Sunlight phototransformation of furosemide in the presence of natural photosensitizers: Experimental measures and modelling.

Phototransformation is an important attenuation process for pharmaceuticals in the aquatic environment. However, there are unknowns regarding the kinetics, products, and mechanisms of the sensitized photolysis of many pharmaceuticals induced by natural sensitizers. In this study, we systematically investigated the photochemical behavior and fate of the diuretic furosemide (FUR) under natural sunlight in the presence of nitrate (NO3-) and dissolved organic matter (DOM). Results show that NO3- significantly promoted the photodegradation of FUR through photosensitization, with photolysis rate constant in the presence of 1 mM NO3- being approximately 1.6-fold higher than that of direct photolysis. The photolysis of FUR predominantly generated hydroxylated product, whose generation and degradation were both facilitated by NO3-. DOM exhibited inhibitory effects on NO3--sensitized photolysis of FUR by light screening and radical quenching. The photolysis pathways of FUR in the presence of NO3- included decarboxylation, C-N bond cleavage, dechlorination, hydroxylation, ring opening, dehydrogenation, and desulfonamidation. Most photoproducts were predicted to be less ecotoxic, persistent, and bioaccumulative than FUR. Model predictions with APEX indicate that direct photolysis and reaction with carbonate radical were the primary photochemical processes for FUR dissipation in natural waters. The photolysis half-lives of FUR were significantly affected by dissolved organic carbon and water depth. Results of this study fulfill the knowledge gaps in the photochemical behavior and fate of the diuretic drugs in sunlit surface waters.