Low band-gap energy photocatalytic membrane based on SrTiO3-Cr and PVDF substrate: BSA protein degradation and separation application

A series of ultrafiltration (UF) membranes based on mixed matrix of chromium-doped strontium titanate (SrTiO3-Cr) photocatalyst and polyvinylidene difluoride (PVDF) were systematically prepared by non-solvent induced phase inversion technique. Porous PVDF membrane and mixed matrix membranes (MMMs) incorporated with 3-10 wt% SrTiO3-Cr were produced in the UF range using polyethylene glycol as a porogen additive. The crystal morphology and electronic property of as-synthesized Cr-doped SrTiO3 nanoparticles were characterized by XRD and UV-Vis DRS. We found that the SrTiO3-Cr nanoparticles are cubic perovskite structure of approximately 30-40 nm in size with a low band gap energy of 2.05 eV. The morphology, surface roughness, hydrophilicity and textural properties of SrTiO3-Cr/PVDF MMMs were comprehensively characterized using field emission-scanning electron microscope with energy-dispersive X-ray spectroscopy, atomic force microscope, sessile drop technique, and capillary flow porometer. The SrTiO3-Cr was observed to be homogeneously dispersed in all the MMMs and provided additional anti-fouling properties from Bovine Serum Albumin (BSA) protein. The membrane UF property and tri-cycling performance were evaluated under three distinct protocols in (1) Dark, (2) constant UVA exposure, and (3) UVA during post-treatment clean only. By increasing the SrTiO3-Cr concentration, the MMMs displayed an improved homogeneity and macropore distribution with enhanced water permeability given by 51-110 LMH bar(-1) pure water flux and 37 to 45 LMH bar(-1) BSA water flux in the Dark. A consistently high BSA rejection of 95% and an improved anti-fouling property under constant UVA irradiation were achieved due to a combination of photo-induced hydrophilic and photocatalytic effect. Also, a significant improvement in UF performance in terms of membrane flux recovery was observed in the 3rd protocol signifying that the effect of protein fouling on membrane filtration was further reduced due to a lower degree of BSA fragment deposition in the membrane pores. The findings in this study dramatically lower the operational constraints (transmembrane pressure, membrane cleaning, use of UVA) in the protein filtration process and offer an innovative membrane material for the research of photo-induced, anti-fouling membranes for protein separation applications.