Preparation of high-flux γ-alumina nanofiltration membranes by using a modified sol-gel method

Membrane technology has become a low-cost and high-efficiency separation technology for industrial processes over the past decennia. The colloidal route is an environmentally benign process for the preparation of ceramic membranes by using water as solvent. A stable, colloidal boehmite sol with an average particle size of 16 nm was prepared from aluminum tri-sec-butoxide using water as a solvent in the pH range of 3-4. Acetic acid was used as the peptizing agent instead of nitric acid. The stable sol was used to fabricate γ-alumina nanofiltration membranes on α-alumina microfiltration substrates by a dip-coating approach. The effects of the peptizing agents on the properties of the sols and the membranes were investigated. γ-alumina nanofiltration membranes with an average membrane thickness of 1.8 μm were obtained after sintered at 600 °C for 2 h. The molecular weight cut-off of the γ-alumina nanofiltration membranes, as obtained by filtration of an aqueous PEG solution, was approximately 1 kDa. This membrane also showed a typical retention for monovalent ions and divalent ions. Additionally, the nanofiltration membranes exhibited a relatively high pure-water flux (>20 L/(m² h bar)), which is four times higher than that of the γ-alumina nanofiltration membranes reported in the literature. The contribution ratios of different microstructure factors on pure-water flux were calculated. The change in tortuosity with a contribution ratio of 75.6% was the main factor for the improvement of pure-water flux.