Co-activation of peroxymonosulfate activation with sunlight and tailored catalytic MnFe2O4/MWCNTs membrane to mitigate membrane fouling caused by NOM and synergistic oxidation mechanism analysis

Membrane fouling has been a major factor hindering the development of ultrafiltration membranes. Herein, a novel in situ oxidation system was constructed via introducing MnFe₂O₄/MWCNTs into polyvinylidene fluoride (PVDF) membranes, utilizing sunlight to synergistically activate persulfate (PMS) to mitigate ultrafiltration membrane fouling. The mechanism of mitigating membrane fouling in MnFe₂O₄/MWCNTs-PVDF ultrafiltration membranes was systematically explored under four system (single filtration, single sunlight irradiation, single PMS oxidation and sunlight co-activated PMS). The MnFe₂O₄/MWCNTs-PVDF membranes exhibited different fouling characteristics under the four systems, with the sunlight and MnFe₂O₄/MWCNTs membrane co-activated PMS filtration system showing the highest humic acid (HA) removal efficiency of 90.2 %, as well as the lowest Rr (0.2108 × 10¹²m⁻¹) and Rir (0.4525 × 10¹²m⁻¹). To further evaluate the practicality and effectiveness of the sunlight-MnFe₂O₄/MWCNTs-PMS system, the secondary effluent was selected to verify the treatment effect on natural organic matter (NOM) of actual water. By observing the microscopic morphology of the fouled membrane surface, it was evident that, compared with the other three filtration systems, the filter cake layer on the MnFe₂O₄/MWCNTs membrane surface of the sunlight co-activated PMS system was obviously reduced, and the structure of the cake layer was more loose. It can be concluded that the principle behind the synergistically activated system to alleviate the membrane fouling was to accelerate the mineralization rate of HA molecules, oxidize the HA into a smaller particle size that can pass through the membrane pores. The main reactive oxygen species and HA degradation mechanism in the synergistic activation system were further elucidated by electron paramagnetic resonance (EPR) analysis and density functional theory calculations (DFT). The results indicated that the degradation of HA by the synergistically activated PMS system involved a combination of free radicals (·O₂⁻、SO₄^·− and ·OH) and non-free radicals (¹O₂). Overall, the in-situ oxidation system provided an alternative way to alleviate ultrafiltration membrane fouling.