Visible-light-driven photocatalytic degradation of diclofenac in water by nanoparticulate Cu2ZnSnX4 (X = S, Se) kesterites with kinetics, transformation products and ecotoxicity.

Diclofenac (DCF) is a prevalent aquatic contaminant poorly removed by conventional wastewater treatment plants. This study investigates the photocatalytic degradation of DCF under simulated sunlight using nanoparticulate kesterite semiconductors Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe), focusing on efficiency, stability, degradation pathways, and environmental implications. CZTSe exhibited superior photocatalytic performance, achieving 83% DCF removal within 60 min with a pseudo-first-order rate constant of 3.0 × 10- 2 min- 1, whereas CZTS reached 68% removal with 1.7 × 10- 2 min- 1. Both photocatalysts maintained stable activity after four reuse cycles. Total organic carbon analysis confirmed progressive DCF mineralization, i.e., the conversion of organic carbon into CO2 and inorganic species. HPLC-MS analysis identified six transformation products generated by DCF degradation, corresponding to the conversion of the parent molecule into intermediate compounds. In silico ecotoxicity assessment using predictive computational models indicated reduced bioaccumulation potential of DCF after treatment; however, some transformation products exhibited higher environmental persistence and retained or increased predicted toxicity. Reactive species experiments demonstrated that superoxide radicals (•O2⁻) were the main reactive species, followed by hydroxyl radicals (•OH). These findings demonstrate that CZTS and, particularly, CZTSe are efficient and reusable visible-light photocatalysts for DCF removal while providing new insights into degradation pathways, mineralization, reactive oxygen species, and transformation-product ecotoxicity, underscoring the importance of assessing degradation by-products in advanced oxidation processes.