Phosphate modification enables high efficiency and electron selectivity of nZVI toward Cr(VI) removal

In this study, we demonstrate that surface phosphate modification of nZVI (p-nZVI) could enhance the electron selectivity of Cr(VI) reduction from 6.1% to 31.3%, as estimated by XANES and XPS, and thus promote the Cr(VI) removal efficiency by 4 folds. DFT calculation and experimental results revealed that phosphate groups were adsorbed on the surface of p-nZVI via a monodentate mononuclear model to inhibit the reaction with oxygen and/or water via the pendant protons of phosphate, accounting for high electron selectivity of p-nZVI. More importantly, surface phosphate modification shifted the binding configuration of Cr(VI) from a monodentate mononuclear model on nZVI to bidentate binuclear one on the p-nZVI surface, thus strengthening the Cr(VI) adsorption ability and favoring the subsequent Cr(VI) reduction. This study provides a facile strategy to enhance the electron selectivity towards metal remediation, and highlights the vital influence of surface structure on nZVI reactivity at the molecular level.