Influence of salts on the rejection of polyethyleneglycol by an NF organic membrane: Pore swelling and salting-out effects

Abstract

Rejection rate measurements were carried out with a single polyethyleneglycol (PEG) solution and mixed PEG/inorganic salt solutions for a nanofiltration polyamide membrane. It was observed that PEG rejection is significantly lower in mixed-solute solutions, i.e. when a salt is present (KCl, LiCl, MgCl 2 and K 2SO 4), and that rejection rate drop increases with salt concentration. Similar nanofiltration experiments were conducted with an NF ceramic membrane in previous works. The rejection of PEGs was also found to decrease in the presence of salts and this phenomenon was imputed to the partial dehydration of PEG molecules by surrounding ions (salting-out effect). Unlike results obtained with the ceramic membrane, the lowering of PEG rejection rates observed with the organic membrane does not follow the Hofmeister series. It means that the salting-out effect cannot solely explain the rejection lowering found for the organic membrane. The additional hypothesis which was considered in the present work is that the rejection lowering might be caused by an increase in the effective pore size (pore swelling) due to accumulation of counterions inside pores resulting from an increase in the surface charge density in the presence of ions. In order to evaluate the significance of the pore swelling phenomenon, the experimental data of rejection rates obtained with the organic membrane were used to compute the variation in the mean pore radius in the presence of the various salts (the Stokes radii of PEG molecules in the presence of the various salts having been determined previously from rejection rates obtained with the ceramic membrane). It was shown that the effective pore size depends on the salt nature and increases with salt concentration. The hypothesis of pore swelling was supported by electrokinetics charge density data calculated from tangential streaming potential measurements: the more the membrane surface charge density increases, the higher the effective average pore size becomes. Finally, the contribution of pore swelling and salting-out to the overall decrease in the rejection rate of PEG was evaluated.