Solvent flux behavior and rejection characteristics of hydrophilic and hydrophobic mesoporous and microporous TiO2 and ZrO2 membranes

This paper describes solvent flux and rejection behavior of different hydrophilic TiO2 membranes and a hydrophobic ZrO2 membrane. Solvents and solutes used range from polar to non-polar. Additionally, the temperature effect on permeability and rejection of solutes through these membranes was examined.It was found that the solvent permeabilities through microporous hydrophilic membranes with MWCO 275 and 1400 Da do not obey the viscous flow transport mechanism, in contrast to the permeability through mesoporous TiO2 membrane with MWCO 7000 Da. Decrease of the pore size increased deviation from the viscous flow mechanism. Obtained aqueous and polar solvent fluxes were high. This raises opportunities for industrial applications. Moreover, permeabilities of the non-polar solvents can be increased to acceptable levels by working at elevated temperatures through the mechanism of activated permeability. The hydrophobic membrane showed very high non-polar solvent fluxes and remarkably low water permeabilities. Here, no surface interactions can be considered, due to the lack of hydroxyl groups on a membrane surface. Thus, transport through this membrane is related with viscosity decrease at higher temperature and is closer to viscous behavior than for the hydrophilic membranes with similar pore sizes.Rejections of the solutes did not change with temperature and were high when the hydrophilic membranes were used. The non-polar solute bromothymol blue in toluene showed high rejections, unlike some other literature data. The hydrophobic membrane showed much higher fluxes, but rejections were lower than expected.The results of this study are representative for the ceramic nanofiltration membranes currently available on the market.