Assessing the photochemical lifetimes of the sulfonamide antibiotic sulfamethoxazole, of significance for sunlit freshwaters

Photochemical modeling was used to evaluate the photodegradation kinetics of sulfamethoxazole (SMX) in sunlit natural freshwater environments, utilizing available data on SMX absorption spectrum, direct photolysis quantum yield, and second-order reaction rate constants with photochemically produced reactive intermediates (PPRIs). The anionic form of SMX is considered here, which prevails in the majority of surface-water conditions (pH > 5.6). SMX would be photodegraded quite rapidly in well-mixed waters under sunny weather, with direct photolysis serving as the predominant pathway in most cases. Among the water parameters, depth and the dissolved organic carbon (DOC) primarily influence the photodegradation of SMX, which would be faster in shallow waters with low DOC. This feature enabled the extension of SMX photochemical lifetime prediction to lakes located within the 60°S-60°N latitude belt, where data on DOC, water depth and sunlight irradiance are available. The fastest photodegradation of SMX (annual average) is expected in the tropical belt, where the combination of high irradiance and low DOC would be most favorable for direct photolysis. With fair weather and good water mixing, typical SMX lifetimes would range from days to some weeks depending on the specific conditions. The lifetimes predicted in shallow waters are consistent with several literature reports of SMX photodegradation kinetics in the laboratory.