Competitive adsorption between phosphate and arsenic in soil containing iron sulfide: XAS experiment and DFT calculation approaches

Abstract The competitive adsorption between phosphate and arsenic (As) in natural soil and mineral systems is important for controlling subsurface contaminants. Phosphate is known to adsorb more strongly than As in Fe(III) oxide-based mineral systems. Here, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) were employed to understand the fundamental reaction mechanisms of As and phosphate on an FeS surface. The competitive effect of phosphate in As(V)-contaminated soil systems differed with changing FeS concentrations. In soil batches with high FeS contents, As(V) was more strongly adsorbed than phosphate, and no additional As release was observed in the aqueous phase, in the presence of phosphate. Through the XAS measurement of laboratory batch samples, the reaction mechanisms could be comprehensively understood, including the reduction of As(V) to As(III), the secondary precipitation of sulfide minerals, and the adsorption mechanism of As(V) on the surface of FeS. The DFT calculations revealed the relative adsorption strengths on the FeS surface were As(V) > phosphate > As(III). Comparing the adsorption energies revealed that As(V) reacted more strongly with FeS than phosphate and As(III), and that the effect of phosphate co-occurrence was negligible in the FeS–As system, in line with the experimental measurements.