Synergistic Removal of Cr(VI) Utilizing Oxalated-Modified Zero-Valent Iron: Enhanced Electron Selectivity and Dynamic Fe(II) Regeneration

To address the challenges of environmental adaptability and passivation in nanoscale zero-valent iron (nFe0) systems, we developed oxalate-modified nFe0 (nFeoxa) through a coordination-driven synthesis strategy, aiming to achieve high-efficiency Cr(VI) removal with improved stability and reusability. Structural characterization (STEM and FT-IR) confirmed the formation of a FeC2O4/nFe0 heterostructure, where oxalate coordinated with Fe(II) to construct a semiconductor interface that effectively inhibits anoxic passivation while enabling continuous electron supply, achieving 100% Cr(VI) removal efficiency within 20 min at an optimal oxalate/Fe molar ratio of 1/29. Mechanistic studies revealed that the oxalate ligand accelerates electron transfer from the Fe0 core to the surface via the FeC2O4-mediated pathway, as evidenced by EIS and LSV test analyses. This process dynamically regenerates surface Fe(II) active sites rather than relying on static-free Fe(II) adsorption. XPS and STEM further demonstrated that Cr(VI) was reduced to Cr(III) and uniformly co-precipitated with Fe(II/III)-oxalate complexes, effectively immobilizing chromium. The synergy between the protective semiconductor layer and the ligand-enhanced electron transfer endows nFeoxa with superior reactivity. This work provides a ligand-engineering strategy to design robust nFe0-based materials for sustainable remediation of metal oxyanion-contaminated water.