A theoretical study on toluene oxidization by OH radical

Abstract Toluene, a critical constituent of Volatile Organic Compounds (VOCs), poses significant adverse effects on human health and the environment. The hydroxyl radical (•OH) is an efficient reactive species that actually carries out the oxidation in Advanced Oxidation Processes (AOPs) for VOCs removal. In this study, we employed computational quantum mechanical methods and experiments to systematically investigate the process of toluene degradation by •OH. Theoretical studies suggest that •OH may oxidize toluene via three possible reaction pathways: formation of phenol after hydrogen abstraction of the phenyl ring and capture of •OH, addition of •OH to the phenyl ring followed by ring-opening reaction, and side-chain oxidation of toluene to benzoic acid followed by addition of •OH and ring-opening reaction. The latter two oxidation pathways enable toluene's ring-opening via •OH addition, which significantly reduces the energy barrier of the ring-opening process. Therefore, we speculate toluene could be efficiently degraded via these two pathways. The computational results align well with the experimental outcomes derived from GC-MS characterization of intermediates in the post-reaction solution. Experimental results indicate that the predominant free radical species in the UV-H2O2 system are •OH and 1O2, with •OH being the principal active species in the degradation of toluene.