Chemosphere 158 (2016) 37e49
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Review
The global menace of arsenic and its conventional remediation - A critical review Arpan Sarkar, Biswajit Paul* Department of Environmental Science & Engineering, Indian School of Mines, Dhanbad 826004, Jharkhand, India
h i g h l i g h t s Presence of arsenic in the groundwater has threatened the human health around the world. Dissolution of arsenic into the groundwater is influenced by various physicochemical and biological activities. Long term exposure to arsenic can cause cancer and other deadly diseases. It has been found that the As(III) is more toxic than the As(V). Various techniques have been employed to remediate arsenic from groundwater to make the water safe for drinking.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 January 2016 Received in revised form 7 May 2016 Accepted 13 May 2016
Arsenic is a ubiquitous element found in the earth crust with a varying concentration in the earth soil and water. Arsenic has always been under the scanner due to its toxicity in human beings. Contamination of arsenic in drinking water, which generally finds its source from arsenic-containing aquifers; has severely threatened billions of people all over the world. Arsenic poisoning is worse in Bangladesh where As(III) is abundant in waters of tube wells. Natural occurrence of arsenic in groundwater could result from both, oxidative and reductive dissolution. Geothermally heated water has the potential to liberate arsenic from surrounding rocks. Inorganic arsenic has been found to have more toxicity than the organic forms of arsenic. MMA and DMA are now been considered as the organic arsenic compounds having the potential to impair DNA and that is why MMA and DMA are considered as carcinogens. Endless efforts of researchers have elucidated the source, behavior of arsenic in various parts of the environment, mechanism of toxicity and various remediation processes; although, there are lots of areas still to be addressed. In this article, attempts have been made to lay bare an overview of geochemistry, toxicity and current removal techniques of arsenic together. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: X. Cao Keywords: Arsenic Arsenic geochemistry Arsenic affected areas Arsenic toxicity Arsenicosis DMA MMA Aqueous arsenic remediation Adsorption Coagulation Membrane filtration
Contents 1. 2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Arsenic in relation to the environment: a brief overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.1. Arsenic in relation to air, water and soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.1.1. Arsenic in air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.1.2. Arsenic in relation to water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 2.1.3. Arsenic in relation to soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
* Corresponding author. E-mail addresses:
[email protected] (A. Sarkar),
[email protected] (B. Paul). http://dx.doi.org/10.1016/j.chemosphere.2016.05.043 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
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3. 4. 5.
6.
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2.2. Use of arsenic in human society . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Arsenic-affected areas in the world . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Toxicity of arsenic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Conventional removal processes of arsenic from groundwater: a brief overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.1. Coagulation-precipitation and filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 5.2. Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.3. Ion exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 5.4. Membrane filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
1. Introduction In modern days, with deteriorating environmental quality, human life is exposed to numerous types of threats. The food for appeasing hunger, water for quenching thirst and air for breathing, have all become unsafe these days. Immedicable diseases have proved the daily hazards of human life. There are number of harmful chemicals around us; ingestion, breathing or any kind of exposure to them can affect the human health tremendously. Arsenic, being a toxic element has made the human life miserable in the areas with a higher concentration of it in soil, water and to some extent, in air. The global attention is mainly focused on the presence of arsenic beyond its safe limit in water (0.01 mg/L) (World Health Organization, 2010), specifically in groundwater, as the major part of global population from the countries like India, Bangladesh, China, Nepal, Thailand, Brazil, United States, Canada, England relies on groundwater to meet the purpose of drinking. Arsenic contamination in groundwater is generally caused by the natural sources. Mining, smelting of arsenic-bearing minerals are among the anthropogenic sources of arsenic contamination in the environment. Arsenic exists in the environment in different forms; III, 0, þIII and þV depending on the prevailing physicochemical conditions of the environment. Incessant consumption of arsenicrich water causes different types of chronic diseases like the Blackfoot disease, pigmentation, keratosis, nausea and most importantly cancer in human. Inorganic arsenic compounds are found to have more toxic effects than the organic forms. Methylated arsenic acids are the organic forms of arsenic and believed to have the carcinogenic effect in human. Scientists and researchers have been working to discover more advanced and apt ways to mitigate this problem since it has risen in the global horizon. A number of measures are already been in practice for treating the arsenic contaminated water by chemicals, activated carbon, reverse osmosis, adsorption on nanomaterials etc. With the emergence of nanotechnology, various nanomaterials have been used for treating water and it lived up to the expectation. In this article we shall seek to discuss briefly the geochemistry, sources of arsenic, the fate of arsenic in soil and groundwater, global view of the arsenic affected area, effects of arsenic exposure in the human body. We shall also concentrate our efforts to depict the current scenario of arsenic mitigation in lights of various removal techniques.
the major mineral forms can be categorized as the arsenates (~60%), the sulfides and the sulfo-salts (~20%) of arsenic. The rest are the silicates and native arsenic (Nriagu, 1994). The arsenate minerals are those, which contain oxyanions of As such as AsO3 4 , AsO3(OH)2. Orpiment (arsenic trisulfide As2S3), realgar (tetraarsenic tetrasulfide As4S4), a and b-dimorphite (tetraarsenic trisulfide As4S3), tetraarsenic pentasulfide (As4S5) are some well known arsenic sulfide minerals, whereas, arsenopyrite (FeAsS2), cobaltite (CoAsS), are two examples of the mixed sulfides of arsenic [M(II) AsS]. In the environment, among the oxidation states of arsenic, þIII and þV are the mostly encountered in soil and water environments. As(V) is the dominant species in an oxidative environment whereas, As(III) is the dominant one in the reducing environment. The sulfides are least resistant to weathering; arseniccontaining sulfides easily get weathered compared to the other arsenic minerals. Oxidation of arsenic sulfides allows arsenic to spread out into the various components of the environment such as soil, water and air (Murdoch and Clair, 1986; Welch and Stollenwerk, 2007). 2.1. Arsenic in relation to air, water and soil
2. Arsenic in relation to the environment: a brief overview
2.1.1. Arsenic in air In the air, the ambient arsenic concentration is generally very marginal. It is lesser in rural areas and higher in urban areas. In rural areas, the aerobic arsenic concentration is reported to range between 0.02 ng/m3 and 4 ng/m3, whereas in urban areas it is found to be lying in between 3 ng/m3 and 200 ng/m3 which is 50e150 times more than that of rural areas (Chappell et al., 2001). The atmospheric arsenic concentration is always subjected to local level contamination magnitude. In many cases, arsenic in the air is contributed by the volcanic eruption, mining and smelting of arsenic-bearing minerals and burning of fossil fuels, especially coal. Coal samples of some mines have been reported to contain arsenic upto1.5 g/kg arsenic where the average arsenic content of average coal samples is around 13 mg/kg (Bissen and Frimmel, 2003). The arsenic species in air are greatly adsorbed by the particulate matters. Polynucleated Aromatic Hydrocarbons (PAH) having a size range from 1 to 20 mm provide the site to the metals and metalloids present in the atmosphere to get adsorbed on it. Arsenic species in air, are generally washed out by rain or get precipitated by various physicochemical processes of the atmosphere.
Arsenic is a metalloid with atomic number 33, atomic mass 74.92 and electronic configuration 4s23d104p3, found as a trace element in the earth’s crust. It is the twentieth most abundant element in the earth’s crust. In the continental crusts, the average arsenic content generally varies from 2 mg to 3 mg/kg (Tanaka, 1988; Cullen and Reimer, 1989). Out of 200 mineral forms of As,
2.1.2. Arsenic in relation to water Arsenic concentration in water varies from one water body to another water body. We have observed a great variation in the aqueous arsenic concentration reported by various scholars in their articles. Chappell et al. (2001), reported the oceanic arsenic concentration to lie between 1 and 2 mg/L; Welch et al. (1988) reviewed
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the same to range between 0.15 and 6 mg/L; while Nriagu et al. (2007) reviewed a wide variation in oceanic arsenic contents; the deep oceanic As(V) concentration at 3.6% salinity is 1.7 mg/L, mean oceanic arsenic concentration varies from 1 to 8 mg/L and the global median of arsenic in seawater is 3.7 mg/L. They reviewed the actual arsenic level in the sea to be less than 1 mg/L except some alteration found in the Antarctic and south-west Pacific oceans, where the concentration is 1.1and 1.2 mg/L respectively. Similarly, geographical speciation shows a great diversity in arsenic contents of rivers, lakes and groundwater around the world. The background arsenic concentration in fresh water is difficult to define as various researchers have reported a widely diverse data contradicting each other. Here, any generalized background arsenic level for fresh water is not given as it would be quite misleading. However, fresh water contaminated with arsenic can contain from hundred microgram to thousands of microgram of arsenic per liter of water (Chappell et al., 2001; Mandal and Suzuki, 2002; Bhattacharya et al., 1997; Mukherjee et al., 2006; Barringer and Reilly, 2013), while the maximum permissible limit for drinking water arsenic is 0.01 mg/L or 10 ppb (World Health Organization, 2010). The concentration of arsenic in groundwater is greatly influenced by the mineralogical characteristics of the aquifer. Arseniccontaining aquifers can release arsenic compounds into the water by being weathered. The degree of weathering is controlled by the physicochemical properties of the groundwater as water plays the key role in weathering of aquifers. Arsenic can find its way in groundwater by reductive dissolution of arsenic-rich iron oxyhydroxides present in dispersed phases in aquifers (Nickson et al., 2000). Arsenic has been found to be present in reduced conditions in the alluvium aquifers of West Bengal (India) and Bangladesh (Ahmed et al., 2004; Anawar et al., 2003). However, it should not be the ultimate mechanism of arsenic dissolution in groundwater everywhere in the earth. Fig. 2 shows that the major part of the world contains the oxidized forms of arsenic in groundwater. Thus, the mechanism of dissolution of arsenic in groundwater should not be the reduction of arsenic-rich iron minerals there; it could be oxidation of arsenic-rich minerals. Some time geothermal activities can contribute arsenic to the water. Arsenic along with other elements like antimony (Sb), boron (B), mercury (Hg), thallium (Tl), selenium (Se), lithium (Li) forms a special group of elements which do not fit easily in the lattice of common rock minerals. Together with hydrogen sulfide, these are found in plenty in high-temperature hydrothermal vents (Barringer and Reilly, 2013). Hot magma from the mantle comes out to the overlying rocks and gets confined in it. Heat radiated from the intrusive magma heats up the surrounding rocks and which indeed heats up the water mass present above or within the rock. The hot water liberates minerals from the rock body and thus becomes rich in minerals. In this way, warm subterranean water can gain arsenic from arsenic-containing rocky aquifers or from underlying impermeable bed rocks. Waters of some well known hot springs like the Yellowstone National Park in U.S.A., the Meager Creek hot spring in British Columbia and some of the Japanese hot springs; contain a higher amount of arsenic (Koch et al., 1999). Arsenic level in geothermal fluids has been found to be ranging from 0.1 to 50 ppm (Ballantyne and Moore, 1988). Unlike other oxyanion-forming elements and heavy metals (e.g. Se, Cr, Sb, Mo, U, V, etc.), arsenic shows quite different sensitivity to mobilization at pH values usually found in groundwater. It, in addition, shows a different character in oxidizing and reducing environments than the other oxyanion-forming metals/metalloids show. The concentration of heavy metal cations (Pb2þ, Cu2þ, Zn2þ, Ni2þ, Co2þ and Cd2þ) in groundwater gets limited by precipitation and/or co-precipitation at a neutral or near neutral pH of
39
groundwater or they are adsorbed by hydrous metal oxides, clay matter, etc. In contrast, the oxyanion-forming metal/metalloids hardly precipitate at near neutral pH and are not adsorbed by hydrous metal oxides or clay matter at an increased pH. It explains the relative abundance of oxyanion-forming elements like As, Cr, Se, U, etc. in groundwater even as these elements do not have the abundance in aquifer forming rocks (Smedley and Kinniburgh, 2001). However, in comparison to other oxyanion-forming elements, arsenic is the most problematic to the environment due to its relative mobility over a large range of redox conditions. Other oxyanion-forming elements like Se, Cr mobilizes and immobilize in different oxidative and reducing conditions, but the arsenic compounds do not immobilize in any of these conditions. Both As(III) and As(V) are soluble in water over a broad range of pH and Eh (redox potential) (Welch et al., 1988; Bell, 1998). The Eh-pH diagram provides the view of stability area of metal/ metalloid species in a solution depending on the solution pH and Eh. Positive Eh values indicate towards an oxidative condition; higher the values stronger the oxidative condition. Whereas, negative Eh represents a reducing condition; more the negative value, stronger the reducing condition. Arsenic in groundwater can 3 be present as oxyanions; arsenate (AsO3 4 ) and arsenite (AsO3 ) depending upon the pH and Eh of groundwater (Welch et al., 1988). For the aqueous solution of arsenic, the Eh-pH diagram (Fig. 1) ¼ suggests that at pH < 6.9, H2AsO¼ 4 species dominates, HAsO4 dominates at higher pH (>6.9), at extreme acidic and alkaline conditions, H3AsO04 and AsO3 4 predominates respectively. At the same time, due to slow kinetics, both arsenite and arsenate can coexist in either redox condition (Hendricks, 2006). Among the arsenites, H3AsO03 exists at a wide range of pH from very low to about 9.2. High alkaline conditions support the existence of HAsO¼ 3 and AsO3 3 in water (Smedley and Kinniburgh, 2001). However, in the presence of extremely reduced sulfur, formation and precipitation of the sulfides of arsenic like As2S3, As4S4, As4S3, etc. is favoured. Therefore, free sulfur-containing water is not expected to have high arsenic content (Moore et al., 1988); but in contradiction
Fig. 1. Stability fields of dissolved As in water with respect to water pH and Eh at 25 C temperature and 1atmospheric pressure (Welch et al., 1988; Smedley and Kinniburgh, 2001).
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to that Kim et al. (2000) had shown that carbonation of arsenic sulfide minerals in anaerobic conditions is the major source of arsenic dissolution in groundwater. The arseno-carbonate com þ plexes like As(CO3) 2 , As(CO3)(OH)2 , AsCO3 are supposed to be formed and are assumed to be stable in groundwater. The controlling factors of groundwater arsenic may include adsorption/desorption on or from metal ions like iron, manganesemineral precipitation or dissolution, existing water pH and Eh, physicochemical properties like organic matter content, the presence of interfering ions, etc. of the aquifer, reaction kinetics, etc. Some of the probable reactions involving arsenic species occurring in the natural environment are given in Table 1. These reactions influence the forms of arsenic present in the natural environment. 2.1.3. Arsenic in relation to soil The background arsenic concentration in soil typically ranges from 1 mg/kg to 40 mg/kg with an average of 5 mg/kg (Garelick and Jones, 2008; Beyer and Cromartie, 1987; Bowen, 1979). Alteration of these values is likely to arise in many instances, as the arsenic concentration is strongly influenced by the chemical composition of local rock bodies and other bio-geological factors. Human activities can alter the common soil-arsenic content to a great extent. Chappell et al., (2001) reviewed some high concentrations of arsenic in soil with arsenic levels 27 g/Kg, 35 g/kg, contaminated with mine and smelter waste deposits and discharges of arsenic pesticide manufacturing company respectively. An amount of 52,000e112,0000 ton of arsenic has been estimated to discharge into the environment annually from anthropogenic sources (Liao et al., 2005). Mining activities have been found to have contaminated soil and water in parts of England, China, Thailand, Ghana, Zimbabwe, Mexico, Canada, U.S.A. and Brazil (Bissen and Frimmel,
2003; Li and Thornton, 1993; Asante et al., 2007; Borba et al., 2003). The global discharge of arsenic from smelting activities has been estimated to 6200 ton per year whereas smelting of copper along contributes roughly 80% of global arsenic discharge from smelting in the environment annually (Bissen and Frimmel, 2003). Smelting of copper, nickel, lead, zinc and gold mineral has been identified as the major anthropogenic source of arsenic into the environment (Bissen and Frimmel, 2003; Borba et al., 2003). Both, the inorganic and organic forms of arsenic in oxidation states As(III) and As(V) can exist in the soil. In the aerobic oxidizing states, As(V) dominates and in the anoxic reducing soil, As(V) gets reduced to As(III) resulting in the relative dominance of As(III) in the anoxic soil. Matschullat (2000) reviewed that the soil rich in organic carbon has a greater concentration of arsenic and it could be vindicated by the potential of dissolved organic matter to inhibit the adsorption of arsenic species on iron oxides, iron hydroxides, alumina, kaolinite or quartz present in soil (Grafe et al., 2002; Bauer and Blodau, 2006; Sharma and Sohn, 2009). In the oxidizing conditions, soil microbes can methylate inorganic arsenic into monomethyl arsenic acid (MMA), dimethyl arsenic acid (DMA), trimethyl arsenic oxides (TMAsO). Alkylation of arsenic with other alkyl group is also common (Bentley and Chasteen, 2002). In anaerobic conditions, these could be easily reduced to form volatile arsenic compounds (Mandal and Suzuki, 2002). An example of this is the formation of highly toxic arsine gas (AsH3) by the reduction of As(III) to As(III) by the anaerobic microbes (Duker et al., 2005). The demethylation process converts the methylated organo-arsenic compounds again to the inorganic forms of arsenic in soil (Manyes et al., 2002). Not all the forms of arsenic are equally accessible to the microbes; sulfides of arsenic, Mn/Fe/Al oxides and hydroxides of arsenic are relatively stable and resist the microbial activities (Sarkar et al., 2007).
Table 1 Probable chemical reactions involving arsenic in the natural environment (Welch et al., 1988; Bentley and Chasteen, 2002). Conversion of arsenic species
Chemical reaction (not balanced)
þ n H3AsO3 / nHþþAsOn 3 / H3AsO4 / nH þ AsO4 þ n H3AsO4 / nHþþAsOn 4 / H3AsO3 / nH þ AsO3 H3AsO3/H3AsO4 / AsH3 H3AsO3 / FeAsS H3AsO4 / FeAsO4, 2H2O H3AsO4 / FeO(OH)x H3AsO3 / AsS, As2S3 As/AsS/As2S3 / As2O3, As4O6, As2O5 H3AsO3 þ CH3 / CH3As(OH)2 þ OH [MMAIII] CH3As(OH)2 þ CH3 / (CH3)2As(OH) þ OH [DMAIII] (CH3)2As(OH) þ CH3 / (CH3)3As þ OH [TMAIII] H3AsO4 þCH3 / CH3AsO(OH)2 þ OH [MMAV] CH3AsO(OH)2 þ CH3 / (CH3)2AsO(OH) þ OH [DMAV] (CH3)2AsO(OH) þ CH3 / (CH3)3AsO þ OH [TMAV] As2O3 þ CH3 / (CH3)3As [TMAIII] Ethylation of arsenic H3AsO3 þ C2H5 / C2H5As(OH)2 C2H5As(OH)2 þ C2H5 / (C2H5)2As(OH) (C2H5)2As(OH) þ C2H5 / (C2H5)3As H3AsO4 þ C2H5 / C2H5AsO(OH)2 C2H5AsO(OH)2 þ C2H5 / (C2H5)2AsO(OH) Mixed alkylation CH3As(OH)2 þ C2H5 / CH3As(OH)C2H5 (C2H5)2As(OH) þ CH3 / (C2H5)2As CH3 CH3AsO(OH)2 þ C3H7 / CH3AsOC3H7 Dealkylation CH3As(OH)2 þ OH / H3AsO3 þ CH3 (CH3)2As(OH) þ OH / CH3As(OH)2 þ CH3 C2H5As(OH)2 þ OH / H3AsO3 þ C2H5 Addition of þn valent metal(M) to MMA and DMA CH3As(OH)2 þ Mnþ / (CH3AsO2)nM2 CH3As(OH)2 þ Mnþ / (CH3AsO2)n-1HM (CH3)2As(OH) þ Mnþ / (CH3AsO)nM CH3AsO(OH)2 þ Mnþ / (CH3AsO3)nM2 CH3AsO(OH)2 þ Mnþ / (CH3AsO3)n-1HM (CH3)2AsO(OH) þ Mnþ / (CH3AsO2)nM
Arsenous acid to arsenic acid Arsenic acid to arsenous acid Arsenous/arsenic acid to arsine Arsenous acid to arsenopyrite Arsenic acid to acrodite Arsenic acid to FeO(OH)x Arsenous acid to arsenic sulfides Oxides of arsenic Methylation of arsenic
Reaction nature
Reaction augmented by
Oxidation Reduction Reduction Reduction Oxidation Precipitation/adsorption Oxidation Alkylation
Redox condition Redox condition Microbial activities pH > 4/1.5 Fe2þ, H2S Fe3þ Fe H2S O2 Methanogenic bacteria
Alkylation
Bacterial activities
Alkylation
Microbial activities
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2.2. Use of arsenic in human society Humans have been using arsenic for various purposes for quite a long time. Usage of arsenic for medicine, manufacturing of semiconductor devices is quite common. Usages of arsenic as wood preservative, herbicides, pesticides, fungicides and livestock feeder were also in practice in the last century until the severity of arsenic toxicity came in the daylight. 3. Arsenic-affected areas in the world Arsenic is largely found in the alluviums of the Indian states of West Bengal, Assam, Bihar, Jharkhand, Uttar Pradesh and in Bangladesh. Parts of other countries like Nepal, China, Mongolia, Myanmar, Thailand, Taiwan, Vietnam, Cambodia, Sri Lanka, Pakistan, Afghanistan, Japan, Hungary, Ghana, Mexico, Bolivia, and Argentina are fairly affected by arsenic toxicity. Reports of availability of arsenic in soil or water beyond the safe limit have also come from parts of Iran, Australia, New Zealand, parts of European Union, Iceland, Brazil, Canada and U.S.A (Mukherjee et al., 2006; Barringer and Reilly, 2013; Garelick and Jones, 2008). Globally over 150 million people are affected due to consumption arsenic contaminated drinking water (Ministry of Water Resources and River Development and Ganga Rejuvenation, 2014). Alluviums of West Bengal and Bangladesh are exposed to severe arsenic toxicity. The scenario is worst in Bangladesh. Almost half of the Bangladeshi citizens are at risk of consuming arsenic contaminated water from tube wells. A study carried out in Bangladesh in 2001, revealed that approximately 9100 deaths and 125,000 disability-adjusted life years (DALYs) occurred due to consumption of arsenic contaminated water (World Health Organization, 2010). A map of global probability of getting arsenic in reducing and oxidative states in groundwater is given in Fig. 2 and the approximate number of people affected by groundwater arsenic contamination in various countries has been tabulated in the Table 2. 4. Toxicity of arsenic Numerous researches have been carried out on the toxicity of arsenic so far. The toxicity of As(III) has found to be greatest among the species of arsenic and the inorganic forms of arsenic are much more toxic than its organic forms. The severity of As(III) poisoning could be realized by the number of deaths and DALYs occurred in Bangladesh due to arsenic poisoning, where the probability of getting more than 10 mg/L As(III) in drinking water is 0.75e1 (Fig. 2). Arsenic finds its way into the human body in various ways. The most common way has been ingestion, followed by inhalation and dermal absorption. Arsenic can be absorbed by human lungs and skin. Consumption of arsenic-contaminated food and drinks as part of the daily diet and inhalation of arsenic has been the main source of arsenic exposure for most of the affected people throughout the ages. The total dietary intake of total arsenic for male and females of U.S.A. as reported by Meacher et al. (2002) has been 1.8e11.4 mg/day and 1.3e9.4 mg/day respectively. Schoof et al. (1999) estimated the daily dietary arsenic intake for U.S. adults to be 3.2 mg/day with range 1e20 mg/day. However, the daily dietary intake varies with different geographical locations due to the different food habits of people and the level of arsenic contamination in food and drinks. The average dietary arsenic consumption in general population typically ranges from 20 to 300 mg/day (Compounds, 2001). Workers of arsenic-related industries have a high risk of getting exposed to arsenic by inhalation and by skin contact. It is now been well established that the seafoods contain arsenic and sometimes the concentration is much higher than the safe
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limit. The arsenic content was found to lie between 0.401 mg/g and 7.032 mg/g (401e7032 mg/kg) in the sea foods of Muang district in Thailand (Kerdthep et al., 2009). In a study carried on sea foods (fishes, shrimp and bivalve) in Shandong, China, Wu et al. (2014) estimated the average arsenic content in fishes and bivalve to be 0.037 ± 0.014 mg/g (~37 mg/kg) and 3.4 ± 0.4 mg/g (~340 mg/kg) respectively. The organic forms of arsenic found in these sea foods are generally considered to have lower toxicity; although the toxicity may vary from one form of chemical to other. Monomethylarsonic acid (MMAV), dimethylarsinic acid (DMAV) arsenochlorin, arsenosugar, arsenobetaine, arsenolipids are some of the most abundant organo-arsenics found in the sea foods (Hughes et al., 2011). Soluble inorganic arsenic species are easily absorbed (over 90%) in the human body. Organic arsenic of sea foods are also absorbed at a high rate (over 80%) but other forms of organic arsenic are absorbed only about 15e40% (Uthus, 1994). Metabolism of arsenic plays a decisive role in the determination of arsenic toxicity. It has been observed in many cases that the metabolic capacity and the toxic effects of arsenic vary from one community to other and it even varies within the family members of the same family (Mazumder, 2008). Studies carried out on the villagers of remote villages in Andes in Argentina, have revealed some stunning facts of the development of arsenic resistance capacity in the village dwellers’ body exposed to high level of arsenic contamination (Schlebusch et al., 2015; Nicole, 2013). The effects of any toxic substance on human or any living organism are generally of two kinds; acute and chronic toxicity. Acute toxicity describes the effects of any toxin that have appeared from a single exposure or multiple exposures within a short period of time; whereas, the chronic toxicity describes the toxic effects of any toxin appeared as a result of a long-term exposure. Generally, acute toxicity of arsenic occurs from accidental ingestion or inhalation of arsenic compounds or from intentional ingestion for suicide. A dose of less than 5 mg arsenic compound at a time may cause vomiting and diarrhea and can be resolved within 12 h without any treatment (Ratnaike, 2003), though different arsenic compounds produce different kinds of toxicity. Inorganic As(III) and As(V) are more toxic than their common methylated forms MMA, DMA (Benramdane et al., 1999). The estimated lethal dose of arsenic for an adult human is 1e3 mg/kg (Hughes, 2002). The mostly encountered manifestations of acute arsenic toxicity include nausea, vomiting, severe abdominal pain, profuse watery diarrhea (often bloody), excessive salivation, etc (Mazumder, 2008; Ratnaike, 2003). The human health effects of chronic arsenic toxicity are known as arsenicosis (Mazumder, 2008). Different body organs or systems like skin, liver, nervous system, respiratory system, renal system, are vulnerable to chronic arsenic poisoning (Mazumder, 2008; Ratnaike, 2003). One of the prominent manifestations of chronic arsenic poisoning is skin pigmentation and keratosis (Mazumder, 2008; Hughes, 2002). In Taiwan, a cardiovascular disease known as the Blackfoot Disease (BFD) had broken out in the early 20th century among the people who had been taking arsenic contaminated water for a long time (Tseng, 2005). In recent years, DNA impairment, inhibition of enzymatic activities, production of reactive oxygen species, tumor promotion are some of the often quoted effects of chronic arsenic poisoning and are of serious concern. Although the mechanism behind these anomalies are not conclusively defined; but researchers have found a strong correlation between the occurrence of these abnormalities to the chronic arsenic exposure (Shi et al., 2004). The inorganic arsenic when ingested, is absorbed in the gastrointestinal tract and subsequently gets methylated; mostly in forms of MMA, DMA in body cells (Hughes, 2002; Loffredo et al.,
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Fig. 2. Modeled global probability of getting geogenic contamination in groundwater in A) reducing conditions and B) in oxidizing conditions (Amini et al., 2008).
Table 2 Groundwater arsenic concentrations and approximate number of affected people in some countries (Mukherjee et al., 2006; Ministry of Water Resources and River Development and Ganga Rejuvenation, 2014; Nordstrom, 2002; Kouras et al., 2007). Country/region
Groundwater As concentration in mg/L (1 mg/L ¼ 0.001 mg/L)
Approximate size of population at risk
India Bangladesh China (Mainland China) Vietnam Thailand Taiwan Inner Mongolia Argentina Chile Mexico Hungary, Romania Greece Spain U.K. U.S.A., Canada Ghana
