Understanding the impact of poly(allylamine) plasma grafting on the filtration performances of a commercial polymeric membrane

Commercial membranes often exhibit difficulties in rejecting specific ionic species, and especially multivalent cations, due to their usual negative charge. To face this drawback, it is proposed here to functionalize the membrane surface by allylamine plasma polymerization. The impact of this modification on both permeation flux and ion rejection is investigated for single salt solutions and ion mixtures. It is shown that the membrane behaves like a positive membrane from the point of view of cation rejection but the negative charge (and the corresponding electrical field) inside pores leads to high rejection of divalent anions like a negative membrane. This allows a high selectivity between divalent and monovalent ions irrespective of their sign. However, in the cases of ionic mixtures, the selectivity between cations and anions is reduced as the grafting duration increases. This outstanding impact on performances was obtained although the membrane surface is perhaps partially covered by plasma poly(allylamine). Indeed, the overall apparent zeta-potential was found to be still slightly negative or slightly positive for 5 and 10 min of polymerization, respectively. Consequently, a definitive conclusion about the mechanisms governing the impact of grafting on rejection cannot still be drawn and it requires an in-depth characterization of the membrane before and after modification so as to consider the changes in physicochemical properties induced by poly(allylamine) grafting.