 8.28 Amperometric I-t curves of the light-hematite-PAO1 system with different conditions under light on/off cycles (Ren et al. 2017)

8 Interactions Between Semiconducting Minerals and Microbes

The results indicated that only a few electrons were probably donated by the swimming cells. Ulteriorly, cyclic voltammetry (CV) and surface-enhanced Raman scattering (SERS) measurement were also used to investigate the performance of soluble redox mediators from live cell culture. These results well matched the results of pyocyanin in birnessite-PAO1. On the one hand, as a more moderate semiconductor, it is interesting that hematite preformed a harmonious relationship with different microorganisms. On the other hand, there was thereby an urgent need which was still a significant challenge to study many more bacteria in addition to Geobacter and Shewanella. Based on the results, the proposed schematic mechanism of visible light enhanced extracellular electron transfer between hematite and PAO1 is shown in Fig. 8.30. During the process of metabolism, bioelectrons were produced and transferred to the extracellular conductive components. Moreover, soluble electron shuttles were biosynthesized and diffused in solution. In dark conditions, bioelectrons were injected into the Fe 3d conduction band on the hematite surface and were delivered to the FTO substrate. Under light illumination, the semiconducting property of hematite was activated leading to better conductivity generating holes in the O 2p valence band. At the same time, the photoexcited holes in the valence band O 2p became free-flowing, which could more easily combine with the bioelectrons because of a more positive potential. Then, a more efficient electron transfer process occurred on the surface of the hematite which resulted in a higher current output. Most of the bioelectrons come from pyocyanin and a small proportion of them from the direct transfer route. When the hematite electrode was excited by light, the bioelectrons reacted with the photoexcited holes. With the help of holes on the hematite surface, hematite no longer acted as the electron acceptor. Finally, the bioelectron delivered to the external circuit and was protected rather than dissolving which was confirmed by the results of the Raman, Fe concentration test and LSV of the hematite electrodes. Next part of contents was based on previous researches of changing the solution and electrodes continuously, and then we applied the gene knockout method to compare the interaction of mutant strains with anatase and the wild type PAO1 with anatase.

8.3.2.1 Concentration Control of PAO1 and Mutant Strains Through OD600 To ensure the concentration of three strains inoculated into the experiment systems remained consistent, UV–Vis was used to decide the absorbance under OD600. After the first activation, OD600 of PAO1, Dphz1Dphz2 and DpilA were all about 0.53 (±0.01). Over the second time of activation, OD600 of three strains totally increased to about 1.15 (Table 8.6). In this case, the same value of OD600 guaranteed

8.3 Regulation and Influence of Mineral-Microorganism Electron …

193

Fig. 8.29 a Light microscopy picture of bio-photoanode; b ESEM picture of bio-photoanode (Ren et al. 2017)

PAO1 and Dphz1Dphz2 was fuzzy with dispersion shaped, while the outer contour of DpilA knocked out the pili gene was more distinct (Fig. 8.32). All three strains had prominent flagella, which was more intuitive under AFM (Fig. 8.32). Considering the comprehensive results of AFM, ESEM, and the previous research (Liu et al. 2019), it can be determined that the pili gene was completely knocked out in DpilA, which can be used in subsequent experiments.

Fig. 8.30 Schematic diagram of the electron transferring process between hematite and PAO1 under visible light irradiation (Ren et al. 2017) Table 8.6 OD600 of PAO1, Dphz1Dphz2 and DpilA OD600

P. aeruginosa PAO1

Dphz1Dphz2

DpilA

First Time

0.521

0.538

0.541

Second Time

1.152

1.150

1.150

the number and concentration of bacteria inoculated into the experiment systems were parallel.

8.3.2.2 Morphology Characteristic of PAO1 and Mutant Strains by AFM and ESEM The external morphology of pure strains of PAO1, DpilA and Dphz1Dphz2 after the dilution and separation were observed and distinguished under AFM and ESEM, respectively. It can be observed that the size of single cells of three strains was all about 1–3 µm with rod-shaped (Figs. 8.31 and 8.32). The surface of each single bacterium of PAO1 and Dphz1Dphz2 was rough and plicated, but DpilA’s surface was flatter and smoother under ESEM (Fig. 8.31). Wherein, the peripheral contour of every single

8.3.2.3 Identification of Biosynthesized Pyocyanin by UV–Vis and SERS The pyocyanin was isolated from the live cell culture by using CHCl3 and HCl and then was demonstrated by UV– Vis and SERS analysis. The strains of PAO1 and DpilA generated the purple liquid of the pyocyanin-HCl. By contrast, the strain of Dphz1Dphz2, which was lack of the pigment gene, formed the colorless transparent liquid of no pyocyanin-HCl (Fig. 8.33 inset picture). As shown in Fig. 8.33, UV–Vis was used to test the absorbance of the pyocyanin-HCl phase within the range of 300–700 nm. UV– Vis absorption spectra testified that the pyocyanin from PAO1 and DpilA had three characteristic absorption peaks in 371, 387 and 521 nm, which was exactly consistent with the characteristic absorption peaks of pyocyanin in P. aeruginosa reported in the literature (Watson et al. 1986). The absorbance value of pyocyanin from PAO1 and DpilA was analogously close, stating the strain of DpilA without pili gene had no effect to generate pigment. On the contrary, no absorption peaks within the range of 300–700 nm were detected which indicated that the strain of Dphz1Dphz2 didn’t produce pyocyanin. In brief summary, PAO1 and DpilA produce pyocyanin as usual, and the pigment gene of mutant strain Dphz1Dphz2 was knocked out thoroughly, which proved all strains can be used in the subsequent experiments. The gained purple liquid pyocyanin-HCl of PAO1 was dropped on the slide and then measured by SERS. The result well matched the result of pyocyanin in birnessite-PAO1.

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8 Interactions Between Semiconducting Minerals and Microbes

Fig. 8.31 ESEM of PAO1, DpilA and Dphz1Dphz2 (Liu et al. 2021a, b)

Fig. 8.32 AFM 3D of PAO1, DpilA and Dphz1Dphz2 (Liu et al. 2021a, b)

“light-anatase-PAO1”, the technology of gene knockout was applied to knock out the pyocyanin biosynthesis gene operons phz1 and phz2, and the pili structure gene pilA in PAO1, respectively. Sequentially, the mutant strains of Dphz1Dphz2 and DpilA were added into the experiments to establish the systems of “anatase-Dphz1Dphz2”, “anatase-DpilA”, “FTO-Dphz1Dphz2” and “FTO-DpilA”. It can be testified that there was an enhancement of electron transfer emerging in experimental systems. Undoubtedly, the current density generated by the “anatase-PAO1” system was the most prominent among all the systems. Furthermore, the enhanced electron transfer and metabolic efficiency were related to the pili gene and pigment gene.

Fig. 8.33 UV-vis of extractliquor of PAO1, DpilA, and Dphz1Dphz2 (Inset picture: Liquid–liquid extraction of pyocyanin in live cell culture of PAO1, Dphz1Dphz2, and DpilA by CHCl3 and HCl) (Liu et al. 2021a, b)

8.3.2.4 The Distinguishing Efficiency of EET in Light-Anatase-PAO1 Systems Ulteriorly, to probe into the mechanism of electron transfer between semiconducting anatase and P. aeruginosa PAO1 under the simulated solar light, we constructed an “anatase-PAO1” system and analyzed its performance with and without the light. In the meanwhile, we compared the performances of the “anatase-LBNS”, “FTO-PAO1” and “FTO-LBNS” as the control experiment systems. To further explore which part impacted in electron transfer of

8.3.2.5 Analysis Different EET Mechanism of Light-Anatase-PAO1 Systems Electrochemical measurements of I-t curves testified (Fig. 8.34; Table 8.7) that under 0.2 V, the average photocurrent density of “anatase-PAO1” was about 9.34 µA/cm2, much higher than the sterile system of “anatase-LBNS” under light (2.14 µA/cm2), which only had the semiconducting mineral without PAO1. Then, a weak photocurrent value (0.46 µA/cm2) was produced in the bacteria system without mineral “FTO-PAO1”. What’s more, “FTO-LBNS” without anatase and PAO1, produced the photocurrent density value of 0.26 µA/cm2, which was the lowest among all the systems and almost can be negligible. According to the results (Table 8.7), the average photocurrent density of “anatase-Dphz1Dphz2” (2.77 µA/cm2) was distinctly lower than “anatase-DpilA” (3.99 µA/cm2) and both of them were much lower than

8.3 Regulation and Influence of Mineral-Microorganism Electron …

195

EET. In a word, the indirect electron transfer process was more dominant, which may be chiefly attributed to the pyocyanin biosynthesized by PAO1. Overall, the results indicated that there was distinct EET between anatase and PAO1 under the light, as well as stating phz1phz2 gene had a crucial effect on extracellular electron transfer.

Fig. 8.34 Dark/light amperometric I-t curves for anatase-PAO1 (Liu et al. 2021a, b)

“anatase-PAO1” (9.34 µA/cm2). The average photocurrent density of “FTO-Dphz1Dphz2” (0.29 µA/cm2) was a little lower than “FTO-DpilA” (0.30 µA/cm2), both of them only had mutant bacterium and were nearly at the same level with slight or even little current. In addition, the photocurrent density generated by all systems under the light was more exceeded the dark current density to some extent. Notably, the increment of the average photocurrent density to the dark current density in “anatase-PAO1”, “anatase-DpilA” and “anatase-Dphz1Dphz2” was 8.20, 3.61 and 2.52 µA/cm2 respectively, which was increased significantly under the light. Consequently, it can be revealed through the electrochemical contrast experiments as follows: at first, anatase can coordinate and promote the EET of PAO1. Besides, the photoelectric response ability of anatase was stimulated under the simulated solar light, and more extracellular electrons were immensely transferred from microorganisms in the anode chamber. What’s more, this process of EET with anatase enhanced the metabolism of microorganisms. Accordingly, anatase and PAO1, which are generally distributed on land surface and in marine euphotic zone, are exposed widely to the sunlight. Photogenerated electrons and holes may be generated, and the spatial separation of them may promote the photoresponse. The EET process could occur in natural environment under sunlight. Moreover, the effect of phz1phz2 gene knockout on the EET was more significant. The mutant strain Dphz1Dphz2 was unable to produce pyocyanin due to the deletion of the pigment gene, thereby inhibiting the formation of electron shuttles and hindering the efficiency of EET. On the other hand, the mutant strain DpilA could not produce pili without the pili gene, which restrained its movement and thus influenced the

8.3.2.6 Mechanism of the Solar Light Drove and Enhanced EET Between Anatase and PAO1 The mechanism of enhanced EET between semiconducting anatase and PAO1, anatase and DpilA, anatase and Dphz1Dphz2 driven by the solar light was illustrated by a schematic diagram (Fig. 8.35). Under solar light irradiation, the semiconducting properties of anatase can be activated, generating photoelectrons and photoholes. In the meanwhile, bio-electrons were produced and transferred extracellularly during the metabolic processes of PAO1. The photoholes took more positive potentials and were easily combined with bio-electrons. By this time, soluble bio-electron shuttles, such as pyocyanin, from PAO1 and DpilA were synthesized and distributed widely in the interaction system. In addition, PAO1 and Dphz1Dphz2 which both had pili and flagellum can move to the surface of anatase electrodes to form biofilms and complete the electron transfer process. Certainly, bio-electrons were continuously pumped out from both the biosynthesized extracellular shuttles and the biofilms. Therefore, anatase not only acted as an electron acceptor but also reacted with bio-electrons and delivered the electrons to the external circuit, achieving the promoted electron transfer driven by the solar light between semiconducting anatase and P. aeruginosa PAO1.

8.3.3 Photoelectron Energy Utilized by Microbes to Accelerate Metabolism Photoautotrophy was once believed to be the ultimate energy-harvesting pathway for all living organisms on earth until the discovery of chemoautotrophy, in which organisms harness chemical energy for growth (Winogradsky 1949). Chemoautotrophs synthesize biomass from carbon dioxide (CO2) and gain chemical energy from the oxidation of inorganic compounds such as hydrogen gas (H2), ammonium (NH4+), nitrite (NO2−), reduced sulfur compounds, and Fe2+ (Stevens and McKinley 1995; Shively et al. 1998; Nealson et al. 2005). Organotrophic organisms rely on organic carbon synthesized from either phototrophs or chemoautotrophs. In this part, we provide evidence in favor of a novel pathway, in which solar energy is converted to chemical energy by photocatalysis of semiconducting minerals to support and stimulate the growth of non-phototrophic

196 Table 8.7 Current density of anatase-PAO1 under light and dark (voltage: 0.2 V)

8 Interactions Between Semiconducting Minerals and Microbes Potential 0.2 V

Photocurrent density (µA/cm2)

Dark current density (µA/cm2)

Increment (µA/cm2)

Anatase-PAO1

9.34

1.14

8.20

Anatase-DpilA

3.99

0.38

3.61

Anatase-Dphz1Dphz2

2.77

0.25

2.52

Anatase-LBNS

2.14

0.21

1.93

FTO-PAO1

0.46

0.45

0.01

FTO-DpilA

0.30

0.22

0.08

FTO-Dphz1Dphz2

0.29

0.21

0.08

FTO-LBNS

0.26

0.20

0.06

Fig. 8.35 Schematic diagram of extracellular electron transfer between anatase and PAO1, anatase and DpilA, anatase and Dphz1Dphz2 driven by the solar light (Liu et al. 2021a, b)

microorganisms. Under the light, photocatalysis of semiconducting minerals rutile, sphalerite, and goethite produced photoelectrons that supported the growth of representative non-phototrophic microorganisms, such as chemoautotrophic and acidophilic A. ferrooxidans. Two experiments were performed to demonstrate the light-induced growth of A. ferrooxidans. The first experiment was primarily designed to test the role of rutile photocatalysis in cell growth. Generation of photoelectrons from rutile on irradiation by a simulated sunlight source (visible light, continuous wavelength, 380–760 nm) was demonstrated by time-course production of Fe2+ (green diamond, Fig. 8.36a) in a bacteria-free and Fe2+-free m9K medium. When bacteria were added to the C-chamber but without visible light in the P-chamber (that is, under the dark condition), cell concentration increased from 3.5
105 cells/mL at time zero to 3.6
107 cells/mL in 4 days (orange stars, Fig. 8.36a) using the released energy from the oxidation of Fe2+ initially present in the m9K medium (7500 cells/mL Fe2+) (green stars, Fig. 8.36a). When the graphite anode in the P-chamber was not coated with rutile (rutile-free), visible light could not produce any photoelectrons, and cell growth solely depended on the energy from

the oxidation of Fe2+ initially present in the m9K medium. Rapid cell growth (orange squares, Fig. 8.36a) corresponded to the rapid oxidation of Fe2+ (green squares, Fig. 8.36a) between day 1 and day 2.5, but the cell growth rate quickly decreased because of depletion of the medium-Fe2+ to a negligible level. In contrast to the above treatments, maximal cell growth occurred when both rutile and visible light were present in the P-chamber. The photoelectrons generated in the P-chamber were spontaneously transported to the C-chamber owing to the potential difference between the two chambers. In the C-chamber, the photoelectrons reduced Fe3+ to Fe2+, which was then oxidized by A. ferrooxidans to support its growth. In contrast to the experiment without rutile, the growth of A. ferrooxidans in the presence of rutile and light continued beyond 2.5 days, even when the Fe2+ concentration remained low (* 4000 mg/L). This sustained growth was ascribed to the regeneration effect of Fe2+ from Fe3+ reduction by photoelectrons as evidenced by a positive correlation between the net cell growth in the C-chamber and the number of photoelectrons transferred from the P-chamber to the C-chamber (Fig. 8.36b). In addition, photoelectrons significantly shortened the lag phase of A.

8.3 Regulation and Influence of Mineral-Microorganism Electron …

197

Fig. 8.36 Relationship between A. ferrooxidans growth and Fe2+ concentration and electric charge transfer. a Time-course changes of Fe2+ (green) and A. ferrooxidans concentrations (orange) using rutile as a semiconducting mineral under different treatments. The P-chamber was either in the dark or irradiated with visible light (continuous wavelength 380–760 nm and * 13.5 mWcm). The cathode electrolyte is either the m9K medium (experiments with A. ferrooxidans) or the Fe2+-free m9K medium (A. ferrooxidans-free experiment). The Fe2+free m9K medium was made by replacing 41.7 g/L FeSO4 ⋅ 7H2O with 6.1 g/L FeCl3 (equivalent to 2.1 g/L Fe3+ conc.). The slightly lower Fe2 + concentration at time zero (that is, 7500 mg/L) than the expected value from 41.7 g/L FeSO4 ⋅ 7H2O (Fe2+ 8382 mg/L) for the treatments with A. ferrooxidans was probably due to oxidation of Fe2+ and precipitation of ferric oxide. The difference in cell concentration

between the rutile-free reaction [rutile-free/A. ferrooxidans (light, open circuit)] and the rutile-catalyzed process [rutile/A. ferrooxidans (light)] is about fivefold, and this difference is reproducible from multiple experiments. Ascorbic acid was used in all these treatments for the sole purpose of quenching positively charged holes remaining in the valence band of rutile. The role of ascorbic acid in promoting bacterial growth was expected to be minimal, as bacterial growth was not stimulated beyond that by the amount of Fe2+ initially present in the m9K medium for the “open-circuit” and dark controls. Even if there was any effect, this effect would have been present in all treatments and should not affect the relative comparison between them; b a significant positive correlation between the net growth of A. ferrooxidans in the reactor and net electric charge transferred from the P-chamber to the C-chamber (p value < 0.001) (Lu et al. 2012b)

ferrooxidans (compare the growth curves between 0 and 0.5 days, Fig. 8.36a). Other semiconducting minerals, such as sphalerite (Fig. 8.37a) and goethite (Fig. 8.37b), showed a similar photocatalytic effect in promoting the growth of A. ferrooxidans. Significantly higher cell growth was observed in the presence of both the semiconducting mineral and visible light under the condition of a closed circuit than those under the other three conditions (Fig. 8.37). The Fe2+ concentration and cell growth profiles in the rutile-, sphalerite- and goethite-catalyzed systems differed slightly under comparable experimental conditions. In the rutile-catalyzed system, the Fe2+ concentration was nearly constant in the first 0.5 day and then gradually decreased (Fig. 8.36a), indicating that the rate of Fe2+ oxidation by A. ferrooxidans was initially equal to the rate of Fe3+ reduction by photoelectrons, but the rate of Fe2+ oxidation exceeded the rate of Fe3+ reduction after 0.5 day, possibly because of the increased cell population. In contrast, in the sphaleritecatalyzed system (Fig. 8.37a), the Fe2+ concentration rapidly decreased, indicating that the rate of Fe2+ oxidation by A. ferrooxidans well exceeded the rate of Fe3+ reduction by photoelectrons. In the goethite-catalyzed system, the Fe2+ concentration initially increased (from 0 to 2 days, Fig. 8.37 b), implying that the rate of Fe3+ reduction by photoelectrons was higher than the rate of Fe2+ oxidation by A. ferrooxidans. After this period, cell growth seemed to enter the logarithmic phase, and the Fe2+ concentration decreased owing to the increased rate of Fe2+ consumption by A. ferrooxidans. In summary, these differences in the Fe2+ concentration profiles

and corresponding cell growth curves in the systems containing the three semiconducting minerals were ascribed to their respective different photovoltaic efficiencies. Also, mineral photocatalysis was light-dependent in a dose-dependent manner, and the spectrum of light-induced bacterial growth closely matched the light absorption spectrum of the minerals. The results showed different photon-to-biomass conversion efficiencies from the three semiconducting minerals (rutile, sphalerite, and goethite), ranging from 0.13 to 0.18‰ for the rutile-A. ferrooxidans system to 0.25–1.9‰ for the sphalerite-A. ferrooxidans system. In comparison, the energy efficiency of photosynthesis, defined as the energy content of biomass that can be harvested annually divided by the annual solar irradiation over the same area, does not exceed 10‰ for crop plants, although short-term measurements can be higher (Blankenship et al. 2011). The biomass conversion efficiencies were calculated based on energy flux from specific wavelengths instead of the full solar spectrum; therefore, the calculated conversion efficiencies in our system is not directly comparable to the value from photosynthesis (Blankenship et al. 2011). Nonetheless, we presume the light-induced and mineral-mediated pathway for light-derived microbial growth is probably less efficient than photosynthesis. Our results demonstrated the effect of the photocatalytic process on the growth of various bacteria. These results are in contrast to those previously reported about photocatalytic disinfection of bacteria by semiconductors, such as TiO2based photocatalysts (Akhavan 2009). In those

198

8 Interactions Between Semiconducting Minerals and Microbes

Fig. 8.37 Time course changes of Fe2+ and A. ferrooxidans concentrations using sphalerite or goethite as a semiconducting mineral under different conditions. The cathode electrolyte is either the m9K medium (experiments with A. ferrooxidans) or a modified Fe2+-free m9K medium (A. ferrooxidans-free experiments) by replacing 22.4 g/L FeSO4 ⋅ 7H2O with 6.1 g/L FeCl3 (equivalent to 2.1 g/L Fe3+ conc.). The slightly lower Fe2+ concentration at time zero (that is, 4250 mg/L in (b)) than the expected value from 22.4 g/L FeSO4 ⋅ 7H2O (4500 mg/L) for the treatments with A. ferrooxidans was probably due to oxidation of Fe2+ and precipitation of ferric iron oxide. a In the presence of sphalerite in the P-chamber, photoelectrons reduced Fe3+, initially present in the modified m9K medium. When bacteria were added to the C-chamber in the dark, cell concentration increased (orange stars) using energy from the oxidation of Fe2+ (green stars initially present in the m9K medium. In visible light, sphalerite photocatalysis occurred but when the circuit was open, cell concentration only reached 8.0
107 cells/mL (orange

squares). In the presence of both sphalerite and cells, the photoelectrons stimulated the cell growth due to the continuous generation of Fe2+ from the reduction of Fe3+ by photoelectrons (orange and green circles for cell growth and Fe2+ concentration, respectively); b in the presence of goethite, photoelectrons reduced Fe3+ in the modified m9K medium. When bacteria were added to the C-chamber in the dark, cell concentration increased from 3.5
105 cells/mL to a maximum of 1.2
106 cells/mL (orange stars) by consuming energy released from the oxidation of Fe2+ (green stars) present in the m9K medium. In visible light, goethite photocatalysis occurred but when the circuit was open, cell concentration only reached 5.1
105 cells/mL (orange squares), and the cell growth occurred at the expense of the m9K medium—Fe2+ (green squares). In the presence of both goethite and cells, the photoelectrons stimulated the cell growth (orange and green circles for cell growth and Fe2+ concentration, respectively) (Lu et al. 2012b)

photocatalysts, photogenerated reactive oxygen species (ROS) were believed to be the “killers” of bacteria, where the cell wall of bacteria adsorbed onto semiconductors could be quickly damaged by ROS through photocatalytic oxidation reactions (Malato et al. 2009; Dalrymple et al. 2010). Because of the short lifetime of ROS, only those microorganisms with immediate contact with semiconductors are threatened (Hoffmann et al. 1995). However, to date, there is no research showing if photogenerated electrons can kill microorganisms. Spatial separation of the semiconducting minerals and the bacteria in our design would minimize any chance of damage to the bacteria by ROS, and, instead, photoelectrons would actually sustain microbial growth. The results from this part were relevant to natural processes because our experimental system simulated the natural environment. The semiconducting minerals, such as oxides and sulfides used in this part, commonly exist on earth’s surface (Vaughan 2006; Wigginton et al. 2007). Compounds of reducing potentials, such as ascorbic acid, humic acid and sulfides, are formed by decomposition of biomass or dissolution of minerals and are widespread in nature (Vaughan 2006; Smirnoff and Wheeler 2000; Hayase and Tsubota 1983). They can all serve as scavengers of positively charged holes (Peral and Mills 1993; Bems et al. 1999; Yanagida et al. 1990) generated during mineral photocatalysis.

A model experimental system consists of a P (photocatalytic)- and a C (chemotrophic)- chamber (Fig. 8.38). This dual-chambered design isolates photocatalysis in the P-chamber from microbial metabolism in the C-chamber and allows the measurement of current density and the calculation of the photon-biomass conversion efficiency. The P-chamber consists of a graphite electrode coated with one of the three semiconducting minerals, that is, rutile, sphalerite, or goethite. On irradiation by simulated solar energy (a Xe lamp), pairs of negatively charged electrons and positively charged holes are generated from the semiconducting mineral. The electrons are excited from the valence band to the conduction band, leaving the holes in the valence band. A reductant (ascorbic acid) is used to scavenge positively charged holes in the P-chamber. The photoelectrons are then transferred to the C-chamber through an externally connected wire to support the growth of A. ferrooxidans, a chemoautotrophic and acidophilic bacterium. This bacterium was chosen for its known ability to derive energy through the oxidation of Fe2+. Chemoautotrophic organisms, which have once been excluded from the development of universally applicable CO2 fixation technology, also have the ability to fix CO2. Being different from photoautotrophs, the energy used by chemoautotrophic organisms to drive the Calvin cycle and fix CO2 is not harvested from the oxidation of water by light

8.3 Regulation and Influence of Mineral-Microorganism Electron …

Fig. 8.38 A schematic diagram of the experimental system. The system consists of two chambers of 350 mL each in size. The P-chamber contains rutile or other semiconducting minerals (anode). On irradiation by a Xe lamp, the electrons are excited from the valence band (VB) with a reduction potential of 2.70 V to the conduction band (CB) with a reduction band of − 0.50 V, whereas the positively-charged holes remain in the valence band. The holes were neutralized by the chemical reductant ascorbic acid. The redox couple of the reduced and oxidized ascorbic acid has a reduction potential of − 0.28 V; thus, the transfer of electrons from ascorbic acid to the valence band of rutile is a spontaneous process. The photoelectrons are

but rather by the oxidation of reducing species (electron donors), such as oxidation of Fe2+ to Fe3+, S2− to S0. Effective CO2 fixation by chemoautotrophic organisms requires fast growth of cells to sustain the cell concentration at an abundant level. In the case of A. ferrooxidans, the cell needs to oxidize 22.4 mol of Fe2+ to fix 1 mol of CO2 (Ingledew 1982), suggesting that the growth of A.f. may be limited by the concentration of Fe2+ if the media contain inadequate amount of Fe2+ as the sole electron donor. Meanwhile, a high concentration of Fe2+ in the culture medium seemed to inversely inhibit the growth of A.f. (Lizama and Suzuki 1989). Therefore, the concentration of Fe2+ could be kept at an adequate level to well support the cell growth. Herein we describe a hybrid system where electrons generated from semiconducting mineral photocatalysis participated in the cycling and regeneration of electron donors and supported the growth of chemoautotrophic bacteria (exampled by A. ferrooxidans). In this synergistic manner, a significant enhancement of CO2 fixation rate by chemoautotrophic bacteria is demonstrated to be achieved. Experiments showed how extraneous amendment of electrons supported the growth of the chemoautotrophic bacterium (A.f.). As presented in Fig. 8.39, cell population and Fe2+ concentration significantly increased at an applied cathodic potential of 0.0 V versus SCE. Neither the cell growth nor the Fe2+ concentration was increased at the potential of 0.6 V versus SCE. The trends of both cell

199

transferred to the C-chamber through an externally connected wire, and the resulting current is determined by measuring the voltage through a 1000 Ω external resistor using a data logger (ADC-16, Pico Technologies, UK). The electrons in the C-chamber are captured by oxidants such as Fe3+ that is produced by oxidation of Fe2+ by A. ferrooxidans. This electron capture process is spontaneous because the reduction potential of graphite (slightly more positive than − 0.50 V owing to the potential drop across the resistor) is more negative than the reduction potential of Fe3+/Fe2+ at pH 2 (0.77 V). The proton exchange membrane (PEM) is used to allow exchange of protons across it to achieve charge balance between the two chambers (Lu et al. 2012b)

Fig. 8.39 Effects of applied cathodic potentials on the bacterial growth and Fe2+ concentration. Bacterium: A.f., medium: Fe2+-free m9K (initially containing 37.5 mM FeCl3), pH: 1.8 (Li et al. 2013)

growth and Fe2+ concentration at a cathodic potential of 0.6 V versus SCE were similar to the cases of no bias control. The reduction of Fe3+ to Fe2+ only can occur at a lower cathodic potential, where the cathodic electrode served as the electron donor. The shortage of Fe2+ supply limited the bacterial growth at applied potential of 0.6 V versus SCE and no bias control. At the applied cathodic potential of 0.0 V versus SCE, the concentration of Fe2+ increased with increasing cell growth, which suggested the electron energy supplied by the electrochemical production of Fe2+ exceeded

200

Fig. 8.40 Voltage variation between rutile anode and sterile cathode (Li et al. 2013)

that needed for maintaining the cell growth. Results indicated that the growth of the bacterium could be supported by the indirect electron transfer from the electrode to the bacterial, in which the soluble Fe2+ acted as the electron-transfer mediator. The photoelectrons generation in the anode chamber and the resultant iron reduction in the cathode chamber were tested in the absence of bacteria. Electrons flow in the reactor was verified by measuring the voltage between the anode and cathode (Fig. 8.40). The voltage increased and decreased in turn as the light source was switched on and off, which confirmed the photocatalysis of natural rutile improved the electron transfer rate to the cathode. As a result, the reduction of Fe3+ to Fe2+ in the sterile cathode chamber was an indicator to evaluate the efficiency of electrons flowing between the two chambers. During the 72-h test, the concentration of Fe2+ in controls with light but without rutile (no photoelectrons generation) increased from 13 to 115 mg/L. In the presence of both light irradiation and rutile, concentration of Fe2+ increased from 18 mg/L to 215 mg/L within 72 h, showing a significantly enhanced Fe3+ reduction. The enhancement of iron reduction improved the electron transfer rate between the two chambers, which was driven by the photocatalysis of rutile under light irradiation. The relationship between the light and semiconducting mineral rutile catalyzed electron transport in the anode chamber and A.f. catalyzed iron oxidation in the cathode chamber were investigated under three different treatments. The bacterial population in the rutile treatment increased from 1.3
106 cell/mL to 4.0
107 cell/mL and remained constant. It appears that bacteria in the rutile treatment reached logarithmic growth starting at 60 h. In comparison, the time required for the bacteria in the no-rutile treatment to reach the logarithmic growth is longer than 96 h. At 96 h, the population was at 4.0
107 cell/mL and 4.5
106 cell/mL in the rutile and no-rutile treatments, respectively. Cell density in the control treatment increased slightly from the initial concentration of 1.3
106 cell/mL (Fig. 8.41). Since all treatments contained adequate Fe2+ (38.39 mM) at the beginning, there was no shortage of electron donors in

8 Interactions Between Semiconducting Minerals and Microbes

the no-rutile treatment that might have caused an inhibition of cell growth. Therefore, the sole mechanism that can explain the observably higher cell growth in the rutile treatment is the electron supplied by semiconducting mineral photocatalysis modulated the Fe2+ concentration at a proper level. These results are consistent with other research on A.f. involving electron supplementation, in which cell density increased by 3 to 50 folds when extraneous electrons were supplied (Yunker and Radovich 1986; Nakasono et al. 1997; Matsumoto et al. 1999). The background decrease of Fe2+ in the control was 1.02 mM within 96 h of incubation, representing a 2.6% decline from the starting concentration of 38.39 mM. The concentration of Fe2+ decreased by 4.18 mM (10.9%) in the no-rutile treatment over the same time frame, with an average oxidizing rate of approximately 1.04 mM/d. Based on the background oxidation in the control, only 8% of the Fe2+ decrease in the no-rutile treatment was caused by microbially-mediated oxidation. However, in the rutile treatment, a rapid decline in Fe2+ concentration was observed, and within 96 h of incubation the Fe2+ concentration decreased by 30.71 mM (80%). Furthermore, the decline in Fe2+ accelerated after 60 h, corresponding to the start of logarithmic bacterial growth (Fig. 8.41). The presence of rutile on the anode generated electron transfer to the cathode for Fe3+ reduction to Fe2+ in the cathode chamber. However, this effect was masked in the rutile treatment because the rate of Fe2+ oxidation well exceeded Fe3+ reduction, resulting in a substantial decline in Fe2+ concentration. Again, this observation correlated with our results regarding the relationship between rutile photocatalysis and bacterial growth (Fig. 8.41), suggesting an enhancement in bacterial growth due to electron transfer from the cathode to A.f. and/or an abundance of Fe2+ in the system.

Fig. 8.41 Fe2+ concentration (open symbols) and A.f. concentration (gray symbols) in the sterile, no rutile (△); no rutile, A.f. inoculated (○), and rutile, A.f. inoculated (□) treatments (Li et al. 2013)

8.4 Environmental Effects and Application …

8.4

Environmental Effects and Application of Pollutant Treatment

8.4.1 Light Fuel Cell Tech for Pollution Treatment by Semiconducting Minerals Cooperating with Extracellular Electron Transform Recently, considerable attention has been paid to clean, efficient and renewable sources of energy throughout the world. Microbial fuel cell (MFC), which is a bio-electrochemical device, can convert chemical energy contained in wastes such as wastewater into electrical power (Logan et al. 2006). However, the practical application of MFCs is presently in the early stages of research. There are many factors limiting the MFC performance, such as reactor configuration (Fan et al. 2007), pH (Gil et al. 2003; Raghavulu et al. 2009), microbial species (Bond and Lovley 2003; Kim et al. 2002), temperature (Liu et al. 2005), electrode material (Park and Zeikus 2003), electron acceptor (Logan et al. 2006), etc. Among the factors that govern the overall energy conversion efficiency of MFC, the choice of cathodic catalyst is crucial (Cheng et al. 2006; Rinaldi et al. 2008). The finding that semiconducting mineral of rutile could be used as an efficient cathodic catalyst in MFCs (Ding et al. 2010; Li et al. 2009a; Lu et al. 2010) has important implications for enhancing the performance and power energy recovery of MFCs. On the other hand, research on microbe-mineral interactions always focused on minerals with redox sensitive elements like Fe, Mn (Lovley et al. 2004) and S (Moser and Nealson 1996). Bacteria harvest energy from the oxidation or reduction of solid minerals, thereby driving their metabolic system. The idea of solar energy should be incorporated into the above-mentioned energy cycle, stimulated an effort to couple the microbe-mineral interactive process with solar energy uptake. The well-known solar energy absorption and conversion event occurred in nature is semiconducting mineral photocatalysis, which was considered as a typical and universally occurring solar-powered geocatalytic process on the earth’s surface (Schoonen et al. 1998). Previous studies indicated that semiconducting minerals such rutile and sphalerite could be stimulated by solar energy to generate photoelectron-hole pairs (Li et al. 2008, 2009b; Lu et al. 2007). Photogenerated electrons or holes could react with redox species present in the natural environment, therefore achieving an energy conversion process from solar to chemical energy. However, whether the photoelectrons generated from the photocatalysis of semiconducting minerals could mediate the electron transport process from bacteria to redox chemicals still remained unknown. It is

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therefore of significant interest to learn the possibly existing mechanism of solar energy-induced photo bio-chemical processes between bacteria and semiconducting minerals. The aim of this study is to investigate the relationship between bacterial metabolism and semiconducting mineral photocatalysis. Despite the traditional spectroscopy and mapping techniques provided an effective means to study the electron transfer mechanism between bacteria and minerals (Geesey et al. 2008; Gorby et al. 2006), we still face major challenges in unraveling and discovering the solar energy-driven biogeochemistry, because the transduction from photons to electrons could not be screened. Here, we employed photoelectrochemical approaches by using MFC-based equipment to create a quantitative model characterizing the mechanisms and efficiency of energy conversion among microbes, minerals and light. To distinguish from traditional MFC, the novel system used in this work is named light fuel cell (LFC). A thorough investigation of parameters affecting the LFC performance was conducted. Equipment configuration, pH, electron acceptors, electrode preparing method and the photocatalytic efficiency of semiconducting minerals may all be factors of significant importance. The potential application of LFC in environmental treatment was further prospected. Based on our study, we believe the exploration of LFC will help in discovery of natural solar-driven biogeochemical process and development of interdisciplinary techniques in environmental remediation.

8.4.1.1 Current Generation in LFC Figure 8.42a shows the current generation of the LFC with a rutile-coated cathode and O2 as an electron acceptor during its start-up period. After incubation, the system was firstly run in the dark to establish the basic MFC operation. The current density increased with time to a stable value of 6.1 A/m3 after about 240 h. The exoelectrogenic strains enriched from the lake sediments were considered to form stable biofilm on the anode electrode. As the same as in traditional MFCs, the anodic microorganisms in LFCs oxide electron donors (acetate in this study) with an electrode serving as the sole electron acceptor, and obtain energy to support their growth from such an electron transfer process. Electrons released from the oxidative reactions were transferred to the anode, through the circuit to the cathode, thus producing current. When the lamp was turned on to illuminate the rutile-cathode, an instantaneous increase of current density to about 7.0 A/m3 was observed. These preliminary observations suggested the performance of the rutile-cathode LFC operated under visible light irradiation exhibited a higher electron transfer rate as compared with an LFC operated in the dark.

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8 Interactions Between Semiconducting Minerals and Microbes

Fig. 8.42 a Current generation of an LFC equipped with a rutile-cathode during the start-up period; b power generation of the same LFC in light and dark. membrane: CEM; cathode binder: Nafion solution; electrode space: 4 cm; oxygen-saturated catholyte (Lu and Li 2012)

8.4.1.2 Evaluation of LFC Performance It is certain that in traditional bioelectrochemical devices such as MFCs, the energy level of electrons when entering the external circuit is much lower than its initial form documented in the electron donors because the energy level is believed to be gradually lowering along the electron transport chain. Differently, the energy loss of the electrons in LFCs might be compensated at the semiconducting mineral photocathode by absorbing and converting solar energy. To demonstrate this speculation, two parallel experiments were conducted under different light controls (light and dark). In an LFC equipped with a visible light-irradiated rutile cathode and saturated oxygen as the terminal electron acceptor, the anodic potential reached − 528 mV (vs. SCE), and the cathodic potential reached − 98 mV (vs. SCE) after 4 h operation. As expected, the volumetric power density of LFC was 12.1 W/m3, 1.6 times higher than that obtained in the dark (7.5 W/m3) (Fig. 8.42b). Experimental results demonstrated that the power in LFC was improved by a light-driven catalytic process, as that operated in the dark produced less power than that in the light. The EIS for the cathode of LFC under different irradiation conditions (Fig. 8.43) at their corresponding open circuit potentials (OCPs) were analyzed by using the one-time constant model (OTCM), in which the solution resistance (Rs) was in series with a parallel combination of the capacitance of the electrode (C) and its polarization resistance (Rp). The results showed that cathodic polarization resistance of LFC in light was 196, while that operated in the dark was 2820. Obviously, the photocatalysis of rutile-cathode in LFC increased the transport efficiency of electrons at the cathode by lowering the cathodic overpotential. Therefore, the activation loss in the cathodic reactions could be reduced and the resultant reaction rate and energy recovery efficiency was expected to be improved by the photocatalysis of semiconducting minerals. Because both the LFCs and the previously reported solar-powered MFCs by using photosynthetic

Fig. 8.43 Impedance spectra of the rutile-cathode under light and dark controls. The solution resistance (Rs) and polarization resistance (Rp) were obtained by fitting the spectra to equivalent electrical circuits using one-time constant model (OTCM) (Lu and Li 2012)

bacteria in the anode were solar energy dependent, the performance comparison between them is reasonable to estimate the developing room of LFCs. Although the reactor configuration, cathodic photocatalyst, and the cathode fabrication methods have not been optimized, the power

8.4 Environmental Effects and Application …

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densities obtained with an LFC were comparable with densities reported by using photosynthetic bacteria in MFCs. Rosenbaum et al. (2005) reported a power density of 7.3 W/m3, in which R. sphaeroides was used as the biocatalyst and ferricyanide was used in the cathodic chamber. Cho et al. (2008) reported a maximum power density of 2.9 W/m3 in a single-chambered MFC by using the same photosynthetic bacteria in the anode. Although parameters were not optimized, the maximum power density achieved by the LFC is 12.1 W/m3, which showed the same level of power output as compared to the available data on the performance of the so-called solar-powered MFCs. Further modifications of LFCs will be expected to result in a significant improvement in power generation.

8.4.1.3 Influence of the Semiconducting Minerals It is also essential to compare different minerals for a fundamental understanding of semiconducting mineral-cathode systems. Here, two typical semiconducting minerals, rutile (TiO2) and sphalerite (ZnS), both of which were previously reported to have good photocatalytic activity in visible light (Li et al. 2008; Lu et al. 2007), were tested and compared with each other for their catalytic efficiency in the cathodic O2 reduction process. From Fig. 8.44, it can be obtained that the maximum power density achieved by the rutile-cathode LFC was 12.1 W/m3, yet that obtained by the sphaleritecathode LFC was 10.6 W/m3. The difference in the power output when using rutile-cathode versus sphalerite-cathode was mainly due to the different potentials of the conduction bands and thus the interfacial electron transfer efficiency. According to the energy positions of the conduction bands of rutile and sphalerite (Fig. 8.45), the conduction band electrons of both minerals were sufficiently energetic to reduce

Fig. 8.44 Effect of cathodic semiconducting material (rutile and sphalerite) on LFC performance. membrane: CEM; cathode binder: Nafion solution; electrode space: 4 cm; oxygen-saturated catholyte (Lu and Li 2012)

Fig. 8.45 Conduction band energy levels of natural rutile (TiO2) and sphalerite (ZnS) and the redox potentials (vs. NHE) of relevant species as a function of pH (Lu and Li 2012)

O2. However, the interfacial electron transfer at the semiconductor/electrolyte interface is very complicated (Xu and Schoonen 2000), that the relative energy difference between the semiconducting mineral and the redox species constraints the interfacial electron transfer kinetics. The larger energy difference between the redox potential of O2/ H2O and the conduction band of sphalerite than that of rutile resulted in a slower electron transfer kinetic and lower energy conversion efficiency. However, finding the optimum semiconductor cathode material is imperative for achieving the maximum efficiency of the system. Further research should focus on the optimization of electrode materials with high photon-to-electron conversion efficiency and minimization of the ohmic resistance loss between the semiconductor and supported conductor substrate. First, the photon-to-electron transfer efficiency of the irradiated semiconductor should be high. Second, although increasing the energy difference in redox potentials between the conduction band of semiconductor cathode and the final redox couples will increase the energy output of LFC, but it will also take disadvantage for the interface electron transfer process. So, the energy level of the conduction band should closely match the redox potential of the cathode reaction, thus facilitating the photogenerated conduction band electrons transfer at the electrode interface.

8.4.1.4 Considerations for Applications of LFC Although the performance of LFC was still evaluated in terms of power output and internal resistance, it is very important to declare that power generation is not the only purpose of developing an LFC device. For us, the power

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output of LFC was just used as a technique parameter for performance evaluation, as the energy efficiency is ultimately reflected and can be directly measured by electrons flowing in the system. From the viewpoint of energy budget, LFC offered a low-cost technology by using a cheap biocatalyst in the anode and a natural semiconducting mineral photocatalyst in the cathode with significant potential for harvesting power from solar energy and other resources such as wastewater. The most attractive advantage of the LFC is the improvement in the reduction ability of cathodic electrons by capturing and converting solar energy to chemical energy. Our results indicated that the cathodic overpotential was significantly reduced by using a semiconducting mineral photocatalyst, which will facilitate the interfacial electron transfer reactions. The environmental significance of LFC is the simultaneously biocatalytic oxidization of pollutants in the anode and photocatalytic reduction of pollutants in the cathode chamber. Although conventional MFCs are capable to do the same thing, the cathodic semiconductor photocatalysis in LFC promised higher applicable value, as some pollutants that are impossible to be reduced in an MFC might be easily reduced in an LFC. Thus, from a photochemistry perspective, there is room for improving the cathodic photocatalysts used in LFCs to maximize the photoreducing efficiency of pollutants. The main concern with respect to an LFC is that the photocatalytic chamber needs to allow sufficient light penetration, so wastewater with deep color or suspended particles possibly blocking light is inappropriate to be treated by this approach. Based on the steady development of fuel cells over the last few years, it is reasonable to expect further improvements in equipment configuration, cathodic photocatalysts, and cathode fabrication method, with the special purpose of minimizing ohmic resistances, maximizing solar to chemical energy conversion efficiency, and enhancing the rates of photocatalytic reaction, will finally lead to significant improvements and practical applications of LFCs in wastewater treatment.

8.4.1.5 Cr(VI) Reduction at Rutile-Catalyzed Cathode in Microbial Fuel Cells Analogous to MFCs, photoelectrochemical (PEC) cells can also destruct toxic chemicals (More and Deshmukh 2003). For example, decomposition of organic compounds by using TiO2 as a photocatalyst was successfully accomplished, and electrical energy was generated during the process (Connolly 2012; Reber and Meier 1984). In this work, we incorporated the PEC mechanism into a novel MFC design, in which a biocatalytic anode and a semiconductor photocatalytic cathode were integrated for the first time. Considering that UV light is inhibitive to microorganisms, natural rutile with visible light-responsiveness was selected as the cathodic catalyst in the MFCs (Lu et al. 2007). Water containing Cr

8 Interactions Between Semiconducting Minerals and Microbes

Fig. 8.46 Voltages and Cr(VI) reduction over time in MFCs containing rutile-coated cathode under visible light and in the dark (Li et al. 2009a)

(VI) was filled into the cathode chamber. Two sets of control devices were constructed, with one set for light control (dark) and the other for microorganism control (sterile). Power generation and Cr(VI) reduction were monitored throughout the study. Preliminary mechanisms of baseline mineral- and light-irradiated photocatalyzed electron transfer by rutile-coated cathode were also discussed.

8.4.1.6 Power Generation and Cr(VI) Reduction Under Light and in the Dark Two parallel sets of MFCs were operated under irradiation and non-irradiation conditions (Fig. 8.46). Our previous studies demonstrated that natural rutile catalyst in lieu of noble metal catalyst such as platinum can facilitate equivalent MFC operations and generate a substantial voltage (3.4 higher than that with graphite cathode and 20% lower than Pt/C cathode and power generation, unpublished data in a separate manuscript under revision). Expectedly, the voltages in both treatments gradually decreased along with the depletion of Cr(VI) in the cathode chamber. The depletion of Cr(VI) was apparently attributed to the reduction by electrons from the cathodes. Noticeably the overall voltages from MFCs with rutile-coated cathode under light were substantially higher than those poised in the dark, indicating a photocatalytically enhanced power generation in these MFCs. Current effort is ongoing to further increase the light-induced power generation by modifying the coating method of rutile to graphite plates. Consistent with the voltage values, the reduction efficiency of Cr(VI) was 1.6-fold higher in the light (88% within 22 h) than that in the dark (65% within 22 h) (Fig. 8.46). Since Cr(VI) was the only constituent of concern in the cathode medium and sorption of Cr(VI) to electrodes was highly unlikely, the change in Cr(VI) content was assumed to be reduced by the electrons that were transferred

8.4 Environmental Effects and Application …

from the anode or excited at the cathode. After 26 h of operation, about 97% of Cr(VI) was completely reduced in the irradiated rutile-cathode MFCs, while Wang et al. (2008) reported more than 50 h was needed to reach this efficiency in an MFC with a graphite cathode, similar concentration of Cr(VI) (28 mg/L), and pH value (pH 2.0) in the cathode chamber. The reduction of Cr(VI) in MFCs containing rutile-coated cathode suggests that rutile-catalyzed mass transfer of electrons from the cathode to Cr(VI) for its reduction. The cyclic voltammogram for MFCs indicated the potential of the cathode was 0.8 V (vs. SCE) and 0.55 V (vs. SCE) in presence and absence of light, respectively. This suggests that visible light-induced photocatalysis might have increased the cathodic potential, favoring Cr(VI) reduction, and resulting in the observed higher Cr(VI) reduction efficiency in the light-irradiated MFCs.

8.4.1.7 Influences of Anodic Microorganisms on Power Generation and Cr(VI) Reduction The anodic chamber of the MFCs contained acetate as the substrate. Under light irradiation, power generation was observed in all MFCs, including the control sets with sterile anode medium (see Fig. 8.47). Apparently, the power generation from the sterile controls was attributed to the light excitement of electrons at the cathode. The current was observably greater in MFCs containing live microorganisms, which added electrons to the power originating from cathode excitement under light. Correspondingly, Cr(VI) reduction efficiency was greater in the same sets of MFCs (Fig. 8.47). These results demonstrate that greater efficiency of electron transfer was achieved when microbes were active in the anode chamber (biocatalytic anode). Data from this study indicates that a collaborative and synergistic process between the biocatalysis (anode) and photocatalysis (cathode) in the MFCs may offer the highest efficiency in system performance.

Fig. 8.47 Reduction of Cr(VI) and current generation in MFCs containing live and sterile anode chambers (Li et al. 2009a)

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8.4.1.8 Mechanisms for Cr(VI) Reduction at the Cathode The uniqueness of an MFC with a rutile-coated cathode is that electrons are transferred via an excited semiconductor mediator rather than directly to the electron acceptor in the cathode chamber. In this study, the visible light excitation of rutile-coated cathode produced electrons and holes in the conduction band and valence band, respectively; then the holes were ‘‘filled” with electrons transferred from the anode, facilitating photogenerated electrons to reduce Cr(VI) at the cathode. Since the lifetime of photogenerated electron– hole pairs is within 10–15 s (Linsebigler et al. 1995), the transfer rate of photoexcited electrons from the conduction band of the semiconductor to the oxidant must be considerably high for a high kinetics of Cr(VI) reduction to take place at the cathode. This photoexcitement did not occur in the dark controls, although a background non-photocatalytic voltage/ current was generated (Fig. 8.46). Therefore, light irradiation is integral to the photocatalysis at the cathode and the resultant Cr(VI) reduction in the MFCs. The primary mechanisms for photocatalytically assisted Cr(VI) reduction at the irradiated rutile-coated cathode may be depicted with equations below (Eqs. 8.1–8.3): Cathode: Irradiated rutile  cathode
þ ! electron  hole pairs hvb þ e cb

ð8:1Þ


þ   þ e Electron  hole pairs hvb cb þ e ! ecb

ð8:2Þ

2 þ 6e ! 2Cr3 þ þ 7 H2 O cb þ Cr2 O7 þ 14H

ð8:3Þ

In addition, the activation energy required to reduce compounds in the cathode chamber will cause potential loss in the cathode (Rismani-Yazdi et al. 2008). The cathodic activation loss appeared to be more obvious in a non-photocatalytic cathode MFC than in an irradiated rutile-cathode MFC, as indicated by the steeper decline of the voltage curve as shown in Fig. 8.46. However, according to the energy conversion law, a portion of the cathodic energy loss could be compensated by absorbing light energy from the rutile-coated cathode. Both previous studies (Linsebigler et al. 1995) and results from this work indicate that reducing the cathodic activation loss can improve the cathodic reaction rates and power output in MFCs; and light excitement of the rutile-coated cathode may offer a solution.

8.4.1.9 Photocatalytically Improved Azo Dye Reduction in a Microbial Fuel Cell with Rutile-Cathode One of the bottlenecks for using an MFC in reducing azo dye is the high cathodic overpotential that impacts the system efficiency (Mu et al. 2009). Semiconductor materials such as TiO2 were previously used for photocatalytic

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reduction of azo dyes (Nasr et al. 1997; Zita et al. 2009) and were recently attempted by our team in MFCs to enhance cathodic reactivity by lowering its overpotential (Li et al. 2009b). Herein, we reported a bioelectrochemically assisted photocatalytic reduction of a model azo dye, methyl orange (MO), in an MFC equipped with a semiconductor mineral (TiO2)-coated cathode. Considering that UV light is inhibitive to microorganisms, natural rutile (TiO2) with visible light-responsiveness was selected as the cathodic catalyst in the MFCs (Lu et al. 2007). The aim of this study was to develop a novel bio-electrochemical system for wastewater treatment by using semiconductor minerals (rutile) as the photocathode. Control experiments were carried out in the same system without irradiation or under open circuit conditions. Electrochemical impedance spectroscopy (EIS) was used to determine if the reduction overpotential could enhance MO decolorization. Kinetics analyses at different MO concentrations and studies of mechanism involved in the bio-photoelectrochemical decolorization were also attempted.

8.4.1.10 MO Decolorization and Electricity Generation In order to investigate the performance of a semiconductor cathode on MO decolorization in an MFC, control experiments were carried out using either rutile-coated graphite cathode or graphite cathode of the same geometric surface area. As expected, MO was gradually decolorized by accepting electrons at the graphite cathode (Fig. 8.48a), and power was generated simultaneously (Fig. 8.48b). By replacing the graphite electrode with the rutile-coated graphite electrode, the decolorizing efficiency of MO increased from 37.8% (graphite electrode) to 47.4% within 24 h, while the output current density also increased. This increase was more significant when the rutile-coated cathode was irradiated by visible light. As shown in Fig. 8.48a, the efficiency of MO decolorization increased from 47.4 to 73.4% within 24 h as the light was on. The overall current density increased concomitantly (Fig. 8.48b). Although the increase Fig. 8.48 Profiles of a MO decolorization efficiency and b current density in MFCs containing graphite or rutile-coated graphite cathode in presence and absence of light (Ding et al. 2010)

8 Interactions Between Semiconducting Minerals and Microbes

of the MO decolorization efficiency at the irradiated graphite cathode could be partially attributed to the photolysis of the dye (Chibisov 1967), the photolysis played no roles in the MFC current increase based on the control data. The result demonstrated that the photoelectrochemical decolorization of MO was much more rapid than an arithmetic compounding of electrochemical process and direct photolysis. The highest current density and MO decolorization efficiency were obtained in the irradiated rutile-cathode MFC. By contrast, the MO decolorization efficiency and power output were the lowest in the non-irradiated graphite-cathode system. The results indicated that the highest rate of MO decolorization in such an MFC required the presence of both rutile and light. The effect of anodic electron supply on MO decolorization efficiency was investigated by disconnecting external wires between the anode and cathode. This set up served as a control MFC. Only 17.8% of MO was decolorized within 24 h under open circuit conditions which was much lower than that in the closed circuit experiment (73.4%). Since there were no electron donors at the cathode when the circuit was disconnected, the reduction of MO by the electrons from the anode was impossible even in the presence of photocatalyst rutile. The slight decolorization of MO in the open circuit experiment can be ascribed to the effects of adsorption and direct photolysis. The results indicated that a microbially driven electron transfer from the anode to the cathode in an MFC was crucial to accomplishing the half-cell photo electrochemical reaction in the cathode. Since the electrons consumed in the cathodic reaction are at an equimolar ratio to those flowing in the external circuit, there should be a correlation between the current density and MO removal efficiency. As demonstrated by the control experiments, the trend of the current generation in the external circuit was consistent with that of MO decolorization efficiency. The higher capability of electricity conversion of an irradiated rutile-cathode resulted in higher power output in the MFC. The comparison of Coulombic efficiency of MO reduction at the rutile-coated cathode in the dark and light was estimated

8.4 Environmental Effects and Application …

Fig. 8.49 Comparison of electrons usage ratio at the rutile-coated cathode in presence and absence of light (Ding et al. 2010)

and plotted as the ratio of the electron required to break down the azo bond and the current flowing through the external circuit. As shown in Fig. 8.49, the Coulombic efficiency under different irradiation conditions could be qualitatively evaluated from the slope of the curve. In general, the trend shows that photocatalysis of rutile at the cathode can enhance the electron usage efficiency, as evidenced by the higher Coulombic efficiency for MO decolorization at the rutile cathode when under visible light irradiation.

8.4.1.11 Kinetic Analysis The biological photo electrochemical reduction of MO at different initial concentrations was investigated to obtain the cathodic reaction kinetics. As shown in Fig. 8.48a, no linear dependence of MO concentration on the irradiation duration was found, indicating that the reduction rate was not in zero order with respect to MO concentration in the cathode chamber. The reduction of MO with the biological photo electrochemical process better fits a first-order reaction, as described in Kusvuran et al. (2004). Figure 8.50 displayed the plot of ln(C0/C) versus irradiation time (t), which shows a strong linear pattern with a slope of 0.059 at high MO concentration (20 mg/L) and 0.269 at low MO concentration (10 mg/L). The results indicate pseudo-first-order kinetics for the MO reduction at different initial concentrations. Hypothetically, if the rate-determining step for MO reduction at the irradiated rutile-cathode was the transfer of electrons from the conduction band of rutile to MO molecules, the reduction rate would increase with the initial MO concentration. However, the rate constant of MO reduction at a low concentration seemed to be higher than that at a high MO concentration. The results clearly suggest that the electron transfer process at the cathode was not the time-limiting step for MO

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Fig. 8.50 Decolorization of MO in MFCs with rutile-coated cathode (Ding et al. 2010)

breakdown, and the concentration of the cathodic electron acceptor was not the determining factor for the MO reduction rate. It seems that the number of electrons initially produced from the anode was insufficient to meet the cathodic electron acceptance. Since the reduction of MO at a higher initial concentration requires more electrons, an inadequate supply of anodic electrons would result in lower MO reduction efficiency. This result also suggests that MO reduction can be further improved by optimizing the anode reaction efficiency. The possible decolorizing mechanism of MO in our MFCs is presented in Fig. 8.51. At the anode, electrochemically active microorganisms extract electrons from the microbially catalyzed acetate (electron donor) oxidation, and the electrons are transferred via the electrode and external circuit to the cathode. At the cathode-electrolyte interface, electrons and holes are produced at the conduction and valence band of rutile under light, respectively. The electron–hole pairs produce a space charge region with the field pointing to the bulk semiconductor. In this case, holes move to the interior of the electrode and electrons move toward the

Fig. 8.51 Schematic representation of species (MO) reduction mechanism in an MFC containing an irradiated semiconductor (rutile)-coated cathode (Ding et al. 2010)

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interface and react with MO (electron acceptor). Since the reported conduction band potential of rutile (− 0.14–0.059 pH vs. SCE) (Sene et al. 2003) is much more negative than that of MO (− 0.058 pH vs. SCE) (Zang et al. 1995), the reduction of MO is thermodynamically favorable. In a unique MFC equipped with an irradiated rutile-cathode, enhanced reduction of a model azo dye MO was achieved. The enhanced MO reduction was ascribed to the decreased cathodic polarization resistance (Rp). Kinetics analysis indicates that further enhancement of MO decolorization could be achieved by optimizing the anodic electron transfer. The mechanisms of MO reduction at an irradiated rutile-cathode can be interpreted as the participation of photogenerated electrons catalyzing the cathodic reaction in an MFC. The results provided insightful information of integrating bio-electrochemical with photoelectrochemical processes, offering an innovative remedy for treating aromatic wastewater and generating electricity during the process.

8.4.2 SSC Enhanced LFC System for Wastewater Treatment In this study, we focused on resolving the anode limitation in traditional MFCs. We constructed a novel SSC-equipped double anodes MFC by using a conventional carbon felt bioanode and a simply synthesized one-dimensional TiO2/ Fe2O3 photoanode (Fig. 8.52). Among a wide variety of semiconductors, TiO2/Fe2O3 was chosen due to its improved visible light response after forming a heterostructure. Furthermore, hematite has a suitable band gap for solar light absorption and it is biocompatible with most bacterial communities. To elucidate the transfer mechanisms of electrons, the performances of various control experiments are simultaneously compared. Then, to gain more insights into the results, the electricity-generating performance and the removal of hexavalent chromium (Cr(VI)) from the aqueous solution with the novel MFC was measured. Such

Fig. 8.52 The diagram of the experimental installation (Ren et al. 2018b)

8 Interactions Between Semiconducting Minerals and Microbes

an environment-friendly system proposed in this paper opens new possibilities for designing MFCs that can harvest solar energy and produce more electricity, as well as treat wastewater simultaneously.

8.4.2.1 Comparison of Power Generation Abilities of Different MFCs The power density and polarization curves for the novel MFC hybrid system, MFC, MFC + TiO2/Fe2O3 and Abiotic MFC + SSC are shown in Fig. 8.53. The OCV of MFC was 478.5 mV, which was similar to that of MFC + TiO2/Fe2O3 (480.2 mV), while the OCV of the new MFC (1052 mV) was close to the sum of SSC (601 mV) and MFC. An interesting phenomenon was observed that the maximum power density was increased from 84.2 to 113.4 mW/m2 when a TiO2/Fe2O3 photoanode was employed in the traditional MFC. A new balanced condition was generated and the improvement was ascribed to the photoelectrons transferred from the semiconducting photoanode under light irradiation. The maximum power density was only 29.7 mW/m2 in the control experiment of “Abiotic MFC + SSC”, suggesting SSC itself couldn’t make effective electron transfer in abiotic MFC. It was important to highlight that the maximum power density reached to 638.3 mW/m2 in the novel MFC hybrid system, this value was approximately 7.6 times higher than that in the general MFC. In addition, TiO2/ Fe2O3 photoanode was stable and no Fe(II) detected by ferrozine-based colorimetric assay in the anode chamber. The results indicated that an efficient cooperation for SSC, MFC and photochemical catalysis process was formed instead of a simple summation. These results suggest that remarkably enhanced electron transfer efficacies were realized in both anodes and the extra photoelectrons effectively surpassed the limitations of traditional MFC anodes at the source. A limiting current density about 1325 mA/m2 was observed in Fig. 8.53 when cathodes were bubbled with air. Cathode is generally believed to be an MFC design element with a slow oxygen reduction reaction on its surface, which

8.4 Environmental Effects and Application …

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Fig. 8.53 Power density and polarization curves for MFC, MFC + TiO2/Fe2O3, Abiotic MFC + SSC, and novel MFC (Ren et al. 2018b) Fig. 8.54 Reduction efficiency of Cr(VI) in different systems. The initial concentration of Cr(VI) was 50 mg/L and the solution pH was 2.0 (Ren et al. 2018b)

commonly limits overall performance (Ter Heijne et al. 2011). As such, an interesting question would be whether the limiting condition is caused by the cathode, SSC, or anode. To answer this question, the catholyte solution of 0.1 M KCl was replaced with K2Cr2O7 solution. An excellent maximum power density of 1302 mW/m2 was achieved, and the current density reached 2460 mA/m2 as well, which was much higher than that reported by Zhang et al. (2012). Therefore, the limiting current was ascribed to the electron acceptors in the cathode, suggesting that sufficient electrons have been transferred from the anode chamber and the anodic efficiency was certainly enhanced significantly.

acceptor in the cathode, the greater rapid reduction of Cr(VI) was attributed to the larger number of electrons transferred from the anode chamber. Therefore, the novel MFC constructed in this study could efficiently treat similar cases of electron-acceptor metal-laden wastewater and simultaneously generate electricity. This scenario suggests that photoanode and bioanode coexist in harmony, and the configuration can be further enhanced by utilizing solar energy with low-cost SSC.

8.4.2.2 Treating Cr(VI) Wastewater with the Novel Hybrid System Given its enhanced electric generation, the novel MFC is likely to exhibit broad applicability in wastewater treatment utilization. The ability of the novel MFC to treat wastewater containing Cr(VI) was demonstrated. The reactors were operated in the close-circuit condition with an external resistance of 600 X under light irradiation. Figure 8.54 shows a minimal decrease of Cr(VI) in the first hour in open circuit, in which the adsorption of Cr(VI) by graphite electrodes is negligible. At the end of the 13.5 h operating period, a significantly reduction efficiency of Cr(VI) reached 90.9%, which was 3.6-fold and 2.3-fold higher than those of the common MFC (25.6%) and “MFC + TiO2/Fe2O3” (39.2%), respectively. The reduction rate of the novel MFC in this study was 3.67 g/(m3 h), which was higher than the value range of 0.4–2.7 g/(m3 h) for MFCs with different electrode materials in the literature (Gupta et al. 2017). The cathode in our system was an ordinary graphite electrode. In general, graphite electrodes are cheaper than precious metal catalysts or activated carbon microfibers (nanofibers), and they can meet the requirements of having environmentally friendly attributes. Given that Cr(VI) was the only electron

8.4.2.3 Mechanism Analysis for the Novel MFC The novel MFC was demonstrated to have approximately 7.6-fold higher electron transfer efficiency than that of the general MFC. It should be pointed out that the SSC itself only offers a few electrons based on the result of “Abiotic MFC + SSC”. Under light irradiation, it generates negative and positive terminals by photoelectric conversion instead of absolute potential values. The positive terminal was connected to the photoanode and bioanode. On the one hand, the dynamics of anodic bio-oxidation were enhanced by the “force” from SSC’s positive terminal. Subsequently, the accelerated metabolic rate could facilitate the generation of additional bioelectrons, and it could partly result in improved MFC performance, which accords well with the previous study (Ding et al. 2014). On the other hand, when the TiO2/Fe2O3 photoanode was excited by light, the photoelectrons at high-energy level would thermodynamically transfer to the conduction band of rutile TiO2, and photogenerated holes accumulated in the valence band of Fe2O3. Notably, the SSC’ positive terminal connected with the photoanode significantly accelerated the photoelectron transfer rate and prevented the recombination of photoelectron-hole pairs. Furthermore, more bioelectrons

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and photoelectrons originated from the anode chamber and flowed into the circuit. The SSC helped overcome the limitations of the bioanode through coupling with the photo electrocatalysis process. In external circuit, a build-in electric field was formed in SSC after converting light energy to electric energy, which overcame the thermodynamic barrier for electron transfer in MFC. Given that the SSC’s negative terminal was linked to the cathode, the potential became more negative and the reducing capacity was enhanced at the same time. Hence, prominent oxygen and Cr(VI) reduction were realized in the novel MFC. Overall, the increased performance of the novel MFC should be correlated with the combined effects of the SSC and photoanode/bioanode. SSC was paired with photoanode and conventional MFC bioanode successfully. Consequently, apart from the increase in the absolute quantity of electrons in the circuit with the addition of bioelectrons and photoelectrons, the thermodynamic barrier during electron transfer was resolved by SSC. We believe this environment-friendly and low-power consumption strategy can facilitate improved MFC development and offers exciting opportunities for the utilization in various environmental applications in future. A novel SSC combined MFC hybrid system with TiO2/ Fe2O3 photoanode and bioanode was constructed to break through traditional MFC anode’s limitation for the first time. Under light illumination, an efficient generation of electricity and a significantly high Cr(VI) reduction rate (3.67 g/ (m3 h)) were realized simultaneously in this novel MFC. The hybrid system produced the maximum power density of 638.3 and 1302 mW/m2 with 0.1 M KCl and Cr(VI) catholyte respectively, which were higher than the value in previous studies. In summary, owing to the cooperation of SSC, MFC and photoelectrocatalytic process, the number of bioelectrons and photoelectrons was remarkably increased. Moreover, the electron transfer barrier was resolved by SSC’s photoelectric conversion in the system. Such an energy-clean, cost-effective novel system provides new directions for designing MFC and have more hopeful prospects in wastewater treatment and environmental remediation.

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214 Zhang B, Feng C, Ni J et al (2012) Simultaneous reduction of vanadium (V) and chromium (VI) with enhanced energy recovery based on microbial fuel cell technology. J Power Sources 204:34–39 Zhang J, Yang G, Zhou S et al (2013) Fontibacter ferrireducens sp. nov., an Fe(III)-reducing bacterium isolated from a microbial fuel cell. Int J Syst Evol Microbiol 63(Pt_3):925–929

8 Interactions Between Semiconducting Minerals and Microbes Zita J, Krýsa J, Mills A (2009) Correlation of oxidative and reductive dye bleaching on TiO2 photocatalyst films. J Photochem Photobiol, A 203(2–3):119–124 Zuo Y, Xing D, Regan JM et al (2008) Isolation of the exoelectrogenic bacterium Ochrobactrum anthropi YZ-1 by using a U-tube microbial fuel cell. Appl Environ Microbiol 74(10):3130–3137

9

Human Pathological Mineral Features

Mineralization is a subsystem of the life system, which is one of several events that constitute the life process of the human body. Functional mineralization is a controlled process that occurs in a specific part of the human body and is completed strictly according to the specific composition, structure and degree. The minerals formed have a special high-level structure and assembly mode. Pathological mineralization, also known as abnormal mineralization, is an out-of-control process and often closely associated with disease. It occurs in places where it should not occur (ectopic mineralization) (Cui et al. 2008). Pathological mineralization can occur in many parts of the human body, such as various organ stones, gout, cardiovascular system calcification, tumor calcification foci and so on. Among them, stones and tophi are easier to obtain samples that scholars have conducted in-depth researches on them for many years and have obtained a more systematic understanding (Ouyang and Zhou 2004). Calcification of the cardiovascular system is the main risk factor for the cardiovascular diseases, and calcification in tumor lesions is also considered to be closely related to the development and diagnosis of tumors. Therefore, the study of pathological mineralization may possibly provide guidance for the prevention and treatment of diseases. Human pathologic mineral is the product of the interaction between human body and the surrounding environment. It is the carrier to record the evolution of the human body and the external environment, which contains a lot of information reflecting the changes in the human and surrounding environments. This information is contained in the morphology, microstructure, chemical composition, physical and chemical properties, spectral characteristics, and distribution laws of minerals. Therefore, to find out the etiology and pathology of diseases related to mineralization, it requires in-depth study of the minerals formed in the lesions. This chapter mainly describes the characteristics of pathological minerals in several tumor mineralization foci and the cardiovascular system. © Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_9

9.1

Mineralization Characteristics of Psammoma Body Mineralization in Meningioma

Meningioma which originates from a meninx and meningeal gap, accounts for 19.2% of all intracranial tumors (Wang 1998). Meningioma is difficult to be completely cured, and has a high recurrence rate. Mineralization is usually a part of the physiological processes that occur in a meningoma. Although previous studies have shown that the mineralized deposits in meningiomas have a typical concentric layered structure (Kubota et al. 1986; Kirschvink et al. 1992; Han et al. 1996; Kiyozuka et al. 2001), the mineralogical methods for identifying them are unsatisfactory, and the mechanisms underlying their formation are poorly understood. As we were particularly interested in the mineral de posits in psammoma bodies (PBs) in meningiomas, we performed both an in-situ and separated characterization of the PBs in terms of chemical compositions microstructure, and morphological features by using scanning electron microscopy (SEM environmental scanning electron microscopy, ESEM), high-resolution transmission electron microscopy (HRTEM energy dispersive X-ray analysis, EDAX), and electron probe microanalysis (EPMA). Based on those results, we discussed and proposed the mechanism of the formation of PBs in meningiomas.

9.1.1 Morphology and Composition of Psammoma Body Mineralization in Meningioma The polarizing optical micrograph of a meningioma sample showed a large number of PBs after H&E staining (Fig. 9.1; 200 amplification). The tumor cells, which were nest-, cord-, and eddy-like shapes with different sizes, were rich in the cytoplasm and had no obvious borders or oval-shaped nuclei. Unequal amounts of fibrous interstitials were present 215

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Fig. 9.1 Light micrograph of meningioma tissue accompanied by a large amount of PBs (Wang et al. 2011)

among the cell nests. Some PBs with obvious concentric layered structure and gaps between layers showed light purple H&E (hematoxylin–eosin) staining (thin arrows in Fig. 9.1); the others with dense concentric layered structure showed deep purple H&E staining (thick arrows in Fig. 9.1). Under ESEM, the morphology of the different developing stages of PBs were clearly observed and could be distinguished from each other. At the initial formation stage of PBs, shown in Fig. 9.2, the typical structure of concentric layers had not formed, but many nanoballs of different sizes were deposited on collagen fibers. The following HRTEM results demonstrated that these nanoballs had already mineralized. The cross-sectional morphology of a PB (Fig. 9.3) shows a mineralized inner core with collagen fibers surrounding around its outer and internal surface. Some mineralized balls deposited on collagen fibers in the marked region, corresponding to PBs with light blue HE staining and obvious gaps between different layers are shown in Fig. 9.1. These features indicated that the PBs were at the developing stage of mineralization. These mineralized balls could continue to grow to form PBs that can be easily observed under a polarizing microscope. The whole profile of a completely calcified PB shows a ball-like shape with very rough surface (Fig. 9.4). The magnified section of the calcified PB in the lower region of Fig. 9.4 shows that the center of the PB is fiber-like mineralized particles arranged as eddy-like with no obvious nuclear structure (Fig. 9.5). Granular mineralized particles can be seen in the marked circle area. These observations suggest that PBs were formed from continuous mineralization of the spiral-shaped arrangement of collagen fibers. Mineralization caused a staggered arrangement of fibrous and granular crystals with collagen fiber pseudomorphs, discounting previous speculation that the PB’s center is formed in necrotic cells followed by de position of

Fig. 9.2 ESEM image of nano-mineralized PBs on collagen fibers (Wang et al. 2011)

Fig. 9.3 ESEM image of the cross-section of the body in the developing stage (Wang et al. 2011)

mineralized particles in multiple layers, or that the PB presents a cylindrical shape. Furthermore, as shown in Fig. 9.6a, a PB appearing as a hollow ball was observed under HRSEM. It can be clearly seen that the surface of the PB is surrounded by collagen fibers. A large amount of dense mineralized particles is present in its interior region, and these are linked by interwoven collagen fibers. The enlarged image (Fig. 9.6b) shows that the collagen fibers in the hollow region, as well as the PB, were assembled by mineralized nanoballs.

9.1 Mineralization Characteristics of Psammoma Body …

217

The Ca/P (At%) ratio ranged from 1.364 to 1.663, which differed significantly among different species. In terms of calcium phosphate minerals, the mineral with the highest Ca/P (At%) ratio was hydroxyapatite (1.667). Thus, the lower Ca/P (At%) ratio indicated the existence of other mineral phases such as octacalcium phosphate. Furthermore, although the back scattered electron image showed the PB of No.16 had an obvious concentric layered structure (Fig. 9.7), with each layer having a different composition contrast, it seemed that the Ca/P (At%) ratio did not present any monotonous increasing or decreasing trend. The brighter layer appeared to have the higher Ca/P atomic ratio.

9.1.2 Characterization of Morphology, Chemical Composition and Microstructure of Separated PBs Fig. 9.4 The whole profile of a PB showing its spherical shape (Wang et al. 2011)

Fig. 9.5 The enlarged image of the complete mineralized PB’s section (Wang et al. 2011)

As PBs were formed by the aggregation of nanoparticles and the particles were smaller than the smallest beam spot diameter of the probe, the exact chemical composition of the PBs could not be obtained. However, the values of Ca/P (At %) ratio (Table 9.1) within the coverage area of the probe beam spot (1 lm) were determined by energy spectrum analysis, which was helpful in obtaining information of the mineral phase and finding the evolutionary trend of mineral composition from the center to the outer edge of the PBs.

To exclude the interference of the surrounding organisms and to get a direct understanding of the mineral deposits in meningioma, the mineralized PBs were separated from selected samples. It was very interesting that the morphology of separated PBs differed significantly from each other. As shown in Fig. 9.8, a separated PB was wound in collagen fibers, indicating an intimate relationship between them. Figure 9.9 shows a PB consisting of many mineralized balls in an oolitic shape. Some mineralized nanoballs are dispersed in the hollow core. Under HRTEM, mainly 2 types of mineral deposits in PBs are presented. The first was a round particle whose diameter was less than 10 nm (Fig. 9.10a) and the second was a fibrous crystal whose length was several tens of nanometers (Fig. 9.12a). The degree of crystallinity of the round nanoballs was low (Fig. 9.10b), suggesting they were at an initial stage of PBs mineralization in meningiomas. The high-resolution lattice fringe (Fig. 9.11) of one round particle showed the interplanar distanced values were consistent with carbonate-hydroxyapatite (ICDD12-0529 card). The selected diffraction patterns of the area in Fig. 9.11 shows a polycrystalline ring (Fig. 9.11b), indicating the existence of nano polycrystals. The square ratio of the diffraction ring’s diameter was approximately 3: 4: 7: 9: 12, which followed the law of hexagonal system. Moreover, the d values were similar to that of carbonate-hydroxyapatite in the ICDD12-0529 card. The diffraction ring’s inner circle was 2 symmetric short arcs, indicating that the crystal prefers to be arranged in the direction of (002) corresponding to the short arcs. The high-resolution lattice fringes of the fibrous crystals showed unclear one-dimensional lattice images (Fig. 9.12b), indicating that the sample was poorly crystallized. The selected diffraction pattern of this area presented as a dispersed poly-crystal ring (Fig. 9.12c). However, only a few rings could be clearly observed, indicating the

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Fig. 9.6 HRSEM images of hollow PBs in meningioma tissue. a A PB appearing as a hollow ball. b the enlarged image (Wang et al. 2011)

Table 9.1 Ca/P (at. %) results of psammoma annulus in meningioma

Sample number

1

2

3

4

5

6

7

8

16(4–10)

1.631

1.644

1.573

1.609

1.557

1.627

1.639

1.544

17(4–11)

1.557

1.482

1.570

1.442

1.473

1.444

1.435

1.406

18(4–12)

1.660

1.663

1.649

1.652

1.660

1.579

1.656

1.491

19(4–13)

1.512

1.390

1.547

1.364

1.390

1.513

1.491

1.434

Fig. 9.7 EPMA backscattered electron Image of Sect. 16 of fully calcified PBs in meningioma (Wang et al. 2011)

8

2 7

5 4

presence of poorly crystallized or amorphous mineral particles. It is very difficult to determine the phase of these particles. Although the EDAX of TEM could not determine the exact composition of such small particles, we used it to obtain the approximate Ca/P ratio, which helped to identify

3 1

6

the mineral phase. The result of EDAX showed the value of Ca/P (At%) was 1.473, much lower than that in hydroxy apatite. Therefore, it could be estimated that octacalcium phosphate or other minerals of calcium phosphate series with lower Ca/P ratios were present in this area.

9.1 Mineralization Characteristics of Psammoma Body …

Fig. 9.8 ESEM image of PBs with the surface (Wang et al. 2011)

Fig. 9.9 ESEM image of PBs entangled collagen fibers on with oolitic convex surface (Wang et al. 2011)

9.1.3 Discussion on the Formation Mechanism of Calcification Hypotheses on nucleation sites of mineralization have been suggested by previous pathological and biochemical researchers on the mechanism of formation of human calcifications. The possible nucleation sites include matrix vesicles, cytoplasmic mitochondria, parasites and damaged

219

cell walls. Macromolecules that control and inhibit the growth of crystals are also different, including various phosphatases, collagen fibers, pyrophosphates, as well as some metal ions. Observations of the morphology of the PBs indicated that at the initial stage of development of the PBs in meningioma, mineralized nanoballs attached to collagen fibers and gradually aggregated, leading to the mineralization of collagen fibers, which then formed a spiral vortex. The size and location of mineralized nanoballs might imply the PBs in meningiomas were mineralized extracellularly rather than by intracellular nucleation. Collagen fibers played an adhesion role. It can be concluded that nucleation occurred outside the cell in light of the sizes and deposition sites of the mineral balls. Studies on bone formation (Weiner and Wagner 1998) indicated that the minimal content of matrix could lead to nucleation and storage of Ca2+ and PO43−. As meningioma has a rich matrix, which can lead to nucleation and maintenance of Ca2+ and PO43−, it may be inferred that the nucleation site of PB mineralization was the matrix vesicle, and that collagen fiber controlled the crystal growth. Further, chemical analysis indicated the presence of carbonate hydroxyapatite and other calcium phosphate minerals like octacalcium phosphate. Therefore, the mineralization process may be similar to that proposed by Weiner and Wagner (1998), that is, Ca2+ and PO43− were attached firstly to matrix to form octacalcium phosphate nanocrystals, and the nanocrystals continuously aggregated on the organic ligand, which led to the succeeding mineralization of the organic ligand. Moreover, octacalcium phosphate gradually transformed to carbonate hydroxyapatite, with the participation of CO32−, which restricted the growth of the nanocrystal. However, in meningioma, the nucleation sites of mineralization were matrix vesicles and octacalcium phosphate nanoballs deposited and grown from collagen fibers. The mineralization process was not the simple precipitation of inorganic crystals, but was accompanied by the aggregation of nanocrystals, and continuously mineralizing collagen fibers arranged in a spiral vortex, resulting in the concentric layered structure. Concentric vortexes in meningioma arranged by spindle tumor cells, epithelial cells, and fibroblasts, are also named meningioma vortexes. Many membrane structures and gaps existing in those vortexes provide nucleation sites for mineralization. Previous research showed that the concentration of anions and cations in the humoral environment was not sufficient for spontaneous nucleation of crystals, so other specific nucleation sites were necessary for aggregating ions and conducting crystallization. It can be seen clearly that PBs in meningioma are the result of its specific features, and not only because of cell necrosis.

220 Fig. 9.10 a HRTEM image. b lattice image of round mineralized particles (Wang et al. 2011)

Fig. 9.11 a HRTEM high-resolution lattice image of granular mineralized particles. b selected area electron diffraction pattern (Wang et al. 2011)

Fig. 9.12 a HRTEM image. b lattice image. c. selected area electron diffraction pattern of fibrous mineralized particles (Wang et al. 2011)

9

Human Pathological Mineral Features

9.2 Characteristics of Cardiovascular Mineralization

9.2

Characteristics of Cardiovascular Mineralization

Mineralization in the human cardiovascular system is often manifested as calcification formed by calcium-containing phosphate minerals. This section describes the characteristics of mineralization in the human cardiovascular system from the perspective of mineralogy and pathology, and introduces the current medical understanding of the mineralization of calcium-containing phosphate minerals.

9.2.1 Cardiovascular System Mineralization Calcification of the human cardiovascular system tends to occur in mechanical stress concentration and atherosclerotic sites, such as innominate arteries, aortic arch, and abdominal aorta (New and Aikawa 2011). According to the location of calcification, it can be divided into valve calcification and vascular calcification (Fig. 9.13). The latter includes aortic calcification and coronary artery calcification (CAC). The most common sites of calcification are the aorta, followed by the coronary arteries, and then the aortic valve, mitral valve, and tricuspid valve. There is an organic connection between CAC and coronary artery disease, which is a reliable indicator for judging coronary atherosclerosis. The detection of CAC can provide a reliable basis for early diagnosis of coronary artery disease and prediction of the occurrence of coronary artery disease, so it has become one of the current research hotspots in the cardiovascular field.

Fig. 9.13 Calcification in the human cardiovascular system. a A cross-sectional view of the heart, showing calcification at the coronary artery opening and valves (including aortic valve and mitral valve). b heart structure and major macrovascular system. c aortic cut-away view, showing extensive aortic intima Fat streaks and atherosclerotic plaques and calcifications associated with atherosclerotic plaques

221

9.2.2 Mineralogical Characterization of Calcification in Cardiovascular Aortic Atherosclerotic Plaque Cardiovascular diseases (CVDs) are becoming a worldwide health concern. It is estimated that 30% of all global deaths (*17.3 million) were caused by CVDs in 2008 and one third of people older than 45 years live with vascular calcification. Calcification in the cardiovascular system is a common pathological mineralization in the human body. However, previous research on calcification is mainly from the perspective of physiology rather than mineralogy (Bobryshev et al. 1995). The mineralogical characterization of cardiovascular calcification would help to better understand the occurrence and development of calcification.

9.2.2.1 Distribution and Morphology of Calcification An image of the calcification of aortic atherosclerotic plaque in an artery is shown in Fig. 9.14a. The calcification (indicated by a black arrow) was sticky and inelastic. It protruded from the vascellum, forming a discontinuous plane on the surface of the tunica interna. Fig. 9.14b and c are HE-stained sections at different stages. Under POM, the calcification appears more evident (dark regions shown in Fig. 9.14b and c). The dark blocks (white arrow) in Fig. 9.14b suggest that atherosclerotic plaque was in the later stages of plaque development, while the correlation of calcification with collagen (grey arrows) in Fig. 9.14c probably represents the initial calcification process (Jeziorska et al. 1998).

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Fig. 9.14 a Sectional view of the aorta, showing atherosclerotic plaque and calcification (arrow) associated with the plaques. b photomicrograph of calcification and amorphous materials under the polarizing microscope (HE staining). c photomicrograph of calcification stippling in the tissues (HE staining) (Li et al. 2014)

9.2.2.2 The Phase Composition of Calcification The XRD patterns of calcifications (Fig. 9.15) for samples A and B fit both HAP (PDF #09–0432) and CHAP (PDF #15– 0100) well. They all share three characteristic peaks at d = 3.444 Å (002), 2.806 Å (211) and 2.712 Å (300) (marked with a cross in sample A and B in Fig. 9.15). These three peaks might also indicate the existence of octacalcium phosphate (OCP, PDF #79–0423), which is believed to be a precursor to HAP (Xin et al. 2006) and shares many peaks with HAP. However, the characteristic peak of OCP at d = 18.64 Å was not detected due to analysis limitation. The calculated unit-cell parameters were a = b = 9.417 Å, c = 6.892 Å. Brown (1962) has also reported that OCP and HAP in cardiovascular plaque could hardly be distinguished in XRD patterns due to the poor crystallinity and partial hydrolysis of OCP. The XRD data suggest only that the mineral phase might be HAP, CHAP or OCP. The mineral particles in sample A are either spherical, with diameters of * 200 nm (Fig. 9.16, black arrow), or elongate, with dimensions of * 100 nm (Fig. 9.16a, white arrow). A crystal lattice (Fig. 9.16b) could be observed clearly at the edge of the circular particle (Fig. 9.16a). One area (white square in Fig. 9.16b) was processed with Digital Micrograph software to reduce background noise and obtain a clear lattice image (Fig. 9.16c). The lattice spacing was 2.82 Å, which is close to HAP (211) d spacing (2.81 Å), CHAP (211) d spacing (2.82 Å) or OCP (530) d spacing

Fig. 9.15 lXRD spectra of calcification particles in samples A and B and the synthesized HAP (Li et al. 2014)

(2.83 Å), suggesting it might be CHAP, HAP or OCP. The SAED analysis of the spherical particle produced a regular hexagonal reciprocal lattice with diffraction rings (Fig. 9.16d). The regular hexagonal reciprocal lattice

9.2 Characteristics of Cardiovascular Mineralization

223

Fig. 9.16 TEM and SAED images of calcification. a An overview of calcification (black arrow: circular particle; white arrow: elongated particle). b the TEM image of the circular particle in a, the white square is the area selected for IFFT processing. c inverse fast Fourier transform images of white square area in b. d SAED of the circular particle, 1–5 are five rings in SAED patterns (Li et al. 2014)

represents the CHAP or HAP (P63/m) diffraction pattern of the b = [0001] zone axis. As OCP has a triclinic crystal structure, it would be impossible for it to show such SAED patterns. The d-spacing values of the three diffraction closest spots to the center were 5.39 Å, 3.17 Å and 2.71 Å, consistent with the (101), (102) and (300) of HAP (PDF #09– 0432) and CHAP (PDF #15–0100). Diffraction rings are also evident in the SAED pattern, suggesting that the spherical particle included aggregates of small particles with different orientations. Five rings could be labelled from the SAED patterns and the Ri2 ratio was: 1:3.84:6.68:9.26:15.9, close to the ratio of the hexagonal crystal system: 1:4:7:9:16. The SAED results excluded the existence of OCP and indicated the existence of HAP or CHAP.

9.2.2.3 The Chemical Composition of Calcification In order to understand the spatial distributions of the functional groups (amide, carbonate and phosphate) and the relationships between organic tissues and minerals in the aorta, FTIR microscopic mapping was used to scan the

section in the yellow frame (50 lm  50 lm) shown in Fig. 9.17a. The bright section is a blank area which resulted from the incision of the ultramicrotome. The distribution of organic tissues is represented by an amide I band at 1638 cm−1 (Fig. 9.17b), which mainly involves C = O stretching displacements of peptide backbone contributed by the CN and NH vibrations. The distribution of calcification is represented by the inorganic groups as CO32− at 1413 cm−1 (Fig. 9.17c) and PO43− at 1037 cm−1 (Fig. 9.17d). It seems that the distributions of organic tissues and calcifications are uneven and spatially separated. Notably, the distribution patterns of carbonate and phosphate in the calcification area were different. It was observed that carbonate was abundant in the (1a, 1b) area, (1d, 3f) area, (6e, 7f) area and (3 h, 6i) area (labelled with black boxes in Fig. 9.17c), while phosphate was mainly distributed in the (1c, 2d) area, (3e, 5 g) area and (4 h, 7i) area (labeled with red boxes in Fig. 9.17d). Interestingly, the (1c, 2c) area, (4e, 5 g) area and (6 h, 7i) area had an abundance of phosphate but very small carbonate fractions. If the whole calcification area was homogeneous B-type

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Fig. 9.17 a Photograph of aortic atherosclerotic plaque tissue on the BaF2 window. b FTIR microscopic images produced by integrating different vibrations, including: the amide I band at 1638 cm−1. c CO32− at 1413 cm−1; and d. PO43− at 1037 cm−1 for the same section. Yellow square in a is 50 lm  50 lm (Li et al. 2014)

CHAP, high-absorption areas of carbonate and phosphate should be well fitted, but the high-absorption areas of phosphate (red boxes, Fig. 9.17d) were different to those of carbonate (black boxes, Fig. 9.17c). As the substituting ratio of phosphate by carbonate in B-type CHAP varied in a range from 2.3 wt.% to 8 wt.% (Barralet et al. 1998) and the incorporation of carbonate into CHAP is a multistepprocesses involving mineral dissolution and re-precipitation, the uneven distribution of carbonate and phosphate groups indicates different substituting degrees in different calcification areas or, alternatively, different mineralization periods.

9.2.2.4 Chemical Environment of Ca in the Calcification To detect the local environment of Ca in the calcification, XANES at the Ca K-edge was carried out. Eight distinct peaks (A to H in Fig. 9.18a) were observed relating to different chemical environments and electron activities (Eichert et al. 2005). For Ca K-edge XANES, the standard chemicals and sample A had the same peak A at a pre-edge position at * 4043 eV, indicating that the effective charge and occupation site symmetry of the Ca ion changes in compounds (Eichert et al. 2005). OCP, a-TCP, HAP and sample A share the same shoulder B at * 4048 eV, which was assigned to the 1 s-4 s transition (Eichert et al. 2005). The

principal peak in all compounds was composed of two sub peaks (C1 and C2, at * 4052 eV and * 4054 eV) corresponding to the 1 s-4p transition and the relative intensities of peak C1 and C2 relate to the type of Ca in the structure (Eichert et al. 2005). At higher energies, shoulder D at * 4062 eV is related to the transition to unoccupied states mainly from 5 s states. Peaks E to H were due to multiple scatterings (Eichert et al. 2005). When considering the position and shape of the two principal peaks (C1 and C2), sample A is similar to HAP rather than a-TCP or OCP. Moreover, as reported by Eichert et al. (2005), compared with OCP, HAP has an obvious absorbance at 4062 eV (shoulder D). In this present study, sample A showed an absorbance at 4062 eV, also indicating that sample A is similar to HAP rather than OCP. The spectra of HAP and CHAP are similar in peak positions and the only difference is the peak intensities. Thus, it was difficult to distinguish HAP and CHAP in calcification minerals. The Ca K-edge XANES revealed that the calcification contained HAP-like minerals and had no OCP. Combined with XRD, TEM, FTIR and Ca-XANES results, it can be concluded that the minerals in the calcification of sample A are a mixture of HAP and B-type CHAP, while the FTIR mapping results of sample B indicate that the calcification is composed of unevenly distributed HAP and B-type CHAP.

9.3 Characteristics of Psammoma Bodies in Ovarian Tumors

Fig. 9.18 Ca K-edge XANES spectra of a-TCP, HAP, OCP and sample A (a-TCP: alpha Tri-Ca Phosphate; HAP: Hydroxylapatite; OCP: octacalcium phosphate; sample A: calcification powder of

9.3

Characteristics of Psammoma Bodies in Ovarian Tumors

Psammoma bodies (PBs) is a type of pathological mineralization (calcification), which have been found in a great variety of neoplastic and non-neoplastic conditions. PBs are most commonly observed in papillary thyroid carcinoma (PTC), meningioma and papillary serous cystadenocarcinoma of ovary (Das 2009). In a review of worldwide literature (Fadare et al. 2004), 38% of patients with PBs have an associated malignancy or ovarian borderline tumour. Specifically, PBs are confined mainly to serous papillary adenocarcinoma, and 56% ovarian cystic teratoma contains tooth or calcification. The 5- and 10-year survival rates were significantly different between the serous carcinomas with less than 5% area of PBs and those at least 5% area. Although PBs in ovarian tumours have been recognized for decades, the understanding of their formation mechanism remains poorly. Until this century, PBs had been recognized as a reflection of dystrophic calcification resulting from cell necrosis (Ferenczy et al. 1977). In 2001, Kiyozuka et al. (2001) found the formation of PBs in ovarian cancer was closely related to BMP-2 and type-IV collagen. Silva et al. (2003) found hormone can affect the formation of PBs. Besides, nanobacteria may also get involved in the crystallization of PBs. All previous studies indicated that PBs were not merely the consequence of dystrophic calcification. Unfortunately, few studies were conducted to recognize the PBs in tumours from the viewpoint of mineralogy. We believe mineralogical studies may help uncover the nature of PBs and the relationship between PB formation and tumour generation, and do necessary help for medical diagnosis and treatment. However, it is difficult to conduct formal

225

atherosclerotic plaque). a Ca K-edge XANES spectra of a-TCP, HAP, OCP and sample A. b Ca K-edge XANES spectra of HAP and sample A (Li et al. 2014)

mineralogical measurements on PBs due to the small quantity and tiny size. Besides, the intergrowth of organic tissues and PBs had been a tough problem until the separation method was proposed. Xylene and sodium hypochlorite solution was used to remove the organics. Although mineralogical studies of ovarian serous cancer have been conducted with this procedure, only one sample was sufficient enough for traditional XRD test. Besides, few mineralogical researches have been conducted on calcification in teratoma up till now. In this section, we established a mineralogical study of PBs in ovarian serous cancer and teratoma, to compare the mineralogical characteristics of PBs in these diseases, and to discuss the mechanism of PB formation and development.

9.3.1 Morphology and Distribution of Psammoma Bodies in Ovarian Tumors All eight tumours contained numerous PBs with the diameter varying from several lm to 30–70 lm. The PBs presented a glassy appearance and constant extinction under cross-polarized light. These dark stained spherules were often encountered in the stroma including stromal histiocytes (Fig. 9.19a, e, f), fibrocytes (Fig. 9.19b) and cancer cell nest (Fig. 9.19c), whereas only a very few PBs occur around epithelial cells (Fig. 9.19d). As far as it could be judged from the nuclear structure and staining property, the cells around PBs did not appear to be degenerated. PBs were usually found in groups, especially in the stromal histiocytes where collagen fibers are abundant (Fig. 9.19a). ESEM observations demonstrated the roughly spherical PBs, either concentric layered or clumpy, were usually covered with collagen fibers (Fig. 9.20a). Spherules with

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Fig. 9.19 a–e. The PBs (HE) in ovarian serous carcinoma; f. mature cystic teratoma (Thin arrows: psalms; thick arrows: a collagen fibers. b epithelial cells. c fibroblasts. d blood vessels. e cancer cell nests) (Meng et al. 2015)

diameters of 40–130 nm were observed in the center (Fig. 9.20d) and periphery (Fig. 9.20b) of PBs. A single PB in ovarian serous cancer was no more than 35 lm in diameter, while PBs in teratoma could be as large as 70 lm. Besides, there were some confluent spherules with a diameter of 30–50 lm which had two crystalline cores (Fig. 9.20e). The layered PBs in teratoma (Fig. 9.20e) showed more complicated layers than that in ovarian serous cancer. However, PBs in both types of tumours could be distinguished with four layers. Adjacent to a cluster of PBs, blood red cells were encompassed by collagen fibers (Fig. 9.20f).

Fig. 9.20 a–d. ESEM images of PBs in ovarian serous carcinoma; e–f. mature cystic teratoma (Meng et al. 2015)

9.3.2 The Mineral Composition and Fine Structure of Psammoma Bodies in Ovarian Tumors EDX analysis revealed the existence of Ca, P, C, O and a small amount of Mg and Na in PBs. Although quantitative analysis was not achieved, EDX result of a single PB reflected subtle fluctuation between different layers. The mineral phase of PBs in all cases was identified by l-SRXRD (Fig. 9.21a) as hydroxyapatite (Ca5(PO4)3(OH), PDF#09–0432) or carbonate hydroxyapatite (Ca5[PO4,

9.3 Characteristics of Psammoma Bodies in Ovarian Tumors

227

Fig. 9.21 a X-ray diffraction (l-SRXRD). b micro-FT-IR spectra of PBs in ovarian cancer (top) and teratoma (bottom) (Meng et al. 2015)

CO3]3(OH), PDF#19–0272), which have similar diffraction characteristics. Further, micro-FT-IR spectra confirmed the existence of carbonate hydroxyapatite (Fig. 9.21b). The strong peak around 1040 cm−1 is associated with m3(PO43−), while the peak at 963 cm−1 is due to m1(PO43−). IR bands around 1400–1500 cm−1 and 880–870 cm−1 are assigned to the m3, m2 mode of the carbonate group, respectively. The broad bands between 2800 cm−1 and 3600 cm−1 are attributed to stretching vibration of OH (*3000–3600 cm−1) and m1(HPO42−) (*2850–2950 cm−1). Amide I and the H–O-H bending mode of water result in the band at * 1662 cm−1. After fitting procedure in the 900–800 cm−1spectral range, the intensity of 872 cm−1 and 880 cm−1 was used to calculate the percentage of A type hydroxyapatite (replacement of CO32− for OH) and B type hydroxyapatite (replacement of CO32− for PO43−). The results showed the predominance of B substitution in all cases with an average B substitution/A substitution ratio of 7.1 ± 3.3. TEM analysis was performed on one case of ovarian serous cancer and one case of teratoma. In ovarian serous cancer, minerals with two different morphologies were observed. One morphology was columnar crystal (Fig. 9.22 a), which was 10–15 nm in length and about 5 nm in width, and with a Ca/P (At%) ratio of 1.76. Its diffraction pattern (Fig. 9.22j) showed dispersed polycrystalline diffraction rings, and the lattice photograph (Fig. 9.22b) showed small crystalline domains, indicating the selected area is the aggregate of nanocrystals. A region of the lattice photograph was indexed after FFT, inverse FFT and calibration (Fig. 9.22c). Take the diffraction pattern (Fig. 9.22j) into consideration, the interplanar spacing d values of 0.35 nm and 0.28 nm can be assigned to the crystal faces of (002) and (112) of carbonate hydroxyapatite (ICDD, 19–0272), respectively. The single crystalline diffraction behavior of (002) indicated the preferred orientation on this face (Fig. 9.22j). The other morphology was granulated or dumbbell-shaped with 20–100 nm in diameter (Fig. 9.22d). The lattice photography (Fig. 9.22e) showed some areas of

one-dimensional lattices (circles), which might be assigned to approximately 5 nm globules (arrow) in the center of granules. The d value of the one-dimensional lattices is approximately 0.35 nm, which can be attributed to the crystal face of (002) in either hydroxyapatite (3.44 Å) or carbonate hydroxyapatite (3.46 Å). Its Ca/P (At%) ratio was 1.87, which was also assigned to be carbonate hydroxyapatite. In teratoma, the morphology of minerals (Fig. 9.22g) is similar as the columnar crystals in ovarian serous cancer. But the size of single crystal was slightly smaller, which was 5–12 nm in length and about 5 nm in width. Although the lattice photograph (Figs. 9.22 h, i) showed similar interplanar spacing d values with that in Fig. 9.22c, its diffraction pattern (Fig. 9.22k) showed more complicated diffraction rings and dots. The mineral phase in teratoma was identified as hydroxyapatite (ICDD, 09–0432) after calibration and indexed in Fig. 9.22k. It was distinguished from carbonate hydroxyapatite by visible split (circle) among (211) (2.81 Å), (112) (2.78 Å) and (300) (2.72 Å). Preferred orientation was also observed with the arching behavior of the (002) diffraction. In summary, mineralogical investigations were conducted on the microcalcification in PB form within human ovarian serous cancer and teratoma. The optimized separating procedure proved to be feasible to pick out the pathological calcification from the human tissues, thus overcoming the interference from the organics when investigating the mineralogical characteristics of microcalcification. Micro-area analysis, such as micro-FT-IR, TEM, l-SRXRD and l-SRXRF, was of great importance in measuring the PBs which are often in small quantity and tiny size. The dominant mineral phase of the PBs was identified as AB-type carbonate hydroxyapatite (Ca10[(PO4)6-x–y(CO3)x (HPO4)y][(OH)2−u(CO3)u] with 0  x,y,u  2) with trace elements of Na, Mg, Zn and Sr. Collagen fibers may not only be adjacent to the PBs, but also play a crucial role on the preferred orientation of (002) face and the inside-out growth pattern of PBs. The crystal size and concentric structure can be important clues for physiological and pathological

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Fig. 9.22 TEM morphology, lattice image and selected area electron diffraction of PBs in ovarian cancer and teratoma. a A short columnar crystal with a length of 10–15 nm and a width of about 5 nm. b, c lattice images of a. d a granular or dumbbell-shaped aggregate with a diameter of 20-100 nm. e, f lattice images of d. g a nano-column about 5–12 nm long and 5 nm wide. h, i lattice images of g. j the diffraction pattern of a. k the diffraction pattern of g (Meng et al. 2015)

changes in the ovary. The size of columnar crystals was smaller and the concentric layer was more complicated in teratoma. The reveal of extra cations and anionic groups in

the microcalcification provides new clues for pathological diagnosis and detailed information of crystallization will help with either promotion or inhibition of the crystals.

9.4 Carbonate and Cation Substitution …

9.4

Carbonate and Cation Substitution in Hydroxyapatite in Breast Cancer Micro-Calcifications

Various techniques have been used to study the mineralogical characteristics of HA and its substitutions. HA with Ca/P ratios as low as 1.51 and as high as 1.71 may produce almost indistinguishable patterns in powder X-ray diffraction (XRD). Although the structure of HA with isomorphic substitutions showed certain distinctions after refinement by the Rietveld method, identification of the exact substituting ions (groups) in apatite samples remained difficult using XRD analyses alone. FT-IR, Raman spectroscopy, and thermogravimetric analyses (TGA) coupled with differential thermal analysis (DTA) are often employed to study CO32− in HA. Cations can be detected directly by energy-dispersive X-ray analysis (EDAX), electron-probe microanalyses (EPMA), laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and X-ray fluorescence (XRF). However, bulk methods such as traditional XRD, TGA-DTA, ICP-MS, FT-IR pellet methods, and traditional XRF are not appropriate for doing analyses of BCCs (breast cancer microcalcifications) due to their micro- or nano-particle size and extremely limited amounts. Furthermore, the spot sizes used in EPMA and LA-ICP-MS are not small enough to allow analyses of BCCs. Analyses of calcifications using conventional X-ray absorption spectroscopy are also difficult due to small sample sizes and their uneven compositions. As a result of these limitations on sample size, quantity, and uniformity, mineralogical studies on BCCs have not yet been extensively and thoroughly conducted. In this study, we established a comprehensive range of micro-regional research methods to characterize BCCs, focusing mainly on isomorphic substitutions to HA in the calcifications, and we compared the characteristics of BCCs with samples of synthetic and natural HA.

229

9.4.1 Mineral Phase and Crystal Structure The BCCs appeared dark purple in HE-stained sections under optical microscope (OM) (Fig. 9.23). Their profiles were either irregular (Fig. 9.23a) or spherical (Figs. 9.23b, c). The irregular calcifications were always greater than 200 lm across and they were usually associated with necrosis. The spherical calcifications were similar to psammoma bodies and had an average size of 30 lm. Both irregular and spherical calcifications appeared to be generally present in breast cancer for stages T1, T2 and T3. The stages were judged according to the size of the tumor, following World Health Organization protocols. The mineral phases of the calcifications were identified using SR-XRD. As shown in Fig. 9.24, the diffraction pattern for the calcifications were indexed as HA (Ca5(PO4)3(OH), ICDD database, PDF #09–0432). Compared to the standards for HA and carbonated HA (CHA, PDF# 19–0272), slight shifts and widening of the peaks corresponding to the (002), (211) and (202) faces were evident, suggesting poor crystallinity, or interference from organic residues. These factors made it difficult to determine whether the sample best matched HA or CHA. The fine structure of separated calcifications was examined using TEM. In all cases, the dominant morphology of the aggregates was short columnar or dumbbell-shaped crystals with widths of 10–15 nm and lengths of 20–50 nm (Fig. 9.25a, dotted line). Both lattice fringe images and (Fig. 9.25b) and diffraction patterns (Fig. 9.25c) showed the two dominant interplanar distances as 3.36 and 2.89 Å, which are matched well to the (002) and (211) planes of HA (PDF #09–0432), respectively. Notably, compared with lattice parameters of HA, standard CHA (PDF #19–0272) has slightly larger c and smaller a, resulting in its larger (002) spacing (3.46 Å) and smaller (211) spacing (2.79 Å). Therefore, the main phase of most aggregates observed in

Fig. 9.23 Calcifications (indicated by arrows) in OM images of breast carcinoma. a Irregular calcifications in a stage T1 breast carcinoma. b Spherical calcifications in a stage T2 breast carcinoma. c Spherical calcifications in a stage T3 breast carcinoma (Zhang et al. 2021)

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Fig. 9.24 SR-XRD pattern of a breast cancer calcification and comparison with standards. The mineral phase was indexed as HA (Ca5(PO4)3(OH), PDF #09–0432) (Zhang et al. 2021)

Fig. 9.25 a, d, e. TEM morphologies; b. lattice photographs; c, f. diffraction patterns of calcifications in breast cancer. a Short columnar (dotted line, left) or dumbbell-like crystals (dotted line, right) with width of 10–15 nm and length of 20–50 nm. b lattice photograph of the area selected in panel a. c diffraction pattern from the area in panel b. d spherical calcifications, approximately 1 lm in diameter. e enlarged view of area selected in panel d. f diffraction pattern from the area of panel e (Zhang et al. 2021)

TEM should be HA, some minor CHA might co-exist and cannot be ruled out. The multiple domains in the lattice photograph and the rings in the diffraction pattern show that these short columnar or dumbbell-like crystals are polycrystalline aggregates.

Spherical aggregates (Fig. 9.25d) in a sample from an 86-year-old woman were larger than 1 lm in diameter, similar to the psammoma bodies. The surfaces of the particles were amorphous and were thinner than the center. An enlarged view showed that the surface of the spherical

9.4 Carbonate and Cation Substitution …

231

aggregates consisted of fibrous nanocrystals whose longitudinal axis was oriented toward the center of the sphere (Fig. 9.25e). Two dominating diffraction patterns for fibrous nanocrystals, belonging to (002) and (211) planes, also confirmed the presence of HA as the main crystallized phase (Fig. 9.25f). Notably, the degree of crystallinity for the fibrous nanocrystal aggregates was poor relative to the crystallinity of the columnar and dumbbell-shaped crystals.

9.4.2 Carbonate Substitution According to the EDAX results (Table 9.2) for section samples on silicon wafers, carbon (C), oxygen (O), calcium (Ca) and phosphorus (P) were the major elements in the calcifications. The Ca/P ratio (atomic percentage) varied from 1.13 to 2.35. Figure 9.26 shows the Raman spectra, Raman mappings, and FT-IR spectra of the BCCs, respectively. According to previous researches (Antonakos et al. 2007), the single Raman band for m1(PO43−) near 960 cm−1, the two bands near 432 and 445 cm−1 for m2(PO43−), and the m1(CO32−) band near 1071 cm−1 are specific to B-type CO32 − substitution (PO43− substituted by CO32−) (Fig. 9.26a). A comparison of the 432 and 445 cm−1 bands for the BCCs Table 9.2 EDAX results for breast cancer calcifications (Zhang et al. 2021)

Element

C

O

(Fig. 9.26b) with those of reference spectra (Fig. 9.26d) showed that the CO32− substitution was mainly of the B type, and a comparison of the band near 1071 cm−1 in the experimental (Fig. 9.26c) and reference (Fig. 9.26e) spectra confirmed that B-type CO32− substitution was present in the HA of the calcification. The distributions of CO32− and PO43− were obtained from Raman scattering mappings of tissue sections. Calcifications appeared as bright white regions in OM images (Fig. 9.27a), and organic tissues appeared dark in the images. According to the actual spectral characteristics of the Raman imaging test, the substitution amount of PO43− was examined at its peak intensity at 958 cm−1 (denoted as I958), and the substitution amount of CO32− was examined at its peak intensity at 1074 cm−1 (denoted as I1074). PO43− (I958, Fig. 9.27b) and CO32− (I1074, Fig. 9.27c) are generally consistent with the distribution of mineralization in Fig. 9.27a, and some areas (indicated by the arrow in Fig. 9.27b) exceed the mineralization area under ordinary light microscope (Fig. 9.27a), illustrating an interrelationship between organic tissues and minerals through PO43− and CO32−. Because the CO32− content in some areas is low (arrows in Fig. 9.27c), the I958/I1074 intensity ratio was calculated to observe the uniformity of PO43−/CO32− in the

Na

Mg

Si

P

S

Ca

Totals 100.00

Weight (%)

14.15

41.97

0.42

0.89

5.33

14.07

0.18

22.98

Atomic (%)

23.20

51.64

0.36

0.72

3.74

8.94

0.11

11.29

Fig. 9.26 Raman spectra for breast cancer calcifications. a 400– 1600 cm−1 range. b m2 and m4 modes of PO43−. c m1 mode of CO32−. d, e reference spectra from Penel et al. (1998), where A indicates 5.8 wt.%

A-type CO32− substitution, and 10B indicates 10 wt.% B-type CO32− substitution (Zhang et al. 2021)

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Fig. 9.27 Raman scattering maps of a breast cancer tissue section. a Light-microscope image (50  ). b distribution of PO43− (I958). c distribution of CO32− (I1074). d PO43−:CO32− ratio (I958/I1074). Intensities from low to high are represented by color brightness (Zhang et al. 2021)

sample. The results (Fig. 9.27d) show that the distribution of this ratio is not significantly different from the distribution of calcification itself (Fig. 9.27a), indicating that the distribution of CO32− in mineralization is broad and uniform. The strongest peak in the FT-IR spectra (Fig. 9.28) was near 1042 cm−1, and it was assigned as m3 (PO43−). The m1 (PO43−) peak at 961 cm−1 was very weak, as is common in bioapatite, and the m4 (PO43−) peak at 605 cm−1 indicates that the main group of calcifications was PO43− (Antonakos et al. 2007). The peaks between 1409 cm−1 and 1457 cm−1 were typical for substitution of PO43− by CO32−. After fitting, three peaks at approximately 860 cm−1, 872 cm−1, and 880 cm−1 were observed in the range of 900–800 cm−1. The peaks at 872 cm−1 and 880 cm−1 were assigned to B-type

Fig. 9.28 FT-IR spectra of breast cancer calcifications and fitted results in the 900–800 cm−1 range (Zhang et al. 2021)

HA (substitution of PO43− by CO32−) and A-type HA (replacement of OH− by CO32−), respectively. The results demonstrate that CO32− substituted into both PO43− and OH− sites of HA, as in the A-B type substitution (Fleet and Liu 2003; Antonakos et al. 2007). Ouyang and Zhou (2004) reported that the intensity ratio of m3 (CO32−) to m3 (PO43−) was linear with CO32− percentage. The percentage ratio of B- to A-type substitution has been estimated based on the intensity ratio of I872/I880. The CO32− content varied from 1.1% to 14.5% with a mean value of 7.3%, which was consistent with typical values for bioapatite (Antonakos et al. 2007). In all cases, B-type substitution was the dominant type of substitution, and the ratio of B-type to A-type substitution was as high as 18.

9.4 Carbonate and Cation Substitution …

233

Fig. 9.29 Fitting curves of SR-XRF results. a Breast cancer calcifications. b blank 3 M film. c HA standard (Black line: experimental results; red line: fitting curve) (Zhang et al. 2021)

9.4.3 Cation Substitution Figure 9.29 compares the SR-XRF results of the calcifications, a blank 3 M film, and an HA standard. Compared to major Ca, the proportions of Zn, Fe and Sr were less than 1%. After data fitting for Ka peak areas, the presence of trace amounts of Cu, Mn and As were verified. In addition, EDAX also indicated the presence of Na and Mg in the calcifications, although neither Na nor Mg was found in amounts greater than 1 wt%. The distributions of Ca, P, Mn, Fe, Cu, Zn, As, and Sr were examined for a 0.1  0.1 mm2 area sample of

breast cancer tissue (Fig. 9.30). Ca and P, the main elements in HA, were distributed roughly in accordance with the contours of the calcification. Remarkably, the distribution of Zn showed a higher concentration around the periphery of the calcification than in the center. Cu, Mn and As were distributed in much smaller amounts within the profile of the calcification, which makes it difficult to compare their distribution patterns with those for Ca and P. Fe was distributed mainly outside the calcification (determined according to the distribution of Ca), and it was associated with the surrounding organics.

Fig. 9.30 Distribution of Ca, P, Mn, Zn, Sr, Fe, and Cu within a 0.1  0.1 mm2 area in breast cancer. Colors from red to blue represent the highest to lowest counts. The maximum counts of each element are

listed in the red box in the upper-right corner of each image. The intensities from low to high are represented by different colors (Zhang et al. 2021)

234

9.4.4 Diagnostic Significance and Implications Isomorphic substitution is very common in bioapatite and has been extensively studied in bones and teeth, but it has rarely been investigated in BCCs. This work firstly observed that cations like Na+, Mg2+, Zn2+, Fe3+, Sr2+, Cu2+, Mn2+ and As5 + could coexist in the calcifications, and CO32− substitution was dominated by B-type substitution. Thus, the calcifications seem to be more chemically active than pure HA. Interactions between organics and calcifications may prove interesting. For example, macromolecules and interstitial fluids adjacent to the calcifications may influence the sites of CO32− substitution, as evidenced both by the failure to synthesize B-type CHA under conditions similar to those within the human body (Barralet et al. 1998) and by the large difference between the CO32− content found in enamel and BCCs. The calculations of CO32− content and the ratio of Bto A-type substitution in BCC provide quantitative data that can be used to determine the characteristics of breast cancer from a mineralogical perspective. The identification of trace elements such as Zn, Fe, Sr, Cu, Mn and As may serve as a useful and promising diagnostic for BCCs. Previous studies found that only the elements Ca, P, Mg, S, Na, Cl and K were found in BCCs, mainly due to the limitations of the measurements. The presence of other trace elements, including the Cu and Zn that we have observed, has great value for breast cancer research. Our findings of Cu and Zn in BCCs with a uniform distribution show that HA could be one of the most important sinks of these abnormal elements in the body. In addition, breast cancer tumors were found to have a significantly lighter Zn isotopic composition (−0.6‰ to − 0.9‰ of d66Zn) than healthy breast tissue (−0.3‰ to − 0.5‰) as well as blood and serum (−0.1‰ to + 0.3‰) in both groups. Because the trace elements present in BCCs are in a relatively stable state, their detection in calcifications will be more meaningful clinically than their detection in the organic tissues. In turn, the development of calcifications can change the local environment as Ca, phosphate ions, and trace elements are accumulated at the calcification site (Cui et al. 2008). This process can be considered a type of detoxification, because relatively high levels of Zn, Fe, and Ca in benign breast tissue may be associated with a modest increase in risk for subsequent breast cancer (Cui et al. 2008). The concentrations or ratios of trace elements in BCCs also have promise for building corresponding diagnostic pools. However, we have not yet found a way to establish a diagnostic pool of this type using current data. Further data is needed and we encourage more mineralogists and materials scientists to join this field to allow sufficient data to be gathered to create a diagnostic pool. The in situ

9

Human Pathological Mineral Features

micro-mapping of elements based on synchrotron radiation light source as well as micro-Raman and FT-IR are an effective method to investigate the distribution and substitution of elements in biogenic microcalcifications.

References Antonakos A, Liarokapis E, Leventouri T (2007) Micro-Raman and FTIR studies of synthetic and natural apatites. Biomaterials 28:3043–3054 Barralet J, Best S, Bonfield W (1998) Carbonate substitution in precipitated hydroxyapatite: An investigation into the effects of reaction temperature and bicarbonate ion concentration. J Biomed Mater Res 41(1):79–86 Bobryshev YV, Lord RSA, Warren BA (1995) Calcified deposit formation in intimal thickenings of the human aorta. Atherosclerosis 118(1):9–22 Brown WE (1962) Octacalcium phosphate and hydroxy apatite: crystal structure of octacalcium phosphate. Nature 192:1048–1050 Cui FZ, Wang Y, Cai Q et al (2008) Conformation change of collagen during the initial stage of biomineralization of calcium phosphate. J Mater Chem 18(32):3835–3840 Das DK (2009) Psammoma body: a product of dystrophic calcification or of a biologically active process that aims at limiting the growth and spread of tumor? Diagn Cytopathol 37(7):534–541 Eichert D, Salome M, Banu M et al (2005) Preliminary characterization of calcium chemical environment in apatitic and non-apatitic calcium phosphates of biological interest by x-ray absorption spectroscopy. Spectrochimica Acta Part B-Atomic Spectroscopy 60 (6):850–858 Fadare O, Chacho MS, Parkash V (2004) Psammoma bodies in cervicovaginal smears: significance and practical implications for diagnostic cytopathology. Adv Anat Pathol 11(5):250–261 Ferenczy A, Talens M, Zoghby M et al (1977) Ultrastructural studies on the morphogenesis of psammoma bodies in ovarian serous neoplasia. Cancer 39:2451–2459 Fleet ME, Liu X (2003) Carbonate apatite type A synthesized at high pressure: new space group (P3) and orientation of channel carbonate ion. J Solid State Chem 174(2):412–417 Han J, Daniel JC, Pappas GD (1996) Expression of type IV collagen in psammoma bodies: inmlunofluorescence studies on two fresh human meningiomas. Acta Cytol 40(2):177–181 Jeziorska M, Mccollum C, Woolley DE (1998) Calcification in atherosclerotic plaque of human carotid arteries: association with mast cells and macrophages. J Pathol 185(1):10–17 Kirschvink JL, Kobayashi-Kirschvink A, Woodford BJ (1992) Magnetite biomineralization in the human brain. Proc Natl Acad Sci USA 89(16):7683–7687 Kiyozuka Y, Nakagawa H, Senzaki H et al (2001) Bone morphogenetic protein-2 and type IV collagen expression in psammoma body forming ovarian cancer. Anticancer Res 21(3B):1723–1730 Kubota T, Yamashima T, Hasegawa M et al (1986) Formation of psammoma bodies in meningocytic whorls: ultrastructural study and analysis of calcified material. Acta Neuropathol 70(3–4):262– 268 Li Y, Li Y, Wang X et al (2014) Mineralogical characterization of calcification in cardiovascular aortic atherosclerotic plaque: a case study. Mineral Mag 78(4):775–786 Meng FL, Wang CQ, Li Y et al (2015) Psammoma bodies in two types of human ovarian tumours: a mineralogical study. Mineral Petrol 109(3):357–365

References New SEP, Aikawa E (2011) Molecular imaging insights into early inflammatory stages of arterial and aortic valve calcification. Circ Res 108(11):1381–1391 Ouyang JM, Zhou N (2004) Research progress of biomineralization in liposomes. J Syn Cryst 33(6):898–904 Penel G, Leroy G, Rey C et al (1998) Micro raman spectral study of the PO4 and CO3 vibrational modes in synthetic and biological apatites. Calcif Tissue Int 63(6):475–481 Silva EG, Deavers MT, Parlow AF et al (2003) Calcifications in ovary and endometrium and their neoplasms. Mod Pathol 16(3):219–232 Wang CQ, Yang RC, Li Y et al (2011) A study on psammoma body mineralization in meningiomas. J Mineral Petrol Sci 106(5): 229–234

235 Wang ZC (1998) Neurosurgery. Hubei Science and Technology Press, Wuhan, pp 397–399 Weiner S, Wagner HD (1998) The material bone: structure-mechanical function relations. Annu Rev Mater Sci 28(1):271–298 World Health Organization (2017) Guide to cancer early diagnosis. World Health Organization. https://apps.who.int/iris/handle/10665/ 254500 Xin R, Leng Y, Wang N (2006) In situ TEM examinations of octacalcium phosphate to hydroxyapatite transformation. J Cryst Growth 289(1):339–354 Zhang Y, Wang CQ, Li Y et al (2021) Carbonate and cation substitutions in hydroxylapatite in breast cancer microcalcifications. Mineral Mag 85(3):321–331

Infrared Effect of Minerals

Fourier transform infrared spectroscopy technology has played an important role in various fields, including food safety detection, identifying the fingerprint area of substances, and revealing the internal vibration structure of minerals. Concerning the complete electromagnetic radiation spectrum, infrared is an important component of it and lies between the red edge of visible and the short edge of terahertz spectral bands (Fig. 10.1; Vatansever and Hamblin 2012). As photons enter a mineral, some are reflected from grain surfaces, some pass through the grain, and some are absorbed. Emitted photons are subject to the same physical laws of reflection, refraction, and absorption to which incident photons are bound. The variety of absorption processes and their wavelength dependence allows us to derive information about the chemistry of a mineral from its reflected or emitted light. Infrared spectroscopy has been used successfully by chemists and physicists to resolve problems in the structural arrangement of organic and inorganic substances. This approach provides a versatile method for analyzing mineral structures at the molecular level (Adler and Kerr 1963). Also, laboratory FTIR spectroscopy is used by geochemists to determine the minerals’ structure (Farmer 1974; Van der Marel and Beutelspacher 1976), to quantify volatile element concentrations, isotopic substitutions, and structural changes in natural and synthetic minerals, glasses, and melts (Farmer 1974; King et al. 2004; McMillan and Wolf 2018; Rossman 2018a), to examine the origin of color and properties of minerals (Clark 1999; Rossman 2018b), to examine thermodynamic and transport properties of geologic materials (Hofmeister 2004), to examine geologic materials in situ during heating, pressurization and/or deformation (Hofmeister 2004; McMillan and Wolf 2018), and to examine geologic surfaces, such as those that have undergone chemical or biogenic processes (Hirschmugl 2002a, b).

© Science Press and Springer Nature Singapore Pte Ltd. 2023 A. Lu et al., Introduction to Environmental Mineralogy, https://doi.org/10.1007/978-981-19-7792-3_10

10.1

10

The Theory of Infrared Spectra

Vibrational spectroscopy is based on the principle that minerals vibrate at quantized frequencies, dependent upon their chemical structure. When energy propagates within a dense medium, such as a mineral, the electromagnetic waves will be altered by interaction with the molecular structure of the mineral. The vibration of a molecule caused is the placement of the atoms, and therefore the associated electrons, from their equilibrium positions and results in the formation of an oscillating dipole. Thermal radiation emitted from a mineral contains information about the molecular vibrations (and crystal structure) that are manifested as absorption features in emission spectra, provided the oscillations produce an offset in the electric dipole moment (Lane and Christensen 1997). The absorption or emission spectrum arising from the rotational and vibrational motions of a molecule that is not electronically excited is mostly in the infrared region. A small molecule having an electric moment emits and absorbs light of frequency below about 250 wavenumbers because of its rotational motion. Molecules that are absorbing or emitting 1 quantum of vibrational energy show bands in the region from about 200 to 3500 cm−1, while bands due to several vibrational quantum jumps are detected from a few hundred to many thousand wavenumbers. As a result, the direction of wave propagation of either ray for the crystal structure orientation may or may not excite any specific vibrational modes of the molecules. When the direction and frequency of the molecular vibrational mode match the direction and frequency of the propagating wave component, energy will be converted for the vibration, resulting in the frequency-dependent variation in emissivity shown in the spectra. A classic review of mineral vibration theory and thermal emission by Wenrich and Christensen has demonstrated that thermal radiation

237

238

10 Infrared Effect of Minerals

Fig. 10.1 The electromagnetic spectra

emitted from a mineral contains information about molecular vibrations such as bending or stretching, which were manifested as absorption features in emission spectra (Wenrich and Christensen 1996; Lane and Christensen 1997). These fundamental principles, therefore, provided related references to explore the emission spectra of minerals, and enable us to make band assignments in emission spectra according to those bands in the infrared absorption (Michalski et al. 2005, 2006; Hardgrove et al. 2016). These provide support for us to analyze the emission spectra of natural spectra. Major rock-forming minerals have fundamental molecular vibrational modes that produce spectral features in the range from 6 to 50 lm, which is the spectral range considered for most of the works. A wealth of excellent laboratory spectra that provide a basic understanding of thermal IR spectral features such as been obtained (Lyon 1965; Farmer Farmer 1974; Salisbury et al. 1987; Crisp et al. 1990; Salisbury and D’Aria 1992). These data have demonstrated the systematic variations in thermal IR spectral features produced by the vibrational motions of atoms within crystalline materials. The frequencies (or wavelengths) of these absorption features vary with mineral composition and crystal structure and provide an excellent means for determining composition. These earlier studies form the foundation for modeling the complicating effects found on natural surfaces, including particle size variations, intimate mixtures with various compositions and particle size distributions, coatings and varnishes, and geometry (Christensen and Harrison 1993). The emissivity is a necessary parameter to balance the radiation performance, and was calculated as a function of temperature, and it has a different wavelength- dependent emissivity function (e(k)) in the spectral region of interest (Yousef et al. 2000; Honner and Honnerova 2015): e¼

R1

0

eðkÞWk dk 1 1 Z ¼ 4 eðkÞWk dk rT W dk k 0 0

R1

ð10:1Þ

where k, r, and T represented wavelength, Stefan Boltzmann constant, and absolute temperature, respectively, and e(k) as the source of spectral emissivity, Wk as the spectral radiant flux density, the numerator in the fraction represented the radiation energy from samples at a specific temperature, and the denominator was the total radiation energy of the black body at the same temperature.

10.2

Thermal Emission Spectra of Carbonate Minerals

On the earth’s surface, 70% of rocks are composed of sedimentary rocks. The relationship between the mineral structure of carbonates and their spectroscopic characteristics has become the key issue in the past three decades (Clayton and Mayeda 1988; Chatzitheodoridis et al. 1990; Gooding et al. 1991; Treiman et al. 1993; Mckay et al. 1996; Bell et al. 2000; Jovanovski et al. 2002; Bandfield et al. 2003; Rivkin et al. 2006; Chang et al. 2020). Molecular vibration of carbonate minerals related to internal atoms was revealed with the development of infrared spectroscopy (Gunasekaran et al. 2006; Frost et al. 2008a, b; Brusentsova et al. 2010). Many factors can affect the integrated performance of infrared emission materials. In particular, intramolecular vibrations of molecules in the crystal lattice at specific frequencies can change the activity of lattice vibrations and thus essentially impact the infrared radiation performance (Farmer 1974; Wang et al. 2010; Huang et al. 2011). Some proper substituted elements in lattice structure as dopants like transition metal and rare earth ions can significantly improve the infrared radiation performance (Lupei et al. 2009; Wang et al. 2011; Zhang and Wen 2012; Liu et al. 2015). Heat capacity influences infrared emission properties by controlling radiative transfer, and the high specific heat capacity of components presents a positive relationship with the emittance (Makarounis 1967; Jaworske 1993; Hapke 1996). The natural substitution of foreign elements in minerals and multiple mineral assemblages in different rocks, on the one hand, can make them too complicated to build the relationship between their structure (or texture) and infrared emission properties. On the other hand, make these natural and earth-abundant substances be the potential infrared functional materials for multipurpose applications. However, little research so far has concerned the contribution of mineral assemblages (especially accessory minerals) and lattice impurities inside individual minerals to the whole infrared performance of rocks. The crystal lattice, physicochemical properties, and mineral structure of the natural carbonate minerals were examined by both mineralogical and spectroscopy techniques. The relationship between crystal chemistry and the

10.2

Thermal Emission Spectra of Carbonate Minerals

infrared radiant performance of carbonate minerals was established and discussed.

10.2.1 The Characteristics of the Natural Carbonate Minerals XRD pattern of the four natural carbonate minerals at room temperature suggested they were all pure minerals (Fig. 10.2), and no other impure phases can be identified within the current detection standard. The observed strongest peaks of calcite, rhodochrosite, siderite, and magnesite were located at 29.5°, 31.5°, 32.0°, and 32.6° respectively, all corresponding to the lattice diffraction of (104) plane. As expected, the decreased radius of cations from 1.12 Å for Ca2+ to 0.89 Å for Mg2+ caused the shrinkage of unit cells of carbonate minerals, leading to the shift of diffraction peaks with the same miller index to a higher angle. XRF analysis indicated that calcite and magnesite have high purity for their cations, which contained only 3‰ of Mg and 3% of Ca, respectively (Tables 10.1 and 10.2). By contrast, rhodochrosite and siderite were less pure. Minor Mn (8%) in rhodochrosite was substituted by Fe, Ca, and

239

Mg, while 14% of Fe in siderite was replaced by the above cations. The chemical compositions of these natural samples accorded well with those reported data in American Mineralogist Crystal Structure Database (http://rruff.geo.arizona. edu/AMS/amcsd.php). More importantly, these impurity cations with less than 20% of substitution would not notably affect IR absorption bands of the whole carbonate minerals, according to Lane and Christensen (1997). The heat capacity of natural carbonate minerals was determined by differential scanning calorimetry (DSC) (Fig. 10.3). There was a positive correlation between heat capacity and temperature. The heat capacity ranges of four samples (calcite, rhodochrosite, siderite, magnesite) were 88.68–102.83, 84.64–98.35, 83.78–93.46 and 76.46– 90.46 J/(molK) respectively, and their average heat capacity was 94.67, 90.44, 88.65, 82.13 J/(molK), respectively, at the temperature range of 50–140 °C. Above heat capacity results were almost consistent with previous research within the range of systematical error (about 2%) (Jacobs et al. 1981; Robie et al. 1984; Krupka et al. 1985).

10.2.2 Infrared Absorption Spectroscopy In infrared spectra of four natural carbonate minerals, the out-of-plane bending (m2), the asymmetric stretching (m3), and the in-plane bending (m4) modes of CO2 were 3 infrared-active; while the in-plane and symmetric stretch of C–O bond mode (m1) were inactive (i.e., no resulting absorption feature) in the thermal infrared region because there was no net offset of the dipole moment (Herzberg 1945b; Dubrawski et al. 1989; Lane 1997; Lane and Christensen 1997; Frost and Palmer 2011). It was apparent that the spectra were characterized essentially by two very sharp (m2 and m4) and a broadband (m3) (Fig. 10.4). And the asymmetric internal stretching mode m3 was characteristically broad for many carbonates (Jones and Jackson 2012). The observed vibrational bands of carbonate samples accord well with those values in references (Huang and Kerr 1960; Adler and Kerr 1963; Elderfield and Chester 1971; Farmer 1974; Dubrawski et al. 1989; Santillán and Williams 2004) (Table 10.3), clearly showing that the asymmetric stretching (m3) and in-plane bending vibration (m4) of four minerals shifted to high frequency (large wavenumber) as the radius of cation decreased, while m2 vibration (out-of-plane bending) did not show a similar result.

10.2.3 Infrared Emission Spectroscopy Fig. 10.2 XRD pattern of samples at room temperature (The notations of hkl index represented the observed peaks) (Zhu et al. 2021)

Substantial research has reported that the phase of carbonate minerals keeps stable within 200 °C except for the minor

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10 Infrared Effect of Minerals

Table 10.1 The origins and chemical compositions of carbonate mineral samples (Zhu et al. 2021) Mineral

Composition

Origin

Calcite

Ca0.997Mg0.003CO3

Zhejiang Province, China

Rhodochrosite

Mn0.929Ca0.04Fe0.018Mg0.013CO3

Hunan Province, China

Siderite

Fe0.865Mg0.105Mn0.025Ca0.005CO3

Jiangsu Province, China

Magnesite

Mg0.967Ca0.033CO3

Liaoning Province, China

Table 10.2 X-ray fluorescence spectroscopy (XRF) data of carbonate minerals (wt%) (Zhu et al. 2021)

a

Sample

CaO

Calcite

MgO

Fe2O3a

MnO

Na2O

K2O

Al2O3

BaO

SiO2

Cr2O3

SrO

TiO2

CO2 (LOI)

Total

55.35

0.11

0.00

0.02

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

43.68

99.16

Rhodochrosite

2.13

0.49

1.40

62.41

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

33.37

99.81

Siderite

0.27

4.05

66.07

1.69

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

26.94

99.03

Magnesite

2.35

48.67

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

48.91

99.94

Means total iron as Fe2O3

Fig. 10.3 Heat capacity of samples as a function of temperature. The black dotted line represented the values of the references. a Calcite (Jacobs et al. 1981); b rhodochrosite (Jacobs et al. 1981); c siderite (Robie et al. 1984); d magnesite (Robie and Hemingway 1994)

thermal expansion of lattice (Cuthbert and Rowland 1947; García et al. 2020; Zhu et al. 2020). Some temperature points ranging from 55 to 145 °C were set to investigate the dependence of radiant energy and emissivity on temperature.

The radiant energy of all samples (Fig. 10.5) revealed a positive correlation with the temperature, which followed Stefan-Boltzmann’s law that a higher temperature will cause higher radiant energy. The temperature-dependent emissivity

10.2

Thermal Emission Spectra of Carbonate Minerals

Fig. 10.4 Infrared emission spectrum (Zhu et al. 2021)

within the selected wavelength range can reflect the overall radiation. Based on this, the average emissivity of these carbonate minerals: calcite, rhodochrosite, siderite, and magnesite were calculated to be 0.954, 0.934, 0.907, and 0.883, respectively (Table 10.4). Obviously, calcite presented the highest average emissivity (0.954), while magnesite was the lowest (0.883). To better manifest the emissivity stability of minerals, thus we calculated the emissivity standard deviation of four carbonate minerals over the whole measured temperature (Table 10.4). It distinctly showed that the emissivity of samples (calcite and

Table 10.3 Wavenumbers of infrared absorption bands of carbonate minerals (cm−1) (Zhu et al. 2021)

CaCO3

MnCO3

241

rhodochrosite) were more stable (with a smaller standard deviation of 0.003 and 0.004, respectively) than siderite and magnesite (0.006 and 0.011, respectively) in the temperature range of 50–145 °C (323–428 K) (Table 10.4; Fig. 10.6). The emissivity variation ranges of samples used were displayed by the colored band under the curve, whose upper edge and lower edge correspond to the maximum and minimum value of each curve, respectively. The width of each band thus can represent the deviation of emissivity values obtained at different temperature points. The average value of emissivity is marked on the right side (Fig. 10.6). Notably, the emissivity of calcite was the most stable with the smallest variation range than that of other measured minerals at the whole temperature and its average emissivity value was the highest, while the magnesite showing a diametrically opposite performance (Fig. 10.6). These basic findings were consistent with the change of radiant energy (Fig. 10.5). To illustrate spectral characteristics and radiation performance varying with minerals composition, the observed characteristic bands of carbonate minerals in emission spectroscopy were listed in Table 10.5, and the infrared emission spectra and radiant energy spectra of samples were all shown in Fig. 10.7. According to the emissivity of samples varied with wavenumbers (Fig. 10.7a), the minima of the absorption band corresponded to the lowest emissivity of minerals, which were assigned as 0.76 (1414 cm−1), 0.70 (1422 cm−1), 0.68 (1428 cm−1), and 0.65 (1466 cm−1) for calcite, rhodochrosite, siderite, and magnesite, respectively (Fig. 10.7a). Besides, the main characteristic absorption band related to the symmetric CO2 3 stretching (v3) located in the range of 1441–1473 cm−1 (calcite), 1434–1492 cm−1 (rhodochrosite), 1435–1607 cm−1 (siderite) and 1431– 1650 cm−1 (magnesite) (Fig. 10.7a). And a secondary narrow absorption band was associated with out-of-plane bending of CO2 group (m2), located in 844–895 cm−1, 3 −1 837–906 cm , 835–908 cm−1, and 881–972 cm−1, respectively (Fig. 10.7a). The radiation valleys in radiant energy spectra (Fig. 10.7b) are also located at the same positions as those in emission spectra, according well with formula (10.1).

FeCO3

MgCO3

Assignments

712

725

733

746

m4: In-plane bend of CO2 3

876

864

866

887

m2: Out-of-plane bend of CO2 3

1420

1422

1426

1443

m3: Asymmetric stretch of CO2 3

1799

1799

1811

1832

m2+m4

2513

2492

2496

2532

2m2+m4

2867

2849

2852

2920

2m3

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10 Infrared Effect of Minerals

Fig. 10.5 Infrared radiation energy spectrum of carbonate minerals. a Calcite; b rhodochrosite; c siderite; d magnesite from 50 °C to 145 °C (Solid lines of different colors represented the radiant energy at different temperatures)

Table 10.4 Carbonate minerals emissivity from 50 to 145 °C (Zhu et al. 2021)

Temperature/ °C

CaCO3

MnCO3

FeCO3

MgCO3

50

0.952

0.937

0.906

0.896

65

0.955

0.930

0.911

0.895

80

0.959

0.931

0.917

0.890

95

0.953

0.931

0.911

0.885

110

0.955

0.933

0.906

0.881

125

0.955

0.935

0.904

0.874

140

0.953

0.937

0.901

0.871

145

0.950

0.941

0.900

0.868

Average emissivity

0.954

0.934

0.907

0.868

Standard deviation

0.003

0.004

0.006

0.011

10.2.4 The Effect of Crystal Chemistry on Characteristic Vibrations Carbonate minerals have anisodesmic structure (Klein and Hurlburt 1977), whose weak cation–anion bonds vibrate at a lower frequency than bonds within the CO2 3 group. Nevertheless, the fundamental bond vibrations of CO2 3 groups can be subtly affected by different cations in the crystal structure. Different cations in carbonates thus distinctly influence the

behavior of the vibrating CO2 3 group, primarily resulting in the shift of absorption bands and the change of corresponding absorption and radiation energy. To better understand the role of crystal chemistry in the vibration of characteristic absorption bands, the ionic radius of cation (Lide 1990), M–O bond length (the denotation M referred to metals), and C–O bond length (Effenberger et al. 1981; Reeder 1983), and cell volume (Effenberger et al. 1981) against characteristic vibration (m2, m3, and m4) were plotted in Fig. 10.8. It

10.2

Thermal Emission Spectra of Carbonate Minerals

243

Fig. 10.6 The emissivity of four samples varying with temperature from 50 to 145 °C (Different widths presented the emissivity variation range of samples and the average value of emissivity marked on the right side) (Zhu et al. 2021)

Table 10.5 Wavenumbers of infrared emission bands of carbonate minerals (cm−1) Samples

m2

m4

m3

CaCO3

871

712

1414

MnCO3

876

724

1420

FeCO3

877

733

1428

MgCO3

908

747

1466

indicated that ionic radius of cation, M–O bond, and C–O bond length, as well as volume all, presented a negative correlation relationship with the vibration frequency of three bands (Fig. 10.8). The bigger radius of Ca2+ gave rise to the longer length of M–O bond, C–O bond, and bigger lattice cell, which was found to have the stronger vibration frequencies (m2, m3, and m4) of CO2 group. Especially, the 3 characteristic bands were all blue-shifted to a higher frequency as M–O and C–O bond length decreased, illustrating the vibration energy was constrained by C–O and M–O bond (Fig. 10.8b, c). It is easily concluded that the change of C–O bond length in carbonate minerals was also affected by cations around CO2 3 group. Therefore, different cations and corresponding crystal chemistry were dominant factors of spectral features of carbonate minerals in our study. These characteristic bands in absorption and emission spectra, in turn, were the foundation for distinguishing the specific carbonate mineral phase.

10.2.5 Infrared Radiation Properties of Minerals 10.2.5.1 The Positive Effect of Heat Capacity on Radiant Energy The theories about the vibrational and thermodynamic behavior of minerals have been studied for dozens of years and related research demonstrated that the heat capacity will

Fig. 10.7 Infrared emission spectrum. a Radiation energy spectrum; b at 80 °C (353 K) (The blue, green and red color dotted line represented the asymmetric stretching (m3), the out-of-plane bending (m2) and the in-plane bending (m4) mode of CO2 3 respectively) (Zhu et al. 2021)

244

10 Infrared Effect of Minerals

Fig. 10.8 Values of absorption band position (in cm−1) versus a the ionic radius of cation; b M–O bond length; c C–O bond length; d cell volume; respectively. (Solid lines of green, blue and red color presented the m2, m3, and m4 respectively. M–O bond length presented the distance

between the cation and the carbonate anion. The ionic radius of cation values from Lide (1990), the values of M–O and C–O bond length from Effenberger et al. (1981) and Reeder (1983), and the values of cell volume from Effenberger et al. (1981) (Zhu et al. 2021)

affect their infrared radiation properties (Goldsmith 1959; Makarounis 1967; Kieffer 1979a; Hapke 1996; Jacobs and de Jong 2009; Korabel’nikov 2018). Several related models were proposed and have suggested that radiant energy was closely linked to heat capacity (Debye 1912; Jaworske 1993; Hapke 1996). To illustrate the close relationship between heat capacity and radiant energy of samples, the plot of heat capacity against radiant energy of the four samples was shown in Fig. 10.9, indicating that the higher heat capacity would result in stronger radiant energy. According to Eq. 10.1, the emissivity of samples was determined by radiant energy. The heat capacity of minerals, therefore, also dominated their emissivity. Such speculation was found to be correct in this case since the order of increasing average emissivity from calcite to magnesite was exactly by their rising order of heat capacity value. It thus can be concluded that the heat capacity of carbonate minerals is one of the factors affecting their infrared radiation performance, and high heat capacity may improve infrared emission properties.

10.2.5.2 The Close Relationship Between CO2 3 Group and Emissivity Thermal infrared spectra of minerals have been proved to be governed by intramolecular vibrations within the crystal that offset the electric dipoles from their equilibrium positions (Wenrich and Christensen 1996). The oscillating dipole moments, manifested as absorption features in emission spectra, will give rise to the emission of energy (Lane and Christensen 1997; Lane 1999; Hamilton 2000). Note that the structure of carbonate minerals determined that the vibration of chemical bonds in the CO2 3 group was primarily located in the mid-infrared range, which was stronger than those related to cations outside CO2 3 group (Keller et al. 1952; Huang and Kerr 1960; Adler and Kerr 1963; Klein and Hurlburt 1977). Both Figs. 10.4 and 10.7a confirmed this and showed the characteristic peaks in 400–4000 cm−1 of the absorption and emission spectra were significantly associated with CO2 3 group. Therefore, it was necessary to further discuss some influence factors of different CO2 3 groups in four carbonate minerals on their emissivity.

10.2

Thermal Emission Spectra of Carbonate Minerals

Fig. 10.9 Relationship between heat capacity and radiant energy of samples (Zhu et al. 2021)

First, we tested the relationship between the average emissivity and the length of C–O bonds of the four carbonate minerals. It indicated that a shorter C–O bond length can result in a smaller average emissivity, whose coefficient of determination (R2) was fitted as 0.721 (Fig. 10.10a). It

Fig. 10.10 Values of average emissivity versus a the C–O bond length; b vibration bands range of C–O bond (in cm−1); c emissivity minima; d the absorption band minima (in cm−1) (C–O bond length

245

should be pointed out that the v3 vibration band was overwhelmingly dominant in IR emission spectra (Figs. 10.3, 10.7a). Therefore, some factors related to that band like vibration range, the minimum of emissivity, and corresponding vibration frequency at minimum were further examined with their relationship with average emissivity values of samples. The vibration range (detailed calculation in Fig. 10.11) in emission spectra against average emissivity of four samples was shown in Fig. 10.10b, indicating an obvious negative correlation relationship (R2 = 0.916) that a narrower range of v3 vibration band would result in a higher emissivity. In other words, the vibration range of the C–O bond can control the infrared radiation properties of carbonates to some extent. Absorption band minima would also influence the average emissivity of samples. On the one hand, as expected, smaller emissivity at the minimum can also bring about smaller average emissivity since the v3 vibration band was the most dominant signal (Fig. 10.10c, R2 = 0.910). On the other hand, it was surprising to find that a stronger vibration frequency at a minimum can give rise to lower average emissivity (Fig. 10.10d, R2 = 0.754). For example, MgCO3 had the strongest vibration frequency (1466 cm−1) at its v3 vibration band but possessed the lowest

values were from Effenberger et al. (1981), the black oblique line represented the fitting result) (Zhu et al. 2021)

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10 Infrared Effect of Minerals

materials can be absorbed by the vibration of chemical bonds, and the range of vibration bands would decide the absorbed energy when ignoring the interference of ambient conditions. In the system of carbonates based on what we have found, a longer length of C–O bond can result in a lower vibration frequency and narrower absorption band range of C–O bond (mainly v3), thus be conducive to higher radiation energy and more emissivity in the process of thermal radiation. According to this regular rule, we can compare the emissivity of carbonate minerals according to C–O bond length in CO2 3 group as well as the minima and range of v3 absorption band.

Fig. 10.11 The emission spectrum of four samples at the range of 1300–1700 cm−1 (The vibration bands of C–O bond was calculated by the difference between the value of the arrow point)

average emissivity (0.883) at the same time, while CaCO3 behaved oppositely. The above correlativity between CO2 3 group and average emissivity indicated that the vibration of the C–O bond was affected by the bond length and resulted in the change of vibration frequency, absorbed energy, and emissivity. In the thermal infrared spectrum, the energy radiated from

Fig. 10.12 a Values of average emissivity versus the ionic radius of cations; b the relationship between the length of M–O bond and average emissivity (Ionic radii values were from Lide (1990); the M–O

10.2.5.3 A Larger Cationic Radius Causes Stronger Emissivity Now that the most essential difference in the four carbonate minerals in our case was their cations, which indeed determined vibration frequencies of all IR active bands in the CO2 3 group (Fig. 10.8). It was also easy to find that there was a positive correlation between cation radius and the bond length of C–O for these selected minerals (Fig. 10.12). According to the above discussion about the effect of the CO2 3 group on emissivity, the strong association between cations and emissivity was apparent. Figure 10.8 presented an obvious positive correlation relationship between the average emissivity of four carbonates and their corresponding cationic radius as well as bond length (R2 = 0.872 and 0.751, respectively). Based on the above discussion about the relationship between vibration frequencies and cationic radius as well as M–O bond length (Fig. 10.12a, b), it can be concluded that a larger cationic radius and longer M–O bond can result in lower vibration frequencies CO2 3 group, which reduced absorbed energy for the vibration of chemical bonds and thus display a higher emissivity (as well as infrared performance).

bond length values were from Effenberger et al. (1981); the black oblique line represented the fitting results) (Zhu et al. 2021)

The Middle and Far-Infrared Spectroscopy Characteristics …

247

Previous research showed that generally no single factor can be directly responsible for the shifted absorption band in the spectra of different carbonates (Keller et al. 1952; Adler and Kerr 1963; Gaffey 1987; Lane and Christensen 1997). All the presented results in this work suggested that complex interactions within the crystal structure were able to affect the fundamental vibrational frequencies and infrared performance of carbonate minerals. Generally, we speculated that a larger cationic radius (as well as the long bond length of M–O) can be the dominant and direct reason for the improvement of emissivity. In particular, larger cations can also lengthen C–O bonds, which weaken the vibration of the CO2 group, 3 decrease the absorbed energy, and thus enhance the emissivity (emission properties) of carbonate minerals. Carbonate minerals (MCO3, M=Mg, Ca, Mn, Fe) in the range of 400–2000 cm−1 both in infrared absorption spectroscopy and thermal infrared emission spectroscopy showed three CO2 3 -related features (v2, v3, and v4), which all were blue-shifted as the cationic radius and bond length of C–O decreased. The average emissivity of calcite, rhodochrosite, siderite, and magnesite, obtained at 50–145 °C, were 0.954, 0.934, 0.907, and 0.883, respectively, influenced and controlled by multiple factors including some crystal chemistry parameters (cationic radius and bond length of C–O), v3 band features (emissivity minima, vibrational range, and frequency) and heat capacity. Particularly, a higher emissivity of selected carbonate minerals can be attributed to the larger cationic radius, and longer bond length (C–O and M– O), which gave rise to higher emissivity minima, and lower vibrational frequency, and narrower vibration range of the v3 band. All of these were conducive to reducing absorbed energy and enhancing the infrared performance of carbonate minerals. Moreover, there was a positive correlation between heat capacity and radiant energy, indicating that higher heat capacity would also result in stronger infrared radiant energy and more brilliant radiation properties. These understandings help distinguish different carbonate minerals through the revealed relationship between mineralogy, spectroscopy, and infrared radiation properties. Based on some spectroscopic methods proposed in this work, future research on more minerals can be carried out to explore the influence of crystal structure features on infrared performance and spectral characteristics.

by paleontologists and stratigraphers (Dickinson and Suczek 1979; Argast and Donnelly 1987; Fralick and Kronberg 1997). Remarkable spectral differences occur for different carbonates in the far IR region and may serve as fingerprints for mineral identification and are more useful identifiers of carbonate species than those in any other infrared range (Brusentsova et al. 2010). Results found that the spectral characteristics of magnesite, calcite, and dolomite are closely related to the crystal structure and element composition from previous studies. The study of spectroscopy plays an important role in understanding the internal molecular structure of carbonate minerals and the vibration of internal groups. However, the results show that the far-infrared spectrum range is generally limited to 70–400 cm−1, and there is no comprehensive spectral analysis and systematic research on the middle and far-infrared spectrum (30– 4000 cm−1) of calcite, dolomite, and magnesite. Compared with the far-infrared absorption spectrum, severe interference of background radiation results in less research on the emission spectra of minerals in the mid-infrared region. Nevertheless, it has long been recognized since the work of Coblentz, that the thermal emission spectral behavior of materials is dependent on the intramolecular vibrations of molecules present near the surface of the material, and proposed that there is a good correspondence between the absorption spectrum and emission spectrum of a substance. This provides an insight to further study the emission spectrum and radiation characteristics of carbonate minerals.

10.3

10.3

The Middle and Far-Infrared Spectroscopy Characteristics of Calcite, Dolomite and Magnesite

Magnesite, calcite, and dolomite all are important sedimentary minerals in the earth, the geological and sedimentary environment in which they exist has been extensively studied

10.3.1 Mineral Characteristics and Infrared Absorption Spectroscopy The selected samples were considered to be pure carbonate minerals (Fig. 10.13a). The three strongest diffraction peaks of carbonate minerals shifted to high angles as the magnesium content increased in the mineral, which was caused by the shrinkage of the unit cell after the introduction of a small radius of Mg. Raman spectrum of samples was presented in Fig. 10.13b, the characteristic vibrations of calcite, magnesite, and dolomite in the high frequency were 1087 cm−1, 1097 cm−1, and 1098 cm−1 respectively, ascribing to the stretching vibration of CO2 3 (m1), the intermediate frequency 712 cm−1, 724 cm−1 and 741 cm−1 attributed to the in-plane bending symmetry vibration mode of CO2 (m4). Fig3 ure 10.13c showed the middle-infrared spectrum of samples. It was apparent that the spectra were characterized essentially by two very sharp (m2 and m4) and a broad band (m3). The dominated peaks of calcite, magnesite, and dolomite were 1423, 876, 712 cm−1 and 1439, 879, 727 cm−1, as well as 1443, 887, 748 cm−1, attributed to m3, m4 and m2 modes of

248

10 Infrared Effect of Minerals

Fig. 10.13 The spectroscopy analysis results of samples. a XRD pattern; b Raman spectra; c mid-infrared absorption spectra; d far-infrared absorption spectra (Zhu et al. 2022a, b)

CO2 3 respectively. Far-infrared spectra of carbonate minerals were situated at lower than 400 cm−1, as Fig. 10.13d presented, so all absorption bands reported here may be attributed to lattice vibrations (Angino 1967; Brusentsova et al. 2010).

10.3.2 Mid-Infrared Thermal Emission Spectroscopy According to the relationship between radiation energy and wavenumber in Fig. 10.14 (left), calculated the emissivity is listed in Table 10.6. Calcite showed the highest emissivity

(0.951), corresponding to high radiant energy, however, magnesite presented the lowest emissivity (0.895), showing the lowest radiant energy, and dolomite was 0.938. A review of mineral vibration theory as it relates to thermal emission may be found in the work by Wenrich and Christensen (1996). And before this research, carbonate minerals have been analyzed in the thermal infrared region predominantly by the transmission technique (Chester and Elderfield 1967; Scheetz and White 1977; Salisbury et al. 1987), this provided related references for our study to explore the emission spectra of carbonate minerals. Therefore, we were able to make mineralogical band assignments for the emission spectra from the infrared absorption band

10.3

The Middle and Far-Infrared Spectroscopy Characteristics …

249

Fig. 10.14 Radiant energy patterns (left) and emission spectra (right) at 80ºC (353 K) of minerals. a Calcite; b calcite; c dolomite; d dolomite; e magnesite; f magnesite (Zhu et al. 2022a, b) Table 10.6 Emissivity of carbonate minerals (Zhu et al. 2022a, b) Sample

Calcite

Dolomite

Magnesite

Emissivity

0.951

0.938

0.895

assignments (Michalski et al. 2003, 2005). These fundamental internal vibrational modes of carbonates are specifically assigned to m values as follows: the in-plane, symmetric

stretch of the C−O bond (m1), the out-of-plane bend (m2), the asymmetric stretch (m3), and the in-plane bend (m4) (Herzberg 1945a). However, the in-plane, symmetric stretch of C−O bond mode (v1) was inactive in the thermal infrared region (i.e., there was no resulting absorption feature) because there was no net offset of the dipole moment (Lane 1997). As Fig. 10.14 showed, both the radiation valley and absorption band of emission spectrum in the sample radiation

250

10 Infrared Effect of Minerals

energy spectrum were located in a wide range. Overall, the absorptions were stronger in transmission spectra than in emission spectra. The shape of the absorption peak was sharp and the half-width was narrow (Fig. 10.13c). Calcite had the main absorption band in the range of 1360–1462 cm−1 and was related to vibrational positions of CO2 3 (Fig. 10.14a). The lowest emissivity (0.81) was located at 1431 cm−1, and these two peaks (1431 cm−1 and 1169 cm−1) represented the stretching and bending vibration modes of the C–O bond, respectively (Christensen et al. 2000). A secondary narrow absorption band located in 775–889 cm−1 may relate to the deformation vibration of the C–O bond (Christensen et al. 2000). The average absorption band positions located at 881 cm−1 (v2) and 1414 cm−1 (m3) and 710 cm−1 (m3), and the lowest emissivity (0.76) located at 1414 cm−1 (Fig. 10.14b). The average absorption band positions of dolomite and magnesite are 897 (m2), 1445 (m3), 723 cm−1 (m4) (Fig. 10.14c) and 908 (m2), 1464 (m3), 747 cm−1 (m4) (Fig. 10.14d), respectively. Their lowest emissivity (0.71 and 0.65) is located at 1445 and 1464 cm−1 respectively, and the main characteristic absorption band is related to vibrational −1 positions of CO2 3 located in the range of 1370–1606 cm −1 and 1365–1637 cm respectively. Besides, the deformation

vibration related to the C–O bond is located in 866– 930 cm−1and 881–972 cm−1 respectively (Christensen et al. 2000). Compared with the infrared emission spectroscopy, we found the results of the radiant energy spectrum could lead to a similar conclusion that the radiation valleys also located at the same positions of the emission spectrum corresponded to low radiation energy (Fig. 10.14). Overall, it can be found that these lowest emissivity values are all located in the range of the lowest absorption band which revealed a close connection among molecular structure, emission spectrum, and radiation energy.

Fig. 10.15 The relationship between the relative atomic mass of cations and vibration modes of spectrums. a Raman spectroscopy; b mid-infrared spectroscopy of carbonate ion; c far-infrared spectroscopy of metallic bond; d mid-infrared emission spectroscopy (The

cations relative atomic mass of dolomite was given in average the Ca and Mg; LV, LV-1, LV-LV-3 represented lattice vibration) (Zhu et al. 2022a, b)

10.3.3 Mass of Metal Atoms Affects the Spectral Vibration Characteristics The spectroscopic characteristics of calcite, dolomite, and magnesite showed that the spectral structure presented a regular variation as the magnesium content increased. The relationship between the relative mass of metal atoms and their spectral structure was shown in Fig. 10.15. Both Raman spectroscopy and infrared spectroscopy showed a similar variation tendency, that is, the vibration positions of

The Middle and Far-Infrared Spectroscopy Characteristics …

251

the spectrum all shifted to the high-frequency region as the relative atomic mass decreased (Fig. 10.15), similar to those reported previously (Morandat et al. 1967; Lane 1997; Edwards et al. 2005; Brusentsova et al. 2010). Morandat et al. pointed out that Ca with a larger mass than Mg will affect the antisymmetric stretching vibration (m3) and out-of-plane bending vibration (m2) of CO2 3 , causing the vibration position to shift to low-frequency (Fig. 10.15a) (Morandat et al. 1967). The infrared absorption spectrum in this study was consistent with Morandat et al., which showed that the m2, m3, and m4 vibrations of CO2 were 3 affected by the magnesium content increased in the mineral, and the absorption peaks were all shifted to high-frequency (Fig. 10.15b). Besides, the vibration of the Ca–O bond will absorb less energy than the Mg–O bond as the mass number increases, resulting in the far-infrared spectral position of calcite in a lower frequency region than magnesite (Fig. 10.15c) (Michalski et al. 2005), consistent with the variation in the mid-infrared region. It was worth noting that the absorption spectrum in the mid-infrared band and emission spectrum have the same vibration mode in the high, medium, and low-frequency regions. The infrared absorption in this study, however, due to the suppression effect of the remaining reflection band on the radiation and the anharmonicity of the polar vibration, the absorption band in the emission spectrum does not correspond to the strong absorption band of the fundamental frequency vibration in the absorption spectrum, on the contrary, corresponded to the medium absorption intensity of two-photon combined absorption band or including three, four, etc. multi-phonon combined absorption band. As the mass of the metal atom varied, the peak in the emission spectrum and absorption spectrum showed a consistent change trend (Fig. 10.15d). Moreover, the Raman spectra and infrared spectra of the sample showed a consistent change trend. The characteristic vibrations all shift to the low-frequency region as the

magnesium content increases. We can fast identify carbonate minerals and then reveal the evolution process and genesis of the minerals according to the spectroscopy characteristics.

10.3

10.3.4 Effect of Antisymmetric Stretching Vibration of C–O Bond on the Emissivity of Carbonate Minerals Based on the above theoretical background, combined with Figs. 10.13c and 10.14, the results show that the characteristic peaks of the absorption spectrum and the position of the absorption band of the emission spectrum are significantly affected by the CO2 3 fundamental frequency. Therefore, this article speculates that the infrared emission characteristics of the three carbonate minerals are related to the C–O bond strong absorption band corresponding to the minimum emissivity and the strongest vibration position (m3). The radiation energy spectrum and emission spectrum of calcite, dolomite, and magnesite show that the strong absorption band formed by the antisymmetric stretching vibration of the C–O bond is located in the high-frequency region of the spectrum (1300–1650 cm−1) (Fig. 10.14). Figure 10.16a shows that there is a negative correlation between the absorption band range formed by the antisymmetric stretching vibration of the C–O bond and the emissivity, indicating that the wide absorption band range of the C–O bond will cause the emissivity to decrease. Also, Fig. 10.16b shows that the antisymmetric stretching vibration of the C–O bond has a negative correlation with the sample emissivity and the lowest emissivity value. It is revealed that the emissivity of minerals is controlled by the vibration of the strongest chemical bond. In general, the absorption of C–O bonds is shown as radiation valleys in the radiation spectrum and appears as a relatively low absorption band in the emission spectrum. The infrared

Fig. 10.16 a Values of the asymmetric stretching vibration bands of C–O bond (in cm−1) versus the emissivity; b the relationship between the asymmetric stretching vibration of C–O bond (in cm−1) and emissivity (Zhu et al. 2022a, b)

252

spectrum (absorption and emission) characteristics of carbonate minerals are mainly affected by the vibration of carbon–oxygen bonds. Influence (Lane and Christensen 1997; Michalski et al. 2005, 2006). The narrower the absorption band formed by the antisymmetric stretching vibration of the C–O bond, the higher the radiant energy and emissivity, and the better the radiation performance. These findings are consistent with previous studies on the vibration of chemical bonds (Farmer 1974; Moenke 1974). According to the theory of vibrational spectroscopy, when positive and negative ions move out of phase, as long as there is a net dipole moment, energy can be absorbed in the wavelength corresponding to the frequency of the vibrational motion (Wenrich and Christensen 1996; Lane and Christensen 1997; Hamilton 2000). When ignoring the influence of environmental conditions in infrared spectroscopy, part of the energy radiated by minerals can be regarded as being absorbed by the vibration of the chemical bond, and the range of the vibration band will determine how much energy the sample absorbs. In this study, the antisymmetric stretching vibration of the C–O bond occurred at a lower frequency and a narrower absorption band, resulting in higher radiant energy.

10 Infrared Effect of Minerals

10.3.5 Influence of Crystal Structure on the Radiation Characteristics of Minerals The vibration spectrum is generated by the vibration movement at a specific frequency inside the crystal lattice, which is directly related to the crystal structure and element composition. Therefore, each mineral has different vibration absorption characteristics, forming a unique spectrum in the thermal infrared region (Farmer 1974; Wilson et al. 1980). The fundamental frequency vibration of geological samples is usually lower than 2000 cm−1. The exact frequency, shape, intensity, and number of characteristic peaks of the mineral spectrum depend on the relative mass, radius, bond angle, and bond strength between atoms of the sample (Reeder 1983; Jovanovski et al. 2002). Figure 10.17 reveals the relationship between the cation radius (dolomite takes the average of Ca and Mg) (Lide 1990), metal–oxygen bond length (dolomite is the average bond length of Ca–O and Mg–O) (Reeder 1983; Jovanovski et al. 2002), C–O bond length (Busing and Levy 1964), unit cell volume (Valenzano et al. 2007) and emissivity, exploring the relations between crystal structure and mineral radiation characteristics and

Fig. 10.17 Samples emissivity varying with a cation radius. b M–O bond length; c C–O bond length; d cell volume. Cation radius and M–O bond length of dolomite were the average value Ca and Mg (Zhu et al. 2022a, b)

10.4

Thermal Emission Spectra of Silicate Minerals

help to accurately determine the characteristic position of chemical bond vibration in the infrared emission spectrum. The ionic radius, bond length, unit cell volume, and emissivity in Fig. 10.17a–d all show a positive correlation. Calcite Ca has the largest atomic radius, the longest bond length between atoms (Ca–O and C–O bonds), and the largest unit cell volume, but its absorption band range is the smallest (Fig. 10.16a) and the C–O bond has the lowest antisymmetric stretching vibration position. (Fig. 10.16b). It shows that the antisymmetric stretching vibration of the C–O bond is affected by the larger metal ion radius, resulting in its vibration frequency appearing in the lower region of the spectrum (Lane and Christensen 1997), thereby reducing energy absorption, increasing radiant energy, and exhibiting a higher emissivity. The structural parameters of magnesite and calcite are opposite. The small ion radius, short chemical bond length, and small unit cell volume cause the antisymmetric stretching vibration of the C–O bond to appear in the higher frequency region, which increases the energy absorption and reduces the radiant energy. Thus exhibiting low emissivity. These data show that the complex interactions that occur in the crystal structure affect the fundamental vibrational frequencies of the chemical bonds of carbonate minerals. In the spectroscopic studies of different carbonate minerals, it is shown that the single physical property of the cation does not directly affect the absorption band shift (Keller et al. 1952; Adler and Kerr 1963; Gaffey 1987; Lane and Christensen 1997). Therefore, this paper found that the crystal structure of the sample (ionic radius, bond length, and unit cell volume) will affect its radiation characteristics, and large ion radius, bond length, and unit cell parameters will affect the antisymmetric stretching vibration of the C–O bond, thereby reducing Absorption of energy to enhance radiation characteristics.

10.4

Thermal Emission Spectra of Silicate Minerals

Silicate minerals are the commonest accessible materials on earth and well over 90% of the crust is made up of silicates. Thus, the spectral characteristics of silicate minerals are another important way for us to further understand the mineral evolution of silicate earth. Representative researchers have skillfully obtained mineralogical and chemical features (e.g., Si/O ratio, cation substitution, chemical bonds, vibration mode assignments, constitutional water) of different nesosilicates, cyclosilicates, inosilicates, phyllosilicates, and tectosilicates through infrared absorption spectroscopy (Launer 1952; Yariv and Heller-Kallai 1975). Within silicates, the remarkable feature was the different polymerization of SiO4 4 groups, forming versatile nesosilicates, cyclosilicates, inosilicates, phyllosilicates, and

253

tectosilicates, which should be distinguishable in middleinfrared spectroscopy. Different from the frequently-used absorption mode studied on the thermal emission spectra for identifying phases, chemical composition and bonds were limited. With decreasing Si/O ratio, the Si–O stretching vibration systematically shifted to lower wavenumbers. The ratio of Si to O, as the simple proxy of polymerization, seems one of the significant factors affecting mid-infrared emission spectral features. But obviously, more types of silicates are required besides phyllosilicates to deeply understand infrared emission spectroscopy and evaluate its resolution in silicate minerals. The aims of this work were to find out the connection among spectral characteristics, crystal structure, and infrared radiation properties of silicate minerals, and also to provide a reference for the spectroscopic study of minerals on the planetary surface beyond the earth.

10.4.1 Infrared Spectroscopy Five types of natural silicate minerals with different connection modes of silicon-oxygen tetrahedral were selected as samples, including forsterite and pyrope as nesosilicates, tourmaline, and beryl as cyclosilicates, jadeite, and tremolite as inosilicates, serpentine and montmorillonite as phyllosilicates, albite, and quartz as tectosilicates (Table 10.7). Infrared absorption spectra and specific band assignments of ten natural silicate minerals were shown in Fig. 10.18 and Table 10.8, respectively. There were several strong absorption bands regarding Si–O stretching vibration in the high-frequency region of 800–1200 cm−1 and most of them had a strong band near 1000 cm−1. In particular, the strongest absorption region nesosilicates (forsterite and pyrope) located at below 1000 cm−1, while other minerals were slightly higher. Some minor absorption bands within 650– 800 cm−1 are also related to the stretching of Si–O bonds of beryl, tourmaline, jadeite, tremolite, and quartz. Below 650 cm−1, absorption bands can be mainly assigned to Si–O bending vibration (near 460 cm−1). As for phyllosilicates, absorptions related to M–OH or M–M–OH deformations in the octahedral sheets (where M = Al3+, Fe3+, Fe2+, or Mg2+) occur from 450 to 950 cm−1. Absorptions related to M–O–Si deformation between the tetrahedral and octahedral sheets were located from 470 to 550 cm−1 (Stubičan and Roy 1961; Farmer 1974; Michalski et al. 2005). The emission spectra of all samples were also shown in Fig. 10.18, and the assignment of each emission band referring to absorption spectra was given in Table 10.8 according to previous research (Lyon 1965; Hamilton 2000; Wyatt et al. 2001; Kloprogge and Frost 2005; Michalski et al. 2005, 2006; Hecker et al. 2010). It was worth noting that there were few absorptions in the infrared emission

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10 Infrared Effect of Minerals

Table 10.7 Five types of natural silicate minerals and their origins Subclass of silicate

Minerals

Formula

Nesosilicates

Forsterite

(Mg1.816Fe0.174Mn0.005)1.995(Si0.993Al0.013)1.006O4

Henan, China

Pyrope

þ (Mg1.514 Fe21:181

Xinjiang, China

Tourmaline

þ (Na0.895K0.096)0.991(Fe22:549 Mg0.202Ti0.085Al0.145)2.981Al6.000(Si5.995Al0.005)6.000O18(BO3)3(OH)4

Hunan, China

Beryl

(Na0.357K0.019)0.376(Be2.818Li0.162)3.000(Al1.945Fe3+0.053)1.998(Si5.991Al0.009)6.000O18

Xinjiang, China

Jadeite

þ þ (Na0.991K0.003)0.994(Al0.885 Cr30:081 Fe30:039 )1.005(Si1.998Al0.002)2.000O6

Zhejiang, China

Tremolite

þ (Ca1.867Mg0.134)2.001(Mg4.961 Fe30:039 )5.000(Si7.967Al0.036)8.003O22(OH)2

Xinjiang, China

Serpentine

þ (Mg2.875 Fe20:102 Al0.030)3.007(Si1.954Al0.046)2.000O5(OH)4

Liaoning, China

Montmorillonite

(Na0.267Ca0.069K0.016)0.352(Al1.432Mn0.001Mg0.355 þ Fe30:215 )2.003(Si3.966Al0.034)4.000O10(OH)2

Jilin, China

Albite

(Na0.663Ca0.158K0.184)1.005(Al1.172Si2.829)4.001O8

Henan, China

Quartz

SiO2

Guangxi, China

Cyclosilicates

Inosilicates

Phyllosilicates

Tectosilicates

Origin

þ Ca0.276Mn0.037)3.008(Al1.993 Fe30:007 )2.000(Si2.992Al0.008)3.000O12

spectra after 1200 cm−1, which were mainly caused by the instrument and background environment (Christensen and Harrison 1993). A very large number of silicate minerals occurred in nature, which are generally difficult to be decomposed within 200 °C due to their chemical stability (Lentz 1964; Granquist 1966; Koike et al. 2006). A temperature range from 55 to 140 °C was set to investigate the dependence of radiant energy and emissivity on temperature. The radiant energy of each sample was found to have a positive correlation with the temperature (Fig. 10.19), which followed Stefan-Boltzmann's law that a higher temperature will cause higher radiant energy. The average emissivity of each sample was the arithmetical mean of emissivity values obtained at different temperatures (Table 10.9; Fig. 10.20), which was able to reflect the overall radiation within the selected wavelength range. In particular, pyrope presented the highest average emissivity (0.981), while quartz was the lowest (0.913). The emissivity varying with temperatures confirmed that the infrared emission properties of nesosilicates were prominent and more stable than other silicate minerals in this study (Table 10.9; Fig. 10.20). These findings were consistent with the change of radiant energy (Fig. 10.19).

10.4.2 Comparison of Absorption and Emission Bands of Silicate Minerals Mineralogically, infrared absorption spectra provided the same information about materials as emission spectra, but

the positions of absorption bands were shifted because absorption spectral features fundamentally depend only on the absorption coefficient (k) whereas emission spectral features depend on both k and the index of refraction (n) (Koike et al. 1989; Wenrich and Christensen 1996; Michalski et al. 2005). Therefore, there must be a difference in wavenumbers between absorption bands and corresponding emission bands. Based on the well-known absorption band assignments, we were able to make similar assignments for the emission spectra (Table 10.8). To obtain a clearer comparison between absorption and emission bands, their wavenumbers which belong to the same vibration features were compiled into Fig. 10.21. As expected, a good linear relationship (R2 = 0.996) between them suggested emission bands of silicate minerals have great correspondence with their absorption bands. The obtained slope of the fitting line (0.93), less than 1, indicated wavenumbers in absorption spectra were slightly more than corresponding wavenumbers in emission spectra for silicate minerals. Within silicates, the exact placement of the absorption bands was a complex function of the chemistry of SiO4 tetrahedrons and the symmetry of crystal structure (Michalski et al. 2005). And the primary spectral absorptions (reststrahlen bands) ranging from 800 to 1300 cm−1 were due to the stretching and bending motions of Si–O bonds in silica tetrahedrons, while other absorption features lower than 800 cm−1 result from metal–oxygen and lattice vibrations (Kieffer 1979b). These abundant theories made the connection of structural information to both absorption and

10.4

Thermal Emission Spectra of Silicate Minerals

255

Fig. 10.18 Infrared spectra of silicate minerals (Black and gray solid curves represented the infrared absorption and emission spectra, respectively) (Zhu et al. 2022a, b)

256

10 Infrared Effect of Minerals

Table 10.8 Band assignments for absorption spectra and emission spectra in wavenumbers Samples

Absorption abs

Emission abs

Absorption assignment

References

Forsterite

984

1055

Si–O stretching

885

947

Si–O stretching

Launer (1952), Hofmeister et al. (1987), Koike et al. (2006)

839

876

Si–O stretching

505

519

Si–O bending

417

–

Si–O bending

961

979

Si–O stretching

897

898

Si–O stretching

872

866

Si–O stretching

573

574

Si–O bending

482

506

Si–O bending

416

–

Si–O bending M–O vibrating

Pyrope

Tourmaline

Beryl

Jadeite

Tremolite

388

–

1086

1097

Si–O stretching

1043

1041

Si–O stretching

981

987

Si–O stretching

819

850

Si–O stretching

716

706

Si–O stretching

505

519

Si–O bending

433

–

Si–O bending

1202

1219

Si–O stretching

1020

1024

Si–O stretching

960

986

Si–O stretching

810

852

Si–O stretching

740

745

Be–O stretching

676

683

Si–O bending

592

597

Si–O bending

524

531

Si–O bending

492

495

Si–O bending

436

447

[SiO4] rotating

1067

1140

Si–O stretching

991

1084

Si–O stretching

926

968

Si–O stretching

856

856

Si–O stretching

665

745

O–Si–O stretching

592

667

Si–O bending

516

528

Si–O bending

465

476

Si–O bending

397

408

M–O translating

1107

1143

Si–O stretching

1065

1120

Si–O stretching

1003

1040

Si–O stretching

951

959

Si–O stretching

758

850

Si–O stretching

687

745

Si–O stretching

511

520

Si–O bending

465

466

Si–O bending

392

403

M–O translating

Omori (1971)

Reddy et al. (2007)

Hofmeister et al. (1987)

Saksena (1961)

Patterson and O’Connor (1966)

(continued)

10.4

Thermal Emission Spectra of Silicate Minerals

257

Table 10.8 (continued) Samples

Absorption abs

Emission abs

Absorption assignment

References

Serpentine

1074

–

Si–O stretching

Farmer (1974), Yariv and Heller-Kallai (1975)

988

1038

Si–O stretching

885

851

Si–O stretching

Montmorillonite

Albite

Quartz

621

641

O–H bending

559

582

Si–O bending

447

475

Mg–OH

402

425

Si–O bending

1055

1070

Si–O stretching

914

918

Si–O stretching

872

853

Si–O stretching

799

806

O–H bending

610

619

O–H bending

527

530

Si–O bending

467

474

Si–O–Si bending

–

425

Si–O bending

1147

1169

Si–O stretching

1094

1113

Si–O stretching

1038

1040

Si–O stretching

997

1000

Si–O stretching

787

846

Si–O stretching

760

771

Si–Si stretching

650

656

Si–O bending

592

598

Si–O bending

532

536

Si–O bending

463

480

Si–O–Si bending

418

422

Si–O–Si(Al) bending

376

–

M–O stretching

1173

1179

Si–O stretching

1084

1116

Si–O stretching

787

800

Si–O stretching

690

646

Si–O bending

517

540

Si–O bending

467

478

Si–O bending

401

417

Si–O bending

emission bands become a reality. Vibrational absorption spectra of five different types of silicate minerals with different degrees of polymerization exhibited significant variations in absorption and emission bands due to both structural and compositional differences (Fig. 10.18; Table 10.8). Two dominant and representative bands in the range of 850–1200 and 400–650 cm−1, related to stretching and bending vibration of Si–O, respectively, were investigated for their variation with the polymerization degree of SiO4 tetrahedrons. In Fig. 10.22a, it was obvious that the absorption bands of ten samples in this study linearly shift to the high-frequency region as the increment of the ratio of Si

Lerot and Low (1976), Bishop et al. (2002)

Chihara and Koike (2017)

Salisbury et al. (1987)

to O, which was a proxy of the polymerization degree of SiO4. Hence, the position of Si–O stretching and bending vibration in the SiO4 tetrahedron can be controlled by the Si/O ratio to some extent, and a higher polymerization degree of SiO4 gave rise to enhanced energy of Si–O bonds and thus stronger vibration of Si–O bond. The same conclusion can be drawn for emission spectra according to Fig. 10.22b. For example, the most dominant Si–O stretching vibration in the emission spectrum of quartz (Si/O = 0.5) centered at 1116 cm−1, whereas that for serpentine (Si/O = 0.4), jadeite (Si/O = 0.33), and forsterite (Si/O = 0.25) was 1038, 968, and 947 cm−1, respectively. In turn, the rough

258 Fig. 10.19 Infrared radiation energy spectrum of silicate minerals from 50 to 140 °C (Solid lines of different colors represented the radiant energy at different temperatures)

10 Infrared Effect of Minerals

10.4

Thermal Emission Spectra of Silicate Minerals

259

Table 10.9 Silicate minerals emissivity from 50 to 140 °C Temperature/ °C

Forsterite

Pyrope

Tourmaline

Beryl

Jadeite

Tremolite

Serpentine

Montmorillonite

Albite

Quartz

50

0.9827

0.9787

0.9796

0.9740

0.9606

0.9513

0.9467

0.9317

0.9215

0.9160

65

0.9816

0.9826

0.9760

0.9791

0.9627

0.9524

0.9472

0.9339

0.9264

0.9109

80

0.9823

0.9809

0.9795

0.9815

0.9656

0.9580

0.9448

0.9356

0.9209

0.9131

95

0.9776

0.9803

0.9729

0.9834

0.9683

0.9571

0.9463

0.9407

0.9159

0.9155

110

0.9802

0.9785

0.9731

0.9784

0.9610

0.9555

0.9455

0.9404

0.9142

0.9100

125

0.9768

0.9840

0.9725

0.9795

0.9681

0.9561

0.9386

0.9391

0.9149

0.9182

140

0.9778

0.9786

0.9767

0.9811

0.9633

0.9503

0.9394

0.9310

0.9181

0.9044

Average emissivity

0.9799

0.9805

0.9757

0.9796

0.9642

0.9544

0.9441

0.9361

0.9189

0.9126

Standard deviation

0.0023

0.0020

0.0028

0.0027

0.0029

0.0028

0.0033

0.0037

0.0040

0.0043

10.4.3 Effect of Vibrating SiO4 Tetrahedron on Infrared Radiation Properties

Fig. 10.20 The emissivity of ten samples (five subclasses) varying with temperature from 50 to 140 °C (Zhu et al. 2022a, b)

Fig. 10.21 The wavenumber comparison of absorption and emission bands of silicate minerals used in this work (Zhu et al. 2022a, b)

prediction or identification of polymerization degree for SiO4 could be realizable according to linear relation in Fig. 10.22 Similar results were obtained in previous work using fewer silicate minerals (Michalski et al. 2005).

A precise model of radiative and conductive energy transfer was presented by Hapke that further described the relationship among several variates including spectral radiance, thermal conductivity, wavelength, spectral irradiance from the source, volume extinction coefficient, spectral volume absorption coefficient, etc. (Hapke 1996). As for silicate minerals, particularly, the effect of SiO4 tetrahedron on infrared radiation properties must be dominated because it formed a fundamental framework in its spectral structure (Hamilton 2000; Michalski et al. 2006). Therefore, several factors about vibrating SiO4 tetrahedron as critical internal causes were considered their roles in controlling average emissivity. The polymerization degree of SiO4 tetrahedrons, obviously, is worthy of consideration because it’s the essence of the difference between one type of silicate mineral and the other type. Moreover, the polymerization degree of silicate minerals has confirmed its tightly close relationship with absorption and emission bands, according to Fig. 10.21. The Si/O ratio of each mineral and corresponding average emissivity were plotted in Fig. 10.23a. It illustrated that a higher polymerization degree of silicate minerals brought about lower emissivity. For example, SiO4 tetrahedrons in nesosilicate pyrope were entirely depolymerized (Si/O = 0.25), which had the highest average emissivity of 0.981 (Fig. 10.23a). According to the positive correlation between polymerization degree and absorption/emission bands (Fig. 10.22), a lower polymerization degree can result in weaker Si–O bonds, smaller vibrational frequencies, and lower absorbed energy for vibration. It was obvious that if less energy for the vibration of chemical bonds requires, more energy will be emitted. It should be pointed out that Si–O stretching absorption bands in the region of 800–1300 cm−1 were overwhelmingly

260

10 Infrared Effect of Minerals

Fig. 10.22 The Si/O ratio versus dominant stretching and bending bands. a Absorption spectra (R2 equals 0.86 and 0.65 for stretching and bending vibration, respectively); b emission spectra (R2 equals 0.80 and

0.77 for stretching and bending vibration, respectively) (Zhu et al. 2022a, b)

Fig. 10.23 Average emissivity of silicate minerals linearly varied with a Si/O ratio; b dominant stretching vibration in emission spectra (cm−1); c vibration range of Si–O stretching; d vibration range of Si–O

bending. Black solid line represented the fitting results, whose R2 was 0.86, 0.88, 0.78 and 0.98, respectively(Zhu et al. 2022a, b)

dominant, which gave rise to an emissivity minimum in IR emission spectra (Fig. 10.8). Here, to present the effect of the Si–O stretching absorption band on emissivity, two factors including the wavenumber at dominant stretching vibration and the vibration range were chosen to investigate

their correlation with emissivity. It showed that the silicate with weaker Si–O stretching vibration (e.g., *950 cm−1 for nesosilicates) had higher emissivity (Fig. 10.23b), which could be interpreted that less absorbed energy for Si–O vibration brought about more emitted energy. The average

10.4

Thermal Emission Spectra of Silicate Minerals

261

Fig. 10.24 The emission spectrum of ten samples at the range of 800–1300 cm−1 (The stretching vibration bands of Si– O bond was calculated by the difference between the value of the arrow point)

emissivity was plotted versus the vibration range of Si–O stretching in Fig. 10.23c, which illustrated that wider absorption ranges for Si–O stretching vibration negatively affected the emission of energy. A narrower vibration range of Si–O bond (like nesosilicates less than 300 cm−1, see details in Fig. 10.24) would result in a higher emissivity than those with a wider range (like tectosilicate near 440 cm−1). A narrow band range indicated that molecular vibrations absorbed a small amount of energy. Therefore, vibration range, similar to vibration frequency, could have a counter-regulation effect on emitted energy. The bending vibration was another concern that could play a role in radiation performance since it was secondary energy (400–650 cm−1) that should be considered besides the most dominant stretching vibration (Fig. 10.25). Figure 10.23d showed the same negative relationship between vibration range for Si–O–Si bending and corresponding emissivity (The determination of that range was shown in

Fig. 10.25). As discussed above, a narrower band suggested less absorbed energy, which was conducive to an enhanced emitted energy.

10.4.4 Geologic Implications Silicate minerals vastly exist on solid planets, especially a large number of silicate minerals that have been discovered on Mars from the Thermal Emission Spectrometer (TES) in recent years (Christensen et al. 2000; Hamilton et al. 2001; Michalski et al. 2005, 2017). The TES aboard the Mars Global Surveyor spacecraft had returned global thermal infrared data that provided mineralogical information about surface materials, showing there were various silicate minerals dominated by clay minerals. Furthermore, a wealth of excellent laboratory spectra that provide a basic understanding of thermal IR spectral features has been obtained from

262

10 Infrared Effect of Minerals

Fig. 10.25 The emission spectrum of ten samples at the range of 400–650 cm−1 (The bending vibration bands of Si–O bond was calculated by the difference between the value of the arrow point)

researchers (Lyon 1965; Farmer 1974; Walter and Salisbury 1989; Salisbury and D’Aria 1992). It has always been known that thermal IR spectroscopy has been used to study the composition of geologic materials that produce spectral features in the range from 6 to 50 lm, with particular emphasis on remotely obtained emission spectra of natural surfaces (Kahle and Goetz 1983; Gillespie et al. 1984; Crisp et al. 1990; Weitz and Farr 1992). And therefore the mineralogical characteristics of silicate minerals were most important for remote geological studies using thermal emission spectra (Christensen et al. 1992). This work, firstly systematically compared the mid-infrared absorption and emission spectra of five different silicate samples and determined their emissivity values as well as influence factors. According to present data, the relationship of absorbed and emitted energy, which were adjusted by polymerization degrees and SiO4 tetrahedrons and vibrating chemical bonds inside, has been revealed to some extent. This provided a wider understanding of the spectral characteristic (infrared absorption spectrum and infrared emission spectrum) of various silicate rocks and would help in better recognizing the composition of planetary surfaces and potentially determine physical, chemical conditions, and petrogenetic processes of planetary surfaces.

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