Physical UV-blocker TiO2 nanocomposites elevated toxicity of chemical sunscreen BP-1 under UV irradiation

Physical UV-blocker TiO2 nanocomposites and chemical sunscreen benzophenones (BPs) have been frequently detected in the aquatic environment, their combined fates and potential risk should be concerned. In this study, the typical cosmetic grade UV-blocker TiO2 nanocomposites (labeled as CG-TiO2) and 2,4-dihydroxy benzophenone (BP-1) were targeted, and the effects of CG-TiO2 on the transformation of BP-1 during UV irradiation were explored. It was found that CG-TiO2 destruction did not form toxic products, while they could promote the chemical reactions of BP-1 that will lead to highly toxic products during UV irradiation. More importantly, UV irradiation could break the organic coatings on CG-TiO2, the broken CG-TiO2 exhibited stronger catalytic activity on BP-1 and form highly toxic products during UV irradiation. Total of 24 transformation products were tentatively identified, including hydroxyl BPs, hydroxyl dibenzo-p-dioxins, hydroxyl benzoyl benzoquinones, etc. The main transforamtion mechanisms of BP-1 included hydroxylation, radical coupling, oxidation, decarboxylation, Norrish type I reaction, and ring closure through electrophilic substitution. This study highlighted that physical UV-blocker CG-TiO2 promoted the transformation of chemical sunscreen BP-1 and generated higher toxicity during UV irradiation. It was expected that these results would provide scientific data for evaluating the fates and potential risk of sunscreens in the environment.