COMPETITIVE ADSORPTION OF ARSENATE AND PHOSPHATE AT THE FERRIC HYDROXIDE-WATER INTERFACE

Single and competitive adsorption of arsenate (As_(V)) and phosphate (P_i) on ferric hydroxide were investigated using adsorption isotherms, adsorption envelopes, surface complexation modeling, adsorption kinetics, and As K-edge EXAFS spectroscopy. This research was aimed at evaluating the selectivity and mechanism of As_(V) and P_i adsorption on ferric hydroxide. Adsorption isotherms at pH 4, 8, and 11 were constructed for both oxyanions with initial concentrations of 0.07 mM to 6.67 mM in single as well as competitive settings. The empirical results indicated that the adsorption of these oxyanions on ferric hydroxide is largely controlled by pH, generally decreasing with increasing pH. The triple layer model simulations using bidentate surface complexes slightly under-predicted adsorption at lower and upper pH regions, and over-predicted adsorption in the intermediate pH range. Kinetic experiments showed early rapid, followed by slower, adsorption/desorption of As_(V) and P_i at the ferric hydroxide-water interface. During competitive adsorption experiments, As_(V) was preferentially adsorbed over P_i. The As K-edge EXAFS spectra showed that the corner-sharing bidentate binuclear complex is the major surface complex of As_(V) on ferric hydroxide. Similarly, the edge-sharing bidentate mononuclear complex of As_(V) was also identified in all samples at pH 4, and in samples where both As_(V) and P_i were loaded at pH 8. It was found that the presence of P_i enhanced the formation of As_(V) bidentate mononuclear surface complexes, more at pH 8 than at pH 4. Adsorption isotherms, envelopes, kinetics, and As K-edge EXAFS analysis suggest that the adsorption preference of As_(V) is related to its ability to form edge-sharing bidentate complex at higher surface coverage and in the presence of P_i. Arsenate was found to compete with P_i for adsorption at the corner sharing sites, in addition to possibly noncompetitive edge-sharing sites, thereby resulting in greater uptake of As_(V) than P_i during competitive adsorption. The research findings indicate that both As_(V) and P_i compete for adsorption on ferric hydroxide and each showed a capacity to desorb the other. Therefore, excessive input of P_i to contaminated soils and sediments due to the overuse of fertilizers could release As through competitive desorption.