A study of arsenate adsorption on goethite (α-FeOOH) in relation to selected ions
Infrared spectroscopy was used to show that the arsenate anion is adsorbed on the surface of synthetically prepared goethite, replacing two singly coordinated (A-type) surface OH groups. In this respect, arsenate is analogous to phosphate, but its larger size causes it to interact more strongly with other OH groups that remain on the surface. This has suggested an alternative infrared band assignment for these OG groups. Potentiometric titrations performed on 'CO2'-free goethite gave a value for the point of zero charge (p.z.c.) of 9.15. The p.z.c. shifted to more acid values for phosphate- and arsenate-treated goethite indicating an increased negative surface charge. For sulphate-treated goethite the positive surface charge increased slightly. The results were interpreted as indicating that phosphate and arsenate were adsorbed by a chemisorption (ligand-exchange) mechanism. The exact nature of the sulphate interaction was still unclear. Solution studies on the goethite-aqueous solution interface demonstrated the adsorption of arsenate by a binuclear bridging mechanism, confirming the findings from the infrared study. The results indicated a 2:1 stoichiometry between ligands exchanged and arsenate adsorbed over the pH range 4-10. No such stoichiometry was observed for sulphate adsorption though the reaction between sulphate and goethite consumed protons (i.e. released hydroxyl). A brief study to ascertain the effect of certain competitors on arsenate adsorption was performed. Results showed considerable reduction in arsenate adsorption when vanadate ions were added simultaneously with arsenate to goethite-aqueous electrolyte suspensions. The results also indicated that goethite previously coated with small quantities of humic acid reduced arsenate adsorption. The presence of sulphate in solution at up to five times the concentration of arsenate had no effect on arsenate adsorption over the pH range 5 to 9.