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

• As(V) and Pi showed early rapid and late slow adsorption/desorption kinetics.
• As(V) and Pi showed similar adsorption in single ion adsorption experiments.
• As(V) was adsorbed preferentially on ferrihydrite in competitive settings.
• Both oxyanions showed limited ability to desorb the pre-equilibrated co-oxyanion.

Metal hydroxides (e.g. ferrihydrite) present in geomedia play significant roles in regulating the environmental mobilities of arsenate (As(V)) and inorganic phosphate (Pi) 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 Pi on ferrihydrite at pH 4 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 Pi. Both oxyanions showed similar behavior in single ion adsorption experiments. However, when both oxyanions were loaded together, more As(V) was adsorbed than Pi. Furthermore, more pre-equilibrated Pi was desorbed by sequentially added As(V) than vice versa. Interactions of As(V) and Pi with ferrihydrite slowed down after the initial rapid adsorption/desorption. The experimentally determined adsorption/desorption kinetic data for As(V) and Pi 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 Pi into the system. A higher proportion of Fe CN associated with 2E As(V) surface complexes decreased after the addition of Pi, compared to Fe CN associated with 2C As(V) surface complexes. The competitive desorption study indicates that the excessive input of Pi due to the overuse of fertilizers could mobilize As(V) from contaminated geomedia. Furthermore, insights into Pi-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.