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WSN 10 (2015) 111-139 EISSN 2392-2192

Environmental, economic and health perspectives of arsenic toxicity in Bengal Delta

Sayan Bhattacharya1,2,*, Uday Chand Ghosh2 1Department of Environmental Studies, Rabindra Bharati University, Kolkata, India

2Department of Chemistry and Biochemistry, Presidency University, Kolkata, India

*E-mail address: [email protected] , [email protected]

ABSTRACT

Arsenic is a metalloid of great environmental concern because of its highly toxic nature and

wide abundance. Arsenic contamination in groundwater has been reported in Bangladesh, India,

China, Taiwan, Vietnam, USA, Argentina, Chile and Mexico. The clinical symptoms of arsenic

toxicity in human body include skin itching to sun rays, burning and watering of the eyes, weight loss,

loss of appetite, weakness, fatigue, limited physical activities and working capacities, chronic

respiratory problems, moderate to severe anemia etc. The Bengal basin is regarded to be the most

acutely arsenic infested geological province in the world. Heavy withdrawal of groundwater for

fulfilling the needs of the increasing population in Bengal Basin resulted in increased arsenic level in

the groundwater. Mobilization of arsenic in Bengal delta is further interfered by microbial activities

and interactions. Different microbial strains have been isolated from Bengal Delta which can tolerate,

transform and resist arsenic. The use of arsenic contaminated groundwater for irrigation purpose in

crop fields elevates arsenic concentration in surface soil and in the plants grown in these areas. Several

plant species have been studied for their ability to accumulate arsenic in the Bengal Delta. Rice is

generally grown in submerged flooded condition, where arsenic bioavailability is high in soil. As

arsenic species are very much toxic to plants and can execute oxidative stresses, they can also affect

the overall production of rice and other vegetables, and can affect the agricultural and economic

development of Bengal Basin. Cattle population also consume arsenic infested water in those areas

and usually eat edible plants contaminated with arsenic, which, in turn, can further increase the

toxicity level in their bodies and also can increase the arsenic bioaccumulation in meat and milk. In the

World Scientific News 10 (2015) 111-139

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rural areas of the Bengal Delta, arsenic contamination raised a number of social problems which are

continuously weakening the structural integrity of rural society. Uses of phytoremediation and

microbial remediation technologies, rainwater harvesting, use of arsenic resistant plant varieties for

cultivation are some sustainable methods which can be applied for arsenic remediation. Besides, nano-

agglomerates of mixed oxides have been synthesized and successfully employed for arsenic removal

from aqueous solutions. Intensive investigation on a complete food chain is urgently needed in the

arsenic contaminated zones, which should be our priority in future researches.

Keywords: Arsenic; Bengal Delta; Toxicity; Bioaccumulation Groundwater; Phytoremediation

1. INTRODUCTION

Arsenic (As) is a metalloid (atomic no. 33) of great environmental concern because of

its highly toxic nature and colossal abundance. Since hundreds of years, people have worked

and lived with this dangerous element, many people have died of its toxicity and even more

have been sickened by it. Today, arsenic still remains a part of our daily lives and millions of

people are being chronically exposed to elevated doses of arsenic through their food, air,

water, and soils with unknown long-term health consequences (Mondal and Suzuki, 2002).

Since its isolation in 1250 A.D. by Albertus Magnus, this element has been a center of

controversy in human history. Its therapeutic use started from about 400 B.C. and many

reports on arsenic are documented by Hippocrates, Aristotle, Dioscorides and Pliny the Elder

(Gontijo and Bittencourt, 2005). Since then, arsenic has been the object of many studies and

imaginary legends and traditions.

Arsenic ranks 20th

in abundance in the earth’s crust, 14

th in the seawater and 12

th in the

human body (Mondal and Suzuki, 2002). Arsenic naturally occurs in over 200 different

mineral forms, of which around 60% are arsenates, 20% are sulfides and sulfosalts and the

rest 20% are arsenides, arsenites, oxides, silicates and elemental arsenic (Bissen and Frimmel,

2003). Major arsenic containing primary minerals are arsenopyrite (FeAsS), realgar (As4S4),

and orpiment (As2S3). Realgar (As4S4) and orpiment (As2S3) are the two common reduced

forms of arsenic whereas the oxidized form is the mineral arsenolite (As2O3). Other naturally

occurring arsenic bearing minerals include loellingite (FeAs2), safforlite (CoAs), niccolite

(NiAs), rammelsbergite (NiAs2), arsenopyrite (FeAsS), cobaltite (CoAsS), enargite

(Cu3AsS4), gerdsorfite (NiAsS), glaucodot ((Co,Fe)AsS), and elemental arsenic (Greenwood

and Earnshaw, 1989).

Exposure to sufficiently high concentrations of inorganic arsenic in natural environment

has proved to be harmful to the organisms. The main pathways of arsenic exposure to the

human beings include ingestion of drinking water and consumption of foods and, to a lesser

extent, inhalation of air. In addition to the natural occurrence, herbicides, insecticides,

desiccants, feed additives, wood preservatives, chemical warfare agent, constituents of

organic and inorganic pigments and drugs are significant sources of arsenic (Bissen and

Frimmel, 2003). The geological contamination is till date the most alarming source of arsenic

in many countries. Arsenic contamination in groundwater has been reported in Bangladesh,

India, China, Taiwan, Vietnam, USA, Argentina, Chile and Mexico (Figure 1). In many of

these places the concentration has exceeded the permissible limit of 50 ppb recommended by

WHO (WHO, 2001).


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2. ARSENIC TOXICITY IN HUMAN BODY

The first observation on the possible carcinogenic action of arsenic was reported in

1822, when Paris detected that cattle grazing close to foundries developed cutaneous

neoplasms in the hips and attributed the cause of these tumors to arsenic-containing expelled

gases (cited in Gontijo and Bittencourt, 2005). There is a long list of internal malignant

neoplasms related to arsenic toxicity, comprising tumors in the digestive, genital and urinary

tracts, lungs and upper respiratory tracts (Gontijo and Bittencourt, 2005). The most

characteristic manifestations are palmar and plantar keratoses, which are usually multiple,

symmetric and punctiform, preferably located lateral aspects of palms, base and sides of

fingers, soles, heels and toes (Gontijo and Bittencourt, 2005). Arsenic is a protoplasmatic

poison that exerts its toxicity by inactivating approximately 200 enzymes, especially those

involved in the production of cell energy and those related to DNA synthesis and repair

(Ratnaike, 2003). Recent studies have been able to detect chromosomal aberrations and sister

chromatid exchange in individuals exposed to arsenic and also highlight the action of reactive

oxygen species produced by arsenic compounds in inducing skin tumors (Mahata et al., 2003;

Matsui et al., 1999).

Arsenic is mainly excreted via urine and feces, although there are some minor routes of

excretion (e.g. in sweat) (WHO, 2001). Since arsenic can accumulate in keratin-containing

tissues (Benko et al., 1971), skin, hair and nails can also be considered as potentially minor

excretory routes. Several studies indicate that arsenic can also be excreted in human milk,

although the levels are low (Grandjean et al., 1995; Concha et al., 1998). Normally inorganic

arsenic is very quickly cleared from human blood. For this reason blood arsenic is used only

as an indicator of very recent and/or relatively high level exposure (e.g. in poisoning cases) or

in cases of chronic stable exposure (i.e. from drinking water) (Ellenhorn, 1997). Studies show

that in general blood arsenic does not correlate well with arsenic exposure in drinking water,

particularly at low levels (Bissen and Frimmel, 2003).

The chemical forms and oxidation states of arsenic are more important as regards to

toxicity. Toxicity also depends on other factors such as physical state, gas, solution, or

powder particle size, the rate of absorption into cells, the rate of elimination, the nature of

chemical substituents in the toxic compound, and, of course, the pre-existing state of the

patient. While it is generally accepted that methylation is the principal detoxification pathway,

recent studies have suggested that methylated metabolites may be partly responsible for the

adverse effects associated with arsenic exposure (Gontijo and Bittencourt, 2005).

Various types of skin manifestation and other arsenic toxicity were observed from

melanosis, keratosis, hyperkeratosis, dorsal keratosis, and non-pitting oedema to gangrene and

cancer (Ghosh and Singh, 2010). Overall prevalence of clinical neuropathy was observed in

different populations of South 24 Parganas, North 24 Parganas, Murshidabad, Nadia and

Malda districts of West Bengal and in the states of Bihar, Uttar Pradesh, Jharkhand and

Chhattisgarh (Bhattacharya et al., 2007). The children in the affected areas are more prone to

contamination than the adults. The following features are commonly noted from the arsenic

affected populations of India:

1. Skin itching to sun rays, burning and watering of the eyes, weight loss, loss of appetite,

weakness, fatigue, limited physical activities and working capacities.


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2. Chronic respiratory complains are also common. Chronic cough with or without

expectoration was evident in more than 50% affected populations.

3. Gastrointestinal symptoms of anorexia, nausea, dyspepsia, altered taste, pain in abdomen,

enlarged liver and spleen, and ascites (collection of fluid in abdomen).

4. Moderate to severe anemia was evident in some cases,

5. Conjunctival congestion, Leg edema was less common (Ghosh and Singh, 2010).

3. ARSENIC CONTAMINATION IN INDIA

In India, since the groundwater arsenic contamination was first detected from West-

Bengal in 1983, a number of other states, namely; Jharkhand, Bihar, Uttar Pradesh in flood

plain of the Ganga River; Assam and Manipur in flood plain of the Brahamaputra and Imphal

rivers, and Rajnandgaon village in Chhattisgarh state have chronically been exposed to

drinking arsenic contaminated hand tube-wells water above permissible limit of 50 μg/L

(Bhattacharya et al., 2007). Many more North-Eastern Hill States in the flood plains are also

suspected to have the possibility of arsenic in groundwater. Even with every additional

survey, new arsenic affected villages and people suffering from arsenic related diseases are

being reported. All the arsenic affected river plains have the river routes originated from the

Himalayan region. Over the years, the problem of groundwater arsenic contamination has

been complicated, to a large variability at both the local and regional scale, by a number of

unknown factors.

Since groundwater arsenic contamination was first reported in year 1983 from 33

affected villages in four districts in West Bengal, the number of villages has increased to 3417

in 111 blocks in nine districts till 2008 in West Bengal alone (Bhattacharya et al., 2007). In

1999, the arsenic groundwater contamination and its health effects in Rajnandgaon district of

Chattisgarh state were also detected. In 2002, two villages, Barisban and Semaria Ojhapatti, in

Bhojpur district, located in the western part of the Bihar state, were reported having

contamination exceeding 50 μg/L. As of 2008, out of 38 districts of Bihar, 57 blocks from 15

districts having total population of nearly 10 million have been reported to be affected by

arsenic groundwater contamination above 50μg/L (Bhattacharya et al., 2007). During 2003,

25 arsenic affected villages of Ballia district in Uttar Pradesh and people suffering from skin

lesions came into limelight. During 2003-2004, the groundwater arsenic contamination and

consequent suffering of hundreds of people were reported from 17 villages of the Sahibgunj

district of Jharkhand state, in the middle Ganga plain (Ghosh and Singh, 2010). In 2004,

arsenic concentration was also reported from Assam in pockets of 2 districts (Bhattacharya et

al., 2007). In 2007, Manipur state, one of the seven North-Eastern Hill States, had also come

into limelight of arsenic contamination in groundwater.

Till 2008, West Bengal, Jharkhand, Bihar, Uttar Pradesh in flood plain of Ganga River;

Assam and Manipur in flood plain of Brahamaputra and Imphal rivers, and Rajnandgaon

village in Chhattisgarh state have so far been exposed to drinking arsenic contaminated hand

tube-wells water (Ghosh and Singh, 2010). The area and population of these states are 529674

km2

& approx. 360 million respectively, in which 88688 km. and approximately 50 million

people have been projected vulnerable to groundwater arsenic contamination (Bhattacharya et

al., 2007).


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Figure 1. World map showing known occurrences of groundwater with elevated

concentrations of geogenic arsenic (adapted from Nriagu et al., 2007).

The affected zones are highlighted.

4. THEORIES OF ARSENIC MOBILIZATION

There are several hypotheses of arsenic mobilization (Ravenscroft et al., 2001; Nickson

et al., 2000). The authors assumed that arsenic is adsorbed under oxidizing conditions as

pentavalent arsenic (arsenate) and becomes mobile as the more toxic trivalent arsenic under

reducing conditions (arsenite). Therefore, three types of release mechanisms have been

discussed:

4. 1. Oxidation of arsenic-bearing sulfides

In connection with the drawdown of the water table, caused by massive extraction of

irrigation water, oxygen enters the anoxic aquifer. The oxidation mainly of pyrite is

anticipated and yields a release of sorbed As(III) as As(V) (Ravenscroft et al., 2001).

However, neither researches showed a relation between the distribution of arsenic pollution

and that of irrigation nor the sediments contain adequate amounts of arsenical pyrite to be

important for this mechanism. Furthermore, the release of sulfate during the oxidation of

pyrite should correlate with the dissolved arsenic. In reality, iron and sulfate are mutually

exclusive in solution (Ravenscroft et al., 2001; Kinniburgh and Smedley, 2001). Lastly, it

should be mentioned that arsenic pollution is uncommon in hand-dug wells (< 10 m) which

are shallowest and therefore most of them are exposed to atmospheric oxygen but they are

commonly arsenic free (Ravenscroft et al., 2001).


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4. 2. Competitive exchange of phosphate from fertilizer

This hypothesis is based on the greatly increased use of phosphate fertilizer over the

past 15 years in Bangladesh. But there is no causal link between arsenic and the application of

fertilizer. Among other reasons experiments showed that only 2 μg/L of arsenic would be

desorbed by 5 μg/L phosphorus concentration (upper limit in Bangladesh groundwater) in

groundwater (Ravenscroft et al., 2001).

4. 3. Reduction and dissolution of FeOOH

This explanation is the most widely accepted one at the moment. Arsenic pollution

occurs because FeOOH is reduced by microbes driven oxidation of organic matter and caused

the release of arsenic, which is generally attached on the FeOOH particles (Bhattacharya et.

al., 2007). Sorbed As(V) is generally reduced to As(III) by this mechanism. Arsenic

accumulates as a result of an extremely low regional hydraulic gradient that caused low

flushing rates (Kinniburgh et al., 2003).

5. GROUNDWATER ARSENIC CONTAMINATION IN BENGAL BASIN

The sources of arsenic in Bengal Delta is geogenic and related to the sediments

deposited by the rivers Ganges, Brahmaputra, and Meghna (Nickson et al., 2000). Bengal

delta is one of the largest deltas of the world and accommodates an enormous volume of

sediments deposited during the Tertiary and Quaternary periods (Bhattacharya et al., 2007).

During the late Holocene period, marshy or swampy lowlands developed in several parts of

the Bengal Delta Plain and formed the peat and other sediments rich in organic matter

(Umitsu, 1993). The pool of arsenic-contaminated groundwater is hosted by the sediments

deposited by the meandering river channels during the late Quaternary or the Holocene age.

Lithology of these sedimentary successions includes sand, representing the channel facies,

and the overbank facies, comprising predominantly silt and clay, and exhibit a typical fining

upward character (Bhattacharya et. al., 2007). The fine-grained overbank facies are rich in

organic matter (Nickson et al., 1998; Bhattacharya et al., 1999; Mazumdar, 2000).

In groundwater, inorganic arsenic commonly exists as As(V) (arsenate) and As(III)

(arsenite), among which the trivalant state is considered to be more mobile and toxic for

living organisms (Bhattacharya et al., 2007). The mobility of arsenic is mainly determined by

the adsorption capacity on the mineral surfaces, which is controlled by geochemical

parameters such as pH, Eh, ionic composition, and mineral type (Bissen and Frimmel, 2003).

However, redox processes in the aquifers trigger the mobility through dissolution of the Fe

oxides that transfer substantial amounts of arsenic into the aqueous phases (Bhattacharya et

al., 2007).

The Bengal basin is regarded to be the most acutely arsenic affected geological province

in the world (Mukherjee et al., 2008). The Bengal arsenic disaster is possibly the worst

environmental disaster in the history of human civilization, and much more serious than

Bhopal Gas Tragedy or Chernobyl Nuclear disaster. The Bengal basin was formed by the

sedimentation of the river Ganga, Brahmaputra and Meghna, along with their tributaries and

distributaries (Mukherjee et al., 2008). The densely populated areas of Bangladesh and India,

although traversed by two of the world’s largest river systems and recipient of several meters


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of rainfall yearly, faces unparalleled water-supply problems (Roychowdhury, 2008). More

than 100 million people are living in the arsenic affected districts of India and Bangladesh.

In West Bengal, during 1980’s, some cases of arsenical dermatosis in the districts of

North 24 Parganas, South 24 Parganas, Nadia, Murshidabad and Burdwan were reported. By

the end of December 2001, this problem spreads from few villages to 2065 villages of 75

blocks in 8 districts. 9 districts out of 19 in West Bengal, 78 blocks and around 3150 villages

are affected with arsenic-contaminated groundwater (Chakraborti et al., 2002). Groundwater

is regularly used for agricultural and household purposes in these areas. As rainwater is

insufficient to support the water demand of the increasing population and intensive

agricultural system of West Bengal, thousands of shallow tube-wells were installed for

irrigation in last 40-45 years (Roychowdhury, 2008). A vast majority of these tube wells have

been installed privately with locally available expertise, without any check of the quality and

yield of the water that originates from them. Heavy withdrawal of groundwater, especially

during that lean period, resulted in oxygenated decomposition of pyrites forming Fe2+

and

Fe3+

sulphates and sulphuric acid, which in turn are responsible for arsenic mobilization

(Bhattacharya et al., 2007). Due to different hydro-chemical behaviors of aquifers in the

Bengal delta, the redox potential, pH, temperature, Ca-HCO3 or Ca-Mg-HCO3 water type

vary substantially which control the arsenic dynamics in the area. Chakraborti and his group

analyzed over 1,25,000 water samples for studying arsenic contamination in the Bengal basin

and reported that arsenic levels in the affected areas of the basin vary between <1 and 1300

mg/l (Chakraborti et al., 2002, 2004).

6. ACCUMULATION, BEHAVIOUR AND DISTRIBUTION OF ARSENIC IN SOIL

The sources of arsenic and other trace elements may be anthropogenic or they can

naturally occur as minor constituents in soil compartments (Kabata-Pendias and Pendias,

1984). Generally, arsenic is deposited on the top soils from burning of fossil fuels, mining and

smelting activities, incineration of wastes, cement production, application of arsenic contained

fertilizers and pesticides, disposal of sludge containing arsenic etc (Bhattacharya et al., 2007).

Besides, the use of arsenic contaminated groundwater for irrigation purpose in crop fields

elevates arsenic concentration in surface soil and in the plants grown in those areas (Meharg

and Rahman, 2003). Many millions of cubic meters of underground water are used for

agricultural irrigation. Much of this groundwater is contaminated with arsenic, which is

deposited in the soil in contact with the irrigation water throughout the year. In Bangladesh

and West Bengal most of the arsenic added from contaminated irrigation water in rice

cultivation remains on the surface soil (Norra et al., 2005). The reason behind it is the soil

used for paddy cultivation is deliberately puddled to hold water on the surface and they

develop a compacted, impervious ploughpan (Brammer, 1996). Ferric hydroxide plays an

important role in controlling the concentration of arsenic in soils and in aqueous media, and

the phenomenon can be expressed by the following reaction:

Fe3+

+ 3H2O ↔ Fe(OH)3 + 3H+ (Naidu and Bhattacharya, 2006).

Arsenic is associated with the primary sulfides; (hydro) oxides of Al, Fe and Mn; clays;

sulfates; phosphates and carbonates in the Bengal Delta Plain (Foster, 2003).


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Arsenic is released in the soil by weathering of arsenopyrite (FeAsS) and sulfide

minerals. The grain sizes of soil particles play an important role in controlling the distribution

and mobility of arsenic. The surface area of the fine-grained particles is large and hence can

adsorb more arsenic (Bhattacharya et al., 2007). For this reason, clay minerals, Fe-, Al-, and

Mn (hydro) oxides are important sinks for arsenic in the aquifers and sediment layers in

Bengal Delta. Arsenic in soil normally occurs in pentavalent state under oxidizing conditions

while under reducing conditions trivalent As(III) species prevail that is more mobile and

bioavailable (Bhattacharya et al., 2012a).

Arsenic also can get biomethylated (i.e. addition of CH3 to arsenic through biological

activity) in the soil–water, sediment-water interfaces through the activity of bacteria (such as

Escherichia coli, Flavobacteriun sp, Methanobacterium sp) and fungi (such as Aspergillus

glaucus, Candida humicola). In the course of biomethylation, As(III) is oxidized to As(V) and

CH3O+ is reduced to CH3− and stable arsenic oxy-species was formed (Bhattacharya et al.,

2007) (Figure 2).

Figure 2. Arsenic cycle in the environment (Mukhopadhyay et al., 2002).


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Soil arsenic levels are very much related with local well water arsenic concentration,

which suggests that the source of soil contamination is the irrigation water (Bhattacharya et

al., 2012a). The absorption of arsenic by plants is influenced by the concentration of arsenic in

the soil. In Bangladesh, where irrigation is carried out with arsenic contaminated

groundwater, soil arsenic level can reach up to 83 mg/kg (Roychowdhury et al., 2005a).

Except in the rainy season, the agricultural land has been exposed to irrigated groundwater

round the year. Sometimes the farmers used to run the shallow tube wells in the rainy season

due to insufficient rain. Most of the vegetables and other crops, used by the villagers were

cultivated in this area and entered the local market.

7. MICROBIAL TRANSFORMATION OF ARSENIC IN THE ENVIRONMENT

Microorganisms have remarkable ability to evolve the necessary molecular mechanisms

to cope with and even benefit from high concentrations of toxic metals in the environment.

Mobilization of arsenic in natural ecosystems is predominantly controlled by microbial

activities and interactions. Microorganisms can control the toxicity of arsenic by either

actively exporting it out of the cell or by chemically altering it to a less toxic form. Some

microorganisms can use arsenic to their advantages and can involve it in energy generating

reactions (Mukhopadhyay et al., 2002). Generally, microbial transformations of arsenic can

be divided into two broad categories: redox conversions of inorganic forms and conversions

from inorganic to organic form and vice versa (by methylation and demethylation). The genes

that encode the proteins involved in arsenic resistance in bacteria are either of plasmid or

chromosomal origins, and have been best studied in Escherichia coli. The arsenate reductase

genes (arrA and arrB) have been identified in a number of bacteria, which can reduce arsenic

from pentavalant to trivalent form (Mukhopadhyay et al., 2002). Microbial processes

involved in arsenic reduction and mobilization are many times faster than inorganic chemical

transformations. Arsenate reductases have been found in phylogenetically diverse groups of

microorganisms. These genes are often found on plasmids, making them available for lateral

gene transfer and distribution within arsenic bearing environments. Arsenite oxidase catalyzes

the oxidation of highly toxic trivalent arsenic to less toxic pentavalant arsenic

(Mukhopadhyay et al., 2002). Although detoxification and energy generating processes are

the most common mechanisms involved in the transformations of arsenic, both prokaryotic

and eukaryotic organisms can also methylate arsenic. Some organisms can utilize methylated

arsenic compounds as carbon and energy sources in a process known as demethylation

(Oremland and Stolz, 2005). Bacteria from the genera Alcaligenes, Pseudomonas, and

Mycobacterium have been found to demethylate arsenic compounds. So, microbiological

processes can either solubilize arsenic, thereby increasing their bioavailability and potential

toxicity, or can immobilize arsenic, and thereby reduce the bioavailability.

Several strains of microorganisms (like Bacillus arsenicus, Bacillus indicus,

Pseudomonas sp., Acinetobactor sp., etc.) have been isolated from Bengal delta which can

resist or tolerate arsenic. 16s rRNA sequencing revealed unique characters of these organisms

isolated from the region (Islam et al., 2004; Shivaji et al., 2005). These biotransformation

processes are important components of biogeochemical cycles and may also be used in

bioremediation of arsenic. Arsenic holds a number of varying valence states, which highly

differ in toxicity and mobility. A thorough understanding of microbial populations, their


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ecological behavior and interactions in the natural environment is very much important and

relevant for using these organisms in arsenic bioremediation.

8. ARSENIC BIOACCUMULATION AND TOXICITY IN PLANTS

8. 1. Molecular mechanisms of arsenic biotransformation

The arsenic concentrations in the edible parts of a plant depend on the availability of the

soil arsenic and the accumulation and translocation ability of a plant (Huang et al., 2006).

Several plant species such as ferns have been studied for their ability to accumulate arsenic.

The Chinese brake fern (Pteris vittata) was reported as the hyperaccumulator of arsenic and

can remove arsenic from soil (Ma et al., 2001). Arsenic(V) is also reduced by other plant

species such as Indian mustard (Brassica juncea) and Arabidopsis thaliana (Tripathi et al.,

2007). However, these species have a lower arsenic accumulation capacity compared to P.

vittata. High amount of arsenic can be accumulated in vegetables including arum, Kachu sak

(Colocasia antiquorum) and Ipomea sp. (kalmi) (Das et al., 2004). Other plants like maize,

barley and ryegrass, Spertina alterniflora can also accumulate arsenic efficiently into plant

bodies (Sadiq, 1986; Jiang and Singh, 1994; Carbonell et al., 1998).

Studies on the impact of the arsenic-contaminated groundwater irrigation on crops have

attracted attention recently (Roychowdhury et al. 2002; Norra et al., 2005; Huang et al., 2006;

Rahman et al., 2007). These works document the enrichment of arsenic in soil and a limited

uptake by different plants, including rice, cereals, and vegetables. Mandal and Suzuki (2002)

have reported that the arsenic concentration in plants varied from less than 0.01 to about 5.0

mg kg-1

. Roychowdhury et al. (2002), studying arsenic affected areas of Murshidabad, West

Bengal, India, found that the accumulation of arsenic in various food composites (potato skin,

leaves of vegetables, rice, wheat, cumin, turmeric powder, and cereals) ranged between

<0.0004 and 0.693 mg·kg-1

. It has also been proved that arsenic can accumulate in the rice

grains upto 7.5 mg/kg even when the source of contamination is 2 km. away (Roychowdhury

et al., 2002).

Plants rarely accumulate arsenic at concentrations that can create health hazards,

because phytotoxicity usually occurs before reaching such concentrations (Walsh et al.,

1977). However, to assess the risk posed by arsenic in the diet, arsenic speciation must be

ascertained, since inorganic arsenic (arsenate and arsenite) is more toxic than methylated

organic (MMA and DMA) forms found in plants (Meharg and Hartley-Whitaker, 2002;

Abedin et al., 2002).

Besides the anthropogenic sources of arsenic species such as sodium arsenate or sodium

salts of methylarsonate acid (weed killer), calcium arsenate (herbicide), lead arsenate

(insecticide), etc, methylated arsenic species are formed in soils by biomethylation processes.

The inorganic and methylated forms can reach the rice grains though an absorption

mechanism (Pizarro et al., 2003).

In general, plants uptake and metabolize As(V) through the phosphate transport

channels (Tripathi et al., 2007). Because of their chemical similarity, arsenic competes with

phosphate for root uptake and interferes with metabolic processes like ATP synthesis and

oxidative phosphorylation (Tripathi et al., 2007). Arsenate is taken up by phosphate

transporter in plants grown on aerobic soils. The presence of phosphate has a negative effect

on the bioaccumulation of pentavalant arsenic in plants (Liu et al., 2004).


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If arsenate has lower concentration than phosphate level in soil, phosphate is a much

better competitor for uptake than arsenate (Meharg et al., 1994). In contrast, arsenite is the

predominant species of arsenic in soil under anaerobic conditions (Masscheleyn et al., 1991).

Recent studies have shown that in rice plants, arsenite is taken up at high rates of influx which

follows the Michaelis-Menten kinetics (Abedin et al., 2002). In rice, arsenic uptake shares the

same pathway as silicon uptake. Rice is a strong accumulator of silicon which may allow

efficient uptake and translocation of arsenite in the shoots (Hodson et al., 2005). The

aquaporin Lsi1 (protein) which is responsible for the influx of silicon (as silicic acid) into the

root cells, is permeable to arsenite (Ma et al., 2008). The reason behind it is the similarity

between the size and coordination chemistry of silicic acid and arsenite. Inside plant tissues,

arsenic is reduced from pentavalent to trivalent state or is biotransformed to less toxic organic

compounds such as DMA, MMA (Ma et al., 2008). Trivalent arsenic can form complex with

thiol groups inside the plant tissues (Zhao et al., 2010). Many plants can synthesize arsenic

reductase, which can convert the pentavalant arsenic into trivalent form (Zhao et al., 2010).

Interestingly, trivalent arsenic is the predominant species in the plant tissues and 50-65 % of

the total arsenic accumulated in stem or leaf parts is trivalent (Peralta-Videa et al., 2009). The

arsenic detoxification in plants involves arsenic mobilization from roots to aerial parts of the

plant (translocation). This movement is controlled by the external arsenic concentration (Zhao

et al., 2010). Plants generally have a low efficiency of arsenic translocation from roots to

shoots, may be due to the formation of complexes of arsenite and thiol compounds and

subsequent sequestration in the root vacuoles, or because of the strong efflux of arsenite to the

external medium (Zhao et al., 2009).

Toxicity limit and mobility of arsenic in soil depend on the properties of soil like

particle size, texture, mineral nutrient content, pH, presence of other ions, moisture regimes,

transformations by microbes and the chemical form of arsenic (Bhattacharya et al., 2007).

Arsenic is more mobile and bio-available in sandy soil than in clayey soil. The effect of

arsenic toxicity in plants increase in low pH, but the uptake mechanism can enhance in higher

pH soil. These properties of soil are very much relevant to evaluate the influence of arsenic on

its accumulation and distribution in plants. Arsenic accumulation in soil irrigated by arsenic

contaminated water and its transfer into rice may vary depending on the soil types, cropping

pattern, arsenic concentration in irrigation water, distance from the water source, depth of

water source and duration of the monsoon flood (Hossain et al., 2008). The typical transfer

factor for arsenic varied from 0.01 to 0.1 (Kloke et al., 1984). The volume of water used for

the irrigation of a specific crop varies considerably depending not only on climatic factors, but

also on the permeability of soil. The water demand of rice is particularly high; the volume of

water used for irrigation of Boro rice in the Indo-Gangetic Plain are in the range between

1000 and 1800 mm/a (Gupta et al., 2002; Huq et al., 2003).

In seasonally flooded soil, soil change between the oxidized state in the dry season and

the reduced state in the wet season when the soil is submerged (Bhattacharya et al., 2012). So

arsenic may present in the soil in different form and in different concentration in different

times of the year. However, flooding leads to fundamental changes in the soil. The overlying

floodwater inhibits oxygen movement into the soil and the remaining oxygen is depleted

within a short time (Bhattacharya et al., 2007). Rice plants are generally grown in submerged

soil condition, where arsenic bioavailability is generally high (Bhattacharya et al., 2007). Rice

roots can constitutively form aerenchyma and waterlogging or O2-deficient conditions can

enhance the process (Colmer, 2003a; Colmer, 2003b; Colmer et al., 2006).


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In roots, O2 is essential for respiration to provide sufficient energy for growth,

maintenance, and nutrient uptake processes, and up to 30–40% of the O2 supplied via the root

aerenchyma is being lost to the soil by the process of radial oxygen loss (Colmer, 2003a;

Colmer, 2003b). Formation of aerenchyma and radical oxygen loss can enhance the

waterlogging tolerance mechanisms of rice plants (Colmer, 2003a; Colmer, 2003b). There is a

significant correlation between radical oxygen loss and arsenic tolerance and accumulation in

rice plants (Mei et al., 2009).

It is known that rice maintains relatively high redox potentials in the rhizosphere, by a

continuous flux of O2 from the shoots toward the roots. The release of O2 enables the

accumulation of Fe-oxyhydroxides (typically lepidocrocite, goethite, ferrihydrite) in the

rhizosphere of living plants (Norra et al., 2005). Consequently, facultative and obligate

anaerobes use oxidized forms in soil for respiration, e.g., iron hydroxide is reduced to Fe2+

and simultaneously arsenic dissolves into soil solution (Inskeep et al., 2002). Further, rice

plant can carry oxygen from the air through stems and can distribute it into the root zone. The

oxidized zone created around the root area helps to form oxidized iron plaque. Iron plaque can

efficiently bind arsenic and can reduce its translocation to the above ground tissues (straw,

husk and grain) of the plant (Liu et al., 2006). It has also been reported that the accumulation

of arsenic in rice plants is highest in the root zones and decease significantly in the upper parts

of the plant (Bhattacharya et al., 2009). Arsenate has a high affinity for iron plaque and can

react with Fe (III) to give the highly insoluble iron arsenate. The concentration of arsenic in

rice roots can reach as high as 160 mg/kg due to the formation of iron plaques (Bhattacharya

et al., 2007). So, iron plaque can affect arsenic dynamics significantly in the rhizosphere

zones (Norra et al., 2005). Arsenate behaves as a phosphate analogue, and like phosphate is

relatively immobile in soil (Liu et al., 2004). Arsenic phytotoxicity is generally greater in

sandy soils than that of clayey soils as sandy soils contain comparatively low amounts of iron

oxides and clays (Sheppard, 1992).

8. 2. Arsenic toxicity in plant seeds

Seed is one of the most vital components of the world’s diet. Cereal grains alone, which

comprise around 90% of all cultivated seeds, contribute up to half of the global per capita

energy intake (Bewley, 1997). Not surprisingly then, seed biology is one of the most

extensively researched areas in plant physiology. With the seed, the independence of the next

generation of plants begins. The seed, containing the embryo as the new plant in miniature, is

structurally and physiologically equipped for its role as a dispersal unit and is well provided

with food reserves to sustain the growing seedling until it establishes itself as a self-sufficient,

autotrophic organism. In comparison, seed germination frequency and the early seedling

growth are more sensitive to metal toxicity because some of the plants’ defense mechanisms

have not yet developed and hence effects at early stages of plant development can be very

useful for toxicity assessment (Xiaoli et al., 2007). Seed germination is the first physiological

process affected by metals (Shanker et al., 2005).

Heavy metals including arsenic have been reported to stimulate the formation of free

radicals and reactive oxygen species leading to oxidative stress (Zhang et al., 2003; Azevedo

et al., 2005). Interestingly, some recent reports have provided experimental evidence that

arsenic-induced generation of free radicals can cause cell damage and death through

activation of oxidative sensitive signaling pathways (Kamat et al., 2005). Requejo and Tena

(2005) proved that the induction of oxidative stress is the main process underlying arsenic


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toxicity in plants. Plants respond to oxidative stress by increasing the production of

antioxidant enzymes such as superoxide dismutase (SOD), ascorbate peroxidase (APX) or

peroxidase (POD) (Bhattacharya et al., 2012b, Bhattacharya et al., 2013a). The increase in

lipid peroxidation, superoxide dismutase (SOD) activity (Hartley-Whitaker et al., 2001a) and

peroxidase (POD) activity (Miteva and Peycheva 1999) were correlated with increasing

arsenic stress. According to them, arsenic accumulated in the plant tissue stimulates

peroxidase synthesis during the early phases of plant development, long before the visible

changes take place. Bengal delta is mainly agriculture dependent and the effects of arsenic

toxicity in seed germination can in turn affect the agricultural economy of that area.

Figure 3. Simplified scheme summarizing the effects of metals and metalloids on seeds; the

images of Oryza sativa (rice) are used to represent seeds and plants generally.

Kranner and Colville, 2011).

8. 3. Arsenic contamination in rice

Globally, over 400 million metric tons of milled rice is consumed each year, which

accounts for around 50% of total cereal consumption of the world (IRRI, 2007). Rice is the

main crop in West Bengal and 73% of the calorific intake of the people is from rice. Huge

amount of water is needed for rice cultivation. Rice is generally grown in submerged flooded

condition, where arsenic bioavailability is high in soil (Duxbury and Panaullah, 2007). Rice is

much more efficient at assimilating arsenic into its grain than other staple cereal crops

(Williams et al., 2007). Arsenic accumulation in soil irrigated by arsenic contaminated water

and its transfer into rice may vary depending on the soil types, cropping pattern, arsenic

concentration in irrigation water, distance from the water source, depth of water source and


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duration of the monsoon flood (Hossain et al., 2008). As(ІІІ) and MMA (Monomethyl arsenic

acid) are phytotoxic to rice plants and the degree of arsenic uptake in rice followed as

As(ІІІ)>MMA>As(V)>DMA (Meharg and Rahman, 2003). The root, shoot and leaf tissue of

rice plant contain mainly inorganic As (III) and As (V) while the rice grain contain

predominantly DMA (85 to 94%) and As (III) (Liu et al., 2005).

Drinking water is responsible for 13% of total arsenic intake where cooked rice can

contribute up to 56% of total intake which indicates rice can contribute most of the daily

arsenic intake (Ohno et al., 2007). Rice can contribute 51-60% of the dietary intake of arsenic

in Bangladesh if rice grains contain 0.5-0.7 mg/kg of arsenic and drinking water contains 100

ppb of arsenic in average (considering 2 litre/day water consumption by each person) (Meharg

and Rahman, 2003). According to a study, rice grain can contribute about 95% of arsenic,

with respect to the dietary intakes of arsenic from the food samples (Roychowdhury, 2008).

Infants and young people generally have higher exposure to arsenic through rice on a body

mass basis, when compared to the adults (Meharg et al., 2008). The median increased lifetime

cancer risk from drinking water, rice and cooking of rice was found to be 48%, 44% and 8%

respectively (Mondal and Polya, 2008). It has been reported that the concentration of arsenic

in cooked rice was higher than that of raw rice. The concentration of arsenic in rice is

increased when it was cooked with arsenic contaminated groundwater and the gruel was not

discarded after cooking. The arsenic was absorbed in the cooked rice from water, thus

increased the concentration (Roychowdhury, 2008).

Arsenic concentration in boro rice is much higher than in aman variety, because much

higher amount of water is needed for boro cultivation compared to aman cultivation

(Williams et al., 2006). Some researchers found a much higher transfer factor of arsenic from

soil to the shoots of rice plants compared to wheat and barley (Su et al., 2010). Additionally,

unpolished rice has been reported to contain higher arsenic concentrations compared to

polished rice samples because the milling and polishing process significantly reduces arsenic

concentration in rice grains. Arsenic also highly accumulates in the grain surface, which is

one of the main causes of such difference. Modern health conscious people generally prefer

whole grain diet, which, in turn can increase the toxicity level in human body if the grains are

contaminated with arsenic.

Rice bran is a byproduct of polishing whole grain rice. Stabilized rice bran extract,

which is also known as bran soluble, is sold as a “natural superfood” and “premier heath food

product”, because of its high food value. A number of companies supply this product to

malnourished children as daily ration in many countries. The supplement has already been

used in Malawi, Guatemala, and El Salvador, with plans to expand further into Latin America,

India, and the Caribbean. Proper risk assessment has not been done yet on this product, which

can be a source of arsenic contamination (Sun et al., 2008). A wide range of other rice

products are used as baby foods such as cereal dusts, noodles, biscuits- all of which can be

significant sources of arsenic contamination in infants and babies (Bhattacharya et al., 2012).

Apart from rice, arsenic has been found in some vegetables in Bengal Delta including data

sak and lal sak, kalmi (Bengali name), Indian mustard, maize, barley etc. (Huq et al., 2006).

As arsenic species are very much toxic to plants, they can affect the overall production

of rice and other vegetables, and can affect the economy of a country as whole. Arsenic

toxicity can reduce the rate of photosynthesis in rice plants, which, in turn can reduce the

chlorophyll content, and can affect the growth and yield of rice (Rahman et al., 2007). Rice

yield has been reported to decrease by 10% when the concentration of arsenic in soil is as


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high as 25 mg/kg (Xiong et al., 1987). It has been shown that in T-Aman rice (Oryza sativa

L.), the grain and straw yields were significantly reduced by the artificial introduction of

arsenic in soil (Azad et al., 2009). Experiments proved that arsenic in the growth medium of

rice seedlings caused quantitative changes in the level of RNA, soluble proteins, free amino

acids and proline and inhibits the activities of RNase and protease. Further, arsenite toxicity

can lead to change in isoform pattern of RNase in growing rice seedlings. The impact of such

responses could be visible in the form of decreased growth and vigour of rice seedlings in

arsenic-polluted environments (Mishra and Dubey, 2006). Figure 3 shows the adverse effects

of arsenic and other heavy metal toxicity on rice plants.

The major portion of the population at risk of arsenic poisoning also suffer from

malnutrition in the Bengal delta of India and Bangladesh, and there is a direct link between

malnutrition and the risk of arsenic poisoning in human body (Milton et al., 2010). The long

term effects are slowly executed by more cancers, stillbirths, defective childbirths,

hypertension and lots of other diseases (Milton et al., 2010). Several studies showed that

arsenic accumulation significantly affects body weight, biological and intellectual

development in children (Majumder et al., 2000). This, in turn, can affect the social and

economic structure of a family as a whole. Rural populations generally eat locally-grown rice

in arsenic affected areas and the chances of arsenic toxicity are much more than the urban

populations. In urban areas, middle class and rich people usually eat wheat, vegetables and

potatoes from mixed or diverse sources, which contain lesser amount of arsenic in general.

Difference in locations and economic classes should be examined properly in dietary studies

to identify the problem in details.

The accumulation of arsenic in plants occurs primarily through the root systems and the

highest arsenic concentrations have been reported in plant roots and tubers. The edible parts

of the tuber crops are exposed continuously to soil and irrigation water contaminated with

arsenic, which, in turn, can increase arsenic level in the tuber part (Bhattacharya et al.,

2012a).

Rice straw is often used as a cattle feed in South Asia. This represents another entry

route of arsenic into the food chain, as rice straw typically contains much higher amount of

arsenic than grains .Cattle population also used to drink water contaminated with arsenic in

those areas, which, in turn, can further increase the toxicity level. But the toxicity mechanism

is highly dependent on the nature of arsenic species in the straw and on the metabolism of the

cattle (Abedin et al., 2002). Cattle manure is often used as fuel in household purposes, which

can also increase the contamination risk (Pal et al., 2007). Besides, the dry straw often been

used by people as fuel, which can release arsenic in air as oxides, and can cause pollution and

health hazards.

8. 4. Mitigation policies for arsenic contamination in rice

When arsenic is accumulated in the rice grains, it is very difficult to remove it properly.

Cooking with high volume of arsenic free water can help to some extent, but, it can also

remove beneficial vitamins and thus can decrease the food value. Levels of toxicity also

depend on the cooking methods which vary in different regions. For example, in Hungary,

rice is generally cooked with an excessive amount of water and the water remaining is

discarded; on the other hand, in China, rice is generally cooked with aliquots of water in order

to absorb it all (Mihucz et al., 2007).


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Aerobic cultivation of rice can effectively reduce arsenic bioaccumulation because in

anaerobic submerged field condition, the chance of arsenic bioavailability is high (Xu et al.,

2008). Rainwater can be an effective source which can be used in irrigation in large scale,

through proper water management systems, which, in turn can reduce the use of arsenic

contaminated water in irrigation in respective contaminated zones. Rainwater can also reduce

the concentration of arsenic in surface soil, which, in turn, can reduce the accumulation of

arsenic in rice and other edible plants. The annual average rainfall in India is 4000 billion

cubic meters, but the annual water requirement of India is only 450 billion cubic meters (Rao,

1995). It clearly indicates that the main problem of Indian water resources is mismanagement

and unsustainable use of water, which, in turn, is making the whole situation paradoxical. We

should focus on the proper management of rainwater, which, in turn can reduce the

groundwater consumption in agricultural sectors. In West Bengal and Bangladesh, the surface

water bodies like rivers, wetlands, flooded river basins and oxbow lakes are among the largest

in the world (Roychowdhury et al., 2005a). Due to our negligence most of these water bodies

is now contaminated with waste materials and pollutants. If we can use these resources

properly for drinking, cooking, agricultural irrigation and other purposes, then we can save

the possible arsenic contamination from groundwater. Pisciculture, duckery, vegetable

cultivation at the bank of these water bodies can also improve the economic condition of the

local people. Proper watershed management and villager participation are needed to assist in

the utilization of these huge bodies of water. Where soils are contaminated with arsenic and

an alternative safe irrigation supply cannot easily be provided, farmers should be encouraged

to increase crop production under rainfed conditions where this is practical. Another

management system can be done by providing the affected population with proper protein diet

and protein supplements which can create a shield against arsenic poisoning (Maiti and

Chatterjee, 2000).

Phytoremediation is an emerging sustainable technology which can be successfully used

to remove heavy metals like arsenic from the contaminated soil, sediments, groundwater and

surface water (Alkorta and Garbisu, 2001). The plants which can accumulate arsenic in their

bodies should be identified properly so that they can be used in a large scale to remove arsenic

from soil effectively. Recently, researchers from the University of Georgia have established a

new technique by introducing genetically modified plants which can tolerate arsenic and have

the capacity to remove arsenic from contaminated sites (Dhankher et al., 2002). As some

specific plants like the Chinese brake fern have the capacity to accumulate arsenic, there is a

possibility of existence of some genes which are responsible for the detoxification

mechanism. Further research can be done for the isolation of these genes, which can be used

effectively in arsenic bioremediation process. Genetic variation is an important factor for

controlling arsenic uptake, speciation and reaction to arsenic stress (Tripathi et al., 2007). The

accumulation of arsenic in rice plants varied with different variety of rice (Norton et al.,

2009). For example, arsenic speciation analysis showed that brown rice can accumulate

higher amount of inorganic arsenic than white rice (Meharg et al., 2008). Experiments

showed that Indian basmati rice varieties has 5-fold lower total arsenic content, and 2.75-fold

lower inorganic arsenic content than the US rice varieties (Williams et al., 2005).

Arsenic resistant genotypes of rice varieties should be considered to reduce the

accumulation rate, which, in turn, can solve the problem partly. Some genotypes which can

accumulate lesser amount of arsenic, specially the trivalent toxic form, can be used in

breeding programmes and genetic researches for identifying the beneficial genes which can


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decrease trivalent arsenic in grains. Production of rice grains that possess a high

organic/inorganic arsenic ratio could help to reduce inorganic arsenic contamination level.

Development of a toxicity database is very much essential for different rice cultivars

and other crops for setting standards for arsenic in flooded and non-flooded soils. The low

levels of inorganic arsenic detected in Chinese rice could be important in rice cultivation to

reduce the dietary exposure (Williams et al., 2006). Phytoexcluder crops for arsenic can also

be developed through laboratory techniques or through proper identification of existing plant

species for growing crops in arsenic contaminated soils without absorbing arsenic (Hossain et

al., 2008). A search should be made for locally-adapted wetland plants that could extract

arsenic from soils during the monsoon season when deeply flooded.

9. Use of nanotechnology in arsenic mitigation

Nanomaterials are fast emerging as potent candidates for water treatment in place of

conventional technologies which, notwithstanding their efficacy, are often very expensive and

time consuming. This would be in particular, immensely beneficial for developing nations

like India and Bangladesh where cost of implementation of any new removal process could

become an important criterion in determining its success. Qualitatively speaking

nanomaterials can be substituted for conventional materials that require more raw materials,

are more energy intensive to produce or are known to be environmentally harmful. Employing

green chemistry principles for the production of nanoparticles can lead to a great reduction in

waste generation, less hazardous chemical syntheses, and an inherently safer chemistry in

general (Shah and Ahmed, 2011). However, to substantiate these claims more quantitative

data is required and whether replacing traditional materials with nanoparticles does indeed

result in lower energy and material consumption and prevention of unwanted or unanticipated

side effects is still open to debate. There is also a wide debate about the safety of

nanoparticles and their potential impact on the environment. There is fervent hope that

nanotechnology can play a significant role in providing clean water to the developing

countries in an efficient, cheap and sustainable way. On the other hand, the potential adverse

effects of nanoparticles cannot be overlooked either. For instance the catalytic activity of a

nanoparticle can be advantageous when used for the degradation of pollutants, but can trigger

a toxic response when taken up by a cell. So this Janus face of nanotechnology can prove to

be a hurdle in its widespread adoption. However as mentioned before nanotechnology can

step in a big way in lowering the cost and hence become more effective than current

techniques for the removal of contaminants from water in the long run (Shah and Ahmed,

2011). In this perspective nanoparticles can be used as potent sorbents as separation media, as

catalysts for photochemical destruction of contaminants; nanosized zerovalent iron used for

the removal of metals and organic compounds from water and nanofiltration membranes

(Savage and Diallo, 2005).

Two vital properties make nanoparticles highly lucrative as sorbents. On a mass basis,

they have much larger surface areas compared to macro particles. They can also be enhanced

with various reactor groups to increase their chemical affinity towards target compounds.

These properties are increasingly being exploited by workers to develop highly selective and

efficient sorbents for removal of organic and inorganic pollutants from contaminated water.

Many materials have properties dependent on size. Hematite particles with a diameter of 7

nm, for example, adsorbed Cu ions at lower pH values than particles of 25 or 88 nm diameter,

indicating the enhanced surface reactivity for iron oxides particles with decreasing diameter.


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Peng et al. (2005) have developed a novel sorbent with high surface area (189 m2/g)

consisting of cerium oxide supported on carbon nanotubes (CeO2-CNTs). They showed that

the CeO2-CNT particles are effective sorbents for As(V). This goes to show that how

chemically modified nanomaterials can enhance the adsorption capacity of a traditional

substance. Deliyanni et al. (2003) have also synthesized and characterized a novel As(V)

sorbent consisting of akaganeite [β-FeOOH] nanocrystals. For the removal of metals and

other inorganic ions, mainly nanosized metal oxides. Equilibrium adsorption of As(III) and

As(V) by nanocrystalline TiO2 occurred within 4 hours and the adsorption followed pseudo-

second-order kinetics in experiments conducted by Peng et al. (2005). Bang et al. (2005)

reported that equilibrium was reached in 63 minutes for adsorption of As(V) adsorption and

240 minutes for adsorption of As(III). Manna et al. (2004) investigated the removal of As(III)

using a synthesized crystalline hydrous titanium dioxide. They found that 70% of As(III)

adsorption occurred within the first 30 minutes of contact time. Nano-agglomerates of mixed

oxides such as iron–cerium, iron–manganese, iron–zirconium, iron–titanium, iron–chromium,

cerium-manganese etc. have been synthesized and successfully employed for pollutant

removal (i.e. arsenic, fluoride etc.) from aqueous solutions (Gupta et al., 2011, Gupta et al.,

2012, Bhattacharya et al., 2013b). Metals such as zinc and tin possess similar reduction

capabilities of iron (Boronina, 1995). Like iron, these metals are converted to metal oxides in

the decontamination process. Other metals have been combined with iron as well to produce

similar results. Both iron-nickel and iron-copper bimetallic particles have been demonstrated

to degrade trichloroethane and trichloroethene (Lien, 2001).

Another example is iron-platinum particles, which possess similar capabilities in

degrading chlorinated benzenes (Lien, 2001). The photo-oxidation of As(III) to As(V) in the

presence of TiO2 and light and subsequent adsorption into TiO2 has also been reported. Bissen

et al. (2001) have showed that photo-oxidation of As(III) to As(V) occurs within minutes. No

reverse reaction of As(V) to As(III) was observed, and while As(III) was oxidized by UV

light in the absence of TiO2, the reaction was way too slow to be feasible in water treatment.

The reaction rate did not depend on the pH of the solution. Peng et al. (2005) reported that

rapid photo-oxidation of As(III) to As(V) occurred in the presence of sunlight, nanocrystalline

TiO2, and oxygen. In natural groundwater, Peng et al. (2005) believed that oxidation of

As(III) to As(V) and subsequent adsorption of As(V) onto TiO2 would completely eradicate

arsenic at slightly acidic pH values.

Nanoparticles in particular will have important impacts on various fields of

environmental technology and engineering not least in water treatment. However most of

techniques for the treatment of wastewater involving nanotechnology so far have only been

investigated in laboratory scale and not all of them are likely to be feasible alternatives for

existing treatment technologies mainly perhaps due to economic reasons (Gupta et al., 2011).

This makes it difficult to predict what the future holds for us at this stage concerning this

nascent technology. Also the incorporation of nanomaterials into existing water purification

systems is another key challenge. Membrane processes such as RO, NF are becoming the

standardised water purification techniques for public utilities and industry because they are

flexible, scalable, modular and relatively easy to operate and maintain (Shah and Ahmed,

2011). Thus further laboratory investigations and pilot scale testing will be needed to integrate

novel nanostructured membranes into existing water purification systems. Also the

environmental fate and toxicity of a material are areas of concern in material selection and

design for water purification.


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10. CONCLUSIONS

In the rural areas of the Bengal Delta, arsenic contamination raised a number of social

problems which are continuously weakening the structural integrity of rural society. Married

women affected by arsenic are sent back to their parents with their children. Affected children

are prevented from attending school, and many young men are deprived of jobs when

employers find black spots on their body and hands. Affected young unmarried women and

men are advised to stay unmarried. They are not allowed even to go to their neighbors to fetch

water. (Biswas et al., 1998; Samanta et al., 1999).

The overall scenario can reflect that the contamination of arsenic in water, soil as well

as in the food chain in Bengal Delta must be addressed properly to understand the importance

of arsenic exposure from food sources. The importance of rice in food security in India and

Bangladesh and the high dietary intake of rice indicate that the impact of arsenic in

groundwater on rice productivity and quality should continue to be carefully monitored

(Roychowdhury et al., 2005a). Arsenic may serve as essential role in growth and nutrition in

minute concentration, but excess intake can be lethal for a population. Arsenic is an element,

not a compound, so it is not possible to break or metabolize it chemically or biologically. The

only thing we can do is to remove it or to prevent it from accumulation and contamination.

Intensive investigation on a complete food chain is urgently needed in the arsenic

contaminated zones, which should be our priority in future researches (Bhattacharya et al.,

2012). Differences in environmental and socio-economic conditions within the contaminated

regions and within countries need to be considered in a sustainable manner.

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( Received 12 May 2015; accepted 30 May 2015 )

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