Insights into the Restoration of Micropolluted Groundwater by Pyrite: The Contribution of Fe(IV) and Outer-Sphere Electron Transfer

Pyrite has been investigated for its potential to modulate the redox microenvironment of groundwater porous media through self-activation. However, the self-purification process of the contaminants by pyrite after their migration from surface water to groundwater has been neglected. This process is accompanied by a decrease in pollutant concentration with a transition from aerobic to anaerobic environments. Here, we selected sulfamethoxazole (SMX), a micropollutant frequently detected in groundwater, as a modeled micropollutant for the investigation. The findings indicate that pyrite could degrade micropollutants SMX (20 μg/L) by self-activation with nearly 100% degradation efficiency under anaerobic conditions. It was also found that ^•OH was not the primary reactive oxygen species (ROS), but rather the longer-lived and more stable Fe(IV) generated by ^•OH_ad-mediated oxidation of structural Fe(III). Additionally, SMX can be degraded by outer-sphere electron transfer with dissolved Fe³⁺ in the system. Whereas, the reduction product Fe²⁺ facilitates the supply of electrons to pyrite and promotes the production of Fe (IV). The contributions of Fe(IV) and outer-sphere electron transfer to SMX degradation were 67.5% and 32.5%, respectively. Furthermore, pyrite self-activation exhibited selective oxidation of electron-rich pollutants under anaerobic conditions. This finding provides a new insight into the self-purification of micropollutants in groundwater environments.