SiO2-ZrO2 nanofiltration membranes of different Si/Zr molar ratios: Stability in hot water and acid/alkaline solutions

SiO2-ZrO2 nanofiltration membranes were fabricated by coating different molar ratios of SiO2, SiO2-ZrO2 (9/1, 7/3, 5/5, 3/7), and ZrO2 sols onto alpha-alumina porous tubes and firing at 200 and 550 degrees C. The SiO2 and SiO2-ZrO2 (9/1, 7/3, 5/5) membranes fired at 200 and 550 degrees C showed pore diameters ranging from 0.65 to 0.80 nm, while SiO2-ZrO2 (3/7) and ZrO2 membranes fired at 550 degrees C showed larger pores than those fired at 200 degrees C due to the formation of crystalline structures in the ZrO2. SiO2-ZrO2 membranes with a zirconia content larger than 50 mol% showed high hydrothermal stability in hot water (90 degrees C). After treating SiO2-ZrO2 (5/5) membranes in hot water, the water permeability (L-p) increased dramatically while the rejection was unchanged. On the other hand, the L-p for SiO2-ZrO2 (3/7) membranes decreased while the rejection increased. This can be ascribed to the balance between the dissolution of Si and the generation of OH groups, which amounts to pore sizes and hydrophilicity. Moreover, SiO2-ZrO2 (5/5, 3/7) membranes showed stable water permeability and molecular weight cut-off values for as long as 4 weeks at pH values of 2 and 12, confirming a high level of chemical stability in strongly acid and alkaline solutions.