Effect of competing anions on the adsorption of arsenate and arsenite by ferrihydrite

The competitive adsorption of arsenate and arsenite and the effect of phosp hate and sulfate on adsorption of arsenate and arsenite by ferrihydrite wer e investigated in the pH range of 3 to 10 and at varying initial ligand con centrations. In dual anion systems, arsenate retention was greater at low p H compared with greater arsenite retention at high pH. In systems with arse nate and arsenite concentrations less than or equal to 2.08 mol(As) kg(fer) (-1) each, the effect of arsenate on arsenite sorption was more pronounced than vice versa, On the contrary, at arsenate and arsenite concentrations o f 3.47 mol(As) kg(fer)(-1) each, arsenate did not influence arsenite sorpti on but arsenite substantially reduced arsenate adsorption, The different so rption behavior of arsenite at Low and high arsenite concentrations might b e 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 th e same surface sites, with a moderate preference for arsenate adsorption. T here was evidence of the presence of some surface sites that exhibited much higher affinity for arsenite than phosphate. The presence of sulfate did n ot influence arsenate adsorption but resulted in a considerable reduction i n arsenite adsorption below pH 7.0, with the largest reduction at the lowes t pH.