Chemistry of inorganic arsenic in soils: II. Effect of phosphorus, sodium, and calcium on arsenic sorption

There are more than 10 000 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 PO43- and Na+ and Ca2+ on the sorption of As-v and As by an Oxisol, a Vertisol, and two Alfisols. The presence of P (0.16 mmol L-1) greatly decreased As-v sorption by soils containing low amounts of Fe oxides (<100 mmol kg(-1)), indicating competitive adsorption between P and As-v for sorption sites. In contrast, the presence of a similar amount of P had little effect on the amount of As-v adsorbed by soils with high Fe content (>800 mmol kg(-1)). However, As-v 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 As-v 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 As-III sorption was observed in the low sorbing Alfisol and high affinity Oxisol. However, the amount of As-III sorbed by the Oxisol was much greater than the Alfisol for all treatments. The presence of Ca2+ increased the amount of As-v sorbed compared with that of Na+ and was manifested through changes in the surface charge characteristics of the soils. A similar trend in As-III sorption was recorded with changes in index cation, although the effect was not as marked as recorded for As-v.