1

ORIGIN OF THE PROBLEM In the process of human evolution, some of the issues confronting today’s society are safe

guarding the natural environment and maintaining a good quality of life. Access to

Safe Drinking Water is essential to health, a basic human right and a component of

effective policy for health protection. However, a slight imbalance in any equilibrium in the

environment is bound to manifest itself in the form of pollution. Pollution of water is

increasing steadily due to rapid population growth, industrial exploitation, urbanization,

increasing living standard and wide spheres of human activities.

Toxic metals/metalloids are the important member of dirty dozen clubs of pollutants.

Arsenic, a metalloid, is found in virtually all the eco-systems of the environment. Due to its

high toxicity and discreetness, arsenic has been called “The King of Poisons” and is

classified as a human carcinogenic substance of group 1 by IARC (International Agency for

Research on Cancer). Arsenic toxicity causes skin lesions, damage mucous membranes,

nervous system, gastrointestinal, cardiovascular, mutagenic etc. Considering the lethal

impact of arsenic on human health, environmental authorities have taken a more stringent

attitude towards the presence of arsenic in water. The scale of the problem of arsenic

poisoning is illustrated by the frequently used term “Mass Poisoning”. WHO and EPA

re-established a guideline of 0.01mg/L as the maximum allowable arsenic content. Arsenic,

therefore, is under strict watch, globally.

The conventional methods used for abatement of arsenic are often restricted because of

technical, economical and environmental constraints. Arsenic has also been removed by

means of the various simple methods viz precipitation with lime, alum, lanthanum, sodium

sulfide/hydrogen sulfide and co-precipitation with ferric sulfate. These methods involve the

use of large amount of chemicals and suffer from a post treatment problem regarding the

sludge, containing large amount of water having chances of leaking of arsenic back to

environment after exposure to water and air. Further, recent research findings have raised

strong doubts about the use of synthetic coagulants for water purification.

The speciation of arsenic in environmental materials is of interest because of the differing

levels of toxicity exhibited by the various species. The toxicological, physiological and

geochemical behaviour depends on its oxidation state. Toxicity is especially due to inorganic

arsenic. Inorganic arsenic species, i.e. arsenite and arsenate, predominantly found in natural

water, are more toxic than organic species such as monomethylarsonic acid and

dimethylarsinic acid. Thus, it is important to determine each of the arsenic species rather

than the total arsenic. The speciation of arsenic has been attempted using non degradable

inorganic chemical species. Not much attention has been paid towards the use of

biomaterials to alleviate current problems of arsenic from water bodies.

2

“The implementation of the local environment tactics for achieving the

environmental goals in a localized region of environment is Green

Environmental Strategy and the need of the day”

To combine technology with environmental safety is one of the key challenges of the new

millennium. There is a global trend of bringing technology into harmony with natural

environment, thus aiming to achieve the goals of protection of ecosystem from the

potentially deleterious effects of human activity and finally improving its quality. The

challenges of treating and diagnosing environmental problems require discovery of newer,

more potent, specific, safe and cost effective synthetic or natural molecules. The magic

plants/products/wastes are around and waiting to be discovered and commercialized.

The emerging concept of “Clean Production and Process” is a new principle guiding the

next generation products and processes. “Bioremediation: A Green Solution to

Pollution”, involves processes that reduce overall treatment cost through the application of

indigenous agricultural wastes which are particularly attractive as they lessen reliance on

expensive water treatment chemicals, negligible transportation requirements and offer

genuine, local resources as appropriate solutions to tackle local issues of water quality.

Regeneration of the biomaterials increases the cost effectiveness of the process thus,

warrants its future success. Use of plant materials with an aim to effectively alleviate the

economic aspects allowing, further extension of water supply to rural areas of developing

countries.

Recently, agricultural wastes exhibiting potential for the decontamination of toxic

metals/metalloids from water bodies have been found to have drawbacks of less sorption

efficiency and stability, restricting their commercial use. Current research is oriented

towards the Structural Modifications onto the biomaterials leading to the Enhancement

of Binding Capacity, Selectivity and Environmental Stability in terms of their

Reusability are, therefore, in great demands. A search for low cost easily available

adsorbents for abatement and chemical speciation of arsenic has led to the investigation of

biomaterial of agricultural origin and becomes an area of current research.

Keeping above views in mind, the present work explores the unexploited property of some

agricultural wastes [Zea mays corn cob & Leucaena leucocephala seeds] as a

bioremedial approach for the abatement of arsenite and arsenate from water bodies and

their modifications leading to the formation of “Novel Biomaterials” with enhanced

sorption efficiency and environmental stability reaching to the level of their

commercialization.

3

ARSENIC: ENVIRONMENT TERRORIST Water is undeniably the most valuable natural resource existing on our planet. Without this

invaluable compound, the life on the Earth would be non-existent. The vitality of water is

mandatory to all forms of life and fundamental for environmental health and management.

However, due to Technological Development, Agricultarization, Urbanization and

Industrialization, the levels of pollutants have been steadily rising. So, the pollution

problem of industrial waste water is becoming more and more serious in the world.

Among all of the pollutants, heavy metals/metalloids have been a major cause of the

environmental pollution. Effluents from textile, leather, tannery, electroplating, galvanizing,

pigment, dyes, metallurgical, paint industries and other metal processing and refining

operations at small and large-scale sector contains considerable amounts of toxicants

(Gonzaga et al., 2006; Vahidnia et al., 2007; Malik et al., 2009). Lead, Arsenic, Cadmium,

Chromium and Mercury are the examples of heavy metals/ metalloids that have been

classified as priority pollutants by the U.S. Environmental Protection Agency (USEPA, 2003).

Widespread contamination of drinking water by arsenic has recently come to the forefront of

world attention (Bose et al., 2011). Arsenic may change chemical form in the environment,

but it does not degrade (Smedly and Kinniburg, 2002). The World Health Organization

(WHO) estimated that about 130 million people worldwide are exposed to arsenic

concentrations (WHO, 2006). Over 70 million people in Eastern India, Bangladesh, Vietnam,

Taiwan and Northern China have been victims of arsenic poisoning (Wang and Wai, 2004;

Sohel et al., 2009; Mahmood and Halder, 2011). Therefore, a complete understanding about

noxious effects caused by the release of toxic metals/metalloids into the environment and

the emergence of more severe environmental protection laws, have encouraged studies

about removal/recovery of them from aqueous solutions. Over a few decades, scientific

community is developing concentrated efforts for the treatment and removal of arsenic in

order to combat this problem.

ARSENIC: GENERAL CHEMISTRY

The name Arsenic is derived from the Greek word “arsenikon”, which means “yellow

Orpiment”. Arsenic compounds have been mined and used since ancient times. The

extraction of the element from arsenic compound was first reported by Albertus Magnus in

1250 AD. Arsenic ranks 47th most abundant element in nature (1.8 mg/kg) and 20th in

earths crust (Wikipedia, 2010). It exhibits metallic as well as non-metallic characteristics. In

the Victoria era, arsenic was mixed with vinegar and chalk and eaten by women to improve

the complexion of their faces, making skin fairer. During the Bronze Age, arsenic was often

included in the bronze (mostly as an impurity), which made the alloy harder. Arsenic is one

of the oldest human poisons known to mankind.

4

Physical Properties of Arsenic

Arsenic is a semi metallic element. It has an atomic number of 33 and atomic weight of

74.92. It exists in three allotropic forms: yellow (alpha), black (beta) and gray (gamma).

Arsenic also exists in black and yellow amorphous forms. The metallic gray form is the

stable form of arsenic. Arsenic appears in Group 15 in the periodic table in the 4th Period.

Elemental arsenic has a specific gravity of 5.73, sublimes at 613 ºC and has a very low

vapour pressure of 1 mmHg at 373 ºC. Many of the inorganic arsenic compounds occur as

white, odourless solids with a specific gravity ranging from 1.9 to 5. The physical properties

of arsenic are summarized in table 1.01.

Arsenic, a metalloid, has been classified as a human carcinogenic substance

of group 1 by IARC (International Agency for Research on Cancer).

Ranked first in a list of 20 hazardous substances by the Agency for Toxic

Substances and Disease Registry and United States Environmental

Protection Agency.

Arsenic has been found in at least 784 of the 1,662 National Priority List of

hazardous substance identified by the Environmental Protection Agency.

5

Table 1.01: Physical properties of arsenic

Name Arsenic

Symbol As

Atomic no. 33

Electronic shell [Ar] 3d10 4s2 4p3

Electronegativity 2.0

Chemical series Metalloid

Atomic weight 74.92 g/mol

Melting point 817°C (1090K, 1503°F)

Boiling point 613°C

Critical temperature 1,400°C

Heat of vaporization 11.2 Kcal/g-atm

Critical pressure 22.3 Mpa

Density (at 14°C) 5.727g/cm3

Isotopes 8

Most stable isotope 75As

Covalent Radius 1.19 A°

Atomic Radius 1.39 A°

Ionic Radius 2.22 A°

Vapour Pressure 1mm (375°C)

(Source: ATSDR, 2010)

Environmentally Relevant Forms of Arsenic

Arsenic is found in four oxidation states, -3 (arsine), 0 (arsenic) +3 (arsenite) and +5

(arsenate) and in the aquatic environment both the organic and inorganic forms are found

(Mandal and Suzuki, 2002). The two dominant oxidation states for inorganic arsenic are the

trivalent arsenite at moderately and strongly reduced conditions and the pentavalent

arsenate in the aerobic environment. Many of the chemical behaviours of arsenic are linked

to the ease of conversion between +3 and +5 valence states (NRC, 1999). The valence state

affects the toxicity of arsenic compounds. Arsenic occurs naturally as a constituent of a

number of different compounds in both marine and terrestrial environments.

6

Inorganic Arsenic

Inorganic arsenic with +5 (arsenate) and +3 (arsenite) oxidation states, is more prevalent

in water than organic arsenic (Irgolic, 1994). The dominant arsenic species depends on pH

and redox conditions. In general +5 predominate under oxidizing conditions and +3

predominate under reducing conditions (Smedley and Kinniburgh, 2005). Inorganic arsenic

compounds found in the environment include oxides [As2O3, As2O5, R3(AsO)n, RnAsO(OH)3-n

(n=1, 2)] and sulfides [As2S3, AsS, HAsS2, HAsS33-] (Cullen and Reimer, 1989). Inorganic

arsenic species which are stable in oxygenated waters include arsenic acid [As (V)] species

[H3AsO4 and H2AsO4-, HAsO4

2- and AsO43-]. Arsenous acid [As (III)] is also stable as H3AsO3

and H2AsO3- under slightly reducing aqueous conditions.

Organic Arsenic

If carbon is present with the compound it is considered to be an organic arsenic species.

Organic arsenic compounds such as Monomethyl arsonic acid (MMAA), Dimethyl arsinic acid

(DMAA), Trimethylarsine (TMA) and Trimethylarsine oxide (TMAO) are generally associated

with terrestrial settings, however, some are found in water (NRC, 1999). Organic arsenic is

produced naturally in the environment in the natural gas (ethyl methyl arsines), shale oil, in

water when microorganisms metabolize inorganic arsenic, and in the human body, as a

result of enzyme activity in the liver (Berger and Fairlamb, 1994). Table 1.02 includes a

summary of both organic and inorganic species of arsenic which may be found in food and

water.

Table 1.02: Organic and inorganic species of arsenic

Name Abbreviation Chemical Formula

Arsenous acid As (III) H3AsO3

Arsenic acid As (V) H3AsO4

Monomethylarsonic acid

MMA (V) CH3AsO(OH)2

Methylarsonous acid MMA (III) CH3As(OH)2

Dimethylarsinic acid DMA (V) (CH3)2AsO(OH)

Dimethylarsinous acid DMA (III) (CH3)2AsOH[((CH3)2As)2O]

Trimethylarsine TMA (CH3)3As

(Source: Mass et al., 2001)

7

Chemical Properties of Arsenic

Arsenic is stable in dry air but the surface oxidizes slowly in moist air to give a bronze

tarnish and finally a black covering to the element. When heated in air, arsenic ignites to

form “arsenic trioxide” – actually tetra arsenic hexaoxide (As4O6). When heated in oxygen,

arsenic ignites in oxygen to form “arsenic pentaoxide” – actually tetra arsenic decaoxide

(As4O10 and As4O6).

4 As (s) + 5 O2 (g) As4O10 (s)

4 As (s) + 3 O2 (g) As4O6 (s)

Arsenic reacts with fluorine (F2) to form penta fluoride arsenic (V) fluoride.

2 As (s) + 5 F2 (g) 2 AsF5 (g)

[Colourless]

Arsenic reacts under controlled conditions with the halogens fluorine (F2), chlorine (Cl2),

bromine (Br2) and iodine (I2), to form the respective trihalides arsenic (III) fluoride (AsF3),

arsenic (III) chloride (AsCl3), arsenic (III) bromide (AsBr3) and arsenic (III) iodide (AsI3).

2 As (s) + 3F2 (g) 2 AsF3 (l) [colourless]

2 As (s) + 3Cl2 (g) 2 AsCl3 (l) [colourless]

2 As (s) + 3Br2 (g) 2 AsBr3 (l) [colourless]

2 As (s) + 3I2 (g) 2 AsI3 (l) [colourless]

Occurrence of Arsenic

Arsenic has two sources one of them is natural and the other is manmade. Natural sources

include earth crust, sedimentary rocks and weathered volcanic rocks. Fossil fuels, mineral

deposits, mining wastes and geothermal are other natural sources through which arsenic

can occur in ground water (Smith et al., 1998). Some of the naturally occurring minerals of

arsenic are listed in table 1.03.

Anthropogenic sources include man-made such as fertilizers, drugs, feed additives,

insecticides, wood preservatives and herbicides (Adriano, 2001). Significant anthropogenic

sources include combustion of fossil fuels, leaching from mining wastes and landfills,

8

mineraling, metal production, timber treatment, cattle and sheep dips and arsenical

pesticides (Eisler, 2004). Precipitation from the atmosphere and application of a range of

agricultural by products such as poultry manure, can also contribute to large quantities of

arsenic contamination on land. Some volatile organic compounds (VOCs) in ground water

may facilitate the release of arsenic from aquifer materials to ground water (Christen,

2001). Ground water affected by VOCs like petroleum products and other landfill wastes

may be sufficiently reduced to result in elevated dissolved iron-oxide concentrations. Under

these reducing conditions, aquifer materials may be a source of dissolved arsenic in ground

water (Ogden, 1990).

Table 1.03: Naturally occurring minerals of arsenic

(Source: Yan-Chu, 1994)

Environmental Concentrations

Arsenic is the 20th most abundant element in the earth’s crust (NAS, 1977). Concentrations

of arsenic in the earth’s crust vary but average concentrations are generally reported to

range from 1.5 to 5 mg/Kg (Cullen and Reimer, 1989).

Arsenic in Soil

Arsenic contamination source in soil is the parent rock from where the soil is obtained. Its

concentrations in soil were examined in a wide range of soils and reported an average of

5-6 mg/kg (may vary with the geological history of the region) for uncontaminated soils

(Mandal and Suzuki, 2002). Sandy soil and granites has the lowest concentration of arsenic.

On the other hand alluvial and organic soils have higher concentrations of arsenic

(Smedly and Kinniburg, 2002).

Name of minerals Chemical Formula

Arsenopyrite FeAsS

Cobaltite CoAsS

Orpiment As2S3

Lollingite FeAs2

Realgar As4S4

Niciolite NiAs

9

Arsenic in Atmosphere

High temperature processes such as coal-fired power generation, smelting, burning

vegetation and volcanism results in emission of arsenic into the atmosphere. Naturally

occurring low temperature biomethylation of arsenic species and microbial reduction process

also release arsenic to the atmosphere. In this process microorganisms forms volatile

methylated derivatives of arsenic under both aerobic and anaerobic conditions (Le, 2002).

Microorganisms can also reduce these methylated compounds to release arsine gas. Arsenic

is released to the atmosphere primarily as As2O3 or less frequently as one of several volatile

organic compounds. In air, arsenic exists predominantly absorbed on particulate matters

and is usually present as a mixture of arsenite and arsenate, with being of negligible

importance except in areas of arsenic pesticide application or biotic activity (Liu et al.,

2007).

Arsenic in Water

Arsenic is the 10th most abundant element in sea water with an average value of 2.0 μg/L

(Vahidnia et al., 2007). In general, concentration of arsenic is relatively stable in sea water

but some seasonal variations can occur due to biological uptake of surface sea water.

Inorganic As (V) is the major arsenic species because arsenic can be stabilized as a series of

pentavalent oxyanions [H3AsO4, H2AsO4-, HAsO4

2- and AsO43-] at high redox potential

conditions.

However, under most reducing (acid and mildly alkaline) conditions and lower redox

potential, the trivalent arsenite species (H3AsO3) predominate. As (0) and As (3-) are rare in

aquatic environment. As (III), MA and DMA are present at lower levels, usually accounting

to 10% of the total arsenic concentration (Le, 2002). In ground water, inorganic arsenic

commonly exists as As (V) and As (III), the latter is considered to be more mobile and toxic

for living organisms.

Arsenic in Food

Trace elements are important but they are more important in food because they can have

adverse health effects. Arsenic, one of the trace elements has no known biological function

and human body higher levels may cause cancer. Arsenic, if in its inorganic form, can

damage DNA and is carcinogenic. However, arsenic present in food is in less harmful

(organic) forms (Edmonds and Francesconi, 1993).

Inorganic arsenic species found in food is not more than 1 to 3 percent of the total arsenic

presents (Food Standards Agency, 2004). Levels of arsenic are higher in the aquatic

10

environment than in most areas of land as it is mostly water-soluble. It could be washed out

of arsenic bearing rock and sea foods arsenic enters into the daily consumption of human

being. Table 1.04 presents a list of arsenic concentrations in environmental media. Arsenic

in igneous and sedimentary rocks is listed in table 1.05.

Table 1.04: Arsenic concentrations in environmental media

(Source: ATSDR, 2005)

Table 1.05: Arsenic in igneous and sedimentary rocks

Rocks Arsenic Conc. range (mg/kg) Average

Igneous Rocks

Ultra basic 0.3 – 16 3.0

Basalts, gabbrous 0.06 – 113 2.0

Granitic 0.2 - 13.8 1.5

Sedimentary Rocks

Limestones 0.1 – 20 1.7

Sandstones 0.6 – 120 2.0

Shales and clays 0.3 – 490 14.5

(Source: Mok and Wai, 1994)

Environmental Media Arsenic Conc. Range Units

Air from remote and rural areas Air from urban areas Air near industrial areas

0.02 – 4.0 3.0 - 200 >1000

ng/m3 ng/m3 ng/m3

Rain from terrestrial air 0.46 μg/L

Rivers 0.20-264 μg/L

Lakes 0.38-1,000 μg/L

Ground water <1.0- >1,000 μg/L

Sea water 0.15-6.0 μg/L

Soil 0.1-1,000 mg/Kg

11

Arsenic Toxicity

Inorganic arsenic is more toxic than organic arsenic (Bose et al., 2011). The toxicity of

inorganic arsenic compounds depend both on the oxidation state of arsenic. Arsenite

[As (III)] and arsenate [As (V)] are considered to be most toxic forms and classified as

human carcinogen substances (USEPA, 2005). Moreover, they are the predominant forms

found in water. One of the most toxic arsenic compounds is arsine gas, AsH3, with -3

oxidation state. Trivalent arsenic species are more toxic than inorganic As (V), MA (V) and

DMA (V) in vitro (Styblo et al., 2000; Andrew et al., 2003). This may be related to more

efficient uptake of trivalent methylated arsenic species than of pentavalent arsenic species

by micro vessel endothelial cells and CHO (Chinese Hamster Ovary) cells (Dopp et al., 2004;

Hirano et al., 2004). The toxicity of trivalent arsenic is related to its high affinity for the

sulfurydyl groups of biomolecules such as glutathione (GSH) and lipoic acid and the cysteinyl

residues of many enzymes (Aposhian and Aposhian, 2006). The formation of inorganic

As (III)–sulfur bonds results in various harmful effects by inhibiting the activities of enzymes

such as glutathione reductase, glutathione peroxidases, thioredoxin reductase and

thioredoxin peroxidise (Schuliga et al., 2002).

Arsenic Lesions

Arsenic is extremely poisonous. IARC (International Agency for Research on Cancer, 2004)

has classified arsenic as a human carcinogenic substance, group 1. Long-term intake of

drinking water with elevated arsenic concentrations can cause the development of

arsenicosis, the collective term for diseases caused by chronic exposure to arsenic. Arsenic

toxicity causes skin lesions, damage mucous membranes, nervous system, gastrointestinal,

cardiovascular, genotoxic, mutagenic and carcinogenic effects (Hymer et al., 2004; Tofail et

al., 2009; Rivas and Aguirre, 2010; Hamadani et al., 2010; Bose et al., 2011). Hyper-

pigmentation, an excess of skin pigmentation, is most often the first visible symptom.

Considering the lethal impact of arsenic on human health, environmental authorities have

taken a more stringent attitude towards the presence of arsenic in water.

Arsenic is under strict watch globally. The European Commission/U.S.

Environment Protection Agency has revised the maximum contaminant limit of

arsenic from 0.05 mg/L to 0.01 mg/L in drinking water.

12

Remedy for Arsenic Contamination

Figure 1.01: Arsenic lesion

(Source: Bissen and Frimmel, 2003)

Arsenic Lesions in Eye Keratosis on Hand

Arsenic Lesions on Head Bowen's Disease

Skin Cancer Blackfoot Disease

13

ARSENIC POISONING: A WORLD WIDE PROBLEM

The most well-known and severe case of arsenic poisoning through drinking water is

currently ongoing in Bangladesh. The scale of the problem is illustrated by the frequently

used term “mass poisoning” (Halem et al., 2009). Two-thirds of the tube wells installed

over the last three decades, roughly three million in total, have been shown to contain

arsenic concentrations above the permissible level set by the WHO (BGS/DPHE, 2001).

These wells were installed with the firm conviction that they would contribute a secure and

reliable drinking water supply, in order to put an end to various contagious diseases caused

by the use of (unsafe) surface water (Figure 1.02). It is therefore, a bitter observation that

this approach has led to widespread arsenic poisoning through drinking water. It is

estimated that 37 to 100 million people are at risk of drinking arsenic-contaminated drinking

water (WHO, 2001; Chowdhury et al., 2006). World Health Organization (WHO) estimated

that long-term exposure to arsenic in groundwater, at concentrations over 500 μg/L, causes

death in 1 in 10 adults in Bangladesh (Smith et al., 2000).

Figure 1.02: Arsenic poisoning in Bangladesh

(Source: http://www.banglapedia.org/httpdocs/HT/A_0308.HTM)

The problem of arsenic contaminated source water is, however, not confined to the Eastern

countries. In China the reducing conditions in the subsurface are the cause of arsenic

mobilization. In Vietnam and Cambodia, arsenic concentrations were also observed to be

high (up to 1,340 μg/L) due to reductive dissolution of young sediments (Buschmann et al.,

2007; Buschmann et al., 2008). Arsenic mobilization caused by mineral dissolution has been

14

found in active volcanic areas of Italy (Aiuppa et al., 2003) and inactive volcanic regions in

Mexico (Armienta and Segovia, 2008). Volcanism in the Andes has lead to arsenic

contamination of ground water in Chile and Argentina (Smedley and Kinniburgh, 2002). Also

mining activities have been found to contribute to arsenic contamination in Latin American

ground water (Smedley and Kinniburgh, 2002). More recently, ground water in Burkino Faso

was found to be contaminated by arsenic with concentrations up to 1,630 μg/L, caused by

mining activities (Smedley et al., 2007). Furthermore, Gunduz et al. (2009) reported

elevated arsenic levels (<560 μg/L) due to mining and geothermal influenced ground water

in Turkey. In the Netherlands, arsenic concentrations have also been observed in the ground

water (including, peaty soil and dune area), but concentrations exceeding the WHO guideline

have never been reported in the drinking water (Stuyfzand et al., 2006).

Arsenic Poisoning In India

The first case of arsenic poisoning was revealed in West Bengal in early 1980’s, the

widespread contamination was not recognized until 1995. Arsenic in ground water above the

WHO permissible limit has been found in six districts of West Bengal covering an area of

34,000 km2 with a population of 30 million (Michael and Voss, 2008). At present, 37

administrative blocks by the side of the River Ganga and adjoining areas are affected. More

than 8 lakh people from 312 villages are drinking arsenic contaminated water and amongst

them at least 1.75 lakh people show arsenical skin lesions (Table 1.06).

Table 1.06: Districts of West Bengal affected by arsenic poisoning

(Source: Harvey et al., 2002)

District No. of blocks

affected

No. of

villages

Population of

the village

Malda 7 229 696822

Murshidabad 18 354 1343866

Nadia 17 541 1743889

North 24 Paraganas 19 472 1884676

South 24 Paraganas 9 409 964431

Howrah 2 4 107951

Hooghly 1 18 37678

Bardhaman 2 38 101171

Total 75 2065 6970484

15

Thousands of tube-well in these six districts have been analysed for arsenic species. As such

chronic arsenic toxicity in West Bengal due to drinking contaminated water are reported

from eight districts from Malda to 24-South Parganas through Murshidabad, Nadia and 24-

North Paraganas on the eastern bank of Bhagirathi river and Bardhman, Haora and Hugli

districts from western bank of river (Hira-Smith et al., 2007). All 75 blocks have been

affected by arsenic contamination.

Unfortunately, today similar mistakes are being repeated in UP, Bihar, Jharkhand and

Assam, where still the villagers are drinking contaminated water. The Times of India

explores the effect of arsenic poisoning on the villagers in Ballia, UP (Figure 1.03). All the

310 villages in Ballia have arsenic above that limit, with 94 having concentrations greater

than 100 mg/L. In Gazipur and Varanasi, arsenic concentration was found above 10 mg/L in

100 villages (Dixit, 2011). In June 2002, first arsenic contamination was reported in Bihar in

middle Ganga plain (Chakraborti et al., 2004; Acharya and Shah, 2004). Today, on the basis

of analyzing 15,000 samples from Bihar, arsenic concentrations was found above 50 µg/L in

12 districts, 32 blocks and 201 villages (Mukherjee et al., 2006). The districts, Bhagalpur,

Khagaria, Munger, Begusarai, Lakhisarai, Samastipur, Patna, Baishali and Buxar were

identified hundreds of subjects with arsenical skin lesions (Chakraborti et al., 2008;

Mukherjee et al., 2012).

The administration in UP and Bihar is yet to learn from the mistakes committed in

a neighboring state of West Bengal.

Figure 1.03: Arsenic poisoning in UP

(Source: http://www.indiawaterportal.org/node/10925)

16

ARSENIC REMEDIATION TECHNIQUES The constantly increasing degree of industrialization and rising standards of living are

strongly impacting on the use of available water sources. Controlling heavy metal discharges

and removing toxic heavy metals from aqueous solutions have become a challenge for the

21st century. The commonly used procedures for removing metal ions from aqueous streams

include Oxidation, Ion Exchange, Reverse Osmosis, Nanofiltration, Adsorption and

Bucket Treatment. The basic principle, procedural details and commercially available

instrumentation based on above phenomenon have been described in brief as below:

Oxidation Process

Arsenic is present in groundwater in As (III) and As (V) forms in different proportions. Most

treatment methods are effective in removing arsenic in pentavalent form and hence include

an oxidation step as pre-treatment to convert arsenite to arsenate. Arsenite can be oxidized

by oxygen, ozone, free chlorine, hypochlorite, permanganate, hydrogen peroxide and

Fulton’s reagent but atmospheric oxygen, hypochloride and permanganate are commonly

used for oxidation in developing countries.

H3AsO3 + ½ O2 = H2AsO4-

H3AsO3 + HClO = HAsO4- + Cl- + 3H+

3 H3AsO3 + 2 KMnO4 = 3 HAsO4- + 2MnO2

+ + 2 K+ + 4 H+ + H2O

Air oxidation of arsenic is very slow and can take weeks for oxidation (Pierce and Moore,

1982) but chemicals like chlorine and permanganate can rapidly oxidize arsenite to arsenate

under wide range of conditions.

Solar Oxidation

SORAS are a simple method of solar oxidation of arsenic in transparent bottles to reduce

arsenic content of drinking water (Wegelin et al., 2000). Ultraviolet radiation can catalyze

the process of oxidation of arsenite in presence of other oxidants like oxygen (Lara et al.,

2006). Experiments in Bangladesh show that the process on average can reduce arsenic

content of water to about one-third.

Ion Exchange

Ion exchange is an exchange of ions between two electrolytes or between an electrolyte

solution and a complex (Figure 1.04).

17

Figure 1.04: Ion exchange process

2R-Cl + HAsO4-- = R2HAsO4 + 2Cl-

Arsenic exchange

R2HAsO4 + 2Na+ + 2Cl- = 2R-Cl + HAsO4-- + 2Na+

Regeneration

Where, R stands for ion exchange resin.

The ion exchange process is less dependent on pH of water. The efficiency of ion exchange

process is radically improved by pre-oxidation of As (III) to As (V) but the excess of oxidant

often needs to be removed before the ion exchange in order to avoid the damage of

sensitive resins. Development of ion specific resin for exclusive removal of arsenic can make

the process very attractive.

Tetrahedron ion exchange resin filter tested under rapid assessment program in Bangladesh

(BAMWSP, DFID and Water Aid, 2001) showed promising results in arsenic removal. The

system needs pre-oxidation of arsenite by sodium hypochloride. The residual chlorine helps

to minimize bacterial growth in the media. The saturated resin requires regeneration by re-

circulating NaCl solution. The liquid wastes rich in salt and arsenic produced during

regeneration require special treatment.

Membrane Techniques

Membrane techniques like reverse osmosis, nanofiltration and electro dialysis are capable of

removing all kinds of dissolved solids including arsenic from water.

In most cases the term is used to denote the

processes of purification, separation,

decontamination of aqueous and other ion-

containing solutions with solid polymeric

exchangers. The process is similar to that of

activated alumina just the medium is a synthetic

resin of more well defined ion exchange capacity.

The process is normally used for removal of

specific undesirable cation or anion from water.

As the resin becomes exhausted, it needs to be

regenerated. The arsenic exchange and

regeneration equations with common salt solution

as regeneration agent are as follows:

18

Ultra Filtration

Figure 1.06: Ultra filtration process

MRT-1000 and Reid System Ltd.

Jago Corporation Limited promoted a household reverse osmosis water dispenser MRT-1000

manufactured by B & T Science Co. Limited, Taiwan. This system was tested at BUET and

showed arsenic (III) removal efficiency more than 80%. A wider spectrum reverse osmosis

system named Reid System Limited was also promoted in Bangladesh. Experimental results

showed that the system could effectively reduce arsenic content along with other impurities

in water. The capital and operational costs of the reverse osmosis system would be relatively

high.

Natural osmosis occurs when solutions with

two different concentrations are separated

by a semi-permeable membrane (Figure

1.05). Pure water is driven from the

concentrated solution and collected

downstream of the membrane. Because RO

membranes are very restrictive, they yield

very slow flow rates. RO also involves an

ionic exclusion process, only solvent is

allowed to pass through the semi-permeable

RO membrane, while virtually all ions and

dissolved molecules are retained (including

salts and sugars).

Ultra Filtration (UF) is a variety of membrane

filtration in which hydrostatic pressure forces a

liquid against a semipermeable membrane.

Suspended solids and solutes of high molecular

weight are retained while water and low

molecular weight solutes pass through the

membrane (Figure 1.06). This separation process

is used in industry and research for purifying and

concentrating macromolecular (103-106 Da)

solutions, especially protein solutions. Ultra

filtration is not fundamentally different from

reverse osmosis, microfiltration or nanofiltration,

except in terms of the size of the molecules it

retains.

Figure 1.05: Reverse osmosis process

19

Low Pressure Nanofiltration

Figure 1.07: Nanofiltration process

Adsorption Processes

Water treatment with coagulants such as alum, Al2(SO4)3.18H2O, ferric chloride, FeCl3 and

ferric sulfate Fe2(SO4)3.7H2O are effective in removing arsenic from water. Ferric salts have

been found to be more effective in removing arsenic than alum on a weight basis and

effective over a wider range of pH. In both cases pentavalent arsenic can be more effectively

removed than trivalent arsenic. In the coagulation-flocculation process aluminium sulfate, or

ferric chloride, or ferric sulfate is added and dissolved in water under efficient stirring for one

to few minutes. The water is then gently stirred for few minutes for agglomeration of micro-

flocs into larger easily settable flocs. During this flocculation process all kinds of micro

particles and negatively charged ions are attached to the flocs by electrostatic attachment.

Arsenic is also adsorbed onto coagulated flocs. As trivalent arsenic occurs in non-ionized

form, it is not subject to significant removal. Oxidation of As (III) to As (V) is thus required

as a pre-treatment for efficient removal. This can be achieved by addition of bleaching

powder (chlorine) or potassium permanganate. The possible chemical equations of alum

coagulation are as follows:

Al2(SO4)3.18 H2O = 2 Al+++ + 3 SO4+++ + 18 H2O

Alum dissolution

2 Al+++ + 6H2O = 2 Al(OH)3 + 6 H+

Aluminium precipitation (acidic)

Oh et al., (2000) applied reverse osmosis and

nanofiltration membrane processes for the

treatment of arsenic contaminated water

applying low pressure by bicycle pump. A

nanofiltration membrane process coupled with a

bicycle pump could be operated under condition

of low recovery and low pressure range from 0.2

to 0.7 MPa (Figure 1.07). Arsenite was found to

have lower rejection than arsenate in ionized

forms and hence water containing higher

arsenite requires pre-oxidation for reduction of

total arsenic acceptable level. In tube well water

in Bangladesh the average ratio of arsenite to

total arsenic was found to be 0.25.

20

H2AsO4- + Al(OH)3 = Al-As (complex + other products)

Co-precipitation (Non-stoichiometric, non-defined product)

Arsenic adsorbed on aluminium hydroxide flocs as Al-As complex is removed by

sedimentation. Filtration may be required to ensure complete removal of all flocs. Similar

reactions take place in case of ferric chloride and ferric sulfate with the formation of Fe-As

complex as end product which is removed by the process of sedimentation and filtration.

The possible reactions of arsenate with hydrous iron oxide are shown below where

[≡ FeOH°] represents oxide surface site (Mok and Wai, 1989; Hering et al., 1996).

Fe(OH)3 (s) + H3AsO4 FeAsO4. 2H2O + H2O

≡ FeOH° + AsO43- + 3 H+ ≡ FeH2AsO4 + H2O

≡ FeOH° + AsO43- + 2 H+ ≡ FeHAsO4

- + H2O

Immobilization of arsenic by hydrous iron oxide requires oxidation of arsenic species into

As (V) form for higher efficiency. Arsenic removal is dependent on pH. In alum coagulation,

the removal is most effective in the pH range 7.2-7.5 and in iron coagulation; efficient

removal is achieved in a wider pH range usually between 6.0 and 8.5 (Ahmed and Rahman,

2000).

Bucket Treatment Unit

The Bucket Treatment Unit (BTU), developed by DPHE – Danida Project is based on the

principles of coagulation, co-precipitation and adsorption processes. It consists of two

buckets, each 20 litre capacity, placed one above the other. Chemicals are mixed manually

with arsenic contaminated water in the upper red bucket by vigorous stirring for about 90

seconds. The mixed water is then allowed to flow into the lower green bucket via plastic pipe

and a sand filter installed in the lower bucket. The flow is initiated by opening a valve fitted

slightly above the bottom of the red bucket to avoid inflow of settled sludge in the upper

bucket. The lower green bucket is practically a treated water container.

The DPHE – Danida project in Bangladesh distributed several thousand BTU units in rural

areas of Bangladesh. These units are based on chemical doses of 200 mg/L aluminium

sulfate and 2 mg/L of potassium permanganate supplied in crushed powder form. The units

were reported to have very good performance in arsenic removal in both field and laboratory

conditions (Sarkar et al., 2000; Kohnhorst and Paul, 2000). In many cases, the units under

rural operating conditions fail to remove arsenic to the desired level of 0.05 mg/L in

Bangladesh. Poor mixing and variable water quality particularly pH of ground water in

21

different locations of Bangladesh appeared to be the cause of poor performance in rapid

assessment. The method Bucket Treatment Unit used zero-valent iron. This method had

between 68% to 100% arsenic removal efficiency with a 36 L/day flow rate of filtrate

(Awuah et al., 2010).

Stevens Institute Technology: A Modified Version

This technology also uses two buckets, one to mix chemicals (reported to be iron sulphate

and calcium hypochloride) supplied in packets and the other to separate flocs by the process

of sedimentation and filtration. The second bucket has a second inner bucket with slits on

the sides as shown in the figure 1.08 to help sedimentation and keeping the filter sand bed

in place. The chemicals form visible large flocs on mixing by stirring with stick. Rapid

assessment showed that the technology was effective in reducing arsenic levels to less than

0.05 mg/L in case of 80 to 95% of the samples tested (BAMWSP, DFID, Wateraid, 2001).

The sand bed used for filtration is quickly clogged by flocs and requires washing at least

twice a week.

Figure 1.08: Stevens Institute Filtration Technology

22

BUET Activated Alumina

Alcan Enhanced Activated Alumina

ARU of Project Earth Industries, Inc., USA

Apyron Arsenic Treatment Unit

Sorptive Filtration Media

Several sorptive media have been reported to remove arsenic from water. These are

activated alumina, activated carbon, iron and manganese coated sand, kaolinite clay,

hydrated ferric oxide, activated bauxite, titanium oxide and many synthetic media. The

efficiency of sorptive media depends on the use of oxidizing agent as an aid to sorption of

arsenic. Saturation of media by different contaminants and components of water takes place

at different times of operation depending on the specific sorption affinity of the medium to

the given component. Saturation means that the efficiency in removing the desired

impurities becomes zero.

Activated Alumina

Activated alumina, Al2O3, having good sorptive surface is an effective medium for arsenic

removal (Kim et al., 2004). When water passes through a packed column of activated

alumina, the impurities including arsenic present in water are adsorbed on the surfaces of

activated alumina grains. Eventually the column becomes saturated, first at its upstream

zone and later the saturated zone moves downstream towards the bottom end and finally

the column get totally saturated. Regeneration of saturated alumina is carried out by

exposing the medium to 4% caustic soda, either in batch or by flow through the column

resulting in high arsenic contaminated caustic waste water. The residual caustic soda is then

washed out and the medium is neutralized with a 2% solution of sulfuric acid rinse. During

the process about 5-10% alumina is lost and the capacity of the regenerated medium is

reduced by 30-40%. The activated alumina needs replacement after 3-4 regeneration. Like

coagulation process, pre-chlorination improves the column capacity dramatically. Some of

the activated alumina based sorptive media used in Bangladesh include:

The BUET and Alcan activated alumina have been extensively tested in field condition in

different areas of Bangladesh under rapid assessment and found very effective in arsenic

23

Sono 3-Kolshi Filter

Granet Home-made Filter

Chari Filter

Adsorptive Filter

Shafi Filter

Bijoypur clay

removal (BAMWSP, DFID, WaterAid, 2001). The Arsenic Removal Units (ARUs) of Project

Earth Industries Inc. (USA) used hybrid aluminas and composite metal oxides as adsorption

media and were able to treat 200-500 Bed Volume (BV) of water containing 550 g/L of

arsenic and 14 mg/L of iron (Ahmed et al., 2000).

Granular Ferric Hydroxide

M/S Pal Trockner (P) Ltd, India and Sidko Limited, Bangladesh installed several Granular

Ferric Hydroxide based arsenic removal units in India and Bangladesh. The Granular Ferric

Hydroxide is arsenic selective adsorbent developed by Technical University, Berlin, Germany.

The unit requires iron removal as pre-treatment to avoid clogging of filter bed. The

proponents of the unit claim to have very high arsenic removal capacity and produces non-

toxic spent granular ferric hydroxide.

Indigenous Filters

There are several filters available in Bangladesh which uses indigenous material as arsenic

adsorbent. Red soil rich in oxidized iron, clay minerals, iron ore and iron scrap or fillings are

known to have capacity for arsenic adsorption. Some of the filters manufactured using these

materials include:

The Sono 3-Kolshi Filter uses zero valent iron fillings and coarse sand in the top kolshi, wood

coke and fine sand in the middle kolshi while the bottom kolshi is the collector of the filtered

water (Khan et al., 2000). Earlier Nikoliadis and Lackovic (1998) showed that 97% arsenic

can be removed by adsorption on a mixture of zero valent iron fillings & sand and

recommended that arsenic species could have been removed through formation of co-

24

Chiyoda Arsenic Removal Unit, Japan

Cool mart Water Purifier, Korea

precipitates, mixed precipitates and by adsorption onto the ferric hydroxide solids. The Sono

3-Kolshi unit has been found to be very effective in removing arsenic (BAMWSP, DFID and

Water Aid, 2001). The one-time use unit becomes quickly clogged, if groundwater contains

excessive iron.

The Granet home-made filter contains relatively inert materials like brick chips and sand as

filtering media. No chemical is added to the system. Air oxidation and adsorption on iron-

rich brick chips and flocs of naturally present iron in ground water could be the reason for

arsenic removal from ground water. The unit produced inadequate quantity of water and did

not show reliable results in different areas of Bangladesh and under different operating

conditions. The Chari filter also uses brick chips and inert aggregates in different Charis as

filter media. The effectiveness of this filter in arsenic removal is not known.

The Shafi and Adarsh filters use clay material as filter media in the form of candle. The Shafi

filter was reported to have good arsenic removal capacity but suffered from clogging of filter

media. The Adarsh filter participated in the rapid assessment program but failed to meet the

technical criterion of reducing arsenic to acceptable level (BAMWSP, DFID and WaterAid,

2001). Bijoypur clay was also found to adsorb arsenic from water (Khair, 2000).

Cartridge Filters

Filter units with cartridges filled with sorptive media or ion-exchange resins are readily

available in the market. These units remove arsenic like any other dissolved ions present in

water. These units are not suitable for water having high impurities and iron in water.

Presence of ions having higher affinity than arsenic can quickly saturate the media requiring

regeneration or replacement. Two household filters were tested at BUET laboratories. These

are:

The Chiyoda Arsenic Removal Unit could treat 800 BV meeting the WHO guideline value of

0.01 mg/L and 1300 BV meeting the Bangladesh Standard of 0.05 mg/L when the feed

water arsenic concentration was 0.30 mg/L. The Coolmart Water Purifier could treat only

20L of water with an effluent arsenic content of 0.025 mg/L (Ahmed et al., 2000). The initial

and operation costs of these units are high and beyond the reach of the rural people.

25

DRAWBACKS OF THE EXISTING REMEDIATION TECHNIQUES

These currently practiced technologies for removal of pollutants from industrial effluents

appear to be inadequate, creating often-secondary problems with metal bearing sludge,

which are extremely difficult to dispose off. Further, research findings have clearly raised

strong doubts about the advisability of the use of synthetic coagulants used for metal

removal. Table 1.07 explains the advantages and disadvantages of the conventional

techniques used for arsenic remediation.

Table 1.07: A Comparison of arsenic remediation technologies

(Source: www.freedrinkingwater.com/water-education/quality-water filtration-method.htm)

Technologies Advantages Disadvantages

Oxidation/precipitation

Air oxidation Relatively simple, low cost but slow process

The process remove only a part of arsenic

Chemical oxidation

Relatively simple and rapid process Oxidizes other impurities and kills microbes

Capital costs and operating costs are typically high.

Coagulation co-precipitation

Alum Coagulation

Relatively low cost Relatively simple

Produces toxic sludges Low removal of As(III)

Iron Coagulation

Common Chemicals available

Pre-oxidation may be required

Sorption Techniques

Activated Alumina

Relatively well known and commercially available

Produces toxic solid waste

Iron Coated Sand

Well defined technique Replacement/regeneration required

Ion Exchange

Resin

Development of scope High tech operation and

maintenance

Other sorbents Relatively high cost

Membrane Techniques

Nanofiltration Well defined and high removal efficiency

Very high capital and running cost

Reverse osmosis

No toxic solid wastes produced

High tech operation and maintenance

Electro dialysis Capable of removal of other contaminants

Toxic wastewater produced

26

GREEN TECHNOLOGIES Unfortunately, the Science particularly Chemistry, despite numerous contributions to the

well being and progress of humanity, has been blamed for the present ills of the world. In

fact, it is not Chemistry or Science or Technology but our past mistakes of increasing the

production only without considering the simultaneous generation of large amount of side

products or waste which have underlined us as culprit. Basically unscientific and careless

rapid urbanization, industrialization and agricultralization are major threat to environment.

Chemists, since 1990 have started addressing complicated environmental issues in safe and

an economically profitable manner under the various names like Clean Chemistry,

Environmentally Benign Chemistry, Sustainable Chemistry, Come Back to Nature,

Grey to Green Chemistry, Green Technologies, Eco-friendly Techniques, Green

Processes and more popularly as Green Chemistry.

Green Chemistry is a special contribution of chemists to the conditions for sustainable

development, incorporating an environmentally benign by design approach to all aspects of

chemical industry. The word Green Chemistry was jointly coined by Prof. Paul T. Anastas

and Prof. John C. Warner, which means “The invention, design and application of

chemicals products and processes to reduce or to eliminate the use and generation

of hazardous substances”.

To combine technology with environmental safety is one of the key challenges of the new

millennium. There is a global trend of bringing technology into harmony with natural

environment, thus aiming to achieve the goals of protection of ecosystem from the

potentially deleterious effects of human activity and finally improving its quality. The

challenges of safe and various treating and diagnosing environmental problems require

discovery of newer, more potent, specific, safe and cost effective synthetic or natural

molecules.

The magic plants are around and waiting to be discovered and commercialized.

They are now recognized and accepted as storehouses of infinite and limitless benefits to

human beings. These natural systems are often referred to as “Green Technologies”, as

they involve naturally occurring plant materials. Biosorption is one such important

phenomenon, which is based on one of the twelve principles of Green Chemistry “Use of

Renewable Resources”. It has garnered a great deal of attention in recent years due to

rise in environmental awareness and consequent severity of legislation regarding the

abatement of arsenic ions from water bodies.

27

Figure 1.09: Twelve principles of Green Chemistry

Prof. Paul T. Anastas Prof. John C. Warner Prof. Paul T. Anastas (Green Chemistry Institute, American Chemical Society, Washington) and Prof. John C. Warner (University of Massachusetts, Washington) have given twelve principles of Green Chemistry.

LESS HAZARDOUS CHEMICALS SYNTHESIS

SYNTHESIS

ATOM ECONOMY

SAFER CHEMISTRY

PREVENTION IS BETTER THAN

CURE

DESIGN FOR DEGRADATION REDUCED

DERIVATIVES

REAL TIME ANALYSIS FOR

POLLUTION PREVENTION

USE OF SELECTIVE CATALYST

DESIGNING SAFER

CHEMICALS

DESIGN FOR ENERGY

EFFICIENCY

SAFER SOLVENTS AND

AUXILIARIES

USE OF RENEWABLE FEED STOCKS

GREEN CHEMISTRY

28

BIOSORPTION: A GREEN SOLUTION During the 1970’s, increasing environmental awareness and concern led to a search for new

techniques capable of inexpensive treatment of polluted waste waters with

metals/metalloids. The search for new technologies involving the removal of toxic pollutants

from waste waters has directed attention towards Biosorption, based on binding capacities

of various agricultural waste/biological materials. The assessment of the metal/metalloid

binding capacity of biomaterials has gained momentum since 1995 (Volesky and Holan

1995a). Till date, research in the area of biosorption suggests it to be an ideal alternative for

abatement of metal/metalloid containing effluents.

Biosorption is a rapid phenomenon of passive metal sequestration by the

non-growing biomass/adsorbents. It can be defined as “a non-directed physicochemical

interaction that may occur between metal/metalloid/radionuclide species and

microbial cells”. The biosorption process involves a solid phase (sorbent or biomaterial;

adsorbent; biological material) and a liquid phase (solvent, normally water) containing a

dissolved species to be sorbed (adsorbate, metal/metalloid). Due to the higher affinity of the

adsorbent for the adsorbate species, the latter is attracted and bound by different

mechanisms. The process continues till equilibrium is established between the amount of

solid bound adsorbate species and its portion remaining in the solution. The degree of

adsorbent affinity for the adsorbate determines its distribution between the solid and liquid

phases.

Biomaterials: A Forecast for the Future

Biomaterials are attractive since naturally occurring biomaterials/adsorbents or spent

biomaterial can be effectively reused. Natural materials that are available in large quantities

or certain waste from agricultural operations may have potential to be used as low cost

adsorbents, as they represent unused resources, widely available and are environment

friendly (Hench, 1998; Park et al., 2010; Narukawa et al., 2012). Availability is a major

factor to be taken into account to select biomaterial for clean up purposes (Volesky and

Holan, 1995b). Some biomaterial can bind and remove a wide range of metals/metalloids

with no specific priority, where as other are specific for certain types of metals/metalloid.

When choosing the biomaterial for metal biosorption experiments, its origin is a major factor

to be taken into account (Keith et al., 1979). In Biosorption, the use of non living

biomaterials containing metal binding compounds would have the advantage of not requiring

tremendous care and maintenance as well as being useful in remediating toxic high levels of

contaminants that would otherwise kill live system (Basso et al., 2002; Narukawa and Chiba,

2010; Egila et al., 2011). However, live biological system work well for low concentration;

they cannot survive the high levels that are found in seriously contaminated areas and

industrial effluents (Fourest and Roux, 1992).

29

Biomaterials can come from

Industrial waste, which should be free of charge.

Plants easily available in large amount in nature and of quick growth.

Agricultural waste products.

Organisms of quick growth, especially cultivated for biosorption process.

Advantages of biosorption process

Biosorption clearly shows that from most perspectives, plants are ideal for environmental

clean up: capital cost is low, ongoing operational costs are minimal, implementation is easy

and non-invasive and public acceptance is high (Veglio and Belochini, 2001; Volesky, 1999).

All this shows that biosorption is a new and vibrant technology having great potential. To

realize this, it will be necessary to understand the various processes that are involved in it.

This may require a multidisciplinary approach and diverse fields of plant biology.

Cost-effectiveness

High efficiency

Minimization of chemical and or biological sludge

Regeneration of biomaterial

Possibility of metal/metalloid recovery

Competitive performance

Metal/Metalloid selectivity

(Volesky, 2003)

30

BIOSORPTION: MECHANISTIC ASPECTS

The complex structure of plant materials and microorganisms implies that there are many

ways for the metal to be taken by the biomaterial. Numerous chemical groups have been

suggested to contribute to biosorption metal binding by either whole organisms or by

molecules. These groups comprises hydroxyl, carbonyl, carboxyl, sulphaydryl, thioether,

sulphonate, amine, amino, imidazole, phosphonate and phosphodiester etc. The importance

of any given group for biosorption of certain metals by plant biomaterial depends on factors

such as number of sites in the biomaterial, the accessibility of the sites, the chemical state

of the sites (availability) and affinity between site and metal (Volesky et al., 1999).

Adsorption and desorption studies invariably yield important information on the mechanism

of metal biosorption. This knowledge is essential for understanding of the biosorption

process and it serves as a basis for quantitative stoichiometric considerations, which

constitute the foundation for mathematical modeling of the process (Yang and Volesky,

2000).

Biosorption Mechanism

Various metal-binding mechanisms have been postulated to be active in biosorption process

and presented in figure 1.10.

Figure 1.10: Mechanism of biosorption

Due to the complexity of the biomaterials used, it is possible that at least some of these

mechanisms are acting simultaneously to varying degrees, depending on the biomaterial and

the solution environment.

BIOSORPTION

MECHANISMS

INTRACELLULAR

ACCUMULATION

TRANSPORT

ACROSS CELL MEMBRANES

CELL SURFACE

ADSORPTION AND PRECIPITATION

ION EXCHANGE

COMPLEXATIONPHYSICAL

ADSORPTION

EXTRACELLULAR

ACCUMULATION PRECIPITATION

PRECIPITATION

31

Chemisorption

It is the adsorption in which the forces involved are valence forces of the same kind as those

operating in the formation of chemical compounds. Some features, which are useful in

recognizing chemisorption, include:

The phenomenon is characterized by chemical specificity.

Since the adsorbed molecules are linked to the surface by valence bonds, they will

usually occupy certain adsorption sites on the surface and only one layer of

chemisorbed molecules is formed (monolayer adsorption).

The energy of chemisorption is of the same order of magnitude as the energy

change in a chemical reaction between a solid and a fluid.

Chemisorption is irreversible.

Chemisorption is of three types: Ion Exchange, Chelation and Co-ordination (Complexation).

Ion Exchange

Ion exchange is a reversible chemical reaction wherein an ion in a solution is exchanged for

a similarly charged ion attached to an immobile solid particle. These solid ion-exchange

particles are either naturally occurring inorganic zeolites or synthetically produced organic

resins. Synthetic organic resins are the predominant type used today because their

characteristics can be tailored to specific applications. Ion exchange reactions are

stoichiometric, reversible and as such they are similar to other solution-phase reactions. For

example, in the reaction the nickel ions of the nickel sulfate (NiSO4) are exchanged for the

calcium ions of the calcium hydroxide Ca(OH)2 molecule.

NiSO4 + Ca (OH)2 Ni(OH)2 + CaSO4

Chelation

The word chelation is derived from the Greek word chele, which means claw and is defined

as the firm binding of a metal ion with an organic molecule (ligand) to form a ring structure.

The resulting ring structure protects the mineral from entering into unwanted chemical

reactions. Examples include the carbonate (CO32–) and oxalate (C2O4

2–) ions:

32

Coordination (Complex Formation)

A coordination complex is any combination of cations with molecules or anions containing

free pairs of electrons. Bonding may be electrostatic, covalent or a combination of both; the

metal ion is coordinately bonded to organic molecules. Example of the formation of a

coordination compound is:

Cu2+ + 4H2O [Cu (H2O)]42+

Cu2+ + 4Cl– [CuCl4]2–

Where, coordinate covalent bonds are formed by donation of a pair of electrons from H2O

and Cl– (Lewis base) to Cu2+ (Lewis acid).

In general, biosorption of toxic metals and radionuclide is based on non-enzymatic processes

such as adsorption. Adsorption is due to the non-specific binding of ionic species to

polysaccharides and proteins on the cell surface or outside the cell. Bacterial cell walls and

envelopes, and the walls of fungi, yeasts and algae, are efficient metal biomaterials that

bind charged groups. The cell walls of gram-positive bacteria bind larger quantities of toxic

metals and radionuclide than the envelopes of gram-negative bacteria. Bacterial sorption of

some metals can be described by the linearized Freundlich adsorption equation:

log S = log K + n log C

where, S is the amount of metal absorbed in µmol/g, C is the equilibrium solution

concentration in µmol/L, K and n are the Freundlich constants.

Biomaterial deriving from several industrial fermentations may provide an economical source

of biosorptive materials. Many species have cell walls with high concentrations of chitin, a

polymer of N-acetyl-glucosamine that is an effective biomaterial.

Biosorption uses biomaterial raw materials that are either abundant (e.g., seaweeds) or

wastes from other industrial operations (e.g., fermentation wastes). The metal-sorbing

performance of certain types of biomaterial can be more or less selective for heavy metals,

depending on the type of biomaterial, the mixture in the solution, the type of biomaterial

preparation, and the chemical-physical environment.

Physisorption

In physisorption, physisorbed molecules are fairly free to move around the sample. As more

molecules are introduced into the system, the adsorbate molecules tend to form a thin layer

33

that covers the entire adsorbent surface. Physiosorption takes place with the help of Vander

Waal’s forces. Some features, which are useful in recognizing physisorption, include:

The adsorbate molecules are held by comparatively weaker Vander Waal’s forces,

thus resulting into lower activation energy.

The process is, however, reversible as the substance adsorbed can be recovered

from the adsorbent easily on lowering the pressure of the system at the same

temperature.

Physisorption may extend beyond a monolayer also, since the physical forces can

operate at any given distances.

Physical adsorption is not specific in nature because it involves Vander Waal’s

forces, which exist among the molecules of every two substances.

Factors Affecting Biosorption

Temperature seems not to influence the biosorption performances in the

range of 20O-35OC.

pH seems to be the most important parameter in the biosorption process. It

affects the solution chemistry of the metals/metalloids, the activity of the

functional groups in the biomaterial and the competition of metallic ions.

Biomaterial concentration in solution seems to influence the specific uptake.

An increase in biomaterial concentration leads to interference between the

binding sites.

Biosorption is selective also. This aspect has to be investigated in detail.

Biosorption is mainly applied to treat waste water containing metal ions

and the removal of one metal may be influenced by the presence of other

metals/metalloids.

(Fourest and Roux, 1992)

34

Desorption Involves

BIOSORPTION: APPLICATION STRATEGIES Over the past few years, intensifying research into metal biosorption elucidated the

principles of this effective metal removal phenomenon. Biosorption can be cost effective,

particularly in environmental applications where low cost of the metal removal process is

most desirable. Some efficient natural biomaterials have been identified that require little

modification in their preparation. It is particularly in ecological aspects where biosorption

can make a difference due to its anticipated low cost. The application makes the research

and development work in this novel area exciting and worthwhile. Biosorption process could

be used even with a relatively low degree of understanding of its metal binding mechanisms

better understanding will make it more effective and optimized applications.

If the biosorption processes were to be used as an alternative in the waste water treatment

scheme, the regeneration of the biomaterial may be crucially important for keeping the

process cost down and to open the possibility of recovering the metal/metalloid extracted

from liquid phase. For this process, it is desirable to desorb the sorbed metals and to

regenerate the biomaterial for another cycle of application.

Yield the metals in a concentrated form

Restore the biomaterial to close to the original conditions for effective reuse

Undiminished metal uptake

No physical change or damage

Extensive “desorption” work may be necessary for assessing whether this is possible and

under what conditions. Desorption and sorbent regeneration studies might require

somewhat different methodologies. While the regeneration of the biomaterial may be

accomplished by washing the metal laden with an appropriate solution, the type and

strength of the solution would depend on just how the deposited metal has been bound.

Dilute solutions of mineral acids (hydrochloric acid, nitric acid, sulphuric acid and acetic acid)

can be used for metal desorption from the biomaterial (Pagnanelli et al., 2002).

Due to different affinities of metal ions for the predominant sorption site (under the solution

conditions), there will be a certain degree of metal selectivity by the sorbent on the uptake.

Similarly, selectivity may be achieved upon the elution-desorption operation (Jessen et al.,

35

2005). Advantage could be taken of this selectivity on the desorption side of the operation

which can contribute to the separation of metals from one another, if desirable. The overall

capacity of sorption process is to concentrate the sorbate metals. This is assessed by

expressing a simple overall process parameter, the concentration ratio (CR). Obviously, the

higher the CR, the better is the overall performance of the sorption process making the

eventual recovery of the metal more feasible as it becomes more concentrated in the small

volume of the eluent solution.

The feasibility of applying the biosorption process into wastewater purification would best be

assessed based on a stage-wise approach. A considerable amount of research on

biomaterials has developed solid basis of knowledge and indicated their enormous potential.

The highest priority at the early stage would be the preliminary and approximate

assessment of the commercial potential and practicality of application of the new technology

based on the family of new biomaterial products. The preliminary assessments that should

be carried out simultaneously as part of a better quantitative estimation of this technology

are as follows:

Assessment of the Competiting Technologies

The current costs and market share of the established conventional processes for metal

removal/recovery from dilute solutions or waste waters have to be summarized or assessed.

As the emission standards tighten the conventional methods for metal detoxification are

becoming progressively more inadequate or prohibitively costly for use of water treatment.

Better and effective metal removal technologies are invariably more costly and often just not

feasible for that purpose. The search is on for efficient and particularly cost effective

remedies. Biosorption promises to fulfill the requirements. Its overall performance and

process application modes justify a comparison with the other existing techniques.

Assessment of the Market Size

It is known that the environmentally-based market for metal removal /decontamination of

metal containing (industrial) effluents is enormous, the actual figure to support this

generally prevailing perception would be most convincing for commercialization. A

quantitative review of the potential clientele for the biosorption metal-removal process

needs to be carried out for different countries where applications of biosorption technology

would be considered. Comparison of costs between the traditional and new technology

establish the feasibility of biomaterial applications and their competitiveness in the market

place. As the application of biosorption proves cheaper, it is anticipated that new

applications, otherwise perhaps not feasible, will significantly increase the size of the current

market and scope of potential clients for biosorption technology.

36

Biosorption can be viewed as

Water treatment process

Significant cost saving process in comparison with existing competing

techniques.

Effective in terms of technical performance, operational qualities and

chemical properties.

Commonly usable having low sensitivity to environmental and impurity

factors.

Additional cost reducing because of possible recovery of heavy metals.

Cost effective, obviously reinforced by a higher market value of recovered

metal and lower costs of biomaterial.

Assessment of Costs of New Biomaterial

Approximate costs of different types of raw biomaterial need to be ascertained, as well as

the costs of processing the biomaterial into applicable biomaterials maintaining their high

efficiency. Preliminary technical work needs to be carried out on the processing, necessary

for biomaterial formulation into a product suitable for use. Different raw biomaterial would

require specific treatment for their optimal formulation into finished ready to use products.

This part would entail specifically planned small-scale laboratory work resulting in an

efficient biomaterial. The most compelling reasons for using biosorption technology, based

on renewable or waste raw materials, are that it is effective and inexpensive. The initial

information gathered in preliminary economic feasibility studies, leads to following main

conclusions regarding the application of bioremediation technology.

Ideally, all these preliminary assessments should be carried out simultaneously as part of a

better quantitative estimation of the venture feasibility. They could also be carried out

simultaneously with the technically oriented pilot-plant efforts. These assumptions lead to

considering the low cost of the biomaterials as the primary significant difference factor

between the biosorption and other processes. For this reason, the study of the biomaterial

sources and costs are particularly important and will allow a measurement of the economic

performance of the process.

37

BIOMATERIALS USED SO FAR

Bioremediation as a variant and Green Technology becomes promising to remediate the

environmental pollutants. It has many advantages, i.e., environmental friendly,

excellent performance, possible recycling and low cost for remediation of arsenic from

contaminated water. A range of microorganisms (Table 1.08) and plant biomaterials (Table

1.09) have been reported for efficient remediation of arsenic from water bodies. However,

there is still a strong challenge in developing economical and commonly available

biomaterial for arsenic removal.

Table 1.07: List of microorganisms used for arsenic removal

Microorganism Arsenic species Reference

Penicillium purpurogenum

As (III) Ridvan et al., 2003

Brown rot fungi

As (V) Kartal & Imamura, 2004

Aspergellus fumigatus

As (III) SathishKumar et al., 2004

Waste tea fungal

biomaterial

As (III) & As (V) Murugesan et al., 2006

Bacillus subtilis As (III) Hossain & Anantharaman, 2006

Heat resistant fungi

As (III) Cernansky et al., 2007

Aspergillus clavatus As (III) & As (V) Urik et al., 2007

Fungal biomass As (III) & As (V) Pokhrel & Viraraghavan, 2007

Thiobascillus As (V) Malik et al., 2009

Algae As (V) Llorente-Mirandes et al., 2010

Lessonia nigrescens AS (V) Ruiz-Chancho et al., 2010

Escherichia coli As (III) Wu et al., 2010

Lactobacillus acidophilus As (III) Singh & Sharma, 2010

Aspergilus flaves As (III) Maheshwari & Murgusen, 2011

Turbinaria conoides As (V) Sivaprakash & Rajamohan, 2011

38

Table 1.08: List of plant biomaterials used for arsenic removal

Plant Biomaterial Arsenic Species Reference

Water lettuce As (V) Basu et al., 2003

Orange waste As (III) & As (V) Ghimire et al., 2003

Garcinia cambogia

As (III) Kamala et al., 2005

Coconut fiber As (V) Igwe & Abia, 2006

Rice husk As (III) & As (V) Amin et al., 2006

Sorghum biomass As (V) Haque et al., 2007

Jute leaf powder (JLP), sugarcane powder

As (V) Islam et al., 2007

Banana peel As (V) Memon et al., 2008

Picea abies As (V) Urik et al., 2009

Pteris vittata As (III) & As (V) Wang et al., 2011

Seaweed As (III) & As (V) Llorente-Mirandes et al., 2011

Water hyacinth As (III) & As (V) Mahamadi, 2011

Farm Rice Total As Rezaitabar et al., 2012

39

CHEMICAL SPECIATION OF ARSENIC

The element exists in different oxidation states and in different chemical form in the

environment, which is called as species. An element can have different species and these

species can have different property and toxicity between each other. The doses of these

elements are also important, higher doses cause toxicity and lower doses cause deficiency.

As a result, total concentration of an element does not give enough information about its

toxicity, biotransformation and other biochemical properties of the element to be analyzed

(Jain and Ali, 2000).

Speciation is essential to understand the distribution, mobility, toxicity and bioavailability

of chemical elements in natural systems. Speciation of arsenic is an important consideration

because toxicity is mainly species-dependent and not well correlated with the total arsenic

concentration (Kumarasen and Riyazudin, 2001). Determination of total arsenic in a sample

is of limited value because the result does not usually reflect the true level of hazard of that

element. Hence, speciation of arsenic is highly relevant in providing meaningful risk

assessment data to assess the appropriate hazard level (Caumette et al., 2011). Inorganic

arsenic is considered to be the most toxic form of the element (Quaghebeur and Rengal,

2004). Inorganic arsenic species, i.e. As (III) and As (V), predominantly found in natural

water, are more toxic than organic species (Smith et al., 2009). The speciation of arsenate

and arsenite is determined by the pH as both (arsenate and arsenite) protonate/deprotonate

depending on the pH of the solution. The trivalent arsenic species exists in a non-ionic form

(H3AsO3) in the pH range 2–7 and in the anionic (H2AsO3-1, HAsO3

-2) form in the pH range

7–10 (Moriarty et al. 2009). The pentavalent arsenic species exists in the monovalent

(H3AsO4– 1) and divalent anion (H2AsO4

2–) in the pH range 2–9 as shown in figure 1.11.

Figure 1.11: Speciation of arsenite and arsenate as a function of pH

(Source: http://ldambies.free.fr/arsenicchemistry.html)

40

TECHNIQUES USED FOR DETECTION AND SPECIATION OF ARSENIC

Several analytical techniques have been used for the detection and speciation of arsenic

species in aqueous system like Capillary Zone Electrophoresis (Timerbaev et al., 2000,

Timerbaev et al., 2001), Mass Spectroscopy (Mcsheehy et al., 2001), Solvent Extraction

(Vela et al., 2001; Galagher et al., 2002), HPLC (Ali et al., 2002), Electrophoresis (Sun et

al., 2003), Gas Chromatography (Bednar et al., 2004), HPLC-NAA (Youqing et al 2004),

Sonoelectrochemical (Simm et al., 2005), Spectrophotometry (Jain and Ali, 2000; Butcher,

2007), Hydride Generation (Del Razo et al., 2001; Hashemi and Modasser, 2007), Ion

Exclusion Chromatography (Yuan and Le, 2009), Atomic Fluorescence Spectroscopy (Liu et

al., 2009), Microwave assisted sequential extraction (Jamali et al., 2009; Rahman et al.,

2009), HPLC-ICP-MS (Miyashita et al., 2009; Caumette et al., 2011) and Ion Exchange

Chromatography (Ammann, 2011; Chavan et al., 2011).

Voltammetric Technique

Voltammetry is an electrochemical method in which current is measured as a function of the

applied potential. This technique is based on the principle of measurement of the diffusion

controlled current flowing in an electrolysis cell in which one electrode is polarisable (Fifield

and Kealey, 2000). A time dependent potential is applied to an electrochemical cell, and the

current flowing through the cell is measured as a function of the potential. A plot of current

which is directly proportional to the concentration of an electroactive species as a function of

applied potential is called a voltammogram. The voltammogram provides quantitative and

qualitative information about the species involved in the oxidation or reduction reaction or

both at the working electrode.

The advantage of this technique include high sensitivity where quantitative and qualitative

determination of metals, inorganic and organic compounds at trace levels (Fifield and

Kealey, 2000), selectivity towards electro active species (Barek et al., 2001), a wide linear

range, portable and low-cost instrumentation, speciation capability and a wide range of

electrode that allow assays of many types of samples such as environmental samples

(Zhang et al., 2002; Ghoneim et al., 2003; Buffle et al., 2005), pharmaceutical samples

(Abdine et al., 2002; Hilali et al., 2003 and Carapuca et al., 2005 ), food samples (Ximenes

et al., 2000; Volkov and Mwesigwa, 2001) and forensic samples (Pournaghi-Azar and

Dastangoo, 2000; Woolever Dewald, 2001).

Various advances during the past few years have pushed the detectability of voltammetric

techniques from the submicromolar level for pulse Voltammetric techniques to the

subpicomolar level by using an adsorptive catalytic stripping voltammetry (Cavicchioli et al.,

2004). Stripping technique is one of the most important and sensitive electrochemical

41

techniques for measuring trace metals and organic samples. The term stripping is applied to

a group of procedures involving pre-concentration of the determinant onto the working

electrode, prior to its direct or indirect determination by means of a potential sweep (Wang,

2000). The pre-concentration (or accumulation) step can be adsorptive, cathodic or anodic.

Its remarkable sensitivity is attributed to the addition of an effective pre-concentration step

with advanced measurement procedures that generate an extremely favourable signal-to-

noise ratio as reported by Blanc et al., (2000).

Square Wave Voltammetry (SWV) is a large-amplitude differential technique in which a

waveform composed of a symmetrical square wave, superimposed on a base staircase

potential, is applied to the working electrode. The current is sampled twice during each

square wave cycle, once at the end of forward pulse and another at the end of the reverse

pulse. Since, the square-wave modulation amplitude is very large, the reverse pulses cause

the reverse reaction of the product of the forward pulse.

Among the various voltammetric techniques, Square Wave Anodic Stripping Voltammetry

(SWASV) has several advantages like excellent sensitivity, rejection of background currents

and speed (Salaun et al., 2007). This speed, coupled with computer control and signal

averaging, allows for experiments to be performed repetitively and increases the signal to-

noise ratio. Applications of square-wave voltammetry include the study of electrode kinetics

with regard to preceding, following, or catalytic homogeneous chemical reactions and

determination of some species at trace levels. Electroanalytical techniques particularly

square wave stripping voltammetry is highly popular for speciation of metals/metalloids in

aqueous system (Brusciotti and Duby 2007; Mays and Hussam, 2009; Bose et al., 2011).

42

LACUNA IN THE PRESENT STATE OF KNOWLEDGE The vigorous survey of the relevant literature indicates the following points:

Abatement of arsenic has been largely attempted using inorganic coagulants. Not

much attention has been paid on decontamination of arsenic using organic

biomaterials.

The potential of chemically modified biomaterials for cationic metal species has been

explored. Information on the ability of chemically modified biomaterials to remove

anionic metal species from waste water is still highly restricted.

Little attention has been paid on the physico-chemical interaction of arsenic and

plant biomaterial which is strongly required for explaining the mechanistic aspects of

arsenic sorption.

The methods used for arsenic estimation are sensitive to total arsenic rather than

individual arsenic species, while the toxicological, physiological and geochemical

behaviour depends on its oxidation state.