Identification and ecotoxicity of the diclofenac transformation products formed by photolytic and photocatalytic processes

Several studies were carried out to monitor and reduce the presence of diclofenac in the environment. Some of them reported an abatement of the drug of only 20-40%, whereas others based on advanced oxidation processes or photocatalytic methods mediated by TiO₂ often highlighted high energy cost requirements and the formation of byproducts whose toxicity was higher than that of diclofenac. This study concerns both the photolytic and the photocatalytic degradation of diclofenac in aqueous media to evaluate its partial or total elimination and the formation of the transformation products, whose toxicity has been investigated. For this purpose, an ultra-high-performance liquid chromatography coupled with a high-resolution quadrupole time-of-flight-mass spectrometry method was used to elucidate the transformation products following a non-targeted approach based on mass data-independent acquisition. Twelve different transformation products were identified, two of them (TP1 and TP4) for the first time, and accordingly elucidated. Based on the accurate tandem mass spectrometry information acquired, a photodegradation mechanism was proposed. A toxicity assessment of the photoproducts was performed by in silico method. Although UV-vis photolysis degradation alone can abate the diclofenac after 120 min of irradiation, the formed transformation products persist at the end of the degradation, and they are mainly carbazole derivatives with comparable toxicity and similar persistence in the environment to that of diclofenac. On the contrary, the use of quaternary chalcogenide nanoparticles (Cu₂ZnSnS₄) is a successfully promising process for the drug photodegradation, resulting in the elimination of diclofenac and the formation of less toxic products than diclofenac at the end of degradation.