Sources and Effects of Contamination

Arsenic is a highly prevalent metalloid element present all over the world, and in most uncontaminated soils, it is present in concentrations of less that 10 mg/kg. (Fitz and Wenzel 2002) High levels of arsenic are often natural, from sources such as silicate or sulfidic ore deposits, volcanic activities, and sea salt sprays. (Fitz and Wenzel 2002) Arsenic may even be essential to animal nutrition in pathways such as the metabolism of the methionine amino acid. (Pickering et.al 2000)

However, in higher concentrations, it is an extremely toxic metalloid pollutant. Inorganic arsenic is classified as a group A human carcinogen, causing skin lesions, kidney, liver and lung cancers as well as nervous system damage. (Dhankher et.al 2002) It has also been known to cause mutations and birth defects, and has been associated with diabetes. (www.edenspace.com) Perhaps arsenic's toxicity is due to the similarity of its oxidized form, arsenate, and the phosphate ion. (Bennett et.al 2001) This allows it to enter cells via the phosphate transporter, where it accumulates, quickly becoming toxic to the cell. (Bennett et.al 2001) The source of arsenate's toxicity is believed to be its ability to uncouple oxidative phosphorylation, thereby halting all metabolism within the cell. (Francesconi, et.al 2002)

Much of the arsenic contamination in the environment is due to human activities such as mining, smelting, and agriculture. It is present in some insecticides, algicides, herbicides, fungicides, sheep dips, dyestuffs, wood preservatives, and tapeworm medications. (Fitz and Wenzel 2002) Photo from http://icelandiscool.com

Although arsenic can be present in food source plants at high levels, such as watercress in New Zealand, (Robinson et.al 2003) the main pathway of exposure to humans is through arsenic-contaminated drinking water. (Fitz and Wenzel 2002) The most seriously contaminated sites are in southern and southeastern Asia. The soil and groundwater of the Ron Phiburn district of Thailand is grossly contaminated with arsenic as the result of a hundred years of tin mining in the area. (Francesconi et.al 2002) The people of this area require the groundwater for domestic use, and many problems have arisen as a result, including about a thousand cases of skin disorders. (Francesconi et.al 2002) The largest arsenic-related calamity occurred in Bengladesh, where millions of people rely on arsenic-contaminated drinking water. (Fitz and Wenzel 2002) Other arsenic-polluted sites include Australia, Western Europe, and the United States, where forty-one percent of the EPA-designated superfund sites are contaminated with arsenic. (Fitz and Wenzel 2002)

Arsenic in the environment

Arsenic can be present in several forms in the environment. The most common forms are the inorganic forms, arsenate (AsV) which is the most oxidized form of arsenic, and arsenite (AsIII) the more reduced form. Arsenate is generally the most prevalent form in soil under aerobic conditions, as this is an oxidizing environment. (Fitz and Wenzel 2002) Arsenite is more common under anaerobic, or flooded conditions, as these are more reducing. (Fitz and Wenzel 2002) This tendency for arsenite to dissolve makes it more mobile than the more oxidized form. Two organic forms are also present in nature, albeit in much smaller concentrations. Monomethylarsonic acid and dimethylarsinic acid are generally produced under anaerobic conditions and are less toxic than the inorganic species. (Fitz and Wenzel 2002)

Soil particle size has a great effect on arsenic availability. Smaller particles, such as those found in clay and silt have a greater total surface area, and therefore have more binding sites for arsenic. (Fitz and Wenzel 2002) The pH of the soil also has a great effect on the availability of arsenic as well as the species available. Normal soil has a pH between 3-8, and within this range arsenate is more soluble at more basic levels, and arsenite is more available at more acidic levels. (Fitz and Wenzel 2002) This property of arsenic can be used to manipulate its bioavailability. One technique used is known as bioleaching. Elemental sulfur is added to the soil and acts as substrate for and indigenous bacterium, Thiobacillus spp. which lower the pH of the soil, and solubilizing up to 80% of the arsenic in the soil. (Seidel et.al 2002) In one type of polluted soil, bioleaching solubilized 667 mg/kg arsenic, whereas without bioleaching only 3.5 mg/kg arsenic was solubilized. (Seidel et.al 2002) Since the solubilization is due to decreased pH (as low as 1.5) it is not surprising that arsenite was the main species dissolved. (Seidel et.al 2002) Plants also change the pH of soil in order to increase nutrient uptake. Phosphorous-deficient plants often exude carboxylic acids such as citric acid to decrease soil pH. (Fitz and Wenzel 2002) These additional hydrogen ions are intended to displace phosphorous from binding sites on soil particles, but will do the same for arsenate, due to the chemical similarity between arsenate and phosphate. (Fitz and Wenzel 2002) Also, due to this similarity, arsenate and phosphate compete for sorption sites on soil particles, so increased phosphate concentration in the soil leads to increased arsenic availability. (Fitz and Wenzel 2002)

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