Oxidation of acetylsalicylic acid in water by UV/O3 process: Removal, byproduct analysis, and investigation of degradation mechanism and pathway

In this study, the removal of acetylsalicylic acid (ASA) by the UV/O3 process was investigated. The byproducts were identified, the factors responsible for degradation were revealed, the initial reaction positions were explored and a possible degradation pathway was deduced. The results showed that the UV/O3 process exhibited higher efficiency toward ASA mineralization than the ozonation process. The optimal pH value for ASA mineralization was determined to be 10.0. The inhibition function of t-butanol addition demonstrated that HO• significantly contributed to ASA degradation via the UV/O3 process. Eleven types of byproducts of the UV/O3 process were detected, including five aromatic byproducts and six chain byproducts. The analysis of the transformation trends of all byproducts revealed that the high mineralization achieved through the UV/O3 process was attributed to the higher transformation rates of salicylic acid (SA). Frontier electron density analysis indicated that through the UV/O3 process, SA was mainly produced via the attack of the benzene ring by HO•, differing from the oxidation of the ester group by O3 molecules. Finally, a possible reaction pathway was proposed: ASA was firstly transformed to SA through the hydrolysis reaction or the hydroxyl substitution reaction on the benzene ring. SA was further oxidized, and possible reactions involved the hydroxylation and decarboxylation of the benzene ring. Finally, the benzene ring was opened, and the formed chain byproducts were further oxidized and mineralized.