Co-adsorption of arsenite and arsenate on mixed-valence Fe(II,III) (hydr)oxides under reducing conditions

Abstract

The reduction of Fe(III) in iron hydroxides plays a significant role in the geochemical cycling of arsenic. However, the adsorption behavior of arsenic (As) at mixed-valence Fe(II,III) (hydr)oxides/water interface remains unclear. This work presents the kinetics, isotherms, and pH edge of arsenite (As(III)) and arsenate (As(V)) co-adsorption on the surface of Fe(II,III) (hydr)oxides as a function of Fe(II)/Fe(III) molar ratio and pH under anoxic environment. The XRD and Raman results indicated that by using Fe(II) and Fe(III) mixed solution as precursors, the coprecipitated products at pH 9 possibly were the mixture of ferrihydrite, goethite, magnetite and green rust, leading to a significantly smaller surface area than ferrihydrite. The adsorption of both As(III) and As(V) decreased significantly as the Fe(II)/Fe(III) molar ratio in the synthetic Fe(II,III) (hydr)oxides increased from 0 to 4. The adsorption of both As(III) and As(V) on the Fe(II,III) (hydr)oxides was strongly pH-dependent and showed completely opposite trends. As(V) was more easily adsorbed on the Fe(II,III) (hydr)oxides under acidic conditions, whereas an increasing amount of As (III) was adsorbed on Fe (hydr)oxides under alkaline conditions, possibly due to the competitive adsorption. Our findings may contribute to the understanding of the interfacial behavior of arsenic in the reducing environments.