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, <i>p</i>-benzoquinone, furfuryl alcohol, <i>tert</i>-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.