Generation of hydroxyl radicals during photodegradation of chloroacetic acids by 254 nm ultraviolet: A special degradation process revealed by a holistic radical determination methodology

Upon ultraviolet (UV) irradiation, aqueous contaminants may undergo direct and/or indirect photolysis. Direct photolysis refers to transformation of contaminants by UV photon, and indirect photolysis refers to degradation of contaminants by UV-induced reactive species in the presence of photosensitizers. Because hydroxyl radical (•OH) was unexpectedly observed during chloroacetic acids photolysis without using photosensitizer, a question arises regarding whether direct photolysis-induced indirect photolysis (DPIP) was present and how it originated and evolved along the process. To answer these questions, this study employed multiple different yet complementary •OH detection approaches (i.e., probe, scavenger, electron paramagnetic resonance, and hydroxylation products) to prove the presence and role of •OH. Given that hydrogen peroxide (H₂O₂) was produced only in oxygenated water but not in deoxygenated water, we revealed that •OH was mainly generated by reduced oxygen. Meanwhile, several photolysis products like formate, glycolic acid, and glyoxylic acid were able to yield H₂O₂ too, suggesting that they can all trigger formation of •OH under 254 nm UV. In addition to evidences of DPIP phenomenon, this study is also novel in demonstrating a holistic methodology to prove and identify the presence and sources of radicals, which might help enhance understandings of UV processes.