Effect of major anions on arsenate desorption from ferrihydrite-bearing natural samples

Abstract

Abstract The influence of background electrolytes (Na2HPO4 · 2H2O, NaHCO3, Na2SO4, NaNO3 and NaCl) on arsenate (As(V)) desorption from 3 environmental samples (a tailings sample, a stream-bed sediment and a top soil) containing ferrihydrite as the main As-bearing phase has been studied by means of kinetic batch experiments and geochemical simulations. The experimental results indicate that As(V) release increases greatly in the presence of dissolved phosphate and carbonate species. Similarly to PO 4 3 + , a strong surface interaction of inner-sphere type between ferrihydrite and aqueous carbonate species is suggested. Nitrate and Cl− have negligible effects on the As(V) desorption reaction, whereas SO 4 2 - exhibits intermediate behavior depending on its dissolved concentration that probably influences the type of surface complex (i.e. outer-sphere or inner-sphere). The process of As(V) release follows the first-order rate equation of Lagergren modified for desorption; most values of the desorption rate constant kdes are in the range of 0.0012–0.0030 min−1. Modeling of the desorption experiments with PHREEQC, with ferrihydrite as the main As-bearing phase, indicates that the influence of pH is notably less important than the displacement action of carbonate species in determining the amount of As(V) released to solution. Simulation of As(V) desorption totally fails when the carbonate surface complexes are excluded from the model. In the NaHCO3 experiments with the tailings sample the best match between observed and calculated data is obtained also including dissolution of scorodite and arsenopyrite in the model. Moreover, modeling has stressed the poor quality of the adsorption constants for sulfate species that leads to strong overestimation of As(V) desorption at pH 4 and underestimation at pH 7.5. Although the findings of this study are consistent with the results of recent studies from other authors, they cannot be generalized or directly applied to natural systems. However, environmental implications concerning As mobility, as well as possible application in various fields (e.g. irrigation agriculture, soil decontamination, water treatment and mine site remediation), might be derived from these findings.