Molybdate modified nano zero-valent iron via green synthesis enhances Cr (VI) reduction during their cotransport in water-saturated porous media

The in situ remediation of hexavalent chromium (Cr(VI)) pollution in groundwater and soil using nano zerovalent iron (nZVI) has garnered great attention. However, Cr(VI) remediation, particularly its reduction remains limited in the subsurface due to the poor migration of nZVI materials to contaminated sites. Herein, we successfully prepared a series of Mo/nZVI@GT materials with varying Mo/Fe synthesis molar ratios using green tea extracts and molybdate and efficiently improved the transportability of Mo/nZVI@GT. The pronounced sizeselective retention of Mo/nZVI@GT in water-saturated porous media also favors for the effective reaction with Cr (VI) during their cotransport. Moreover, Cr(VI) reduction capacity of Mo/nZVI@GT was remarkably improved at pH 6.0, especially in the presence of phosphate. Mechanistically, X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) revealed that tea polyphenols promoted the production of Mo(IV) species in Mo/nZVI@GT, which enhanced the conversion of Cr(VI) to Cr2O3 rather than Cr(OH)3. Additionally, the increased Mo(IV) and Fe(II) active sites might mitigate phosphate disturbance. This study elucidated the mechanisms of Cr(VI) reduction and removal by green synthesized nZVI@GT with Mo modification during their cotransport and highlighted the potential application of Mo/nZVI@GT composites in efficient in situ remediation of Cr(VI)-contaminated groundwater and soil.