Mechanistic Insights into Increased Reduction and Removal of Cr(VI) by Clay-Mediated nZVI during Their Cotransport in Soil-Water Systems

The efficiency and stability of in situ remediation of Cr(VI) contamination in underground water and soil are still challenging. In this study, a green tea (GT) synthesized nano-zerovalent iron (nZVI) nanocomposite supported by diatomite (DE) (nZVI@GT/DE) via tea polyphenols was designed. The synergetic effect of DE support and tea polyphenols wrapping increased the Fe(II) component in nZVI@GT/DE and its transportability. Compared to nZVI@GT/Mont (supported by Mont with sandwich structure), nZVI@GT/DE exhibited a notably higher capacity (>100 mg g(-1)) for immobilizing Cr(VI) and a more pronounced ability to release iron ions in solution (corrosion parameter: K approximate to 1.046). This was attributed to the combined effect of the unwrapping process of tea polyphenols coverage and the distinctive columnar porous structure of DE, which increased the exposure of alpha-Fe-0 core for enhanced Cr(VI) reduction during their cotransport in the water-saturated soil system. X-ray absorption fine structure and X-ray photoelectron spectroscopy analyses confirmed that nZVI@GT/DE released more Fe(II) ions to reduce Cr(VI) to bidentate binuclear Cr(OH)(3) (vs FeOCr2O3) complexes. This study unraveled the molecular mechanism and highlighted the Cr speciation transformation and physiochemical property changes of nZVI@GT/DE during their cotransporting when applied in situ remediation in complex subsurface of Cr(VI)-contaminated groundwater and soil system.