There are more than 10000 arsenic (As) contaminated sites in Australia. The ability of soils at these contaminated sites to sorb As is highly variable and appreciable amounts of As have been recorded in the subsurface soils. The potential risk of surface and ground water contamination by As at these sites is a major environmental concern. Factors that influence adsorption capacity of soils influence the bioavailability and subsequent mobility of As in soils. In the present study we investigated the effect of PO³⁻ ₄ and Na⁺ and Ca²⁺ on the sorption of Asⱽ and Asᴵᴵᴵ by an Oxisol, a Vertisol, and two Alfisols. The presence of P (0.16 mmol L⁻¹) greatly decreased Asⱽ sorption by soils containing low amounts of Fe oxides (<100 mmol kg⁻¹), indicating competitive adsorption between P and Asⱽ for sorption sites. In contrast, the presence of a similar amount of P had little effect on the amount of Asⱽ adsorbed by soils with high Fe content (>800 mmol kg⁻¹). However, Asⱽ sorption substantially decreased from 0.63 to 0.37 mmol kg⁻¹ as P concentration was increased from 0.16 to 3.2 mmol L⁻¹ in selected soils. This suggests increased competition between P and Asⱽ for soil sorption sites, through either the higher affinity or the effect of mass action of the increasing concentration of P in solution. A similar effect of P on Asᴵᴵᴵ sorption was observed in the low sorbing Alfisol and high affinity Oxisol. However, the amount of Asᴵᴵᴵ sorbed by the Oxisol was much greater than the Alfisol for all treatments. The presence of Ca²⁺ increased the amount of Asⱽ sorbed compared with that of Na⁺ and was manifested through changes in the surface charge characteristics of the soils. A similar trend in Asᴵᴵᴵ sorption was recorded with changes in index cation, although the effect was not as marked as recorded for Asⱽ
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