CuO and TiO₂ particles generated more stable and stronger EPFRs in dark than under UV-irradiation

Environmentally persistent free radicals (EFPRs) have recently attracted a great deal of research attention because of their significant toxicity and ubiquitous occurrence in the environment. The information is still very limited on how to estimate the intensity of EPFRs under ambient circumstances. This study is designed to specifically compare EPFRs generation during catechol degradation in dark and UV light irradiation. CuO and TiO₂ were selected as model metal oxides to coat on silica at 1% CuO has a large electron exchange capacity, which may mediate catechol degradation in dark, while TiO₂ possesses strong photocatalytic property and could accelerate catechol degradation under UV light. Under UV light irradiation, EPFRs were generated very quickly and reached the maximum value in 4 d, which was related to the photocatalytic property of the particle. However, these EPFRs dissipated quickly in 14 d. On the contrary, the intensities of EPFRs generated in dark were 2 times higher, and stabled for over 2 months. Therefore, the environmental impacts of EPFRs in dark may be widespread and long-lasting, which should be monitored more carefully. It should be noted that for CuO-coated silica, a significant amount of EPFRs (20% of the maximum) survived the UV-light irradiation and stabled during the experimental period (45 d). Stronger EPFRs were associated with more abundant dimer structures, suggesting the dimer structures were related to EPFRs formation during catechol degradation. Monitoring the generation of dimer structures in the degradation of organic chemicals may provide useful information to estimate EPFRs generation and risks.