Degradation of sulfamethoxazole by ionizing radiation: Identification and characterization of radiolytic products

Abstract Sulfamethoxazole (SMX) is a widely prescribed pharmaceutical compound to treat bacterial infections in both human and animals. As an alternative treatment process for non-degradable pharmaceuticals by conventional water treatment processes, radiolysis using gamma radiation has been applied as one of the radical-based advanced oxidative processes. However, further information was limited with regard to the production mechanism and fate of radiolytic products after treatment. Therefore, the degradation characteristics of SMX using ionizing radiation were investigated in this study. In addition, some radiolytic products of SMX were identified, and a degradation pathway as a result of the radiolysis of SMX was proposed. The radiolytic products were analyzed using liquid chromatography quadrupole time-of-flight mass spectrometry, liquid chromatography tandem mass spectrometry, and ion chromatography. Molecular structures of the radiolytic products were elucidated by the interpretation of MS 2 fragmentation patterns of each product. In total, fifteen products were elucidated as a result of ionizing radiation treatment of aqueous SMX in the range 0.1–5.0 kGy. Hydroxylation, bond-cleavage, and the combined mechanism of cleavage and transformation were proposed as the predominant mechanisms, inducing the various radiolytic products. In particular, based on the comparison of the relative intensity and the quantified concentration using authentic standards, RP270-1 (hydroxylated SMX) and RP172 (sulfanilic acid), RP99 (3-amino-5-methylisoxazole), and RP96 (sulfate) were the most abundant products. Chromatographic profiles of radiolytic products also revealed the change of major products with increasing absorbed doses, from compounds that have high molecular weight (MW), to relatively lower MW. The results of this study lead to an understanding of the role of ionizing radiation on the fate of the parent compound and its degradation products when applied to pharmaceutical pollutants.