Preparation and nanofiltration properties of thin supported meso-porous ceria membranes

Abstract Thin film meso-porous ceria membranes were prepared by colloidal deposition of ceria precursor nanoparticles on ~2 mm thick macro-porous α-Al2O3 supports, followed by rapid thermal processing at 600 °C or conventional thermal processing at 500 °C. Stable ceria precursor nanoparticles with a diameter of 4–6 nm were made by ultrasound-assisted precipitation and stabilization with bicine. The ceria precursor concentration, poly-vinyl alcohol concentration in the deposition dispersion, and the coating times were varied to obtain the optimum membrane deposition. The effect was investigated of deposition conditions on membrane thickness, water permeation and salt rejection at ambient circumstances. Meso-porous ceria membranes made on monolithic macro-porous α-Al2O3 supports with a bulk pore size of 100 nm and a porosity of 30% exhibited high salt rejection for aqueous solutions with low salt concentrations. For instance at a pH of 4 and a trans-membrane pressure of 0.68 MPa, a ceria membrane had stable maximum rejections of 83% and >95% for 1 mol/m3 NaCl and CaCl2, respectively. At a concentration of 1 mol/m3, >81% Na2SO4 was rejected at a pH of 10. These salt-rejections are caused by surface charging of the porous ceria membrane structure by chemi-sorption of H+, Ca2+ and SO42−. Ceria membrane structures with just monolithic α-Al2O3 supports had a volumetric water permeance of 5.9 L/(m2 h bar). To increase the overall permeance, a ceria membrane was deposited onto a ~10 μm thick macroporous α-Al2O3 layer, in turn supported by a 2 mm thick and more permeable macroporous α-Al2O3 support. With this three-layer structure a salt rejection of 73% was obtained for 1 mol/m3 NaCl at pH = 4 and 25 °C with an overall volumetric water permeance of 43 L/(m2 h bar). During the experiments no obvious changes in transport properties occurred while the membranes were also fully stable during 6 weeks exposure to ultrapure water at 60 °C. This indicates a complete stability in aqueous solutions at room temperature and moderate pH.