Implanting titanium-oxo clusters into PVDF membrane matrix for photocatalytic self-cleaning in a continuous flow-through process

The photocatalytic ultrafiltration membranes have been demonstrated to be efficient for the treatment of various wastewaters under mild conditions at low cost, but it remains challenging to maintain their high efficiency due to irreversible membrane fouling, low mass transfer efficiency in the photocatalyst layer, and low stability of the membrane to ultraviolet (UV) radiation. To this end, negatively-charged amphiphilic polyvinylidene fluoride (PVDF) ultrafiltration membranes were prepared, and polyoxotitanium clusters (PTCs) with broad visible light response were incorporated into them to develop a self-cleaning membrane. High resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and corresponding energy dispersive X-ray spectrometry (EDS) verify that the sub-nanometer PTCs are uniformly implanted into the membrane pore. The hydraulic test shows that the pores are not blocked by the implanted photocatalysts. The optimized composite photocatalytic membrane (PM3) benefits from the electrostatic enrichment of the basement membrane and the efficient mass transfer in a nanoscale confined channel. This allows the membrane to self-clean by degrading dyes under visible light irradiation. Compared with the original membrane, PM3 could maintain high permeate flux and dye rejection rate with negative fouling ratio during 5 cycles. This design enabled efficient removal of Rhodamine B (RhB, > 85%) during 200 min of continuous separation, whereas it could only maintain less than 50 min on the original membrane. This work provides a common and facile process to develop photocatalytic ultrafiltration membranes with catalysts confined in nanoscale channels for efficient wastewater treatment.