New insights on Cr(VI) retention by ferrihydrite in the presence of Fe(II).

Hexavalent chromium [Cr(VI)] contamination poses a significant environment hazard due to its high toxicity, solubility, and mobility. Fe(II) is often found to co-exist with iron oxide minerals (IOMs) that are naturally occurring in soil and groundwater. However, the mechanism by which Cr(VI) is retained by IOMs in the presence of Fe(II) remains unclear. Ferrihydrite, the precursor of various iron oxides, is a representative IOM. In this study, the mechanism of Cr(VI) retention by ferrihydrite in the presence of Fe(II) was elucidated. Results from this study showed that Fe(II) adsorbed on ferrihydrite had a significant effect on the Cr(VI) retention process. FTIR analysis demonstrated that Cr(VI) adsorbed on a mineral surface, including outer-spheres and coordinated compounds, can be desorbed. XPS analysis further revealed that non-desorbable Cr includes reduced Cr(III) and partial Cr(VI), which were found to be incorporated into the Cr(III)-Fe(III) co-precipitation within the iron mineral. We also carried out XRD, HRTEM, and TG-DSC measurements in order to determine that the crystal structure, morphology, and physicochemical properties of IOMs were not changed significantly before and after Cr(VI) adsorption. The insights provided by this study aid in the development of a clear understanding of the effects of ferrihydrite in the presence of Fe(II) on the fate of Cr(VI) in both water and soil.