Improvement of sulfamethoxazole (4-amino-N-(5-methylisoxazol-3-yl)-benzenesulfonamide—SMX) biodegradability using a modified Fenton's reaction has been studied. The modification consists of replacing hydrogen peroxide with atmospheric air and adding copper sulphate as a reaction promoter. Two series of experiments were carried out. The first (Series 1) was conducted using only the catalysts with aeration. In the second series (Series 2), cycles of UVA radiation and aeration were used. During UVA radiation, the removal of sulfamethoxazole proceeds less rapidly than in only aerated solution. After 1.5 h of these two processes, the SMX degradation was 23% in Series 2 and 59% in Series 1. The opposite trend was observed for mineralization and the removal of DOC was about 5% higher in Series 2 than in Series 1. The FTIR spectra of the extracts of reaction products yielded by four organic solvents of varying polarity revealed a wide diversity of functional groups in the post-reaction mixture in comparison to the extracts from sulfamethoxazole solution. Based on FTIR analysis, several oxidation products of sulfamethoxazole are proposed. Apparently, hydroxyl radicals initially attack sulphonamide bonds, resulting in the formation of sulfanilic acid and 3-amino-5-methylisoxazole. Irrespective of the reference organism used in toxicity tests, the post-reaction mixture in the Series 2 was more toxic than the post-reaction mixture in Series 1. In contrast, the biodegradability calculated as BOD5/DOC ratio, was higher for post-reaction mixture 2 and amounted to 0.43.

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