Facet-Dependent Hematite Reactivity in Cr(Ⅵ) Removal with Fe(Ⅱ)

Hematite displays diverse crystal structures and often coexists with Fe(Ⅱ), both of which are crucial in controlling the fate and mobility of Cr(Ⅵ). However, the mechanisms underlying Cr(Ⅵ) removal in the presence of Fe(Ⅱ) on various hematite facets remain elusive. This study aims to elucidate the facet-dependent reactivity of hematite nanocrystals in conjunction with Fe(Ⅱ) for the removal Cr(Ⅵ) from aqueous solutions. Hematite nanoplates (HNPs), predominantly composed of {001} facets, and nanorods (HNRs), exposing both {001} and {110} facets, were synthesized and characterized. Their Cr(VI) removal capabilities were evaluated in hematite-Cr(VI) and hematite-Fe(II)-Cr(VI) systems, as well as the Fe(II)-Cr(VI) system. The adsorption of Fe(Ⅱ) and Cr(VI) on hematite surfaces was highly dependent on the crystal facets and pH, with HNRs demonstrating superior Cr(Ⅵ) adsorption over HNPs, especially under acidic conditions. Neutral pH favored Fe(II)-Cr(VI) redox reactions and Fe(II) adsorption. The hematite-Fe(Ⅱ) couple displayed a synergistic effect in removing Cr(Ⅵ) under acidic conditions, which was not observed under neutral conditions. The presence of Fe(Ⅱ) notably enhanced Cr(Ⅵ) adsorption onto hematite, and bound Fe(Ⅱ) facilitated electron transfer, accelerating Cr(Ⅵ) reduction. HNRs-Fe(Ⅱ) exhibited higher Cr(Ⅵ) removal efficiency than HNPs-Fe(Ⅱ) due to their lower free corrosion potential and improved electron transport properties. This research underscores the potential of facet engineering in optimizing hematite nanocrystals for environmental remediation, specifically in Cr(Ⅵ)-contaminated environments.