Photoinduced transformation pathways of the sulfonamide antibiotic sulfamethoxazole, relevant to sunlit surface waters

Here we report that sulfamethoxazole (SMZ) is expected to undergo photodegradation in sunlit surface waters, due to direct photolysis and indirect photochemistry. The direct photolysis of SMZ is triggered by absorption of UV-B radiation and is mostly relevant to shallow water bodies, where a non-negligible part of the water column is illuminated by short-wavelength radiation. In deeper water layers, where the UV-B intensity is much lower, indirect photochemistry processes could take on importance. In particular, upon irradiation of natural water samples, we got evidence that triplet-sensitized degradation of SMZ might be an important or even the main reaction pathway in brackish-water environments, in the presence of elevated concentrations of humic substances. The direct UV-B photolysis of SMZ would produce transformation products that are comparably toxic or less toxic than SMZ itself, thereby resulting in overall decontamination. In contrast, longer-wavelength (λ > 340 nm) irradiation of SMZ in natural water matrices could produce a potentially toxic compound.