Effect of firing temperature on the water permeability of SiO2–ZrO2 membranes for nanofiltration

Abstract

SiO2–ZrO2 membranes were successfully prepared by coating SiO2–ZrO2 (molar ratio 5/5) sols on cylindrical α-alumina porous supports with average pore sizes of 2.1, 2.9 and 3.6μm followed by firing at 550°C. The pore sizes of the SiO2–ZrO2 membranes, which were evaluated by nanopermporometry using hexane, were 1.20 and 0.65nm after coating with SiO2–ZrO2 sols of 35 and 19nm in diameter, respectively. The membrane pore sizes were not affected by the pores of the supports, but, instead, were controlled by the colloidal sizes of the SiO2–ZrO2 sols that made up the top layer. The average pore sizes of SiO2–ZrO2 membranes fired at 200, 300, 400 and 550°C increased slightly from 0.60 to 0.70nm with an increase in the firing temperature while water permeability (Lp) tended to decrease with increases in the firing temperature that ranged from (3.3–0.8)×10−12m3/(m2sPa). The decreased water permeability was ascribed to chemical and physical changes by firing temperature such as hydrophilicity/hydrophobicity, porosity, etc. The water permeabilities of SiO2–ZrO2 membranes showed stable flux due to the addition of zirconia into the silica sol, showing improved stability in water. Nanofiltration performance was evaluated using aqueous solutions and showed molecular weight cut-offs ranging from 200 to 350.