Unveiling the Activity Difference Cause and Ring-Opening Reaction Routes of Typical Radicals Induced Degradation of Toluene

This study employs five UV-AOPs (PMS, PDS, H2O2, NaClO and NaClO2) to produce radicals (•OH, SO4•-, ClO•, O2•- and 1O2) and further comparatively studies their activity sequence and activity difference cause in toluene degradation. The toluene mineralization efficiency as a descending order is 73% (UV-PMS) > 71% (UV-PDS) > 70% (acidified-UV-NaClO) > 55% (UV-H2O2) > 36% (UV-NaClO) > 35% (UV-NaClO2); that of conversion efficiency is 99% (acidified-UV-NaClO) > 95% (UV-PMS) > 90% (UV-PDS) > 74% (UV-H2O2) > 44% (UV-NaClO) > 41% (UV-NaClO2). Acidic pretreatment significantly boosts the reactivity of UV-NaClO. ESR combined with radical quenching tests reveals the radicals' generation and evolution, and their contribution rates to toluene conversion, i.e. ClO• > SO4•- > O2•- > 1O2 > •OH. Theoretical calculations further unveil the ring-opening reaction routes and the nature of the activity difference of different radicals. The minimum energy required for ring-opening reaction is 116.77, 150.63, 168.29 and 191.92 kJ/mol with respect to ClO•, SO4•-, 1O2 and •OH, and finding that the ClO•-HO• pair is the best for toluene mineralization. The difficulty for eliminating typical VOCs by using UV-AOPs method is determined as toluene > chlorobenzene > benzene > ethyl acetate.