Natural sunlight-driven phototransformation of the expectorant drug ambroxol in freshwaters

Ambroxol (AMB), a widely used expectorant drug, has been ubiquitously detected in aquatic environments due to its limited metabolism in the human body. Herein, we systematically investigated the photodegradation of AMB in waters upon natural sunlight irradiation. AMB has a pK_a value of 8.42, as determined by spectroscopic titration. A dibromoaniline moiety was identified as the primary chromophore responsible for its light absorption. AMB underwent relatively fast photodegradation, with a near-surface half-life (t_1/2) of 2.1-3.0 h at noon on a clear midsummer day at 32°N. The quantum yield (Φ) of direct sunlight photolysis was actinometrically determined to be 0.015 in a neutral aqueous solution. Natural organic matter (NOM) played dual roles in the sunlight-induced photolysis of AMB, serving as a light screener and photosensitizer. Product analysis suggested that the direct photolysis of AMB proceeded mainly through photoionization and photonucleophilic substitution. Kinetic modeling predicted half-lives ranging from 2 to 4 days in well-mixed lake water columns (depth = 3 m, [DOC] = 5 mg C L^-1). These findings highlight the significant role of direct photolysis in determining AMB's environmental fate in sunlit surface waters and provide insights into the photochemical behavior of pharmaceuticals in aquatic environments.