Superiority of solar Fenton oxidation over TiO₂ photocatalysis for the degradation of trimethoprim in secondary treated effluents

The overall aim of this work was to examine the degradation of trimethoprim (TMP), which is an antibacterial agent, during the application of two advanced oxidation process (AOP) systems in secondary treated domestic effluents. The homogeneous solar Fenton process (hv/Fe²⁺/H₂O₂) and heterogeneous photocatalysis with titanium dioxide (TiO₂) suspensions were tested. It was found that the degradation of TMP depends on several parameters such as the amount of iron salt and H₂O₂, concentration of TiO₂, pH of solution, solar irradiation, temperature and initial substrate concentration. The optimum dosages of Fe²⁺ and H₂O₂ for homogeneous ([Fe²⁺] = 5 mg L⁻¹, [H₂O₂] = 3.062 mmol L⁻¹) and TiO₂ ([TiO₂] = 3 g L⁻¹) for heterogeneous photocatalysis were established. The study indicated that the degradation of TMP during the solar Fenton process is described by a pseudo-first-order reaction and the substrate degradation during the heterogeneous photocatalysis by the Langmuir–Hinshelwood kinetics. The toxicity of the treated samples was evaluated using a Daphnia magna bioassay and was finally decreased by both processes. The results indicated that solar Fenton is more effective than the solar TiO₂ process, yielding complete degradation of the examined substrate within 30 min of illumination and dissolved organic carbon (DOC) reduction of about 44% whereas the respective values for the TiO₂ process were ∼70% degradation of TMP within 120 min of treatment and 13% DOC removal.