Characterization of Sulfamethoxazole Direct Photodegradation through Multi-Element Compound-Specific Isotope Analysis

The direct photodegradation of sulfamethoxazole (SMX) represents a significant dissipation process in wetlands. However, distinguishing photodegradation from concurrent processes such as microbial and plant degradation in these environments presents a challenge. Therefore, our objective was to employ novel isotope concepts to characterize and differenciate the specific mechanisms involved in photodegradation processes. The GC-IRMS method developed for SMX includes carbon, hydrogen, and nitrogen isotope analysis, while the GC-MC-ICP-MS method specifically caters to sulfur isotope analysis. SMX exhibits varying protonation states at different pH levels, significantly affecting its degradation kinetics. We conducted direct photodegradation of SMX in simulated sunlight (>280nm) at pH 3 and pH 7. Degradation was faster at pH 3 than at pH 7. We observed normal carbon and sulfur isotope fractionation, yielding carbon isotope fractionation values (ε_C) of -1.9 ± 0.2 at pH 7 and -2.7 ± 0.4 at pH 3. The sulfur isotope fractionations (ε_34S) were -3.7 ± 0.5 at pH 7 and -6.3 ± 0.5 at pH 3, while ε_33S values were -6.4 ± 1.2 at pH 7 and -7.5 ± 1.1 at pH 3. In contrast, an inverse nitrogen isotope fractionation was observed, with ε_N = 3.0 ± 0.2 at pH 7 and 3.6 ± 0.1 at pH 3. These results support the idea of an involvement of carbon, nitrogen, and sulfur in the bond cleavage during the rate-limiting step. However, insignificant changes in the hydrogen isotopic compositions of SMX during degradation suggest that either hydrogen was not significantly involved in the bond cleavage or the transformation realted to the hydrogen bond cleavage played a minor role in the overall degradation process. Overall, the isotope data highlighted distinct transformation pathways during direct photodegradation at different pH levels. At pH 7, the dominant transformation products were sulfonilic acid, 3-amino-5-methylisoxazole (3A5MI) from N-S bond cleavage, 5-methylisoxazol-3-yl)sulfamate from C-S bond cleavage, and sulfanilamide from C-N bond cleavage, which aligned with the observed isotope fractionation data. Conversely, at pH 3, different dominant transformation products, including sulfonilic acid, 3A5MI, N4-hydroxylation of sulfanilamide and SMX isomerization, suggest that transformation pathways differed from those observed at pH 7. Altogether, the specific isotope fractionation signatures derived from multi-element CSIA for direct photodegradation of SMX represent a unique reference enabling future comparisons with other degradation pathways.