Although the incidence of arsenic poisoning in groundwater is worldwide and includes Bangladesh and India, Taiwan, Vietnam, Chile, China, North America, and Finland, the area of the highest demand for a resolution of the problem is Bangladesh. The source seems to be geological, for arsenic has been found in tube well water used for drinking and irrigation, although the geochemistry is not completely understood. As many thousands of boreholes have been produced to support modern irrigation systems, the underground aquifers are aerated, which causes transformation of anaerobic conditions to aerobic conditions. The presence of oxygen in this way decomposes arsenopyrite-releasing arsenic acid. At low pH, this arsenic dissolves in water and hence leads to water contamination. The arsenic content of sediments is high relative to crustal concentrations. The biogeochemical cycling of arsenic and iron are coupled in deltaic systems; iron oxyhydroxides act as a carrier for the deposition of arsenic in sediments. From there, it can be mobilized by bicarbonate, which can extract arsenic from sediments under both aerobic and anaerobic conditions.
Arsenic also becomes a pollutant as a result of various industrial uses and activities. Arsenic is a metalloid, and its primary usage has been in agriculture, in formulating herbicides, especially for controlling weeds in cotton fields. Sodium arsenite has been used as an insecticidal ingredient in sheep-dips. In industry, arsenic has found use in glass manufacture and a new role in the semiconductor industry. Copper smelting releases significant amounts into soils.
Arsenic and the Geography of Bangladesh[]
Bangladesh, 85% of which is deltaic and alluvial plain, is situated in the lower end of three large river systems, the Ganges, the Brahmaputra, and the Meghna, whose catchment area is about 600,000 square miles. The sediments produced in the catchment areas are very high and expose the underlying rocks, including arsenicbearing rocks.
Arsenic pollution became a live issue in Bangladesh as recently as 1993, following a warning by the World Health Organization (WHO) that levels of arsenic in groundwater above the permissible limit of 0.05 mg/L had been reported in seven districts of adjoining West Bengal in India. The Department of Public Health Engineering of Bangladesh was invited to test water samples from the adjoining eight Bangladeshi districts that have the same geographical continuity and aquifers as the West Bengal districts; this yielded the result that slightly more than 20% of the samples contained arsenic at levels ranging from 0.01 to 0.4 mg/L. Ten million people populate these areas and hence are at risk of arsenic toxicity.
Since that time, it has been shown that there is groundwater contamination in more than 40 districts that endanger in excess of 50 million people. The problem has been described in The Lancet as the world's worst episode of arsenic poisoning; more than 220,000 people reportedly suffer from arsenic-related diseases. In a recent study of 27 districts in Bangladesh, 58% of the water samples were unsuitable for drinking. The worst case was in Nawabganj district, where one well contained 60 times the WHO maximum permissible level.
Toxicity and Disease[]
Arsenic occurs principally in the forms of organic arsenic (methyl arsonic acid, dimethyl arsonic acid, arsenobetaine, and arsenocholine) and inorganic arsenic (trivalent and pentavalent arsenic). Of these, the trivalent form is the most toxic to humans (20 times more so than the pentavalent form) and is the most difficult to remove chemically from water.
Arsenic is a suspected carcinogen and has many acute effects on human health. But at the concentrations present in drinking water, it has no immediate side affects. The latency (i.e., the time from first exposure to manifestation of disease) for arsenic-caused skin lesions, in particular keratoses, is typically of the order of 10 years, and so a major increase in the number of cases of arsenic-caused diseases can be projected into the future.
Exposure to arsenic in this way can lead to latent or manifest clinical symptoms through even low-level exposure over a period of time. This can result in an accumulation of this toxicant in various organs and systems, affecting their normal functioning, including the kidney and nervous system. Arsenic causes skin cancers and internal cancers such as lung and bladder cancer. The most common manifestations in afflicted people in Bangladesh are melanosis (93.5%), keratoses (68.3%), hyperkeratosis (37.6%), and dipigmentation (leucomelanosis) (39.1%). Cancers are found in 0.8% of the afflicted population. Preliminary work indicates that there may be several factors triggering arsenic-related diseases, but experts generally feel that poor nutrition may be a primary cause. Studies in Taiwan have shown that there is an increased occurrence of diabetes in the population exposed to arsenic via drinking water.
Recent studies have shown that arsenic is also a teratogen. Further, at the 5th International Conference on Arsenic held in Dhaka, 2004, one of the key messages and cause for increased concern is that there is very good evidence that the environmental contaminant is getting into the food chain, thus putting even more lives at risk.
Drinking Water Standards[]
The World Health Organization has set 10 (ig/L as the allowable level for arsenic in drinking water. On January 22, 2001, the U.S. EPA adopted this standard, and public water systems must comply by January 23, 2006.
Detecting Arsenic in Drinking Water[]
In the modern analytical laboratory, arsenic is quantified by soluble arsenic assaying, preferable with GF-AAS (graphite furnace-atomic absorption spectrometry) for detection levels of less than 50 [ig/L. However, given the highly dispersed nature of tube wells in Bangladesh, the transport of the many samples to central laboratories is logistically impossible. Field techniques are more important, so that samples can be processed as they are taken. In Bangladesh, this requires inexpensive and completely portable techniques. At present, a chemical test kit is being used. The disadvantage of this approach is that the sensitivity of the chemistry (poor below 100 |xg/L) is not compatible with the levels of contamination that need to be detected (50 [ig/L and less). Often, at best, the presence or absence of arsenic can be inferred, but not the level of contamination. In addition, such testing is slow and can take about 6 months to cover some 2000 villages in a district.
Bioavailability Biosensors for Detecting and Quantifying Arsenic[]
A few strains of bacteria are resistant to arsenate, arsenite, and antimonite through the action of the gene products of the ars operon. The ars operon consists of five genes that code for three structural and two regulatory proteins. Two structural genes in the ars operon, arsA and arsB, code for proteins that form an efflux pump that transports arsenite and antimonite out of cells.
A means of measuring available arsenicals would be to construct a gene fusion plasmid in which part of the ars operon is fused upstream of a reporter gene system, such as the bacterial lux operon, which results in the production of light. A transcriptional gene fusion has been done (1) that consists of E. coli arsB :: lux AB. The detection limit of arsenic is of the order of 10 ng/L. Moreover, bioluminescence may be inducible in a concentration-dependent manner (Fig. 4) by arsenic salts; high concentrations result in higher bioluminescence, so that such biosensors may be able to
- Quantify arsenic within the required range of drinking water in Bangladesh
Removal of Arsenic from Drinking Water[]
Coprecipitation of arsenate with ferric (Fe 3+ ) ion is currently the most effective and practical method of arsenic removal. Optimum stability of the FeAs04 precipitate occurs at Fe/As molar ratios of >4; this ratio increases significantly, in practice, depending on water turbidity, slime levels, dissolved solids, and the presence oron-consuming species. However, Fe 3+ ion coprecipitation of arsenite (AsC>3 3 ~) is moderately effective at pH ~7.0. The trivalent As(III) species must be oxidized to As(V) for complete precipitation with Fe 3+ ion. Oxidation may be achieved through aeration or by adding oxidizers such as hypochlorite, permanganate, peroxide, and ozone. The application of other technologies, including alum and lime precipitation together with activated alumina adsorption, are not fully effective.
In Bangladesh, for geographical and financial reasons, there is likely to be a preference for local treatment rather than large-scale treatment plants. The ideal solution would be to modify each tube well at low cost for arsenic removal by, for example, ion exchange.
Sources[]
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Harvard University Arsenic Project Website Natural Resources Defense Council — FAQs: Arsenic in drinking water
U.S. Agency for Toxic Substances and Disease Registry —
ToxFAQs: Arsenic U.S. Environmental Protection Agency — Arsenic Standard pages
and Q & A's: Occurrence of Arsenic in Ground Water West Bengal and Bangladesh Arsenic Crisis Information Centre World Health Organisation http://www.who.int/mediacentre/factsheets/fs2 10/en