Phototransformation study of the antidepressant paroxetine in surface waters

In this work, the photochemistry of the antidepressant paroxetine (PXT) and its photochemical fate in surface waters were investigated. The direct photolysis quantum yield, as well as the second-order rate constants of the reactions between PXT and the photochemically produced reactive intermediates (i.e., HO•, 3CDOM* and 1O2) were assessed with steady-state irradiation experiments, while the reaction rate constant with CO3•− was known from the literature. Using these results, the PXT photochemical fate (i.e., pseudo-first order photodegradation rate constant and half-life time) was modelled in a surface-water scenario by varying the chemical composition of water and its depth. Nine transformation products were identified, formed upon PXT direct and indirect photolysis, and a photodegradation pathway was proposed that is initiated by photohydrolysis of PXT to 4-(4-fluorophenyl)-3-(hydroxymethyl)piperidine. The results showed that PXT reacts with HO• radicals approaching a diffusion-controlled kinetics, while the direct photolysis quantum yield is ΦPXT = (4.03 ± 0.25) × 10−2. Direct photolysis, together with reactions with HO• and CO3•− radicals would be the main photodegradation pathways for PXT in surface waters. Reaction with CO3•− is particularly important for low amounts of dissolved organic matter (DOM), high pH and high inorganic carbon, which are for instance typical of surface waters in Slovenia. Finally, PXT reaction with 3CDOM* is little important because it is significantly inhibited in the presence of antioxidant compounds, which reduce the partially oxidized intermediates back to the parent PXT.