Effect of pH, phosphate and oxalate on the adsorption/desorption of arsenate on/from goethite

We studied (i) the competitive adsorption of arsenate and phosphate on goethite as affected by pH, anion concentration, and order of anion addition; (ii) the effect of increasing amounts of arsenate or phosphate on the desorption of phosphate or arsenate from the surfaces of goethite; and (iii) the influence of oxalate on the desorption of arsenate and phosphate from goethite. Similar amounts of phosphate and arsenate were adsorbed on goethite samples at pH 3.0-8.5. By adding arsenate and phosphate as a mixture (As + P systems; arsenate/phosphate molar ratio of 1), the amounts of arsenate and phosphate adsorbed on goethite were similar at pH > 6.0, but at pH < 6.0 slightly more arsenate than phosphate was adsorbed. When arsenate was added before phosphate (As before P systems), the efficiency of arsenate in inhibiting the adsorption of phosphate on goethite was higher than that of phosphate in inhibiting the adsorption of arsenate, when phosphate was added before arsenate (P before As systems). Furthermore, we found that the decrease in adsorption of phosphate in the presence of increasing concentrations of arsenate was greater than that of arsenate in the presence of increasing concentrations of phosphate. There is much evidence indicating that arsenate and phosphate compete primarily for a similar set of surface sites on goethite, but they are adsorbed on some sites uniquely specific for arsenate and phosphate. In the pH range of 3.0 to 8.5, much more phosphate previously adsorbed on goethite was desorbed by arsenate (mainly at PH < 6.0) than arsenate by phosphate. The desorption of both the ligands was influenced by time. Finally, the presence of oxalate ions in solution desorbed higher amounts of phosphate than arsenate ions, adsorbed previously on goethite, mainly in acidic systems.