Effect of Competing Anions on the Adsorption of Arsenate and Arsenite by Ferrihydrite

Abstract The competitive adsorption of arsenate and arsenite and the effect of phosphate and sulfate on adsorption of arsenate and arsenite by ferrihydrite were investigated in the pH range of 3 to 10 and at varying initial ligand concentrations. In dual anion systems, arsenate retention was greater at low pH compared with greater arsenite retention at high pH. In systems with arsenate and arsenite concentrations ≤2.08 mol As kg −1 fer each, the effect of arsenate on arsenite sorption was more pronounced than vice versa. On the contrary, at arsenate and arsenite concentrations of 3.47 mol As kg −1 fer each, arsenate did not influence arsenite sorption but arsenite substantially reduced arsenate adsorption. The different sorption behavior of arsenite at low and high arsenite concentrations might be due to surface polymerization of adsorbed arsenite at high concentrations. The presence of phosphate resulted in a significant reduction in arsenate and arsenite adsorption by ferrihydrite, with strong dependence on pH and phosphate concentration. The effect of phosphate on arsenate adsorption was greater at high pH than at low pH, whereas the opposite trend was observed for arsenite. Results indicated that arsenate and phosphate compete for the same surface sites, with a moderate preference for arsenate adsorption. There was evidence of the presence of some surface sites that exhibited much higher affinity for arsenite than phosphate. The presence of sulfate did not influence arsenate adsorption but resulted in a considerable reduction in arsenite adsorption below pH 7.0, with the largest reduction at the lowest pH.