phytoremediation. biomethylation arsenobetaine inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolized through a process of methylation. for example, the mold scopulariopsis brevicaulis produces trimethylarsine if inorganic arsenic is present. the organic compound arsenobetaine is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. the average person's intake is about 10–50 µg/day. values about 1000 µg are not unusual following consumption of fish or mushrooms, but there is little danger in eating fish because this arsenic compound is nearly non-toxic. environmental issues exposure naturally occurring sources of human exposure include volcanic ash, weathering of minerals and ores, and mineralized groundwater. arsenic is also found in food, water, soil, and air. arsenic is absorbed by all plants, but is more concentrated in leafy vegetables, rice, apple and grape juice, and seafood. an additional route of exposure is inhalation of atmospheric gases and dusts. during the victorian era, arsenic was widely used in home decor, especially wallpapers. occurrence in drinking water main article: arsenic contamination of groundwater extensive arsenic contamination of groundwater has led to widespread arsenic poisoning in bangladesh and neighboring countries. it is estimated that approximately 57 million people in the bengal basin are drinking groundwater with arsenic concentrations elevated above the world health organization's standard of 10 parts per billion (ppb). however, a study of cancer rates in taiwan suggested that significant increases in cancer mortality appear only at levels above 150 ppb. the arsenic in the groundwater is of natural origin, and is released from the sediment into the groundwater, caused by the anoxic conditions of the subsurface. this groundwater was used after local and western ngos and the bangladeshi government undertook a massive shallow tube well drinking-water program in the late twentieth century. this program was designed to prevent drinking of bacteria-contaminated surface waters, but failed to test for arsenic in the groundwater. many other countries and districts in southeast asia, such as vietnam and cambodia, have geological environments that produce groundwater with a high arsenic content. arsenicosis was reported in nakhon si thammarat, thailand in 1987, and the chao phraya river probably contains high levels of naturally occurring dissolved arsenic without being a public health problem because much of the public uses bottled water. in pakistan, more than 60 million people are exposed to arsenic polluted drinking water indicated by a recent report of science. podgorski's team investigated more than 1200 samples and more than 66% exceeded the who minimum contamination level. since the 1980s, residents of the ba men region of inner mongolia, china have been chronically exposed to arsenic through drinking water from contaminated wells. a 2009 research study observed an elevated presence of skin lesions among residents with well water arsenic concentrations between 5 and 10 µg/l, suggesting that arsenic induced toxicity may occur at relatively low concentrations with chronic exposure. overall, 20 of china's 34 provinces have high arsenic concentrations in the groundwater supply, potentially exposing 19 million people to hazardous drinking water. in the united states, arsenic is most commonly found in the ground waters of the southwest. parts of new england, michigan, wisconsin, minnesota and the dakotas are also known to have significant concentrations of arsenic in ground water. increased levels of skin cancer have been associated with arsenic exposure in wisconsin, even at levels below the 10 part per billion drinking water standard. according to a recent film funded by the us superfund, millions of private wells have unknown arsenic levels, and in some areas of the us, more than 20% of the wells may contain levels that exceed established limits. low-level exposure to arsenic at concentrations of 100 parts per billion (i.e., above the 10 parts per billion drinking water standard) compromises the initial immune response to h1n1 or swine flu infection according to niehs-supported scientists. the study, conducted in laboratory mice, suggests that people exposed to arsenic in their drinking water may be at increased risk for more serious illness or death from the virus. some canadians are drinking water that contains inorganic arsenic. private-dug–well waters are most at risk for containing inorganic arsenic. preliminary well water analysis typically does not test for arsenic. researchers at the geological survey of canada have modeled relative variation in natural arsenic hazard potential for the province of new brunswick. this study has important implications for potable water and health concerns relating to inorganic arsenic. epidemiological evidence from chile shows a dose-dependent connection between chronic arsenic exposure and various forms of cancer, in particular when other risk factors, such as cigarette smoking, are present. these effects have been demonstrated at contaminations less than 50 ppb. arsenic is itself a constituent of tobacco smoke. analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable increase in risk for bladder cancer at 10 ppb. according to peter ravenscroft of the department of geography at the university of cambridge, roughly 80 million people worldwide consume between 10 and 50 ppb arsenic in their drinking water. if they all consumed exactly 10 ppb arsenic in their drinking water, the previously cited multiple epidemiological study analysis would predict an additional 2,000 cases of bladder cancer alone. this represents a clear underestimate of the overall impact, since it does not include lung or skin cancer, and explicitly underestimates the exposure. those exposed to levels of arsenic above the current who standard should weigh the costs and benefits of arsenic remediation. early (1973) evaluations of the processes for removing dissolved arsenic from drinking water demonstrated the efficacy of co-precipitation with either iron or aluminium oxides. in particular, iron as a coagulant was found to remove arsenic with an efficacy exceeding 90%. several adsorptive media systems have been approved for use at point-of-service in a study funded by the united states environmental protection agency (us epa) and the national science foundation (nsf). a team of european and indian scientists and engineers have set up six arsenic treatment plants in west bengal based on in-situ remediation method (sar technology). this technology does not use any chemicals and arsenic is left in an insoluble form (+5 state) in the subterranean zone by recharging aerated water into the aquifer and developing an oxidation zone that supports arsenic oxidizing micro-organisms. this process does not produce any waste stream or sludge and is relatively cheap. another effective and inexpensive method to avoid arsenic contamination is to sink wells 500 feet or deeper to reach purer waters. a recent 2011 study funded by the us national institute of environmental health sciences' superfund research program shows that deep sediments can remove arsenic and take it out of circulation. in this process, called adsorption, arsenic sticks to the surfaces of deep sediment particles and is naturally removed from the ground water. magnetic separations of arsenic at very low magnetic field gradients with high-surface-area and monodisperse magnetite (fe3o4) nanocrystals have been demonstrated in point-of-use water purification. using the high specific surface area of fe3o4 nanocrystals, the mass of waste associated with arsenic removal from water has been dramatically reduced. epidemiological studies have suggested a correlation between chronic consumption of drinking water contaminated with arsenic and the incidence of all leading causes of mortality. the literature indicates that arsenic exposure is causative in the pathogenesis of diabetes. chaff-based filters have recently been shown to reduce the arsenic content of water to 3 µg/l. this may find applications in areas where the potable water is extracted from underground aquifers. san pedro de atacama for several centuries, the people of san pedro de atacama in chile have been drinking water that is contaminated with arsenic, and some evidence suggests they have developed some immunity. hazard maps for contaminated groundwater around one-third of the world's population drinks water from groundwater resources. of this, about 10 percent, approximately 300 million people, obtains water from groundwater resources that are contaminated with unhealthy levels of arsenic or fluoride. these trace elements derive mainly from minerals and ions in the ground. redox transformation of arsenic in natural waters arsenic is unique among the trace metalloids and oxyanion-forming trace metals (e.g. as, se, sb, mo, v, cr, u, re). it is sensitive to mobilization at ph values typical of natural waters (ph 6.5–8.5) under both oxidizing and reducing conditions. arsenic can occur in the environment in several oxidation states (−3, 0, +3 and +5), but in natural waters it is mostly found in inorganic forms as oxyanions of trivalent arsenite or pentavalent arsenate . organic forms of arsenic are produced by biological activity, mostly in surface waters, but are rarely quantitatively important. organic arsenic compounds may, however, occur where waters are significantly impacted by industrial pollution. arsenic may be solubilized by various processes. when ph is high, arsenic may be released from surface binding sites that lose their positive charge. when water level drops and sulfide minerals are exposed to air, arsenic trapped in sulfide minerals can be released into water. when organic carbon is present in water, bacteria are fed by directly reducing as(v) to as(iii) or by reducing the element at the binding site, releasing inorganic arsenic. the aquatic transformations of arsenic are affected by ph, reduction-oxidation potential, organic matter concentration and the concentrations and forms of other elements, especially iron and manganese. the main factors are ph and the redox potential. generally, the main forms of arsenic under oxic conditions are h3aso4, h2aso4−, haso42−, and aso43− at ph 2, 2–7, 7–11 and 11, respectively. under reducing conditions, h3aso4 is predominant at ph 2–9. oxidation and reduction affects the migration of arsenic in subsurface environments. arsenite is the most stable soluble form of arsenic in reducing environments and arsenate, which is less mobile than arsenite, is dominant in oxidizing environments at neutral ph. therefore, arsenic may be more mobile under reducing conditions. the reducing environment is also rich in organic matter which may enhance the solubility of arsenic compounds. as a result, the adsorption of arsenic is reduced and dissolved arsenic accumulates in groundwater. that is why the arsenic content is higher in reducing environments than in oxidizing environments. the presence of sulfur is another factor that affects the transformation of arsenic in natural water. arsenic can precipitate when metal sulfides form. in this way, arsenic is removed from the water and its mobility decreases. when oxygen is present, bacteria oxidize reduced sulfur to generate energy, potentially releasing bound arsenic. redox reactions involving fe also appear to be essential factors in the fate of arsenic in aquatic systems. the reduction of iron oxyhydroxides plays a key role in the release of arsenic to water. so arsenic can be enriched in water with elevated fe concentrations. under oxidizing conditions, arsenic can be mobilized from pyrite or iron oxides especially at elevated ph. under reducing conditions, arsenic can be mobilized by reductive desorption or dissolution when associated with iron oxides. the reductive desorption occurs under two circumstances. one is when arsenate is reduced to arsenite which adsorbs to iron oxides less strongly. the other results from a change in the charge on the mineral surface which leads to the desorption of bound arsenic. some species of bacteria catalyze redox transformations of arsenic. dissimilatory arsenate-respiring prokaryotes (darp) speed up the reduction of as(v) to as(iii). darp use as(v) as the electron acceptor of anaerobic respiration and obtain energy to survive. other organic and inorganic substances can be oxidized in this process. chemoautotrophic arsenite oxidizers (cao) and heterotrophic arsenite oxidizers (hao) convert as(iii) into as(v). cao combine the oxidation of as(iii) with the reduction of oxygen or nitrate. they use obtained energy to fix produce organic carbon from co2. hao cannot obtain energy from as(iii) oxidation. this process may be an arsenic detoxification mechanism for the bacteria. equilibrium thermodynamic calculations predict that as(v) concentrations should be greater than as(iii) concentrations in all but strongly reducing conditions, i.e. where so42− reduction is occurring. however, abiotic redox reactions of arsenic are slow. oxidation of as(iii) by dissolved o2 is a particularly slow reaction. for example, johnson and pilson (1975) gave half-lives for the oxygenation of as(iii) in seawater ranging from several months to a year. in other studies, as(v)/as(iii) ratios were stable over periods of days or weeks during water sampling when no particular care was taken to prevent oxidation, again suggesting relatively slow