Hydrophobic cordierite-based capillary membranes applied to desalination

Abstract

Hydrophobic ceramic membranes are widely used in water treatment, increasingly in desalination using membrane distillation process. Indeed, due to their ability to perform in harsh pH, temperature, and pressure conditions, they may replace hydrophobic polymer membranes. Many examples exist in the literature using porous ceramic membranes in a tubular configuration; however, a few of them reports on capillary porous membranes that may be further applied to desalination by membrane distillation. The development of membrane supports based on low cost powders instead of more expensive alumina, titania or other oxides will help decrease the production cost of porous capillary membranes. Novel cordierite capillary membrane supports were fabricated by extrusion using various cost efficient and bio-sourced pore-forming agents such as corn starch, maize flour, and rice husk. The capillary supports were further coated with one to three filtration layers, namely zirconia (8 m2/g), zirconia (43 m2/g), and γ-alumina to yield membranes with an active layer pore size of 9 nm. The different membranes were finally chemically modified using three alkoxysilanes, namely 1H,1H,2H,2H-perfluorooctyltriethoxysilane, 1H,1H,2H,2H-perfluorodecyltriethoxysilane and dodecyltriethoxysilane to bring hydrophobicity to the membranes to be applied to air gap membrane distillation (AGMD). The chemical modification of the mineral capillary membranes was evidenced by infrared spectroscopy, water contact angle measurement, and N2 and water vapors sorption experiments. Thermogravimetric and gas sorption analyses allowed us to approximate the number of grafted molecules and their temperature resistance. The pure water permeability of the hydrophilic non-modified corn starch-based membrane was 597 L/(m2.h.bar) that was higher than the membranes prepared with maize flour (143 L/(m2.h.bar)) and rice husk (302 L/(m2.h.bar)). AGMD of a NaCl solution (35 g/L) yielded a permeate flux between 3–8 L/(m2.h). The membranes grafted with 1H,1H,2H,2H-perfluorodecyltriethoxysilane gave a higher rejection of 95 % due to higher contact angle and Liquid Entry Pressure of water (LEPw).