Porous inorganic membranes made from sintered ceramic oxides are considered for water purification applications. In particular, inorganic membranes are attracting attention because of their chemical, thermal and mechanical stability. These characteristics imply that ceramic membranes can be subject to large pressure differences, high temperatures and rigorous cleaning procedures, and have operational lifetimes on the order of decades. Inorganic membranes are considered for wastewater treatment, desalination, and recycling of produced water.
In this work, synthesis and properties of mesoporous γ-alumina, titania, tin oxide and gadolinium-doped cerium oxide (CGO) nano-filtration membranes are presented. “Mesoporous” refers to pore sizes of 2…50 nm and “nano-filtration” refers to removal of 1 to 2 nm species. The membranes were synthesized by dip-coating nanoparticle sols onto smooth, macro-porous α-alumina supports followed by thermal consolidation. Synthesis conditions were optimized for increased ion rejection.
Single layer γ-alumina membranes with 800 nm thickness and a typical pore diameter, Øp, of 4 nm were made and studied for aqueous NaCl, CaCl2, AlCl3, NaCl+1%AlCl3, and CaCl2+1%AlCl3 nano-filtration. It was found that a small amount of AlCl3 in the solution can suppress dissolution of the γ-alumina membrane. This leads to a major improvement in the reproducibility of mechanical permeance measurement so that, for the first time, osmotic effects could be unmistakably demonstrated. The nearly 100% rejection values for CaCl2, CaCl2+AlCl3 and AlCl3 solutions indicate that virtually no connected pore defects with Ø > 10 nm are present in the supported membranes.
Mesoporous titania membranes were studied because of their excellent aqueous stability. Thin double-layer membranes of 250 nm thickness were made, and their transport properties were studied with solutions of NaCl, CaCl2, MgCl2, Na2SO4 and CaSO4. It was found that for solutions containing Cl- ions, the maximum ion rejection occurs in acidic solutions. In contrast, titania membranes have minimal rejection at ~pH 5.5 for solutions containing SO42- ions and maximum rejection in basic, pH = 10 environments. Analysis of prolonged water purification measurements confirmed a complete water stability of titania membranes after weeks of aqueous exposure.
The ion rejection ability of other inorganic membranes was examined. Tin oxide nano-particle sols were used to synthesize <100 nm thick membranes that showed 62% rejection of 1 mol/m3 NaCl at pH 4 and 0.518 MPa trans membrane pressure. CGO membranes were prepared by dip-coating an α-Al2O3 support with a suspension of 4 wt% CGO and 1 wt% PVA. Very high and stable aqueous rejection values of 82.4% and 98.7% were found for 1 mol/m3 NaCl and CaCl2 solutions at pH 4.
All membrane structures were consolidated by rapid thermal processing to diminish synthesis time, obtain a finer micro-structure, and to demonstrate the possibility of a cost-effective synthesis process. The flow resistance of the experimental supported membranes was dominated by the macro-porous support resistance. These supports were used because of their well-developed synthesis and smooth surface. More recently, it was demonstrated that a much lower overall resistance could be attained with the same deposition properties by making graded support structures. The bottom carrier layer has ~700 nm pores while the thin top layer with 40…80 nm pores provides a smooth surface for membrane deposition. Application of mesoporous inorganic membranes developed on new multi-layer supports demonstrates viability for large scale water purification.
Great progress has been made in the past decades with the micro-structural optimization of supported inorganic membranes. However, very high ion selectivities are currently only obtained for low concentrations. This is caused by limitations in the membrane surface charge density. High selectivity at high concentrations will be obtained only by increasing the surface density of structurally optimized membranes. Future studies must focus on the preparation of semi-conducting membrane materials such as doped SnO2 and CeO2 or on applying an external charge to the membrane.