Concentration-Dependent Enhancing Effect of Dissolved Silicate on the Oxidative Degradation of Sulfamethazine by Zero-Valent Iron under Aerobic Conditions.

Abstract

Dissolved silicate, as a ubiquitous inorganic component in natural waters, is reported to depress the reactivity of zero-valent iron (ZVI) for reductive reactions under anoxic conditions, but it is unclear if the same inhibitory effect occurs for a ZVI/O2 system. In this study, the role of dissolved silicate for the reactivity of micron-sized ZVI (mZVI) was revisited under aerobic conditions, and different observations were found. Silicate had a volcano-type enhancing effect on the performance of the ZVI/O2 system for sulfamethazine (SMT) degradation. The results showed that, under a circum-neutral or alkaline pH condition (pH 6.0-9.0), the presence of dissolved silicate could significantly enhance the degradation of SMT because silicate coordinated with ferrous ions and further led to the generation of reactive oxygen species (ROS). This study suggests that silicate can act as both a ligand and corrosion inhibitor in a ZVI/O2 system: the coordination of silicate and ferrous iron accelerated the oxidative degradation of organic pollutants in an oxic aqueous solution, while the corrosion inhibitory effect of surface-bound silicate at higher concentrations may decrease the reactivity of the ZVI/O2 system, thereby offsetting the enhancing effect from the silicate-coordinated ferrous iron. This study not only redefines the role of naturally occurring silicate for a ZVI reaction system but also gives clues to develop high-efficiency ZVI/O2 technologies for water remediation.