Arsenic Contamination of Groundwater, Blackfoot ... - ACS Publications

Chapter 16

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Arsenic Contamination of Groundwater, Blackfoot Disease, and Other Related Health Problems 1

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Chien M. Wai, Joanna Shaofen Wang , and M. H. Yang 1

Department of Chemistry, University of Idaho, Moscow, ID 83844 Department of Nuclear Sciences, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China

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Arsenic contamination of groundwater can occur by natural leaching of minerals and by human activities. In aquatic environments, arsenic usually exists in +3 and +5 oxidation states, both as inorganic and organometallic species. Inorganic arsenite is more toxic than arsenate which in turn is more toxic than monomethylarsonic acid and dimethylarsinic acid. Deep well waters often have arsenic concentrations far greater than the current maximum contaminant level of 10 ppb and with arsenite/arsenate ratios >1. The Blackfoot disease found in southwest Taiwan nearly half a century ago was related to the drinking of deep well waters containing high concentrations of arsenic with high fractions of arsenite by local villagers. Similar arsenic poisoning problems were later found in Inner Mongolia, Bangladesh and India, all related to the drinking of groundwaters contaminated with arsenic. This global arsenic contamination problem is perhaps one of the most serious environmental problems facing human beings today.

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© 2002 American Chemical Society In Biogeochemistry of Environmentally Important Trace Elements; Cai, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Introduction Arsenic is a naturally occurring and ubiquitous element found in the earth's crust. It is classically considered as a metalloid and shares many of its toxic attributes with the other heavy metals such as lead and mercury. Contamination of groundwater with arsenic is a global environmental problem because arsenic can enter groundwater systems from weathering of minerals in rocks and soil. The arsenic standard set by World Health Organization (WHO) is 50 ppb (0.05 μg/mL) and WHO's guideline is 10 ppb in drinking water. In the U.S.A. the current maximum contaminant level (MCL) of arsenic in water is 50 ppb, a level established in 1942 by the U.S. Public Health Service (1). This is also the permissible level of arsenic in bottled water according to the U.S. Code of Federal Regulations (CFR) (2). However, analyses (3,4) suggest that the current standard of 50 ppb has a substantial increased risk of cancer and is not sufficiently protective of public health. The U.S. Environmental Protection Agency (EPA) is required by the Safe Drinking Water Act Amendments of 1996 to propose a new standard of arsenic by January 2000 and to finalize that regulation by January 2001(5), but in 2000 Congress extended it to June 22, 2001. Based on the accumulating scientific information and data on the health effects of arsenic, EPA in January 2001 issued regulations that set a MCL level of 10 ppb arsenic standard for drinking water (6). The EPA estimated that the new standard would affect around 13 million people, mainly in the West, Midwest, and New England where arsenic levels in many well waters are greater than 10 ppb. The current Bush Administration withdrew the 10-ppb standard in March 2001, three days before it was to take effect. In October of 2001, EPA affirmed the appropriateness of the MCL and reinstated 10 ppb as the new M C L for arsenic in drinking water. Water systems must meet this standard by January 2006. The chemical form of arsenic in drinking water is not specified by the CFR, although it is well established that the toxicity of arsenic depends on its chemical form. Arsenic exists in natural waters in different oxidation states depending on the redox environment. The trivalent inorganic species arsenite is more toxic to the biological systems than the pentavalent species arsenate (7,8). Organoarsenicals such as monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) also exist in the natural environments, but their toxicities are lower than the inorganic arsenic species pathophysiologically (7,8). The trivalent arsenic is more toxic because it can bind to thio groups in biological systems. Information on the distribution of arsenic species and speciation of arsenic is therefore important to assess its toxicity in drinking water. Since arsenic is an ubiquitous element in the earth's crust and it can be concentrated in well waters, arsenicosis has become an emerging epidemic in many areas of the world. Blackfoot disease(BFD), an arsenic related disease,

In Biogeochemistry of Environmentally Important Trace Elements; Cai, Y., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


212 was first observed in Taiwan in the 1930s and peaked in the 1950s (9). The disease was found to correlate to the high arsenic contents in the groundwaters consumed by local inhabitants in several villages in southwest Taiwan (10-12). The symptoms of BHD start with spotted discoloration on the skin of the extremities, especially on the feet. The spots change from white to brown and eventually to black, hence the name BFD. The affected skin gradually thickens, cracks and ulcerates. Amputation of the affected extremities is often the final resort to save the BFD victims. After 20-30 years of exposure to high levels of arsenic, internal cancers may also appear. Arsenic related diseases were later reported to occur in other areas of Asia including Inner Mongolia of China (13), Bangladesh (14), and India (75). A large number of populations in Bangladesh and India are currently affected by arsenic in drinking water obtained from subsurface sources. Other countries such as Vietnam (76,77), Chile (18,19), Argentina (20), Finland (27), and the United States (22) also have groundwater arsenic contamination problems. This global arsenic contamination problem is getting considerable attention from the scientific communities today. This paper summarizes current information concerning arsenic contamination in groundwaters of some selected areas and related environmental and health problems.

Environmental Arsenic Distribution and Toxicity In order to understand the global arsenic problem, it is necessary to understand the chemistry of arsenic. The toxicity, bioavailability, bioaccumulation, and transport of arsenic are often dependent upon its species in the system under investigation. Because each arsenic species possesses unique physical and chemical properties and causes specific effects in living systems, measurement of the total concentration of arsenic provides little information about its toxicity or bioavailability. Toxicity tests have shown that the most toxic form of arsenic is arsenite, which is as much as 60 times more toxic than arsenate, due to its ability to react with enzymes in human metabolism, and several hundred times more toxic than M M A or DMA (23). Arsenite is also significantly more mobile in groundwaters than arsenate. Penrose (23) compiled the approximate toxicity order of various arsenic compounds, which, in decreasing order, is: arsines > arsenite > arsenoxides > arsenate > pentavalent arsenicals > arsonium compounds > metallic arsenic. The chemical formulas of different arsenic species are shown in Table I. The natural abundance of arsenic in soil is of great importance both for the assessment of environmental quality and for devising countermeasures against soil pollution. The levels of arsenic may be much higher in soils contaminated by human activities. The levels of arsenic in soil of various countries have been


213 reported in 1979 to range from 0.1 to 40 ppm (mean 6 ppm) by Bowen (24), and from 1 to 50 ppm (mean 6 ppm) by Backer and Chesnin in 1975 (25). The total concentrations of arsenic in soils from a number of countries were also described by Huang in 1994 (26).


Table I. Chemical Forms of Arsenic Species 1. Arsine 2. Arsenoxide 3. Cacodylic Acid 4. Dimethylarsine 5. Dimethylarsinic Acid (DMA) 6. Methylarsine 7. Monomethylarsonic Acid (MMA) 8. P-Arsanilic Acid 9. Phenylarsine Oxide 10. Phenylarsonic Acid 11. Arsenite 12. Arsenate 13. Trimethylarsine 14. Triphenylarsine Oxide

AsH As(0)(OH) (CH ) HAsO (CH ) AsH (CH ) As(0)(OH) (CH )AsH CH As(0)(OH) NH C6H4As(0)(OH) C H AsO C H As(0)(OH) As0 As0 " (CH ) As (C6H ) AsO 3

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In aquatic environments, arsenic can exist in several oxidation states, both as inorganic and organometallic species (27). Arsenic in surface river waters is present primarily as an inorganic ion, arsenate. Reduced arsenic (arsenite) and methyl arsenicals (MMA and DMA) are also occasionally present (28,29). In contaminated rivers, sediments can contain substantial amounts (100-300 ppm or higher) of arsenic that are potentially mobile during water-sediment interactions. The mobility of arsenic in these sediments is affected by the physical and chemical forms of the arsenic species as well as by environmental conditions. Dissolved arsenic species can be adsorbed on or co-precipitated with suspended solids and carried down to the river sediments. On the other hand, a build-up of arsenic compounds in the bottom sediments of a river may subsequently be released to the overlying water (30). For water with arsenic levels less than 0.2 ppm (31), the major health concern is an increased chance of getting some types of cancer such as skin, bladder, lung and possible liver and kidney cancers. As arsenic levels in water become greater than 0.2 ppm and length of water use becomes longer than a year, the following health effects may occur on the skin including: a "pins and needles" sensation in your hands and feet, skin changes in color appearing as a



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fine freckled or "raindrop" pattern in the trunk, hands, and feet, and unusual skin growth (wart-like) on the palms and soles. Several years of low level arsenic exposure can also cause various skin lesions. Hyperpigmentation (dark spots), hypopigmentation (white spots) and keratoses of the hands and feet will appear. After a dozen or so years, skin cancers are expected (31). Arsenic is present in most foodstuffs in concentrations of less than 1 μg/g (lppm). However, marine fish may contain arsenic concentrations up to 5 ppm wet weight and concentrations in some crustacean and bottom-feeding fish may reach several tens of ppm, predominantly in the form of organic arsenic (31). In both animal and man, organic arsenic compounds ingested via fish and crustacean are readily absorbed from the gastrointestinal tract and 70 % - 80 % are eliminated within a week, mainly in the urine. Urine is a suitable indicator for assessment of exposure to inorganic arsenic, since most studies show that the elimination of arsenic, in both animals and man, takes place mainly via the kidneys. Arsenic levels in the hair of unexposed human adults are usually below 1 ppm. Levels up to about 80 ppm have been recorded in subject with chronic arsenic poisoning caused by ingestion of contaminated well waters (31). In humans, the highest arsenic concentrations are found in skin, hair, and nail, all tissues rich in keratin. Data from mice and rabbits show the highest arsenic concentrations are found in liver, kidney, lung, and intestinal mucosa at short times after a single exposure to trivalent or pentavalent inorganic arsenic. However, arsenic in the trivalent form generally causes higher concentrations in tissue levels than the pantavalent form. Inorganic arsenic is methylated in the body mainly to M M A and DMA. In humans exposed to low doses of trivalent or penatavalent inorganic arsenic, the urinary excretion consists of about 20 % M M A and 60 % DMA, the rest being inorganic arsenic.

Arsenic Species in Groundwater and Analytical Methods In aquatic environments, arsenite can be converted to arsenate under oxidizing conditions (e.g. aerated surface water). Likewise, arsenate can also become arsenite under reducing conditions (e.g. anaerobic groundwater). However, the conversion in either direction is quite slow, so the reduced species can be found in oxidized environments and vice versa. Microbes, plants and animals can also convert these inorganic arsenic species into organic compounds involving carbon and hydrogen atoms, such as M M A and DMA. These compounds are much less commonly found in natural waters (32). The major species of interest for most analytical work are inorganic arsenic, M M A and DMA. In most environmental water samples these will be the predominant forms of arsenic. Other arsenic species that have been assayed include arsine and methylarsines, triphenylarsine oxide (TPAO), arsanilic acid,



Figure 1. Feet of a BFD victim. Adapted with permission from reference 86. Copyright 1993 Kaohsiung Medical College Press.


216 phenylarsonic acid (PAA), and arsenate mononucleotide complexes (Table I). Most of the speciation techniques determine different arsenic species rather than valence states. Total arsenic in water can be measured directly by flame or graphite furnace atomic absorption spectrophotometry (AAS or GFAAS). The most commonly used techniques for the preconcentration of arsenic involve its transformation into arsine. Subsequent measurements of arsine can be carried out using spectrophotometry, flames and electrothermal devices for AAS (33), atomic fluorescence (AFS)(34) or atomic emission spectroscopy (AES)(35). Other separation methods include solvent extraction(56j, ion exchange chromatography (TEC)(37), liquid chromatography (LC)(33), gas chromatography (GC) (38) and eletrochemistryfJ9). Neutron activation analysis (NAA) using radiochemical separation is a very sensitive method for the determination of arsenic, with detection limits near 1 ng (40,41). Current speciation methods for arsenic rely mainly on separations based upon the principles of selective hydride generation, chromatography, solvent extraction, and electrochemistry. Often, one separation method is used to isolate or concentrate some arsenic species before applying the second technique for final separation and quantification as in the common employment of a solvent extraction step prior to separation by chromatography. Hydride generation (HG) techniques for the speciation of arsenic involve selective reduction of the hydride-forming arsenic species to the corresponding arsines (42). The arsines are generated in a reaction chamber by reduction with sodium tetrahydroborate (NaBEL*) at different pH and separated either by GC, HPLC, or by sequential volatizaion. At pH 5-7, arsenite can be reduced to arsine and at pH

Arsenic Contamination of Groundwater, Blackfoot ... - ACS Publications

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