Direct Photolysis Determines the Environmental Fate of Furosemide and Its Metabolite Saluamine in Sunlit Surface Waters

Furosemide (FUR) and its metabolite saluamine (SLA) are emerging contaminants that undergo rapid phototransformation in sunlit surface waters. The chloroaniline moiety of FUR serves as a chromophore responsible for the absorption of natural sunlight. The quantum yields for sunlight photolysis of FUR and SLA were determined to be 2.08 × 10^–2 and 2.44 × 10^–2 mol E^–1, respectively. Under midspring noon sunlight, their near-surface half-lives were approximately 17.3–25.3 and 27.3–40.0 min, respectively, indicating nonpersistence in photic surface waters. The phototransformation of FUR occurs primarily through photonucleophilic substitution, generating a hydroxyl-dechlorination product. No significant difference in the photolysis rate constants was observed at environmentally relevant pH. The presence of dissolved organic matter suppressed the direct photolysis of FUR by competing to absorb photons. Kinetic modeling revealed that FUR and SLA accumulate in the dark hypolimnion during lake stratification, but exhibit half-lives of merely 266–554 and 450–981 min within 3 m depths under well-mixed conditions. These findings underscore the importance of direct photolysis in determining the environmental fate of FUR and SLA in sunlit surface waters or well-mixed photic zones.