Revealing the crucial role of Carbon-Centered radicals in UV/PAA process for trace amounts of organic contaminants removal.

[Display omitted] • OH and CH 3 C(O)OO were identified as the main reactive species in UV/PAA process. • CH 3 C(O)OO exhibited lower reaction rates but higher concentration compared to OH. • RAF was proposed as the main initial reaction mechanism in UV/PAA process. • CH 3 C(O)OO possessed higher selectivity than OH towards TrOCs. Peracetic acid (PAA) is a promising peroxyacid to combine with ultraviolet (UV) to eliminate trace amounts of organic contaminants (TrOCs), with growing importance in water treatment. In spite of a large number of research on UV/PAA process, the underlying mechanism still require further investigation, due to the difficulties in analyzing the carbon-centered radicals. Here we demonstrated the crucial roles of the carbon-centered radicals (namely CH 3 C(O)OO), via electron paramagnetic resonance (EPR) and kinetic models, with concentration of 4 orders magnitude higher than that of OH. Then, the theoretical calculation using density functional theory (DFT) showed that CH 3 C(O)OO and OH tended to attack phenol mostly through the route of radical adduct formation. The generated CH 3 C(O)OO and OH adducts may undergo further reactions to produce catechol and hydroquinone. Further, the environmental applicability of UV/PAA process for TrOC abatement was evaluated, including the effects of pH, PAA dosage, water constituents, as well as the variation of acute toxicity. Different from the OH-dominated process, kinetic analysis revealed that CH 3 C(O)OO was almost immune to the presence of water constituents, leading to the outperformance of UV/PAA process in actual water samples. Our present study could inform the application of UV/PAA process in water and wastewater treatment. [ABSTRACT FROM AUTHOR]