Vanadate complexation to ferrihydrite: X-ray absorption spectroscopy and CD-MUSIC modelling

Abstract

Environmental contextVanadium, a metal pollutant from fossil fuels and slags, may be toxic, thereby necessitating an understanding of its environmental chemistry. One important factor that controls the mobility and bioavailability of vanadium is its binding to iron oxides. This study focuses on the characterization and modelling of vanadium adsorption onto ferrihydrite. The new model can be used to simulate the transport and bioavailability of vanadium in the environment. AbstractThe mobility of vanadium in the environment is influenced by sorption to metal (hydr)oxides, especially those containing iron. The aim of this study is to understand the adsorption behaviour of vanadium on poorly ordered (two-line) ferrihydrite (Fh). A further objective was to determine the binding mechanism of vanadate(V) to ferrihydrite surfaces in aqueous suspension to constrain the CD-MUSIC surface complexation model. Vanadium adsorption to ferrihydrite was evaluated by batch experiments which included series with different Fh-to-V ratios and pH values. Vanadate(V) adsorption was also evaluated in the presence of phosphate to compete with vanadate(V) for the available surface sites on ferrihydrite. In agreement with earlier studies, vanadate(V) was strongly adsorbed to ferrihydrite and the adsorption increased with decreasing pH. In the presence of phosphate, less vanadate(V) was adsorbed. Analysis by X-ray absorption near-edge structure spectroscopy revealed that the adsorbed vanadium was tetrahedral vanadate(V), VO4, regardless of whether vanadate(V) or vanadyl(IV) was added to the system. Spectra collected by extended X-ray absorption fine structure spectroscopy showed that vanadate(V) is bound primarily as an edge-sharing bidentate complex with V⋯Fe distances around 2.8Å. Based on this information, a surface complexation model was set up in which three bidentate vanadate(V) complexes with different degrees of protonation were included. The model provided a satisfactory description of vanadate(V) adsorption over most of the pH and concentration ranges studied, also in the presence of competing phosphate ions.