Self-Accelerated Photodegradation of 2,4-Dihydroxybenzophenone in Water: Formation of Photoactive Products and Implications for the Transformation of Coexisting Organic Contaminants

Benzophenone (BP) UV filters are a group of contaminants of emerging concern. In this study, we found that 2,4-dihydroxybenzophenone (BP1) was more liable to photodegradation than the other BPs. Particularly, an accelerated degradation of BP1 was observed under UV365nm irradiation. Singlet oxygen (1O2) was identified as the key reactive species leading to its degradation. Generation of 1O2 was mostly ascribed to certain photodegradation products of BP1. These products have an aromatic ketone structure and can be excited to the activated triplet state, giving rise to 1O2. Intriguingly, the reaction between BP1 and 1O2 generated more photochemically reactive products. Thus, the removal of BP1 appeared to be accelerated. A total of four such products, that is, 2-methylenenaphthalen-1(2H)-one, (E)-2-(1-oxonaphthalen-2(1H)-ylidene)-acetic acid, (1-oxo-1H-benzo[c]oxepin-3-ylidene)-acetic acid, and 2-oxo-3-(1-oxo-1H-benzo[c]oxepin-3-ylidene)-propionic acid, were identified. In addition to sensitizing the photodegradation of the parent BP1, these products can also lead to the indirect photodegradation of other organic contaminants, such as isoproturon. To the best of our knowledge, such photochemical activity of BP1 has not been reported. It indicates that the presence of BP1 can influence the transformation of other organics in sunlit surface waters or UV-based water treatment processes, which comprises an important aspect of its environmental behavior and merits further study.