Overlooked Pathway of UV Filter Degradation in the UV/H2O2: The Important Role of Triplet State UV Filter

The large production and usage of UV filters resulted in pervasive contamination in the aquatic environment. UV/H2O2 is a widely used advanced oxidation process (AOP) to eliminate contaminants including UV filters relying on the generated HO center dot. In this study, the degradation of UV filters in the UV/H2O2 AOP was investigated by using benzophenone (BP) as a representative. A previously overlooked pathway for BP degradation and HO center dot formation was newly identified. The triplet state species generated from BP photosensitization (i.e., 3BP*) was found to react with H2O2 to produce HO center dot, and this pathway contributed to 31% of BP degradation. The second-order rate constant of 3BP* with H2O2 was determined to be 8.7(+/- 1.0) x 107 M-1 s-1 at pH 7.0 by using a laser flash photolysis system. 3BP* acted as a reductant, and it was transformed into a radical cation (BP center dot+). The further hydrolysis of BP center dot+ produced hydroxylated BPs as products. This pathway can be barely influenced by pH and inorganic ions in the real water matrix. This work not only reported an unrecognized pathway for pollutant degradation in the UV/H2O2 AOP but also inspired ideas to develop novel technologies to abate pollutants by taking advantage of their triplet states.