Silicate Enhances the Long-Term Dechlorination Performance of Sulfidized Zero-Valent Iron: Trade-Off between Passivation and In Situ Oxidation.

Sulfidized zero-valent iron (S-ZVI) holds promise in the remediation of chlorinated hydrocarbons. However, S-ZVI is susceptible to corrosion in aquifers with elevated dissolved oxygen (DO) levels. This study demonstrates, for the first time, that a trade-off between the passivation and oxidative corrosion of aged S-ZVI can be achieved in the presence of silicate to promote its dechlorination performance on trichloroethylene. The electron transfer from aged S-ZVI to target contaminants and DO can be promoted to generate reactive oxygen species (ROS), simultaneously enhancing reductive and oxidative dechlorination. Silicate interactions contribute up to an 18.9% decrease in the electron resistance, a 23.3% increase in oxidative dechlorination, a 61.1% increase in the dechlorination rate constant, and an 85.7% increase in the degradation capacity of aged S-ZVI as compared to the control. Furthermore, the decrease in the reductive sulfur species over an extended aging time promotes the depolymerization of silica polymers into monomer silicate on S-ZVI, which is conducive to mitigating silicate passivation and promoting the release of dissolved silicate in aged S-ZVI systems. This process, in turn, facilitates ferrous iron dissolution and ROS production, thereby enhancing the redox reactivity of aged S-ZVI. This study provides new insights into the effect of silicate on the aging of S-ZVI, which will help develop novel strategies for remediation using S-ZVI-based techniques.