Reduction and adsorption mechanisms of selenate by zero-valent iron and related iron corrosion

Abstract

The mechanisms of selenate [Se(VI)] removal by zero-valent iron (ZVI) were investigated using X-ray absorption spectroscopy (XAS) including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Batch experiments were performed to measure the ZVI removal of Se(VI) as a function of Se(VI) concentration, pH, and dissolved oxygen (DO) concentration; these experiments demonstrated that low Se(VI) concentration, low pH, and the presence of DO increased the kinetics of Se(VI) removal. XAS showed that the main factor affecting Se(VI) reduction and adsorption as well as ZVI corrosion was the ratio between Se(VI) concentration and ZVI. At high Se(VI) concentrations (>50mg/L), lepidocrocite (γ-FeOOH) was the principal ZVI corrosion product. During ZVI corrosion, Se(VI) was not completely reduced to Se metal [Se(0)]/selenide [Se(-II)] with 1g/L ZVI, although Se(VI)/selenite [Se(IV)] was adsorbed onto the surface of the lepidocrocite. At a low Se(VI) concentration (10mg/L), Se(VI) was reduced to Se(0)/Se(-II) with 1g/L ZVI, which indicated that reduction by ZVI was the principal mechanism of Se(VI) removal, with magnetite (Fe3O4) being the primary corrosion product in the absence of absorbed Se(VI) ions. Therefore, the reduction capacity and iron corrosion coating of ZVI depended on Se(VI) concentration (i.e., the Se(VI)/ZVI ratio) and less extensively on pH and DO.