Mechanistic Insights into Triplet Excited-State Organic Contaminants: Unrecognized Role of Chemical Probes and Scavengers in UV-Based Advanced Oxidation Processes

In UV-based advanced oxidation processes (AOPs), chemical probes and scavengers are widely used to identify radical species responsible for organic contaminant removal due to their simplicity and selectivity. A prevailing assumption is that these reagents selectively target radicals without interacting with triplet excited-state contaminants generated under UV irradiation. Here, we challenge this assumption by demonstrating that triplet-state contaminants can directly interact with commonly used probes and scavengers, revealing an overlooked transformation pathway that biases radical identification and mechanistic interpretation. Using naproxen as a model compound, we show that seven representative reagents (isopropanol, p-benzoquinone, furfuryl alcohol, tert-butanol, methanol, ethanol, and HCO₃⁻) efficiently quench triplet-state naproxen, with bimolecular rate constants ranging from 1.0 × 10⁶ to 7.1 × 10⁸ M⁻¹ s⁻¹. The quenching process followed saturation kinetics and can be well described by the Hanes-Woolf model. Transient absorption spectroscopy and quantum chemical calculations reveal feasible electron transfer and proton-coupled electron transfer pathways. Further, in a UV/H₂O₂ system, we found that isopropanol promotes naproxen degradation with triplet-state interactions, leading to significant underestimation of •OH contribution. These results highlight a critical interference pathway in AOPs and underscore the need for caution when using probes and scavengers for kinetic and mechanistic evaluations.