Degradation of ibuprofen in a solar-activated, electrically produced hypochlorite system catalyzed by N-TiO2: Its synergistic mechanism, free radical cycling, and microbial effects

To address the low solar activation and high charge carrier complexation rates of TiO 2 catalysts, we modified TiO 2 through nitrogen (N) doping, and combined the resultant N-TiO 2 with free chlorine recycling, photocatalysis, and the electrochemical advanced oxidation process (EAOP) to construct a solar/N-TiO 2 /EAOP system for improved ibuprofen (IBP) degradation. The incorporation of N-TiO 2 significantly enhanced chlorine recycling, compared to that of undoped TiO 2 . Active chlorine (AC) produced by the EAOP was activated by hydrated electrons (e aq – ) on the N-TiO 2 surface to generate hydroxyl radicals (•OH) and chloride ions (Cl – ), which formed AC by repetition of the EAOP via the photoelectric cycle. Surface valence band holes (h vb + ) in N-TiO 2 improved its affinity for hypochlorous acid (HClO) and accelerated its conversion to •OH via the photocycle. The system is consistent with the theory of double-cycling to produce highly oxidizing radicals, with a synergy factor of 4.35, and its degradation efficiency was much higher than that achieved by stacking single processes. The IBP removal rate of the solar/N-TiO 2 /EAOP system was 87 % within 30 min of operation, compared to 15 %, 26 %, and 52 % for the EAOP, solar/N-TiO 2 , and solar/TiO 2 /EAOP systems, respectively. We also investigated the effects of different reaction parameters and anions on the efficiency of the system in degrading IBP, and explored its application in environmental water samples. Furthermore, we examined the effect of effluents treated by the proposed system on microbial communities in these water sources. Our findings provide new insights into IBP pollution treatments and their ecological impacts. • The constructed Solar/ N-TiO₂/EAOPs system eliminates the need for AC dosing and reduces operational risk. • N-TiO₂ significantly enhances the utilization of sunlight and reduces the charge complexation rate, thus enhancing the synergistic efficiency of the double cycle. • Double-cycle mechanism: N-TiO 2 catalyzes the formation of Cl - ions from NaClO, which is re-oxidized to HClO through the electrode, allowing for the efficient use of NaClO and the generation of continuous hydroxyl radicals. • To explore the key parameters affecting the IBP removal efficiency, to further validate the rationality and stability of the dual-cycle mechanism, and to confirm its applicability in real water bodies. • Evaluation of the ecotoxicity of the treated effluent to reveal the environmental safety of the system in natural water discharges.