Fabrication of temperature-responsive ZrO2 tubular membranes, grafted with poly (N-isopropylacrylamide) brush chains, for protein removal and easy cleaning

Fouling of membranes by proteins is a major problem in membrane filtration operations. To reduce protein-fouling, an easy-clean surface was designed by grafting temperature-sensitive poly (N-isopropylacrylamide) (PNIPAAm) brushes onto the surface of zirconium oxide (ZrO2) membranes via chemical grafting. The grafting density of PNIPAAm brushes and polymer thickness were controlled by changing the NIPAM concentration. The water fluxes of the PNIPAAm-grafted-ZrO2 (PNIPAAm-g-ZrO2) membranes were highly dependent on the solution temperature, and increased at higher solution temperature. An increase in the water fluxes was evident at approximately 32–34°C. Compared with the ZrO2 membranes, the fluxes for the PNIPAAm-g-ZrO2 membrane declined more gradually at both 25°C and 35°C, and a high initial rejection of 89.4%, along with a suitable steady flux of 66.2Lm⁻²h⁻¹, were achieved at 25°C. The PNIPAAm-g-ZrO2 membrane exhibited good anti-fouling properties and improved membrane performance during the filtration of bovine serum albumin (BSA). In particular, due to the reversible volume phase transition of PNIPAAm around the lower critical solution temperature (LCST), an environmentally-friendly membrane cleaning method by means of temperature-change water elution around the LCST of the PNIPAAm brushes was proposed. Following the alternate temperature-change (35°C/25°C) cleaning, a flux recovery of about 80% was obtained for the PNIPAAm-g-ZrO2 membrane (the flux recovery of the ZrO2 membrane was only about 14% at 25°C). Moreover, when the experiments were repeated four times, the anti-fouling and easy-cleaning properties were maintained. The hydrophilicity and the rapid stretching of PNIPAAm chains were considered responsible for loosening and shaking off the adsorbed BSA on the graft membrane surface.