Removal of arsenate by ferrihydrite via surface complexation and surface precipitation

Abstract

In this study, macroscopic and spectroscopic experimental methods accurately modeled the sorption process of arsenate on ferrihydrite. EXAFS, X-ray diffraction and infrared (IR) spectroscopy indicated that the behavior of As(V) adsorption onto ferrihydrite took place mainly via surface complexation and surface precipitation at acidic pH (3.0–6.0), while the surface precipitation was dominated at longer time intervals and higher Fe3+ concentration. The macroscopic competitive adsorption experiment between arsenate with phosphate indicated two types of adsorption sites existing on the surface of ferrihydrite, i.e., non-exchangeable sites, which are responsible for a rapid surface complex formation; and exchangeable sites for a slow build-up of surface precipitates. In the slow build-up precipitates, the As(V) surface coverage (mmol/g) exhibited a good linear relationship (R2=0.952) with the amount of dissolved Fe3+. Three steps are involved during the process of surface precipitation, i.e., (1) an initial uptake of As(V) via surface complexation; (2) re-adsorption of Fe3+ leaching from ferrihydrite on the surface complex; and (3) As(V) adsorption via surface complexation again and finally forming the surface precipitate.