Modulating d-Orbital Electronic Configuration of Magnetic Iron Sulfide Nanocrystals for Maximized Treatment Efficiency of Chromium-Contaminated Water

Abstract

Hexavalent chromium (Cr(VI)) is one of the most rigorously regulated contaminants frequently detected in surface and ground water. Magnetic iron sulfides are naturally abundant, environmentally friendly materials ideal for the removal of Cr(VI) from contaminated water, but the high-spin states of Fe(III) ions limits their adsorption affinity. Herein, we develop a heteroatom-doping approach to boost the Cr(VI) removal efficacy of greigite. Compared to pristine greigite, cobalt doping significantly enhances the capability of greigite to adsorption Cr(VI) by decreasing electron occupancy in the e_g orbitals and reducing the spin state of Fe ions. With a combination of electrochemical characterizations and theoretical calculations, we confirm that cobalt doping significantly enhances its reduction capacity toward Cr(VI) by elevating the d-band center and increasing electron transfer rate. We corroborate the finding by showing that Cu-doping, which has the opposite effects on d-orbital electron configures, compromises the efficacy of greigite. Of note, the magnetic properties of greigite remain largely unaffected upon heteroatom doping, allowing easy separation and recovery of the materials from the aqueous solutions. This work provides valuable mechanistic insights for nanomaterial design in contaminant removal and may inform the development of nanotechnology for green remediation of contaminated sites.