Contamination of groundwater by arsenic: a review of occurrence, causes, impacts, remedies and membrane-based purification

Abstract

The contamination of groundwater by leached out arsenic has assumed an alarming proportion in several countries. Continued and prolonged ingestion even at a very low level can lead to serious arsenic-related diseases. Strong epidemiological evidence of arsenic carcinogenicity has forced the World Health Organization (WHO) to lower the maximum permissible contaminant limit (MCL) in drinking water to 10 ppb from earlier limit of 50 ppb. This has thrown a big challenge to the scientific community to devise efficient methods to purify contaminated water to such a high level. Though literature abounds in occurrence of groundwater contamination by arsenic and its removal from drinking water by laboratory techniques, millions of people continue to suffer, particularly in the developing countries like India (Bangladesh, West Bengal). Through a comprehensive review of the existing literature on the occurrence, causes, impacts and remedial measures, this article finds out what has gone wrong and what is to be done with special emphasis on membrane-based separation that seems to be highly promising in purifying arsenic-contaminated groundwater to a WHO-prescribed level.

Keywords:

• arsenic contamination
• health hazards
• arsenic removal
• membrane separation
• nanofiltration

1. Introduction

Arsenic contamination of groundwater has now resulted in world-wide human health problems affecting millions of people across a large number of countries like Argentina (Smedley and Edmund 2002; Heredia and Cirelli 2009; Morgada et al. 2009), Bangladesh (Islam et al. 2004; Zhenga et al. 2004; Hafeman et al. 2005; Harvey et al. 2006; Klump et al. 2006; Sengupta et al. 2008; Chen et al. 2009), India (Pal et al. 2007b; Sengupta et al. 2008; Bhattacharjee et al. 2005; Smith et al. 2000; Chakraborti et al. 2003), Nepal (Panthi et al. 2006; Pokhrel et al. 2009), Pakistan (Nickson et al. 2005), Thailand (World Bank Technical Report 2004), China (Ding et al. 2001; Xia et al. 2007), USA (Schreiber et al. 2000; Wickramasinghe et al. 2004), Vietnam (Berg et al. 2001; Tong 2002), Mexico (Romero-Schmidt et al. 2001), Mongolia (Smedley et al. 2003), and Taiwan (Tseng 2003; Hsieh et al. 2008). In some places in Bangladesh, the concentration of arsenic in groundwater is as high as 1000 μg/l (Harvey et al. 2002). The largest population at risk among the 21 countries with known groundwater arsenic contamination is in Bangladesh, followed by West Bengal in India (Das et al. 1994, 1995; Mandal et al. 1996; Chowdhury et al. 2000; Rahman et al. 2002).

Though literature abounds regarding the arsenic contamination problem of groundwater in several parts of the world and the laboratory-based treatment options for purifying contaminated water, millions of people continue to suffer and a comprehensive review of the problems and the latest developments is very much essential to find out what is still needed to guide region-specific policy formulation as socio-economic conditions and availability of technology and alternative options are of varied nature across the globe. Thus, the prescribed maximum contaminant level (MCL) of arsenic in drinking water ( Table 1) is found to vary from country to country (Choong et al. 2007). While the value is 50 μg/l in the developing countries like Bangladesh, India, China, Taiwan, it is 10 μg/l in the developed countries like the USA, Germany, and Japan, 25 μg/l in Canada and, 7 μg/l in Australia. This study is an attempt to review the latest developments from almost all major angles so as to arrive at a concrete region-specific solution of the enormous arsenic-contamination problem. Though arsenic contamination problem has been reported from several countries, the gravity of the situation varies from country to country. The largest population affected lives in the Bengal delta basin comprising Bangladesh (50 million) and India (West Bengal, 6 million) followed by China (3 million) and the USA (3 million). The average concentration of arsenic in groundwater also varies across the countries, the highest being in India (0.003–3700 μg/l) followed by China (220–2000 μg/l) and Bangladesh (10–1000 μg/l). Thus, while finding out the root causes, the extent of health hazards and region-specific solutions, our major emphasis is on the Bengal delta basin which is the largest affected part of the world.

Table 1. Permissible MCL of arsenic set by different countries.

Countries/others	Permissible value (ppb)
WHO/USPEA/European Union	10
Germany	10
Australia	7
France	15
India, Bangladesh, Vietnam, Mexico	50
Malaysia	50–10

2 Occurrence and causes of arsenic contamination of groundwater

Arsenic may occur naturally in some 200 minerals in varying degrees as elemental arsenic, arsenides, sulphides, oxides, arsenites and arsenates (World Bank Technical Report 2004). The highest concentrations of arsenic are, however, associated with sulphide minerals and metal oxides, especially iron oxides. The problem of arsenic contamination can result in places of abundant occurrence of these minerals only if the geochemical conditions favour the release of arsenic from these minerals. The geochemical conditions which are widely accepted as reasons for the contamination of groundwater are pH, an aerobic or reducing environment, groundwater flow and transport. The most abundant arsenic ore mineral is pyrite (FeS₂) followed by chalcopyrite, galena and marcasite where the arsenic concentrations can be as high as 10 weight percentage. Besides being an important ore component, pyrite can also be formed in sedimentary environments under reducing conditions.

Under aerobic conditions, pyrite may get oxidized to iron oxides with the release of sulphate, acidity, and arsenic along with trace elements. The pyrite oxidation reactions take place following the equation given below.

Thus, human activities around coal mining are often blamed for arsenic problems in coal mine areas. Fortunately, under most circumstances, mobilization of arsenic to groundwater and surface water is low because of the high retention of arsenic species in the associated minerals. Arsenic can also be released from arsenopyrite (FeSAs) under aerobic conditions and in many aquifers around the world; the lowering of the water table has been held responsible for creating an aerobic environment through introduction of atmospheric oxygen (Das et al. 1994; Shreiber et al. 2000; Smith et al. 2000; Chakraborty et al. 2003; Bhattacharjee et al. 2005; Pal et al. 2007b). The other school of thought (Akai et al. 2004; Islam et al. 2004; vanGeen et al. 2004; Stute et al. 2007; Sengupta et al. 2008) believes that arsenic leaching which is caused by bio-mediated reductive dissolution of arsenic-bearing ferric-oxyhydroxide is mainly responsible for the problem of arsenic contamination in the Bengal delta basin. Reduction of oxyhydroxides (FeOOH) in alluvial aquifers needs organic matter (OM), the source of which may be anthropogenic (unsewered sanitation, surface soils) or authigenic. Though OM drives reduction, the surface source of such OM is almost ruled out (Sengupta et al. 2008) contrary to the observations of Nickson et al. (2005) in the case of arsenic-pollution of groundwater in Pakistan's Muzzafargarh. In the Bengal delta basin, organic-rich fluvio-deltaic sediments that were deposited during the high-stand setting of the mid Holocene age (Acharyya 2002) are found to be associated with a major arsenic contamination problem. Authogenic sulphide minerals containing arsenic can be formed under strongly reducing conditions in lakes, oceans, and aquifers. Oxidation conditions that often cause dissolution of arsenic from sulphide minerals may happen in shallow aquifers and not in deep aquifers. However, groundwater may remain in oxic conditions for 5000 years when the associated sediment itself is organic-poor as in the Sherwood sandstone aquifer in UK (Smedley and Edmund 2002). In the Terai region of Nepal, the source of arsenic is believed to be geogenic (Pokhrel et al. 2009) where arsenic contamination of groundwater is attributed to reductive dissolution of ferro-oxyhydroxide.

In controlling the redox conditions of reducing aquifers, the role of OM has been widely suggested (DPHE 1999; McArthur et al. 2001; Smeldey and Kinniburgh 2001; Harvey et al. 2002) though there remain some disputes on the nature of OM. It is suggested by many researchers (Ormland et al. 2002; Islam et al. 2004) that rapid burial of OM along with sediments facilitates microbial activities which generate reducing conditions favourable to the formation of sulphide minerals containing arsenic. Nickson et al. (2005) in their investigation on the causes of shallow groundwater (<30 m deep) contamination in Muzaffargarh District of Pakistan blamed sewage, animal and human wastes (anthropogenic OM) for the reduction of hydrous ferric oxide and the release of sorbed arsenic into groundwater, though the surface source of OM in driving such reduction processes has been considered as extremely unlikely (Sengupta et al. 2008). However, reducing conditions in deep (>30 m) aquifers seem to be due to naturally occurring OM. In various parts of Asia, the onset of reducing conditions in the sediment and later conditions of oxidation in the aquifers have largely been held responsible for the problem of groundwater contamination by arsenic. After the release of arsenic from crystal lattice and its dissolution in water, accumulation of arsenic in the aquifer may continue unless it is flushed out by moving groundwater over time. Slow groundwater movement (due to a low recharging rate) has been blamed for many high arsenic aquifers in South East Asian countries. Thus, low arsenic concentrations in deep and coastal aquifers in Bangladesh and elsewhere are attributed to high groundwater movement and a high rate of recharging.

In some geologically recent and poorly flushed arid and semi-arid regions of the world like the inland basins of Argentina and Southwest USA, high pH conditions have resulted in desorption of arsenic from mineral surfaces (Robertson 1989; Welch et al. 2000; Smedley and Kinniburgh 2001). Mineral weathering and high evaporation lead to high pH conditions. It is well established that under aerobic and low to neutral pH regime, adsorption of arsenic, especially as As(V) on iron oxides, is very strong, aqueous concentrations are low and arsenic desorption is favoured at high pH.

Arsenic can occur in the environment in various forms and oxidation states (−3, 0, +3, +5) but in natural water arsenic occurs mainly in inorganic forms as oxyanions of trivalent arsenite or as pentavalent arsenate (Nordstrom 2002). The two oxidation states common in drinking water in the form of arsenate and arsenite are part of the arsenic (H₃AsO₄) and arsenous (H₃AsO₃) acid systems, respectively. These two forms depend upon oxidation–reduction potential and pH of the water (Schnoor et al. 1996; Yan et al. 2000). At typical pH values of 5.0–8.0 in natural waters, the predominant arsenate species are H₂AsO₄ ⁻ and HAsO₄ ²⁻, and the arsenite species is H₃AsO₃. Under oxidizing conditions, HAsO₄ ²⁻ dominates at high pH regime whereas H₃AsO₄ predominates at low pH regime. H₂ASO₄ ⁻ predominates at low pH (<6.9). This means that As(III) remains as a neutral molecule in natural water. Arsenates are stable under aerobic or oxidizing conditions, while arsenites are stable under anaerobic or mildly reducing conditions (Choong et al. 2007). In reducing waters, arsenic is found primarily in the trivalent oxidation state in the form of arsenious acid that ionizes according to the following equations:

The acid base dissociation reactions of arsenic acid can be described as:

Surface water is also found to be contaminated with arsenic by anthropogenic sources to various degrees as arsenic is also used in agriculture (pesticide), industrial applications, mining activities and feed additives.

3 Effect of arsenic-contamination of drinking water on human health and medicinal remedies

Today, arsenic poisoning has become one of the major environmental worries in the world as millions of human beings have been exposed to excessive arsenic through contaminated drinking water. One of the most serious consequences of chronic arsenic toxicity is the carcinogenic influence of arsenic on humans, as the element is classified as a Group 1 carcinogenic substance based on epidemiological evidence (IARC 1987). The developments have necessitated stringent regulation of arsenic concentration in drinking water. The World Health Organization (WHO) has been forced to revise the earlier guideline on the maximum allowable concentration limit (MCL) of arsenic in drinking water from 50 μg/l to 10 μg/l (WHO 1996). To combat this problem is now a big challenge to environmentalists, doctors, scientists, engineers and the policy makers. The toxicology and carcinogenicity of arsenic depend on its oxidation states and chemical forms. While inorganic arsenic is more toxic than organic arsenic, the trivalent form is more hazardous than the pentavalent form (Mascher et al. 2002). Arsenic cannot be easily destroyed and can only be converted into different forms or transformed into insoluble compounds in combination with other elements, such as iron. Many impurities such as lead, iron and selenium may be mixed up together with arsenic waste and make it uneconomical to remove. The contaminants like iron, calcium, magnesium, bicarbonate, chloride and sulphate are found to be associated with arsenic in the groundwater of these countries. Inorganic arsenic, As(V) and As(III) in the form of Na₂HAsO₄ and NaAsO₂, respectively, are toxic to man and plants (Choong et al. 2007). Inorganic arsenic is always considered a potent human carcinogen, not only associated with the cancer of the urinary bladder, lungs, skin, kidney, nasal passages, liver and prostrate owing to long term exposure to arsenic in drinking water, but also non-cancerous effects including cardio-vascular, pulmonary, immunological, neurological and endocrine (e.g. diabetes) disorders. Besides its tumorigenic potential, arsenic has been shown to be genotoxic (Ning 2002).

The toxicology of arsenic can be classified into acute and sub-acute types. Arsenic poisoning requires prompt medical attention and usually occurs through ingestion of contaminated food or drink. The major early manifestations due to acute arsenic poisoning include burning and dryness of the mouth and throat, dysphasia, colicky abnormal pain, projectile vomiting, profuse diarrhea, and haematuria. India (mainly in the West Bengal state) and Bangladesh are long known for having suffered from the problem of arsenic contaminated ground water and claim it as the biggest calamity in the world. According to some estimates, arsenic in drinking water will cause 200,000–270,000 deaths from cancer in Bangladesh (Smith et al. 2000). The much hidden nature of groundwater poisoning by leached out arsenic surfaced in an acute form in the Bengal delta basin when hundreds of people started showing symptoms of arsenic-related diseases. Reportedly, the problem manifested itself after some two decades of continuous consumption of arsenic-contaminated water (Roy et al. 2008). Arsenic concentration of over 60 mg/l is lethal for human consumption (Wickramasinghe et al. 2004). Therefore, in the last few decades, increasing attention has been focussed on the possible long-term health risks associated with ingestion of low levels of arsenic in drinking water. To prevent arsenic-related diseases, the maximum contaminant level (MCL) in drinking water has been set by different countries as in Table 1 (Choong et al. 2007).

Arsenic contamination in ground water is a worldwide problem and has become an important issue and challenge for the world environmentalists, scientists and even the policy makers. For example, chronic arsenic toxicity due to drinking arsenic-contaminated water has been one of the worst environmental health hazards affecting nine districts of West Bengal since the early 1980s. Detailed clinical examination and investigation of 248 such patients revealed protean clinical manifestations of such toxicity. Over and above, hyper pigmentation and keratosis, weakness, anaemia, burning sensation of eyes, solid swelling of legs, liver fibrosis, chronic lung disease, gangrene of toes, neuropathy, and skin cancer are some of the other manifestations (Majumdar 2002). Naturally occurring arsenic, adsorbed from rocks through which water passes, is present in some 4000 sites in the US, mainly in the southwest and northeast states. Utilities supplying water complied with earlier EPA standards of a 50 μg/l maximum contaminant level, but the revised compliance levels that reduced this to 10 μg/l represented a big change. In Bangladesh, 2000 villages have been identified as containing arsenic above 50 μg/l and over 50 million people are at the exposure of arsenic poisoning. In India over 2700 villages are affected by arsenic poisoning in ground water. Above 6 million people are consuming arsenic-contaminated water and there are over 30,000 reported cases who are already affected by arsenic (Smedley et al. 2002; Bhattacharjee et al. 2005).

Jain and Ali (2000) reported comprehensively on the occurrence and toxicity of arsenic. The toxicology of arsenic is a complex phenomenon and generally classified into acute and sub-acute types. Acute arsenic poisoning requires prompt medical attention. It usually occurs through ingestion of contaminated food or drink. The major early manifestations due to acute arsenic poisoning includes burning and dryness of the mouth and throat, dysphasia, colicky abnormal pain, projectile vomiting, profuse diarrhoea and haematuria. Muscular cramps, facial oedema, cardiac abnormalities and shock can develop rapidly as a result of dehydration (Jain et al. 2000). In general, there are four recognized stages of arsenicosis or chronic arsenic poisoning (Romero-Schmidt et al. 2001) and these are preclinical, clinical, complications and malignancy. In the preclinical stage, the patient shows no symptoms, but arsenic can be detected in urine or body tissue samples. In the clinical stage, various effects can be seen on the skin. Darkening of the skin (melanosis) is the most common symptom, often observed on the palms. Dark spots on the chest, back, limbs or gums have also been reported. Oedema (swelling of hands and feet) is often seen. A more serious symptom is keratosis, or hardening of skin into nodules, often on palms and soles. The WHO estimates that this stage requires 5–10 years of exposure to arsenic. In the complications stage, clinical symptoms become more pronounced and internal organs are affected. Enlargement of the liver, kidneys and spleen have been reported. Some research indicates that conjunctivitis (pinkeye), bronchitis and diabetes may be linked to arsenic exposure at this stage. Tumours or cancers (carcinoma) affect skin or other organs in the malignancy stage. The affected person may develop gangrene or skin, lung or bladder cancer.

The results of clinical findings for arsenic poisoning from drinking arsenic contaminated water show the presence of almost all the stages of arsenic clinical manifestation (Karim 2000). Diseases caused by arsenic poisoning are no longer news but reported worldwide. Exposure to arsenic via drinking water (groundwater) has been reported to cause a severe disease of blood vessels leading to gangrene, known as black foot disease in Taiwan (Tseng et al. 2003). According to some estimates, arsenic in drinking water will cause 200,000–270,000 deaths from cancer in Bangladesh alone (Smith et al. 2000).

After absorption, inorganic arsenic accumulates in the liver, spleen, kidneys, lungs and gastrointestinal tract. It is then rapidly cleared from these sites but leaves a residue in keratin-rich tissues such as skin, hair and nails. Arsenic, particularly in its trivalent form, inhibits critical sulfhydryl-containing enzymes. In the pentavalent form, the competitive substitution of arsenic for phosphate can lead to rapid hydrolysis of the high-energy bonds in compounds such as ATP. Chronic exposure to high levels of arsenic concentration in drinking water has been associated with cancers of skin, lung, liver, kidney in different arsenic-affected parts of the world (Brown et al. 1989; Hertz-Picciotto and Smith 1993; Buchet and Lison 1998). Arsenic poisoning has also been blamed for several cardiovascular, cerebrovascular endocrine-disrupting and neuro-developmental diseases (Chen et al. 1995, 1996; Chiou et al. 1997; Tsai et al. 2003; Tseng 2003; Wasserman et al. 2004). Even at low to moderate dose of arsenic poisoning adverse health effects in the form of pre-malignant skin lesions, high blood pressure, and neurological dysfunctions have been reported in the arsenic longitudinal study in Bangladesh by Chen et al. (2009). Poor health and hygiene, relatively low affordability of the greater majority of population living in these zones and the lack of awareness of the possible consequences of arsenic intoxication make this problem more complex (Mallick et al. 2003).

The vastness of this problem calls for a tremendous all-out effort to bring the situation under control. Chronic exposure generally leads to various ailments and the dysfunction of several vital organs like the liver, kidney, lungs, tremor-producing effects, neurological disorders, etc. (Guha Majumder et al. 1997; Hafeman et al. 2005), more often when there is an accompanying nutritional/dietary deficiency (Murphy 2006). Most of the affected people in general complain of muscle and joint pains and are highly depressed with various gastric problems and general weakness (USEPA 2001). Skin and nail changes, e.g. arsenical dermatosis, melanosis, keratosis, oedema, gangrene, sensory and motor polyneuritis, hepatitis/chronic liver disease, chronic diarrhea aplastic anaemia, hyperostosis, portal hypertension, toxic optic neuropathy(atrophy), skin cancer, cancers of the lung, liver (angiosarcoma), bladder, kidney and colon (Niu et al. 1997) are found to occur. The role of antioxidants has been found to be very successful in combating arsenic-related diseases (Patrick 2003). Verret et al. (2005) studied the efficacy of vitamin E and selenium supplementation in treating patients with arsenic-induced skin lesions. However, orthodox medicines (e.g. chelating agents like dimercaptosuccinic acid (DMSA), diethylenetriamine-pentaacetic acid (DTPA), etc. and some antioxidants) have been most unsuccessful (Murphy 2006). With such a background, besides water purification it should also be our aim to find suitable antagonists of arsenic poisoning, which should be (i) easy to administer, (ii) effective in low doses, (iii) inexpensive and (iv) without any toxic effects of their own. Studies by Mallick et al. (2003) convincingly demonstrate that the potentized homeopathic drug, Arsenicum Album, not only has the ability to help removal of arsenic from the body, but these drugs in micro doses appear to have the ability to detoxify the ill effects produced by arsenic. Many evidence-based researches on human beings and animals have shown the efficacy of homoeopathic drugs in combating this problem (Braunwald et al. 2001).

4 The remedies through better water management

As geological disturbances resulting from over-withdrawal of groundwater have largely been held responsible for arsenic contamination of groundwater, remedial measures directed towards reducing the pressure of such withdrawal are likely to be successful. Therefore, it is suggested that the remedy should be sought through better management of surface water so that the geological disturbance of the underground aquifers can be minimized in a regime of controlled groundwater use. As the major portion of the groundwater in the Bengal delta basin is used in rice-dominated, highly water-intensive agriculture, development of surface water-based irrigation can effectively hold control on the withdrawal of groundwater. Region-specific solutions through better supply and demand side managements and through institutional mechanisms can be found in Roy et al. (2008).

5 Remedies through purification of contaminated water

5.1 Available purification options

There are many conventional physical–chemical methods used for arsenic removal. These methods include precipitation, adsorption, ion exchange, membrane technology. Other precipitation methods have been studied for arsenic removal using hydrogen peroxide, calcium oxide, ferric sulphate and Portland cement as the precipitation agents.

Over the last few decades, several studies have been conducted on the removal of arsenic from drinking water. Adsorption, chemical coagulation–precipitation and membrane separation have been established as the broad technology options in the purification of arsenic contaminated drinking water. In adsorption-based studies (Kazuo and Toshio 1998; Balaji et al. 2000; Elizalde-Gonzalez et al. 2001; Grafe et al. 2001; Xu et al. 2002; Morgada et al. 2009) several adsorbents have been examined for assessing the effectiveness of arsenic separation from water in small scale. Physico-chemical separation through chemical coagulation and precipitation has been demonstrated by several researchers (Edwards and Benjamin 1989; Brewster 1992; Cheng et al. 1994; Edwards 1994; Hering et al. 1996; Hering 1997; Hatice et al. 2004; Wickramasinghe et al. 2004). Pal et al. (2007b) have demonstrated that the physico-chemical treatment of contaminated water on a large scale could be a viable option particularly in far-flung arsenic-affected areas where alternate sources of water supply are limited. In recent years, several membrane-based purification studies (Beolchini et al. 2007; Hsieh et al. 2008; Lin et al. 2008; Pagana et al. 2008; Fogarassy et al. 2009; Geucke et al. 2009; Nguyen et al. 2009; Qu et al. 2009) have been reported.

5.1.1 Chemical precipitation

In the precipitation process, anions are combined with cations. Three processes are well known: alum coagulation, iron coagulation and lime softening. The disadvantages of the chemical precipitation process include: (a) it requires a large amount of chemicals and creates a volumetric sludge; (b) Arsenic III sulphide, calcium arsenate or ferric arsenate formed are unstable under certain conditions (Shih 2005). Several researchers over the last few decades have successfully demonstrated arsenic removal by chemical precipitation (Edwards and Benjamin 1989; Brewster 1992; Cheng et al. 1994; Edwards 1994; Hering et al. 1996; Hering et al. 1997; Gecol et al. 2004; Wickramasinghe et al. 2004; Pal et al. 2007b).

5.1.1.1 Alum precipitation

This process is effective for the removal of solids and dissolved metals. Chemicals required for the process are chlorine, acid, alum and caustic soda. Ninety per cent removal of arsenic from water containing 300 μg/l of arsenic using 30 mg/l of alum is achieved if the pH is maintained at 7 or less and an oxidizing agent, such as chlorine, is added ahead of the flocculator and clarifier (Kartinen and Martin 1995). In the absence of chlorine, the removal of arsenic is reduced to only about 10% (Choong et al. 2007). Chlorine is added to oxidize arsenic at the head end of the plant. Acid is required to maintain pH at the desired level. To increase pH to an acceptable level in the post-treatment of the clarified water, caustic soda (NaOH), for example, would be added. The alum sludge generated in the clarifier contains arsenic removed from the water. Yuan et al. (2003) investigated the coagulation process through orthogonal array experiment to arrive at optimal doses of the coagulants and other additives.

5.1.1.2 Iron precipitation

In this process, a ferric salt (for example, FeCl₃ and Fe₂(SO₄)₃) and chlorine, as an oxidizing agent, are added. The arsenic combined with the iron forms a precipitate that settles out in the clarifier. The particles of iron/arsenic that are not settled out in the clarifier are removed by employing a filter, followed by a clarifier. Ninety-five per cent removal of arsenic from water containing 300 μg/l of arsenic using 30 mg/l of alum is achieved at a pH of less than 8.5 with chlorine. Under the same conditions, arsenic removal is reduced to about 50% or more in the absence of chlorine. The pH adjustment does not appears to be as important as with the alum precipitation process. This process is well known for its simplicity, versatility, selectivity and low cost. Wickramasinghe et al. (2004) made a comparative study of arsenic removal by chemical precipitation using Bangladesh water and water from the USA. They used FeCl₃ and FeSO₄ coagulants and suggested that the technique could be used in both cases successfully with prior pH adjustment. Roberts et al. (2004) studied the removal of arsenic by co-precipitation with iron. Leupin et al. (2005) attempted oxidative arsenic removal using zero valent iron in which they used filter columns filled with iron fillings. Leupin and Hug (2005) also used sand filters along with an iron adsorption bed. Berg et al. (2006) made a similar attempt to remove arsenic from water using a sand filter. Pal et al. (2007a) developed ARSEPPA, a visual basic software tool for arsenic separation plant performance analysis where the progress of the arsenic co-precipitation with iron can be observed in a Microsoft Excel environment. The software allows optimization of major operating variables of the iron coagulation process.

5.1.1.3 Lime softening

In this process, arsenic is removed with other particles from water other than hardness (calcium and magnesium ions). The chemicals required for the process are chlorine, lime and acid. Chlorine is needed to oxidize the arsenic. Acid is necessary to lower the pH of the treated water to acceptable drinking water levels. The arsenic removal efficiencies of this process highly depend on the pH and the presence (or absence) of chlorine. In the absence of chlorine, arsenic removal from water is achieved about 15% up to pH 10.5 and removal efficiency is increased to almost 80% at pH 11.0 (Pal et al. 2007b). However, arsenic removal in the presence of chlorine is always more than without chlorine. The limitation of the lime softening process is that it is economical when softened water is required (Choong et al. 2007). The produced sludge containing arsenic has no added value, and can limit the use of technology. For this reason, treatment in two stages is justified: lime softening followed by arsenic removal.

5.1.1.4 Co-precipitation

This process is applied to remove arsenic along with iron (and/or manganese) from arsenic and iron (and/or manganese) contaminated water. The principle of separation is oxidizing the iron and/or manganese from their soluble state (oxidation state 2+) to a higher oxidation state to form iron and/or manganese precipitates. The arsenic is apparently removed as an iron/arsenic or manganese/arsenic precipitates, which is backwashed off of the filter media. Then the precipitates can be filtered. The most important chemical used in this process is chlorine as an oxidizing agent. Other chemicals such as ferric chloride, sulphur dioxide, potassium permanganate, polymeric aluminium silicate sulfate (PASS) may or may not be needed depending on water chemistry and process employed. Two processes are employed for the removal of arsenic combined with iron (and or manganese). One of the processes involves a proprietary media. In one variation of this process, chlorine is injected into the raw water containing iron and/or manganese in reaction vessel for one or two minutes. Then sulphur dioxide may also be injected into the water and allowed to react with the iron and/or manganese for a short period of time. The water is then discharged into one or more filter vessels that contain the proprietary media. Arsenic reductions of perhaps 50% can be obtained with this process (Kartinen and Martin 1995). In another process, three additional chemicals – ferric chloride, potassium permanganate, polymeric aluminum silicate sulfate – are needed. In addition to the two reaction vessels and filters described in the preceding paragraph, a flocculator and clarifier are also used. Arsenic removal rates of more than 90% may be obtained in the second process.

5.1.2 Adsorption

The technology of adsorption is a process by using materials that have a strong affinity for dissolved arsenic. Arsenic is attracted to the sorption site on the adsorbent's surface and is removed from water. This process is efficient for arsenic removal from drinking water. Activated carbon is the well-known widely used adsorbent. However, activated carbon still remains an expensive material.

Morgada et al. (2009) studied the removal of arsenic from drinking water by commercial iron nanoparticles and observed that the main mechanism of arsenic removal was adsorption on iron corrosion phases promoted by reactive oxygen species and the same could be enhanced by UV-irradiation. They were successful in removing arsenic well up to 10 μg/l from arsenic-contaminated groundwater of the Chacopampean Plain of Argentina (Tucuman province). Activated alumina has been applied in many instances as an adsorbent to remove arsenic. When the alumina surface becomes sufficiently saturated with arsenic, it is necessary to regenerate the alumina. Arsenic adsorbed on the alumina surface layer and absorbed into that layer is removed by contacting the saturated alumina with caustic soda. The alumina is then neutralized with a sulphuric acid and reused. As 20–30% capacity is lost after two or three regenerations. However, the main advantage of alumina is that its low cost and relatively high capacity for arsenic makes the process economical. Lin and Wu (2001) investigated the kinetics of arsenic adsorption by activated alumina. Kunzru and Chaudhury (2005) used manganese amended activated alumina for 84–89% arsenic removal from groundwater in an adsorption column.

The arsenic removal rate from water by using alumina as an adsorbent depends on the oxidation states of arsenic and pH of feed water. As(V) removal from water is more efficient compared to the removal of As(III). Chlorine or an other oxidizing agent is necessary to obtain the higher arsenic removal rates. Keeping the pH of the feed water in the 5.5–6.0 range is more effective for arsenic removal (Kartinen and Martin 1995).

The well-known low cost adsorbents for water purification and waste water treatment include agricultural wastes like rice husk, coconut husk, amine modified coconut coir, carbonized wood powder, sawdust, orange juice residues and waste tea fungal biomass.

5.1.3 Ion exchange

Ion exchange is the process by exchanging arsenic anions for chloride or other anions at active sites bound to a resin. An ion exchange resin, attached with chloride ions at the exchange sites, is placed in a vessel. The arsenic containing water is passed through the resin bed and the chloride ion is exchanged by arsenic anions. The water coming out from the resin bed is lower in arsenic but higher in chloride than the water entering the vessel. When all or most of the exchange sites are occupied by arsenic or other anions by replacing chloride ions, the resin gets exhausted. The exhausted resin is regenerated with salt (sodium chloride). During the regeneration process there are substantial concentrations of sodium and chloride in the wastewater as well as the arsenic. The resins prefer sulphate ions to arsenic anions when arsenic contaminated water containing sulphate ions is treated in the ion exchange process. As a result, the sulphate ions are exchanged for chloride ions before the arsenic ions.

Ion exchange resin can only exchange anions from water; that is, arsenic in the form of anions can only replace chloride ions attached to the resin. The pH of the feed water is maintained above about 7.5 because most of the arsenic (V) can be expected to be present either in the form of HAsO₄ ²⁻ or H₂AsO₄ ⁻. H₃AsO₃ remains in the neutral form at above about 7.5; therefore, As(III) in water is required to be converted into As(V) by a suitable oxidizing agent, such as chlorine. There is a risk of degradation of the resin by the oxidizing agents during the oxidation of the arsenic (+3) to achieve a (5+) oxidation state. For the effective removal of arsenic, the suitable conditions are that the arsenic has a (5+) oxidation state and that the pH be at least 7.5. Experimental works show that ion exchange can achieve arsenic reductions of more than 95%. Greenleof et al. (2006) have claimed to have successfully removed arsenic from water using newly developed ion exchange fibers (IX fibers) that contain dispersed hydrated ferric oxide nanoparticles. The advantage of this ion exchange material is that it can be easily regenerated using 2% NaOH and 2% NaCl solutions. The ion exchange process has the disadvantage of releasing noxious chemical reagents used in the resin regeneration into the environment.

The major disadvantages of the precipitation, adsorption and ion exchange methods are the requirements of multiple chemical treatments, pre- and/or post-treatment of drinking water, disciplined/trained operation, high running/capital cost and more importantly, regeneration of the medium and handling of arsenic contaminated sludge. Disposal of the sludge will probably pose a problem in most cases.

While adsorption-based processes are often suitable for domestic water purification or at the most for a small community, it has the associated problem of frequent replacement as regeneration at such level is practically impossible. In the study region, a large number of activated alumina adsorbent-based community water filters installed earlier have turned defunct. Large-scale physico-chemical treatment plants could be very effective for supply of arsenic-free water to a large community (Pal et al. 2007) but this often needs government level initial investment and continuous operating costs. Membranes with high selectivity have the potential to produce totally arsenic-free water due to the small molecular weight (<150 Da) of most dissolved species of arsenic. In water arsenic occurs mostly either as trivalent arsenite or as pentavalent arsenate in natural water and these are part of arsenic acid (H₃AsO₄) and arsenious acid (H₃AsO₃) systems respectively protonation of which depends on the pH of the aqueous system. At typical pH conditions of 6.5–8.0, As(V) remains as an anion and As(III) as a neutral molecule (Pal 2001). Thus, membranes have the potential to remove arsenic from drinking water. Particularly, charged nanofiltration membranes are expected to effectively separate the anionic forms by charge repulsion (Donnan effect). Trivalent arsenic removal by membranes takes place basically through the sieving mechanism. Ultra, nano and reverse osmosis membranes can purify arsenic-contaminated water well up to the WHO-prescribed limit. A major advantage of membrane purification is that such membranes can remove not only arsenic but all such impurities including pathogenic microorganisms. The region receives abundant rainfall (Goswami 2002; Roy et al. 2008) but water quality remains a major concern as is evident in recurrent water-borne enteric diseases in large scale. And this is where membrane-based treatment processes can play a very effective role. Membrane-based processes produce very little sludge which is often a big problem in physico-chemical treatment. However, associated costs, membrane fouling, and replacement requirements should be critically examined. In the next section we, therefore, review such membrane-based processes.

5.1.4 Membrane-based processes for purification of arsenic-contaminated water

Given the size distribution and chemical speciation of arsenic in groundwater, nanofiltration (NF) and reverse osmosis membranes (RO) with solution diffusion mechanisms for solute transport have the highest potential of producing safe drinking water from the arsenic-contaminated groundwater against the possibility of limited success of microfiltration (operating range being 0.08–2.0 μm) and ultrafiltration membranes (0.005–0.02 μm) where the separation mechanism is based on sieving. For example, while trying to separate arsenic by ultrafiltration alone, Lin et al. (2008) achieved only 10% arsenic removal. However, they observed that the combination of chitosan (an environment-friendly biopolymer), humic acid (from naturally occurring humic substances or DOM) and arsenic can cause an effective 65% separation of arsenic from water. Chitosan has weak affinity for arsenic but can strongly adsorb DOM. DOM with an average molecular weight of 35,000 Da or greater can cause chelation of arsenic and its subsequent removal by UF membranes. The technique however, cannot bring down arsenic concentration to 10 μg/l level and with a small charge on the surface, ultrafiltration membranes can separate arsenic only to some extent whereas microfiltration (pore size being of the order of 0.05 μm) can do so only with prior physico-chemical treatment like coagulation and flocculation as has been reported by Hering and Elmelech (1995) and Han et al. (2002). Hsieh et al. (2008) applied electric voltage (25 V) on an ultrafiltration membrane to achieve about 79% arsenic separation with reasonably high flux that resulted from applied electric voltage. However, the technique is still short of target reduction down to 10 μg/l. Studies on hybrid processes combining microfiltration and ultrafiltration with chemical precipitation or adsorption have been reported (Beolchini et al. 2007; Lin et al. 2008; Pagana et al. 2008). Pagana et al. (2008) combined adsorption with ultrafiltration in removing arsenic as well as chromium from water and brought down arsenic concentration from 1 ppm to 10 μg/l using Fe₂O₃ adsorbent nanoparticles. In this process, adsorbent nanoparticles are totally removed in subsequent ceramic ultrafiltration membranes. But the process does not consider removal of trivalent arsenic and frequent membrane fouling necessitates frequent regeneration of the ceramic membranes. Shorney et al. (2001) studied such arsenic separation with prior coagulation of arsenic-bearing surface water with ferric chloride (FeCl₃) and ferrous sulphate (Fe₂(SO₄)₃). Floch and Hideg (2004) reduced arsenic concentration in water from 200 to 300 μg/l to below 10 μg/l in a similar membrane filtration study using a ZW-1000 hollow fibre membrane module with prior oxidation by potassium permanganate (KMnO₄), and coagulation with ferrous and ferric sulphates. Fogarassy et al. (2009) have also successfully reduced arsenic concentration of water to below 10 μg/l using nanofiltration and reverse osmosis techniques with prior oxidation of trivalent arsenic to pentavalent ones with KMnO₄ oxidation. Their suggested process is in fact a hybrid type that consists of nanofiltration/reverse osmosis in the first stage, followed by chemical treatment with Ca (OH)₂ and H₂S where the basic objective of the study was to reduce the volume of high arsenic-bearing wastewater. Qu et al. (2009) used a direct contact membrane distillation (DCMD) method in the separation of arsenic from water. Though the polyvinylidene membrane (PVDF) of the DCMD system could separate arsenic up to the WHO-prescribed level (10 μg/l), the flux generated was low at around 20 LMH (litre/m² h). Geucke et al. (2009) tested different Dow-made thin composite membranes on a small commercial desalination unit for removal of arsenic from water. Though they succeeded in reducing the concentration to the WHO-prescribed level, the reported flux (25–28 LMH) was low and membrane filtered water needed further polishing particularly for trivalent arsenic. Nguyen et al. (2009) observed a dramatic increase in the arsenic separation efficiency of micro and nanofiltration membranes (Nitto Denko Corp., Japan) upon the addition of a small amount of zero valent iron to the feed water that contained 500 μg/l arsenic. A marked difference was noted, however, in separation efficiency with respect to trivalent and pentavalent arsenic species.

Li et al. (2005) explored separation of arsenic and humic substances (HSs) on a laboratory scale electro-ultrafiltration (EUF) system. As negatively charged species, arsenate (V) was readily removed after applying voltage to the EUF cell. Arsenite (III) was removed via EUF after the pH of the water had been adjusted. Meanwhile, the rejection of humic substances increased due to the presence of an electric field. This study also showed that the removal of arsenite (III) from water relies primarily on electrostatic and non-electrostatic mechanisms. In the presence of HSs, arsenate (V) complexed with the HSs and was then able to be removed by EUF. This study demonstrates that EUF is a highly promising means of removing arsenic from water. Sklari et al. (2008) proposed a novel separation scheme of combined adsorption–ultrafiltration processes for the removal of arsenic from water. For the removal of As(V) a combined adsorption–ultrafiltration process has been proposed based on Fe₂O₃ adsorbent and c-Al2O₃ ultrafiltration membranes. The experimental results showed that 0.2 wt% (w/v) Fe₂O₃ adsorbent reduces the As(V) concentration in water from 1 ppm down to 10 ppb. Brandhuber and Amy (1998, 2001) studied ultrafiltration of river water after spiking the same with 15–30 μg/l arsenic (III) and arsenic (V) and achieved only a 47.2% rejection of arsenic against a 95% rejection by RO and NF membranes. Beolchini et al. (2007) used a high molecular weight cut-off (MWCO 100 kDa) PES ultrafiltration membrane to reduce the volume of highly concentrated arsenic-bearing wastewater (6–10 mg/l) and they achieved 93–98% As(V) removal but As(III) removal was only 70–74%. Combination of a high MWCO ultrafiltration membrane and a low surfactant concentration was claimed to lead to low membrane surface requirement and better economies.

Most of the arsenic removal studies are concentrated on RO and NF membranes (operating range being 0.0001–0.001 μm). US-EPA and WHO (2001) reports show that 40–99% removal of arsenic has been achieved in both bench and pilot scale RO plants studies. Geucke et al (2009), Fagarassy et al. (2009), Brandhuber and Amy (1998) have demonstrated the effective removal of arsenic from drinking water using a reverse osmosis membrane. Brandhuber and Amy (1998, 2001) have shown that up to 95% arsenic removal is possible using RO and NF membranes. In their studies, it is observed that NF can yield much higher flux than RO for the same rate of arsenic removal.

An RO membrane with a dense pore structure (pore size of about 0.0005 microns) has the capability of production of largely arsenic-free water but high pressures are required to cause water to pass across the membrane from a concentrated to dilute solution. In general, driving pressure increases as selectivity increases. Both in RO and NF, it is desirable to achieve the required degree of separation at the maximum specific flux (membrane flux/driving pressure). Nanofiltration is based on the use of membranes constructed of a porous inert layer of polysulphone and a negatively charged hydrophobic rejection layer.

Vrijenhoek and Waypa (2000), in their study with synthetic water, have shown that both RO and NF can achieve 99% rejection of arsenic. They achieved comparable separation of As(V) and As(III) with no preferential rejection of As(V) over As(III) indicating governing role of size exclusion in their separation. In their study on the effect of pH on the rejection of arsenic, Urase et al. (1998) found that electrically charged membranes generally had a higher rejection for charged solutes than for non-charged solutes. Arsenic(V) is removed more efficiently than As(III). They could explain their results in terms of extended Nernst–Planck equation.

Oh et al. (2000) applied reverse osmosis and nanofiltration membrane processes for the treatment of arsenic applying a low pressure (0.2–0.7 MPa) bicycle pump indicating the potential for this application of nanofiltration membranes in rural areas. Kang et al. (2000) observed the same trend as Urase et al. (1998) in comparative separation of As(III) and As(V). They observed that removal of As(V) was much higher than As(III) over the pH range 3–10. The effect of solution pH on the removal of arsenic using RO membranes was strongly affected by the solution pH, especially As(IlI). Almost the same kind of difference (30% lower for arsenite) in AS(III) and As(V) removal by NF was observed by Seidel et al. (2001) where up to 90% As(V) could be removed. Similar observations (75% arsenite and 95% arsenate removal) have been reported by Sato et al. (2002). Shih (2005) in their literature overview of arsenic removal by pressure-driven membrane has concluded that efficiency of AS(V) removal by membranes is much higher than that of As(III) removal and that use of an oxidizing agent can substantially improve overall arsenic removal by NF. But use of chemical oxidants like KMnO₄ or chlorine-based ones has the potential danger of damaging membranes. In such a scenario, the option of microbial oxidation of trivalent arsenic to pentavalent arsenic may be examined further. Over 30 such microbial strains from at least nine different genera have been identified (Oremland and Stolz 2003). Though several studies on microbial oxidation of As(III) to As(V) have been reported (Oremland et al. 2002; Katsoyiannis and Zouboulis 2004), efficiency of such oxidation has been reported to be limited to maximum 80% only. Though genetic modifications of the microbes can enhance this oxidation efficiency this has inherent potential of damaging ecosystem (Chatterjee 2000).

Ning (2002) had reviewed the removal mechanism of RO and concluded that arsenic in the commonly high oxidation states of (V) is very effectively removed by RO. With further attention to the removal of the weakly acidic arsenic (III) species in waters by the operation of RO at sufficiently high pH values made possible by the newer anti-scalants. He has concluded that practical processes can be developed with RO to remove all major species of arsenic from water. Removal of arsenic(V) from water using surfactant micelles and ultrafiltration membranes has been studied by Hatice et al. (2004). Polyethersulphone (PES) membranes without the surfactant micelles were found to be ineffective for arsenic removal. Regenerated cellulose (RC) membranes provide better removal than PES membranes due to their negatively charged surface at the experimental conditions. However, the arsenic concentration in permeate water is not reduced to 10 μg/l. With the addition of 10 mM cationic surfactant cetyl pyridinium chloride (CPC) to the feed water, both PES and RC membranes reduce the arsenic concentration in permeate water well below the MCL. The process does not take care of trivalent arsenic which is almost always present in groundwater. Separation of As(V) from synthetic solution by NF-300 membrane was studied by Saitua et al. (2005). Rejection was found to be between 93 and 99% from synthetic water containing 100–382 μg/l As(V). It is also observed that the rejection of As(V) was independent of transmembrane pressure, cross flow velocity and temperature. Košutić et al. (2005) observed the As separation by NF270 membrane. They showed that the separation of As in nanofiltration is not much less than RO and the nanofiltration membrane showed a better permeation rate. The membrane material and pore size distribution influence the uncharged organic molecules rejections. In China, Xia et al. (2007) investigated As(V) and As(III) removal with synthetic water. The results show that there is a large difference in the removal of As(V) and As(III). As(V) was almost fully removed, while As(III) was removed about 5%. The removal of As(V) was higher than that of As(III) with different humic acid concentrations. The presence of additional salts has been shown to have an impact on the rejection of As(V). The arsenic rejection by the membrane increased with increasing pH. That nanofiltration or reverse osmosis can very effectively remove arsenic from drinking water is well established through several studies but it does not imply its automatic adoption in the affected areas. It depends on availability of the technology, device, membrane, scale up confidence and economics. Though pilot testing of nanofiltration and reverse osmosis has been reported (Brandhuber and Amy 1998; Vrijenhoek and Waypa 2000) from the developed world such testing has hardly been reported from the developing world like the Bengal delta region.

6 Conclusion

The vastness of the arsenic contamination problem calls for an all-out effort to combat it on war footing. Because of the very insidious nature of the problem and the absence of effective monitoring we do not know how many people are daily joining the list of arsenic victims. Though a lot of work has been done to provide a geological explanation of arsenic contamination, little work has been done to find out the causes of the outbreak of arsenic poisoning mainly over the last two decades. A few studies have been taken up to enlighten people on how to give relief to the people already suffering from arsenic-related diseases. That the water use pattern of a growing population could be a root cause of the problem has failed to attract the attention of researchers. Literature abounds in adsorption-based purification, but a major weakness of the technology has not been highlighted. Large-scale physico-chemical treatment plants for treating the contaminated groundwater have not been studied much. Neither has pilot testing of such plants been reported from the worst affected region. Membranes have been proved to be most effective in producing almost arsenic-free water. But, pilot testing of the technology, in the Bengal delta basin where the majority of the arsenic affected people live has hardly been taken up. Though nanofiltration in such a region could be the most potential technology, scale up confidence is still very limited in the absence of adequate data on successful operation. Hardly any field level plant has been reported to be operational in this region. A multipronged approach consisting of better surface water management thus significantly minimizing the geological disturbance of the underground aquifers, effective medication of the affected people and large scale adoption of membrane based purification technology seem to be the solution.

Both NF and RO can very effectively remove arsenic from drinking water. But NF can even outperform RO in terms of higher flux for the same rate of removal of arsenic while permitting filtration at a lower pressure than is necessary for RO. Low pressure operation is again a great advantage vis-à-vis RO types in the affected rural areas where power supply is a major constraint. Though most of the NF studies show that removal efficiency for As(V) is higher than that for As(III), wide variations have been reported with respect to the extent of such separation. That As(V) removal is much better than As(III) removal points to the necessity of use of oxidation technique prior to membrane filtration if presence of As(III) in water is significant. There is little doubt as to the efficiency of chemical precipitation, adsorption and membrane-based purification of arsenic-contaminated groundwater. But economies and applicability of the techniques in various parts of the world and especially in the South East Asian countries where the largest population has been affected have not been studied to the extent which is essential to develop scale-up confidence and installation of large-scale treatment plants. This is evident from the fact that millions of people in these arsenic-affected areas continue to suffer and instead of use of modern technology of water purification, the mitigation approach has been on supply of arsenic-free water from alternate sources like the river bodies at the most. Such provisions often involve very high capital investment in laying long pipe lines in addition to physico-chemical treatment plant to take care of industrial pollutants and pathogens in surface water. Because of the involvement of huge capital, implementation of such projects in the arsenic-affected areas often take long time forcing thousands of people to continue to drink contaminated water.