Technological options for the removal of arsenic with special reference to South East Asia

Abstract

Arsenic contamination in ground water, used for drinking purpose, has been envisaged as a problem of global concern. However, arsenic contamination of ground water in parts of South East Asia is assuming greater proportions and posing a serious threat to the health of millions of people. A variety of treatment technologies based on oxidation, co-precipitation, adsorption, ion exchange and membrane process are available for the removal of arsenic from ground water. However, question remains regarding the efficiency and applicability/appropriateness of the technologies, particularly because of low influent arsenic concentration and differences in source water composition. Some of these methods are quite simple, but the disadvantage associated with them is that they produce large amounts of toxic sludge, which needs further treatment before disposal into the environment. Besides, the system must be economically viable and socially acceptable. In this paper an attempt has been made to review and update the recent advances made in the technological development in arsenic removal technologies to explore the potential of those advances to address the problem of arsenic contamination in South East Asia.

Introduction

Arsenic contamination in ground water, used for drinking purpose, has been envisaged as a problem of global concern. It has been reported from many parts of the world like Argentina, Australia, Bangladesh, Bolivia, Cambodia, Chile, China, Finland, Greece, Germany, Ghana, Hungary, India, Italy, Japan, Mexico, Mongolia, Myanmar, Nepal, Peru, Philippines, Romania, Spain, Taiwan, Thailand, USA and Vietnam (Robertson, 1986, 1989; Moncure et al., 1992; Schlottmann and Breit, 1992; Frost et al., 1993; Das et al., 1994, 1995; Chatterjee et al., 1995; Mandal et al., 1996; Ahmad et al., 1997; Berg et al., 2001; BGS, 2001; Tandukar et al., 2001; Alam et al., 2002; Mandal and Suzuki, 2002; Nordstrom, 2002; Smedley and Kinniburgh, 2002; Shrestha et al., 2003; Fytianos et al., 2004; Sun, 2004; Xia and Liu, 2004; Stranger et al., 2005; Mukherjee et al., 2006; Buschmann et al., 2007; Katsoyiannis and Katsoyiannis, 2006; Katsoyiannis et al., 2007). However, the natural arsenic contamination of ground water in parts of South and East Asia has assumed greater proportions and posing a serious threat to the health of millions of people (Berg et al., 2001; WHO, 2001; Chakraborti et al., 2002; Mandal and Suzuki, 2002; Ng et al., 2003; Polya et al., 2005; Mukherjee et al., 2006). The countries affected in the region include Bangladesh (the worst affected), India, Myanmar, Nepal and Pakistan (South Asia); and Cambodia, China (including Taiwan), Lao People's Democratic Republic and Vietnam (East Asia). An estimated 60 million people are at risk from high levels of naturally-occurring arsenic in ground water, and at least 700,000 people in the region have thus far been affected by arsenicosis (World Bank, 2005a, World Bank, 2005b). Bangladesh has the highest percentage of contaminated shallow tube wells and an estimated 30 million people are dependent on those wells for domestic purposes (Heikens, 2006; Heikens et al., 2007). Most arsenic affected areas in South East Asia are reported from Bangladesh and West Bengal-India. Nine districts in West Bengal-India and 47 districts in Bangladesh have arsenic level in ground water above 50 μg L⁻¹. The WHO guideline for arsenic in drinking water is 10 μg L⁻¹. The area and population of the 47 districts in Bangladesh and 9 districts of West Bengal are 112407 km² and 93.4 million, and 38.865 km² and 42.7 million, respectively (Bhattacharya et al., 1997; Chowdhury et al., 1997; Chowdhury et al., 2000a, Chowdhury et al., 2000b; BGS and DPHE, 2001; Mukherjee and Bhattacharya, 2001; Rahman et al., 2001; Chakraborti et al., 2002, 2003, 2004; Singh, 2006).

The high levels of arsenic in ground water in the affected countries are predominantly of geogenic origin. Reductive dissolution of iron (hydr)oxides (FeOOH) stimulated by microbial activity and organic materials is regarded as the most important mechanism releasing arsenic into the aquifer (Nickson et al., 1998, 2000; Ravenscroft et al., 2001; Smedley and Kinniburgh, 2002; Smedley et al., 2003; Ahmed et al., 2004; McArthur et al., 2001, 2004; Zheng et al., 2004). Anthropogenic sources of arsenic include various industrial activities, pesticides, herbicides and fertilizers.

Over the past several years, numerous toxicological and epidemiological studies have been conducted to ascertain health risks associated with low-level exposure to arsenic ingestion. Ingestion of inorganic arsenic can result in both cancer and non-cancer health effects (NRC, 1999). Arsenic interferes with a number of essential physiological activities, including the actions of enzymes, essential cations and transcriptional events in cells (NRC, 1999). The US Environmental Protection Agency (USEPA) has classified arsenic as a Class ‘A’ human carcinogen. Chronic exposure to low arsenic levels has been linked to health complications, including cancer of the skin, kidney, lung and bladder, as well as other diseases of the skin, neurological and cardiovascular system (EPA, 2000a). The USEPA in 2001 adopted a new standard for arsenic in drinking water at 10 μg L⁻¹ (EPA, 2001), replacing the old standard of 50 μg L⁻¹.

Arsenic occur in the environment in different oxidation states and form various species, viz., As(V), As(III), As(0) and As(-III) (Braman and Foreback, 1973; Andreae, 1977; Shaikh and Tallman, 1978). The toxicity of different arsenic species varies in the order: arsenite [As(III)] > arsenate [As(V)] > monomethylarsonate (MMA) > dimethylarsinate (DMA) (Penrose, 1974; Lewis and Tatken, 1978; Stugeron et al., 1989). The valency state of arsenic plays an important role for the behavior of the element in the aqueous system. For example, toxicity of arsenic in trivalent state [As(III)] is higher than that of their pentavalent [As(V)] species (Berman, 1980; Gesamp, 1986). The valency state of an element also determines the sorption behavior and consequently the mobility in the aquatic environment. Jain and Ali (2000) have reported the occurrence, toxicity and speciation techniques for arsenic while Duker et al. (2005) described the toxic effects of arsenic as well as its mobilization in the natural environment.

In natural water, arsenic is mostly found in trivalent [As(III)] or pentavalent [As(V)] states. The distribution of arsenic species [As(III), As(V)] in natural waters is mainly dependent on redox potential and pH conditions (Cullen and Reimer, 1989). Under oxidizing conditions i.e., surface waters, the predominant species is pentavalent arsenic, which is mainly present in the oxyanionic forms (H₂AsO₄⁻, HAsO₄²⁻) with pK_a1 = 2.19, pK_a2 = 6.94 respectively. On the other hand, under mildly reducing conditions such as in anoxic ground waters, As(III) is the thermodynamically stable form, which at the pH values of most natural waters is present as non-ionic arsenious acid (H₃AsO₃, pK_a1 = 9.22) (Cullen and Reimer, 1989). Similar findings were also reported by other researchers (Bose and Sharma, 2002; Kim et al., 2002; Ryu et al., 2002; Smedley and Kinniburgh, 2002). Recently, Katsoyiannis et al. (2007) explored arsenic speciation and concentrations to geological conditions and correlated with various redox indicative parameters and chemical components with their implications on ground water treatment. A significant correlation between Eh and arsenic speciation was observed with predominance of As(V) at higher Eh values (oxidizing conditions) and As(III) at lower Eh values (reducing conditions). A strong correlation was also observed between uranium concentrations and arsenic speciation, depicting their use as a possible indicator of ground water redox conditions (Katsoyiannis et al., 2007).

The removal of arsenic from water sources is generally accomplished by the application of conventional treatment methods such as oxidation, co-precipitation, adsorption, ion exchange and membrane process (Shen, 1973; Jekel, 1994; Hering and Elimelesh, 1995; Kartinen and Martin, 1995; Driehaus et al., 1998; Zouboulis and Katsoyiannis, 2002a, Zouboulis and Katsoyiannis, 2002b). The specific methods adopted for the treatment of arsenic-contaminated water include flotation (Zhao et al., 1996), coagulation (Hering et al., 1997; Sancha, 2000), enhanced coagulation (Cheng et al., 1994), precipitation with sulfide (Bhattacharya et al., 1979), oxidation of As(III) followed by removal of total arsenic using ferric hydroxide, ferric chloride (Hering et al., 1996), iron-oxide coated sand (Joshi and Chaudhuri, 1996) and greensand filtration (Viraraghavan et al., 1996). A detailed review of arsenic removal technologies has been presented by Sorg and Logsdon (1974). Jekel (1994) has documented several advances in arsenic removal technologies. In view of the lowering the drinking water standards by USEPA, a review of arsenic removal technologies was made to consider the economic factors involved in implementing lower drinking water standards for arsenic (Chen et al., 1999). Many of the arsenic removal technologies have also been discussed in details in American Water Works Association reference book (Pontius, 1990). A comprehensive review of low-cost, well-water treatment technologies for arsenic removal has also been compiled by Murcott (2000).

During the last few years many small scale and community based arsenic removal technologies have been developed, field-tested and used in various countries. However, question remains regarding the efficiency and applicability/appropriateness of the technologies, particularly because of low influent arsenic concentration and differences in source water composition. Some of these methods are quite simple, but the main disadvantage associated with them is that they produce large amounts of toxic sludge, which needs further treatment before disposal into the environment. Besides, the system must be economically viable and socially acceptable.

Speciation of arsenic and redox kinetics plays an important role in development and design of arsenic removal technologies. Therefore, a better understanding of arsenic speciation and redox kinetics is urgently needed for development of simple and reliable water treatment procedures. Most of the conventional methods for arsenic removal are not efficient in the removal of As(III) and therefore include a pre-oxidation step to achieve finished waters with total arsenic concentrations below 10 μg L⁻¹ (Katsoyiannis et al., 2004, 2007). Keeping in view the above facts, an attempt has been made in this paper to review and update the recent advances made in the technological development in arsenic removal technologies to explore the potential of those advances in addressing the problem of arsenic contamination in ground water of South East Asia.