Coupling-promoted oxidative degradation of organic micropollutants by iron oxychloride (FeOCl) with dual active sites

Heterogeneous Fenton has emerged as a profitable solution for contaminant removal via advanced oxidation processes (AOPs). Besides the dominant hydroxyl radicals (HO•), some weaker oxidants, such as ferryl-oxo species (Fe(IV)=O) species, can also be produced during the surface H2O2 activation, but its function is not well understood. In this study, we developed a vanadium-etched iron oxychloride (V-FeOCl) catalyst that simultaneously incorporates Fenton-like sites and peroxidase-like (Fe(IV)=O) sites. The derived V-FeOCl material showed 2.8–5.4 times enhancement of the pseudo-first-order rate constant for various recalcitrant organic micropollutants. Most importantly, the activity demonstrated an intriguing induction period for the TOC removal as well as a rocketed kinetics after the induction. This induction period was further attributed to the oxidative coupling of the organic monomers, as revealed by the identification of dimers using UPLC-MS. The coupling intermediates were demonstrated to be more susceptible to HO• radical attack via the high-throughput prediction of the HO• radical rate constants of 94 possible coupling intermediates using machine learning. These findings clarified the key role of Fe(IV)=O in the HO•-based oxidation process and points to a novel coupling-enhanced degradation pathway, which could potentially pave a new avenue of oxidative transformations for catalytic and environmental applications.