Silanization of tubular ceramic membranes for application in membrane distillation

Membrane distillation is a promising thermally driven separation process that can be used for water desalination. The primary requirement of this technique is the hydrophobicity of the membrane, so polymeric membranes are mainly used as the ceramic ones are intrinsically hydrophilic. In this work, in order to modify the water wettability, Al2O3 commercial tubular membranes with different pore sizes were functionalized using methyltrichlorosilane. The main aim of the work was to verify in depth the effects and the success of this hydrophobizing treatment. The effect of silanization on pore size and geometry was evaluated using scanning electron microscopy as well as gas permeation in order to highlight possible morphological changes in the membrane structure. No evident modification was detected. The hydrophobicity was assessed by measuring water contact angles which increased from about zero for the pristine supports up to 145° for the modified membranes. After treatment, the water entry pressures ranged between 0.7 and 2.1 bar. Further analyses were carried out in order to elucidate the amount of silicon grafted to the Al2O3 particles and its distribution. Finally, the modified membranes were tested in a vacuum membrane distillation plant at different temperatures (50, 70 and 90 °C) using as feed both deionized water and a highly concentrated NaCl solution (90 g/L). Very satisfactory performances were obtained. The permeate fluxes were comparable – or even higher – to those registered for a polypropylene membrane used as a reference. The separation properties were evaluated during the NaCl solution tests and were close to the full rejection of salt in any operating condition.