Chapter 9 Arsenic

Abstract

Arsenic (As), an ubiquitous element known for its toxicity to biota naturally occurs in several oxidation states between –III and +V. Total As concentrations in the soil solid phase range between 0.1 and 55 mg kg−1 in uncontaminated soils but may be as high as several percent in soils contaminated by mining, smelter and other industrial activities. In aerobic mineral soils, As is primarily associated with iron (Fe) (oxy)hydroxides whereas As sulphide minerals may precipitate in anaerobic conditions. In the presence of Fe (oxy)hydroxides only minor amounts of As are associated with (oxy)hydroxides of aluminium (Al) or manganese (Mn), clay minerals or organic matter. There is no evidence for significant association of As with calcium (Ca) minerals below pH 9.5. Binding of As to organic matter appears to be important in organic soils such as forest floors of peat, but the nature of this association is largely unknown. The main control of both As(III) and As(V) solubility in soils is sorption to Fe (oxy)hydroxides, mainly as inner-sphere bidentate (mononuclear and binuclear) and monodentate surface complexes, with a greater share of monodentate complexes at low As loads and increasing pH (at pH values >8), and predominance of binuclear bidentate complexes at highest As loads. The main As species in soil pore water are arsenate in aerobic soil and undissociated arsenous acid in anaerobic conditions, with little contribution of organic forms. Soil pore water in organic soils may also contain substantial amounts of organic As, primarily in methylated forms. Biological transformations of As include oxidation – reduction and methylation – demethylation reactions triggered by soil bacteria and fungi. Plant roots take up As(V) via phosphate transporters and As(III) by aquaporins and release As(III) back into their rhizospheres after internal reduction of As(V) to As(III). Arsenic pollution is a global phenomenon with a major contribution of anthropogenic emissions to the global As cycle. Large-scale risks to ecosystems and human health arise not only from (ancient) mining and smelter activities, but more recently also from of the use of As-contaminated water as a source of drinking water and for irrigation of crops, in particular paddy rice.