Photocatalytic oxidation of sulfamethoxazole in the presence of TiO2: Effect of matrix in aqueous solution on decomposition mechanisms

As water matrix, inorganic anions and natural organic matter (NOM) may impact the removal of pharmaceuticals not only on the kinetics but also on the transformation products formation. In this study, the effects of initial pH, inorganic anions and NOM on the transformation pathways of sulfamethoxazole (SMX) during photocatalytic decomposition were investigated. Three pathways including hydroxylation, hydrolysis and isoxazole ring oxidation were found during SMX photocatalysis. SMX in deionized water was degraded by hydroxylation principally in alkaline situation, while hydrolysis was the main pathway in acidic condition. Almost all the pathways for SMX decomposition were suppressed when anions were added, with the inhibitory effect followed the order of HPO42- > HCO3- > SO42- > Cl-> H2PO4-. The formation of products with larger energy barrier was affected more significantly when anions were added. The products formation was also impacted by the presence of NOM isolates including "Suwannee River Humic Acid" (SRHA), "Suwannee River Fulvic Acid" (SRFA) and "Suwannee River NOM" (SRNOM). The mechanisms of hydroxylation and isoxazole ring oxidation were inhibited by NOM, with the order of SRFA > SRHA > SRNOM, due to their hydroxyl radical scavenging abilities which were related to their aromaticity degree. Most of the hydrolysis products were favored in the presence NOM, and the enhancement effect was in accordance with the sequence of SRNOM > SRFA > SRHA due to the different photoinductive abilities of the NOM isolates.