ARSENIC CONTAMINATIONOF GROUNDWATER

Mechanism, Analysis, and Remediation

Edited by

SATINDER AHUJA

A JOHN WILEY & SONS, INC., PUBLICATION

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ARSENIC CONTAMINATIONOF GROUNDWATER

ARSENIC CONTAMINATIONOF GROUNDWATER

Mechanism, Analysis, and Remediation

Edited by

SATINDER AHUJA

A JOHN WILEY & SONS, INC., PUBLICATION

Copyright © 2008 by John Wiley & Sons, Inc. All rights reserved.

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Ahuja, Satinder, 1933–Arsenic contamination of groundwater : mechanism, analysis, and remediation

/ Satinder Ahuja.p. cm.

Includes index.ISBN 978-0-470-14447-3 (cloth)

1. Groundwater pollution. 2. Arsenic—Environmental aspects. I. Title.TD427.A77A48 2008628.1′6—dc22

2008021429

Printed in the United States of America

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CONTENTS

Contributors vii

Preface xi

1 The Problem of Arsenic Contamination of Groundwater 1Satinder Ahuja

2 Fate of Arsenic in Irrigation Water and Its Potential Impacton the Food Chain 23S. M. Imamul Huq

3 Microbial Controls on the Geochemical Behavior of Arsenic inGroundwater Systems 51Farhana S. Islam

4 Molecular Detection of Dissimilatory Arsenate-RespiringBacteria in North Carolina Groundwater 83Holly Oates and Bongkeun Song

5 Biogeochemical Mechanisms of Arsenic Mobilizationand Sequestration 95Kate M. Campbell and Janet G. Hering

6 Geomicrobiology of Iron and Arsenic in Anoxic Sediments 123Carolina Reyes, Jonathan R. Lloyd, and Chad W. Saltikov

7 Development of Measurement Technologies for Low-Cost,Reliable, Rapid, On-Site Determination of Arsenic Compoundsin Water 147Julian F. Tyson

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vi CONTENTS

8 Field Test Kits for Arsenic: Evaluation in Terms of Sensitivity,Reliability, Applicability, and Cost 179Jörg Feldmann and Pascal Salaün

9 Mucilage of Opuntia ficus-indica for Use as a Flocculantof Suspended Particulates and Arsenic 207Kevin A. Young, Thomas Pichler, Alessandro Anzalone, and Norma Alcantar

10 Prediction of Arsenic Removal by Adsorptive Media:Comparison of Field and Laboratory Studies 227Malcolm Siegel, Alicia Aragon, Hongting Zhao, Shuguang Deng,Melody Nocon, and Malynda Aragon

11 Arsenic Remediation of Bangladesh Drinking Water UsingIron Oxide–Coated Coal Ash 269Ashok Gadgil, Lara Gundel, and Christina Galitsky

12 Development of a Simple Arsenic Filter for Groundwaterof Bangladesh Based on a Composite Iron Matrix 287Abul Hussam and Abul K. M. Munir

13 Community-Based Wellhead Arsenic Removal Unitsin Remote Villages of West Bengal, India 305Sudipta Sarkar, Anirban Gupta, Lee M. Blaney, J. E. Greenleaf,Debabrata Ghosh, Ranjan K. Biswas, and Arup K. SenGupta

14 Water Supply Technologies for Arsenic Mitigation 329M. Feroze Ahmed

15 Solutions for Arsenic Contamination of Groundwater 367Satinder Ahuja

Index 377

CONTRIBUTORS

M. Feroze Ahmed, Department of Civil Engineering, Bangladesh University ofEngineering and Technology, Dhaka, Bangladesh

Satinder Ahuja, Ahuja Consulting, Calabash, North Carolina

Norma Alcantar, Department of Chemical Engineering, University of SouthFlorida, Tampa, Florida

Alessandro Anzalone, Department of Chemical Engineering, University of SouthFlorida, Tampa, Florida (currently at Department of Industrial Engineering, Poly-technic University of Puerto Rico, San Juan, Puerto Rico)

Alicia Aragon, Geochemistry Department, Sandia National Laboratories, Albu-querque, New Mexico

Malynda Aragon, Geochemistry Department, Sandia National Laboratories,Albuquerque, New Mexico

Ranjan Biswas, Bengal Engineering and Science University, Howrah, India

Lee M. Blaney, Department of Civil and Environmental Engineering, LehighUniversity, Bethlehem, Pennsylvania

Kate M. Campbell, California Institute of Technology, Pasadena, California(currently at U.S. Geological Survey, Menlo Park, California)

Shuguang Deng, Department of Chemical Engineering, New Mexico State Uni-versity, Las Cruces, New Mexico

Jörg Feldmann, Department of Chemistry, University of Aberdeen, Aberdeen,UK

Ashok Gadgil, Lawrence Berkeley National Laboratory, Berkeley, California

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viii CONTRIBUTORS

Christina Galitsky, Lawrence Berkeley National Laboratory, Berkeley, Califor-nia

Debabrata Ghosh, Bengal Engineering and Science University, Howrah, India

J.E. Greenleaf, Department of Civil and Environmental Engineering, LehighUniversity, Bethlehem, Pennsylvania

Lara Gundel, Lawrence Berkeley National Laboratory, Berkeley, California

Anirban Gupta, Bengal Engineering and Science University, Howrah, India

Janet G. Hering, California Institute of Technology, Pasadena, California(currently at Swiss Federal Institute of Aquatic Science and Technology,Dübendorf, Switzerland)

Abul Hussam, Department of Chemistry and Biochemistry, George Mason Uni-versity, Fairfax, Virginia

S. M. Imamul Huq, Department of Soil, Water and Environment, University ofDhaka, Dhaka, Bangladesh

Farhana S. Islam, Department of Molecular and Cellular Biology, College ofBiological Science, University of Guelph, Guelph, Ontario, Canada

Jonathan R. Lloyd, School of Earth, Atmospheric and Environmental Sciences,The University of Manchester, Manchester, UK

Abul K. M. Munir, Manob Sakti Unnayan Kendro, Kushtia, Bangladesh

Melody Nocon, Department of Environmental Engineering, University ofCalifornia–Berkeley, Berkeley, California

Holly Oates, Center for Marine Sciences, University of North Carolina–Wilmington, Wilmington, North Carolina

Thomas Pichler, Department of Geology, University of South Florida, Tampa,Florida

Carolina Reyes, Department of Environmental Toxicology, University of Cali-fornia–Santa Cruz, Santa Cruz, California

Pascal Salaün, Department of Chemistry, University of Aberdeen, Aberdeen,UK; Department of Earth and Ocean Sciences, University of Liverpool, Liv-erpool, UK

Chad W. Saltikov, Department of Environmental Toxicology, University of Cal-ifornia–Santa Cruz, Santa Cruz, California

Sudipta Sarkar, Department of Civil and Environmental Engineering, LehighUniversity, Bethlehem, Pennsylvania

Arup K. SenGupta, Department of Civil and Environmental Engineering,Lehigh University, Bethlehem, Pennylvania

CONTRIBUTORS ix

Malcolm Siegel, Radiological Consequence Management Department, SandiaNational Laboratories, Albuquerque, New Mexico

Bongkeun Song, Center for Marine Sciences and Department of Biology andMarine Biology, University of North Carolina–Wilmington, Wilmington,North Caroloina

Julian F. Tyson, Department of Chemistry, University of Massachusetts,Amherst, Massachusetts

Kevin A. Young, Department of Chemical Engineering, University of SouthFlorida, Tampa, Florida

Hongting Zhao, University of Wyoming, Laramie, Wyoming

PREFACE

Arsenic contamination has been found in regional water supplies in Argentina,Bangladesh, Cambodia, Canada, Chile, China, Ghana, Hungary, India, Laos,Mexico, Mongolia, Nepal, Pakistan, Poland, Taiwan, Thailand, the UK, the UnitedStates, and Vietnam. Even in advanced countries such as the United States, nearly10% of groundwater resources exceed arsenic levels of 10 ppb. Recognizing thefact that inorganic arsenic is a documented human carcinogen, the World HealthOrganization (WHO) set a standard at no more than 10 μg/L (or 10 parts per bil-lion) of arsenic (As) in drinking water in 1993. This standard was finally adoptedby the United States in 2006; however, 50 μg/L (50 ppb or 0.05 mg/L) is themaximum contamination level (MCL) considered acceptable in Bangladesh. Thepopulation at risk approaches 100 million in Bangladesh at the MCL set bythe WHO. Some experts estimate that as many as 500 million people could beaffected by this problem worldwide.

Groundwater can be contaminated with arsenic from a variety of sources,such as pesticides, wood preservatives, glass manufacture, and other miscella-neous uses of arsenic. These sources can be monitored and controlled. However,the contamination from naturally occurring arsenic in the ground is difficult tocontrol. The worst case of this problem was discovered in Bangladesh, wherea large number of shallow (10 to 40 m) tube wells installed in the 1970s werefound in the 1980s to be contaminated with arsenic. It is estimated that as manyas 100 million people in Bangladesh may be exposed to high levels of arsenic,exceeding the WHO guidelines. It should be noted that these guidelines do notconsider different species of arsenic, even though it is already well establishedthat the toxicity of arsenic can vary enormously with its speciation. The effects ofthe oxidation state on chronic toxicity are confounded by the redox conversion ofAs(III) and As(V) within human cells and tissues. Clinical symptoms of arseni-cosis may take about six months to two years or more to appear, depending onthe quantity of arsenic ingested and also on the nutritional status and immunitylevel of the individual. Arsenic ingestion causes various effects on the skin, suchas dark spots on the chest, back, and limbs; and enlargement of the liver, kidneys,

xi

xii PREFACE

and spleen. Later, patients may develop nephropathy, hepatopathy, gangrene, orcancers of the skin, lung, or bladder.

The book has been planned to improve our understanding of the horrificproblem of groundwater contamination with arsenic and offers some meaningfulsolutions. The focus is primarily on groundwater pollution from natural sources,and the nature and scope of the problem is discussed in the first two chapters.Chapter 1 provides a broad overview of the problem. Accumulation of arsenicin various crops that are irrigated with arsenic-rich water and its consequenceson dietary intake are covered in Chapter 2. Various remedial measures to combatarsenic accumulation in soils and crops are also discussed.

In West Bengal, India and in Bangladesh, aquifer sediments containing arsenicare derived from weathered materials from the Himalayas. Arsenic typicallyoccurs at concentrations of 2 to 100 ppm in these sediments, much of it sorbedonto a variety of mineralogical hosts, including hydrated ferric oxides, phyllosil-icates, and sulfides. The mechanism of arsenic release from these sediments hasbeen a topic of intense debate, and both microbial and chemical processes havebeen invoked. The oxidation of arsenic-rich pyrite has been proposed as onepossible mechanism. Other studies have suggested that reductive dissolution ofarsenic-rich Fe(III) oxyhydroxides deeper in the aquifer may lead to the release ofarsenic into the groundwater. A large number of studies that have been conductedin Asia, the United States, and the United Kingdom to improve our understand-ing of the mechanism of groundwater contamination, covered in Chapters 3 to6, favor the microbial processes.

Detection and quantification limits for arsenic down to ultratrace levels (below 1ppm) are possible with inductively coupled plasma mass spectrometry (ICP-MS).However, the speciation of arsenic requires separations based on solvent extrac-tion, chromatography, and selective hydride generation. High-performance liquidchromatography coupled with ICP-MS is currently the best technique available fordetermination of inorganic and organic species of arsenic; however, the cost of theinstrumentation is prohibitive. For underdeveloped countries confronting this prob-lem, reliable, low-cost instrumentation and reliable field test kits are desperatelyneeded. To address this issue, various low-cost analytical methods and kits that canbe used to monitor arsenic contamination in water are described in Chapters 7 and 8.

A large number of approaches have been investigated for removing arsenicfrom drinking water. The basic chemistry for these processes is discussed inChapters 9 to 14. A number of remediation methods that utilize natural or rela-tively inexpensive materials to purify the water have been discussed. The devicesthat earned the first and second Grainger Challenge Prizes of $1 million and$200,000, respectively, from the National Academy of Engineering, Washington,DC, are described in this volume.

Finally, potential solutions to this devastating problem are provided inChapter 15. Piped potable surface water should be given the desired priority;this will require total commitment from local governments and funding agencies.Other surface water options, such as rainwater harvesting, sand water filters, anddug wells, should be tapped as much as is reasonably possible. The next best

PREFACE xiii

option is deep tube wells. They should be installed properly such that surface con-taminants cannot get into them. Furthermore, the water should be tested properlyto assure that they do not contain other harmful contaminants. Arsenic removalsystems can work for a family or for small communities; however, their reliabil-ity initially and over a period of time remains an issue. Other contaminants inwater can affect their performance. The education and training of local scientistsshould be encouraged in the underdeveloped countries so that they can addressthese problems.

It is believed that this book will be of interest to numerous scientists work-ing in the field of geochemistry, hydrology, analytical chemistry, environmentalchemistry and engineering, and separation science and technology. Academicand regulatory personnel working in these fields, along with aid agencies (WHO,World Bank, UNICEF, etc.) and nongovernmental organizations are also likelyto find a lot of significant information of interest to them. Furthermore, it isanticipated that this book will encourage scientists, environmentalists, engineers,and other well-wishers to rise to the occasion and explore the interesting sci-ence involved in the mechanism of arsenic contamination, develop low-pricedinstrumentation for analysis, and find suitable methods for remediation of theproblem.

Satinder Ahuja

January 5, 2008

1THE PROBLEM OF ARSENICCONTAMINATION OFGROUNDWATER

Satinder AhujaAhuja Consulting, Calabash, North Carolina

INTRODUCTION

Nature and Scope of the Problem

Groundwater contamination by arsenic (As) can occur from a variety of sources,such as pesticides, wood preservatives, glass manufacture, and miscellaneousother arsenic uses. These sources can be monitored and controlled. However,this is not the case with naturally occurring arsenic. The natural content of Asin soil is mostly in a range below 10 mg/kg; however, it can cause major havocwhen it gets into groundwater. The worst case of this problem was discovered inBangladesh, where a large number of shallow tube wells (10 to 40 m) installed inthe 1970s were found in the 1980s to be contaminated with arsenic [1]. Arsenicscreening of 4.73 million tube wells showed 1.29 million wells to be abovethe 50-μg/L level, the acceptable limit in Bangladesh. Since the estimated totalnumber of tube wells in Bangladesh is 8.6 million, it may be concluded thatmore than 2 million wells in Bangladesh are likely to be contaminated. It hasbeen estimated that as many as 100 million people in Bangladesh are exposed tohigh levels of arsenic, exceeding the World Health Organization (WHO) standardof 10 μg/L, or 10 ppb. The maximum contaminant level (MCL) of 10 μg/L fordrinking water has been approved by many countries in the world and has beenenforced since the beginning of 2006 in the United States. It should be mentionedthat the guidelines do not consider different arsenic species, even though it is

Arsenic Contamination of Groundwater: Mechanism, Analysis, and Remediation,Edited by Satinder AhujaCopyright © 2008 John Wiley & Sons, Inc.

1

2 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

already well established that the toxicity of arsenic can vary enormously with itsspeciation (see the discussion below).

Skin lesions and cancers related to arsenic were rare and ignored until newevidence emerged from Taiwan in 1977. The serious health effects of arsenicexposure, including lung, liver, and bladder cancer, were confirmed shortly there-after by studies of exposed populations in Argentina, Chile, and China. In 1984,K. C. Saha and colleagues at the School of Tropical Medicine in Kolkata, India,first attributed lesions observed on the skin of villagers in the state of West Bengalin India to the elevated arsenic content of groundwater drawn from shallow tubewells. As noted in the Preface, the countries affected by elevated arsenic con-centrations in groundwater include Argentina, Bangladesh, Cambodia, Canada,Chile, China, Ghana, Hungary, India, Laos, Mexico, Mongolia, Nepal, Pakistan,Poland, Taiwan, Thailand, the UK, the United States, and Vietnam. Figure 1shows groundwater contamination in the United States; over 31,000 samplesanalyzed over an almost 30-year period revealed that a large number of statesare affected by it. Of various countries affected by this contamination, Bangladesh(see Figure 1 in Chapter 2) and West Bengal are experiencing the most seriousgroundwater arsenic problem, and the situation in Bangladesh has been describedas “the worst mass poisoning in human history.” In addition to consumptionthrough drinking water, arsenic can also be taken up via the food chain. Directconsumption of rice irrigated with arsenic-rich waters is a significant source ofarsenic exposure in areas such as Bangladesh and other countries where rice isthe staple food and provides the main caloric intake.

Arsenic is a semimetal or metalloid that is stable in several oxidation states(−III, 0, +III, +V), but the +III and +V states are the most common in naturalsystems. Arsenic is a natural constituent of the Earth’s crust and ranks twentiethin abundance in relation to other elements. Table 1 shows arsenic concentra-tions in various environmental media. Arsine(−III), a compound with extremelyhigh toxicity, can be formed under high reducing conditions, but its occurrencein gases emanating from anaerobic environments is relatively rare. Arsenic is awell-known poison with a lethal dose in humans of about 125 mg. Most ingestedarsenic is excreted from the body through urine, feces, skin, hair, nails, andbreath. In cases of excessive intake, some arsenic is deposited in tissues, caus-ing the inhibition of cellular enzyme activities. The relative toxicity of arsenicdepends mainly on its chemical form and is dictated in part by the valence state.Trivalent arsenic has a high affinity for thiol groups, as it readily forms kineticallystable bonds to sulfur. Thus, reaction with As(III) induces enzyme inactivation,as thiol groups are important to the functions of many enzymes. Arsenic affectsthe respiratory system by binding to the vicinal thiols in pyruvate dehydroge-nase and 2-oxoglutarate dehydrogenase, and it has also been found to affectthe function of glucocorticoid receptors. Pentavalent arsenic has a poor affinitytoward thiol groups, resulting in more rapid excretion from the body. However,it is a molecular analog of phosphate and can uncouple mitochondrial oxidativephosphorylation, resulting in failure of the energy metabolism system. The effects

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4 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

TABLE 1 Arsenic Concentrations in Environmental Media

Environmental Medium Arsenic Concentration Range

Air (ng/m3) 1.5–53Rain from unpolluted ocean air [μg/L (ppb)] 0.019Rain from terrestrial air (μg/L) 0.46Rivers (μg/L) 0.20–264Lakes (μg/L) 0.38–1000Groundwater (well) (μg/L) 1.0–1000Seawater (μg/L) 0.15–6.0Soil (mg/kg) 0.1–1000Stream/river sediment (mg/kg) 5.0–4000Lake sediment (mg/kg) 2.0–300Igneous rock (mg/kg) 0.3–113Metamorphic rock (mg/kg) 0.0–143Sedimentary rock (mg/kg) 0.1–490

Source: Ref. 2.

of the oxidation state on chronic toxicity are confounded by the redox conversionof As(III) and As(V) within human cells and tissues. Methylated arsenicals suchas monomethylarsonic acid (MMAA) and dimethyl arsenic (DMAA) are lessharmful than inorganic arsenic compounds. Clinical symptoms of arsenicosismay take about six months to two years or more to appear, depending on thequantity of arsenic ingested and the nutritional status and immunity level ofthe person. Untreated arsenic poisoning results in several stages: for example,various effects on the skin with melanosis and keratosis; dark spots on the chest,back, limbs, and gums; and enlargement of the liver, kidneys, and spleen. Later,patients may develop nephropathy, hepatopathy, gangrene, or cancers of the skin,lung, or bladder.

Arsenicosis now seriously affects the health of many people in Bangladesh,India, China, Nepal, and a number of other countries worldwide. In the UnitedStates, nearly 10% of groundwater resources exceed the MCL. Arsenic toxic-ity has no known effective treatment, but drinking arsenic-free water can helparsenic-affected people at the preliminary stage of their illness rid themselvesof the symptoms of arsenic toxicity. Hence, provision of arsenic-free wateris urgently needed for mitigation of arsenic toxicity and the protection of thehealth and well-being of people living in acute arsenic problem areas in thesecountries.

A national policy and implementation plan for arsenic mitigation was devel-oped in Bangladesh in 2004, along with protocols for installation of alternativewater supply options, disposal of arsenic-rich sludge, diagnosis of arsenicosiscases, and water management. However, provision of alternative arsenic-safewater supplies has thus far reached approximately 4 million citizens, only 2.9%of the population of Bangladesh.

INTRODUCTION 5

In this book we focus primarily on groundwater pollution resulting from natu-ral sources. Until recently it was generally believed that arsenic is released in thesoil as a result of weathering of arsenopyrite or other primary sulfide minerals.Currently, it is believed that arsenic pollution of groundwater is a by-product ofthe microbes that metabolize organic matter, a process that releases arsenic, iron,phosphate, bicarbonate, and other species to groundwater. The precise microbialecology responsible for arsenic release is still not fully understood and is thesubject of further investigation. In an attempt to improve our understanding ofthis horrific problem, this book has been planned to improve our understandingand to offer some meaningful solutions:

• Nature and scope of the problem (Chapters 1 and 2)• Mechanism of groundwater contamination (Chapters 3 to 6)• Low-cost analytical methods and testing kits (Chapters 7 and 8)• Remediation methods (Chapters 9 to 14)• Workable solutions (Chapter 15)

Fate of Arsenic in Irrigation Water and Its Potential Impacton the Food Chain

The observation that arsenic poisoning in the world’s population is not consistentwith the level of water intake has raised questions regarding possible pathways ofarsenic transfer from groundwater to the human system. Even if an arsenic-safedrinking water supply could be ensured, the same groundwater will continueto be used for irrigation purposes, leaving a risk of soil accumulation of thistoxic element and eventual exposure to the food chain through plant uptake andanimal consumption. Studies on arsenic uptake by crops indicate that there is greatpotential for the transfer of groundwater arsenic to crops. Chapter 2 deals withthe fate of arsenic in irrigation water and its potential impact on the food chain,particularly as it occurs in Bangladesh and similar environments. Contaminationof the irrigation water by arsenic; the retention, release, distribution, and buildupof arsenic in soil, accumulation in various crops that are irrigated with arsenic-richwater, the result of arsenic accumulation in crops and its consequence on dietaryintake, and possible remedial measures to combat arsenic accumulation in soilsand crops are discussed.

Of the various crops and vegetables analyzed, green leafy vegetables werefound to act as arsenic accumulators, with arum (kochu), gourd leaf, Amaranthus ,and ipomoea (kalmi) topping the list. Arum, a green vegetable commonly grownand used in almost every part of Bangladesh, seems to be unique in that theconcentration of arsenic can be high in every part of the plant. Arsenic in riceseems to vary widely. Speciation of arsenic in Bangladesh rice shows the presenceof As(III), DMAV, and As(V); more than 80% of the arsenic recovered is in theinorganic form. It has been reported that more than 85% of the arsenic in riceis bioavailable, compared to only about 28% of arsenic in leafy vegetables. It is

6 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

thus pertinent to assess the dietary load of arsenic from various food materialsotherwise contaminated with arsenic. A person consuming 100 g [dry weight(DW)] of arum daily with an average arsenic content of 2.2 mg/kg, 600 g (DW)rice with an average arsenic content of 0.1 mg/kg, and 3 L of water with anaverage arsenic content of 0.1 mg/L would ingest 0.56 mg/day, which exceeds thethreshold value calculated based on the U.S. Environmental Protection Agency(EPA) model.

MECHANISM OF ARSENIC CONTAMINATION OF WATER

Until recently it was generally believed that arsenic is released in the soil asa result of weathering of the arsenopyrite or other primary sulfide minerals.Important factors controlling this phenomenon are:

• Moisture (hydrolysis)• pH• Temperature• Solubility• Redox characteristics of the species• Reactivity of the species with CO2/H2O

It has been reported that weathering of arsenopyrite in the presence of oxygenand water involves oxidation of S to SO42− and As(III) to As(V):

4FeAsS + 13O2 + 6H2O ↔ 4SO42− + 4AsO43− + 4Fe3+ + 12H+ (1)

Although there are both natural and anthropogenic inputs of arsenic to the envi-ronment, elevated arsenic concentrations in groundwater are often due to naturallyoccurring arsenic deposits. Whereas the average abundance of arsenic in theEarth’s crust is between 2 and 5 mg/kg, enrichment in igneous and sedimentaryrocks, such as shale and coal deposits, is not uncommon. Arsenic-containingpyrite (FeS) is probably the most common mineral source of arsenic, although itis often found associated with more weathered phases. Mine tailings can containsubstantial amounts of arsenic, and the weathering of these deposits can liberatearsenic into the surface water or groundwater, where numerous chemical and bio-logical transformations can take place. Arsenic can also be released directly intothe aquatic environment through geothermal water, such as hot springs. Anthro-pogenic sources of arsenic include pesticide application, coal fly ash, smeltingslag, feed additives, semiconductor chips, and arsenic-treated wood, which cancause local water contamination.

In Bangladesh and West Bengal, India, where the problem has received themost attention, the aquifer sediments are derived from weathered materials fromthe Himalayas. Arsenic typically occurs at concentrations of 2 to 100 ppm in

MECHANISM OF ARSENIC CONTAMINATION OF WATER 7

these sediments, much of it sorbed onto a variety of mineralogical hosts, includ-ing hydrated ferric oxides, phyllosilicates, and sulfides. The mechanism of arsenicrelease from these sediments has been a topic of intense debate, and both micro-bial and chemical processes have been invoked. The oxidation of arsenic-richpyrite has been proposed as one possible mechanism. Other studies have sug-gested that reductive dissolution of arsenic-rich Fe(III) oxyhydroxides deeper inthe aquifer may lead to the release of arsenic into the groundwater. Additionalfactors that may add further complications to potential arsenic-release mecha-nisms from sediments include the predicted mobilization of sorbed arsenic byphosphate generated from the intensive use of fertilizers, by carbonate producedvia microbial metabolism, or by changes in the sorptive capacity of ferric oxy-hydroxides.

A large number of experts from various countries, including Bangladesh andIndia, who participated in a workshop in Dhaka and in symposia in Atlanta, Geor-gia [3–5], generally agreed that sedimentary arsenic is being carried downstreamto Bangladesh by the Ganges–Padma–Meghna river system. Bacteria have beenimplicated in the desorption and dissolution of arsenic in the anaerobic, reducingenvironment in the subsoil. The results from a study conducted in Araihazar,Bangladesh indicate that the accumulation of arsenic in groundwater and sedi-ment release rate in the uppermost 20 m of the Holocene aquifer appear to befairly constant within a range of favorable geochemical conditions. This sug-gests that the groundwater flow regime controls much of the spatial variabilityof dissolved arsenic concentrations in shallow aquifers; however, this needs tobe tested elsewhere in Bangladesh.

Role of Microbes in the Geochemical Behavior of Arsenicin Groundwater Systems

In Chapter 3 a brief review of high arsenic concentrations in the groundwaterand proposed mechanisms for the release of arsenic into groundwater systemsis provided, with particular significance to the possible role of metal-reducingbacteria in arsenic mobilization into the shallow aquifers of the Ganges delta.The bacterial effects on arsenic behavior in anoxic sediments and the variousinteractions between mineral, microbes, and arsenic, which have a significantimpact on arsenic mobilization in groundwater systems, are also discussed.

Throughout evolution, microorganisms have developed the ability to survivein almost every environmental condition on Earth. Their metabolism dependson the availability of metal ions to catalyze energy-yielding reactions and syn-thetic reactions and on their aptitude to protect themselves from toxic amountsof metals by detoxification processes. Furthermore, microorganisms are capableof transforming a variety of elements as a result of (1) assimilatory processes inwhich an element is taken up into cell biomass, and (2) dissimilatory processesin which transformation results in energy generation or detoxification. Arsenic iscalled an “essential toxin” because it is required in trace amounts for growth andmetabolism of certain microbes but is toxic at high concentrations. However,

8 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

it is now evident that various types of microorganisms gain energy from thistoxic element, and subsequently, these reactions have important environmentalimplications.

Bacterial reduction of As(V) has been recorded in anoxic sediments, whereit proceeds via a dissimilatory process. Dedicated bacteria achieve anaerobicgrowth using arsenate as a respiratory electron acceptor for the oxidation oforganic substrates or H2, forming arsenite quantitatively as the reduction prod-uct. The reaction is energetically favorable when coupled with the oxidation oforganic matter, because arsenate is electrochemically positive; the As(V)/As(III)oxidation–reduction potential is +135 mV. To date, at least 19 species of organ-isms are known to respire arsenate anaerobically, and these have been isolatedfrom freshwater sediments, estuaries, hot springs, soda lakes, and gold mines.They are not confined to any particular group of prokaryotes, and they are dis-tributed throughout the bacterial domain. These microbes are collectively referredto as dissimilatory arsenate-reducing prokaryotes (DARPs), and there are otherelectron acceptors used by these organisms which are strain-specific, includingelemental sulfur, selenate, nitrate, nitrite, fumarate, Fe(III), dimethyl sulfoxide,thiosulfate, and trimethyamine oxide. For example, Sulfurospirillum barnesii (for-merly strain SES-3), a vibrio-shaped gram-negative bacterium isolated from aselenate-contaminated freshwater marsh in western Nevada, is capable of grow-ing anaerobically using As(V) as the electron acceptor, and it can also supportgrowth from the reduction of a variety of electron acceptors, including selenate,Fe(III), nitrate, fumarate, and thiosulfate. The gram-positive sulfate-reducing bac-terium Desulfotomaculum auripigmentum , isolated from surface lake sediments ineastern Massachusetts, has been found to reduce both As(V) and sulfate. DARPscan oxidize a variety of organic and inorganic electron donors, including acetate,citrate, lactate, formate, pyruvate, butyrate, fumarate, malate, succinate, glucose,aromatic hydrogen, and sulfide. Two gram-positive anaerobic bacteria, Bacillusarsenicoselenatis and B. selenitireducens were also isolated from the anoxic,muds of Mono Lake, California. Both grew by dissimilatory reduction of As(V)to As(III), coupled with oxidation of lactate to acetate plus CO2.

Detection of Dissimilatory Arsenate-Respiring Bacteriain North Carolina Groundwater

Dissimilatory arsenate-reducing bacteria (DARB) are considered to be highlyinvolved in arsenic mobilization in anoxic environments. To determine their con-tribution to arsenic levels found in underground aquifers, DARB communities intwo drinking water wells (labeled D and R) located in western North Carolinawere examined by molecular detection methods and enrichment culture tech-niques (Chapter 4). The genes encoding arsenate respiratory reductase (arrA)were amplified with newly developed polymerase chain reaction (PCR) primersfrom the DNA extracted from groundwater samples and were used to examinethe diversity of DARB communities. The enrichment cultures with groundwa-ter samples were established to measure arsenate reduction activities. The arrA

MECHANISM OF ARSENIC CONTAMINATION OF WATER 9

genes detected from both well waters were closely related to the gene found inGeobacter uraniumreducens . A higher diversity of the arrA genes was foundin well R, where a higher amount of arsenic was found. In addition, higherarsenate-reducing activities were measured in the R well water. This might implythat more diverse DARB communities have higher reduction activities, whichlead to higher levels of dissolved arsenic found in groundwater. Using molecularand microbial tools, this study demonstrates the significance of DARB in arseniccontamination in the North Carolina underground aquifers.

Biogeochemical Mechanisms of Arsenic Mobilization and Sequestration

Sediment diagenetic processes are the biogenic and abiotic changes that occur toalter the sediment during and after deposition (see Chapter 5). Sediment diagene-sis involves chemical, physical, and biological processes, including (1) deposition,(2) diffusion, (3) reductive dissolution (and other redox changes), and (4) sec-ondary mineral precipitation. Diagenesis is driven primarily by the mineralizationof organic carbon and the subsequent changes in redox potential with depth. Asthe sediments become more reducing, the redox equilibrium of various chemicalspecies in the sediment shifts. However, it is important to recognize that thekinetics of these reactions is variable and sensitive to environmental parameterssuch as microbial activity. Thus, it is common to observe As(III) and As(V) orFe(III) and Fe(II) co-occurring under a variety of redox conditions because ofkinetic factors.

Reductive Fe(III) oxide dissolution is controlled by a complex interplay ofmany different parameters, such as pH, redox state, mineralogy, biological activ-ity, and solution chemistry. Biologically mediated reduction depends strongly onthe bacterial consortia present in the sediments, as well as substrate availabil-ity (e.g., organic carbon) and Fe oxide crystallinity. The rate of dissolution can,in turn, affect the mineral transformation products, which have the potential tosequester arsenic in more crystalline lattice structures, or the release of arsenicto pore waters as surface binding sites are lost.

A case study illustrates how arsenic partitioning between the solid and dis-solved phases can be affected simultaneously by arsenic redox cycling, sedimentdiagenesis, and pore water composition. Ultimately, the mobility of arsenic insurface and groundwater systems is determined by (1) the arsenic redox state,(2) associations with the solid phase, (3) transformation of the solid phase dur-ing diagenesis, and (4) pore water composition, which can also change as aresult of diagenetic processes. Many of these parameters are driven by microbialprocesses. This interplay of biogeochemical mechanisms makes understandingthe processes responsible for arsenic mobilization in the environment inevitablycomplex.

Geomicrobiology of Iron and Arsenic in Anoxic Sediments

Microbially mediated reduction of assemblages comprising arsenic sorbed toferric oxyhydroxides is gaining consensus as the dominant mechanism for the

10 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

mobilization of arsenic into groundwater. For example, a recent microcosm-basedstudy provided the first direct evidence for the role of indigenous metal-reducingbacteria in the formation of toxic mobile As(III) in sediments from the Gangesdelta (see Chapter 6). This study showed that the addition of acetate to anaerobicsediments, as a proxy for organic matter and a potential electron donor for metalreduction, resulted in stimulation of microbial reduction of Fe(III), followed byAs(V) reduction and the release of As(III). Microbial communities responsiblefor metal reduction and As(III) mobilization in the stimulated anaerobic sedimentwere analyzed using molecular (PCR) and cultivation-dependent techniques. Bothapproaches confirmed an increase in numbers of metal-reducing bacteria, princi-pally Geobacter species. However, subsequent studies have suggested that mostGeobacter strains in culture do not possess the arrA genes required to supportthe reduction of sorbed As(V) and mobilization of As(III). Indeed, in strainslacking the biochemical machinery for As(V) reduction, Fe(II) minerals formedduring respiration on Fe(III) have proved to be potent sorbants for arsenic presentin the microbial cultures, preventing mobilization of arsenic during active ironreduction. However, the genomes of at least two Geobacter species (G. unra-niumreducens and G. lovleyi ) do contain arrA genes , and interestingly, genesaffiliated with the G. unraniumreducens and G. lovleyi arrA gene sequences havebeen identified recently in Cambodian sediments stimulated for iron and arsenatereduction by heavy (13C–labeled) acetate using a stable isotope-probing tech-nique. Indeed, the type strain of G. unraniumreducens has recently been shownto reduce soluble and sorbed As(V), resulting in mobilization of As(III) in thelatter case. Thus, some Geobacter species may play a role in arsenate releasefrom sediments. However, other well-known arsenate-reducing bacteria, includ-ing Sulfurospirillum species, have also been detected in 13C–amended Cambo-dian sediments and hot spots associated with arsenic release in sediments fromWest Bengal. Although the precise mechanism of arsenic mobilization in South-east Asian aquifers remains to be identified, the role of As(V)-respiring bacteria inthe process is gaining support. Indeed, recent studies with Shewanella sp. ANA-3and sediment collected from the Haiwee Reservoir (Olancha, California) havesuggested that such processes could be widespread, but not necessarily driven byAs(V) reduction, following exhaustion of all bioavailable Fe(III). In this study,arsenate reduction started before Fe(III) reduction and ceased after 40 to 60 hours.During part of the experiment, arsenate and Fe(III) were reduced simultaneously.

ANALYTICAL METHODS

It is not difficult to determine arsenic at 10 ppb or an even lower level in water.A number of methods can be used for determining arsenic in water at the ppblevel:

• Flame atomic absorption spectrometry• Graphite furnace atomic absorption spectrometry

ANALYTICAL METHODS 11

• Inductively coupled plasma-mass spectrometry• Atomic fluorescence spectrometry• Neutron activation analysis• Differential pulse polarography

Very low detection limits for arsenic down to 0.0006 μg/L can be obtainedwith inductively coupled plasma mass spectrometry (ICP-MS). The speciationof arsenic requires separations based on solvent extraction, chromatography, andselective hydride generation. High-performance liquid chromatography (HPLC)coupled with ICP-MS is currently the best technique available for the determi-nation of inorganic and organic species of arsenic; however, the cost of theinstrumentation is prohibitive. For underdeveloped countries confronting thisproblem, the development of reliable, low-cost instrumentation and reliable fieldtest kits would be very desirable (see the discussion below).

Development of Low-Cost Measurement Technologies for On-SiteArsenic Determination

Hydride generation (HG) has been known for many decades and has the advan-tage that arsenic may be determined by a relatively inexpensive atomic absorptionspectrometer or an even cheaper atomic fluorescence spectrometer (AFS) atsingle-digit μg/L concentrations (see Chapter 7). Its generation is prone to infer-ence from other matrix components, so every “new” matrix can represent a newanalytical problem. In this technique, arsenic compounds are converted to volatilederivatives by reaction with a hydride transfer reagent, usually tetrahydroborateIII (also known as borohydride), whose sodium and potassium salts are relativelystable in aqueous alkalis. HG can be quite effective as an interface betweenhigh-performance liquid chromatographic separation and element-specific detec-tion. In fact, it is possible to get the same performance from HG-AFS as fromICP-MS. Therefore, as the former detector represents a significant saving in bothcapital and operational costs over the latter, there is considerable interest in this“niche” use of AFS. The disadvantage of any method in which an atomic spec-trometry instrument is involved is that the procedure has to be carried out in alaboratory setting in which appropriate supplies of electricity, gas, power, andsometimes cooling water are available.

Accurate, fast measurement of arsenic in the field remains a technical chal-lenge. Even though the technological advances in a variety of instruments havemet with varying success, the central goal of developing field assays that reli-ably and reproducibly quantify arsenic has not yet been achieved. The exquisiteselectivity of the hydride generation process is very tempting as an integral pre-treatment stage, but always comes with the associated issues of safety unlessall of the arsine generated ends up bound to a solid surface somewhere in theapparatus. However, it will be quite some time before the Gutzeit method isobsolete.

12 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

There are clearly many prospects for the further development of chemical mea-surement technologies for the determination of arsenic in environmental watersdown to single-digit μg/L concentrations. In this scenario, techniques such asatomic fluorescence spectrometry are good candidates for such a “lab” facility.The quartz crystal microbalance, a device whose interface is more robust thanan electrode for stripping voltammetry, also holds promise, especially as themeasurement incorporates an inherent preconcentration step (the accumulationof arsenic at the surface of the oscillating crystal).

Reliability of Test Kits

In a comprehensive study (see Chapter 8) using three field kits (Merckoquant,NIPSOM, and GPL), more than 290 wells were tested against reference methods.At arsenic concentrations below 50 μg/L, the false positive results were accept-ably low: 9.2% (NIPSOM) and 6.5% (GPL). In the range between 50 and 100μg/L, false positives increased to 35% and 18%, respectively. As a result, usingthe NIPSOM kit, 33% of the unsafe tube wells were colored green (i.e., safe).The false negative results reported between 50 and 100 μg/L were 57% and 68%,which means that up to two-thirds of the wells painted red were safe. Above 100μg/L, the percentage of false negatives was still considerable (26% for NIPSOMand 17% for GPL). The mislabeling of 45% of the wells is unacceptable butcannot be explained easily by possible variability of arsenic concentration, a lackof quality assurance/quality control, or operator error. This study clearly demon-strates the need for a stringent testing procedure to generate reliable data and theneed to develop more reliable analytical systems that would make costly retestingunnecessary. Emphasis should be given to new developments of electrochemicalmethods and their potential to form the basis of a “new generation” of field kitsthat satisfy all requirements for reliable arsenic detection in the field.

Two main approaches are used at present for the on-site analysis of arsenic.By far, the most widely used systems are those based on a colorimetric principle.These systems require few reagents, are supposedly easy to use, and give resultsthat should be straightforward. The second approach is based on electroanalysisand on the possibility of reducing or oxidizing arsenic species. Although moredifficult to operate, the detection limits obtained with such devices can be muchlower than those obtained by colorimetry. The EPA supports an environmen-tal technology verification (ETV) program to facilitate the implementation ofinnovative new technologies for environmental monitoring. The ETV works inpartnership with recognized standards and testing organizations, with vendors anddevelopers, and with stakeholders for potential buyers. The ETV tested severalcommercially available kits in July 2002 and added four more in August 2003under field conditions with trained and untrained operators [6]. An assessmentof the new generation of field testing kits is provided in Table 2 of Chapter 8.

In contrast to colorimetric field tests kits, which are selective for arsenic only,voltammetric systems can be tuned to detect other metals simultaneously. Forexample, at a gold electrode, metals such as Cu, Hg, Zn, Mn, or Sb can be detected

REMEDIATION OF ARSENIC-CONTAMINATED WATER 13

along with arsenic. For instance, antimony has been shown to modulate the toxic-ity of arsenic [7] and was found in samples containing high levels of arsenic [8];35% of the arsenic-contaminated groundwater samples from Bangladesh alsocontained levels of manganese above the WHO value of 500 ppb [9]. Knowl-edge of the concentrations of other metals brings insights into the toxicity of thesample and is the basis for efficient removal technologies. Field systems shouldtherefore be as selective as possible.

The performance of field testing kits for arsenic is unsatisfactory overall,although the new generation of kits have become much more reliable. The reportof false negative and false positive results of over 30% is not unusual, althoughthe latest seem encouraging, and more reliable measurements can be done inthe field. However, these studies were using a water standard of 50 μg/L as adecisive concentration. If the new WHO guideline of 10 μg/L is adopted as adecision-making criterion, the sensitivity of most arsenic testing kits based oncolorimetric methods will not be sufficient. This is particularly the case for kitsthat are battery powered and also for electronic systems. Although some reportssurprisingly suggest that in some cases untrained operators produce more reliableresults, the training aspect of the operator should not be underestimated. Voltam-metric sensors should be ideally suited for on-site analysis of arsenic. However,the need for a chemical reduction step seems to be the major problem, limitingpotential applications both in the field and in sample throughput. Systems usingvoltammetric reduction of arsenate are probably easier to implement for on-siteanalysis, although the SafeGuard system developed by TraceDetect Inc. hasremarkable performance rates. Potential problems in the voltammetric determi-nation of arsenic are numerous because of the sample matrix effect. To be highlyefficient for on-site analysis with unqualified operators, these voltammetric sys-tems should therefore be as sensitive as possible to allow dilution of the originalsample. For such purposes, the use of microelectrodes is preferred. In addi-tion, specifically designed software and hardware are required to guide the userthrough the simple actions required to make the analytical system more accessibleto unqualified operators. Although promising results have been obtained usingvoltammetric systems, more work is needed to develop methods toward thesearsenic species. The most promising development in direct arsenic speciation isby electrochemical detectors, but they still have to be tested in the field.

REMEDIATION OF ARSENIC-CONTAMINATED WATER

A large number of approaches have been investigated for removing arsenic fromdrinking water. Several useful reviews of the techniques for removing arsenicfrom water supplies have been published [10–13]. Existing and emerging arsenicremoval technologies include:

• Coagulation with ferric chloride or alum• Sorption on activated alumina

14 THE PROBLEM OF ARSENIC CONTAMINATION OF GROUNDWATER

• Sorption on iron oxide–coated sand particles• Granulated iron oxide particles• Polymeric ligand exchange• Nanomagnetite particles• Sand with zero-valent iron• Hybrid cation-exchange resins• Hybrid anion-exchange resins• Polymeric anion exchange• Reverse osmosis

Reverse osmosis is essentially a nonselective physical process for exclud-ing ions with a semipermeable membrane The basic chemistry for the rest ofthe processes includes either or both of the following interactions: (1) As(V)oxyanions are negatively charged in the near-neutral pH range and therefore canundergo coulombic or ion-exchange types of interactions; and (2) As(V) andAs(III) species, being fairly strong ligands or Lewis bases, are capable of donat-ing lone pairs of electrons. They participate in Lewis acid–base interactions andoften show high sorption affinity toward solid surfaces that have Lewis acidproperties.

Flocculation of Arsenic with the Mucilage of Opuntia ficus-indica

The design of a benign and sustainable water purification technology based onnatural products is gaining interest because of the inherently renewable charac-ter, low cost, and nontoxicity. The use of mucilage, derived from nopal cactus,Opuntia ficus-indica, provides a reliable technology to treat drinking water sup-plies that have been contaminated with particulates and toxic metals (Chapter 9).The long-term goal is to deploy the optimized design in rural and underdevel-oped communities in Mexico, where drinking water supplies are contaminatedwith toxic metals, the nopal cactus is readily available and amenable to sustain-able agriculture, and where access to conventional technologies is limited. Thiswork shows how cactus mucilage is extracted from the nopal cactus and usedas a flocculant to remove particulates and heavy metals from drinking water.Mucilage efficiency to reduce arsenic and particulates from drinking water hasbeen determined by light scattering, jar tests, atomic absorption, and hydridegeneration–atomic fluorescence spectroscopy. Comparisons against a syntheticflocculant [i.e., aluminum sulfate, alum, Al2(SO4)3] show the high efficiencyof that cactus mucilage to separate particulates and arsenic from drinking water.These flocculation studies prove that mucilage is a much faster flocculating agentthan alum, with the efficiency increasing with mucilage concentration. Jar testsreveal that lower concentrations of mucilage provide the optimal effectiveness forsupernatant clarity, an important factor in determining the potability of water. Thiswork has established a systematic approach for providing clean water that can beexpanded to communities of other underdeveloped or even developed countries