Unexpected Effects of Alcohol-Based Quenchers on Inhibiting Direct Photolysis of Organic Micropollutants in UV Processes

Alcohol quenchers such as tert-butanol (TBA) are widely used for assessing the role of hydroxyl radicals in UV-based processes. One critical assumption is that alcohols have little effect on the direct photolysis of organic micropollutants (OMPs). However, this work demonstrated that TBA can inhibit direct photolysis of OMPs upon irradiation by UV. First, 5 mM TBA decreased the direct photolysis rate of OMPs at 222 and 254 nm for carbamazepine, caffeine, and diethyltoluamide, with a stronger inhibition of 47%–66% at 222 nm. TBA’s impact was minor for the other eight tested OMPs. Second, N2 purging and probe experiments suggested that the inhibition was not caused by dissolved oxygen or self-generation of •OH from OMPs. Third, model compounds with carbonyl groups showed strong inhibition by TBA at 222 and 254 nm. Further tests indicated that the inhibition was potentially related to interactions between alcohol and excited states of OMP. Overall, this work suggested that employing alcohols as quenchers for •OH may overestimate its role in OMP degradation in UV photolysis and UV/AOP processes. Fundamental mechanisms indeed warrant further exploration for the roles of different excited states, wavelength dependence, and structural influence.