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 HCO 3 – ) efficiently quench triplet-state naproxen, with bimolecular rate constants ranging from 1.0 × 10 6 to 7.1 × 10 8 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/H 2 O 2 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.