Hydroxyl Radical-Involving p-Nitrophenol Oxidation during Its Reduction by Nanoscale Sulfidated Zerovalent Iron under Anaerobic Conditions.

Sulfidated zerovalent iron (S-ZVI) has been extensively used for reducing pollutants. In this study, the oxidation process in the reductive removal of p-nitrophenol (PNP) by S-ZVI was confirmed under anaerobic conditions. We revealed that a PNP oxidation process involving •OH resulted from the H2O2 activation by surface-bound Fe(II) in S-ZVI, in which H2O2 was generated via a surface-mediated reaction between water and FeS2. Only the PNP reduction process occurred for ZVI. Herein, efficient PNP degradation by S-ZVI resulted from two functions: reduction into p-aminophenol due to enhanced electron transfer and PNP oxidation into p-benzoquinone by •OH radicals from the interaction of surface-bound Fe(II) and in situ generated H2O2, the contributions of the oxidation and reduction processes to PNP degradation over S-ZVI were 10 and 90%, respectively. Sulfur in S-ZVI suppressed the pH increase in the reaction media and produced more surface-bound Fe(II) than ZVI for •OH generation via the heterogeneous Fenton reaction process. Since different degradation pathways could lead to different effects on the water environment, such as toxicity, our findings suggest that the oxidizing process induced by S-ZVI during groundwater decontamination should be considered.