The roles of pyrite for enhancing reductive removal of nitrobenzene by zero-valent iron

Zero-valent iron (ZVI) is a popular reductant that has been successfully applied for remediation of groundwater contaminated with various pollutants, but it still suffers from surface passivation and pH increase in the reaction media. In this study, pyrite, a ubiquitous sulfide mineral in anaerobic environment, was adopted to enhance the reactivity of ZVI for removal of nitrobenzene. The synergetic effect between pyrite and ZVI was observed for nitrobenzene reduction, and the rate constant k_obs at the initial pH (pH₀) 6.0 was enhanced by 8.55–23.1 folds due to the presence of pyrite with pyrite/ZVI mass ratio ranging from 1.0 to 6.0. Moreover, nitrobenzene could be removed effectively at pH₀ ranging from 5.0 to 10.0 in the presence of pyrite, while negligible removal of nitrobenzene by ZVI (0.5 g/L) alone was observed at pH₀ ≥7.0. ZVI sample recovered from the reacted ZVI/pyrite mixture was also more effective for nitrobenzene degradation than pristine ZVI. The mechanism study revealed that pyrite could suppress the pH increase in reaction media, boost the production of reactive Fe²⁺, and activate the ZVI surface through replacing partially the passive oxide film with iron sulfide (FeS). In particular, the formation of highly reactive FeS@Fe in the reaction system of ZVI/pyrite mixture was proved by XRD, Mössbauer, XANES, XPS and SEM-EDS analyses, which provides a facile way for in-situ sulfidation of ZVI and for enhancing the removal of contaminants with ZVI technology.