Long-lasting organics removal via •OH adsorbed transition metal flocs: Electron transfer-mediated H-bond and van der Waals force

Homogenous advanced oxidation processes (AOPs) based on transition metal catalysts toward the activation of H₂O₂ to hydroxyl radical (•OH) have been widely applied to organic pollutants removal, such as Fenton and Fenton-like processes. These transition metal catalysts mostly flocculate as the pH increases. It's worth noting that the formed transition metal flocs are complex heterogeneous aggregations with active substances, providing diverse reaction spaces and interfaces. However, it is a challenge to distinguish the roles of transition metal flocs in the organic pollutants removal from homogeneous catalytic reactions. Herein, we unveiled a pathway for the long-lasting removal of organic pollutants via Cr flocs adsorbed with •OH (HO^•-Cr flocs) using a stepwise method. First, adsorbed •OH (•OH_ads) within the HO^•-Cr flocs was proved to be the active site forming hydrogen bond (H-bond) and van der Waals force with organic pollutants. Then, the presence of switchable electron transfer between Cr and OH groups within the HO^•-Cr flocs was revealed, contributing to the persistent existence of •OH_ads and consequently ensuring the long-lasting organics removal. Further, this removal pathway of organic pollutants was confirmed during the leather wastewater treatment. These findings will complement a different pathway for organic pollutants removal via transition metal flocs and extend the lifetime of homogeneous AOPs based on transition metal catalysts, providing significant implications for their design and optimization.