Sulfamethoxazole Transformation by Heat-Activated Persulfate: Linking Transformation Products Patterns with Carbon and Nitrogen Isotope Fractionation

Sulfamethoxazole (SMX) is a frequently detected antibiotic in groundwater, raising environmental concerns. Persulfate oxidation is used for micropollutant removal. To investigate SMX transformation by persulfate, experiments were conducted using heat-activated persulfate at pH 3, 7, and 10. TP269a (SMX-hydroxylamine) and TP178 were identified as the dominant TPs across the pH levels. The exclusive formation of 4-nitroso-SMX, 4-nitro-SMX, and TP518 at pH 3 highlighted the role of SO₄^•- in attacking the NH₂. At pH 7 and 10, 3A5MI emerged as the dominant TP. Carbon isotopic fractionation (ε_C = -1.3 ± 0.5‰, -1.1 ± 0.4‰, and -1.1 ± 0.3‰ at pH 3, 7, and 10) remained consistent across pH levels, caused by the formation of TP178 involving C-S bond cleavage. An inverse nitrogen isotope fractionation at pH 3 (ε_N = +0.68 ± 0.11‰) was associated with SO₄^•--induced single-electron transfer. Conversely, normal nitrogen isotope fractionation at pH 10 (ε_N = -0.27 ± 0.04‰) was associated with N-H bond cleavage by H abstraction through HO^• and N-S bond cleavage. The inverse nitrogen isotope fractionation at pH 7 indicated that the dominant pathway involved SO₄^•- reactions, accounting for 74%. Overall, the results highlight the potential of CSIA to elucidate SMX oxidation pathways.