As dust, forest fires and human activities (Pacyna,

As distribution in environment is generally associated with its geological origin but sometimes anthropogenic contamination can be a major source. As occurs naturally in soil and is the 20th most abundant trace element in earth’s crust (NRC 1977). As concentration in soil varies geographically depending on the geological origin of the region and the chemical properties of the soil (Wild, 1993). Average As content in Earth’s continental crust vary between 2 – 3mg As/kg (Tanaka, 1988; Cullen and Reimer, 1989). As gets into natural systems and biogeochemical cycles through weathering of the parent rock and sediments, geothermal activities, dissolution and mobilization through ground water, volcanoes, wind-blown dust, forest fires and human activities (Pacyna, 1986; Nordstrom, 2000). The strong interaction of the water with the rock and sediments containing As is the primary reason for As recycling through biosphere. The global average concentration of As in soil is about 5 mg/Kg. Uncontaminated soils typically contain <10 mg/Kg total As, but the concentration can reach hundreds or thousands of mg/Kg in contaminated environments and the highest concentration is seen in alluvial and organic soils (IARC 2004, Kabata-Pendias, 2010). As exist in environment in 4 oxidation states, +5 (Arsenate), +3 (Arsenite), 0 (As) and -3 (Arsine). Redox potential, pH, and biotic factors (microorganisms) mediates the speciation and transformation of As in the environment (Zhao et al., 2010; Nearing et al., 2014). As occurs in more than 200 mineral forms in soil and are generally seen as arsenates (60%), sulfides and sulfosalts (20%) and the remaining 20% includes arsenides, arsenites, oxides, silicates and elemental As (As) (Onishi, 1969). As is often associated with weathering resistant sulphide minerals such as realger (As4S4) and orpiment (As2S3). As precipitates as ferric arsenate or arsenopyrites (FeAsS) in soil horizons rich in iron (Mandal et al., 2002). Leaching of As from the parent rock and sediments lead to their occurrence in soil and ground water. As speciation and transformation depends on the types of sorbent present in soil, pH and redox conditions of the environment and is essential to understand its behaviour in an environment (Mandal and Suzuki, 2002). In aerobic environment arsenates are stable under oxidising condition while arsenites occur predominantly under reducing conditions (Bhattacharya et al., 2002). When stable, arsenate gets absorbed into the clay particles, iron and manganese oxides/hydroxides and organic matter (Naidu and Bhattacharya, 2006; Pokroviski et al., 1996; Reichert and Trelles, 1921). pH also influences the adsorption ability and mobility of different As species (Pantsar-Kallio and Manninen, 1997). As adsorption experiments done by Goldberg et al. (2002), showed that arsenate adsorption on oxides and clays was maximal at low pH and decreased with increasing pH above pH 9 for Al oxide, pH 7 for Fe oxide and pH 5 for clays. Under anaerobic conditions these can be reduced to volatile and easily oxidized methylarsines (Fordyce et al., 1995). 

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