Silica-coated nanosized zero-valent iron/persulfate system for sequential reduction-oxidation remediation of...

Sequential reduction and oxidation processes are promising for mineralizing recalcitrant contaminants with high oxidation states. The reduction step transforms such contaminants into oxidizable intermediates that are subsequently mineralized in the oxidation step. In this study, we assessed a sequential reduction-oxidation system using silica-coated nanosized zero-valent iron particles (nZVI@SiO₂) and persulfate to establish in situ reactive zones (IRZs) for groundwater remediation. A sequential column and a physical aquifer model were used to simulate groundwater environments in the field. The silica coating made the nZVI@SiO₂ markedly more mobile and selective than bare nZVI. In the column reactor, strong positive correlations were found between the consumption of nZVI@SiO₂ (being a consumable reductive material) and the amounts of nitrobenzene reduced and aniline formed. The sequential reduction-oxidation process induced by the combination of the nZVI@SiO₂ reductive column and the persulfate oxidative column markedly enhanced contaminant mineralization, which could not be achieved effectively through only reduction or only oxidation. A more practical test of the treatment process for in situ reactive zone remediation was performed using a three-dimensional physical aquifer model reactor. The removal efficiencies achieved using nZVI@SiO₂ in reactors of different sizes were determined, and extrapolations were performed to determine a quantitative baseline for designing effective nZVI@SiO₂ treatment systems for field applications.