A surface wettability-driven and photo-catalytic self-cleaning TiO₂/Al₂O₃ ceramic membrane for oily wastewater remediation

Membrane fouling and flux deterioration during long-term filtration present major challenges in the membrane-based oil-water emulsion separation. In this work a multifunctional ceramic membrane with water-passing, oil-repellent and photocatalytic self-cleaning properties was fabricated to mitigate this problem. The membrane was fabricated by drop-casting nanostructured titanium oxide (TiO₂) coating onto a porous alumina (Al₂O₃) surface followed by calcination process. Structural, morphological, and compositional analyses using XRD, SEM, EDX, and elemental mapping confirmed the formation of a thin uniform layer of TiO₂ on Al₂O₃ without altering the bulk crystal structure or blocking membrane pores. The characterization of surface wettability of nano-TiO₂-coated Al₂O₃ membrane demonstrated superhydrophilicity and superoleophilicity in air, along with underwater superoleophobicity. This unique surface wettability of the nano-TiO₂-coated Al₂O₃ membrane is attributed to the high surface energy of TiO₂, and the hierarchical nano/microstructure surface roughness. The fabricated nano-TiO₂-coated Al₂O₃ membrane was tested for its performance on oil-in-water emulsion separation in the cross-flow filtration system. The membrane showed a nearly linear increase in the permeate flux with increasing applied pressure, while maintaining >99% separation efficiency that was constant in the 0.5–2 bar pressure region. The long-term stability of the membrane was also examined by the continuous operation for 840 min, and it was found that permeate flux declined approximately to ∼70% of the initial value due to organic (oil) fouling, whereas the separation efficiency remained unaffected. Importantly, by evoking the inherent photocatalytic self-cleaning property of TiO₂, by subjecting it to UV irradiation for 45 min, the accumulated organic foulants/pollutants were effectively removed and ∼90% of the initial flux after UV-assisted photocatalytic cleaning was achieved.