Photodegradation of Expectorant Drug Ambroxol in Seawater Environment: Seasonal Impact on a Global Scale

The ocean is a globally important zone for the photochemical transformation of various organic contaminants. Its unique chemical composition, especially the high concentration of halide ions, can significantly alter the photochemical reactivity and fate of these contaminants. Herein, we systematically investigated the effects of marine constituents on the sunlight-driven photolysis of ambroxol (AMB), an expectorant drug extensively used following COVID-19 pandemic, and assessed its spatiotemporal persistence on a global scale. The results revealed that marine matrix significantly promoted AMB photodegradation in a halide-specific manner, where chloride drove such acceleration via photonucleophilic substitution, while bromide exerted an inhibitory effect. Differing from the pathways in pure water, the marine photolysis of AMB was dominated by photonucleophilic substitution, generating the primary chlorinated product with a substantial formation yield of ∼50.5%. Toxicity evaluation suggested that the halogen exchange reaction reduced the toxicity of the photoproducts, whereas other transformation pathways heightened the environmental risks of AMB. Utilizing the global-scale photolysis model GCSOLAR, this study further indicated that near-surface environmental persistence of AMB was strictly governed by geographic latitude and seasonal dynamics, where environmental half-lives remained minimal (1.24–1.83 h) in equatorial zones while rising to 34.77 h in high-latitude regions during winter. These findings highlighted the critical role of the marine matrix and global spatiotemporal variations in collectively reshaping the transformation landscape and intensifying the ecological risks of typical emerging contaminants.