Presentation: 2025 ND EPSCoR Annual conference 

October 21, 2025, NDSU Memorial Union, Fargo, North Dakota

Quenching Effects on Direct Photolysis of Organic Micropollutants under UV-222 and UV-254

Advanced oxidation processes (AOPs) rely on the generation of hydroxyl radicals for the degradation of organic micropollutants (OMPs) in water. Quenchers such as tert-butyl alcohol (TBA) are well known to suppress radical-mediated pathways in AOPs but are generally assumed to have little or no effect on direct photolysis. In this work, we demonstrate that quenching can also play a significant role during direct photolysis. We examined the degradation of several environmentally relevant OMPs, including carbamazepine (CBZ), caffeine (CAF), N,N-Diethyl-m-toluamide (DEET), sulfamethoxazole (SMX), naproxen (NAP), atrazine (ATR), diclofenac (DIC), and 4-chlorophenol (4-CP), under UV irradiation at 222 nm and 254 nm. Kinetic experiments were performed with and without 5 mM TBA in buffered aqueous solutions. Pseudo-first-order rate constants revealed that photolysis was consistently suppressed in the presence of TBA, with the strongest inhibition observed for amide-containing compounds such as CBZ, CAF, and DEET. For example, the degradation of CBZ was reduced by more than 60% at 222 nm and by ~25% at 254 nm in the presence of TBA. These findings suggest that TBA interacts not only with hydroxyl radicals but also with excited triplet states of OMPs, thereby quenching their direct photolysis pathways. Our study provides new evidence that quenching effects extend beyond radical-based AOPs to direct photolysis. Recognizing the role of quenchers in both processes is essential for accurately assessing micropollutant removal and for optimizing UV-based water treatment strategies.