Humic acid removal and easy-cleanability using temperature-responsive ZrO₂ tubular membranes grafted with poly(N-isopropylacrylamide) brush chains

New poly(N-isopropylacrylamide) brushes grafted with ZrO₂ (PNIPAAm-g-ZrO₂) composite membranes, which had been prepared in our laboratory, were used for humic acid (HA) removal. We found that the fluxes associated with such membranes, when compared to those obtained from unmodified ZrO₂ membranes, declined slightly at both 25 °C and 35 °C. The PNIPAAm-g-ZrO₂ membrane achieved a high rejection, of 98.0%, at a suitable steady flux of 111.9 L m^-2 h^-1 at 25 °C. This membrane exhibited good anti-fouling properties as well as improved membrane performance during filtration of HA. The important role of pH and Ca²⁺ concentration in HA removal was also investigated. Lower adsorption fouling and a higher rejection were obtained at higher pH levels. The Ca²⁺ ions reduced the electrostatic exclusion and played a cross-linking role between HA and the PNIPAAm-g-ZrO₂ membrane surface. Fouling was severe in the presence of Ca²⁺. These tests led to the development of an environment-friendly membrane cleaning method, by means of temperature-change water elution around LCST of PNIPAAm-brushes. After the alternate temperature-change (25 °C/35 °C) cleaning, a flux recovery of 98.2% was obtained for the PNIPAAm-g-ZrO₂ membrane. Moreover, after four repeated experiments, the anti-fouling and easy-cleaning properties were still maintained. It is implied that PNIPAAm-brushes were firmly " stuck" to the membrane surface, and could not easily be removed by water cleaning or HA filtration. The PNIPAAm-g-ZrO₂ membranes exhibited good stability and great potential for HA removal.