Interaction of electrolytes and non-electrolytes in nanofiltration

The performance of a series of membrane systems in separating a range of organic and inorganic solutes has been investigated. The performance of one such membrane XN45 (Trisep) was unexpected, and previously unreported effects were observed with respect to the dependence of the observed rejections of the organic species on the inorganic compounds. The rejection of the inorganic salts used, sodium dihydrogen orthophosphate showed a consistent pattern with increasing concentration, a small initial increase in rejection being followed by a sustained gradual decline. The same pattern was observed for the non-electrolytes in response to the increasing salt concentration. It is proposed that the effect is linked to the polarisability of the organic species leading to interactions with negatively charged membranes.