Chemistry of arsenic in soils: I. Sorption of arsenate and arsenite by four Australian soils

Ten soils varying widely in chemistry and mineralogy were used to examine the physical and chemical properties that influence As sorption processes in soils. The sorption of As was measured using a batch technique with As-V and As-III concentrations added in background solutions of NaNO3 (0.00-0.3 mol L-1) and a range of pH values (2.0-8.5). The soils generally sorbed more As-V than As-III at equivalent As concentrations. In general, highly oxidic soils sorbed three times more As-V than soils containing small amounts of oxidic minerals. The effect of pH on As sorption varied considerably among the soils. In soils with low concentrations of oxidic minerals, increasing pH had little affect on the amount of As-V sorbed while in highly oxidic soils, sorption of As-V decreased with increasing pH. This decrease was attributed to two interacting factors, the increasing negative surface potential on the plane of sorption and increasing amount of negatively charged As-V species present in soil solution. In contrast to As', sorption of As-III increased with increasing pH. The effect of ionic strength on As sorption varied bet between the As species. In general, there was a pH (similar to 3) below which As-V sorption decreased with increasing ionic strength and above which the reverse occurred. For As-III species, increasing ionic strength had little effect on the amount of As sorbed.