Effect of salts on the retention of polyethyleneglycol by a nanofiltration ceramic membrane

Retention properties of a nanofiltration ceramic membrane were investigated with single polyethyleneglycol (PEG) solutions and mixed PEG/inorganic salt solutions. The rejection rate of PEGs was found to decrease in the presence of ions. It was shown that the effect of ions on the retention of neutral solutes increases with the salt concentration. This salt effect phenomenon is in accordance with former observations showing that the retention of uncharged solutes by nanofiltration organic membranes decreases in the presence of salts [J. Membr. Sci. 258 (2005) 123–132; J. Membr. Sci. 247 (2005) 11–20]. It has been postulated that the decrease in the rejection rate of neutral solutes could result (i) from pore swelling, i.e. the increase in the mean pore radius due to repulsion forces between counterions in the electrical double layer at the pore walls and/or (ii) from Hofmeister effects, that is a partial dehydration of neutral molecules in the presence of ions because water would preferentially solvate ions. Since the matrix of ceramic membranes is rigid, our findings show that Hofmeister effects are probably responsible for the observed decrease in the rejection rate of PEG molecules when salts are added in solution (of course it does not mean that pore swelling does not contribute to the lowering of neutral solute rejection rates observed with organic membranes in mixed-solute solutions). This assumption is confirmed by the lowering of the PEG rejection rates which follows the Hofmeister series, i.e. Mg2+> Li+> K+ and C6H5O73−> SO42−> Cl−. Experimental data were used to compute the resulting decrease in the Stokes radius of PEG molecules in the presence of the various salts.