Competitive Adsorption of Arsenate in the Presence of Phosphate and Silicate on Gibbsite and Hematite.

Arsenate adsorption on the aluminum oxide, gibbsite, and the iron oxide, hematite, was investigated as a function of solution pH (3-11) an equilibrium solution arsenate, phosphate, or silicate concentration. Adsorption of all three anions decreased with increasing pH. Competitive adsorption experiments were carried out at equimolar and 10x equimolar concentrations. The competitive effect for all three anions was greater at the 10x equimolar concentrations. Arsenate adsorption in the presence of silicate was reduced only slightly, while the reduction in adsorption was more pronounced presence of phosphate. In each case, the competitive effect was more pronounced above pH 8. The ability of the constant capacitance model to describe adsorption in single anion and competitive systems was evaluated. The ability of the constant capacitance model to predict the competitive systems using surface complexation constants determined for single anion systems was tested.