Beyond the reaction kinetics: Interpretable machine learning reveals unique pathways of sulfate and carbonate radicals.

Sulfate radical (SO4•-) mediated advanced oxidation process (AOP) represents a cutting-edge and energy-efficient technology for wastewater treatment with carbonate radical (CO3•-) as coexisting species in alkaline waters. The reaction mechanism of both radicals with trace organic contaminants (TrOCs) has long been accepted to occur mainly through single electron transfer (SET). However, recent studies suggested a more complex mechanism than previously thought. In this study, we conduct a thorough investigation into their reaction mechanisms, and our results reveal that the reactivity of both radicals with benzene derivatives aligns with electrophilic substitution patterns, with the presence of electron-donating groups enhancing the trend. Extending beyond benzene derivatives, we formulate linear free-energy relationships (LFERs) for a broader range of TrOCs by means of machine learning. Our results highlight that SET pathway is the rate-determining step for the reactions with a more pronounced effect in reactions initiated by CO3•- than those by SO4•-, as other pathways are as equally important as SET when SO4•- reacts with TrOCs with hydroxyl, amine, and carbonyl groups. These in-depth insights not only help to elucidate the origin of the discrepancies but also enable a better control over reactive radicals, holding the potential to the controllable byproduct transformation.