Kinetics of competitive adsorption/desorption of arsenate and phosphate at the ferrihydrite-water interface

Metal hydroxides (e.g. ferrihydrite) present in geomedia play significant roles in regulating the environmental mobilities of arsenate (As(V)) and inorganic phosphate (P_i) because of their high adsorption affinities for these oxyanions. In this study, results are presented of experiments aimed at determining individual and competitive adsorption/desorption kinetics of As(V) and P_i on ferrihydrite at pH4 and 8. Selected samples were also subjected to As K-edge EXAFS study for understanding the changes with time in As(V) complexation on ferrihydrite in the presence/absence of P_i. Both oxyanions showed similar behavior in single ion adsorption experiments. However, when both oxyanions were loaded together, more As(V) was adsorbed than P_i. Furthermore, more pre-equilibrated P_i was desorbed by sequentially added As(V) than vice versa. Interactions of As(V) and P_i with ferrihydrite slowed down after the initial rapid adsorption/desorption. The experimentally determined adsorption/desorption kinetic data for As(V) and P_i showed good compliance with pseudo-second order, Elovich, and power-function equations. Both oxyanions competed for adsorption on ferrihydrite, and each of them showed a limited capacity to desorb the other. EXAFS analysis of selected samples indicated the presence of mononuclear (2E) and binuclear (2C) bidentate As(V) surface complexes. The Fe coordination numbers (CN) increased with increasing time and decreased with addition of P_i into the system. A higher proportion of Fe CN associated with 2E As(V) surface complexes decreased after the addition of P_i, compared to Fe CN associated with 2C As(V) surface complexes. The competitive desorption study indicates that the excessive input of P_i due to the overuse of fertilizers could mobilize As(V) from contaminated geomedia. Furthermore, insights into P_i-induced desorption of As(V) could also provide an opportunity for developing chemical treatment methods to intercept the mobilized As(V) by co-precipitation in apatite-like phases.