UV-chlorine combined treatment on composite sunscreen BP-1-TiO2 accelerates toxicity enhancement

Both of chemical sunscreen 2,4-dihydroxyl benzophenone (BP-1) and physical UV-blocker titanium dioxide (TiO 2 ) nanocomposites are frequently found in aquatic environment, their environmental fates and potential risks should be concerned. The toxicity variation of the composite sunscreen BP-1-TiO 2 during UV-chlorine combined treatment was investigated in this study. It was found that the reaction mixtures exhibited dramatically increased acute toxicity after UV-chlorine treatment of BP-1-TiO 2 , implying the formation of toxic products. Totally, 21 transformation products were tentatively identified, including chlorinated/hydroxylated benzophenones (BPs), hydroxylated benzoyl benzoquinones, phenols, and benzoic acids. We proposed the plausible transformation pathways of BP-1, mainly including hydroxylation, oxidation, chlorination, decarboxylation and ring closing via electrophilic substitution, and Norrish I cleavage. Notably, cosmetic-grade TiO 2 (CG-TiO 2 ) nanocomposites promoted BP-1 transformation and toxic products formation during UV-chlorine combined treatment. The present results highlight that the coatings of CG-TiO 2 nanocomposites are not stable during UV-chlorine combined treatment, the naked TiO 2 would act as photocatalyst to promote BP-1 transformation and generate toxic products. These findings provide valuable insights for assessing potential environmental risks of sunscreen products.