Nanoengineering in the beverage industry 9780128172841, 0128172843

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Nanotechnology in the Beverage Industry

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Nanotechnology in the Beverage Industry Fundamentals and Applications Edited by

Abdeltif Amrane Susai Rajendran Tuan Anh Nguyen Aymen Amine Assadi Ashraf Mahdy Sharoba

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/ permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-819941-1 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Simon Holt Editorial Project Manager: Charlotte Rowley Production Project Manager: Prem Kumar Kaliamoorthi Cover Designer: Greg Harris Typeset by SPi Global, India

Contents Contributors ............................................................................................................xxi

PART 1 Nanomaterials in water treatment CHAPTER 1 TiO2-based nanomaterials for wastewater treatment .......................................................................3 R. Parimaladevi, M. Umadevi, T.N. Rekha, and A. Milton Franklin Benial 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

Introduction.....................................................................................3 Photocatalytic activity ....................................................................4 Photocatalytic mechanism ..............................................................5 Photocatalyst TiO2 ..........................................................................6 Synthesis of TiO2 composites ........................................................7 Structural studies ............................................................................7 Vibrational analysis ........................................................................9 Photocatalytic activity measurement..............................................9 Photocatalytic investigation of TiO2 composites.........................11 References.....................................................................................21 Further reading .............................................................................24

CHAPTER 2 Groundwater treatments using nanomaterials ........... 25

2.1 2.2

2.3 2.4

2.5

Saravanan Ramiah Shanmugham, Gautham B. Jegadeesan, and V. Ponnusami Nanotechnology in groundwater treatment..................................25 Adsorption using nanomaterials ...................................................26 2.2.1 Nano-zerovalent iron treatment..........................................27 2.2.2 Carbonaceous materials......................................................29 2.2.3 Metal oxides .......................................................................34 2.2.4 Metal-organic framework ...................................................36 2.2.5 Adsorption kinetics and equilibrium..................................37 Photocatalysis ...............................................................................38 Implementation of nanotechnology for water treatment .............40 2.4.1 Ex situ .................................................................................40 2.4.2 In situ ..................................................................................41 Health implications on the use of nanotechnology in groundwater treatment ..................................................................41 References.....................................................................................42

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CHAPTER 3 Copper-based ternary metal sulfide nanocrystals embedded in graphene oxide as photocatalyst in water treatment ........................................................... 51 3.1 3.2

3.3

3.4

3.5

Olalekan C. Olatunde and Damian C. Onwudiwe Introduction...................................................................................52 Wastewater treatment technologies..............................................55 3.2.1 Adsorption...........................................................................55 3.2.2 Membrane separation .........................................................56 3.2.3 Advanced oxidation processes ...........................................56 Copper-based ternary metal sulfide nanocrystals (CBTS) ..........69 3.3.1 Classification of copper ternary metal sulfides .................69 3.3.2 Synthesis of copper-based ternary metal sulfides.............. 77 3.3.3 Applications of copper metal ternary sulfides ...................81 Graphene, its derivatives and photocatalysis ...............................85 3.4.1 Synthesis of graphene oxide...............................................86 3.4.2 Graphene oxide in wastewater treatment...........................88 3.4.3 GO/semiconductor composites...........................................89 3.4.4 GO/metal chalcogenide nanocomposite photocatalysts......................................................................90 3.4.5 GO/copper-based ternary metal sulfide nanocomposite photocatalysts ............................................90 3.4.6 Mechanism of action of GO-supported photocatalysts......................................................................92 3.4.7 Future perspective...............................................................93 Conclusion ....................................................................................94 Acknowledgments ........................................................................94 References.....................................................................................94

CHAPTER 4 Nanosensors for water quality control..................... 115 Adriana Marcia Graboski, Janine Martinazzo, Sandra Cristina Ballen, Juliana Steffens, and Clarice Steffens 4.1 4.2 4.3 4.4

Introduction.................................................................................115 Nanosensors ................................................................................116 4.2.1 Types of nanosensors .......................................................117 Applications of nanosensors in water quality control ...............119 Conclusion ..................................................................................124 Acknowledgments ......................................................................124 References...................................................................................124

Contents

CHAPTER 5 Nanostructured membranes for water treatments ................................................................. 129 Rozita M Moattari and Toraj Mohammadi 5.1 5.2 5.3

5.4

5.5 5.6

Introduction.................................................................................129 Concept .......................................................................................131 Nanostructured membranes ........................................................132 5.3.1 Nanoporous polymeric membranes..................................132 5.3.2 Nanostructured ceramic membranes ................................135 Nanomaterial-incorporated membranes .....................................139 5.4.1 Carbon nanotubes .............................................................140 5.4.2 Graphene ...........................................................................141 5.4.3 Zeolites..............................................................................143 Challenges...................................................................................144 Conclusions.................................................................................145 References...................................................................................145 Further reading ...........................................................................150

CHAPTER 6 Nanomaterials for fouling-resistant RO membranes .......................................................... 151 Zahra Shabani, Soheil Zarghami, and Toraj Mohammadi 6.1 6.2 6.3

6.4

6.5

6.6

Introduction.................................................................................152 Reverse osmosis: Fundamentals and principals.........................153 RO membrane fabrication strategies ..........................................154 6.3.1 CA membranes .................................................................155 6.3.2 TFC membranes ...............................................................155 6.3.3 Polyelectrolyte membranes ..............................................156 6.3.4 MMMs ..............................................................................157 6.3.5 Biomimetic membranes....................................................157 RO membranes fouling types .....................................................157 6.4.1 Colloids .............................................................................158 6.4.2 Organic fouling.................................................................158 6.4.3 Inorganic fouling ..............................................................159 6.4.4 Biofouling .........................................................................159 RO fouling control strategies .....................................................160 6.5.1 Feed pretreatment .............................................................160 6.5.2 Membrane cleaning ..........................................................161 6.5.3 Membrane modification ...................................................162 Utilization of nanomaterials for preparation of antifouling RO membranes ........................................................163

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6.7

6.6.1 Carbon-based nanoparticles enabled RO membranes ........................................................................163 6.6.2 Titanium dioxide-based nanoparticles enabled RO membranes .................................................................167 6.6.3 Silica-based nanoparticles enabled RO membranes ........168 6.6.4 Silver-based nanoparticles enabled RO membranes........170 6.6.5 Other nanoparticles enabled RO membranes...................173 Conclusion ..................................................................................176 References...................................................................................177 Further reading ...........................................................................184

CHAPTER 7 Nanomaterials in wastewater treatments ................ 185

7.1 7.2 7.3 7.4 7.5

7.6

7.7

Bapun Barik, Pratap Sagar Nayak, and Priyabrat Dash Introduction.................................................................................185 Wastewater and its sources ........................................................187 Wastewater treatment processes.................................................188 Nanomaterials .............................................................................189 Modified metal oxide nanomaterials..........................................191 7.5.1 Graphene oxide-supported metal oxide nanomaterials ....................................................................191 7.5.2 Polymer-supported metal oxide nanomaterials................193 Graphene oxide and polymer-supported metal oxide nanomaterials for wastewater treatment.....................................194 7.6.1 Graphene oxide-supported metal oxide nanomaterials for wastewater treatment ..........................195 7.6.2 Polymer-supported metal oxide nanomaterials for wastewater treatment ..................................................196 Conclusions and future perspectives ..........................................199 References...................................................................................200

CHAPTER 8 Nanomembranes for water treatment ....................... 207 Faisal Rehman, Khalid Hussain Thebo, Muhammad Aamir, and Javeed Akhtar 8.1 8.2

Introduction.................................................................................207 Synthetic techniques ...................................................................209 8.2.1 Drop casting......................................................................209 8.2.2 Vacuum filtration..............................................................209 8.2.3 Spin coating ......................................................................211 8.2.4 Langmuir-Blodgett (LB) method .....................................211

Contents

8.3

8.4

8.5 8.6

Some common types of membranes ..........................................212 8.3.1 TMDC membranes ...........................................................212 8.3.2 MXene membranes...........................................................213 8.3.3 hBN membranes ...............................................................214 8.3.4 MOFs membranes.............................................................214 8.3.5 Zeolite membranes ...........................................................215 Desalination process via 2D membranes ...................................216 8.4.1 Graphene ...........................................................................216 8.4.2 Assembled 2D material laminates....................................227 Dye separation via 2D membrane..............................................228 Conclusion and future prospects ................................................235 References...................................................................................236

CHAPTER 9 The use of nanocatalysts (and nanoparticles) for water and wastewater treatment by means of advanced oxidation processes ............................ 241 9.1

9.2 9.3 9.4 9.5

Vincenzo Vaiano, Diana Sannino, and Olga Sacco Introduction to nanocatalysts and nanomaterials for pollutant removal ........................................................................241 9.1.1 Nature of nanomaterials applied to wastewater treatment ...........................................................................242 Advanced oxidation processes (AOPs) for water and wastewater treatment ..................................................................245 Fenton and photo-Fenton processes for water and wastewater treatment ..................................................................247 Heterogeneous photocatalysis for water and wastewater treatment .....................................................................................252 Conclusion and future perspectives............................................257 References...................................................................................257

CHAPTER 10 Fe-doped TiO2 nanomaterials for water depollution ................................................................ 265

10.1

10.2

Maria Cris¸ an, Adelina-Carmen Ianculescu, Dorel Crișan, Nicolae Dra˘gan, Ligia Todan, Ines Nit¸oi, and Petrut¸a Oancea Introduction.................................................................................266 10.1.1 General overview............................................................266 10.1.2 Objectives .......................................................................267 State of the art regarding undoped and Fe-doped TiO2 sol-gel nanomaterials..................................................................267

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10.3

10.4

10.2.1 Photocatalytic effect .......................................................268 10.2.2 Influence of iron dopant on photocatalytic activity ............................................................................270 Undoped and Fe-doped TiO2 sol-gel nanopowders...................280 10.3.1 Short consideration of sol-gel method for TiO2-based nanopowders preparation ............................280 10.3.2 Our studies regarding undoped and Fe-doped TiO2 nanopowders ..........................................................281 Undoped and Fe-doped TiO2 sol-gel films................................291 10.4.1 Sol-gel films ...................................................................291 10.4.2 Our studies regarding undoped and Fe-doped TiO2 films .......................................................................291 References...................................................................................305

PART 2 Smart nanocapsules/nanocarriers in drinks CHAPTER 11 Nanoencapsulation of flavors for beverage manufacturing ........................................................... 317 Mohebbat Mohebbi 11.1 11.2 11.3

11.4 11.5 11.6

11.7

Flavor ..........................................................................................317 Flavor perception ........................................................................318 Flavor release..............................................................................320 11.3.1 Models for flavor release ...............................................320 11.3.2 Controlled release ...........................................................324 Flavor in emulsion beverages.....................................................325 Nanotechnology and flavor encapsulation .................................327 Methods for flavor nanoencapsulation.......................................328 11.6.1 Emulsification...............................................................328 11.6.2 High-pressure homogenization.....................................328 11.6.3 Ultrasonication..............................................................328 11.6.4 Microfluidization ..........................................................329 11.6.5 Phase inversion emulsification.....................................329 11.6.6 Spray-drying .................................................................330 11.6.7 Spray chilling................................................................330 11.6.8 Molecular inclusion ......................................................331 11.6.9 Freeze-drying ................................................................332 11.6.10 Electrospraying/electrospinning ...................................332 Summary .....................................................................................333 References...................................................................................333

Contents

CHAPTER 12 Antioxidant-loaded nanocarriers for drinks ............. 337

12.1

12.2

12.3

12.4

Mariana Veiga, C
elia Costa, Maria Joa˜o Carvalho, Eduardo M. Costa, Sara Silva, and Manuela Pintado Introduction.................................................................................337 12.1.1 Antioxidant compounds..................................................338 12.1.2 Diversity of antioxidant compounds ..............................339 12.1.3 Antioxidant compounds and health................................342 12.1.4 Antioxidant compounds as additives .............................343 Nanoencapsulation of bioactive compounds..............................344 12.2.1 Nanomaterials and delivery systems..............................344 12.2.2 Nanoencapsulation techniques .......................................348 12.2.3 Incorporation in beverages .............................................348 Legislative framework ................................................................359 12.3.1 Nanotechnology ..............................................................360 12.3.2 Functional ingredients: Nutrition and health claims ..............................................................................361 12.3.3 Technological ingredients: Food additives ....................362 Conclusion ..................................................................................363 References...................................................................................363

CHAPTER 13 Nanocarriers loaded with nutraceuticals and bioactive ingredients (vitamins and minerals) ................................................................... 373 Berta N. Estevinho 13.1 13.2

13.3 13.4

Introduction.................................................................................374 Importance of the nutraceuticals in the food industry...............375 13.2.1 Historical context............................................................375 13.2.2 Nutraceutical categories .................................................376 New solutions for nutraceuticals—Delivery systems ................378 Nutraceuticals and bioactive ingredients ...................................381 13.4.1 Probiotics ........................................................................381 13.4.2 Flavors.............................................................................382 13.4.3 Bioactive lipids ...............................................................382 13.4.4 Antioxidants and natural compounds.............................383 13.4.5 Vitamins..........................................................................383 13.4.6 Bioactive proteins, peptides, and enzymes ....................386 13.4.7 Minerals ..........................................................................386 13.4.8 Dyes and colors ..............................................................387 13.4.9 Stabilizers........................................................................387

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13.5

13.6

Encapsulation..............................................................................388 13.5.1 Encapsulation techniques ...............................................388 13.5.2 Encapsulating agents ......................................................388 13.5.3 Controlled release mechanisms ......................................394 13.5.4 Evaluation of the bioavailability ....................................396 Conclusions.................................................................................397 Acknowledgments ......................................................................398 References...................................................................................398 Further reading ...........................................................................412

CHAPTER 14 Multifunctional drinks from all natural ingredients ................................................................ 413

14.1

14.2

R. Dorothy, K Sneka Latha, RM Joany, T Sasilatha, Susai Rajendran, Gurmeet Singh, and S. Senthil Kumaran Introduction.................................................................................414 14.1.1 Remarkable reasons for drinking fresh fruit juice daily .....................................................................415 14.1.2 Healthiest beverages we should be drinking ............... 416 14.1.3 Most unhealthy beverages to be avoided.....................416 14.1.4 We can drink green juice every day ............................417 14.1.5 Drinks we can have besides water ...............................417 14.1.6 Natural ingredients in energy drinks............................417 14.1.7 Fresh natural drinks for everybody ..............................417 14.1.8 Smoothies......................................................................417 14.1.9 Goodness and badness of coffee ..................................418 14.1.10 Coffee and digestion.....................................................418 14.1.11 Decaffeinated coffee is harmful ...................................418 14.1.12 Prebiotic foods ..............................................................418 14.1.13 Probiotics ......................................................................418 Recent trends on multifunctional drinks from natural ingredients...................................................................................418 14.2.1 Tannins from Trapa taiwanensis hulls...........................419 14.2.2 Categorization of food ingredients.................................419 14.2.3 “Ultraprocessed foods” for the youth.............................420 14.2.4 Cosmeceutical effect of ethyl acetate fraction of Kombucha tea ............................................................420 14.2.5 Functional drinks made from ginger extracts ................421 14.2.6 Cocoa- and carob-based drink powders from foam mat drying .............................................................421 14.2.7 Physicochemical properties of Rambutan (Nephelium lappaceum L.) fruit sweating..........................................422

Contents

14.3

14.2.8 Investigation of functional properties of cocoa waste from concentrated cocoa drink ..........................422 14.2.9 Analysis of the lobbying arguments and tactics of stakeholders in the food and drink industries ..............423 14.2.10 Nonnutritive sweeteners possess a bacteriostatic effect and alter gut microbiota in mice........................423 14.2.11 Alginate as a functional food ingredient......................424 14.2.12 A sour milk beverage ...................................................424 14.2.13 Antimicrobial evaluation of Foeniculum vulgare leaves extract ingredient of ethiopian local liquor ....................................................................425 14.2.14 Influence of spices on the content of fluoride and antioxidants in black tea infusions ...............................426 14.2.15 Antiaging effects of guarana (Paullinia cupana) in Caenorhabditis elegans............................................426 14.2.16 Influence of dried apple powder additive on physical-chemical and sensory properties of yoghurt ..........................................................................427 14.2.17 Analysis of natural carbonated drinks..........................427 14.2.18 Influence of a microencapsulated Amazonic natural ingredient with potential interest as a functional product.........................................................428 14.2.19 Modern technologies in beverage processing ..............428 14.2.20 Homogenization and physical properties of model coffee creamers stabilized by quillaja saponin ..........................................................................429 Conclusion ..................................................................................429 References...................................................................................430

PART 3 Applications of nanotechnology for hygiene of drinks CHAPTER 15 Nanodevices for the detection of pathogens in milk ....................................................................... 435 P. Priyadharshini, V. Amalnath, Anand Babu Perumal, J.A. Moses, and C. Anandharamakrishnan 15.1 15.2 15.3

Introduction.................................................................................436 Microbial contamination in milk................................................436 Major pathogens in milk ............................................................437 15.3.1 Listeria monocytogenes ..................................................437 15.3.2 Salmonella ......................................................................437

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15.4

15.5 15.6 15.7

15.8

15.3.3 E. coli..............................................................................438 15.3.4 Campylobacter species ...................................................438 15.3.5 Shigella species...............................................................438 15.3.6 Brucella species ..............................................................438 Conventional methods used for detection of pathogen in milk.........................................................................................439 15.4.1 Polymerase chain reaction..............................................439 15.4.2 Loop-mediated isothermal amplification .......................441 15.4.3 Nucleic acid sequence-based amplification ...................442 15.4.4 Flow cytometry...............................................................443 15.4.5 Spectroscopy techniques.................................................444 15.4.6 Multisensory techniques .................................................446 15.4.7 Biosensors .......................................................................447 Limitations in the conventional methods...................................448 Nanotechnology in pathogen detection......................................449 Existing nanodevices in pathogen detection in milk .................450 15.7.1 Immunosensing methods combined with nanotechnology ...............................................................450 15.7.2 Nanoparticle-based detection .........................................456 15.7.3 SERS-based detection.....................................................459 15.7.4 Sensor-based detection ...................................................463 Conclusion ..................................................................................463 References...................................................................................463

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva with presence of beverage ............. 471

16.1 16.2

16.3

16.4

A. Christy Catherine Mary, J. Jeyasundari, V.R. Nazeera Banu, R. Dorothy, Susai Rajendran, S. Senthil Kumaran, and A. Peter Pascal Regis Introduction.................................................................................471 Experimental...............................................................................472 16.2.1 Materials .........................................................................472 16.2.2 Methods...........................................................................473 Results and discussion ................................................................474 16.3.1 Analysis of polarization curves ......................................474 16.3.2 AC impedance spectra study ..........................................481 16.3.3 Investigation of the film formed on metal surface .............................................................................493 Conclusions.................................................................................503 Acknowledgment ........................................................................503 References...................................................................................503

Contents

CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva with presence of fragrant drink additives .......................................................... 505

17.1 17.2

17.3

RM Joany, A Anandan, S Gowri, Susai Rajendran, Bhawna Chugh, S. Senthil Kumaran, and Gurmeet Singh Aroma compounds used in foods and beverages.......................505 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva with presence of fragrant drink additives: A case study...............................................................506 17.2.1 Artificial saliva (AS) ......................................................507 17.2.2 Polarization study ...........................................................507 17.2.3 AC impedance spectra....................................................510 17.2.4 Contact angle measurement ...........................................518 17.2.5 AFM images ...................................................................520 Conclusion ..................................................................................522 References...................................................................................522 Further reading ...........................................................................523

CHAPTER 18 Nanofiltration in beverage industry ......................... 525 Carmela Conidi, Roberto Castro-Mun˜oz, and Alfredo Cassano 18.1 18.2 18.3

18.4

Introduction.................................................................................525 Properties of nanofiltration membranes .....................................526 Application of NF membranes in beverage industry.................529 18.3.1 Wine and beer.................................................................529 18.3.2 Fruit juice processing .....................................................535 18.3.3 Whey and milk ...............................................................539 Conclusions and future trends ....................................................544 References...................................................................................544 Further reading ...........................................................................548

CHAPTER 19 Chromatographic nano-column technology and its application in beverage analysis................. 549 €kaltun, Kemal C¸etin, Sarah Alharthi, and Aslıhan Go Cemil Aydog˘an 19.1 19.2 19.3

Introduction to capillary nano-columns for beverage analysis...........................................................................................549 Capillary nano-column technology ............................................550 Chromatographic nanoseparation techniques.............................551 19.3.1 Capillary/nano-liquid chromatography ..........................551 19.3.2 Capillary electrophoresis/electrochromatograph............553

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19.4

19.5

Applications of beverage analysis..............................................553 19.4.1 Nano-monoliths applications..........................................553 19.4.2 Packed columns applications..........................................556 19.4.3 Open-tubular (OT) columns ...........................................562 19.4.4 Miscellaneous applications.............................................566 19.4.5 Chip LC, CE, and CEC ..................................................569 Conclusions and perspectives.....................................................573 References...................................................................................574

PART 4 Applications of nanotechnology for packaging of drinks CHAPTER 20 Active nanoenabled packaging for the beverage industry ..................................................... 587 Hajar Shekarchizadeh and Fatemeh Sadat Nazeri 20.1 20.2 20.3

20.4 20.5 20.6 20.7 20.8 20.9

20.10 20.11

20.12

Nanotechnology ..........................................................................588 Packaging....................................................................................589 Biobased packaging ....................................................................590 20.3.1 Starch and its derivatives ...............................................591 20.3.2 Polylactic acid.................................................................591 20.3.3 Polyhydroxybutyrate.......................................................591 20.3.4 Polycaprolactone.............................................................592 Beverage packaging....................................................................592 Nanotechnology in beverage packaging ....................................592 Active packaging ........................................................................593 Active packaging and nanotechnology ......................................595 Nanocoatings and nanolaminates ...............................................596 Nanoenabled active packaging materials ...................................597 20.9.1 Nanotubes and nanofibers ..............................................597 20.9.2 Metallic and metallic oxide nanoparticles .....................597 20.9.3 Clay nanoparticles and nanocrystals ..............................597 20.9.4 Nanocomposites..............................................................598 Industrial applications of active nanopackaging in beverages.....................................................................................598 Nanotechnology and environment and health concerns ............600 20.11.1 Environmental impact ..................................................600 20.11.2 Human health impact....................................................601 20.11.3 Nanotechnology and food safety..................................602 Active packaging: Legal issue and safety concern....................602 20.12.1 Safety of nanoactive packaging ...................................603

Contents

20.13

20.12.2 Food nanopackaging regulations and legislations ....................................................................603 Conclusion ..................................................................................604 References...................................................................................605

CHAPTER 21 Biodegradable nanomaterials for drink packaging ................................................................. 609 Maricica Stoica 21.1 21.2

21.3

21.4 21.5

Introduction.................................................................................609 Biopolymers for drink packaging bionanomaterials..................610 21.2.1 PHAs: PHB and PHBV ..................................................611 21.2.2 PLA .................................................................................613 PHB-, PHBV-, and PHA-based material with nanofillers.........614 21.3.1 PHB- and PHBV-based materials with nanofillers........614 21.3.2 PLA-based materials with nanofillers............................617 Biodegradable nanomaterials in contact with drink ..................621 Conclusions.................................................................................621 References...................................................................................622 Further reading ...........................................................................632

CHAPTER 22 Polymer nanocomposites for drink bottles .............. 633 22.1 22.2

22.3

22.4

Maricica Stoica Introduction.................................................................................634 Polymers for drink packaging ....................................................635 22.2.1 Common polymers used in drink packaging materials..........................................................................635 22.2.2 Polymer package-drink-environment potential interactions......................................................................636 Polymer nanocomposites ............................................................640 22.3.1 Inorganic nanoparticles...................................................640 22.3.2 Nanoparticles migration from PNCs ..............................644 Final considerations ....................................................................648 References...................................................................................649

CHAPTER 23 Powdered alcohol ..................................................... 657 S. Christina Joycee, S. Santhana Prabha, Suresh Jancyrani, S. Senthil Kumaran, B. Narayanaswamy, and Susai Rajendran 23.1 23.2

Production process......................................................................658 Recent developments on powdered alcohol...............................659 23.2.1 Sorghum grain tea-rich in phenolic compounds ............659

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23.3

23.2.2 Efficacy of the powder form of coconut inflorescence sap...........................................................659 23.2.3 Powder made of dried cranberry squash......................660 23.2.4 Caffeine level in home-made coffee liqueur ............... 660 23.2.5 Cereal liquor “Parshot” as a staple food in Dirashe special woreda, southern Ethiopia................................661 23.2.6 Detection of small differences in the acceptance of a new healthy beverage between males and females ...................................................................661 23.2.7 Effect of some beverages on the human dental enamel ...........................................................................661 23.2.8 Comparison of diet pills, powders, and liquids ........... 662 23.2.9 Spectrophotometric analysis of water, chili powder, chili sauce and tomato sauce samples..........................663 23.2.10 Adsorption of patulin from apple juice by inactivated yeast powder ..............................................663 23.2.11 Pyromellitic dianhydride as modified biosorbent by waste beer yeast powder .........................................663 23.2.12 Influence of Guardian Angel powder on blood alcohol level..................................................................664 23.2.13 Vinegar from Japanese liquor and the antioxidant activity ..........................................................................664 23.2.14 High amount of phenolics extracted from coconut shell by ultrasound assisted extraction technology.....................................................................665 23.2.15 Identification of carbohydrates, carboxylic acids, alcohols, and metals in foods .......................................665 23.2.16 Incalculable health consequences of alcopops in powder form..................................................................666 23.2.17 Alcohols in food and beverages ...................................666 23.2.18 Aqueous kava extracts and liver function....................666 Palcohol.......................................................................................667 References...................................................................................667

CHAPTER 24 Powdered wine ......................................................... 669

24.1

P. Shanthy, R. Joseph Rathish, J. Maria Pravina, B. Narayanaswamy, and Susai Rajendran Introduction.................................................................................670 24.1.1 Wine powder...................................................................670 24.1.2 Red wine powder and alcohol........................................670 24.1.3 Wine is more expensive .................................................670

Contents

24.2

24.1.4 Chaptalization ...............................................................671 24.1.5 Cheap wine makes one sick .........................................671 24.1.6 More expensive wine is better .....................................671 24.1.7 Aging of cheap wine ....................................................671 24.1.8 Sicking by a bad bottle of wine ...................................671 24.1.9 Wine and headache.......................................................671 24.1.10 Wine spoilage ...............................................................671 24.1.11 Benefits of wine............................................................672 Recent developments on powdered wine...................................672 24.2.1 Detection of micro- and nanoparticles in drinks and foods.......................................................................672 24.2.2 Detection of arsenic in wine and beer .........................672 24.2.3 Discoloration of red wine.............................................673 24.2.4 Processing technology of compound dandelion wine..............................................................674 24.2.5 Rice wine and palm wine of Cambodia.......................674 24.2.6 Opinions of males and females on new healthy beverage ...........................................................674 24.2.7 Detection of mycotoxin ................................................675 24.2.8 Korean traditional rice wine takju................................676 24.2.9 Schisandra wine ............................................................676 24.2.10 Detection of major and trace elements in food ...........676 24.2.11 Colored beverages and the color parameters of a resin composite ..........................................................678 24.2.12 Curcuminoid coloring principles in commercial foods..............................................................................678 24.2.13 Enhanced microbial production of organic acids ..............................................................................679 24.2.14 Cocoa-containing and chocolate products rank second after red wines..........................................679 24.2.15 Whey beer and whey wine ...........................................680 24.2.16 Improvement of wine bloom susceptibility .................680 24.2.17 Analysis of carbohydrates, carboxylic acids, alcohols, and metals in foods .......................................681 24.2.18 Alcohols in the food and beverages.............................681 24.2.19 Red wine does not reduce mature atherosclerosis in apolipoprotein E-deficient mice...............................681 24.2.20 Supplementation with wine phenolic compounds increases the antioxidant capacity of plasma...............682 24.2.21 Determination of inorganic anions and cations in wine ..........................................................................683

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24.2.22 Sprouting rice wine ......................................................683 24.2.23 Asbestos fibers in wine samples ..................................683 References...................................................................................684

CHAPTER 25 Instant beer ............................................................... 687 R. Betsy Clarebel, S. Charles Edison, A. Peter Pascal Regis, K. Kavi Priya, and Susai Rajendran 25.1 25.2 25.3 25.4 25.5

25.6 25.7

25.8

25.9

25.10

Introduction.................................................................................688 What is beer? ..............................................................................688 The classification of beer ...........................................................689 Chemistry of beer .......................................................................690 Positive and negative effects of beer .........................................690 25.5.1 Good effects....................................................................690 25.5.2 Bad effects ......................................................................690 Instant beer..................................................................................691 Attempt on instant beer production............................................691 25.7.1 Danish brewery invents instant craft-beer powder.............................................................................691 Preparation of instant beer..........................................................692 25.8.1 Tablet or powder for producing a carbonated beer beverage ..................................................................692 25.8.2 The popular methods that are used to make instant beer using maize supplied by South African farmers...............................................................695 25.8.3 The following methods illustrate the invention ............. 695 Nanomaterials for instant beer ...................................................699 25.9.1 Smart nanocontainers .....................................................699 25.9.2 Nanofiltration and active nanoenabled packaging......... 699 Conclusion ..................................................................................699 References...................................................................................700

Index ......................................................................................................................703

Contributors Muhammad Aamir Materials Laboratory, Department of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan Javeed Akhtar Materials Laboratory, Department of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan Sarah Alharthi Department of Chemistry, Taif University, Taif, Saudi Arabia V. Amalnath Computational Modeling and Nano Scale Processing Unit, Indian Institute of Food Processing Technology (IIFPT), Ministry of Food Processing Industries, Government of India, Thanjavur, Tamil Nadu, India A Anandan SKV Higher Secondary School, Namakkal, India C. Anandharamakrishnan Computational Modeling and Nano Scale Processing Unit, Indian Institute of Food Processing Technology (IIFPT), Ministry of Food Processing Industries, Government of India, Thanjavur, Tamil Nadu, India Cemil Aydog˘an Department of Food Engineering, Bing€ ol University, Bing€ol, Turkey Sandra Cristina Ballen Department of Food Engineering, URI Erechim, Erechim, Brazil Bapun Barik Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India A. Milton Franklin Benial Department of Physics, NMSSVN College, Madurai, Tamil Nadu, India Maria Joa˜o Carvalho Universidade Cato´lica Portuguesa, CBQF—Centro de Biotecnologia e Quı´mica Fina—Laborato´rio Associado, Escola Superior de Biotecnologia, Porto, Portugal Alfredo Cassano Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Rende, Italy Roberto Castro-Mun˜oz Monterrey Institute of Technology, Toluca, Mexico

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xxii

Contributors

Kemal C¸etin Department of Chemistry, Faculty of Science, Necmettin Erbakan University, Konya, Turkey A. Christy Catherine Mary Department of Chemistry, Parvathy’s Arts and Science College, Dindigul, India Bhawna Chugh Department of Chemistry, Netaji Subhas Institute of Technology, New Delhi, India R. Betsy Clarebel Department of Chemistry, St Joseph’s College, Tiruchirappalli, India Carmela Conidi Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Rende, Italy C elia Costa Universidade Cato´lica Portuguesa, CBQF—Centro de Biotecnologia e Quı´mica Fina—Laborato´rio Associado, Escola Superior de Biotecnologia, Porto, Portugal Eduardo M. Costa Universidade Cato´lica Portuguesa, CBQF—Centro de Biotecnologia e Quı´mica Fina—Laborato´rio Associado, Escola Superior de Biotecnologia, Porto, Portugal Maria Cris¸ an “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Bucharest, Romania Dorel Crișan “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Bucharest, Romania Priyabrat Dash Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India R. Dorothy Department of EEE, AMET University, Chennai, India Nicolae Dra˘gan “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Bucharest, Romania S. Charles Edison Department of Chemistry, St Joseph’s College, Tiruchirappalli, India Berta N. Estevinho LEPABE, Departamento de Engenharia Quı´mica, Faculdade de Engenharia da Universidade do Porto, Porto, Portugal Aslıhan G€ okaltun Department of Chemical Engineering, Hacettepe University, Ankara, Turkey

Contributors

S Gowri Corrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India Adriana Marcia Graboski Department of Food Engineering, URI Erechim, Erechim, Brazil Adelina-Carmen Ianculescu Department of Oxide Materials Science and Engineering, Politehnica University of Bucharest, Bucharest, Romania Suresh Jancyrani Department of Chemistry, MVM Government Arts College for Women, Dindigul, India Gautham B. Jegadeesan Biomass conversion and Bioproducts Laboratory, Center of Bioenergy, School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur, Tamil Nadu, India J. Jeyasundari PG and Research Department of Chemistry, SVN College, Madurai, India RM Joany Department of ECE, Sathyabama University, Chennai, India S. Christina Joycee Corrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India K. Kavi Priya Department of Chemistry, PSNA College of Engineering and Technology, Dindigul, Tamil Nadu, India K Sneka Latha Corrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India Janine Martinazzo Department of Food Engineering, URI Erechim, Erechim, Brazil Rozita M Moattari School of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Science and Technology, Iran University of Science and Technology (IUST), Tehran, Iran Toraj Mohammadi School of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Science and Technology; Research and Technology Centre of Membrane Separation Processes, School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran Mohebbat Mohebbi Department of Food Science and Technology, Ferdowsi University of Mashhad, Mashhad, Iran

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Contributors

J.A. Moses Computational Modeling and Nano Scale Processing Unit, Indian Institute of Food Processing Technology (IIFPT), Ministry of Food Processing Industries, Government of India, Thanjavur, Tamil Nadu, India B. Narayanaswamy Department of Science & Humanities, Jeppiaar Maamallan Engineering College, Sriperumbudur, India Pratap Sagar Nayak Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India V.R. Nazeera Banu Department of Chemistry, The American College, Madurai, India Fatemeh Sadat Nazeri Department of Food Science and Technology, College of Agriculture, Isfahan University of Technology, Isfahan, Iran Ines Nit¸oi National Research and Development Institute for Industrial Ecology, ECOIND, Bucharest, Romania Petrut¸a Oancea Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Bucharest, Romania Olalekan C. Olatunde Material Science Innovation and Modelling (MaSIM) Research Focus Area; Department of Chemistry, School of Physical and Chemical Sciences, Faculty of Natural and Agricultural Sciences, North-West University, Mafikeng Campus, Mmabatho, South Africa; Department of Industrial Chemistry, Ekiti State University, Ado Ekiti, Ekiti State, Nigeria Damian C. Onwudiwe Material Science Innovation and Modelling (MaSIM) Research Focus Area; Department of Chemistry, School of Physical and Chemical Sciences, Faculty of Natural and Agricultural Sciences, North-West University, Mafikeng Campus, Mmabatho, South Africa R. Parimaladevi Department of Physics, Mother Teresa Women’s University, Kodaikanal, Tamil Nadu, India Anand Babu Perumal Computational Modeling and Nano Scale Processing Unit, Indian Institute of Food Processing Technology (IIFPT), Ministry of Food Processing Industries, Government of India, Thanjavur, Tamil Nadu, India A. Peter Pascal Regis Department of Chemistry, St Joseph’s College, Tiruchirappalli, India

Contributors

Manuela Pintado Universidade Cato´lica Portuguesa, CBQF—Centro de Biotecnologia e Quı´mica Fina—Laborato´rio Associado, Escola Superior de Biotecnologia, Porto, Portugal V. Ponnusami Biomass conversion and Bioproducts Laboratory, Center of Bioenergy, School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur, Tamil Nadu, India S. Santhana Prabha PSNA College of Engineering and Technology, Dindigul, India J. Maria Pravina Corrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India P. Priyadharshini Computational Modeling and Nano Scale Processing Unit, Indian Institute of Food Processing Technology (IIFPT), Ministry of Food Processing Industries, Government of India, Thanjavur, Tamil Nadu, India Susai Rajendran Corrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India R. Joseph Rathish PSNA College of Engineering and Technology, Dindigul, India Faisal Rehman Department of Electrical Engineering, The Sukkur IBA University, Sukkur, Pakistan T.N. Rekha Department of Physics, Lady Doak College, Madurai, Tamil Nadu, India Olga Sacco Department of Chemistry and Biology “A. Zambelli”, University of Salerno, Fisciano, Salerno, Italy Diana Sannino Department of Industrial Engineering, University of Salerno, Fisciano, Salerno, Italy T Sasilatha Department of EEE, AMET University, Chennai, India S. Senthil Kumaran School of Mechanical Engineering, VIT University, Vellore, India Zahra Shabani School of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Science and Technology; Research and Technology Centre of Membrane Separation Processes, School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

xxv

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Contributors

Saravanan Ramiah Shanmugham Biomass conversion and Bioproducts Laboratory, Center of Bioenergy, School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur, Tamil Nadu, India P. Shanthy Department of Chemistry, Sri Meenakshi Government Arts College for Women, Madurai, India Hajar Shekarchizadeh Department of Food Science and Technology, College of Agriculture, Isfahan University of Technology, Isfahan, Iran Sara Silva Universidade Cato´lica Portuguesa, CBQF—Centro de Biotecnologia e Quı´mica Fina—Laborato´rio Associado, Escola Superior de Biotecnologia, Porto, Portugal Gurmeet Singh Pondicherry University, Puducherry, India Clarice Steffens Department of Food Engineering, URI Erechim, Erechim, Brazil Juliana Steffens Department of Food Engineering, URI Erechim, Erechim, Brazil Maricica Stoica Cross-Border Faculty, “Dunarea de Jos” University of Galati, Galati, Romania Khalid Hussain Thebo University of Chinese Academy of Sciences, Beijing, People’s Republic of China Ligia Todan “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Bucharest, Romania M. Umadevi Department of Physics, Mother Teresa Women’s University, Kodaikanal, Tamil Nadu, India Vincenzo Vaiano Department of Industrial Engineering, University of Salerno, Fisciano, Salerno, Italy Mariana Veiga Universidade Cato´lica Portuguesa, CBQF—Centro de Biotecnologia e Quı´mica Fina—Laborato´rio Associado, Escola Superior de Biotecnologia, Porto, Portugal Soheil Zarghami School of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Science and Technology; Research and Technology Centre of Membrane Separation Processes, School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

PART

Nanomaterials in water treatment

1

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CHAPTER

TiO2-based nanomaterials for wastewater treatment

1

R. Parimaladevia, M. Umadevia, T.N. Rekhab, A. Milton Franklin Benialc a

Department of Physics, Mother Teresa Women’s University, Kodaikanal, Tamil Nadu, India Department of Physics, Lady Doak College, Madurai, Tamil Nadu, India cDepartment of Physics, NMSSVN College, Madurai, Tamil Nadu, India

b

Chapter outline 1.1 Introduction ........................................................................................................ 3 1.2 Photocatalytic activity ......................................................................................... 4 1.3 Photocatalytic mechanism ................................................................................... 5 1.4 Photocatalyst TiO2 ............................................................................................... 6 1.5 Synthesis of TiO2 composites ............................................................................... 7 1.6 Structural studies ................................................................................................ 7 1.7 Vibrational analysis ............................................................................................. 9 1.8 Photocatalytic activity measurement .................................................................... 9 1.9 Photocatalytic investigation of TiO2 composites .................................................. 11 References .............................................................................................................. 21 Further reading ........................................................................................................ 24

1.1 Introduction Human civilization requires clean water for existence and growth. In fact, it is necessary not only for the mankind but also for all the other living beings. Because of extensive water pollution, the availability of clean drinking water has become a major problem in the recent years. Dye manufacturing and textile industries are the chief contributors to the problem of water pollution. Textile pigmentation and treatment processes contribute around 17%–20% to the global industrial water pollution. Approximately 7
105 tons of dyestuffs are produced worldwide annually, and 10%–15% of that is released into the environment during the process of synthesis and dyeing [1]. Textile wastewater is characterized by intense color, high chemical oxygen demand, high biooxygen demand, and fluctuating pH. The release of such untreated wastes into the water bodies is resulting in the destruction of aquatic life Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00001-8 # 2020 Elsevier Inc. All rights reserved.

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

and hence in turn disturbing the ecological balance while causing various diseases and health problems in human beings. Water purification methods are commonly classified into two broad categories: physical and chemical methods. Physical methods include boiling, filtration, sedimentation, distillation, desalination, reverse osmosis, and irradiation with UV light. Chemical method includes coagulation, flocculation, and chlorination. Physicochemical method is basically the photocatalytic degradation of dissolved water pollutants under irradiation and has become very popular over the past decade for wastewater treatment. As the degradation of water pollutants using light-absorbing materials under solar radiation is more economic and highly efficient way of wastewater treatment, this method is promising excellent solution to the pollution problem. The harmful organic dyes present in the wastewater can be easily removed through photocatalysis, utilizing the solar radiation. Since photocatalysis plays a vital role in addressing the environmental problem of water pollution, it has attracted the interest of the science community, and many studies are being carried out. The term photocatalyst is a combination of photochemistry and catalysis [2], and in the process, light and a catalyst are necessary to accelerate the chemical transformation.

1.2 Photocatalytic activity In the photocatalytic process a nontoxic, biologically and chemically stable, less expensive semiconductor is commonly used, which is capable of retrieval and extended use without loss of catalytic ability. This semiconductor is also capable of pollutant management and controls the contaminants in air under ambient conditions [3]. An advanced study intended in understanding and developing semiconductor photocatalysts was reported by Fujishima et al. in 1972 [4], illustrating simultaneous oxidation and reduction of water into O2 and H2 upon UV illumination of a TiO2 electrode in the aid of a small electrochemical bias. This significant discovery paved the way for extensive research works on the production of hydrogen from water as an alternate source of clean energy using sunlight. A semiconductor photocatalyst has two sets of closely spaced energy levels that form the valence and the conduction bands. The bandgap of a semiconductor is the difference in energy between the electrons populated valence band and the almost vacant conduction band. Bandgap energy is a measure of the wavelength of the light required to excite the semiconductor upon illumination. When subjected to bandgap excitation through illumination with light of energy greater than the bandgap, the semiconductor undergoes charge separation in which an electron is promoted from the valence band to the conduction band, leaving an electronic vacancy termed as a “hole” at the valence band edge [5]. Ideally, these excited charges should initiate further interfacial electron transfer or chemical reactions to its adsorbate, reactant, or the surface-bound hydroxyl group. However, since

1.3 Photocatalytic mechanism

most of the semiconductor photocatalysts are of nanosize to submicron size that introduces a high density of grain boundaries, coupled with the insufficient electron-hole pair lifetime and electron diffusion length, only a fraction ( 5%) of these charges can be accessed for redox reactions at the interface before they undergo recombination [6].

1.3 Photocatalytic mechanism For a semiconductor to be an active photocatalyst in the degradation of organic pollutants, the redox potential of the photogenerated holes should be sufficiently positive to generate the OH radicals. Further the redox potential of the photogenerated electrons must be sufficiently negative to be able to reduce the adsorbed O2 to superoxide. The redox potentials of the H2O/•OH and O2/O 2 couple together with the bandgap positions of semiconductors. The mechanism of semiconductor photocatalysis, for example, in TiO2, can be illustrated as follows: Excitation Trapping

Recombination

TiO2 ! e + h+ • h+ + OH! OH • + h + H2O! OH + H+ h+ + Rads !R+ads e + O2 !O 2 e + h+!heat

The photogenerated electron-hole pairs involve three major processes: (i) migration of photogenerated electrons and holes on the surface of semiconductor, (ii) capturing of the photogenerated electrons-holes on the semiconductor surface, and (iii) recombination or release of energy in the form of heat. The last two processes are not involved in photocatalytic reaction and are termed as deactivation processes. Only the photogenerated charges that reach surface of semiconductor will be available for photocatalytic reaction. The defect sites on the surface of the semiconductor act as the recombination centers for photogenerated carriers, which will decrease the efficiency of photocatalytic processes. Photocatalytic activity is mainly dependent on the efficient charge separation, which can be improved through various strategies. For example, synthesis of photocatalyst at high temperature will result in high crystallinity and reduction in the formation of defect-free sites. Formation of different kinds of nanostructures such as nanowires [7–10] and nanosheets [11–14] will also promote charge separation efficiency. Further the creation of junctions with built-in electric fields or chemical potential difference also decreases the charge recombination (Fig. 1.1).

5

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

FIG. 1.1 Photocatalytic mechanism of semiconductor oxide.

1.4 Photocatalyst TiO2 Until recently the heterogeneous semiconductor materials like ZnO [15], Fe2O3 [16], CdS [17], ZnS [18], TiO2 [19], SnO2 [20], WO3 [21], and LiNbO3 [22] are used as photocatalysts. The phase structure of TiO2 is one of the most important factors in determining the photocatalytic application. TiO2 occurs in nature in three different crystalline phases, anatase, brookite, and rutile, with rutile being the most abundant and thermodynamically stable phase. Rutile can be normally obtained from annealing the other two polymorphs at elevated temperatures. Yet, anatase exists as the most photoactive phase because of its improved charge-carrier mobility and the higher number of surface hydroxyl groups [23]. Heterogeneous photocatalysis using titanium dioxide is a safe, nonhazardous, and eco-friendly process, which does not produce any harmful by-products. Extensive studies using TiO2 had clearly indicated the necessity to retain the charge separation states in achieving higher photoconversion efficiencies. As a wide bandgap semiconductor photocatalyst (Eg  3.0 eV), TiO2 can absorb only the light in the UV region, and that is one of the biggest drawbacks of TiO2. The photocatalytic performances of a semiconductor can be improved by (i) extending the photoresponse of the UV-active semiconductor (particularly TiO2) into the visible region through bandgap engineering or introduction of photosensitizers on surface of semiconductor, (ii) suppressing excited electron-hole recombination, and (iii) promoting the forward reaction through localizing the reactants adjacent to the active sites. The strategies include coupling with a narrow bandgap semiconductor [24–27], metallic and nonmetallic doping [26, 27], dye sensitization [28, 29], depositing noble metal nanoparticles [30–32], and hybridized structure with carbonaceous materials [33–39]. At present, significant attention is being given toward the use of carbonaceous nanomaterials, such as activated carbon [40, 41] and carbon nanotubes (CNTs) [42–46]. Fullerene [47] and graphene [48] have been recently utilized with TiO2 to form hybrids for enhanced photocatalytic activities of semiconductors.

1.6 Structural studies

Incorporating carbon nanostructures in composites of photocatalysts not only suppresses charge recombination but also facilitates a hydrophobic microenvironment for localization of concentrated reactant neighboring the active sites [49]. Recent investigations have also indicated that the graphitic carbon, when it forms an effective complex interface with TiO2, can increase photocatalytic activity [50] or act as a sensitizer [51] or as impurity energy levels through the formation of TidOdC bonds [52] to extend the light absorption region. Thus the carbonaceous materials exhibit great potential in fulfilling all the three strategies in improving photocatalytic activities.

1.5 Synthesis of TiO2 composites Analar grade TiO2 is to be powdered initially and heated to 500°C for 5 h in a muffle furnace. Fluorine doped TiO2 can be synthesized by mixing 1 wt% of TiO2 with ammonium fluoride (2 wt%). The mixed salt is further grained to powder and is heated at 500°C for 5 h. Subsequent to the heating the powder will turn yellow in color. Carbon nanocone and disc composites added with fluorine doped TiO2 are synthesized by mixing 3 wt% of composites of carbon nanocones and disc added with fluorine doped TiO2 of 2 wt%. The resultant salt is then grinded to powder and heated at 500°C for 5 h. After heating the powder will turn into gray color. Multiwalled carbon nanotubes added with fluorine doped TiO2 composites are further synthesized by mixing 3 wt% of composites of multiwalled carbon nanotubes with fluorine doped TiO2 of 2 wt%. The mixed salt is then grinded to powder and is heated at 500°C for 5 h. Subsequent to heating the powder will turn to gray in color.

1.6 Structural studies XRD patterns were used to determine the crystalline forms of the samples. Fig. 1.2(i) shows the XRD pattern of undoped TiO2 (PT), carbon cone and disc-doped TiO2 (CCDT), fluorine doped TiO2 (FT), and composite of carbon nanocone and discfluorine-codoped TiO2 (CCDFT) nanocomposites. Fig. 1.2(ii) shows XRD patterns of PT, MWCNTsT, and MWCNTs-FT nanocomposites. All the samples exhibit eight distinct peaks (2θ ¼ 25.4, 36.9, 37.9, 38.7, 48.1, 54.0, 55.2, and 62.9 degree), which represent tetragonal structure with the indices of (101), (103), (004), (112), (200), (105), (211), and (213) planes of anatase TiO2. The diffraction peaks of the doped samples are slightly broader and stronger than pure TiO2. As expected the broad diffraction peak in the doped samples was also observed, which can be attributed to the decrease in crystalline size with the destruction of crystalline structure. Moreover, the doped carbon and fluoride atoms do not cause any shift in peak positions of TiO2. Furthermore, it is indicated that the doped TiO2 samples exhibited typical structure of TiO2 crystal without any detectable dopant related peaks. The reason could be due to the fact that the doped carbon and fluorine have moved into either the

7

(105) (211)

(200)

(112)

(004)

(103)

(D)

Intensity (a.u.)

(101)

CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

(C)

(B)

(A) 20 (i)

40

60

(105) (211)

(200)

(004)

2θ (degree) (101)

(D)

(C) Intensity (a.u.)

8

(B)

(A) 20 (ii)

40 2θ (degree)

60

FIG. 1.2 (i) XRD patterns of (A) PT, (B) CCDT, (C) FT, and (D) CCDFT. (ii) XRD patterns of (A) PT, (B) FT, (C) MWCNTsT, and (D) MWCNTsFT. Part (i) reprinted with permission from M. Sangari, M. Umadevi, M. Jeyanthinath, K. Anitha, J. P. Pinheiro, Mater. Sci. Semicond. Process. 31 (2015) 543 and (ii) from M. Sangari, M. Umadevi, M. Jeyanthinath, J.P. Pinheiro, Spectrochim. Acta A, 139 (2015) 290.

1.8 Photocatalytic activity measurement

interstitial positions or the substitutional sites of the TiO2 crystal structure. Moreover nonmetal doping could slightly restrain the growth of the crystallite, which might be due to the creation of surface oxygen vacancies; these oxygen atoms directly involved and improved in the photocatalysis activity [53].

1.7 Vibrational analysis FTIR measurements of PT, CCDT, FT and CCDFT were recorded in the range of 400–4000 cm1, and the corresponding results are shown in Fig. 1.3(i). The peak at 3200–3600 cm1 is attributed to the stretching vibration of the hydroxyl group [54]. The peak at 400–700 cm1 is due to TidO and TidOdTi bridging stretching vibration [55]. A weak peak corresponding to the vibration of surface dOH group is observed over the PT, CCDT, and FT. However, relatively strong peaks of dOH group appear over the CCDFT samples suggesting the presence of rich surface dOH groups. The peak at 702 cm1 corresponds to TidF vibration, and that confirms that the fluorine atoms were incorporated into the TiO2 crystal lattice. Further, it is noted that the hydroxyl groups are broader in the CCDFT composites and can lead to the enhancement of the photocatalytic activity. The hydroxyl groups on the surface contribute to the improvement of the photocatalytic activity by their interactions with photogenerated holes that support better charge transfer by inhibiting the recombination of electron-hole pairs [56]. Therefore an improvement in photocatalytic activity is expected over the doped TiO2 composites because of the presence of several surface dOH groups. FTIR measurements of PT, FT, MWCNTsT, and MWCNTs-FT were recorded in the range of 400–4000 cm1, and the corresponding results are shown in Fig. 1.3(ii). The figure shows the effect of introducing MWCNTs, F, and MWCNTs-F to the TiO2 doping. In the MWCNTS-FT spectrum the band 3217 cm1 is attributed to OdH stretching. The peak at 748 cm1 is due to TidF vibration. The peak at 1124 cm1 is associated with CdO stretching vibrations band of MWCNTs, which may result from the strong interaction between TiO2 nanoparticles. The MWCNTsFT/MWCNTsT composite spectrum is sharper than that of pure and fluorinated doped TiO2, which may be attributed to the changing of crystallite sizes and shape.

1.8 Photocatalytic activity measurement The photodegradation of MO was employed to evaluate the photocatalytic activity. The photocatalytic activity of composites of carbon nanocones and disc-fluorine codoped TiO2 and multiwalled carbon nanotube-fluorine codoped TiO2 was studied in the photocatalytic degradation of methyl orange in aqueous solution under UV irradiation (254 nm) and visible light irradiation. In this experiment, 0.0125 mM of composites of TiO2 was added into 10 mL of MO (0.0327 g). Experiments were performed at different UV and visible light irradiation time.

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

FIG. 1.3 (i) FTIR spectra of (A) PT, (B) CCDT, (C) FT, and (D) CCDFT. (ii) FTIR spectra of (A) PT, (B) FT, (C) MWCNTsT, and (D) MWCNTsFT. Part (i) reprinted with permission from M. Sangari, M. Umadevi, M. Jeyanthinath, K. Anitha, J. P. Pinheiro, Mater. Sci. Semicond. Process. 31 (2015) 543 and (ii) from M. Sangari, M. Umadevi, M. Jeyanthinath, J.P. Pinheiro, Spectrochim. Acta A, 139 (2015) 290.

1.9 Photocatalytic investigation of TiO2 composites

1.9 Photocatalytic investigation of TiO2 composites The photocatalytic activities of pure TiO2 (PT), pure composites of carbon nanocones and disc (PCCD), composites of carbon nanocones and disc-doped TiO2 (CCDT), fluorine doped TiO2 (FT), and composites of carbon nanocone and discfluorine codoped TiO2 (CCDFT) were measured by the degradation of methyl orange (MO) aqueous solution under UV (254 nm) and visible light irradiation. Fig. 1.4 presents the optical absorption spectra of MO with PT, PCCD, CCDT,

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.5

1.0

0.5

300

400

500

600

700

1.0

0.5

0.0

(A)

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.5 Absorbance(a.u.)

Absorbance(a.u.)

2.0

(B)

0.0 300

400

Wavelength(nm) 1.6

600

700

1.6

1.4

1.0 0.8 0.6 0.4

Absorbance(a.u.)

1.4 Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.2 Absorbance(a.u.)

500 Wavelength(nm)

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.2 1.0 0.8 0.6 0.4 0.2

0.2 0.0 0.0

(C)

300 300

400

500

600

700

400

(D)

500 Wavelength(nm)

600

700

Wavelength(nm) 1.6

Absorbance(a.u.)

1.4

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.2 1.0 0.8 0.6 0.4 0.2 0.0

(E)

300

400

500

600

700

Wavelength (nm)

FIG. 1.4 Optical absorption spectra of MO with (A) pure TiO2, (B) pure composites of carbon nanocones and disc, (C) composites of carbon nanocones and disc-doped TiO2, (D) fluorine doped TiO2, and (E) composites of carbon nanocone and disc-fluorine codoped TiO2 in different UV light irradiation time.

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

FT, and CCDFT in different UV irradiation times. The optical absorption spectra indicate that the intensity of the absorption is decreasing gradually with time, inferring degradation of methyl orange during the photocatalytic reaction under UV irradiation. This probably implies that the electrons in the excited states of MO would not shift to the conduction band of TiO2. MO solution has been suggested to be decomposed by hydroxyl radicals •OH through TiO2 catalysis under UV light excitation. The variation in photocatalytic degradation of dye with reaction time is determined using the linear transforms equation of the first order: ln ðAt =A0 Þ ¼ Kapp

where A0 and At are the absorbance value of MO at t ¼ 0 and t, respectively, and k is the pseudo-first order rate constant. Fig. 1.5 shows the photocatalytic reaction kinetics of MO degradation using PT, PCCD, CCDT, FT, and CCDFT versus irradiation time. The Kapp value of PT, PCCD, CCDT, FT, and CCDFT are 0.09, 0.14, 0.15, 0.17, and 0.2 min1, respectively. CCDT, FT, and CCDFT photocatalysts have good stability in photocatalytic reactions. F ions play a very significant role in the catalytic process. The photocatalytic activity of fluorine doped TiO2 has been enhanced, and the formation of oxygen vacancies has also been improved. This oxygen is directly involved in the photocatalysis processes. Although fluorine doping gives an adverse effect on the textured property of TiO2, causing a small variation in crystalline size, much higher activity is obtained over the fluorine doped TiO2. Therefore, in this case, the separation

FIG. 1.5 Rate of decomposition of MO uses UV light with PT, PCCD, CCDT, FT, and CCDFT. Reprinted with permission from M. Sangari, M. Umadevi, M. Jeyanthinath, K. Anitha, J. P. Pinheiro, Mater. Sci. Semicond. Process. 31 (2015) 543.

1.9 Photocatalytic investigation of TiO2 composites

efficiency of the photogenerated electrons and holes and the physical-chemical property of the catalysts might become the key factors to cause an increase in the photocatalytic degradation rate. It is found that the photocatalytic activity of CCDFT was higher than that of PT, PCCD, CCDT, and FT, under UV irradiation. CCDFT nanocomposites can provide higher activity, due to their small crystalline size. Carbon can form a new state that lies just above the valence band, which could enhance surface hydroxyl group density of TiO2 and make the catalyst to absorb UV efficiently. In both cases pure TiO2 nanoparticles exhibit low decomposition rate as compared with PCCD, CCDT, FT, and CCDFT clearly indicating that these carbonaceous species lead to new active sites, which are responsible for the high photocatalytic activity. Thus the higher photocatalytic activity of CCDFT may be attributed to the buildup of carbonaceous species rather than the excitation of the intrinsic absorption band of the bulk TiO2. Further, it is also identified that it has enhanced adsorbent activity than PT, PCCD, and CCDT, as the methyl orange is absorbed onto the photocatalyst more quickly resulting in a higher degradation rate. It can also capture the photogenerated holes (h+) and transform into •OH radicals, which are the main reactive species responsible for the decomposition of organic molecules. Therefore the carbonate species and fluorinated TiO2 can produce more •OH radicals, which greatly improve the photocatalytic degradation rate. The codoping with carbon and fluorine improves the photocatalytic efficiency of TiO2, promotes the separation of the photogenerated electrons and holes, and accounts for the improved photocatalytic oxidizing species. The optical absorption spectra of MO with PT, PCCD, CCDT, FT, and CCFT in different visible light irradiation time are provided in Fig. 1.6. Fig. 1.7 shows the photocatalytic reaction kinetics of MO degradation using PT, PCCD, CCDT, FT, and CCDFT versus visible light irradiation time, respectively. The Kapp value of PT, PCCD, CCDT, FT, and CCDFT are 0.013, 0.015, 0.017, 0.03, and 0.07 min1, respectively. It was found that the photocatalytic activity of CCDFT was higher than that of PT, PCCD, CCDT, and FT under visible light irradiation. The observed values indicate that the CCDFT has a highest decomposition rate in UV irradiation than visible light irradiation. Zhang et al. [57] reported that the photocatalytic activity increases with decrease in particle size and the XRD result also confirmed the enhanced photocatalytic activity of CCDF-doped TiO2 nanoparticles. The FTIR study reveals that the carbon nanocones and disc-fluorine co-doped TiO2 nanocomposites have a large amount of hydroxyl group on the surface. The hydroxyl groups on the particle surface contribute to the improvement of the photocatalytic activity by their interactions with photogenerated holes resulting in better charge transfers, which inhibit the recombination of electron-hole pairs. The observed higher photocatalytic activity of carbon nanocones and disc-fluorine codoped TiO2 nanocomposites was further supported by FTIR study. This CCDFT shows higher photocatalytic activity, which may be due to smaller particle size, crystalline anatase phase, cubic morphology, and intense absorption in the UV light radiation.

13

CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

Absorbance(a.u.)

1.0

0.5

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.5 Absorbance(a.u.)

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.5

1.0

0.5

0.0

(A)

300

400

500

600

0.0

700

(B)

Wavelength(nm)

300

400

1.8 Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.4 1.2 1.0 0.8 0.6 0.4

500 Wavelength(nm)

600

700

1.6

Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.4 Absorbance(a.u.)

1.6

Absorbance(a.u.)

1.2 1.0 0.8 0.6 0.4 0.2

0.2

0.0

0.0 300

(C)

400

500

600

300

700

400

(D)

Wavelength(nm)

500

600

700

Wavelength(nm)

1.6 1.4 Pure MO 0min. 10mins. 20mins. 30mins. 40mins. 50mins. 60mins. 70mins. 80mins. 90mins. 100mins. 110mins. 120mins.

1.2 Absorbance(a.u.)

14

1.0 0.8 0.6 0.4 0.2 0.0

(E)

300

400

500

600

700

Wavelength(nm)

FIG. 1.6 Optical absorption spectra of (A) MO with pure TiO2, (B) pure composites of carbon nanocones and disc, (C) composites of carbon nanocones and disc-doped TiO2, (D) fluorine doped TiO2, and (E) composites of carbon nanocone and disc-fluorine codoped TiO2 in different visible light irradiation time.

The photocatalytic activities of multiwalled carbon nanotube-doped TiO2 and multiwalled carbon nanotube-fluorine codoped TiO2 nanocomposites were measured by the degradation of methyl orange (MO) aqueous solution under UV light irradiation (254 nm). The optical absorption spectra of MO with pure, fluorine doped, and multiwalled carbon nanotube-fluorine codoped TiO2 were recorded in different irradiation times (Fig. 1.8). The optical absorption spectra show that the intensity of the absorption is decreasing gradually with the increase

1.9 Photocatalytic investigation of TiO2 composites

FIG. 1.7 Rate of decomposition of MO uses visible light with PT, PCCD, CCDT, FT, and CCDFT. Reprinted with permission from M. Sangari, M. Umadevi, M. Jeyanthinath, K. Anitha, J. P.k Pinheiro, Mater. Sci. Semicond. Process. 31 (2015) 543.

in time. This indicates that methyl orange is suffering degradation during the photocatalytic reaction under UV light irradiation. This probably implies that the electrons in the excited states of MO would not shift to the conduction band of TiO2. MO solution has been suggested to be decomposed by hydroxyl radicals •OH through TiO2 catalysis under UV light excitation. The optical absorption spectra of pure and fluorine doped TiO2 nanoparticles are reported in our article [56]. Fluorine doped TiO2 exhibited a clear absorption peak than pure TiO2 nanoparticles in its UV spectrum. Fig. 1.9 shows the photocatalytic reaction kinetics of MO degradation by using pure, multiwalled carbon nanotube-doped TiO2 and multiwalled carbon nanotubefluorine codoped TiO2 nanocomposites versus irradiation time, respectively. The Kapp values of MWCNTsT and MWCNTs-FT nanocomposites are 0.10 and 0.20 min1, respectively. It is found that the photocatalytic activity of MWCNTsFT nanocomposites is higher than that of the other doped TiO2 under UV light irradiation. MWCNTs-FT nanocomposites have better photocatalytic activity than the previously reported fluorine doped TiO2 nanoparticles. MWCNTs-FT nanocomposites have a higher surface area and can possibly prevent recombination of electron + (e cb) and hole (hvb) pairs. MWCNTs-FT nanocomposites can provide more activity species, due to small crystalline size, which accounts for the improved photocatalytic activity. Multiwalled carbon nanotube-fluorine codoped TiO2 nanocomposites can

15

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

FIG. 1.8 Optical absorption spectra of MO with (A) multiwalled carbon nanotubes, (B) multiwalled carbon nanotube-doped TiO2, and (C) multiwalled carbon nanotube-fluorine codoped TiO2 in different UV light irradiation time.

1.9 Photocatalytic investigation of TiO2 composites

FIG. 1.9 Rate of decomposition of MO using UV light with pure MWCNTS, MWCNT doped TiO2, multiwalled carbon nanotube-fluorine codoped TiO2. Reprinted with permission from M. Sangari, M. Umadevi, M. Jeyanthinath, J.P. Pinheiro, Spectrochim. Acta A, 139 (2015) 290.

capture the photogenerated holes (h+) and transform to •OH radicals, which are the main reactive species for the decomposition of organic molecules. Therefore the multiwalled carbon nanotubes and fluorine codoped TiO2 nanocomposites produce more • OH radicals, which greatly improve the photocatalytic degradation rate. It obviously indicates that the multiwalled carbon nanotube-fluorine codoped TiO2 nanocomposites have enhanced photocatalytic activity and act as an adsorbent, dispersing agent, and electron reservoir facilitating the separation of the photogenerated electron-hole pairs at the TiO2/MWCNT interface leading to the faster rates of photocatalytic oxidation. The optical absorption spectra of MO with MWCNTsT and MWCNTs-FT in different visible light irradiation times were recorded (Fig. 1.10). Fig. 1.11 shows the photocatalytic reaction kinetics of MO degradation using MWCNTsT and MWCNTs-FT nanocomposites versus visible light irradiation time, respectively. The Kapp values of MWCNTsT and MWCNTs-FT nanocomposites are 0.01 and 0.1 min1, respectively. It was found that the photocatalytic activity of MWCNTsFT nanocomposites was higher than that of the other doping composites under visible light irradiation (Fig. 1.12).

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

FIG. 1.10 Optical absorption spectra of MO with (A) multiwalled carbon nanotubes, (B) multiwalled carbon nanotube-doped TiO2, and (C) multiwalled carbon nanotube-fluorine codoped TiO2 in different visible light irradiation time.

1.9 Photocatalytic investigation of TiO2 composites

3.0 MWCNTS MWCNTS doped TiO2 MWCNTS- F-co-doped TiO2

2.8 2.6 2.4

Equation y = a+b*x Adj. R-Squa 0.92075 0.9350 0.93524 Value Standard Err 0.03954 Intercept 0.3222 B 0.0066 5.59163E–4 Slope B 0.06826 Intercept 0.0208 C 0.0127 9.65359E–4 Slope C 0.10675 Intercept 0.4946 D 0.00151 0.0199 Slope D

2.2

InAt/A0

2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0

20

40

60 Time(mins)

80

100

120

FIG. 1.11 Rate of decomposition of MO using visible light with pure MWCNTs, MWCNT-doped TiO2, and multiwalled carbon nanotube-fluorine codoped TiO2. Reprinted with permission from M. Sangari, M. Umadevi, M. Jeyanthinath, J.P. Pinheiro, Spectrochim. Acta A, 139 (2015) 290.

UV light irradiation of multiwalled carbon nanotubes-fluorine codoped TiO2 nanocomposites has a higher decomposition rate than that of the visible light irradiation (Fig. 1.12). UV light source was used for the application of heterogeneous photocatalysis. The FTIR study reveals that the multiwalled carbon nanotube-fluorine codoped TiO2 nanocomposites have a large amount of hydroxyl group on the surface. The hydroxyl groups on the particle surface contribute to the improvement of the photocatalytic activity by their interactions with photogenerated holes giving better charge transfers inhibiting the recombination of electron-hole pairs. The observed higher photocatalytic activity of multiwalled carbon nanotube-fluorine codoped TiO2 nanocomposites was further supported by FTIR study [58]. This multiwalled carbon nanotube-fluorine codoped TiO2 nanocomposites show higher photocatalytic activity, which may be due to the smaller particle size, anatase phase, tube morphology and intense absorption, dispersing agent, and electron reservoir in the UV light radiation. Thus the application of emerging multiwalled carbon nanotube-fluorine codoped TiO2 in wastewater treatment can create tremendous opportunities to provide healthy living environment for the generations to come.

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CHAPTER 1 TiO2-based nanomaterials for wastewater treatment

FIG. 1.12 Rate of decomposition versus different sample under (A) UV light irradiation and (B) visible light irradiation.

References

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[52] B. Qiu, Y. Zhou, Y. Ma, X. Yang, W. Sheng, M. Xing, J. Zhang, Facile synthesis of the Ti3+ self-doped TiO2-graphene nanosheet composites with enhanced photocatalysis, Sci. Rep. 5 (2015) 8591. [53] M. Umadevi, R. Parimaladevi, M. Sangari, Synthesis, characterization and Photocatalytic activity of fluorine doped TiO2 nanoflakes synthesized using solid state reaction method, Spectrochim. Acta A Mol. Biomol. Spectrosc. 120 (2014) 365–369. [54] X. Li, H. Zhang, X. Zheng, Z. Yin, L. Wei, Visible light responsive N-F-codoped TiO2 photocatalyst for the degradation of 4-chlorophenol, J. Environ. Sci. 23 (2011) 1919–1924. [55] C. Yu, J.C. Yu, M. Chan, Sonochemical fabrication of fluorinated mesoporous titanium dioxide microspheres, J. Solid State Chem. 182 (2009) 1061–1069. [56] M. Umadevi, M. Sangari, R. Parimaladevi, A. Sivanantham, J. Mayandi, Enhanced photocatalytic, antimicrobial activity and photovoltaic characteristics of fluorine doped TiO2 synthesized under ultrasound irradiation, J. Fluor. Chem. 156 (2013) 209–213. [57] Z. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, Role of particle size in nanocrystalline TiO2-based photocatalysts, J. Phys. Chem. B 102 (1998) 10871–10878. [58] M. Sangari, M. Umadevi, M. Jeyanthinath, J.P. Pinheiro, Photocatalytic degradation and antimicrobial applications of F-doped MWCNTs/TiO2 composites, Spectrochim. Acta A 139 (2015) 290–295.

Further reading [59] P.S.M. Dunlop, J.A. Byrne, N. Manga, B.R. Eggins, The photocatalytic removal of bacterial pollutants from drinking water, J. Photochem. Photobiol. A Chem. 148 (2002) 355–363. [60] W.J. Huang, G.C. Fang, C.C. Wang, The determination and fate of disinfection by-products from ozonation of polluted raw water, Sci. Total Environ. 345 (2005) 261–272. [61] H.J. Kool, C.F. Keijl, J. Hrubec, Water Chlorination: Chemistry, Environmental Impact and Health Effects, Lewis, Chelsea, MI, USA, 1985. [62] W.J. Masschelin, Ultraviolet Light in Water and Wastewater Sanitation, Lewis, Boca Raton, FL, USA, 2002. [63] J.M.C. Robertson, P.K.J. Robertson, L.A. Lawton, A comparison of the effectiveness of TiO2 photocatalysis and UV photolysis for the destruction of three pathogenic microorganisms, J. Photochem. Photobiol. A Chem. 175 (2005) 51–56. [64] M. Sangari, M. Umadevi, M. Jeyanthinath, K. Anitha, J.P. Pinheiro, Photocatalytic and antimicrobial activities of TiO2-carbon nano-cones and disc composites, Mater. Sci. Semicond. Process. 31 (2015) 543–550.

CHAPTER

Groundwater treatments using nanomaterials

2

Saravanan Ramiah Shanmugham, Gautham B. Jegadeesan, V. Ponnusami Biomass conversion and Bioproducts Laboratory, Center of Bioenergy, School of Chemical & Biotechnology, SASTRA Deemed University, Thanjavur, Tamil Nadu, India

Chapter outline 2.1 Nanotechnology in groundwater treatment .......................................................... 25 2.2 Adsorption using nanomaterials ......................................................................... 26 2.2.1 Nano-zerovalent iron treatment .........................................................27 2.2.2 Carbonaceous materials ...................................................................29 2.2.3 Metal oxides ....................................................................................34 2.2.4 Metal-organic framework ..................................................................36 2.2.5 Adsorption kinetics and equilibrium ..................................................37 2.3 Photocatalysis .................................................................................................. 38 2.4 Implementation of nanotechnology for water treatment ........................................ 40 2.4.1 Ex situ ............................................................................................40 2.4.2 In situ ............................................................................................41 2.5 Health implications on the use of nanotechnology in groundwater treatment ......... 41 References .............................................................................................................. 42

2.1 Nanotechnology in groundwater treatment Population growth and industrialization, use of pesticides and fertilizers, and discharge of domestic and industrial wastes had led to contamination of groundwater with various chemicals. The groundwater contaminants can be classified as organic (e.g., pesticides like carbofuran, dibromochloropropane, endothall, glyphosate, oxamyl, picloram, lubricants, solvents, and petrochemical residues), inorganic (e.g., heavy metals like lead, arsenic, chromium, mercury, cadmium, copper, and zinc), and radioactive contaminants (e.g., uranium and radium) [1]. Recently, pharmaceuticals (e.g., ciprofloxacin, sulfamethoxide, ibuprofen, diclofenac, and naproxen), personal care products (PCPs), and endocrine-disrupting compounds (EDCs) are also found in groundwater supplies [2, 3]. Since the effect of pharmaceuticals, PCPs, and Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00002-X # 2020 Elsevier Inc. All rights reserved.

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CHAPTER 2 Groundwater treatments using nanomaterials

EDCs is not clearly understood, these are known as “contaminants of emerging concern” or “emerging contaminants” [4]. It has been reported that in United States about 90% of the oral drugs administered ultimately find their way to water bodies. Similar results have been seen in India as well [5]. Lack of sufficient rain, nonavailability of surface water, and overdependence of groundwater for human consumption further aggravate the issue as all of these lead to concentration of the pollutants in the groundwater. In many places the concentrations of pollutants have gone beyond specified safe limits. In the recent past, there is a growing interest on nanotechnological solutions for remediation of contaminated soil and groundwater. Nanotechnology deals with phenomena or structures at the nanometer scale. The US NNI defines it as follows: “nanotechnology is the understanding and control of matter at dimensions of roughly 1 to 100 nanometers.” It is fairly well established that nanomaterials show intermediate behavior between a macroscopic solid and an atomic (or molecular) system. Controlling materials at the nanolevel can accelerate the production of nanomaterials with improved properties and functionalities. As a result, nanotechnology is revolutionizing our ability to tackle problems in the medical, manufacturing, chemical, military, space exploration, human hygiene, energy production, water purification, and other fields. Various promising applications of such engineered nanomaterials have already been reported for cosmetics, biomaterials, solar cells, aerospace materials, and environmental catalysts [6]. In the area of groundwater treatment, various nanomaterials such as graphene sheets (for contaminant adsorption), photocatalytic metal oxides, zerovalent metals, or nanosilver-coated membranes (for disinfection) have been developed for the removal of organic, inorganic, and radioactive contaminants from water. Some of these technologies have been commercialized, but many are still in their developmental stages. The mechanisms with which the nanomaterials are able to remove groundwater contaminants are via adsorption or catalysis (i.e., degradation of the contaminant to alternate, nontoxic forms). This chapter discusses the use of such nanomaterials for groundwater treatment with focus on specific organic and inorganic contaminants.

2.2 Adsorption using nanomaterials Numerous nanosize adsorbents had been examined in the recent past for the removal of heavy metals. This includes carbonaceous adsorbents (e.g., activated carbon, carbon nanotubes [CNTs], and graphene) and meal oxides (Fe3O4, MnO2, TiO2, MgO, etc.). Major advantages of nanosorbents over other conventional adsorbents are as follows: (i) They possess high specific surface area (m2/g) and (ii) their surface can be functionalized to improve their affinity toward specific contaminants such as metal ions. Additionally, some of these materials are also environmentally benign, have high selectivity toward target contaminant, and are easily recyclable [7].

2.2 Adsorption using nanomaterials

2.2.1 Nano-zerovalent iron treatment Among the various adsorbents/catalysts developed till date, nano-zerovalent iron (nZVI) systems are extensively used as they offer efficient contaminant removal for a variety of contaminants, compared with traditional adsorption-based techniques at low costs. When nZVI is exposed to water and oxygen, corrosion of iron occurs as Fe0 is converted to Fe2+/Fe3+, its most preferred state [8, 9]. Metal corrosion is an electrochemical process in which the reduced form of metal gets oxidized to form more stable species. During the corrosion process of metallic iron, electrons are given up along with the formation of ferrous ions. While the electrons released are available for reduction of oxidized species such as some organic species, etc., ferrous ions undergo precipitation reactions. The iron hydroxides thus formed will subsequently transform to form stable iron oxides, which act as a suitable adsorbent for contaminant removal. Groundwater remediation using zerovalent iron (Fe0) has been used to effectively degrade organic contaminants and adsorb oxyanions such as selenium, arsenic, and chromium [10–18]. Chromium exists in its oxyanionic form in aqueous solutions, Cr 3+ (VI) and Cr(III), as Cr2O2
7 and Cr , respectively. Chromium concentrations up to 370 μg/L have been detected in Indian rivers, with far higher concentrations in groundwater. The primary source of chromium contamination is the indiscriminate discharge of untreated leather/tanning wastewater discharges, resulting in high concentration of Cr(VI) in groundwater and surface water. Laboratory studies have demonstrated that elemental iron can effectively remove chromium, that is, hexavalent and trivalent chromium, from solution. Upon exposure to water and oxygen, corrosion of iron occurs, and electrons are given up along with the formation of ferrous ions. While the electrons released are available for reduction of oxidized species, ferrous ions undergo precipitation reactions to form iron oxyhydroxides. The general mechanism is the reduction of Cr(VI) to Cr(III) upon oxidation of Fe to Fe(II), followed by its precipitation as iron hydroxychromite [12–14, 16–18]. Table 2.1 highlights some of the work done till date on chromium removal using nZVI, with and without supports. nZVI has also been effective in removing other oxyanions such as arsenic and selenium from groundwater. Arsenic removal is usually via adsorption/coprecipitation on the oxidized Fe0. Arsenic, either as arsenic (III) or arsenic (V), adsorbs onto iron oxides via a ligand exchange mechanism (formation of a monodentate or bidentate complex) [29, 30]. Though it has been theorized that As(V) to As(III) can happen thermodynamically, evidence of the process in the presence of Fe is lacking. The pathways for iron-mediated reactions involve the following: (1) spontaneous oxidation of metallic iron in aqueous solutions leading to the generation of protons (H +) and electrons and (2) direct electron transfer from the metal to the surface. At neutral pH the reduction of arsenate to As0 by Fe0 has also been ideated but only seen in electrochemical studies. Table 2.2 highlights some of the work done till date on arsenic removal using nZVI (nano-zerovalent iron), with and without supports.

27

28

CHAPTER 2 Groundwater treatments using nanomaterials

Table 2.1 Summary on chromium removal using nZVI. Sl no. 1.

Material

2.

Acidic cation exchange resin-nZVI Sepiolite-supported nZVI

3.

Humus-supported nZVI

4.

Chitosan-stabilized nZVI

5.

Activated carbonsupported nZVI/Ag bimetallic Agar-nZVI

6.

7. 8. 9.

10.

Silica fume-supported nZVI Multiwalled carbon nanotube nZVI nZVI/Fe3O4-embedded polyvinyl alcohol/sodium alginate (nZVI) assembled on magnetic Fe3O4/ graphene

Chromium removal efficiency (%)

Capacity

Reference

84.4%

41.3 mg/g

[19]

98.5% (pH 4.0) 30.7% (pH 9.0) 99.9% (8 min, Si ¼ 40 mg/L) 30.0% (3 min, Si ¼ 200 mg/L) 74.3% (180 min, Si ¼ 40 mg/L) 97.5% (60 min, Si ¼ 4 mg/L pH 3) 100% (2 h, Si ¼ 50 mg/L, pH 3) 88% (2 h, Si ¼ 40 mg/L) 100% (pH 5)

43.6 mg/g

[20]

40.4 mg/g

[21]

148.06 mg/g

[22]

100 mg/g

[23]

NA

[24]

NA

[25]

NA

[26]

99.3% (pH 3)

50 mg/g

[27]

83.8% (pH 3.0)

101.0 mg/g

[28]

NA, data not available.

Previous research has established the effectiveness of zerovalent iron particles in Se remediation as well. Table 2.3 highlights some studies on selenium removal using nZVI, with the mechanism similar to that for arsenic and chromium removal. However, as the reaction/adsorption process progresses, the metal becomes passive if it substantially resists corrosion in a given environment. The surface passivation is from the buildup of an oxide coating that limits the transport of corrosion products from the bulk iron to the surface and the transport of corrosive components from the bulk solution to the iron metal surface [9, 12, 13, 16]. In addition to the oxide layer formation, nZVI tends to aggregate in solution, resulting in a reduction in the active surface area and chemical reactivity. The chemical reactivity of nZVI is also limited by the growth of an oxide film (aqueous corrosion). To improve catalytic efficiencies, bimetallic nanoparticles (BNPs) are usually developed [9, 12, 13, 16]. When a second metal is galvanically coupled with base

2.2 Adsorption using nanomaterials

Table 2.2 Summary on arsenic removal using nZVI. Sl no. 1.

Material

6.

Supported nZVI on activated carbon nZVI: colloidal reactive barrier Montmorillonitesupported nZVI nZVI-stabilized with starch and carboxymethyl cellulose (CMC) nZVI-reduced graphite oxide-modified composites Gas-bubbled nZVI

7.

Biochar-supported nZVI

8.

Honeycomb-like structured nZVI/chitosan composite foams nZVI-mesoporous carbon composites

2. 3. 4.

5.

9.

10.

Fuller’s earth immobilized nZVIs

Arsenic removal efficiency

Capacity

Reference

96.4 (As(III))

18.0 mg/g

[31]

100% (As(V), pH 3, 10 min) 100% (pH 7, 4 h)

NA

[32]

59.9 mg/g

[33]

74.6% (As(V), Starch support, Si ¼ 10 mg/L)

34.6 mg/g

[34]

100% (As(III), pH 4, 60 min)

35.83 mg/g

[35]

100%, (As(V), 60 min, Si ¼ 3 ppm) 90% (As(V), Si ¼ 5.5 ppm) 80% (As(III), 20 min, Si ¼ 200 mg/L) Maximum removal of As(III) at pH 7 76.86% (As(V), pH 3)

125.3 mg/g

[36]

124.5 mg/g

[37]

114.9 mg/g

[38]

26.8 mg/g

[39]

91.42 mg/g

[40]

NA, data not available.

metal, the more noble metal participates in the reaction, while other metal corrodes to galvanically protect the noble metal. This galvanic coupling of two metals increases the catalytic activity of the BNPs, retards oxide film formation on the surface of base metal via an in situ regeneration process, and induces the less nobler metal to release electrons at a faster rate, thereby enhancing the rate of contaminant removal. Use of such bimetallics such as NiFe, PdFe, and CuFe for the removal of oxyanions and heavy metals has been reported in the past [8, 48–50].

2.2.2 Carbonaceous materials 2.2.2.1 Activated carbon Activated carbon (AC) is a highly porous material with a surface area of about 1000 m2/g. It has been widely used as adsorbent for the removal of impurities from drinking water, the removal of dyes and heavy metals from industrial wastewater,

29

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CHAPTER 2 Groundwater treatments using nanomaterials

Table 2.3 Summary on selenium removal using nZVI. Sl no.

Material

1.

Biosynthesized nZVIs

2.

nZVI on carbon nanotubes

3. 4.

nZVI nZVI/C in mesoporous silicaprotected Prussian blue microcubes nZVI under oxic conditions nZVI/Al-bentonite nZVI-confined in polymeric anion exchanger (D201)

5. 6. 7.

Selenium removal efficiency

Reference [41]

90% (pH 3, Si ¼ 2 ppm, 120 min) 95.6% (Se(IV), 120 min) 99.6% (Se(IV), 1 min) 100% Se(IV)

[43] [44]

90% (50 h, Se(VI)) 95.7% (Se(VI)) 84.9%

[45] [46] [47]

[42]

Note: Data on maximum capacity for selenium removal not available in the given references.

and decolorization of fermentation products. Activated carbon is commercially produced from carbonaceous precursors like coal, wood, and coconut shells [7]. Precursor materials should meet the following properties for it to be considered for the production of AC. The precursor should (i) have high carbon content, (ii) have low amount of inorganic ash content, (iii) show potential extent for activation, (iv) show low degradation on storage, (v) have high density and sufficient amount of volatile content, (vi) have a stable supply in producing country, and (vii) be made from inexpensive raw material [51]. To reduce production cost and to achieve better properties, various alternate precursors had been studied in the recent past. A list of low-cost precursors used in the recent past and the properties of the activated carbon synthesized from the precursors is shown in Table 2.4. Properties of AC are influenced by the precursor and synthesis conditions. Typically, carbonaceous materials are incinerated in the absence of oxygen (pyrolysis) at high temperature (controlled variables: temperature and time) to prepare carbon material. The organic precursor is carbonized first at temperatures of 400–700°C. The product from this carbonization stage has a typical carbon content ranging between 25% and 50% on a mass basis, depending on the type of starting material and process conditions used. The carbon obtained from the first stage can be activated either using physical and chemical methods. During physical activation the carbons are activated to form AC using activators such as steam or carbon dioxide (CO2) at high temperatures ranging from 700°C to 900°C. Physical activation has shown to affect the specific surface area, pore volume, pore size distribution, and the oxygen content [58]. Although low cost of activation and simplicity are considered advantageous in physical activation process, low yield of AC obtained during this process is the major disadvantage during industrial scale process. Low yield of AC during physical activation is due to higher temperature and longer activation

2.2 Adsorption using nanomaterials

Table 2.4 Activated carbon from low-cost precursors for groundwater decontamination.

Precursor

Properties of AC

Adsorbate

Adsorption efficiency (mg/g)

Lawsonia inermis wood

BET: 584 m2/g Micropore surface area: 483 m2/g Pore volume: 0.441 cm3/g

Pesticide

169.49

[52]

Heavy metals

64.50 Cu(II) 62.50 Cd(II) 60.90 Cr(III) 68.90 Pb(II) 51.30 Pb(II)

[53]

Heavy metals

17.67 Cu(II) 57.09 Cd(II) 147.53 Pb(II)

[55]

Heavy metals

31.60 Cd(II) 25.90 Co(II)

[56]

Pesticide (carbofuran)

296.52

[57]

Ulva lactuca

Sugarcane bagassesewage sludge (1:2) Olive stones

Diplotaxis harra

Rice straw

BET: 806.5 m2/g Pore size: 9.4 nm Pore volume: 0.678 cm3/g Specific surface area: 1194 m2/g Average pore diameter: 20.72 A Micropores: 0.552 cm3/g Mesopores: 0.009 cm3/g pHzpc 3.4 Iodine number: 1058.8 mg/g Methylene blue index: 280.4 mg/g Specific surface area: 1304 m2/g Average pore diameter: 23.9 A

Heavy metal

Reference

[54]

time. Chemical activation is another alternative method for preparing AC. Chemical activating agents such as KOH [59], NaOH [60], ZnCl2 [61], and H3PO4 [62] can be used for the preparation of AC. During chemical activation, solutions containing chemical activation agents are impregnated to the surface of precursor organic material, dried, and activated at temperatures 400–700°C for 1–4 h under an inert atmosphere. During this stage, pores are developed in the carbon, and functional groups are attached to the surface. Following activation the chemical agent is refluxed away using diluted mineral acid, filtered, and dried. The schematic diagram explains the typical process of activated carbon synthesis (Fig. 2.1).

31

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CHAPTER 2 Groundwater treatments using nanomaterials

Energy crops

Forest residues

Crop residues

Aquatic biomass

Initial Carbonization (400–700°C)

Lignocellulosic biomass (precursor) Physical activation (steam/gas)

Chemical activation (KOH, NaOH, ZnCl 2, H3PO4)

Activated carbon (AC)

FIG. 2.1 Schematic diagram for activated carbon synthesis.

Roman et al. [63] report that pore size distribution can be tuned by controlling the activated temperature. Surface functional groups play a major role on the overall effectiveness of the adsorbent. Activation of carbon synthesized from olive stone with chemical agent ozone and HNO3 had been successful in improving the adsorption capacity of the carbon [64]. Activated carbon fibers (ACF) exhibit a superior adsorption capacity over granular activated carbon. Diameter, pore size distribution, and surface area of ACF affect its adsorption capacity. Typically, ACF has fiber diameter in the order of few micron meters and surface area in the order of 1400 m2/g [65]. A range of organic contaminants including pesticides are removed more efficiently by ACFs than granular AC. ACF synthesized from woven rayon by Faur et al. contained up to 32% mesopores [65].

2.2.2.2 Carbon nanotubes Carbon nanotubes (CNTs) are long cylinders made of graphite sheets (continuous hexagonal network formed by carbon atoms). Typical dimensions of CNTs are as follows: diameters 1–10 nm and length of few microns to several centimeters [66, 67]. Typically, surface area of CNTs is in the range of 150–1500 m2/g [67]. CNTs can be either single-walled (SWCNTs) or multiwalled (MWCNTs). With high surface area, CNTs are superior to granular activated carbon in the removal of natural organic matter, pesticides, and other heavy metals. The inherent mesoporous structure of CNTs makes them a suitable candidate for heavy metal removal [68]. Metals like Cd(II), Co(II), Cr(IV), Cr(VI), Cu(II), Mn(VII), Ni(II), Pb(II), and Zn(II) are efficiently removed by CNTs [7, 66, 69]. CNTs with or without functionalization are also used for the removal of heavy metal ions and organic contaminants.

2.2 Adsorption using nanomaterials

Adsorption capacity of CNTs is improved through surface functionalization. Functional groups like dOH, dCOOH, dNH2, and carbonyl groups can be added to the wall of CNTs through chemical treatment, heat treatment, and endohedral filling. Functionalization further improves the adsorption capacities of CNTs. Sorption of heavy metals/organic contaminant on CNTs mainly occurs due to surface adsorption, ion exchange, and/or electrochemical attraction. Chemical interaction between the charged contaminants and the functional groups also plays a major role in sorption of contaminants on CNTs [67]. CNTs are also used for the removal of emerging contaminants such as antibiotics and its degradation by-products. For example, ofloxacin, a quinolone antibiotic, can be removed by adsorption on functionalized CNTs [70]. Hydroxylized CNTs adsorb ofloxacin effectively due to strong electron donor-acceptor interaction. Ofloxacin acts as π-acceptor due to the presence of fluorine group, which has a strong electron withdrawing ability. Also, it was found that interaction between a π-donor compound and a π-acceptor compound is much stronger compared with donor-donor or acceptor-acceptor pairs [71]. Solubility and solution pH affect adsorption of antibiotics on CNTs. Nanocomposites made by combining CNTs with other supports can improve the utility value of CNTs as they prevent CNTs from entering the environment after use. The major advantage of CNTs is that the contaminant can be easily desorbed from the surface following which the CNTs and the contaminants can be reused and recycled appropriately. Some of the limitations in implementing CNTbased adsorption systems for large-scale applications are (i) higher cost of CNTs, (ii) difficulty in removal of CNTs from treated water [66], (iii) nonselective nature of CNTs, and (iv) concerns over toxicity of CNTs. On the other hand, carbon nanosheets are less toxic and are being widely used for the removal of heavy metals [72]. Table 2.5 summarizes the literature on heavy metal removal using surface functionalized CNTs.

2.2.2.3 Graphene Graphene is a two-dimensional crystal made of layered carbon atoms. Owing to the unique two-dimensional structure of graphene, it possesses efficient mechanical, thermal, and electrical properties. Fox example, at a pressure of 130 GPa, graphene is 300 times stronger than diamond [73]. Graphene and its derivatives are attractive adsorbents for water purification because of their high surface area, tunable properties, and delocalized π-electron system [74]. Metal ions like As(III), As(V), Cd(II), Co(II), Cu(II), and Pb(II) can be removed from aqueous solutions using graphenebased nanomaterials. Different forms of graphene, for example, pristine graphene, graphene oxides (GO), graphene oxide nanosheets (GONSs), layered GONSs, graphene composites (e.g., magnetite-graphene), and functionalized graphene oxides (GO), exhibit different adsorption behaviors [7]. While GOs are good at removal of cationic ions, GONSs are good for removal of both cations and anions. Literature reports suggest that oxygen-containing functional groups play a major role in metal adsorption. Functionalizing GO with organic materials like polymers and multidentate chelating agents introduces multifunctional groups in GO and enhances

33

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CHAPTER 2 Groundwater treatments using nanomaterials

Table 2.5 Functionalized CNTs for heavy metal removal. Heavy metal

Functional group

As(III)

CNT base iron oxide CNT base iron oxide CNT/Al2O3

As(V) Cd(II) Cr(VI) Cr(III)

Cu(II) Cu(II) Ni(II)

Pb(II) Zn(II)

CeO2/ACNT Thermally oxidized MWCNTs CNT/Al2O3 CNT NaOClmodified SWCNTs CNT/Al2O3 NaOClmodified MWCNTs

Adsorption capacity (mg/g) 24.05

Adsorption conditions

Reference

Heavy metal concentration: 1–15 ppm; pH 7.0; 25°C

[108]

Heavy metal concentration: 2–15 ppm; pH 5.0; 25°C 35.3 ppm; pH 7.0; 25°C 1 ppm; pH 7.0; temperature not available

[109]

47.41 8.89 31.55 0.50

[110] [111]

26.59 8.25 47.86

2–25 ppm; pH 5.0; 25°C 43.1 ppm; pH 6.0; 27°C 10–80 ppm; pH 7.0; 25°C

[109] [112] [113]

67.11 32.68

5–40 ppm; pH 5.0; 25°C 10–80 ppm; pH 7.0; 25°C

[109] [114]

adsorption potential [75]. For example, alginate introduces dCOOH and dOH functional groups, and chitosan introduces dNH2 and dNHCO groups. Dendrimer a highly branched monodispersed polymeric macromolecule when combined with GO enhances the adsorption potential of GO significantly. Such a composite had been shown to efficiently remove Se(IV) from contaminated water [75]. Metal ions are typically removed by electrostatic complexation and coordinate approaches. Apart from the functional groups, high negative charge density and hydrophilic characteristics of graphene-based nanomaterials are the other reasons for the metal adsorption on graphene materials [66]. Table 2.6 summarizes the literature on heavy metal and pesticide removal using graphene-based nanomaterials.

2.2.3 Metal oxides For heavy metal removal like Cd(II), Cr(VI), Cu(II), Hg(II), Ni(II), Pb(II), Pd(II), and Zn(II), nano-metal oxides like ferric oxide, manganese oxide, aluminum oxide, titanium oxide, magnesium oxide, and cerium oxides have been used [89]. As iron is nontoxic, nano-iron oxides can be directly pumped to contaminant water supplies without health risk. Goethite, hematite, maghemite, and magnetite are few commonly used nanoferric oxides for heavy metal removal [89]. Magnetite NPs synthesized using ferrous and ferric chloride as precursors were effective in removal of Pb

Table 2.6 Heavy metal sorption by graphene-based nanomaterials. Sorption conditions

Adsorption capacity (mg/g)

Cd(II), Ni(II), Pb(II), Zn(II), Cr (II) Cd(II)

pH 3.0; 25°C

[76]

pH 5.0; 30°C

204.08, 250.00, 555.55, 322.58, 60.24 253.81

Pb(II) Cu(II) Mn(II) Cd(II), Pb(II) Cd(II), Pb(II), Cu(II) Zn(II), Cd(II) Pb(II) Cd(II) Pb(II) Pb(II) Pb(II)

pH 4.5; 30°C pH 4.5; 30°C pH 4.0; 30°C pH 7.1; 27°C pH 5.0; 20°C pH 5.6; 100°C pH 5.6; 100°C pH 6.0; 25°C pH 5.0; 25°C pH 6.0; 60°C pH 5.0; 30°C

568.20 68.70 18.30 167.00, 333.00 177.00, 447.00, 425.00 72.80, 88.90 65.60 33.70 1308.00 1850.00 98.10

[77] [77] [77] [78] [79] [80] [80] [81] [82] [83] [84]

Chlorpyrifos, endosulfan, malathion Cd(II) Hg(II) Hg(II) Triazine pesticides

pH 7.0; 30°C

1200.00, 1100.00, 800.00

[85]

pH 6.0; pH 6.0; pH 6.0; pH 9.0;

234.80 381.00 397.00 6.4–9.80

[86] [87] [87] [88]

Adsorbent

Adsorbate

Graphene oxide (GO)-silica nanocomposite GO/polyamidoamine dendrimers (PAMAMs) GO/PAMAMs GO/PAMAMs GO/PAMAMs Fe3O4/mesoporous silica/GO Chitosan/SH/GO TiO2/GO TiO2/GO Fe3O4/GO/EMIMBF4 Graphene hydrogel Few-layered GO lignosulfonate-GO-polyaniline (LS-GO-PANI) RGO 3-D sulfonated RGO GO/chitosan GO/magnetic chitosan Cellulose-GO nanocomposite

55°C 25°C 25°C 45°C

Reference

[77]

36

CHAPTER 2 Groundwater treatments using nanomaterials

(II) ions from water. Specific studies have 100% removal of Pb(II) that was achieved from water with initial lead concentration of 50 ppm at 25°C and pH 5.0 [90]. The major advantage is that the regeneration of such NPs are easy and they can be reused for several cycles [90]. Rajput et al. used magnetite NPs synthesized by coprecipitation for the removal of Pb(II) and Cr(VI) heavy metals from water [90]. Maximum adsorption capacities of 53.11 and 34.87 g/g, respectively, for Pb(II) and Cr(VI) ions, were observed [91]. Chen et al. [92] used sulfonated magnetic Fe3O4 NPs for the removal of Cd(II) and Pb(II) ions, and maximum adsorption capacities of 80.90 and 108.93 mg/g, respectively, were reported [92]. Kumari et al. used magnetite hollow nanospheres for the removal of Pb(II) and Cr(VI) ions, and maximum adsorption capacities obtained under optimized conditions were 9.0 and 19.0 mg/g, respectively, for Pb(II) and Cr(VI) [93]. Titanium oxide and zinc oxide, well-recognized photocatalysts, can also act as adsorbents for the removal of heavy metals. Various structures of TiO2 and ZnO NPs, such as layered titanates, nanosheets, nanoflowers, nanoplates, and nanowires, have been synthesized and used for heavy metal removal [72]. Cerium oxide nanoparticles remove chromium (VI) by reduction-oxidation (redox) reaction. Cr(VI) after adsorption on the catalyst surface is reduced to Cr (III). CeO2 NPs had a particle size in the range of 12 nm and the surface area in the order of 65 m2/g. CeO2 can be synthesized in different morphological structures like nanorods, nanowires, nanotubes, and hollow nanospheres. Research has shown that ceria hollow nanosphere adsorbs nearly about 70 times more heavy metals compared with bulk ceria [94]. Similarly, manganese oxide nanoparticles (MO-NPs) have much superior adsorption potential compared with bulk manganese oxides [89]. Studies conducted in literature have shown that MO-NPs exhibit good sorption of heavy metals in terms of high capacity and selectivity owing to their large surface area and size quantization effect [89, 95]. Cr(VI), Cd(II), and Pb(II) ions had been efficiently removed by MO-NPs [89]. Alumina, functionalized alumina NPs, and copper oxide NPs are other metal oxides used for the removal of heavy metals like arsenic [89]. The major limitation of using metal oxides at nanoscale is their stability. At nanoscale level, metal oxides lose their stability (due to increased surface energy) and form aggregates (due to van der Waals forces/other interactions) and therefore lose their adsorption capacity. To overcome this issue the metal oxides are typically used as composites by fixing them on porous support materials.

2.2.4 Metal-organic framework Metal-organic frameworks (MOFs) consisting of central metals and organic ligands are gaining importance in water purification applications. It has been recognized that, due to the presence of organic and inorganic components, MOFs possess better chemical stability, mechanical strength, and structural diversity [96]. Chemical bonding, coordination interactions, acid-base interaction, electrostatic interaction, diffusion, and van der Waals force are the mechanisms responsible for adsorption of heavy metals on MOF [96]. Unmodified MOF, modified MOF, and magnetic

2.2 Adsorption using nanomaterials

Table 2.7 Metal-organic framework for heavy metal removal. MOF Zr-MOF Thiol-functionalized MOF MOF UiO-66-NH2

Isonicotinate-based N-oxide MOF

Heavy metal pollutant

Adsorption capacity (mg/g)

Pb(II) Pb(II) Cd(II) Pb(II) Cd(II) Hg(II) Cr(III) Cr(VI)

135.00 313.00 90.70 232 49 769 117 145.00

Reference [115] [116] [117]

[118]

MOF composites have been examined for water decontamination. Table 2.7 presents a brief summary of literature of MOFs used for heavy metal removal.

2.2.5 Adsorption kinetics and equilibrium Understanding of kinetics and equilibrium is essential for effective design of adsorption systems. While adsorption rate (mg/g min) depends on process variables like temperature, solution pH, and net driving force, the maximum possible contaminant loading (mg/g) depends on thermodynamic equilibrium between the adsorbate and adsorbent. Most common isotherm and kinetic models are briefly discussed here. For more detailed discussion on the types of kinetics and isotherms used, readers are advised to refer to other sources. Typically, most adsorption systems follow either pseudo-first order or pseudo-second order kinetics. The Lagergren pseudo-first order model and Ho’s pseudo-second order model are the two reaction-based kinetic models that are widely used by the researchers in explaining the kinetics adsorption. The pseudo-first order rate expression suggested originally by Lagergren based on solid capacity is expressed in Eq. (2.1): dqt ¼ k1 ðqe
qt Þ dt

(2.1)

Pseudo-second order model is expressed by Eq. (2.2). dqt ¼ k2 ðqe
qt Þ2 dt

k1: pseudo-first order rate constant, min
1 k2: pseudo-second order rate constant, g/g min qe: solid-phase dye concentration at equilibrium, g/g qt: solid-phase dye concentration at time t, g/g t: time, min

(2.2)

37

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CHAPTER 2 Groundwater treatments using nanomaterials

Similarly a number of isotherm models are available to explain adsorption equilibrium. However, the two most commonly used models Langmuir and Freundlich models (Eqs. 2.3 and 2.4) are capable of fitting the data for most of the cases. Langmuir model: qe ¼

qm KL Ce 1 + KL Ce

(2.3)

Freundlich model: qe ¼ KF Ce1=n

(2.4)

Ce: equilibrium dye concentration in liquid phase, g/m3 qm: maximum dye adsorbed per unit mass of adsorbent (mono layer capacity), g/g KF: Freundlich constant, g
1 m3 g1
1/n KL: Langmuir adsorption constant, m3/g Aqueous-phase concentration is measured experimentally, and the solid-phase concentration is calculated from aqueous-phase concentration by making use of mass balance equation: qt ¼

C0
Ct D

(2.5)

where C0: aqueous-phase dye concentration at time ¼ 0, g/dm3 Ct: aqueous-phase dye concentration at time ¼ t, g/dm3 D: adsorbent dosage, g/dm3 The rate constants determined by fitting the experimental data to Eq. (2.1) or (2.2) along with the equilibrium constants determined from Eq. (2.3) or (2.4) together help in understanding the mechanism of the process and decide whether a particular adsorbent is suitable for removing a specific contaminant. Invariably, adsorbents are porous materials. Therefore knowledge of pore diffusion is required for designing adsorption equipment, which uses engineered porous materials. In such cases mass transfer models based on Fickian diffusion are frequently employed [97]. A combination of kinetic-based, equilibrium (isotherm)-based, and mass transfer-based models helps in designing good adsorption units for large-scale water treatment.

2.3 Photocatalysis Photocatalysts absorb photons of suitable wavelength and get excited to create electron-hole pair on the surface of the catalyst. Most commonly used photocatalysts are TiO2 and ZnO. Desirable characteristics of the photocatalysts are high surface area, narrow bandgap energy, resistance to photocorrosion, thermal stability, mechanical strength, among others. Morphology and structure of the catalyst also

2.3 Photocatalysis

play a major role in photocatalysis. Apart from TiO2 and ZnO, various other photocatalysts such as CeO2-, bismuth-, and tungsten-based oxides have also been examined in the recent past for degradation of organic compounds. New photocatalysts are designed to (i) improve the absorption of light in visible range, (ii) prevent recombination of electron hole pair, and (iii) make them photocorrosive resistant. Halogenated organic compounds, PCPP, and other volatile organic compounds (VOCs) can be effectively removed from water by photocatalysis. However, at low concentration of VOCs, often, it is used in combination with other treatments [98]. Among three different allotropic forms of TiO2, namely, brookite, rutile, and anatase, anatase is found to be more efficient for photocatalytic applications. TiO2 can oxidize range of contaminants like organic acids, pesticides, herbicides, and some heavy metals such as arsenic. TiO2 has been synthesized in different forms like nanotubes, nanorods, nanowires, among others [99]. Cerium titanate nanorods synthesized through hydrothermal method have been shown to have a narrow bandgap of 2.65 eV, which make them highly effective even in the visible light region. Metal and nonmetal doping (e.g., Fe, Ag, and Au), noble metal deposition, coupling with other semiconductors, sensitization of TiO2 with dyes, etc. increase absorption in visible range [100]. Indium- and cerium-codoped TiO2 had shown a better activity under visible light. Substitution of In3+ leads to large number of structural defects in TiO2 and results in increased surface Bronsted activity [99]. Ag/AgCl@TiO2 nanocomposites also have shown improved photocatalytic activity. A sample irradiated for 20 min reportedly exhibited superior photocatalytic performance with higher reaction rate when compared with Ag/TiO2 and N/TiO2. Presence of Ag/AgCl results in surface plasmon resonance, which increased the reaction rate by 6.8 times [100]. Another TiO2-based nanocomposite TiO2-graphene nanocomposite has been used for the reduction of Cr(IV) to Cr(III) [68]. A composite of BiVO4 with 5 wt% silica has shown improved oxidation of organic contaminants. Addition of silica improves adsorption capacity of BiVO4 and reduces negative conduction band potential of BiVO4 [101]. Use of coreduction catalysts like Pt, Pd, or CuO to semiconductor catalysts helps to minimize one electron oxygen reduction and results in enhanced oxidation rate of organic molecules [101]. Addition of Pt to BiVO4 promotes catalytic activity through multielectron reduction. However, loading too much of Pt, beyond optimal level, results in decrease in activity [101]. TiO2 and other photocatalysts generate highly reactive radicals like hydroxyl radicals (•OH
) and superoxide radicals (O2∗
), which are responsible for catalyzing various redox reactions. Particularly the hydroxyl radical has sufficient potential to catalyze oxidation of organic contaminants such as pesticide residues [102]. In a report, it has been stated that hydroxyl radical accelerates the oxidation of pesticides and a reaction rate constant is typically around 109 L/(mol s) for the catalyzed reaction. The major advantage is that these reactions yield CO2 and H2O as end products without releasing toxic intermediates. Photocatalyst concentration is an important factor that affects the efficiency of the process. At too low catalyst concentration, part of the light is transmitted through the suspension instead of being absorbed on catalyst surface leading to poor photonic efficiency. Since emerging pollutants are

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present at very low concentrations (in μg/L or ng/L), catalysts are used at low concentration requiring a careful choice of reactor dimensions. As photonic efficiency increases with increase in reactor diameter, tubular reactors with large diameters to provide sufficient optical thickness are preferred, in particular for the treatment pollutants at μg/L or ng/L levels [103].

2.4 Implementation of nanotechnology for water treatment 2.4.1 Ex situ 2.4.1.1 Batch processes with suspended nanoparticles Despite the advantages, technological limitations inhibit the applicability of the use of nanomaterials in a large scale [104]. The primary advantage of nanomaterial use in batch processes (i.e., in small laboratory scale setting) is that the entire surface of the nanoparticle is available for the reaction, and therefore the surface is more efficiently used. However, in a batch process with suspended nanoparticles, the solid fraction is usually very low, and therefore the probability of contact between the liquid and solid phases is lower, leading to poor utilization of the nanoparticle. Stability of the nanoparticle suspension is another important factor affecting the efficiency of the process. Nanoparticles in suspended form can elute out of flow-through processes, thus becoming unavailable for use. Possible solutions to this issue with the nanomaterials are the use of (i) supported nanoparticles and nanocomposites in fixed beds and (ii) supports like nanomembranes, porous materials, and graphene oxide. To facilitate separation of particles, they can be supported on magnetic cores to form core-shell structure. Magnetic core is typically made from magnetic elements (e.g., iron and nickel) or their oxides, alloys of ferromagnetic or supermagnetic elements. Alternatively, to remove other functionalized nanomaterials after their use, magnetic carbon nanomaterials had been tried [105]. Reduced graphene oxide-Fe3O4 NPs were used by Shih et al. to collect gold NPs used for adsorption Hg2+ from seawater [105].

2.4.1.2 Packed bed processes In a packed bed with a void volume fraction of 0.4–0.5, the fluid follows plug flow pattern, and the contact between fluid and solid is more efficient here. However, careful control of fluid velocity is essential to prevent flow irregularities like channeling and flooding. Optimum particle size is another important factor. Too low packing size provides high surface-to-volume ratio but causes high pressure drop across the bed. On the other hand, large packing size provides less surface-to-volume ratio but minimizes pressure loss. Thus a trade-off is very critical in efficient operation of fixed bed. By supporting nanoparticles on suitable porous support matrix, packing size can be tuned to desired level. Meanwhile, while supporting nanoparticles on support matrix, fraction of the active surface becomes unavailable for the intended purpose, and the particles are underutilized. Major advantage of the packed bed process is that there is no need for the separation of nanoparticles and thus the treated water is free of nanoparticles. Second, regeneration and reuse of the catalyst

2.5 Health implications on the use of nanotechnology in groundwater treatment are convenient and easy. Unlike batch process, in fixed bed, almost complete removal of groundwater contaminants is possible.

2.4.2 In situ In situ treatment of most inorganic and organic contaminants can be done by employing reactive barriers along the flow of groundwater. These are called permeable reactive barrier (PRB), which contains a catalyst or an adsorbent, depending on the type of contaminant [104]. Use of nZVI in PRBs has been well documented and has been applied in the field [12, 16–18]. PRBs are zones of reactive material in a wall-type construction that extends below the water table and treats any contaminant it comes in contact with. It is often considered to be an effective alternative to the regular pump and treat option used for groundwater remediation, because of its high effectiveness and low costs. The key concern with the use of PRBs for groundwater remediation is its longevity. Longevity of PRBs is usually determined by either its reactivity (or lack or) or permeability of water across the barrier. A field study was done to evaluate selenium removal using PRBs. It was observed that selenium removal suffered due to the rusting of iron, which resulted in solidification of iron bed, thereby inhibiting the reduction reaction (loss of reactivity). It was also found that column systems using zerovalent iron worked well for anaerobic systems; however, they plug up when exposed to aerobic solutions [12]. The formation of a passivating film of magnetite and maghemite (different forms of iron oxide) on the surface of corroded Fe0 particles could affect the behavior of the metal-contaminant interaction, thereby reducing the life of the PRB. Other studies have looked into the high flow rates and presence of nontoxic ions (nitrate and phosphate) as reasons for lower longevity of PRBs. In many cases the precipitation of iron oxyhydroxides (resulting from the corrosion of nZVI) was observed to cause issues with permeability of the system, resulting in reduced effectiveness of the PRBs. For these reasons alone, use of PRBs has been limited to some sites in the United States where pump and treat methods are costly. Nevertheless, PRBs offer a sustainable, and with improved technology, low-cost alternative toward groundwater remediation.

2.5 Health implications on the use of nanotechnology in groundwater treatment As outlined in the earlier section, the use of nanomaterials in environmental remediation has significant benefits. With growing consumer use of nanomaterials, large amounts of nanomaterials are produced from point and nonpoint sources, distributed to users, and finally released to the environment without notice [106]. As a result, fundamental research on full-cycle lifetime assessment of the transport and fate of nanomaterials should be addressed to respond to potential risks of acute or chronic contamination events related to the release of nanomaterials. Concerns that the unique properties of engineering nanomaterials could potentially lead to threats to human health and the environment have risen in recent years, particularly with

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respect to ingestion and inhalation of nanomaterials. Transport of engineering nanomaterials to aquatic streams via wastewater discharges and/or groundwater flow can also potentially result in human health and environment risks. Toxicology studies with vapors, fibers, and ultrafine nanoparticles report serious damages to tissues, such as interstitial fibrosis for inflammation, biochemical response to produce reactive oxidizing species (ROSs), and material-related mutagenicity to generate oxidative stress [107]. The extent of pulmonary inflammation and cytotoxicity with the use of nanomaterials is often associated with free radical activity. Insoluble nanomaterials stuck in the lungs are known to cause oxidative stress, leading to inflammation, fibrosis, and even cancer. Nanomaterial-based water purification techniques are becoming a cause of concern because of the release of toxic metals into the water after longer duration of use. It has also been observed that it is very difficult to filter/ remove nanoparticles after use, and therefore they are released into the environment and find their way back to water supply, and therefore newer technologies for the nanomaterial removal are required. Further the toxicity of such nanomaterials such as silver and graphene sheets is still in debate. Adsorption-based water purification techniques, even on nanomaterials, suffer from the same drawback as those with microsize materials, including large amount of used sorbent for disposal. Given the concerns over the general use of nanomaterials, it is prudent to research on the environmental risks and health effect of nanomaterials in parallel with the development and application of nanomaterials.

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CHAPTER

Copper-based ternary metal sulfide nanocrystals embedded in graphene oxide as photocatalyst in water treatment

3

Olalekan C. Olatundea,b,c, Damian C. Onwudiwea,b a

Material Science Innovation and Modelling (MaSIM) Research Focus Area, Faculty of Natural and Agricultural Sciences, North-West University, Mafikeng Campus, Mmabatho, South Africa, b Department of Chemistry, School of Physical and Chemical Sciences, Faculty of Natural and Agricultural Sciences, North-West University, Mafikeng Campus, Mmabatho, South Africa, c Department of Industrial Chemistry, Ekiti State University, Ado Ekiti, Ekiti State, Nigeria

Chapter outline 3.1 Introduction ...................................................................................................... 52 3.2 Wastewater treatment technologies .................................................................... 55 3.2.1 Adsorption ......................................................................................55 3.2.2 Membrane separation .......................................................................56 3.2.3 Advanced oxidation processes ...........................................................56 3.3 Copper-based ternary metal sulfide nanocrystals (CBTS) ..................................... 69 3.3.1 Classification of copper ternary metal sulfides ....................................69 3.3.2 Synthesis of copper-based ternary metal sulfides ................................77 3.3.3 Applications of copper metal ternary sulfides .....................................81 3.4 Graphene, its derivatives and photocatalysis ...................................................... 85 3.4.1 Synthesis of graphene oxide .............................................................86 3.4.2 Graphene oxide in wastewater treatment ............................................88 3.4.3 GO/semiconductor composites ..........................................................89 3.4.4 GO/metal chalcogenide nanocomposite photocatalysts ........................90 3.4.5 GO/copper-based ternary metal sulfide nanocomposite photocatalysts ..90 3.4.6 Mechanism of action of GO-supported photocatalysts .........................92 3.4.7 Future perspective ...........................................................................93 3.5 Conclusion ........................................................................................................ 94 Acknowledgments .................................................................................................... 94 References .............................................................................................................. 94 Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00003-1 # 2020 Elsevier Inc. All rights reserved.

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CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

3.1 Introduction Although water occupies about 70% of the earth’s surface, large volumes are trapped in soils, icebergs, oceans, or the atmosphere as vapor. Only about 0.3% of its total volume is available in usable form for human consumption. Recent estimates showed that about 1.2 billion of the world’s population are without clean drinking water [1]. Water pollution is a threatening global issue, and it is one of the major causes of illness and diseases in the world. It accounts for approximately 14,000 deaths everyday [2]. According to the world water development report by the United Nations, about 2 million tons of sewage, industrial, and agricultural wastes are discharged into water bodies daily [3]. In developed countries, 80% of industrial wastewater is released directly into the environment without prior treatment, while this is up to 95% in developing countries [4]. The increased wastewater generation occasioned by increased agricultural and industrial activity has led to the discharge of different heavy metals, aromatic pollutants, and industrial dyes into the already scarce water resources, thus aggravating the unavailability of potable water and deepening the global water crisis [2, 5]. Consequently, there is a need for a globally sourced approach to the desalination, decontamination, and disinfection of wastewater in order to enhance the reuse and recycle of wastewater. This would bridge the everincreasing gap between the demand for water and its supply. Wastewater treatment has been dominated by various traditional processes such as filtration, sedimentation, centrifugal separation, flocculation, aerobic and anaerobic treatments, and coagulation. However, with the increase in complexity of pollutants and the volume of wastewater generation, highly effective and efficient technologies are required. Recent advancements in material synthesis championed by the field of nanochemistry and nanotechnology have opened the door to advanced wastewater treatment technologies such as membrane technology, adsorption, reverse and forward osmosis, ultrafiltration, microfiltration, ion exchange, and advanced oxidative processes [6]. However, some of these processes suffer a few setbacks. For example, about 3 KWh of energy is required to produce 1 m3 of drinking water using the reverse osmosis desalination technology. The process suffers from fouling, making it energy intensive and expensive [7]. Also, despite the huge advantages of adsorption processes, the risk of reintroducing pollutants into the environment is very high as there exist no suitable disposal means for the used adsorbents. Recent foray into the use of advanced oxidation process (AOP) for wastewater treatment, which attempts to mineralize pollutants, has opened new opportunities for technologies with suitable operating conditions. AOP employs in situ generated, highly reactive, and nonselective reactive oxygen species (ROS) such as H2O2, HO%, O3%
, and O2%
for complete mineralization of pollutant to inorganic acids, water, and carbon dioxide [8]. One major advantage of AOP over other wastewater treatment technologies is that the materials could be used to achieve water detoxification, purification/remediation, and disinfection, leading to water with qualities that fulfill standard regulations for potability [9]. AOPs can be classified into two broad groups

3.1 Introduction

Vacuum ultraviolet process Photolytic process

X-ray irradiation process Electron beam irradiation process

UV/S2O82–process UV/S O32–process Photochemical process

UV/O3 process

UV/H2O2 process

Photo-Fenton process

Photocatalytic process

Photon-Fenton like process

Advanced oxidation process Semiconductorbased process Peroxone process

Sonication process

Non photochemical process

Electrochemical process Nonthermal plasma process Photoelectrochemical process

Hybrid process

Photoelectro Fenton process Sonoelectro Fenton process

FIG. 3.1 Classification of advanced oxidation process.

depending on the activation mode as photochemical and nonphotochemical. The processes are made up of either a combination of oxidants, radiation, or photocatalysts. Combination of two different processes gives a hybrid process [10]. Fig. 3.1 shows the classification of AOPs and the existing technologies.

53

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CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

Recently the use of photocatalysts in wastewater treatment has gained much attention due to its environmental friendliness, economic viability, and ease of incorporation in water treatment plants. The photocatalyst may be dissolved in the liquid phase to give a homogeneous process or be supported on a solid matrix or suspended in a liquid to give a heterogeneous process. Homogeneous photocatalytic AOP processes include photo-Fenton [11], photocatalytic ozonation (O3/UV) [12], hydrogen peroxide photolysis (UV/H2O2) [12], peroxone (O3/H2O2) [13], and vacuum ultraviolet (VUV) [14] process. Heterogeneous processes are preferable to homogeneous process, mainly because of the ease of separating the catalyst from the reaction system. In heterogeneous photocatalytic AOPs, semiconductors with suitable bandgaps capable of generating electrons that interact with water molecules on their surfaces to produce ROS are not of the same phase with the reaction system. In some cases the semiconductor photocatalyst could also be supported on suitable material. To be suitable as a catalyst in AOPs, the semiconductor must be readily available, economical, show low toxicity, good photocatalytic activity, and must be chemically inert and stable [15]. TiO2 is the most explored semiconductor in AOPs because of its suitable properties. However, its large bandgap (3.4 eV) means it can only be employed in systems with ultraviolet radiation source. In order to make AOPs more economical and environmentally friendly, sunlight, which is a renewable energy, is the best source of energy to be considered in the design system. Different techniques including doping [16], dye sensitization [17], and compositing [18] have been explored to make TiO2 visible light responsive [19]. The quest for the synthesis of visible light-responsive semiconductor materials has, however, focused mainly on the synthesis of new photocatalytic materials with suitable bandgap for absorbing photons in the visible region of the electromagnetic spectrum. Other semiconductors that have been explored as suitable replacement for TiO2 in AOPs include SnO, ZnO, ZnS, CdS, ZrO2, WO3, Fe2O3, and SrTiO3. However, the use of these semiconductors has been hampered either by their solubility in water or toxicity [20]. Metal chalcogenides have dominated the research for novel semiconductors, due to their physical, optical, electronic, and magnetic properties that are controllable by their composition and stoichiometry. Among the chalcogenides, copper-based chalcogenides have been of utmost interest because [21] of the following: i. They are made from relatively earth-abundant metal atoms with reduced toxic concerns compared with lead- and cadmium-based materials. ii. They are compositionally, structurally, and stoichiometrically versatile and are able to exhibit numerous nonstoichiometric phases. iii. They possess excellent functional properties like high carrier concentrations, direct bandgaps, low thermal conductivity, and plasmonic properties. In line with the development of semiconductors for AOPs, research has also focused on the development of suitable supports that can enhance the activity of semiconductors. Feng et al. [22] reported the synthesis of iron-based metal-organic framework (MIL-53Fe) supported Fe3O4 photocatalyst in the degradation of rhodamine B dye.

3.2 Wastewater treatment technologies

Other support materials used for AOPs include carbon nanotubes, carbon nanofibers, zeolite, activated carbon, and graphene [23]. Graphene has gained much attention as support for semiconductors because of its good adsorption and electronic properties. This accounted for the application of graphene-supported semiconductors in a wide range of applications such as sensing [24], medicine [25], supercapacitor [26], fuel cells [27], batteries [28], and photocatalysis [29]. Despite different reports and reviews that exist on the application of graphenesupported semiconductors, only very limited literatures are found on the use of graphene-supported copper chalcogenides and almost none on copper-based ternary metal sulfide nanocrystals supported on graphene as photocatalysts in AOPs. This may be related to the great challenge posed by the development of synthetic approaches to ternary semiconductor materials of the right stoichiometry that are reproducible and controllable. This chapter, therefore, explores the potentials in the utilization of these materials as photocatalysts in wastewater treatment.

3.2 Wastewater treatment technologies The need to deal with more complex, recalcitrant molecules, which cannot be removed by conventional treatment methods, has served as impetus for the development of more efficient treatment technologies. Though each of these technologies has advanced the field of wastewater treatment, they also suffer from significant drawbacks. In this section, focus will be placed on advanced oxidation processes while only slightly highlighting other very common and significant technologies such as adsorption and membrane technology.

3.2.1 Adsorption Adsorption involves the concentration of species (adsorbates) on the surface of solid materials (adsorbents). It is a surface phenomenon, which could either be chemisorption or physisorption, depending on the nature of the interaction between the adsorbent and the adsorbate [30]. The type of interaction depends mainly on the adsorbate and adsorbent’s surface functionalization and charge characteristics. If the force involved in the interaction between the adsorbate and adsorbent is a weak one like the van der Waals forces, the process is physisorption, while adsorptions involving the formation of chemical bonds like π–π interactions and electrostatic force of attractions are chemisorption processes. The adsorption process is influenced by various process parameters like adsorbent’s size and morphology, surface characteristics, pH, temperature, and adsorbent dose. Adsorption is one of the preferred wastewater treatment technology because it is cost effective, simple, and highly efficient in the removal of inorganic and organic pollutants in trace level from water [31]. It has also been explored in the disinfection of wastewater [32]. A large group of materials have been explored

55

56

CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

as adsorbents including activated carbon and other carbon forms like carbon nanotubes [33], cellulose nanomaterials [34], clay minerals [35], chitosan [36], and composites [32].

3.2.2 Membrane separation Membrane separation involves the selective separation of solutes using the pores of a semipermeable membrane, which act as physical barriers, allowing water to pass through while retaining the solute via diffusion and sieving. The force applied on the feed stream separates it into the permeate and retentate streams, which are the purified water and rejected salts, respectively [37]. Membrane technology is explored mainly in the treatment of sea and brackish water. The process of extracting salts and mineral constituents, which are usually present as negatively or positively charged ions from solution, is referred to as desalination. The process is employed in the generation of potable water (salinity UV/H2O2 > UV/PMS with 86%, 73%, and 45% tyrosol removal, respectively, at pH of 6.8 and 320 mJ/cm2 UV irradiation. Three principal factors were noted to be responsible for the difference in efficiency: (i) effect of pH on radical generation, (ii) reactivity of the sulfate and hydroxyl radical toward tyrosol oxidation and its concentration, and (iii) the oxidation system’s quantum yield. The photochemical mineralization of benzene, toluene, ethylbenzene, and xylenes (BTEX) using UV (254 nm)/H2O2 and VUV (185 nm)/H2O2 was reported by BustilloLecompte et al. [78]. Under acidic pH the mineralization efficiency was 80% and 90% for UV (254 nm)/H2O2 and VUV (185 nm)/H2O2, respectively, at the optimum oxidant concentration. To develop a more eco-friendly photochemical AOP, pulsed light (PL) has been employed as energy source for AOP. The main advantage of PL sources such as xenon over other UV light sources like light-emitting diode, XeBr, and KrCl is the reduced time required for the process, as high energy is generated for photolysis of oxidants within a very short time. Martı´nez-Lo´pez et al. [79] employed PL as the

3.2 Wastewater treatment technologies

light source in a PL/H2O2 and PL/H2O2/ferrioxalate processes for the decolorization of Congo red dye. Over 50% decolorization of Congo red dye was achieved after just 9 s. The AOP degradation and mineralization of sulfamethoxazole was studied using O3 and UV/O3 by Martini et al. [80]. While the O3 process was the most efficient for degradation, higher mineralization of about 83% was obtained from the UV/O3 process after 3 h. However, a significant increase in toxicity was observed in the treated solution, indicating the formation of a more toxic intermediate compound, than the sulfamethoxazole.

3.2.3.4 Photocatalysis (PCAOP) Photocatalytic AOPs are often classified among the photochemical processes; however, the process involves a different and much complex mechanism compared with the other photochemical processes. The large volume of research interest in this process implies that it could carve out its own niche among AOPs. Photocatalysis AOP comprises mainly three components: (i) a light source of appropriate wavelength, (ii) a catalyst, and (iii) an oxidant (usually oxygen) [81]. Photocatalysis AOPs can be classified as either homogeneous or heterogeneous photocatalysis. Homogeneous PCAOPs involve a situation in which both the reactants and the photocatalysts exist in the same phase. The use of ozone and photo-Fenton systems (Fe+ and Fe+/H2O2) remains the most common homogeneous photocatalyst. This system involves the use of Fe2+ as catalyst and inorganic oxidants like H2O2 under light irradiation (reaction 3.23). The process is often referred to as the photo-Fenton process. The edge that this process has over the conventional Fenton process arises from the regeneration of the Fe2+ catalyst from Fe3+ with additional generation of %OH radicals (reaction 3.24), thus reducing the required iron salt concentration and the iron sludge produced. The efficiency of the process is greatly influenced by the pH of the system. The pH range for the process is restricted within the range of 2.5–5.0, and so, it must be monitored and controlled throughout the process. However, chelating the Fe2+ with ligands like humic acids, nitrilotriacetate, and ethylenediaminetetraacetic acid can facilitate the operation of the process near neutral pH [81]. Other Fenton-like processes involving the use of different forms of iron such as iron nanoparticles, microparticles, solid iron, and other oxi
dants like S2O2
8 , HSO5 have been explored [82]: Fe3 + + H2 O + hv
!Fe2 + + OH + H +

(3.23)

Fe2 + + H2 O2
!Fe3 + + OH +
OH

(3.24)





In heterogeneous PCAOPs the catalyst and the reactants are in a different phase. The catalyst is a semiconductor material with a suitable bandgap, which generates electron-hole pairs on irradiation by light of suitable wavelength. The primary ROS formed in this process are the hydroxyl and superoxide anion radicals [9]. In a semiconductor photocatalyst (SP)/UV system, the photogeneration of reactive species is according to the reaction steps in reactions (3.25)–(3.37) [83]:

63

64

CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

+ SP + hvðUVÞ ! SP e
CB + hVB




(3.25)


 + SP hVB + H2 O ! SP + H + + OH

(3.26)


 + SP hVB + OH
! SP + OH

(3.27)



 SP e
CB + O2 ! SP + O2

(3.28)

+ O
2 + H ! HO2

(3.29)

2HO2 ! H2 O2 + O2

(3.30)

H2 O2 ! 2OH

(3.31)

Dye + OH ! CO2 + H2 O

(3.32)


+ ! Oxidation productus Dye + SP hVB

(3.33)


Dye + SP e
CB ! Oxidation productus

(3.34)











In the presence of ozone, superoxide radical can react with O3 to generate ozonide radicals in a process called photocatalytic ozonation (reactions 3.30–3.32) [84]: O3 + e

!O3


(3.35)

O3
+ H + >HO3

(3.36)

HO3
!O2 + HO

(3.37)









The major focus in heterogeneous PCAOPs is the development of novel catalysts, with improved properties and characteristics to overcome present limitations in terms of bandgap, mass transfer limitation, and electron-hole recombination. This is being vigorously pursued by exploring different synthetic routes and materials’ composition. Though complete mineralization of pollutants is the major aim of AOP, the formation of intermediate compounds has been reported. These intermediates have been suggested to impact on the overall mechanism, kinetics, and effectiveness of the photocatalytic process. It is however very important to ensure the intermediates formed by the process are not also toxic as the overall aim of the process is then defeated. Yu et al. [85] studied the intermediate compounds formed during the photocatalytic degradation of ciprofloxacin and proposed a reaction mechanism for its degradation as shown in Fig. 3.2. The frontier orbital theory was employed in

3.2 Wastewater treatment technologies

FIG. 3.2 Degradation intermediates and pathway for the photocatalytic degradation of ciprofloxacin by Zn-doped Cu2O. Adapted from Yu et al. [85].

determining the frontier electron densities (FED) of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the ciprofloxacin and the intermediates obtained. The pathway was then proposed based on the frontier orbital theory principle that 2FED2HOMO and FED2HOMO + FED2LUMO values can be used to deduce the sites of attack by %OH and h+. While h+ will attack atoms with large 2FED2HOMO, the preference of %OH is for atoms with high FED2HOMO + FED2LUMO. A detailed explanation of how the pathway was constructed using data from HPLC-MS/MS spectra of intermediates from the process can be obtained from the publication for further consultation. The significant contribution of h+ and %OH to the photoactivity of the material was confirmed by carrying out by trapping the active species using radical scavengers. The holes, superoxide, and hydroxyl radicals were scavenged using potassium iodide, nitrogen, and isopropanol, respectively. A reduction in activity from 94.5% to % radicals were 16.2%, 32.0%, and 61.2% was observed when h+, OH, and O
2
% removed, respectively. This shows that O2 also played a relatively significant role in the activity of the photocatalyst and the proposed mechanism may not have accounted for all the reaction taking place in the system. The mechanism for methyl red degradation as proposed by Mahmoud et al. [86] is shown in Fig. 3.3. The major radical species identified responsible for the photoca% radicals. Dehydrogenated radical species were talytic activity was %OH or O
2 % formed by OH attack on methyl red. This dehydrogenated intermediate can either combine with %OH to form hydroxyl compounds via ring opening or undergo further decomposition to yield smaller-molecular weight compounds.

65

66

CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

FIG. 3.3 Proposed mechanism for the degradation of methyl red dye. Adapted from Mahmoud et al. [86].

3.2 Wastewater treatment technologies

3.2.3.5 Basic principles and mechanism for heterogeneous photocatalytic degradation of pollutants Since TiO2 is the most studied photocatalyst, most studies on the mechanism of photocatalytic degradation have been carried out using TiO2 photocatalysts. Depending on the final product(s) of a photocatalytic process, the reaction may be classified as photomineralization, photodegradation, or photodecolorization [87]. Photomineralization [88] involves complete decomposition of pollutants to H2O, CO2, N2, etc., while photodegradation [89] entails the conversion of the pollutant into stable and nontoxic products. Sometimes photodecomposition is used to refer to both photomineralization and photodegradation. Photodecolorization [90] simply implies the photoreduction or photooxidation of dyes. Fig. 3.4 shows the steps involved in the photocatalytic treatment of dye. The first step in a photocatalytic reaction is the absorption of light photon by the semiconductor to generate photoelectrons (e
CB) that are promoted to the empty conduction band from the filled valence band of the semiconductor and the generation of holes (h+VB) in the valence band (reaction 3.25). The photogenerated electrons and holes are together called electron-hole pairs. For generation of the electron-hole pair to be achieved, the light source must supply energy equal or higher than the bandgap of the semiconductor. UV light source of different sources has been used as light source for photocatalytic reactions especially for TiO2-based processes; however, with the development of techniques to produce TiO2 materials with reduced bandgap and the development of new semiconductor materials with suitable bandgap, light source emitting in the visible region of the electromagnetic radiation can now be used (of particular interest is the use of sunlight as energy source). These generated charge carriers could then subsequently undergo any of the three steps: (i) Recombine and release the imputed energy as heat, (ii) get attached to available metastable states on the surface, or (iii) undergo reaction with electron acceptors and donors adsorbed/ bound at the solid-liquid interface [91]. Since the holes and electron generated are powerful oxidizing and reducing agents, respectively, they can directly drive the oxidation and reduction of pollutants (reactions 3.33–3.34) [92]. Alternatively the photogenerated charge carriers could react with adsorbed species like O2, OH
, and H2O. The photogenerated holes react with bound water or OH
to generate %OH (reactions 3.26 and 3.27), while the electron reacts with oxygen to produce % (reaction 3.28). These generated ROS then react with polsuperoxide radical O
2 lutants to achieve mineralization of the pollutants. Further generation of OH. is % to produce HO.2 (hydroperoxyl radical) and achieved by the protonation of O
2 H2O2 subsequently, which then dissociate to give %OH (reactions 3.29–3.31). Due % % to this contribution of O
2 to OH generation, it is generally believed that the photo% catalytic efficiency of semiconductor is strongly dependent on the rate of O
2 and %OH generation [92]. Thus, from the previously described mechanism for the photocatalytic degradation of pollutants, an effective photocatalyst must satisfy some basic criteria, which include [93] the following: (i) The catalyst must possess low bandgap for easier generation of electron-hole pair, (ii) the catalyst must also exhibit high photon

67

Red

Violet

Radiation intensity (amount)

Ultravlolet

Visible light 7%

44%

Near infared

Far intared 11%

37%

0.4 0.7 1.0 Wavelength (μm)

1.5

Microwave TV waves Less than 1%

0.001 Wavelength (m)

FIG. 3.4 Mechanism of photocatalytic degradation of dye.

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

absorptivity for large electron-hole pair generation, (iii) the charge carrier recombination must be minimal in the catalyst to achieve high quantum efficiency, (iv) the catalyst must possess sufficient surface area to act as reaction sites, and (v) the catalyst must exhibit good chemical and physical stability to enhance mass transfer (Table 3.2).

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS) Ternary metal sulfide nanomaterials have become subject of great interest for researchers in the field of nanotechnology and materials science. This is due to their unique structural, chemical, and physical characteristics, which have made them potential materials in numerous optoelectronic devices including sensors, biomarkers, superconductors, and solar cells and also as photocatalysts and sensitizers. They are formed by substituting the metal in the binary system with two metals while still maintaining equal total charge [103]. Compared with binary compounds the addition of a third element makes ternary sulfides more interesting due to increased stoichiometric variation, access to inaccessible bandgaps, and synergistic effect [104]. For instance the performance of CuFeS2 nanosheet was reported to be much higher than those of FeS2 and CuS in the hydrogen evolution reaction [105]. A wide variety of ternary metal sulfides have been identified; however, CBTS have gained more attention because of its abundance and low toxicity.

3.3.1 Classification of copper ternary metal sulfides CBTS compounds are obtained by incorporating transition metals (Ga, Zn, Cr, and Fe) or main-group metals (In, Sn, Bi, and Sb) into binary CuS system, resulting in wide variety of stoichiometries as shown in Fig. 3.5. Although diamond-like structure is the main crystal structure in Cu ternary chalcogenides, the lower enthalpy of 2+
2+ formation of defect pairs like 2Cu2
In + InCu and 2VCu + InCu accounts for the occurrence of ordered nonstoichiometric compounds like CuIn3S5, CuIn5S8, and CuIn7S11 [21]. CBTS can be classified on the basis of the position of the second metal in the periodic table. Identified classes include group I-III-VI2, group I-IV-VI, and group I-V-VI. The main advantage of these materials is that their morphology, crystal structure, size, stoichiometry, and thus applications can be controlled through synthesis conditions like precursor type and ratio, temperature, capping agent/solvent, and the time of reaction (Table 3.3). Table 3.4 gives an overview of some reported CBTS and their properties.

3.3.1.1 Group I-III-VI2 compounds (CuInS2 and CuGaS2) This class of semiconductor materials is suitable as thin film absorbers in photovoltaic cells due to their direct bandgap, good electrical and radiation stability, and high absorption coefficient (10
5 cm
1) [112]. They are also suitable materials in energy

69

Table 3.2 Parameters and efficiency of some photocatalytic process.

Photocatalyst

Light source

Radiation intensity (W)

PET-TiO2

Solar light

500

Pillared clay/ H2O2 TiO2-CF Pr6MoO12

UV

795.8

UV UV

500 NA

TiO2-LDH BiOBr/ Bi12O17Br12 Ag4P2O7

UV Solar light UV

8.85 500 W 300 15 W

DyVO4 TiO2 Zn-doped Cu2O

Visible Visible Visible

400 500 500

Pollutant Lincomycin Moxifloxacin Trimethoprim Sulfadiazine Sulfamethoxazole Isoniazid Metronidazole Real winery waste Rhodamine B Methylene blue Acid red 92 Cr(VI) Resorcinol NO Rhodamine B Rhodamine 6G Erythrosine Anthraquinone Ciprofloxacin

PET, poly(ethylene terephthalate); NA, not available.

Catalyst concentration

Pollutant concentration

Time (min)

Efficiency

Rate constant (min21)

50 mg/L

1 mg/L each

98 mM H2O2

250 mg C/L

120 120 360 360 360 120 120 240

100  100 90 93 98  100  100 79.3

0.024 0.065 0.007 0.009 0.015 0.026 0.037 0.0038

1.5 g 0.05 g

10 mg/L 20 mg/L

60 80

100 mg 0.03 g 0.06 g 50 mg

20 mg/L 10 mg/L 400 ppb 0.01 mM

150 30

0.05 g 40 mg 30 mg

5 mg/L 100 mg/L 20 mg/L

97 95 68 96.81 93 57 98 99 88 88 94.5

70 25 180 240 240

No of recycle

References

5 cycles

[95]

3 cycles

[96]

NA 0.038 0.015 0.012 NA

10 cycles NA

[97] [98]

NA NA

[99] [100]

0.057 0.184 NA 0.009 0.004

3 cycles

[101]

5 cycles NA 5 cycles

[102] [103] [86]

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

FIG. 3.5 Copper ternary sulfides.

storage devices [113] and medical imaging [114]. Copper ternary sulfides in this class include CuInS2 (CIS) and CuGaS2 (CGS). The CIS is the most studied of these two compounds. The three crystal phases wurtzite, zinc blende, and chalcopyrite phases are identified for CIS (Fig. 3.6). In bulk materials, only the chalcopyrite phase is stable at room temperature, while the wurtzite and zinc blende phases are only stable at high temperatures (>980°C). However, all three phases are stabilized in nanomaterials. Its direct bandgap, defect tolerance, good radiation hardness, and absorption coefficient make it a highly potent material in photovoltaics [103]. The high defect tolerance makes large dopant loading possible and tuning of bandgap via defect site concentration. The photoluminescence (PL) property of CIS is also greatly influenced by its complex crystal structures and nonstoichiometric compositions. CIS exhibits a multiexponential PL decays that is broadband (with full width at half maximum of approximately 200–300 meV), strong stokes shift of 300 meV, and a long decay constant. However, pristine CIS nanocrystal gives a PL quantum yield of within 5%–10% [115]. By forming heterostructures like core-shell compounds (e.g., CIS/ CdS and CIS/ZnS), PL quantum yield of 70%–85% and a triple fold increase in the emission life time can be obtained [116]. Despite numerous studies on the PL properties of CIS, the process is still yet to be fully understood. The donor-acceptor pair (DAP) recombination mechanism is one of the many plausible explanations for photoluminescence in CIS. It proposes the

71

Table 3.3 Properties and applications of selected Cu ternary sulfide nanoparticles. Compound

Bandgap (eV)

Crystal structure

Lattice parameters (A˚)

Absorption coefficient (cm21)

CuFeS2

0.35

Tetragonal

a ¼ 5.29, c ¼ 10.42

10
5

CuInS2

1.45–1.53

Chalcopyrite

a ¼ 5.52, c ¼ 11.12

Wurtzite

a ¼ b ¼ 3.897, c ¼ 6.441 a ¼ 5.41 a ¼ 6.525, b ¼ 7.523, c ¼ 37.662 a ¼ 6.02, b ¼ 3.80, c ¼ 14.50 a ¼ 7.723, b ¼ 10.395, c ¼ 6.716 a ¼ 5.347, c ¼ 10.474 a ¼ 3.777, c ¼ 5.248 a ¼ 5.309

Cu3SnS4

1.22–1.46

Zinc blende Orthorhombic Tetragonal

CuSbS2

0.90–1.38

Orthorhombic

Cu3BiS3

1.2–1.5

Orthorhombic

CuGaS2

1.0–2.2

Chalcopyrite Tetragonal Zinc blende

Applications

References

Thermoelectric; energy storage Photovoltaics; catalysis

[118, 120] [118, 121]

104

104

Photovoltaics; catalysis

[122, 123]

104

Photovoltaic; energy storage Photovoltaic; thermoelectric

[124]

Catalysis

[126]

104 105

[125]

Table 3.4 CBTS synthesis parameters and properties.

Material

Synthesis method

CuInS2

Solvothermal

CuInS2

Solvothermal

CuInS2 CuInS2

Solvothermal Hydrothermal

CuInS2 CuInS2

Solvothermal/ templating Hydrothermal

CuInS2

Solvothermal

CuGaS2

Solvothermal

CuGaS2 CuGaS2

Hot injection Heat up

CuInS2

Temperature (°C)

Solvent/ capping agent

240

ODE

240

Morphology

Phase

Size (nm)

References

Wurtzite

8.0

[144]

ODE

Irregular nanodisk Particles

Chalcopyrite

5.0

[144]

240 150

ODE H2O/MPA

Particles Quantum dot

Zinc blende –

–

[144] [145]

230

–

Nanowire

Chalcopyrite

200

[146]

150

MPA

Quantum dots

Chalcopyrite

2.2

[147]

130

DMF

–

–

[148]

240

OLA

Quantum dots Nanocrystals

Chalcopyrite

[121]

GaCl3/Cucl/S GaCl3/CuCl/SC (NH2)2

180 240

DDT OLA

Nanocrystals Nanowire

Chalcopyrite Wurtzite

Pyrolysis

(PPh3)2CuIn(SEt)4

200

DOP/HT

Nanocrystal

–

CuGaS2

Thermolysis

200

–

Nanocrystal

Cu2SnS3

Sputtering/ sulfurization

[(iPr3PCu)2(Me2Ga)2 (SCH2CH2S)2] Sn/CuSn

–

–

Thin film

Zinc blende/ wurtzite Monoclinic

5.6– 10.9 3.0 4 μm  50 nm 2.1– 4.0 200– 300 2.57μm thick

Precursors Cu(acac)2/In(NO3)3/ DDT Cu(acac)2/In(acac)3/ DDT CuCl/In(acac)3/DDT CuCl2  2H2O/ InCl  4H2O/CS(NH2)2 CuCl/InCl3/DETA CuCl 2H2O/ InCl3 4H2O/CS (NH2)2 Cu(acac)2/In(acac)3/ TG GaCl3/Cucl/S

[121] [121]

[149] [122] [150]

Continued

Table 3.4 CBTS synthesis parameters and properties—cont’d

Material

Synthesis method

Cu2SnS3 CuSbS2

Ball milling Thermolysis

CuInS2

Thermolysis

Cu3BiS3 Cu3BiS3

Sulfurization Microwave

CuSbS2 Cu3SbS3 Cu3SbS4 CuGaS2

Hot injection Hot injection Hot injection Heat up

CuInS2 CuInS2

Solvothermal Sonochemical

Precursors

Temperature (°C)

Solvent/ capping agent

Morphology

Phase

Size (nm)

References

Cu/Sn/S Cu(CS2CO(C5H6)) (CH3)2/Sb(CS2CO (C5H6)(CH3)2 Cu(CS2CO(C5H6)) (CH3)2/In(CS2CO (C5H6)(CH3)2 CuO/Bi2O3/S BiCl3/CuCl/ C3H7NO2S CuCl/SbCl3/S CuCl/SbCl3/S CuCl/SbCl3/TMS CuCl/GaCl3/DT

380 250–300

– Chlorobenzyl/ n-hexylamine

Thin film Thin film

Chalcopyrite Orthorhombic

7.81 50– 100

[151] [152]

25

OLA

Nanoparticles

Chalcopyrite

2–3

[153]

420 700 W

– Cysteine

Thin film Dendrites

Orthorhombic Orthorhombic

– –

[154] [155]

260 240 200 270

OLA ODE/OLA OLA ODE

Nanoparticles Nanoparticles Nanoparticles Nanocrystal

Orthorhombic Tetragonal Monoclinic Chalcopyrite

[156] [156] [156] [157]

CuCl/Incl3/TAA Cu(NO3)2/In(NO3)3/ TAA

160 65

DMF/EG Ethanol

Nanocrystal Nanoparticle

Chalcopyrite Tetragonal

14.3 10.8 10.5 70– 350 – 9.2– 14.8

[158] [159]

DETA, diethylenetriamine; MPA, mercaptopropionic acid; DMF, N,N-dimethylformamide; TG, 1-thioglycerol; OLA, oleylamine; DDT, 1-dedecanethiol; ODE, 1-ocadecene; DOP, dioctyl phthalate; HT, hexanethiol; TMS, bis(trimethyl)sulfide; DT, 1-dodecanethiol; TAA, thioacetamide; EG, ethanediol.

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

Chalcopyrite Zinc blende Wurtzite

Chalcopyrite Zinc blende

Normalized Intensity (a.u.)

Wurtzite

c c a

c a

b a

b

(B)

(C)

(A)

b

= Cu = In =S

20

(D)

30

40

50

60

70

2θ (degrees)

FIG. 3.6 CuInS2 crystal phases and their XRD patterns. Adapted from Leach and Macdonald [117].

presence of deep acceptor level (which could either be a Cu vacancy (Vcu) obtained under In3+-rich synthesis conditions or indium vacancy (Vin)/Cuin defects obtained under Cu+-rich synthesis conditions) and a somewhat shallow donor level that could be interstitial indium, vacant sulfur, or interstitial copper [118]. CuGaS2 (CGS) is a less studied material because of the difficulty associated in obtaining single-phase growth. Like CuInS2, its properties are also influenced by its propensity for intrinsic defect formation, which may arise due to VCu or substitution of Cu by Ga atoms [119]. CuGaS2 possess high absorption coefficient in the range 105–106 cm
1 and a bandgap of 2.49 eV [120]. CGS has been reported to crystallize in four different crystalline phases as shown in Fig. 3.7.

3.3.1.2 Group I-IV-VI2 compounds (Cu2SnS3 (CTS) and Cu3GeS3) Cu2SnS3 (CTS) and Cu3GeS3 are the stoichiometric members of this group. The interest in this class of material is due to unique properties and their potential for application in thermoelectric, nonlinear optics, energy storage, and photovoltaic cells. The most studied is the Cu2SnS3, and it is a p-type semiconductor with bandgap that is greatly influenced by the preparation method. Bandgap range of 0.93–1.15 eV has been reported using methods like direct evaporation, DC magnetron sputtering, and electrodeposition [122]. This varying bandgap is due to different Cu2SnS3 stable phases like orthorhombic, cubic, monoclinic, tetragonal, and wurtzite phases [123]. The temperature at which each phase is formed or their exact bandgaps are still subjects of contention among researchers [124]. At temperatures between the range 300°C and 400°C, the tetragonal phase is the stable phase [125], while, at temperatures >400°C, Cu2SnS3 may crystallize in both the cubic or monoclinic phase and the bandgap varies between 0.95 and 1.11 eV [126]. Above 520°C a pure monoclinic phase with a lower bandgap of 0.93 eV and another observed optical transition at 1.0 eV are obtained [127]. The monoclinic phase has the highest solar energy conversion compared with the other phases in solar structures due to transitions arising from the valence band to conduction band and not defect sites. This confirms that the

75

76

CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

FIG. 3.7 Crystal structures of CuGaS2 (A) chalcopyrite phase, (B) zinc blende phase (C), orthorhombic phase, and (D) wurtzite phase. Adapted from Kluge et al. [121].

monoclinic phase is less defective and thus has few electronic defects resulting in a greater voltage potential in an open circuit [124, 128]. Off-stoichiometric forms of the Cu-Sn-S system include Cu4SnS4, Cu4Sn7S16, and Cu4Sn15S7. All these compounds have Cu in the oxidation state of +1. However, other off-stoichiometric forms such as Cu10Sn2S13, Cu4SnS4, and Cu5Sn2S7 with Cu in the +2 oxidation state also exist [129].

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

3.3.1.3 Group I-V-VI (Cu-Sb and Cu-Bi chalcogenides) Cu-Sb and Cu-Bi chalcogenides present avenue for the production of photovoltaic absorber materials that have low toxicity and are earth abundant. The ns2 lone pair electrons on the Bi and Sb ions impart novel properties on its materials. In Sb and Bi compounds, off-stoichiometric compounds are also formed due to the presence of antibonding orbitals that are compensated for by the inclusion of distortions into the compound’s crystal lattice, which results in broken symmetry, thus leading to complex crystal structures [130]. Among the studied Cu-Sb systems, CuSbS2 is the most explored absorber material because of its ideal optoelectronic character. It is a p-type conductor because of its grain growth at low temperature and high VCu [131]. It has a bandgap of 1.49 eV and an optimum bandgap range of 1.1–1.6 eV from the Shockley-Queisser analysis. It, thus, absorbs within the visible range with a high absorption coefficient (>10 cm
1) [132]. The tunable hole concentration and mobility range between 1016 and 1018 cm
3 [133] and 0.1–1.0 cm2/VS [134]. Synthesis of CbSbS2 with a pure phase requires great care due to the presence of many stable off-stoichiometric compounds like Cu3SbS4, CuSbS3, and Cu12Sb4S13 [135]. This can however be achieved by varying the Cu/Sb ratio during synthesis [136]. Cu-Bi system is also made up of a group of compounds that are promising p-type semiconductors. It also shows great potential in photovoltaic devices as solar absorbers [137]. Cu3BiS3 is the most researched of the materials in the group. It exhibits good thermal stability and an optical absorbance within the visible region [138]. The absorption coefficient is around 105 and has a direct bandgap in the range 1.10–1.72 [139]. Like Cu-Sb systems, all its elements are earth abundant and nontoxic, making it environmentally friendly and less costly. The carrier concentration is in the order of 1016 cm
1, and it possesses a Hall mobility and thermoelectric power of 4 cm2/VS and 0.73 mV/K, respectively [137b]. The crystal phase of CBS and CAS is shown in Fig. 3.8 Both CuSbS2 and Cu3BiS3 crystallize in the orthorhombic phase, with each unit cell comprising tetrahedral CuS units and distorted square pyramidal (Sb, Bi)S5 units [140]. Off-stoichiometric compounds for both Cu-Sb and Cu-Bi systems also exist. Cu3SbS3, Cu12Sb4S13, CuBi3S5, and Cu4Bi4S9 are some of the reported stoichiometric compounds [130]. Fig. 3.9 shows the XRD pattern, HR-TEM images, and SAED patterns for CuSbS2 and other off-stoichiometric systems.

3.3.2 Synthesis of copper-based ternary metal sulfides The synthetic route to copper-based ternary metal sulfides influences the morphology, crystal phase, optical properties, and composition of the prepared compounds. In the synthesis of these materials, the choice of precursors, solvent/capping agent, and synthesis conditions, most importantly temperature, are parameters used to control the synthesis of copper-based ternary systems. Table 3.4 is a list of some synthetic routes to CBTS and properties of the produced materials. Multiple precursor methods have been explored widely in the synthesis of ternary materials; the

77

78

CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

c

a

b

(A)

(B)

FIG. 3.8 Normal configuration (A) and polyhedral configuration (B) of the crystal structure of Cu(Sb,Bi)S2 (Kumar and Persson [140]).

precursor could either be elemental, inorganic salts, or organometallic precursors of the required element. In multiple precursor synthesis carried out in solution, there is a need to achieve a balance in the reactivity of the metal ions toward S2
through the use of stabilizers. The use of multiple precursors is desirable because it involves mild process conditions. However, there are difficulties in stoichiometry control and contamination [142] and often involves the use of toxic pyrophoric precursors. Recently the use of single molecule or single-source precursors (SSP) containing the elements required in a ternary semiconductor has been explored to eliminate the use of many precursors. Numerous complexes have been synthesized as SSPs such as dithiocarbamate, xanthate, and thiocarboxylate. With SSP, cheaper precursors that result into pure nanoparticles could be obtained. To prepare ternary semiconductor materials through SSP, two approaches are possible: (i) A single precursor containing the two metals and the chalcogen may be thermally decomposed in the presence of a capping agent, and (ii) to exert more control over the stoichiometry of the ternary nanoparticles, two single-source precursors (dual source precursor) containing each of the metals and a chalcogen in its molecule are decomposed thermally in the presence of a capping agent.

3.3.2.1 Synthesis of group I-III-VI2 compounds (CIS group) Due to the large interest in this class of compounds, a wide range of synthetic routes have been explored in their synthesis, resulting in compounds with unique

40

60 2q (degree)

80 20

40

60

80 20

2q (degree)

(A²)

(B²)

(C²)

(A²¢)

(B²¢)

(C²¢)

Cu3SbS3

(124)

(JCPDS card No. 82-0851)

(242) (324) (–226)

(311)

(200) (004) (212) (114) (131)

Intensity (a.u.)

(622)

(440) (611)

(431) (521)

(220)

Intensity (a.u.)

(119)

(215) (304) (203)

(015) (212) (213)

(013)

20

(C¢)

Cu12Sb4S13 (JCPDS card No. 42-0561)

(321) (400) (411)

(111)

(222)

(B¢)

CuSbS2 (JCPDS card No. 65-2416)

(004) (104) (200)

Intensity (a.u.)

(A¢)

(–132)

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

40

60 2q (degree)

80

FIG. 3.9 XRD pattern of CuSbS2 (A), Cu12Sb4S13 (A0 ) and Cu3SbS3 (A00 ); HR-TEM images of CuSbS2 (B), Cu12Sb4S13 (B0 ), and Cu3SbS3 (B00 ); and SAED patterns of CuSbS2 (C), Cu12Sb4S13 (C0 ), and Cu3SbS3 (C00 ). Adapted from Xu et al. [141].

morphology and properties. These compounds have been produced via many synthetic routes including solvothermal reactions [159], seed catalytic growth [112], and hot solvent methods [112]. The use of multiple precursors has been widely explored in synthesis of Cu-In-S. However, Cu2+ reacts faster with S2+ than In3+ because they are both soft Lewis acid, while In3+ is a hard Lewis acid. Due to this difference in reactivity of Cu+ and In3+ toward S2
, there is the need for controlled release of the metal ions during the reaction, which is achieved by the use of stabilizing agents like thiols and carboxylic acid [160]. Alternatively, simultaneous release of the metal ions can be achieved by employing single-source precursors (SSPs). CuInS2 was obtained by heating stoichiometric quantities of elemental Cu, In, and S in an evacuated quartz ampoule loaded with iodine as the transport agent [161].

79

80

CHAPTER 3 Copper-based ternary metal sulfide nanocrystals

The use of metal salts and chalcogenide salts in suitable solvents/capping agent is a highly explored synthesis route. Copper (II) chloride dihydrate, indium (III) sulfate anhydrous, and sodium sulfide nonahydrate precursors in deionized water and employing thioglycolic acid as the capping agent produced CuInS2 at 80°C after stirring for 48 h under atmospheric condition. Phase control between the chalcopyrite and wurtzite phase could be achieved through altering the reaction solution pH [162]. A one-step electrodeposition technique using aqueous mixture of CuCl2, InCl3, and Na2S2O3, followed by calcination in N2/H2 gas mixture at 500°C, produced chalcopyrite CuInS2 thin films. One-pot thermolysis of CuCl, GaCl3, and 1-dodecanethiol in a noncoordinating 1-octadene solvent at 270°C produced CGS nanoplates, with a polytypic phase consisting of 60% wurtzite and 40% zinc blende phases [156]. Using the heat-up method and 1-dedecane as capping agent, diethyl dithiocarbamate complexes of Cu and In were employed in the synthesis of uniform and highly crystalline CuInS2, which was in the wurtzite phase [108a]. Cowen et al. reported the thermal decomposition of [(LR)2Cu(SR0 )21n(SR0 )2] (L ¼ P, As, and Sb; E ¼ S; R0 and R ¼ alkyl and aryl) precursors under anaerobic condition in the deposition of thin films of CuInS2 in low temperature/pressure spray chemical deposition technique. The films obtained were single phase with no presence of impurity observed [163].

3.3.2.2 Synthesis of group I-IV-VI2 compounds (CTS group) Various group I-IV-VI2 compounds have been prepared using different methods such as direct evaporation, annealing of superimposed precursors [122b], magnetron sputtering [122a, 123c], electrodeposition of precursors [160], and spray pyrolysis [164] employing different precursor types. The properties (electronic and physical) of these materials are also controlled by the synthetic methods. Elemental Cu, Sn, and S coevaporated and then annealed at 570°C in the presence of lump sulfur under N2 atmosphere were employed in the synthesis of CTS films as absorber layers in solar cells [165]. CTS was also prepared by crystallization from stacked NaF/Cu/Sn precursors, through the precursor annealing in a sulfur/tin for 30 min at 570°C. Control on the grain size of the CTS films could be exerted by altering the ratio of NaF/Cu mole ratio in the precursor. It was observed that an increase in the NaF/Cu mole ratio resulted in a decreased grain size [166]. Jia et al. [164] reported the spray pyrolysis of CuCl22H2O, SnCl2, and thiourea precursors dissolved in distilled water. The spray pyrolysis was carried out under N2 atmosphere with temperature range of 325–400°C. Two stoichiometries of CTS, Cu2SnS3 and Cu4SnS4, were prepared by depositing thin film of copper sulfide over thin films of tin sulfide via sequential chemical deposition. Annealing of the layers in N2 atmosphere at 350°C and 400°C gave the Cu2SnS3 and Cu4SnS4 materials, respectively [167].

3.3.2.3 Synthesis of group I-V-VI compounds This class of materials can be prepared from the elements using methods such as ball milling and solvothermal synthesis or binary compounds using coevaporation, chemical bath deposition, sputtering, sulfurization of metal films, and single source precursors.

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

Solvothermal synthesis of Cu3BiS3 nanorod using solvent as a means of aspect ratio control was reported by Chen et al. [138]. The precursors CuCl2  2H2O, BiCl3, and thiourea were dissolved in three different solvents: ethanol, ethylene glycol, or glycerine in an autoclave maintained at 160°C. CBS thin film deposited using the chemical bath process post annealed at temperatures of 573 and 673 K showed an increase in crystallinity with increase in annealing temperature [168]. Thermal annealing of metal precursors Cu and Bi followed by magnetron sputtering with thermoevaporated S layer heated at 250°C in Ar atmosphere for 30 min produced Cu3BiS3 with the orthorhombic phase [169]. Coevaporation of metallic precursors in a sulfur environment generated through the evaporation of elemental sulfur produced Cu3BiS3 thin films in a two-stage process [170]. A two-stage process of magnetron sputtering and thermal evaporation using elemental precursors was also reported in the synthesis of Cu3BiS3 thin films. About 0.3-μm-thick layers of Cu and Bi was initially deposited on Mo-coated soda-lime glass, which was then coated by a 1.5-μm-thick layer of sulfur via thermal evaporation in Ar atmosphere at 250°C for 30 min [171]. By varying the Cu/Bi ratio, the ratio of capping agent and the reaction temperature, cothermodecomposition of dual source single source precursor of Bi(Dtc)3 and Cu(Dtc)3 (diethyldithiocarbamate [Dtc]) was reported in the synthesis of CuBiS3 nanosheets, CuBiS3 nanoparticles, Cu4Bi4S9 nanowires, and CuBi4S9 nanoribbons [108a]. Nanocrystals of CBS was prepared via the hydrothermal decomposition process, by heating a mixture of CuCl dissolved in thiourea and BiCl3 dissolved in thiourea in an autoclave at 120°C for 10 h under autogenous pressure [172]. The ability to exert control over the morphology and composition through synthetic routes of CAS is still in the preliminary stage, and there is a continuous search for more facile routes. Cothermodecomposition of dual source single source precursors M(Dtc)3 (M ¼ Cu, Sb) was employed in the synthesis of three different stoichiometries of CAS (trigonal pyramidal Cu12Sb4S13, rectangular CuSbS2, and rhombic Cu3SbS3) by varying the solvent substituents on the thiocarbamate metal complex [141]. The dual source precursor decomposition of dithiocarbamate metal complexes of antimony and copper was also reportedly employed in the synthesis of offstoichiometric Cu12Sb4S13 CAS material in the cubic phase [151].

3.3.3 Applications of copper metal ternary sulfides CMTS compounds have found applications in numerous fields due to their unique properties and the tunability of these properties to meet specifications for different fields. CMTS have been employed in photovoltaics as solar cell absorber layers, thermoelectric devices, and catalysts in water splitting and pollutant degradation.

3.3.3.1 Photovoltaic devices Photovoltaics are devices based on semiconductors, which generate electricity directly from sunlight. The bandgap of a semiconductors determines its efficiency when employed as absorbers in a PV cell. Presently, silicon is the commonly employed commercial absorber due to its good reliability, low cost, and high

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efficiency (>20%) [173]. However, due to its indirect bandgap and low coefficient of absorption, thick layers (100 μm) of silicon is usually required as absorber layers leading to long diffusion paths before charge carriers can reach electrical contacts. This usually results to loss of photogenerated charge carrier and the need to ensure that high purity silicon is used, which accounts for the manufacturing processes’ high energy consumption and a resultant high cost of production [174]. To overcome these limitation, there is the need to develop absorber materials that have good absorption coefficient and with economical cost of production. Thin film absorbers of CBTS have been identified as suitable candidates that could overcome the limitations of Si-based solar cells. They can be synthesized easily using various solution-based methods. Their high absorption coefficient, high charge carrier concentration, and their constituents (which are from earth-abundant elements) make them viable alternatives to silicon solar cells. The major concern over these materials is their lower power conversion efficiency, compared with silicon absorber. For example, while the best efficiency reported for CIS-based solar cell is 12.5%, the efficiency of Si cells almost doubles the value [175]. The quest for improved efficiency CBTS materials has made them object of intense research. The efficiency of an absorber layer is greatly influenced by the composition, the presence of secondary phases, and the crystal structure of the absorber. Efficiencies of 10.2% and 12.5% have been reported for absorber layers obtained from Cudeficient and Cu-rich precursors, respectively [176]. A study on the efficiencies of CIS absorber with a mixed chalcopyrite and tetragonal phase and a Cu/In ratio of 1.09–1.34 showed the highest efficiency was obtained at Cu/In ratio of 1.22 [176b]. Improving the purity of absorber material is another effective way to improve the activity and efficiency of solution-processed material in photovoltaic cells. The removal of organic residues like melamine, ketones, and esters formed during hightemperature processing was achieved via solvent treatment, resulting in about 40% improvement in the efficiency of the device [177]. CBTS can also be employed in hybrid solar cell devices. A polymer-nanocrystal hybrid solar cell using an absorber layer made of poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5thiophenediyl] (PCDTBT) polymer and CuInS with PCE of 2.1% and open circuit voltage of 550 mV was reported [178]. Rath et al. [179] also reported a polymernanocrystal hybrid solar cell of p-DTS(FBTTh2)2 and CuInS2 with a PCE of 1.3%. Other polymer-nanocrystal hybrid solar cells that have been reported include poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV)/CuInS2 [180] and poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3benzothiadiazole-4,7-diyl-2,5-thiophenediyl]/CuInS2 [181]. CBTS are also employed as counter electrode materials in solar cells. Guo et al. [182] reported a ligand exchanged CuInS2 nanocrystals employed as counter electrode in dye-sensitized solar cells, with improved electrocatalytic activity and electrical conductivity. Also, CuInS2/ZnS nanocrystals chemically bounded to NiO photocathode have been used as a sensitizer to improve the photovoltaic properties of the cathode material [183].

3.3 Copper-based ternary metal sulfide nanocrystals (CBTS)

Evaluation of the photovoltaic application of CAS as absorber layer in solar cells was explored by Pietro et al. [184]. Orthorhombic Cu3SbS3 films deposited and annealed on Mo/glass substrate at 500°C showed a quantum efficiency of 4% in the photon range of 1.1–3.0 eV. Banu et al. [185] evaluated the efficiency of CuSbS2-based hybrid ink prepared via nonvacuum methods and reported a conversion efficiency of 3.22%. The photoconversion efficiencies of CTS thin films in the range 0.08–6.01 has been reported with the efficiency greatly influenced by the composition and the phase purity of the thin film [186]. A detailed review on the photovoltaic application of CTS can be found in the publication by Lokhande et al. [187]

3.3.3.2 Thermoelectric device (TED) The conversion of thermal energy directly to electricity can be achieved through thermoelectric materials. TED works on the principle of the Seebeck effect, which explores the current generated (thermocurrent) due to temperature difference (Δ T) at the point of intersection of two different materials, which results in an electromotive force in the connected circuit (thermoelectromotive force) [188]. With TED, waste heats can be captured and harnessed for electricity generation using devices with no requirements for moving parts or maintenance. TEDs are composed of both p-type and n-type semiconductor units that are connected in parallel thermally and in series electrically [189]. TEDs can either be thermoelectric generator (TEG) or a thermoelectric cooler (TEC) [190]. The efficiency of a TE material at a given temperature is defined by a dimensionless parameter called figure of merit (Z), which is given by the equation ZT ¼

 2  S σ T, κ

where S is the thermopower, σ is the electrical conductivity, κ is the elecrical conductivity, and T is the absolute temperature [191]. A good TE material should therefore possess high electrical conductivity (or power factor), high Seebeck coefficient, and low thermal conductivity. For semiconductor materials, wide bandgap semiconductors are the best TE materials [192]. Most materials employed as TE materials include bulk alloys like CoSb3, PbTe, Bi2Te, and SiGe, with Bi2Te3 the most explored. However, these materials have ZT values 300°C) with ZT value of 0.6 reported for the undoped material [194]. Stabilizing the tetrahedrite phase by incorporating dopants like Zn, Mn, and Ni has resulted in materials with ZT values near 1.

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Weller et al. [195] studied the effect of Zn doping on the thermoelectric activity of tetrahedrite. The doped and undoped materials were prepared using a low-energy modified polyol process that produced nanoparticles in the 50–200 nm range. Evaluation of the thermopower of the material within temperature range of 323–723 K showed the highest ZT value of 0.66 and 1.09 for the undoped and doped materials, respectively. It was observed that in the doped material a reduced thermal conductivity was recorded (0.5 W/mK) compared with the undoped (0.7 W/mK) despite both possessing a lattice thermal conductivity of 0.4 W/mK. This shows the possibility of obtaining highly effective TE materials from this class of materials.

3.3.3.3 Photocatalysis Application of semiconductor materials as photocatalysts in the degradation of dyes is presently a keenly researched area, due to its numerous advantages. This is due to its environmental and economic advantages over other wastewater treatment processes. The photocatalytic efficiency of a photocatalyst is greatly influenced by the light absorption capacity of the material. The light source is also very important as either UV or visible lights could be employed [87]. Photocatalysis has been explored in the treatment of varieties of contaminants such as dyes [196], pharmaceuticals [197], and biological contaminants [198]. CBTS photocatalysts are subject of interest in environmental remediation because of their high light absorptivity coefficient and less toxicity. The photocatalytic activity of Cu3SbS3 obtained via the chemical bath deposition of aqueous solution of CuCl2, SbCl3, and Na2S2O3 was evaluated for the degradation of methylene blue dye under xenon lamp radiation and sunlight. Complete mineralization of the dye to CO2 and H2O was achieved after 3 h of irradiation with 70% and 55% removal under sunlight and xenon lamp, respectively. No loss in the activity was recorded after reuse of the material. Conversion of nitrate to N2 using CuInS2 photocatalyst was reported by Yue et al. [199] under visible light irradiation with conversion rate of 8.32 mg N/N. An adsorption-reduction mechanism was proposed for the process, and the need for the use of sacrificial agent in nitrate mineralization was confirmed. CuInS2 nanocomposite quantum dots were explored in the degradation of rhodamine B under ultraviolet irradiation. The degradation followed the LangmuirHinshelwood mechanism, with the species responsible for degradation being the photogenerated holes and electrons [200]. Li et al. [201] reported the combination of Cu3SbS3 self-doped TiO2 crystal structure to form heterojunction arrays. The composite was used for the photocatalytic degradation of methyl orange. After 120 min, 96.6% degradation of the dye was achieved under sunlight irradiation, and the rate of the pseudo-first order reaction was almost 2.8 times higher than the sample without CuInS2. To study the effect of morphology on the photocatalytic activity of CTS, two CTS materials were prepared using the ball milling and solvothermal method. Morphological studies of both materials showed that, while the ball milling method produced aggregates, the solvothermal process produced flowerlike structures composed of nanosheet. The two samples also showed different optical properties and

3.4 Graphene, its derivatives and photocatalysis

compositions. Evaluation of the photocatalytic activity of the materials revealed a higher activity from the material obtained via solvothermal method compared with the ball milling method. The higher activity of the solvothermal material was attributed to the lower recombination of electron-hole pairs in the solvothermal material [202].

3.4 Graphene, its derivatives and photocatalysis Graphene is a sp2-hybridized single atomic layer of carbon, with hexagonal crystal lattice transverse by π-electrons delocalized about the structure [203]. It has gathered significant research attention due to its unique properties, which include charge mobility of 200,000 cm2/VS, high Young’s modulus of approximately 1000 GPa, thermal conductivity of 5000 W/mK, and high specific surface area record of about 2630 m2/g, coupled with excellent transport phenomenon and magnetism [204]. The most explored route to graphite exfoliation is the oxidation of graphite by strong acids and oxidizing agents to produce graphite oxide. The oxidation process introduces functional groups like hydroxyl, epoxide, carboxylic, and other groups into the layered structure of the graphite oxide leading to a reduction in the van der Waals force holding the layer together in the graphite structure. The degree of oxidation achieved is determined by the method used, precursor, and the conditions of the reaction [204c]. Complete exfoliation leading to the formation of homogeneous graphene oxide (GO) colloid can be achieved either by stirring the aqueous graphene oxide (other solvents like acetone, methanol, and DMF could also be used to produce chemically modified graphenes) for an extended time [205] or by sonication [206]. GO is, therefore, an extensively oxidized single-layer graphite oxide, with better hydrophilicity and negatively charged surface in water that accounts for their stable dispersion in water [204c]. GO and other products obtained from it are electrically insulating due to the interruption of the graphitic framework. Often times, there is a need to convert GO into a conducting material by restoring the graphitic network via GO reduction. Reduction of GO can be achieved by thermal annealing [207], UV radiation [208], electrochemical reduction [209], and chemical method using reductants like hydroquinone [210], dimethylhydrazaine [211], NaBH4 [212], and hydrazine [213]. The reduction of GO, however, does not lead to the complete restoration of the graphitic network of pristine graphene, and defects are significantly introduced in the structure [214]. Therefore the structure and properties of the material produced are often significantly different from that of pristine graphene (Fig. 3.10). To differentiate pristine graphene from the product of GO reduction, the material is often referred to as reduced graphene (rGO). Graphene and its derivatives (GO and rGO) are suitable materials in numerous applications such as polymer composites [215], energy storage [216], gas sensors [217], mechanical resonators [218], and liquid crystal devices [219]. These derivatives are, however, preferable to pristine graphene because they offer routes to overcome the problem of poor solubility [220] and agglomeration [221] in pristine

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Graphene

Oxidation COOH

COOH

OH

OH OH

HOOC

HOOC

OH COOH

COOH OH HO

O

HO

Reduction

O

HO

O OH

HOOC

HOOC COOH

COOH HO HOOC

HO HOOC

HOOC

Graphene oxide (GO)

HOOC

Reduced graphene oxide (rGO)

FIG. 3.10 Structures of pristine graphene and its derivatives (GO and rGO). Adapted from McCoy et al. [223].

graphene, and they can also be produced via simple top-down approach from graphite and other carbon source rather than the more challenging bottom-up approach for pristine graphene [222]. GO is a subject of great interest because of its excellent properties and the ease of functionalization due to the presence of numerous functional groups on its surface. In addition to these advantages, it also possesses a surface area that is comparable with that of graphene. GO is deemed as possessing an amphiphilic structure [224], with hydrophilic properties arising from the functional groups in the basal plane (hydroxy and epoxy groups) and edges (carboxyl group), as indicated in Fig. 3.10, making them highly acidic [225], and hydrophobic properties arising from the carbon layer in the basal plane [226]. The presence of functional groups also influences the electrical, thermal, and mechanical properties of GO. Compared with pristine graphene the thermal conductivity and mechanical strength of GO is lower [227], and while graphene is a conducting material, GO is considered an insulating material [228].

3.4.1 Synthesis of graphene oxide The raw material for GO synthesis is graphite. The graphite source could be both synthetic and natural. However, the major graphite source for chemical synthesis is purified naturally occurring flake graphite mineral due to its possession of lattice

3.4 Graphene, its derivatives and photocatalysis

structure with a lot of localized defects that may act as initiation sites for chemical reactions [204c, 229]. Few layered graphene sheets could be obtained from bulk graphite or other carbon sources like ethylene via mechanical exfoliation, chemical vapor deposition, epitaxial growth on SiC and other electrical insulating surfaces, or chemical synthesis in colloidal systems [230]. Chemical synthesis of graphene is the most suitable route because of the potential for effective large-scale production and suitability for functionalization [176b]. It involves the exfoliation of graphite through processes such as intercalation, oxidation-reduction, chemical derivatization, surfactant addition, thermal expansion, or a combination of any of the processes [213, 231]. GO is prepared from graphite via two major steps. The first step is the oxidation of graphite to graphite oxide, which is readily dispersible in water or any other polar solvent. The choice of oxidant for this step is very important in GO production. Based on the choice of oxidants and acids employed in the oxidation process, three routes to GO production are known: (1) the Brodie’s method [233], (2) Staudenmaier’s method [234], and (3) the Hummers’ method [235]. Fig. 3.11 shows the three methods, the oxidants, and acid employed. The methods involve an initial intercalation of alkali compounds like NaNO3 and KClO3 into the layers of the graphite, subsequently followed by the oxidation of the intercalated graphite with acids such as HCl, HNO3, and H2SO4, to form graphite oxide, which is then hydrolyzed and exfoliated via either mechanical stirring or sonication. Brodie reported the first synthesis of graphite oxide by treating the slurry obtained from graphite powder in fuming HNO3 with KClO3 that were added at once.

KCIO3 & Fuming HNO3 Brodie’s method

COOH

0.335 OH O Conc. H2SO4, KCIO3 & Fuming HNO3

O

OH OH HOOC

Staudenmaier’s method

0.335

O

OH

KMnO4, NaNO3 & Conc. H2SO4

Hummer’s methods

Graphene oxide

Graphite

FIG. 3.11 Methods of GO synthesis. Adapted from Adetayo and Runsewe [232].

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Compositional analysis of the product showed the material contained carbon, hydrogen, and oxygen with percentage composition of 61.04, 1.85, and 37.11, respectively. The long reaction time of about 3–4 days and the release of toxic gases are the major flaws of this method. The Brodie method was improved by Staudenmaier [234] by introducing H2SO4 as additive in the process and also employing excess HNO3. This process allows for the production of highly oxide GO in a single-step process at a much reduced reaction time. However, the addition of the KClO3 is done in aliquots, which may sometimes last a week, and there is a need to purge the chlorine dioxide gas evolved by inert gas making the process more laborious. The risk of explosion is also major hazard concern [236]. The Hummers and Offeman [235] method is the most accepted route to GO synthesis. The process involves the treatment of graphite slurry with NaNO3, H2SO4, and KMnO4, excluding HNO3 and KClO3 from the synthesis. The three stages of the process (i.e., the intercalation, oxidation, and hydrolysis) are carried out at low (88%), liquid entry pressure (>186 kPa), and contact angle (>162 degree).

5.4.3 Zeolites Zeolites are aluminosilicate materials with crystalline lattice containing alkali and alkali earth metals. The fundamental structural characteristic of zeolites is a threedimensional tetrahedral framework in which each oxygen atom is shared by two tetrahedra networks to form regular intracrystalline cavities and molecular-sized channels. The zeolite framework with cavities and internal channels allows easy

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movement of ions and molecules inside and outside the structure. This particular physicochemical property provides the new surface selectivity for zeolites, which makes them suitable for water desalination due to their high adsorption capacity, sieving properties, and ion exchange capacities [96, 97]. Recent studies have proven that zeolite membranes, as a prominent desalination material, utilize the ability of zeolites to remove ions from saline aqueous solution. Dong et al. [98] investigated NaY zeolite-incorporated TFN RO membrane. The results disclosed that at the optimal conditions and the incorporation of 0.15 wt% zeolites, the water flux was risen from 23 to 44 gal/ft2 day and a NaCl salt rejection as high as 98.8% was achieved. Fathizadeh et al. [59] integrated nano-NaX zeolite into the matrix of the PA TFN membrane and observed a superior membrane performance. Cho et al. [99] prepared the NaA zeolite-based membrane and obtained high salt rejection as high as 99.9% with a high water flux (2 kg/m2) at 69°C. Swenson et al. [100] integrated clinoptilolites into the matrix of the membrane for water desalination. The results indicated that a high level of K+ and Na+ rejection (as high as 97.5%) and complete rejection of Mg2+ and Ca2+ could be achieved at 75°C and 1-atm feed side pressure.

5.5 Challenges Although water treatment using NSM has an experienced market, there are several technical challenges that need further investigation and research. Some of the most important NSM challenges are as follows: • • • • • • •

Membrane fouling and scaling should be reduced. Flux needs to increase. Pressure drop and consequently energy consumption should be reduced. Membrane performance need to accurately modeled or simulated. It is better to achieve a uniform pore-size distribution. Chemical and mechanical stability of membrane need to be enhanced. Membrane selectivity should be improved.

Membrane fouling is a very important technical issue. The fouling is the accumulation of materials deposited on the membrane. The fouling results in pore clogging and therefore in decreased flux. Therefore regular and periodic cleaning of the membranes is inevitable. Another problem for NF membranes is membrane performance modeling, which is usually not sufficiently accurate to predict performance in largescale purification units. Laboratory-scale and pilot-scale experiments are recommended because membrane performance depends on different pollutants or other substances present in the particular source of water used. Obviously the selectivity of the membrane to specific materials is very important. Regretfully the rate of rejection in NF and UF is restricted depending on the membrane surface charge and the pore size. Another important issue in industrial wastewater treatment is the membrane resistance to certain substances. Recent research

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has also focused on the development of solvent-resistant NSMs, and major advances have been made in the field. Economic reasons may also stop the extensive installation of NSM units. It should be emphasized that UF is a standard water treatment method for supplying drinking water (for elimination of bacteria or viruses), while NF is much less conventional. In general the water industry operates conservatively and resists against applying new methods as long as current methods are in compliance with rules and regulations. In many cases, conventional methods are inexpensive, and the installation of NSM units needs a completely different and specific maintenance structure. Hence, the NSM market is currently predominantly in new water treatment units and in areas requiring the removal or recovery of specific substances. There is no concern about the social and environmental impacts of NSM. Since the NSMs do not contain nanoparticles in their structure, they are not expected to impose a negative impact on human health and environment. In fact the NSMs can be employed to separate undesirable nanoparticles in wastewater treatment units. Besides, ensuring strict standards for any material in contact with water is essential to guarantee water quality. However, one remaining social concern is the installation of NSM units in developing countries. Inasmuch as membrane maintenance needs experienced experts, water treatment plants are only installed in industrialized countries where trained specialists are present.

5.6 Conclusions This chapter highlighted and discussed the different aspects of nanostructured membranes including concept, classification, performance for water treatment, and possible challenges. The studies showed that the nanostructured membranes are promising materials for efficient purification of water and wastewater. Although nanostructured membranes offer many advantages, a careful evaluation of the limitations of these membranes, including material availability, cost, convenience of processing, adaptability, scalability perspective, and safety hazards associated with nanomaterials, is required. If extensive and focused research is conducted on the design and characterization of new nanostructured membranes, restrictions may be identified. Future investigations can then focus on improving or removing current drawbacks.

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[65] B.S. Karnik, et al., Fabrication of catalytic membranes for the treatment of drinking water using combined ozonation and ultrafiltration, Environ. Sci. Technol. 39 (19) (2005) 7656–7661. [66] N. Ma, et al., Ag–TiO2/HAP/Al2O3 bioceramic composite membrane: fabrication, characterization and bactericidal activity, J. Membr. Sci. 336 (1–2) (2009) 109–117. [67] Y.-L. Zhao, J.F. Stoddart, Noncovalent functionalization of single-walled carbon nanotubes, Acc. Chem. Res. 42 (8) (2009) 1161–1171. [68] K. Balasubramanian, M. Burghard, Chemically functionalized carbon nanotubes, Small 1 (2) (2005) 180–192. [69] A. Abbas, et al., Heavy metal removal from aqueous solution by advanced carbon nanotubes: critical review of adsorption applications, Sep. Purif. Technol. 157 (2016) 141–161. [70] Ihsanullah, et al., Effect of acid modification on adsorption of hexavalent chromium (Cr (VI)) from aqueous solution by activated carbon and carbon nanotubes, Desalin. Water Treat. 57 (16) (2016) 7232–7244. [71] C. Rizzuto, et al., Multiwalled carbon nanotube membranes for water purification, Sep. Purif. Technol. 193 (2018) 378–385. [72] C. Thamaraiselvan, et al., Characterization of a support-free carbon nanotubemicroporous membrane for water and wastewater filtration, Sep. Purif. Technol. 202 (2018) 1–8. [73] J. Saththasivam, et al., A novel architecture for carbon nanotube membranes towards fast and efficient oil/water separation, Sci. Rep. 8 (1) (2018) 7418. [74] T.H. Kim, et al., Biocatalytic membrane with acylase stabilized on intact carbon nanotubes for effective antifouling via quorum quenching, J. Membr. Sci. 554 (2018) 357–365. [75] Y. Wang, et al., Influence of CNT-rGO composite structures on their permeability and selectivity for membrane water treatment, J. Membr. Sci. 551 (2018) 326–332. [76] J. Lee, S. Jeong, Z. Liu, Progress and challenges of carbon nanotube membrane in water treatment, Crit. Rev. Environ. Sci. Technol. 46 (11 12) (2016) 999–1046. [77] R. Das, B.F. Leo, F. Murphy, The toxic truth about carbon nanotubes in water purification: a perspective view, Nanoscale Res. Lett. 13 (1) (2018) 183. [78] P. Wick, et al., The degree and kind of agglomeration affect carbon nanotube cytotoxicity, Toxicol. Lett. 168 (2) (2007) 121–131. [79] Y. Manawi, et al., Can carbon-based nanomaterials revolutionize membrane fabrication for water treatment and desalination? Desalination 391 (2016) 69–88. [80] R. Das, et al., Carbon nanotube membranes for water purification: a bright future in water desalination, Desalination 336 (2014) 97–109. [81] X. Qu, P.J. Alvarez, Q. Li, Applications of nanotechnology in water and wastewater treatment, Water Res. 47 (12) (2013) 3931–3946. [82] C.H. Ahn, et al., Carbon nanotube-based membranes: fabrication and application to desalination, J. Ind. Eng. Chem. 18 (5) (2012) 1551–1559. [83] G. Bounos, et al., Enhancing water vapor permeability in mixed matrix polypropylene membranes through carbon nanotubes dispersion, J. Membr. Sci. 524 (2017) 576–584. [84] S. Li, et al., Removal of humic acid from aqueous solution by magnetic multi-walled carbon nanotubes decorated with calcium, J. Mol. Liq. 230 (2017) 520–528. [85] V. Vatanpour, N. Zoqi, Surface modification of commercial seawater reverse osmosis membranes by grafting of hydrophilic monomer blended with carboxylated multiwalled carbon nanotubes, Appl. Surf. Sci. 396 (2017) 1478–1489.

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[86] J. Zheng, et al., Sulfonated multiwall carbon nanotubes assisted thin-film nanocomposite membrane with enhanced water flux and anti-fouling property, J. Membr. Sci. 524 (2017) 344–353. [87] S. Daer, et al., Recent applications of nanomaterials in water desalination: a critical review and future opportunities, Desalination 367 (2015) 37–48. [88] M.D. Stoller, et al., Graphene-based ultracapacitors, Nano Lett. 8 (10) (2008) 3498–3502. [89] X. Li, et al., Chemically derived, ultrasmooth graphene nanoribbon semiconductors, Science 319 (5867) (2008) 1229–1232. [90] F.M. Kafiah, et al., Monolayer graphene transfer onto polypropylene and polyvinylidenedifluoride microfiltration membranes for water desalination, Desalination 388 (2016) 29–37. [91] Y.H. Teow, A.W. Mohammad, New generation nanomaterials for water desalination: a review, Desalination 451 (2019) 2–17. [92] D. Cohen-Tanugi, J.C. Grossman, Water desalination across nanoporous graphene, Nano Lett. 12 (7) (2012) 3602–3608. [93] Y. Han, Z. Xu, C. Gao, Ultrathin graphene nanofiltration membrane for water purification, Adv. Funct. Mater. 23 (29) (2013) 3693–3700. [94] M. Safarpour, A. Khataee, V. Vatanpour, Thin film nanocomposite reverse osmosis membrane modified by reduced graphene oxide/TiO2 with improved desalination performance, J. Membr. Sci. 489 (2015) 43–54. [95] Y.C. Woo, et al., Water desalination using graphene-enhanced electrospun nanofiber membrane via air gap membrane distillation, J. Membr. Sci. 520 (2016) 99–110. [96] G.J. Dahe, R.S. Teotia, J.R. Bellare, The role of zeolite nanoparticles additive on morphology, mechanical properties and performance of polysulfone hollow fiber membranes, Chem. Eng. J. 197 (2012) 398–406. [97] S.G. Kim, et al., Nanocomposite poly (arylene ether sulfone) reverse osmosis membrane containing functional zeolite nanoparticles for seawater desalination, J. Membr. Sci. 443 (2013) 10–18. [98] H. Dong, et al., High-flux reverse osmosis membranes incorporated with NaY zeolite nanoparticles for brackish water desalination, J. Membr. Sci. 476 (2015) 373–383. [99] C.H. Cho, et al., Pervaporative seawater desalination using NaA zeolite membrane: mechanisms of high water flux and high salt rejection, J. Membr. Sci. 371 (1–2) (2011) 226–238. [100] P. Swenson, et al., Pervaporative desalination of water using natural zeolite membranes, Desalination 285 (2012) 68–72.

Further reading [101] W. Salim, W.W. Ho, Recent developments on nanostructured polymer-based membranes, Curr. Opin. Chem. Eng. 8 (2015) 76–82. [102] N.C. Mueller, et al., Nanofiltration and nanostructured membranes—should they be considered nanotechnology or not? J. Hazard. Mater. 211 (2012) 275–280. [103] J. Kim, B. Van der Bruggen, The use of nanoparticles in polymeric and ceramic membrane structures: review of manufacturing procedures and performance improvement for water treatment, Environ. Pollut. 158 (7) (2010) 2335–2349.

CHAPTER

Nanomaterials for foulingresistant RO membranes

6

Zahra Shabania,b, Soheil Zarghamia,b, Toraj Mohammadia,b a

School of Chemical, Petroleum and Gas Engineering, Center of Excellence for Membrane Science and Technology, Iran University of Science and Technology (IUST), Tehran, Iran, bResearch and Technology Centre of Membrane Separation Processes, School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

Chapter outline 6.1 Introduction ....................................................................................................152 6.2 Reverse osmosis: Fundamentals and principals .................................................153 6.3 RO membrane fabrication strategies .................................................................154 6.3.1 CA membranes ..............................................................................155 6.3.2 TFC membranes ............................................................................155 6.3.3 Polyelectrolyte membranes .............................................................156 6.3.4 MMMs ..........................................................................................157 6.3.5 Biomimetic membranes .................................................................157 6.4 RO membranes fouling types ............................................................................157 6.4.1 Colloids ........................................................................................158 6.4.2 Organic fouling ..............................................................................158 6.4.3 Inorganic fouling ...........................................................................159 6.4.4 Biofouling .....................................................................................159 6.5 RO fouling control strategies ............................................................................160 6.5.1 Feed pretreatment .........................................................................160 6.5.2 Membrane cleaning .......................................................................161 6.5.3 Membrane modification .................................................................162 6.6 Utilization of nanomaterials for preparation of antifouling RO membranes ...........163 6.6.1 Carbon-based nanoparticles enabled RO membranes ........................163 6.6.2 Titanium dioxide-based nanoparticles enabled RO membranes ..........167 6.6.3 Silica-based nanoparticles enabled RO membranes ..........................168 6.6.4 Silver-based nanoparticles enabled RO membranes ..........................170 6.6.5 Other nanoparticles enabled RO membranes ....................................173

Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00006-7 # 2020 Elsevier Inc. All rights reserved.

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6.7 Conclusion ......................................................................................................176 References ............................................................................................................177 Further reading ......................................................................................................184

6.1 Introduction One of the most critical challenges facing the human societies is water scarcity and providing appropriate water resources for various purposes including agriculture and industrial activities and also drinkable water because of crucial problems such as climate change and global warming, growing population, and industries and socioeconomic events [1]. It is anticipated that the needs of water will be increased from 4500 to 6900 billion m3 until 2030 [2]. The water reuse and brackish/seawater desalination are the two main methods for overcoming the aforementioned challenges [3]. Membrane separations are known as the most capable techniques for high-quality water production. The reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) as the pressure-driven membrane processes, the pressureretarded osmosis (PRO) and forward osmosis (FO) as the osmotically driven membrane processes, the membrane distillation (MD) as the thermally driven membrane processes, electrodialysis (ED) as the electrically driven membrane processes, the dialysis, liquid membrane (LM), and molecularly imprinted membranes (MIMs) as the concentration-driven membrane processes have been utilized for wastewater treatment to address the lack of fresh water [4–7]. Among these technologies the RO process is known as an attractive choice due to its unique properties such as lowenergy costs and simple operation [8]. This process has been reported to successfully recover municipal, industrial, and any synthetic wastewaters economically. The production of clean water, asymmetric and chemical- and physical-resistant membranes are required as the heart of RO process [9]. However, despite the various advantages of RO membranes, fouling phenomena arise during operational using of RO membranes creating some problems [3]. Apart from the adverse effects of fouling on the produced water quality and capacity, the more energy consumption increases the operational costs. Also, fouling reduces the membrane life span so that after specific time, the system should be replaced with another module. As the result of existed foulants in the feed streams, their precipitation, accumulation, or adsorption onto the membranes surface or within the membranes matrix will occur [10, 11]. The four prevalent fouling types are precipitated colloidal fouling, biofouling, and organic/inorganic fouling [12]. According to the literature reports, various strategies have been utilized for addressing the membrane fouling, including pretreatment (e.g., coagulation and flocculation), membrane monitoring and cleaning, and surface modification [10]. Incorporation of nanoparticles for modification of RO membranes to enhancement their fouling resistance has become more interested in recent years (Fig. 6.1). In this chapter, current growths in preparation of RO membranes containing different types of nanoparticles are being studied.

6.2 Reverse osmosis: Fundamentals and principals

FIG. 6.1 Number of research papers published in the indexed journals between 2008 and 2019 containing the keyword “reverse osmosis membrane” and “fouling” in the title, abstract, or keywords. Source: Scopus, searched on 08 October 2019.

6.2 Reverse osmosis: Fundamentals and principals The naturally transport or diffusion of water across a semipermeable membrane from the high water potential side to the low water potential side until reaching to an equilibrium of water potential on both sides of the membrane is named as osmosis or forward osmosis. A solution with higher concentration has lower potential in comparison with the lower concentrated solution leading to water passage through the membrane [13]. A schematic illustration of RO, FO, and PRO are presented in Fig. 6.2. In FO membrane process, two sequential steps are needed to reach pure water. First, water permeates from the feed solution (low concentration side) to draw solution (high concentration side). After that the draw solute must be separated from the diluted draw solution to produce water. In one-step RO membrane process, applying hydraulic pressure on the solution with high concentration leads to overcome the osmotic pressure difference (Δπ), the solute can be rejected by the membrane and pure water is produced [14]. PRO membrane process is categorized between FO and RO processes in which the applied hydraulic pressure is less than Δπ, so direction of the net water transfer is still similar to FO process [15].

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Semi-permeable membrane A=Fresh water

B=Salty water

Changes to

Pressure head

FO

Jw

DP< Dp

DP< Dp Changes to

Pressure head

PRO

Jw

DP> Dp Changes to

Jw

DP> Dp

RO

FIG. 6.2 Schematic representations of the osmotic processes. Reproduced with permission from G. Han, S. Zhang, X. Li, T.-S. Chung, Progress in pressure retarded osmosis (PRO) membranes for osmotic power generation, Prog. Polym. Sci. 51 (2015) 1–27. https://doi.org/10.1016/j. progpolymsci.2015.04.005. Copyright 2015, Elsevier Science Ltd., Oxford, UK.

A general equation can be applied in FO, RO, and PRO process for water flux calculation as follow [15]: Jw ¼ A ðσΔπ  ΔPÞ

(6.1)

In the above equation, Jw, A, and σ are water flux, water permeability constant, and reflection coefficient, respectively. Also the osmotic pressure and the applied pressure differences across the membrane are shown by Δπ and ΔP, respectively.

6.3 RO membrane fabrication strategies The membrane characteristics, including high water flux and salt rejection accomplished with antifouling properties, chemical, mechanical, and thermal stabilities and also chlorine resistance that is related to the membrane inherent structure and chemistry, are essential to synthesis an ideal high-performance RO membrane [16]. By reviewing the recent literature studies, it can be found that four categories of RO membranes including cellulose acetate (CA) membranes, thin-film composite

6.3 RO membrane fabrication strategies

(TFC) membranes, mixed matrix membranes (MMMs), and polyelectrolyte membranes have been broadly reported.

6.3.1 CA membranes In about mid-19th century, Loeb and Sourirajan were proposed as a pioneer of RO cellulose acetate membrane fabrication. They developed nonsolvent-induced phase separation (NIPS) for preparation of CA RO membranes [8]. For the phase separation process, first, a homogeneous and bubble-free polymeric solution is prepared. After that, this solution is cast onto a clean glass via an appropriate film applicator with desired thickness. At the end the demixing process between solvent and nonsolvent (commonly water) occurs in a coagulation bath. This process leads to the formation of the polymer-rich phase (membrane body) and also the polymer-lean phase (membrane pores) [17, 18]. The rapid solvent evaporation forms the thin dense top layer onto the surface of porous structure. The variable parameters in the NIPS process are polymeric solution concentration, temperature and composition of coagulation bath, solvent type, the used additives, and the applicator thickness [19]. Both flat sheet and hollow fiber asymmetric membranes can be produced through this methodology [18]. The annealing posttreatment methods are used for enhanced rejection of CA membranes, so that after solution casting, the obtained membrane is immersed in a hot water bath for the desired time duration [20]. High water permeability and solute rejection are necessary factors for achieving high-performance RO membrane separations. Despite the fact that CA membranes have high water permeability, they suffer from rejection of low-molecular weight solutes. By adopting annealing operation the undesirable microspores appear during the phase separation eliminate. In this regard, annealing temperature affects membrane rejection [21]. The inexpensive CA polymer has inherent hydrophilicity so that its membrane fabrication method forms a membrane with low in air water contact angle (WCA), antifouling property, and high water flux. Apart from the aforementioned advantages, CA membranes have faced some limitations. Outside particular pH and temperature ranges, hydrolysis chemical reaction of CA membranes occurs. Besides, bacterial attraction to CA membrane surface is another challenge of these membranes [22, 23].

6.3.2 TFC membranes ’A thin-film composite (TFC) RO membrane is commonly composed of three individual layers. A thin top polyamide (PA) active layer with a thickness of 10–100 nm is placed onto a UF support midlayer [23]. The polysulfone (PSF) and polyethersulfone (PES) are the most polymeric materials used for the preparation of this UF membrane via NIPS process, with thickness in the range of 20–50 μm. The nonwoven fabric finally is used as the third bottom layer with a thickness of 100–200 μm [18]. An optimization process can be done on the TFC membrane fabrication to obtain membranes with high water flux and rejection,

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because both the materials and manufacturing process for the fabrication of the top layer and midlayer are different. The interfacial polymerization (IP) between amine monomers in aqueous solution (e.g., m-phenylenediamine [MPD]) and acid chloride monomers in an organic solution (e.g., trimesoyl chloride [TMC]) forms PA layer onto the surface of UF membrane [24]. For this purpose the as-prepared support layer is immersed in MPD solution for saturation of pores with the aqueous phase. Afterward the excess amount of MPD is removed by a rubber roller or some water uptake filter papers. Then the TMC solution is poured onto the membrane surface for a specific time. After washing with deionized (DI) water, the obtained membrane is dried in an oven and kept in DI water for subsequent usage or modifications [25]. An optimization process can be applied in the IP method based on variation of the influential parameters including monomers/solvents/additive types and concentrations, condition of polymerization, reaction time, and curing process [16, 26]. Also, chemical and morphological properties of the support membrane surface and its pore size affect the PA layer formation. Furthermore, chemical, mechanical, biological, and thermal stabilities are important for a suitable membrane support. Due to the skin layer and high cross-linked PA layer, the TFC RO membranes have higher water permeability and salt rejection in comparison with the CA membranes. The TFC membranes with extensive pH and temperature ranges are highly stable in biological attack. However, the TFC RO membranes are susceptible to chlorine, which is used for biofouling disinfection in wastewater treatment [24, 27].

6.3.3 Polyelectrolyte membranes Polyelectrolyte membranes are synthesized on surface of the charged supports via sequential coating of anionic and cationic polyelectrolytes. This assembly technique named as layer by layer (LbL) is attractive for the preparation of NF and RO membranes, and the obtained dense structure can limit passage of ions through the membranes [28]. In this method, first, the initially charged membrane is soaked in the positive dilute solution of cationic polyelectrolyte. After that the membrane is removed from the solution and rinsed with water for elimination of the unbound molecules. Then the obtained positively charged membrane is immersed in the negative dilute solution of anionic polyelectrolyte followed by water rinsing [29]. In each step, a small content of polyelectrolytes adsorbs on the membrane surface and consequently the previous charge of the membrane reverses. Multiple positive and negative layers onto the membrane surface cause the preparation of polyelectrolyte multilayer membranes. The number of formed polyelectrolyte layers has an essential role in water flux and salt rejection of the polyelectrolyte membranes. The higher number of the layers increases mass transfer resistance so water flux decreases. On the other hand, salt rejection increases with increment of the deposited dense polyelectrolyte layers. It is worth noting that there are an optimum number of layers that determine the membrane performance. The separation performance, thickness,

6.4 RO membranes fouling types

surface hydrophilicity, and charge of the LbL membranes are affected by type, concentration, pH, and the layer number of the polyelectrolytes [30].

6.3.4 MMMs One of the conventional structure about performance-enhanced RO membranes are MMMs. Dispersion of inorganic nanomaterials into polymeric casting solution and incorporation of inorganic nanomaterials into the monomer during PA layer synthesis are two categories of MMMs. The latter is named as thin-film nanocomposite (TFN) membranes [31, 32]. The commonly used polymer for fabrication of the polymeric matrix is PES, PSF, polyelectrolytes, etc. The various types of inorganic nanomaterials are used for RO membrane preparation. For example, Hoek’s group [33] developed TFN membrane by adding Linde Na A (LTA) zeolite nanoparticles during the formation of the PA layer. There are different properties of nanoparticles such as hydrophilicity/superhydrophilicity, size, and type that increase water permeability and salt rejection of RO membranes. Surface charge, morphology, and hydrophilicity are affected by nanoparticles addition into/onto the membrane structure.

6.3.5 Biomimetic membranes Aquaporins (AQPs) that exist in living cells are biological proteins that act as transmembranes. Recently, AQPs have attracted much attention because of their high water permeability and selectivity. These membranes can transport water molecules through their high permeable water channels (109 water molecules/s channel). Apart from high water permeability, their specific property is prevention of ions and other solutes [34]. Biomimetic membranes are composed of AQPs as filler through incorporation or immobilization into the membranes. The most promising method for fabrication of biomimetic membranes is utilization of AQPs into the PA layer. The applied AQPs pore formers in the active layers of TFC membranes facilitate water passage across the membranes [35].

6.4 RO membranes fouling types Fouling phenomenon as an expected problem during membrane processes is the main reason of operational challenges such as water flux decline and makes membrane replacement of the new membrane, and chemical cleaning necessary leading to increment of maintenance costs. Any undesirable molecules and species that cause membrane fouling are named as foulants [36, 37]. The fouling phenomena can be splitted into two types. The foulants may be attached to the membrane surface or into membrane pores in the pressure-driven membrane processes, so that based on the foulants size, the undesired molecules may enter into the membrane pores and/or accumulate onto the membrane surface [6, 38]. According to the dense structure

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of the RO membranes, its main fouling mechanism is the attachment of foulant layers (gel/cake/scaling) on the membrane surface [27, 39]. The membrane surface chemistry and morphological characteristics influence the membrane antifouling properties. The polar interactions of hydrophilic groups onto the membrane surface cause water bonding onto the membrane surface that inhibits the foulants attachments. It was mention when the undesired components are hydrophilic, these foulants are adsorbed onto the hydrophilic membranes surface, and this decreases the membranes antifouling properties. In addition to hydrophilicity the membrane surface electrostatic charge and roughness are significant parameters regarding the membrane fouling. The repulsive interaction between the foulants and the charged membranes decline the fouling phenomena due to the reduced foulant accumulation on the membrane surface. Also the membranes with the rougher topological surface are more susceptible to fouling in comparison with the smoother ones, due to the attachment of foulants onto the valleys of the rougher membranes [27, 39]. Based on the foulant characteristics, fouling can be categorized into colloidal fouling, organic/inorganic fouling, and biofouling [40].

6.4.1 Colloids Colloidal fouling is composed of tiny particles that accumulate onto the membrane surface and form a cake layer. These particles are in the wide range from 1 nm to 1 μm. The final cake acts as a barrier for water transport across the membrane and therefore decreases water flux [41, 42]. One of the most important foulants that impedes the RO membrane performance is colloids. Regarding their molecular sizes, they can pass through MF and UF membranes during the pretreatment processes. Also, in comparison with the RO membrane pores size, they are adequately large to accumulate onto the RO membrane surface. In contrary to the small colloids, the large ones can be removed from the membrane surface using operational backwashing. The influential parameters on the colloidal fouling are chemical feed conditions (pH, ionic interactions, and strength); foulant properties such as charge, shape, and size; and physicochemical properties (hydrophilicity, surface roughness and charge, etc.) of the RO membranes. The cross flow velocity and water flux also influence the adsorbing and sweeping of the colloidal foulants [43, 44].

6.4.2 Organic fouling Organic matter (OM) with high adhesion tendency to the membrane surface and formation of a sticky layer is problematic due to limitation of water passage through the membrane [37]. There are three categories related to the OM, namely, (i) natural organic matter (NOM) generated from potable water resources, (ii) chemically synthetic organic matter (CSOM) produced as by-products of wastewater disinfection or added during water consumptions, and (iii) soluble microbial products (SMP) created through wastewater treatment processes as a result of organic compound dissociation [27]. The NOMs are known as the significant type of organic fouling in RO

6.4 RO membranes fouling types

membranes that composed of various hydrophobic and hydrophilic matters with different molecular weights such as amino, humic and fulvic acids, and proteins. Formation of the first layer of organic foulants and covering the fresh membrane surface is determined by the foulant-membrane interactions in a short-time duration. After that, affinity of the foulants to be adsorbed onto the formed cake depends on the foulant-foulant interactions. Formation of thicker and more compacted fouling layer at higher foulant-foulant interactions can cause higher hydraulic resistance to water flux behavior [40, 45, 46].

6.4.3 Inorganic fouling Inorganic mineral pollutants that precipitate onto the membrane surface cause inorganic fouling (scaling) on the membranes surface. The enhanced of the dissolved salts concentration is known as one the most challenges in desalination-related membrane processes (e.g., NF, RO, and FO). Increasing the salts concentration above the critical saturation level causes salt crystal deposition onto the membrane surface. Scaling in RO membranes is commonly formed by minerals that exist in the feed media such as CaSO4, CaCO3, and Ca3(PO4)2. The RO membrane performance is influenced by two crystallization behaviors simultaneously, including homogeneous (bulk) and heterogeneous (surface) crystallizations. In the surface crystallization, after heterogeneous nucleation, the scalants grow onto the membrane surface and form strong adhered scaling leading to pore clogging and thus the flux decline [27, 42, 47]. Bulk crystallization happens by deposition of crystals mostly formed in the bulk solution onto the RO membrane surface. Reducing the saline solution pH can mostly diminish the scaling but not always. Antiscalant chemicals intervene in the crystallization process. This preventative additives dissolve crystals near the RO membranes surface and reduce the fouling degree [48].

6.4.4 Biofouling Biological fouling, which also named as biofouling, is very hard to control in comparison with the other fouling types. However, reduction of the microorganism amount in the feed phase is effective, but it is a hard method for alleviating biofouling in RO processes. Apart from the other fouling types, large amounts can be reduced using the pretreatment processes, biofouling reduction needs more proceedings [49, 50]. Adhesion of microorganisms onto the membrane surface and their growth by available nutrients in the feed system develops extracellular polymeric substances (EPSs) during cell proliferation. EPSs are natural polymers with high-molecular weight that can attach to hydrophobic and hydrophilic surfaces (Fig. 6.3). Colonization and accumulation of various types of bacteria (alive, inactive, and dead) along with EPSs form biofilm thin layer on the membrane surface [51–53]. The EPSs attach to the membrane surface by physical, chemical, and electrostatic interactions [54]. The biofilm acts like as a second membrane and hinders the

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FIG. 6.3 Adhesion of EPSs on surface of (A) hydrophobic and (B) hydrophilic membranes. Reproduced with permission from H. Lin, M. Zhang, F. Wang, F. Meng, B.-Q. Liao, H. Hong, J. Chen, W. Gao, A critical review of extracellular polymeric substances (EPSs) in membrane bioreactors: characteristics, roles in membrane fouling and control strategies, J. Membr. Sci. 460 (2014) 110–125. https://doi.org/10.1016/j. memsci.2014.02.034. Copyright 2015, Elsevier Science Ltd., Oxford, UK.

membrane performance such as (i) permeate flux reduction, (ii) concentration polarization increment, (iii) membrane module ΔP increment, (iv) polymeric membrane biodegradation, (v) energy consumption enhancement, and produced water quality reduction [51, 54]. Characteristic of the membrane surface, interactions between bacteria and the membrane surface, and feed and operating conditions control the microorganism adhesion onto the membrane surface [38].

6.5 RO fouling control strategies Fouling effects mitigation can be achieved by minimization and remediation approaches. Hence, various strategies including feed pretreatment, membrane cleaning, and membrane modification can be developed to address the membrane fouling challenges.

6.5.1 Feed pretreatment The feedwater can include various effluents. Utilization of the pretreatment steps before the standalone RO unit should remove or decrease the four fouling types. Using this, not only the RO membrane lifetime increases, but also the cost of operation decreases [44, 55]. The main impact of the pretreatment processes is the production of high-quality water, regardless of feed water pollution dosage [56]. There are two categories of pretreatment technologies. These processes may be conventional such as coagulation/flocculation, flotation, adsorption, UV radiation, acidification, and scale inhibition or nonconventional membrane-based pretreatment process including MF, UF, and NF. A hybrid of pretreatment processes may be used regarding the feed initial conditions (Fig. 6.4) [57].

6.5 RO fouling control strategies

Membrane pretreatment: MF/UF/NF Initial: conventional pretreatment

RO Post-treatment

Seawater Potable water

Concentrated waste and brine

FIG. 6.4 Example of a typical SWRO setup using more than one pretreatment technique. Reproduced with permission from S.F. Anis, R. Hashaikeh, N. Hilal, Reverse osmosis pretreatment technologies and future trends: a comprehensive review, Desalination. 452 (2019) 159–195. https://doi.org/10.1016/j. desal.2018.11.006. Copyright 2019, Elsevier Science Ltd., Oxford, UK.

Among the aforementioned pretreatment processes, MF, UF, and coagulation/ flocculation have been more interested in the literatures [44]. Studies have been also performed to use FO and NF processes as the pretreatment of the RO systems to diminish fouling in the RO units. In comparison with the conventional pretreatment processes, application of the membrane-based pretreatment processes reduces chemical usage, floor space, and also maintenance costs [53, 56].

6.5.2 Membrane cleaning Pretreatment methods are effective approaches for prevention of RO membrane fouling, but cannot eliminate all existed foulants in the feed solutions completely [23]. Cleaning, “a process where material is relieved of a substance which is not an integral part of the material,” is considered regarding RO membranes when permeate flux and rejection decline continuously insofar as higher ΔP is required to reach the preferable water flux [45, 58]. In physical cleaning, hydraulic, electrical, and mechanical forces are applied for enhanced removal of foulants/particles from the RO membrane surface. It is worth mentioning that efficiency of this method is less than that of chemical cleaning method. Although, utilization of physical cleaning (chemicals free) method leads to less membranes surface degradation [59, 60]. Chemical cleaning technique consists of various functions (Table 6.1) using chemical agents for removal of strongly attached foulants followed by foulant transfer from the fouling layer toward the bulk solution. Membrane chemical cleaning can be classified into cleaning in place (CIP) that the feed reservoir is replaced with chemical agents or cleaning out of place (COP) that membranes are removed from the main flow and immersed in chemicals for foulants elimination [59, 61]. The long-time chemical cleaning processes need chemical agents that may degrade the membranes. Selection of proper chemical agents depends on the feed properties [58, 62]. Air pulse technique can be applied for cleaning of MF and UF membranes, but NF and RO membranes need backwashing operation (flow reversing) for membrane

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Table 6.1 Common cleaning agents and possible interactions between cleaning agents and foulants. Family

Examples

General functions

Acids

Strong: HCl, HNO3 Weak: H3PO4, Citric

Alkalis

Strong: NaOH, KOH Weak: Na2CO3

Oxidants Surfactants

Chelants

NaClO, H2O2 Anionic: SDS Cationic: CTAB Nonionic: Tween 20 EDTA

pH regulation, dissolution of inorganic precipitates, acidic hydrolysis of certain macromolecules pH regulation, alteration of surface charges, alkaline hydrolysis of proteins, catalyzing saponification of fats Oxidation of organics, disinfection Dispersion/suspension of deposits

Enzymes

Proteases, lipases

Complexion with metals, removal of mineral deposits Catalyzing lysis of specific substrates (e.g., proteins and lipids)

Reproduced with permission from X. Shi, G. Tal, N.P. Hankins, V. Gitis, Fouling and cleaning of ultrafiltration membranes: a review, J. Water Process Eng. 1 (2014) 121–138. https://doi.org/10.1016/j. jwpe.2014.04.003. Copyright 2014, Elsevier Science Ltd., Oxford, UK.

cleaning. These techniques are useful for the reversible fouling elimination. However, to overcome the irreversible fouling, utilization of suitable chemicals are necessary [30].

6.5.3 Membrane modification Wettability of the membranes plays an essential role in the membrane fouling tendency. Some hydrophobic polymers such as PSF, PES, polyvinylidene fluoride (PVDF), and polyether ketone (PEK) are commercially used for preparation of membranes. It is crucial to reduce the undesired interactions between the foulants and the membrane surface for elimination of the membrane fouling. Synthesis of the membranes with hydrophilic property leads to the formation of water layer onto the membranes surface so that adsorption of foulants can be reduced [37, 63]. A variety of methods have been used for enhancement of antifouling properties such as surface coating, plasma grafting, and thermal treatment [64]. Among the different techniques used for the membrane fouling reduction, incorporation of nanoparticles such as multi-walled carbon nanotubes (MWCNTs), TiO2, SiO2, zeolites, and other nanomaterials into the RO membranes polymeric casting solutions as nanofillers and preparation of MMMs have been interested (as further discussed in Section 6.6). After implementation of the nanoparticles, it is anticipated that the membrane performance such as water flux and rejection as well as antifouling and physicochemical properties of the RO membranes are improved.

6.6 Utilization of nanomaterials

6.6 Utilization of nanomaterials for preparation of antifouling RO membranes 6.6.1 Carbon-based nanoparticles enabled RO membranes Carbon nanotubes (CNTs) have been reported as a good candidate for enhancement of the antifouling properties and water transport (through its channels) of the RO membranes [65]. Zhao et al. synthesized TFC RO membranes by addition of MWCNTs into the PA layer. Ca(HCO3)2 and bovine serum albumin (BSA) were used as inorganic foulant and model protein, respectively, for evaluation of antifouling properties of the membranes. The fresh and recovered membranes displayed reduction in their flux-time behaviors for the Ca(HCO3)2 and BSA foulants as shown in Figs. 6.5 and 6.6, respectively. Although flux decline over time was observed for the membranes, the polyamide/MWCNT membrane showed higher water flux than the neat TFC membrane for the both Ca(HCO3)2 and BSA foulants [66]. In the literature it was reported that CNT contact with bacteria can cause the bacteria damaged and inactivated [67].

1.00 TMC/MPD membrane TMC/MPD with modified MWCNTs membrane 0.98

Decline of flux

0.96 0.94 0.92 0.90 0.88 0

2

4

6 8 Operation time (d)

10

12

FIG. 6.5 Comparison of flux decline due to the presence of Ca(HCO3)2 as foulant for polyamide and polyamide/MWCNT membranes. The MWCNT loading in the tested membranes was 0.1 wt%. Reproduced with permission from H. Zhao, S. Qiu, L. Wu, L. Zhang, H. Chen, C. Gao, Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes, J. Membr. Sci. 450 (2014) 249–256. Copyright 2014, Elsevier Science Ltd., Oxford, UK.

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1.0

TMC/MPD membrane TMC/MPD with modified MWCNTs membrane

0.9 0.8 Decline of flux

164

0.7 0.6 0.5 0.4 0.3 0

2

4

6 8 Operation time (d)

10

12

FIG. 6.6 Comparison of flux decline due to the presence of BSA as foulant for polyamide and polyamide/MWCNT membranes. The MWCNT loading in the tested membranes was 0.1 wt%. Reproduced with permission from H. Zhao, S. Qiu, L. Wu, L. Zhang, H. Chen, C. Gao, Improving the performance of polyamide reverse osmosis membrane by incorporation of modified multi-walled carbon nanotubes, J. Membr. Sci. 450 (2014) 249–256. Copyright 2014, Elsevier Science Ltd., Oxford, UK.

Tiraferri et al. [68] functionalized surface of a TFC membrane by modified single-walled carbon nanotubes (SWCNTs). After binding cytotoxic SWCNTs the membrane could inactivate 60% of the attached bacteria in 1 h contacting. Kim et al. [69] employed filtration method for deposition of carbon nanotubes (CNTs) onto the PA layer of a TFC RO membrane. After that the obtained membrane was coated with polyvinyl alcohol (PVA) solution with various concentrations (for CNT stabilization) and named as PA-CNT-PVA. For comparison, they prepared membranes without depositing of CNTs. The low percentage of PA-CNT-PVA membrane cell viability (80%. Better antifouling properties were obtained during the Na2SO4/BSA filtration, resulted mostly from the hydrophilic property of the silica nanoparticles [82]. Ahmad et al. [83] explored possibility of utilizing silica nanoparticles (1%– 5%, w/v) into the CA/polyethylene glycol (PEG)-doped solutions to prepare RO membranes via thermally induced phase inversion (TIPS) method. Enhanced water flux (from 0.35 to 2.46 L/m2 h) and salt rejection (from 81.5% to 90%) were observed regarding the modified membrane. Peyki et al. [84] developed a series of TFC membranes using the PA layer embedded with various SiO2 loadings (0.005–0.5 wt%). The SiO2 modified TFC RO membranes showed notable enhancement in water flux at the optimum concentration (0.1 wt%) because of increment in the membranes surface hydrophilicity. Increasing the nanoparticle concentration reduced the flux decline under fouling measurements. However, at the highest nanoparticle concentration (0.5 wt%), the mass transport resistance was dominant, and at this concentration the obtained flux was reduced.

6.6.4 Silver-based nanoparticles enabled RO membranes By adopting nanoparticles with antibacterial properties onto the membrane surface, inhabitation of biofilm formation can be obtained successfully. Silver nanoparticles with biocidal activity was used for fabrication of membranes with the antibacterial surface [85]. Facile in situ loading of silver nanoparticles onto the commercial TFC membranes was developed by the research group of Elimelech. Three types of bacteria including E. coli and P. aeruginosa as gram negative and Staphylococcus aureus (S. aureus) as gram positive were used for biofouling examination. By introducing the silver nanoparticles onto the membrane surface, the number of attached live bacteria colonies (colony-forming unit, CFU) decreased in comparison with the neat membrane (Fig. 6.13). Also, biovolume related to the EPS, dead and live bacteria, decreased by 25%, 38%, and 73%, respectively, after surface functionalization with silver nanoparticles [86]. In situ immobilization of silver nanoparticles using tannic acid (TA)-ferric ionpolyethylenimine (PEI) intermediate layer was performed by Dong et al. [87]. Silver immobilization increased water flux, salt rejection, and bacteria death. Antibiofouling properties of the membranes were evaluated using E. coli and Bacillus subtilis (B. subtilis) foulants. As displayed in Fig. 6.14 the neat and the TA-Fe-PEI-modified membranes show 35% and 30% water flux reduction in comparison with their initial flux values, respectively. This value is about 11% and 17% for the TA-Fe-PEI/Agmodified membrane after biofouling evaluation by E. coli and B. subtilis, respectively. The observed flux reduction of the membranes was related to the biofilm formation that increased the water permeability resistance. In a study by Chung’s group, four functionalities including carboxylic acid, amine, PEG, and silver nanoparticles were applied for the surface modification of TFC membranes, and the membranes antifouling properties were compared.

6.6 Utilization of nanomaterials

FIG. 6.13 Number of attached live bacteria colonies (CFU) on neat (gray) and in situ Ag nanoparticlemodified membranes (green) for gram-negative (E. coli and P. aeruginosa) and gram-positive (S. aureus) bacteria. Experimental conditions: neat- and in situ-modified (2:2 AgNO3:NaBH4) membranes were contacted and incubated with isotonic solution (0.15 M NaCl, 20 mM NaHCO3 as buffer, pH 8.2) with bacteria (OD600: 0.15  0.01) for 5 h at 37°C. After incubation the membranes were sonicated with sterile isotonic solution, and the detached bacteria were plated on LB agar. Reproduced with permission from M. Ben-Sasson, X. Lu, E. Bar-Zeev, K.R. Zodrow, S. Nejati, G. Qi, E.P. Giannelis, M. Elimelech, In situ formation of silver nanoparticles on thin-film composite reverse osmosis membranes for biofouling mitigation, Water Res. 62 (2014) 260–270. Copyright 2014, Elsevier Science Ltd., Oxford, UK

The fouling reduction of the silver-PEGylated dendrimer nanocomposite coated TFC membrane reached to the maximum value of 99.8%. The small adhesion forces for PEG and modified membranes with silver nanoparticles caused low protein fouling [88]. In another research a new strategy was developed for immobilization of silver nanoparticles onto the PA layer of a TFC membrane (Fig. 6.15). The surface of the TFC membrane was functionalized with silver nanoparticle-SiO2 hybrid particles (silver nanoparticles @SiO2) (grown silver nanoparticles [30 nm] on SiO2 particles [400 nm]) using cysteamine covalent linker. Utilization of the silver nanoparticles @SiO2 with relatively large size led to minimum aggregation, effective distribution, and low resistance against water transport through the membrane so that water flux and salt rejection did not decrease considerably in comparison with the neat TFC membrane. Also, deposition of the hybrid nanoparticles improved the TFC membrane antibacterial ability [89]. Sun et al. [90] fabricated the GO-silver nanoparticles composite onto the CA membrane using the filtration method (Fig. 6.16). The antifouling properties of the new developed membranes were evaluated using filtration of feed water containing E. coli, and the results showed lower flux reduction (46%) in comparison with the neat CA membrane (88%). Contacting the modified membrane with E. coli bacteria made almost all of them dead after 4 h.

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CHAPTER 6 Nanomaterials for fouling-resistant RO membranes 70

Water flux before bacteria growth Water flux after E. coli growth Water flux after B. subtilis growth

60 50 Water flux (L/(m2 h))

172

40

30 20 10 0 TA-Fe-PEI-modified TA-Fe-PEI/Ag-modified

Virgin

FIG. 6.14 The water fluxes of the membranes before and after being immersed in E. coli and B. subtilis suspensions (1  109 and 1.5  1010 CFU/mL, respectively) and cultured at 37°C for 72 h. Reproduced with permission from C. Dong, Z. Wang, J. Wu, Y. Wang, J. Wang, S. Wang, A green strategy to immobilize silver nanoparticles onto reverse osmosis membrane for enhanced antibiofouling property, Desalination 401 (2017) 32–41. Copyright 2017, Elsevier Science Ltd., Oxford, UK.

Bacteria

Released Ag+ s

s HN

O

HN

O

TFC RO membrane FIG. 6.15 Schematic illustration of the membrane modification of the TFC RO membrane by immobilization of silver nanoparticles @SiO2. Reproduced with permission from S.-H. Park, Y.-S. Ko, S.-J. Park, J.S. Lee, J. Cho, K.-Y. Baek, I.T. Kim, K. Woo, J.-H. Lee, Immobilization of silver nanoparticle-decorated silica particles on polyamide thin film composite membranes for antibacterial properties, J. Membr. Sci. 499 (2016) 80–91. Copyright 2015, Elsevier Science Ltd., Oxford, UK.

6.6 Utilization of nanomaterials

Modified Turkevich method

Modified Humers method

Dispersion of GO-AgNPs

(A) (C)

(D)

CA membrane Ceramic support 1mm

(B) FIG. 6.16 (A) Possible mechanism for the formation of the GO-Ag nanoparticles. (B) Preparation procedure of the GO-Ag nanoparticle membrane. (C) Digital image and (D) SEM image of the GP-Ag nanoparticle membrane. Reproduced with permission from X.-F. Sun, J. Qin, P.-F. Xia, B.-B. Guo, C.-M. Yang, C. Song, S.-G. Wang, Graphene oxide-silver nanoparticle membrane for biofouling control and water purification, Chem. Eng. J. 281 (2015) 53–59. Copyright 2015, Elsevier Science Ltd., Oxford, UK.

6.6.5 Other nanoparticles enabled RO membranes Metal-organic frameworks (MOFs), a novel kind of porous materials, with broad range of topologies and sizes, have attracted attention in different fields such as separation, catalysis, and drug delivery. The different sizes of ZIF-8 MOF particles (50, 150, and 400 nm) were added to the PA layer of TFC membranes, and the effect of particles size in salty water desalination was investigated. Also the studies regarding the salty feed, contained BSA organic foulant, were performed for the assessment of antifouling properties of the fabricated membranes. It was demonstrated that incorporation of ZIF-8 MOF particles leads to lower flux decline ( 10%) compared with the neat TFC membrane ( 34%). Under the best condition with the 50-nm particle size of ZIF-8 MOF, good dispersion in the PA layer was obtained leading to fabrication of the high-performance TFC RO membrane [91]. Alumina (Al2O3) nanoparticles are known as stable and low-cost inorganic materials widely used in the membrane fabrication. IP of MPD aqueous phase and TMC organic containing Al2O3 nanoparticles was performed for fabrication of TFC RO membranes. The salty water filtration of the nanocomposite membranes was improved due to the positive effect of the alumina nanoparticles on the PA layer cross-linking [92].

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Other conventional inorganic nanoparticles also used in designing RO membranes are zeolites. These nanoparticles have subnanometer pores that not only create extra channels for water passage but also exclude the salty ions [93]. Safarpour et al. [94] developed new TFC membranes adding the plasma-treated natural hydrophilic clinoptilolite zeolites into the PA layer of RO membranes. They evaluated the modified membranes prepared with different concentrations (0, 0.005, and 0.01 wt%) and plasma gas pressures (0, 0.1. 0.5, and 1 Torr). At the optimum condition (0.01 wt% clinoptilolite and 1 Torr oxygen), the best performance of the membrane (39% water flux improvement and 88% fouling recovery ratio) was obtained. At this condition the minimum contact angle and surface roughness were obtained. Halloysite nanotubes (HNTs) with chemical composition of Al2Si2O5(OH)42H2O received increasing interest in wastewater treatment due to their biocompatibility, large specific surface area, and plentiful hydroxyl groups [95]. The effect of HNTs addition with various concentrations into the PA layer of TFC RO membranes was investigated by Ghanbari et al. [96]. They reported that the higher amount of nanoparticles increases the membrane WCA, surface roughness, and water flux. The maximum water flux and salt rejection obtained were 36 L/m2 h (at 15 bar) and 95.6%, respectively, at 0.05 wt/v% of the HNTs. It was claimed that the negative surface charge and the surface hydrophilicity of the modified membrane inhibit the attachment of BSA protein during the fouling measurement. Copper nanoparticles with antimicrobial properties are known as inexpensive alternatives for silver nanoparticles. Recent studies have shown a growing interest in the membranes preparation with antimicrobial functionality [97]. Ben-Sasson et al. [98] loaded the surface of TFC membrane with copper nanoparticles using the PEI capping agent via dip-coating method. The functionalization was performed based on irreversible binding between the positive charge of the copper nanoparticles and the negative charge of the membrane surface. The number of viable model bacteria including E. coli, P. aeruginosa, and S. aureus after 1 h contact onto the surface of the copper nanoparticle-modified TFC membrane decreased by 87.0%, 96.0%, and 79.5%, respectively. Although, the membrane surface positive charge has affinity to the negatively charged bacteria, the reason of reduced live bacteria onto the membrane surface is related to the toxicity of the loaded copper nanoparticles. In another study regarding the copper nanoparticle loading, Ma et al. [99] mentioned some disadvantages for the dip-coating method. This is time-consuming, and controlling the number of the attached nanoparticles is difficult. They developed spray- and spin-assisted layer-by-layer assembly (SSLbL) technique (Fig. 6.17) to address the disadvantages of the dip-coating method. The time consumed for the preparation of a uniform bilayer is about 1 min, which is much shorter than that required for preparation of layer via dip-coating method (25–60 min). Similar to the previously discussed study, inhabitation of bacteria deposition was obtained for both static bacterial inactivation and membrane filtration. Cerium oxide (CeO2) is a kind of inorganic nanoparticles that being used for modification of the PA layer of TFC RO membranes due to their properties such

6.6 Utilization of nanomaterials

FIG. 6.17 Schemes of (A) preparation of the PEI-coated Cu nanoparticles via the wet chemical reduction method; (B) coating the Cu nanoparticles on the membrane surface via the layerby-layer self-assembly method; (C) SSLBL self-assembly process. Reproduced with permission from W. Ma, A. Soroush, T.V.A. Luong, G. Brennan, M.S. Rahaman, B. Asadishad, N. Tufenkji, Spray-and spin-assisted layer-by-layer assembly of copper nanoparticles on thin-film composite reverse osmosis membrane for biofouling mitigation, Water Res. 99 (2016) 188–199. Copyright 2016, Elsevier Science Ltd., Oxford, UK.

as large surface area, low-cost and etc. Different amounts of hydrophilic CeO2 nanoparticles (50, 100, 150, 200, and 400 mg/L) were added to the PA layer for improving the performance of the TFC RO membrane. The best results were obtained about 100-ppm concentration of incorporated nanoparticles. The fouling analysis by BSA and HA foulants also showed good fouling resistance [100]. The iron and iron oxide nanoparticles (Fe nanoparticles and FeOx nanoparticles) are kind of low-cost inorganic nanoparticles considered as antibiofouling agents for modification of the TFC membranes. In this regard, Armenda´riz-Ontiveros et al. immersed a TFC membrane in an aqueous suspension of GO-Fe nanoparticles (Fig. 6.18) and fabricated the GO-Fe nanoparticle membranes. They compared the three types of membrane performance (neat, Fe nanoparticles, and GO-Fe nanoparticle membranes). However, after the modifications, the membrane fluxes were reduced, while their fouling resistances were improved compared with the neat

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FIG. 6.18 Schematic illustration of the GO-Fe-coated RO membranes. Reproduced with permission from M.M. Armenda´riz-Ontiveros, A.G. Garcı´a, S. de los Santos Villalobos, G.A.F. Weihs, Biofouling performance of RO membranes coated with iron NPs on graphene oxide, Desalination 451 (2019) 45–58. Copyright 2019, Elsevier Science Ltd., Oxford, UK.

membrane. Regarding the Fe nanoparticles and GO-Fe nanoparticles membranes, the obtained optical density were reduced by 40% and 48%, the total cell count were reduced by 67% and 40%, and total organic were reduced by 91% and 98%, respectively, in comparison with the neat membrane [101]. The enabled PA layer using the natural-based nanoparticles opened new avenues for the development of cost-effective RO membranes. The cellulose nanocrystals (CNCs) that also named the nanocrystalline cellulose (NCC) can be obtained by acid treatment of plentiful and sustainable biomass resources. These low-cost, safe, and nontoxic nanoparticles have reactive surface with hydroxyl groups that are able to be further functionalized. Utilization of the 0.1% (w/v) of CNCs into the PA layer of TFC membrane could increase its water flux from 30 to 63 L/m2 h (at pressure of 20 bar) without reduction of its high salt rejection (97.8%). The BSA fouling measurements (1200-min time duration) showed the higher fouling resistance of the modified membrane with about 0.85 normalized water flux, which was better than that of the neat TFC membrane with normalized water flux of about 0.74 [102].

6.7 Conclusion Large-scale and industrial applications of RO membranes for desalination and wastewater treatment depend on the synthesis of fouling-resistant RO membranes with high lifespan. Also, the membranes with antifouling properties reduce the costs of produced clean/drink water. Development of the novel and antifouling membranes is possible based on the incorporation of some materials into the

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membranes that mitigate the different types of fouling including colloids, organic fouling, inorganic fouling, and biofouling that may reduce the RO membranes performance. Recently, impressive studies have been reported regarding the fabrication of antifouling /antibiofouling-resistant RO membranes by nanoparticles incorporation into the PA selective layer of TFC membranes or their embedment into the polymeric matrix. However, more exploration is needed for the synthesis of the nanoparticles, which can enable the membranes to mitigate all four fouling types that exist in any aqueous feed media simultaneously. Also, assessment of these membranes in long-time fouling measurements can overcome the challenges in practical applications.

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CHAPTER

Nanomaterials in wastewater treatments

7

Bapun Barik, Pratap Sagar Nayak, Priyabrat Dash Department of Chemistry, National Institute of Technology, Rourkela, Odisha, India

Chapter outline 7.1 7.2 7.3 7.4 7.5

Introduction ....................................................................................................185 Wastewater and its sources .............................................................................187 Wastewater treatment processes ......................................................................188 Nanomaterials .................................................................................................189 Modified metal oxide nanomaterials .................................................................191 7.5.1 Graphene oxide-supported metal oxide nanomaterials .......................191 7.5.2 Polymer-supported metal oxide nanomaterials ..................................193 7.6 Graphene oxide and polymer-supported metal oxide nanomaterials for wastewater treatment ......................................................................................194 7.6.1 Graphene oxide-supported metal oxide nanomaterials for wastewater treatment .....................................................................195 7.6.2 Polymer-supported metal oxide nanomaterials for wastewater treatment .....................................................................196 7.7 Conclusions and future perspectives ................................................................199 References ............................................................................................................200

7.1 Introduction Water is one of the most essential and basic ingredient to sustain life on earth. To describe the importance of water, once, Leonardo da Vinci quoted, “Water is the driving force in nature,” but a tremendous development in industrialization and urbanization ignited by upsurge in population dynamics has vastly declined the natural water quality. Several industries such as leather and cloth manufacturing, dye, fertilizers, agricultural, mining, and metallurgical industries are the major contributors to the wastewater causing environmental pollution [1, 2]. The toxic discharges from these industries cause increase in heavy metals, dyes, pesticides, pharmaceuticals, phenol, and organic matter concentration in water [3]. These water Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00007-9 # 2020 Elsevier Inc. All rights reserved.

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contaminants are hazardous to both human beings and environment. They can cause dangerous harmful effects, which may lead to mutagenesis and carcinogenesis in human beings and aquatic creatures [4]. With ever-growing demand for water supply toward sustainable development, it is required to harvest the unconventional water sources such as contaminated fresh water, blackish water, seawater, and wastewater. In past few decades, various techniques have been reported by several scientific groups to decontaminate these aqueous media pollutants, which include electrochemical and thermal decomposition, solvent extraction, gravity separation, evaporation, micro- and ultrafiltration, chlorination, ozonation, photooxidation, electrodialysis, adsorption, reverse osmosis, coagulation, ion exchange, and membrane separation. [5, 6] In recent times, nanomaterials have gained immense popularity toward environmental restoration for wastewater treatment with a wide range of reports related to its successful purification. Nanoscale materials are generally known as materials whose at least one dimension is in the range of 1–100 nm, which are either synthesized by bottom-up or top-down approach [7]. They usually carry unique properties like appropriate dispersibility, porosity, high surface area, and tunable activity. These multifunctional properties have opened wide opportunities in the field of chemistry, physics, electronics, biology, device storage, medicine, and environmental betterment [8]. Among these, water purification for restoration of aquatic ecosystem is one of the most harnessing fields of nanomaterials. In some of the traditional techniques, the wastewater treatment required some centralized facility, infrastructures, tedious procedures, and big finances [9], but introduction of efficient, tunable, and multifunctional nanomaterials to water purification technology has empowered the aquatic ecosystem by reducing its dependence on large frameworks and expenses. Among all the available nanomaterials, metal oxides have evolved as the go-to materials for water scientists because it eliminates most of the difficulties in the existing treatment process and brings new advanced methods that could improve the water quality and water supply [10]. Moreover, nanosized metal oxide-based water treatment process has the potential to completely remove or convert the pollutants into environmentally suitable forms [11]. All these comprehensive advantages of metal oxides are due to their attractive properties that can be modified or controlled depending upon the requirement of application. Specially, in wastewater treatment, these nanomaterials have shown their dominance extensively [12]. Properties of these metal oxide nanomaterials are generally driven by the size, shape, structure, and phase change in the nanomaterials [13]. Moreover, their surface modification with supporting materials such as graphene and natural polymers has encouraged several unique properties in these nanomaterials such as high surface-to-volume ratio, hydrophilicity, enhanced reactivity, strong sorption capacity, and separation behavior, and negligible waste formation ability has benefited the use of nanomaterials in the field of wastewater treatment [14, 15]. Higher porosity and active surface properties of the nanomaterials reinforce the rapid and more efficient binding with the wastewater pollutants without causing any secondary contamination. For these reasons, scientific interests have massively increased to explore more sustainable and active nanoscale materials as

7.2 Wastewater and its sources

FIG. 7.1 Overview on the use of nanomaterials for wastewater treatment.

a new alternative for wastewater treatment. In recent scholarly articles, a variety of graphene and polymer-based metal oxide nanomaterials are investigated thoroughly for wastewater purification. As summarized in Fig. 7.1, in this chapter, we present the recent advancement in the field of nanomaterials for treatment and purification of wastewater with the noble vision of restoration of aquatic ecosystem.

7.2 Wastewater and its sources In general, wastewater can be defined as the combined water mixed with wastes released due to mankind activities from domestic usage, institutions, commercial and industrial establishments, etc. [16] Recent developments in rapid industrialization influenced by the extensive use of automobiles, population explosion, enhanced use of chemicals in the field of agriculture, and textile manufacturing have increased the environmental contamination dramatically. Every anthropogenic source including industrial, domestic, commercial, or agricultural process results with production of large amount of toxic and harmful wastewater. Domestic wastewater includes materials added by a community during or after its use. Thus, it contains human body wastes and sullage; other wastewater resulted due to personal washing, laundry, kitchen usage, etc. [17] Domestic wastewaters have generally objectionable appearance and hazardous consequences. This may be due to the fact that they contain a lot of pathogenic microorganisms. Industrial processes due to large-scale products and

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production methods emit a wide range of different wastewater with toxic and dangerous carcinogenic contaminants [18]. Industries like food processing, aluminum, steel plants, coal mines, dyeing, textile, and pharmaceutical industries are generally the most important wastewater resources that excrete substantial amount of contaminated water to the environment. Moreover, every industry has its respective wastewater flow fraction terms and conditions with provided consequences to that. As discussed, industrial wastewater contains numerous variety of components such as obstructing compounds (glass powders, plastic parts, cigarette leftovers, etc.), insoluble and suspended solids, organic compounds, nutrient salts (NH4+, SO4 2 , PO4 3 , etc.), corrosion substances (alkali, solvents, oils, etc.), cleaning agents, lubricants, disinfectants, and other hazardous substances (hydrocarbons, chlorinated molecules, organic halogen compounds, cyanides, heavy metals, etc.) [19]. Another major source of wastewater is agricultural activities all over the world. Usually, agricultural effluents have much higher biological oxygen demand (BOD) and nitrate contents than sewage, which cause significant water contamination. Substantial deterioration of water quality due to increase in carbon and nitrogen content causes serious environmental threat, which should be reduced before discharging these effluents to water sources. [20]. These agricultural by-products are very strong and really difficult to cleanse from wastewater. Toxic chemicals in fertilizers, pesticides, waste of animals, and irrigation residues are the main components of agricultural wastewater, which diversely affect the surface water. Real-world wastewater also includes many other wastewater sources such as dairy plants, hospitals, and animal confinements [21]. Nowadays, it is a big challenge for mankind to save the environment by treating and monitoring this wastewater with efficient and high-end purification methods. The increasing presence of pollutant macromolecules in wastewater demands introduction of newer technologies to degrade these macromolecules into smaller nontoxic molecules.

7.3 Wastewater treatment processes In last few decades, numerous physical, chemical, and biological treatment processes have been developed for wastewater treatment. The most common technologies adopted are coagulation, solvent extraction, sedimentation, gravity separation, flotation, precipitation, micro- and ultrafiltration, oxidation, evaporation, distillation, electrodialysis, membrane filtration, ion exchange, reverse osmosis, photocatalysis, adsorption, etc. [22] Among all the treatment processes, adsorption and photocatalysis have gained a great amount of importance due to their higher efficiency and tremendous large-scale potential. Adsorption is one of the most followed techniques for wastewater treatment due to its economic and easy processability. A wide range of adsorbents were already investigated for this purpose with excellent success rate toward numerous soluble, insoluble, biological, organic, and inorganic water pollutants [23]. Adsorption provides an additional advantage of source reduction and reclamation for portable

7.4 Nanomaterials

industrial wastewater treatment. This process is mainly dependent on the surface area and amount of active sites present on the conventional adsorbents, which decide their efficiency to decontaminate wastewater. An ideal adsorbent for wastewater treatment purposes should satisfy some criteria such as environmental suitability, high sorption capacity, and selectivity in low concentration surface regeneration property [24]. Again, it has certain drawbacks such as limited commercial success, lack of selectivity, and required kinetics. Moreover, a single adsorbent couldn’t show required efficiency for removing all kinds of wastewater pollutants. In this direction, composite adsorbent provides significant improvement with their enhanced specific surface area and associated active sites. Again, short intraparticle distance, tunable pore size, super surface chemistry, and most importantly the potential of surface modification make the combination of nanomaterials and adsorption process the most promising solution for future generation. Photocatalysis is an advanced oxidation process under light irradiation for efficient decontamination of wastewater from several toxic organic pollutants and microbial pathogens [25]. It is a polishing treatment for hazardous and nonbiodegradable water contaminants to encourage their biodegradability. Still, the barrier to large-scale application of conventional photocatalytic procedure is its slow kinetics and limited light-harvesting nature [26], but photocatalysis with unique properties of nanomaterials has enhanced its range of applications such as degradation of pollutant macromolecules from water and atmosphere. Several metal oxides, semiconductors, and metal nanoparticles have shown potential in photocatalysis, which can be credited to their improved and controllable optical properties [27]. The next section of the chapter focuses on the different types of nanomaterials used in adsorption and photocatalytic applications.

7.4 Nanomaterials Research on nanomaterials has gained enhanced interest from scientists and material engineers worldwide. The term “nanomaterial” is usually referred as materials with external dimensions or an internal structure measured in nanoscale range [28]. These materials possess unique size-dependent characteristics totally diversified from their bulk counterparts, many of which have been explored for various potential applications. Nanomaterials display unique optical, mechanical, magnetic, conductive, and sorption properties that are better than the same material in wider scale [29]. The better understanding on the properties of nanomaterials paves way to synthesize novel materials, which have the potential to improve the quality of life. The sizedependent characteristics of nanomaterials are normally due to their high specific surface area, high reactivity, fast dissolution, and strong sorption property [30]. Other interesting properties of nanomaterials are superparamagnetic behavior, quantum confinement effect, and localized surface plasmon resonance (SPR). In recent times, nanomaterials are becoming more commercialized, beginning to emerge as

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daily life commodities, and used in several innovative applications, including a broad range of consumer products [30]. There are different types of nanomaterials such as organic, inorganic, carbon, and composite-based nanomaterials, which display a variety of physical, chemical, and biological characteristics. In recent past, a variety of potential nanomaterials have gained attention in wastewater treatment. Some mostly explored nanomaterials are metal oxides, magnetic nanomaterials, carbon nanotubes (CNT), metal nanoparticles, metal-organic frameworks (MOFs), layered double hydroxides (LDHs), graphene-based nanomaterials, and polymer-based nanomaterials. Organic nanomaterials include CNT, fullerenes, single-walled carbon nanotubes (SWCNT), multiwalled carbon nanotubes (MWCNT), and graphene and its derivatives. Most of the organic nanomaterials are carbon-based nanomaterials [31], while metal and metal oxide-based nanomaterials such as aluminum, zinc, copper, iron, aluminum oxide, iron oxide, titanium oxide, and ZnO are categorized as inorganic nanomaterials [32]. Composite nanomaterials are the combination of organic-organic nanomaterials, organic-inorganic nanomaterials, and inorganic-inorganic composite materials [33]. Among all available nanomaterials, nanoscale metal oxides like cerium oxides, magnesium oxides, titanium oxides, aluminum oxides, and iron oxides have proved as the suitable ones for different environmental applications [34]. This may be due to the large surface area and high size quantization effect. Moreover, particle size of metal oxides expected to influence the properties mainly in two aspects: first one is the change in structural characteristics, such as the lattice symmetry and cell parameters, while the other one is the presence of undercoordinated atoms (like corners or edges) or O vacancies in an oxide [35]. These undercoordinated atoms or O vacancies produce atomic arrangements different from that in the bulk material. At the same time, the occupied electronic states are located above the valence band, enhancing the chemical activity of the system. Recent studies suggested that the high sorption ability and selectivity of metal oxide nanoparticles play an important role for removal of toxic pollutants from wastewater to meet the large-scale demand [36, 37]. However, due to reduced size from micrometer levels to nanometer levels, the surface energy enhances leading to poor stability. Additionally, conventional metal oxides possessing magnetic character have increasing demand as they can be easily separated from the aqueous system without any secondary contamination [38]. Such facile separation possibility improves the operation efficiency and simultaneously consumes time for wastewater treatment, but metal oxides are very much prone to agglomeration because of van der Waal’s force of interaction resulting dramatic minimization or complete loss of sorption capacity and selectivity [39]. Size- and shape-dependent properties of metal oxide can be controlled by adopting suitable synthesis protocol. More efficient, highly active, and monodispersed metal oxides have been studied in last few years. These synthesis protocols include several physical (ball milling, ultrasound, gas condensation, etc.) and chemical approaches (coprecipitation, pulse electrode deposition, gas-phase deposition, liquid-phase deposition, etc.) with many successful results [40]. The approaches

7.5 Modified metal oxide nanomaterials

are widely used and easily scalable with high yields. A suitable fabrication technique for metal oxides can enhance its activity for desired synthetic application. Nevertheless, these metal oxide samples have some inherent disadvantages like agglomeration, difficult separation, and dramatic pressure drops when aimed to large-scale synthesis. The most effective approach for synthesizing this metal oxide nanoparticles by overcoming these drawbacks is to modify the nanosized metal oxides with suitable support materials [41]. In this direction, to improve the wastewater applicability of metal oxide nanomaterials, these are impregnated onto different support materials of similar or larger scale to obtain better activity and stability.

7.5 Modified metal oxide nanomaterials Most of the individual metal oxides couldn’t accomplish all requirements for the development of newer technical studies and find the solution to the world’s immediate problems. This is due to the inherent brittleness and high agglomeration tendency, which have been a major reason to force the scientific community to prepare novel and more efficient materials [39]. In this direction, design of composite materials involving these nanomaterials on suitable support material has been found as a suitable solution. Several factors such as shape, size, dispersion, interaction, and alignment play an important role for designing a novel nanocomposite material. Other important factors that drive the efficiency of the composite nanomaterial are orientation, composite ratio, volume fraction, synergy, etc. Such metal oxide-support composite materials provide several operational benefits such as addition of single chemical and use of one experimental system rather than conventional two operational units. This favorable condition suits their large-scale application in wastewater treatment process[42]. In recent times, various composite nanomaterials involving graphene, CNT, fullerene, natural polymer, and silica have found potential applications [43, 44]. The most accepted support materials used for wastewater treatment are natural polymers, synthetic polymer hosts, activated carbon, graphene and its derivatives, etc. Among these support materials, graphene derivatives (graphene oxide and reduced graphene oxide) and natural polymers (chitosan and cellulose) show more effective and specific contaminant removal property for wastewater treatment technology [41, 45].

7.5.1 Graphene oxide-supported metal oxide nanomaterials Graphene is known as the “rising star” material in current generation. When it is readily synthesized via scotch tape method in 2004 from graphite powder, the scientific community is blessed with a future solution to many worldwide problems [46]. The super exciting properties of graphene, which include high surface area, tunable bandgap, superior electrical, thermal, and conducting properties, gained immense attention toward this sp2 honeycomb array of carbon atoms [47]. There are several derivatives of graphene like graphene oxide that attracted interest from the research

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community for their potential achievements in real-world applications [48]. From the structural and functional aspects of graphene, the most common materials related to graphene are graphene oxide and reduced graphene oxide. Graphene oxide (GO) has been emerged as an excellent support material for a broad range of nanosized materials toward potential applications in the field of many environmental applications. GO is a 2D carbon sheet that is highly oxidized to have several oxygenated functional groups like carbonyl, carboxylic, epoxide, and hydroxyl on its surface [49]. Due to the hydrophilic nature, they are dispersible in wide range of solvents. It is generally synthesized from graphite powder or graphite oxide after dispersion in a suitable solvent to exfoliate the carbon layers by solvent molecules [50]. This provides better surface availability and activity to the GO sheets. Not only GO but also the other derivative of GO, that is, reduced graphene oxide (rGO), is also very popular among the scientific community for its potential electronic properties. It is the reduced form of GO, which is generally prepared following harsh chemical methods resulting deoxygenation of surface functional groups. Further, during synthesis, both GO and rGO achieve many defects and vacancies within the sp2 carbon lattice [51]. Considering all the benefits associated with GO and rGO, many researchers have focused on revealing more potential application of these materials without affecting its poor crystallinity. Having so many advantages associated with GO and rGO, the major drawback is their tendency to get folded and prone to agglomeration, which reduces their efficiency [52], so combining GO and rGO with metal oxide nanomaterials have opened new door to the materials science world. Their incorporation with GO and rGO enhances their mechanical strength and affordability for large-scale applications. Such GO- and rGO-based metal oxide nanomaterials have shown their extensive applicability in catalysis, photocatalysis, energy storage, sensing, and water purification. In the case of water purification, GO and rGO modified with metal oxides have shown major achievements with potential future possibilities (Fig. 7.2). Since the 2D array of GO and rGO has larger surface and a wide range of surface modification possibilities, it can be used as a selective and versatile material for

FIG. 7.2 (A) Graphene oxide and (B) reduced graphene oxide.

7.5 Modified metal oxide nanomaterials

surface binding of different wastewater contaminants. The presence of oxygenated functional groups on the GO surface brings several water favorable qualities to these materials such as polarity, solubility, better reactivity, selectivity, and biocompatibility. These 2D sheets can rapidly interact with wastewater contaminants like heavy metals, pesticides, and organic dyes via many possible interactions such as π-π interactions, π-π stacking, hydrogen bonding, and covalent and electrostatic interaction. The available literatures suggest remarkable wastewater pollutant removal ability of GO- and rGO-based composites, which are being outlined in the following section.

7.5.2 Polymer-supported metal oxide nanomaterials Polymeric materials are potential alternative to many traditional materials due to their mechanical rigidity, controllable surface chemistry, porous arrangement, and regeneration under mild conditions. Polymers have been famous for trapping many environmental pollutants such as organic acids, aromatic hydrocarbons, phenols, alkanes, and dyes. Additionally, the binding of pollutants can be extended to many highly water soluble compounds, heavy metal ions, and agricultural wastes [53]. Among polymers, natural polymers are the most admired because of their environmentally friendly nature, and also, they satisfy the principles of green chemistry. Among all available polymeric materials, chitosan and cellulose have gained tremendous success particularly in the field of wastewater treatment. Chitosan, a deacetylated form of parent chitin natural polymer, is a biocompatible, biodegradable, nontoxic, and biorenewable amino polysaccharide [54]. It has also unique structure, multidimensional behavior, and unique functionalities that made it suitable for binding of pollutant molecules on its surface. Similarly, cellulose is another very interesting biopolymer that has gained popularity among the researchers for development of novel and sustainable materials for wastewater treatment. Cellulose is a naturally occurring biopolymer with unique structural, mechanical, and optical characteristics [55]. Moreover, high functionality, sustainability, high strength, and biocompatibility benefit the chitosan and cellulose materials for real-world applications. However, there are some major limitations of pristine chitosan and cellulose polymers such as low adsorption capacity and relatively hydrophilic in nature. To address these issues, surface modification or functionalizations are proved to be an effective process as they can provide specific interaction with the target pollutants. Recently, polymeric materials combined with metal oxide nanomaterials emerged as a potential solution to the wastewater treatment [56]. Polymer-based advanced nanocomposites are designed to meet the required challenge toward specific water purification applications. By tuning the chemical structures and physicochemical properties such as hydrophilicity, thermal and mechanical stability, porosity, and charge density along with proper modification of functionalities, the desired efficiency of the polymeric nanomaterials for photocatalytic, adsorptive, and antimicrobial applications can be achieved [57]. Some of the recent reports suggest the excellent development in the

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FIG. 7.3 (A) Chitosan, (B) cellulose, and (C) hydroxypropyl methylcellulose (HPMC).

field of polymeric metal oxide nanomaterials for wastewater treatment with potential large-scale production that are presented in the later part of this chapter (Fig. 7.3).

7.6 Graphene oxide and polymer-supported metal oxide nanomaterials for wastewater treatment Removal of toxic pollutants from wastewater is necessary for health and environmental protection. In this purpose, numerous materials were already used with some noteworthy achievements. Wastewater treatment with metal oxide-based composite nanomaterials is receiving increased attention due to their exceptional property modulation and superior performance compared with traditional organic or inorganic nanomaterials. Particularly, when the metal oxide nanoparticles are incorporated with graphene sheets or polymer matrixes, they show better activity with least environmental side effects. In comparison with conventional nanomaterials, the composites’ nanostructures show much higher efficiencies and faster adsorption rates in wastewater treatment [58]. A variety of efficient, low-cost, and eco-friendly composite nanomaterials with unique functionalities have been proposed for potential applications in detoxification of industrial, domestic, and agricultural effluents [59]. In recent years, many studies have proved that the metal oxides such as ferric oxide (Fe3O4) [60], manganese oxide (MnO2) [61], titanium oxide (TiO2) [62], magnesium oxide (MgO) [63], and zinc oxide (ZnO) [64] supported on graphene or polymer supports can satisfy most of the requirements for wastewater treatment. The nanomaterials may be successfully used as efficient, cost-effective, and environmentally friendly adsorbents or photocatalysts for the removal of various toxic substrates from wastewater such as heavy metals, azo dyes, pesticides, and other organic matters.

7.6 Graphene oxide and polymer-supported metal oxide

7.6.1 Graphene oxide-supported metal oxide nanomaterials for wastewater treatment With the developments with GO- and rGO-based wastewater treatment, some researchers have cited the less activity of pristine GO due to its inherent property of agglomeration leading to minimization of desired activity. So, modification of GO with nanoscale materials brings new possibilities to improve the efficiency toward numerous environmental applications. In addition to the economic benefits, GO-based nanomaterials in environmental implication act as a prompt factor toward development of graphene-based nanotechnologies. GO, when modified with various nanomaterials, reinforces the physicochemical properties of the composite materials. To get better performance benefits, a careful optimization of synthesis protocol is very necessary. In combination with metal oxide nanomaterials, GO shows improved performance due to their synergistic interaction between the GO sheets and the metal oxides attached to the sheets. These potential GO-based nanomaterials are extensively used in wastewater treatment for a wide variety of water pollutants such as inorganic heavy metals, organic molecules, waterborne microorganisms, pesticides, and pharmaceutical pollutants. Baranik et al. reported GO/CeO2 composite nanomaterial for adsorptive removal of As(III), As(V), Se(IV), Cu(II), and Pb(II) from aqueous solution with an excellent 6 to 30 mg/g adsorption capacity [65]. Huang et al. analyzed the photocatalytic dye removal efficiency of rGO-CeO2 with successful minimization of electron-hole pair combination. They also noticed enhanced photocatalytic degradation of MB dye by rGO-CeO2 than pristine CeO2 nanoparticles [66]. Similarly, Yu and his coworkers prepared phenylamine-rGO-TiO2 photocatalyst for selective degradation of anionic dye from the mixture of cationic dye and anionic dye where MB and MO are taken as model dyes [67]. Fugetsu and his coworkers synthesized sodium hydrosulfide reduced GO for enhanced adsorption of acridine orange from aqueous media. A maximum adsorption capacity of 3.3 g/g was obtained, which is much higher than the pristine GO (1.4 g/g) [68]. Bradder et al. showed excellent MB and malachite green (MG) dye adsorption with layered GO, which has expanded graphene structure and enhanced surface oxygen functionalization. This plays a significant role in MB and MG dye adsorption with adsorption capacity of 351 and 248 mg/g, respectively [69]. Pradeep and his coworkers investigated the pesticide degradation capability of graphene oxide with an unprecedented adsorption capacity [70]. They noticed adsorption capacity for chlorpyrifos (CP), endosulfan (ES), and malathion (ML) around 1200, 1100, and 800 mg/g, respectively. The authors reported GO as a pH-insensitive and reusable photocatalyst for degradation of these particular pesticides. Perreault et al. reviewed the essentiality of GO-based nanomaterials as the next-generation water treatment and decontamination technology [71]. Chandra et al. synthesized a magnetic graphene oxide with only 10-nm particle size for over 99.9% removal of arsenic within 1 ppb from water. They also demonstrated the room-temperature supermagnetic nature of the nanocomposite with practical usable separation of arsenic [72]. Another report by Wang et al. in 2011 describes Fe3O4-GO composite as a promising material for preconcentration

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and separation of Co(III) ions from aqueous media [73]. They noticed the sorption process to be ionic strength independent and pH dependent. At low pH, inner-sphere complexation was the major factor, whereas at higher pH simultaneous precipitation and inner-sphere surface complexation dominate the sorption mechanism. They also found the sorption process to be endothermic and spontaneous in nature. Sood and his coworkers fabricated a novel GO-based MnFe2O4 nanoadsorbent for efficient removal of Pb(II), As(III), and As(V) from contaminated water [74]. A maximum adsorption capacity of 673, 146, and 207 mg/g was noticed for Pb(II), As(III), and As(V), respectively. Fast kinetics, enhanced surface area, magnetic separation, low cost, and reusability are some attractive properties that made the nanoadsorbent more favorable for practical application. In another report, Rokhsat et al. presented ZnO/GO composite nanomaterial for UV-treated photocatalytic degradation of MB dye in 450 min [75]. GO-grafted titanium oxide was synthesized via a simple solvothermal method by Lee and his coworkers [76]. They investigated its photocatalytic activity for MB and MO under UV light radiation and compared the results with bare TiO2 nanoparticles. The TiO2/GO binary nanocomposite showed maximum degradation efficiency 100% and 84% for MB and MO in a neutral solution within 25 and 240 min, respectively. The increased photocatalytic performance was due to increased light absorption and reduced charge recombination after embedment of GO. Besides, Jiao et al. presented a novel rGO-based Ag nanoparticle (rGO/PEI/Ag) containing composite hydrogel as a dye catalyst for wastewater treatment [77]. The rGO/PEI/Ag hydrogel exhibited excellent photocatalytic degradation rates for rhodamine B (RhB) and MB dye in aqueous environment. They also showed the efficient separation of the catalyst from the aqueous media that suggested the potential large-scale application for wastewater treatment. Gao et al. made a composite of water filtration membrane surface modified with GO-TiO2 composite for significant photocatalytic MB degradation with enhanced kinetics under UV (60%–80% faster) and sunlight (3–4 times faster) than GO-TiO2 nanocomposite [78]. Moreover, the surface modification with metal oxides introduced a promising route to the synthesis of high-performance photocatalytic membranes toward wastewater treatment. So, the reported literatures suggest the significant achievements of different GO-supported metal oxide nanomaterials for wastewater treatment, which can be attributed to the extended oxygenated functional groups on GO surface and also for the synergistic binding between the nanomaterials and the GO sheets. Again, the photoexcited electrons captured by GO sheets increase the electron flow, and the combination of GO with nanomaterials acts as an electron transfer channel minimizing the charge recombination possibility [79]. Some of other major reports of GO-supported metal oxide nanomaterials in wastewater treatment are outlined in Table 7.1.

7.6.2 Polymer-supported metal oxide nanomaterials for wastewater treatment Numerous reports suggest the efficiency of chitosan nanocomposite toward the removal of the pollutant macromolecules for wastewater treatment technology. For example, Hosseini et al. studied the photocatalytic properties of heterostructured

7.6 Graphene oxide and polymer-supported metal oxide

Table 7.1 GO-supported metal oxide nanomaterials for wastewater treatment. Material

Removal method

Pollutants

References

Magnetite-rGO δ-MnO2-graphene GO@Fe3O4/ZnO/SnO2 GO-ZnO TiO2/rGO-Fe3O4

Adsorption Adsorption Photocatalysis Photocatalysis Adsorption and photocatalysis Photocatalysis Photocatalysis Adsorption

As(III) Ni(II) Azithromycin MB MB and As(III)

[72] [80] [81] [75] [82] [83] [84] [85]

Photocatalysis Photocatalysis

Chlorpyrifos MB Pb(II) and As(III) ions RhB RhB

Photocatalysis

MB, Congo red

[88]

Photocatalysis

RhB, MB, MO, and phenol

[89]

CoFe2O4@TiO2-RGO α-Fe2O3/GO Mesoporous SiO2-GO RGO-mesoporous TiO2 Cu2O nanoparticles wrapped by rGO Graphene oxideSnO2-TiO2 Graphene oxideCuFe2O4-ZnO

[86] [87]

Sb2S3-CeO2 on chitosan-starch composite [90]. The photocatalytic properties of the nanocomposites for efficient degradation of paraquat as a toxic organic compound were analyzed under UV light. They found the optimum effect at pH 7 at 30 min contact time. Again, Yang and his coworkers reported selective adsorption of Pb2+ removal from wastewater with nano-TiO2-cellulose composite fiber. The adsorption process was regenerable with 0.1M HCl solution [91] and also showed better selectivity toward Pb2+ adsorption than Ca2+ with superior adsorption capacity and faster adsorption. Chattopadhyay et al. documented the efficient photocatalytic property of CuO/Chitosan nanocomposite toward degradation of RhB [92]. The combined synergistic effect of CuO nanosphere with chitosan beads provides higher efficiency up to 99% removal of dye molecules from the contaminated water. This excellent result can be attributed to the slow electron-hole pair recombination rate between CuO and biopolymer chitosan. Iftekhar et al. studied cellulose intercalated with Zn/Al LDH for highly efficient separation of rare earth elements [93]. From the kinetic experiments, separation equilibrium was achieved for Y3+, La3+, and Ce3+ within 10 min. The adsorption capacities were 102.25, 92.5, and 96.25 mg/g for Y3+, La3+, and Ce3+, respectively. The composite showed a good selectivity and reusability up to five cycles. Chang and his coworkers investigated the super adsorbent property of cellulose-clay hydrogel nanocomposite for efficient dye removal capacity [94]. They adopted chemical cross-linking procedure for cellulose, carboxymethyl cellulose, and intercalated clay in NaOH/urea aqueous solution, which possesses high adsorption capacity for MB. Maximum removal efficiency of hydrogel samples for MB were achieved with a concentration of 10 and 100 mg/L, which

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were 96.6% to 98%. Similarly, Marinho et al. proposed the photocatalytic reduction of Cr(VI) with TiO2-coated cellulose acetate nanocomposite [95]. They used citric acid as hole scavenger and photo-reduced Cr(VI) to Cr(III) by simple dip coating method under continuous irradiation of natural solar light. They demonstrated that a six-layer TiO2-coated TiO2-P25-cellulose acetate monolithic (CAM) showed efficient activity during 10 consecutive Cr(VI) reduction cycles. In another report, Gan et al. fabricated a biorenewable and recyclable cellulose-based carbon microspheres combined with CoFe2O4 and demonstrated its photocatalytic properties for degrading RhB dye [96]. They followed a one-step hydrothermal method (CMS) and twostep hydrothermal and pyrolysis process (pCMS) where material synthesized via pCMS has better photocatalytic performance due to higher graphitization degree than CoFe2O4. Moreover, magnetic separation of the polymeric nanocomposite was possible, which makes the sample easy separable and a potential wastewater treatment material for achieving the goal of clean water supply. Liu and his coworkers designed BiOBr/regenerated cellulose photocatalysts by utilization of pulp boards as the cellulose source [97]. The results showed that BiOBr nanoparticles are bound to the cellulose through strong interaction between hydroxyl groups and nanoparticles via hydrogen bonding. The photocatalyst demonstrated remarkable photocatalytic efficiency for degradation of 25 mg/L RhB concentration within only 50 min, again, a maximum degradation percentage of 99% and 88.6% after four cycles. Several other reports claimed the profound efficiency for wastewater treatment which are provided in Table 7.2.

Table 7.2 Polymer-based metal oxide nanomaterial wastewater treatment. Material

Removal method

Pollutants

References

Chitosan-bound Fe3O4 EDTA-modified chitosan/ SiO2/Fe3O4 Chitosan-TiO2 Phthalocyanine/chitosanTiO2 Cu2O/chitosan-Fe3O4

Adsorption Adsorption

Cu(II) Cu(II), Pb(II), and Cd(II) MB Aniline

[98] [99]

[102]

Cellulose-templated TiO2/Ag Cu2O functionalized cellulose Cellulose/Fe3O4/activated carbon TiO2-cellulose Al2O3-chitosan-HPMC

Photocatalysis Photocatalysis

X-3B decoloration RhB and salicylic acid MB

[104]

Adsorption

Congo red

[105]

Adsorption Adsorption

Pb2+ F and methyl orange

[106] [107]

Photocatalysis Photocatalysis Photocatalysis

[100] [101]

[103]

7.7 Conclusions and future perspectives

7.7 Conclusions and future perspectives Design of novel nanomaterials with unprecedented characteristics has gained immense attention in last few decades. Applicative analysis of these nanomaterials in wastewater treatment processes has been a primary focus of the recent scientific community globally. The exciting properties of these nanomaterials have opened many future perspectives for introduction of more prominent and active large-scale nanomaterials. Also, the recent studies reveal that coordination of nanomaterials with GO and its derivatives provides better activity and economic large-scale production possibility for wastewater treatment. GO, ever since its discovery, has dominated the materials science research due to its unique properties such as high surface area, tunable electrical and optical properties, and favorable physicochemical characteristics. Moreover, the presence of multiple oxygenated functional groups opened new possibilities for synthesis of GO-based composite nanomaterials for the purpose of better and more efficient wastewater treatment. This book chapter is focused on the recent developments in the field of nanomaterial and supported nanomaterials for various wastewater treatment applications. Combination of different supporting materials with nanoscale materials has better performance benefits due to synergistic interaction between the two individual components. The initial section of this book chapter explains various wastewater sources that are the major contributor to the water toxification. The narratives then continued with the progressive nanomaterials and their higher activity with further modification with supported materials. GO and polymers are some of the highly explored supporting materials for the nanoparticles applied in various fields along with wastewater purification. The next section highlights about the importance and developments of GO- and polymer-based nanomaterials used for wastewater purification. The section also highlights advantages after compositing the nanomaterials with their supports that improves their physicochemical properties related to wastewater treatment. Recent examples of such GO- and polymer-supported nanoscale materials and their efficiency toward wastewater purification technology are presented. Till date, numerous composite materials were reported for environmental technology development, but still, their applicability has been constrained due to some technical drawbacks, which include less large-scale production possibility and inefficiency in optimized material development. In this direction, several researchers are trying to further modify the supported nanomaterials with other semiconductor materials for enhanced performances. This compositing benefits materials to promote their synergistic improvement in surface properties. The potential use of sophisticated characterization techniques and computational tools is preferentially recommended for future research perspective. The selectivity of the nanomaterials toward the target pollutants has to be improved with better efficiency toward a wide range of contaminants. Some developed material fabrication technique needs to be designed at molecular and electronic level along with potential use in versatile conditions. This will benefit the commercial sectors both in terms of economy and greener environment. The current developing world is searching for greener and

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sustainable nanomaterials to be in action for environmental treatment. In this regard, the design of novel nanomaterials or modified nanomaterials requires more attention and proper investigations in these aspects.

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CHAPTER

Nanomembranes for water treatment

8

Faisal Rehmana, Khalid Hussain Thebob, Muhammad Aamirc, Javeed Akhtarc a

Department of Electrical Engineering, The Sukkur IBA University, Sukkur, Pakistan bUniversity of Chinese Academy of Sciences, Beijing, People’s Republic of China cMaterials Laboratory, Department of Chemistry, Mirpur University of Science and Technology (MUST), Mirpur, Pakistan

Chapter outline 8.1 Introduction ....................................................................................................207 8.2 Synthetic techniques .......................................................................................209 8.2.1 Drop casting ..................................................................................209 8.2.2 Vacuum filtration ...........................................................................209 8.2.3 Spin coating ..................................................................................211 8.2.4 Langmuir-Blodgett (LB) method ......................................................211 8.3 Some common types of membranes ..................................................................212 8.3.1 TMDC membranes .........................................................................212 8.3.2 MXene membranes ........................................................................213 8.3.3 hBN membranes ............................................................................214 8.3.4 MOFs membranes ..........................................................................214 8.3.5 Zeolite membranes ........................................................................215 8.4 Desalination process via 2D membranes ...........................................................216 8.4.1 Graphene ......................................................................................216 8.4.2 Assembled 2D material laminates ...................................................227 8.5 Dye separation via 2D membrane .....................................................................228 8.6 Conclusion and future prospects ......................................................................235 References ............................................................................................................236

8.1 Introduction Sustainable and cost-effective membrane technology (MT) for water purification is an active area of research now a days. Two-dimensional (2D) material-based membranes such as graphene, metal-organic framework (MOFs), zeolite, transition metal dichalcogenides (TMDCs), MXenes, graphitic carbon nitride (g-C3N4), Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00008-0 # 2020 Elsevier Inc. All rights reserved.

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and hexagonal boron nitride (hBN) have shown brilliant potential application in various separation processes owing to their supreme properties [1–4]. Although, the development of simple, versatile strategies for the fabrication of 2D-derived membranes is of increasing scientific interest, but challenges exist in understanding suitable fabrication methods and mechanisms. Currently, 2D material-based membranes can be fabricated in two basic forms, that is, pristine and modified membranes. In general the pristine nanosheets consists of a monolayer or a few layers of 2D material with intrinsically uniformly sized pores such as nanoporous graphene, zeolite, and MOFs for selective permeation. Two-dimensional nanosheets are generally synthesized from top-down (such as mechanical forceassisted, chemically liquid exfoliation and ball milling) methods from bulk materials or bottom-up (chemical vapor deposition (CVD) and hydro-/solvothermal) methods assembling from small molecules. Further, nanosheets can be assembled into laminar membranes by wet chemical methods, such as direct evaporation or drop casting, spraying coating, spin coating, vacuum filtration, and LangmuirBlodgett assembly methods [1, 2]. 2D material-based modified membranes have also been fabricating by using same wet chemical approach. These membranes derived from 2D materials show superior molecular separation properties in various separation and filtration processes [2]. Although extraordinary separation efficiency has been achieved in pristine nanosheet membranes, however, the fabrication of large area and integrated nanosheets with uniform nanopores remains a challenge for scientific community. In this case, laminar membranes assembled from 2D materials provide a more practical approach for using 2D materials for separation and exchange applications. 2D interlayer channels between nanosheets could yield fast and selective transport of small molecules. To date, various 2D materials including GO, TMDCs, and MXene have been fabricated as laminar membranes with several advantages. GO is a pioneer member of 2D family with unique properties and has been widely studied for their separation application since few years [5–9]. Pristine GO and its derivatives can be easily fabricated into a freestanding and supported laminates. Chemical exfoliation is a good approach to fabricate GO-based membranes with several advantages [10]. The chemically exfoliated GO and its derivatives can then be easily dispersed in either organic or inorganic solutions due to the presence of plenty of oxygen-containing groups, such as hydroxyl, carboxyl, carbonyl, and epoxy groups, on the edges and basal planes. The presence of intrinsic defects and pores within GO nanosheets, which significantly affect performance in separation, need to be assembled into laminated membranes for enhanced separation performance with better selectivity. High-resolution TEM studies suggest that, for single-layer GO nanosheet, there are three types of regions: holes, pristine graphene, and oxidized regions with the percentages of 2%, 16%, and 82%, respectively. Further, freestanding GO membranes can be easily fabricated by directed assembly of GO nanosheets, resulting in tightly packed interlocking structures in a parallel manner that subtly undulate along the membrane surface and robust mechanical properties. Moreover the as-prepared GO membrane can be easily peeled off from the substrate and

8.2 Synthetic techniques

can be easily transferred [6]. These exceptional features ensure GO is a versatile platform for fabricating well-defined laminar nanostructures. For instance the stacking of GO nanosheets into laminar (layered) structures has been considered as a powerful and scalable approach to fabricate 2D laminar membranes. With these several advantages of a high aspect ratio structure and easily water dispersible properties, GO nanosheets can be assembled readily into laminar membranes by wet chemical methods, such as direct evaporation or drop casting, spin or spray coating, dipcoating, Langmuir-Blodgett assembly, vacuum filtration, and electrophoretic deposition [1, 2, 11].

8.2 Synthetic techniques In the succeeding text, we will discuss the fabrication techniques and related the issues with regard to lamellar structure membranes.

8.2.1 Drop casting Drop casting is the most simple method to fabricate GO membranes and thin films. First, GO colloidal suspension was drop cast onto a substrate with a smooth surface such as silica or paper and then dried at room temperature. A piece of freestanding and uniform GO membrane was subsequently peeled off from the underlying substrate. Water tends to be a poor solvent for drop casting due to the low vapour pressure and large surface tension. In some cases alcohol (polar solvent) can be a substitute of water, while other organic solvents such as hexane, toluene, or halogenated solvents are also very good choices for nanoparticles with hydrophobic capping ligands. Drop-casting technique is highly favored by many researchers due to its simplicity and ease of preparation and quick and accessible method to prepare thin films on relatively small substrates. Besides this, main drawback of this technique is that, even under near ideal conditions, differences in evaporation rates across the substrate or concentration gradients in the liquid phase can lead to variations in film thickness or internal structure [6].

8.2.2 Vacuum filtration Vacuum filtration (Fig. 8.1A) is the most common and straight forward route for the large-scale fabrication of self-supported GO membranes. It is worth noting that this process does not change the physiochemical properties of GO because interactions between the very large aspect ratio compliant GO sheets include electrostatic repulsion, Van der Waals attractive forces, and hydrogen bonding, but not covalent bonding [6, 12]. GO membranes likewise bear a certain number of negative charges and exhibit excellent hydrophilic properties. Typically the thickness of the GO membranes largely depends on the volume of GO suspension introduced into the cell of the vacuum filtration system. Usually, GO membranes prepared by

209

FIG. 8.1 Fabrication of MoS2 nanosheet membrane. (A) Schematic process to produce MoS2 nanosheets from layered bulk materials in the CO2/ethanol/ water system environment by vacuum filtration method. (B) Preparation of MoS2 nanosheets by lithium-intercalated MoS2 dispersion and vacuum filtration. (C) Schematic representation of synthesis of a 2D Ti3C2Tx membrane from Ti3AlC2 and photograph and cross-sectional SEM image of a Ti3C2Tx membrane. (D) 2D lamellar MXene membrane nanosheets on AAO support. (E) Freestanding BN-laminated membrane, photo of the colloidal solution of few-layer BN. Photo of a piece of freestanding BN membrane with size
15 mm  4 mm. Cross-sectional SEM image of a BN membrane, with the inset showing the lamellar structure of the BN membrane constructed by few-layer BN sheets and AFM image of fewlayer BN nanosheets on a mica substrate, with the corresponding height profile (inset) along the blue line showing thicknesses and lateral sizes of the nanosheets. (A) Reproduced with permission from Y. Qi, et al., A green route to fabricate MoS2 nanosheets in water-ethanol-CO2. Chem. Commun. 51(31) (2015) 6726–6729. (B) Reproduced with permission from A. Achari, S. Sahana, M. Eswaramoorthy, High performance MoS2 membranes: effects of thermally driven phase transition on CO2 separation efficiency. Energy Environ. Sci. 9(4) (2016) 1224–1228. (C) Reproduced with permission from C.E. Ren, et al., Charge- and size-selective ion sieving through Ti3C2Tx MXene membranes. J. Phys. Chem. Lett. 6(20) (2015) 4026–4031. (D) Reproduced with permission from L. Ding, et al., A two-dimensional lamellar membrane: MXene nanosheet stacks. Angew. Chem. Int. Ed. 56(7) (2017) 1825–1829. (E) Reproduced with permission from S. Qin, et al., High and stable ionic conductivity in 2D nanofluidic ion channels between boron nitride layers. J. Am. Chem. Soc. 139(18) (2017) 6314–6320.

8.2 Synthetic techniques

filtration are usually deposited on microporous a polymeric membrane, which is selected to optimize both surface roughness and wettability to achieve a homogeneous GO membrane [13].

8.2.3 Spin coating Spin coating is a film deposition technique already introduced in a variety of industries such as microelectronics and coating technologies, whose benefits include accurate thickness control, high deposition uniformity, fast process time, and low equipment cost [9]. However, substrate size and shape are the principal drawbacks to implement the technique at industrial scale. Nowadays, it is widely used by many to prepare high-quality GO membranes. GO suspensions have been deposited onto opaque substrates, such as silicon dioxide and silicon nitride and transparent materials like glass, different polymeric support, and copper foil to fabricate a thin and uniform GO membrane. For example, a GO suspension on substrate was spinning with different speed depending on requirements, and procedure was repeated until the uniform membranes were obtained. Self-supported GO membranes are then obtainable by removing substrates using etchants corresponding to different substrates. Finally the membranes can be cleaned by deionized water and dried on a hot plate at moderate temperature ( Mn > Cd, but the size variation has a trend Mn > Cd > Cu > Na. Further, author also studied the barrier separation properties and demonstrated that sodium salts can be separated from copper salts and also from the organic molecules using GO membrane. In the first case of NaCl and CuSO4, feed has the color of CuSO4, which does not affect the color of filtrate due to the separation. The observed effect is due to the wet condition of GO membrane allowing any ions to pass through. Once the membrane is wet, the capillaries become wider to accommodate hydrated Na+ ions. Nair et al. [7] have investigated ion permeation through GO membrane by controlling d-spacing with the help of physical confinement (Fig. 8.4A–D). Authors ˚ , providhave demonstrated membranes with d-spacing ranging from
9.8 to 6.4 A ing the accurate and tunable ion sieving, smaller than that of diameter of hydrated ions. Authors have cutted the GO laminates into  100 μm thick rectangular strips

Wet GO

Dry GO O O

d-spacing: distance between centers of carbon planes

OH OH

O ? nm

HO O

O

HO

CI

–

Water

GO

Permeation rate (mol h–1 m–2)

Epoxy encapsulant

Na

O

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+

K+

K+

10–2

Li+

Na+ 9.0 Å

10–3

OH

9.8 Å

7.9 Å 8.6 Å

10–4

Ca2+

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6 Water 4

2 10–6

(B)

8

10–4

Mg2+

6.4 Å

Na+

Water permeance (L h–1m–2 bar–1)

d-spacing = 0.34 nm

10

10–2 OH

HO

Permeation rate (mol h–1 m–2)

Graphene

(C)

7 6.4

7.2

8.0

Hydrated diameter (Å)

8

9

10

11

8.8

(D)

Interlayer spacing (Å)

FIG. 8.4 (A) Schematic illustration of d-spacings of graphene, dry GO, and GO soaked in water. (B) Schematic illustrating the direction of ion/water permeation along graphene planes (PCGO membranes), photograph of a PCGO membrane glued into a rectangular slot within a plastic disk of 5 cm in diameter along with optical micrograph of the cross-sectional area, which shows 100-μm-thick GO laminates (black) embedded in epoxy and cross-sectional area by SEM. (C) Permeation rates through PCGO membranes with different interlayer distances. The salts used were KCl, NaCl, LiCl, CaCl2, and MgCl2. (D) Permeation rates for K+ and Na+ depend exponentially on the interlayer distance. (A) Reproduced with permission from S. Zheng, et al., Swelling of graphene oxide membranes in aqueous solution: characterization of interlayer spacing and insight into water transport mechanisms. ACS Nano. 11(6) (2017) 6440–6450. (D) Reproduced with permission from J. Abraham, et al., Tunable sieving of ions using graphene oxide membranes. Nat. Nanotechnol. 12(6) (2017) 546–550.

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(4 mm  10 mm) and kept them at different humidity from 1 to 2 weeks. The result˚ as RH changes from 0% to ing interlayer d-spacing was changed from  6.4 to 9.8 A ˚ . Then indi100%. GO membrane soaked in liquid water showed d  13.7  0.3 A vidual GO strips with desirable d were encapsulated and stacked together using epoxy (Fig. 8.4B), to increase the available cross section for filtration to
1 mm. The stacked GO laminates, embedded in the epoxy, are referred to as physically confined GO (PCGO) membranes because the epoxy mechanically restricts the swelling of the laminate on exposure to RH or liquid water. Finally, they are successful to fabricate graphene-based membranes with limited swelling ration, which exhibit superior separation efficiency for NaCl (
97%). Recently, Chen et al. [89] demonstrated experimentally that highly efficient and selective ion rejection by GO membranes can be readily achieved by controlling the interlayer spacing of GO membranes using cations (K +, Na +, Ca2+, Li+, and Mg2+) ˚ , and themselves. The interspacing can be controlled with precision as small as 1 A GO membranes controlled by one kind of cation can exclude other cations with a larger hydrated volume, which can only be accommodated with a larger interlayer spacing. Authors have also used first principle calculations and reveal that the strong noncovalent cation-π interactions between hydrated cations in solution and aromatic ring structures in GO are the cause of this unexpected behavior. Although considerable progress has been achieved for graphene-derived membrane for desalination and water purification applications, there are still several promising topics that are worth addressing such as the precise design of usercontrolled GO membranes including the pattern of oxidized regions, the interlayer space, the pore size, modification of pore chemistry, and the number of graphene layers that is still not entirely successful. When pore diameter is larger than 0.8 nm, graphene-derived membrane shows higher water flux than CNT membranes due to higher velocity in the center region. There have been efforts to tune the interlayer spacing. For example, it can be widened to increase the permeability by intercalating large nanomaterials and by cross-linking large and rigid molecules. RGO membranes can lead to sharp decrease of interlayer spacing but to be highly impermeable to all gases, liquids, and aggressive chemicals. It remains difficult both to reduce the interlayer spacing sufficiently and to exclude small ions while keeping this separation constant against the tendency of GO membranes to swell when immersed in aqueous solution. This limits the potential of such membranes for separating ions from bulk solution or sieving ions of a specific size range from a mixed salt solution, such as the most common ions in seawater and those in the electrolytes of lithium-based batteries and supercapacitors. In addition, the swelling of GO in aqueous solutions also needs serious attention to achieve required selectivity [90]. The presence of abundant oxygenated functional groups on GO makes the material very hydrophilic and thus endows it with a high tendency to absorb water and swell in humid or aqueous environments, greatly deteriorating its targeted performance [90]. The aqueous-phase separation capability of a layer-stacked GO membrane can be significantly affected by its natural tendency to swell, that is, absorb water into the GO channel and form an enlarged interlayer spacing. Accurate characterization

8.4 Desalination process via 2D membranes

and a fundamental understanding of GO swelling are expected to provide key information for the optimal synthesis of GO membranes with enhanced performance. Besides chemical stability of GO-based membrane, mechanical stability is also very important for many industrial processes under high-pressure environment. In this regard, laminar membranes based on other 2D materials are excellent candidates for applications in desalination and water filtration due to the formation of nanocapillaries between individual crystals that can exhibit a molecular and ionic sieving effect while allowing high water permeance. Recent work has suggested that other 2D-layered materials beyond graphene, such as the MXene and TMDCs, could be suitable for desalination and water purification application [2, 91]. Despite this, there have been only few reports available on MXene-, TMDC-, and LDH-based membranes for desalination application yet. Recently, Gogotsi et al. [30] have fabricated freestanding and PVDF-supported Ti3C2Tx (MXene) 2D-based membranes by using vacuum filtration. The membrane was demonstrated for selective sieving of alkali, alkaline earth (Li+, Na+, K+, Mg2+, and Ca2+), transition and other metal (Ni2+ and Al3+), and methylthioninium+ (MB+) dye cations through Ti3C2Tx membranes (Fig. 8.5A–D). These membranes showed good water permeability of 37.4 L m2 h1 bar1. Metal ions with a larger charge and hydration radii smaller ˚ ) showed little bit slower permeation than the interlayer spacing of MXene (
6 A as compared with single-charged cations. Furthermore, in another report, the antibacterial properties of single- and few-layer Ti3C2Tx MXene flakes in colloidal solution have been investigated. Ding et al. [29] have fabricated new kind of lamellar membrane based on a stack of 2D MXene nanosheets. The MXene membrane was demonstrated for the separation of different molecules and showed rejection rate (over 90%) for those molecules that have size larger than 2.5 nm with excellent water permeance up to 1000 L m2 h1 bar1 (Fig. 8.5E and F). This permeance was better than reported before. Furthermore, separation performances of MXene can be improved by incorporating MXene into another 2D materials or some suitable cross-linking reagent to form composite nanostructures. Until recently, this Ti3C2Tx MXene 2D material was only used in ion separation membranes based on chargeand size-selective permeation. Similar to GO laminar membranes, the hydrophilic nature of Ti3C2Tx with water within the intergalleries promoted water flow. The MXene membrane rejected cations with hydration radii larger than the interlayer spacing (0.6 nm). Moreover, cations with a larger charge showed an order of magnitude slower permeation compared with single-charged cations. As for TMDCs, there have been few reported studies of the filtration properties of these TMDC materials. The existing reports for MoS2- and WS2-based membranes demonstrate excellent improvements in water flux, up to 2–5 times greater than GObased membranes of comparable thickness. These TMDC membranes also show promising rejection (>80%) of large (
1 nm) organic molecules, but their ionic rejection properties remain largely unexplored and require further investigation to understand the behavior of the nanocapillary channels formed between the TMDC layers. There have been promising theoretical investigations into the performance of individual pores in MoS2 layers that suggest that defect sites, with exposed Mo

225

4000

4 3000 3 2000 2 1000 38.5 0 600 900 1200 Filtered thickness (nm)

1500

NaCl 45.7 H2O 37.5

30

MgCl2 24.2

20

AlCl3 18.0

10

4 3 2

0

(B)

1

2 Time (h)

0

1 2 Cation charge

3

3

4

1200

100

800

90

80

400

(E)

0 M1

M2

70 MXene membrane

Na+

1

Li+

Ca2+ Ni2+

Mg2+

Ti3C2Tx

Al3+

GO

3.2

(C)

Permeance Rejection

0

(D)

K+

1

0.1

1 0

Permeance (L m–2 h–1 bar–1)

300

50 40

1 0

0

(A)

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37.4

5

Rejection (%)

5

Na+ Li+ K+ Mg2+ Ni2+ Ca2+ Al3+ MB+

3.4

3.6

3.8

4.0

4.2

4.4

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Permeance Rejection

100

1200

90 800 80 400

0

(F)

0.1

Rejection (%)

6

Permeation rate (mol h–1 m–2)

5000

Permeance (L m–2 h–1 bar–1)

7

6

Permeated ions (mol m–2)

7

Filtration time (h)

Water flux (L bar–1 h–1 m–2)

6000

70

EB (1.2×3.1 nm1)

60 Cyt.c (2.5×3.7 nm1)

Gold (5×5 nm1)

FIG. 8.5 Water flux through Ti3C2Tx membranes; (A) water flux with varying thicknesses; (B) number of cations permeated through the membranes against time for 0.2 mol L1 feed solutions; (C) the permeation rates of cations against their hydration radii; (D) water flux of water and salt solutions as a function of the cation’s charge; and (E and F) comparison of the performance of the different MXene membranes for the separation of EB molecules at room temperature and separation performance of the MXene membranes for different molecules with different sizes. (D) Reproduced with permission from C.E. Ren, et al., Charge- and size-selective ion sieving through Ti3C2Tx MXene membranes. J. Phys. Chem. Lett. 6(20) (2015) 4026–4031. (E and F) Reproduced with permission from L. Ding, et al., A two-dimensional lamellar membrane: MXene nanosheet stacks. Angew. Chem. Int. Ed. 56(7) (2017) 1825–1829.

8.4 Desalination process via 2D membranes

atoms, yield water permeation rates 2–5 orders of magnitude greater than commercially available materials. Recently, Bissett et al. [92] fabricated chemically functionalized MoS2 membranes. As-prepared membranes exhibit excellent ionic sieving (
99%) for all major cationic components commonly found in seawater (e.g., Na+, K+, Ca2+, and Mg2+) while maintaining water fluxes significantly higher (
5 times) than those reported for GO membranes. These functionalized MoS2 membranes exhibit excellent long-term stability over 6 months with no swelling and consequent decrease in ion rejection, when immersed in water for periods exceeding 6 months. Similar stability is observed when exposed to organic solvents, indicating that they are ideal for a variety of technologically important filtration applications. A substantial effort has been devoted to fabricate highly efficient 2D material membranes for desalination applications to improve antimicrobial properties, increase permeability, and enhance mechanical properties. Most of the studies have attributed to graphene-based material, while very limited work has been done for other 2D materials. GO-based membranes showed very promising antifouling properties and permeability due to high hydrophilicity induced by the functional groups at the basal plane, and terminals of GO sheets should also have contribution to high water permeation and antifouling owing to the low interfacial energy between a surface and water. By adjusting the pore size and nanochannels via induction and modification of surface oxygen-containing groups, these membranes can be oriented toward narrow pore size distribution, which will be used for the precise sieving based on size exclusion mechanism. Other 2D materials such as black phosphorus, graphyne, graphane, and silicene based have been theoretically investigated for the water purification and desalination applications since more than one decade but still have not achieved any experimentally breakthrough. Technological challenges such as the incorporation of 2D materials to increase selectivity and to decrease hydrophobicity at the surface of the membranes, scale up fabrication of ion channel, increase in pore density per area of the active layer, and low cost membrane fabrication. Still the costs of such membranes could eventually be affordable with future improvements in 2D material synthesis and membrane processing.

8.4.2 Assembled 2D material laminates Similarly the permeation behaviors of bulk 2D-laminated membranes usually depend on the microstructure of nanosheets. On account of the sp2 hybridization of C and N atoms, the structure of g-C3N4 is analogous to graphite whose individual layers stack together by Van der Waals force. There are two types of structural models for g-C3N4 such as condensed s-triazine or tri-s-triazine unites. For tri-s-triazine-type C3N4 the planar tertiary amino groups connect every tri-s-triazine subunits with a periodic array of vacancies in the lattice. The g-C3N4 nanosheets were partially exfoliated from bulk g-C3N4 with intrinsic synthetic nanopores and unstrapped fragments, which acted as the self-supporting spacers for water transport pathway. However, the g-C3N4 nanosheets consisting of lateral size under several

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CHAPTER 8 Nanomembranes for water treatment

hundred nanometers seem hard to form regular 2D nanochannels like graphene oxide but stack together distractedly. Thus, accounting for the separation mechanism of gC3N4 membrane, the water molecules were transported through the spacer of stacked nanosheets and partially exfoliated fragments and the synthetic nanopores. Apart from GO-based membrane, the TMDC nanosheets process limited chemical defects and dangling bonds, thus providing smooth and clean 2D nanochannels for water transport. According to the previous report, the interplanar distance is acknowledged by the characterization of XRD at the peak (002). However, the inter˚ may be not the correct pore size for 2D channels. First the planar distance of 6.3 A intrinsic interlamellar spacing of bulk MoS2 sows extremely approximate with the spacing mentioned earlier, which can be regarded as the interlamellar spacing of not completely exfoliated MoS2 nanosheets. Second the effective thickness of each ˚ , and the void distance of 0.3 A ˚ cannot permit the transport of MoS2 sheet is about 6 A ˚ single water molecule with the diameter of 4 A. Third, with the presence of nanowrinkles and random stack, there is a broad spacing distribution for interlamellar spacing actually. Similarly, like GO membrane, the water molecules can transport through 2D nanocapillary network with the formation of vapor or liquid. For organic vapors the permeation behavior in MoS2 membrane follows the molecular sieving mechanism, and the critical threshold for permeation can be observed at the molec˚ . The diffusion of ions follows the Fickian diffusion characular diameter of 5.7 A teristic, and most dye molecules can be blocked by the 2D channels owing to the steric hindrance. While as compared with flexible GO membranes, the water flux for other 2D material (i.e., MoS2, WS2, and g-C3N4)-based rigid bulk membranes presents a linear growth with the pressure increasing. According to the HagenPoiseuille equation, the viscous flow indicates the rigidity of MoS2 nanosheets, which can resist the external pressure up to 0.55 MPa. Moreover, different from the abundant oxygen-containing groups of GO, there is no strong electrostatic repulsion interaction for disengaging, and the membrane could stably maintain at different pH with their Van der Waals binding. Hence the change of pH makes no obvious influence on the water permeation performance for TMDCs, MXene, g-C3N4, or other 2D-based membranes beyond GO, even though the zeta potential of g-C3N4 presents different conditions.

8.5 Dye separation via 2D membrane Molecular (dye) separations are energy-intensive processes that can be used in a wide spectrum of areas, ranging from emerging biotechnologies to more classical fuel and chemical production. The global move toward greener production is necessarily coupled with the need for more energy-efficient, and hence novel, separations. An ideal membrane for molecular separation should be as thin as possible to maximize its solvent flux, be mechanically robust to prevent it from fracture, and have welldefined pore sizes to guarantee its selectivity. Recently, 2D material membranes have been confirmed to be an excellent platform for developing size-selective

8.5 Dye separation via 2D membrane

molecular separation membranes because of its atomic thickness, high mechanical strength, and chemical inertness [93, 94]. In this portion, we review the recent progress on 2D material-based membranes for molecular separation application. Graphene-based materials can be applied for membrane separation in two ways: drilling holes in single- or few-layer graphene membranes and fabricating GOMs with layered structures. Both theoretical and experimental studies have demonstrated that nanoporous graphene membranes are highly efficient for molecular separation, mainly due to their atomic thicknesses. But the fabrication of large-area singlecrystal graphene monolayer is very expensive. Furthermore the defect-free transfer of large-area CVD graphene and generating high-density pores with precisely controlled sizes are still technically challenging. Before these problems are cheaply and properly addressed, GOMs are a good compromise choice. GOMs can be cheaply fabricated at large scale, and the nanochannels between GO sheets can act as a precise molecular sieve by blocking all the species with larger sizes. The rich oxygencontaining functional groups not only endow GO sheets with good dispersibility and processability in water but also make GOMs unstable in aqueous media. Therefore GOMs without modification are not suitable for molecular separation. Fortunately, controlled reduction or chemical cross-linking enables the application of GOMs for water separation [94]. More importantly the sizes of GO nanochannels can be engineered by intercalating nanoparticles or nanowires between GO sheets, thereby tuning the selectivity and permeability of GOMs [93]. Finally the good stability of GOMs in various organic solvents makes them promising for molecular separation in organic solvents. Rapid progresses in industrialization, population expansion, and urbanization have largely contributed to the growing number of inorganic and organic exhausts: heavy metals, dyes, crude oil, petroleum products, organic solvents, and other gases [3, 4, 95]. Number of industries including textile, dyestuff, paper, and laundry eliminate a huge amount of organic dyes into water, which causes severe environmental and ecological problems each year. Therefore removal of these organic dyes from industrial wastewater using different methods and technologies remains exciting area for researcher. Among them, membrane technologies are considered more efficient and low-cost method to the separation of organic dyes. Recently, 2D materialbased membranes have grown rapidly for their applications in dye separation. Up to date, various 2D materials including GO, TMDC, and MXene have been utilized for the separation of these toxic dyes from industrial wastewater. Initially, Huang et al. [12] fabricated ultrafiltration NSC-GO membranes with numerous nanochannels with diameters of 3–5 nm. As-prepared GO membranes showed a superior separation performance of small molecules and ultrafast water permeation (695  20 L m2 h1 bar1). Authors have also used these membranes for separation of rhodamine B (RB) and Evans blue (EB) dye molecule, but unfortunately, membranes exhibit less rejection toward RB (87  3%) and EB (83  1%) molecule. This finding was not suitable for separation of dye molecule. Further reported membranes only show a favorable behavior for the species with one type charge. Authors observed that the negatively charge GO membrane can trap the positively charged species

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only. Peng et al. [96] have studied the filtration properties of GO nanochannels. They have tuned the membrane properties by tuning pH, salt concentration, and pressure. Author observed 85% of rejection rate for Evans blue (EB) and noticed a water flux of 71 L m2 h1 bar1 for some of their membranes. According to their explanation the nanochannel network in membrane that is actually the intersheet distance blocks some molecules. It can be concluded from their work that the membrane filtration depends on the type and concentration of charge near GO membrane. This result was backed by solution pH dependence of membrane properties. Due to the protonation of carboxylic acid, the electrostatic repulsion between GO sheets weakens at lower pH. As a consequence of this, the flux rate decreases, and the rejection of EB molecules takes place. Mi et al. [63] fabricated ultrathin GO membranes (10–30 nm) via layer-by-layer deposition of GO nanosheets, which were cross-linked by 1,3,5-benzenetricarbonyl trichloride, on a polydopamine-coated polysulfone support. The cross-linking not only provided the stacked GO nanosheets with the necessary stability to overcome their inherent dispensability in water environment but also fine-tuned the charges, functionality, and spacing of the GO nanosheets. These membranes were tested against organic dyes such as methylene blue (MLB) and rhodamine-WT (RWT) dye rejections. But unfortunately, these membranes showed moderate rejection (46%–66%) for MLB and a high rejection (93%–95%) for rhodamine-WT dye. Water permeability was measured under a trans-membrane pressure of 50 psi (0.34 MPa). Water permeance through the polydopamine-coated membrane was 1340  40 L m2 h MPa, which decreases to 80–276 L m2 h MPa after the membrane was coated with 5–50 layers of GO. It is interesting to observe that water permeance does not decrease monotonically as the number of GO layers increases. This observation possibly suggests that the water resistance of GO coating does not linearly depend on the thickness of a GO membrane. They attributed to the water transport properties through the GO nanochannels. Recently, Akbari et al. [17] have prepared large-area GO nanofiltration (shear-aligned) membranes on porous substrates by using a gravure printer with doctor blade. As-prepared membrane was evaluated for the retention of different probe molecules with different charges and hydrated radii such as methyl viologen (VB), methyl orange (MO), MLB, orange G (OG), RB, methyl blue (MB), brilliant blue (BB), and rose Bengal (RB) (Fig. 8.6A and B). Pressure-driven transport data demonstrated high rejection (>90%) for ˚ , while modest charged and uncharged molecules with a hydrated radius above 5 A (30%–40%) retention has been achieved for monovalent and divalent salts. Authors have also compared separation performance (water flux vs pressure measurements) for three different varieties of membrane: SAM, vacuum filtered, and a commercial membrane (NF270 membrane). The newly fabricated SAM had a water permeability of 71  5 L m2 h1 bar1, which is almost seven times better than the membrane prepared with vacuum filtration membranes (10  2 L m2 h1 bar1). On the other side, rGO membranes also have better separation for the fluids with outstanding stability in aqueous solution. Many authors fabricated rGO membranes for various nanofiltration for separation of dye molecule. Han et al. [62] have designed and fabricated ultrathin (22–53-nm thick) rGO membranes with 2D

80

70 65 60 55 50 45 40 MV

MR

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MnB OG RosB Ru Probe molecules

RB

MB

100 3 80 2

60 40

1

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(C)

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Positively charge probe molecule Negatively charge probe molecule Neutral probe molecule

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MO

95

Permeance (103 L m–2 h–1 bar–1)

Retention (%)

RosB

OG

100

RB

MLB

MB

EB

FIG. 8.6 (A and B) Separation performances of 150  15-nm-thick shear-aligned membrane: water permeability vs thickness (A) and retention performance as a function of hydrated radius, for probe molecules with different charges and sizes (B). (C and D) Permeability and separation efficiency of GO-TA (150 nm) and GO-TH (60 nm) membranes, respectively. (A and B) Reproduced with permission from A. Akbari, et al., Large-area graphene-based nanofiltration membranes by shear alignment of discotic nematic liquid crystals of graphene oxide. Nat. Commun. 7 (2016) 10891. (C and D) Reproduced with permission from K.H. Thebo, et al., Highly stable graphene-oxide-based membranes with superior permeability. Nat. Commun. 9(1) (2018) 1486.

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nanochannels by the simple filtration-assisted assembly strategy and successfully applied them as nanofiltration membranes for water purification. The efficiency of the membranes was evaluated by using dead-end filtration device, and the efficiency for the water treatment and the pure water flux reached up to 21.8 L m2 h1 bar1. These membranes exhibited high rejection (>99%) against organic dyes. The integration of high-performance, low-cost, and simple solutionbased fabrication process promises graphene nanofiltration membranes great potential application in practical water purification. Recently, Thebo et al. [97] have reported a class of rGO membranes with enlarged interlayer distance fabricated by using GO sheets and tannic acid (TA) and theanine amino acid (TH) as reducing agent and cross-linker. Such membranes show ultrahigh water permeability and good separation efficiency (
100%) for various organic dyes including RB, MLB, and MB with excellent stability. These membranes show water permeance over 10,000 L h1 m2 bar1, which is 10–1000 times higher than those of the reported GOMs and commercial membranes [97]. The water permeance of GO-TA and GOTH membranes is over 100 times higher than that of GOMs for all the dyes. Because of different charge states, the GO-TA and GO-TH membranes show a bit lower rejection to negatively charged MB and EB but a bit higher rejection to positively charged RB and MLB than the GOMs. For instance the GO-TH membranes show a rejection of 100% and permeance of 8526  30 L m2 h1 bar1 for MLB and a rejection of 71  5% and permeance of 10,602  30 L m2 h1 bar1 for MB. Since the GO-TA and GO-TH membranes are negatively charged at neutral pH, positively charged molecules can be easily taken up by the membranes via electrostatic interactions. As a result the nanochannels could be partially blocked, leading to a high rejection for cationic dyes. The higher rejection and permeance are attributed to four different roles played by TA and TH molecule: reduce the GO sheets to increase pristine graphitic regions as reducing agent, link the neighboring rGO sheets as cross-linker, enlarge the interlayer distance between rGO sheets as spacer, and block the solutes together with the stacked rGO sheets. Further authors have demonstrated a green method to fabricate such membranes by simply using GO sheets and green tea extractives (GT) that contain TA and TH. These membranes also showed
100% rejection for RB and MLB [97]. As compared with graphene, TMDCs can offer clean 2D channels with limited undesirable chemical defects or inherent intercalation. However, besides these advantages, very limited studies have been carried out for molecular separation. Sun et al. have first time fabricated laminar separation membrane from atom-thick micrometer-sized MoS2 membranes through the vacuum filtration [20]. Authors have measured the water permeances and separation of organic dyes EB through these MoS2 membranes. As-prepared membrane exhibited a water permeance of 245 L h1 m2 bar1, which was 3–5 times higher than that of GO membranes without degradation of the rejection ratio (89%) for EB molecules. Additionally, as-prepared membrane exhibits good chemical stability under harsh conditions. Recently, Mi et al. [98] fabricated MoS2 membranes with improved permeability (30–250 LMHB). The separation capability of layer-stacked MoS2 membranes

8.5 Dye separation via 2D membrane

was evaluated using representative organic dyes using model compounds, including negatively charged RWT and positively charged methylene blue. Typically the removal of RWT by the 500-nm-thick MoS2 membrane was as high as 90%. The increase in MoS2 membrane thickness (from 200 to 900 nm) did not improve the rejection significantly, further confirming the negligible role of adsorption in the separation of RWT molecules. The rejection of RWT by these membranes is similar to the rejection of Evans blue obtained in a previous study [20], although RWT has a smaller molecular weight (487 vs 961) and a smaller Stokes radius (1.1–1.2 nm, as calculated using eq S8, vs 2.8 nm) [73, 74]. The capability of removing smaller molecules was possibly due to the compaction of loose MoS2 structure by high pressure after membrane preparation in the study. The rejection of positively charged methylene blue by the 500-nm-thick MoS2 membrane was initially 100% but decreased to a stable level of
40%. The initial high rejection was due to physical adsorption on the anionic MoS2 nanosheets. At the end of the test, methylene blue was also concentrated in the retentate, indicating a size exclusion mechanism after the saturation of membrane adsorption capacity. We believe that the overall separation mechanisms for organic dye separation by a MoS2 membrane include both size exclusion and electrostatic repulsion. In another study, Sun et al. [21] have prepared ultrafast WS2 membrane and used for separation of small molecules with size of about 3 nm. The ultrathin WS2 membranes (300 nm) demonstrated good rejection over 90% for EB molecule with water permeance of 730 L m2 h1 bar1. This water permeance is higher than of pristine GO-based membrane and MoS2 laminar membranes. The water permeance is enhanced by two fold from the duplicated channels, which come from the ultrathin nanostrands as templates without loss of the rejection efficiency. Pressure loading test indicates that water flow through the corrugations on the nanosheets in the membrane obeys the viscous flow law, and the fluidic channel size is not changed under pressure. The pressure loading-unloading curve suggests that the channels arising from ultrathin nanostrands are cracked between 0.3 and 0.4 MPa and result in a further two times increase of the flux without significantly degrading the rejection for 3-nm molecules. It is exciting that the crack produces new fluidic nanochannels and further results in water flux four times higher than that of the as-prepared WS2 membrane. Further, authors have used WS2 membrane with thickness 500 nm for rejection EB, and membrane showed good rejection about 93% for EB, with water permeance of 450 L m2 h1 bar1. Further the permeability of membrane was increased up to 1380 L m2 h1 bar1 after insertion of nanostrands, but unfortunately the rejection rate for EB was degraded by 32% and 23%, respectively, due to the bigger channels created by the overlapped nanostrands. When nanostrands were removed, the nanostrand-channeled membrane exhibits a water permeance of 930 L m2 h1 bar1, which is double that of the as-prepared WS2 membrane. It is interesting that, after removing the nanostrands, the rejection of EB is recovered up to 83% and 91%, respectively. This discovery indicates that the newly developed nanochannels are successfully constructed by the nanofibrous templates. Authors have also claimed that these membranes have two orders of magnitude higher separation performances than that of commercial membranes with

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similar rejections and hold promising potential for highly efficient liquid separation. Despite the unique molecular transport and clear-cut sieving properties, there are challenges around the TMDCs. For instance, both mechanical strength and chemical stability of the TMDCs lamellar membrane have not been characterized yet. The separation mechanism for dye separation is still clear. Besides there is serious need to improve the water permeance as compared with GO-based membranes and other commercial-based membranes. As for the MXene, Ding et al. [29] have fabricated a new kind of lamellar membrane based on a stack of 2D MXene nanosheets, that is, Ti3AlC2, on a porous support by vacuum filtration. The MXene membrane supported on AAO substrate shows excellent water permeance (more than 1000 LMHB) and favorable rejection rate (over 90%) for molecules with sizes larger than 2.5 nm. As-prepared MXene membrane was applied in water purification, and it was firstly evaluated with the EB (1.2 nm  3.1 nm) solutions at room temperature. It has to be noted that the vacant AAO support with uniform pore size around 200 nm gives a water permeance of 4500 LMHB and no rejections for EB molecules. From studies, it can be found that the MXene membrane with thickness of 400 nm exhibits a water permeance of 1084 LMBH and a high rejection rate of 90% for EB molecules. Recently, another layered material g-C3N4 nanosheets have been modified into laminated membranes. Wang et al. [99] have fabricated 2D g-C3N4 nanosheet membrane with artificial nanopores and self-supporting spacers by assembly of 2D g-C3N4 nanosheets in a stack with elaborate structures. As-prepared membranes were tested for the separation of different organic dye and other species from water. The artificial nanopores in the nanosheets and the spacers between the partially exfoliated g-C3N4 nanosheets provide nanochannels for water transport, while bigger molecules (organic dyes) are retained. The 160-nm-thick membrane was evaluated in a number of separation tests. Different types of dye molecules, such as RB (1.8  1.4 nm2) and EB (1.2  3.1 nm2), were also filtered on the g-C3N4 membranes. The membrane exhibited rejection rates of 75.5% and 87.2% for RB and EB respectively, but shows a better water permeance of 29 L m2 h1 bar1. Further the self-supported nanochannels in the g-C3N4 membrane are very stable and rigid enough to resist environmental challenges, such as changes to pH and pressure conditions. Permeation experiments and molecular dynamics simulations indicate that a novel nanofluidic phenomenon takes place, whereby water transport through the g-C3N4 nanosheet membrane occurs with ultralow friction. The findings provide new understanding of fluidics in nanochannels and illuminate a fabrication method by which rigid nanochannels may be obtained for applications in complex or harsh environments. It is first time that g-C3N4 has been used for the separation membranes. However, it still needs more focus to fabricate ultrathin membranes based on g-C3N4 with high separation efficiency and permeability. The 2D membranes are of great interest in high-performance separation applications in a variety of fields such as selective biological molecular sieving, selective ion penetration, gas separation, and water purification [1, 2, 20, 85, 100]. In particular the separation of biological molecules with similar sizes is one of the key

8.6 Conclusion and future prospects

challenges in the purification of biomaterials. It requires precise control over the pore size, long-term mechanical stability, film uniformity, and mass productivity. Bioseparation is very important application of several industries including food, medical, and pharmaceutical industries. Thus far, substantial research efforts have been carried out to improve the performance of existence membranes. The atomic thinness, stability, and electrical sensitivity of 2D materials motivated us to investigate the potential use of 2D material-based membranes and nanopores for the selective biological molecular sieving [101–103]. In this regard, graphene therefore opens up new opportunities for nanopores such as new analytical platforms to detect, for example, local protein structures on biopolymers or sequencing with single-base resolution. Indeed, theoretical calculations of DNA translocation through a nanopore in graphene have already indicated the possibility for single-base resolution by probing the translocating molecule electrically in the transverse direction by the use of the intrinsic conductive properties of graphene. Besides this, these membranes and nanopores have been widely explored for separation of several other biomolecules including protein, DNA, BSA, humic acid adamantane, g-cyclodextrin, cytochrome c, citric acid, and TMPyP. The nanochannels between 2D nanosheets may allow water to pass through while rejecting larger molecules, similar to the sieving mechanism of membranes. However, there can be a strong trade-off between water permeability/ flux of 2D membrane and solute removal. Recently, Choi et al. [100] fabricated graphene-based nanosieve and its application in the separation of similar-size proteins. A suspended rGO nanosieve with ultrathin, large-area, well-ordered, and dense 15-nm-sized pores was fabricated using block copolymer lithography. The fabricated ultrathin nanosieve (5 nm) with an area of 200 μm  200 μm (an ultrahigh aspect ratio of
40,000) endured pressure up to 1 atm and effectively separated hemoglobin (Hb) from a mixture of hemoglobin and immunoglobulin G, the common proteins in human blood, in a highly selective and rapid manner. The use of the suspended rGO nanosieve is expected to provide a simple and manufacturable platform for practical biomolecule separation offering high selectivity and a large throughput.

8.6 Conclusion and future prospects Fabrication of 2D membranes of various materials show promising results for water purification and dye separation purposes. The selectivity and scalability are core issue in membrane technology. Developing selectivity in membranes is a paramount factor for improved and quality separation of water. Strength and durability of synthetic membranes are another desirable property for making the next-generation highly efficient membranes. Moreover, it is high time to put research efforts to design multifunctional membranes for wider applications. Thus graphene oxide and MOF-based composite materials have potential due to their intrinsic higher surface area and ability to tune their selectivity through functionalization.

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CHAPTER

The use of nanocatalysts (and nanoparticles) for water and wastewater treatment by means of advanced oxidation processes

9

Vincenzo Vaianoa, Diana Sanninoa, Olga Saccob a

Department of Industrial Engineering, University of Salerno, Fisciano, Salerno, Italy bDepartment of Chemistry and Biology “A. Zambelli”, University of Salerno, Fisciano, Salerno, Italy

Chapter outline 9.1 Introduction to nanocatalysts and nanomaterials for pollutant removal ...............241 9.1.1 Nature of nanomaterials applied to wastewater treatment ..................242 9.2 Advanced oxidation processes (AOPs) for water and wastewater treatment .........245 9.3 Fenton and photo-Fenton processes for water and wastewater treatment ............247 9.4 Heterogeneous photocatalysis for water and wastewater treatment ....................252 9.5 Conclusion and future perspectives ..................................................................257 References ............................................................................................................257

9.1 Introduction to nanocatalysts and nanomaterials for pollutant removal With the start of nanotechnology age that could be individuated around 1990, nanomaterials have attracted numerous studies due to their fascinating and enhanced properties. In catalysis, nanostructure-induced effects depend both on the modification of the electronic structure due to their quantum size and the increased specific surface due to the reduction of material dimensions. The nanostructured materials have attracted intense attention: a first classification is based on their dimensions as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) nanomaterials. According to the nanoscale effect, nanoparticles present an increased chemical reactivity together with a change of the physical properties. Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00009-2 # 2020 Elsevier Inc. All rights reserved.

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Nanomaterials are the subject of extensive researches in the last decade due to their features such as promoted catalysis, absorption surface, and high reactivity and demonstrated good results in treating wastewater. In fact, nanomaterials are able to convert several toxic and persistent chemicals into less hazardous ones, acting either as a reactant or as a catalyst or sorbent. Nanomaterials in the most advanced processes for wastewater treatment can be easily classified based on their ability into three major types: nanoadsorbents, nanocatalysts, and nanomembranes. Nanoadsorbents including atoms of chemically active elements demonstrate high absorption capacity due to the high surface area; some examples are activated carbon, silica, clay materials, metal, and metal oxides [1]. The nanosize of these materials results in a highly enhanced chemical activity and absorption capacity that confers, for instance, to pollutants such as heavy metals to coordinate to the surface of these nanoadsorbents. Nanocatalysts are the second class of nanomaterials that are, among others, found in metal oxides and semiconductors. Remarkable activity against heavy metals, persistent organic pollutants, radionuclides, bacteria, and viruses is shown by nanometals and metal oxides. The purification of pollutants in wastewater uses different types of nanocatalysts such as photocatalysts, Fenton-based catalysts, and antimicrobial catalysts [2,3]. Nanomembranes, applied in pressure-based treatment, are used on a larger scale for wastewater treatment, since they combine small pores and low cost with high efficiency and easy use [4]. Nanomembranes enable new functionalities, such as high permeability, catalytic reactivity, and fouling resistance and give effective disinfection with low costs. Nanomembranes can be characterized by nanometal or nonmetal particles and carbon-based nanomaterials [5].

9.1.1 Nature of nanomaterials applied to wastewater treatment The nanoparticles are now a new frontier in the application to environmental sector, as demonstrated by the studies in the field of the wastewater treatment of iron-based nanoparticles, like zerovalent nanoiron and magnetite (Fe3O4), metal and metal oxide engineered nanomaterials that include other oxides such as TiO2, Ag, and ZnO and carbon-based nanomaterials. Starting from the applications regarding the nanoscale zerovalent iron (nZVI) [6,7], an almost high number of studies in groundwater remediation have been developed, due to low toxicity and high compatibility with the environment. Nanoscale zerovalent iron not only offers a high surface area that makes this material suitable of use as adsorbent for heavy metals but also presents combined chemical transformation with adsorption and coprecipitation processes. In fact, nZVI can donate electrons, transforming high valence of metals to less or nontoxic forms. Moreover, high activity in the removal of polychlorinated compounds [8–10] has been demonstrated.

9.1 Introduction to nanocatalysts and nanomaterials for pollutant removal

nZVI surface can be modified with functional groups or with further metals yielding in bimetallic nZVI nanoparticles, operations that confer increased selectivity toward certain pollutant removal. The efficiency of nZVI nanoparticles is affected by the limitation due to their aggregation, and their stabilization is required both during the application time and preserving time. In fact, because of strong interparticle attraction, nZVI may form agglomerate of larger dimensions, which could reach also several microns in size [11]. To solve this problem the surface of the nanoparticles is coated with organic compounds or polymers to obtain a stabilization of the nanoparticle dispersion. Again, based on iron, nanomagnetite (Fe3O4) particles have been largely studied as nanoadsorbents; in particular, they give good results in heavy metals and dye removal. The advantage in their use is related to the fact that they are easily recovered by the application of a magnetic field [12–16]. The goal is to maintain their phase; in fact, Fe3O4 nanomagnetic particles can be subjected to a phase change as function of the operating conditions, such as low pH, and loose the magnetic characteristics with subsequent aggregation. Of course the pollutant removal efficiency and rate of reaction of the magnetic nanoparticles are related to their size. The surface area of particles increases up to a thousand times when the particle size decreases to nanoscale, and the obtained high surface area-to-volume ratio induces an increase of the number of active sites for the reactions, allowing the reduction of the mass required for treatment processes [17]. Commercial zerovalent iron nanoparticles are nanocomposites of Fe(0) (14–18 wt%) and Fe3O4 with a core/shell-like structure composed by an Fe(0)-rich core surrounded by a magnetite-rich shell. Alternatively to the Fe3O4, magnetic nanoparticles as spinel ferrites (MFe2O4, M ¼ Mn, Cu, Co, Ni, Zn, Mg, Ca, etc.) and their composites of designed size, composition, and structure are used in water treatment applications [18]. These compounds can be used in the adsorption of pollutants, such as metal ions, dyes, and pharmaceuticals. With regard to the nanometal oxides, nanophotocatalysts too are extensively investigated, exploring their formulation and electronic, physical, and chemical characteristic control. Among them, nanoporous TiO2, very effective for water purification of organic pollutants, is to be focused. Nano-TiO2 can be obtained in a nanoparticulate form or in nanostructures; in particular, titanium dioxide nanotubes have improved performance in photovoltaic. Nanoparticulate forms of TiO2 can be obtained by different methods such as sol-gel, micelle and inverse micelle, hydrothermal, solvothermal, direct oxidation, chemical vapor deposition, flame spray pyrolysis electrodeposition, sonochemical, and microwave methods [19]. In particular the sol-gel method offers advantages due to its relatively low cost and great flexibility. Four basic steps are involved in the sol-gel method starting from metallorganic precursor—hydrolysis, polycondensation, drying, and thermal treatment—and it is influenced by several parameters, such as type and concentration of precursor, solvent, water content,

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pH, and temperature. The properties of the resulting materials, including the crystal structure, particle size, shape and crystallinity, specific surface area, surface properties, aggregation, and adsorption ability, are influenced by the conditions of the synthesis. Bundles and arrays of nanoTiO2 nanotubes with different qualities can be synthesized by different techniques, for instance, by template-assisted, sol-gel, hydrothermal, electroanodization, chemical vapor deposition, and physical vapor deposition [20]. Since nanoTiO2 is a wide bandgap semiconductor, meaning that it is activable by UV light (bandgap 3.2 eV), several approaches have been conducted to promote the visible light photoactivity that includes the sensitization of semiconductors with organic molecules and metal and nonmetal ion doping, nanocompositing or coupling with small bandgap semiconductors. TiO2 doped with nonmetallic elements such as nitrogen (N), sulfur (S), and carbon (C) gives high activity with visible light. ZnO is another nanoxide that demonstrates high photocatalytic activity in the removal of organic contaminants. It is a wide bandgap semiconductor photocatalyst with characteristics of high chemical reactivity, low cost mild reaction conditions, economicity, and low toxicity. ZnO can be found in nanostructured forms, as films, or as nanowires, or in combination with other oxides like ZnO/ZnAl2O4, in doped forms (Mn-doped ZnO), and also in Ag2O/ZnO nanorod heterostructure, and applied for the degradation of organic contaminants. Other interesting nanomaterials are the perovskites that demonstrated promoted photoactivity with visible light. In combination with the purification of glucose-containing wastewater, a relevant hydrogen production can be obtained on LaFeO3 prepared through combustion synthesis [21]. Nanometals are well known in water disinfection, and in particular, silver materials have shown high activity in the microbial inactivation. Also in this case, material nanoengineering helps to improve the performances of silver. Since the interactions with DNA, membrane, and enzymatic activity of cells, nanosilver expresses promoted activity due its relevant surface area, from which the formation of reactive oxygen species or radicals starts [22–25]. Furthermore, their morphology is very relevant [26] with respect to the antibacterial properties, truncated triangular shaped nanosilver plates are more active in comparison with rod-shaped silver nanoparticles [27]. Nanosilver can be added to polymers or into membranes being an important component in the production of low-cost microfilters applied for water potabilization in decentrated areas. Noble metal nanoparticles of other expensive materials have been investigated in the environmental field, such as Pd nanoparticles. In combination with gold, metal nanoalloys have shown high activity in the trichloroethylene hydrodechlorination. Carbon nanomaterials have now been stated as new generation of adsorbent materials, with increased adsorption capacity and are allotropic forms of carbon that can be listed as single-walled carbon nanotubes, multi-walled carbon nanotubes, carbon nanofibers, fullerenes, graphene and derivatives, and amorphous carbonaceous composites [28].

9.2 Advanced oxidation processes

Recently, g-C3N4 has received much attention for environmental depollution [29]. The graphite-like carbon nitride (g-C3N4) is a metal-free polymer n-type semiconductor, with many promising characteristics, such as unique electric, optical, structural, and physiochemical properties. Carbon nanomaterials demonstrated high adsorption affinity for organic and inorganic pollutants due to exceptionally high surface area. Nanostructuring of such nanomaterials has been realized; for instance, they can form aligned nanostructures to obtain efficient filters or be mixed in membranes for removal of pollutants.

9.2 Advanced oxidation processes (AOPs) for water and wastewater treatment The fast industrialization of the last century has emerged a very severe problem linked to the intense water and air pollution. More specifically the progressive accumulation of organic compounds in waters is mainly due to the development of chemical technologies aimed to organic synthesis and processing [30]. Additionally, the main causes for surface water and groundwater contamination are industrial discharges, excess use of pesticides, fertilizers, and landfilling domestic wastes. The most common pollutant substances include solvents, volatile organics, chlorinated volatile organics, dibenzofurans, pesticides, antibiotics, polychlorinated biphenyls, and chlorophenols. As a consequence, it is mandatory to develop effective technologies for water and wastewater treatment, also considering that water use and reuse have become a major concern. The treatment processes of different types of effluents must guarantee the elimination or recuperation of the pollutant to reach the strict authorized levels for the discharge of these effluents. The levels of pollutants allowed in discharge waters are directly related with the type of present pollutant in the effluent. Generally, wastewater treatment processes are based upon various mechanical, biological, physical, and chemical steps. In particular a combination of many operations like filtration, flocculation, and chemical sterilization and the elimination of particles in suspension are employed. More specifically the most used process is the biological oxidation process (natural decontamination), but it is only effective in the removal of biodegradable contaminants. On the other hand the physical-chemical processes (coagulation and/or flocculation) use different chemicals (such as aluminum chloride, ferric chloride, and polyelectrolytes) and induce the formation of large amounts of sludge. For this reason the increasing demands for water quality indicators and drastic change regulations on water and wastewater disposal require the development of more efficient and more effective processes (such as ion exchange, ultrafiltration, reverse osmosis, and electrochemical technologies). However, each of these treatment methods has advantages and disadvantages. In addition, new regulations and emission limits are imposed, and industrial activities are required to search and to develop new methods and technologies able to effectively remove the pollution loads and to reduce the wastewater volume. In this perspective, AOPs

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for wastewater treatment are required to eliminate pollution. AOPs are widely used for the removal of recalcitrant organic constituents from industrial and municipal wastewater. In this sense, AOP type procedures can become very promising technologies for treating wastewater containing nonbiodegradable or hardly biodegradable organic compounds with high toxicity. These procedures are based on generating highly oxidative, such as HO• radicals in the reaction medium [31–33]. The main advantage of AOPs over all the existing chemical and biological processes is that they are “environment-friendly” as they don’t transfer pollutants from one phase to the other (as in chemical precipitation and adsorption), and they don’t produce hazardous sludge [34–36]. The enhanced degradation performances of AOPs are due to the in situ generation of various reactive oxygen species (ROS) via different processes such as sonolysis, ozonation, UV, and Fenton processes [33]. These ROS are generally characterized by a very high reactivity, and therefore they are able to remove various categories of pollutants [33]. The most common AOPs for treating water and wastewater containing organic compounds fall within one of the following categories:         

H2O2 + UV (direct photolysis) [37] H2O2 + Fe2+/3+ (classic, homogeneous Fenton) [38] H2O2 + Fe/support (heterogeneous Fenton) [39] H2O2 + Fe2+/3+ + UV (photo-Fenton) [40] O3 (direct ozone feeding) [41] O3 + UV (photo-ozone feeding) [42] O3 + catalysts (catalytic ozone feeding) [43] H2O2 + O3 [44] TiO2 + UV (photocatalysis) [45].

The degradation efficiency of each AOPs generally depends both on the rate of ROS generation and the extent of contact between the radicals and the organic compound [46]. The combination of single AOPs should result in better degradation performances as compared with individual processes because of the similarity between the mechanisms of destruction (radical oxidation) of the different AOPs and the enhancements in the rate of ROS generation using combined methods [47]. However, with regard to the AOPs based on O3, it must be considered that the oxidation products produced by ozonation are generally characterized by high toxicity [33]. These toxic compounds include formaldehyde, ketones, phenols, nitromethanes, and carcinogens such as bromates that are formed when ozone molecules and HO• radicals react with bromide in the aqueous phase [48,49]. Moreover the high cost of ozone as oxidizing agent is another major limiting factor in the implementation of this technology [33]. On the other hand the direct photolysis of H2O2 may result in expensive high energy requirements to have high removal efficiencies [46,50]. Additional drawbacks of H2O2 + UV processes include the low molar absorption coefficient of H2O2 and a low quantum yield of HO• production at 254 nm, which is the required wavelength for UV sources to be used in these technologies [51]. On the contrary, in the homogeneous Fenton and photo-Fenton

9.3 Fenton and photo-Fenton processes

processes, the reagents are nonthreatening to the environment, and they are safe to handle [46]. Additionally, for these types of AOPs, complicated apparatus are not required, and therefore the transition from laboratory to a large-scale application can be easily realized [52]. A main disadvantage of homogeneous Fenton and photo-Fenton processes is the strong dependence on the aqueous solution pH since acidic conditions (pH in the range 2–4) are required to achieve high degradation performances and minimize the sludge production [46,52]. These main drawbacks could be overcome by heterogeneous catalytic systems [53,54]. Among the different AOPs, heterogeneous photocatalysis, also called the “green” technology, represents one of the main challenges in the field of treatment and decontamination of water [55]. Heterogeneous photocatalysis is based on the simultaneous action of the light source and a catalyst (in most cases TiO2) that induces the oxidative degradation of water pollutants [56]. The main advantages of UV/TiO2 photocatalysis include operation at low temperature and pressure, low cost, and significantly low energy consumption [45]. On the basis of these observations, the following sections of the chapter are focused on heterogeneous catalysts for Fenton and photo-Fenton processes and on photocatalysis for the removal of water pollutants.

9.3 Fenton and photo-Fenton processes for water and wastewater treatment The Fenton reaction and its modification with the irradiation (photo-Fenton process) is of large interest for the purification of the wastewater. The Fenton reaction was firstly studied in the late 1800s [57]; meanwhile the mechanism was elucidated 40 years later. In the Fenton reaction that uses hydrogen peroxide as oxidizing agent instead of molecular oxygen, a transition metal is present to promote the formation of hydroxyl radical that is the effective oxidative agent for the oxidation of organic compounds. The Fenton reaction can be outlined as follows (Eq. 9.1): Mn + + H2 O2 ! Mðn + 1Þ + + OH + OH



(9.1)

where M is a transition metal as Fe or Cu. The metal acts like a catalyst, and the hydrogen peroxide reduces again the metal according to the following reaction (Eq. 9.2): Mðn + 1Þ + + H2 O2 ! Mn + + HO2 + H + 

(9.2)

The two reactions constitute the Fenton catalytic cycle. The most used metal for the oxidative decomposition and transformation of organic substrates by H2O2 is the Fe2+ (known as Fenton’s reagent). Hydrogen peroxide is cheaper than other oxidants, and iron is largely abundant on the earth. In water, it is present as ferric or ferrous ions, forming complexes with water and hydroxyl ions in dependence by pH, concentration, and temperature. At 25°C, Fe2+

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exists in solution up to a pH level around 8. The concentration of Fe3+ in solution in relation to the pH at 25°C is low over a pH of 3.2 because this is the solubility limit. Therefore, in Fenton conditions, the Fe3+ will be in solution if pH is lower than 3.2. At pH 3 the solubility of Fe3+ is around 4 mg/L. According to the literature [58], in spite of the low solubility of Fe3+ in the presence of phenol in the reaction medium, ferric hydroxide does not precipitate throughout oxidation because it is transformed into Fe2+ due to the reaction of Fe3+ with some products of phenol oxidation. Because at high pH ferric ions precipitate as hydroxides, the drawback of the Fenton method is related to the necessity to work at low pH, usually below 4. The scheme of reaction in the presence of Fe2+ can be described according to the reactions in Fig. 9.1. Fenton reaction involves the formation of HΟ• radicals, as evidenced many techniques, including EPR spectroscopy [59]. In the dark the rate-limiting step of the Fenton reaction is the conversion of Fe3+ to Fe2+. Therefore it has been considered to use UV light (λ < 400 nm) to promote the reduction of ferric ions. In particular the reaction is (Eq. 9.3). Fe3 + + H2 O + hν ! Fe2 + + HO + H +

(9.3)

•

That demonstrates that additional HO are formed, increasing the oxidizing power. Moreover the light is able to decompose the hydrogen peroxide in further HO•: H2 O2 + hν ! 2 HO

Fenton and photo-Fenton are largely studied reactions, since these constitute a promising technology for the complete mineralization of persistent organic pollutants with respect to the conventional physicochemical and biological treatment processes. The initial concentration of H2O2 is very relevant in the oxidation of organic compounds and affects the operating costs of such treatment procedures; as a Fe2+ + H2O2

Fe3+ + HO– + HO•

Fe3+ + H2O2

Fe2+ + H+ + HOO•

Fe2+ + HO•

Fe3+ + HO–

HO• + H2O2

HOO• + H2O

Fe2+ + HO2•

Fe3+ + HOO–

Fe3+ + HO2•

Fe2+ + H+ + O2

2 HO•

H2O2

FIG. 9.1 Scheme of mechanism for Fenton reaction.

9.3 Fenton and photo-Fenton processes

consequence the optimization of dose of this reagent is necessary. Increased addition of H2O2 leads to a higher production of hydroxyl radicals, but at high peroxide concentrations, the reaction between H2O2 and the HO• species produces HO2• radicals that are less reactive [60]. On the other hand, in the presence of low concentration of hydrogen peroxide, unwanted intermediate products can be formed. Thus it is common to observe the existence of an optimal dose of oxidant both in Fenton and in photo-Fenton processes. This can be overcome by the use of a continuous dosage [53]. The homogeneous catalysis of Fenton and photo-Fenton processes has some drawbacks: the restricted range of operative pH to avoid the precipitation of iron hydroxide that reduces the availability of catalyst for reaction, the production of sludge, and the necessity to remove the iron at the end of the reaction, since the final content of iron permitted for the discharge of a treated wastewater is only 2 mg/L of iron directly to the environment. The disadvantages of homogeneous Fenton can be overcome by the use of iron-based or other heterogeneous catalysts. One example is the application of monolith structured perovskite catalyst in heterogeneous photo-Fenton reaction of acetic acid that enlarges the pH range of operation and allows the separation of the catalyst from the reaction medium without any further treatment [53]. These catalysts have been proved in the removal of simulated winery wastewater and MTBE [61]. In this case the process is also called Fenton-like. The nanotechnology has brought new insights with regard to Fenton advanced oxidation process. Innovative applications of nanomaterials in the field water treatment are reported [62] to resolve the water quality issues and enhance the performance of existing treatment processes, owing the high catalytic ability due to their important surface area, which yields in a large number of accessible reaction sites on the surface [17] so limiting less the loss of activity with respect to the homogeneous Fenton catalysis. In primis heterogeneous forms of iron, such oxides and hydroxides, have been applied in heterogeneous Fenton-like processes. Nanoparticles of magnetite (Fe3O4), hematite (Fe2O3), goethite (α-FeOOH), and lepidocrocite (γ-FeOOH) have been used as unsupported nanocatalysts, overcoming the limitation in the range of operative pH and giving easier separation after the use. Nanomagnetite with Fe2+ and Fe3+ in octahedral sites permits reversible oxidation and reduction of iron species, has a permanent magnetization, and has been proved successfully for the degradation of phenol [63]. The crystal facets have a relevant role in determining the catalytic performance of hematite nanoparticles. The preferential exposure of (1 0 4), (1 1 3), and (0 0 1) planes gives a direct correlation between the surface facets and the photocatalytic performance. Heterogeneous photo-Fenton process under visible light irradiation for the degradation of methylene blue dye proceeds more on hematite nanoparticles if the surface facets (1 1 3) are predominant [64].

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Amorphous Fe2O3 nanoparticles can be obtained by controlled decomposition of iron (II) oxalate dihydrate and thermally crystallized to hematite (α-Fe2O3), yielding in nanopowders of different surface area and particle crystallinity as function of the reaction time. The highest efficiency in H2O2 decomposition was achieved on sample exhibiting a prevailing content of crystalline α-Fe2O3 phase [65]. The catalytic efficiency of Fe hydroxides in Fenton-like processes is strongly influenced by factors such as the Fe oxidation state, surface area, isomorphic substitution of Fe by other cations, pH, and temperature and resulted in a highly efficient generation of reactive species such as hydroxyl radicals, even at room temperature and under atmospheric pressure [38]. The process of separating nanoparticles from the aqueous phase after the reactions is however complex [66]. Moreover, hematite, goethite, and lepidocrocite, applied as catalysts in their pure forms or doped with other cations, can aggregate during the reactions, and so supporting them on other materials leads to a preservation of the nanosize. For instance, composites with carbon, alumina, and zeolites, among others, have been considered. Nanocarbon-supported nanocatalysts have the advantages of profiting of the support nanodimensions, of surface functionalities, and of synergic interactions [67]. Multi-walled carbon nanotubes (MWCNT), graphene (G), and graphene oxide (GO) have been used to stabilized nanocatalysts. Fe3O4/MWCNT was used as catalyst in the heterogeneous photo-Fenton degradation of bisphenol A, methyltestosterone, and acid orange II [62]. FexOy/MWCNT [68], Fe2O3/MWCNT [69], and Fe3O4/GO [70] were tested successfully in Fenton-like reaction for the degradation of dyes and organic molecules while GO/Fe2O3 [71], GO/NiFe2O [72], and G/Fe3O4 [73] in the photo-Fenton process. In photo-Fenton degradation of tetracycline α-Fe2O3@g-C3N4, nanocatalyst was under visible light (420 nm) at pH ¼ 5.5, showing at the optimal composition 92% of removal. Stabilized with the nanosupports, metal nanoparticles provide better stability and less leaching of iron in the resulting solution from the nanoparticles, according to the iron limits for the discharge in the environment (2 mg/L). In the remediation of water and air, clay-based nanocatalysts have been applied with relevant results due to their unique properties, structures, and surface chemistry [74,75]. The synthesis of nanoclay impregnated with layer graphene (NFLG) was successful. Pillared clay materials, possessing mesoporous structures, surface properties, and low cost, have been also used. Laponite and bentonite clay–supported iron (Fe2O3) and silicate-based (Fe2Si4O10(OH)2) nanocatalysts were effective in photo-Fenton degradation of dyes. The iron-loaded nanoclays performed well in a large pH range. Some authors worked with magnetically separable manganese ferrite and silicate nanocomposites (MnFe2O4/SiO2) for the decomposition of H2O2. These Fenton-like nanocatalysts of performed well in the concentration range of 0.005–3 MH2O2. They worked well in wide range of pH 6.0–13.0.

9.3 Fenton and photo-Fenton processes

Regarding the application of nanozerovalent iron (nZVI) particles in an heterogeneous Fenton process, they have been used in the treatment of a synthetic tannery wastewater (STW). The experimental tests demonstrated that, as function of initial pH (3–7) and nanocatalyst/oxidant ratio, the overall efficiency of the process can be optimized. A reduction of 70% of initial COD (1200 ppm) and a total Cr (VI) removal can be reached at optimal operating parameters 35% of nanocatalyst with respect to oxidant (H2O2) and a pH ¼ 3 [76]. CuO-doped ZnO composite nanomaterials were reported as efficient catalysts for H2O2 photodecomposition of humic acid by solar irradiation, having rod shape and being approximately 50–200 nm in size. In the photo-Fenton the realization p-n semiconductors heterojunction behaves favorably because of faster electron transfer to OH radical. Dispersed nanoparticles of gold on a support yield into an active catalyst. Generally the synthesis process of the gold nanocatalysts proceeds from colloidal gold suspension, by reducing Au3+ to Au0 [77] using different reducing agents (such as alcohols, ascorbic and citric acid, amines, citrate, hydrazines, and toluene) [77,78] and stabilizers, like amines, quaternary alkyl ammonium ions, phosphine, carboxyl acids, and thiols. However, the catalytic processes, favored from gold particles, have smaller sizes, between 2 and 10 nm. This is attained by the reduction with a strong reducing agent, such as NaBH4. The catalyst synthesis is completed by deposition of gold on a support. Colloid formation and deposition on the support can be conducted simultaneously through several methods, deposition/precipitation, coprecipitation, impregnation, vaporphase deposition, grafting, sol-gel, and ion exchange, among others [79]. One example of application is represented by phenol oxidation in the presence of hydrogen peroxide on gold supported on CeO2, TiO2, carbon, and Fe2O3 [80], obtaining the higher performances on the catalysts with smaller gold particle size ( Cu-TiO2 > Co-TiO2 > Fe-TiO2 > Cr-TiO2 > V-TiO2, which is in accordance with increase of photoelectron-hole recombination rate constants, for different doped metal ions. This behavior is ascribed to the photogenerated bandgap electrons and holes, which can migrate to the surface and govern redox reactions with adsorbed substrates, in competition with their disappearance due to mutual recombination. Because certain surface sites can trap the photoexcited electrons before their recombination with the holes, the increase lifetime of these charges favors the pollutant oxidation by holes or by radical species (•OH, •O• 2 ) generated on the surface of catalyst due to holes and electrons reactions with water, OH, and O2, respectively. Substitutional doping of TiO2 with Fe has a controversial influence on the catalyst photoactivity. Some papers show that Fe (III) behaves like an electron/hole recombination center leading to increase of kr value [76,77]. Other studies indicate that doping with proper concentration of Fe(III) can drastically increase the charge lifetime, which can be extended to minutes and even hours [78]. Studies regarding the influence of Fe content in Fe-TiO2-doped catalyst used for the degradation of 4-nitrophenol revealed that photocatalytic activity is decreasing with increasing of Fe(III) load between 1% and 8% onto TiO2 at the beginning of irradiation, while after 45 min it increases by further increasing of Fe(III) content, except for the 1% Fe-TiO2 [79]. This behavior is in relation with high iron ions leaching into solution. In spite of this aspect, all doped catalysts show to be more photoactive than bare TiO2, and the optimal amount of iron is 1%. After 60 min of irradiance, using this catalyst, the pollutant and total organic carbon (TOC) removal

10.2 Undoped and Fe-doped TiO2 sol-gel nanomaterials

efficiencies are 100% and 67.5%, respectively. The beneficial effect of iron presence on photocatalytic activity can be explained by the following reactions that emphasize that transition metal ions are acting as hole-electron traps, especially at low concentration level [80–83]: TiO2 + hν ! e + h +

(10.1)

Fe3 + + e ! Fe2 + electrons trapping

(10.2)

Fe2 + + O2ðadsÞ ! Fe3 + + O2  electron release

(10.3)

Fe2 + + Ti4 + ! Fe3 + + Ti3 +

(10.4)

Fe3 + + hVB + ! Fe4 + hole trapping

(10.5)

Fe4 + + OH ! Fe3 + +  OH hole release

(10.6)

The TiO2 irradiation leads to generate electron-hole pairs (Eq. 10.1). By means of a transfer of photogenerated electrons from TiO2 to Fe3+ (Eq. 10.2), Fe2+ ions result relatively instable due to the loss of d5 (half-filled high spin) electronic configuration and tend to return to Fe3+ (d5). Subsequently Fe2+ could be oxidized to Fe3+ by transferring electrons to absorbed O2 on the surface of TiO2 with electron release, and hyperoxide (O•2) appears (Eq. 10.3). The Fe2+/Fe3+ energy level lies close to Ti3+/Ti4+ level. As a consequence, the trapped electron in Fe2+ can also be easily transferred to a neighboring surface Ti4+ (Eq. 10.4), which then leads to interfacial electron transfer. That is to say, Fe3+ can be an effective electron trap in anatase. Meantime, Fe3+ can also serve as hole trap (Eq. 10.5), due to the energy level for Fe3+/Fe4+ above the valence band edge (EVB) of TiO2 anatase [82]. Fe4+ is transferred to the catalyst surface and release holes; Fe3+ appears again together hydroxyl radical (•OH) (Eq. 10.6). As it was mentioned before, iron ions can also act as recombination centers for the hole-electron pairs when their concentration is high, according to the following reactions (Eqs. 10.7, 10.8) [80,82]: Fe3 + + e ! Fe2 +

(10.7)

Fe2 + + hVB + ! Fe3 +

(10.8)

It has been also proved that Fe doping induced a bathochromic shift by the decrease of bandgap or introduction of intra-bandgap states, resulting in more visible light absorption, with a positive effect on photocatalytic activity and also on operation cost savings for practical application of photocatalysis in water treatment process using solar light as irradiation source. This advantage is limited by the level of used transition metal, because the excess of deposited iron on TiO2 can form Fe(OH)2+ species, with a great absorbance of incident light in UV-Vis domain in respect to bare TiO2. This higher adsorption is responsible (together with other factors) for the decrease of the photocatalytic activity of Fe-doped catalyst [84].

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A large number of studies have described also the effect of catalyst concentration on the pollutant degradation efficiency. Zhao et al. [79] reported the influence of this parameter in the case of 4-nitrophenol photocatalytic degradation on 1% Fe-TiO2-doped catalyst at 1 h irradiation. The degradation yield increased to 95% by increasing catalyst concentration up to 0.1 g/L, and then it was nearly constant from 0.1 to 0.4 g/L. Further increasing of catalyst concentration above 0.4 g/L resulted in a decrease to 75% of the degradation yield. It was observed that a very small dose of doped catalyst is sufficient for advanced degradation of 1.4  104 M pollutant concentration. The photocatalytic degradation of the same pollutant using 0.2–4 g/L dose of Degussa P25 TiO2 was reported by Chan and Ray [85]. They found an optimal catalyst concentration of 2 g/L, which emphasized that among other factors (reactors geometry and UV light sources applied), the higher photoactivity of doped catalyst compared with undoped TiO2 has an important influence on the pollutant degradation efficiency. The Degussa P25 TiO2 was also used for degradation of 1.6  104 M nitrobenzene, and the optimum catalyst level reported was 3 g/L [86,87]. The different catalyst loadings requested for the two nitrocompound degradation are due to the high sensitivity of nitrophenol to the •OH radicals attack compared with nitrobenzene. The profile of pollutant degradation efficiency versus catalyst concentration is a consequence of the increased number of catalyst particles that will lead to the increase of absorbed photons and pollutant molecule number. In this context, the degradation efficiency will be enhanced with increasing catalyst concentration due to the increase of total surface area available for contaminant adsorption. Excess catalyst concentration is leading to water suspension opacity, which decreases light penetration. It is also reported that above certain catalyst level, the number of TiO2 surface active sites may become almost constant due to decreased light penetration, increased light scattering, and loss in surface area because catalyst aggregation (particle-particle interaction). The occurrence of all these phenomena has resulted in the decrease of pollutant degradation efficiency [88,89]. For any application, the photocatalytic reactor should be operated at optimum catalyst concentration to avoid excess catalyst and to ensure efficient photon absorption and pollutant degradation. It has been demonstrated that catalyst characteristics and concentration, nature and concentration of target pollutants, light intensity and wavelength, oxygen concentration, oxidants addition, pH of aqueous medium, and the pollution matrix are the main parameters affecting the efficiency and rate of pollutant degradation [14]. Reported data suggest that ensuring advanced degradation of recalcitrant pollutants as polynitroaromatic derivates imposes the use of catalyst with increased photocatalytic activity like transitional metal-doped TiO2 or/and prolonged irradiation time for a given light intensity, catalyst loading, and pollutant initial concentration. Nowadays, Fe-doped TiO2 materials (powders and films) have become an environmental decontamination photocalalyst for a large variety of organics, including nitroaromatic compounds as it is presented in Table 10.2.

10.2 Undoped and Fe-doped TiO2 sol-gel nanomaterials

Table 10.2 Application of Fe-doped TiO2-assisted photocatalysis on pollutant degradation. Molar ratio Fe/TiO2 Dopant concentration (mol%, wt%, and at.%)

Pollutant

Fe/ TiO2 catalyst

1,2-Dichloroethane

Nanoparticles

2,4Dichlorophenoxyacetic acid

Nanoparticles

2,4-Dichlorophenol

Nanoparticles

2,4Dichlorophenoxyacetic acid

Nanoparticles

2-Propanol

Nanoparticles

0.08-0.12-1.82 (wt%) Fe

2-Propanol

Nanoparticles

4-Nitrophenol

Nanoparticles

4-Nitrophenol

Nanoparticles

Acetophenone

Nanoparticles

The amounts of Fe ions implanted (107 mol/ gcat): (a) 0, (b) 2.2, (c) 6.6, (d) 13.2, and (e) 22.0 1.0 mol of dopant metal ions over 100 mol of dopant and titanium ions 1, 3, 5, and 8 wt % Fe-doped TiO2 catalysts 0.5 wt% Fe

Fe/Ti (mol%)0.001-0.010.05 0.025-0.050.10 wt% Fe2O3-TiO2 composites 0.1-0.5-1.03.0-5.0 wt% Fe/TiO2 0.1-0.25-0.50.75-1.0 (mol%) Fe/Ti

Synthesis method

Ref.

Sol-gel

[90]

Sol-gel

[91]

Sol-gel

[92]

Impregnation and photodeposition methods Doping by chemical vapor deposition of nano-TiO2 obtained hydrothermal The metal (Fe) ion-implantation method on nano-TiO2 sol-gel

[93]

[94]

[95]

The incipient wet impregnation method

[75]

Impregnation method

[79]

Sol-gel and impregnation methods

[96]

Continued

275

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CHAPTER 10 Fe-doped TiO2 nanomaterials for water depollution

Table 10.2 Application of Fe-doped TiO2-assisted photocatalysis on pollutant degradation—cont’d

Pollutant

Fe/TiO2 catalyst

Molar ratio Fe/TiO2 Dopant concentration (mol%, wt%, and at.%)

Alachlor

Nanoparticles

0.1 wt% Fe/Ti

Ammonia (as gaseous indoor pollutant) Benzene

Thin films on glass support Thin films on glass support Catalyst immobilized onto fiberglass cloth Thin films on glass fibers

0.25-0.75-1.01.25 mol% Fe 0.25-0.75-1.01.25 mol% Fe 0.1 mol% Fe

Benzene

Benzene

Benzene

Benzoic acid

Thin film on glass support Nanostructured catalyst; FeTiO2 on active carbon Nanoparticles

Bisphenol A

Nanoparticles

CHCl3-chloroform

Nanoparticles

Diazinon

Nanoparticles

Dichloromethane

Nanoparticles

Benzoic acid

Synthesis method

Ref.

Colloidal impregnation method Sol-gel

[97]

Sol-gel

[98]

Solvothermal method

[99]

Fe/Ti ratio: 0.005, 0.01, 0.05, 0.1, 0.5 0.25-0.75-1.01.25 mol% Fe Fe:TiO2 (wt/wt): 1:99

Sol-gel

[99]

Sol-gel

[98]

Sol-gel

[100]

1.0 mol of dopant metal ions over 100 mol of dopant and titanium ions Fe/Ti (molar ratio): 0.005, 0.01, 0.02, 0.03 0.02-0.05-0.10.2-0.5-1.0 Fe3+ dopant concentration (at.%) Fe/TiO2 (wt%): 1-1.5-2.0

The incipient wet impregnation method

[75]

Sol-gel

[101]

Sol-gel

[102]

Mild hydrothermal conditions Sol-gel

[103]

Fe/Ti (mol%): 0.05-0.5-5.0

[98]

[104]

10.2 Undoped and Fe-doped TiO2 sol-gel nanomaterials

Table 10.2 Application of Fe-doped TiO2-assisted photocatalysis on pollutant degradation—cont’d

Pollutant

Fe/ TiO2 catalyst

Estriol

Nanoparticles

Ethanoic acid

Nanostructured catalyst

Ethylbenzene

Catalyst immobilized onto fiberglass cloth Thin films on glass fibers

Ethylbenzene

Formaldehyde Malachite green dye

Thin films on glass support Thin film on glass support

Methanoic acid

Nanoparticles

Methyl orange Methyl orange

Nanoparticles Nanoparticles

Methyl orange

Nanoparticles

Molar ratio Fe/TiO2 Dopant concentration (mol%, wt%, and at.%)

Synthesis method

Ref.

0.3-0.6-1.0 (at.%) Fe-TiO2 1.0 mol of dopant metal ions over 100 mol of dopant and titanium ions 0.1 mol% Fe

Hydrothermal sol-gel synthetic The incipient wet impregnation method

[105]

Solvothermal method

[106]

Fe/Ti ratio: 0.005, 0.01, 0.05, 0.1, 0.5 0.25-0.75-1.01.25 mol% Fe Fe3+/Ti(O-iPr)4 ratios (mol/mol) 0.003 and 0.07 1.0 mol of dopant metal ions over 100 mol of dopant and titanium ions Ti0.75 Fe0.25 O2 0.3, 0.5, 0.8, and 1.0 mol% Fe 0.05-0.1-0.20.4-0.6 wt% Fe-TiO2

Sol-gel

[99]

Sol-gel

[98]

Hydrothermal process

[107]

The incipient wet impregnation method

[75]

Sol-gel Sol-gel

[108] [109]

The hydrolysis of Ti(OC4H9)4 via an esterification reaction between acetic acid and ethanol followed by hydrothermal treatment

[110]

[75]

Continued

277

278

CHAPTER 10 Fe-doped TiO2 nanomaterials for water depollution

Table 10.2 Application of Fe-doped TiO2-assisted photocatalysis on pollutant degradation—cont’d

Pollutant Methyl orange

Methyl orange

Methylene blue

Methylene blue

Fe/TiO2 catalyst Thin film on glass or silica plate Nanoparticles

Fly ashsupported TiO2: Fe3+ powder Supported on HY zeolite

Molar ratio Fe/TiO2 Dopant concentration (mol%, wt%, and at.%)

Synthesis method

Ref.

0,1, 2, 4 wt% Fe

Sol-gel dip coating method

[64]

0.5-1.0-3.05.0 wt% Fe

Molten salt method using a solid mixture of TiO2 powder and FeCl3 precursor Sol-gel

[111]

[112]

Sol-gel

[113]

Sol-gel Sol-gel

[114] [115]

Sol-gel

[116]

Sol-gel and impregnation methods Sol-gel and impreganation methods Method involving the calcination of a composite cross-linked titania and structuredirectingsurfactant material; irongrafted mesoporous titania Modified sol-gel method

[96]

0.1-0.3-0.50.7 wt% Fe

Methylene blue Methylene blue

Nanoparticles Nanoparticles

Methylene blue

Nanoparticles

Nitrobenzene

Nanoparticles

3.0–10.0 wt% Fe-TiO2/HY zeolite 6 wt% Fe 1.0-3.0-5.0 mol % Fe 3.0-5.0-7.010.0 mol% Fe 0.5 wt% Fe

Nitrobenzene

Nanoparticles

0.47 wt% Fe

Norfloxacin

Nanoparticles

1.0-1.5-2.13.0 wt% Fe2O3TiO2

Oxalic acid

Nanoparticles

5.0 at.% Fedoped TiO2

[96]

[117]

[118]

10.2 Undoped and Fe-doped TiO2 sol-gel nanomaterials

Table 10.2 Application of Fe-doped TiO2-assisted photocatalysis on pollutant degradation—cont’d

Pollutant

Fe/ TiO2 catalyst

o-Xylene

Thin films on glass fibers

o-Xylene

Nanoparticles

Paper-making effluent

Nanoparticles

Para-nitrophenol

Nanoparticles

Paraquat (N,N0 dimethyl-4,40 bipyridinium dichloride) Paraquat (N, N0 -dimethyl4,40 -bipyridinium dichloride) Paraquat (N,N0 dimethyl-4,40 bipyridinium dichloride) Phenol

Nanoparticles

Phenol

Nanoparticles

Phenol

Nanoparticles

Phenol

Nanoparticles

Phenol

Nanostructured catalyst

Molar ratio Fe/TiO2 Dopant concentration (mol%, wt%, and at.%)

Synthesis method

Ref.

Sol-gel

[99]

Sol-gel

[72]

Sol-gel

[119]

Sol-gel

[120]

Hydrothermal

[121]

Impregnation

[121]

Nanoparticles

Fe/Ti ratios: 0.005, 0.01, 0.05, 0.1, 0.5 0.5 at.% FeTiO2 0.01-0.03-0.050.07-0.1 wt% Fe Molar ratio Fe/Ti ¼ 0.025, 0.05, 0.075, 0.1 0.5-1.0-2.04.0-6.0 wt% Fe/TiO2 1 wt% Fe/TiO2

Nanoparticles

1 wt% Fe/TiO2

Sol-gel

[121]

Nanoparticles

Atomic ratio % Fe/Ti, 0.006 and 0.034 Fe2O3/TiO2 wt %: 0.2-0.270.41-0.5 0.5-1.0-1.55.0-10.0 wt% Fe Fe-doped TiO2 sample, FexTi1 x O2 (x ¼ 0, 0.002, 0.005, 0.008, 0.01) 0.1-0.6-1.23.0-6.0 and 10.0 at.% Fe

Sol-gel

[122]

Sol-gel

[123]

Microemulsion method

[124]

The calcination of FexTiS2

[125]

Coprecipitation and hydrothermal methods

[80]

Continued

279

280

CHAPTER 10 Fe-doped TiO2 nanomaterials for water depollution

Table 10.2 Application of Fe-doped TiO2-assisted photocatalysis on pollutant degradation—cont’d

Pollutant

Fe/TiO2 catalyst

p-Xylene

Transparent thin films on pyrex tube

Reactive red 198

Nanoparticles

Organic dye Reactive Black 5

Nanoparticles

Toluene

Catalyst immobilized onto fiberglass cloth Nanoparticles

Toluene

Toluene

Thin films on glass fibers

Xylene

Catalyst immobilized onto fiberglass cloth

Molar ratio Fe/TiO2 Dopant concentration (mol%, wt%, and at.%) 0.025, 0.05, 0.10, 0.50, 1.00, and 2.00 mol% Fe2O3/TiO2 0, 0.1, 1, 5, and 10 wt% Fe 0.5-1.0-2.0 Fe/TiO2 mol ratio 0.1 mol% Fe

0.3-0.5-0.71.0-1.5 at.% Fe/TiO2 Fe/Ti ratios: 0.005, 0.01, 0.05, 0.1, 0.5 0.1 mol% Fe

Synthesis method

Ref.

Sol-gel

[70]

Sol-gel

[126]

Nonhydrolytic sol-gel method

[127]

Solvothermal method

[106]

Sol-gel

[128]

Sol-gel

[99]

Solvothermal method

[106]

10.3 Undoped and Fe-doped TiO2 sol-gel nanopowders 10.3.1 Short consideration of sol-gel method for TiO2-based nanopowders preparation The sol-gel prepared titania can be obtained both as amorphous gel and crystalline material (anatase, rutile, or brookite). The temperature corresponding to the anatase transformation into rutile is dependent on many factors: the coordination of the metal in the precursor, the length of the metal-oxygen (MdO) bond in the precursor gel, the method of preparation, the presence or the absence of the impurities, and the texture and the size of the primary particles of anatase [129]. The sol-gel process can provide submicron, monodisperse TiO2 powders, no matter of the used precursor (inorganic or organic). The hydrolysis of alkoxides (the organic method) represents

10.3 Undoped and Fe-doped TiO2 sol-gel nanopowders

the most common way for the preparation of TiO2 particles. The chemical reactions can be schematically presented as follows: TiðORÞ4 + xH2 O ! TiðORÞ4x OHx + xROH ðhydrolysisÞ TiðORÞ4x OHx + TiðORÞ4 ! ðORÞ4x TiOx TiðORÞ4x + xROH ðcondensationÞ

(10.9) (10.10)

where R is an organic radical, such as ethyl, propyl, and i-propyl, n-butyl. The sol-gel process involves the formation of a gel as a result of the condensation of the partially hydrolyzed species into a three-dimensional polymer lattice. As the hydrolysis and the polycondensation take place in the same time, being in competition, there is possible to change in some manner their relative rate. There are many factors that influence the process. The most studied are the alkoxy groups belonging to the alkoxides, the reactants concentration, the solvent, the quantity of water used for hydrolysis, the pH of the solution, the temperature of hydrolysis, the nature of catalyst, and the presence of additives [130]. Any factors that affect these reactions can have an impact on the properties of the resulted gel. The motivation for sol-gel processing is primarily the potentially higher purity and homogeneity due to the quality of the available precursors and the lower processing temperature that allows the densification of the “pre”-inorganic network formed in solution, to inorganic oxide solids. For the multicomponent systems the following specific advantages can be mentioned: the ability to control both structure and composition at molecular level, the possibility to introduce several components in a single step, and the power to impose kinetic constraints on a system and thereby stabilize metastable phases. Furthermore, the controlled shape and size (usually, monodisperse) and the nanometer size of the particles must be mentioned.

10.3.2 Our studies regarding undoped and Fe-doped TiO2 nanopowders 10.3.2.1 Sample preparation Both pure TiO2 and Fe-doped TiO2 samples have been synthesized using the alkoxide route of the sol-gel method. The TiO2 precursor was tetraethyl orthotitanate, Ti (OC2H5)4 and the Fe source was iron nitrate, Fe(NO3)39H2O, both supplied by Merck. Four Fe mass concentrations related to TiO2 content have been used: 0.5, 1, 2, and 5 wt%. The solvent was the absolute ethanol, C2H5OH (Riedel de Hae¨n). The hydrolysis of titanium alkoxides took place with a water excess in noncatalyzed conditions, for all dopant concentrations, except 5 wt% Fe, in which case a few drops of ammonia have been used. Then the solutions were maintained 2 h under stirring, at room temperature. The final pH value was 6. The obtained sols were concentrated and dried at 80°C, and the resulted xerogels were subjected to thermal analysis measurements in order to establish the thermal schedule for further investigations. Thus, for structural and morphological studies, the powders were thermally treated at three temperatures: 300°C, 400°C, and 500°C with 1 h plateau each. The heating rate of the samples up to final temperature was 1°C/min. The samples were

281

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CHAPTER 10 Fe-doped TiO2 nanomaterials for water depollution

denominated: “T” for pure TiO2 sample and “TF” followed by concentration of the dopant and by the temperature of the thermal treatment values for Fe-doped TiO2 samples (e.g., TF0.5_300, TF1_400, and TF2_500).

10.3.2.2 Results and discussion A detailed XRD analysis, based on an own calculus program, has established the lattice parameters, the average size of the crystallites (D), the unit cell volume (UCV), and the average lattice strains (S), which can give some information about the structural disorder. The structural changes due to the thermal treatment and the iron dopant are mentioned. The photocatalytic activity of the prepared nanopowders has been tested in the degradation of nitrobenzene from water. The relation between structure and photocatalytic activity of samples was established. XRD results are presented in Table 10.3 [131]. Table 10.3 The structural parameters from XRD data, magnetic susceptibilities, and photocatalytic activities variation with temperature for undoped TiO2 (T) and Fe-doped TiO2 (TF) samples. TF Temperature (°C)

Structural parameters a (A˚) c (A˚)

300

UCV (A˚3) hDi (A˚) hSi  103 ηNB% χ  106 (cm3 g1) a (A˚) c (A˚)

400

UCV (A˚3) hDi (A˚) hSi  103 ηNB% χ  106 (cm3 g1) a (A˚) c (A˚)

T

TF0.5

TF1

TF2

TF5

3.7812 (13) 9.5148 (46) 136.04 (16) 138(4) 0.06(10) 50.07 0.04 (DM) 3.7809 (6) 9.4993 (21) 135.80 (7) 166(6) 1.27(14) 54.14 –

3.7858 (7) 9.4721 (25) 135.75 (9) 165(5) 0.75(14) 56.95 1.07 (PM) 3.7811 (3) 9.4786 (10) 135.51 (4) 236(6) 1.19(7) 84.91 0.59 (PM) 3.7809 (1) 9.4950 (4)

3.7700 (18) 9.4700 (64) 134.60 (22) 183(8) 1.22(13) 50.33 2.08 (PM) 3.7847 (6) 9.4716 (23) 135.96 (8) 249(9) 0.97(13) 74.77 2.49 (PM) 3.7829 (2) 9.5015 (6)

3.7808 (15) 9.4815 (53) 135.53 (18) 114(4) 0.53(41) 68.47 4.44 (PM) 3.7792 (15) 9.5178 (55) 135.93 (19) 176(7) 0.67(12) 71.77 4.34 (PM) 3.7833 (6) 9.4973 (20)

3.7845 (14) 9.0986 (88) 130.31 (22) 0.89; drug transport mechanism, super case-II transport; and rate as a function of time, tn1 – Sphere form  release exponent, n < 0.43; drug transport mechanism, Fickian diffusion; and rate as a function of time, t0.57  release exponent, 0.43 < n < 0.85; drug transport mechanism, anomalous transport; and rate as a function of time, tn1  release exponent, n ¼ 0.85; drug transport mechanism, case-II transport; and rate as a function of time, zero-order release  release exponent, n > 0.85; drug transport mechanism, super case-II transport; and rate as a function of time, tn1 In studies of the release in pharmaceutical systems, the Hixson-Crowell equation (Eq. 13.5) is also applied. KS is the constant that incorporates the relation between the area and the volume of the form of pharmaceutical doses: 1

1

Q30  Q3t ¼ KS t

(13.5)

Kopcha et al. [198] proposed an empirical equation (Eq. 13.6) to fit release data of optimized batches. 1

M ¼ At2 + Bt

(13.6)

395

396

CHAPTER 13 Nanocarriers loaded with nutraceuticals and bioactive ingredients where M is the amount of drug released at time t, while A and B are, respectively, the diffusion and erosion (physical and chemical) terms. According to this equation for M  70% in a given time t, if A/B ¼ 1, then the release mechanism involves both diffusion and erosion. If A/B > 1, the diffusion prevails, and if A/B < 1, the erosion prevails [193]. The Weibull model (Eq. 13.7) is more useful for comparing the release profiles of matrix-type drug delivery [58, 199]: 2 Mt ¼ M∞ 41  e

 

tt0 τd

β 3 5

(13.7)

where Mt is the dissolution/release (%) at time t (min), M∞ is the dissolution/release (%) at infinite time, and t0 the lag time (min) of the dissolution (normally t0 ¼ 0). β represents the shape parameter of the curve and τd the time (min) when 63.2% of M has been dissolved/released [58, 199]. For β ¼ 1, the shape of the dissolution/release curve corresponds exactly to the shape of an exponential profile. When β > 1, the shape of the curve gets sigmoidal with a turning point. For β < 1, the shape of the curve would show a steeper increase than the one with β ¼ 1. Other models have been developed further from the main models, for example, the Baker-Lonsdale model was developed from the Higuchi model [58]. The Baker-Lonsdale model (Eq. 13.8), which represents drug release from spherical monolithic dispersions, was developed by Baker and Lonsdale [200] from the Higuchi model and describes the drug release from spherical matrices by using the following equation [201, 202]: 2 3  2 34 Mt 3 5 Mt  f1 ¼ 1  1  ¼ kt 2 M∞ M∞

(13.8)

where the release rate constant, k, corresponds to the slope [58, 202, 203]. The mathematical models can provide a scientific knowledge related to mass transport mechanisms that are involved in the control of substances release. This information is useful and necessary to design new systems, and it can be used to simulate the effect of the design parameters (geometry and composition) on the resulting release kinetics [40]. The applicability of mathematical models to the controlled release of compounds is increasing in food industry permitting the development of new food systems. Sophisticated instrumentation, modern mathematical models, and computational power have revolutionized the entire process of formulation and development of drug and food delivery systems [52].

13.5.4 Evaluation of the bioavailability One of the items that is necessary to analyze with special attention is the bioavailability of the bioactive compound after encapsulation. Bioavailability may be assessed by in vitro and in vivo studies.

13.6 Conclusions

In vitro tests consist in the release study under simulated gastrointestinal conditions, which can also include tests with cell cultures to determine permeability and flux and also tests to determine microparticles adhesion of the intestinal tissue [204]. In a first stage, it is essential to perform these types of tests to obtain preliminary results about the bioactive ingredient release mechanisms, rate, and extent. In vitro studies do not consider physiological variability, metabolic responses, compounds active transport, and the effect that other foods may have during consumption and digestion [205]. Therefore in vivo studies are necessary to determine release location and patterns, tissue biodistribution, and physiological responses [205]. The fact that the bioactive compound is generally delivered simultaneously with other foods increases the complexity of these kind of tests due to possible interactions of the organism with the food [206]. To avoid food interaction complexity, in vivo tests are usually performed with fasted individuals [206]. However, till the final commercialization of the food products, supplementary studies are necessary to evaluate the effect of food interactions.

13.6 Conclusions Nowadays the consumers are forcing the food manufacturers to look for food products with specific or general health benefits. The role of food in the modern lifestyle exceeds its basic nutritional needs like satisfying hunger and providing essential nutrients for humans. The development of nutraceutical-fortified foods and beverages containing multiple kinds of microencapsulated bioactive agents is being investigated for their potential additive or even synergistic health benefits. These bioactive compounds can include vitamins, minerals, and other nutraceuticals. However, some of these compounds are very sensitive to ambient or industrial process conditions. The nutraceutical and bioactive compound loss during the processing or storage of foods is a very common occurrence. So, all the recent developments in the application of nano-/microdelivery systems in food products and beverages are proving to be a combination of factors, interests, and expectations of the researchers, food manufacturers, and consumers regarding possibility of enhancing the functionality of bioactive compounds within the fortified food and beverage products. In this direction, nano /microdelivery systems have been produced to stabilize and enhance the biological activity of the bioactive compounds. This chapter intended to present and discuss the importance of bioactive compounds and nutraceutical in food industry (food products and beverages) and the different stages involved in the development of microencapsulation processes, since the selection of the raw material till the characterization of the microparticles and controlled release studies and finally the commercialization of the food products containing these microparticles.

397

398

CHAPTER 13 Nanocarriers loaded with nutraceuticals and bioactive ingredients Considerations about the design of experiments (DOE) and the “delivery by design,” which facilitates the creation of delivery systems suitable for commercial applications, were also made. The global nutraceutical market is increasing in an exponential way and is expecting to reach $285.0 billion by 2021 and at the same for the functional beverages market that should reach $105.5 billion by 2021. Most of the nutraceutical compounds are added to food products and in the beverages in microencapsulated formulations. Finally and as mentioned before, food is for our sustenance, nourishment, and enjoyment, but more than this, people are increasingly looking to nonnutrient food components for added benefits from their food and beverages, which may play a role in health promotion, disease prevention, and performance improvement.

Acknowledgments This work was financially supported by project UID/EQU/00511/2019 Laboratory for Process Engineering, Environment, Biotechnology, and Energy (LEPABE) funded by national funds through FCT/MCTES (PIDDAC); project POCI-01-0145-FEDER-028715 (microdelivery— development of controlled delivery functional systems by microencapsulation of natural and active compounds with therapeutic, nutritional, and technological interest), funded by FEDER funds through COMPETE2020 Programa Operacional Competitividade e Internacionalizac¸a˜o (POCI) and by national funds (PIDDAC) through FCT/MCTES; and project “LEPABE-2-ECO-INNOVATION” NORTE-01-0145-FEDER-000005, funded by Norte Portugal Regional Operational Program (NORTE 2020), under PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). Anto´nia Gonc¸alves acknowledges Fundac¸a˜o para a Ci^encia e a Tecnologia (FCT) for the award of doctoral research grant (SFRH/BD/129207/2017). Also, Berta Estevinho acknowledges FCT for the contract based on the “Lei do Emprego Cientı´fico” (DL 57/2016).

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c, et al., Encapsulation of polyphenolic antioxidants from medicinal plant extracts in alginate–chitosan system enhanced with ascorbic acid by electrostatic extrusion, Food Res. Int. (Internet, cited 2011 Aug 8). 44 (4) (2011) 1094–1101. Available from: http://linkinghub.elsevier.com/retrieve/pii/S0963996911001876. [181] B. Muru´a-Pagola, C.I. Beristain-Guevara, F. Martı´nez-Bustos, Preparation of starch derivatives using reactive extrusion and evaluation of modified starches as shell materials for encapsulation of flavoring agents by spray drying, J. Food Eng. (Internet, cited 2012 Apr 17). 91 (3) (2009) 380–386. Available from: http://linkinghub.elsevier.com/ retrieve/pii/S0260877408004573. [182] M.O. Eraso, A. Herrera, Use of starches and milk proteins in microencapsulation, Int. J. Veg. Sci. (Internet). (2013) 1–29. Available from: http://www.tandfonline.com/doi/ abs/10.1080/19315260.2013.803181. [183] P. Sun, M. Zeng, Z. He, F. Qin, J. Chen, Controlled release of fluidized bed-coated menthol powder with a gelatin coating, Dry. Technol. (Internet). 31 (13–14) (2013) 1619–1626. Available from: http://www.tandfonline.com/doi/abs/10.1080/07373937. 2013.798331. [184] A. Gharsallaoui, G. Roudaut, O. Chambin, A. Voilley, R. Saurel, Applications of spray-drying in microencapsulation of food ingredients: an overview, Food Res. Int. (Internet, cited 2012 Feb 29). 40 (9) (2007) 1107–1121. Available from: http:// linkinghub.elsevier.com/retrieve/pii/S0963996907001238. [185] B. Krajewska, Application of chitin- and chitosan-based materials for enzyme immobilizations: a review, Enzym. Microb. Technol. (Internet, cited 2012 Mar 15). 35 (2–3) (2004) 126–139. Available from: http://linkinghub.elsevier.com/retrieve/pii/ S0141022904001231. [186] A. Madene, M. Jacquot, J. Scher, S. Desobry, Flavour encapsulation and controlled release—a review, Int. J. Food Sci. Technol. (Internet, cited 2013 Jan 31). 41 (1) (2006) 1–21. Available from: https://doi.org/10.1111/j.1365-2621.2005.00980.x.

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[187] S. Freiberg, X.X. Zhu, Polymer microspheres for controlled drug release, Int. J. Pharm. (Internet, cited 2012 Mar 14). 282 (1–2) (2004) 1–18. Available from: http://www.ncbi. nlm.nih.gov/pubmed/15336378. [188] R. Cortesi, C. Nastruzzi, S.S. Davis, Sugar cross-linked gelatin for controlled release: microspheres and disks, Biomaterials (Internet). 19 (18) (1998) 1641–1649. Available from: http://www.ncbi.nlm.nih.gov/pubmed/9839999. [189] C.S. Favaro-Trindade, A.S. Santana, E.S. Monterrey-Quintero, M.A. Trindade, F. M. Netto, The use of spray drying technology to reduce bitter taste of casein hydrolysate, Food Hydrocoll. (Internet, cited 2012 Apr 17). 24 (4) (2010) 336–340. Available from: http://linkinghub.elsevier.com/retrieve/pii/S0268005X09002161. [190] X. Huang, C.S. Brazel, On the importance and mechanisms of burst release in matrixcontrolled drug delivery systems, J. Control. Release (Internet). 73 (2–3) (2001) 121–136. Available from: http://www.ncbi.nlm.nih.gov/pubmed/11516493. [191] R. Ito, B. Golman, K. Shinohara, Controlled release with coating layer of permeable particles, J. Control. Release (Internet, cited 2012 Apr 17). 92 (3) (2003) 361–368. Available from: http://linkinghub.elsevier.com/retrieve/pii/S0168365903003638. [192] W. Guo, P. Quan, L. Fang, D. Cun, Sustained release donepezil loaded PLGA microspheres for injection: preparation, in vitro and in vivo study, Asian J. Pharm. Sci. (Internet). 10 (5) (2015) 405–414. Available from: https://doi.org/10.1016/j.ajps.2015.06.001. [193] M.G. Sankalia, R.C. Mashru, J.M. Sankalia, V.B. Sutariya, Reversed chitosan-alginate polyelectrolyte complex for stability improvement of alpha-amylase: optimization and physicochemical characterization, Eur. J. Pharm. Biopharm. (Internet, cited 2012 Apr 17). 65 (2007) 215–232. Available from: http://www.ncbi.nlm.nih.gov/pubmed/16982178. [194] A. Sood, R. Panchagnula, Design of controlled release delivery systems using a modified pharmacokinetic approach: a case study for drugs having a short elimination half-life and a narrow therapeutic index, Int. J. Pharm. (Internet, cited 2012 Mar 19). 261 (1–2) (2003) 27–41. Available from: http://linkinghub.elsevier.com/retrieve/pii/ S0378517303002679. [195] Y. Cao, L. Huang, J. Chen, J. Liang, S. Long, Y. Lu, Development of a controlled release formulation based on a starch matrix system, Int. J. Pharm. (Internet, cited 2012 Apr 17). 298 (1) (2005) 108–116. Available from: http://www.ncbi.nlm.nih. gov/pubmed/15905051. [196] C. Ferrero, D. Massuelle, E. Doelker, Towards elucidation of the drug release mechanism from compressed hydrophilic matrices made of cellulose ethers. II. Evaluation of a possible swelling-controlled drug release mechanism using dimensionless analysis, J. Control. Release (Internet, cited 2012 Mar 28). 141 (2) (2010) 223–233. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19766681. [197] M.A. Holgado, A. Iruin, J. Alvarez-Fuentes, M. Ferna´ndez-Ar
evalo, Development and in vitro evaluation of a controlled release formulation to produce wide dose interval morphine tablets, Eur. J. Pharm. Biopharm. (Internet, cited 2012 Apr 17). 70 (2) (2008) 544–549. Available from: http://www.ncbi.nlm.nih.gov/pubmed/18588973. [198] M. Kopcha, K.J. Tojo, N.G. Lordi, Evaluation of methodology for assessing release characteristics of thermosoftening vehicles, J. Pharm. Pharmacol. (Internet). 42 (11) (1990) 745–751. Available from: http://www.ncbi.nlm.nih.gov.ezproxy.lib.utexas.edu/pubmed/ 1982296. [199] I. Antal, R. Zelko´, N. Roczey, J. Plachy, I. Ra´cz, Dissolution and diffuse reflectance characteristics of coated theophylline particles, Int. J. Pharm. 155 (1997) 83–89.

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CHAPTER 13 Nanocarriers loaded with nutraceuticals and bioactive ingredients [200] R.W. Baker, H.S. Lonsdale, in: A.C. Tanquary, R.E. Lacey (Eds.), Controlled Release of Biologically Active Agents, Plenum Press, New York, 1974. [201] K.H. Ramteke, P. Dighe, A.R. Kharat, S. Patil, Mathematical models of drug dissolution: a review, Sch. Acad. J. Pharm. 3 (5) (2014) 388–396. [202] T. Terada, M. Tagami, T. Ohtsubo, Y. Iwao, S. Noguchi, S. Itai, Sustained-release microsphere formulation containing an agrochemical by polyurethane polymerization during an agitation granulation process, Int. J. Pharm. (Internet). 509 (1–2) (2016) 328–337. Available from: https://doi.org/10.1016/j.ijpharm.2016.05.061. [203] J. Siepmann, F. Siepmann, Mathematical modeling of drug delivery, Int. J. Pharm. (Internet). 364 (2) (2008) 328–343. Available from: http://www.ncbi.nlm.nih.gov/pubmed/ 18822362. [204] E. Acosta, Testing the effectiveness of nutrient delivery systems, in: Delivery and Controlled Release of Bioactive in Foods and Nutraceuticals, Woodhead Publishing Limited, 2008, pp. 53–106. [205] M.A. Augustin, L. Sanguansri, Challenges in developing delivery systems for food additives, nutraceuticals and dietary supplements, in: Encapsulation Technologies and Delivery Systems for Food Ingredients and Nutraceuticals, Elsevier Masson SAS, 2012, pp. 19–48. [206] A. Mackie, Interaction of food ingredient and nutraceutical delivery systems with the human gastrointestinal tract, in: Encapsulation Technologies and Delivery Systems for Food Ingredients and Nutraceuticals, Elsevier Masson SAS, 2012, pp. 49–70.

Further reading [207] C. Wandrey, A. Bartkowiak, S.E. Harding, Materials for encapsulation. in: N.J. Zuidam, V.A. Nedovi
c (Eds.), Encapsulation Technologies for Active Food Ingredients and Food Processing, Springer, New York, NY, 2010, pp. 31–100, https://doi.org/ 10.1007/978-1-4419-1008-0_3.

CHAPTER

Multifunctional drinks from all natural ingredients

14

R. Dorothya, K Sneka Lathab, RM Joanyc, T Sasilathaa, Susai Rajendranb, Gurmeet Singhd, S. Senthil Kumarane a

Department of EEE, AMET University, Chennai, India bCorrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India cDepartment of ECE, Sathyabama University, Chennai, India dPondicherry University, Puducherry, India eSchool of Mechanical Engineering, VIT University, Vellore, India

Chapter outline 14.1 Introduction ..................................................................................................414 14.1.1 Remarkable reasons for drinking fresh fruit juice daily .................. 415 14.1.2 Healthiest beverages we should be drinking ................................. 416 14.1.3 Most unhealthy beverages to be avoided ...................................... 416 14.1.4 We can drink green juice every day .............................................. 417 14.1.5 Drinks we can have besides water ............................................... 417 14.1.6 Natural ingredients in energy drinks ............................................ 417 14.1.7 Fresh natural drinks for everybody ............................................... 417 14.1.8 Smoothies ................................................................................. 417 14.1.9 Goodness and badness of coffee ................................................. 418 14.1.10 Coffee and digestion ................................................................ 418 14.1.11 Decaffeinated coffee is harmful ................................................ 418 14.1.12 Prebiotic foods ........................................................................ 418 14.1.13 Probiotics ............................................................................... 418 14.2 Recent trends on multifunctional drinks from natural ingredients .....................418 14.2.1 Tannins from Trapa taiwanensis hulls .......................................... 419 14.2.2 Categorization of food ingredients ............................................... 419 14.2.3 “Ultraprocessed foods” for the youth ........................................... 420 14.2.4 Cosmeceutical effect of ethyl acetate fraction of Kombucha tea ..... 420 14.2.5 Functional drinks made from ginger extracts ................................ 421 14.2.6 Cocoa- and carob-based drink powders from foam mat drying ........ 421 14.2.7 Physicochemical properties of Rambutan (Nephelium lappaceum L.) fruit sweating ..................................... 422

Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00014-6 # 2020 Elsevier Inc. All rights reserved.

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14.2.8 Investigation of functional properties of cocoa waste from concentrated cocoa drink ........................................................... 422 14.2.9 Analysis of the lobbying arguments and tactics of stakeholders in the food and drink industries .................................................. 423 14.2.10 Nonnutritive sweeteners possess a bacteriostatic effect and alter gut microbiota in mice ...................................................... 423 14.2.11 Alginate as a functional food ingredient ..................................... 424 14.2.12 A sour milk beverage ................................................................ 424 14.2.13 Antimicrobial evaluation of Foeniculum vulgare leaves extract ingredient of ethiopian local liquor ............................................ 425 14.2.14 Influence of spices on the content of fluoride and antioxidants in black tea infusions ............................................................... 426 14.2.15 Antiaging effects of guarana (Paullinia cupana) in Caenorhabditis elegans ............................................................ 426 14.2.16 Influence of dried apple powder additive on physical-chemical and sensory properties of yoghurt .............................................. 427 14.2.17 Analysis of natural carbonated drinks ........................................ 427 14.2.18 Influence of a microencapsulated Amazonic natural ingredient with potential interest as a functional product ............................ 428 14.2.19 Modern technologies in beverage processing .............................. 428 14.2.20 Homogenization and physical properties of model coffee creamers stabilized by quillaja saponin ...................................... 429 14.3 Conclusion ....................................................................................................429 References ............................................................................................................430

14.1 Introduction Apart from water, we take many juices from natural resources and also synthetic juices. Natural juices are nontoxic and good for health. Caffeine, present in coffee, usually boosts energy, alertness, and athletic performance. Many researches have been carried out on multifunctional drinks from natural ingredients, plant-based prebiotic foods, and plant-based probiotic foods [1–22]. Prebiotic ingredients include inulin, fructo-oligosaccharides, galacto-oligosaccharides, mannan-oligosaccharides, and polydextrose that are used in a wide range of beverages as they have multifunctional properties that most prominently improve digestive health. They also preserve intestinal flora and simulation of intestinal transit, modify colonic microflora to prevent diarrhea and constipation, help eliminate glucose and cholesterol, support the immune system, and contribute to controlling obesity. Additionally, they decrease the risk of osteoporosis, reduce blood pressure, and act as an anticarcinogen. The need for functional beverages is increasing across the globe. Beverages are the most active functional foods among the functional food category. This is due to the convenience of the product type, ease of distribution, easy refrigeration to improve shelf life, and availability in different packaging sizes and shapes, and they also have a

14.1 Introduction

high potential to incorporate desirable nutrition ingredients. The diverse types of commercially obtainable products include dairy-based beverages, fruit and vegetable beverages, and sports drinks. Probiotics are good bacteria, living in our gut. Prebiotics are the food for the good bacteria (Table 14.1, Figs. 14.1 and 14.2). Previous chapter focuses on the “Antioxidant-loaded nanocarriers for drinks.” In the following parts, multifunctional drinks from natural ingredients are presented.

14.1.1 Remarkable reasons for drinking fresh fruit juice daily Drinking a glass of fresh juice every morning helps in detoxifying our body. If we do not have a fruit every day, the easiest alternative is to make fresh juice a part of your diet. Nevertheless, we must stay away from those packaged fruit juices by all means. A glass of fresh juice contains all the enzymes, minerals, and vitamins that are easily Table 14.1 Probiotic foods and prebiotic foods.

FIG. 14.1 Plant-based probiotic foods.

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FIG. 14.2 Plant-based prebiotic foods.

absorbed in our body. The regular intake of fresh fruit juice keeps us away from ailments. Enriched with vitamins a glass of juice promotes weight loss. Juice is a good way to prevent food cravings. However, anything in excess is harmful! We must avoid consuming fruit juices as it contains sugar, which could increase our blood sugar levels if consumed excessively.

14.1.2 Healthiest beverages we should be drinking There are five healthy beverages we should drink daily. They include water, green tea, hibiscus tea, water with apple cider vinegar, and white tea. There are hundreds of reasons why turmeric may be the world’s most important herb. The extract of turmeric improves memory apart from curing most of the diseases known including HIV and AIDS. It is interesting to state that there are 70 health reasons to eat more flaxseed.

14.1.3 Most unhealthy beverages to be avoided The following beverages are considered unhealthy, and it is recommended that they may be avoided: • • • • • • • •

Juice and juice drinks: Juice can spoil your diet. Sports drinks: Most people do not need sports drinks. Soda: Soda is a definite no-no. Zero-calorie beverages: Stay away from diet beverages. Energy drinks: Energy drinks are bad news. Purchased smoothies: A store-bought smoothie is a no-go. Flavored coffee drinks. Cocktails.

14.1 Introduction

14.1.4 We can drink green juice every day It is best to include both drinks and whole food vegetables into your diet. To the question “Can we drink green juice every day?”, the answer is a big yes. Drinking green juice every day is an easy way to replenish the nutrients our immune system needs.

14.1.5 Drinks we can have besides water When we have diabetes, we can drink the following, apart from water. No doubt, water is the perfect drink. We can take chocolate milk. This treat may remind us of the school lunchroom, but it is a good calcium-rich choice for grown-ups as well. We can take sweet tea, orange juice, chai latte, lemonade, hot chocolate, or apple cider.

14.1.6 Natural ingredients in energy drinks The following ingredients are found in energy drinks. Caffeine is present in small quantities. Caffeine may boost energy, alertness, and athletic performance. Other items present are ginseng, vitamins, sugar, taurine, green tea extract, guarana, and green coffee extract.

14.1.7 Fresh natural drinks for everybody Fresh natural drinks are useful for everybody. Let us consider ginger fizz with mint. It can be prepared as follows. Peel and grate a piece of fresh ginger and squeeze the juice from it with your hands. Next, let us consider Greek yoghurt. Peel one cucumber and extract about 100 mL of juice from it. Other natural drinks are green tea with lemon and honey, spicy tomato juice, watermelon juice with chocolate, apricot juice with cinnamon, and almond milk with honey.

14.1.8 Smoothies Smoothie is a smooth, thick drink made with pur
eed fresh fruit and yoghurt, ice cream, or milk or with blended vegetables. It is bad to drink smoothies every day. Some smoothies, especially the ones you make at home from whole fruits and vegetables, are high in vitamins, minerals, and many other beneficial nutrients, but for losing weight, smoothies tend not to be a good choice because they are liquids. The addition of sweet ingredients is what usually makes a smoothie unhealthy. By choosing your fruits wisely; skipping the store-bought fruit juice; and adding protein, plant milks, and good fats, you can make healthy smoothies that are just as tasty and satisfying as the sugary, high-calorie ones.

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14.1.9 Goodness and badness of coffee Like so many foods and nutrients, too much coffee can cause problems, especially in the digestive tract, but studies have shown that drinking up to four 250 mL cups of coffee per day is safe, but sipping coffee in reasonable amounts just might be one of the healthiest things you can do.

14.1.10 Coffee and digestion Contractions in the colon push contents toward the rectum, which is the final section of our digestive tract. Research has shown that caffeine makes the colon 60% more active than water and 23% more active than decaf coffee (coffee without caffeine). Nonetheless, studies have shown that decaf coffee can also stimulate the urge to poop.

14.1.11 Decaffeinated coffee is harmful Decaffeinated coffee may be harmful to heart. Decaffeinated coffee may have a harmful effect on the heart by increasing the levels of a specific cholesterol in the blood, researchers say. Their explanation is that caffeine-free coffee is often made from a type of bean with a higher fat content.

14.1.12 Prebiotic foods Prebiotics are fibers and natural sugars that stimulate the good bacteria in the gut. Many prebiotic foods are appropriate for vegans and people on other diets to eat. These foods include almond, chicory, garlic, and chickpea. Prebiotic fiber is a nondigestible part of foods like bananas, onions and garlic, Jerusalem artichoke, the skin of apples, chicory root, and beans. Prebiotic fiber goes through the small intestine undigested and is fermented when it reaches the large colon.

14.1.13 Probiotics When it comes to digestive concerns, probiotics can actually offer relief a little more quickly. One might have more regular bowel movements and fewer irritable bowel syndrome (IBS)-type symptoms after taking probiotics for a short period of time. The IBS symptoms include pain and cramping, diarrhea, constipation, changes in bowel movements, gas, and bloating.

14.2 Recent trends on multifunctional drinks from natural ingredients Many researches have been undertaken on multifunctional drinks from natural ingredients. They are presented in this section.

14.2 Recent trends on multifunctional drinks from natural ingredients

14.2.1 Tannins from Trapa taiwanensis hulls The fruit and hulls of the water caltrop (Trapa taiwanensis Nakai) are used as hepatoprotective herbal tea ingredients in Taiwan. The strength of hydrolyzable tannins in herbal drinks has rarely been reported. In their study, two hydrolyzable tannins, tellimagrandin II (TGII) and 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranose (PGG), were isolated by Wang et al. [1] from water caltrop hulls. The stability of the two compounds was evaluated by treatment with various pH buffer solutions, simulated gastric fluid and intestinal fluid, different temperatures, and photoirradiation at 352 nm in different solvents. Results are given that TGII and PGG were more stable in a pH 2.0 buffer solution (with 91.88% remaining) and in a water solution with 352 nm irradiation (with 95% remaining). TGII and PGG were more secure in methanol or ethanol solutions (with >93.69% remaining) than in an aqueous solution (with .05) the physicochemical properties of the sweatings. High concentrations of sugars and organic acids allow the sweatings to have a balance of sweet and sour taste, and they are suitable to be used in the manufacture of syrup, soft drinks, jam, jelly, marmalade, and vinegar [7].

14.2.8 Investigation of functional properties of cocoa waste from concentrated cocoa drink Recovery of phenolic content from waste is one of the main concerns for any possible applications. This work of Hussain et al. [8] was carried out to investigate and explore the functional characteristics of cocoa waste (CW) from espresso cocoa production. The total phenolic content (TPC), total flavonoid content (TFC) 2,2-diphenyl-1-picrylhydrazyl scavenging assay (DPPH), and metal (Fe2+) chelating activity were calculated. Parameters include water holding capacity (WHC), oil holding capacity (OHC), swelling capacity (SWC), and proximate compositions; total dietary fiber (TDF), insoluble dietary fiber (IDF), and soluble dietary fiber (SDF) were also found. As a reference, spent coffee ground (SCG) was also measured under all parameters. Two solvents, ethanol and water, were used to extract the bioactive

14.2 Recent trends on multifunctional drinks from natural ingredients

compounds from CW. The ethanol-CW extract was created to contain significantly the highest (P < .05) TPC and TFC with 52.3 mg GAE/mL sample and 84.36 mg quercetin/mL sample, respectively. This was correlated to its high (P < .05) antioxidant activities in DPPH (IC50 3.58
0.07 mg/mL) and metal chelating activity (IC50 2.32
0.09 μg/mL). Positive correlations ranging from r2 ¼ 0.82 to r2 ¼ 0.98 were recognized between the phytochemicals and antioxidant activities of all extracts. All samples displayed significantly (P < .05) high WHC and SWC, in relevance to their high (P < .05) TDF, which were over 60% of 100 g dry matter. CW exhibited significantly high (P < .05) IDF/SDF ratio, in contrast to SCG with also high protein content of 13%. In this study, Hussain et al. [8] have indicated that CW has a potential as a source of natural antioxidant and phytochemical in functional food growth and intermediate food ingredient [8].

14.2.9 Analysis of the lobbying arguments and tactics of stakeholders in the food and drink industries € Tselengidis and Ostergren [9] have investigated the lobbying actors of the food and drink industry (FDI), their web lobbying arguments used in the sugar taxation debate, and the tactics deployed when facing legislative restrictions on their products to curb the burden of noncommunicable diseases in Europe. A stakeholder investigation was performed to identify the FDI’s actors lobbying against sugar taxation within the EU Platform for Action on Diet, Physical Activity and Health in December 2015. Qualitative content examination was applied to assess the FDI’s web lobbying claims related to three main concepts (sugar as a product, sugar’s association with noncommunicable diseases, and sugar taxation) guided by a framework for corporate political activity. The website content of a front organization and six FDI lobbyists was analyzed. Some new strategies emerged alongside known corporate strategies (“questioning the effectiveness of regulation and promoting benefits of a withdrawal,” “promoting sugar’s good traits and shift the blame away from it,” and “establishing relationships with trade unions”). The lobby tactics were comparable with those earlier applied by the tobacco industry in Europe, although the argument that sugar is a natural ingredient in many foods was unique to the FDI. The observed tactics and arguments presented by the FDI in opposition to sugar taxation have striking similarities with those previously used by the tobacco industry. An improved understanding of the stakeholders’ mandate and resources and their most important tactics will strengthen the position of public health experts when debating sugar taxation with the FDI, which may add to improving population health [9].

14.2.10 Nonnutritive sweeteners possess a bacteriostatic effect and alter gut microbiota in mice Nonnutritive sweeteners (NNSs) are extensively used in various food products and soft drinks. There is growing proof that NNSs contribute to metabolic dysfunction and can influence body weight, glucose tolerance, appetite, and taste sensitivity.

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Several NNSs have also been revealed to have major impacts on bacterial growth both in vitro and in vivo. Here, we studied the influence of various NNSs on the growth of the intestinal bacterium, Escherichia coli, and the gut bacterial phyla Bacteroidetes and Firmicutes, the balance between which is associated with gut health. Wang et al. [10] have established that the synthetic sweeteners acesulfame potassium, saccharin, and sucralose all exerted strong bacteriostatic effects. Wang et al. [10] have got that rebaudioside A, the active ingredient in the natural NNS stevia, also had similar bacteriostatic properties, and the bacteriostatic effects of NNSs varied among different E. coli strains. In mice fed a chow diet, sucralose increased Firmicutes, and Wang et al. [10] have obtained a synergistic effect on Firmicutes when sucralose was provided in the context of a high-fat diet. In review, it is observed that NNSs have direct bacteriostatic effects and can change the intestinal microbiota in vivo [10].

14.2.11 Alginate as a functional food ingredient Alginate is a natural polymer extracted from brown seaweed. Its exclusive biophysical properties are highly valuable in the development of functional food products. As a food ingredient the applications of alginate are based on three main characters: thickening, gelling, and film forming. Qin et al. [11] have explained the main physical, chemical, and biological effects of alginate as a functional food ingredient and summarized the historical and recent developments of the functional benefits of various types of food and drink products containing alginate. The exceptional gelling abilities at low temperatures alongside good heat stability make alginate ideal for use as thickeners, stabilizers, and restructuring agents. In addition, alginate is increasingly used in a myriad of newer applications, from encapsulating active enzymes and live bacteria to acting as the carrier for protective coatings of prepacked, cut, or prepared fruits and vegetables. With definite chemical and biological modifications to alter its dimensions and properties, there are possibilities of novel applications of specific alginates in the food industry that have high bioactivities at low concentrations. Consequently, food producers, alginate producers, and food and nutrition scientists will need to work more closely together to develop more novel uses of alginate in the food and drink industry [11].

14.2.12 A sour milk beverage Nagovska et al. [12] have discovered that, to improve consistency during storage of sour milk beverages, it is necessary to ensure the binding of free moisture through the use of natural stabilizers, thickeners, and the substances that perform a similar function. Among many tested ingredients of this group of substances, the stabilizing methods based on natural components of plant and animal origin were chosen for implementation and preferred for usage. Analysis of the information sources gives lack of data on the use of wheat bran in the technologies of sour milk beverages. That

14.2 Recent trends on multifunctional drinks from natural ingredients

is why there is an objective need to make new kinds of sour milk beverages, specifically kefir, with the use of wheat bran. Consumption of such functional products guarantees the elimination of malnutrition and replenishment of the organism with required components. The influence of wheat bran on quality indicators of the sour milk beverage was investigated. It was found that the sour milk drink with wheat bran with fat content of 2.5% by physical and chemical indicators meets the requirements of standard DSTU 4417:2005 Kefir. Technical specifications. Studying the organoleptic indicators of the beverage using wheat bran revealed its clean sour milk taste and smell. As reported the total amount of amino acids in the drink with wheat bran increased by 15.08%, the amount of necessary amino acids by 10.57%, and that of nonessential amino acids by 18.24%. The identified changes in the amino acid composition of the drink with wheat bran indicate that the use of wheat bran in manufacturing sour milk beverages allows to increase their nutritional and biological value of the protein component. The sour milk drink with wheat bran is a medical and prophylactic product because it contains dietary fibers, which are a valuable energy additive [12].

14.2.13 Antimicrobial evaluation of Foeniculum vulgare leaves extract ingredient of ethiopian local liquor Medicinal plants are of great interest to the researcher in the field of biotechnology, as natural products, together with medicinal plants, account 25% of prescribed drugs. Plants are sources of fragrances, drink colors, and flavors in several countries including Ethiopia. All parts of Foeniculum vulgare were usually used as antispasmodic, aromatic, carminative, digestive, galactagogue, stomach, and kidney ailment. F. vulgare leaf extract was studied for its phytochemicals and antimicrobial effects. The petroleum ether, CHCl3, CHCl3/CH3OH (1:1), and CH3OH crude extract were subjected to phytochemicals screening test, which exposed that it is rich in any primary and secondary metabolites such as steroids, tannins, flavonoids, cholesterol, terpenoids, saponins, phenols, cardiac glycosides, carbohydrates, and proteins. The necessary oil of the plant leaves was investigated by GC-MS and was found to have (64.92%) anethole as a major constituent followed by (30.88%) estragole and (3.21%) fenchyl acetate. The crude extracts, oil, and the isolated mix were tested against four bacterial species (gram-negative bacteria, E. coli and Shigella flexneri, and gram-positive bacteria, Staphylococcus aureus and Streptococcus pyrogenes) and two fungal species (Fusarium oxysporum and Aspergillus niger) using paper disc diffusion method. Tests of antimicrobial activity showed that all crude extracts and isolated pure compound were lively against all the tested bacterial and fungal species. Nevertheless the hydrodistillation extract was found to have no antibacterial activity toward the tested bacterial species but active against the two fungal species, and thus the present report supported the conventional claims of the plant [13].

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14.2.14 Influence of spices on the content of fluoride and antioxidants in black tea infusions Black tea and spices are a precious source of natural bioactive ingredients. One of the factors affecting the antioxidant status and the content of bioactive compounds in tea infusions is their enrichment with various additives and consumption in the form of blends and chemical composition. Additives to teas are often herbs, flowers, and spices. Janda et al. [14] have determined the content of polyphenols, fluoride, and antioxidant potential in infusions to examine the correlation between these parameters. In addition, the effect of added spices on the aforementioned parameters was confirmed. The research material consisted of infusions of black tea with the addition of spices prepared at 80°C. The content of polyphenols and antioxidant activity were determined by the spectrophotometric method. Fluoride concentrations were measured by the potentiometric method. The maximum content of polyphenols occurred in tea with cloves (345.49 ppm), while the lowest was in the infusion with cardamom (343.59 ppm). The strongest antioxidant properties were illustrious in the infusion with cloves (79.11% of DPPH inhibition), and the weakest ones were found in the infusion with turmeric (72.7% of DPPH inhibition). The highest content of fluoride was in the tea with turmeric (0.809 mg/L), while the lowest one appeared in the infusion with cloves (0.358 mg L1). The result indicates that the spices and fluoride content affect the antioxidant properties of infusions and their content of polyphenols. High fluoride levels reduce the antioxidant capacity of tea infusions [14].

14.2.15 Antiaging effects of guarana (Paullinia cupana) in Caenorhabditis elegans Guarana (Paullinia cupana) is usually ingested by people in the Amazon region and is a key ingredient in different energy drinks consumed worldwide. Extension in longevity and low prevalence of chronic age-related diseases has been connected to habitual intake of guarana. Antiaging potential of guarana was also demonstrated in Caenorhabditis elegans; nevertheless the mechanisms involved in its influence are not clear. Arantes et al. [15] have investigated the putative pathways that regulate the effects of guarana ethanolic extract (GEE) on life span using C. elegans. The major known longevity pathways were analyzed through mutant worms and RT-qPCR assay (DAF-2, DAF-16, SKN-1, SIR-2.1, and HSF-1). The possible participation of purinergic signaling was also investigated. This study has established that GEE acts through antioxidant activity, DAF-16, HSF-1, SKN-1 pathways, and human adenosine receptor ortholog (ADOR-1) to extend life span. GEE also downregulated SKN-1, DAF-16, SIR-2.1, and HSP-16.2 in 9-day-old C. elegans, which might reflect less need to activate these protective genes due to direct antioxidant effects. Our results contribute to the comprehension of guarana effects in vivo, which might be helpful to prevent or treat aging-associated disorders, and also recommend purinergic signaling as a plausible therapeutic target for longevity studies [15].

14.2 Recent trends on multifunctional drinks from natural ingredients

14.2.16 Influence of dried apple powder additive on physicalchemical and sensory properties of yoghurt The benefit of dried fiber products is that they are perceived by consumers as natural ingredients in fruits and vegetables. Due to this positive perception, the dried powdered fruit has become increasingly significant as a potential source of fiber for the food industry. The purpose of the study was to find out the effect of applying two doses of dried apple powder—1.5% and 3%—on the total acidity, color, syneresis, texture, and sensory features of yoghurt shaped using an apple powder and the estimation of its quality after 7 days of refrigerated storage. The assessed parameters were as follows: pH, total acidity, syneresis, color parameters (L*, a*, and b*), texture with the use of TPA, and sensory features. The dried apple powder additive considerably affected the reduction of syneresis, acidity increase, and yoghurt hardness reduction. Adding dried apple powder in the amount of 1.5% caused the release of whey to be 4% reduced, while growing the quantity of dried apple powder added to 3% caused the syneresis effect to be reduced as much as c. 6.5%. Moreover, increasing the amount of dried apple powder added from 1.5% to 3% significantly increased the intensity of red and yellow color of the yoghurt. The dried apple powder added caused the yoghurt color to adversely grow dark, but at the same time, it imparted a distinct apple flavor and aroma to the yoghurt, the ones preferred by consumers. The dried apple powder added caused the intensity of the milky-cream flavor to be reduced and the intensity of the sour taste of yoghurt to increase. The dried apple powder did not impart any foreign aroma and foreign flavor to the yoghurt, and this additive was apparent by consumers as a natural, fruity flavor. The dried apple powder is a natural fruit product that should be applied to a greater extent in the production of milk-based drinks owing to its high fiber content, optimal sugar content, and very good taste and aroma. This additive fits well into the “clean label” trend in the organic food segment [16].

14.2.17 Analysis of natural carbonated drinks Choi and Hong [17] performed a research to examine the market for natural carbonated drinks with health-improving lifestyle. A survey was conducted enrolling adults over the age of 20 years. Data analysis of 544 valid samples was performed using SPSS 18.0. Participants were segmented based on their degree of interest in a health-improving lifestyle as follows: “high-interest,” “mid-interest,” and “low-interest.” In the “high-interest” cluster, 30.4% was in their 50s, and 74.1% had a higher monthly income than ￦4,000,000. The “high-interest” group showed statistically significant higher fulfillment and importance in all items except price appropriateness compared with other clusters. Safe ingredients, taste, price appropriateness, and expiration date indication showed high importance, while low sugar, thirst relief, natural flavor, and coloring showed high satisfaction by written order. The “high-interest” cluster also showed statistically significant higher purchase intention for natural carbonated drinks than other clusters. In the “mid-interest”

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cluster, 27.4% was in their 50s, and 26.9% was in their 20s. In addition, 58.9% had a monthly income higher than ￦4,000,000. The “mid-interest” cluster considered safe ingredients to be the most significant. In the “low-interest” cluster, 28.9% was in their 20s, and 32.0% was in their 30s. 51.6% of all “low-interest” participants earned more than ￦4,000,000. This group considered price appropriateness to be the most significant. The study shows that it can be used as baseline data to establish market segmentation strategies for natural carbonated drinks according to healthimproving lifestyle [17].

14.2.18 Influence of a microencapsulated Amazonic natural ingredient with potential interest as a functional product Dı´az et al. have [18] focused on fruit pulp microencapsulation by spray-drying with the aim to develop the basic ingredient for an instant beverage with phytochemicals from asai (Euterpe precatoria) and flavor compounds from cupuassu (Theobroma grandiflorum), both outstanding Amazonic fruits. Asai has a high content of polyphenols but a very mild flavor, while cupuassu has favorable sensory properties, making it an interesting food matrix to innovate and incorporate phytochemicals for producing functional drinks. Therefore asai and cupuassu were mixed and microencapsulated with the aim to preserve bioactive compounds from the oxidation process and obtain a stable natural ingredient. The main objective compounds to protect were asai anthocyanins. Prior to mixing, cupuassu pulp was treated with pectolytic and amylolitic enzyme applications to improve pulp extraction from the seed-pulp mass and with filtration to divide fruit remaining starch and insoluble fiber fraction; this process increased the sugar and organic acid contents as compared with the untreated pulp, improving flavor. The asai pulp was filtered; this procedure reduced the insoluble fiber and fat with no significant influence on the polyphenols. Microencapsulation by spray-drying was performed with 35% maltodextrin as the wall material. The asaı´:cupuassu ratio (1:1) was fixed to enhance the asai flavor without masking it, following a previous sensory analysis performed on pulp mixtures. The obtained powder was assessed for antioxidant properties (DPPH ∗ ABTS ∗ inhibition) and total polyphenols, total anthocyanins, and sugar and organic acid contents. The nutritional examination of the obtained ingredient showed a high potential for a functional food product [18].

14.2.19 Modern technologies in beverage processing In the 21st century the beverage industry has faced challenging times with new developments as a response to consumers’ demand to attain new and more differentiated food products of high superiority and with guarantee of safe values. The nonalcoholic beverage market itself is composed of various segments, such as juices, vegetable blends, teas, dairy drinks, and another beverage. The drinks of today need specific facts of exotic ingredients, novel processing techniques, and a variety of functional ingredients. With such diverse group of commodities, a considerable

14.3 Conclusion

research effort in the application of innovative technologies for manufacturing beverages has been completed in the recent years. Numerous new processing technologies have emerged with regard to beverage processing. A number of novel thermal and nonthermal technologies have become available to ensure high-quality food retention levels while extending the products’ shelf life. On the other hand the increasing demand for natural ingredients, improving health and appearance, is also attracting nonalcoholic beverages as the fastest rising segment on the functional food market. Consumer demands “miracle beverages” that are not only safe and nutritious but also natural, economical to manufacture, convenient, great tasting, environmentally friendly, and improve health and well-being. Advances in food science and technology are presenting exciting opportunities for the beverages sector [19].

14.2.20 Homogenization and physical properties of model coffee creamers stabilized by quillaja saponin There is an increasing demand for the use of natural ingredients in food developed. Chung et al. [20] have utilized a natural emulsifier, quillaja saponin (1%), to fabricate nondairy model creamer emulsions (containing 10% medium chain triglycerides oil). Varying homogenization conditions, ranging from a high-shear mixer to passing through a microfluidizer at 20,000 psi, were applied to fabricate emulsions. The influence of particle size on the appearance, tristimulus color coordinates, and electrical characteristics of the model creamers and white coffee drinks were investigated. The standard droplet size varied from 0.2 to 16 μm. All model creamers had whitish milk-like appearance, and the white coffee solutions had light brown color. All systems were actually stable except for the systems with largest oil droplets (1.8 and 16 μm), which had creaming. The lightness, L* (whiteness), of the model creamer and the white coffee increased with decreasing oil droplet size as smaller droplets scatter more light. Reducing the oil droplet size led to lower zeta potential (from 73 to 54 mV) due to lesser negative charge group accumulated on the interfacial layer of the droplets. The oil droplets were also established to be stable to aggregation in hot acidic coffee solutions prepared using model hard water. In general, this study found that oil droplets stabilized with natural plant-based surfactant have potential for application in liquid coffee creamers, and their stability and whitening power were dependent on the droplet size [20].

14.3 Conclusion Manufacturers of food and beverage products look toward original natural ingredient solutions such as prebiotics for the health-conscious population without compromising on product integrity, quality, and nutritional value. The awareness about prebiotics has attracted much attention. This has stimulated scientific and industrial interest. Prebiotics have been used as food ingredients to maintain or restore a “healthy” gut microflora. They also have other health benefits. For illustration, they

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help prevent inflammatory bowel disease, diarrhea, colon cancer, and obesity and promote calcium absorption and bone health, skin health, and immunity [21, 22]. To maintain good health, it is essential to drink probiotic and prebiotic juices of fruits and other food items, which are available in plenty in nature.

References [1] C.-C. Wang, H.-F. Chen, J.-Y. Wu, L.-G. Chen, Stability of principal hydrolysable tannins from Trapa taiwanensis hulls, Molecules 24 (2) (2019) 365. [2] J. Aschemann-Witzel, P. Varela, A.O. Peschel, Consumers’ categorization of food ingredients: do consumers perceive them as ‘clean label’ producers expect? An exploration with projective mapping, Food Qual. Prefer. 71 (2019) 117–128. [3] A. Aguirre, M.T. Borneo, S. El Khori, R. Borneo, Exploring the understanding of the term “ultra-processed foods” by young consumers, Food Res. Int. 115 (2019) 535–540. [4] N. Pakravan, E. Mahmoudi, S.-A. Hashemi, M. Rahimzadeh, V. Mahmoodi, Cosmeceutical effect of ethyl acetate fraction of Kombucha tea by intradermal administration in the skin of aged mice, J. Cosmet. Dermatol. 17 (6) (2018) 1216–1224. [5] S. Yasni, Development technology of functional drinks made from ginger extracts as products model for developing small-medium enterprises, IOP Conf. Ser. Earth. Environ. Sci. 196 (1) (2018) 012017.  [6] M. Benkovi
c, K. Radi
c, D. Vitali Cepo, M. Morkunaite, S. Srecec, Production of cocoa and carob-based drink powders by foam mat drying, J. Food Process Eng. 41 (6) (2018) 12825. [7] K.F. Chai, N.M. Adzahan, R. Karim, Y. Rukayadi, H.M. Ghazali, Effects of fermentation time and turning intervals on the physicochemical properties of Rambutan (Nephelium lappaceum L.) fruit sweating, Sains Malays. 47 (10) (2018) 2311–2318. [8] N. Hussain, B.A.P. Agus, A.Z.M. Dali, H.W. Teng, Determination of functional properties of cocoa waste from concentrated cocoa drink, J. Food Meas. Charact. 12 (3) (2018) 2094–2102. € [9] A. Tselengidis, P.O. Ostergren, Lobbying against sugar taxation in the European Union: analysing the lobbying arguments and tactics of stakeholders in the food and drink industries, Scand. J. Public Health 47 (5) (2019) 565–575. [10] Q.-P. Wang, D. Browman, H. Herzog, G. Gregory Neely, Non-nutritive sweeteners possess a bacteriostatic effect and alter gut microbiota in mice, PLoS One 13 (7) (2018) e0199080. [11] Y. Qin, J. Jiang, L. Zhao, J. Zhang, F. Wang, Applications of alginate as a functional food ingredient, in: Biopolymers for Food Design, Academic Press, 2018, pp. 409–429. [12] V. Nagovska, Y. Hachak, B. Gutyj, O. Bilyk, N. Slyvka, Influence of wheat bran on quality indicators of a sour milk beverage, East.-Eur. J. Enterp. Technol. 4 (11–94) (2018) 28–34. [13] M. Seid, A. Dekebo, N. Babu, Phytochemical investigation and antimicrobial evaluation of Foeniculum vulgare leaves extract ingredient of ethiopian local liquor, J. Pharm. Nutr. Sci. 8 (1) (2018) 20–28. [14] K. Janda, K. Jakubczyk, K. Woz´niak, J. Wolska, I. Gutowska, Does the addition of spices change the content of fluoride and antioxidants in black tea infusions? J. Elem. 23 (2) (2018) 599–609.

References

[15] L.P. Arantes, M.L. Machado, D.C. Zamberlan, M. Aschner, F.A.A. Soares, Mechanisms involved in anti-aging effects of guarana (Paullinia cupana) in Caenorhabditis elegans, Braz. J. Med. Biol. Res. 51 (9) (2018) e7552. [16] A. Znamirowska, D. Kalicka, M. Buniowska, P. Roz˙ek, Effect of dried apple powder additive on physical-chemical and sensory properties of yoghurt (Zywnosc. Nauka. Technologia. Jakosc), Food Sci. Technol. Qual. 25 (2) (2018) 71–80. [17] H.-R. Choi, W.S. Hong, Market segmentation analysis of natural carbonated drinks according to health improving lifestyle, J. Korean Soc. Food Sci. Nutr. 46 (12) (2017) 1539–1549. [18] R.O.S. Dı´az, M. Carrillo, M.S. Herna´ndez, M. Lares, J.P. Ferna´ndez-Trujillo, Development of a microencapsulated Amazonic natural ingredient with potential interest as a functional product, Acta Hortic. 1178 (2017) 123–128. [19] I. Aguilo´-Aguayo, L. Plaza, Innovative Technologies in Beverage Processing, John Wiley & Sons Ltd, 2017, pp. 1–314. [20] C. Chung, A. Sher, P. Rousset, D.J. McClements, Influence of homogenization on physical properties of model coffee creamers stabilized by quillaja saponin, Food Res. Int. 99 (2017) 770–777. [21] https://apfoodonline.com/industry/prebiotic-ingredients-multifunctional-and-natural/. [22] https://www.google.com/search?q¼prebiotic+ingredients+in+foods&oq¼prebiotic+ ingredients&aqs¼chrome.1.69i57j0l5.8844j0j8&sourceid¼chrome&ie¼UTF-8.

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Applications of nanotechnology for hygiene of drinks

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Nanodevices for the detection of pathogens in milk

15

P. Priyadharshini, V. Amalnath, Anand Babu Perumal, J.A. Moses, C. Anandharamakrishnan Computational Modeling and Nano Scale Processing Unit, Indian Institute of Food Processing Technology (IIFPT), Ministry of Food Processing Industries, Government of India, Thanjavur, Tamil Nadu, India

Chapter outline 15.1 Introduction ..................................................................................................436 15.2 Microbial contamination in milk .....................................................................436 15.3 Major pathogens in milk ................................................................................437 15.3.1 Listeria monocytogenes .............................................................. 437 15.3.2 Salmonella ................................................................................ 437 15.3.3 E. coli ....................................................................................... 438 15.3.4 Campylobacter species ............................................................... 438 15.3.5 Shigella species ........................................................................ 438 15.3.6 Brucella species ........................................................................ 438 15.4 Conventional methods used for detection of pathogen in milk ..........................439 15.4.1 Polymerase chain reaction .......................................................... 439 15.4.2 Loop-mediated isothermal amplification ...................................... 441 15.4.3 Nucleic acid sequence-based amplification .................................. 442 15.4.4 Flow cytometry .......................................................................... 443 15.4.5 Spectroscopy techniques ............................................................ 444 15.4.6 Multisensory techniques ............................................................. 446 15.4.7 Biosensors ................................................................................ 447 15.5 Limitations in the conventional methods .........................................................448 15.6 Nanotechnology in pathogen detection ...........................................................449 15.7 Existing nanodevices in pathogen detection in milk ........................................450 15.7.1 Immunosensing methods combined with nanotechnology .............. 450 15.7.2 Nanoparticle-based detection ..................................................... 456 15.7.3 Sers-based detection ................................................................. 459

Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00015-8 # 2020 Elsevier Inc. All rights reserved.

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15.7.4 Sensor-based detection .............................................................. 463 15.8 Conclusion ....................................................................................................463 References ............................................................................................................463

15.1 Introduction Milk is a colloidal matrix produced from the mammary glands of mammals. India is the world’s largest milk-producing country followed by the United States, China, Pakistan, and Brazil [1]. Generally, cattle contribute about 85% of the total milk followed by buffalo (11%), goat (2.3%), sheep (1.4%), and camel (0.2%) [2]; among these, cow milk is widely consumed by people worldwide. The chemical composition of milk includes water (87%), lactose (4.5%), protein (3%), fat (3%–4%), minerals (0.8%), and vitamins (0.1%) [3]. Water: Water is the major component present in the milk. It has substantial effect on the physical, chemical, and microbiological stability of milk [4]. Fat: Milk fat contains triglycerides of about 98% with traces of free fatty acids, mono- and diglycerides, phospholipids, sterol, and hydrocarbons. Protein: Milk protein is mainly classified into soluble and insoluble protein. Soluble protein (e.g., whey protein) contributes to about 20% of the milk protein. Insoluble protein (e.g., casein) contributes to about 80% of the total milk protein. Consuming 1 L of milk can provide 32 g of protein [5]. Lactose: Lactose is a disaccharide found in milk that provides primary source of energy required for infants [4]. Vitamins: Milk contains fat-soluble and water-soluble vitamins. Fat soluble includes A, D, E, and K, and water soluble includes B complex such as thiamine and riboflavin [3]. Minerals: Calcium and phosphorus are the most important minerals in milk [3]. The constituents of milk are derived from the blood of animals. The species, breed, stage and time of lactation, interval between lactation, season, feed, and condition of animal are some of the factors affecting the composition of milk. Milk is an easily digestible substance, and it provides the required nutrients for the growth and development of neonates.

15.2 Microbial contamination in milk Milk is a highly perishable substance. The high nutritive value, neutral pH, and high water activity are the factors that facilitate the growth of microorganism in milk [6]. Milk may get contaminated either when the milk comes in direct contact with the contaminated sources in the environment or from the udder of an infected animal [7]. To reduce the contamination of microorganisms in raw milk, it is necessary to maintain a clean environment around the animals and to maintain sanitation during the milking process [8].

15.3 Major pathogens in milk

The microorganism that contaminates the raw milk may be of spoilage-causing microbes and pathogenic microorganism. Spoilage-causing microorganisms in dairy products results undesirable odors, physical defects, and secondary metabolite toxicity. Spoilage-causing microorganisms in milk include Pseudomonas fluorescens, Pseudomonas fragi (Gram-negative bacteria), and Bacillus cereus and Staphylococcus aureus (Gram-positive bacteria). Mostly, spoilage-causing microorganism is present in the area of milking, equipment used for milking, and the processing area. So, contamination of milk by microorganism is unavoidable. To avoid contamination, milkers have to do some control measures to maintain the milking environment free of spoilage-causing microorganisms. The beneficial microorganism present in milk includes Lactococcus, Streptococcus, Propionibacterium, and Leuconostoc. The most common pathogenic microorganisms present in milk include Listeria species, Salmonella species, Escherichia coli, Campylobacter species, Shigella species, and Brucella species [9]. Heat sterilization is the most effective means of destroying the pathogens in milk. Since it involves more loss of nutrients, it cannot be applied for milk and dairy products. So, pasteurization is used to minimize the pathogenic contamination in milk. Pasteurization is done in three ways. Vat pasteurization involves heating of milk at 63°C for 30 min. High-temperature short-time (HTST) pasteurization involves heating of milk at 72°C for 15 s. Ultrapasteurization involves heating of milk at 138°C for 2 s. Immediately after pasteurization the milk is cooled to 10°C to avoid the loss of nutrients [8]. Unpasteurized milk has the maximum risk of pathogenic contamination [10].

15.3 Major pathogens in milk 15.3.1 Listeria monocytogenes Listeria monocytogenes is a Gram-positive bacterium that belongs to Listeriaceae family. It is a nonspore forming bacterium, and it is facultative anaerobic in nature. It is a rod-shaped bacterium that requires an optimum temperature of 30–37°C, pH of 4–9.5 and water activity of 0.90–0.97 for its growth. L. monocytogenes is responsible for a number of diseases including meningitis, encephalitis, septicemia, and intrauterine infections. Infective dose of this species includes approximately 1000 cells [11].

15.3.2 Salmonella Salmonella is a Gram-negative bacterium that belongs to a family Enterobacteriaceae. It is a rod-shaped bacterium that grows at an optimum temperature of about 32– 37°C. It is a motile, facultative anaerobic bacterium that does not form spores [12]. Salmonella bacteria are responsible for salmonellosis in human. Symptoms of salmonellosis include nausea, vomiting, fever, headache, diarrhea, abdominal cramps, and blood in feces [13]. The major species associated with Salmonella bacteria include

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S. dublin, S. typhimurium, S. muenster, S. give, S. heiselberg, and S. enteritidis [14]. As per Centers for Disease Control and Prevention (CDC), Salmonella is the leading cause of foodborne infections in the United States with approximately 1.2 million illness and 450 thousand deaths per year. In Africa, 3.4 million illnesses and 680 thousand deaths have been reported due to nontyphoid Salmonella in 2010 [15].

15.3.3 E. coli E. coli are Gram-negative bacteria that belong to Enterobacteriaceae family. E. coli appear as single straight rods that grow at an optimum temperature of about 35–40°C. The optimum pH and water activity for the growth of E. coli include 6–7 and 0.995, respectively. They are facultative anaerobic in nature. E. coli may contain specific strain of “o-lipopolysaccharide” antigens on its cell wall and flagella and H antigens. E. coli 0157:H7 is the most important serotype of E. coli. They may be motile or nonmotile [12].

15.3.4 Campylobacter species Campylobacter species are Gram-negative bacteria that belong to the Campylobacteraceae family. They are curved, rod-shaped bacteria that grow at an optimum temperature of 37–42°C. They are motile that requires an optimum pH of 6.5–7.5 for its growth. They can be inactivated at temperature above 48°C since they are susceptible to heat and they are also susceptible to antibiotics such as erythromycin and gentamicin [13]. They are responsible for a number of diseases including acute enteritis, extraintestinal infections such as bacteremia, abscess, meningitis, and postinfectious complications [16].

15.3.5 Shigella species Shigella species are Gram-negative bacteria that belong to Enterobacteriaceae family. They are facultatively anaerobic in nature, and they stay alive for 72 h at 4°C in milk and multiply rapidly at 15–37°C. They are rod-shaped bacteria that are responsible for the disease shigellosis/bacillary dysentery. Symptoms of shigellosis/bacillary dysentery include fever, bloody and mucopurulent stool emission that lead to dehydration, and intestinal discomfort caused by cramps and tenesmus. The four major species of Shigella include S. dysenteriae, S. boydii, S. flexneri, and S. sonnei. Infective dose of this bacterium is 10–100 bacterial cells. They are nonmotile in nature [17].

15.3.6 Brucella species Brucella is a Gram-negative bacterium that belongs to the Brucellaceae family. It is a nonspore forming bacterium that needs optimum incubation temperature of about 36–38°C. They occur as coccobacilli or short rods that grow at an optimum pH of

15.4 Conventional methods used for detection of pathogen in milk

about 6.6–7.4 pH. They are facultatively intracellular, oxidative, and aerobic in nature. Brucella abortus, B. melitensis, B. suis, and B. canis are the four major species of Brucella. B. abortus is responsible for brucellosis in cattle. Temperature, moisture, pH, time of maturation of milk products, storage conditions, and effect of bacteria are some of the factor affecting the tenacity of the Brucella in milk. They are sensitive to heat and fairly sensitive to ionizing radiation. They are nonmotile in nature [18].

15.4 Conventional methods used for detection of pathogen in milk Traditionally, preparation of growth media, plating, and biochemical screening are used in the pathogen detection. But, nowadays, many methods have been developed such as Polymerase chain reaction (PCR), Loop-mediated isothermal amplification (LAMP), Nucleic acid sequence-based amplification (NASBA), flow cytometry, spectroscopic techniques, multisensory techniques, and biosensors (Table 15.1).

15.4.1 Polymerase chain reaction It was developed by Dr. Kary Mullis in the 1980s. He was awarded Nobel Prize in 1993 for the development of PCR. PCR is a technology that is capable of producing exponentially growing amount of DNA from the tiny starting material [21].

15.4.1.1 Components of PCR (1) DNA template: DNA template is defined as the DNA containing the target sequence. (2) DNA polymerase: It is the enzyme that can synthesize new DNA complementary strands to target sequence. (3) Primers: They are starting point for DNA polymerase to create new complementary strand. (4) Nucleotide: They are the single units of bases A, T, G, and C that serve as building blocks for the new DNA strands.

15.4.1.2 Steps in PCR Each cycle of PCR comprises three steps: (1) Denaturation: The temperature of the reaction mixture is brought to about 94–96°C to unwind the double helix of DNA by breaking down the hydrogen bond.

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Table 15.1 The pathogen detection in milk by the conventional methods. Methods

Bacteria detected

Detection limit

Reference

10 CFU/mL 0.1 CFU/mL 1 CFU/mL

[6]

Nucleic acid amplification methods PCR

LAMP

NASBA

a) L. monocytogenes b) M. avium subsp. paratuberculosis c) E. coli O157 a) L. monocytogenes b) Salmonella a) S. aureus

186 CFU/mL 35 CFU/ 250 mL 10 CFU/mL

Flow cytometry Immunoreagent-based detection

a) L. monocytogenes b) S. typhimurium

Stain-based detection

a) E. coli b) S. aureus a) Pseudomonas spp

104 cells/mL 104 cells/mL

a) L. monocytogenes

108 CFU/mL

[20]

a) P. aureofaciens b) B. cereus a) P. fluorescens

103 CFU/mL 104 CFU/mL 102 and 1010 CFU/mL

[6]

Nucleic acid hybridization-based detection

2
106 cells/mL 103–2
104 cells/mL

[19]

Spectroscopy methods Raman spectroscopy Multisensor methods Electronic nose Electronic tongue CFU, colony-forming unit.

(2) Primer annealing: The reaction mixture is cooled to about 45–65°C. This results in the combination of forward and reverse primers with opposite strands of DNA through complementary base pairing. (3) Extension: The reaction mixture is again heated to 72°C. Here the polymerase enzyme binds with primer-template hybrid complex to form a new complementary strand by using the free nucleotides in the reaction mixture. After extension the reaction again returns to denaturation step, and the PCR cycle continues. Each cycle doubles the amount of DNA, resulting in an exponential increase of DNA [22].

15.4 Conventional methods used for detection of pathogen in milk

15.4.1.3 Phase of PCR PCR amplification comprises four phases. (1) Initial amplification: It is a stage where amplification process begins. (2) Exponential amplification: The number of amplicons gets doubled in each cycle. (3) Leaving off: The amplification process slows down due to limiting amount of substrates, DNA polymerase activity, and the accumulation of amplicons. (4) Plateau: In this stage the amplification stops [22]. Wang et al. [23] reported on the capabilities of quantitative PCR (qPCR) and digital droplet PCR (ddPCR) in detecting the S. typhimurium in milk. Both qPCR and ddPCR are highly sensitive and efficient in pathogen detection. Compared with qPCR, ddPCR is suitable for detecting and analyzing the zero tolerance bacteria, and it can save up to 2 h compared with qPCR. The quantitative PCR have been used in the detection of S. aureus in the bovine milk sample with a detection limit of 1.04
101 CFU/mL [24].

15.4.2 Loop-mediated isothermal amplification LAMP is a rapid, accurate, and sensitive means of detecting pathogens in milk [25]. LAMP was developed by Notomi in 2000. The first one who used LAMP to detect bacteria was Maryuma [26].

15.4.2.1 Principle The target gene was amplified at 65°C within 1 h. One type of polymerase enzyme and four primer sets were employed to amplify the target gene. The four primers in LAMP include forward inner primer, backward inner primer, forward outer primer, and backward outer primer (Table 15.1). They can amplify the target gene sequence of six specific region [26]. A multiplex LAMP assay has been established for the simultaneous detection of Salmonella and Shigella in milk [27].

15.4.2.2 Advantages of LAMP over PCR (1) (2) (3) (4) (5)

LAMP technology is 10–100 times more sensitive than PCR. It consumes less time than PCR, which can save about 1 h. It is cost efficient. Operation is simple and easy. In LAMP, we can avoid the separate bacterial cultivation and slow nucleic acid extraction step, which is done during PCR.

In LAMP, we can observe the absence of target gene through naked eye by producing the white precipitate of magnesium pyrophosphate [26].

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15.4.3 Nucleic acid sequence-based amplification It was first developed by J Compton in 1991. It is also known as “self-sustained sequence replication (3SR)” or “transcription-mediated amplification (TMA)” [28]. It is an isothermal amplification technique that is designed for the amplification of RNA. It uses three enzymes: avian myeloblastosis virus reverse transcriptase (AMV RT), RNase H, and T7 DNA-dependent RNA polymerase (DdRp) (Fig. 15.1). It also contains two oligonucleotide primers complementary to RNA

P1

Reverse transcriptase

RNAse H

P2

Reverse transcriptase

T7 RNA polymerase P2

Reverse transcriptase

Reverse transcriptase

RNAse H P1

FIG. 15.1 Schematic representation of NASBA process [28].

15.4 Conventional methods used for detection of pathogen in milk

region: deoxyribonucleotide triphosphate for the activity of AMV RT, ribonucleoside triphosphate for the activity of AMV RT, and ribonucleoside triphosphate for the activity of T7 RNA polymerase [29]. In NASBA the amplification process takes place in two phases: noncyclic phase and cyclic phase [29]. In the noncyclic phase a double-stranded DNA product with a reorganization sequence for T7 RNA polymerase is obtained. RNA is exponentially produced in the cyclic phase [29]. It usually takes place at 41°C. NASBA is a highly sensitive method with a detection limit of 1 CFU/mL without the need of preenrichment culture [6]. O’Grady et al. [30] used NASBA to amplify transfer-messenger RNA (tmRNA) to detect S. aureus in 1-mL raw milk. Twenty milk samples (comprise both naturally and artificially contaminated milk samples) from a farm in Ireland were tested for the detection of S. aureus by culture-based method and NASBA method. S. aureus was detected efficiently by means of real-time NASBA assay. The detection limit of artificially contaminated milk samples is 1–10 CFU/mL [30]. A molecular beacon-based real-time NASBA assay has been developed for the rapid detection of the Mycobacterium avium subsp. paratuberculosis in water and milk [31].

15.4.4 Flow cytometry Flow cytometry was first described in 1947. But, until 1953, flow cytometry based on fluid sheath flow principle was not described, and only core sheath-based flow cytometry was explained. The flow cytometry consists of three components: a fluid component, optical component, and an electronic component for data collection and analysis [19]. The liquid sample is allowed to flow in between the sheath fluid (fluid component). At particular point the flow of sample is interrupted by laser (optical component); the light gets scattered in two directions, forward and at right angle; and the stained cells may emitted fluorescent signals at different wavelength (Fig. 15.2). Data collection from the individual stained cells and degree of uptake of particular strain helps in the distinguishing the cells into discrete subpopulation [32].

15.4.4.1 Main advantages (1) (2) (3) (4)

Rapid data generation takes place within 1–2 min. Minimal sample volume is required. Less labor and space when compared with plating technique. Stain multiplicity is available to examine different aspect of cell viability [32].

15.4.4.2 Application Rapid flow cytometry enumeration method for bacteria in UTH-treated milk/raw milk was reported by Gunasekera, Attfield, Veal, and Veal [33]. Flow cytometry on enumeration of total bacterial count in raw bulk tank milk was reported by Holm [34]. In addition, Flint et al. [35] reported on the development of flow

443

Fl uo d e re s te ce ct n or ce

CHAPTER 15 Nanodevices for the detection of pathogens in milk

Sheath fluid Mixture of cells Dichroic mirrors Core

Fl uo de res te ce Fl ct n uo or ce de res te ce ct n or ce s S de ca ide te tte ct r or

Flow

444

Forward scatter detector

Light source(s)

Cell sorter

FIG. 15.2 Schematic representation of flow cytometry [19].

cytometry-based assay for total viable count of bacteria in milk and milk products [32] (Table 15.1). Flow cytometry was used to detect and distinguish Gram-positive and Gramnegative bacteria in mastitis milk samples [36]. Flow cytometry combined with immunomagnetic separation was used in the detection of L. monocytogenes [37].

15.4.5 Spectroscopy techniques Spectroscopy is the study of interaction between electromagnetic radiation and matter [38]. Numerous spectroscopy techniques are available for the pathogen detection. Among all those, Raman spectroscopy is the most promising technique for detecting the pathogens in milk.

15.4 Conventional methods used for detection of pathogen in milk

15.4.5.1 Raman spectroscopy Raman spectroscopy was named after C.V. Raman, who invented it. C.V. Raman and K.S. Krishnan were the first to publish paper on this technique. Raman spectroscopy is a scattering technique based on Raman effects. It is a nondestructive tool as there is no need for the addition of dyes or labels for bacterial identification. Compared with Fourier transform infrared spectroscopy, Raman spectroscopy can efficiently collect the inelastically scattered light of molecules by laser excitation [39].

Principle The principle of the technique is based on the inelastic scattering of the incident radiation by its interaction with molecules. When a monochromatic radiation is allowed to strike the entire sample, it interacts with the sample molecules and gets scattered in all direction. Much of the scattered radiation has a frequency that is equal to that of the frequency of the incident radiation, this constitutes Rayleigh scattering. Only a small fraction of the scattered radiation has a frequency different from that incident radiation. This corresponds to Raman scattering when the frequency of the incident radiation is higher than the frequency of the scattered radiation, it corresponds to stokes lines. When the frequency of the incident radiation is lower than the scattered radiation, it corresponds to the anti-Stokes lines [38]. The scattered radiation having a different frequency from that of the incident light is used in the construction of Raman spectrum [40]. This Raman spectrum displays the molecular composition of the target sample. The application of Raman spectroscopy is in the detection of the food borne pathogens in milk and meat samples, which was reported by Meisel and coworkers [41] (Table 15.1). Confocal micro-Raman spectroscopy combined to chemometric analysis have been developed for the detection of L. monocytogenes in milk with a detection limit of 108 CFU/mL [39].

15.4.5.2 Fourier transform infrared spectroscopy (FTIR spectroscopy) Molecular spectroscopy was introduced in the 1960s. At that time, frequent use of spectroscopy method was not practical due to the limitation in instruments and lack of integrated computational analysis. In the late 1980s and 1990s, with the arrival of FTIR spectroscopy and computational analysis, FTIR methods were reintroduced by Naumann et al. [42]. The infrared region of the electromagnetic spectrum has three regions, namely, near infrared (NIR), mid infrared (MIR), and far infrared (FIR). Most commonly MIR is used in analysis, since all the molecules have characteristic absorbance frequency and primary molecular vibrations in this range when absorbing some specific wavelength so that vibration such as stretching, contracting, and bending takes place in the chemical bonds of the material. The basic components of FTIR spectroscopy include an interferometer commonly Michelson interferometer with a beam splitter, stationary mirror, and a moving mirror. Based on the interference pattern, interferometer measures the wavelength of light accurately (Fig. 15.3). When the IR radiation is passed through the sample, some of the radiation is absorbed, and the rest of them is transmitted to the detector. The interferogram from

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CHAPTER 15 Nanodevices for the detection of pathogens in milk

Computer

Electronic links Moving mirror

Beamsplitter Stationary mirror

Sample

Detector

Interferometer Source

FIG. 15.3 Schematic representation and components present in FTIR spectrometer [43].

all the different wavelength of IR is measured by means of detector. A mathematical function Fourier transform can convert the interferogram to an IR spectrum (interferogram is intensity vs time spectrum, whereas IR spectrum is intensity vs frequency spectrum). The advantages of IR spectroscopy include the following: i. Very little sample is required for spectral acquisition. ii. The bacterial cell remains intact during the analysis [43]. Nicolaou et al. [44] reported on the use of FTIR spectroscopy and Raman spectroscopy in the rapid detection, enumeration, and growth interaction of S. aureus and Lactococcus lactis spp. in milk.

15.4.6 Multisensory techniques Multisensors such as electronic nose and electronic tongue are used in pathogen detection. This is a secondary method where the collected data have been analyzed separately.

15.4.6.1 Electronic nose The concept of artificial nose system was proposed at the University of Warwick by Persaud [45]. The term “artificial or electronic nose” appeared at the beginning of 1990s [46]. E-nose is an instrument that is designed for mimicking the sense of smell of biological system by using the nonselective sensors that interact with odor molecules. The working principle of e-nose is based on the operation principle of human nose.

15.4 Conventional methods used for detection of pathogen in milk

The e-nose tried to mimic the human nose’s structure and tried to reduce its disadvantage. The components of e-nose include sensors, electronic components, pumps, air conditioner, flow controller, software for the hardware monitoring, data processing, and statistical analysis. The volatile compounds in the sample are analyzed by the mean of sensor array, and the generated signals were sent to data recognition system that stimulates brain function [47]. Gardner and Bartlett [48] defined e-nose as “an instrument that comprises an array of electrochemical sensor with partial specificity and an appropriate pattern recognizing sample or complex odors.” In another study, Magan et al. [49] reported on the use of e-nose (which contains 14 conducting polymer sensors) for the detection of volatile profiles in the uninoculated or inoculated (with bacteria or yeast) skimmed milk and the separation of spoiled and unspoiled milk by using discriminant function analysis (DFA) technique. Similarly, Haugen et al. [50] reported on the use of e-nose in the correlation between secondary volatile metabolites from milk and the microbes that produced those compounds.

15.4.6.2 Electronic tongue According to IUPAC, electronic tongue is defined as “a multisensory system, which consists of number of low selective sensors and uses advanced mathematical procedures for signal processing based on pattern recognition and/or multivariate data analysis” [51]. Electronic tongue sensor has features differ from traditional chemical sensors. Electronic tongue sensors provide overall selectivity that will be obtained as global information about the solution. The global information is applied as digital fingerprint. The electronic tongue is not specific and nonselective in nature [52]. A voltammetric electronic tongue has been developed to monitor the quality and storage time of unsealed pasteurized milk [52].

15.4.7 Biosensors Biosensors are defined as the analytical devices that transform the biological response to a measurable signal by incorporating the biorecognition element and the biological material a physical transducer. The biorecognition elements include antibodies, enzymes, and nucleic acid. Transducer is used in the transformation of biorecognition signals into measurable signals based on signal transduction; transducers are classified into three basic group such as electrochemical, optical, and mass sensitive. In the analysis of milk sample, electrochemical transducers are most commonly used followed by optical and mass-sensitive transducers [53].

15.4.7.1 Electrochemical biosensors These are less expensive and not affected by the milk turbidity. Based on parameters like current, impedance, potential, and conductance, electrochemical biosensors are further classified into amperometric, impedimetric, potentiometric, and conductometric biosensors, respectively [53]. Li
ebana et al. [54] reported on the

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CHAPTER 15 Nanodevices for the detection of pathogens in milk

amperometric biosensors for the detection of S. aureus in 50 μL of raw milk with a detection limit of 1 CFU/mL and assay time of 2 h. Laczka et al. [55] reported on the amperometric biosensors for the detection of E. coli in 100 μL of milk sample with a detection limit of 100 cells/mL and assay time of 1 h. ´ vila et al. [56] reported on the amperometric biosensors for the detection of SalA monella spp. in 5 mL of skimmed milk with a detection limit of 0.1 CFU/mL and assay time of 50 min.

15.4.7.2 Optical biosensors Compared with other biosensors, optical biosensors are considered highly sensitive. But the factors such as milk turbidity and protein fouling limit its application in milk testing. Zhu et al. [57] reported on the optical biosensors combined with cellphone technology for the detection of E. coli 0157:H7 in fat-free milk with a detection limit of 10 CFU/mL and assay time for an hour. Waswa et al. [58] reported on the two label-free surface plasmon resonance optical biosensors for the detection of S. enteritidis and E. coli in 2 mL of 23–25 cells/mL and assay time of 1 h.

15.4.7.3 Mass-sensitive biosensors In the analysis of milk sample, mass-sensitive transducers such as piezoelectric quartz crystal microbalance and magnetoelastic transducers were applied to the biosensors. Lakshmanan et al. [59] applied the magnetoelastic biosensors for the detection of S. typhimurium in 1 mL of fat-free milk with a detection limit of 5
103 CFU/mL and assay time of 20 min. In addition, Shen et al. [60] reported on the detection of E. coli 0157:H7 in 10 μL of sterilized milk sample by means of piezoelectric quartz crystal microbalance with a detection limit of 53 CFU/mL and assay time of 4 h.

15.5 Limitations in the conventional methods Though the conventional methods have many advantages in the pathogen detection, it also has some limitations that facilitate the search of an alternative approach for the pathogen detection. Such limitations are as follows. Colony-counting method is considered to be the gold standard technique for pathogen detection, but nowadays, it is not widely used because it is a time-consuming process (requires long incubation time) and it requires more labor [61]. PCR: PCR is a highly sensitive method. It can even detect a single gene. But it is unable to distinguish viable and nonviable cells, because both the viable and nonviable cells have the amplification target [61]. Another disadvantage is that the assay volume of PCR is on microliters, so the sample has to concentrate to microliter range. And PCR is not suitable for onsite diagnosis, since it requires technical equipment and laboratory step [62].

15.6 Nanotechnology in pathogen detection

NASBA: Though the amplification reaction in NASBA is isothermal (41°C), a simple melting step is required prior to amplification reaction to enable the annealing of the primers to the target. The specificity of NASBA depends on thermolabile enzymes. The length of the RNA target sequence to be amplified should be in the range of “120–250 nucleotides.” If the length is beyond the range, the amplification process is not efficient [28]. Flow cytometry: Though it is a sensitive and rapid technique for the pathogen detection, its uses have been limited in food industry due to its high cost and signal-to-noise ratio in complex matrix [6]. Raman spectroscopy: It is an inefficient process since the signals produced in the Raman spectroscopy are very weak. So, it is difficult to detect the trace amount of pathogens. FTIR spectroscopy: The major limitation of FTIR is that the environment conditions around the FTIR instrument may cause variation in the spectra, so it requires background and multiple scans of the same sample. Additionally, it requires bacterial separation/purification steps since misinterpretation of results take place in complex sample like mixture of bacteria [43]. LAMP: The LAMP process is susceptible to contamination that results in the false-positive rate of detection [26].

15.6 Nanotechnology in pathogen detection Nanotechnology is an emerging technology in agriculture and food science field. National Nanotechnology Initiative (NNI) states that “nanotechnology is the understanding and control of matter at dimensions between 1 and 100 nm, where unique phenomena enable novel applications.” Nanotechnology is a multidisciplinary technological and scientific field. In food sector the market of nanotechnology-derived products has reached US $1 billion, and it is expected to rise more than US $20 billion in next decade. In recent years, nanotechnology seems to be an alternative technology in pathogen detection due to the limitations in conventional methods. Novel system that provides specific and selective information on presence and amount of pathogens has been developed by combining nanotechnology with bioscience, electronics, and software engineering. Nanotechnology is a fascinating approach in pathogen detection because of the unique properties of nanoscale materials. The properties include larger surface area, enhanced surface reactivity, quantum confinement effect, enhanced electrical conductivity, and enhanced magnetic properties. Since the surface-to-volume ratio of nanostructure is larger than that of the macroscopic-sized materials, it is possible to detect very small amount of analyte with increased sensitivity. The basic properties of different kinds of nanostructures facilitate its use in pathogen detection and integration with biomolecules. Besides their use in bioassay and sensors, several kinds of nanostructure fabricated from materials such as copper, silver, and gold/tellurium in various configuration including nanotubes, nanowires, nanoparticles, and nanoarrays have been developed for

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CHAPTER 15 Nanodevices for the detection of pathogens in milk

their antimicrobial activity. Employing nanomaterials in biosensors offer high level of sensitivity and other novel attributes. Recently the development of sensors and smart labels as indicators of toxicity and multifunctional system that combines the capture detection and inactivation function has become an important area in research [63, 64].

15.7 Existing nanodevices in pathogen detection in milk 15.7.1 Immunosensing methods combined with nanotechnology 15.7.1.1 Dual mode immunochromatographic assay Lateral flow assay (LFA) is a paper-based method that is rapid, easily operated, and less costly. But it has some limitations like low accuracy and poor sensitivity. So horseradish peroxidase (HRP-based colorimetric method is used to retain the essential benefits of LFA. Nanoenzyme-based LFA was used in the detection of Campylobacter jejuni. They can be detected based on color signal and SERS signal. For C. jejuni detection, gold nanorod coated with platinum (AUNR@Pt), a kind of nanoenzyme with peroxidase-like catalytic activity, was introduced into the LFA. When compared with gold nanorods, platinum-coated gold nanorods have significantly higher catalytic activity and strong electromagnetic field (Table 15.2).

Working When the sample is added to the sample pad, the analytes (C. jejuni) will flow in the longitudinal direction under capillary force. When the C. jejuni comes to the conjuration pad, they will be conjugated with signal probes (i.e., anti-C. jejuni monoclonal antibody and four MBA modified AUNR@Pt nanoparticles). Then, complex comes to the flow through the nitrocellulose membrane. When they pass through the detection area, the immobilized capture probe anti-C. jejuni polyclonal antibody will capture them to form sandwich-like structure. Here, bimetallic platinum-coated gold nanorods were used as signal amplifier instead of gold nanoparticles to make the captured signal probes more readable. Now, C. jejuni can be detected in two modes: When 3,30 ,5,50 -tetramethylbenzidine (TMB) and H2O2 are added to the sandwich structure, a rapid catalytic oxidation reaction will take place, producing a blue color, which is stronger than the red color produced by the AUNPs. The generation of blue color indicates the presence of C. jejuni in the milk sample. When laser beam of about 785 nm is passed through the sandwich stricture, a SERS spectrum was obtained as a result of laser excitation (Fig. 15.4). Hence, we can quantify the concentration of C. jejuni in the sample based on the color intensity and SERS signal intensity, since both of them were proportional to

Table 15.2 shows the pathogen detection in milk by the nanoparticle. S. no

Methods

Type of nanomaterial used

Detected pathogen

1

Dual mode immunochromatographic assay

Gold nanorod coated with platinum

C. jejuni

2

Nanoporous membrane-based immunochromatic assay Disposable amperometric immunosensing Colloidal gold immunochromatic strip with double monoclonal antibodies Recombinase polymerase amplification combined with unmodified gold nanoparticles Combining biofunctional magnetic nanoparticles and ATP bioluminescence Combining amino-modified magnetic nanoparticles and polymerase chain reaction

Alumina nanoporous membrane Gold nanoparticles Gold nanoparticles

8

Nuclear magnetic resonance by using biofunctional magnetic nanoparticles

9

10

SERS-based lateral flow strip combined with recombinase polymerase amplification SERS combined with aptasensor

11

SERS integrated with LAMP

12

Quartz crystal microbalance aptasensor

3 4 5

6

7

Detection limit

Reference [65]

E. coli

50 CFU (for SERS mode) 75 CFU (for color mode) tea.

Tafel plots — Ni / Ti alloy immersed in various test solutions –6.0 –6.5

log(Current/A)

–7.0 –7.5 –8.0 –8.5

(B) Tea

–9.0

(C) AS + Tea –9.5

(A) AS –10.0 –10.5 –0.10

–0.20

–0.30

–0.40

–0.50

–0.60

–0.70

–0.80

Potential (V)

FIG. 16.2 Polarization curve of Ni-Ti alloy immersed in various test solution (A) AS, (B) tea, and (C) AS + tea.

475

Table 16.2 Corrosion parameters of alloys immersed in artificial saliva (AS) in the absence and presence of tea obtained by polarization study. Metal

System

Ecorr (mV/SCE)

bc (mV/decade)

ba (mV/decade)

LPR (Ω cm2)

Icorr (A/cm2)

Ni-Ti alloy

AS Tea AS + tea AS Tea AS + tea AS Tea AS + tea AS Tea AS + tea AS Tea AS + tea

375 553 638 327 601 863 343 391 657 454 672 701 501 466 622

178 143 129 187 161 111 193 176 162 164 174 158 158 171 142

302 359 310 221 233 195 302 273 296 317 230 255 315 212 280

3,277,970 342,232 4,697,814 1,334,259 5,274,536 2,079,093 2,638,460 2,101,240 964,243 4,177,473 2,313,358 3,946,969 7,636,745 14,851,401 8,369,940

1.488  108 1.300  107 8.420  109 3.301  108 7.857  109 1.478  108 1.942  108 2.210  108 4.717  108 1.127  108 1.860  108 1.074  108 5.775  109 2.767  109 4.890  109

22 Carat gold

SS 18/8 alloy

SS316L alloy

Thermoactive alloy

16.3 Results and discussion

This study reveals that people having orthodontic wire made of Ni-Ti alloy need not hesitate to take tea. The reason for it is because in this medium the corrosion resistance of Ni-Ti alloy increases.

16.3.1.2 22 Carat gold The polarization curves of 22 carat gold, immersed in various test solutions, are shown in Fig. 16.3. The corrosion parameters, namely, corrosion potential (Ecorr), Tafel slopes (bc,ba), corrosion current (Icorr), and linear polarization resistant (LPR), are given in Table 16.2. When 22 carat gold is immersed in artificial saliva (AS), LPR value is 1,334,259 ohm cm2. The corrosion current (Icorr) is 3.301108 A/cm2. The corrosion potential (Ecorr) is 327 mV versus SCE. When 22 carat gold is immersed in tea, LPR value increase from 1,334,259 to 5,274,536 ohm cm2. The corrosion current (Icorr) decreases to 7.857  109 A/cm2. This indicates that 22 carat gold is more corrosion resistant in tea. Further the corrosion potential (Ecorr) value is 601 mV versus SCE. A protective film is formed on the metal surface. When 22 carat gold is immersed in the system consisting of AS +tea, the LPR value decreases to 2,079,093 ohm cm2. The corrosion current increases to 1.478  108 A/cm2 when compared with tea. This indicates that 22 carat gold is less corrosion resistant in AS + tea system than in tea system but more corrosion resistant in AS + tea system than in AS system.

Tafel plots — Gold 22 carat immersed in various test solutions –5.5 –6.0

log(Current/A)

–6.5 –7.0 –7.5 –8.0 –8.5

(B) Tea

–9.0 –9.5

(A) AS

(C) AS + Tea

–10.0 0

–0.10

–0.20

–0.30

–0.40

–0.50

–0.60

–0.70

–0.80

Potential (V)

FIG. 16.3 Polarization curve of 22 carat gold immersed in various test solution (A) AS, (B) tea, and (C) AS + tea.

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CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

Further the corrosion potential shifts to 863 mV versus SCE. Thus the polarization study leads to the conclusion that, when 22 carat gold is immersed in various test solution, the decreasing order of the corrosion resistance of 22 carat gold is as follows: tea > AS + tea > AS. This study reveals that people having orthodontic wires made of 22 carat gold need not hesitate to take tea. The reason for it is because in this medium the corrosion resistance of 22 carat gold increases.

16.3.1.3 SS 18/8 alloy The polarization curves of SS 18/8 alloy, immersed in various test solution, are shown in Fig. 16.4. The corrosion parameters, namely, corrosion potential (Ecorr), Tafel slopes (bc,ba), corrosion current (Icorr), and linear polarization resistant (LPR), are given in Table 16.2. When SS 18/8 alloy is immersed in artificial saliva (AS), LPR value is 2,638,460 ohm cm2. The corrosion current (Icorr) is 1.942  108 A/cm2. The corrosion potential (Ecorr) is 343 mV versus SCE. When SS 18/8 alloy is immersed in tea, LPR value decreases to 2,101,240 ohm cm2. The corrosion current (Icorr) increases to 2.210  108 A/cm2. This indicates that corrosion resistance of SS 18/8 alloy decreases in tea when compared with the saliva system. When SS 18/8 alloy is immersed in AS and tea

Tafel plots — SS18/8 alloy immersed in various test solutions –6.0 –6.4 –6.8

log(Current/A)

478

–7.2 –7.6 –8.0 –8.4 –8.8 –9.2

(A) AS

(B) Tea

(C) AS + Tea

–9.6 –10.0 –0.10

–0.20

–0.30

–0.40

–0.50

–0.60

–0.70

–0.80

Potential (V)

FIG. 16.4 Polarization curve of SS 18/8 alloy immersed in various test solution (A) AS, (B) tea, and (C) AS + tea.

16.3 Results and discussion

system, the LPR value decreases to 964,243 ohm cm2, and the corrosion current (4.717  108 A/cm2) increases. This indicates that corrosion resistance of SS 18/8 alloy decreases in AS + tea system. Thus the polarization study leads to the conclusion that, when SS 18/8 alloy is immersed in various test solution, the decreasing order of corrosion resistance of SS 18/8 alloy is as follows: AS > tea > AS + tea. This study suggests that people having orthodontic wires made of SS 18/8 alloy should avoid taking tea orally. The reason for it is because in this medium the corrosion resistance of SS 18/8 alloy decreases.

16.3.1.4 SS316L alloy The polarization curves of SS316L alloy, immersed in various test solution, are shown in Fig. 16.5. The corrosion parameters, namely, corrosion potential (Ecorr), Tafel slopes (bc,ba), corrosion current (Icorr), and linear polarization resistant (LPR), are given in Table 16.2. When SS316L alloy is immersed in artificial saliva (AS), LPR value is 4,177,473 ohm cm2. The corrosion current (Icorr) is 1.127  108 A/cm2. The corrosion potential (Ecorr) is 454 mV versus SCE. When SS316L alloy is immersed in tea, LPR value decreases to 2,313,358 ohm cm2. The corrosion current (Icorr) increases to 1.860  108 A/cm2. This indicates that corrosion resistance of SS316L alloy decreases in tea system than in AS system.

Tafel plots — SS316L alloy immersed in various test solutions –6.0

log(Current/A)

–7.0 –8.0 –9.0

(C) AS + Tea

–10.0 –11.0 –12.0 –0.10

(A) AS –0.20

–0.30

–0.40

(B) Tea –0.50

–0.60

–0.70

–0.80

Potential (V)

FIG. 16.5 Polarization curve of SS 316L alloy immersed in various test solution (A) AS, (B) tea, and (C) AS + tea.

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CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

When SS316L alloy is immersed in AS and tea system, the LPR value increases to 3,946,969 ohm cm2, and the corrosion current value decreases to 1.074  108 A/cm2. This indicates that corrosion resistance of SS 316L alloy increases in AS + tea system. Thus the polarization study leads to the conclusion that, when SS 316L alloy is immersed in various test solution, the decreasing order of corrosion resistance of SS 316L alloy is as follows: AS > AS + tea > tea. This study suggests that people having orthodontic wires made of SS 316L alloy should avoid taking tea orally. The reason for it is because in this medium the corrosion resistance of alloy SS 316L alloy decreases.

16.3.1.5 Thermoactive alloy The polarization curves of thermoactive alloy, immersed in various test solutions, are shown in Fig. 16.6. The corrosion parameters, namely, corrosion potential (Ecorr), Tafel slopes (bc ¼ cathodic and ba ¼ anodic), linear polarization resistance (LPR), and corrosion current (Icorr), are shown in Table 16.2. When thermoactive alloy is immersed in artificial saliva (AS), linear polarization resistance (LPR) value is 7,936,745 ohm cm2. The corrosion current (Icorr) is 5.775  109 A/cm2. The corrosion potential (Ecorr) is 501 mV versus SCE. When thermoactive alloy is immersed in tea, linear polarization resistance (LPR) value increases from 7,936,745 to 14,851,401 ohm cm2. The corrosion

Tafel plots — Thermoactive alloy immersed in various test

solutions –6.5 –7.0 –7.5

log(Current/A)

480

–8.0 –8.5 –9.0 –9.5

(C) AS + Tea

–10.0

(B) Tea

–10.5

(A) AS

–11.0 0

–0.10

–0.20

–0.30

–0.40

–0.50

–0.60

–0.70

–0.80

Potential (V)

FIG. 16.6 Polarization curve of thermoactive alloy immersed in various test solution (A) AS, (B) tea, and (C) AS + tea.

16.3 Results and discussion

current (Icorr) decreases from 5.775  109 to 2.767  109 A/cm2. This indicates that corrosion resistant of thermoactive alloy increases in tea when compared with that in saliva system. The corrosion potential (Ecorr) value shifts to 466 mV versus SCE. When thermoactive alloy is immersed in AS and tea system, the LPR value further is 8,369,940 ohm cm2, and the corrosion current value is 4.890  109 A/cm2. This indicates the corrosion resistance of thermoactive alloy that further decreases in AS + tea system than in tea system, but not for AS. Thus the polarization study leads to the conclusion that when thermoactive alloy is immersed in various test solution, the decreasing order of corrosion resistance of thermoactive alloy is as follows: tea > AS + tea > AS. This study suggests that people having orthodontic wires made of thermoactive alloy should not hesitate to take tea orally because in this medium the corrosion resistance of thermoactive alloy increases.

16.3.2 AC impedance spectra study AC impedance spectra of Ni-Ti alloy, 22 carat gold, SS 18/8 alloy, SS316L alloy, and thermoactive alloy immersed in AS in the absence and presence of tea have been recorded. The results are presented and discussed in this section.

16.3.2.1 Ni-Ti alloy The AC impedance spectra of Ni-Ti alloy immersed in various test solutions are shown in Figs. 16.7 (Nyquist plot) and 16.8A–C (Bode plots). The corrosion parameters are charge transfer resistance (Rt), double-layer capacitance (Cdl), impedance (log(Z/ohm)), and phase angle given in Table 16.3. When Ni-Ti alloy is immersed in AS, the charge transfer resistance is (Rt) 12,278 ohm cm2. The double-layer capacitance (Cdl) value is 4.1538  1010 F/cm2.

FIG. 16.7 AC impedance spectra for Ni-Ti alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Nyquist plots).

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CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

log (Z/ohm)

4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 51.0 48.0 45.0 42.0 39.0 36.0 33.0 30.0 27.0 24.0 21.0 1.8

2.1

2.4

2.7

3.0

(A)

3.3

3.6

log (Freq/Hz)

log (Z/ohm)

4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

log (Freq/Hz)

–Phase (deg)

482

38.0 36.0 34.0 32.0 30.0 28.0 26.0 24.0 22.0 20.0 18.0 16.0

(B)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.8 AC impedance spectra for Ni-Ti alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Bode plots).

When Ni-Ti alloy is immersed in tea, the Rt value decreases from 12,278 to 9891 ohm cm2. The Cdl value increases from 4.1538  1010 to 5.194  1010 F/cm2. This indicates that corrosion resistance of Ni-Ti alloy further decreases in tea system than in AS.

log (Z/ohm)

16.3 Results and discussion

4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 63.0 60.0 57.0 54.0 51.0 48.0 45.0 42.0 39.0 36.0

(C)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.8, cont’d

Table 16.3 Corrosion parameters of alloys immersed in artificial saliva (AS) in the absence and presence of tea obtained by AC impedance spectra. Nyquist plots

Bode plot

Metal

System

Rt (ohm cm2)

Cdl (F/cm2)

Impedance log (Z/ohm)

Phase angle

Ni-Ti alloy

AS Tea AS +tea AS Tea AS + tea AS Tea AS + tea AS Tea AS + tea AS Tea AS + tea

12,278 9819 56,387 14,014 11,618 51,994 14,789 28,160 6961 12,468 2482 9554 27,941 28,861 28,761

4.1538  108 5.194  1010 2.0803  1010 3.639  1010 4.3897  1010 9.809  1010 3.4484  1010 1.811  1010 7.326  1010 4.0905  1010 2.0541  1010 5.3376  1010 1.8253  1010 1.767  1010 1.773  1010

4.203 4.130 4.094 4.409 4.330 4.088 4.348 4.477 4.196 4.443 4.582 4.128 3.871 4.617 4.259

50.09 36.97 62.68 67.30 49.39 68.05 55.44 33.20 63.51 64.64 41.04 58 58.45 55.34 67.99

22 Carat gold

SS 18/8 alloy

SS 316L alloy

Thermoactive alloy

483

484

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

When Ni-Ti alloy is immersed in AS + tea system, the charge transfer resistance (Rt) value further increases to 56,387 ohm cm2, and double-layer capacitance (Cdl) value decreases to 2.0803  1010 F/cm2. This indicates that the corrosion resistance of Ni-Ti alloy in AS + tea system further increases. Thus AC impedance spectra lead to the conclusion that the corrosion resistance of Ni-Ti alloy immersed in various test solutions decreases in the following order: AS + tea > AS > tea. Thus AC impedance spectra lead to the conclusion that people having orthodontic wires made of Ni-Ti alloy need not hesitate to take tea orally.

16.3.2.2 22 Carat gold The AC impedance spectra of 22 carat gold immersed in various test solutions are shown in Figs. 16.9 (Nyquist plot) and 16.10A–C (Bode plots). The corrosion parameters are given in Table 16.3. When 22 carat gold is immersed in AS, the charge transfer resistance is (Rt) 14014.4 ohm cm2. The double-layer capacitance (Cdl) value is 36.3911  1011 F/cm2. When 22 carat gold is immersed in tea, the Rt value is 11,618 ohm cm2. The Cdl value is 4.3897  1010 F/cm2. The impedance value is 4.330. This indicates that 22 carat gold is more corrosion resistant in tea than in AS. When 22 carat gold is immersed in AS + tea system, the charge transfer resistance (Rt) value tremendously increases to 51,994 ohm cm2, and double-layer capacitance (Cdl) value decreases to 9.809  1011 F/cm2. This reveals that the corrosion resistance of 22 carat gold in AS + tea system increases. Thus AC impedance spectra lead to the conclusion that the corrosion resistance of 22 carat gold immersed in various test solutions decreases in the following order: tea > AS + tea > AS.

FIG. 16.9 AC impedance spectra for 22 carat gold immersed in (A) AS, (B) tea, and (C) AS + tea (Nyquist plots).

log (Z/ohm)

4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

log (Freq/Hz)

–Phase (deg)

70.0 60.0 50.0 40.0 30.0 20.0 10.0 1.8

2.1

2.4

2.7

3.0

log (Z/ohm)

(A)

3.3

3.6

log (Freq/Hz) 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3

1.8

2.1

2.4

2.7

3.0

3.3

3.6

–Phase (deg)

log (Freq/Hz) 51.0 48.0 45.0 42.0 39.0 36.0 33.0 30.0 27.0 24.0 21.0 18.0

(B)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

log (Z/ohm)

4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 70.0 65.0 60.0 55.0 50.0 45.0 40.0 35.0 30.0

(C)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.10 AC impedance spectra for 22 carat gold immersed in (A) AS, (B) tea, (C) AS + tea (Bode plots).

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

Thus AC impedance spectra lead to the conclusion that people having orthodontic wires made of 22 carat gold need not hesitate to take tea orally. AC impedance spectra (Bode plots) of 22 carat gold immersed in AS.

16.3.2.3 SS 18/8 alloy The AC impedance spectra of SS 18/8 alloy immersed in various test solutions are shown in Figs. 16.11 (Nyquist plot) and 16.12A–C (Bode plots). The corrosion parameters are given in Table 16.3.

FIG. 16.11

log (Z/ohm)

AC impedance spectra for SS 18/8 alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Nyquist plots).

4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8

1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

log (Freq/Hz)

–Phase (deg)

486

(A)

56.0 52.0 48.0 44.0 40.0 36.0 32.0 28.0 24.0 20.0 1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.12 AC impedance spectra for SS 18/8 alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Bode plots).

log (Z/ohm)

16.3 Results and discussion

4.50 4.40 4.30 4.20 4.10 4.00 3.90 3.80 3.70

1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 34.0 32.0 30.0 28.0 26.0 24.0 22.0 20.0 18.0 16.0 14.0

log (Z/ohm)

(B)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8

1.8

2.1

2.4

2.7

3.0

3.3

3.6

–Phase (deg)

log (Freq/Hz)

(C)

65.0 60.0 55.0 50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.12, cont’d

When SS 18/8 alloy is immersed in AS, the charge transfer resistance is (Rt) 14789.3 ohm cm2. The double-layer capacitance (Cdl) value is 3.4484  1010 F/cm2. When SS 18/8 alloy is immersed in tea, the Rt value increases from 14789.3 to 28,160 ohm cm2. The Cdl value decreases from 3.4484  1010 to 1.811  1010 F/cm2. The impedance value increases from 4.348 to 4.743. This indicates that SS 18/8 alloy is more corrosion resistant in tea than in AS.

487

488

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

When SS 18/8 alloy is immersed in AS + tea system, the charge transfer resistance (Rt) value decreases to 6961.4 ohm cm2, and double-layer capacitance (Cdl) value increases to 7.326  1010 F/cm2. The impedance (log(Z/ohm)) value is 4.196. This reveals that the corrosion resistance of SS 18/8 alloy in AS + tea system further decreases. Thus AC impedance spectra lead to the conclusion that the corrosion resistance of SS 18/8 alloy immersed in various test solutions decreases in the following order: AS > tea > AS + tea. Thus AC impedance spectra lead to the conclusion that people having orthodontic wires made of SS 18/8 alloy should avoid taking tea orally.

16.3.2.4 SS316L alloy The AC impedance spectra of SS316L alloy immersed in various test solutions are shown in Figs. 16.13 (Nyquist plot) and 16.14A–C (Bode plots). The corrosion parameters are given in Table 16.3. When SS316L alloy is immersed in AS, the charge transfer resistance is (Rt) 12,468 ohm cm2. The double-layer capacitance (Cdl) value is 4.0905  1010 F/cm2. The impedance (log(Z/ohm)) value is 4.443. When SS316L alloy is immersed in tea, the Rt value decreases from 12,468 to 2482.8 ohm cm2. The Cdl value is 2.0541  1010 F/cm2. This indicates that SS316L alloy is less corrosion resistant in tea than in AS. When SS316L alloy is immersed in AS + tea system, the charge transfer resistance (Rt) value is 9554.73 ohm cm2, and double-layer capacitance (Cdl) value is 5.3376  106 F/cm2. This reveals that the corrosion resistance of SS316L alloy in AS + tea system decreases.

FIG. 16.13 AC impedance spectra for SS316L alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Nyquist plots).

16.3 Results and discussion

log (Z/ohm)

4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

log (Freq/Hz) –Phase (deg)

65.0 60.0 55.0 50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0 10.0

1.8

2.1

2.4

2.7

3.0

log (Z/ohm)

(A)

3.3

3.6

log (Freq/Hz)

4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6

1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 42.0 39.0 36.0 33.0 30.0 27.0 24.0 21.0 18.0 15.0

(B)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.14 AC impedance spectra for SS316L alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Bode plots). (Continued)

489

log (Z/ohm)

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

log (Freq/Hz)

–Phase (deg)

490

(C)

58.0 56.0 54.0 52.0 50.0 48.0 46.0 44.0 42.0 40.0 38.0 1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.14, cont’d

Thus AC impedance spectra lead to the conclusion that the corrosion resistance of SS316L alloy immersed in various test solutions decreases in the following order: AS > AS + tea > tea. Thus AC impedance spectra lead to the conclusion that people having orthodontic wires made of SS316L alloy should avoid taking tea orally.

16.3.2.5 Thermoactive alloy The AC impedance spectra of thermoactive alloy immersed in various test solutions are shown in Figs. 16.15 (Nyquist plot) and 16.16A–C (Bode plots). The corrosion parameters are given in Table 16.2. When thermoactive alloy is immersed in AS, the charge transfer resistance is (Rt) 27,941 ohm cm2. The double-layer capacitance (Cdl) value is 1.8253  1010 F/cm2. The impedance (log(Z/ohm)) value is 3.871. When thermoactive alloy is immersed in tea, the Rt value increases from 27,941 to 28,861 ohm cm2. The Cdl value is 2.3329  1010 F/cm2. The impedance (log(Z/ohm)) value is 4.617. This indicates that corrosion resistance of thermoactive alloy increases in tea system than in AS. When thermoactive alloy is immersed in AS + tea system, the charge transfer resistance (Rt) value is 28,761 ohm cm2, and double-layer capacitance (Cdl) value is 1.773  1010 F/cm2. The impedance (log(Z/ohm)) value is 3.335. This indicates

16.3 Results and discussion

FIG. 16.15

log (Z/ohm)

AC impedance spectra for SS316L alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Nyquist plots).

4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 55.0 50.0 45.0 40.0 35.0 30.0 25.0 20.0 15.0

(A)

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.16 AC impedance spectra for SS316L alloy immersed in (A) AS, (B) tea, and (C) AS + tea (Bode plots). (Continued)

that the corrosion resistance of thermoactive alloy in AS + tea system increases when compared with AS system. Thus AC impedance spectra lead to the conclusion that the corrosion resistance of thermoactive alloy immersed in various test solutions decreases in the following order: tea > AS + tea > AS.

491

log (Z/ohm)

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2 1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

–Phase (deg)

log (Freq/Hz) 56.0 52.0 48.0 44.0 40.0 36.0 32.0 28.0 24.0 20.0 1.8

log (Z/ohm)

(B)

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4 2.2

1.8

2.1

2.4

2.7

3.0

3.3

3.6

3.9

4.2

4.5

4.8

5.1

3.9

4.2

4.5

4.8

5.1

log (Freq/Hz) –Phase (deg)

492

(C)

69.0 66.0 63.0 60.0 57.0 54.0 51.0 48.0 45.0 42.0 39.0

1.8

2.1

2.4

2.7

3.0

3.3

3.6

log (Freq/Hz)

FIG. 16.16, cont’d

Thus AC impedance spectra lead to the conclusion that people having orthodontic wires made of thermoactive alloy need not hesitate to take tea orally.

16.3.2.6 Section conclusion The corrosion inhibition of Ni-Ti alloy in AS + tea system, furnished the highest LPR (4,697,814 ohm cm2) with slightest Icorr (8.420  109 A/Cm2) and highest Rct (56,387 ohm cm2) with least Cdl (2.0803  1010 F/cm2) values as given in

16.3 Results and discussion

Tables 16.2 and 16.3, respectively. Hence, this is considered as one of the best electrode for this system. Further surface analysis was done using Ni-Ti alloy in AS and AS + tea system to confirm the results obtained by electrochemical studies.

16.3.3 Investigation of the film formed on metal surface 16.3.3.1 UV-visible absorption and fluorescence spectra The UV-visible absorption spectra confirmed the protective film formed on the metal surface. The UV-visible absorption spectra and fluorescence spectra of various test solutions are given in Figs. 16.17–16.19. The UV-visible absorption spectrum of artificial saliva is shown in Fig. 16.17. A peak appears at 344 nm. The intensity is 0.042. The UV-visible absorption spectrum of tea is shown in Fig. 16.18. Peaks appear at 358.4, 437.6, 509.6, 596, and 682 nm. The maximum peak appears at 437.6 nm with an intensity of 0.920. A small piece of orthodontic wire made of Ni-Ti alloy was immersed in an aqueous solution containing AS and tea for 1 day. After 1 day the wire was removed from the solution. The UV-visible absorption spectrum of this solution is shown in Fig. 16.19. Peaks appear at 362, 437.6, 509.6, 596, and 686 nm. The maximum peak appears at 437.6 nm with an intensity of 0.928. These peaks are due to the formation of complexes between the metal ions present in Ni-Ti alloy and the ingredients of AS and tea.

0.216 0.138 0.060 Abs

1 –0.019 –0.097 –0.175 200.00

319.52

439.04

558.56

Wave length (nm)

FIG. 16.17 UV-visible absorption spectrum of solution containing AS.

678.08

797.60

493

494

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

0.970 0.791 0.612 3

Abs 0.433 0.254 0.075 200.00

319.52

439.04

558.56

678.08

797.60

Wave length (nm)

FIG. 16.18 UV-visible absorption spectrum of solution containing tea.

0.976 0.795 0.614 3

Abs 0.433 0.252 0.070 200.00

319.52

439.04

558.56

678.08

797.60

Wave length (nm)

FIG. 16.19 UV-visible absorption spectrum of solution containing AS + tea + Ni-Ti alloy.

16.3.3.2 Fluorescence spectra

The fluorescence spectra of solution containing AS (λex ¼ 344 nm) are shown in Fig. 16.20A. A peak appears at 353 nm. The fluorescence spectrum of the film formed on the metal surface (Ni-Ti) after immersion in AS for 1 day is shown in Fig. 16.20B. A peak appears at 344.5 nm.

16.3 Results and discussion

98.331

Fluorescence (mV)

78.664 (353.0, 58.339)

58.998

39.332

19.666

0.000 300.0

360.0

420.0

480.0 WL (nm)

540.0

600

(A)

89.902

Fluorescence (mV)

71.922 (344.5, 56.384)

53.942

35.962

17.982

0.000 300.0

360.0

420.0

480.0

540.0

600.0

WL (nm)

(B) FIG. 16.20 Fluorescence spectrum of solution containing AS (A), film formed on alloy (B), and combined spectra (C). (Continued)

495

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

67.090 Quant yield : 1.044674

53.672 Fluorescence (mV)

496

40.255

26.838

13.421

(C)

0.000 300.0

360.0

420.0

480.0

540.0

600.0

WL (nm)

FIG. 16.20, cont’d

The fluorescence spectrum of tea (λex ¼ 459 nm) is shown in Fig. 16.2A. A peak appears at 468.5 nm. The fluorescence spectrum of the film formed on the metal surface (Ni-Ti alloy) after immersion in tea for 1 day is shown in Fig. 16.21B. A peak appears at 459 nm. A small piece of orthodontic wire made of Ni-Ti alloy was immersed in an aqueous solution containing AS and tea for 1 day. After 1 day the wire was removed from the solution. The fluorescence spectrum of this solution (λex ¼ 455 nm) is shown in Fig. 16.22A. A peak appears at 464.5 nm. This peak is due to the complexes formed in solution between the metal ions of the Ni-Ti alloy and the active principles present in AS and tea. The fluorescence spectrum of the film formed on the metal surface (Ni-Ti alloy) after immersion in this solution for 1 day is shown in Fig. 16.22B. A peak appears at 455 nm. This peak is very close to that of the complexes in solution formed between the metal ions of the Ni-Ti alloy and the active principles present in AS and tea. Thus it is concluded that the protective film formed on the orthodontic wire after immersion in the solution containing AS and tea consists of complexes formed between the metal ions of the Ni-Ti alloy and the active principles present in AS and tea.

16.3.3.3 Scanning morphology study The two-dimensional representation of the surface gives the nature of the protective film formed on metal surface. Fig. 16.23A and B gives the SEM images of polished surface of Ni-Ti alloy exposed for 1 day in AS + tea solution.

FIG. 16.21 Fluorescence spectrum of solution containing tea (A), film formed on alloy (B), and combined spectra (C).

498

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

FIG. 16.22 Fluorescence spectrum of solution containing Ni-Ti + AS + tea (A), film formed on alloy (B), and combined spectra (C).

16.3 Results and discussion

(A)

(B)

FIG. 16.23 Scanning electron microgram of (A) polished Ni-Ti alloy (50 μm) and (B) after being exposed to AS + tea.

The SEM image of polished Ni-Ti alloy is shown in Fig. 16.23A. The surface appears very smooth. The SEM image of Ni-Ti alloy immersed in AS + tea is shown in Fig. 16.23B. The formation of a protective film is seen on Fig. 16.23B.

16.3.3.4 Energy-dispersive analysis of X-rays (EDAX) study The graphical representation of distribution of elements on metal surface for the polished Ni-Ti alloy is shown in Fig. 16.24A. The graphical representation of distribution of elements on metal surface for the Ni-Ti alloy immersed in AS + tea system is shown in Fig. 16.24B. It is observed from EDAX spectra that Fe, Cr, Ni, and C are present on metal surface for both solutions (Tables 16.4 and 16.5). The weight percentage of Ni-Ti has increased from 69.59, 15.20, and 11.13 to 72.09, 16.03, and 9.86, respectively, as weight percentage of C has decreased from 9.79 to 8.79 after being immersed as containing tea. The intensity of Ni peak is considerably suppressed for AS + tea system as compared with polished surface, as the active ingredient of tea (caffeine) would get coordinated with metal ions such as Ni2+, Ti2+, and Fe2+ that resulted in the formation of insoluble metal caffeine complex. The weight percentage of carbon has no effect on inhibitive performance.

16.3.3.5 Atomic force microscopy (AFM) study AFM is the most powerful technique for nanoscale study of sample. It can image both 2D and 3D topography. It is a scanning probe microscopy with very high resolution. It is usually used to obtain average roughness (Sa), root-mean-square roughness (Sq), and maximum peak-to-valley (Sy) height value.

499

500

CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

cps (eV)

14

(A)

12 10 8 6

Fe Cr T Mn C O Ni

Si

Ti

Cr

Mn

Fe

Ni

4 2 0 2

1

3

4

5 keV

6

7

8

9

10

cps (eV)

14

(B)

12 10 8 6

Ti Na Ca Fe Zn Cl Cr Cu C O Ni

Cl

Al Si

Ca

Cr

Ti

Fe

Ni

Cu

Zn

4 2 0 1

2

3

4

5 keV

6

7

8

9

10

FIG. 16.24 EDAX spectra of (A) polished Ni-Ti alloy (50 μm) and (B) after being exposed to (A) AS and (B) AS + tea.

Table 16.4 Spectrum parameters for polished Ni-Ti alloy. El

AN

Series

Unn. C (wt%)

Norm. C (wt%)

Fe Cr Ni C Ο Cu Zn

26 24 28 6 8 29 30

K-series K-series K-series K-series K-series K-series K-series

151.06 32.99 24.17 3.02 1.19 1.91 1.76

69.59 15.20 11.13 1.39 0.55 0.88 0.81

Atom. C error (at.%) 64.94 15.23 9.89 6.04 1.78 0.72 0.65

(1 Sigma) (wt%) 4.63 1.07 1.02 1.15 0.44 0.24 0.26

16.3 Results and discussion

Table 16.4 Spectrum parameters for polished Ni-Ti alloy—Cont’d El

AN

Series

Unn. C (wt%)

Norm. C (wt%)

Atom. C error (at.%)

(1 Sigma) (wt%)

Si Al Ca Ti CI Na

14 13 20 22 17 11

K-series K-series K-series K-series K-series K-series Total:

0.42 0.27 0.13 0.14 0.00 0.00 217.07

0.19 0.13 0.06 0.06 0.00 0.00 100.00

0.36 0.24 0.08 0.07 0.00 0.00 100.00

0.07 0.06 0.05 0.05 0.00 0.00

Table 16.5 Spectrum parameters of Ni-Ti alloy after its exposure to AS containing tea. El

AN

Series

Unn. C (wt%)

Norm. C (wt%)

Atom. C error (at.%)

(1 Sigma) (wt%)

Fe Cr Ni C Μn O Si Ti

26 24 28 6 25 8 14 22

K-series K-series K-series K-series K-series K-series K-series K-series Total:

177.99 39.56 24.35 2.21 2.02 0.47 0.29 0.00 246.89

72.09 16.03 9.86 0.89 0.82 0.19 0.12 0.00 100.00

68.94 16.46 8.97 3.97 0.80 0.64 0.22 0.00 100.00

5.44 1.28 1.05 0.99 0.17 0.27 0.06 0.00

Line fit 14 mm

50 mm

(B)

0 mm

X*

16 mm

Topography range

0 mm

0 mm

Topography range

Y*

Y*

(A)

Topography — Scan forward Line fit

Line fit 209 nm

16 mm

Topography — Scan forward Line fit

0 mm

X*

49.5 mm

FIG. 16.25 Two-dimensional AFM images of the surface of (A) polished Ni-Ti alloy and (B) after being exposed to AS + tea.

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CHAPTER 16 Corrosion behavior of orthodontic wires in artificial saliva

FIG. 16.26 Three-dimensional AFM images of the surface of (A) polished Ni-Ti alloy and (B) after being exposed to AS + tea.

The two-dimensional and three-dimensional AFM morphologies and AFM crosssectional profile of Ni-Ti alloy polished surface are represented in Figs. 16.25A and B, 16.26A and B, and 16.27A and B, respectively. Table 16.6 gives the summary of average roughness (Sa), root-mean-square roughness (Sq), and maximum peakto-valley (Sy) high values. The values of average roughness (Sa), root-mean-square roughness (Sq), and maximum peak-to-valley (Sy) height value for the polished Ni-Ti alloy are 110.74, 142.3, and 1924.9 nm, respectively.

FIG. 16.27 AFM cross-sectional images of the surface of (A) polished Ni-Ti alloy and (B) after being exposed to AS + tea.

References

Table 16.6 AFM data for polished Ni-Ti alloy after being exposed to AS + tea.

Samples

Root-mean-square (RMS) (Sq) roughness (nm)

Average (Sa) roughness (nm)

Maximum peakto-valley height (nm) (Sy)

Pure Ni-Ti alloy AS + tea + Ni-Ti alloy

110.74 69.735

142.3 89.025

1924.9 715.05

The values of average roughness (Sa), root-mean-square roughness (Sq), and maximum peak-to-valley (Sy) height value for the Ni-Ti alloy immersed in AS + tea system are 69.735, 89.025, and 715.05 nm, respectively.

16.4 Conclusions The present study leads to the following conclusions: •

•

•

•

•

Corrosion resistance of Ni-Ti alloy, 22 carat gold, SS 18/8alloy, SS316L alloy, and thermoactive alloy in artificial saliva (AS) in the absence and presence of tea has been evaluated by polarization study, AC impedance spectra, UV-visible absorption, and fluorescence spectra SEM, AFM, and EDAX. For Ni-Ti alloy, 22 carat gold, SS 18/8 alloy, SS316L alloy, and thermoactive alloy, polarization study and AC impedance spectra lead to the result that the Ni-Ti alloy has more corrosion resistance. Linear polarization resistance, corrosion current, charge transfer resistance, double-layer capacitance, and impedance were utilized to evaluate the corrosion inhibition of the tea. UV-visible absorption spectroscopy (UV), fluorescence spectroscopy, scanning electron microscopic (SEM) studies, electron-dispersive X-ray spectroscopy (EDAX), and atomic force microscopy (AFM) give the evidence of better surface condition of the orthodontic wire. The decreasing order of the corrosion resistance for Ni-Ti alloy is AS + tea > AS > tea. People fixed with orthodontic wire made of Ni-Ti alloy can take tea orally without any hesitation.

Acknowledgment The authors are thankful to their respective managements for their constant help and encouragement.

References [1] R. Mayer, D.C. Smith, Biodegradation of the orthodontic bracket system, Am. J. Orthod. Dentofacial Orthop. 90 (1986) 195–198.

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[2] K.L. Baker, L.G. Nieberg, A.D. Weimer, M. Hanna, Frictional changes in force values caused by saliva substitution, Am. J. Orthod. Dentofacial Orthop. 91 (1987) 316–320. [3] C. Cox, T. Nguyen, L. Koroluk, C.C. Ko, In vivo force decay of NiTi closed coil springs, Am. J. Orthod. Dentofacial Orthop. 145 (2014) 505–513. [4] T. Duerig, A. Pelton, D. St€ockel, An overview of Nitinol medical applications, Mater. Sci. Eng. A 273 (1999) 149–160. [5] D. Mantovani, Shape memory alloys: properties and biomedical applications, JOM 52 (2000) 36–44. [6] K. Otsuka, X. Ren, Recent developments in the research of shape memory alloys, Intermetallics 7 (1999) 511–528. [7] S.M. Toker, D. Canadinc, H.J. Maier, O. Birer, Evaluation of passive oxide layer formation–biocompatibility relationship in NiTi shape memory alloys: geometry and body location dependency, Mater. Sci. Eng. 36 (2014) 118–129. [8] W. Buehler, J. Gilfrich, R.C.W. Wiley, Effect of low-temperature phase changes on the mechanical properties of alloys near composition TiNi, J. Appl. Phys. 34 (1963) 1475–1477. [9] G.F. Andreasen, T.B. Hilleman, An evaluation of 55 cobalt substituted Nitinol wire for use in orthodontics, J. Am. Dent. Assoc. 82 (1971) 1373–1375. [10] O. Prymak, A. Klocke, B. Kahl-Nieke, M. Epple, Fatigue of orthodontic nickel–titanium (NiTi) wires in different fluids under constant mechanical stress, Mater. Sci. Eng. A 378 (2004) 110–114. [11] C.J. Hwang, J.S. Shin, J.Y. Cha, Metal release from simulated fixed orthodontic appliances, Am. J. Orthod. Dentofacial Orthop. 120 (4) (2001) 383–391. [12] E. Al-Waheidi, Allergic reaction to nickel orthodontic wires: a case report, Quintessence Int. 26 (1995) 385–388. [13] S. Rajendran, J. Paulraj, P. Rengan, J. Jeyasundari, M. Manivannan, Corrosion behaviour of metals in artificial saliva in presence of spirulina powder, J. Dent. Oral Hyg. 1 (2009) 1–8. [14] S. Rajendran, V. Uma, A. Krishnaveni, J. Jeyasundari, B. Shyamaladevi, M. Manivannan, Corrosion behavior of metals in artificial saliva in presence of D-Glucose, Arab. J. Sci. Eng. 34 (2C) (2009) 147–158. [15] S. Rajendran, P. Chitradevi, S. Johnmary, A. Krishnaveni, S. Kanchana, L. Christy, R. Nagalakshmi, B. Narayanasamy, Corrosion behaviour of SS 316 L in artificial saliva in presence of electral, Zasˇtita Materijala 51 (2010) 149–158.

CHAPTER

Corrosion resistance of orthodontic wires in artificial saliva with presence of fragrant drink additives

17

RM Joanya, A Anandanb, S Gowric, Susai Rajendranc, Bhawna Chughd, S. Senthil Kumarane, Gurmeet Singhf a

Department of ECE, Sathyabama University, Chennai, India bSKV Higher Secondary School, Namakkal, India cCorrosion Research Centre, Department of Chemistry, St Antony’s College of Arts and Sciences for Women, Dindigul, India dDepartment of Chemistry, Netaji Subhas Institute of Technology, New Delhi, India eSchool of Mechanical Engineering, VIT University, Vellore, India f Pondicherry University, Puducherry, India

Chapter outline 17.1 Aroma compounds used in foods and beverages ..............................................505 17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva with presence of fragrant drink additives: A case study ..........................................506 17.2.1 Artificial saliva (AS) ................................................................... 507 17.2.2 Polarization study ...................................................................... 507 17.2.3 AC impedance spectra ............................................................... 510 17.2.4 Contact angle measurement ....................................................... 518 17.2.5 AFM images .............................................................................. 520 17.3 Conclusion ....................................................................................................522 References ............................................................................................................522 Further reading ......................................................................................................523

17.1 Aroma compounds used in foods and beverages An aroma compound is also known as an odorant, aroma, fragrance, or flavor. An aroma compound is a chemical compound that has a smell or odor. Usually a chemical compound has a smell or odor when it is sufficiently volatile to be transported to the olfactory system in the upper part of the nose. In general, molecules meeting this Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00017-1 # 2020 Elsevier Inc. All rights reserved.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

specification have molecular weights of less than 300. Flavors affect both the sense of taste and smell. On the other hand, fragrances affect only smell. Flavors tend to be naturally occurring, and fragrances tend to be synthetic. Various types of natural aroma chemicals can be used in foods and beverages [1]. It was reported in literature that orthodontic bracket corrosion can occur in the oral environment [2–6]. For the stimulated orthodontic appliances, their ion release (nickel and chromium) and corrosion rate have been evaluated [7, 8]. In addition, many kinds of aroma compounds can be used as corrosion inhibitors [9, 10]. Aroma compounds are found in food, wine, spices, floral scent, perfumes, fragrance oils, and essential oils. For instance, many form biochemically during the ripening of fruits and other crops. In wines, most form as by-products of fermentation. In addition, many of the aroma compounds play a major role in the production of flavorants that are used in the food and drink industry to flavor, improve, and generally increase the appeal of their products. An odorizer may add a detectable odor to a dangerous odorless substance, like propane, natural gas, or hydrogen, as a safety measure. Aroma compounds such as vanilla, rose, pineapple, orange, and lemon essences are used in various food items such as ice cream and payasam; these food items are orally taken by people who have been clipped with orthodontic wires made of various alloys such as SS316L, SS18-8, thermoactive NidTi, and 22 karat gold. In the oral environment, in the presence of saliva, these wires will undergo corrosion. Further in the presence of these aroma compounds, these wires may undergo corrosion. An interesting study is undertaken to investigate if an orthodontic wire made of SS18-8 undergoes corrosion when the aroma compounds vanilla, rose, pineapple, orange, and lemon essences are taken orally along with food items. The corrosion rate has been evaluated by polarization study and AC impedance spectra. The hydrophobicity of the protective film formed on the metal surface has been analyzed by contact angle measurement. The surface morphology at nanolevel has been investigated by AFM technique.

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva with presence of fragrant drink additives: A case study Beautiful smile attracts everyone. Well-arranged teeth lead to beautiful attractive smiles. However, God creates ill-arranged teeth in the mouth of the less fortunate people. To regulate the arrangement of teeth, people meet the dentists. These doctors regulate the growth of teeth with the help of orthodontic wires made of various metals and alloys such as 22 karat gold, SS18-8, and SS316L. After having clipped with orthodontic wires, people take orally different types of food, juice, tablets, etc. During these processes the orthodontic wires that are already in the oral environments (saliva) will undergo corrosion. The corrosion resistance of orthodontic wires

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

in artificial saliva in the absence and presence of various ingredients, such as various tablets, juices, spirulina, teas, and coffees, has been investigated by polarization study and AC impedance spectra. The present study is undertaken to investigate the corrosion resistance of orthodontic wire made of SS18-8, in artificial saliva (AS), in the absence and presence of various fragrant food additives such as vanilla, orange, lemon, pineapple, and rose essences. The corrosion resistance was measured by polarization study and AC impedance spectra. These electrochemical studies were carried out in a CHI 660 A work station model. A three-electrode cell assembly was used. SS18-8 was the working electrode. Platinum was used as counterelectrode. SCE was used as reference electrode. The commercial fragrant essences were used as such.

17.2.1 Artificial saliva (AS) Artificial saliva (AS) was prepared by dissolving the following chemicals in 1 L of double distilled water (Table 17.1).

17.2.2 Polarization study The polarization curves of SS18-8 immersed in artificial saliva in the absence and presence (2 mL each) of various fragrant essences shown in Figs. 17.1–17.6. The corrosion parameters such as corrosion potential (Ecorr), corrosion current (Icorr), Tafel slopes anodic ¼ ba and cathodic ¼ bc, and linear polarization resistance (LPR) value were derived from the Tafel plots. These values are given in Table 17.2. In general, when corrosion resistance of a system increases, the corrosion current decreases, and LPR value increases. When the corrosion potential shifts to the anodic side, anodic reaction is controlled predominantly. On the other hand, when the corrosion potential shifts to the cathodic side, cathodic reaction is controlled predominantly. These ideas are used throughout the investigation. It is observed from Table 17.2 that, when SS18-8 is immersed in AS, the corrosion potential is 276 mV versus SCE. The LPR value is 554,386 Ω cm2. The corrosion current value is 9.655  108 A cm2. When 2 mL of vanilla essence is added, to AS, the LPR value increases, and the corrosion current value decreases. Table 17.1 Composition of artificial saliva. Chemical

Quantity (g L21)

KCl NaCl CaCl22H2O NaH2PO42H2O Na2S9H2O Urea

0.4 g 0.4 g 0.906 g 0.690 g 0.005 g 1g

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

FIG. 17.1 Polarization curve of SS18-8 immersed in artificial saliva.

FIG. 17.2 Polarization curve of SS18-8 immersed in artificial saliva + vanilla.

These observations indicate that, in the presence of vanilla essence, the corrosion resistance of SS18-8 that is immersed in AS increases. It implies that people clipped with orthodontic wire made of SS18-8 need not hesitate to take food items containing vanilla essence. It is observed also from Table 17.2 that similar is the case with other fragrant essences also. That is, in the presence of all the essences studied, the corrosion resistance of SS18-8 alloy increases. Hence, it is concluded that people clipped with orthodontic wire made of SS18-8 need not hesitate to take food items containing vanilla essence and other essences under investigation. However, the corrosion

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

FIG. 17.3 Polarization curve of SS18-8 immersed in artificial saliva + orange.

FIG. 17.4 Polarization curve of SS18-8 immersed in artificial saliva + lemon.

resistance of SS18-8 alloy in artificial saliva in the presence of these essences decreases in the following order: AS + vanilla > AS + orange > AS + lemon > AS + pineapple > AS + rose > AS

Fig. 17.7 compares the LPR values of various systems. Corrosion inhibition efficiencies of various fragrant materials offered to SS18-8 alloy, immersed in artificial saliva, are shown in Fig. 17.8 (Table 17.3). The corrosion inhibition efficiencies (IE) were calculated from the equation IE ¼ (LPR1 LPR2)/LPR1, where LPR1 is LPR value in the presence of inhibitor (in this case vanilla) and LPR2 is LPR value in the absence of inhibitor.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

FIG. 17.5 Polarization curve of SS18-8 immersed in artificial saliva + pineapple.

FIG. 17.6 Polarization curve of SS18-8 immersed in artificial saliva + rose.

17.2.3 AC impedance spectra The AC impedance spectra of SS18-8 immersed in artificial saliva in the absence and presence (2 mL each) of various fragrant essences are shown in Figs. 17.9–17.20. The corrosion parameters such as charge transfer resistance (Rt), double-layer capacitance value (Cdl), and impedance value [log(Z/Ω)] derived from the spectra are given in Table 17.4. In general, when corrosion resistance increases, charge transfer resistance (Rt) increases, double-layer capacitance value (Cdl) decreases, and impedance value [log(Z/Ω)] increases.

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

Table 17.2 Corrosion parameters of SS18-8 immersed in artificial saliva (AS), in the absence and presence of various fragrant essences obtained by polarization study.

System

Ecorr (mV/ SCE)

bc (mV/ decade)

ba (mV/ decade)

LPR (Ω cm2)

Icorr (A cm22)

AS AS + vanilla AS +orange AS + lemon AS +pineapple AS + rose

276 313 409 271 254 208

167 149 269 177 173 179

465 76 144 347 299 272

554,386 3,889,397,248 30,272,318 5,060,043 1,679,805 1,417,692

9.655  108 5.646  1012 1.349  109 1.006  108 2.832  108 3.309  108

FIG. 17.7 Comparison of LPR values of SS18-8 alloy immersed in various systems.

It is observed from Table 17.4 that, when SS18-8 is immersed in AS, charge transfer resistance (Rt) value is 26,051 Ω cm2. The double-layer capacitance value (Cdl) is 1.958  1010 F cm2. The impedance value is 4.670. When 2 mL of vanilla essence is added to AS, the charge transfer resistance increases, and the double-layer capacitance value (Cdl) decreases. These observations indicate that, in the presence of vanilla essence, the corrosion resistance of SS18-8 is immersed in AS increases.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

FIG. 17.8 Corrosion inhibition efficiencies (IE%) offered by various systems.

Table 17.3 Corrosion inhibition efficiencies (IE%) offered by various systems. IE (%) AS AS + vanilla AS + orange AS + lemon AS + pineapple AS + rose

FIG. 17.9 Nyquist plot of SS18-8 immersed in artificial saliva.

– 99.99 98.17 89.40 66.99 60.90

FIG. 17.10 Nyquist plot of SS18-8 immersed in artificial saliva + vanilla.

FIG. 17.11 Nyquist plot of SS18-8 immersed in artificial saliva + orange.

FIG. 17.12 Nyquist plot of SS18-8 immersed in artificial saliva + lemon.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

FIG. 17.13 Nyquist plot of SS18-8 immersed in artificial saliva + pineapple.

FIG. 17.14 Nyquist plot of SS18-8 immersed in artificial saliva + rose.

It implies that people clipped with orthodontic wire made of SS18-8 need not hesitate to take food items containing vanilla essence. It is also observed from Table 17.4 that similar is the case with other fragrant essences also. That is in the presence of all the essences studied, the corrosion resistance of SS18-8 alloy increases. Hence, it is concluded that people clipped with orthodontic wire made of SS18-8 need not hesitate to take food items containing vanilla essence and other essences under investigation. However, the corrosion resistance of SS18-8 alloy in artificial saliva in presence of these essences decreases in the following order: AS + vanilla > AS + orange > AS + lemon > AS + pineapple > AS + rose > AS

Fig. 17.21 compares the charge transfer resistance (Rt) values of various systems.

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

FIG. 17.15 Bode plots of SS18-8 alloy immersed in artificial saliva.

FIG. 17.16 Bode plots of SS18-8 alloy immersed in artificial saliva + vanilla.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

FIG. 17.17 Bode plots of SS18-8 alloy immersed in artificial saliva + orange.

FIG. 17.18 Bode plots of SS18-8 alloy immersed in artificial saliva + lemon.

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

FIG. 17.19 Bode plots of SS18-8 alloy immersed in artificial saliva + pineapple.

FIG. 17.20 Bode plots of SS18-8 alloy immersed in artificial saliva + rose.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

Table 17.4 Corrosion parameters of SS18-8 immersed in artificial saliva (AS), in the absence and presence of various fragrant essences obtained by AC impedance spectra. System

Rt (Ω cm2)

Cdl (F cm22)

Impedance log (Z/Ω)

Phase angle degree

AS AS + vanilla AS + orange AS + lemon AS + pineapple AS + rose

26,051 4,416,000 1,600,360 233,840 212,730 115,152

1.958  1010 1.155  1013 3.187  1012 2.181  1011 2.397  1011 4.429  1011

4.670 6.684 7.694 5.521 5.408 5.103

50.45 160; 44.61 101.2 54.80 39.93; 25.57 57.41

FIG. 17.21 Comparison of Rt values of SS18-8 alloy immersed in various systems.

17.2.4 Contact angle measurement The hydrophobicity nature and smoothness of the metal surface before and after immersion in inhibitor environment can be investigated by contact angle measurement (Figs. 17.22–17.24). For the polished SS18-8 alloy, the contact angle is 110.5 degrees. When the alloy is immersed in artificial saliva medium, it has undergone corrosion, and the surface has become rough. Hence the contact angle is 8.5 degrees. When the alloy is immersed in the medium containing AS and vanilla essence, protective film is formed on the metal surface. The contact angle of this surface is 99.8 degrees (Table 17.5). Because of this protective film, the surface has become more

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

FIG. 17.22 Contact angle of polished SS18-8 alloy surface.

FIG. 17.23 Contact angle of polished SS18-8 alloy surface immersed in AS.

FIG. 17.24 Contact angle of polished SS18-8 alloy surface immersed in AS + vanilla.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

Table 17.5 Contact angles of various systems. System SS18-8 alloy SS18-8 alloy+ AS SS18-8 alloy+ AS+ vanilla essence

Contact angle

Hydrophobicity

Wettablity

110.5 degree 8.5 degree 99.8 degree

Highest Lowest In between

Lowest Highest In between

hydrophobic than the metal surface in AS only. Hence, SS18-8 is more corrosion resistant in the AS +vanilla system, than in AS system only. The hydrophobicity of various systems are as follows: Polished alloy SS18-8 ðcontact angle 110:5 degreeÞ > SS18-8 + AS + vanilla essence ð99:8 degreeÞ > SS18-8 alloy + AS ð8:5 degreeÞ:

It is interesting to record that the corrosion resistance is also in the same order, that is, SS18-8 in AS and vanilla essence system > SS18-8 in AS system

It is observed that, when contact angle increases, the hydrophobicity increases. That is, water molecules cannot reach the metal surface. Hence, corrosion-resisting nature increases.

17.2.5 AFM images Two-dimensional and three-dimensional images of surfaces give valuable information about the roughness of the surfaces. RMS roughness (Rq), average roughness (Ra), and maximum peak-to-valley height are derived from the AFM images. For polished metal surface the RMS roughness (Rq) will be very low. For polished metal surface, immersed in the corrosive medium (artificial saliva in the present case), the RMS roughness (Rq) will be very high. For inhibitor system, this value will be in between the aforementioned two systems. This is due to the formation of a protective film on the metal surface, in the presence of inhibitor system (in the present case vanilla essence). Similar is the case with other two parameters. The results are given in Table 17.6 and Figs. 17.25–17.27. Table 17.6 AFM images of SS18-8 surface immersed in various environments. Sample Polished SS18-8 SS18-8 in AS and vanilla SS18-8 in AS

RMS (Rq) roughness (nm)

Average (Ra) roughness (nm)

234.77

184.19

156.5

348.62

268.98

1743.7

442.06

343.24

381.2

Maximum peak-tovalley height (nm)

17.2 Corrosion resistance of orthodontic wire SS18-8 in artificial saliva

FIG. 17.25 AFM images of SS 18-8 alloy.

FIG. 17.26 AFM images of SS 18-8 alloy immersed in AS.

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CHAPTER 17 Corrosion resistance of orthodontic wires in artificial saliva

FIG. 17.27 AFM images of SS 18-8 alloy immersed in AS and Vanilla.

17.3 Conclusion Corrosion behavior of SS18-8 alloy in artificial saliva with or without the presence of five fragrances (viz., vanilla, orange, lemon, pineapple, and rose) has been investigated by polarization study and AC impedance spectra. The hydrophobicity of the surfaces has been analyzed by contact angle measurement. The surface morphology of the protective film has been analyzed by AFM. The corrosion protection efficiencies of various systems are as follows: vanilla > orange > lemon > pineapple> rose. All these systems offer better corrosion protection to orthodontic wire made of SS188 alloy in artificial saliva. Hence, it is implied that people clipped with orthodontic wire made of SS18-8 alloy need not hesitate to take orally food items flavored with the investigated fragrances.

References [1] D.J. Rowe, Chapter 10—Natural aroma chemicals for use in foods and beverages, in: D. Baines, R. Seal (Eds.), Natural Food Additives, Ingredients and Flavourings, Food Science, Technology and Nutrition, Woodhead Publishing, 2012, pp. 212–230, https:// doi.org/10.1533/9780857095725.1.212. [2] R. Maijer, D.C. Smith, Corrosion of orthodontic bracket bases, Am. J. Orthod. 81 (1) (1982) 43–48. [3] K. House, F. Sernetz, D. Dymock, J.R. Sandy, A.J. Ireland, Corrosion of orthodontic appliances—should we care? Am. J. Orthod. Dentofac. Orthop. 133 (4) (2008) 584–592.

Further reading

[4] M. Nakagawa, S. Matsuya, T. Shiraishi, M. Ohta, Effect of fluoride concentration and pH on corrosion behaviour of titianium for dental use, J. Dent. Res. 78 (1999) 1568–1572. [5] N. Schiff, B. Grosgogeat, M. Lissac, F. Dalard, Influence of fluorinatied mouthwashes on corrosion resistance of orthodontic wires, Biomaterials 25 (2004) 4535–4542. [6] R. Patel, S. Bhanat, D. Patel, B. Shah, Corrosion inhibitory ability of Ocimum Sanctum Linn (Tulsi) rinse on ion release from orthodontic brackets in some mouthwashes: an in vitro study, Natl. J. Community Med. 5 (1) (2014) 135–139. [7] H. Kerosuo, G. Mobe, A. Hensten-Pettersen, Salivary nickel and chromium in subjects with different types of fixed orthodontic appliances, Am. J. Orthod. Dentofac. Orthop. 111 (1997) 595–598. [8] R.D. Barrett, S.E. Bishara, J.K. Quinn, Biodegradation of orthodontic appliances. Part 1. Biodegradation of nickel and chromium in vitro, Am. J. Orthod. Dentofac. Orthop. 103 (1993) 8–14. [9] P.B. Raja, M.G. Sethuraman, Natural products as corrosion inhibitor for metals in corrosive media—a review, Mater. Lett. 62 (1) (2008) 113–116. [10] B.E. Amitha Rani, B.B.J. Basu, Green inhibitors for corrosion protection of metals and alloys: an overview, Int. J. Corros. 2012 (2012) 380217. 15 pages, https://doi.org/10. 1155/2012/380217.

Further reading [11] C.J. Hwang, J.S. Shin, J.Y. Cha, Metal release from simulated fixed orthodontic appliances, Am. J. Orthod. Dentofac. Orthop. 120 (2001) 383–391. [12] H. Kerosuo, G. Moe, E. Kelven, In vitro release of nickel and chromium from different types of simulated orthodontic appliances, Angle Orthod. 65 (1995) 111–116. [13] H.Y. Park, T.R. Shearer, In vitro release of nickel and chromium for stimulated orthodontic appliances, Am. J. Orthod. 84 (1983) 156–159.

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CHAPTER

Nanofiltration in beverage industry

18

Carmela Conidia, Roberto Castro-Mun˜ozb, Alfredo Cassanoa a

Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Rende, Italy bMonterrey Institute of Technology, Toluca, Mexico

Chapter outline 18.1 Introduction ..................................................................................................525 18.2 Properties of nanofiltration membranes ..........................................................526 18.3 Application of NF membranes in beverage industry .........................................529 18.3.1 Wine and beer ........................................................................... 529 18.3.2 Fruit juice processing ................................................................. 535 18.3.3 Whey and milk .......................................................................... 539 18.4 Conclusions and future trends ........................................................................544 References ............................................................................................................544 Further reading ......................................................................................................548

18.1 Introduction Food and beverage industry is one of the major contributors to the growth of all economies. In European Union, it constitutes the largest manufacturing sector in terms of turnover, value added, and employment [1]. This growth is due to the recognition of the key role of foods and beverages in the prevention and treatment of different diseases. Thus the production and consumption of functional foods have gained much importance as they provide health benefit beyond the basic nutritional functions. At present, beverages are considered the most active functional food category because of convenience and possibility to meet consumer demands for container contents, size, shape, and appearance, as well as ease of distribution and storage for refrigerated and shelf-stable products. Moreover, they are an excellent delivering means for nutrients and bioactive compounds including vitamins, minerals, antioxidants, fiber, prebiotics, and probiotics [2, 3]. New processing technologies are gaining popularity worldwide producing beverages with high nutritional and sensory quality and ensuring at the same time food safety and shelf life extension. In this context, membrane Nanotechnology in the Beverage Industry. https://doi.org/10.1016/B978-0-12-819941-1.00018-3 # 2020 Elsevier Inc. All rights reserved.

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separation processes represent a valid alternative to traditional technologies due to their low operating and maintenance costs, mild operating conditions of temperature and pressure (therefore preserving the functional properties of food products), nonuse of chemical agents or solvents, and, consequently, possibility to avoid product contamination [4, 5]. In particular, pressure-driven membrane operations such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are today well-established technologies in different fields of food and beverage industries [6]. Among these processes, NF is particularly adequate for the purification, fractionation, and concentration of several food products and by-products and for the recovery of value-added compounds and solvent from food wastes [7]. NF membranes are at relatively recent development in membrane technology with separation capabilities falling between two well-established technologies: UF and RO. One of the most relevant characteristics of NF membranes is their ability to permeate monovalent ions while rejecting divalent and multivalent ions and higher flux compared with RO membranes. This flexibility opens up many possibilities in the development of specific applications in different fields of the food industry as fruit juice, beverages, dairy, sugar, and vegetable oil processing [8, 9]. The growing interest toward NF membranes is one of the key emerging trends in the market of membranes for food applications. According to data reported by Allied Market Research [10], the global NF membranes market was valued at $643.22 million in 2017 and is expected to reach at $954.65 million by 2025, registering a compound annual growth rate (CAGR) of 5.4% from 2018 to 2025. After describing the main aspects of NF membranes, this chapter attempts to give a comprehensive overview of the applications of this technology in different fields of the beverage industry. It focuses more specifically on recent advances and potentialities in wine, beer, fruit juice processing, and milk and dairy industry.

18.2 Properties of nanofiltration membranes NF membranes are considered as the most selective membranes within the category of pressure-driven membrane technologies. Their pore size ranges typically between 0.5 and 2 nm corresponding to molecular weight cutoffs (MWCO) of 100–1000 Da. Typical operating pressures (in the range of 3–30 bar) are lower than those used for RO membranes. In principle the separation capability of NF membranes depends on their MWCO especially for neutral solutes (size-based exclusion); however, the MWCO is not an absolute barrier for the separation of micro- and macrosolutes. For instance, the asymmetric property of the membrane pores does not usually display a narrow MWCO range [11]: this is generally attributed to the manufacture protocol of commercial UF and NF membranes mainly based on the wet-phase inversion process [12, 13]. Fig. 18.1 illustrates a typical view of an asymmetric membrane. Such asymmetric characteristic is obtained when the polymer dope solution comes spontaneously in contact with the nonsolvent in the coagulation bath: a fast outflow of the solvent from the polymer solution to the coagulation bath produces the aggregation

18.2 Properties of nanofiltration membranes

FIG. 18.1 Cross-sectional view of an asymmetric porous membrane. Adapted from N. Ali, H. Sofiah, A. Asmadi, A. Endut, Preparation and characterization of a polysulfone ultrafiltration membrane for bovine serum albumin separation: effect of polymer concentration, Desalin. Water Treat. 32(1–3) (2011) 248–255.

of the polymer molecules at the top layer, the so-called thin or skin layer, which importantly contributes to enhance the solute rejection. Polymeric NF membranes contain ionizable groups as carboxylic or sulfonic acid groups, which result in a surface charge in the presence of a feed solution. Therefore, although the molecular sieving is the main separation mechanism of NF membranes, the charge effect plays a significant role on membrane selectivity. The classical Donnan effect describes the equilibria and membrane potential interactions between charged species and the interface of the charged membrane [14]. Furthermore, membrane properties such as hydrophobicity/hydrophilicity and surface features display a strong influence on membrane-solute interactions and therefore on the separation performance of such membranes. Commonly used NF membranes are polymeric in nature, including cellulose acetate, polyamide, polyimide, polysulfone, and polyethersulfone or ceramic such as zirconia, titania, silica-zirconia, and alumina. Compared with polymeric NF membranes, ceramic membranes generally show a higher chemical, structural, and thermal stability. Polymeric membranes generally present a thin film composite structure (made of polyamides or polyethersulfone) that combine a high selectivity with a high permeability [15]. Commercially, NF membranes are commonly prepared with an additional layer aiming at obtaining high rejection rates; the deposition of this layer is carried by coating a top layer on the membrane surface. Some specifications of commercial NF membranes with different types of composite polymer materials are reported in Table 18.1. In general, hydrophilic nature composites are the most used within NF membranes. As for other pressure-driven membrane operations, the use of NF is limited by fouling, which reveals itself as a decrease in flux with time of operation. Fouling is an irreversible and time-dependent phenomenon generated by binding, accumulation, or absorption of materials on the surfaces of the membrane and/or within the porous structure; it is strongly influenced by membrane material and solute-solute and solute-membrane interactions that cause an irreversible decline of the permeate flux, which can only be recovered by the chemical cleaning of the membrane.

527

Table 18.1 Specification of some commercial NF membranes. Membrane type Supplier

Desal 5 DL GE Osmonics

N30F NADIR

UTC20 Toray

NF-400 Filmtec

150–300 –

NTR7450 NittoDenko 600–800 –

MWCO (Da) MgSO4 rejection (%) Max. temperature (°C) pH range Top layer

400 –

180 –

90

40

95

1–11 Cross-linked aromatic PA

2–14 Sulfonated PES

0–14 Hydrophilic PES

PES, polyethersulfone; PA, polyamide; PPA, polypiperazineamide.

Desal 51 HL GE Osmonics

– 98

NF90 GE Osmonics – 99

35

45

50

50

3–10 PPA

3–10 PPA composite

2–11 PA

3–9 Cross-linked aromatic PA

150–300 –

18.3 Application of NF membranes in beverage industry

Therefore the selection of a suitable membrane in terms of MWCO, material (functional groups, charge, and hydrophobicity), and morphology (i.e., surface roughness) is very important in determining the success of a specific application. Specific intrinsic membrane properties, including hydrophilicity, surface topography, charge and pore dimensions are all factors influencing the performance of NF membranes since they affect the fouling behavior of membranes. It has been documented that membrane fouling is promoted by rougher surfaces [16]. The presence of protuberances on the surface (e.g., in polyamide-based membranes) contributes also to membrane fouling, capturing suspended organic and inorganic matter. On the other hand, cellulose acetate membranes reveal smoother surfaces and therefore are less susceptible to fouling. In addition, most membranes possess a net negative charge that may produce electrostatic forces when treating solutions enriched in charged particles. It is worthy to mention that the surface charge on the membrane depends on the kind of membrane material; however, the chemical nature (e.g., pH and ionic strength) of the feed bulk solution also contributes. This surface charge becomes more crucial when the feed solutions contain charged molecules like proteins [17]. Another important intrinsic property, such as hydrophobicity/hydrophilicity, is important as well. Apparently, hydrophilic membranes seem to display better performance when treating feed bulks based on water as the primary solvent. All aforementioned membrane properties and the physicochemical properties of the feed bulk solutions can lead to specific interactions, including coulombic and hydrophobic interactions, which takes place between the solutes and membrane surface. Importantly, beverages enriched in phenolic compounds (e.g., fruit juices and wine) are prone to form interactions like phenolics-membrane and phenolics-phenolics [18]. Such interactions can contribute to the solute rejection [19, 20]. In addition, some process parameters like equipment design, temperature, feed concentration, flow, and pressure can also contribute to membrane fouling. Based on all aforementioned characteristics, NF membranes are nowadays displaying the highest performance in separating, recovering, and concentrating lowmolecular weight molecules (e.g., anthocyanins, low-molecular weight phenolics, low-molecular weight sugars, and some other derivatives) from their original sources. Thanks to this success, NF has been currently used and therefore consolidated in the beverage industry. Thereby the following section gives an overview of the main applications of NF membranes in the field.

18.3 Application of NF membranes in beverage industry 18.3.1 Wine and beer The use of NF in wine processing is of interest for different applications including the control of alcohol content of wines, the partial removal of sugars from musts, and the reduction of volatile acidity in wines and wine acidification [21].

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In the last years the demand for low-alcohol beverages has risen in several countries as a result of health and social concerns. In addition, the alcoholic content has a strong impact on the product quality affecting the volatility of aroma compounds [22]. The ethanol removal through conventional dealcoholization methodologies, such as distillation or evaporation, is characterized by the undesired loss of sensorial and organoleptic compounds. This is because generally the aromatic and organic compounds of alcoholic beverages display affinity for ethanol; therefore such aromas are concurrently removed out together with the ethanol. In this context, NF seems to be a more promising technology for obtaining lowalcohol wines [23]. NF membranes permeate water and ethanol and retain bioactive compounds in wine more efficiently than MF and UF membranes. In addition, the NF process provides higher alcohol flow rates together with greater permeation rates than RO. It can be carried out at low pressures and temperatures, thus preserving the sensorial properties of the original product. A brief summary of some NF applications in beverage dealcoholization is presented in Table 18.2. Catarino and Mendes [26] evaluated four polyamide NF membranes with similar MWCO (200 Da) (NF99 HF, NF99, and NF97 from Alfa Laval and YMHLSP1905 from GE Osmonics) for removing ethanol from red wine containing 12% alcohol by volume (ABV). NF was operated under 16 bar and 30°C of feed pressure and operating temperature, respectively. All selected membranes showed higher effectiveness in alcohol removal from wine, due to their good permeability to ethanol Table 18.2 Application of NF membranes in beverage dealcoholization. Membrane type

Beverage

Polyamide, XN45 (Trisep)

Red wine

NF200, DOW Filmtec

Tokaji Ha´rslevelu˝ wine Red wine

Polyamide, NF99 HF, Alfa Laval Polyamide, XN45 (Trisep)

Red wine

Polyamide, HC50 (DDS)

Rose` wine

Polyamide, HC50 (DDS)

Red wine

Operating conditions C0, 12.8%; P, 20 bar; T, 30°C C0, 13.1%; P, 20 bar; T, 25°C C0, 12%; P, 16 bar; T, 30°C C0, 10.7%; P, 15 bar; T, 25°C C0, 10.7%; P, 15 bar; T, 25°C; C0,11.1%; P, 15 bar; T, 25°C;

Results

Reference

Alcohol concentration in product, 9.8%; cost, 4.8 €/m3 Alcohol concentration in permeate, 9.72%; alcohol rejection, 17.06% Alcohol concentration in product, 9.11% (120 h)

[24]

Alcohol concentration in product, 7.3% (reconstitution) Alcohol concentration in product, 7.3% (reconstitution) Alcohol concentration in product: 8% (reconstitution)

[25]

[26]

[27]

[28]

[28]

18.3 Application of NF membranes in beverage industry

(ethanol rejection 7%–10%) and high rejection toward aroma compounds, resulting in dealcoholized wine samples with promising organoleptic properties. Selected NF membranes were used to produce dealcoholized wines (up to ca. 5 vol.%) that were blended with the original wine to produce reconstituted wine samples. YMHLSP1905 and NF99 membranes produced the dealcoholized wine samples with the most equilibrated analytical aroma profile and sensorial analysis. A pervaporation (PV) step was also implemented to recover aroma compounds from the original wine, and the aroma extract (0.3 vol%) was added to the wine samples produced by the two methods, single-step dealcoholization and reconstituted method, improving the aroma and taste profile of these samples. Another approach to reduce the alcohol content of wines is based on the reduction of the sugar content of grape must before its fermentation. At this purpose, Salgado et al. [29] evaluated the performance of a single- and two-stage NF process in the reduction of sugar from both white and red must using an experimental device equipped with a spiral-wound membrane of 200 Da (KMS SR3 from Koch Membrane Systems). The first stage produced a permeate with medium sugar content (P1) and a retentate (R1) enriched in sugar from untreated must (C). The retentate stream also possessed the major content of high-molecular weight solutes, including polysaccharides, polyphenols, and proteins. The NF permeate (P1) was submitted to a second NF stage providing a retentate (R2) and a permeate (P2) with a lower sugar content. For both musts, red and white, the permeate P2 was blended with the first retentate (R1) in a suitable ratio with the aim to achieve a moderate reduction in the alcoholic degree of the final wine (the corresponding wines showed a 1–2° alcohol reduction). The blend preserved specific features of the grape attributed to the high-molecular weight compounds retained in R1. This approach allowed the production of wines with similar sensorial and chemical properties in comparison with wines produced by traditional fermentation protocols. Results showed that the two-stage NF process promoted a higher recovery of polyphenols and less volume losses. The schematic representation of both single- and two-stage NF procedure applied for the white must and ulterior fermentation is depicted in Fig. 18.2. The two-stage NF process was also combined with a PV unit for the elaboration of full-flavored low-alcohol white wines [30]. The recovery of the aroma compounds from grape must was carried out prior to NF in a pilot plant-scale PV unit equipped with a polydimethylsiloxane (PDMS)-based membrane in spiral-wound configuration (PV-SR1 SWM, from Pervatech). The mixture of musts obtained from the integrated process showed an aroma content more similar to the original grape must with the exception of benzaldehyde and 1-hexanol. The ethanol content of the final product was of about 10.2–10.5 vol%. It is well known that the traditional production usually produces white wine containing ca. 12 vol%, which means that a small decrease in the alcohol content was acquired. A schematic representation of the integrated process is depicted in Fig. 18.3. Garcı´a-Martı´n et al. [31] analyzed the retention characteristics of different UF and NF membranes toward glucose and fructose in synthetic mixtures, with the aim

531

FIG. 18.2 Graphical depiction of the NF procedures developed for the dealcoholization of wines. (A) Two-stage and (B) single-stage nanofiltration [29].

18.3 Application of NF membranes in beverage industry

Aroma compounds

PV White must

Mixture

NF

Alcoholic fermentaon

White wine

NF Retentate

FIG. 18.3 Integrated dealcoholization process used for the production of low alcoholic wines (PV, pervaporation; NF, nanofiltration). Adapted from C.M. Salgado, E. Ferna´ndez-Ferna´ndez, L. Palacio, A. Herna´ndez, P. Pra´danos, Application of pervaporation and nanofiltration membrane processes for the elaboration of full flavored low alcohol white wines, Food Bioprod. Process. 101 (2017) 11–21.

of controlling the sugar in grape musts. Experimental results indicated higher rejections for NF membranes toward polyphenols, anthocyanins, and tartaric acid in comparison with UF membranes. A two-step NF process for the production of wines with a low amount of alcohols but enriched of low- and high-molecular weight compounds was also proposed [32, 33]. In this approach the must obtained from the NF treatment was mixed with the untreated must or with the retentate of the first NF stage in an adequate proportion to reduce the alcohol content of the resulting wines by 2°. The method was considered inexpensive in terms of production costs. In addition, its profitability can be increased by using the NF retentate of the second step for the production of sweet wines, liquors, or additives for functional foods. Valuable compounds of red wines were concentrated by using an experimental apparatus equipped with a piperazine-based NF membrane with a MWCO of 300–500 Da (Trisep XN45, from MICRODYN-NADIR) [24]. Both water and ethanol permeated across the NF membrane producing a permeate with an alcohol content of 10.75 vol%, which can be reused as raw material in the alcohol industry. The NF retentate had a similar alcoholic content of the original wine, with about two times higher concentration of valuable components. The addition of water to the retentate was suggested as a method to produce a wine with a decreased alcohol content. In another study, Banvolgyi et al. [27] evaluated the effects of operating parameters (temperature and transmembrane pressure difference) on the NF membrane performance in terms of permeate flux and retention of anthocyanins and resveratrol.

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CHAPTER 18 Nanofiltration in beverage industry

Experimental results indicated that the highest retention of anthocyanin and resveratrol was achieved at 20°C and that reduction of volume by a factor of 4 led to 2.5–3 times more anthocyanins and resveratrol in the wine concentrates. An increasing in pressure from 10 to 20 bar caused a twofold increase in permeate flux at the same temperature. The final products obtained by using various forms of reconstitution of the concentrated wine had low alcohol content (4–6 vol.%), and their sensory attributes were similar to those of the original wine. A process modeling in the production of low-alcohol content wines by direct concentration and diafiltration using NF membranes was developed by Taka´cs et al. [25]. Diafiltration experiments were performed by using a relatively dense NF membrane with a MWCO of 200 Da (NF 45, from Dow Filmtec), while for simple wine concentrations a membrane developed for organic components rejection (NF200, from Dow Filmtec) was investigated. The model described the filtration efficiency as a function of the operational parameters. The developed relation allows to select a specific membrane on the basis of the knowledge of the wine constants, which is a primary aspect in planning and creating the process optimal. An integrated NF process to reduce the alcohol content of alcoholic beverages was patented by Gonc¸alves and De Pinho [28] in 2004. In this approach, aromatic compounds are retained by the NF membrane, while the permeate stream, a mixture of water and ethanol, is distilled to remove the ethanol. A low alcoholic grade beverage is produced through the recombination of the distillation-based product with the NF retentate. Currently the production of nonalcoholic beer (alcohol content below 0.5 vol.%) and low-alcohol beer (alcohol content from 0.6 to 1.2 vol.%) is a topic of growing interest in the brewing industry. Similarly to wine processing the use of NF deals with the ethanol removal from the original beer. Since polymers have a lower cost in comparison with inorganic materials, the use of NF polymeric membranes in PV processes has been expected to extend their application. At this purpose, Verhoef et al. [34] compared the performance of three different PV membranes with a PDMS-based top layer with that of a hydrophobic NF membrane (SolSep 3360) in the treatment of ethanol/water mixtures and alcoholic beverages including lager beer and white wine. Fluxes and permeances resulted higher for the NF membrane in comparison with two PDMS-based PV membranes, while the separation factor and selectivity were comparable or even better. Differences between NF and PV membranes were attributed to the influence of swelling and interactions between permeating molecules and selected membranes. Therefore NF membranes were considered useful for PV processes aiming at the removal of ethanol from alcoholic beverages. More recently, polyamide NF membranes with MgSO4 rejection of 98% (NF, NF10, and NF99HF, from Alfa Laval) have been tested for beer dealcoholization [35]. Such commercial membranes exhibited a sevenfold higher flux (up to 0.717 kg m
2 min
1) and twice the ethanol permeability compared with RO membranes. The NF99HF membrane was found to be the best performing membrane in

18.3 Application of NF membranes in beverage industry

terms of flux and ethanol permeability. The primary drawback of the NF processes was the difficulty in removing ethanol below 0.5% ABV at low cost. In Denmark the legal limit for alcohol-free beer labeling has been recently changed from 0.1% ABV to 0.5% ABV making membrane processes a viable alternative for future alcoholfree beer production.

18.3.2 Fruit juice processing In recent years, one of the strategies that has been adopted by the food industry to develop new products is the implementation of new food technologies, which provide a series of benefits in terms of food safety, shelf life extension, and increased nutritional and sensory quality [36]. The fruit juice industry is one of the food sectors that has invested the most in the implementation of new technologies for production of clarified and concentrated juices able to retains as much as possible the peculiarity of the fresh fruit and its high content in biologically active compounds. At this purpose, research on fruit juice processing have been moving from conventional thermal treatments toward nonthermal processing techniques. Traditional thermal sterilization processes lead to a degradation of sensorial and nutritional characteristics with partial loss of aroma and nutrients, induction of cooked taste due to the formation of 5-hydroxymethyl-2-furfural (HMF), and browning due to Maillard reactions [37]. In addition, a large amount of energy is needed for the removal of water. Membrane processes represent a relevant alternative to the traditional unit operations in fruit juice production thanks to their capacity to operate at room temperatures, preserving the functional properties of thermolabile compounds and color and nutritional properties, avoiding phase changes and the use of chemical additives [38]. Additional advantages include high separation efficiency, easy scale-up and low energy consumption. These features are becoming very important factors in the production of new fruit juices with natural fresh taste and additive free [39]. Fruit juice clarification, stabilization, depectinization, and concentration are typical steps where membrane processes as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and osmotic distillation (OD), also in integrated systems, have been successfully utilized. In particular, NF has been found to be extremely efficient in the clarification, concentration, and deacidification of fruit juices in association with UF or MF or traditional technologies as pretreatment steps [7]. NF offers a better retention of MF and UF processes for lowermolecular weight molecules as sugars, organic compounds, and ions. At the same time, concentration of fruit juices with NF presents many advantages if compared with RO in terms of better juice quality due to the lower pressure required and lower cost due to the less energy consumption [40]. Several applications of NF in fruit juice concentration have been reported in literature. An integrated process based on the use of NF and RO membranes for the production of highly concentrated fruit juice was developed by Nabetani [41]. Fruit juices were concentrated from 10 to 45°Brix with energy savings of 1/8 and 1/5 in comparison with evaporation and freeze concentration, respectively.

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CHAPTER 18 Nanofiltration in beverage industry

Warczok et al. [42] investigated the concentration of apple and pear juice at low pressures (8–12 bar) by using two tubular membranes (AFC80, PCI Membranes, and MPT-34, Koch) and two flat-sheet membranes (Desal-5DK, Osmonics, and MPT-34, Koch). According to the experimental results, the Desal-5DK membrane exhibited the highest permeate flux, sufficiently high retention, and a higher concentration degree than the MPT-34 membrane. Ba´nv€ olgyi et al. [43] investigated the concentration of blackcurrant juice by using a NF membrane in flat-sheet configuration with a salt rejection of 78.11%. The juice was concentrated in selected operating conditions of pressure (20 bar), temperature (30°C), and feed flow rate (400 L/h) up to a volume reduction factor (VRF) of 2.23. In these conditions, permeate fluxes of about 18 L m
2 h
1 and retention of total extract of 96.72% were obtained. More recently, Popovic et al. [44] studied the concentration of chokeberry juice at different pressures (45, 50, and 55 bar) and temperatures (with cooling and without cooling of the retentate) by using a plate-and-frame module equipped with six polyamide thin-film composite NF membranes (DSS A Tetra Pak from Alfa Laval) having a sodium chloride rejection higher than 98%. Operating conditions of 55 bar without cooling of the retentate produced the highest productivity (permeate fluxes between 9.61 and 19.43 L m
2 h
1) and the highest retention of phenolic compounds, while aromatic components were mostly retained at 55 bar with cooling. NF membranes have been also successfully employed for the purification, fractionation, and concentration of biologically active compounds extracted from different natural sources including fruit juices and vegetables. Gilewicz-Łukasik et al. [45] investigated the use of NF for the concentration of anthocyanins from aronia (black chokeberry) juice. The juice was previously clarified by cross-flow UF with a polysulfone membrane and then processed in a dead-end cell (SEPA ST Osmonics) equipped with a flat-sheet NF membrane (BQ01, from Osmonics Inc.) composed of a polysulfone support coated with a layer of sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO). The NF membrane retained about 91% of anthocyanins. The observed retention increased (more than 99%) in the presence of sodium sulfate(IV). In a similar approach, Ghosh et al. [46] investigated the recovery of anthocyanidins from Indian blackberry (Jamun) juice. After extraction the juice was clarified with a 50-kDa UF membrane and then concentrated by NF with a polysulfone spiral-wound NF membrane having a MWCO of 300 Da. Table 18.3 shows the physicochemical characterization of permeate and retentate streams of the NF process. Anthocyanidins were completely retained by the NF membrane; rejection rates of protein, polyphenol, and antioxidants were of 63%, 74%, and 40%, respectively. Accordingly the NF process was considered suitable for the production of a concentrated product with possible use in the beverage and pharmaceutical industry. NF membranes with MWCO of around 200 Da resulted efficient in concentrating anthocyanins and ellagitannins from blackberry juice [47]. Among the investigated membranes a semiaromatic polypiperazineamide thin-film composite membrane with a NMWCO of 150–300 Da (NF270, from Dow Filmtec) showed the highest rejection toward target compounds (100% of anthocyanins and ellagitannins) and

18.3 Application of NF membranes in beverage industry

Table 18.3 Physicochemical composition of clarified Jamun juice before and after NF. Parameter

Feed

Permeate

Retentate

TSS (°Brix) Turbidity (NTU) Clarity (%T) pH Protein (mg/g) Polyphenols (mg GAE/g) Antioxidant (% DPPH) Cyanidin chloride (mg/10 g) Malvidin chloride (mg/10 g) Delphinidin chloride (mg/10 g)

16.07  0.34 0.03  0.001 89.33  0.41 3.6  0.23 109.43  0.87 125.74  0.98 34.22  0.12 3.8  0.01 7.1  0.01 2.8  0.01

14.32  0.21 0.03  0.001 94.4  0.57 3.52  0.11 40.63  0.34 32.27  0.26 3.61  0.02 n.d. n.d. n.d.

19.36  0.32 0.17  0.011 70.43  0.37 3.56  0.16 120.42  0.96 90.71  0.67 89.45  0.53 5.9  0.02 20.8  0.01 3.6  0.01

highest permeate flux in the selected operating conditions. This membrane was also considered useful for juice deacidification since, at high pressures, sugars were completely retained and retention of acids was under 90%. Tundis et al. [48] evaluated the performance of three commercial NF membranes with different MWCO (400 and 1000 Da) and polymeric material (composite fluoropolymer and polyethersulfone) for the concentration of different classes of polyphenols from elderberry (Sambucus nigra L.) juice. The antioxidant and hypoglycemic properties of both permeate and retentate fractions were also studied. Among the investigated membranes the NP030 (a polyethersulfone membrane with a MWCO of 400 Da, from MICRODYN-NADIR) exhibited the highest rejection toward phenolic compounds. Accordingly the retentate fraction exhibited the highest antioxidant activity. Recently, NF has been evaluated for the concentration of procyanidins from the grape juice [49]. Grape juice was pretreated by MF to remove suspended solids and then processed by using a spiral-wound polyamide membrane with a MWCO of 800 Da (NFG-2B-1812, Synder Filtration) under different concentrations and pH. Experimental results indicated that the mass transfer coefficient increased with the increase in the procyanidin concentration due to the solution-diffusion effect and the charge repulsion effect. The solute rejection increased by increasing the pH from 3.0 to 8.0 due to the Donnan effect between procyanidins and membrane surface charge. The NF process resulted also efficient in concentrating the phenolic content of both microfiltered and natural strawberry juice [50]. Juices were processed with a polyvinylidene fluoride membrane with a MWCO of 150–300 Da (GE Osmonics) at operating pressure of 6 bar and a temperature of 20°C. The use of UF and NF spiral-wound membranes with MWCO ranging from 150 to 3500 Da was tested for the recovery, purification, and concentration of two important classes of flavonoids with statin-like principles (bruteridin and melitidin) from clarified

537

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CHAPTER 18 Nanofiltration in beverage industry

bergamot juice [51]. The performance of the selected membranes was compared in terms of permeate fluxes, retention toward sugars (glucose, fructose, and sucrose), and selected flavonoids. As expected a decreasing MWCO produced an increase of the rejection coefficient toward both bruteridin and melitidin. According to the obtained results, a polyamide-TFC membrane (Desal GE, from GE Osmonics) with a MWCO of 1000 Da exhibited the best performance in terms of separation between sugars and flavonoids in the clarified juice with higher retentions toward flavonoids (>67% for melitidin and >82% for bruteridin) and lower retention toward glucose (21.9%) and fructose (10.6%). These results confirm that a combination of UF and NF membranes is an attractive alternative to produce at low temperature and without use of organic solvents concentrated flavonoid fractions from bergamot juice of interest for the production of fortified fruit juices with hypocholesterolemic activity. In another study, tubular ceramic membranes with a MWCO of 450 Da (from Inopor) exhibited the best separation factor between flavonoids and sugar in the fractionation of depectinized and ultrafiltered bergamot juice [52] with retention coefficients toward flavonoids and sugars of 95.4% and 48.7%, respectively. The proposed process results in a clear solution enriched in sugars and organic acids (NF permeate) and a fraction enriched in phenolic compounds with high antioxidant activity (NF retentate) (Fig. 18.4). Bergamot juice (10°Brix)

Depectinization

UF 100 kDa

Suspended solids

Clarified juice (9.8°Brix)

NF 450 Da

Flavonoids (narirun , hesperidin, naringin)

Permeate (4°Brix)

FIG. 18.4 Recovery of phenolic compounds from bergamot juice by integrated membrane process (UF, ultrafiltration; NF, nanofiltration). Adapted from C. Conidi, A. Cassano, E. Drioli, A membrane-based study for the recovery of polyphenols from bergamot juice, J. Membr. Sci. 375(1–2) (2011) 182–190.

18.3 Application of NF membranes in beverage industry

Recently, Magalhaes et al. [53] evaluated the performance of a sequential membrane filtration process including MF, UF, and NF for the clarification and concentration of pequi fruit extract. The juice was previously clarified with MF membranes of 0.22 μm (from Millipore) and then treated by UF by using a polyethersulfone membrane with a MWCO of 5 kDa (from MICRODYN-NADIR); finally the UF permeate was processed by NF with a polypropylene membrane of 500–600 Da (from MICRODYN-NADIR). The recovery of phenolic compounds in the integrated process was compared with that of direct UF and NF processes. Results indicated that direct UF and NF offered the greatest retention factors and yield for bioactive compounds although permeate fluxes were lower than those measured in the sequential process. A mass balance of bioactive compounds referred to a treatment of 1000 L of pretreated feed stream for both sequential and direct processes is depicted in Fig. 18.5. The yields of polyphenols in the NF retentate stream for the sequential and direct processes resulted in 5.5% and 85.7%, respectively. Similar yields were reported for phenolic compounds of pomegranate juice processed with UF and NF membranes [54]. More recently, Tamba et al. [55] proposed an integrated membrane-based process to purify, fractionate, and concentrate betacyanins from cactus pear juice. The selected method consisted of a first clarification step by MF and a second step for solute separation by using UF or NF membranes with different MWCO (in the range 200–4000 Da). Experimental results showed that betacyanins and major solutes (total dry matter, TDM) were completely retained by a NF membrane in polyethersulfone with a MWCO of 200 Da (NP030, from MICRODYN-NADIR) working at low transmembrane pressure (5 bar). At the same operating pressure, a thin film composite membrane with a MWCO of 1 kDa (MPS 36, from Koch) allowed fractionation of betacyanins limiting the total dry matter losses. Therefore, by selecting different membrane/pressure conditions, it was possible to promote either the concentration of all the betacyanins or their purification from total dry matter. The integration of different unit operations, including NF, resulted an attractive alternative to produce, at low temperature, concentrated and purified betacyanin extracts from cactus pear juice without thermal damage.

18.3.3 Whey and milk The market for functional foods and dairy-based functional beverages represents a growing segment of the global food industry. Whey is a by-product of cheese and casein production with a low content of solids (up to 5%–6%) and high biological oxygen demand (BOD5 ¼ 30–50 g/L), which make its disposal difficult and costly [56]. With the improvement of processing technologies, it has become a valuable raw material for the food, pharmaceutical, and biotechnological industries [57]. Several investigations have been performed to develop whey-based beverages. These products consist of a fermented or nonfermented beverages that have a minimum milk-based concentration of 51% and can be added with fruit preparations [58].

539

Pretreated pequi extract

1000 L Microfiltration 0.22 µm FC = 1.7

TPC = 193.8 g GA = 31.45 g EA = 11.78 g CA = 0.70 g

MF retentate V = 588 L

412 L

Ultrafiltration 5 kDa FC = 2.0

206 L

UF retentate V = 206 L

TPC = 56.13 g GA = 9.39 g EA = 4.07 g CA = 0.29 g

1000 L

Ultrafiltration 5 kDa FC = 2.0

UF retentate V = 500 L

TPC = 16.18 g GA = 1.79 g EA = 0.31 g CA = 0.09 g

Nanofiltration 103 L 400 Da FC = 2.0

NF retentate V = 103 L

TPC = 5.52 g GA = 0.28 g EA = 0.09 g CA = 0.00 g

500 L

TPC = 36.26 g GA = 3.19 g EA = 1.10 g CA = 0.01 g

1000 L

Nanofiltration 400 Da FC = 2.0

NF retentate V = 500 L

500 L

TPC = 27.63 g GA = 1.95 g EA = 0.60 g CA = 0.03 g

FIG. 18.5 Clarification and concentration of pequi extract by membrane-based operations. Mass balance of bioactive compounds for sequential and direct processes (TPC, total polyphenol content; GA, gallic acid; EA, ellagic acid; CA, p-coumaric acid) [53].

18.3 Application of NF membranes in beverage industry

Actually, whey-based fruit beverages are receiving notable attention as their market potential is increasingly improving. In addition to their appetizing feature, these beverages are extremely nutritious and energetic [59]. At this purpose a variety of processing technologies have been studied to increase their functional properties [60]. For instance, a possible method of improving functionality is to enrich beverages with functional food components obtained by the biotransformation of whey constituents. Recently, Pa´zma´ndi et al. [61] investigated a multistep process based on membrane filtration and subsequent enzymatic conversion of wheyderived lactose into prebiotic oligosaccharides. In particular, partially demineralized whey was first concentrated and diafiltered by UF with a spiral-wound membrane having an active layer made of polyethersulfone and MWCO of 20 kDa (SM type, Synder Filtration) to obtain whey protein isolates. Then the UF permeate was further concentrated by NF at different concentration factors by using a spiral-wound polymeric membrane with a MWCO of 150–300 Da (DK type, General Electrics) to obtain streams with various concentrations of lactose. Finally a systematic study was performed to investigate the hydrolytic and transgalactosylation activity of Biolacta N5, a Bacillus circulans-derived β-galactosidase, on whey-derived substrates at different concentrations of lactose. The investigated process resulted well suited for generating whey protein and lactose. NF was efficient in concentrating the UF permeate up to moderate lactose levels. A key feature of the enzymatic conversion is the competition between hydrolysis and transgalactosylation. It was shown that lactose hydrolysis is the predominant mechanism when using UF permeates, and transgalactosylation is particularly pronounced when using NF concentrates as substrates in the catalytic step. A flowchart of the investigated process is depicted in Fig. 18.6. In a previous work, Benedetti et al. [62] studied a potential use of tofu whey concentrated by NF to obtain functional fermented lactic beverages. Tofu whey was first submitted to a MF process; then the MF permeate was concentrated by NF up to a VRF of 4.5 by using an organic polyimide membrane (from PAM Membranas Seletivas) in hollow fiber configuration. The concentrated fraction was used to produce fermented lactic beverages at two different whey concentration (10% of concentrated whey + 90% of milk and 20% of concentrated whey + 80% of milk). Results showed that the content of total isoflavones was greater for beverage with concentrated whey at 20% evidencing that the content of concentrated tofu whey has a direct influence on the total isoflavone content. The use of the NF process to concentrate tofu whey and the use of the concentrate in the production of a functional fermented lactic beverage offer an opportunity for an entirely new approach in the use of tofu whey. NF membranes have a high permeability for monovalent salts (NaCl and KCl) but low permeability for organic compounds (lactose, proteins, urea, etc.): therefore they allow concentration and partial demineralization of the whey in one step [63, 64]. Additional advantages offered by NF over conventional processes include the reduction of overall costs; the lower energy consumption; and the production of a product with a better taste, properties, and viscosity [65].

541

542

CHAPTER 18 Nanofiltration in beverage industry

Feed V = 20 L [Pro] = 7.4 g/L [Lac] = 46.9 g/L

Deionized water V = 16 L

UF

DF

20 kDa VRF = 5 UF permeate V = 16 L [Pro] = ferulic). This effect, however, was totally lost after widespread dialysis. The study reveals that the enhancing effect of the RWPC supplementation on Pl-AOC may be due to a

24.2 Recent developments on powdered wine

phenolic-compound action both in the aqueous phase of plasma and at the surface of lipoprotein particles. Surface location probably explains the enhancing-sparing effect of supplementation on LDL vitamin E and the absence of effect on dialysed-LDL oxidizability [28].

24.2.21 Determination of inorganic anions and cations in wine A mixed-bed column was filled with anion-exchange resin ICS-A23 and cationexchange resin CH1. The chromatographic behavior of organic acids, inorganic anions and cations on the mixed-bed column have been investigated. The method urban in this study was used for the simultaneous resolve of organic acids, inorganic anions and cations in wine, Japanese sake and immediate coffee powder lacking any special pretreatment system [29].

24.2.22 Sprouting rice wine Rice grain (Oryza sativa var. Japonica, Reiho) was drenched in running water overnight and incubated at 25°C for 7–8 days to prepare sprouting rice. A novel and/or early alcoholic beverage designated as sprouting rice wine was correctly made from various starchy materials using sprouting rice powder as a saccharifying agent. Sprouting rice wine containing 9%–15% (v/v) ethanol had a characteristic aroma just like Japanese sake and a rather sour flavor as judged by organoleptic testing. The rice wine was colorless or faintly yellow and the acidity was around 5.0. Larger amounts of volatile compounds such as higher alcohols and esters were found to be limited in the rice wine by gas chromatographic analysis. As the starchy materials fed to the initial mash were augmented, the amounts of ethanol and dextrin contained in the rice wine also increased, and the excellence of the rice wine was improved. Though the origin of ancient Japanese sake is still unclear and the evidence for the production of sprouting rice wine in ancient times is deficient, the legendary and inexplicable alcoholic beverage of antiquity prepared with sprouting rice was reproduced [30].

24.2.23 Asbestos fibers in wine samples For nearly a century asbestos has been used in filtration techniques. Clarification of wines is normally done by two procedures involving the hazard of infectivity of the filtrates by fibers of asbestos or other minerals: either filtration on plates made up of woven asbestos, cellulose or compact Kieselguhr (infusorial earth), or filtration by packing where the wine is mixed with a powder containing Kieselguhr supplemented or not with asbestos or cellulose. The filtering layer is established by depositing inorganic particles on wire mesh. The risk of cancer is still under debate. Nevertheless, among asbestos workers likely to inhale and swallow asbestos fibers a significant increase in cancer of the gastrointestinal tract has been shown. Moreover, the option of migration of ingested asbestos fibers into the majority of the organs has been

683

684

CHAPTER 24 Powdered wine

demonstrated in animals, and recently a noteworthy number (30%) of cancers of various organs (kidney, liver, brain, lungs) in rats consuming food containing asbestos filters has been found. With the discovery in the United States of asbestos fibers in various beverages (except wine), the authors investigated whether these fibers were present in samples of popular wines from different localities. The ultrafiltered suspended particles, experiential in the electron microscope, were subjected to crystallographic analysis. Numerical assessment showed fibers in gout of 22 samples, with a numerical concentration of 64, 40, 16, 15, 13, 11, 4 and 2  104 fibers per liter, correspondingly; fibers were of the chrysotile type with a diameter