Fluoride: Properties, Applications and Environmental Management: Properties, Applications and Environmental Management [1 ed.] 9781620818404, 9781612093932

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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science Publishers,

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

FLUORIDE: PROPERTIES, APPLICATIONS AND ENVIRONMENTAL MANAGEMENT

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Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

FLUORIDE: PROPERTIES, APPLICATIONS AND ENVIRONMENTAL MANAGEMENT

STANLEY D. MONROY Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

EDITOR

Nova Science Publishers, Inc. New York

Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Fluoride : properties, applications, and environmental management / [edited by] Stanley D. Monroy. p. cm. Includes index. ISBN: (eBook) 1. Fluorides--Environmental aspects. 2. Goundwater--Pollution. I. Monroy, Stanley D. TD427.F54F58 2010 363.738'49--dc22 2010051531

Published by Nova Science Publishers, Inc.  New York

Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

CONTENTS Preface Chapter 1

Growth of Single and Mixed Alkali Earth Fluoride Crystals with Wide Application: Control of Crystallization Zone Contamination and Stability of Melt-Crystal Interface J.T. Mouchovski and B. Mullin

Chapter 2

Fluoride Content in Central and Southeast Argentinean Groundwaters M. L.Gomez and O. M. Quiroz Londoño

Chapter 3

Fluoride in Groundwater: Causes, Implications and Mitigation Measures K. Brindha and L. Elango

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vii

Chapter 5

Occurrence, Distribution and Mechanism of Fluoride Release in Ground Water: a Case Study Mitali Sarkar, Aparna Banerjee, Partha Pratim Pramanick and Asit R. Sarkar Fluoride Contamination of Water: Origin, Health Effects and Remediation Methods Sujata Mandal and S. Mayadevi

1 93

111

137

159

Chapter 6

Fluoride in Groundwater in Nigeria: Origin and Health Impact U.A. Lar, H. Dibal and K. Schoeneich

Chapter 7

Hydrochemical Characterization of Fluoride Rich Groundwater: A Case Study Mouna Ketata, Moncef Gueddari and Rachida Bouhlila

183

The Reliability of the Grab Sample for the Determination of Fluoride in Potable Water Supplies J. A. Armstrong and S. A. Katz

207

Pulsed Laser Ablation and Deposition of Thin Films of Rare Earth Ions-Doped Fluorides P. Bicchi, M. Anwar-ul-Haq and S. Barsanti

215

Chapter 8

Chapter 9

Index Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

177

241

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PREFACE This new book presents topical research in the study of fluoride and its applications, properties and in environmental management. Topics discussed include advances in the use of alkali-earth metal fluorides as optical materials; the fluoride content in the groundwaters of central and southeast Argentina and Nigeria; the reliability of the grab sample for the determination of fluoride in potable water supplies and the production of thin films of Nd3+ doped fluorides via the pulsed laser deposition technique Chapter 1- The advance and current state for alkali-earth metal fluorides – single or solid solutions with a diverse range of compositions – as optical materials used for manufacturing of various components in large gamma of devices with wide applicability from deep ultra violet to middle infrared optics are reviewed by outlining the basic technological issues related to production of crystal with minimal structural defects and excellent physicalchemical properties. The control of the two key factors determining crystal quality – the shape and position of crystallization front (CF) and the purity of molten material and its environmental – are put under discussion. It is shown such complex control can be implemented effectively by applying a combined purification/growing technique that provides appropriate growing conditions in crystallization zone (CZ) with minimum contamination. The technique is especially effective when fluorspar is used as starting material since it allows to be reduced substantially the level of rare earth impurities embedded into fluorite lattice, minimizing at the same time the oxygen containing contaminants in the CZ. The high-T purification precedes and continues during the growing itself. Specially designed crucible is utilized for the purpose wherein the molten portions of material are being imposed to temperature head (overheating) initiating intensive melts vaporization. At that the final products are several impoverished to RE boules and crystallized deposit enriching to RE. The release of the CZ from oxygen contaminants are guarantee by using optimized amount of scavenger (PbF2) added to starting material while their penetration from the ambient are marginalized by appropriate choice of the sizes for the system of channels related the crucible cameras to evacuated environment. The efficiency of purification is being attained owing to strict mathematical description of interrelating phenomena: ionic adsorption, evaporation, and phase transition; melt boiling and deposition; mass-transport processes in gas-vapor ambient. The requirements for normal growth by slightly convex CF-shape are attained by effective regulation of the temperature field into the load for compensating the different CF-shift in particular cameras during crucible withdrawal throughout a sufficiently broadly designed adiabatic furnace zone (AdZ). The preciseness of CF-adjustment is provided by introduction

Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

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Stanley D. Monroy

of additional system of molybdenum shields into the furnace unit that simplifies the derived model equation. Appropriate modes are derived to provision a simultaneous growth of boules with tailored properties. The exploration is supported by plenty of experimental data. Definite advantages of the technique are discussed that show it may complement the existing ones or replaced them effectively. Substantial advantages are a reduced risk for operators’ health and minimum contamination of the environmental. Chapter 2- Naturally occurring fluoride in groundwater is an important aspect in the central and southeast sector of Argentine. Former investigations have demonstrated that volcanic glass dissolution disseminated in the loess-like sediments is the main source of fluoride in the Argentine pampas. Nevertheless groundwater fluoride distribution is erratic and the controlling factors of it are not well known. Rural and urban activities in these zones rely exclusively on the exploitation of groundwater and in many cases it is the only source of drinking water. For these reasons, fluoride content in groundwater is a sanitary problem which needs special attention since several fluorosis cases have been detected in Córdoba Province. The aim of this study is to analyze the geochemical conditions associated with the presence of fluoride (F-) in the phreatic aquifer in three areas in the central and southeast sectors of the Argentine Chacopampean plain. Two study areas are located in the south of Córdoba province, involving 1,040 km2. Aquifers in these zones are mainly composed of silty sand sediments of aeolian origin, typically loess-like sediments of Holocene, and are situated near igneous-metamorphic basement rocks of the Paleozoic. The other study area is located in the north west of the intermountainous plain at Buenos Aires province, involving more that 2,760 km2. It extends between two low hills ranges of Precambrian metamorphic rocks and sedimentary Paleozoic rocks, and it is filled by a thick sequence of Cenozoic sediments, mainly silts and silty-clayed, with sand layers intercalated. High concentrations of F (0 - 18 mg.l-1) in groundwater were detected in the three study areas. More than 80 % of domestic wells exceed the drinking water limit of Argentine Law (1.3 mg.l-1). Hydrogeochemical data indicates a high relationship between sodium bicarbonate waters and the highest pH values. There was a high correlation between F- and As(Total), and F- and Na+/Ca+2 ratio. Saturation indexes indicate that dissolution are the main processes that control F contents. Fluorite saturation index showed that fluorite saturation is reached just in few samples. In some areas F- distribution appears to be mainly controlled by a general salinity increase and the proximity of Paleozoic rocks containing minerals with F- contents. Sediment compositions and hydrogeochemical conditions are the main factors in determining the F concentration. The composition and texture of loess, low permeability and hydraulic gradients, mineralogical composition of the basement rocks together with sodium bicarbonate watertypes are proper conditions for fluoride mobilization in groundwater in central and east sectors of Argentina. Chapter 3- Groundwater is the major source for various purposes in most parts of the world. Presence of low or high concentration of certain ions is a major issue as they make the groundwater unsuitable for various purposes. Fluoride is one such ion that causes health problems in people living in more than 25 nations around the world. Fluoride concentration of atleast 0.6 mg/l is required for human consumption as it will help to have stronger teeth and bones. Consumption of water with fluoride concentration above 1.5 mg/l results in acute to chronic dental fluorosis where the tooth become coloured from yellow to brown. Skeletal

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Preface

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fluorosis which causes weakness and bending of the bones also results due to long term consumption of water containing high fluoride. Presence of low or high concentration of fluoride in groundwater is because of natural or anthropogenic causes or a combination of both. Natural sources are associated to the geological conditions of an area. Several rocks have fluoride bearing minerals like apatite, fluorite, biotite and hornblende. The weathering of these rocks and infiltration of rainfall through it increases fluoride concentration in groundwater. Fluoride which is present in high concentration in volcanic ash is readily soluble in water and forms another natural source. Anthropogenic sources of fluoride include agricultural fertilisers and combustion of coal. Phosphate fertilisers contribute to fluoride in irrigation lands. Coal which is a potential source of fluoride is used for combustion in various industries and in brick kilns. The aerial emission of fluoride in gaseous form during these activities reaches the surface by fall out of particulate fluorides and during rainfall they percolate with the rainwater thus reaching the groundwater table. Also the improper disposal of fly ash on ground surface contributes to fluoride in groundwater. Since ingestion of high fluoride has a long term effect on human health it is essential to monitor its concentration in groundwater used for drinking periodically and take steps to bring them within the permissible range of 0.6 to 1.5 mg/l. There are several methods available for the removal of fluoride from groundwater which is insitu or exsitu. To dilute the groundwater contaminated with fluoride, artificial recharging structures can be built in suitable places which will decrease its concentration. Rainwater harvesting through existing wells also will prove effective to reduce the groundwater fluoride concentration. Exsitu methods which are conventional treatment methods like adsorption, ion exchange, reverse osmosis, electrodialysis, coagulation and precipitation etc can be practiced at community level or at households to reduce fluoride concentration before ingestion. But the choice of each method depends on the local conditions of the region such as the quality of groundwater and the source of contamination whether it is natural or anthropogenic. Fluoride contamination being a prominent and widespread problem in several parts of the world and as causes for this are mostly natural and unpreventable, educating the people and defluorinating the groundwater before consumption are essential for a healthy world. Chapter 4- Fluoride is a persistent and non-degradable poison that accumulates in soil, plants, living organisms and acts as a potential environmental hazard. It is an essential micro nutrient and known to prevent tooth decay up to a certain concentration. However, prolonged ingestion in moderate to high dose results damage to human biological systems, even at molecular level leading to serious health disorders known as fluorosis. The physiopathology is believed to be complex one and is cumulative in respect to level and duration of exposure as well as sex and age. Several hundred million people in the world, at present, are either suffering from fluorosis or estimated to be at risk. The occurrence of fluoride at elevated concentrations in different environmental compartments in almost all parts of the world poses a real threat to life on the earth. Fluoride may release into the aquatic environment by anthropogenic and natural sources. Ground water percolating through fluoride-bearing rocks and minerals becomes fluoride enriched. In India, fluoride appears mostly in the hard rock areas, both in shallow and deeper water zones. The governing mechanisms are decomposition/dissociation, dissolution/enrichment and leaching/mobilization. The present report highlights the occurrence, distribution of fluoride in ground water in general and correlation of elevated fluoride level with ground water quality in a typical fluoride prone study area.

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Chapter 5- Fluorine is the 13th most abundant element in the earth’s crust. It exists in trace amounts in ground water all over the world. Drinking water is the primary source through which fluoride enters the human body, especially in regions where fluoride concentrations in groundwater and/or surface water are high. It is estimated that more than 200 million people worldwide depend on drinking water with fluoride concentration exceeding the present World Health Organization (WHO) guideline (Maximum contaminant level of 1.5 mg/l). Fluoride bearing foodstuffs and fumes from burning of coal also significantly contribute to the daily intake of people in some regions. Prolonged consumption of excess fluoride may lead to different types of fluorosis (dental and skeletal) depending on the level and period of exposure. Presence of fluoride in drinking water above permissible level has been related to increased incidence of fluorosis among the people all over the world including China, India, Australia, Mexico, Argentina, Egypt and Kenya. In India, the problem of excessive fluoride in groundwater was first reported in 1937 in the state of Andhra Pradesh. More than 6 million people all over India are known to be seriously affected by fluorosis and another 62 million are exposed to it. The best choice for combating fluorosis is to have alternative source of water with low fluoride level. In absence of alternative source of water, defluoridation of excess fluoride in water is the only option. Different methods are available for defluoridation of water. But the selection of the appropriate method for achieving a sustainable solution to the fluorosis problem is very important. Defluoridation of drinking water by adsorption is the most simple and effective technology that can work in household as well as community level drinking water treatment. It is also the most widely studied method for defluoridation of water, as the fluoride concentration in groundwater is usually very low ( 10 mg/l). A wide variety of adsorbents have been explored for this purpose. Synthetic layered double hydroxides are comparatively new materials examined for the adsorption of fluorides and they exhibit good fluoride adsorption capacity. A discussion on various defluoridation methods, adsorbents for defluoridation and recent developments are presented in this chapter Chapter 6- Until recently, the mottling and staining of teeth (dental caries) was believed to be an identity of certain ethnic groups or communities in Nigeria. Those born and reared locally within such communities had mottling teeth and fluoride as the causal factor was not known then. It was sooner discovered that dental caries extended beyond tribal or communal barriers. Even foreigners that came from far away Asia presented this disease condition. Dental caries is endemic and spreads over a large range of superficial area mainly the north eastern half of Nigeria both in the crystalline basement and sedimentary areas. The few data available on fluoride in drinking water clearly establishes the relationship between dental caries and environmental fluoride in drinking water. With the failure of the water supply systems in most parts of Nigeria to meet the demand of the increasing human population, about 90% of people use groundwater (well and borehole) for drinking and other domestic purposes. Studies have shown that, fluoride values of 0.2 – 8 mg/l above the 1.5 mg/l WHO admissible value have been recorded in the groundwater from the crystalline Basement aquifer (consisting of granites, gneisses, and migmatites). In the sedimentary aquifer, fluoride values of between 1mg/l to 4 mg/l have been recorded and especially in the Benue trough, there is a direct link between these fluoride values and the incidence of dental caries all along the 1000 m N-S long trough. It is difficult to detect the presence of fluoride in drinking water because it has no taste. Its presence is revealed only when it has caused a significant spread of the disease fluorosis. Thus, a lot of awareness campaign still needs to be done on the health

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implications of drinking of fluoride-rich waters and to debunk the belief of its association to certain tribes or communities. Chapter 7- Serious problems are faced in several parts of the world due to the presence of high concentration of fluoride in drinking water. Fluoride is known to have both beneficial and adverse effects on humans, depending on the total intake. Fluoride can be beneficial in helping to prevent dental caries at drinking water concentrations of about 1 mg/L but it has also been shown to cause dental mottling and adverse effects on bone, including increased risk of fracture at concentrations in excess of 1.5 mg/L, with the risk gradually increasing with the total intake of fluoride. Naturally occurring high fluoride levels in groundwater is a complicated issue for drinking water providers in many regions of the world. Presence of fluoride bearing minerals in the host rock, the chemical properties like decomposition, dissociation and dissolution and their interaction with water is considered to be the main cause for fluoride in groundwater. Chemical weathering under arid to semi-arid conditions with relatively high alkalinity favours high concentration of fluoride in groundwater. According to the WHO health-based guideline waters containing more than 1.5 mg/L are unsuitable for drinking use. In Tunisia dental and skeletal fluorosis are prelevant particulary in the south part of the country, wich can be related to the usage of high fluoride groundwater for drinking. In recent years the acquisition of considerable additional data on the hydrogeochemical behavior of fluoride in groundwaters of Tunisia has been made possible by extensive groundwater sampling campaigns as well as by improvements in analytical techniques. The study is an attempt to assess the hydrogeochemistry of groundwater in Gabes-south aquifer located in the southeastern part of Tunisia with a focus on fluoride occurrence. In order to reach this objective, this chapter presents a synthesis of the data so far obtained on the sources and distribution of fluoride in southeastern Tunisia, examines the extent of fluorine toxicity and puts forward recommendations to combat or minimize the problem. Results of the chemical data of the groundwater suggest that the considerable number of groundwater samples collected from the studied aquifer show fluoride content greater than that of the safe limit prescribed for drinking purpose. Fluoride may essentially be from a natural origin. Limestone (chalky) and marly formations contain significant quantities of fluoride which can be liberated by the water–rock interaction. In general, the mineralogy of the bedrock is the primary source of fluoride in groundwater and is responsible for the difference of fluoride concentration of groundwater between different bedrock types. Dissolution of Fluorite (CaF2) is a plausible source of fluoride ion in groundwater. The suggested remedial measures to reduce fluoride pollution in groundwater It is recommended that water with high fluoride levels should be defluoridated or alternative sources with low fluoride should be identified. Chapter 8- Water samples were collected three times each week for five weeks from the potable water supplies at domestic sites in five Southern New Jersey communities. The sampling regimen was designed to reflect periods of high and low water usage. None of the water supplies was fluoridated, but fluoride is known to be a naturally – occurring water contaminant in some Southern New Jersey domestic water supplies. The fluoride ion concentrations of the samples were determined by ion selective electrode potentiometry without prior distillation. The analytical error associated with the method was evaluated by repetitive measurements on reference samples, and the sampling errors were identified by statistical analysis of the results obtained from the samples.

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The statistical analysis of the data showed the fluoride concentrations varied from site to site and from time to time. In some cases, the time – to – time variations exceeded the “two sigma” limits at a given site. This observation raises a serious question about the reliability of a grab sample for the determination of fluoride, and possibly other contaminants, in potable water supplies. Chapter 9- This chapter describes the production of thin films of Nd3+-doped fluorides of high optical quality, to be used as devices in non linear optics, via the pulsed laser deposition technique. The innovative aspect of this research is the use of the same monocrystalline undoped fluoride as the substrate for the deposition. The technique is briefly recalled and the experimental apparatus is described in some detail. Two different kinds of Nd3+-doped fluoride films are described, namely Nd3+:YF3 and Nd3+:LiYF4, can be obtained on a pure LiYF4 substrate from the ablation of a Nd3+:LiYF4 bulk crystal by changing some ablation/deposition parameters such as the substrate temperature and the presence or absence of a buffer gas in the ablation/deposition chamber. The onset of a film on the substrate is checked by interferometric measurements. The optical characterization of the films includes both polarized laser induced fluorescence spectroscopy analysis, which testifies of the kind of fluoride film produced, and the 4F3/2 Nd3+ manifold lifetime measurement, which is related to the Nd3+ ions concentration. Both the laser induced fluorescence spectra and the lifetime measurements are compared with the corresponding ones obtained in the bulk crystal. Some data on the films thickness and on their morphology are also presented together with some checks on the ablation plume constituents and on their expansion dynamics.

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In: Fluoride: Properties, Applications and Environmental … ISBN: 978-1-61209-393-2 Editor: Stanley D. Monroy, pp. 1-92 © 2011 Nova Science Publishers, Inc.

Chapter 1

GROWTH OF SINGLE AND MIXED ALKALI EARTH FLUORIDE CRYSTALS WITH WIDE APPLICATION: CONTROL OF CRYSTALLIZATION ZONE CONTAMINATION AND STABILITY OF MELT-CRYSTAL INTERFACE J.T. Mouchovski and B. Mullin Institute of Mineralogy and Crystallography, ‘‘Acad. Ivan Kostov’’, Bulgarian Academy of Sciences, ‘‘Acad. G. Bonchev’’ Str., Bl. 107, 1113 Sofia, Bulgaria

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ABSTRACT The advance and current state for alkali-earth metal fluorides – single or solid solutions with a diverse range of compositions – as optical materials used for manufacturing of various components in large gamma of devices with wide applicability from deep ultra violet to middle infrared optics are reviewed by outlining the basic technological issues related to production of crystal with minimal structural defects and excellent physical-chemical properties. The control of the two key factors determining crystal quality – the shape and position of crystallization front (CF) and the purity of molten material and its environmental – are put under discussion. It is shown such complex control can be implemented effectively by applying a combined purification/growing technique that provides appropriate growing conditions in crystallization zone (CZ) with minimum contamination. The technique is especially effective when fluorspar is used as starting material since it allows to be reduced substantially the level of rare earth impurities embedded into fluorite lattice, minimizing at the same time the oxygen containing contaminants in the CZ. The high-T purification precedes and continues during the growing itself. Specially designed crucible is utilized for the purpose wherein the molten portions of material are being imposed to temperature head (overheating) initiating intensive melts vaporization. At that the final products are several impoverished to RE boules and crystallized deposit enriching to RE. The release of the CZ from oxygen contaminants are guarantee by using optimized amount of scavenger (PbF2) added to starting material while their penetration from the ambient are marginalized by appropriate choice of the sizes for the system of channels related the

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2

J.T. Mouchovski and B. Mullin crucible cameras to evacuated environment. The efficiency of purification is being attained owing to strict mathematical description of interrelating phenomena: ionic adsorption, evaporation, and phase transition; melt boiling and deposition; mass-transport processes in gas-vapor ambient. The requirements for normal growth by slightly convex CF-shape are attained by effective regulation of the temperature field into the load for compensating the different CF-shift in particular cameras during crucible withdrawal throughout a sufficiently broadly designed adiabatic furnace zone (AdZ). The preciseness of CF-adjustment is provided by introduction of additional system of molybdenum shields into the furnace unit that simplifies the derived model equation. Appropriate modes are derived to provision a simultaneous growth of boules with tailored properties. The exploration is supported by plenty of experimental data. Definite advantages of the technique are discussed that show it may complement the existing ones or replaced them effectively. Substantial advantages are a reduced risk for operators’ health and minimum contamination of the environmental.

PACS: 81.10.Eq; 42.70-a; 61.72.Ji Keywords: A1. Characterization; A1. Contamination control; A2. Melt growth control; B1. Single and mixed fluoride crystals; B2. Optical materials.

1. INTRODUCTION

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1.1. Single Alkali Earth Fluorides: CaF2 Synthetic optical crystals became necessary when the measurement and discrimination of electromagnetic radiation moved beyond the visible spectrum (390-770 nm) into the ultraviolet (UV) and the infrared (IR). The single fluorides of the alkali-earth metals (CaF2, BaF2 and SrF2) and their mixed compounds (Ca1-xSrxF2 and Ca1-yBayF2) possess a high cubic crystal symmetry structure that determines the isotropic optical properties of these materials. Amongst these crystals, artificial crystalline calcium fluoride was distinguished, soon after its discovery in the late 1930s, by virtue of its unique optical, physical, and mechanical characteristics: a broad range of light transmission from the far UV up to the middle IR, within which range the refractive index varies less than that of the other fluoride crystals. During this period optical CaF2 was developed with superior dispersion characteristics, controllably low stress-induced birefringence, negligible solubility in water and insolubility in many acids, adequate hardness and a fair thermal conductivity. A high resistance to radiation at a low luminescence level completes the superior characteristics of CaF2 and determines its wide exclusive applicability for manufacturing various optical elements. The products, windows, lenses, mirrors, prisms and others find application in vacuum UV- (VUV-), UV-, IR-, and laser optics, holography and dosimetry, lithography, materials processing and other uses. An important circumstantial aspect for CaF2 gaining serious advantage over the other two alkali-earth fluorides with fluorite structure (BaF2 and SrF2) appears to be its availability from natural sources in a usable optical form as the mineral fluorite [1,2]. As a consequence it is understandable that natural calcium fluoride was developed and evolved into the major production infrastructure as long ago as the mid-1970s. After long-lasting production development research in the USA [3,4] and in the former SSSR [5-8], the second half of

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

3

1990s marked a trend for new application of UV CaF2 lenses in their use in semiconductor exposure systems [9,10]. This stimulated the big companies, such as the leaders in crystal growth Bicorn, Schott Litotes, Schott Glass, Carl Zeus, SCT, to carry out intensive research programs in order to improve the yield and the quality of optical CaF2, so that it satisfied the firm requirements of optical micro-lithography, namely, a stress induced birefringence of less than 1 nm/cm and a homogeneity better than 1 ppm [11]. However, a serious issue emerges related to the appearance of a large intrinsic birefringence in the far VUV that causes significant ‘‘spacial dispersion’’ in CaF2 lenses [12]. The inherent nature of intrinsic birefringence makes it pointless to consider its reduction by any improvement in the optical quality through the growth of CaF2 crystals. It is for this reason that expensive and sophisticated techniques for 157-nm lithography are developed in the optical system of the steppers. This involves appropriately positioned lenses and mirrors made from CaF2, which allow a reduction of the intrinsic birefringence in conformity with industrial requirements. Besides the attempts in simplifying these techniques to reduce costs, an alternative approach was also applied. It is based on the experimentally established opposite dependence of intrinsic birefringence in the crystal cubic direction for the couples CaF2-BaF2 and CaF2-SrF2. Both couples may form solid solutions according to their phase diagram and following growth under appropriate conditions one can obtain crystals with an extremely low intrinsic birefringence [13,14]. A precise control of all optical characteristics is required as well, which demands a very high level of homogeneity, which in turn requires an extreme purity of crystal structure. Besides the reviewed applications the alkali earth metal (AEM) fluorides MF2 (M = Ca, Sr or Ba), should be very attractive host crystals for Ln and transition metal laser activators since they possess a broad optical transparency region, moderate Raman gain coefficients, relatively small linear and non-linear refractive indices, and rather flat curvature of the refractive index dispersion over a wide wavelength range in the near IR. Under such unique optical properties and rather reasonable physical characteristics one may expect high peak laser action. That is why these single AEM fluorides doped by some rare earth (RE) trivalent ions appeared objects of intensive investigations from the very beginning of the last decade of 20th century. The ambitious purpose was to be established whether these materials were suitable for use as effective laser-pumped amplifier media. CaF2 and SrF2 were preferable for use as single host crystals in the earlier stage of laser research [15] while BaF2 jointed them after single crystals reached at last the required optical quality and sizes following significant improvement of a technique [16] for its growth.

1.2. Mixed Alkali Earth Systems The AEM single fluorides MF2 may form tri- or four-component solid solutions of type Ca1-xSrxF2, Ca1-yBayF2 or Ca1-x-yBaxSryF2, which retain the cubic symmetry of the fluorite lattice. Complete solubility of the starting single fluorides has proved to be possible only for the first system [17,18,19], and its properties appear intermediate between those of CaF2 and SrF2, which highlights possible advantages of this mixed fluoride system. Thus, the higher resistance to oxidation with heating and deep UV (DUV) radiation, as well as the higher mechanical strength and lower optical stress coefficient of SrF2 as compared to CaF2 single crystals [20] outline the possible benefits for the utilization of Ca1-xSrxF2 crystals everywhere

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where an intermediate properties’ balance is needed. Besides, some compositions of this or other mixed fluoride crystals have proved very promising for eliminating the spacial birefringence effect, which appears intolerably high in the case of single AEM fluorides at wavelengths below 193 nm (in the VUV) [21]. The mixed AEM fluoride crystals have been studied mostly for their promising lasing properties after doping/co-doping by appropriate trivalent Ln-ions with eventual chargecompensation by univalent alkali ions. For Ln3+-based lasers these properties depend strongly on both the symmetry and strength of the crystal field so they may be purposely controlled by appropriate modification of the host material.

2. REVIEW OF GROWING TECHNIQUES

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2.1. Artificial Fluorite After the 1930s artificial fluoride was grown from melt that possesses substantial advantages over growth from solutions (back diffusion and hydrothermal growth). It was Shamovski [22] who firstly recognized the deteriorating role of melt hydrolysis on crystal quality. Stockbarger [1,3] discussed in detail this issue and formalized the necessity for high vacuum conditions being sustained in the furnace chamber. In this case, a basic difficulty arises for keeping the physicochemical growth conditions that are in conformity with both, the relative chemical instability of CaF2 at the m.p. and high chemical activity of the fluorine at temperatures over the m.p. For this reason, no matter how the crystallization arises - from nucleus or spontaneously - the apparatuses for melt growth of optical CaF2 have to satisfy definite requirements as regards an efficient decontamination of the vacuum chamber. The developed methods/techniques and apparatuses pertain to two principal crystallization mechanisms: indirect or spontaneous crystallization of molten material in a container and direct crystallization by withdrawal of a crystal from seeding. A characteristic feature of the first one is uniform translation of the melt/crystal interface (IFM/Cr) along a vertically (with axial symmetry) or horizontally positioned container accomplished by moving the container or the furnace (Bridgman method [23]). Alternatively, the container may be fixed in the furnace under impact of thermal field with an appropriate configuration, changed controllably by lowering the furnace power supply [24,25]. A complication for any method of spontaneous crystallization appears with the use of container which, on the one hand, represents a potential source of melt contaminants initiating eventually structural defects, whilst on the other hand, may cause significant lattice deformations as a result of a difference between the coefficients of thermal volume expansion of crystal and container material. Besides, the observation or pyrometric control of the growing process by visual observation through the obligatory drilled channel in the container lid is impossible because of the need to avoid a large cross-sectional area for this channel. Such small sizes are based on the necessity to minimize the evaporation and decomposition of the melt whilst maintaining a relevant pressure over it, so as to determine a slow Knudsen type gas-vapor diffusion in the channel. Considering the above inconveniences, the indirect or spontaneous crystallization methods possess substantial advantages over the direct methods of crystallization by seeding, developed by Chochralski and Nakkin-Kyropulos [26]. These

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advantages consist in: the crystallization of the whole melt volume; the ability to obtain crystals of a fully dissociable melt; to provide a thermal field configuration in the furnace with a sufficiently steep temperature gradient for compensating a partial supercooling of the melt with negligible convection effects; a small free surface area of the melt; relative technical simplicity with the possibility of the growth crystals with a desired form and size. Even though synthetic fluorite has been grown by the Chochralski and Nakkin-Kyropulos methods [2], the success of the development of industrial technology for optical CaF2 is based on the conventional vertical Bridgman (B) technique and its permanently improved modifications whereby cylindrical ingots (boules) with differently shaped bottom regions are easily obtained. The B-technique is preferred to the Stober/Tamman one [24,25] which requires a tighter temperature control using a thermocouple, traversed vertically along a cylindrical container (crucible), as well as a more precise control of the growth rate [27]. Nevertheless, the conventional B-technique has also been shown to have definite shortcomings: the impossibility of ensuring a sufficiently high vertical temperature gradient in the crystallization zone (CZ), which leads to melt supercooling in a thin layer just ahead of the crystallization front (CF) resulting in segregation of impurities therein and relevant fluctuations of the defects density [28]. Besides this, a constructively predetermined significant vertical temperature gradient is set up along the lower 2/3 of the furnace unit that initiates macroscopic stripes caused by transverse slippage of the plastic crystal during its gradual solidification. At this junction, the efforts of researchers were directed at improving the B-technique for obtaining a thermal field with an optimal configuration, capable of ensuring normal growth during the crystallization of nearly the whole melt volume. Stockbarger [1,3] was a pioneer in this field, introducing a second heater in the furnace unit, thus differentiating a lower zone, called as well ‘‘cold’’. Since the two heaters are independently power supplied, one may vary correspondingly the temperature profile to ensure a much steeper vertical gradient in the zone of separating diaphragm (SD) set up between the upper and lower zones, compared to the case of single-zone Bridgman type furnaces that reduces the melt supercooling ahead the CF. Moreover, the single-zone furnaces, by displaying a temperature profile with a clearly expressed parabolic form, resulted in the non-uniform distribution of impurities in the grown crystals, even if the lower end of the furnace was being additionally cooled so as to increase the vertical temperature gradient therein. Despite these evident advantages of two-zone furnaces over single-zone ones, their use may cause problems for growing fluorite mono-crystals since a significant heat flux penetrates into the lower zone due to the intensive emission from the very hot diaphragm and the moving crucible. Thence the temperature gradient setup vertically in SD zone is less steep compared to the theoretically predicted one [4], causing a higher mean absolute temperature therein. Simultaneously applying forced cooling of the diaphragm and the lower zone leads steadily to the maintenance of the CF within SD zone. A successful modification of BS technique represents a furnace unit with auxiliary annular heater mounted just above the diaphragm aimed at the establishment of a more homogeneous thermal field in the upper zone and of a steeper vertical temperature gradient below it [5,6,29,30]. Intensive explorations were accomplished for improving the thermal conditions in the load and for optimizing the thermodynamic solutions in CZ in order to obtain high quality crystals with large diameter [2,4]. Besides the thermal conditions the crystal quality depends substantially on melt purity within the CZ that is controlled via the purity of the starting fluorspar, the gaseous/vapor

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environment, and the graphite crucible (container). Since the principal contaminants deteriorating the fluorite lattice are water vapors and oxygen ions [1,5], the growth techniques for single (as well as for mixed fluoride crystals) suppose an environment essentially free from any form of oxygen-containing contaminants – molecular and ionized oxygen and/or water vapor. In keeping with this requirement, the growth by Bridgman-Stockbarger (BS) technique and its various combinations is performed either high vacuum (HV) [1,5] or in a reactive atmosphere (RA) [31]. When growing CaF2 in HV, besides an intensively evacuated furnace chamber to compensate for the inevitable leakages, a small amount of metal fluoride is usually added to the starting material – chemically purified grained fluorspar – for scavenging mostly the oxygen contaminants deeply adsorbed in its particles, whose dimensions, principally below 1 mm, determine a high specific area of the material. As an alternative to HV growth technology there was developed the RA technology by using HF or other fluorine-containing gaseous materials. An advantage of such technology is the possibility of the use of raw materials with relatively high impurity content, especially as regards the oxygen contaminants. The method was developed intensively in the USA for growing extremely pure crystals of alkali earth, rare earth and some other metal fluorides [3239]. Despite the variety of devices and apparatuses that were invented, they mostly share one common limitation: the crystals were grown in a container whose inside was connected to the chamber atmosphere so that the melt and crystal grown in this way were exposed to gaseous contaminants that are released continuously from the heaters, baffles, shields, insulations, and the container itself. To avoid the use of ineffective HF for formation of a fluorine-containing atmosphere, other researchers propose saturating the space above the melt with fluorine based on decomposition of flour-carbon polymers [40-42]. The problem in this case was the difficulty in controlling the pressure of the decomposed fluorine above the melt. A new approach separates the purification and crystal growth processes [43-46] as the purification is carried out in a special chamber apparatus involving the use of solid, liquid or gaseous reactive substances in order to target impurity removal and provide a higher grade of start-up crystalline material. This plate growth technique is promising in providing excellent control and conservation of melt/crystal stoichiometry at growth rate up to 30 mm/h.

2.2. In-Batch Growth of CaF2-SrF2 Solid Solutions The mixed fluoride compounds with uniform composition are usually grown from a congruently molten mixture of the starting single fluorides by using the BS method [23,1,3], as well as by the Gradient Freeze (GF) technique [47] or the Temperature Gradient Technique (TGT) [48]. The promising Single Crystal Technology (SCT) [46] is also applied but it does not cover simultaneous production of several parallel boules that differ in the proportion of the involved metal components. In this case a specific issue that BS-growers have to solve appears implementation of effective control over the position and shape for the CF in all cameras loaded with CaF2-SrF2 mixtures with different proportions that prerequisites different temperatures of congruent melting [49]. The efficiency of such control should increase considerably around the region where the slope of solidus/liquidus phase diagram curves changes insignificantly [50]. Of great research interest is the growth of Ca1-xSrxF2 crystals with a largely varying alkaline earth proportion as a batch of axis-symmetrically disposed boules. Such crystals pose

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a severe technological challenge for the reliable production under identical conditions of uniform high-grade optical material with controllably altered characteristics. The challenge comes from the necessity to solve several issues related to the purity of the starting materials and growth atmosphere as well as to the implementation of efficient control over all growth parameters. The present survey demonstrates how one may solve these complex issues. Its specific tasks appear: (i) development of reliable procedures (technique) for minimizing the oxygen contamination within them CZ and reducing substantially the content of REimpurities that present, as a rule, in CaF2 component when using a fluorspar; (ii) obtaining a simple model relationship between the growth conditions and crystal quality assessed by transmissivity t and absorption losses measured over the whole spectral range (DUV - near IR (NIR)); (iii) reveal the influence of melt supercooling on the optical parameters and microstructure of the grown boules by using the suppercooling criterion for interface instability as well as the criterion for establishing normal growth. The results are anticipated being useful in providing significant improvements in the growth technology for reliable production of several high optical quality boules with different compositions under identical in practical conditions that appear perfect objects for scientific exploration. In turn such exploration should enhance the study of non-linear properties of the system for its application for cleaning of femtosecond laser pulses as well as for the precise measurement of short high intensive laser pulses in the UV. The investigation can be enhanced as well for production of doped/co-doped Ca1-xSrxF2 with unique characteristics determining these systems as promising laser materials.

3. FACTORS INFLUENCING THE CRYSTAL GROWTH

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3.1. Oxygen Contaminants It is well known that the optical properties of CaF2 crystals significantly improve on lowering the oxygen-containing impurities contaminating the CZ. Two sources of oxygen contamination can be clearly distinguished. Firstly, it is the starting material, the particles of which possess a high specific area that favors a deep adsorption of moisture on their surface. Because of the high energy of binding for water molecules to the lattice, their total removal from the starting material is difficult to carry out. A number of methods have been proposed for solving this issue [51]. Some of them are based on evacuating the chamber to a very low pressure, below 10-3 N/m2, at a temperature below the m.p. of the used fluorspar. Since this methodology turned out to be insufficiently efficient, a metal scavenger has been added to the starting material. Using this procedure the water vapor and/or the final product of its reaction with CaF2 (CaO) can give rise to several chemical reactions [37,39,51], whose products are volatile metal oxides and gasses such as HF and CO2 which are easily removed under high vacuum and high-temperature conditions. The most commonly used metal scavenger is PbF2, whose optimal amount is differently quoted in the literature. It varies from 0.05 wt.% [53], 0.1-0.2 wt.% [2], 0.25 wt.% [5], 0.25-2 wt.% [54], 1-2 wt.% [55], 1.5 wt.% [56], up to 2 wt.% [1,57-60]. Amongst several other metal fluorides testified as scavengers, ZnF2, alone has been reported to possess a significantly higher reactivity regarding the water vapor compared to the commonly used PbF2 and to the ecologically harmful CdF2 [53]. Such a result is a

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consequence of an order of magnitude lower saturation vapor pressure of ZnF2 compared to that of PbF2 at temperatures exceeding 1600 K, which determines a more prolonged action of ZnF2 and hence its higher effectiveness. Secondly, residual oxygen-containing ions/molecules can move from evacuated furnace chamber inside crucible interior under the developed gradients in partial pressure along the relating system of channels; in quasi-equilibrium these gradients are opposite directed to the established gradient of total pressure that determines the fluorspar vapor fluxes throughout the channels. In case the growing is carried out in static or dynamic atmosphere of highly purified inert gas, the physical ground that determines the mass transport in gas-vapor phase remain the same as concern the path of oxygen contamination. Thus, independently of the value of the equilibrated total pressure inside the chamber, water vapor and oxygen molecules will steadily penetrate inside the chamber, under their own inward partial pressure gradients, that develop in the regions of the inevitable existing apparatus leaks. That is why any vacuum system, coupled to an inert-gas line, should be capable only of equilibrating the gaseous oxygen-containing impurities to a certain acceptable level, never removing them completely. The use of argon has been shown to be a convenient way of creating an inert atmosphere inside the chamber since a static state can be established by switching off the pumping system to allow the introduction of extremely purified gas 99.9999%. In this case also, the required extremely low concentration level for the gaseous oxygen contaminants is not maintained in the chamber because of very steep inward partial pressure gradients, initiating back Knudsen diffusion throughout the apparatus leaks. For instance, an air leak of only 0.0006 ml/min will increase the oxygen content to 0.2 ppm in a 180-l chamber filled with 5 nines purity argon, flowing at 500 ml/min. Once entering the cameras, the thermally ionized oxygen-containing molecules dissolve into the melt in accordance with Henry’s law, proportionally to the relevant partial pressure established over the liquid phase. Nevertheless, the diffusivities of the dissolved OH- and, especially of O2- (of the order of 10-9 m2/s [61]), are very low compared to that of F-. At this junction, only hydroxyl diffusion into the melt is deep enough to cause sufficient change in their activity for chemical reaction with Ca to produce as a final product CaO. This way, a two-phase system (CaF2-CaO) will be formed that begins to propagate from melt surface into the bulk. Thus, an oxygen-contaminated zone is distinguished as its width depends on the hydroxyl activity in the very top of the melt, and on the presence of natural and/or forced convection in the bulk. The forced convections can be initiated either as a result of intensive bubbling due to evaporation/boiling or by relatively fast crucible rotation, and they should equalize, in a very short time, any OH--concentration gradients that arise in the melt. This way it is thought the mass-transport resistance of the liquid phase being insignificant and only the mass-transport resistance of the gas-vapor phase has to be considered.

3.2. Impurities in Natural Fluorite A key goal of the survey appears how to remove efficiently mostly non-isomorphic impurities and, especially, the isomorphic RE impurities embedded into fluorite lattice. It has been proved both impurities to disturb the growth, this way deteriorating the optical properties of the grown crystals.

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The impurity content before and after the obligatory chemical purification of the starting material as well as that remaining within the grown crystals was precisely determined using appropriate methods whose application has been reviewed thoroughly by Detcheva and Havezov [62]. They have shown the combined methods of direct solid sample graphite furnace atomic absorption analysis (SS-GF-AAS) and electrothermal vaporization inductively coupled plasma optical emission spectroscopy (ETV-ICP-OES) turned out most reliable [6265] for the determination of Cu, Pb, Cd, Zn, Fe, which were found in some fluorite samples in trace concentrations (0.1n ppm.) It appears ETV-ICP-OES has a better relative standard deviation and wider working range than SS-GF-AAS. The lowest limits of detection were obtained by means of SS-GF-AAS: 0.04 ng – for Cu, 0.0009 ng – for Cd, 0.001 ng – for Zn, and 0.005 ng – for Pb. Even in such negligible concentrations of later element its bi-valence cations, embedded in fluorite lattice on the places of Ca2+, form OAC of light-absorption in the vacuum UV. Thus, SS-GF-AAS was especially appropriate for detection of trace elements in high-purity artificial fluorite crystals thus to assess their ability being used for manufacture of lenses used in optical system devices for micro-photolithography and UV-laser technology. A general disadvantage natural fluorite to be used for growing UV grade CaF2 or any mixed fluoride systems appears the presence of RE impurities in starting fluorspar. It is considered the entire removal of these impurities was either impossible or related to considerable technological complications that turn the processes involved highly unprofitable. An original approach for solving the problem is trashed out in the present survey.

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3.3. Melt Supercooling: Criterion for Interface Stability Structural and chemical perfection of fluoride type crystals grown from melt by BS or Stober methods is determined by the concentration of trapped impurities during crystallization which, in turn, depends on melt supercooling in a layer ahead the CF and on CF- shape. Despite the crystallization is non-equilibrium process the conventional theory considers an existence of local quasi-equilibrium on CF grounded on extremely fast ionic exchange on this interface. At usually imposed by BS methods crystallization rate (CR) for growing of artificial fluorite, the ions accomplish approximately 108 temporary captures before being incorporated into the lattice of growing crystal. The RE cations presenting in molten fluorspar should be isomorphic embedded into the lattice of growing crystal when their equilibrium segregation coefficient is below 1 as is the case for mostly lanthanides [66]. However the segregation can be significantly accelerated if any reason the CR exceeds a definite level, whereat liquid to solid quasi-equilibrium turns out de-balanced owing to alteration of interrelation between CR and the effective diffusion coefficient for impurities ions in melt layer nearby the growth surface. As a result the concentration of captured impurities ions in just forming crystalline layer can exceed significantly that predicted according to the value of equilibrium coefficient for impurities segregation between crystal and melt [67]. Besides, the enhanced concentration of these and/or other impurities in the narrow zone ahead the CF may cause their re-distribution. This can occur at the absence of sufficiently intensive convection into the melt capable to equalize the concentration gradients of any impurities in the bulk. At the low rate for diffusion of heterogeneous impurities in liquid phase (being approved of the order of 10-10 - 10-11 m2/s) they, depending on their dissolution

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proportion in the melt and crystal, may either enriching (at effective segregation coefficient Kseg < 1) or impoverishing (at Kseg > 1) the boundary molten layer ahead the IFM/Cr under the effect of segregation mechanism. Both cases this causes a change in melt temperature in this layer with wideness not exceeding 1 mm at usually set up CR for CaF2 crystal growth. The theory predicts that for impurities with Kseg < 1 the boundary layer is super-cooled and these impurities, being rejected by the freezing solid, can be built up in a way the equilibrium freezing temperature in the melt adjacent to the CF to remain above the actual temperature [68]. Owing to this nucleolus centers appear simultaneously that breaks the morphological IFM/Cr stability and the normal growth is being replaced by high rate bulk (dendroidal) crystallization with formation of cellular sub-structure (dendrites, cavities, poly-crystallinity). Besides, the trapped impurities act as light-scattering centers. Arhang’elskaya and co-worker [61] have shown such light-scattering centers in the grown CaF2 crystals represent a number of fine crystals forming a zone of impurities enrichment (ZIE). When the melt supercooling is not very high these crystalline particles are well shaped as cube-octahedrons sized to 10 m. At deeper melt supercooling the strong T-dependence of viscosity leads to its significant increase in the ZIE whereat the inclusions are frozen in the shape of whiskers and/or fine colloform eutectics CaF2+СаО. The role of these eutectics is insignificant since their total quantity is negligible while the sizes are smaller than the wavelength for reflecting light. The formation of these light-scattering inclusions is a consequence of several chemical-physical processes. Initially, at T > 880 K a fast rate reaction of CaF2 with traces of ionized water vapor or/and molecular oxygen produces some amount of CaO which, being not isomorphic with CaF2, cannot be solved in growing structure to form a solid solution. The CaO enriches the ZIE up to maximum concentration determining by the ratio No/Kseg where No is the mean quantity for particular impurity in the melt bulk while Kseg  1 is the coefficient of its segregation between liquid and solid phases. Under equilibrium the width of ZIE for impurity under consideration, im (i = CaO) depends on the proportion between the mean impurity diffusion coefficient, Dim, and the maximum linear crystallization rate accepted equal to set up speed of crucible withdrawal Vcr along the imposed temperature field. The impurity enrichment ahead the CF leads to decrease of crystallization temperature Tcrys by Тcrys, defined by formula: Тcrys = (RТcrys2/Hsl)[No(Kseg-1-1)]

(3-1)

where R is the Universal Gas Constant equal to 1.986 cal/K.mole, and Hsl is the latent heat liberates during crystallization. When CaF2 is grown by BS technique, it has been found out [61,69]: (RТcr2/Hsl)  792 K at Тcrys  1683 К and the latent heat of crystallization Hsl  7100 cal/mol. Besides, it is approved fact that CaO in insoluble in practical into fluorite lattice, that is, Kseg(CaO)  1 and (Ksep-1-1)  1. This way, even at relatively small concentration of the impurity No, the product [No(Ksep-1 -1)] may attain significant value exceeding of two order of magnitude No. Using Ksep  0.01 and No = 5.10-4 mole parts, from (3-1) was calculated Тcrys = 40 K. On the other hand, Dim for О2- dissolved in CaF2 melt is too low ( 10-11 m2/s) so that these anions may consider being in practical immobile in comparison with F- anions which – at nearly the same effective ionic radius – bear twice less negative electrical charge. In such a case, for usually set up speed of crucible withdrawal (2–10 mm/h) [1,2,5], approved for ¾ of the

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grown boule being very close to established CR, the ZIE width stays of order of 10-3 cm (10 m). In such super-cooled melt layer unsteady conditions arise that initiates a single crystal growth from the heterogeneous region of CaF2 – CaO phase diagram. As a result, it appears a great number of fine fluorite crystals with higher temperature of crystallization than Tcrys at the CF acting as light-scattering centers. To avoid their formation it is necessary to be provided stable growing conditions according to criterion:

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Тcr/im = (RТcr2/Hsl)[No(Ksep-1-1)](Vcr/D im)

(3-2)

For Vcr = 1 cm/h, from (3-2) follows: Тcr/ im  105 K/cm or 10 K/m. Such steep gradient predetermines a drastic lowering the temperature ahead the CF that will surely destabilize any mono-crystal growth. Since Тcr/im is proportional to Vcr (= CR) by lowering Vcr one can stabilize crystal growth. Evidently however, Vcr could be lowered up to reasonable level so that the growth not being prolonged excessively long which to cause considerable evaporation and/or decomposition of molten material, and following break in stoichiometry owing to loss of F- from anionic sub-lattice. Hence, other approach should be searched for lowering the melt suppercooling ahead the CF. From (3-2) it results that Тcr/im stays negligible when either No  0 or Ksep  1. The first opportunity can be implemented if from the starting fluorspar or/and its melt are being removed the weakly dissolved into the lattice impurities, as CaO appears itself. This can be attained if the growing is carried out in reactive fluorine containing medium or atmosphere enriched to HF(gas) whereat CaO will react with F2 (HF) giving a product of CaF2. Good result for minimizing the oxygen contamination is obtained as well by adding to starting fluorspar some small amount of solid metal fluoride with efficient scavenger effect [49]. The other opportunity (Ksep  1) can be accomplished by introducing into the fluorspar/melt some substances that may considerably enhancing CaO solubility. Such additives turn out small amounts of rare earth fluorides, whose tri-valence ions, Ln3+, replace Ca2+ into the lattice while their excessive positive charge being compensated during a simultaneously proceeding replacement of F- to O2-. Unfortunately the presence of firmly incorporated into anion sub-lattice O2- leads to specific light-absorption bands localized within the whole available spectral t-range (VUV - NIR). On the other hand, if it is technically possible to be attained sufficiently steep vertical temperature gradient along the AdZ, the considered, opposite in sign constitutional temperature gradient ahead the CF Тcr/im may be compensated. However such approach can be effective only if lowering No or/and increasing Ksep as closed to 1 as possible. One should take into consideration that melt supercooling, not related to the purity in CZ, can arise being caused by appearance and constant rise up of radiation flow throughout the growing transparent crystal, surpassing increasingly the conductive flow towards the cold furnace zone. The performed analysis of supercooling effect shows the presence of RE impurities in ZIE should contribute insignificantly this effect since their content rarely exceeds several hundreds of ppm while their effective segregation coefficient stays closed to 1. Nevertheless the efficient removal of these impurities from the CZ is obligatory for growing UV and laser grade crystals that required excellent transmittance within VUV - UV regions without any specific light-absorption.

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The break of the normal growth as a result of constitutional supercooling (CSup) means the smooth CF to become instable that leads to spontaneous growth of protuberating structures on the IFM/Cr so that the grown crystals turn out with cellular structure. The phenomenon has been put on quantitative analysis firstly by Tiller and co-authors [70] and approved by many others [68,71-73]. This allowed the derivation on thermodynamic basis of CSup criterion for assessing the conditions that lead to IFM/Cr instability. The criterion, despite its possible shortcomings, appears useful and convenient measurement tool in many growth situations even those that concern complex three and four component fluoride systems. Thus, in the case of concentrated mixed fluoride systems of the type MF2-M’F2 and MF2-RF3 (M # M’ = AMF; R = RE) even the simplest one-dimensional microscopic approximation for IFM/Cr-instability (Tiller criterion) can be applied to obtain a real account for the effect of CSup on crystal morphology. It has been derived in the form: G/Vgr ≥ - m.(1-K)Co/D.K

(3-3)

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where: G is the temperature gradient in the melt at the CF; Vgr – the growing rate; K and D – the coefficients of distribution (segregation) and diffusion, respectively, for the second (impurity) component; m – the liquidus slope (tangent of the slope’s angle for the liquidus curve); Co – the concentration of the second (impurity) component in the melt prior to solidification, equal to the concentration in the crystallized solid phase and in the melt sufficiently far from the CF. The criterion was derived suggesting stationary crystallization without stirring the melt and small concentration for the second component so that Co ≤ 1 while K and m can be considered constants, and supposing that just constitutional suppercooling causes the IF instability. Nevertheless, the criterion has been proved being valid for concentrated melts as well [27,28,74]. A useful form of this criterion is: G.D/Vgr ≥ m.∆C

(3-4)

where ∆C = Cs – CL – is the apparent discontinuity in the impurity’s concentration at the CF, and Cs and CL - are the concentrations at the CF in the liquid and solid phases respectively. Taking the distribution coefficient at equilibrium, Ko, the right side of inequality (3-4), denoted by F(C) as a function of stability [75-77], can be estimated easily knowing the corresponding compositional phase diagram; to a first approximation F(C) corresponds to the difference in temperatures between the liquidus and solidus. The function is positive, F(C) ≥ 0, reducing to zero for pure components and at the extremes where the liquidus and solidus have gotten tangential points while in the range of small impurity’s concentrations it approximates a straight line [78]. Applying criterion (3-4) to a real crystallization process described by parameters G, D, and Vgr, it is evident the parameters’ combination G.D/Vgr should lie higher at a given Cs than the corresponding point pertaining to F(C)-dependence so that CF to remain planar and stable. Function F(C) has been investigated in detail for the system MF2-RF3 (M = Ca, Ba, and Sr) [50]. The results show a regular function of F(C) depending on the RE ionic radius with a clear minimum for Ba and Sr that correspond to a transition from K > 1 for the larger cations in the beginning of the Ln-series to K < 1 for the smaller cations pertaining to the yttrium sub-group. In the case where M = Ca there was no

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found such clear regularity that can be related to the fine partitioning of the Ln-series on several sub-groups [79]. The diffusion coefficient D of several RE have been studied as well for the same alkali earth - rare earth fluoride systems and it has been shown D becomes independent on the concentration for La as the second component with the largest ionic radius when its content became greater than 0.16 mol.%. At the same time D has been shown to keep a linear dependence on the Ln-radius, decreasing approximately twice from La to Yb [50]. Knowing D and the stability function F(C) one may estimate easily the crystallization rate for obtaining optically uniform non-cellular crystal with a particular composition for particular vertical temperature gradient being set up. Several simplistic assumptions were used to derive the criterion (3-4), as more precise approach requires to be taken into consideration the stabilizing influence of surface energy [72,73]. The divergence between the results obtained by criterion (3-4) and that considering the surface energy becomes greatest for small concentrations of impurities. Thence for the first highly concentrated component exists a certain boundary impurity concentration, below which even at very fast crystallization rates a cellular structure could not be formed so that the established planar CF will be absolute stable. With increasing the concentration of impurities the contribution of the surface energy lessens rapidly and may be neglected. It is possible that the corrections due to the stabilizing effect of the surface energy become significant close to the extreme points on the melting curves for any mixed fluoride solid solutions. The above analysis of transition from mono-crystalline to cellular growth for solid solutions with fluorite structure is correct only if normal growth proceeds on a rough surface. This suggestion has proved to be correct if the value of Lo/kT is found to be below 2 [80] or below 3.5 [81] where Lo (ΔHmolt) is the heat of fusion, k - the Boltzmann constant and T - absolute temperature. If this criterion fails then the growth proceeds by the formation of two-dimensional nucleation and such high anisotropy leads the distribution coefficient and surface energy for the impurity to depend strongly on the growth direction. In this case besides cells’ formation, a laminar distribution of impurity can occur caused by capturing of melt’s layer adjacent to the growth surface. It is important the IFM/Cr-instability to be apprehended on micro-level as well. This requires to be taken into consideration other factors: the real crystallization rate (not the speed of crucible movement), the difference in diffusion coefficients for a variety of ions and their concentration dependence, the real conditions causing laminar structures in solid solution crystals in the absence of forced stirring of the melt or crucible rotation.

3.4. Temperature Head and Melt Vaporization Under definite temperature head (defined as T-excess over the m.p.) and high vacuum maintained into the furnace chamber, the induced vaporization of molten material inside the crucible accelerates significantly. This leads to fast supersaturation of the free space over the melt surface by fluorspar vapors whose composition will differ increasingly from that of the liquid phase approaching quasi-equilibrium. The changing P-T conditions inside the crucible lead to melt vaporization proceeding under concurring effect of surface evaporation and bulk or surface boiling. Accordingly Duhem-Morgulious law, independently on supersaturation pressure established inside the crucible, the more volatile impurities (including some RE) appear in higher molar fractions in the vapor phase compared to liquid one. Thus, if crucible

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construction provides conditions for condensation of vaporized fluorspar, this condensate should be enriched to those impurities, the partial pressure of which vapors is higher than that of CaF2. The non-vaporized molten part can be transformed either by fast cooling the crucible to polycrystalline/sintered precursor or slowly cooling the crucible – to single boule. Both cases the products should be considerably impoverished to easily volatile impurities escaping from the melt as single or complex ions. Thus, after sufficient number of cycling vaporization – solidification being performed, the concentration of these easily volatile impurities in final product may be reduced under admissible level whereat their presence should not deteriorate noticeably the optical quality of the grown crystals. By applying this approach several metal fluorides have been purified from some not readily volatile impurities compositions but the rate of vaporization was found being too low to ensure considerable yield of pure crystals. In this case, the usage of conventional mono-cameral crucible with low gas-permeability in its unique cover’s channel leads to inadequate results. Firstly, the back Knudsen diffusion of any oxygen-containing gaseous contaminants in the channel should be with little effect for their penetration inside the crucible. At the same time however, the high channel resistance as regards the mass vapor flux towards deeply evacuated chamber determines a high level of supersaturation pressure Pssat being established over the melt. It is estimated when Pssat ≥ 102 N/m2 the ionized at high extend molecules of fluorspar melt would be substantially impeded being desorbed to evaporate from the surface. Accordingly, the vaporization rate (VR) would be low. Besides, when fluorite vapors stay permanently in super-saturated mode they begin to deposit on significantly cooler inner surface of the crucible cover owing to its position into the furnace temperature field with parabolic profile. Moreover, part of vaporized molecules reflects diffusively from the inner surface of the cover this way lowering supplementary the VR in accordance with surface micro-relief. The concentration of not readily volatile impurities in the molten fluorspar (mostly Si and Al oxides) remains, in practical, constant in liquid and vapor phases during vaporization – solidification processing. That is why these oxides have to be removed preliminary from starting material by applying chemical methods [2, 49] till their total content decreases below 0.3 wt.% [51]. This boundary level was considered being sufficiently low the supercooling effect of these impurities could destroy the normal growth so that the optical characteristics of grown boules are being deteriorated unacceptably. From isomorphic embedded into fluorite lattice impurities only strontium – as SrF2 – possesses T-dependence of saturation vapor pressure very close to that of CaF2. Hence, it is not likely a predominant vaporization of Sr to occur during vaporization – solidification processing the fluorspar melts. In general, the ions of any isomorphic embedded impurities, except those of the RE, present in concentrations that cannot deteriorate the general optical characteristics of grown boules above the accepted levels corresponding to Laser Grade (LG) crystals. Moreover, as pointed out by Chern’evskaya [8], adding to the fluorspar of 1 wt.% SrF2 leads to significant improvement of the mechanical endurance and lowers the internal stresses in grown crystals. That is way it has been considered there is not need for removing all types of isomorphic impurities from melt bulk and CZ. The attention should be paid exclusively to removal of the RE, the traces of which cause a selective light absorption in short wave UV, luminescence, and deterioration of radiative resistance of the crystals. In the light of aforesaid analysis, principally it is possible the RE to be removed by bringing the melt into a mode of intensive evaporation. This case the RE is thought to vaporize in gaseous ambient as ionized fluoride compounds, LnF3 or LnF2, since the strength of Ln-cations bonds into fluorite lattice is stronger than that in the molten ionized complexes LnF3+ (LnF2+). It is

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

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known several of thus composed RE fluorides to possess a saturation pressure significantly higher (up to order of magnitude) than CaF2 saturation pressure for established P-T conditions inside the crucible. At this junction the gaseous/vapor ambient over the melt should be enriched to these easily volatile fluoride compounds during the evaporation. Principal issue appears the ability for efficient control of melt impoverishment to RE during its division to liquid and vapor phases so that this purification process being unified with normal crystallization of the remaining non-vaporized part of the melt. Here it should be taken into consideration the yield of highly purified material for precursors’ production and further/simultaneous crystal growth could be hardly expected to exceed 50% in single conventional crucible. Consequently, one cannot anticipate an industrial yield of high quality CaF2 crystals being attained without relevant technical improvements to be carried into effect.

Vaporization Mechanisms The vaporization of fluorspar may proceed either by surface evaporation or by bulk or film boiling. With an increase of temperature head upon the melt the intensity of vaporization rises up rapidly since the evaporation from vapor/melt IF (IFV/M) turns out concurred more and more considerably by newly arising non-stationary processes of phase transition, known as bulk and surface (film) boiling, respectively. At bulk boiling numbers of bubbles nucleate and grow up into the bulk, releasing the containing vapor when coming to the melt surface. Necessary condition the bubbles’ growth to proceed steadily by evaporation inside their cavities appears the vapor pressure in particular bubble to exceed at any moment the sum of ambient pressure, hydrostatic pressure of the melt above the bubble and capillary pressure that depends on curvature of bubble’s surface. Such condition is fulfilled when the vapor and surrounding liquid stay in thermal equilibrium at temperature higher than boiling temperature, Тboil, for which the melt has to be overheated. However, the imposed intensive evaporation towards the bubbles’ interior causes fast cooling of the whole surrounding liquid till new quasi-thermal phase equilibrium is being attained when T falls to saturation temperature, Tsat. At that, the boiling can be easily discontinued if the vapor pressure in the bubbles, being strongly T-dependent, drops down below the ambient pressure thus becoming insufficient to overcome the corresponding hydrostatic pressure. If the liquid contains dissolved gasses or any small solid particles, then – even at relatively low melt overheating – they become nucleolus for vapor bubbles that initiates steady bulk boiling. Very often these principal centers of vaporization arise upon the heated solid surface of the container on the spots of some micro-pores fulfilled with non-desorbed gases as well as other surface non-uniformities, inclusions, films/residues that all act for lowering the molecular cohesion forces between liquid and solid surface. Such centers generate surface boiling for which the vaporization rate is determined by the heat irradiated from the heated surface and absorbed from the liquid. The phenomenon is characterized by formation of full with vapor cavities upon the directly heated walls and may proceed either by bubbling mechanism whereat chains of nucleated bubbles arise upon single surface centers of boiling or by film mechanism whereat the liquid turns out isolated from heated surface by thin film of vapor. When increasing temperature head ΔThd the number of nucleated centers of vaporization augments rapidly so that more and more single bubbles are passing throughout the bulk towards the melt surface, thus causing an intensive stirring of the liquid. This leads to significant increase of the heat flux from heated surface to already boiling liquid. Relevant to that rises up the amount of produced vapor. Since the heat of vaporization is a constant parameter, the number of molecules/ions

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J.T. Mouchovski and B. Mullin

vaporized per unit time is proportional to the heat flow. At a given temperature head the heat flow density attains a maximum critical value (first boiling crisis), after which it decreases sharply and, in conformity with that, the rate of vaporization decreases sharply. This transitional boiling regime is grounded on a permanent enlargement of the heated dry spot area due to the merge of growing bubbles. When the whole heated surface turns out cover with thus formed vapor film the heat flux throughout it will be transposed to the melt bulk via conduction and radiation. At this point the second boiling crisis occurs – the heat flux density drops down substantially due to free convection in the liquid. In case of low vapor pressure established over the melt a direct transition of single-phase convection to film boiling liquid is carried into effect and the third boiling crisis occurs. The bubbling boiling is being recognized one of the most intensive process of heat-exchange where the heat transfer coefficient (Wt/m2. K) can be expressed by power function on heat flux density with index equal to 2.3 [82]. Compared to that, the heat transfer coefficient at film boiling is significantly lower as a result of lower conductivity of the vapor compared to liquid. The vaporization by boiling is influenced thermodynamically on the specific physical properties of the liquid: hidden heat of evaporation and surface tension on vapor/liquid IF. Besides, any non-volatile species dissolved into the liquid decrease its vapor saturation pressure thus rising up its boiling temperature. Boiling may arise not only under heating the liquid at constant ambient pressure, but also at lowering the ambient pressure under a constant temperature. This way an overheating may be initiated at the expense of lowering the saturation temperature. Controlling the melt boiling one may adjust the vaporization rate (VR) to be constantly several times higher than the rate of surface evaporation. Thus boiling appears a unique vaporization mechanism for effective removal of easily vaporized melt components grounding on different dependence for boiling and condensation temperature on liquid content. Principal disadvantage of this essentially distillation technique is the very low yield that can be significantly improved if putting the vapor continuously in exchange with vaporizing liquid by prolonging the path of vapor before its condensation (rectification technique). Anyway a pure component can be produced only if the dependence of vapor pressure on the content does not reveal a maximum. The rectification approach should lead to considerable technical complications when trying to apply it for effective removal of RE impurities from molten fluorspars especially if unifying the purification procedure with crystal growing. Nevertheless, a single melt distillation seams easy to be utilized appropriately for development of efficient technique for obtaining a good yield of high grades CaF2 optical crystals.

4. FUNDAMENTS OF PURIFICATION/GROWING TECHNIQUES The principal suggestion is that emerging during the boiling bubbles become enriched to vapors of those RE fluorides ionic complexes, the saturation pressure of which is higher than that of CaF2. This way, during the liquid – vapor phase transition of the molten fluorspar the RE concentration in non-vaporized part of the melt will decrease appropriately. Besides, the intensive boiling should provide sufficiently fast but uncontrollable in practical masstransport of the RE impurities from the bulk towards the surface. At the same time, in gaseous/vapor phase the RE-transport can be put under control varying the sizes of the

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

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channel into crucible cover this way changing its gas-permeability. This is substantial prerequisite for effective purification/growing technique being developed after taking into consideration several important circumstances. When crystallization proceeds under intensive bulk boiling, the RE distribution in growing crystal cannot attain equilibrium because of permanent impoverishment of the melt as regards these impurities. If their mass-transport from the CZ throughout the boiling bulk to the melt surface proceeds with a rate higher as comparing the crystallization rate (CR) – as it is expected disregarding the diffusion processes in view of the intensive mixing during the boiling, then the axial RE-distribution into the grown boule should not rise progressively that is peculiar for convective transport regime. Instead, the dependence would reveal a clear minimum when the two rates becoming equalized. This minimum could be shifted at the very top of the grown boule if the inevitable increase of the actual CR has been compensated by controllable rise up of the upper, lower or both furnace zones temperatures. In this case the axial RE-distribution would follow a decreasing course towards the top nearly along the whole boule’s height. The RE-distribution could be obtained approximately constant as well if CR and boiling rate (BR) are adjusted any way sufficiently close each other already during the initial stage of growing. Such condition can be defined by investigating experimentally the regularities governing the melt boiling. Choosing appropriately the parameters that influence the BR, the later can be maintained approximately equal to the CR, which could be corrected supplementary by precise alterations of the set up furnace temperature program. It is purposeful before growing to start, the initial RE concentration in the fluorite melt to be lowering continuously by intensive boiling in motionless crucible positioned within the upper furnace zone but without excessive losses of material throughout the channel in crucible cover. This way can be attained not only a constant in practical axial distribution (segregation) for RE impurities along more than 95% of boule’s height but their concentration can be lowered till extremely low levels. Nevertheless, a maintenance of nearly constant BR vboil appears a challenging task since it depends in a complex way on temperature head that, in turn, changes with crucible withdrawal into a furnace zone with steep decreasing T. Besides, the BR depends substantially on the pressure established over the melt, which may change significantly at shifting the quasi-equilibrium for complicated mass-transport throughout the channel and related crucible interior. Thus, vboil should sharply lower when – at a definite increase of temperature head – the vapor bubbles merge to form a thin layer upon the heated wall of the crucible whereat the vaporization mechanism changes from bulk to film boiling. Retardation of vboil is possible as well at definite supersaturation of the space over the melt owing to high mass-transport resistance of the cover’s channel. Anyway initiating lowering of vboil may lead to alteration in segregation mechanism for particular RE-impurity from convective to diffusive that will cause relevant concentration changes along the height of the grown crystal. In case convective type segregation mechanism determines the distribution of particular impurity “i” into the growing boule, its concentration on liquid-solid (melt-crystal) IF (IFM/Cr) Cis is given by [83]: Cis = Ki.Cio(1-H)Ki-1 while at diffusion segregation mechanism the distribution is presented by [83]:

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(4-1)

18

J.T. Mouchovski and B. Mullin Cis = Ki.Cio{1+[(1- Ki)/Ki]exp[-(v/Dim)x']}

(4-2)

In these relationships: Cio is the impurity’s concentration in the melt bulk sufficiently far from the IFM/Cr, H – the crystallized part of the melt; Kieff is the effective coefficient of distribution (segregation) for micro-impurity into the crystal; vCF – the rate of CF; Dim – the diffusion coefficient for impurity into the melt; x' – the distance to CF. Since the grown boule represents a cone cylinder, from geometric consideration follows that H depends on introduced axial coordinate z with origin coinciding to the cone tip via: Hcone = zcone/(hcone + 3hcyl)

(4-3)

in the conical boule’s section, whose height hcone is constant for each one boules from the grown batch while hcyl may alter correspondingly the loaded amount of fluorspar. At the same time, in the cylindrical boule’s section the dependence is: Hcyl = (hcone + 3zcyl)/(hcone + 3hcyl)

(4-4)

Here z = zcone – in the conical section and z = hcone + zcyl – in the cylindrical section. The distance x’ is determined by the difference (h – z) where the total height of the boule, h, represents a sum of the lengths for: conical section, finished cylindrical section, and the remaining upper waste section (considered without optical quality):

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h = hcone + hcyl = hcone + hcyl(boule)+ hcyl(waste)

(4-5)

The above analysis suggests that only on theoretical grounds and models one can not specify conditions for substantial lowering the RE concentrations and uniform distribution of these impurities along nearly the whole boule’s height. The issue becomes more complicated due to necessity of extremely low concentration of any oxygen-containing contaminants remaining or diffusing from the bulk inthe CZ. At this junction any originally developed modifications of boiling RE-purification technique that claim significant industrial efficiency has to be grounded primarily on experimental results that concern the influence of all significant factors on mass-transport processes and related to them kinetic phenomena that proceed in crucible interior before and during growing by BS-technique of single or mixed fluoride boule(s) by using fluorspar containing definite amount of RE impurities. These factors can be distinguished to apparatus (thermal field configuration within furnace unit (FU), crucible mode (construction), partial and total pressures of the residual atmosphere into the growing chamber, position and speed of the crucible) and substance (structure, composition, and quantity of starting fluorspar and eventually added scavenger(s)). The substance factors and crucible construction are constant parameters for particular experimental run whereas the apparatus factors are being widely varied.

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5. THERMODYNAMIC GROUNDS FOR RE PURIFICATION Grounding on thermodynamic considerations, the deepness of purification for particular RE impurity, β(REi), can be expressed as a function of the ratio (Gi/Ginit) = Gpur, representing the purified weight percent from loaded fluorspar portion. For the purpose it is used a formula: ∆μi = (μinit – μend)i = RTln(iCi) (5-1) relating the alteration of chemical potential, i, for given RE-micro-impurity (by i are subscribed the relevant elements from Ln-series) during its crystal-chemical reaction of isomorphic embedment into the lattice of growing crystal to the attained level of impurity’s concentration, Ci. Here i is the activity for the mirco-impurity, T – absolute temperature and R – the Universal Gas constant equal to 1.9859 cal/mole. K). When multicameral crucible with axial-symmetric, equal in sizes, peripherally disposed nests/inserts is being used for performing comparative analysis, the purification of the molten material inside each one insert may consider being proceeded at very similar thermal conditions. Under such consideration, the alteration in chemical potential for given microimpurity along infinitesimal height, ∂h, of the melt in any insert can be described by: ∆μi∂h = R.∂[Tln(iCi)]

(5-2)

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Since fluorspar behaves as strongly diluted solution as regards all RE cations dissolved into its lattice (so called Henry’s range), then, at isothermal process (T = const) and i = const, from (5-2) it follows: (∆μi/RT)∂h = (∂Ci/Ci)

(5-3)

If it is supposed thermodynamic equilibrium for single partitioning element per unit height of the melt, then – for each other “k”-element – will be fulfilled: (∆μi/RT) = ai = const, and (∂Ci/∂h)T,P = aiCi

(5-4)

The constant “ai“ has gotten physical meaning as coefficient of purification intensiveness for particular RE impurity. Its positive sign is determined under consideration that h and Ci will change unidirectional that is just the opposite in case of purification of micro-impurities from solutions by rectification [84]. The relationship (5-4) manifests that for each partitioning element from the melt height the degree of lowering the particular RE-concentration is proportional to local concentration of this impurity. After introduction of dimensionless variable (h/hinit), where hinit is the initial height of the melt inside a given insert, (5-4) takes on a form: (∂Ci/Ci)T,P = aihinit (h/hinit)

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(5-5)

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J.T. Mouchovski and B. Mullin

The integration of the right-hand side of (5-5) is spread over a domain, the upper endpoint of which is a function of the reduced variable (h/hinit) = (hi/hinit) whereat the relevant REi-concentration is denoted by Ci. The lower end-point of the domain is then: (h/hinit) = (hinit/hinit) = 1 whereat C = Cinit. Thus, the integration of (5-5) leads to expression: ln(Ci/Cinit) = ai.hinit [(hi/hinit) – 1] + Ao (5-6) Grounding on definitional formula for the mean melt density, ρm = G/πrins2, that relates the equal cross-section area for the involved inserts to nearly constant melt density ρm inside the studied volume, the height’s ratio h/hinit in (5-6) can be replaced by weight’s ratio Gi/Gin: ln(Ci/Cinit) = ai.hinit [(Gi/Ginit) – 1] + Ao

(5-7)

The integration constant Ao is being taken equal to zero since in the initial moment, when the purification process starts, the integration is spread over the whole height: hi = hinit or Gi = Ginit whereat the concentration for particular RE is equal to its initial concentration, Ci = Cinit. Thus, after anti-logarithm of (5-7) is taken, the deepness of purification for a given RE impurity, βi= Ci/Cinit, can be expressed as a function of purified, non-vaporized part of loaded material, Gpur = (Gi/Ginit): βi = (Ci/Cinit) = [exp(ai .hinit .Gpur)].exp(–ai .hinit)

(5-8)

or, using Gvap = 1 – Gpur, βi becomes a function of vaporized part of fluorspar melt:

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βi = (Ci/Cinit) = exp(–ai hin .Gvap)

(5-9)

The negative sign of exponential functions in (5-8) and (5-9) manifests the direction of alteration in chemical potential for particular RE impurity per unit height of the melt: the deepness of purification increases, that is, βi decreases, with lowering the remaining nonvaporized part of the melt, Gpur, and, respectively, with increasing the vapor quantity (the vaporized part of the substance, Gvap). The ratio between thus separated parts of loaded fluorspar depends on several technological factors (temperature head upon the melts, gas-permeability of the channels in lids’ inserts, initial RE-concentration before temperature to rise up controllably) as each one of them may affect the effectiveness of separation processes for production of one highly purified crystallized (sintered) part and other highly contaminated deposited part. The exponential expressions (5-8) and (5-9) define a definite residual concentration for particular REi impurity: when Gpur → 0 or Gvap → 1, then βires → exp(–ai hinit). This residual quantity lowers exponentially with the increase of the coefficient of purification intensiveness for particular REi, that is, with increasing the alteration in its chemical potential per unit volumetric melt, participating in ai = (∆μi/RT). The parameter ai is determined empirically by following the ln-dependences (6-8) or (6-9) wherefrom the alteration in the chemical potential, μi, is easy to be calculated. Using together (5-8) and (5-9) it is being derived convenient for empirical investigation formula where ln-i/k may be presented by a simple linear equation, the coefficients of

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

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which represent multiplied by (hinit/RT) differences in alterations of chemical potential for relevant pair REs, “i” - “k”, and Gpur appears variable: i = exp[(–i/RT)hinit].exp[(–i/RT)hinitGpur]

(5-10)

k = exp(–k/RT)hinit].exp[(–k/RT)hinitGpur]

(5-11)

Term-wise division of (5-10) to (5-11) leads to: i/k = exp[– (i – k)hinit/RT]•exp[(i – k)hinitGpur/RT]

(5-12)

Taking anti-logarithm of (5-12) it is obtained the linear equation: ln(i/k) = – (i – k)hinit/RT + (i – k)hinitGpur/RT

(5-13)

with a complex variable: Ψi/k = (i – k)hinit/RT

(5-14)

This way (5-13) takes a mode: ln(i/k) = – Ψi/k + Ψi/k .Gpur

(5-15)

or:

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ln(i/k) = – Ψi/k .Gvap

(5-16)

by using the relation Gpur + Gvap = 1. Provided Gev > 0 and hinit > 0, then when (i/k) > 1, i < k и via versa, if (i/k) < 1, then i > k. The alteration in chemical potential for particular pair of REs, notified by “i” and “k”, can be calculated combining Eq. (5-14) with an equation derived by division of two equations following from the basic ln-equation (5-9): lni = ln(Ci/Cinit) = –ai hinit .Gvap

(5-17a)

lnk = ln(Ck/Cinit) = –ak hinit .Gvap

(5-17b)

so that lni/lnk = ai/ak = i/k

(5-18)

Thence, the replacement of k in (5-14) by the product i(lnk/lni) taken from (5-18), gives:

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J.T. Mouchovski and B. Mullin i = (RT/hinit)[lni/(lni – lnk)]Ψi/k

(5-19)

from which, by using (5-18), one can directly calculate the alteration in chemical potential for the other RE impurity: k = (RT/hinit)[lnk/(lni – lnk)]Ψi/k

(5-20),

Important relationship is being derived as well after anti-logarithm of (5-18) is being taken: i = k(ai/ak) = k(i/k)

(5-21)

The investigation of relationship (5-23) as a function of Gpur allows the index ζ =

i/k, presenting the ratio between the alterations in chemical potential per unit height of the grown boule corresponding to particular pair RE micro-impurities, to be found. When ζ > 1 the intensiveness of purification for impurity “i” is higher than that of impurity “k” that will determine a higher deepness of purification for impurity “i” in case of similar initial concentration for the two REs. Opposite to that, if ζ < 1 the “k”-impurity will be purified more intensively and to a high degree comparing to “i”-impurity.

6. RE PURIFICATION PROCESSES: MASS-TRANSPORT AND KINETICS

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6.1. Differential Equations for Gas-Vapor Mass-Transport The mass-transport phenomena that may occur into the free of melt spaces of single or multi-crucible interior when applying high-T RE purification – CaF2 crystal growth technique are explored by means of the strict kinetic molecular theory of gases and liquids [85]. In absence of any turbulent and thermal convection the motion of molecules of gas-vapor mixture into the free space of any cylindrical volume of utilized mode crucible loaded with molten material, including the channels’ volume in general (outer) crucible cover and inner lids is described by the generalized equations for stationary mass-transport of gaseous mixtures [86]. These equations follow from more generalized Nernst - Planck equations cast back into Stefan-Maxwell form [87]. According to this approach the molecules move under a simultaneous effect of partial and total pressure gradients, temperature gradient, attended by convective Stephan motion of the gas-vapor mixture. The mixture’s content determines the number of equations composing the system under solution.

Quasi-Equilibrium: Content of Gas-Vapor Phase After sufficiently long performance of BS vacuum system the molecules in evacuated chamber stay in quasi-equilibrium at the maintained temperature of approximately 280 K. At that the total residual pressure of ≈ 10-2 N/m2 was established inside the chamber while partial pressures measurements, accomplished by incorporated in the BS apparatus quadruple massspectrometer, showed a content of 80% water vapor, 19% – nitrogen, and 1% – oxygen. At this junction, the residual atmosphere into the chamber and the free of melt spaces in crucible

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

23

interior can consider consisting of these three gaseous species and vapors of the used fluorspar, concentrated up to ≈ 99.6 wt.% CaF2. The quasi-equilibrium for mass-transport processes in gas-vapor phase may think being characterized by a constant distribution for all gas-vapor species into a motionless crucible. Such assumption is grounded on the specificity of utilized technique and apparatus and physical processes involved: the vacuum system creates constant partial and total gradients along the length of all free volumes into the chamber, including the cylindrical volumes over the melts and related to them channels. Under the influence of the total gradient it arises a molecular (viscous) flow towards the evacuating environment in the chamber. At low pressure – high temperature conditions the molten material begins to boil steadily thus saturating the spaces over the melts. As a result the partial pressures (concentrations) for partially ionized water vapor, nitrogen and oxygen decrease therein since much heavier ionized molecules of vaporized components (exclusively CaF2) replaced these ions. As a consequence, partial pressure gradients of these three types ionized species will arise along the considered cylindrical volumes, acting in opposite direction to total gradient of gas-vapor mixture. Since the gas-permeability of the channels appears very much lower (minimum of two order of magnitude) than the free cylindrical spaces over the melts, the quasi-equilibrium for mass-transport processes may consider being determined in conjunction with the character of these processes solely into the channels. Thus, a thermodynamic equilibrium may think being attained when the diffusion flows in the channels, caused by acting partial gradients, appear equalized to relevant opposite directed viscous flow components. As a result the effective (total) fluxes for each type inert gaseous species becomes equal to zero. Thence their motion will be restricted to rotational mode by two degrees of freedom and vibrational mode by one degree of freedom while the vector of transitional motion stays zero. This case only vibrational and transitional motion will transmit the energy between the inert ions. The rate for attaining thermodynamic quasi-equilibrium of ionized gas-vapor mixture is determined by the desorption energy level for particular components which is a function on: specific surface area and nature for gas-releasing elements into the chamber where prevalent are elements made of porous graphite, furnace unit temperature, established residual pressure into the chamber (depending on vacuum system performance), and the quantities of sorbed gases. The rate of desorption diminishes rapidly above 600 oC attaining a marginal level after a few hours that makes nonsense of waiting extra time for full degassing the chamber’s elements. In practical, after the melt is being homogenized (≈ 2 hours) the mass-transport processes in gasvapor phase may consider to attain quasi-equilibrium. After that the distribution of ionized molecules along consecutively connected free cylindrical volumes depends solely on the geometric sizes of these volumes and the speed of vacuum system involved. For reducing the number of independent differential equation thus simplifying the mathematics, the residual gaseous/vapor components are treated as uniform molecules with effective molecular weight of 19.7 g per mole, calculated from: (Meff)-1 = 0.8[M(H2O)]-1 + 0.2[M(air)]-1

(6-1)

This way, the total fluxes of CaF2-vapor (J1) and transitionally “immobile” gaseous molecules (J2), all being ionized to a high degree at the maintained temperature head of several dozens of Kelvin over the fluorspar m.p., are expressed by a system of only two

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J.T. Mouchovski and B. Mullin

independent differential equations that, at laminar motion along the vertical axis z, take a form:

J 1   D1

J 2   D2

dn1 n dT d 2 n dP  D T1  x 1 1J  x 1  1 dz T dz 32  dz

(6-2)

dnz n dT d 2 n dP  DT 2  x2 2 J  x2 2 dz T dz 32  dz

(6-3)

The parameters and variables in (6-2) and (6-3) possess relevant physical meanings: - Effective diffusion coefficients, Di, and ratios i and i, (i = 1, 2) are related to coefficients of Knudsen diffusion, Dki, and coefficient of binary normal diffusion, D12, via the definitional relations [86]: 1 D -1i(eff)  D ik1  D12

i 

Di D  (1  12 ) 1 D12 D ik

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 i  1  i  -

-

(6-4)

(6-5)

Di D (1  ik ) 1 D ik D12

(6-6)

Thermo-diffusion coefficients, DT; Linear gradients in partial concentration dni/dz and in total pressure, dP/dz; Viscosity  = η12 of the binary gas-vapor mixture; Total concentration n = P/kT and temperature T of gas-vapor mixture; Mole fraction of participating components, xi = (ni/n); Total flux of gas-vapor mixture, J =J1+J2, where each one of composing total fluxes Ji, represents a sum of diffusion and viscous terms according to Ji = Jdi+xiJv, (i = 1, 2) while J = Jd+ Jv; Diameter of cylindrical volume, d.

In conjunction with kinetic molecular theory [85, 88]:

Dik 

d  8 RT  3  M i

12

1  1.5

d



 i 2   1  1.8525 d  i 2

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(6-7)

Growth of Single and Mixed Alkali Earth Fluoride Crystals …

1 1 12  ) M1 M 2 2 ( 2 .2 )* P 12 12 (T*)

25

1883 . * 10 20 T3 2 ( D12 

12 

1 (1  x1 )  M 1  M 2  1   x 1 12  2M 2  2 12

12

(6-8)



2 x1   M 1  M 2  1   1  x1   2M 1  2 12 2 2

12

(6-9)

where the mean free path of gaseous molecules i in (6-7) is presented by:



RT 2N A  2i (2.2)* (T* )P

(6-10)

while the partial viscosity of gaseous components i in (6-9) – by:

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i 

M i RT 5 16 N A  2i  ( 2.2 )* (T*)

(6-11)

In definitional expressions (6-8) - (6-11) the effective radius of interaction between the two types of molecules is given by the mean value: 12 = 0,5(1 + 2), where i is the radius for corresponding type molecules (i = 1, 2) and the reduced integral of interaction between pair of molecules 12 (T*) are estimated at Lennard - Jones 6-12 potential of interaction and depend on binary interaction constant 12=(1+2)1/2 via the reduced temperature T* = kT/12. (k – is Boltzman constant). Grounding on slightly different physical considerations, W. Heintze [88] has been proposed a multiplier 2/3π instead of 5/16 in (6-11). The origin for the coordinate system (z = 0) is chosen to lie on the lower boundary cross section of considered cylindrical volume that, in case of milticameral crucible with axialsymmetric cameras, means the z-origin for all cameras to lie on one and the same crucible cross-section. The boundary condition for the system (6-2) - (6-3) is: at z = 0, n = nio, n = no, and T = To. The second equation (6-3) of the system is set up equal to zero (J2 = 0) according to the suggestion for quasi-stationary mass-transport of component “2”. Because of the small sizes for utilized crucible CGS-measure system is often preferred to SI-system. The pressures are re-calculated in torr-units when considering useful for comparative analysis.

Assumptions for Analytical Solution Three simplifying assumptions were introduced in the system (6-2) - (6-3) for obtaining analytical solution. First, the Coulomb’s electrostatic forces were disregarded, as their contribution stands negligible in the motion of gas-vapor mixture. Second, the thermodiffusion term in the equations were omitted being its contribution assessed of three orders of magnitude lower than that of concentration diffusion terms. Indeed, the ratio

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26

J.T. Mouchovski and B. Mullin

(D1/DT).(∂x1/∂z)/[( ∂T/T)/∂z] ≈ 103 since (D1/DT) ≈ 10 while the normalized to T temperature gradient remains ≈ 10-2. Third, the diffusion coefficients were taken in their first approximation, that is, it was disregarded their dependence on the content of gas-vapor mixture. Here the maximum error was estimated below 0.1%.

6.2. Solution in Case of Stationary Vapor Flux At quasi-equilibrium along the length l of the cylindrical body, the stationary specific flux for vaporized component (i = 1), J1 (number/s.cm2), is obtained by solving Eq. (6-2) at z=l and J2=0 [89, 90]: J1 = K1t(Po - Pl)/kTl

(6-12)

where K1t represents the total gas-permeability for the cylindrical volume under consideration as regards CaF2 vapor component. The total gas-permeability, K1t, (cm2/s) is related to total gas-conductivity, П1t, (l/s) defined as mass-vapor flux through the cylinder with cross-section area of (π/4)d2 (cm2) per unit pressure gradient, (Po - Pl)/l (din/cm3): П1t = 10-3.(πd2/4l)K1t

(6-13)

This way:

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J1' = П1t (Po - Pl)/kT

(6-14)

where the vapor flow J1' (number/s) is connected to experimentally measured mass-flow, Q (g/h), via a simple relationship: J1' = 3600.(NA/M1)Q

(6-15)

where Avogadro’s number NA = 6.022  1023 mole-1. The total gas-permeability K1t and corresponding total gas-conductivity, П1t, represent sums of two terms: K1t = [Dk1 + KvMm(l)]

(6-16)

П1t = (πd2/4l)[Dk1 + KvMm(l)]

(6-17)

thus unifying simultaneous affect upon vaporized molecules of two different in nature masstransport mechanisms – Knudsen diffusion and viscous flow. The Knudsen term, Dk1, presents a molecular type gas transport by elastic and/or diffuse collisions of molecules upon the wall of the cylindrical body. According to definitional relation (6-7), Dk1 depends on the mean pressure, Pm, of gas-vapor mixture by ratio (d/1) in the quotient [1+1.88(d/1)]/[1+2.322(d/1)], the estimated limits of which are 1 and 0.84 correspondingly

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

27

to 0 and 1 for d/1 ratio. When (d/1)  4 the quotient becomes sufficiently close to 0.84 one to consider the Knudsen term being, in practical, independent on Pm. The second term in (6-16) appears a product of viscous multiplier Kv and an expression depending in a complex way on molecular weight ratio of components M2/M1: Mm(z = l) = (M2/M1)1/2 + [1-(M2/M1)1/2]x1(l)

(6-18)

Kv presents laminar mechanism that induces a fluid flowing by interaction between each two adjacent fluid’s layers. For the binary gas-vapor fluid approximation, Kv is proportional to ratio Pm/12 of the mean total pressure Pm= 0.5(Po+Pl) established in the considered cylinder to mixture’s viscosity 12 presented by (6-9) as the coefficient of proportionality is equal to d2/16 so that Kv = d2Pm/3212. As seen from expressions (6-9) and (6-18) both multipliers, constituting the second (viscous) term of (6-16), depend directly on gas-vapor content. Nevertheless, the dependence is weak within the whole interval for x1 between 0 and 1. Indeed, the calculated by (6-9) - (611) divergence of 12/Mm(l) from 1 = const is less than 7% as an inflexion maximum appears at x1 = 0.3. The strict theory requires Kv to be multiplied by correctional factor f1 depending linearly on (d/1)-ratio. The slope of the relevant line has been found equal to 0.01 [88] so that this correction becomes significant only when argument is higher than 100 units corresponding to d. Pm-product above 9.1 cm.torr. However, conditions for surpassing this limit could never be fulfilled at implemented high vacuum in the growing chamber so that this factor is not further considered being, in practical, equal to 1.

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6.3. Channels’ Effective Gas-Conductivities: End Effects The formulas concerning gas-permeability/gas-conductivities are derived for the case of long channel when the ratio of its length, l, exceeds significantly the channel’s diameter, dlid. However, for relatively short channel, when l and d are of one and the same order of magnitude, the influence of channel’s ends (inlet and outlet) on character and speed of the stream has to be taken into consideration. For the purpose it is convenient to be introduced the effective gas-conductivity for the system of consecutively connected gas-conductivities for particular lid’s channel П1lid, its inlet П1inl and outlet П1out, and the free cylindrical space over the melt П1cyl: П1eff(m)-1 = П1inl(m)-1 + П1lid(m)-1 + П1out(m)-1 + П1cyl(m)-1

(6-19)

П1eff(v)-1 = П1inl(v)-1 + П1lid(v)-1 + П1out(v)-1+ П1cyl(v)-1

(6-20)

where the effective gas-conductivities П1eff(m) and П1eff(v) refer to molecular (Knudsen) and viscous regime of mass-transport, respectively. When predominant is molecular stream, a simple relationship relates gas-conductivities for the inlet and outlet to their circular cross sections [88]:

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28

J.T. Mouchovski and B. Mullin П1inl(m) = (π/4)dinl2√(RT/2πM1)

(6-21)

П1out(m) = (π/4)dout2√(RT/2πM1)

(6-22)

where dinl and dout are the inlet and outlet diameters. Introducing the effective gas-conductivity of the outlet as regards the inlet, П1effout/inl(m), it has been derived a physically grounded relationship [88]: П1effout/inl(m) = (√πRT/32M1)  dout2/[1 – (dout/din)2]

(6-23)

whereat (6-19) becomes: П1eff(m)-1 = + П1lid(m)-1 + П1effout/inl(m)-1 + П1cyl(m)-1

(6-24)

which, for usual case П1cyl(m) >> П1lid(m), is simplified to: П1eff(m)-1 ≈ П1lid(m)-1 + П1effout/inl(m)-1

(6-25)

Using the definitional relationship (6-13), the replacement in (6-25) of expression (6-7) to П1lid(m) and the expression (6-23) to П1effout/inl(m) leads to derivation of convenient for analysis of the limiting cases presentation for effective molecular gas-conductivity of short cylindrical channel which diameter differs from both, inlet and outlet diameters (dout ≠ din): П1eff(m) ≈ П1lid(m)finl/out = П1lid(m){1+1.33[1–(dout/dinl)2]/(l/dlid).(dout/dlid)2}-1

(6-26)

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Two cases appear with practical interest: 1) dout = din (diameter of the outlet is equal to that of the inlet). Then fin/out = 1, which means, the effective inlet/outlet conductivity becomes zero and the molecular vapor flow depends solely on channel gas-conductivity. 2) din = dlid (diameter of the lid’s channel coincides to its inlet diameter). Then flid/out (notified by β’) = {1+1.33[1–(dout/dlid)2]/(l/dlid)}-1, which, for dout> П1lid(v), it follows: П1eff(v) ≈ П1lid(v)/[1+ (П1out(v)/П1lid(v))]

(6-29)

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From (6-19) follows, if П1out(v) and П1lid(v) appear one and the same order of magnitude, an appropriate correction has to be made to П1lid(v), this way taking into account the outlet viscous gas-conductivity. This is conditioned in case the total (opposite) pressure after the outlet stays so low that the passing through the outlet vapor molecules/ions turn out instantly expanded whereat their speed fall down. As a consequence the pressure inside the vapor stream reduces that again accelerates the molecules/ions. As a result the parallel stream, established at relatively high opposite pressure, alters to continuity of swells and knots. A reliable formula has been derived for outlet viscous gas-conductivity, П1out(v), on the base of Prandtl correlation [89] that have been introduced in gas-conductivity definition: П1out(v) = Aout.Ψ

(6-30)

where Aout (cm2) = π(dout2/4) is the area of circular outlet with diameter dout while in the expression:

 Pl

Pl '  2  RT       1   Pl Pl '  1   Pl Pl '    M l  





12

(6-31)

Pl and Pl' are the pressures established just after and before the outlet, γ = cp/cv is the ratio between the specific thermal capacities at constant pressure and constant volume, respectively (for CaF2 γ = 1.3). The replacement in (6-29) of the product (πdlid2/4.l).Kv = dlid4Pm/128lη12 to П1lid(v) and the expression (6-30) to П1out(v) leads to:

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30

J.T. Mouchovski and B. Mullin П1eff(v) ≈ П1lid(v).fvψ =

(6-32)

where: fvψ = [1 + (128.l/dlid2)(dout/dlid)2.(η12.Ψ/Pm)]-1

(6-33)

Following Ψ as a function on (Pl/Pl')-ratio it can be seen the outlet viscous gasconductivity will rise up monotonously as for variables higher than 0.9 the argument exceeds 2 units that causes a fast increase of viscous gas-conductivity, П1lid(v), according to (6-29), that is, the viscous mass-vapor transport turns out in fast increasing control on the rise up of the internal kinetic energy for flowing throughout the channel molecules/ions [88]. It has to be taken into consideration that the estimation of the effective viscous gasconductivity П1eff(v) should be conducted by iteration since in common case the (Pl/Pl')-ratio remains unknown and a target value has to be taken firstly, equal to 1 that corresponds to the case when the outlet correction is being omitted (fvψ = 1).

6.4. Total Pressure Distribution along the Channels The expressions (6-12) - (6-15), together with those representing the correction factors, β” (6-27) and fvψ (6-33), lead to approximately quadratic dependence of Q on pressure difference (Po – Pl) established along the channel since, as a rule, Po >> Pl whereat the derived equation: (Po2 – Pl2)+ b(Po – Pl) + c = 0

(6-34)

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simplifies to square equation: (Po)2 + b(Po) + c ≈ 0

(6-35)

The coefficient b and the free term c are given by: b = (64/3d.β)[η12/Mm(z=l)]um

(6-36);

c = –450(l/d4fvψ)[η12/Mm(z=l)]um2Q(g/h)

(6-37),

while the introduced mean quadratic velocity for vapor molecules um is defined by: um = (8RT/πM1)0.5

(6-38)

Thus conducted simplification manifests an important result: at Po/Pl ≥ 10, Q approximates parabolic function on the total pressure Po established over the melt. This case is mostly fulfilled when geometric sizes of lid’s channel predetermine prevalence of Knudsen diffusion mass-transport mechanism. The more exact solution of Eq. (6-34) requires Pl to be

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Growth of Single and Mixed Alkali Earth Fluoride Crystals …

31

estimated by measuring the pressure established in the chamber before the inlet of the vacuum system. In principal, the determination of (Po – Pl ) from (6-34), respectively of Po – from (6-35), has to be conducted by iteration since the expressions (6-36) - (6-37) contain η12 and Mm(z=l), both expressions of which being depended on gas-vapor content that alters along the length of cylindrical volume under consideration.

6.5. Gas-Conductivity as a Function of Geometric Factor If in definitional relation (6-17) is being introduced the geometric factor Kgeom denoting the ratio d3/l, the first (Knudsen) term in the relation is proportional to this factor, П1k ~ Kgeom. At the same time, the dependence of the second (viscous) term on geometric sizes via Kgeom is more complex, a power-type: П1v ~ (Kgeom)4/3.l1/3 so that: П1t = Kgeom{β’.(πum/12) + (π/256)[fvψ.(Po + Pl)(Mm(z=l))/η12](Kgeom.l)1/3}

(6-39)

At this junction, the vapor flux Q (g/h) may be presented in a form convenient for study as regards Kgeom: Q (g/h) = (Kgeom/450){[β’.um(Po – Pl)/12)] - [fvψ (Po2 – Pl2)(Mm(z=l))/512um2η12]. (Kgeom.l)1/3} (6-40)

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6.6. Mass-Transport at Dominance of Knudsen Diffusion Under high vacuum and high temperature conditions and at sufficiently low gasconductivity in lid’s channel the mass-transport will be strongly dominated by Knudsen diffusion, that is, the vaporized molecules pass throughout the channel striking its wall but not striking each other. In this case Dk1 >> KvMm(l) whereat K1t ≈ Dk1 and П1t ≈ П1k. Under such approximation, from (6-14) and (6-15) it follows a simple linear dependence of Q(CaF2) on pressure difference established along the channel, (Po – Pl): Q (g/h) ≈ (2400/um)(β’d3/l)(Po – Pl) = 1200(πM1/2R)0.5(β’d3/l)(Po – Pl)/T0.5

(6-41)

or, in case the pressure is given in torr-units and 78 g per mole is replaced for M1, Eq. (6-41) becomes: Q (g/h) ≈ 1942.1(β’d3/l)(Po – Pl)/T0.5 = 1942.1β’.Kgeom(Po – Pl)/T0.5

(6-42)

Low gas-permeability in the channel (Dk1>>KvMm(l)) means high resistance effect on molecular motion that leads to significant pressure difference established along the channel. This way Po appears much higher than Pl (Po >> Pl.) and (6-42) can be simplify to: Q (g/h) ≈ 1942.1(β’d3/l)Po/T0.5 = 1942.1β’KgeomPo/T0.5

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(6-43)

32

J.T. Mouchovski and B. Mullin

6.7. Distribution of Inert Gaseous Species The partial pressure gradient of gaseous component “2” established along the length of cylindrical body (channel), (P2l – P2o)/l, can be estimated using the solution of equation (6-3) after the molecular flux J2 is being set up equal to zero: P2l/P2o = exp(Al)

(6-44)

where: Al=[1+Mm(l)/Knk)]/[D12(Po)/D1k]-[Mm’(l)]-1{1- [1+ +4[1+Mm(l)/Knk)](1+(Mm(l)/Knk)]1/2}

(6-45)

The diffusion coefficients D1k and D12, and gas-vapor characteristic expression Mm(z=l) are defined by (6-7), (6-8), and (6-18), respectively, while: Mm'(l) = (M1/M2)1/2 + [1 – (M1/M2)1/2]x1(l)

(6-46)

and so called Knudsen number Knk(Po) is being introduced for the ratio D1k/K1v(Po) [86,89]. By using the expression (6-44), the P2l/P2o-ratio can be followed as a function on the total pressure Po established in any cylindrical volume varying parametrically its gas-permeability. The values for Po are obtained from the solution of Eq. (6-34).

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6.8. Total Pressure in Specialized Crucible: Influence of the Vacuum System When it is used a multicameral crucible with intermediate compartment servicing as a buffer between the cameras and ambient it appears very important to assess reliably the true pressure established in this compartment since it determines the driving forces that initiate the vapor fluxes inside the crucible and the total flux to vacuum ambient in the chamber. The tubular intermediate compartment with annular cross section has sufficiently large volume to determine at quasi-equilibrium uniform pressure Ptub inside the compartment including the space just before the inlet of the general channel. The total gas-conductivity in this channel П1tgcov is, as a rule, very much lower as compared to that in the vacuum chamber, Пcb. At this junction (Ptub/Pl)-ratio is inversely proportional to the ratio of the relevant total gasconductivities wherefrom it follows: Ptub = Pl .(Пcb/П1tgcov)

(6-47)

The assessment of (Пcb/П1tgcov) shows this ratio is approximately equal to 200 for the used Bridgman-Stockbarger Growth System (BSGS) described in Sec. 7 provided the conductivities are being estimated as regards the geometric sizes of the water-cooling chamber and the channel at temperatures 293 K and 1725 K, respectively: Пcb = 150 l/s and П1tgcov = 0.77 l/s. Nevertheless, it has to be taken into consideration that the free space into the chamber is significantly less than estimated geometric volume since it represents a complex

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33

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system of consecutively and in series connected free spaces with irregular geometric volume and indefinite gas-conductivities. Approximating these spaces to different geometric volumes and using corresponding formulas relating their gas-conductivities, it is found Пcb being significantly lower, ≈ 62 l/s whereat (Пcb/П1tgcov) ≈ 80.5. Thence the estimation of Ptub by using Eq. (6-35) instead of Eq. (6-34) leads to relative error of ≈ 1.2% whereat the vapor flux throughout the general channel is presented as parabolic function of the pressure established into the tubular (peripheral) compartment. During the controllable increase of FU-temperature up to 1200 K the saturation pressure for CaF2 over not melted yet grained fluorspar loaded in the crucible does not exceed 10-6 torr (Figure 6.1.) whereat it could not affect the pressure established into tubular compartment, Ptub, that is determined according to actual evacuation speed of the free spaces inside the crucible.

Figure 6.1. Partial pressure of saturated CaF2 vapors over fluorspar melts as a function of temperature T.

For low gas-conductivity of the general channel П1tgcov, if Pl is expressed via the measured pressure, Pp, established in the vacuum chamber, the speed of evacuation Sp on the inlet of the diffusion pump and the net evacuation speed of the chamber, S-1 = (Sp)-1 + (Пcb)-1 [88,92], from (6-47) it is being derived: Ptub= [(Пcb+Sp)/П1tgcov]Pp

(6-48)

Since Sp for utilized BSGS is set up equal to 300 l/s at ultimate pressure of approximately 10-2 N/m2 (7.5 .10-5 torr), then:

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34

J.T. Mouchovski and B. Mullin Ptub = [450/П1tgcov]Pp

(6-49)

If taking the estimated value Пcb-≈ 62 l/s, and S ≈ 51.4 l/s, then: Ptub = (362/П1tgcov)Pp

(6-50)

Taking into account the position of measuring head for used Penning type vacuummeter, an additional correction is made for Pp that lower the measured values by factor of ≈ 28. Thus for calculation the true pressure inside the peripheral compartment at T below 1200 K it is used a precise empirically formula: Ptub ≈ 0.036.(362/П1tgcov)Pp ≈ (13/П1tgcov).Pp

(6-51)

6.9. Deposit in the Common Crucible Compartment The mass flux of the material (jm)subl (g/s.cm2) crystallizing from vapor phase in the peripheral compartment depends on both kinetic of deposition and vapor transport limitations. According to Schonherr [94] (jm)subl, is related to the difference (Psour – Ps) established between the saturation pressure of the source (vaporized CaF2 melt) and the actual vapor pressure at the surface of growing layer via the equation:

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[(jm)subl/kn]1/n + [(jm)subl/kg] = (Psour – Ps)

(6-52)

where kn is a kinetic constant, kg – the coefficient of mass-transport, and the denominator in (1/n)-index is equal to 1.2. The kinetic constant kn (cm/s)-1 is defined by [94]: kn = Yk[M1/2  RT]1/2

(6-53)

where the kinetic mechanism is presented by parameter Yk. while M and T are vapor characteristics. When the growth proceeds by parallel mono-atomic layers distant to yo each other, and in case yo is comparable to the mean free path of vapor species, xs, that hit the surface layer before being adsorbed on it, Yk has been derived in the form: Yk= stanh[yo/(2)1/2xs]/[yo/(2)1/2xs]

(6-54)

For growth layers being disposed close to each other, yo > kg, the transport limitations turn out determining for (jm)dep: (jm)subl = kg(Psour-Ps)

(6-56)

whereas in case kn 6), “ytterbium” (Rc < 0.2) and “intermediate” (0.4 ≤ Rc ≤ 1.5).

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J.T. Mouchovski and B. Mullin

Optical Characterization of Grown Crystals The most sensitive optical characteristic has been recognized the light transmission (transmittance) tλ [49]. It is defined as intensity of the light observable emerging from the crystal to the intensity incident on the crystal. The light transmitting through a given crystal sample diminishes firstly, by reflection at boundary surfaces, secondly, by absorption into the lattice and, thirdly, by internal scattering as a result from: irregularities on the surface (due to bad polishing or/and contamination), any inclusions captured into the crystal and plastic deformation of its structure. According to this demarcation, several characteristic coefficients are defined given below. Light-scattering Lsc (coefficient) represents the intensity of scattering from crystal inlet/outlet surfaces to the intensity incident on inlet surface. In the usual case of a plane polished window with normal light incidence and negligible absorption the coefficient of light-scattering is called equivalent reflectivity rλ' depending on the index of refraction nλ by: (nλ-1)2/(nλ2+1) [96]. Light-absorption Labs represents the intensity of light absorbed in the crystal to intensity incident on the inlet surface. If absorption losses exceed about two orders of magnitude the sum of multiple reflections and internal scattering, Labs is equal to 1 - t = 1 - exp [-αλlwin] where lwin is the window’s thickness (the light path). Light-extinction Lext is the sum of light-scattering and light-absorption, that is, the intensity of extinction of the light transmitting through the crystal to intensity incident on the inlet surface, that is (1/tλ). The Briggs logarithm of coefficient of extinction is called optical density Dλ = log (1/tλ). Extinction index Eλ is determined using Buggert -Lambert - Berr law:

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I = Io (1-rλ)2 exp [-(αλ + ρλ)lwin]/[1-r2exp [-2(αλ + ρλ)lwin]

(7-1)

where αλ is the absorption index, ρλ - the internal scattering index, and rλ = (nλ - 1)2/(nλ + 1)2 is the reflectivity of the optical window. For nλ ≤ 2.5 (which is fulfilled for all alkali earths fluoride crystals) and if the absorption is low enough, the second term in the denominator of (7-1), rλ2 exp [-2(αλ + ρλ)lwin] ≤ 0.03 at lwin ≤ 1 cm. Then: I ≈ Io (1-rλ)2 exp [-(αλ + ρλ)lwin]

(7-2)

Equation (7-2) may be written as a sum of three terms: the first term, Io exp [-(αλ + ρλ)lwin], is absorption-internal scattering term (no reflection losses); the second term, 2rλIo exp[-(αλ + ρλ)lwin], represents twice the reflection loss from the first surface plate; the third term, rλ2Ioexp [-(αλ + ρλ)lwin], is less than 1% compared to the second term for nλ ≤ 2. Hence, neglecting the third term a simplified expression is derived: I ≈ Io (1-2rλ) exp[-(αλ + ρλ)lwin]

(7-3)

Thus, for alkali earth fluoride crystalline materials the external transmission is given by: tλ ≈ (1-2rλ) exp[-(αλ + ρλ)lwin]

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(7-4)

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The natural logarithm of (8-4) gives an expression for the extinction index, Eλ, of a given window: Eλ (L-1) = αλ + ρλ ≈ lwin. ln[(1-2rλ)/tλ)]

(7-5)

If ρλ 110 mm (7-13b) Here, ∆T1(x1) and ∆T2(x1) represent the current T-ramps according to set up temperature programs for the two furnace zones (Figure 7.4). The coupling of expressions (7-13a) and (7-13b) at x1 = 110 mm corresponds to crucible position when the plane, differentiated by the conical tips in the peripheral cameras, reaches a FU cross-section that is distant to 4 mm below the lower boundary section of the AdZ (x1 = 0). At such position a substantial re-distribution of the thermal flow throughout the load may be expected since the crucible bottom is just entering the Z2 where the lateral surface of penetrating crucible starts continuously to enlarge. At this junction, the lateral (radial) heat losses from the load increase rapidly inside Z2. This inevitably leads to progressively faster cooling of the growing boules. As a result of such a heat imbalance, the CF shifts upward to Z1, which could be compensated by relevant increase of T1 or T2 or both temperatures. As has been already shown, the number of R-rings fixed on crucible stem may only shift symmetrically the dependence of CF position along the furnace height in the case of CaF2 boules in a batch growing process [49]. The formulas (3a, b) can easily be unified to retaining their frame of reference for any furnace unit having a similar construction (two zones with a sufficiently large SD) where the described processes are firmly in progress since only the coefficients a1, a2 , b1, b2, and c1 are changed appropriately.

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It is introduced as well a supplementary variable xcon(z) to define the distance passed by the plane section of conical tips in the crucible’s peripheral cameras (where the initial nucleation starts) relative to the lower boundary section of the AdZ. In this way the height representing the crystallized part of the boules is being determined by the difference (xCF – xcon). For simplifying the analysis, both distance parameters, xCF and xcon, are reduced to dimensionless form: xCF* = xCF/lAdZ and xcon* = xcon/lAdZ where lAdZ = 2.4 cm is the thickness of the AdZ. But specifically the x1 variable defined the extent of the total distance the crucible moves in a given run: x1* = x1/x1tot. The currently crystallized volume of the boules is defined by V* = Vcryst/Vcrysttotal. Thus, the crossover points of the xCF*(x1*) curves with the straight line, xcon*(x1*), will specify so-called “nucleation” curve xnucl*(x1*) that manifests the starting positions for crystallization in the particular cameras. As follows from the relationships (3a, b), shifting the CF towards Z2 lead to a reduction of the CR, whereas shifting towards Z1 means the CR is being increase. Thus, knowing the xCF vs. x1 dependence and the CS set up, one can easily calculate the relative positive/negative alterations in the CR with regard to the CS, that is, one can obtain the vreal* = (vreal/vcru) vs. x1* dependence.

8. CONTROL OF RE IMPURITIES VIA HIGH-T PURIFICATION TECHNIQUE

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8.1. Mass Transport in Conventional Multicameral Crucible 8.1.1. Vapor Flux versus Total Gas-Conductivity and Temperature Head The important feature of conventional multicameral crucible type “platform-support” appears that initiated vapor fluxes Q pass throughout lids’ channels directly in chamber ambient. The dependence of these fluxes on varying gas-conductivities of lids’ channels is being followed by using Eqs. (7-39) - (7-40). The obtained family of curves 1, 2, and 3 (Figure 8.1a) demonstrates a theoretically predicted parabolic dependence, the slopes of which remains nearly constant for П1tlid ≤ 0.7 l/s, and can be approximated by straight lines. This result shows a dominance of Knudsen diffusion for the vapor transport in the channels that means, even under a small temperature head of 15 K (curve 1), the free spaces over the melts are being rapidly supersaturated by CaF2-vapor whereat the total pressure Po therein turns out higher than the saturation pressure Ps corresponding to set up constant furnace zones temperature (Figure 6.1). At П1tlid > 0.7 l/s the curves’ slope decreases progressively that manifests gradual attenuation for vaporization intensiveness. The lower position of curves 1a, 2a, and 3a (representing in practical straight lines), obtained from (6-39) - (6-40) replacing therein Po = Ps, as compared to curves 1, 2, and 3 indicates that at Tmelt > 1688 K the evaporation from melts surface contributes insignificantly for vapor flux constitution at the expense of rapid increase of the vaporization by bulk boiling. The sharp increase of curve 3 in its initial linear section as comparing to that of corresponding line 3a shows that, at low channels’ gas-conductivities, a temperature head of 100 K is, more likely, being sufficient to initiate a high rate of vaporization by bulk boiling mechanism, which to exceeds many times the rate of surface evaporation. Here, the vapor flux is being conditioned, in practical, by Knudsen mass-transport mechanism so that the free spaces below the lids are being rapidly

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super-saturated. At П1tlid ≥ 0.5 l/s the slope of curve 3 decreases sharply that should be due to competitive influence of film boiling mechanism whereat the rate of vaporization will decrease correspondingly.

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Figure 8.1a. Fluorspar vapor fluxes throughout the lids’ channels versus gas-conductivities in corresponding lid’s channels at three different melts temperatures: curve 1 () – 1663 K, 24 h; curve 2 () – 1688 K, 24 h and () – 1688 K, 36 hours; curve 3 () – 1764 K, 24 hours. Corresponding curves 1a, 2a, and 3a are estimated from Eq. (7-40) replacing therein Po to CaF2 saturation pressure Ps(Tmelt).

Figure 8.1b. Vapor flux, Q, throughout lids’ channels as a function of melts temperature Tmelt at parametric alteration of total gas-conductivity in the channels, П1tlid (l/s): () – 0.1; () – 0.3; () – 0.5; () – 0.7; () – 1; () – 1.4; () – 2; () – 2.5; () – 3. Dash curves: No. 1 – П1tlid = 0.5 (l/s); No. 2 – П1tlid = 1 (l/s); No. 3 – П1tlid = 3 (l/s).

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For fast alteration in vaporizing mechanism under increasing ∆Thd argues the specific course of the curves presenting Q as a function of melt temperature Tmelt at parametrically varying gas-conductivity in the lid’s channels, П1tlid (Figure 8.1b). As seen the curves’ slope rises up sharply for ∆Thd exceeding 40 K that testifies for fast alteration in vaporizing mechanism from evaporation to vaporization by bulk boiling. Replacement of Ps in (6-41) for Po at П1tlid taken parametrically equal to 0.5, 1, and 3 l/s leads to curves, the slopes of which appear significantly lower than those for corresponding experimentally found curves as the relevant slopes’ difference rises up with Tmelt-increase. This result shows that even at low temperature head the rate of vaporization turns in practical higher than the concurring process of evaporation from melt surface.

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8.1.2. Total Pressure Versus Total Gas-Conductivity This dependence is substantial for clarifying the physical-chemical processes in crucible interior. Since the rate of the vacuum system is constant and the temperature head is being restricted within 50 and 80 K (so that to be avoided excessively evaporation losses and melt decomposition), the geometric parameter of the channels becomes with primarily importance for efficient controlling the total pressure Po established in the free spaces over the melts. The data set for vapor flows Q are used for solving Eq. (6-35) instead of Eq. (6-34). This is conditioned by covered range of relatively low gas-permeability in the lids’ channels whereat Po ≥ 10Pl can be thought being fulfilled. Since the coefficient b and the free term c in (6-35) depend on gas/vapor distribution along the channels they are being determined via iteration. Nevertheless, for the specific case of CaF2-H2O vapor-gaseous mixture the convergency is assessed being very fast –after a single iteration the error reduces to less than 1%. The results lead to experimental dependence of the total pressure, Po, established in free spaces inside the inserts, on total gas-conductivity in the lids’ channels, П1tlid, that is close to theoretically predicted parabolic one (Figure 8.2).

Figure 8.2. Total pressure, Po, established into inserts over the melts at constant temperature (Tmelt = 1688 ± 10 K) as a function of total gas-conductivity in relevant lids’ channels: () – 24 h, () – 36 h run’s duration. Fitting curve equation: Po = 18.594 - 1.4392*Π1tlid +0.6/Π1tlid. Line 1: Saturation vapor pressure of CaF2, Ps(CaF2) = 8.66 N/m2 (0.065 torr). Curve 2: Initial residual pressure, Poinit, into inserts before heating the crucible.

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Figure 8.3. Characteristic ratio, P2l/P2o, between partial pressures of oxygen contaminants that are established in front of and behind the channels in inserts’ lids versus gas-conductivity, Πlid, of the channels at a constant temperature head imposed upon the melts.

As seen, the total pressure Po, starting from 29.3 N/m2 (0.22 torr), reduces further by only 68% when the argument increases by two orders of magnitude. Such behavior gives rise to moderate supersaturation of the free spaces into inserts having low gas-conductivity ( 0.08 cm3/s, the free space over the melts will be supersaturated as it is approved from the picture in Figure 8.2. This condition is not consistent with two experimental points remaining on the left of the line П1tlid = 0.08 cm3/s. Such discrepancy may be explained by taking into consideration interfering effects upon the variation in the position of the cross over point for curve 2 and line 1, and the high relative error in determining the low stream Q through channels with too low gas-conductivity.

Back Diffusion of Chemically Inert Species The boules grown at the conditions analyzed in Sec. 8.1.2 (low temperature head of 40 K at residual pressure in the chamber within 5 and 8 .10-3 N/m2) are found visibly slightly colored. Only the boule, corresponding to maximal losses of material throughout the channel with the largest gas-conductivity (5 l/s), is found milk-white colored. The t-spectra reveal all boules are being completely opaque in the UV, showing shallow bands of light-absorption in the Vis that indicates a presence of color centers in fluorite lattice. The boule, grown in the insert with the largest gas-conductivity of lid’s channel, turns out completely non-transparent in the UV and Vis regions. This means considerably higher degree of oxygen/water vapor contamination for this boule as compared the others. The material’s losses of the boule in question are measured being ≈ 25%. This result suggests that the growth inside this insert has been proceeding under conditions of bulk boiling producing an intensive stirring of the melt and fast convective transport of oxygen-containing ionic complexes whereat mostly of them may enter the CZ and forming CaF2-CaO eutectics that turn out incorporated in the lattice of growing crystal. The low quality of the grown crystals, being proved a result of significant oxygen contamination throughout the lids’ channels, means that the usage of a powerful vacuum system, although capable to compensate to a high degree the inevitable leakages in the chamber thus providing very low residual pressure (below 10-3 N/m2) therein, cannot prevent the penetration of oxygen contaminants inside the crucible from ambient. The reason is that as low is being maintained the residual pressure inside the chamber, as steep partial gradients for oxygen and water vapor along the leakages’ places will be established. Thence, their concentration into the chamber cannot fall below a definite level attained when the speed of evacuation equalizes the rate of back diffusion through the leakages. At this junction condition, the probability of oxygen/water vapor contamination inside the crucible remains considerable and could be restricted only by appropriate increase of gas-transport resistance of the channels, Thence the normal diffusion will be replaced by much slower Knudsen diffusion while the influence of the vacuum system is marginalized and the spaces over the melts are easily supersaturated. The driving force for Knudsen diffusion appears the mean partial pressure gradient for inert (oxygen-containing) gaseous components (P2o - P2l)/llid < 0. At quasi-equilibrium this negative gradient is opposite to that of the total pressure gradient (Pо – Pl)/llid > 0. The relevant vapor-gas distribution is being presented by the dependence of (P2l/P2o)-ratio on channels’ total gas-conductivity Πtlid calculated from the expressions (6-44) and (6-45) at z = l = 0.15 cm and Tmelt= 1415 oC (Figure 8.3). As seen the steepness of the curve increases in

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hyperbolic manner attaining 2.8 units at argument of 5 l/s. Thus, taking P2l ≈ 2.10-4 torr (the residual pressure measured at chamber’s outlet (inlet of the vacuum system), one can estimate the gradient for component “2” equal to 8.6•10-4 torr/cm. The steep curve’s increase for low argument’s values (up to 0.16 l/s) indicates the partial concentration for oxygen-containing species will rise up very fast inside the inserts all the time when the Knudsen diffusion in relevant lids’ channels stays a rate-determining stage for the overall gas-liquid mass-transport for these species. At larger arguments the curve’s steepness drops down sharply since the total pressure, Po, in particular inserts attains relatively low level of quasi-equilibrium and thus does not affect on contribution of the normal diffusion and viscous flow as gas-transport mechanisms. At low gas-conductivity in lid’s channels the spaces over the melts appear supersaturated, that is, the established total pressure stays relatively high, while the normal diffusion – correspondingly low. Under low temperature head (40 K) the experimental data (Figure 8.2) show a highest total pressure of super-saturated vapor in the insert, the lid’s channel of which possessed the lowest gas-conductivity: at Klid = 0.05 l/s, Po ≈ 0.22 torr (29.3 N/m2), whereat D12 ≈ 104 cm2/s. However the vapor flux, Q, throughout this channel (curve 1 in Figure 8.1) is found being insignificant at these conditions. This pre-supposes a negligible total pressure gradient along the free volume over the melt. Consequently, the relevant opposite negative partial pressure gradient for inert component 2 will be insignificant too at quasi-equilibrium vapor-gaseous mass transport. Hence, both Knudsen diffusion and viscous flow should play marginal role for the movement of oxygen-containing species throughout the gas-vapor atmosphere inside the considered insert at the expense of normal diffusion mechanism. The resistance of vapor - liquid phase transition is being assessed too low at relatively low rates of mass-exchange so that it is out of consideration. Considering further the liquid phase mass transport for the dissolved into the melt oxygen-containing species their penetration to the CZ has to be limited because of firstly, low liquid diffusion coefficient (not exceeding 10-9 m2/s) and secondly, unstable concentration gradients owing to the melt vaporization by boiling and destroying effect of forced convection. Analyzing the case when the temperature head ∆Thd is imposed being much higher (above 270 K) it is seen the vapor flux Q grows up increasingly faster through the channels with gas-conductivity > 0.3 l/s (curve 3 in Figure 8.1). Under such conditions the attained equilibrium for total pressure Po in corresponding inserts, calculated within the range of 300 400 N/m2, may provide super-saturation even at relatively high gas-conductivity in the channels. In this case the influence of the vacuum system will drop down compared to the case of weak temperature head but still the normal diffusion remains sufficiently intensive to cause significant penetration of oxygen contaminants in the relevant inserts. Besides too much material will be lost being transferred directly into the vacuum chamber – up to 50% at Klid > 2.5 l/s and even at a moderate temperature head of 100 K. The results of implemented experiment reveal how, by controlling the total pressure Po in the crucible inserts by varying the gas-conductivity in the lids’ channels and adjusting the temperature head imposed over the melts, one may influence on the type of mass-transport mechanism in the channels in a way to reduce substantially the penetration of gaseous oxygen contaminants into the inserts. However any significant lowering of channels’ gasconductivities makes impossible to be provided sufficiently intensive vaporization so that an efficient RE-purification at vapor-melt IF cannot be accomplished. That is way the next series of experiments are being carried out in specially designed multicameral crucible – “tube-

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support” (Figure 7.1b). This mode crucible is thought to allow more strict control upon physical chemical conditions in crucible interior, capable of providing the needed REpurification at minimum oxygen contamination. The crucible consists of central camera surrounded by peripheral tubular compartment with annular cross-section wherein can be fixed in parallel up to 9 inserts. Lids through which have been drilled axial channels cover the loaded inserts. The crucible itself is being isolated from chamber ambient by outer cover, the gas-conductivity in which axial channel, together with the gas-conductivities in inner lid’s channels, constitute a system of parallel and in series connected gas-conductivities, determining the total effective gas-conductivity of the system. The sum of the vapor fluxes throughout lids’ channels gives the total amount of vaporized material passed into the common peripheral tubular compartment thus supersaturating the free space therein. It is supposed mostly supersaturated vapor to be deposited when being in contact with supercooled inner surface of crucible bottom whereupon they condense with following nucleation and crystallization proceeding. Only insignificant quantity of vaporized material will pass throughout the outer channel into the vacuum chamber.

8.2. Mass-Transport and Kinetics in Specialized Multicameral Crucible

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8.2.1. Vapor Flux Throughout Lids’ Channels Versus Geometric Factor The total pressure Po inside the cameras depends on lids’ channels gas-conductivity via the vapor fluxes Q throughout these channels. Thus the accent falls on investigation of Qdependence on gas-permeability for inner lids’ channels influenced by gas-permeability of the general channel (Figure 8.4).

Figure 8.4. Vapor flow throughout the channels in the lids covering the peripheral inserts versus channels’ gas-permeability factor, Kgeomins, at constant temperature head upon the melts, ΔThd = 34 K, for two values of gas-permeability of the general channel in crucible cover: (●) – Kgeom(gcov) = 0.085 cm2; (○) – 0.0014 cm2.

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Figure 8.5. Ratio of attained mean to initial concentration (deepness of purification) for two RE impurities each one appearing as principal in corresponding type of fluorspar – cerium (○) and ytterbium (●).

For relatively low temperature head of 34 K imposed and two values of Kgeom(gcov) (differing each other by factor of ≈ 60) the dependence follows both cases a parabolic course with curvature much more pronounced for the higher gas-permeability of the general channel while the maximums are shifted each other to 1.7 cm2. Such large shift indicates considerable alteration of the proportion for the factors determining the gaseous-vapor quasi-equilibrium inside the crucible. The ascending curves’ branches shows that at not too high gas-permeability for lids’ channels, when the Knudsen term in the relationship (6-40) stays dominant for gaseous-vapor mass-transport, the vapor fluxes’ increase will be accompanied by relevant linear lowering of Pins as suggests the curve in Figure 8.2 that can compensate only partially the stronger linear enlargement of Qins on Kgeomins according to Eq. 6-41. As seen this picture alters under the more and more tangible effect of the viscous flow on the vapor transport throughout the channels by enlarging their gas-permeability; it comes to the fore significantly faster lowering of Poins by the increase of Qins as a result of fast augmentation of the vacuum system effect.

8.2.2. Degree of RE Purification for Particular RE For assessing the efficiency of the developed purification technique as regards particular RE it is being followed the dependence of purification deepness REi in non-vaporized crystallized part of the melts Gpur when “ytterbium” (Rc ≤ 0.2) and “cerium” (Rc > 6) types fluorspars are being used. The gas-conductivities in lids’ channels are being varied so that the vaporized quantities to induce Gpur alteration within 0.76 and 0.98. As seen (Figure 8.5) the studied dependence goes down much steeper in case the cerium type fluorspar has been used as all data points are

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positioned on the right-hand side to those ones concerning ytterbium type fluorspar. This fact shows much higher purification efficiency when the principal RE impurity is cerium.

8.2.3. Purification Deepness and Intensiveness for Fluorspars with Largely Varying Impurities’ Content The dependence (5-8) is being used for assessing the influence of the specific fluorspar impurities’ content on the deepness and intensiveness of purification process as regards a particular RE micro-impurity – cerium – as mostly occurring Ln in selected fluorite probes. The data points for Ce versus Gpur dependence are found to be dispersed on four curves, each one corresponding to definite type of fluorspar (Figure 8.6). The curves follow an exponentially decreasing dependence with origin at 1.0-coordinates and their slope alters in accordance with the type of fluorspar. The definitional Eq. (5-8) is rewritten in a form used for data fitting: Ce = exp(-#A).exp(#A.Gpur)

(8-1)

The constant #A represents the product (ai.hin) that, multiplied by RT gains a definite physical meaning, presenting the alteration in chemical potential for Ce along the height of the residual melts where intensive vaporization and crystallization are proceeding simultaneously.

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Table 8.1. Characteristic parameters of developed purification/growing technique (deepness and intensiveness of purification (alteration of chemical potential at IFV/M) for cerium impurity) by using fluorspars originating from different Bulgarian deposits

Type of fluorspar

“Cerium” (Rc > 6)

Fluorite Deposit

Slavyanka (*)

Optical Element

windows lwiv=10 mm

Rich/Poor to Ln “Intermediate” (0.4≤ Rc≤1.5) Slavyanka (*) Finished cylinders lcyl=3.5 cm

Slavyanka (*) Chiprovtzi (2*) Mihalkovo (3*) Finished cylinders lcyl= 2.5 - 4 cm

Element’s Adjacent to 1 cm above the Adjacent to disposition conical section conical section conical section Corr., coef., 0.9820 0.9836 0.9836 R2 FitStErr 0.0420 0.0420 0.030 #A 4.1918 3.0163 2.0382 aCe (cm-1) 0.9315 0.6703 0.4529 ΔμCe 3252.1 2340.44 1581.9 (cal/mol.cm) βCerem(Gpur=0) 0.0151 0.0490 0.1303 (*) – South-west Bulgaria; (2*) – North-west Bulbaria; (3*) – Central Bulgaria. (4*) – aCe = #A/hin; ΔμCe = aCe.RT; βCerem(Gpur = 0) = exp(-#A).

Rich to Ln – “intermediate” (0.4 ≤ Rc ≤ 1.5), non-isomorphic impurities Chiprovtzi (2*) Mihalkovo (3*) Finished cylinders lcyl= 2.5 - 4 cm

Adjacent to conical section 0.9705 0.035 1.3329 0.2962 1034.1 0.2637

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The data fitting lead to very high coefficient of correlation (R > 0.985) at insignificant standard error, within 0.03 and 0.042, independently on the type of fluorspar (Table 8.1). Hence the applied mathematics describes correctly the basic physical-chemical processes proceeding during the described high-T Ce-purification procedure. Comparing the curves’ slope at Gpur = const, it is being found out a slope’s decrease in the sequence of curves’ number: 1→2→3→4. This fact indicates lowering in this order of the coefficient of purification intensiveness for Ce аСе (cm-1) along one and the same height of such purified boules grown from different types fluorspars. The effect is grounded by relevant alteration in the chemical potential per unit height, ΔμCe= aCe.RT, cal/mole.cm (Table 8.1). The established significant changes in ΔμCe when the total Ln-content varied up to 100 ppm as well as the strong sensitivity of this parameter as regards presenting non-isomorphic impurities (mostly oxides of Si, Al, and Fe) with concentration up to 0.1 wt.% give grounds, ΔμCe to be utilized as reliable criterion for usability of a given type fluorspar to be purified efficiently by the developed technique for obtaining high grades optical fluorite.

Figure 8.6. Residual cerium content, reduced to its initial fluorspar concentration, in non-vaporized (purified) parts of the melts after its simultaneously proceeding crystallization. Curve 1 and 2: “Cerium” type fluorspar (Rc > 6): () – lowest positioned windows, (), () – finished cylinders positioned adjacently above the windows; () – “Intermediate” type fluorspar (Rc≈ 1), poor to Ln (total amount within 7 - 9 ppm). Curve 3: (●) – “Intermediate” type fluorspar (0.4≤ Rc≤1.5) with varying Lncontent. Curve 4: () – rich to Ln-impurities (total amount ≥ 100 ppm.) fluorspar containing some nonisomorphic impurities.

The lower position and the steeper slope at constant Gpur of the curve 1 (windows) as compared to curve 2 (cylinders) can be explained with segregation mechanism influencing the axial distribution of the residual Ce in the grown boules. The found differences are

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appreciable since the equilibrium segregation coefficient has been found equal to 0.88 [66] that is, being sufficiently far from 1. When both, Ce and Yb contents are found being commensurably low (7 - 9 ppm) – Rc≈ 1, the relevant data points () – in case optical windows are being used – lie again on the curve 1 for Gpur < 0.50 whereat βCe drops down below 0.1. This result shows high deepness for Ce purification, independently, in practical, on Yb-content. The shift of some data points () on higher positions (around βCe= 0.2) could be due to alteration in redox conditions during purification and/or crystallization processes. The established high rate of lowering for relative Ce-concentration means high intensiveness of purification process at low initial level of Ce in the fluorspar. Thus, by implemented purification technique it is being attained a mean residual level for Ce-content in the grown boules ≈ 1 ppm, which satisfies the strict requirements for production of laser grade (LG) fluorite. However, the concentration of the other REs has to be also reduced about this level, which, as seen in Figure 8.5, is not realized for Yb in any of the grown boules wherein the minimum residual Yb-content is estimated being still too high (≈7.6 ppm). Hence, when target appears LG crystals to be grown, the developed purification/growing technique (P/Gr-T) is insufficiently effective to be applied upon any “intermediate” type fluorspar even if the initial total Ln-content seams being sufficiently low (within 5 and 10 ppm). Principally such unsatisfactory result can be explained with stronger affinity of Ce-ions to electron’s capture than any other Ln-ions. This means the presence of Ce3+-ions will suppress the oxidation of any bi-valence RE-ions, that is, Ce3+-ions possess reduction effect as regards Yb2+-ions in fluorite lattice whereat the later will stay in its lower valence state. Taking into consideration that any ytterbium fluoride composition with stoichiometry close to YbF2 (YbF2-x, x ≤ 0.1) possesses volatility (saturation pressure T-dependence) similar to that of the basic CaF2 [101], it will be thermo-dynamically non-grounded significant amount of Yb to be transposed from molten to vapor phase in a form of complex bi-valence Yb ions. The formation of easy volatile YbF3 (boiling point of 150 K lower than that of YbF2+x) will determine strongly oxidative conditions in the melt but it seems unlikely to occur inside a graphite crucible at residual atmosphere in the chamber consisting exclusively of water molecules/ions. Indeed, both products of the reaction [49]: C + H2O = CO + 2H

(8-2)

possess strong reduction effect as regards any tri-valence RE-compositions. Only at a definite excess of PbF2 scavenger added to starting fluorspar portions some 2+ Pb , remained non-reacting with oxygen-containing ions and being in good contact to uniformly distributed Yb2+, can reduce to atomic lead, Pbo in accordance with reaction: 2Ln2+ + Pb2+ ↔ 2Ln3+ + Pbo

(8-3)

thus oxidizing the presented Ln2+-ions. In this case ytterbium, in a form of complex ions of YbF3 composition with saturation pressure at given T significantly higher that CaF2, would overcome easily the IFV/M resistance to convert with considerable rate in vapor phase. This way, Yb-content into the purified precursors should drop down correspondingly. The oxidative reaction (8-3) requires, however, very strict dosage of PbF2 that to provide

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effectiveness for both scavenger and oxidative effects. Hence, when poor to Ln-content fluorspar – “intermediate” type – has been used, its purification via described technique will be efficient only as regards the RE-impurity with highest affinity to electron’s capture, that is, as regards cerium. When Ce, Yb and Sm with similar concentrations are distributed in the used fluorspar within interval of several dozens of ppm (0.4 ≤ Rc ≤ 1.5), the mechanism and related efficiency for their purification in final boules have been found to depend substantially on the amounts of several non-isomorphic impurities – mainly of Si (0.13 wt.%), Al (0.02 wt.%), and Fe (0.0004 wt.%) – remained non-melted as SiO2, Al2O3, and some complex minerals containing Fe-oxides. Most of these compositions cannot evaporate at usually imposed temperature head and appear into the structure of grown crystals as randomly distributed micro-particles. The presence of such particles into the melt decelerates the intensiveness of RE-purification as shows the highest position of curve 4 in Figure 8.6. Accordingly, a sharp lowering by factor of three is calculated for the alteration of the chemical potential for Ce-ions, ΔμCe, at their transposition into bubbling melts as compared to the cases concerning fluorspars containing only traces of non-isomorphic impurities (curves 1, 2 and 3 on Figure 8.6). This result correlates with the established higher boiling temperature for fluorspars more contaminated with non-isomorphic impurities. Curve 3 on Figure 8.6 presents the results of simultaneous purification of fluorspars with different genesis wherein the total and partial Ln-concentrations as well as Rc-ratio varied widely. As seen, this curve is positioned between curves 2 and 4 that manifests lower deepness and intensiveness for Ce-purification as compared to the case of poor fluorspar “cerium” type (curve 1 and 2) but higher purification efficiency comparing the case when the fluorspar contained up to 0.2 wt.% total amount of some non-isomorphic impurities (curve 4). Hence, the physical-chemical conditions, providing the efficiency for Ce-reduction depend substantially on the initial concentration and mutual proportions for all elements/compositions presenting into fluorspar in usage. The dispersion of data points around curve 4 reveals some variations in purification intensiveness into particular inserts. Once the high efficiency for Ce-purification are being approved when using “cerium” types fluorspars, the next series experiments with this starting material are carried into effect for studding the influence of the apparatus factors on the deepness of purification process.

8.2.4. Dependence of the Deepness for Ce-Purification on Apparatus Factors “Cerium” type fluorspar with initial Ce-content of 6 ppm is being used. The influence of three substantial apparatus factors – temperature head duration, established melt temperature and gas-permeability of the general channel are being considered one by one (Figure 8.7a-c) varying the gas-permeability of the inner lids’ channels and keeping the other parameters constant. Case 1: moderate temperature head is being imposed; the gas-permeability (geometric factor) of the general channel Kgeom(gcov) is chosen below the interval, within which the geometric factor of the inner lids’ channels Kgeom(ins) is being varied. It is seen (Figure 8.7a) βCe-dependence on tvap decreases progressively as the curves obtained can be approximated by two crossing each other lines at tvap = 10 h, the slopes of which differ significantly; the slopes for tvap ≤ 10 h are appreciable steeper than that for tvap > 10 h. Such character for studied dependence suggests the rate of deposition in the common peripheral crucible compartment to drop down rapidly as a result of entering the vapor/deposit crystal interface (IFV/DCr) in a zone with higher T owing to the relative shift of the IFV/DCr upwards, which causes a relevant

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increase in super-saturation pressure inside the compartment. Nevertheless the effect will be entirely compensated after crucible withdrawal has been started with a constant speed exceeding the rate of IFV/DCr shift upwards.

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a)

b)

c) Figure 8.7. Relative alteration in cerium concentration as a function of: a) temperature head duration at ∆Thd = 64 К (Tmelt(max) ≈ 1658 K) and Kgeom(gcov) = 0.085 cm2. b) maximum melt temperature at 10 hours ∆Thd-duration and Kgeom(gcov) = 0.085 cm2. c) geometric factor of the general cover at 10 hours duration of ∆Thd = 64 K. Kgeom(ins) (cm2): 0.3 – (); 1.8 – (); 2 – (); 4.1 – (); 9.1 – (); 10.5 – (); 6.2 (center) – ().

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The hierarchical curves’ order in Figure 8.7a. can be explained by the influence of Kgeom(ins)-factor: as larger it appears as more vapor passes throughout the channel and as lower is positioned the corresponding curve. It is seen as well the slope of the curves in the first, linearly approximated section (tvap ≤ 10 h) tends to increase on augmentation of Kgeom(ins). This regularity is broken for high sizes (diameter) of the channels wherein the transport mechanism changes from Knudsen diffusion to viscous flow influenced slightly on channel outlet’s effect. As a consequence, the effective gas-conductivity in these channels is found to rise up. Unusually high position for the curve corresponding to the central camera can be explained by considerably larger portion of loaded material as compared to those being loaded in the peripheral inserts. As a result a relative increase of Gpur quantity occurs that increases correspondingly the deepness for Ce-purification, βCe. The curves in Figure 8.7a. reveal that the chosen combination of Kgeom(ins) and Kgeom(gcov) can not provide – at moderate duration of the imposed moderate temperature head – the needed conditions whereat one may expect βCe to attain the acceptable limit of 0.15. Case 2: the same combination of Kgeom(ins) and Kgeom(gcov) are used at moderate duration tvap for step like altering temperature head as variable. It is seen (Figure 8.7b) the slope of the decreasing βCe-dependence on Thd changes noticeably when Thd exceeds 90 K. This fact suggests an alteration in vaporization mechanism from evaporation to bulk or/and film boiling. For two values of Kgeom(ins) – 2 and 10 cm2 – the curves’ slope attains maximum that is evidence for intensive vaporization via bulk boiling. In these cameras/lids assembles the desired low level for high Ce-purification (βСе = 0.15) can be firmly attained after imposing the melts on temperature head within 300 - 350 K during 10 hours in case the gasconductivity in general channel remains at least 20 times lower than the gas-conductivities in inner channels lids. Case 3: moderate temperature head with moderate duration is being imposed, Kgeom(ins) are being varied the same way as in cases 1 and 2 and βCe-dependence is followed at different values for the geometric factor of the general channel. As seen (Figure 8.7c) the dependence goes down rapidly within the set up segment for Kgeom(gcov): 0 – 0.1 cm2 wherein it can be linearly approximated. Such behavior is due to dominance of Knudsen diffusion mechanism when the gas-conductivity in the general channel stays low. The slope of fitted lines increases towards the higher Kgeom(ins) while their order follows an opposite hierarchy as regards this parameter. Such order can be explained with fast lowering of the residual pressure (the supersaturation drops down) in the common compartment as a result of the augmented gasconductivity in the general channel. Under these conditions it becomes possible to be maintained high rate of deposition in this compartment as well as intensive vapor-transport throughout mostly channels in the inserts’ lids. Approximation of the lines till they cross the line parallel to abscissa (βCe = 0.15) manifests that the needed deepness of Ce-purification is attainable at only moderate temperature head (60-70 K) and not long duration for its imposing (10 hours) if the gas-conductivity in general channel (0.085 cm2) is being enlarged to 75% at the expense of only 13% increase of channel’s diameter while Kgeom(ins) stays between 2 and 10.5 cm2. At the same time, the losses of material owing to increased vapor flux throughout the general channel towards the vacuum chamber ambient remain still insignificant as far as at Knudsen diffusion the vapor flux is proportional to Kgeom(gcov). The performed analysis of the interrelation effects of the key apparatus factors on the deepness of Ce-purification and its intensiveness confirms the gas-conductivity in the general

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channel appears the most sensitive parameter for a precise control over the processes proceeding during the implemented purification technique.

8.2.5. Deposition Control via Adjusting the Vapor Pressure Inside the Crucible The triple point for CaF2 is found applying the vapor pressure – T relationships for tetragonal β-CaF2 crystal phase and liquid CaF2: CaF2βcr: lnP(atm) = -53480/T – 4.525lnT + 56.08

(8-4)

CaF2melt: lnP(atm) = -50200/T – 4.525lnT + 53.96

(8-5)

At phase equilibrium: CaF2βcr – CaF2melt – CaF2vap: lnP(CaF2βcr) = lnP(CaF2melt)

(8-6)

from which equality follows at the t.p.: T = 1689 K (1418 oC) and P = 8.35.10-5 atm or 0.0635 torr. Under these P-T conditions the deposition will start in the common crucible compartment. The process can also proceed at any super-saturation pressure Ptub but at relevant higher T. At the same time, when Ptub >> Pcb, the deposition is related by Eq. (6-34) or Eq. (6-35) to deposited crystallized amount Qtub via the effective (summary) gasconductivity for connected in series total gas-conductivities in the lid’s channels, defined by:

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Π1sum = ∑(Π1tlid)i

(8-7)

where i notifies the number of peripheral inserts plus 1 (for the central camera). By measuring Qi(ins) and knowing the gas-conductivity in corresponding lid’s channel, Eq. (6-34) can be used for obtaining the difference (Ptub – Pi(ins)) from which is calculated the super-saturation pressure, Pi(ins), established over each one melt. The found set of values Pi(ins) determine the boiling temperatures being within a definite interval according to gasconductivities variation (Figure 8.8a). The exact dependence of Tboil(i) on (Π1tli)i is given on Figure 8.8b. It is seen the negative slope of the dependence becomes constant for argument higher than 0.4 l/s. At this junction, Π1sum may be controlled varying Π1tlid in the inner channels adjusting it in a way an appropriate super-saturation pressure to be established in the common compartment that to ensure constant deposition to proceed therein. At the same time by this procedure it is put under control the vaporization in all inserts and the central camera via the established boiling temperatures (b.t.) in the melts. Thus being the b.t. crucial for both rates of vaporization and deposition, the construction of the multicameral crucible (sizes and volume proportions between separated compartments, channels’ gas-permeability) turns out a decisive factor for provisioning an efficient purification process to proceed as regards any RE impurities. The rate and mechanism of deposition depend on crucible position within the temperature field created by both furnace heaters and fixed by surrounding screening and shielding systems. The mathematics describing the process has been given in Sec. 6.11. For implemented experiments the difference (Psours- Ps) in Eqs. (6-56) and (6-57) presents the difference between the mean pressures established over the melts in particular inserts/central camera Pins(mean) and the pressure established in the common compartment Ptub. On the other hand, the flux density (jm)subl presents the mass vapor flux Qdep per unit cross-section area of

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the free volume inside the common compartment, that is, (jm)dep= Qdep/Sper. From geometric considerations it follows: Sper=(/4)(deff)2 =(/4){(dext)2 – [(dint)2 + (dins)2]}

(8-8)

where: dext и dint are external and internal diameters of the annular cross-section, respectively; dins – external diameter of the inserts and ν – their number. At this junction, from (6-54) and (6-55) it follows: Qdep = kn'.[Pins(mean) – Ptub]1.2

(8-9)

Qdep = kg'.[Pins(mean) – Ptub]

(8-10)

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where there are introduced the constants: kn' (cm.s) = (/4)(deff)2 .kn

(8-11)

kg' (cm.s) = (M1/RT).П1t(per) = (M1/RT)(П1k(per) + П1v(per))

(8-12)

Here, kg' is expressed as a product of the total gas-conductivity in the peripheral common compartment, П1t(per) = П1k(per) + П1v(per), and M1/RT. Thus, according to (8-9) and (8-10), the mass flux of vapor under deposition, Qdep, is proportional to the difference (Pins(mean) - Ptub) on power 1 or 1.2 depending on whether the vapor transport or kinetics appear rate-determining stage for deposition-crystallization processes. That is why it is convenient to investigate lnQdep dependence on ln(Pins(mean) – Ptub), the slope of which differentiates the contribution of the two limiting the process stages: kinetics and vapor mass transport while from the intercept one can draw conclusions about the growth mechanism and the character of the masstransport in vapor phase, respectively. For standardizing the experimental conditions as regards T, Qdep-data set are multiplied by Tm(max)0.5, that is, it has been supposed kinetic limitations for deposition where kn' is proportional to T0.5. The dependence of ln(Qdep.Tm(max)0.5) on ln(Pins(mean) – Ptub) is found out being linear (Figure 8.11) as the data points are positioned – in accordance with the imposed temperature head – on two shifted one to another lines with rather different slope.

Figure 8.8. a) Ln-dependence of boiling temperature on equilibrium pressure inside the differentiated crucible compartments. b) Boiling temperature of CaF2 melts in crucible inserts versus total gasconductivity in corresponding lid’s channel. Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

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Line 1 corresponds to significant overheating of the melts (ΔThd exceeding 175 K). Its slope is close to theoretical value of 1.2 corresponding to the case when the kinetics is a ratedetermining stage.

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Figure 8.9. Ln-dependence of the product of fluorspar deposit into peripheral crucible compartment and square roof of maximal absolute temperature in the melts on ln-difference of pressures established in the inserts and peripheral crucible compartment. Temperature head ΔThd above 175 К – line 1 (n = 1.31±0.02); ΔThd < 100 К – line 2 (n = 0.45±0.01).

Here the marginal role of mass transport limitation is being expectable since at considerable overheating the melts from physical grounds follow the mass-transport between cameras and peripheral compartment proceeds by viscous flow perturbed by convection as well as by intensive stirring of the melts during their boiling. From the intercept of line 1, by using (6-54), it is being estimated the factor Yk that determines the mechanism of crystal growth from vapor phase. The found value is 0.011, which, in view of the linear course of the function tanhx/x with a slope equal to 1 at low arguments, gives 110 units for the product (yo/20.5 xs). This case has been considered corresponding to growth by parallel steps, distant each other to yo that appears of two orders of magnitude higher than the mean free path xs for the particles hitting the growing surface and adsorbing on it. Line 2 unified the data points relevant to ΔThd less than 100 K. Its slope is considerably lower than that of line 1, which means the mass-transport in vapor phase is a rate-determining stage for overall deposition/crystallization process. Here various explanations can be lay out: significant distance between the source of vapor and the place whereupon it is condensed, low gas-conductivity in lids’ channels, absence of considerable natural convention in vapor phase, not sufficiently intensive vaporization by boiling that to cause significant convection (stirring) in the melts. The disagreement between experimental slope’s value (0.45) and theoretical one (1.0) is far beyond the statistical error but can be easily explained if taking into consideration the strong T-dependence for the total effective gas-conductivity in the peripheral compartment, П1teff(per), determined by its viscous term, П1v(per). Thus, the linear rate of deposition (vc)dep normally to the growth surface, is being estimated from (6-58) to be within 0.4 and 1.6 mm/h at temperature pressure up to 100 K or within 1.9 and 6.4 mm/h – when the imposed ΔThd varied from 175 to 285 K. That way found rates’ intervals are compatible to

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those approved as suitable for CaF2 crystal growing (2 – 10 mm/h) [1,5,49]. This result gains ground for successful conformation of the high-T RE-purification to in-batch growth of CaF2 boules by using “cerium” type fluorspar, providing high efficiency of implemented technique.

8.2.6. Influence of “Substance” Factors on Cerium Purification The optimization of apparatus factors and run conditions for providing highly efficient Ce-purification required being clarified the influence of so called “substance factors” (total content and particular amounts of Ln in starting fluorspar) on the deepness of the process. For the purpose two series experiments have been performed by using: i) uniform fluorspars with widely varying total amount of Ln-impurities where the Ce is being always the prevalent one; ii) fluorspars with similar concentrations of Ce, Sm, and Yb in widely varying total Ln-content. Case 1: The Ce-concentration in the loaded portions of fluorspars varies within 4 and 107 ppm, being at least of order of magnitude higher than the concentration of any other REimpurity. The grown boules, differing in weight, are being 53 in number as the ratio of peripheral to central boules is 9:1 (at a twice larger diameter for the central boules). The deepness of Ce-purification βCe as a function of purified weight portion of loaded material, Gpur = Gi/Ginit is shown in Figure 8.10. The data points are fitted by the definitional Eq. (5-8) presented in the form (8-1). The coefficient of correlation, R, is found being equal to 0.9756 (R2 = 0.9518) at FitStdErr = 0.0547. The intensiveness of Ce-purification, aCe, presents the ratio of the constant #A (3.6403) in (9-3) and the mean boules’ height hinit(mean) = 3.95 cm. Rank 1 Eqn 8001 y=() r2=0.95181111 DF Adj r 2=0.95086623 FitStdErr=0.0546658734 Fstat=+INF

1 0.8 0.6  Ce

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a=-3.6402751

0.4 0.2 0 0

0.2

0.4

0.6

0.8

1

Gpur Figure 8.10. Relative alteration in Ce-concentration in purified partitions finished from boules grown by using “cerium” type fluorspars with widely varying initial Ce-content (from several to over 100 ppm).

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Thus, aCe = 0.9216 cm-1, which – at Tmelt(mean) = 1768 K – gives for the alteration in the chemical potential per unit height of the boules: Се = aCe .RT = 3235.9 cal/mole.cm. This value coincides within experimental error to that (3252.1 cal/mole.cm) calculated after processing much less number of data points obtained by using “cerium” types fluorspars (Table 8.1). Such result confirms the propriety for set up experiments as well as the validity of thermodynamic considerations upon which Eq. (5-8) has been derived.

Distribution of Ce Under Segregation Mechanism The concentration of Ce impurities along the height of the grown boules depends as well on the segregation effect on IFM/Cr. For assessing this effect it is being compared lndependence of βCe on Gpur for two sets of data points (Figure 8.11): line 1 corresponds to optical windows prepared from the lowest cylindrical body of the boules while line 2 – to finished longer cylinders taken adjacently above the windows. Both lines show impressively high coefficient of correlation, R ≥ 0.9856, at FitStErr ≤ 0.0424 (Table 8.2).

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Table 8.2. Comparative analysis for alterations in purification deepness and intensiveness, and chemical potential for Ce-impurity in fluorspar melts crystallized under the influence of segregation/purification mechanisms by applying P/Gr-T and using “cerium” type fluorspar Studied Objects Position along the boule (See Fig. 8-3b) Theoretical expression R2 FitStErr R #A aCe = #A/hin cm-1 (hin = 45 mm) T (K) ΔμCe=aCe.RT cal/mol.cm βCerem(Gpur = 0) = exp(-#A) ΔμCe(cyl)/aΔμCe(wind) Ln-dependence R2 Fitt StErr R a b [(│-a│- b)/0.5(│-a│+ b)]100% #A = 0.5(│-a│+ b)] aCe = #A/hin (cm-1) ΔμCe(ln)= aCe.RT (cal/mol.cm) [ΔμCe-ΔμCe(ln)/ΔμCe(ln)]100% ΔμCe(cyl)/ΔμCe(win)

Optical windows – mean Optical cylinders – mean length of 10 mm length of 35 mm Above the cone’s section Above the windows’ (Line 1) section (Line 2) βCe = exp(-#A).exp(#A.Gpur) 0.97131 0.97476 0.0424 0.0412 0.9856 0.9873 4.1145 3.3107 0.9143 0.7357 1758 1758 3192.2 2568.5 0.0163 0.0365 0.8046 lnβCe = -#A + #A. Gpur = a + b.Gpur 0.98314 0.98814 0.067 0.0534 0.9915 0.9941 -3.7267 -3.0187 3.65774 2.94945 1.9 2.3 3.6922 2.9841 0.8205 0.6631 2864.5 2315.1 10.3 9.9 0.8082

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The coefficients a and b are found to differ insignificantly each other – for cylindrical samples by 2.3% and for windows by 1.9%. Thus the statistics confirms once more the mathematics involved for describing reliable the complexity of physical-chemical processes proceeding during the developed purification technique. The steeper slope for line 1 (windows) corresponds to deeper Ce-purification that grounds on larger ΔμCe (by ≈ 20%) and can be explained by segregation effect at IFM/Cr during the embedment of Ce-ions into the lattice of growing crystal. At this junction one may consider the axial segregation of Ce in growing boules to proceed simultaneously with its exponential impoverishment in still noncrystallized, non-vaporized part of the melts. Thence, according to definitional relationships (4-1) and (4-2), the combined effect of these two phenomena can be expressed by introduction of correctional factor in the principal formulas (5-8) and (5-9) determining the deepness of Ce-purification: Fconc – in case of convective mechanism of segregation and Fdiff – at diffusion segregation mechanism. Fconv = {[1 – (z/Hi)]/[1 – (z/Hin)]}Keff-1

(8-13)

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Fdiff = {1+[(1 – k)/k]exp[–v(Hi – z)/Dm]}/{1+[(1 – Keff)/Keff]exp[–v(Hin – z)/Dm]} (8-14) The functionalities of Fconv and Fdiff for Ce along the height (axial coordinate z) of arbitrarily chosen boule (grown in the peripheral compartment of the used crucible) are being calculated for: Keff (Ce) = 0.88 [66]; v = 10–4 cm/s; and Dm = 2.10–5 cm/s. The current height Hi and the total height Hinit are being estimated using (4-3) or (4-4) in accordance with particular geometric volume (cone or cylinder) where: hcone is being taken equal to 1.24 cm (for inserts) and 2.51 cm (for central camera), hcyl = li or linit, and hwaste(mean) = 0.84 cm. It is seen (Figure 8.12) that for 1.25 ≤ z ≤ 2.25 – an interval relevant to window’s thickness of 1 cm the F-factor for both dependence differs from unity less than 2%. At z > 2.5 the divergence of F from unity becomes considerable for convective mechanism as within the last few millimeters of boule’s height curve 1 goes up steeply, inclining to infinity.

Figure 8.11. Ln-dependence of relative alteration in Ce-concentration on Gpur along the height of the cylindrical section of the boules grown by using fluorspars of “cerium” type, initial Ce-content up to 10 ppm. Line 1 – windows finished from the lowest cylindrical boules’ section; line 2 – parallel plates optical cylinders finished from the section adjacent to the windows.

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Figure 8.12. Correctional multiplier F to the quantities representing the axial Ce-distribution in boule grown by applying purification-growing technique, suggesting different segregation mechanism for this impurity: curve 1 – convection, curve 2 – diffusion.

At the same time, curve 2, corresponding to diffusion mechanism, inclines only to a constant value of 1.136 that is determined via the ratio (1 - Keff)/Keff. Such difference in the course of the two curves suggests convective segregation mechanism for Ce-distribution along the height of investigated boule. Physically it is grounded on appearance of radial temperature gradient, which, according to approved theoretical considerations [102], will be significant just within the peripheral inserts when the crucible passes throughout the AdZ, causing this way natural convection into the melts’ bulk. Complementary, forced convection appears owing to set up crucible rotation. In the central camera however, where the radial gradient will be flatten according to approved theory, the effect of natural thermal convection stays insignificant as well as the effect of forced convection at the imposed slow rotation (5 rpm) for moving downwards crucible. As a consequence the segregation of Ce will proceed by diffusion mechanism in all boules grown in the central camera and the deepness of Cepurification, βCe, will alter insignificantly along the height of these boules. This suggestion is being approved experimentally by using windows finished from adjacent sections along several boules grown in the central camera. Nevertheless, since all 53 experimental data points for βCe dependence on Gpur – windows and optical cylinders pertaining to boules grown in the peripheral inserts and central camera – are fallen within the fitting interval for definitional relationship (5-8) (Figure 8.10), the calculated mean value for Ce may consider reliable to express the average alteration for the chemical potential during Ce-impoverishment in the growing crystals. In practical, one may consider the determination of Ce does not depend on the position of the grown boules inside multicameral crucible in usage. Case 2: When multiform combination of fluorspars (with total amount of Ln-impurities from below 1 ppm to over 100 ppm) has been used for loading the cameras, those boules grown from portions of fluorspars “rich” to some Ln-impurities turn out impoverished in different extent to these impurities. At the same time, enrichment by the same Ln is being found in the boules grown from fluorspars “poor” to Ln-impurities. This phenomenon indicates the adsorption/desorption equilibrium for REs at the IFV/L may be shifted to left or right direction during the described melt dividing process.

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Any RE micro-impurity containing in CaF2 vapor being dissolved into fluorite melt shows growing up entropy whereat its chemical potential becomes more and more negative, inclining to zero. These thermodynamic alterations will continue till relevant positive level for chemical potential is being attained. Similar considerations concern as well the adsorption/desorption equilibrium for Ln at IFM/Cr [103]: when the chemical potential for particular RE in the melt – after its liquid to solid equilibrium has been already established – attains sufficiently high level, its ions will tend to be adsorbed upon the growing surface at the account of corresponding number of Ca2+-ions. The process is characterized by a decrease in free energy for the system as a result of rising up the concentration of the dissolved microimpurity and lowering the concentration of the solvent (CaF2) at the IFM/Cr. As a result a local break in stoichiometry for growing crystal occurs. The free energy difference established in the melt bulk and upon the growing surface for both, the dissolved RE-impurity and the solvent (CaF2) leads to so calls “interface effects”, which cause alteration of equilibrium coefficient of segregation for the considered RE. Exactly variations in the chemical potential for the three basic Ln occurring in the different cameras on both interfaces – vapor/melt and melt/crystal – ground a decrease in their concentration in non-vaporized melts’ portions in some cameras but a relevant concentration increase into the melts in the other cameras. The shift in chemical potential to negative or positive level depends on the initial concentration of particular RE into just molten material (before any temperature head to be continuously imposed) and on its equilibrium chemical potential in the vapor phase. When vaporization proceeds by bulk boiling and bubbling into the melts rich to some Ln, the chemical potential of these REs inclines to definite negative levels and correspondingly, their concentration in the liquid phase decreases. At the expense of that, owing to set up low gasconductivity in the general channel, the concentration of considered Ln into the free space of the common compartment will rise up to quantities determining a shift to positive levels of Ln-chemical potential at the IFV/L inside those cameras that have been loaded by “poor” to Ln fluorspars portions. This phenomenon, being physically well grounded, allows be exploring and explaining the direction, intensiveness and degree of re-distribution for the three key RE impurities (Ce, Yb, and Sm) during high-T division of several multiform fluorspars melts. For the purpose, there are being juxtaposed the changes in βi for each couple of the three REs presented in the testified optical samples at parameter Gpur = cont. At that, βi > 1 corresponds to enrichment of the melt to relevant Ln. The altering proportion in the degree of impoverishment/enrichment for Sm/Ce couple is being studied fitted the empirical data for βSm and βCe by using the definitional Eq. (5-21): Sm = Ce(aSm/aCe) = Ce(Sm/Ce) = Ce#A

(8-15)

The coefficient of correlation for fitted dependence (Figure 8.13a) is found being sufficiently high, R = 0.8519 at FitStErr = 0.1113. The fit constant #A = (Sm/Ce) = 0.3078 shows the chemical potential of Sm undergoes 3.5-times weaker alteration than that of Ce independently on whether the concentration of the two RE decreases or increases caused by relevant break in their adsorption/desorption equilibrium on the IFV/M into particular cameras. This fact appears a prerequisite for considerably higher deepness of RE-purification to be attained in the boules

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grown from “rich” to Ln fluorspar portions as well as of RE-contamination in the boules corresponding to “poor” to Ln fluorspars when one compares Sm to Ce.

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a)

b)

c) Figure 8.13. Relative concentration alterations of: a – Sm to Ce; b – Yb to Ce; c – Sm to Yb along the height of the boules grown by applying purification-growing technique.

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Similar result was obtained for Yb-Ce couple (Figure 8.13.b) by using the equation: Yb = Ce(aYb/aCe) = Ce(Yb/Ce) = Ce#A’

(9-16)

In this case the coefficient of correlation for fitted function is being found equal to 0.9221 at FitStErr = 0.1867 while #A’ = (Yb/Ce) is calculated equal to 0.6052 that determines 1.7-times weaker alteration for the chemical potential of Yb as compared to Ce. For the last couple, Sm-Yb, the studied dependence (Figure 8.13c):

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Sm = Yb(aSm/aYb) = Yb(Sm/ Yb) = Yb#A”

(8-17)

appears significantly flatter juxtaposing it to that for the other two couples (Figures 8.13a and 8.13b), which supposes comparable embarrassments for purification/contamination of the melts by Sm and Yb. The coefficient of correlation is being found R = 0.9246 at FitStErr = 0.0715. The constant #A” = (Sm/Yb) = 0.3981 determines a weaker alteration of the chemical potential for Sm (≈ 2.5-times) as compared to Yb. The established phenomenon of different rate of alteration for chemical potential of the dominant RE impurities – Ce, Yb and Sm – in the used fluorspars can be interpreted as a result of interfering effects of several apparatus factors (crucible construction including gasconductivities of the free volumes and related to them channels, set up thermal field configuration, duration of the imposed temperature head), and various specific “substance” factors related to presented RE impurities: atomic weight, affinity to electron’s capturing that concerns the valence state of the REs, their position into the lattice and bonds’ strength, potential of thermal ionization for all participating species and vapor saturation pressure for possible Re fluoride compositions. The thermodynamic theory suggests the alteration of the chemical potential for any particular RE impurity at its transposition from vapor to liquid phase and via versa to depend on relevant alteration in molar concentrations for the other presenting REs on the IFV/M. At this junction, the difference in activation energy for adsorption/desorption processes for particular RE and the difference in vapor saturation pressure for the produced RE fluorides as well as the level of their ionization may be drawn in jointly as key grounds for explaining the alterations in RE-chemical potentials for proceeding physical-chemical processes and reactions. Various reasons lie behind the possible different valence state of the RE-ions into fluorite lattice. Their embedment in tri-valence state as medium activators turns out non-productive since they fall during the melt vaporization under the additive reduction effect of CaF2 vapor that supersaturates the free space differentiated inside the particular cameras over the melts. This way it is thought to be completed a partial reduction of Ln3+ to bi-valence state. The process may be considered as electron’s transfer from “hole”-centers into fluorine sub-lattice to Ln3+-centers into cation lattice [104]. At that the activation energy for the opposite oxidative reactions will be sufficiently high to be kept a meta-stable state for RE-ions during cooling the load to room temperature. This meta-stability can be de-balanced only after the grown crystal has been heated appropriately whereat the bi-valence Ln-ions incorporated would be oxidized to tri-valence state. The process has been explored applying the method of thermo-illumination of co-doped by RE-fluorides crystals where as first activator was used Ce. The specified results gave opportunity for ranking the lanthanides depending on their

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affinity to electron’s capturing: Eu → Yb → Sm → Tu → (Dy – Lu) → (Ho – Y) → (Nd – La) → (Tb – Gd) → (Er – Ce) → Pr. The “tunnel” effect was thought as possible mechanism for thermo-oxidation of the bi-valence Ln-ions [104]. Speculating further, when the starting fluorspar contains definite dominant RE its bivalence ions can loose electrons, transpose to any of the existing “hole” centers Fo (fluorine atoms) into fluorine sub-lattice, thus produces stable Ln3+-ions. This may occur in case “cerium” type fluorspar is being used for growing boules, the t-spectra of which reveal only specific light-absorption band at 306 nm associated to Ce3+ OAC. In case two RE impurities with similar concentration present in the fluorspar, electron’s transfer may occur from Ln12+ to Ln23+ that grounds relevant lattice degeneration owing to dominant concentration of the cubic tri-valence RE ions, Lncub3+. Electrons migrate to Lncub3+ forming excited Ln12+ as the process effectiveness depends on the level of thermal ionization. After de-activation of Ln12+ they leave off the migration process. It has been considered the changes in valence state for any two types of RE-ions, introduced as activators in CaF2 crystals, to depend primarily on the potential of their thermal ionization [105]. The isothermal kinetics for electrons’ transfer in such double-activated crystals has been thought to possess a pseudo-monomolecular character. Then the direction for the redox reactions is determined on the initial concentration of RE centers in the crystal: reduction occurs when the concentration of cubic Ln3+-centers exceeds at least of order of magnitude that of bi-valence RE-ions. The reverse oxidative reaction can occur via thermotransfer at heating the crystal whereat the valence of the basic RE-activator decreases at the expense of increasing valence of the other RE-activator. The potential of thermal ionization for Ln2+ in CaF2 has been found to vary within 0.9 and 1.3 V. It has been considering the presence of bi-valence RE-ions into fluorite lattice was due to divergence of the system CaF2 - Ln3+ from ideally ionic. It is possible as well a non-local compensation of double extra-charge to occur at Ln3+ centers, which can accelerate significantly the electron’s capturing whereat the RE-ions transfer from tri- to bi-valence state [106]. The meta-stable equilibrium between the two type centers depends on the rate of heating/cooling the crystals as well as on the ambient atmosphere and hence, the equilibrium will be determined according to set up purification/growing conditions. From the review of possible reasons that ground different valence state of RE impurities ions into fluorite lattice follow important consequences for explanation of the experimental results about the efficiency of the developed purification technique. Cerium that appears stable into fluorite lattice in its highest valence (3+) will be facilitated to transfer from liquid to vapor phase via easily volatile composition – CeF3 – possessing high degree of thermal ionization. At this junction, the resistance of Ce V-M phase transition is small while its activation energy of de-sorption on melt surface is low. At the same time Sm and Yb appear both electrons’ acceptors remaining in lower valence state in the melt. Their bi-valence fluoride compositions – comparing to CeF3 – possess considerably higher boiling temperature and enthalpy of liquid to vapor phase transition and lower vapor saturation pressure and energy of thermal ionization [101] as well as higher activation energy for de-sorption for their ionic complexes and molecules. Such characteristics ground for significantly weaker alteration of the chemical potential for Sm and Yb compared to Ce during the induced melt division inside the designed specialized crucible.

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There are no any theoretical considerations to be supposed the presence of other elements from Ln-group – Eu, Dy, Ho, Er that have been identified with trace concentrations in studied types Bulgarian fluorites [107] – to affect significantly the discussed regularities controlling purification-contamination processes as far as these elements are after Ce in any characteristic rows ranking the Ln as regards their affinity to electron’s capture, degree of thermal ionization, vapor saturation pressure of fluoride compositions and activation energy of desorption since all these characteristics appear similar to those for Sm or Yb.

8.3. Kinetic Limitations during RE Purification/Growing Processes

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Comparative analysis is being conducted for checking the dependence of Yb to Ce deepness of purification in case their initial concentrations are similar at coefficient of melts’ division Gpur below 0.4 (Figure 8.14). As has been shown, such condition appears necessary to provide sufficiently high deepness for Ce-purification (Ce within 0.1 - 0.15) whereat the optical properties of the grown crystals may satisfy the high requirements of UV-laser optics [58].

Figure 8.14. Comparison of Yb to Ce deepness of purification in crystallized parts of fluorspar melts at coefficient of melt division Gpur ≤ 0.4.

Surprisingly, the data fitting determine a clear hyperbolic function with R = 0.923 at FitStErr = 0.0371, the asymptotes of which coincide to coordinate axes. The curve manifests that, at βCe below 0.1, the Yb-concentration starts to rise up rapidly whereas Ce-concentration continues to lower the same way as above 0.1-level. Such phenomena can be explained with appearance of supplementary mass-transport and inevitable kinetic limitations arising inside the crucible – a result of progressively shortening of the geometric volumes occupied by the molten portions (owing to their intensive bulk vaporization and simultaneous crystallization) whereat the mean temperature, Tmelt(mean), therein will decrease slowly but gradually. As a consequence the temperature head, Thd, upon the melts drops down correspondingly. Since the intensiveness of bulk boiling has been found being proportional to (Thd)2.3 [82], a considerably larger attenuation for the process can be expected in the shorter melts’ volumes. Thus, the super-saturation vapor pressure over these shorter melts will be reduced and the

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established total pressure gradients along the corresponding lids’ channels become smaller so that the caused vapor fluxes directed outside the cameras are less intensive. On the other hand, with the progress of melts division, the IFV/DCr in the common compartment will shift upwards the higher temperatures if the supporting rate of deposition exceeds correspondingly the speed of crucible withdrawal to the cold zone Z2. One may expect at a given instant the deposition being significantly suppressed in view of which the concentration of Yb impurity in the free space inside the common compartment will start to rise up till quasi-equilibrium being attained whereat a partial pressure gradient for Yb, PYb', will be established along the relevant lids’ channels causing a back diffusion of Yb-ions inside the particular cameras. At that, as larger is being PYb' as higher Yb-saturation over the shorter melts can be expected. After surmounting the resistance of V-M phase transition, the adsorbed Yb-ions will diffuse throughout the melts bulk being imposed to concentration gradients larger in the cameras with smaller amount of residual (non-vaporized) molten material. Owing to that after cooling the crucible, the samples with smaller coefficient of division, Gpur ≤ 0.4 turn out boules found enriched to higher degree of Yb.

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9. CONTROL OF THE SHAPE AND POSITION OF GROWTH INTERFACE The quality of the grown crystal depends substantially on the shape and position of the melt/crystal interface (IFM/Cr). Control of both is needed to provide an unperturbed (normal) growth during crystallization. The indirect QIMC-method applied for determination of IFM/Cr shift from optimized thermal conditions in the FU has been proved reliable for controlling the CF position to stay within a zone with minimum radial gradient and maximum axial gradient (adiabatic zone) in case of in batch growing CaF2 boules by using uniform fluorspars as starting material. However, the control of the CF is being much more complicated if calcium strontium fluoride crystals with widely varying content are grown simultaneously – aimed at developing the theory and specifying several technological details for improving the growing technique. The complications come from the form of liquid-solid phase diagram for this system giving solid solutions within the entire compositional range with pronounced minimum [19].

Calcium Strontium Fluoride Crystals Two growth runs are being carried out under different thermal conditions (Fig. 7-4) in the utilized multicameral crucible loaded with portions of pre-melted CaF2 – SrF2 mixtures with different proportions (the details are given in Sec. 8-4). The reduced positions of the CF and the conical crucible’s tips with crucible movement along the BSGS-furnace unit are followed for each of the runs (Figure 9.1a, b). It is seen that xCF* for all boules shifts equidistantly by a constant slope towards Z2 during approximately 50% of the total distance moved by the crucible, located within Z1 or AdZ. The later situation implies respectively a convex or a planar shape for the CF in particular cameras a condition that is favorable for normal growth favoring minimum inbuilt structural defects for the first half of the grown boules. The second half of the melts crystallize under entirely different thermal conditions as shown by the

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gradually decrease of the negative slope for xCF*-curves (run 1) or the double sign change for the slope of the curves (run 2).

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a)

b) Figure 9.1. a, b Reduced positions of the CF and the conical crucible’s tips along the FU during two growth runs carried out under different thermal conditions in a multicameral crucible loaded with portions of pre-melted CaF2 - SrF2 mixtures with different proportions. a. – run 1: (●) – 0.007, () – 0.021, () – 0.054, (o) – 0.056, () – 0.065, (■) – 0.088, () – 0.113, (black) – 0.189, () – 0.307, () – Centre-0.213, (¤) – xcon*. b.– run 2: (●) – 0.383, () – 0.408, () – 0.436, () – 0.471, (■) – 0.509, () – 0.554, () – 0.608, () – 0.675, (o) – xcon*. The total height of some boules and the height of their dendritic tip section, reduced to dimensionless form, hboule* = hboule/lAdz, and hdend* = hdend/lAdZ are shown as well.

In both cases the resulting peculiarities occur either in the AdZ or near to it in the Z1, indicating a relevant redistribution of the thermal exchange between the complex load and its surroundings. The reason for such effects appears to be a significant enhancement of nonaxially directed heat losses throughout the load. Thus one would expect that the CF-shape

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would vary slightly around planarity, being closer to convex if the CF-position turns out above the middle cross section of the AdZ. For each boule grown under such thermal conditions one may consider that the growth optimum has been achieved which should ensure a perfect optical quality for most of the grown crystals. Physically this is based on the effective avoidance of significant micro-defects (impurities, parasitic nucleus, and others) occurring from a thin layer in front of the CF. As seen, the starting point for xnucl*(x1*)-curves varies widely between -0.27 (run 1, x=0.307) and +4.41 (run 2, x=0.675). This means that significant differences exist in the initial nucleation conditions at the tip of the relevant cameras that will affect the further propagation of normal growth and the IFM/Cr morphological stability. Within these considerations one notes that two of run 1-boules, those with x equal to 0.189 and 0.307, turned out to have had the worst conditions with regard to the initial shape of the initially nucleated CF, since they should be firmly concave within the whole conical section. If so, this would initiate a polycrystalline cellar structure with embedded impurities causing related additional macro- or/and micro-defects. The same interpretation follows from the course of the stability function F(x) in criterion (3-3) in the studied compositional range (Figure 9.2). The curves on the figure present the alteration with x of the critical melt temperature gradient established before the CF, above which the suppercooling effect should be marginalized. Its values are being estimated utilizing liquidus/solidus curves obtained by Klimm et al. [55] but corrected by using a formula (4) taking into account the lower m.p. (1375 oC) for the fluorspar used in the present studies compared to m.p. of chemically produced pure CaF2 (1418 oC) [108]. As seen the shape of the curves is similar, revealing three inflexions – two clear maximums surrounding a minimum, which attitude depends substantially on the CS: as CS is high as larger all inflexions become but the shift of the minimum is much less pronounced.

Figure 9.2. The critical melt temperature gradient, Gcrit, for Ca1-xSrxF2 crystal solid solutions grown simultaneously during two runs (see Fig. 8-5) and crystallization rate taken equal to the CS, vcr: 6 mm/h (□); 3 mm/h (▲); 2.5 mm/h (); 2 mm/h (●); 1 mm/h (o). By straight lines are shown: 1) the maximum axial temperature gradient along the AdZ for run 1 – G1e and run 2 – G2e, (for using an empty fixed crucible); 2) corrected by factor of 2.8 maximum melt temperature gradient before the CF.

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Figure 9.3. The defined (left-hand, middle, and right-hand) and the total compositional intervals, ∆x, for IF stability determined for five CS and two bounding melt temperature gradients corresponding to run1 and run2: G1m = 3.9 K/cm – left (●), middle (▲), right (▼), and total (■); G2m = 7.5 K/cm – left (o), middle (), right (), and total (□).

The area above the relevant curve covers the permissible Gmelt-values for which the IFM/Cr is anticipated to be stable for the chosen CS. As seen for crystals with compositions close to the end-members (Ca and Sr) or around the Gcrit minimum the IFM/Cr stability may be attained without the use of very high temperature gradients for the standard set up CS used (for carried out runs up to 6 mm/h). Nevertheless, for this BSGS apparatus the axially measured temperature gradients using an empty motionless crucible (Figure 9.8) along the AdZ (where it is most appropriate to shift the CF close to planar IFM/Cr-shape) was measured 20.8 K/cm (run 2) and 10.7 K/cm (run 1), which infers definite bounds upon the choice for the crucible speed (CS). Indeed, a CS ≤ 2 mm/h seems to be a sufficiently low limit in order to avoid any significant supercooling effect upon the IFM/Cr stability, which should follow from the fact that the Gcrit(x) curves estimated for 1 and 2 mm/h lie entirely below the relevant lines G2e = 20.8 K/cm and G1e = 10.7 k/cm. At the same time, both lines cross the Gcrit(x) curves estimated for CS = 2.5, 3, and 6 mm/h thus outlining definite compositional intervals, ∆x, for morphological IFM/Cr-instability. For the reliable assessment of IFM/Cr stability one ought to consider the real temperature gradient in the melt in front of moving IFM/Cr; on physical analysis it has been established being significantly lower for a loaded crucible. Indeed, the theoretical model of Jones et al. [4] has shown an average axial temperature gradient approximately 3 times lower over a distance of DSD/10 (DSD is the SD diameter) for a crucible fulfilled with molten CaF2 as compared to the temperature gradient used in a BG furnace. The comparative analysis of experimental data obtained by Sokolov et al. [109] leads nearly to the same result for the ratio (≈ 2.8) of the vertical temperature gradient measured in empty BG furnace unit and to that measured on the wall of a graphite crucible filled with molten CaF2. The later gradient may be assumed to be equal to the melt temperature gradient in front of the CF since there exists quasi-equilibrium for the thermal fluxes passing through the lateral walls of any crucible made from graphite which has a much higher thermal conductivity

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compared with that of any molten single or mixed fluorides. Using these considerations the supercooling criterion analysis is being performed on the basis of a newly introduced real restriction on the bounds for Gcrit, setting G2m = 7.5 K/cm and G1m = 3.9 K/cm, both values obtained by using a reduction factor of 2.8 for the initial G2e and G1e measured in empty crucible. As seen (Figure 9.2) the G2m-line crosses 4 times the Gcrit(x)-curves estimated for CS between 2 and 6 mm/h, as only Gcrit(x)-curve (for 1 mm/h) remains entirely below it. At the same time the G1m-line crosses 4 times all the Gcrit(x)-curves except that one for CS = 6 mm/h, which is crossed twice – on left-hand and right-hand of the compositional scale. Using this correction the length of three compositional intervals, defined for IFM/Cr stability, appear on left-hand, middle, and right-hand of the graph, become larger on lowering the CS as illustrated on Figure 9.3. As seen the slope of the curves decreases progressively on CS, being steeper for run 2 curves due to a value nearly twice as high for the set up than for the corresponding melt temperature gradient. The observed picture is conditioned by the specific thermal distribution in the load and the chosen CS for the two runs. Thus, all three favorable compositional intervals (on left-hand, middle, and right-hand) as well as their sum relevant to run 2conditions appear significantly larger than those of run 1 as the difference for the middle positioned intervals (around the azeotropic m.p. of the studied system) is considerably larger than those for the left and right compositional ends. Since most of the bulk of the run1-boules should be grown by the set up CS of 2 mm/h, the graphs on Figures 9.2 and 9.3 show x ≤ 0.034 as the bounding condition for Sr content. With the above conditions being satisfied the supercooling criterion for IFM/Cr-stability, would indicate only two boules with x = 0.007 and x = 0.021, one may assume, would not be seriously affected by melt supercooling. The remaining 7 boules, for which 0.056 ≤ x ≤ 0.307, are likely being grown with IFM/Cr outside the favorable criterion bounds. This detailed analysis are confirmed being consistent with the optical properties for studied samples – two serious of windows positioned adjacent along the boules’ height (Figure 9.4a-c), showing the compositional dependence on Labs+sc/lwin in large spectral range obtained by applying two different techniques: commonly adopted spectra-photometric (SPh) technique and so called Vapor Laser Irradiation (VLIr) technique developed and confirmed being suitable for the present study [110,111]. This dependence possesses some peculiarities according to the studied spectral region discussed elsewhere [110] showing, however, a roughly similar course to that of Gcrit(x) within the noted xinterval. Nevertheless, the sharp maximum for Labs+sc/lwin(x) function is shifted appreciably on the left at x ≈ 0.08 as compared to the plane maximum for the Gcrit(x) dependence established at x ≈ 0.3 (the later function passes through a nearly plane section within 0.15 ≤ x ≤ 0.3). Such a partial discrepancy could be a result of: first, the slope of melt temperature gradient could alter with crucible withdrawal, x1* (this follows from the result of the axial temperature gradient into the FU (Fig. 8 in [108]); second, the crystallization starts at different x1*positions of the moving crucible (Figure 9.1 a, b). Besides, with the set up stepwise increase in CS: 2 - 2.5 - 3 mm/h (Figure 7.4) should lower the critical value for Gcrit(x) that will sharp up the relevant first maximum for the newly formed curve consisting of three adjacent sections pertaining to the three curves with different CS.

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Figure 9.4. Absorption plus internal light-scattering losses as a part of total losses for Ser. 2 – optical windows – prepared from boules of Ca1-xSrxF2 with different composition, measured in the UV - IR range by using vapour laser irradiation and spectra-photometric techniques. λ = 248.6 nm: (●) - VLIr, (o) - SpPh; λ = 510.6 nm: (▲) - VLIr, (Δ) – SpPh; c – NIR(900 nm) - (▼) - VLIr and MIR (λ = 6.45 μm) - () - SpPh.

The established differences in total optical losses per unit window’s length established between the two boules for which the IFM/Cr stability criterion is found satisfied, that is, those with x = 0.007 and x = 0.021, can be explained in view of eventual failure in the normal growth criterion. Indeed, as shown this criterion applied for studied boules (Figure 9.5), the first boule should be grown under worse conditions. However, according to this criterion ΔHmolt/kT ≤ 0.1 mol.% Sr content, which means it is being satisfied only for three run 1-boules with x: 0.113, 0.189, and 0.307, respectively. As seen from the graphs on Figure 9.4, the optical quality decreases up to the maximum at x ≈ 0.08 but improves quickly after that for the remaining three boules. The latest section for these boules was found full of cavities (Figure 9.6). This shows a severe breakdown in normal growth, this time due to the rapid decrease of the real CR (Figure 9.7). Here the crystallization may even have ceased followed by partial surface melting of the already grown body. Evidently, the correlation between Gcrit(x) and Labs+sc/lwin(x) should be affected at least in the first half of the concentration interval by reasons not directly related to melt supercooling rather than to some circumstances leading to an indirect breakdown in normal growth criterion. The view of the graphs Labs+sc/lwin(x) in Figure 9.4 for run-2 boules shows a minimum at 0.55 (UV-Vis) and 0.6 (near IR) while the minimum of Gcrit(x) function is estimated as being only slightly above 0.4 (Figure 9.2). The increase in Labs+sc/lwin(x) at Sr-concentrations higher than 0.55 mol.% is being found significant but only at λ in the UV that is in accordance with the sharp increase of the criterion function up to x ≈ 0.57. Probably, such deterioration in UV-optical quality is due to related structural growth defects with sizes appropriate to act as light-scattering centers.

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Figure 9.5. Compositional dependence of the factor associated with normal single crystal growth (ΔHmolt/kT) for Ca1-xSrxF2 solid solution system. Corresponding straight lines mark both critical values cited in the literature.

Figure 9.6. Boules grown during run 1 with upper section showing dendritic (bulk) crystallization: F – x = 0.307, H – x = 0.189.

Hence, despite the use of both criteria for preliminary assessment of the growth conditions and the quality of the simultaneously grown boules, one has to take into consideration as well the real temperature distribution in the particular cameras during crystallization and the current shift in the CF-position in each individual camera, both parameters being dependent on the mixture’s composition. As has been found out the local vertical temperature gradient in the furnace undergoes significant changes along the FU (Figure 9.8) that should be taken into consideration when using the criterion (3-3).

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Figure 9.7. Relative alteration of the real crystallization rate during two growing runs carried out under different thermal conditions for a series of mixed calcium-strontium fluoride boules.

Figure 9.8. Alterations in the slope for reduced axial T-profile along the normalized axial furnace coordinate, measured in empty crucibles before the growth of the mixed calcium-strontium fluoride crystals: (●) – run 1; (o) – run 2. Arrows mark the points where crystallization starts for the two runs at the ends of the Sr-compositional interval. (Δ) – difference in the slopes for the two runs (run 2 - run 1).

As seen the gradient decreases with upward movement in the FU until approximately the middle cross section of Z1, which leads inevitably, according to criterion (3-3), to an increase in the possibility of significant melt suppercooling propagating within a thin layer in front of the CF. It follows that the morphological stability of IFM/Cr would break down and even a deterioration in the normal growth could be expected. As a consequence, a rapidly propagating dendritic crystallization would follow [50,61,68]. Being far from the AdZ the CF shifts upwards because of stronger radial inhomogeneities predicted theoretically to arise in the system [30,102] that would additionally disturb the IFM/Cr stability and normal growth itself.

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Figure 9.9. Calcium fluoride and calcium strontium fluoride crystals grown in Crystal Growth Laboratory at the Institute of Mineralogy and Crystallography – Bulgarian Academy of Sciences applying the developed purification/growing technique.

The specific courses of the curves on Figure 9.7 abrupt (run 1) or gradual (run 2) increase in the reduced parameter vreal* when the crucible has been attained a particular position in the FU is caused by very fast increase of the heat flux towards Z2 due to the impact of the presence of the movable SMoSh. At the time when the moving crucible reaches a position where all seven rings R that are fixed on the stem in parallel to the bottom bound plane (run 1), move into a position below the lower plane section of the liner L in Z2, the heat flow throughout the load becomes easier since the constitutional radial part of its effective thermal resistance just disappears. This causes the crystallization rate to exceed the set up crucible speed and vreal* rises abruptly up to 25% above 1. In the case of the use of one single ring R being slipped on the stem (run 2), here the extra-heat release in radial direction starts significantly earlier and vreal* begins to increase when the crucible bottom is adjacent to the lower bound section of the AdZ. The further movement of the crucible downwards is peculiar in approaching approximately constant acceleration of the vreal* as seen on Figure 9.8. Such behavior is a consequence of the imposed T1-rise of 5 K/h (Figure 7.4) whereby additional heat is supplied and flows into the inside of the upper load section. This way the heat losses resulting from the movement into Z2 by a gradually enlarging lateral surface of the crucible are compensated with a surplus that causes a relevant downward shift of the CF. The smooth transition of a constant rate below the crucible speed to a rate slightly in excess of the CS is thought to support a growth of several boules having nearly ideal optical quality (run 2) and highlights the need for a process of continuous normal crystal growth with a planar or slightly convex shape of the CF in order to guarantee a high morphological IFM/Cr stability. The application of another approach to compensate for the heat losses occurred through the movement into Z2 involved applying a gradual rise of T2 of 8.6 K/h (Figure 7.4, run 1); it is found not to be possible to adjust vreal* to a region around 1. The chosen heat increase turns out to be rather high leading to such a fast increase in lowering the CF positions that vreal* declines abruptly to a value far below 1 and even – for a while – stayed below 0 (Figure 9.8,

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.

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run 1). As a result of this procedure the IFM/Cr-stability as well as the normal growth would both be severely disturbed due to the development of a strongly concave shape to the CF causing increased impurity incorporation within the grown boules. The further increase in the rate of crystallization leads to relevant increase in the numbers of nucleation centers. In addition a small portion of the surface of the already grown boules melts again when the vreal* drops below zero, a phenomenon which surely would create additional growth anomalies and failure of the crystal structure. The performed analysis results that the growth conditions for run 2 (0.3833 ≤ x ≤ 0.6745) may be assessed much more favorably for ensuring normal growth with a sufficiently steep vertical temperature gradient and a gradually changing negligible shift throughout for the CF within the AdZ. In turn, this presupposes that the observed insignificant variations of Labs+sc/lwin and Labs+sc/h2-1 [108,110] correlate with high and homogeneous optical perfection for the major part of the boules (Figure 9.9). Impressively low level for absorption + light-scattering losses and high structural homogeneity are being found for the boule with x = 0.554 (Figure 9.4). Such excellent optical quality is entirely consistent with criterion (3-3) for morphological IFM/Cr stability during the growth – a result of achieving optimum CF-position (steepest G, absence of radial heat losses), and optimum CR, Vcr, that eliminates the supercooling effect and should maintain a very efficient purification as result of crystallization. Of course this process should be effectively supported by an intensive partial vaporization of the melt mixtures during set up in the high vacuum. During the evaporation several cationic impurities present in the starting CaF2 component (fluorspar), appear to break their bonds in the fluorite lattice as discussed in detail above. As a result of the detailed study of the growth parameters that helped to be clearly distinguished their partial and interrelated influence on optical parameters controlled, the analysis demonstrates how to efficiently control crystal quality by adjustment of the crystallization rate as a function of thermal adjustment and crucible movement.

CONCLUSION The growth of single CaF2 and solid solution crystals of calcium strontium fluoride having state of the art optical quality has been achieved using an appropriately modified BS type apparatus and utilizing optimized high temperature purification technique for efficiently controlling the contamination within the crystallization zone wherein the oxygen-containing anions to be kept minimum while the RE-impurities’ content to be reduced substantially. The purification control requires a definite level of super-saturation vapor pressure to be established over the molten material thus conditioning an intensive vaporization by melt boiling before and during the growing. At that some RE ions may easily overcome the resistance barrier at liquid/vapor phase transition and their concentration in the vapor increases while in the residual melt – decreases. Only the rare earths, the fluoride compositions of which possess sufficiently high partial pressure, can be removed efficiently from vaporized molten material up to levels determined by the specificity of their cations (valence, ionization, affinity to electron’s capture) and concurring mass-transfer and kinetics limitations. Thence the cerium is being proved to attain easily minimum acceptable level. By means of such combined purification/growing technique one may accomplish an efficient control upon physical-chemical conditions determining quasi-equilibrium for

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proceeding mass-transport and kinetics reactions in each section of load as well as upon the thermal fluxes throughout these sections. Thence the optical properties of the grown boules can be controlled varying the significant technological growth parameters. As a result it has been shown possible to enlarge an efficient control in a simultaneous batch growth of calcium strontium fluoride boules having a wide range of composition of the alkali earth components. The growth control requires a formation of a relatively large adiabatic zone set up in the furnace unit through an introduction of a sufficiently thick separating diaphragm together with a specific shielding system. The optimum growth conditions are established by an appropriate adjustment of the CF-positions in conjunction with fluorspars m.p. and mixtures proportion. The optimum appears when the shift of CF was held within the adiabatic zone, above its middle cross-section, so that the real CR rate to remain steadily slightly above the crucible speed. The grown Ca1-xSrxF2 boules represent solid solutions having a large compositional difference. Their optical characteristics change in a similar way within DUV – NIR range depending on compositional variations and the growth conditions defined by a simple supercooling criterion which models the morphological stability or instability of the interface. The microstructure is ideal for those boules with a composition around the azeotropic minimum of the binary phase diagram having x (mol.%) portions between 0.5 and 0.6, a situation where the applied interface stability function is not far from the inflexion minimum. Melt supercooling, by destroying the IF stability, appears as a decisive factor in causing deterioration in optical quality for grown boules. Additionally a breakdown in the normal growth mechanism is also possible at low Sr-concentrations. Measuring the transmissivity along the boules height and distinguishing the pure absorption from the occurred refractive losses reliably implemented the assessment of optical quality. The in batch growth of Ca1-xSrxF2 single crystals with controllable composition and linear optical properties should enhance the exploration of their non-linear properties. One anticipates the use of definite mixed crystal compositions for cleaning of femtosecond laser pulses as well as for a precise measurement of short high intensive laser pulses in the UV. The developed purification/growing technique can be utilized for any single or mixed alkali earth fluoride crystal system so as to be able to rapidly produce a reliable compositional specification suitably selected to satisfy a given system based on a specific laser with corresponding applications. This technique is also suitable, after adding appropriate dopants to any starting mixture of single alkali earth fluorides, to be used for the preparation of a variety of doped and co-doped multi-components solid solutions. The determination of the host crystals, the type, optimal quantity, and uniformity for distributed dopants is of fundamental interest to material scientists. The newly grown crystalline compounds may possess unique laser and other properties provisioning their wide application. For industrial application the technique allows the save of valuable time at insignificant losses of starting material this way increasing the yield. Besides it appears in practical harmless for the people and does not pollute the environment.

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ACRONYMS AND SYMBOLS Ao – integration constant in Eq. (5-7) A1 – symbol for expression (6-45) Aout – circular area of the channel’s outlet #A fitting constant in Eq. (8-1) representing the product (ai.hin) AdZ - Adiabatic Zone between upper and lower furnace zone (see Fig. 7-2a) AEM - Alkali Earth Metal ai – coefficient of purification intensiveness for particular RE impurity B – Bridgman BR – boiling rate (vboil) BS – Bridgman-Stockbarger BSGS – Bridgman-Stockbarger Growth System (apparatus) Ci – concentration of particular RE impurity CL and Cs – concentrations at the CF in the liquid and solid phase, respectively Co – concentration of impurity component in the melt prior to solidification CREi – molar concentration for particular RE impurity in optical fluorite C(Ce) – absolute concentration of Ce impurity in fluorspar CLn(tot) – total molar concentration of RE-impurities in the fluorspar CF – Crystallization Front (see Fig. 7-2a) CR – Crystallization Rate CS – Crucible Speed towards Z2 CSup - Constitutional melt Supercooling CZ – Crystallization Zone D – coefficient of diffusion for any impurity component into the melt bulk Di– effective diffusion coefficients for considered binary vapor-gas mixture Dki – coefficient of Knudsen diffusion for “1” or “2” component DT – thermo-diffusion coefficient Dλ – optical density (Briggs logarithm of coefficient of extinction D12 – coefficient of binary normal diffusion for vapor-gas couple “1”-“2” Dim – mean impurity diffusion coefficient in the melt DSr – diffusion coefficient for the second component (Sr) DSD – diameter of the separate diaphragm in furnace unit DUV – Deep Ultra Violet spectral region d – diameter for given cylindrical volume dext, dint – external and internal diameters for the annular cross-section of peripheral crucible compartment dins – external diameter of crucible inserts din, dout – inlet and outlet channel’s diameters Eλ – extinction index equal to absorption index plus internal scattering index Eλ(RE) – extinction index per unit optical path at particular wavelength λ(RE) peculiar for definite mode RE-OAC Eλ(RE)o – Eλ(RE) at zero REi-concentrations Fconc, Fdiff – correctional factor to the principal formulas (5-8) and (5-9) in case of convective or diffusion mechanism of segregation

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J.T. Mouchovski and B. Mullin F(C) – function of IF stability F(x)cor, F(x) – corrected and non-corrected stability function ETV-ICP-OES – Electrothermal Vaporization Inductively Coupled Plasma Optical Emission Spectroscopy FU – Furnace Unit of BSGS or any BS (B) growing apparatus (Fig. 7-1a) flid/out – correctional factor notified by β’ or by β” fvψ – outlet correctional factor for viscous flow G – temperature gradient in the melt at the CF Gcrit – critical melt temperature gradient, Gcrit, for Ca1-xSrxF2 crystal solid solutions Gmelt – temperature gradient in the melt G1e, G2e – maximum axial temperature gradient along the AdZ for run1 and run2, measured in empty fixed crucible G1m, G2m – maximum melt temperature gradient before the CF for run1 and run2, estimated multiplying G1e and G2e by factor of (1/2.8) Gmelt – permissible values for the melt temperature gradient established before the CF, for which the IF is anticipated to be stable for the chosen speed of crucible Gi – non-vaporized weight of the molten material Ginit – initial weight of the molten material Gpur – purified weight percent from molten fluorspar portion Gvap – vaporized weight percent from molten fluorspar portion GF - Gradient Freeze technique in crystal growing H – crystallized part of the melt HV – high vacuum hboule , hdend – boule and dendritic (tip) heights (see Fig. 7-2b) hboule* , hdend* – reduced dimensionless heights of hboule and hdend hcone – height of cone section of the boule hcyl – height of cylindrical section of the boule with optical quality hcyl(boule) – total height of cylindrical section of the boule hcyl(waste) – height of tip boule section without optical quality hinit – initial height of the melt inside a particular camera IF – any Interface IFV/M – Vapor/Melt Interface IFV/DCr – Vapor/Deposit Crystal Interface IFM/Cr – Melt/Crystal Interface IR – Infrared spectral region J – total flux density for considered binary gas-vapor mixture total diffusion and total viscous fluxes densities Jd – total diffusion flux density for considered binary gas-vapor mixture Jdi – partial diffusion flux density for considered binary gas-vapor mixture (i = 1, 2) Jv – total viscous flux density for considered binary gas-vapor mixture J1 – total flux density for fluorspar vapor J1’ – total flow for fluorspar vapor J2 – total flux density for effective inert gaseous component (jm)subl– mass flux of sublimating material K – and coefficients of distribution (segregation) for impurity component in the melt Kieff – effective coefficient of distribution (segregation) for impurity in the crystal

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Ko – distribution (segregation) coefficient at equilibrium Kgeom – geometric factor for cylindrical free volumes (channels) equal to d3/l Kseg – coefficient of impurity’s segregation at melt-crystal interface Kgeom(gcov) – geometrical factor for the general outer channel Kgeom(ins) – geometrical factor for the inner lids’ channels K1t – total gas-permeability (sum of Knudsen and viscous terms) Kv – multiplier in the viscous term for the total gas-permeability Knk(Po) – Knudsen number equal to the ratio D1k/K1v(Po) k – Boltzmann constant kn, kg – kinetic and mass-transport constants (coefficients) kn’, kg’ – constants derivative from kn and kg L – molybdenum Liner in SMoSh (see Fig. 7-1a) Labs – light-absorption losses in the crystal Lsc – light-scattering losses in the crystal Labs+sc – pure light-absorption plus light-scattering losses in the crystal Lexp – total losses of the transmitted through the sample monochromatic light Lext – light extinction losses as a result of light scattering and light-absorption losses Labs+sc/lwin – light-absorption plus light-scattering losses per unit optical length equal to window (finished sample) thickness Lo (ΔHmolt) – heat of fluorspar fusion LG – Laser Grade crystal Ln – group of the lanthanide metals lAdZ – measured thickness of the AdZ (see Fig. 7-2a) lwin – thickness of optical window or finished sample (see Fig. 7-2a) M1, M2 – molecular weight in the binary gas-vapor mixture Mm(z=l), Mm'(z=l) – expressions depending on weight ratio M2/M1 or M1/M2, of the components for considered binary gas-vapor mixture Meff– effective molecular weight for inert gaseous components MIR – Middle IR spectral region m – liquidus slope (tangent of the slope’s angle for liquidus curve) minit, mend – initial and end quantities of the loaded portions in crucible cameras mv – measured quantity of vaporized fluorspar NA – Avogadro’s Number No - mean quantity for particular impurity in the melt bulk NIR – Near Infrared spectral region n – total concentration of binary gas-vapor mixture or index in Eqs. (6-56) and (6-57) ni – partial concentrations for components of the binary gas-vapor mixture nCaF2 , nSrF2 - refractive index for the end members CaF2 and SrF2 crystals nmix - refractive index for calcium strontium fluoride crystals OAC – Optical Active Centres in fluorite lattice P/Gr-T – Purification/Growing Technique P – total pressure of considered binary gas-vapor mixture Pl – total pressure established at the outlet of any cylindrical volume in used crucible Plsat – vapor saturation pressure in peripheral crucible compartment Pp – the pressure measured in vacuum chamber Ps – actual vapor pressure at the surface of growing layer

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J.T. Mouchovski and B. Mullin Psour – vapor saturation pressure of the source (vaporized CaF2 melt) Pssat – supersaturation vapor pressure Po – total pressure established at the inlet of any cylindrical volume in used crucible Poins – total pressure established inside crucible inserts Q – measured mass flow of vaporized fluorspar throughout any crucible channel Qins – measured mass flow of vaporized fluorspar throughout the lid’s channels Qdep – measured deposit fluorspar per unit time in the peripheral crucible compartment QI – Quenched Interface presenting a boundary between fast cooled molten part and nonmelted part of loaded in the crucible material (see Fig. 7-3) QIFC – Quenched Interface in a Fixed Crucible (see Fig. 7-3) R – Universal Gas Constant or coefficient of correlation in data fitting or R- batch of rings in SMoSh (see Fig. 7-1a) Rc – fluorspar classification factor equal to the ratio of Ce to Yb molar concentration RA – Reactive Atmosphere RE – Rare Earth metals RV – Rate of Vaporization r – reflectivity of a single plane surface for a given optical window rλ – reflectivity of optical window at a given λ rλ’ – equivalent reflectivity (coefficient of light-scattering) rmix - reflectivity for studied mixed fluoride crystals S – net evacuation speed of the chamber Sp – speed of evacuation on the inlet of diffusion pump SCT – Single Crystal Technology SD – Separating Diaphragm in the FU SMoSh – System of Molybdenum Shields introducing in the FU (Fig. 7-1a) SpPh – Spectra-Photometric Technique STA – Simultaneous Thermal Analysis SS-GF-AAS – Solid Sample Graphite Furnace Atomic Absorption Analysis T – absolute temperature T1 – controllable temperature of the upper (“hot”) furnace zone (see Fig. 7-4) T2 – controllable temperature of the lower (“cold”) furnace zone (see Fig. 7-4) T* – reduced temperature of considered binary gas-vapor mixture (= kT/12) Тboil – boiling temperature Tcrys – crystallization temperature Tmelt – melt temperature Tmelt(mean) – mean melt temperature Tsat. – vapor saturation temperature TGT – Temperature Gradient Technique in crystal growing tλ – external transmittance (transmissivity) of the light in the crystal body tv– duration of vaporization at ≈ constant temperature head um – mean quadratic velocity for vapor molecules Vgr – growth rate equal to maximum linear crystallization rate VIS – visible spectral region VLIr – originally developed Valour Lasers Irradiation technique VR – Vaporization Rate VUV - Vacuum Ultra Violet spectral region

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vCF – rate of the CF vcrucible (vcr) – speed of the crucible (see Fig. 7-2a) (vc)sub, – linear rate of deposition normally to growth surface vreal – real (actual) crystallization rate vreal* – reduced (normalized to vcrucible) real crystallization rate x – molar fraction of Sr in Ca1-xSrxF2 solid solutions xi – mole fraction of gas-vapor components (i=1,2) x' – distance to CF xCF – position of the CF along the height of furnace unit (see the scheme 1 below) xCF* – reduced to dimensionless form position of the CF along the height of the furnace unit x1(z) – axial coordinate with origin on crucible bottom (see Fig. 7-2a) x1max – maximal distance of crucible withdrawal for a given run x1* – extent of the total distance the crucible moves in a given run xcon(z) – distance passed by conical tips’ plane section in the inserts relative to the lower bound section of the AdZ (see Fig. 7-2a) xcon*(z) – dimensionless form of xcon(z) xnucl*(x1*) – “nucleation” points showing the starting position of crystallization yo – growth distance between parallel mono-atomic layers Z1– “hot” (upper) furnace zone in BS apparatus (see Fig. 7-2a) Z2 – “cold” (lower) zone in BS apparatus (see Fig. 7-2a) ZIE – Zone of Impurities Enrichment z – axial coordinate (see Fig. 7-2a) αs – accomodation coefficient αλ –absorption index β – crystal phase in CaF2 β’, β”, β* – different approximations for correctional factor at Knudsen diffusion β(REi) – deepness of purification for particular RE impurity i – activity for particular RE impurity or the difference 1 - i ∆C = Cs – CL - compositional difference as apparent discontinuity in the impurity’s concentration at liquid/solid IF (CF) Hsl – latent heat of crystallization ΔLabs+sc/lwin – relative errors for Labs+sc/lwin i – alteration of chemical potential at melt-vapor IF for particular RE impurity ∆T1(x1), ∆T2(x1) – current T-rises according to the set up temperature programs for two furnace zones (see Fig. 7-4) Тcrys – decrease of the crystallization temperature ΔThd – temperature head ∆x – compositional intervals for alteration of Sr molar proportion in studied solid solutions i – ratio between effective diffusion coefficient for components of considered gas-vapor mixture (i = 1,2) and their coefficient of binary normal diffusion εi – partial interaction constant for considered binary gas-vapor mixture (i=1,2) ε12 – binary interaction constant for considered binary gas-vapor mixture ηi – partial viscosity of gaseous components of considered binary gas-vapor mixture

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J.T. Mouchovski and B. Mullin η12 – viscosity of considered binary gas-vapor mixture λ – wavelength of transmitted light λi – mean free path for components of considered binary gas-vapor mixture ν – the number of the crucible inserts ρcr – density of growing CaF2 crystal, im – width of ZIE for impurity under consideration ρm – mean melt density ρλ – internal scattering index σi – molecular radius for components of for considered gas-vapor mixture σ12 – mean molecular radius for considered gas-vapor mixture χ λ(REi) – coefficient of proportionality in Eq. (7-9) Пcb – total (effective) gas-conductivity in the vacuum chamber П1cyl – total gas-conductivity in the free cylindrical space in the cameras over the melt П1eff(ins/cnest) – effective total gas-conductivity in connected in series free cylindrical spaces in the inserts/central camera – lids’ channels П1effout/inl(m) – effective gas-conductivity of the outlet as regards inlet П1inl, П1out – gas-conductivities for channel’s inlet and outlet П1k(per) – molecular (Knudsen) gas-conductivity in peripheral compartment П1lid – total gas-conductivity in lids’ channels of the used crucible Π1sum – effective (summary) gas-conductivity for connected in series total gasconductivities in all lid’s channels П1tgcov – total gas-conductivity in the general channel of outer crucible cover П1t – total gas-conductivity in anyone free of melt cylindrical volume П1t(per) – total gas-conductivity in peripheral crucible compartment П1v(per) – viscous gas-conductivity in peripheral compartment Yk – parameter presenting the kinetic mechanism Ψi/k– complex variable for given couple RE impurities proportional to (Δμi - Δμk) Ώ12 (T*) reduced integral of molecular interaction for considered binary gas-vapor mixture

ACKNOWLEDGMENTS The author is pleased to take opportunity to express special thanks to Prof. Brian Mullin, Editor-in-Chief, PCGCM, who, agreeing to become editor of this survey, spent a valuable time to foster new versions for several manuscript’s sections and to improve significantly the wording of the text so that to make it more acceptable for larger reading public. I am greatly indebt to him for our long-lasting cooperation during which he provided competent and thorough editing of my manuscripts for they being firmly approved for publication.

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[57] Mouchovski, J. T. Avt Svid 1994, 49,870, IPM-BAN, Sofia, priority 1989, Aug 15 (in Bulgarian). [58] Mouchovski, J. T.; Haltakov, I. V.; Lyutskanov, V. L. J. Cryst. Growth 1996, 162, 7982. [59] Leckebusch, R.; Recker, K. J. Cryst. Growth 1972, 13/14, 276-280 (in Germany). [60] Prakash, V. Ind. J. Technol. 1964, 2 (2), 46-50. [61] Arhang’elskaya, V. A.; Reiterov, V. M.; Smolyanskii, P. L. Neorgan Mater 1976, 12 (9), 1560-1567 (in Russian). [62] Detcheva, A.; Havezov, I. Bulg Chemistry and Industry, 2001, 72 (3), 65-67. [63] Detcheva, A.; Hassler, J. Univ Plovdiv Paissii Hilendarski Bulg Sci. Works Chem. 2001, 30 (5), 31-34. [64] Schron, W.; Detcheva, A.; Dressler, B.; Danzer, K. Fresenius J. Annal Chem. 1998, 361, 106-109. [65] Detcheva, A.; Dressler, B.; Hassler, J.; Schroen, V. In CANAS’97 Colloq Analyt Atomspektroskopie; Vogt, C.; Wennrich, R.; Werner G. Eds.; Univ Leipzig, UFZ Umweltforschungszentrum Leipzig; Kalle GmgH: Leipzig, 1998, 383-389. [66] Nassau, K. J. Appl. Phys. 1961, 32 (10), 1820-1824. [67] Watson, E. L. Geochim. Cosmochim. Acta 1996, 60 (24), 5013-5020. [68] Sekerka, R. F. J. Cryst. Growth 1968, 3/4, 71-81. [69] Dubinchuk, V. T.; Sidorenko, G. A.; Shamovskii, L. M.; Shushkanov, A. D. Izv Akad Nayk SSSR Ser Fizich 1974, tom 38 (7), 1462-1466 (in Russian). [70] Tiller, W.; Jackson, K. A.; Rutter, J. W.; Chalmers, B. Acta Materialia 1953, 1 (4), 428437. [71] Hurle, D. T. J. Solid States Electronics 1961, 3 (1), 37-44. [72] Mullins, W. W.; Sekerka, R. F. J. Appl. Phys. 1964, 35 (2), 444-451. [73] Sekerka, R. F. J. Appl. Phys. 1965, 36 (1), 264-268. [74] Stepanov, I. V.; Vasil’eva, M. A.; Sheftal, N. N. In Rost Kristalov; Akad Nauk SSSR: Moskva, 1961; tom III, 239-243 (in Russian). [75] Sobolev, B. P.; Jmurova, Z. I.; Karelin, V. V. In Post Kristallov; Nauka: Moskwa, 1986; tom 16, (in Russian). [76] Fedorov, P. I.; Fedorov, P. P. Osnovy tehnologii osobo chistyh veshchestv; MIHM MITHT: Moskwa, 1982; 1-95 (in Russian). [77] Turkina, T. M.; Fedorov, P. P.; Sobolev, B. P. Kristallografiya 1986, 31 (1), 146-152 (in Russian). [78] Van-der-Vaal’s, I. D.; Konstamm, F. Kurs Termostatiki; ONTI: Moskwa, 1936; tom 1, 1936, 67-135 (in Russian) [79] Djurinskij, B. F.; Bandurkin, T. A. Izv AN SSSR Neorg mater 1979, tom 15, 10241027 (in Russian). [80] Jackson, A. Am. Soc. metals 1958, 174-186. [81] Alfintzev, G. A.; Ovsienko, D. E. In Post Kristallov; Nauka: Moskwa, 1980; tom 13, 121-123 (in Russian). [82] Fizitzeskii entziklopedicheskii slovar; Prohovov Ed. In chief; Sovetskaya entziklopediya: Moskwa, 1984; (in Russian).

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[83] Mayer A. A. Teoriya i metodyi vyirashchivaniya crisyallov; Moskwa 1971; 97-125. [84] Stepin B. D.; Gorshtein, I. G.; Blyum, G. Z., Kurdyumov, G. M., Ogloblina, I. P. Metodyi polucheniya osobo chistyih neorganicheskih veshchestv; Himiya: Leningrad, 1969; 30-67 (in Russian). [85] Girshf’elder, G.; Kertiss, Ch.; Berd, R. Molekulyarnaya teoriya gazov i jidkostei; Inostran Literatura: Moskwa, 1961; 12-860 (in Russian). [86] Mason, E. A.; Malinauskas, A. P.; Evans III, R. B. J. Chem. Phys. 1967, 46 (8), 31993214. [87] Mehta, G. D.; Morse, T. F.; Mason, E. A.; Daneshpajooh, M. H. J. Chem. Phys. 1976, 64 (10), 3917-3927. [88] Geintze, W. Vvedenie v vakuumnuyu tehniku. Tom I. Fizitzeskie osnovy vakuumnoj tehniki; Gosud Energet Izdat: Moskwa, Leningrad, 1960; 10-167 (in Russian). [89] Muhovski, I. T. Dr Thesis 1982, CLEHIT BAS, Sofia, Chem Library, 8-75. [90] Muhovski, I. T. Izrastvane na optitzeski kristali ot CaF2; NACID CNTB, Sofia, 2006; pp 327, Appendix pp 223 (in Bulgarian). [91] Sechenkov, A. P. Tehnika Fizicheskogo eksperimenta; Energoatomizdat: Moskwa, 1983; 124-127. [92] Weston, G. F. Ultrahigh vacuum practice; Butterworth: London, Boston, Durban, Singapore, Sydney, Toronto, Wellington, 1985, 16-49. [93] Clausing, P. Ann Physik 1932, 12, 448-450; J. Vac. Sci. Technol. 1971, 436-438. [94] Efimov, A. I. Svoistva neorganicheskih soedinenii. Spravochnik; Himiya: Leningrad, 1983; 58-60. [95] Schonherr, E. The Growth of Large Crystals from the Vapor Phase; ??? 1980, 53-118. [96] Huffman D. R.; Norwood, M. H. Phys. Rev. 1960, 117, 709-712. [97] Holter, M. R. Fundamentals of infrared technology; MacMillan: New York, 1962; 9-24. [98] Mouchovski, J. T. Bulg. Chem. Commun. 2007, 39 (1) 3-8. [99] Squires, G. L. Practical Physics; McGraw-Hill: London, 1968, 7-69. [100] Zidarova, B.P. Comp. Rend. Acad. Bulg. Sci. 1992, 45 (8), 73-76. [101] Mouchovski, J. T.; Temelkov, K. A.; Vuchkov, N. K.; Sabotinov, N. V. J .Phys. D: Appl. Phys. 2007, 40, 7682-7686. [102] Gmelin, L. Handbuch der anorganischen chemie; 1976; 8 auflage, teil C3, 39, pp.78, 81, 196, 206. [103] Jasinski, T.; Witt, A. F.; Rohsenow, W. M. J. Cryst. Growth 1984, 67, 173-184 [104] Tiller, W. A. J. Cryst. Growth 1986, 75, 132-137. [105] Btyigov, S. H.; In Spectroskopiya Ktistallow, Mater Symp Spectroskopiya Ktistallow soderjayushchih Redkozem Elementov; Harkow, 1967, 9-14 Oct, 167-169. [106] Arhang’elskaya, V. A.; Kiseleva, M. N.; Shraiber, V. M. Fiz. Tverd. Tela 1969, 11 (4), 869-875 (in Russian). [107] Shcherbina E. V.; Skorobogatov, B. S. In Spectroskopiya Ktistallow, Mater Symp Spectroskopiya Ktistallow soderjayushchih Redkozem Elementov; Harkow, 1967, 9-14 Oct, 172-173. [108] Zidarova, B. P. D Sc Thesis 1989, Inst Appl Min BAS, Sofia, Nat Library, 18-35 (in Bulgarian).

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[109] Mouchovski, J. T.; Temelkov, K. A.; Vuchkov, N. K. Prog. Cryst. Growth Charact. Mater. 2010 (in press). [110] Sokolov, V. A.; Sinev, A. N.; Kvashnin, B. I. Seria Elektrotermiya 1979, vyp. 11 [207], 4-9 (in Russian). [111] Mouchovski, J. T.; Temelkov, K. A.; Vuchkov, N. K.; Sabotinov. N. V. Compt. Rend. Acad. Bulg. Sci. 2009, 62, (6) 687-694. [112] Mouchovski, J. T.; Temelkov, K. A.; Vuchkov, N. K.; Sabotinov. N. V. 2009. Bulg. Chem. Commun. 2009, 41 (3), 253-260.

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In: Fluoride: Properties, Applications and Environmental … ISBN: 978-1-61209-393-2 Editor: Stanley D. Monroy, pp. 93-110 © 2011 Nova Science Publishers, Inc.

Chapter 2

FLUORIDE CONTENT IN CENTRAL AND SOUTHEAST ARGENTINEAN GROUNDWATERS M. L.Gomez 1 * and O. M. Quiroz Londoño2 1 CCT-CONICET Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA). Avda. Ruíz Leal s/n Parque G. San Martín, Mendoza. Argentina. 2 Instituto de Geología de Costas y del cuaternario. Universidad Nacional de Mar del Plata. Buenos Aires, Argentina

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ABSTRACT Naturally occurring fluoride in groundwater is an important aspect in the central and southeast sector of Argentine. Former investigations have demonstrated that volcanic glass dissolution disseminated in the loess-like sediments is the main source of fluoride in the Argentine pampas. Nevertheless groundwater fluoride distribution is erratic and the controlling factors of it are not well known. Rural and urban activities in these zones rely exclusively on the exploitation of groundwater and in many cases it is the only source of drinking water. For these reasons, fluoride content in groundwater is a sanitary problem which needs special attention since several fluorosis cases have been detected in Córdoba Province. The aim of this study is to analyze the geochemical conditions associated with the presence of fluoride (F-) in the phreatic aquifer in three areas in the central and southeast sectors of the Argentine Chacopampean plain. Two study areas are located in the south of Córdoba province, involving 1,040 km 2. Aquifers in these zones are mainly composed of silty sand sediments of aeolian origin, typically loess-like sediments of Holocene, and are situated near igneous-metamorphic basement rocks of the Paleozoic. The other study area is located in the north west of the inter-mountainous plain at Buenos Aires province, involving more that 2,760 km2. It extends between two low hills ranges of Precambrian metamorphic rocks and sedimentary Paleozoic rocks, and it is filled by a thick sequence of Cenozoic sediments, mainly silts and silty-clayed, with sand layers intercalated. High concentrations of F (0 - 18 mg.l-1) in groundwater were detected in the three study areas. More than 80 % of domestic wells exceed the drinking water limit of Argentine Law (1.3 mg.l-1). Hydrogeochemical data indicates a high relationship between sodium bicarbonate waters and the highest pH values. There was a high correlation between F- and As(Total), * E-mail:

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M. L. Gomez and O. M. Quiroz Londoño and F- and Na+/Ca+2 ratio. Saturation indexes indicate that dissolution are the main processes that control F contents. Fluorite saturation index showed that fluorite saturation is reached just in few samples. In some areas F- distribution appears to be mainly controlled by a general salinity increase and the proximity of Paleozoic rocks containing minerals with F- contents. Sediment compositions and hydrogeochemical conditions are the main factors in determining the F concentration. The composition and texture of loess, low permeability and hydraulic gradients, mineralogical composition of the basement rocks together with sodium bicarbonate watertypes are proper conditions for fluoride mobilization in groundwater in central and east sectors of Argentina.

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INTRODUCTION Fluorine is an essential element for human and animal health. Therefore, the F health effects have been recognised in many parts of the world. At low concentrations (1000 ppm (alkalic) (Frencken, 1992). In general, fluorine accumulates during magmatic crystallization and differentiation processes. Consequently, the residual magma is often enriched in fluorine. Groundwater from crystalline rocks, especially (alkaline) granites (deficient in calcium) are particularly sensitive to relative high fluoride concentrations. Such rocks are found especially in Precambrian basement areas (Brunt et al., 2004). Clastic sediments have higher fluorine concentrations due that the fluorine is concentrated in micas and illites in the clay fractions. High contents may also be found in sedimentary phosphate beds (shark teeth) or volcanic ash layers (Frencken et al, 1992). Fluoride is a highly reactive element that combines with other elements by covalent and ionic bonds. It is mainly found in alkaline rocks and alkaline soils, being fluorite the principal component (Saxena et al. 2004). Many hydrochemical studies of F- and its origin in Argentina have been associated to the volcanic glass alteration which is disseminated in the Pampean loess sediments (Nicolli et al. 1989; Carrica and Albouy 1999; Carrica et al. 2002, Smedley et al. 2002). Health problems related to the consumption of water with high F concentrations in Argentina have been registered mostly in the Chacopampean area, particularly in the provinces of Córdoba, Santa Fé, Santiago del Estero, Buenos Aires, Chaco and La Pampa (Biagini et al. 1978; Besuschio et al. 1980).

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Naturally occurring fluoride in groundwater is an important aspect in the central and southeast sector of Argentina (Gomez et al., 2009; Quiroz et al 2008). The WHO sets the limit for F concentration in drinking water between 0.5 and 1.2 mg.l-1 whereas the Codigo Alimentario Argentino (CAA, 1994) establishes a limit that varies according to the mean annual temperature of the place (1.3 mg.l-1 for the study area, which has a mean annual temperature value of 16ºC). High F- contents are detected in many regions of the Chacopampean plain. Three study areas located in the central and southeast sectors of the Argentine Chacopampean plain, involving 3,800 km2 were selected. Rural and urban activities rely exclusively on the exploitation of groundwater and in many cases it is the only source of drinking water. Coronel Moldes obtains its water supply from a confined aquifer located at 150 m in depth, while its surrounding areas and people near to the Tandilia Range are supplied with groundwater from the unconfined aquifer. Fluoride content in groundwater is a sanitary problem which needs special attention since several fluorosis cases have been detected in Córdoba Province (Gomez, 2009). These aquifers are mainly composed of thin or remobilized aeolian materials which constitute the main natural source carrying F- into groundwater. The aim of this study is to analyze the geochemical conditions associated with the presence of F- in the phreatic aquifer in three areas in the central and southeast sectors of the Argentine Chacopampean plain. The contribution of this paper is the enhancement of the knowledge of the geochemical distribution and behavior of F- in the Argentine Chacopampean plain in order to allow more efficient management policies.

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Study Area The study area involves two provinces of Argentina, Córdoba and Buenos Aires (Figure 1) including three areas of the Chacopampean plain. The Area 1 (from 33º 30` and 33º 40` S to 64º 30`and 64º 45`W) and Area 2 (from 33º 24´ and 33º 48´S to 65º 15´ and 64º 52´ W) are located in the south of Córdoba province, with an extension of 440 km2 and 600 km2 respectively. The first one has a mean altitude of 400 masl while Area 2 presents a mean altitude that ranges from 430 to 730 masl. The Area 3 is located to the southeast of Buenos Aires province, between 38° 39´ and 37° 34´S, and 59° 6´ and 58° 16´W, comprises 2,740 km2, with a maximum elevation of 420 m asl. The media rainfall value for the region varies from 800 mm.y-1 in Chajan (Area 2) to 943 mm.y-1 in Buenos Aires (Area 3). Both places are characterized by a similar climate (Humid and Sub-humid) with noticeable seasonality. The 85% rainfalls take place between October and April.

GEOLOGICAL FEATURES The study area is part of the Chacopampean plain, a big basin that received sediments coming from the Andes and that today constitutes the main transit place of those materials toward the Atlantic continental platform and slope (Chebli et al. 1999).

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Figure 1. Location map of the three study areas.

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Area 1. Coronel Moldes The outcropping sediments in this zone are mainly aeolian of Holocene age. These materials were locally remobilized by the dominant NE winds, and have been deposited forming sand dunes, which constitute a geomorphic region called “loess plain” of the south of Cordoba (Cantu and Degiovanni 1987). The relief presents homogeneous features, and only 3 geomorphologic units can be differentiated. Two units were defined in the northeastern: the Aeolian plain with remobilized sand dunes and the Aluvial plain of Sampacho stream and the Typical aeolian plain unit that comprises the greatest part of the area. The general topographic slope is low and uniform toward the SE.

Area 2. Chaján The upper part of Chaján river basin is developed in plutonic and metamorphic rocks (from acid to ultrabasic composition) of the Precambric-Paleozoic (Gordillo y Lencinas, 1979). The crystalline basement is partially covered by a thin layer of Quaternary deposits. The outcrops are thicker in valleys, showing up to 10 m of thickness. They exhibit colluvial and alluvial sequences. The middle and lower part of the basin are composed by Quaternary sediments. These sediments have a fluvial, aeolian and lacustrian origin of Pleistocene-Sup-Holocene ages. The main textures are integrated by sands of varied granulometry and silt.

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Area 3. Buenos Aires It extends between two low ranges of Precambrian metamorphic rocks and sedimentary Paleozoic rocks, and it is filled by a thick sequence of Cenozoic sediments. The ranges are constituted of metamorphites (mainly gneisses, amphibolites and migmatites) basic dikes and granitoids (Dalla and Iñiguez 1979). Lying in disconformity on the Precambrian basement is the Lower Paleozoic cover. It outcrops like a broad range of siliciclastic and carbonitic sedimentary rocks, primarily composed by sand and quartz sabulites associated with sandy levels (Poire and Spalleti 2005). On the other hand, a Cenozoic sediment cover is mainly made up of silts, sands and clays with variable amounts of calcium carbonate. Its compactness is increased due to the abundance of calcium carbonate concretions, locally known as tosca. Aeolian, marine and fluvial processes have developed on these sediments, which prevailed in the upper Pleistocene-Holocene, and have resulted in the current morphology (Kruse et al. 1998).

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HYDROGEOLOGY In the 3 areas the unconfined aquifer in clastic sediments has been studied. It is mainly composed of silty sand sediments of aeolian origin, typically loess-like sediments of Holocene. Particularity the Area 2 is situated near igneous-metamorphic basement rocks of the Paleozoic. In Area 3 the Precambrian metamorphic rocks, which constitute the base of the ranges, limit the zone to the north. Estimated values of hydraulic conductivity in the aquifer are 5 m.D-1. The potentiometrics maps (Figure 2) show a general groundwater flow direction for each area. Areas 1 and 2 have a NW-SE direction with velocities ranging from 0.2 to 1 m.D-1 (Gomez, 2009). An hydraulic conductivity values between 0.5 m.D-1 and 5 - 10 m.D-1 are assumed (Sala, 1975). The general flow direction in the Area 3 is NW to SE with a velocity values varying from 1 to 35 m.D-1 and hydraulic conductivities of 1 to 10 m.D-1 (Quiroz Londoño, 2009).

LOESS TEXTURE AND GEOCHEMICAL FEATURES A dominant grain size mode of very thin sands could be found in the aquifer, with a weight percentage of 52–60%, followed by silts and clays in the order of importance.. Petrographic studies of loess in South Cordoba (Blarasín, 1984) have determined that the main components are light minerals, while the heavy ones are highly subordinated (10%). Among the light minerals, potassic feldspar dominates followed by quartz, volcanic glass (up to 24% of weight) and plagioclase, making up the rock fragments between 6 and 10%. Pyroxenes and amphiboles predominate among the heavy minerals, being tourmaline, zircon, apatite and opaque minerals subordinated. Ferromagnesian minerals, such as ilmenite and magnetite, are altered into clay minerals and Fe and Ti oxides (Smedley et al. 2002). The average chemical composition of these loess sediments closely resembles the dacite, and the composition of volcanic glasses is similar to that of the Rhyolite (Nicolli et al. 1989). Geochemical analyses of loess from different parts of the Chacopampean plain carried out by Nicolli et al. (1989, 2004) show the maximum, minimum and geometric mean concentrations

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of the main trace elements in loess sediments. Its analysis demonstrates that all trace elements (As, Se, U, F, Sb, Mo) are found in the loess of Córdoba province in high concentrations and thus constitute a potential source of contribution to groundwater. Fluoride concentrations with values between 520 and 490 mg.kg-1 were analyzed in sediments and volcanic glass loessic respectively (Rossi, 1996). These concentrations result particularly high when it is compared with the average values of 220 mg.kg-1 determined by Hem (1989) in resistitas (volcanic glass) from different zones in USA. For the north west of the inter-mountainous plain at Buenos Aires province granulometric analysis performed on drilling samples from four wells in the area (Quiroz et al. 2008) yields clayey silt-sands prevalence, where the psammitic fraction accounts for more than 60% in all cases. Very fine sands between 0.125 and 0.062 mm prevail. Mineralogical analyses revealed the abundance of quartz, as well as the presence of calcium carbonate concretions and some lithoclast fragments. Occasionally, plagioclase, obsidian, biotite and gypsum fragments were observed; while ferromagnesian minerals were frequently detected in sandy fractions. The silt fraction contained disseminated CaCO3 in 95% of all samples. The upper outcropping sediments are formed by Plagioclase (65%), quartz (30%), orthoclase, volcanic glass shards (1–25%), and organic opal are the main components of this sandy fraction. Even some heavy minerals, especially iron ores (mainly magnetite), amphiboles and pyroxenes and, rarely, pyrite are found. In the silty fraction, volcanic glass shards are usually detected, followed by plagioclase, quartz, orthoclase, and, more rarely, by gypsum fibrous crystals, in this particular case, antigenic. Finally the clayey fraction is essentially made up of montmorillonite.

Figure 2. Potentiometric contour map. Unconfined aquifer of the three study areas.

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MATERIALS AND METHODS A total of 146 groundwater samples were taken (40 samples in Area 1, 61 samples in Area 2 and 45 samples in Area 3) from December 2004 to March 2008. The collection, preservation and chemical analysis for major ions, Si, F and As of water samples were made following the standard methods given by the American Public Health Association (APHA 1998). Chemical analyses were performed applying standard methods: chloride following Mhor method, sulfate by turbidimetry, calcium and magnesium by complexometric titrations with EDTA, sodium and potassium by flame spectrometry, bicarbonate-carbonate by potenciometric titulation, silica by means of silicomolibdate method, fluorine by zirconyl chloride method and As (total) by atomic absorption spectroscopy Analyst 300 Perkin-Elmer. Water temperature, pH and electrical conductivity were in situ estimated. The selected wells belong to rural inhabitants who obtain water from the aquifer’s top (first 15–40 m). The phreatic aquifer levels were measured by using a piezometric probe. Hydrochemical results have been analyzed by using usual hydrochemical diagrams (Piper). The equilibrium conditions of the solution regarding different mineral species were analyzed from the saturation rates obtained with the program PHREEQC2.0 (Parkhurst and Appelo, 1999).

RESULTS

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Hydrogeochemistry Table 1, 2 and 3 show maximum and minimum values of the ionic components of groundwater in the three study areas. High F- concentrations in groundwater are common characteristics in most of the water samples. In Area 1 the 90% of the samples result with Fconcentration up to 1.3 mg.l-1, while Area 2 result the 100% of the samples collected with Fconcentration up to 1.3 mg.l-1. For the Area 3 more than 60% of the total amount of samples exceeds the regulation value.

Area 1. Coronel Moldes Groundwater is mostly alkaline, with pH values ranging from 7.31 to 8.85. Hydrochemical facies are mostly sodium bicarbonate and sodium bicarbonate sulfate (Figure 3). Sodium bicarbonate water corresponds, in general, to samples with low salinity. Sodium is the dominant cation, with concentrations varying from 200 to 600 mg.l-1 (Table 1), mainly due to cationic exchange processes resulting from the interaction between water and clay minerals (mainly illite in the loess of the area). Moreover, a great amount of sodium is brought by the hydrolysis of the silicate minerals. The concentration of Ca+2 ion is governed both by the equilibrium reaction of calcite dissolution and the cationic exchange. The saturation index (SI) in aragonite and calcite is between 0.01 and 0.28, respectively, indicating equilibrium conditions. High dissolved silica concentrations are observed, between 13.7 and 70 mg.l-1. Similar concentrations have been mentioned in other loess aquifer of the Pampa Region (Martínez and Osterrieth 1999). These concentrations can in general be attributed to the

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dissolution of amorphous silica minerals, such as volcanic glass, whose reaction velocity is faster. Chalcedony and quartz are under oversaturation conditions. SI values in amorphous silica are slightly subsaturated or close to equilibrium conditions (SI between -0.2 and -0.8), which indicates that it is possible to assume its dissolution. About 90% of the analyzed samples show F- values above 1.3 mg.l-1 (CAA limit). The distribution of F- concentrations in groundwater is heterogeneous (Figure 4). There is a significant correlation between F- and As (R = 0.92) (Figure 5). Furthermore, a moderate positive lineal regression between F- and Na+/Ca+2 (R2=0.61) was found. This ionic ratio is highlighted if bicarbonate water is only considered (R2=0.77) (Figure 6). The highest Fconcentrations (>5 mg.l-1) correspond with sodium bicarbonate water. For this watertype, a weak positive lineal regression of R2=0.35 for F- and pH was observed (Figure 7). A negative regression, considering all the samples, was observed between F- with Ca+2 and Mg+2.

Area 2. Chaján Groundwater salinity is very variable; varying from 300 mg.l-1 (upper basin) to 9000 mg.l(in the middle) related to phases changes from bicarbonate (upper basin) to sulfated and/or chloride (lower-middle basin) ones. A normal geochemical evolution of groundwater flow has been observed with local variations according to the geological features of the region. Sodium water types were more common in the area (Figure 3). 1

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Table 1. Groundwater statistic parameters of the analyzed samples in Coronel Moldes area (Area 1)

References: St. Dev., Standard deviation.

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Table 2. Groundwater statistic parameters of the analyzed samples in Chaján area (Area 2)

References: St. Dev., Standard deviation.

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Table 3. Groundwater statistic parameters of the analyzed samples in Buenos Aires area (Area 3)

References: St. Dev., Standard deviation.

Locally calcium bicarbonate waters were found in the upper part of the basin. The fresh waters were associated with mountain valleys, piedmont and volcanic outcrops. High salinity was associated with distal groundwater flow, as a result of the slow circulation’s water in finer sediments at the lower part of the basin. The F- contents (Table 2) is high in almost all samples (1.8 mg.l-1 to 18 mg.l-1). Higher values of this element (> 15 mg.l-1) were more common in mountain valleys and foothills associated with fresh waters (Figure 4). Fluorite

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saturation index showed that fluorite saturation is reached just in few samples. Fluoride distribution appears to be mainly controlled by a general salinity increase and the proximity of Paleozoic rocks containing minerals with F- contents. There is a high correlation between As(total) and F- (R = 0.93) (Figure 8) in groundwaters of mountain and piedmont valleys where water is almosty sodium bicarbonate geochemical type. On the other hand, these relationship for groundwater from the aeolic plain, where sulphate and sodium chloride water type is dominant, has an R value of 0.80 (Figure 9). Samples of the mountain valleys and foothill area correlate well with the F- ratio Na+/Ca+2 (R2=0.78) (Figure 10) and a negatively regression linear with Ca+2 and Mg+2 have been observed. The groundwater in the upper part of the basin has the highest concentrations of Ca+2 and Mg+2.

Area 3. Buenos Aires

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From the hydrochemical point of view groundwater is bicarbonate type, mostly composed of sodium, but some calcium-magnesium members can also be observed. pH is lightly alkaline, with mean values of 7.58 and 8.14 respectively. Major groundwater composition has been plotted in a Piper diagram (Figure 3). The content of dissolved F- in groundwater varies between 0.4 and 3.1 mg.l-1, with an average value of 1.5 mg.l-1, a standard deviation of 0.6 and a coefficient of variation of 0.4. Its spatial distribution is showed in Figure 4.

Figure 3. Piper diagram showing the chemical composition of groundwater from shallow wells of the three study area.

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Fluoride Content in Central and Southeast Argentinean Groundwaters

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Figure 4. Distribution of F- concentrations in groundwater of unconfined aquifer of the three study areas.

Figure 5. Correlation between As and pH in groundwater in Area 1. Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

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Figure 6. Relation between F- and Na+/Ca+2 to the sodium bicarbonate waters.

Figure 7. Relation between F- and pH to the sodium bicarbonate waters.

Figure 8. Correlation between F- and As to the sodium bicarbonate waters in Area 2. Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

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Figure 9. Correlation between F- and As to the sodium sulfate-chloride waters in Area 2.

Figure 10. Relation between F- and Na+/Ca+2 in groundwater in Area 2.

The leaching of volcanic glass in the sediments of the aquifer is assumed to be the main source of this ion in the aquifer. This is consistent with studies conducted by Nicolli et al. (1989) in the southern province of Cordoba and Rossi (1996) for the province of Buenos Aires. The lowest values in the concentration of this anion coincides with one of the areas where there is a decrease of HCO3- and Ca+2, suggesting precipitation of calcite, a process which can be removed F- solution (Turner et al., 2005). All the samples containing more than 1.3 mg.l-1 of F- are bicarbonate sodium watertype, on the other hand, low F- concentration waters (less than 1.3 mg.l-1) correspond to a bicarbonate type, including sodium, calcium and magnesium in their cationic composition. Inspite of the similar tendency in the ionic ratios assessed in these areas, lower correlations in Area 3 were evident.

DISCUSSION AND CONCLUSIONS Natural origin of fluoride pollution is a logical conclusion taking into account the lack of any human source. Fluoride concentrations in groundwater in the Chacopampean plain are

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conditioned by natural sources in the like loess sediments (Nicolli et al. 1989; Smedley et al. 2002; Gomez et al., 2009). Loess constituents with different types of minerals, including fluoride bearing minerals, like volcanic glass, volcanic lithic fragments and the minerals from the rocks that form the crystalline bedrock of the Tandilia hills and the hills of Córdoba are the mainly sources of F- to the studies aquifers. The main chemical processes in groundwater are the dissolution of those minerals in conditions of sodium bicarbonate watertypes and high pH. Volcanic glass shards dispersed in the loess-like sediments have been considered to be the main source of F- in groundwater of the Pampa (Nicolli et al., 1989; Rossi, 1996). Due to the presence of volcanic glass all over the Pampa’s area and the random distribution of F- contents, the processes controlling the shards dissolution and the F- mobility must be identified. Hydrogeochemical data indicates a high relationship between F- and sodium bicarbonate waters and the highest pH values including a negative regression observed between F- with Ca+2 and Mg+2. Other authors previously have identified small affinity between F- with calcium and magnesium in groundwater (Smedley et al., 2002; Bundshchuh et al., 2000; Valenzuela and Ramirez, 2004). The processes that could control the negative relationship between F- and Ca+2, and the positive relationship between F- and bicarbonate would be the balance equation relating calcite and fluorite (Valenzuela and Ramirez, 2004), when both are in contact with water:

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CaCO3(s) + H+ + 2F- = CaF2(S) + HCO3The samples in which F- concentrations are over 5 mg.l-1 are oversaturated with regarding fluorite. Once the fluorite equilibrium is reached, CaCO3 removal due to calcite precipitation allows F- concentration to increase (Kim and Young Jeong 2005). Oxidant conditions and high pH prevail in the aquifer are two features of the geochemical behavior of this element has been found in three areas and they coincide with other records in the region (Nicolli et al. 1989; Smedley et al. 2002; Gomez et al., 2009): a good correlation with the distribution of sodium bicarbonate water. High pH and high bicarbonate concentrations would facilitate the dissolution of volcanic glass, passing F into groundwater cycle, where they constitute anionic complexes. Other minerals, from the rocks that form the crystalline bedrock of the Sierras de Córdoba, and its derived sediments, are also considered probable sources of F (Villalba, 1999). In some areas F distribution appears to be mainly controlled by a general salinity increase and the proximity of Paleozoic rocks containing minerals with F contents. Biotites, present in biotitic gneisses, could be responsible for part of F present in water. On the other hand, F- in the solution could also be derived from the exchange of ions from bearing minerals (apatite, pyroxene, hornblende and titanite among others). Zack (1980) highlights that F- would be released to the environment by an anionic exchange with OH-, more than by the dissolution of source minerals. In this way, dissolution processes, which tend to be very slow, would be subordinated. In alkaline environments, F- is desorbed from clay minerals (Valenzuela and Ramirez, 2004), and in high pH conditions, Fdissolution from bearing minerals, in this case volcanic glass, would be more favorable (Saxena et al., 2004). Laboratory experiments carried out by Turner et al. (2005) in calcite solutions and F- concentrations between 3 and 2,100 mg.l-1 showed that together with CaF2 precipitation processes, F- adsorption processes on the surface of calcite also occur. In this

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manner, CaCO3- precipitation processes would also decrease the concentration of F in the solution. The high correlation between F- and As in the groundwater supports the hypothesis of a common source for these elements, which could be the dissolution of volcanic glass. The highest solubility of amorphous silica is related to sodium bicarbonate water (Marshall and Warakomski 1980). Moreover, the correlation F- with pH and the association between the higher concentrations of the element in sodium bicarbonate water evidence the dissolution process of amorphous silica. The distribution of F- concentrations in groundwater is heterogeneous. This is attributed to the lower scale heterogeneities in the materials that constitute the phreatic aquifer, which would bring about the resulting changes in the geochemical conditions required for the mobilization of those elements. It can therefore be assumed that the sediment compositions and hydrogeochemical conditions are the main factors in determining the F- concentration. The composition and texture of loess, low permeability and hydraulic gradients, mineralogical composition of the basement rocks together with sodium bicarbonate watertypes are proper conditions for Fmobilization in groundwater in central and southeast sectors of Argentina.

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ACKNOWLEDGMENTS Financial support for this project came from SECyT (Secretaría de Ciencia y Técnica) from Universidad Nacional de Río Cuarto. The National Council of Scientific and Technological Research (CONICET) provided a grant for PhD studies to M.L. Gomez, in which course the data for this article were obtained. The authors would like to thank the financial support of the Agencia Nacional de Promoción Científica y Tecnológica (National Agency for Scientific and Technological Promotion) through PICT 2003 7-13891,CONICET (PIP 5668) as well as by the International Atomic Energy Agency (IAEA). Besides, the authors would like to thank the help of Asunción Romanelli for the writing the English version.

REFERENCES Agrawal, V. 1997. Groundwater quality: focus on the fluoride problem in India. CoGeoenvironmental Journal, 10. Besuschio, S.; Desanzo, A.; Perez, A. and M. Croci. 1980. Epidemiological associations between arsenic and cancer in Argentina. Biol. Trace Elem. Res. 2:41–55. Biagini, R.; Rivero, M.; Salvador, M. and M. Cordoba. 1978. Hidroarsenisismo cronico y cancer de pulmon. Arch Argent Dermatol 48:151–158. Blarasín, M. 1984. Hidrología subterránea de la zona de Laguna Oscura. Córdoba. Tesis de Lic. UNRC. Inédito. 150 pág. Brunt, R.; Vasak, L. and J. Griffioen. 2004. Fluoride in groundwater: Probability of occurrence of excessive concentration on global scale. International Groundwater Resources Assessment Centre. Report Nº. SP 2004-2.

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Bundschuh J, Bonorino G, Viero AP, Albouy R. and A. Fuertes. 2000.Arsenic and other trace elements in sedimentary aquifers in the Chaco-Pampean Plain, Argentina: 31st International Geol. Cong., Río de Janeiro, Brazil, 2000, pp 27–32. CAA. Codigo Alimentario Argentino. 1994. Resolucion 494/94. Boletin Oficial No. 27.932, 1ra seccion. Art. 982 modificado. Cantú, M. and S. Degiovanni. 1987. Génesis de los sistemas lagunazas del centro-sur de la provincia de Córdoba. Actas del X Congreso Geológico Argentino, Tucumán. 4 pp. Carrica, J. ; Bonorino, G. and D. Delpino. 2002. Fluoruros en el agua de manantiales en la zona de Zapala, provincia del Neuquen. Argentina. XXXII AIH and VI ALHSUD CONGRESS, Agua Subterranea y Desarrollo Humano, Mar del Plata, Argentina. Carrica, J. and E. Albouy. 1999. Variaciones hidroquı´micas en el agua subterranea de localidades del Partido de Puan, Provincia de Buenos Aires (Groundwater hydrochemical changes in towns of Puan District, Buenos Aires province). Hidrologıa Subterranea. II Congreso Argentino de Hidrogeologia. Santa Fe, Argentina. Ser. Correl. Geol. 13:221– 230. Chebli, G.; Mozetic, M, Rossello, C. and M. Buhler. 1999. Cuencas Sedimentarias de la Llanura Chacopampeana. Instituto de Geologia y Recursos Minerales. Geologia Argentina. Anuales 29(20):627–644. Dalla SL, Iñiguez RM (1979) “La Tinta”, Precambrico y Paleozoico de Buenos Aires [La Tinta, Precambrian and Paleozoic in Buenos Aires]. VII Congr Geol Arg I, Neuquén, Argentina, pp 539–550 Edmunds, W. y P. Smedley. 1996. Groundwater, Geochemistry and Health. British Geological Survey. En Geoindicators, Assessing rapid environmental changes in earth systems. 135-150. Editorial Balkema. Frencken, J. 1992. Endemic Fluorosis in developing countries, causes, effects and possible solutions. Publication number 91.082, NIPG-TNO, Leiden, The Netherlands. Gomez, M.L. 2009. Modelado geoquímico de contaminantes procedentes de efluentes urbanos e industriales en el acuífero clástico del área de Coronel Moldes, Córdoba: Córdoba, Argentina, Universidad Nacional de Río Cuarto, Tesis Doctoral, 290 p. Gomez, M.L.; Blarasin, M. and D. Martínez. 2009. Arsenic and fluoride in a loess aquifer in the central area of Argentine. Environmental Geology, Vol 57:143–155. Gordillo, C. and Lencinas, A. 1979. Sierras Pampeanas de Córdoba y San Luis. II Simposio de Geología Regional Argentina. Ac Nac. de Ciencias. Vol. I:577-650. Hem, J. 1992. Study and interpretation of the chemical characteristics of natural water, U.S. Geological Survey Water-Supply Paper: 2254, cuarta edición, 264 pp. Kim, K. and G. Young Jeong. 2005. Factors influencing natural occurrence of fluoride-rich groundwaters: a case study in the southeaster part of the Korean Peninsula. Chemosfere 58(10):1399–1408. Kruse E, Varela L, Laurencena P, Deluchi M (1998) Efectos del agua subterránea enb la configuración de los cauces de la llanura interserrana de la provincia de Buenos Aires [Groundwater effects on the stream network in the ntermountain plain in the Province of Buenos Aires]. Actas del X Congreso Latinoamericano de Geologia y Vi Congreso Nacional de Geologia Económica, Buenos Aires, vol I, pp 340–344 Marshall, W. and M. Warakomski. 1980. Amorphous silica solubility. II: Effect of aqueous salt solutions at 25˜C. Geochim. Cosmochim. Acta 44:915–924.

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Martinez, D. and M. Osterrieth. 1999. Geoquimica de la Silice Disuelta en el Acuifero Pampeano en la Vertiente Sudoriental de Tandilia. II Congreso Argentino de Hidrogeologia, San Miguel de Tucuman, Argentina Ser Correlac Geol 13:241–250. ISSN 1514-4186. Mgalela, R., 1997. Some aspects of saline groundwaters in Gokwe District. Annals of the Zimbabwe Geological Survey, Harare Zimbabwe. Mjengera, H. and Mkongo, G., 2002. Appropriate technology for use in fluoritic areas in Tanzania, 3rd Waternet/WARFSA Symposium on Water Demand Management for Sustainable Use of Water Resources, University of Dar Es Salaam. Msonda, K; Masamba, W. and E. Fabiano. 2007. A study of fluoride groundwater occurrence in Nathenje, Lilongwe, Malawi. Physics and Chemistry of the Earth, 32, 1178–1184. Naseem, S.; Rafique, T.; Bashir, E.; Bhanger, M.; Laghari, A. and T. Usmani. 2010. Lithological influences on occurrence of high-fluoride groundwater in Nagar Parkar area, Thar Desert, Pakistan. Chemosphere, 78, 1313–1321. Nicolli, H.; Suriano, J.; Gomez Peral, M.; Ferpozzi, L. and O. Baleani. 1989. Groundwater contamination with Arsenic and other trace elements in an area of the La Pampa province of Cordoba, Argentina. Environ. Geol. Water Sci. 14(1):3–16. Nicolli, H.; Tineo, A.; Falcon, C.; García, J.; Merino, M.; Etchichury, M.; Alonso, M. S. and O. Tafalo. 2004. Aguas subterráneas con alto contenido en arsénico en el área de Los Pereyra, Tucumán, Argentina.XXXIII Congress IAH & 7º Congress ALHSUD. Zacatecas México. ISBN: 970-32-1749-4. Parkhurst, D. and A. Appelo. 1999. User´s guide to PHREEQC (Version 2): a computer program for speciation, batch reaction, one-dimensional transport, and inverse geochemical calculations. U.S.G.S. Water-Resources Investigations Report 99-4259, pp 1–312. Poiré D, Spaletti LA (2005) La cubierta sedimentaria Precámbrica- Paleozoica inferior del sistema de Tandilla [The Precambrian. Lower Paleozoic sedimentary cover of the Tandilia system]. X Congreso Geológico Aregntino, relatorio, La Plata, Argetina, 2005, pp 51–68 Qinghai, G., Yanxin, W., Teng, M., Rui, M., 2007. Geochemical processes controlling the elevated fluoride concentrations in groundwaters of the Taiyuan Basin, Northern China. Journal of Geochemical Exploration 93 (1), 1–12. Quiroz Londoño OM, Martínez DE, Dapeña C and Massone H (2008) Hydrogeochemistry and isotope analyses used to determine groundwater recharge and flow in low-gradiente catchments of the province of Buenos Aires, Argenitna. Hydrogeology Journal 16: 11131127. Quiroz Londoño, O. M. (2009) “Hidrogeología e Hidrogeoquímica de las Cuencas de los Arroyos Tamangueyú y El Moro Provincia de Buenos Aires”. Tesis doctoral. Universidad nacional de Río Cuarto. 292 pp. Rafiquea, T.; Naseemb, S.; Usmania, T.; Bashirb, E.; Khana, F. and M. Bhangerc. 2009. Geochemical factors controlling the occurrence of high fluoride groundwater in the Nagar Parkar area, Sindh, Pakistan. Journal of Hazardous Materials, 171, 424–430. Rajagopal, R and Tobin, G. 1991. Fluoride in drinking water: a survey of expert opinions. Environmental geochemistry and health, 13, 3-13.

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Rossi, P. 1996. Evolución hidrogeoquímica del agua subterránea en la cuenca superior del arroyo Chasicó, provincia de Buenos Aires. Tesis Doctoral. Universidad Nacional del Sur, Bahía Blanca. 300 pp. Saxena VK, Sanjeev Kumar and V. Singh. 2004. Occurrence, behavior and speciation of arsenic in groundwater. Curr. Sci. 86(2):281–284. Smedley, P.; Nicolli, H.; Macdonald, D.; Barros, A. and O. Tullio. 2002. Hydrogeochemistry of arsenic and other inorganic constituents in groundwaters from La Pampa, Argentina. Apply Geochemistry 17(3):259–284. Turner B, Binnig P. and L. Stipp. 2005. Fluoride removal by calcite: evidence for fluorite precipitation and surface adsorption. Environ. Sci. Technol. 39(9):9561–9568. Valenzuela, V. and H. Ramirez. 2004. Analisis del comportamiento del Fluor en el acuı´fero que abastece a Hermosillo, Sonora, Mexico. XXXIII Congress IAH & 7 Congress ALHSUD. Zacatecas Mexico. ISBN: 970-32-1749-4. Villalba, G. 1999. Estudio geohidrologico con enfasis en la geoquimica del Fluor de la cuenca del Río Talita, Dpto. Rio Cuarto, Cordoba. Trabajo final de Licenciatura. URC. Inedito 200 pag. Zack, A. 1980. Geochemistry of fluoride in the Black Creek aquifer system of Horry and Georgetown counties, South Carolina and its Physiological implications. Washington, D.C.; Geological Survey water-supply Paper 2067.

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Chapter 3

FLUORIDE IN GROUNDWATER: CAUSES, IMPLICATIONS AND MITIGATION MEASURES K. Brindha and L. Elango* Department of Geology, Anna University, Chennai 600025, India

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ABSTRACT Groundwater is the major source for various purposes in most parts of the world. Presence of low or high concentration of certain ions is a major issue as they make the groundwater unsuitable for various purposes. Fluoride is one such ion that causes health problems in people living in more than 25 nations around the world. Fluoride concentration of atleast 0.6 mg/l is required for human consumption as it will help to have stronger teeth and bones. Consumption of water with fluoride concentration above 1.5 mg/l results in acute to chronic dental fluorosis where the tooth become coloured from yellow to brown. Skeletal fluorosis which causes weakness and bending of the bones also results due to long term consumption of water containing high fluoride. Presence of low or high concentration of fluoride in groundwater is because of natural or anthropogenic causes or a combination of both. Natural sources are associated to the geological conditions of an area. Several rocks have fluoride bearing minerals like apatite, fluorite, biotite and hornblende. The weathering of these rocks and infiltration of rainfall through it increases fluoride concentration in groundwater. Fluoride which is present in high concentration in volcanic ash is readily soluble in water and forms another natural source. Anthropogenic sources of fluoride include agricultural fertilisers and combustion of coal. Phosphate fertilisers contribute to fluoride in irrigation lands. Coal which is a potential source of fluoride is used for combustion in various industries and in brick kilns. The aerial emission of fluoride in gaseous form during these activities reaches the surface by fall out of particulate fluorides and during rainfall they percolate with the rainwater thus reaching the groundwater table. Also the improper disposal of fly ash on ground surface contributes to fluoride in groundwater. Since ingestion of high fluoride has a long term effect on human health it is essential to monitor its concentration in groundwater used for drinking periodically and take steps to bring them within the * Corresponding author: [email protected]; [email protected] Fluoride: Properties, Applications and Environmental Management : Properties, Applications and Environmental Management, Nova Science

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K. Brindha and L.Elango permissible range of 0.6 to 1.5 mg/l. There are several methods available for the removal of fluoride from groundwater which is insitu or exsitu. To dilute the groundwater contaminated with fluoride, artificial recharging structures can be built in suitable places which will decrease its concentration. Rainwater harvesting through existing wells also will prove effective to reduce the groundwater fluoride concentration. Exsitu methods which are conventional treatment methods like adsorption, ion exchange, reverse osmosis, electrodialysis, coagulation and precipitation etc can be practiced at community level or at households to reduce fluoride concentration before ingestion. But the choice of each method depends on the local conditions of the region such as the quality of groundwater and the source of contamination whether it is natural or anthropogenic. Fluoride contamination being a prominent and widespread problem in several parts of the world and as causes for this are mostly natural and unpreventable, educating the people and defluorinating the groundwater before consumption are essential for a healthy world.

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INTRODUCTION It is well known that about 70% of the earth’s surface is covered with water. Most of the water is in the oceans (96.5%) in the unusable form while some of them are frozen (1.74%). Lakes, swamp water and rivers hold 0.014% and soil moisture accounts for 0.001%. Water also exists in the form of vapour in the air (0.001%) and as groundwater beneath the sub surface in the aquifers (1.7%) (Gleick, 1996). World’s water needs are met from surface and groundwater resources. However, use of groundwater is advantageous as it is comparatively fresh and widely distributed unlike the surface water. Threats to groundwater have been increasing everyday due to raise in population and their needs. Thus with increasing demand of groundwater for domestic, industrial and agricultural needs, the pressure on this resource has become enormous. Overexploitation and improper management has also lead to contamination of this resource. The degradation of groundwater may be due to natural or anthropogenic processes. Natural causes are inherent geological conditions while anthropogenic causes include wastewater from sewage treatment plants, discharge from industries, improper solid waste disposal, agrochemicals, runoff from agricultural fields, leakage from underground storage tanks etc. When the chemical composition of groundwater is not within the prescribed standards for drinking or irrigation or industrial water, they become unsuitable. Arsenic, fluoride, nitrate, iron, manganese, boron, most heavy metals and radionuclides are few contaminants that are of great concern if not present within permissible limits. In this chapter, the causes and implications due to presence of increased fluoride in groundwater used for drinking purpose and measures to be adopted to mitigate the problem are discussed.

PROPERTIES OF FLUORIDE Fluoride belongs to halogen family represented as ‘F’ with atomic weight 18.998 and atomic number 9. It occurs as a diatomic gas in its elemental form and has a valence number 1. It is the most electronegative and the most reactive when compared to all chemical elements in the periodic table (Greenwood and Earnshaw, 1984; Gillespie et al., 1989). It has an oxidation state of -1 and occurs as both organic and inorganic compounds. It is the 13th

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most abundant element in the earth’s crust (Weinstein and Davison, 2003). Its natural abundance in the earth’s crust is 0.06 to 0.09% (Fawell et al., 2006) and the average crustal abundance is 300 mg/kg (Tebutt, 1983). Fluoride does not exhibit any colour, taste or smell when dissolved in water. Hence, it is not easy to determine it through physical examination. Only chemical analysis of the groundwater samples can determine the concentration of this ion. The widely used method for the estimation of fluoride in groundwater sample is colorimetric SPANDNS (sodium 2-(parasulfophenylazo)-l,8-dihydroxy-3,6-naphthalene disulfonate) method. The other colorimetric method extensively used is the complexone method. Fluoride concentration can also be quantified using sophisticated instruments like ion chromatograph. Ion selective electrodes are available to measure fluoride concentration in water, which can be used both in the field and in laboratory. Fluoride is one of the important micronutrient in humans which is required for strong teeth and bones. In humans, about 95% of the total body fluoride is found in bones and teeth. WHO (World Health Organisation) (1984) has prescribed the range of fluoride from 0.6 to 1.5 mg/l in drinking water as suitable for human consumption. BIS (Bureau of Indian Standards) (1992) has set a required desirable range of fluoride in drinking water to be between 0.6 and 1.2 mg/l. However, this standard suggests the maximum permissible limit can be extended up to 1.5 mg/l. This required fluoride is supplied to the human body usually through drinking water. Consumption of water with fluoride below or above the prescribed range is detrimental to human health. Hence, it is essential to monitor the groundwater quality regularly which is used directly without treatment as drinking water.

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FLUORIDE IN GROUNDWATER Groundwater is considered as the major source of drinking water in most places on earth. Usually people use groundwater for drinking and other domestic household purposes such as cooking without any physical or chemical treatment. This is not a healthy practice and may lead to number of health disorders. However, this practice cannot be avoided due to lack of treated piped water supply system in several parts of developing countries. Groundwater with fluoride concentration above the permissible limit set by WHO i.e 1.5 mg/l have been recorded in several parts of the world. In 1984, WHO estimated that more than 260 million people living all over the world consume water with fluoride concentration above 1 mg/l (WHO, 1984). The problem of high fluoride in groundwater has been reported by several researchers in India, China, Japan, Sri Lanka, Iran, Pakistan, Turkey, Southern Algeria, Mexico, Korea, Italy, Brazil, Malawi, North Jordan, Ethiopia, Canada, Norway, Ghana, Kenya, South Carolina, Wisconsin and Ohio (Dissanayake, 1991; Gaciri and Davies, 1993; Srinivasa Rao, 1997; Banks et al., 1998; Oruc, 2003; Rukah and Alsokhny, 2004; Kim and Jeong, 2005; Tekle-Haimanot et al., 2006; Valenzuela-Va´squez et al., 2006; Zheng et al., 2006; Chae et al., 2007; Farooqi et al., 2007; Mirlean and Roisenberg, 2007; Msonda et al., 2007; Vivona et al., 2007; Davraz et al., 2008; Messaïtfa, 2008; Moghaddam and Fijani, 2008; Oruc, 2008; Suthar et al., 2008; Desbarats, 2009; Li et al., 2009; Karthikeyan et al., 2010; Keshavarzi et al., 2010; Kim et al., 2010; Looie and Moore, 2010; Naseem et al., 2010; Reddy et al., 2010a; Yidana et al., 2010; Young et al., 2010; Brindha et al., 2011). The other

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possible sources of intake of fluoride apart from drinking water are through food, beverages and dental products like tooth paste. Most of the people affected by high fluoride concentration in groundwater live in the tropical countries where the per capita consumption of water is more because of the prevailing climate. In places like Ghana, people consume 3 to 4 liters of water which is higher than the WHO estimate of 2 l/adult/day (Apambire et al., 1997). The risk of fluorosis is higher in these places. However, incidence of fluorosis in people living in other parts of the world has also been reported. The intensity of fluorosis problem is very serious in the two heavily populated countries of the world namely India and China (Ayoob and Gupta, 2006). In most cases, fluoride in groundwater is contributed by the host rocks which are naturally rich in fluoride. Because of rock water interaction, long residence time and evapotranspiration, the concentration of fluoride increase. Most of the studies indicate the increase in fluoride composition in groundwater with increase in depth from ground surface (Hudak and Sanmanee, 2003; Edmunds and Smedley, 2005; Kim and Joeng, 2005; Valenzuela-Va´squez et al., 2006). But this is not always common (Apambire et al., 1997; Reddy et al., 2010a). The geochemistry of high fluoride groundwater are often associated with neutral to alkaline pH, low calcium concentration and high sodium and bicarbonate concentrations (Handa, 1975; Ramamohana Rao et al., 1993; Kundu et al., 2001; Smedley et al., 2002; Edmunds and Smedley, 2005; Chae et al., 2007). In some cases, there is also occurrence of high nitrate where there is high fluoride in groundwater (Handa, 1975; Reddy et al., 2010b). Saxena and Ahmed (2001) put forth that alkaline conditions with pH ranging between 7.6 and 8.6 are favorable for dissolution of fluorite mineral from the host rocks. Sodium bicarbonate type waters are typical of high fluoride waters (Handa, 1975; Srinivasa Rao, 1997; Chae et al., 2007). Fluoride exhibits a positive relationship with sodium and bicarbonate while it extends an inverse relationship with calcium. Handa (1975) observed that groundwater is generally undersaturated with respect to fluorite but in some cases they are saturated or over saturated (Srinivasa Rao, 1997; Smedley et al., 2002; Chae et al., 2007). There are also cases where both calcite and fluorite saturation occurs in groundwater with high fluoride. Overall, the natural concentration of fluoride in groundwater depends on the geological, chemical and physical characteristics of the aquifer, the porosity and acidity of the soil and rocks, the surrounding temperature, the action of other chemical elements, depth of the aquifer and intensity of weathering (Feenstra et al., 2007).

CASUES FOR FLUORIDE The possible causes and sources through which fluoride exists in the environment are schematically shown in Figure 1.

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Figure 1. Possible casues for fluoride in groundwater.

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Aquifer Material Most of the fluoride in groundwater is naturally present due to weathering of rocks rich in fluoride. Water with high concentration of fluoride is mostly found in sediments of marine origin and at the foot of mountainous areas (WHO, 2001; Fawell et al., 2006). Known fluoride belts on land include: from Syria through Jordan, Egypt, Libya, Algeria, Sudan and Kenya, from Turkey through Iraq, Iran, Afghanistan, India, northern Thailand and China. There are also same kind of belts in the America and Japan (WHO, 2001). Fluorite occurs in igneous and sedimentary rocks. Deer et al., (1983) reported that the occurrence of fluoride in both these rock types is almost similar. Fluoride occurs as sellaite [MgF2], fluorite or fluorspar [CaF2], cryolite [Na3AlF6], fluorapatite [3Ca3(PO4)2 Ca(F,Cl2)], apatite [CaF2.3Ca3(PO4)], topaz [Al2SiO4(F,OH)2], fluormica (phlogopite) [KMg3(Si3Al) O10(F,OH)2], biotite [K(Mg,Fe)3 AlSi3O10(F,OH)2], epidote [Ca2Al2(Fe3+;Al) (SiO4)(Si2O7)O(OH)], amphibole such as tremolite [Ca2Mg5Si8O22(OH)2] and hornblende [Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2], mica, clays, villuanite and phosphorite (Matthess, 1982; Pickering, 1985; Hem, 1986; Handa, 1988; Haidouti, 1991; Gaumat et al., 1992; Gaciri and Davies, 1993; Datta et al., 1996; Apambire et al., 1997; Kundu et al., 2001; Mohapatra et al., 2009). Fluoride minerals such as fluorite and cryolite are not readily soluble in water under normal pressure and temperature. But under alkaline conditions and range of specific conductivity between 750 and 1750 µS/cm, dissolution rate of fluorite minerals increase (Saxena and Ahmed, 2001). Granitic rocks which are a typical source of fluoride rich rocks contain fluoride ranging between 500 and 1400 mg/kg (Koritnig, 1978; Krauskopf and Bird, 1995), which is much higher than any other rock type. The world average content of fluoride in granitic rocks is 810 mg/kg (Wedepohl, 1969). The weathering of these rocks results in increased fluoride content in groundwater. Longer residence time in aquifers with fractured fluoride rich rocks enhance fluoride levels in the groundwater. Naseem et al. (2010) put forth that granitic rock and clay in Pakistan contained average fluoride of 1939 and 710 mg/kg respectively. Granite and granitic gneisses in Nalgonda, India contain fluoride rich minerals such as fluorite (0 to 3.3%), biotite (0.1 to 1.7%) and hornblende (0.1 to 1.1%) (Ramamohana Rao et al., 1993). Mondal et al. (2009) reported from the same area that the rocks contain 460 to 1706 mg/kg of fluoride. Laboratory studies conducted by Chae et al. (2006) showed that leaching of fluoride from granitic rocks contributed 6 to 10 mg/l of fluoride to water.

P

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Volcanic Ash Volcanic rocks are often enriched in fluoride. Hydrogen fluorine is one of the most soluble gases in magmas and comes out partially during eruptive activity (D’Alessandro, 2006). The aerial emission of fluoride in the form of volcanic ash during volcanic eruption reaches the surface by fall out of particulate fluorides and during rainfall. This fluoride from the soil surface will easily reach the groundwater zone along with percolating rainwater. Volcanic eruptions are common in Iceland and fluorosis poisoning in livestock and humans was identified long ago in 1978 from the Laki eruption (Fridriksson, 1983; Steingrímsson, 1998). The fluoride content in ash from Hekla eruption in 2010 was 23-35 mg/kg (Matvælastofnun, 2010). Volcanic ash is readily soluble and thus the risk of fluoride contamination in groundwater is very high. These volcanic sources have also been found to cause fluoride contamination in groundwater of Kenya (Gaciri and Davies, 1993).

Fly Ash

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Like volcanic ash, fly ash from the combustion of fossil fuels also account for high fluoride. More than 100 to 150 million tons of fly ash is produced worldwide annually due to the combustion of coal especially from power plants (Prasad and Mondal, 2006; Piekos and Paslawska, 1998). Inappropriate disposal of this fly ash will result in the leaching of fluoride to groundwater. Churchill et al. (1948) reported that coal contains 40 to 295 mg of fluoride/kg. But the fluoride content of coal depends on the type of coal being burnt. Brick kilns which use coal for burning also account as a source for fluoride pollution (Jha et al., 2008).

Fertilisers Phosphate containing fertilisers add up to the fluoride content in soil and groundwater (Motalane and Strydom, 2004; Farooqi et al., 2007). It is evident that superphosphate (2750 mg of F/kg), potash (10 mg of F /kg) and NPK (Nitrogen Phosphorous Potassium) (1675 mg of F /kg) which are phosphatic fertilisers contain remarkable amount of fluoride (Srinivasa Rao, 1997). Also, fluoride concentration in irrigation water accounts to be 0.34 mg/l. In agricultural areas successive irrigation had lead to the increase in fluoride concentration in groundwater (Young et al., 2010). The amount of water soluble fluoride in the soil near a phosphate fertiliser plant in Brazil was 10 mg/kg. Datta et al. (1996) put forth that if an agriculture field of 1 ha receives 10 cm of irrigation water containing 10 mg/l of fluoride, then the soil can obtain 10 kg of fluoride. This is considered as a potential threat for increase in fluoride concentration in soil and groundwater. Apart from these, industrial processes such as aluminium smelting (Haidouti, 1991), cement production and ceramic firing (WHO, 2002) also lead to release of fluoride into the environment.

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HEALTH IMPLICATIONS

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Intake of fluoride higher than the optimum level is the main reason for dental and skeletal fluorosis. Depending upon the dosage and the period of exposure fluorosis may be acute to chronic. Ayoob and Gupta (2006) quoted that around 200 million people, from among 25 nations all over the world are under the dreadful fate of fluorosis of which India and China, the two most populous countries of the world, are the worst affected. In India 62 million people including 6 million children are estimated to have serious health problems due to consumption of fluoride contaminated water (Andezhath and Gosh, 2000). Endemic fluorosis is prevalent in 29 provinces of China, municipalities and autonomous regions (Wang and Huang, 1995). Dental and skeletal fluorosis was predominant in China due to the indoor burning of coal to make brick tea or for heating purposes (Ando et al., 1998; Watanabe et al., 2000; Ando et al., 2001). Dissanayake (1991) stated that dental fluorosis was predominant in the dry zone where fluoride concentration was high and dental caries was prevalent in wet zones where low fluoride occurs in groundwater of Sri Lanka. Of the 10 million people living in the Ethiopian Rift Valley, more than 8 million are exposed to elevated concentration of fluoride (Rango et al., 2010). The health outcome by consuming fluoride at different concentration was given by Dissanayake (1991) i.e. when fluoride concentration in drinking water is below 0.5 mg/l it causes dental carries; fluoride between 0.5 to 1.5 mg/l results in optimum dental health; 1.5 to 4 mg/l causes dental fluorosis; 4 to 10 mg/l induces dental and skeletal fluorosis while fluoride above 10 mg/l results in crippling fluorosis. However, fluorosis results not only due to the presence of high concentration fluoride in drinking water but also depend on other sources such as the dietary habits which enhance the incidence of fluorosis.

Dental Fluorosis Tooth enamel is principally made up of hydroxyapatite (87%) which is crystalline calcium phosphate (Brudevold and Soremark, 1967). Fluoride which is more stable than hydroxyapatite displaces the hydroxide ions from hydroxyapatite to form fluoroapatite. On prolonged continuation of this process the teeth become hard and brittle. This is called dental fluorosis. Dental fluorosis in the initial stages results in the tooth becoming coloured from yellow to brown to black. Depending upon the severity, it may be only discolouration of the teeth or formation of pits in the teeth. The colouration on the teeth may be in the form of spots or as streaks. Usually these streaks on the teeth are horizontal. Children who are exposed to excess fluoride from childhood show symptoms of fluorosis very often than compared to adults. Hence the fluoride problem in an area may not be decided on the fact that the adults have good teeth with no symptoms of discolouration. Though the main source for dental fluorosis is fluoride ingestion through drinking water, it can also be ingested through toothpastes containing fluoride. It is common for children to swallow toothpastes which has to be avoided to prevent fluorosis. A significant relationship between fluoride intake by water and the prevalence of dental fluorosis has been reported by several researchers (Heller et al., 1997; Viswanathan et al., 2009; Mandinic et al., 2009; Viswanathan et al., 2010).

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Skeletal Fluorosis Exposure to very high fluoride over a prolonged period of time results in acute to chronic skeletal fluorosis. It was stated in 1993 that crippling skeletal fluorosis might occur in people who have ingested 10 to 20 mg of fluoride per day for over 10 to 20 years (National Research Council, 1993). India and China has been largely affected by crippling skeletal fluorosis with 2.7 million people being affected in China. Of the 32 states in India, 17 have been identified as endemic areas with 6 million people affected by skeletal fluorosis. Apart from ingestion of fluoride through drinking water, skeletal fluorosis also may be caused due to indoor use of coal as fuel and by air borne fluoride. Ingestion of fluoride through inhalation in factories and industries is one of the occupational health problems. Skeletal fluorosis does not only affect humans but also animals fed with fluoride rich water and fodder. Fluorosis is also now associated with heavy consumption of tea (Cao et al., 1996; Watanabe et al., 2000; Whyte et al., 2008; Joshi et al., 2010). Early stages of skeletal fluorosis start with pain in bones and joints, muscle weakness, sporadic pain, stiffness of joints and chronic fatigue. During later stages, calcification of the bones takes place, osteoporosis in long bones, and symptoms of osteosclerosis where the bones become denser and develop abnormal crystalline structure. In the advanced stage the bones and joints become completely week and moving them is difficult. The vertebrae in the spine fuse together and the patient is left crippled which is the final stage. Skeletal fluorosis is usually not recognized until the disease reaches an advanced stage.

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Other Effects Other health disorders that occur due to consumption of high fluoride in drinking water to be muscle fibre degeneration, low haemoglobin levels, deformities in RBCs, excessive thirst, headache, skin rashes, nervousness, neurological manifestations, depression, gastrointestinal problems, urinary tract malfunctioning, nausea, abdominal pain, tingling sensation in fingers and toes, reduced immunity, repeated abortions or still births, male sterility, etc (Meenakshi and Maheshwari, 2006). As fluoride is excreted in urine through the kidneys, they affect the effective functioning of the kidneys. They facilitate in the formation of kidney stones. Li et al. (1988) reported that fluoride might have genotoxical effects. Several studies also reported these effects on humans and animals (Sheth et al., 1994; Joseph and Gadhia, 2000; Tripathi et al., 2009). Consumption of drinking water with high fluoride by children may affect their intelligence. Tang et al. (2008) who studied this phenomenon however could not come out with a mechanism by which the IQ of children is lowered. Guan et al. (1999) suggested that when phospholipids and ubiquinone contents gets altered in the brain of rats affected by chronic fluorosis, changes in their membrane lipids may be the cause of this problem. Several other studies carried also comply with this fact (Trivedi et al., 2007; Ge et al., 2010). The presence of excessive fluoride in groundwater has its impact not only on humans but also on soil fertility and plant and animal growth.

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STATUS OF GROUNDWATER FLUORIDE OCCURRENCE IN THE WORLD Dean (1933) reported the prevalence of fluorosis in Arizona, Arkansas, California, Colorado, Idaho, Illinois, Iowa, Kansas, Minnesota, Nevada, New Mexico, North Carolina, North Dakota, Oklahoma, Oregon, South Carolina, South Dakota, Texas, Utah and Virginia. In 1980’s the United States incorporated several recommendations to reduce fluoride ingestion. But Kumar et al. (1998) found that the prevalence of dental fluorosis has not declined even more than ten years after the recommendations. High concentration of fluoride (>3.5 mg/l) was noticed in South Carolina where 40% of the people depend on groundwater for their needs (South Carolina Ambient Groundwater Quality Report, 2003). The groundwater of Lake Saint-Martin area, Manitoba represented geochemical signature of groundwater originating from a deep regional aquifer, which are upwelling in a previously unrecognized discharge zone created by structural uplift associated with the impact event (Desbarats, 2009). It was further found that Na–HCO3–SO4 groundwater type with low chloride and less depleted 18O concentrations represented modern recharge displacing groundwater of Na-mixed anion type with high chloride and highly depleted 18O compositions representing discharge from Winnipeg formation in Manitoba. The volcanic ash deposits in Texas which was the reason behind high fluoride in groundwater up to 6.27 mg/l showed better correlation with well depth (Hudak and Sanmanee, 2003). Emission from phosphate fertiliser production factory which was the cause of high fluoride in groundwater in southern Brazil was found to be influenced by vegetation cover i.e. places with grasslands had higher fluoride than those with eucalyptus plantation (Mirlean and Roisenberg, 2007). In Ireland, from 1964 fluoridated water was supplied to the public in order to meet the fluoride requirement of the human body. The Department of health, Social Services and Public Safety of Ireland put forth that 71% of the population are exposed to this fluoridated water in 2007 (Oral Health Strategy for Northern Ireland, 2007). In 1984, the National Survey of Children’s Dental Health in Ireland examined people until 15 years and found that they had fluorosis from mild to questionable level but higher levels were absent (Whelton et al., 2004). Federation Dentaire International stated that most Germany had low fluoride water (Federation Dentaire International, 1990). In Belgium, drinking water contains fluoride below 0.3 mg/l. However, cases of dental fluorosis reported by Carvalho et al. (1998) in Belgium. Schwartz and Friedrich (1973) assessed the concentration of fluoride in spring and stream waters to determine occurrences of fluorite in Osor district, Spain. Groundwater in Norway had high fluoride up to 8.26 mg/l due to fluoride rich rocks. Fluoride concentrations in groundwater were high in areas of granite while it was low in anorthosites. It was observed that dental fluorosis was detected in parts of Norway which are related to the bedrock source (Banks et al., 1998). Czarnowski et al. (1996) studied the groundwater quality in Poland with respect to fluoride and found that they were well below limits in most places. However, fluoride concentration of 1.38 mg/l was detected around a phosphate industry waste disposal site. Over 90% of the population in Estonia consumed water with fluoride below 1.5 mg/l. Even then fluoride concentrations of about 7 mg/l occur naturally in western Estonia which is due to Silurian-Ordovician aquifer system (Indermitte et al., 2009). The risk of getting affected by dental fluorosis in this region is 4.4 times higher in category of exposure ranging between 1.5 and 2.0 mg/l. Alumina production plants had increased the fluoride concentration

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in Greece. Studies by Haidouti (1991) showed that total soil fluoride collected from near the alumina production plant at depths of 0-5, 5-15 and 15-30 cm decreased with distance from the emission source and reached background levels at about 20 km. Also, total soil fluoride decreased with depth at high impact areas and it was vice versa at low impact areas. This high fluoride in soil may leach through the unsaturated zone during precipitation and increase fluoride in groundwater. Oruc (2008) stated that in Turkey endemic fluorosis related to high fluoride ranging from 1.5 to 4 mg/l in drinking water was observed since 1955. Public drinking water supply system in Isparta, Turkey draws water from lakes and springs from 1962. Springs discharged from volcanic rocks, Golcuk pyroclastic and Miocene clastic rocks contained fluoride between 3.71 and 5.62, 1.81 and 3.95, 0.39 and 1.02 mg/l respectively. This resulted in elevated fluoride concentration in groundwater which resulted in dental fluorosis in south west Turkey (Davraz et al., 2008). North Jordan has very low fluoride concentration (Rukah and Alsokhny, 2004) where it becomes necessary to provide fluorinated water to meet the daily requirement of fluoride by the human body. It is found that the fluoride concentration follows the regional topography in that area. In Saudi Arabia 34% of the water needs are supplied by groundwater. The fluoride concentration in groundwater that is supplied to the water treatment plants before reaching the public contains 0.63 to 1.6 mg/l of fluoride (Alabduláaly, 1997). Thus fluoride concentration is well within limits in drinking water in Saudi Arabia. Moghaddam and Fijani (2008) found that groundwater occurring almost everywhere in basaltic rocks in north western Iran contain fluoride beyond the suitable range. Dental decay was found increasing in southern Algeria. Messaïtfa (2008) reported the concentration of fluoride up to 2.3 mg/l in groundwater in Algeria. About 70% of the fluoride intake for the people of this region is through groundwater used for drinking. Apart from these, dates and tea contribute to 10% and 20% of fluoride intake respectively. Thus the daily intake of fluoride ingested by an adult exceeds the threshold limit of 0.05 to 0.07 mg of fluoride/kg/day (Burt, 1992). Fluoride contents in some rivers (12-26 mg/l), springs (15-63 mg/l) and alkaline ponds and lakes (60-690 mg/l) were found to be very high in Tanzania (Nanyaro et al, 1984). Weathering of fluoride rich nephelinitie and carbonatitic rocks and soils and gaseous emanations through mineral springs were the probable contributing sources. Gaciri and Davies (1993) noticed that in natural waters of Kenya, fluoride concentration was greater in lake water than groundwater and springs which was greater than river water. Evaporation would have been a major cause to increase the concentration of fluoride in lakes of this region. Groundwater studies on fluoride in South Korea show that the concentration of fluoride depends on the residence time (Kim and Jeong, 2005) due to geogenic source of fluoride (Chae et al., 2007; Kim et al., 2010). People living in Ikeno district of Japan were accidentally exposed to drinking water containing 7.8 mg/l fluoride for 12 years (Ishii and Suckling, 1991). After the realization of the problem, they were substituted with water containing 0.2 mg/l of fluoride. Because of this the children in this area developed dental fluorosis. In Japan, as there are number of volcanoes, there is probability of fluoride contamination through volcanic ash. Ash from volcanic explosion of Sakurajima volcano, Japan was found to contain average fluoride concentration of 788.1 mg/kg (Nogami et al., 2006). This forms a potential source of threat to groundwater around this area. Abdelgawad et al. (2009) found that weathering and alteration of granitic rocks was the factor affecting oversaturation of fluoride ion in groundwater in Mizunami area, Japan.

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The incidence of mottled enamel was first reported in 1930 by Anderson and Stevenson (1930) in China. About 26 million people in China suffer from dental fluorosis due to high fluoride consumption through water. Also 16.5 million people suffer from dental fluorosis resulting from coal smoke pollution (Liang et al., 1997). It was estimated that more than 30 million people suffer from chronic fluorosis. Fluoride problems in China occur through drinking water, indoor coal combustion and brick tea. In Taiyuan basin of China, Guo et al. (2007) noticed that in recharge area interactions between groundwater and fluoride containing minerals were the sources for high fluoride whereas in discharge area evaporation and mixing of karst water contributed to high fluoride. Migration and enrichment of fluoride in North west China was studied by Genxu and Guodong (2001). This study concluded that on the basis of spatial distribution the causes for fluoride can be differentiated as dissolution-runoff, evaporation-runoff leaching-evaporative enrichment depending on hydrogeochemical zones. Fertiliser containing leachable fluoride ranging from 53 to 255 mg/kg and coal containing fluoride ranging from 5 to 20 mg/kg were reported to pollute groundwater with high fluoride in east Punjab, Pakistan by Farooqi et al. (2007) where 2 million people are at risk of being exposed to high fluoride. The granitic rocks with average fluoride concentration of 1939 mg/kg in Nagar Parkar area, Pakistan, contain fluoride in kaolin deposits between 468 and 1722 mg/kg and secondary kaolin deposits have 270 mg/kg which are the source of fluoride up to 7.85 mg/l in groundwater in this area (Naseem et al., 2010). Studies on fluoride in groundwater in Sri Lanka carried out by Dissanayake (1991) and Young et al. (2010) shows that the condition has not changed even after about two decades with fluoride above 4 mg/l in groundwater. It was found that high fluoride areas lie within low plains and low fluoride areas were usually highlands. This was because the contact time with the geological material was longer in the plains and there exists slow groundwater movement compared to highlands (Dharmagunawardhane and Dissanayake, 1993). A detailed description on the concentration of fluoride in groundwater and its sources in various regions of the world based on literature are given in Table 1 and Figure 2.

Figure 2. Occurrence of fluoride in groundwater in various parts of the world based on literature.

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Table 1. Concentration of fluoride in groundwater and its sources in various parts of the world based on literature Country (in Source aphabetical order) Algeria Fluorinated minerals Australia Atmospheric Brazil Phosphate fertilizer production emission Canada Fluoride rich rock China, Dissolution of the fluorine minerals and Taiyuan basin evaporation China Fluorine rich minerals and rocks China, Fluorite from Holocene sediments Mongolia China, Limestone Taiyuan basin Estonia Silurian-Ordovician carbonaceous aquifer Ethiopia Geochemical characteristics Ghana, Keta Mineral weathering basin Ghana Fluorine enriched Bongo coarse grained hornblende granite and syenite suite Iran Dolomite and limestone along with gypsum Iran, Isfahan Amphibole and mica group minerals in metamorphic and granitic rocks Iran, Maku Basaltic rocks Jordan Fluorite and calcite solubility Kenya Volcanic activity and chemical weathering

General range of fluoride concentration in groundwater 0.4 to 2.3 mg/l Up to 0.69 mg/l 0.1 to 4.79 mg/l Up to15.1 mg/l >2 mg/1

Messaïtfa, (2008) Petrides and Cartwright, (2006) Mirlean and Roisenberg, (2007) Desbarats, (2009) Li et al., (2009)

2.5 to 10.3 mg/l 2.3 to 9.8 mg/l

Genxu and Guodong, (2001) Zheng et al., (2006)

Up to 6.20 mg/l

Guo et al., (2007)

0.01 to 7.2 mg/l 0.01 to 13 mg/l 0 to 282.29 mg/l

Indermitte et al., (2009) Tekle-Haimanot et al., (2006) Yidana et al., (2010)

0.11 to 4.60 mg/l

Apambire et al., (1997)

0.7 to 6.6 mg/l 0.2 to 9.2 mg/l

Looie and Moore, (2010) arzi et al., (2010)

0.30 to 5.96 mg/l 0.009 to 0.055 mg/l 0.1 to 25 mg/l

Moghaddam and Fijani, (2008) Rukah and Alsokhny, (2004) Gaciri and Davies, (1993)

Korea, Gimcheon Korea Korea Malawi

Up to 2.15 mg/l

Kim et al., (2010)

Pegmatite

Granitic rocks > 5 mg/l Geological 0 to 40.8 mg/l Geological, chemical and physical 1.65 to 7.5 mg/l characteristics of the aquifer Malawi, Weathering of rocks containing biotite, 0.5 to 6.98 mg/l Lilongwe dissolution of hornblende, fluorite and amphibole Mexico, Deep regional flows, heating processes and 0.53 to 7.59 mg/l Sonora fluorite dissolutions in granitic rocks Mexico, San Fluorite 2.10 to 3.65 mg/l Luis Potosí basin Norway Lithological Up to 8.26 mg/l Pakistan, Phosphate fertilizers and coal combustion 0.11 to 22.8 mg/l Punjab Pakistan, Thar Granitic rocks 1.13 to 7.85 mg/l desert Poland Anthropogenic 0.3 to 2.45 mg/l Saudi Arabia -0.42 to 1.8 mg/l Sri Lanka Granitic gneiss 0.01 to 4.34 mg/l Sri Lanka Fluoride in rocks > 5 mg/l Tanzania Fluoride rich nephelinitie and carbonatitic rocks 15 to 63 mg/l Turkey Fluorite in limestone 1.5 to 13.7 mg/l USA, Felsic igneous and equivalent metamorphic 0.01 to 7.60 mg/l Wisconsin rocks USA, Texas Volcanic ash deposits 0.3 to 6.27 mg/l USA, South Cryptocrystalline fluoroapatite > 3.5 mg/l Carolina USA, Utah USA, Ohio

Fluorite rich rocks Bedrock

0.01 to 0.6 mg/l 0.2 to 2.8 mg/l

Reference

Kim and Jeong, (2005) Chae et al., (2007) Sajidu et al., (2008) Msonda et al., (2007)

Valenzuela-Va´squez et al., (2006) Carrillo-Rivera et al., (2002)

Banks et al., (1998) Farooqi et al., (2007) Naseem et al., (2010) Czarnowski et al., (1996) Alabduláaly, (1997) Young et al., (2010) Dissanayake, (1991) Nanyaro et al., (1984) Oruc, (2008) Ozsvath, (2006) Hudak and Sanmanee, (2003) South Carolina Ambient Groundwater Quality Report, (2003) Mayo and Loucks, (1995) Deering et al., (1983)

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India Of the 85 million tons of fluoride deposits on the earth’s crust, 12 million are found in India (Teotia and Teotia, 1994). Hence it is natural that fluoride contamination is widespread, intensive and alarming in India. Endemic fluorosis is prevalent in India since 1937 (Shortt et al., 1937). It has been estimated that the total population consuming drinking water containing elevated levels of fluoride is over 66 million (FRRDF, 1999). Different parts of India where elevated concentration fluoride in groundwater as reported in literature are shown in Figure 3 and a detailed list of concentration of fluoride in groundwater and their sources in different places are given in Table 2. Some regions in north western and southern India are heavily affected with fluorosis (Agarwal et al., 1997; Yadav et al., 1999). About 50% of the groundwater in Delhi exceeds the maximum permissible limit for fluoride in drinking water (Datta et al., 1996). Jacks et al. (2005) observed that high fluoride in groundwater in many parts of India was due to evapotranspiration of groundwater with residual alkalinity. Fluoride content was higher in deeper aquifers of Maharashtra (Madhnure et al., 2007) which was due to long residence time than shallow groundwater. The rocks in southern India are rich with fluoride which forms the major reason for fluoride contamination in groundwater. It is a well established fact that groundwater in Nalgonda district, Andhra Pradesh, has high fluoride due to the inherent fluoride rich granitic rocks. The granitic rocks in Nalgonda district contain fluoride from 325 to 3200 mg/kg with a mean of 1440 mg/kg. The mean fluoride content in Hyderabad granites is 910 mg/kg (Ramamohana Rao et al., 1993). The Nalgonda granties contain much higher fluoride than the world average fluoride concentration of 810 mg/kg (Wedepohl, 1969). In Kurmapalli watershed, rocks are enriched in fluoride from 460 to 1706 mg/kg (Mondal et al., 2009). Calcretes act as a sink for fluoride. Co-precipitation and/or adsorption of fluoride by calcrete deposits in Wailapalli watershed had resulted in high fluoride in groundwater (Reddy et al., 2010a) Brindha et al. (2011) found that when groundwater fluctuation was within 4.5 m below ground level, fluoride concentration was high when the water level was low and the fluoride concentration decreases with the rise in water table. This was due to direct evaporation of groundwater from these wells. If groundwater fluctuation was beyond 4.5 m below ground level the concentration of fluoride measured in groundwater after the monsoonal rains were higher than the preceding months. This was because evaporation resulted in the precipitation of fluoride rich salts on the soil which reached the groundwater along with percolating rainwater. The fluoride rich rocks form the main source for high fluoride groundwater in India. Also agriculture is intensively practiced in most parts of India. Hence it is possible that the fertilisers also add up to the sources of fluoride contamination in groundwater. Thus treatment of groundwater especially for fluoride before using it for drinking purpose is very essential in India.

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Figure 3. Range of fluoride in groundwater in India based on literature.

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Table 2. Concentration of fluoride in groundwater in India and its sources based on literature State, district/place (in

Source

aphabetical order)

Andhra Pradesh, Kurmapalli Fluoride rich rocks watershed Andhra Pradesh, Nalgonda Fluoride rich grantic rocks Fluoride rich granitic rocks

0.1 to 8.8 mg/l

Ramamohana Rao et al., (1993) Brindha et al., (2011)

Pyroxene amphibolites and pegmatites Granitic rocks

Up to 3.4 mg/1

Srinivasa Rao, (1997)

0.6 to 2.1 mg/l

Subba Rao, (2009)

Hornblende, biotite, apatite, fluorite and fluoride rich calcretes Andhra Pradesh, Wailapally Fluorite bearing rocks watershed Andhra Pradhesh and Coal ash Jharkhand Assam, Guwahati Granite

0.5 to 7.6 mg/l

Reddy et al., (2010a)

0.97 to 5.83 mg/l

Reddy at el., (2010b)

0.1 to >4 mg/l 0.18 to 6.88 mg/l

Prasad and Mondal, (2006) Das et al., (2003)

Delhi

0.1-16.5 mg/l

Datta et al., (1996)

0.94 to 2.81 mg/l 1.5 to 5.6 mg /l

Salve et al., (2008) Dhiman and Keshari, (2006) Garg et al., (2009)

Andhra Pradesh, part of Nalgonda district Andhra Pradesh, Vamsadhara river basin Andhra Pradesh, Visakhapatnam Andhra Pradesh, Wailapally watershed

0.4 to 20 mg/1

Haryana, Bhiwani

Irrigation water and brick industries Granite, gneiss and pegmatite Calcite and dissolution of dolomite Rock

Karnataka, Bellary

Apatite, hornblende and biotite 0.33 to 7.8 mg/l

Keral, Palghat

Hornblende and biotite gneiss 0.2 to 5.75 mg/l

Maharashtra, Yavatmal

Amphibole, biotite and fluoroapatite Fluoride bearing host rocks

Gujarat, Mehsana Gujarat, Mehsana

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General range of Reference fluoride concentration in groundwater Up to 21.0 mg/l Mondal et al., (2009)

Rajasthan, Hanumangarh Tamil Nadu, Erode

0.14 to 86 mg/l

Wodeyar and Sreenivasan, (1996) Shaji et al., (2007)

0.30 to 13.41 mg/l

Madhnure et al., (2007)

1.01 to 4.42 mg/l

Suthar et al., (2008)

Uttar Pradesh, Kanpur

Host rocks and weathering of 0.5 and 8.2 mg/l fluorite -0.14 to 5.34mg/l

West Bengal, Hooghly

Super phosphate fertilizer

0.01to 1.18 mg/l

Karthikeyan et al., (2010) Sankararamakrishnan et al., (2008) Kundu and Mandal, (2009)

MITIGATION MEASURES Everybody needs clean water. When high fluoride in the drinking water source has been identified, it is better to avoid that source and look for other sources. But this is not a long lasting solution. Insitu and exsitu methods are available to treat groundwater with high fluoride and bring it to the usable form.

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Insitu-Treatment Methods Insitu method aims at directly diluting the concentration of fluoride (in groundwater) in the aquifer. This can be achieved by artificial recharge. Construction of check dams in Anantapur district, India has helped widely to reduce fluoride concentration in groundwater (Bhagavan and Raghu, 2005). Rainfall recharge also called as rainwater harvesting can be adopted using percolation tanks and recharge pits which may prove helpful. Recharge of rainwater after filtration through the existing wells can also be planned to improve the groundwater quality.

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Exsitu-Treatment Methods Numerous exsitu methods are available for defluoridation of water either at household or community level. Adsorption method involves the passage of water through a contact bed where fluoride is adsorbed on the matrix. Activated charcoal and activated alumina are the widely used adsorbents (Chidambaram et al., 2003; Chauhan et al., 2007). Brick, bone char, fly ash, serpentine, red mud, waste mud, rice husk, kaolinite, bentonite, charfines, ceramic etc. are some of the other absorbents capable of effectively removing fluoride from groundwater (Srimurali et al., 1998; Çengeloğlu et al., 2002; Chidambaram et al., 2003; Yadav et al., 2006; Sarkar et al., 2006; Castillo et al., 2007; Ma et al., 2008; Kemer et al., 2009; Tor et al., 2009; Ma et al., 2010; Ganvir and Das, 2010; Chena et al., 2010). The effective removal of fluoride by these absorbents depends on the initial concentration of fluoride, pH, contact time, type of absorbent and its size. In ion exchange process, when water passes through a column containing ion exchange resin, the fluoride ions replace calcium ions in the resin. Once the resin is saturated with fluoride ions, it is backwashed with solution containing chloride such as sodium chloride. The chloride ions thus again replaces the fluoride ions in the resin and is ready for reuse. But the backwash is rich in fluoride and hence care should be taken in disposing this solution. Similarly in precipitation methods, the disposal of sludge with concentrated fluoride is a great problem. Precipitation involves addition of chemicals such as calcium which results in the precipitation of fluoride as fluorite. Aluminum salts are also used for this process. Nalgonda technique which is a well known technique uses alum, lime and bleaching powder followed by rapid mixing, flocculation, sedimentation and filtration. This was developed in India by National Environmental Engineering Research Institute to serve at community and household levels. The resulting sludge from this process contains high amount of aluminium and fluoride, the disposal of which is yet another problem. These above mentioned ex-situ methods are simple and cost effective. Membrane processes is also an ex-situ technique which includes methods called reverse osmosis and electrodialysis. These are advanced techniques which require high cost input. Both these methods use a semipermeable membrane which removes dissolved solutes from the water when they pass through them. But the negative point is that even the ions which are essential for the human body are also removed. The difference between these techniques is that reverse osmosis works on pressure while electrodialysis works on direct potential. Also reverse osmosis can be practiced at household level whereas electrodialysis involves huge set up and is even more expensive. All these methods have their own advantages and

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disadvantages. Hence it is necessary to evaluate the prevailing local conditions and cost effectiveness before choosing a particular defluorination method for an area. Apart from all these it is essential to create environmental awareness among public regarding the ill effects of high fluoride. Reduction in the use of fertilisers, especially phosphatic fertilisers is important. It is better to adopt organic farming in places of fluoride threat. In countries with high temperature, it is advisable to reduce evapotranspiration by increasing vegetation cover. This will prevent the deposition of fluoride salts on the unsaturated zone which will subsequently reach the groundwater during rainfall. Other way of combating fluorosis is to modify the dietary intake of the people. Food with more calcium and vitamin C can prevent fluorosis to a certain extent. Usage of coal for combustion indoors should be avoided and the resultant fly ash obtained from combustion of fossil fuel in industries has to be disposed cautiously.

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SUMMARY AND CONCLUSION It is evident from studies by several researchers worldwide that fluoride in groundwater has been a potential problem to human society. The main source of fluoride in groundwater is the rocks which are rich in fluoride. Fluoride occurs in sellaite, fluorite, cryolite, fluorapatite, apatite, fluormica, biotite, amphibole and several other rocks. Weathering of these rocks and prolonged residence time leads to high fluoride groundwater. Low calcium, high sodium and high bicarbonate are typical of high fluoride groundwater. Volcanic ash and combustion of coal are the next major source for fluoride. The other sources for fluoride are infiltration of agricultural runoff containing chemical fertilisers, improper disposal of liquid waste from industries, alumina smelting, cement production and ceramic and brick firing. Some amount of fluoride is essential for the human body for healthy teeth and bones. But when they are present above the recommended limit of WHO and BIS i.e. 1.5 mg/l it results in mild dental fluorosis to crippling skeletal fluorosis as the quantity and period of exposure increases. Dental fluorosis is more prevalent in children than in adults. Skeletal fluorosis occurs when an individual is exposed to fluoride of above 10 mg almost every day over a period of one or two decades. Apart from fluorosis there are also several health disorders due to ingestion of drinking water with high fluoride. To remediate the groundwater with high fluoride, defluorination techniques are adopted. They include adsorption, ion exchange, coagulation and precipitation, reverse osmosis and electrodialysis. Of these, reverse osmosis has been considered as the best available technology. Onsite treatment includes artificial recharge methods such as rain water harvesting, constructing check dams, percolation ponds, facilitating recharge of rain water through existing wells etc. Adopting a particular method depends on the initial fluoride concentration, source and cost effectiveness in an area.

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Oruc, N. (2008). Occurrence and problems of high fluoride waters in Turkey: An overview. From the issue entitled "Medical Geology in Developing Countries, Part 2". Environmental Geochemistry and Health, 30(4), 315-323. Ozsvath, D. L. (2006). Fluoride Concentrations in a Crystalline Bedrock Aquifer, Marathon County, Wisconsin Marathon County, Wisconsin. Environmental Geology, 50 (1), 132– 138. Petrides, B., & Cartwright, I. (2006). The hydrogeology and hydrogeochemistry of the Barwon Downs Graben aquifer, southwestern Victoria, Australia. Environmental Geology, 14 (5), 809-826. Pickering, W. F. (1985). The mobility of soluble fluoride in soils. Environ. Pollution Ser. B 9, 281–308. Piekos, R., & Paslawska, S. (1998). Leaching characteristics of fluoride from coal fly ash. Fluoride, 31(4), 188-192. Prasad, B., & Mondal, K. K. (2006). Leaching Characteristics of Fluoride from Coal Ash. Asian Journal of Water, Environment and Pollution, 4(2), 17-21. Ramamohana Rao, N. V., Suryaprakasa Rao, K., & Schuiling, R. D. (1993). Fluorine distribution in waters of Nalgonda District, Andhra Pradesh, India. Environmental Geology, 21, 84–89. Rango, T., Bianchini, G., Beccaluva, L., & Tassinari, R. (2010). Geochemistry and water quality assessment of central Main Ethiopian Rift natural waters with emphasis on source and occurrence of fluoride and arsenic. Journal of African Earth Sciences, 57, 479–491. Reddy, A. G. S., Reddy, D. V., Rao, P. N., & Prasad, K. M. (2010b). Hydrogeochemical characterization of fluoride rich groundwater of Wailapalli watershed, Nalgonda district, Andhra Pradesh, India. Environmental Monitoring and Assessment, 171, 561-577. Reddy, D. V., Nagabhushanam, P., Sukhija, B. S., Reddy, A. G. S., & Smedley, P. L. (2010a). Fluoride dynamics in the granitic aquifer of the Wailapally watershed, Nalgonda District, India. Chemical Geology, 269 (3-4), 278-289. Rukah, Y. A., & Alsokhny, K. (2004). Geochemical assessment of groundwater contamination with special emphasis on fluoride concentration, North Jordan. Chemie der Erde - Geochemistry, 64(2), 171-181. Sajidu, S. M. I., Masamba, W. R. L., Thole, B., & Mwatseteza, J. F. (2008). Groundwater fluoride levels in villages of Southern Malawi and removal studies using bauxite. International Journal of Physical Sciences, 3 (1), 1-11. Salve, P. R., Maurya, A., Kumbhare, P. S., Ramteke, D. S., & Wate, S. R. (2008). Assessment of Groundwater Quality with Respect to Fluoride. Bull Environ Contam Toxicol, 81, 289–293. Sankararamakrishnan, N., Sharma, A. K., & Iyengar, L. (2008). Contamination of nitrate and fluoride in ground water along the Ganges alluvial plain of Kanpur district, Uttar Pradesh, India. Environmental Monitoring and Assessment, 146, 375-382. Sarkar, M., Banerjee, A., Pramanick, P. P., & Sarkar, A. R. (2006). Use of laterite for the removal of fluoride from contaminated drinking water. Journal of Colloid and Interface Science, 302, 432–441. Saxena, V. K., & Ahmed, S. (2001). Dissolution of fluoride in groundwater: a water–rock interaction study. Environmental Geology, 40(9), 1084–1087. Schwartz, M. O., & Friedrich, G.H. (1973). Secondary dispersion patterns of fluoride in the Osor area, Province of Gerona, Spain. Journal of Geochemical Exploration, 2,103-114.

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and nonendemic areas of Dindigul District, Tamil Nadu, South India. Water Research, 44, 6186-6200. Viswanathan, G., Jaswanth, A., Gopalakrishnan, S., & Sivailango, S. (2009). Mapping of fluoride endemic areas and assessment of fluoride exposure. Science of the Total Environment, 407, 1579-1587. Vivona, R., Preziosi, E., Madé, B., & Giuliano, G. (2007). Occurrence of minor toxic elements in volcanic-sedimentary aquifers: a case study in central Italy. Hydrogeology Journal, 15, 1183-1196. Wang, L. F., & Huang, J. Z. (1995). Outline of control practice of endemic fluorosis in China, Social science and medicine, 41(8), 1191-1195. Watanabe, T., Kondo, T., Asanuma, S., Ando, M., Tamura, K., Sakuragi, S., Rongdi, J., & Chaoke, L. (2000). Skeletal Fluorosis from Indoor Burning of Coal in Southwestern China. Fluoride, 33(3), 135-139. Wedepohl, K. H. (Ed) (1969). Handbook of geochemistry. Vol. II-l. Berlin: Springer-Verlag. Weinstein, L. H., & Davison, A. (2003). Fluoride in the Environment, CABI Publishing. Whelton, H. P., Ketley, C. E., McSweeney, F., & O’Mullane, D. M. (2004). A review of fluorosis in the European Union; prevalence, risk factors and aesthetic issues. Community Dent Oral Epidemiol, 32(1), 9-18. WHO (2001). Naturally Occurring Hazards; Water, Health and Human Rights; Floods and Droughts. World Water Day 2001, 1-23. WHO, (1984). Guidelines for Drinking Water Quality. In: Health Criteria and Other Supporting Information, second ed., vol. 2. World Health Organization, Geneva. WHO, (2002) Fluorides, Geneva, World Health Organization. Environmental Health Criteria 227, 268. Whyte, M. P., Totty, W. G., Lim, V. T., & Whitford, G. M. (2008). Skeletal fluorosis from instant tea. Journal of Bone and Mineral Research, 23(5), 759–769. Wodeyar, B. K., & Sreenivasan, G. (1996). Occurrence of fluoride in the groundwataers and its impact in Peddavankahalla basin, Bellary district, Karnataka-A preliminary study. Current science, 70(1), 71-74. Yadav, A. K., Kaushik, C. P., Haritash, A. K., Kansal, A., & Rani, N. (2006). Defluoridation of groundwater using brick powder as an adsorbent. Journal of Hazardous Materials, B128, 289-293. Yadav, S., Khan, T. I., Gupta, S., Gupta, A. B., & Yadava, R. N. (1999). Fluorosis in India with special reference to Rajasthan. In: Proceedings of the International Conference on Water, Environment, Ecology, Socioeconomics and Health Engineering (WEESHE), Seoul National University, 18–21st October, 3–10. Yidana, S. M., Yakubo, B. B., & Akabzaa, T. M. (2010). Analysis of groundwater quality using multivariate and spatial analyses in the Keta basin, Ghana. Journal of African Earth Sciences, 58, 220–234. Young, S. M., Pitawala, A., & Ishiga, H. (2010). Factors controlling fluoride contents of groundwater in north-central and northwestern Sri Lanka. Environmental Earth Sciences, doi:10.1007/s12665-010-0804-z Zheng, L., Luo, H., Bandou, K., Kanai, F., Terasaki, K., Yoshimura, T., Sakai, Y., Kimura S., & Hagiwara, H. (2006). High fluoride groundwater with high salinity and fluorite in aquifer sediments in Inner Mongolia, China. Chinese Journal of Geochemistry, 25(1), 103.

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Chapter 4

OCCURRENCE, DISTRIBUTION AND MECHANISM OF FLUORIDE RELEASE IN GROUND WATER: A CASE STUDY Mitali Sarkar1, Aparna Banerjee2, Partha Pratim Pramanick1 and Asit R. Sarkar1 1

Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India 2 State Water Investigation Directorate, Krishnanagar, West Bengal, India

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ABSTRACT Fluoride is a persistent and non-degradable poison that accumulates in soil, plants, living organisms and acts as a potential environmental hazard. It is an essential micro nutrient and known to prevent tooth decay up to a certain concentration. However, prolonged ingestion in moderate to high dose results damage to human biological systems, even at molecular level leading to serious health disorders known as fluorosis. The physiopathology is believed to be complex one and is cumulative in respect to level and duration of exposure as well as sex and age. Several hundred million people in the world, at present, are either suffering from fluorosis or estimated to be at risk. The occurrence of fluoride at elevated concentrations in different environmental compartments in almost all parts of the world poses a real threat to life on the earth. Fluoride may release into the aquatic environment by anthropogenic and natural sources. Ground water percolating through fluoride-bearing rocks and minerals becomes fluoride enriched. In India, fluoride appears mostly in the hard rock areas, both in shallow and deeper water zones. The governing mechanisms are decomposition/dissociation, dissolution/ enrichment and leaching/mobilization. The present report highlights the occurrence, distribution of fluoride in ground water in general and correlation of elevated fluoride level with ground water quality in a typical fluoride prone study area.

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INTRODUCTION: Elevated concentration of fluoride in soil and ground water arising from both natural and anthropogenic activities are reported in several countries around the world including India [1, 2]. There is a long debate on the effect of fluoride on human health. It is believed that low dose of fluoride is beneficial to protect the tooth decay as well as for mineralization of tooth enamel [3, 4]. However, prolonged ingestion of fluoride in moderate to high dose damages the human biological systems, even at molecular level. Fluoride intoxication primarily results dental, skeletal and non-skeletal fluorosis. As a safeguard, the desirable and permissible limit for fluoride in drinking water, as per World Health Organization (W.H.O), are 0.5 and 1.0 mgdm3 respectively [5] and 1.0 and 1.5 mgdm-3 respectively as per Indian Standard Institute (I.S.I.) [6]. However, it is shown that fluoride concentration has a correlation with temperature [7] as described by the following equation (1):

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fluoride concentration (mgdm-3) = 0.34/E

(1)

where, E = -0.038+0.0062 temperature in oF The harmful effect of fluoride on health is believed to be cumulative with respect to duration as well as its level of exposure. In case of dental fluorosis, the added parameter is the sex and maturity stage of the teeth enamel [8]. As a first symptom, mottling of teeth with brown/black stains is identified. Fluoride at high concentration level is known to affect calcium functioning resulting deformation and abnormalities in the bone structure, the skeletal fluorosis. Fluoride is known to damage a number of organs, viz. central nervous system, respiratory system, reproductive system, excretion system, pinal and thyroid glands affecting various enzymatic functions. The following Chart-1 demonstrates the concentration dependent physiological effect of fluoride on human health.

Chart-1. Effect of fluoride on human health.

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Biochemical Mechanism of Fluoride Intoxication in Living Organism It is observed that soluble fluorides readily and completely absorbed in the gastrointestinal tract whereas bioavailability from dentifrices is 84-100%. Taking single oral fluoride dose of 1.5-10 mg, plasma fluoride level in human, reached 0.06-0.4 mgdm-3 in 30 minutes [9]. Pharmacokinetic data showed a multi exponential elimination of plasma fluoride; the initial rapid phase was followed by a slower phase with a half-life of 2 to 9 hours. Fluoride is excreted in hours to days during normal renal function, while at the end stage of renal disease, the elimination phase may prolong up to 2 years [10]. Fluoride is also excreted with sweat and the rest is deposited in bone. Fluoride, as is not protein bound, occurs as free ion in plasma. Bone deposition of fluoride occurs to the extent of 50% in growing children but only 10% in the adults [11]. Fluoride is known to produce metabolic disorders, affect different glands and alter the blood chemistry together with various other changes in different organs, even at molecular level, in human and animals.

OCCURRENCE OF FLUORIDE IN GROUND WATER Fluoride is ubiquitous in the environment and enters into the ground water via natural or geogenic route, through water-rock interactions. Fluoride concentrations in ground water are generally higher than that of surface water. The earth crust and soil are the main source of fluoride for geochemical leaching to ground water. A considerable amount of fluoride enters into ground water from anthropogenic sources.

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Sources of Fluoride (a) Earth Crust: Natural occurrence of fluoride in the Earth’s crust is to the extent of 0.065% (544 mgdm-3) [12]. It is found in the minerals like fluorite or fluorspar (CaF2), cryolite (Na3AlF6), fluorapatite [3Ca3(PO4)2, CaF2] etc. The fluoride content in different minerals [13] and rocks [14] is given in Table 1. (b) Soil: Almost 90% of the fluoride tends to be insoluble or tightly bound to soil particles. Under acidic conditions (pH ~ 5.3), surface soils tend to be depleted more relative to deeper layers. Thus, leaching of fluoride is assisted by rain water and fluoride is subsequently adsorbed onto soil minerals viz. goethite (FeOOH). Clay soil retains fluoride better than sandy soil. Further, the presence of calcium in soil improves its retention extent. Ion exchange is expected to be the predominant interaction for replacement of hydroxyl ions of clays (gibbsite, kaolinite, halloysite) by fluoride ion [15, 16]. (c) Anthropogenic Sources: Human activities play a significant role contributing fluoride load in the environment. Although fluoride concentration in pristine rainfall is very low(3.5 mgdm–3) and abnormally high (7.0 – 16.0 mgdm–3).

The percentage occurrence of different categories is presented in Figure 3. It is estimated Fig.2. Water quality in respect to fluoride that 40% samples are safe and 2% are abnormally high while 25, 23 and 10% samples belong to category (2), (3) and (4) respectively.

high abnormally high within DL within PL safe

0

10

20

30

40

50

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Sample proportion (%) Figure 2. Water quality in respect to fluoride.

However, fluoride concentration in the samples is found to have seasonal variation. The fluoride level in the summer is found higher compared to that in the monsoon season. The phenomena are characteristic (as fluoride source in the study area arises from leaching underground mineral) and results due to evaporation and transpiration together with water level fluctuation in the underground water table [48]. It is observed that fluoride concentration of water samples of tube wells, deep tube wells and khadan is higher compared to that of wells and dug wells.

Statistical Analysis of Water Quality Data In order to assess the quality as potable water, statistical parameters such as, minimum, maximum, range, mean, median, standard deviation (SD) values, taking cumulative data of all the studied water sources, are evaluated (Table 6).

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Table 6. Statistical parameters for the water quality data

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Parameter Range Mean Median SD pH 5.6-9.2 7.4 7.3 0.32 E.C. 150.00-2946.00 736 652 1215 T.D.S. 78.01-1662.00 893 822 532 F0.21-19.00 3.49 2.21 1.02 Cl10.0-750.7 208.38 197 115 PO430.02-0.75 0.09 0.07 0.04 SO420.59-367.12 112.34 101 47 T.H. 9.9-1095.0 716 629 226 T.A. 60.0-460.0 323 278 118 Na+ 5.96-183.01 70.70 64 88 K+ 0.20-67.01 15.95 11 13 Ca2+ 4.02-432.00 109.46 97 63 Mg2+ 1.02-279.0 186 77 42 Fe 0.34-1.00 0.397 0.21 0.56 All the parameters except pH and E.C .are expressed in mgdm-3, E.C. in μs at 25°C

The pH of ground water samples ranged between 5.6 and 9.2, with an average value of 7.4 indicating that the ground water in this area is slightly alkaline (Table 6). Only two samples showed pH below lower permissible limit of 6.5. The TDS of samples ranged between 78 and 1662 mgdm-3. Results show that 24% of the samples had TDS in excess of the desirable limit (500 mgdm-3) for potable water. None of the samples had TDS in excess of the permissible limit (2,000 mgdm-3). Fluoride concentration in ground water samples ranged from 0.21 to 19 mgdm-3 and 35% of the water samples had fluoride concentrations above the permissible limit of 1.5 mg/L. The other water quality parameters for most of the samples are within the permissible limits for potable water. It is felt that for effective maintenance of water quality, through appropriate control measures, continuous monitoring of quality parameters is essential. However, it is very difficult and laborious task for regular monitoring of all the parameters even if adequate man power and laboratory facilities are available. An alternative approach may therefore be to establish some mathematical relationship between two individual parameters that are mutually and statistically correlated. Statistical correlation is effectively applied to multivariate complex geochemical data to reveal the underlying factors responsible for the observed variations in different parameters [49]. The idea is based on that if good correlation exists between individual parameters then the determination of only few parameters sufficiently describe the overall water quality. Thus, correlation has been used for [50, 51]: (1) the assessment of overall water quality, (2) the determination of range of variability of parameters to indicate extreme of quality, (3) the prediction of validity of any particular removal technique in real samples.

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An attempt has been made in the present investigation for a systematic calculation of correlation coefficient between each pair of water quality parameters in an aim to minimize the complexity and dimensionality of large set of data [52]. The correlation between different water quality parameters such as pH, E.C., T.D.S, fluoride, chloride, sulphate, phosphate, hardness, alkalinity, sodium, potassium, calcium and iron are made in Table 7. The Pearson correlation coefficient significant at 0.01 level (2tailed) and at 0.05 level (2- tailed) is determined. The correlation parameter indicates an appreciable correlation for fluoride with pH, E.C., Na, alkalinity, hardness and Ca2+ and low correlation for fluoride with Cl–, PO43–, Fe and SO42–. It is found that fluoride bears a positive correlation with pH and Na+. Similar trend was reported by Chakrabarty et al [53] for the water quality of Nagaon and Karbi Anglong districts in Assam. Fluoride bears a negative correlation with K+ and alkalinity, in contrary to the report of Das et al. [54] for the water quality data of Assam. Similar to the other reports negative correlation of fluoride with Ca2+, Mg2+ and total hardness is observed with the present set of data [53, 54]. This is thought to be due to the solubility restriction of the precipitated salt of fluoride [55, 56]. Table 7. Correlation of water quality parameters

pH E.C. T.DS . F-

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ClPO43SO42T.H. T.A. Na+ K+ Ca2+ Mg2+

pH 1.0000.084 0.607 -0.087 0.593 0.670 0.000 -0.064 0.693 0.048 0.767 -0.201 0.213 -0.181 0.263 -0.082 0.623 0.134 0.41 -0.266 0.097 0.016 0.924 0.009 0.283

T.D.S.

F-

Cl-

PO43-

SO42-

T.H.

T.A.

Na+

0.995 0.000 -0.179 0.268 0.973 0.000 -0.109 0.505 0.802 0.000 0.931 0.000 0.676 0.000 0.325 0.041

1.000 -0.176 0.279 0.975 0.000 -0.115 0.482 0.825 0.000 0.912 0.000 0.691 0.000 0.322 0.043

1.000 -0.076 0.641 -0.077 0.638 -0.179 0.268 -0.311 0.050 -0.367 0.020 0.341 0.031

1.000 -1.07 0.511 0.826 0.000 0.886 0.000 0.673 0.000 0.352 0.026

1.000 -0.073 0.656 -0.053 0.744 -1.30 0.424 -0.150 0.355

1.000 0.665 0.000 0.510 0.001 0.329 0.038

1.000 0.558 0.000 0.199 0.217

1.000 0.224 0.165

1.000 -

0.433 0.005 0.756 0.000 0.631 0.008

0.431 0.005 0.752 0.000 0.515 0.001

-0.204 0.208 -0.065 0.689 -0.426 0.379

0.429 0.006 0.756 0.000 0.414 0.002

-0.141 0.385 0.069 0.674 0.097 0.308

0.474 0.002 0.658 0.000 0.712 0.011

0.399 0.011 0.748 0.000 0.543 0.000

0.252 0.118 0.393 0.012 0.221 0.017

0.141 0.387 0.314 0.049 0.173 0.044

E.C.

K+

Ca2+

1.000 0.470 0.002 0.270 0.002

1.000 0.738 0.422

1.000-

All the parameters except pH and E.C .are expressed in mgdm-3, E.C. in μs at 25°C. The bold case parameters indicate 0.01 and underlined parameters indicate 0.05 significance level.

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Hydrogeochemical Facies of the Present Study Area Ground Water

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The hydrochemical pattern diagram was initially conceived by Hill [57] and later improved by Piper [58]. Such pattern diagram helps in hydrogeochemical facies classification. The trilinear diagram of this study is classified into different hydrochemical facies based on the dominance of different cations and anions. Figure 3. represents the Piper diagrams for ground water samples. The distribution of data points in lower base triangles in reveals that majority of the samples do not categorize in any dominant cation type. A majority of samples fall in HCO3- type, while the remaining samples do not classify to any particular category. The distribution of data points in the rhomboid of the Hill Piper diagram reveals that the ground water is dominated by alkaline earth metals (Ca2+ and Mg2+) and weak acids (represented by HCO3-) over the alkali metals (Na+ and K+) and strong acids (Cl- and SO42-). Further, 59% of samples from belong to the Ca– HCO3 type and most of the remaining belong to the mixed Ca–Na-HCO3 type. Carbonate hardness (secondary alkalinity) is exhibited by 59% of samples whereas 44% fall into the zone where no cation–anion pair exceeds 50%.

Figure 3. Hill Piper diagram.

P

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HEALTH QUALITY EVALUATION The people in the study area continuously consuming fluoride rich drinking water have several complaints bearing health and social implications. Significant incidence of dental fluorosis is noted among the children between age group of 5-16 years while skeletal fluorosis is prevalent among adults. A typical study conducted among 300 children (subjects) indicates that about 90% subjects show prevailing symptom of dental fluorosis, in some form of other, among which 64% are severely affected. Only 10% of the subjects are detected free from dental problem. In the particular survey on 152 adults (male: 65, female: 87) almost 55-60% have identified to suffer from skeletal fluorosis resulting severe osteosclerosis (abnormal increase bone density), spondylosis (degeneration of intervertibral disks), osteoporosis (bones become abnormally dense, splinter and fracture) and arthritis [48].

RECOMMENDATION Considering the spread of fluorosis among the people in study area supply of fluoride free drinking water is most essential. Therefore, effort for remediation of fluoride from ground water should be made [59, 60]. Hydrogeochemical investigation, to understand the status of ground water quality and identify potential areas of risk, is the vital component of the broad strategy to combat endemic fluorosis.

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CONCLUSION Investigation reveals that origin of elevated fluoride in the ground water of the present study area is natural. High rates of evaporation induced by the semi-arid climate accelerate the precipitation of calcite in neutral to alkaline ground water with a deficiency of calcium ions. This is associated with increased dissolution of fluorite present in the granite and gneissic rocks of the area in order to compensate the requirement of calcium ions towards common ion effect. Further, calcite precipitation is the driving force for the fluoride ion enrichment in the ground water considering the oversaturation with respect to calcite and under saturation with respect to fluorite. The possibility of further increase in fluoride concentration, in future, due to dissolution of fluorite and spread of fluoride can’t be omitted. Suitable short term and long term measures are the urgent need to check the problem.

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Handa, B. K.; Ground Water, 1975, 13, 275. Stanley, V.A.; Pillai, K. S. Poll. Res. 1975, 18, 305. Cerklewski, F.L. Nutritional Res., 1997, 17, 907. Cox, S.D.; Lassiter, M.O.; Miller, B.S.; Doyle, R.J. Biochim. Biophys. Acta, 1999, 1428, 415.

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W.H.O. ‘Guidelines for Drinking Water Quality’ World Health Organization, Geneva, 1993, p. 45. IS : 10500, ‘Indian Standard Code for Drinking Water’, BIS, India. 1983 Galagan, D.J.; Vermillion, J.R.; Publ. Hlth. Rep. Wash., 1957, 72, 491. Weatherell, J.A.; Deutsch, A.D.; Robinson, C.; Hallswarth, A.S. Nature, 1995, 356, 230. Trautner, K.; Ejnwag, L. Archives of Oral Bio., 1998, 33, 543 Melovor, M.E. Drug Safety, 1990, 5, 79. Heifetz, S.; Horowitz, H.S. Pediatrics, 1986, 77, 876. Sarkar, R. ‘General Inorganic Chemistry’ (Part-II). 1st Publication, March 2001, p. 517 Rao, C.N.R. ‘Fluoride and Environment – A Review’, Proc. 3rd Int. Conf. on Environment and Health, Chennai, December 15-17, 2003, India, p. 386. Fleischer, M.; Robinson, W.O. R. Soc. Can. Spec. Publ., 1963, 6, 56. Bower, C.A.; Hatcher, J.T. Soil Sci., 1967, 103, 151. Fluher, H.J.; Polomski, J.; Blaser, P. J. Environ. Qual., 1982, 11, 461. Madhavan, N.; Subramanian, V. Curr. Sci., 2001, 80, 1312. Hem, J.D. USGS Water Supply, 1970, 363. Brown, D.W.; Roberson, C.E. USGS J. Res., 1977, 5, 506. Handa, B.K. Ground Water, 1975, 13, 275. Rao, N.V. Rammohon: Rao, N. Rao, K. Surya Prakash; Schmiling, R.D. Env. Geol. 1993, 21, 84. Apambire, W.B.; Boyle, D.R.; Micrel, T.A. Env. Geol. 1997, 33, 24. Saxena, V.K.; Ahmed, S. Geol. 2001, 40, 1084. Saxena, V.K.; Ahmed, S. Geol. 2003, 43, 731. Pauwels, H.; Ahmed, S. Geosci. 2007, 5, 68. Das, S.; Mehata, B.C.; Das, P.K.; Srivastava, S.K.; Samata, S.K. Poll. Res., 1999, 18, 21. Fluoride in natural waters. In Essentials of Medical Geology. Ed.:O. Selinus, Pub.: Elsevier, 2005, pp. 301-329. W.H.O., Oral Health–Country/Area Profile Program, World Health Organization, Geneva, 1993 29. Susheela, A.K. Curr. Sci., 1999, 77, 1250. Madhaman, N.; Subramanian, V. Curr. Sci., 2000, 80, 1312. Report of UNICEF, 1995. Fluoride Research and Rural Development Foundation Report. Ministry of Rural Development, Govt. of India, 1999. Susheela, A. K. Prevention and control of fluorosis in India (RGDWM, Ministry of Rural Development, New Delhi, Health Aspect) 1993. Madhavan, N., Subramanian, V. J. Environ. Monit., 2002, 4, 821. Chatterjee, M.K., Mohabey, N.K. Environ. Geochem. Hlth., 1998, 20, 1. Jacks, G., Rajagopalan, K., Alveteg, T., Jönsson, M. Appl. Geochem., 1993, (Suppl. 2), 241. Jacks, G., Sharma, V.P. Geoderma, 1995, 67, 203. Bhattacharya, P., Jacks, G., Possible occurrences of sepiolite in soil environment and its role in accumulation and mobility of fluorine in a semiarid region of Nagaur District, Rajasthan, Western India. Nordic Society of Clay Research, Report No. 10, 5–6, 1995.

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[41] Krishnamachari, K.A.V.R. Indian J. Med. Res. 1976, 64, 284. [42] Problems and prospects related to hydrogeology of West Bengal by U. K. Ghosh, Course material for Artificial Ground Water Recharge and Aquifer Management, 5-10 October, 2009, Kolkata L-5/1-21. [43] Sengupta, A. Case studies on sporadic fluoride contamination in drinking water at Nalhati 1 block, district Birbhum, Proc. of Workshop on ground water pollution and its protection with special reference to arsenic contamination, 22 January, 1999, Kolkata, p. 69. [44] Gupta, S.; Banerjee, S.; Saha, R.; Duta, J.K.; Mondal.N. Fluoride, 2006, 39, 318. [45] Bhattacharya, S.; Srivastava, K.K.; Incidence of high concentration fluoride in ground water in Rampurhat and Nalhati blocks, district Birbhum, West Bengal, Proc. of Workshop on ground water pollution and its protection with special reference to arsenic contamination, 22 January, 1999, Kolkata, p. 45. [46] Sarkar, M., Chakrabarty, S; Banerjee, A. Geoenvironmental Reclamation, Eds. A. G. Paithankar, P. K. Jha, R. K. Agarwal, Oxford and IBH Publishers Co. Pvt. Ltd. New Delhi, 2000, p. 249. [47] Pathak, C.S.; Maity, S.K.; Das, B. Combined resistivity and induced-polarization surveys for fluoride contamination-A perspective, Proc. of Workshop on Medicinal Geology, 3-4 February, 2004, Nagpur, India, p. 168. [48] ‘Standard Methods for the Examination of Water and Waste Water’, 20th edition, 1998, APHA, AWWA, WEF, Washington DC, p. 2-27. [49] A.W. Hounslow, ‘Water Quality Data: Analysis and Interpretation’, CRC, 1995, p. 54. [50] Sarkar, M.; Banerjee, A. Annual Set-The Environment Protection, 2003, 5, 123. [51] Tripathy, S; Panigrahi, M.K.; Kundu, N. Env. Geochem. Hlth., 2005, 27, 205. [52] Dasgupta A. M.; Purohit, K. M. Poll. Res., 2001, 20, 227. [53] Sarkar, B.; Biswal S. K.; Pradhan, S. C. Poll. Res., 2001, 20, 481. [54] Sarkar, M.; Banerjee, A.; Pramanick, P. P; Chakraborty, S. J. Ind. Chem. Soc., 2006, 83, 1248. [55] Chakrabarty, D; Chanda, C. R Curr. Sci., 2003, 78, 1424. [56] Das, B; Talukdar, J. Curr. Sci., 2008, 83, 1424, 657 [57] Smedley, .H; Nicolli, H. B; Macdonald, D. M; Barros, J. A. J; Tullio, J. O. Appl. Geochem., 2002,17, 259. [58] Indirabai, W. P. S; George, S. Poll. Res., 2002, 21, 243. [59] Hill, R.A., Trans. Am. Geophy Union. 1994, 21, 46. [60] Piper, A.M., Trans. Am. Geophy. Union 1994, 25, 914. [61] Sarkar, M.; Banerjee, A.; Pramanick, P. P; Sarkar, A. R. Chem. Eng.J., 2007, 131, 329. [62] Sarkar, M.; Banerjee, A.; Pramanick, Indust. Eng. Chem. Res., 2006, 45, 5920.

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Chapter 5

FLUORIDE CONTAMINATION OF WATER: ORIGIN, HEALTH EFFECTS AND REMEDIATION METHODS Sujata Mandal and S. Mayadevi Chemical Engineering and Process Development Division, National Chemical Laboratory, Maharashtra, India

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ABSTRACT Fluorine is the 13th most abundant element in the earth’s crust. It exists in trace amounts in ground water all over the world. Drinking water is the primary source through which fluoride enters the human body, especially in regions where fluoride concentrations in groundwater and/or surface water are high. It is estimated that more than 200 million people worldwide depend on drinking water with fluoride concentration exceeding the present World Health Organization (WHO) guideline (Maximum contaminant level of 1.5 mg/l). Fluoride bearing foodstuffs and fumes from burning of coal also significantly contribute to the daily intake of people in some regions. Prolonged consumption of excess fluoride may lead to different types of fluorosis (dental and skeletal) depending on the level and period of exposure. Presence of fluoride in drinking water above permissible level has been related to increased incidence of fluorosis among the people all over the world including China, India, Australia, Mexico, Argentina, Egypt and Kenya. In India, the problem of excessive fluoride in groundwater was first reported in 1937 in the state of Andhra Pradesh. More than 6 million people all over India are known to be seriously affected by fluorosis and another 62 million are exposed to it. The best choice for combating fluorosis is to have alternative source of water with low fluoride level. In absence of alternative source of water, defluoridation of excess fluoride in water is the only option. Different methods are available for defluoridation of water. But the selection of the appropriate method for achieving a sustainable solution to the fluorosis problem is very important. Defluoridation of drinking water by adsorption is the most simple and effective technology that can work in household as well as community level drinking water treatment. It is also the most widely studied method for defluoridation of water, as the fluoride concentration in groundwater is usually very low ( 10 mg/l). A wide variety of adsorbents have been explored for this purpose. Synthetic layered double hydroxides are comparatively new materials examined for the adsorption

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Sujata Mandal and S. Mayadevi of fluorides and they exhibit good fluoride adsorption capacity. A discussion on various defluoridation methods, adsorbents for defluoridation and recent developments are presented in this chapter.

INTRODUCTION

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Fluorine is highly electronegative and exists in solution as fluoride ions (F-). Fluoride bearing minerals cover about 0.06-0.09 percent of the earth’s crust. Natural weathering of the fluoride bearing minerals leads to dissolution of fluorides in groundwater in the form of fluoride ions. Fluoride and hydroxide ions have the similar charge and radius, and may easily displace each other in mineral structures [1]. Low concentration of fluoride (maximum limit: 1.5 mg/l, WHO 2006 [2]) in drinkingwater has beneficial effects on teeth and bones. Excessive intake of fluoride through drinkingwater, and/or due to exposure to other sources, can lead to many adverse health effects. These range from mild dental fluorosis to crippling skeletal fluorosis depending on the level and period of exposure. Elevated concentration of fluoride in water is known to occur in various parts of the world. Reducing the concentration of fluoride in water in these regions to acceptable levels is imperative to the long term health of the population. There are several defluoridation methodologies available for drinking water treatment; e.g. chemical precipitation, membrane based separation, adsorption and ion-exchange processes. The selection of appropriate treatment technology depends on the fluoride concentration in the water. This paper starts with the origin of fluoride contamination in water, its impact on human health, present global scenario of groundwater fluoride contamination, and the situation in India. It further discusses the different technologies available for defluoridation of drinking water, their suitability, advantages and disadvantages.

FLUORIDE IN WATER-ORIGIN Fluoride is present in almost all natural water in some concentration [2]. Water in rivers typically has fluoride concentration below 0.5 mg/l and fluoride concentration in sea water is about 1 mg/l. Fluoride in water is of geological origin. Fluoride contamination or fluoridation of water occurs primarily due to natural weathering of the local fluoride minerals like fluorapatite [Ca5(PO4)3F], fluorite (CaF2), topaz and cryolite (Na3AlF6), present in the earth’s crust [3]. In ground water, however, low or high concentrations of fluoride can occur, depending on the nature of the rocks and the presence of fluoride-bearing minerals. Concentration in water is limited by fluorite solubility; in the presence of 40 mg/l calcium the limit is 3.1 mg/l [1]. The absence of calcium in solution permits dissolution of higher quantities of fluorite in water [4]. High fluoride concentrations may therefore be expected in ground water from calcium-poor aquifers and in areas where fluoride-bearing minerals are present. Fluoride concentrations may also increase in ground water in which cation exchange of sodium for calcium occurs [4]. Fluoride contamination of water can also occur due to discharge of fluoride containing effluent from various industries, viz. semiconductor manufacturing, glass and ceramic production, uranium refinery, electroplating industry [5-9].

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High ground water fluoride concentrations associated with igneous and metamorphic rocks such as granites and gneisses have been reported in the region from Syria to Jordan, Egypt, Libya, Algeria, Sudan and Kenya, from Turkey through Iraq, Iran, Afghanistan, India, northern Thailand, China, parts of America and Japan [2].

FLUORIDE IN WATER-HEALTH EFFECTS

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Fluorine is an essential micronutrient for animals and human beings for the calcification of bones and teeth. Excess fluoride intake, most commonly through drinking water is detrimental and can cause fluorosis [10], which adversely affects teeth and bones. Paradoxically, low level of fluoride intake (0.5 – 1.0 mg/l) is considered to contribute to bone formation and prevent tooth decay [11, 12]. The maximum permissible level for fluoride in drinking water as regulated by WHO is 1.5 mg/l [13] and in regions with warm climate, the suggested level is below 1 mg./l [14]. Regular in-take of moderate amounts of fluoride (0.9 - 1.2 mg/l) may give rise to mild dental fluorosis [15]. This has been confirmed by numerous subsequent studies, including a large-scale survey carried out in China [16]. The survey revealed that dental fluorosis was detectable in 46% of the population where the drinking-water contained 1 mg of fluoride per litre. As a rough approximation, in regions with a temperate climate, dental fluorosis manifests at a drinking-water fluoride concentration above 1.5–2 mg/l. In warmer regions, dental fluorosis (Fig. 1) occurs at lower fluoride concentrations in the drinking-water because of the larger amounts of water consumed [13, 17-19]. It is also possible that, in regions where fluoride intake via routes other than drinking-water (e.g. air, food) is elevated, dental fluorosis develops at drinking-water fluoride concentrations below 1.5 mg/l [17].

Figure 1. Dental fluorosis [19].

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Fluoride can also have more serious effects on skeletal tissues. Skeletal fluorosis (with adverse changes in bone structure) is observed when fluoride content in drinking-water is 3–6 mg/l. Crippling skeletal fluorosis (Fig. 2) develops where drinking-water contains over 10 mg/l of fluoride [13, 19]. Long term drinking of water having high fluoride content will lead to various diseases such as osteoporosis, arthritis, brittle bones, cancer, brain damage, Alzheimer syndrome and thyroid disorder [20].

Figure 2. Skeletal fluorosis among people in India [19].

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Several epidemiological studies are available on the possible association between fluoride in drinking-water and cancer rates among the population. International Agency for Research on Cancer (IARC) evaluated these studies in 1982 and 1987 and considered that they provided inadequate evidence of relation between fluoride intake and carcinogenicity in humans [21, 22]. The results of several epidemiological studies on the possible adverse effects of fluoride in drinking-water on pregnancy are inconclusive [13, 18, 23]. It is known that persons suffering from certain forms of renal impairment have a lower margin of safety for the effects of fluoride than the average person. The data available on this subject are, however, too limited to allow a quantitative evaluation of the increased sensitivity to fluoride toxicity of such persons [18, 23]. Provision for supply of safe water and nutritional interventions coupled with early detection of fluorosis may mitigate health complaints arising due to fluorosis [24]. It is suggested that there is an essential relation between poverty and fluorosis as malnutrition is found to aggravate the severity [25]. A desk review on the impact of nutrition on fluorosis has been brought out by United Nations’ International Children’s Emergency Fund (UNICEF) in 1998 [26]. It emphasizes the importance of consumption of food rich in calcium, vitamin C, vitamin E and antioxidants on a daily basis. Nutritional intervention along with consumption of safe water can minimize the adverse effects of fluorosis and effect cure in cases of early detection.

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FLUOROSIS - GLOBAL AND INDIAN SCENARIO Presence of fluoride in drinking water above permissible level and increased incidence of fluorosis among the people have been reported all over the world (Fig. 3) including Australia, Mexico, Argentina, Egypt, Kenya, UAE, China, Pakistan, Thailand and India [27-30]. A statistical survey carried out by Amini and co-workers [28] showed that high fluoride concentration in groundwater is found in the belt stretching from North Africa to the middleeast and moves towards Pakistan, Uzbekistan and Kazakhstan. According to the statistical mapping by Amini, densely populated areas of India and China are also severely affected. It is estimated that around 200 million people, from nations throughout the world, are under the dreadful fate of fluorosis [31]. In China, endemic fluorosis has been reported in all 28 provinces, autonomous regions and municipalities except in Shanghai. Both shallow and deeper ground waters are affected; in general the deeper ground waters have higher concentrations [2]. In Sri Lanka, Dissanayake [32] found concentrations of up to 10 mg/l in ground waters in the Dry Zone, and associated dental and possibly skeletal fluorosis. In India, seventeen states including Andhra Pradesh [33 – 35], Haryana [36], Orissa [37], Rajasthan [31], Gujarat [38] and Karnataka (Fig. 4) have been identified as endemic to fluorosis [39]. All over India, more than 6 million people are seriously affected by fluorosis and another 62 million are exposed to it [40]. It is estimated that 65% of India’s villages are exposed to fluoride risk [41].

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Figure 3. Countries with endemic fluorosis due to excess fluoride in drinking water [30].

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FLUOROSIS – PREVENTION/MANAGEMENT Prevention/management of fluorosis requires multi-level approaches at both Government and community level. The most effective method for the prevention of fluorosis is to ensure continuous supply of safe fluoride-free water from an external source, which is not always an economic solution. Nutritional interventions (regular consumption of food rich in calcium, vitamins C and E, and antioxidants) coupled with early detection of fluorosis may mitigate health complaints arising due to fluorosis. The prevention of fluorosis through management of locally available drinking-water source is not an easy task and requires favorable conditions combining knowledge, motivation, prioritization, discipline, and technical and organizational support. Many filter media and several water treatment methods are known to remove fluoride from water. However, many initiatives on defluoridation of water have resulted in frustration and failure [42]. Therefore, any attempt to mitigate fluoride contamination for an affected community should investigate the provision of safe, low fluoride water from alternative sources as the first choice. In cases where alternative sources are not available, defluoridation of water is the only option to prevent fluorosis. There are different defluoridation methods available; but the selection of an appropriate defluoridation method to achieve a sustainable solution to the fluorosis problem is very important.

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Figure 4. Drinking water fluoride contamination and its intensity in different states in India [25].

DEFLUORIDATION UF WATER – DIFFERENT TECHNIQUES Techniques for the removal of fluorides from water can be classified as chemical precipitation (precipitation-coagulation, Nalgonda technique), membrane-based methods (membrane filtration, reverse osmosis, electro-dialysis), adsorption and ion-exchange.

Chemical Precipitation-Coagulation The process of chemical precipitation involves direct addition of an alkaline calcium salt viz. lime or hydrated lime, calcium chloride, calcite, to adjust the water pH to the point where fluoride exhibits the minimum solubility [43, 44]. Fluoride is removed either by precipitation, co-precipitation, or adsorption onto the formed precipitate. The precipitated fluoride is subsequently removed by sedimentation and filtration. Use of alum and a small amount of

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lime has been extensively studied for defluoridation of drinking water. A modification of the method is popularly known as the Nalgonda technique [45], named after the town in India where it was first used at water works level. A description of the technique and its defluoridation mechanism has been illustrated separately in the coming section. CaCl2 is another preferred choice for chemical precipitation of fluoride. The advantages of using CaCl2 as precipitating agent are: less amount of sludge is generated, less chance of clogging of pipes and pumps, and lower risks of industrial hazard from dust [46]. Its major drawback is that very fine CaCl2 precipitate formed remains suspended and requires addition of coagulants to settle. This method of purification is preferred when the fluoride concentration in water is high (e.g. industrial effluents). The primary drawback of the chemical precipitation method is that the process generates huge amounts of a water rich sludge and the disposal of this adds to the cost of purification.

THE NALGONDA TECHNIQUE The Nalgonda process of defluoridation was developed by the National Environmental Engineering Research Institute (NEERI), India, to be used at both the community or household levels. The process involves addition of aluminium sulfate [Al2(SO4)3.18H2O] to the fluoride containing water followed by coagulation-flocculation sedimentation and filtration [47]. The reaction steps involved are presented below: Alum dissolution:

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Al2(SO4)3.18H2O = 2Al3+ + 3SO42– + 18H2O Aluminium precipitation (Acidic): 2Al3+ + 6H2O = 2Al(OH)3 + 6H+ Co-precipitation (non-stoichiometric): F– + Al(OH)3 = Al–F complex Compared to normal drinking-water flocculation, a much larger dosage of aluminium sulfate is normally required in the defluoridation process. As the aluminium sulfate solution is acidic, simultaneous addition of lime is often needed to ensure neutral pH in the treated water and complete precipitation of aluminium. pH adjustment: 6Ca(OH)2 + 12H+ = 6Ca2+ + 12H2O Surplus lime is used as a weighting agent, to facilitate more complete settling during the process. Finally, the treated water is filtered in order to ensure that no sludge particles escape

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with the treated water. Later, the Nalgonda technique was modified to suit the African households also [48]. In spite of the fact that the Nalgonda technique has been introduced in many places, it has not yet been demonstrated to be the method of choice due to the following disadvantages:

   

The treatment efficiency is limited within 70 % and hence the process is limited for medium to high fluoride contamination. Disposal of huge amount of sludge that is generated during the treatment as the sludge contains the removed fluoride in a concentrated form. Large dose of aluminium sulphate is used which resulted into high residual sulphate in the treated water. Proper training is required for the users.

MEMBRANE-BASED METHODS

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Among the membrane filtration processes, nanofiltration is the most common method that has been used by several researchers for defluoridation of water [49-53]. Negatively charged commercial thin film composite membranes have been found to exhibit good performance in the defluoridation of water. The other membrane filtration process, microfiltration, has always been applied to pre-treat water used by one of the methods, like, coagulation/sedimentation, sand filtration, and chlorination [54, 55]. Defluoridation by nanofiltration technique is considered to be a cost-effective alternative for Reverse osmosis process [56]. Membrane-based defluoridation techniques are too expensive to use in household or community level drinking water treatment.

REVERSE OSMOSIS (RO) Reverse osmosis (RO) is an efficient membrane-based defluoridation technique [56-58]. The advantages of this technique are, (i) it is highly effective for fluoride removal and (ii) it can remove other ions as well. The RO-kits come as compact-modular forms, which are easy to use and do not require handling of chemicals for the user. The RO treatment processes are generally expensive and are beyond the affordability of common people. The water from RO process is completely free from fluoride which can be a disadvantage sometimes as fluoride is a micronutrient necessary for health. It also generates a fluoride-rich stream which has to be treated separately.

ELECTRODIALYSIS AND DONNAN DIALYSIS Electrodialysis and Donnan dialysis techniques involve separation of fluoride ions from water by the use of an anion exchange membrane under the influence of electrical potential difference. Different anion exchange membranes namely, DSV, SB-6407, Neosepta-ACM,

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are used by the researchers for defluoridation of water by these methods [59-63]. More than 90 % fluoride could be removed using these membrane-based techniques. The technique is reported to be influenced by the presence of other ions (cations and anions) in the water.

Electrocoagulation (EC) In EC, the anode generates an active coagulant that removes fluoride by in situ precipitation and flotation [64, 65]. Defluoridation by EC using aluminium electrode has been demonstrated to be effective by some researchers [65, 66]. The defluoridation mechanism by EC process involves replacement of the OH- groups from the Aln(OH)3n by F- ions to form AlnFm(OH)3n−m [67, 68]. The EC process is governed by the presence of other anions in the water; specifically, sulphate ions inhibit the process. Compared to traditional chemical coagulation, EC process requires less space and does not require chemical storage, dilution, and pH adjustment. Though the process has been successfully tested for defluoridation of water in pilot plant scale [66], the technique requires further developmental studies before real life application.

Ion-Exchange and Adsorption Based Processes

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Concentration of fluoride in ground water in most cases is very low and lies below 10 mg/l although it can be as high as 30-50 mg/l in volcanic areas [2]. Adsorption and ionexchange are very efficient techniques for the removal of fluoride at such low fluoride concentrations. Hence they are the most widely used techniques for the defluoridation of drinking water.

ION-EXCHANGE Defluoridation by ion-exchange is possible using anion exchange resins. However the selectivity of anion exchange resins for different species follows the order: citrate > SO42−, oxalate > I− >NO3− > CrO42− > Br− > SCN− > Cl− > formate > acetate > F− [69]. Hence these may not be efficient in the removal of fluoride ions in presence of other competing anions in solution. Research work on fluoride removal by ion-exchange employ metal loaded cation exchangers including inorganic cation exchangers such as silica gel, alumina gel, chelating resins modified with high-valence metals such as iron (III), lanthanum (III), cerium (IV), and zirconium (IV) [70]. Ho et al. [71] reported defluoridation by zirconia and silica loaded mesoporous Ti-oxohydroxide. Several organic ion-exchange resins viz., Indion FR 10 (IND), Ceralite IRA 400 (CER), modified Amberlite resin, are also reported for defluoridation of water by ion-exchange [72, 73]. Although a number of studies have been reported on defluoridation of water using different ion-exchangers, none of them has been used for application in household or community level water treatment.

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ADSORPTION A wide spectrum of adsorbents has been studied for their fluoride adsorption properties. Activated and impregnated alumina [74-80], bone charcoal [81-83], carbonaceous materials [84-90], natural and synthetic clays and minerals [91-99], industrial waste materials [100102], natural low-cost materials [103, 104] etc. have been studied for their defluoridation properties. Active research on new adsorbents for defluoridation as well as development of modified forms of the existing adsorbents are in progress all over the world and day by day new adsorbents are being added to the existing list.

ADSORBENTS FOR THE DEFLUORIDATION OF WATER A brief outline of the work on defluoridation of water using some of the adsorbents is presented below:

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Activated Alumina Activated alumina is the most popular adsorbent for the removal of fluoride from water. Activated alumina is aluminium oxide (Al2O3) modified to have an adsorptive surface. The defluoridation process is carried out by passing the water through a packed column of activated alumina when the fluoride gets adsorbed onto the surface of the active alumina grains. The column, after complete saturation with fluoride ions, is regenerated (normally with caustic soda solution). The fluoride adsorption capacity of activated alumina varies between 1 - 15 mg/g. The large difference in defluoridation capacity is due to the difference in the method and degree of activation of the adsorbent. The use of activated alumina for fluoride adsorption is limited primarily due to the difficulties of regeneration and the relatively high price. A large number of chemically modified or impregnated forms of alumina, namely magnesia amended activated alumina, alum impregnated activated alumina, hydrous manganese oxide coated alumina, mesoporous alumina, alumina cement granules, electroactivated alumina etc., have been developed for defluoridation of water [105-111]. However, none of them is being used in applications.

Bone Charcoal Bone charcoal is the oldest known water defluoridation agent. It is a blackish, porous, granular material having calcium phosphate as the major component (57–80 %) along with a little calcium carbonate (6–10 %) and activated carbon (7–10 %). It exists mainly as hydroxyapatite [Ca10(PO4)6(OH)2]. Bone charcoal has the specific ability to take up fluoride from water by replacing one or both the hydroxyl-groups [81-83]. Today bone charcoal defluoridation at waterworks has been replaced by the use of ion-exchange resins and activated alumina. At domestic level, bone charcoal defluoridation seems to work well in

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Thailand and Africa, although there is no report of wide scale implementation [2]. The major objection to the use of bone charcoal defluoridation is related to religious beliefs of some societies and communities.

Carbonaceous Materials, Low-Cost Natural / Industrial Solid Waste Adsorbents Different types of carbonaceous materials viz. activated carbon, carbon nanotubes, activated carbon fibers and activated carbons obtained from different agricultural waste materials, are studied by the researchers for defluoridation of water [84-86]. Chemically modified activated carbons are not very effective for defluoridation of water. Carbon nanotubes and carbon nanofibers have great potential for defluoridation of water, but are too expensive to consider for commercial utilization. Use of low cost materials such as nirmali seeds [104] is not so effective for removing fluoride from water. Fly ash, red mud, bone charcoal etc. are not very effective adsorbents for removing fluoride from drinking water, as the presence of other ions and the pH of the treated water, are also very important.

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Natural and Synthetic Clays Both clay powder and fired clay are capable of sorption of fluoride from water. Clay may be used as a flocculent powder in a batch system. Among naturally occurring clays and minerals, laterite, gibbsite, goethite, kaolinite, bentonite, zeolite(s), alkaline soil, acidic clay, China clay and Fuller’s earth, have been studied and confirmed to adsorb fluoride from water [103, 112-115]. The metal lattice hydroxyl-groups present in the mineral get exchanged with the fluoride ions present in water. In general, the minerals themselves do have some capacity for fluoride removal. The capacity can be increased through “activation” by acid washing, calcination or air drying [116]. None of these minerals can be considered to be a universal defluoridation agent. Should one of them, however, occur adjacent to a fluoride affected area, and thus be available at low or no cost, it could be considered as the medium of choice for that particular area. For example, Padmasiri [117] reported clay to be appropriate for use in Sri Lanka. Clay and similar media can be regenerated; however, it might not be cost effective in most cases. Synthetic anionic clays (also known as layered double hydroxides) have attracted considerable attention due to their versatile applications as catalysts, catalyst precursors, adsorbents for removing organic/inorganic contaminants, support for inorganic-organic hybrid materials having applications in optics, nano-composites, bio-medical science etc. Layered double hydroxides (LDHs) form a class of synthetic clay, and have shown great potential as an adsorbent for defluoridation of water. LDHs have two-dimensional layered structure consisting of bivalent and trivalent metal ion hydroxides having positive surface charge separated by interlayer anions, stacked together by electrostatic interactions. Adsorption of fluoride on LDHs occurs through surface adsorption as well as by exchange of interlayer anions with the fluorides. Because of these dual characteristics, LDHs form a promising adsorbent for defluoridation of water. When these LDHs are calcined at higher

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temperatures (450 - 500 C), they form mixed metal hydroxides with the loss of their characteristic layered structure. An important property of these calcined LDHs is the so-called “memory effect”, that is, the reconstruction of the original layered structure when soaking the mixed oxide in an aqueous solution containing anions. This property has been extensively used to remove fluoride ions from water, instead of direct exchange. LDHs with different compositions and concentrations (calcined Mg-Al-CO3, Zn-Al-Cl), have been studied by researchers for defluoridation activity [94, 95, 97, 118-120]. Calcined LDHs exhibited higher defluoridation capacity compared to un-calcined LDHs, probably due to the combination of memory effect and sorption characteristics of the mixed metal oxides [97, 121]. More than 90% fluoride removal could be achieved using both calcined and un-calcined LDH adsorbents even at very low fluoride concentrations in water (below 10 mg/l). In addition to the high fluoride uptake capacity, LDHs have other advantages like, ease of synthesis and regeneration, low energy requirements, and near neutral working pH range. The major disadvantage of LDH adsorbents is their loss of fluoride adsorption capacity in presence of other co-existing anions in water. It is reported that presence of phosphate and arsenate ions has maximum adverse influence followed by chloride and sulphate ions. Fluoride adsorption by the LDHs is not affected by the presence of nitrate ions in water [94, 97]. Defluoridation of water using LDH as adsorbent has not been implemented so far in household or community level water treatment. The potential of this adsorbent should be studied in more detail in an approach to field application. Mandal and Mayadevi [122] have developed and reported a novel supported LDH for fluoride removal. This supported adsorbent is prepared by supporting the LDH on cellulose, which is a biodegradable material. The supported LDH showed a much higher defluoridation capacity compared to the un-supported LDH. Since cellulose is abundantly available in nature, bio-degradable and inexpensive, the cellulose supported LDH adsorbent is expected to be cost-effective as compared to the pure LDH.

SUMMARY AND CONCLUSION Fluoride is a micronutrient to human beings when present in the range of 0.5 - 1.5 mg F-/l in drinking water, and is considered to be essential for calcification of dental enamel as well as for bone formation. Prolonged consumption of high fluoride containing drinking water can lead to various diseases such as osteoporosis, arthritis, brittle bones, cancer, brain damage, Alzheimer syndrome and thyroid disorder. Presence of fluoride in drinking water above permissible level has been related to increased effect of fluorosis among the people in several countries including China, India, Pakistan and Thailand. In India, the problem of excessive fluoride in groundwater was first reported in 1937 in the state of Andhra Pradesh. Since then, more than 6 million people are known to be seriously affected by fluorosis and another 62 million exposed to it. As fluorosis is a disease with no cure, its prevention by reducing the fluoride content in drinking water has been one of the important mission programs by the Government of India. There are several defluoridation methodologies available for drinking water treatment; e.g. chemical precipitation, membrane based separation and adsorption/ion-exchange processes. The selection of appropriate water treatment technology depends on the fluoride

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concentration in the water. Adsorption is normally used when the fluoride concentration in water is very low. It is the most widely studied method for defluoridation of groundwater as the fluoride concentration in groundwater is usually very low. A wide variety of adsorbents have been examined for the removal of fluoride from groundwater. These include activated and impregnated alumina, carbonaceous materials, natural and synthetic clays. Among all the available adsorbents, activated alumina is the most common adsorbent for defluoridation of drinking water. Defluoridation using activated alumina is effective for community level water treatment plant but international performance standards are yet to be developed. Among carbonaceous materials, bone char and carbon nano-tubes were explored for defluoridation of water. Clay-based adsorbents (natural or synthetic) comprise of chemically modified natural clays like, bentonite, illite and fired clay as well as synthetic hydrotalcite type materials (also known as layered double hydroxides, LDHs). Removal of fluorides by LDHs is by both adsorption and ion-exchange. The ease of synthesis and regeneration, low energy requirements and performance at near neutral pH make them attractive adsorbents for the removal of fluorides from water. They are, however not highly selective towards fluoride ions in presence of other competing anions. Preliminary research on LDHs supported on biodegradable materials such as cellulose has revealed the high fluoride adsorption capacity of these new supported adsorbents. This is a nascent area with a lot of scope for development.

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Chapter 6

FLUORIDE IN GROUNDWATER IN NIGERIA: ORIGIN AND HEALTH IMPACT U.A. Lar1, H. Dibal1 and K. Schoeneich2 1

Department of Geology and Mining, University of Jos, Jos-NIGERIA 2 Ahmadu Bello University Zaria (ABU), Zaria, Nigeria

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ABSTRACT Until recently, the mottling and staining of teeth (dental caries) was believed to be an identity of certain ethnic groups or communities in Nigeria. Those born and reared locally within such communities had mottling teeth and fluoride as the causal factor was not known then. It was sooner discovered that dental caries extended beyond tribal or communal barriers. Even foreigners that came from far away Asia presented this disease condition. Dental caries is endemic and spreads over a large range of superficial area mainly the north eastern half of Nigeria both in the crystalline basement and sedimentary areas. The few data available on fluoride in drinking water clearly establishes the relationship between dental caries and environmental fluoride in drinking water. With the failure of the water supply systems in most parts of Nigeria to meet the demand of the increasing human population, about 90% of people use groundwater (well and borehole) for drinking and other domestic purposes. Studies have shown that, fluoride values of 0.2 – 8 mg/l above the 1.5 mg/l WHO admissible value have been recorded in the groundwater from the crystalline Basement aquifer (consisting of granites, gneisses, and migmatites). In the sedimentary aquifer, fluoride values of between 1mg/l to 4 mg/l have been recorded and especially in the Benue trough, there is a direct link between these fluoride values and the incidence of dental caries all along the 1000 m N-S long trough. It is difficult to detect the presence of fluoride in drinking water because it has no taste. Its presence is revealed only when it has caused a significant spread of the disease fluorosis. Thus, a lot of awareness campaign still needs to be done on the health implications of drinking of fluoride-rich waters and to debunk the belief of its association to certain tribes or communities.

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INTRODUCTION Low fluoride level in drinking water is almost completely absorbed by the human body. The oral intake of the normal fluoride dose of 0.1 – 1.5 mg/l in drinking water reduces dental decay and facilitates proper bone development (WHO, 1996). Fluoride level of > 1.5 mg/l results in dental fluorosis whose manifestations are the staining, weakening and the eventual loss of teeth. Children are the first casualties. For these reasons fluoride has been recommended for pregnant women and children. Higher exposure to fluoride (> 3 mg/l) will manifest as osteosclerosis, which is the hardening and calcification of bones and causes pain, stiffness, and irregular bone growth. More advanced manifestations are crippling skeletal fluorosis resulting in bone deformation and debilitation (Pauwel and Ahmed, 2007). So far, high fluoride levels of upto 8 mg/l have been reported in groundwater from the crystalline basement aquifers in most parts of Nigeria. The great danger associated with fluoride is due to its lack of taste which makes it difficult to detect its presence in water until there is a significant spread of the disease fluorosis. For example all along the N-S stretch of the 1000 km Benue trough of Nigeria, where the groundwater contains appreciable amounts of fluoride, a whole generation of children, suffers from dental fluorosis.

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GEOLOGY AND FLUORIDE LEVELS/DISTRIBUTION IN GROUNDWATER Nigeria is situated in West Africa between the Republics of Benin and Cameroon bordering the Gulf of Guinea. It is precisely located between Latitude N4° and N14° and Longitude E2.30° and E14.30°. Nigeria has a total superficial area of 927,770 km2 made up of a landmass of about 910,770 km2 and water of about 13,000 km2. According to the 2006 Population Census, Nigeria has a population of about 140 million people with a growth rate of 2.9%. Geologically, Nigeria is made up of three major geologic terrains viz: The ProterozoicLower Palaeozoic metamorphic Basement Complex, the Jurassic Younger Granites, the volcanic provinces and the Cretaceous sedimentary terrains. The crystalline Basement complex terrain is granitic, comprising of the metamorphic rocks (gneiss-migmatites, schist and granites associated with amphibolites, charnockites, diorites and serpentinites). The Younger Granite terrain is also granitic in composition and is centered in north central Nigeria. The Tertiary to Quaternary volcanic provinces cover essentially the eastern half of Nigeria. This includes the Jos Plateau, the Biu Plateau, the Longuda Plateau and the Benue valley. The sedimentary terrain comprises of the Niger Delta, the Anambra basin, the Lower, Middle and Upper Benue trough, the Chad basin, the Sokoto basin, the Mid-Niger (BidaNupe) basin and the Dahomey embayment (Figure 1). Fluoride in groundwater is derived from the crystalline rocks aquifer and their derivatives (soils, clays etc), where it occurs in the constituent fluoride bearing minerals (topaz (Al2SiO4(F,OH)2), fluorite (CaF2), fluorapatite (Ca5(PO4)3F), cryolite (Na3AlF6)) and/or in mica muscovite (KAl2(AlSi3)O10 (OH, F) and biotite (MgFeAlK, OH, F) (Lar et al., 2007). The interaction between water and the soil and the rock formations dissolves their constituent fluoride compounds, resulting in the presence of small amount of soluble fluoride in virtually all water sources.

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Figure 1.

Generally, natural fluoride concentration in groundwater in Nigeria varies depending on the types of aquifer. In the crystalline Basement aquifers, fluoride values vary from 1 mg/l to 8 mg/l (Lar et al., 2007) but in sedimentary aquifers it varies between 1 – 4 mg/l (Dibal and Lar, 2005). The fluoride higher values in the basement aquifers have been attributed to the depth from where the water was collected and how long was the resident time. Most of these values exceed the WHO admissible standard.

ORIGIN OF FLUORIDE-RICH GROUNDWATER Investigations have shown that the origin from which fluoride is released into groundwater is mainly from the weathering of the metamorphic basement rocks (granites, migmatite and gneiss), pegmatite veins and their derivatives (soils, clays, and sediments).

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Rock thin sections of these rocks have revealed the presence of some fluoride bearing minerals such as apatite, biotite and amphibole, where fluorine substitutes for hydroxyl positions. The occurrence of fluoride bearing minerals has also been reported in the Jurassic Younger Granites of the Jos Plateau (Badejoko, (1976). In the sedimentary terrains however, fluoride concentration of groundwater is controlled by mainly fluorite (CaF2), fluoroapatite (Ca5(PO4)3F) and clays. A contribution from anthropogenic sources cannot be ruled out especially through the use of phosphatic fertilizers where fluoride is contained as impurities. Also, fluorides are largely present in most pesticides. Analysis has shown that fluoride evolves along groundwater flow path. Thus, its concentration tend to increase progressively in the direction of flow (from recharge to discharge area) (Pauwells and Ahmed, 2007). Higher fluoride values from spring water, which originated from great depth, go to assert that fluoride concentration increases with depth and of course the residence time. The presence of fluoride in groundwater seems to be controlled by essentially the presence of Calcium, and Sodium. The higher the fluoride level, the higher is the sodium level and the lower is that of Calcium (Dibal and Lar, 2005). This may be as a result of the substitution of Na by Ca during the circulation of water in an aquifer or through carbonate precipitation.

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FLUORIDE AND HEALTH Fluorine, when ingested in the correct dose (0.5 – 1.5 mg/l), it reduces tooth decay. Its absence (< 0.5 mg/l) causes tooth decay. Fluoride concentration of > 1.5 mg/l causes the mottling of teeth and dental fluorosis. Higher fluoride dose (> 3 mg/l) is associated with skeletal fluorosis and other physiological disorders in humans (WHO, 1996). Studies have shown that dental fluorosis is endemic areas underlain by the metamorphic Basement rocks, and the Younger granites of north; the basement of the southwestern Nigeria Central Nigeria (with fluoride concentrations between 0.21 – 8 mg/l (Lar et al, 2008)). Other endemic areas are the Benue and the Sokoto sedimentary basins (with fluoride values of up to 4 mg/l (Dibal and Lar, 2005)). Investigation into dental caries particularly within the Benue trough shows an overall prevalence among primary school children (Alakija, 1983).

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Figure 2. Mottled teeth among young children in central Nigeria.

MITIGATION STRATEGIES OF FLUORIDE RELATED PROBLEMS Records have shown that elsewhere in the world where dental caries was endemic, fluoridization of public water supplies was done and that reduced the prevalence of dental carries greatly. Nowadays, the deficiency in fluorine is addressed through the addition of fluorine toothpaste to supplement for the needed fluorine so as to reduce tooth decay. Also, like in most urban centres in Nigeria where there are controlled water supply systems, fluorine is added to water supplies so as to boost the naturally low concentration (Davies and Anderson, 1987; Lererett, 1982). In the case of excess fluoride in drinking water, partial defluoridation is recommended to render the water safe for drinking; the simplest and the most cost-effective of which is direct evaporation technique. Also, the addition of alum into the water reduces fluoride content. Other methods exist but will require skill to operate and could be costly for most Nigerians. The most popular methods include adsorption/ion exchange and precipitation and electro-dialysis or reverse osmosis methods. More needs to be done to create public awareness of the health risk associated with the excess consumption of fluoride by humans.

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CONCLUSION The source from which fluoride is released into groundwater is from the crystalline metamorphic Basement and igneous rocks and rarely by anthropogenic processes. In the sedimentary aquifers, fluoride finds its way into groundwater through the leaching of mainly fluorite or fluoroapatite. Depending on the level in which fluoride is present in the water, it could be beneficial or detrimental to both bone and dental development in human beings. Excessively high fluoride groundwater concentrations are from crystalline aquifer and this increases with depth (old water) where the water had considerable residence time. This work has established the relationship between fluoride content in water and the prevalence of dental caries in especially where it is underlain by the Precambrian Basement rocks e.g. Southwestern, north central and northeastern Nigeria and the sedimentary basins of Sokoto and the Benue. More than 80% of Nigerians depend on groundwater for drinking and other domestic purposes. In view of the health hazards associated with fluoride intake, endemic communities must be made to appreciate the health risk and to understand the need to treat fluoride-rich or poor water destined for human consumption. Therefore, a lot more needs to be done to identify new areas of endemic dental carries. This can only be done by the a close study to map out fluoride endemic areas and to proffer alternative mitigation strategies to be applied directly to water so as to reduce the associated health problems.

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REFERENCES Alakija,W. (1983). Dental caries in primary school children in Nigeria. Journal of Tropical Pediatrics (6) 29 pp317-319. Badejoko, (1976) Bano, A.I., Isichei,W. P., Das,S.U., Morimoto,I. and Nagataki,S.(1987). Common Trace Elements in Potable water in Plateau State of Nigeria and their impact on goiter prevalence in the State. Proceedings of Nigeria-Japan Joint Conference on Trace Metal, Giotre, Dirrhoea, Medical Entomology and Epidemiology pp 15 – 17 Japan Int. Coop. Agency. Davies, T.C. (2003). Some environmental problems of geomedical relevance in East and Southern Africa. In: Geology and Health. Oxford Univ. Press. Elsevier. Pp139-144. Dibal, H.U. and Lar, U.A. (2005). Preliminary survey of fluoride concentrations in the groundwater of Kaltungo area, Gombe State, northeastern Nigeria. Journal of Environmental Sciences, Vol.9 No.2 pp41-52. Edmund and Smedley, (2005) Fluoride in Natural Waters: In Essentials of Medical Geology, O. Sellinus (ed) Elsevier pp 301- 309. Ford, S.O. (1981). The economic mineral resources of the Benue Trough. Earth Evolution Sciences 2 pp154-163. Fordyce, F (2000). Geochemistry and Health. Why geoscience information is necessary. Geoscience and Development No.6, p6-8. Lar, U.A, Dibal, H.U, Daspan, R.I and Jaryum, S.W. (2007). Fluoride occurrence in the surface and groundwaters of Fobur area of Jos East LGA, Plateau State. Journal of Environmental Sciences 11, 2, 99 – 105. Lar,U.A. and Sallau,A. K. (2005). Trace element geochemistry of the Keana brine field, Middle Benue trough Nigeria. International Journal of Env. And Health. Vol4 pp236243. Pauwels, H and Ahmed, S. (2007) Fluoride in groundwater: origin and health impacts. Geosciences et Sante No.5 BRGM’s Journal for Sustainable Earth pp68 -73 Wind , A. (2009). Keeping it real. Geoscientist, The magazine of the Geological Society of London. Vol.19 No.5 pp16-21. World Health Organisation, (2004). Guidelines for drinking water quality. 2nd Ed., Vol.2, Health criteria and other supporting information. Published by International Programme on Chemical Safety, WHO, Geneva.

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Chapter 7

HYDROCHEMICAL CHARACTERIZATION OF FLUORIDE RICH GROUNDWATER: A CASE STUDY Mouna Ketata1-2, Moncef Gueddari1 and Rachida Bouhlila2 1

Laboratory of Geochemistry and Environmental Geology, Departement of Geology, Faculty of Mathematical, Physical and Natural Sciences, University Campus, Tunis, Tunisia 2 Modelling in Hydraulic and Environment Laboratory, National Engineers School of Tunis, Tunis, Tunisia

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ABSTRACT Serious problems are faced in several parts of the world due to the presence of high concentration of fluoride in drinking water. Fluoride is known to have both beneficial and adverse effects on humans, depending on the total intake. Fluoride can be beneficial in helping to prevent dental caries at drinking water concentrations of about 1 mg/L but it has also been shown to cause dental mottling and adverse effects on bone, including increased risk of fracture at concentrations in excess of 1.5 mg/L, with the risk gradually increasing with the total intake of fluoride. Naturally occurring high fluoride levels in groundwater is a complicated issue for drinking water providers in many regions of the world. Presence of fluoride bearing minerals in the host rock, the chemical properties like decomposition, dissociation and dissolution and their interaction with water is considered to be the main cause for fluoride in groundwater. Chemical weathering under arid to semi-arid conditions with relatively high alkalinity favours high concentration of fluoride in groundwater. According to the WHO health-based guideline waters containing more than 1.5 mg/L are unsuitable for drinking use. In Tunisia dental and skeletal fluorosis are prelevant particulary in the south part of the country, wich can be related to the usage of high fluoride groundwater for drinking. In recent years the acquisition of considerable additional data on the hydrogeochemical behavior of fluoride in groundwaters of Tunisia has been made possible by extensive groundwater sampling campaigns as well as by improvements in analytical techniques. The study is an attempt to assess the hydrogeochemistry of

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Mouna Ketata, Moncef Gueddari and Rachida Bouhlila groundwater in Gabes-south aquifer located in the southeastern part of Tunisia with a focus on fluoride occurrence. In order to reach this objective, this chapter presents a synthesis of the data so far obtained on the sources and distribution of fluoride in southeastern Tunisia, examines the extent of fluorine toxicity and puts forward recommendations to combat or minimize the problem. Results of the chemical data of the groundwater suggest that the considerable number of groundwater samples collected from the studied aquifer show fluoride content greater than that of the safe limit prescribed for drinking purpose. Fluoride may essentially be from a natural origin. Limestone (chalky) and marly formations contain significant quantities of fluoride which can be liberated by the water–rock interaction. In general, the mineralogy of the bedrock is the primary source of fluoride in groundwater and is responsible for the difference of fluoride concentration of groundwater between different bedrock types. Dissolution of Fluorite (CaF2) is a plausible source of fluoride ion in groundwater. The suggested remedial measures to reduce fluoride pollution in groundwater It is recommended that water with high fluoride levels should be defluoridated or alternative sources with low fluoride should be identified.

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INTRODUCTION Groundwater is most abundant and readily available sources of freshwater. This resource represents over 90% of the word’s readily available freshwater resource. Population growth and low recharge accompanied by industrial and agricultural development are at the origin of a great and perceptible increase of groundwater exploitation, which contributes significantly to the deterioration of groundwater quality. In recent days, groundwater quality assessment is important to ensure sustainable safe use of water [1]. The lack of clean drinking water is adversely affecting the general health and life expectancy of the people in many developing countries [2]. Indeed, about 80% of the diseases and deaths in the developing countries are related to water contamination [3]. The suitability of groundwater for a particular use depends upon its quality, which is determined by solutes and gases dissolved as well as matter suspended or floating in it [4]. Prevention and control of the groundwater pollution must rely on the reliable information of water quality and identification of pollutant sources [5, 6] The chemical composition of groundwater is impacted by several sources such as atmospheric input, interaction of water with rocks and human activities [7]. Fluoride occurs naturally in groundwater, and is one of the emerging problems in groundwater use. The concentration of fluoride in drinking water is very important because it is an essential component which has both positive and negative effects on human health. Small concentration of fluoride is essential for normal mineralization of bones and the formation of dental enamel [8-11]. The guideline of the World Health Organization (WHO) limits the fluoride concentration for drinking Water Quality to 1.5 mg/L [12]. Fluoride beyond desirable amounts is a major problem in many parts of the world. Around 200 million people from 25 nations have health risks because of high fluoride in groundwater [13]. The main source of fluoride in groundwater is considering being the weathering of minerals containing fluoride such as fluorite, fluoroapatite, and mica is reported as the main source of fluoride in groundwater [14-20]. Apart from natural sources, a considerable amount of fluoride may be contributed through anthropogenic activities. Phosphatic fertilizers, which are

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extensively used in agriculture, often contain fluoride as an impurity that can leach down to the saturated zone [21, 22]. Fluoride contamination of groundwater is a function of many factors such as availability and solubility of fluorine bearing minerals [23, 24], and on the presence or absence of other precipitating or complexing ions [25-27]. Therefore, in high-fluoride groundwater, geological processes and geochemical features capable of increasing its concentration of fluoride should be carefully investigated to reduce related environmental problems. Gabes-south groundwaters have high levels of fluoride, which represents the major national water quality problem. Consequently, groundwater resource management became increasingly important for sustainable development in these regions. As a representative case, this chapter presents the results of 7-year hydrochemical study carried out in Gabes-south deep aquifer using geochemical techniques in order to reveal the distribution of high-fluoride groundwater, to explain the origin of the high fluoride in groundwater, and to identify its evolutional mechanisms. This research could provide useful information for water resources management and development in the studied region.

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FLUORIDE IN GROUNDWATER Fluorine is thirteenth in the order of abundance of elements in the earth’s crust. The fluorine atom has a molar weight of 18.998, a Van Der Waals radius of 1.47 and is esoterically similar to a hydroxyl group. Fluoride is physiologically important and its extremely high electronegativity makes it highly reactive and therefore it occurs in a number of naturally combined forms. It is found mostly in the silicate minerals of the earth’s crust at a concentration of about 650 mg/kg [28, 29]. The geochemistry of the F- (ionic radius 136 pm) is similar to that of the OH- ion (ionic radius 140 pm) and there can be easy exchange between them [30]. Groundwater is a major source of human intake of fluoride, including its subsequent incorporation into food items. The fluoride content of groundwater varies greatly depending on the geological settings and type of rocks. High fluoride concentration in groundwater is common in areas where rocks contain fluoride-bearing minerals such as fluorspar (CaF2), fluor-apatite [Ca5(PO4)3F)], cryolite (Na3AlF6), topaz Al2SiO4(F,OH)2, hydroxyl-apatite and micas (muscovite and biotite) [14-17, 19, 20, 23, 24, 31, 32]. The natural concentration of fluoride in groundwater depends on the geological, chemical and physical characteristics of the aquifer. Igneous and volcanic rocks have a fluorine concentration from 100 ppm (ultramafic) up to >1000 ppm (alkalic) [33]. Groundwaters from crystalline rocks, especially (alkaline) granites (deficient in calcium) are particularly sensitive to relative high fluoride concentrations. Such rocks are found especially in Precambrian basement areas. Sedimentary rocks have a fluorine concentration from 200 ppm (limestone) up to 1000 ppm (shales) [33]. In carbonate sedimentary rocks the fluorine is present as fluorite. Clastic sediments have higher fluorine concentrations as the fluorine is concentrated in micas and illites in the clay fractions. High concentrations may also be found in sedimentary phosphate beds (shark teeth) or volcanic ash layers [33]. Metamorphic rocks have a fluorine concentration from 100 ppm (regional metamorphism) up to more than 5000 ppm (contact metamorphism). In these rocks the original minerals are enriched with fluorine by metasomatic processes [33]. Geological

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structures such as fault or bolide impact structure also play important roles controlling fluoride concentrations [31, 34-36] Presence of dissolved fluoride in groundwater is possible only under favourable physicochemical conditions and with a sufficient residence time [23, 37]. Such groundwaters are usually associated with deep aquifer systems and a slow groundwater movement. Shallow aquifers which contain recently infiltrated rainwater usually have low fluoride [38]. Other geochemical process such as ion exchange and the evaporation and precipitation of calcite can contribute to the enrichement of fluoride in groundwater [16, 31, 39, 40]. In arid regions groundwater flow is slow and the reaction times with rocks are therefore long. The fluoride contents of water may increase during evaporation if solution remains in equilibrium with calcite and alkalinity is greater than hardness. Dissolution of evaporative salts deposited in arid zone may be an important source of fluoride [22, 38]. Other factors including water temperature, pH of the draining solutions, and concentration of calcium and bicarbonate ions seem to have significantly contributed to the fluoride accumulation in groundwater (31, 34, 36, 38). Indeed, High-fluoride groundwaters are mainly associated with a sodium-bicarbonate water type and relatively low calcium and magnesium concentrations. Such water types usually have high pH values (above 7) [38]. Apart from natural sources, considerable amount of fluoride may be contributed through anthropogenic activities. Modern agricultural practices, which involve the application of phosphate fertilizers coupled with pesticides, and industrial activities (clays used in ceramic industries or burning of coals) also contribute to high fluoride concentrations in groundwater [22, 38, 41-43]. Fluoride levels of