Reduction of Se(VI) to Se(-II) by zerovalent iron nanoparticle suspensions

The reaction of selenate (Se(VI)) with zerovalent iron nanoparticles (nano Fe⁰) was studied using both conventional batch equilibrium and X-ray spectroscopic techniques. Nano Fe⁰ has a high uptake capacity for removal of dissolved Se(VI) reaching concentrations as high as 0.10 Se:Fe molar ratio in the solid product mixture. Kinetic studies of the Se(VI) uptake reaction in batch experiments showed an initial reaction rate (0–30 min) of 0.0364 min^?1 which was four times greater than conventional Fe⁰ powder. Analysis of the oxidation state of Se in the solid products by X-ray absorption near edge structure (XANES) spectroscopy showed evidence for the reduction of Se(VI) to insoluble selenide (Se(-II)) species. Structural analysis of the product by extended X-ray absorption fine structure (EXAFS) spectroscopy suggested that Se(-II) was associated with nano Fe⁰ oxidation products as a poorly ordered iron selenide (FeSe) compound. The fitted first shell Se–Fe interatomic distance of 2.402 (±0.004) Å matched closely with previous studies of the products of Se(IV)-treated Fe(II)-clays and zero-valent iron/iron carbide (Fe/Fe₃C). The poorly ordered FeSe product was associated with Fe⁰ corrosion product phases such as crystalline magnetite (Fe₃O₄) and Fe(III) oxyhydroxide. The results of this investigation suggest that nano Fe⁰ is a strong reducing agent capable of efficient reduction of soluble Se oxyanions to insoluble Se(-II).