Commercial membranes often exhibit difficulties in rejecting specific ionic species due to their usual negative charge and are furthermore prone to fouling. To overcome these distinct issues, this study proposes functionalizing the membrane surface via plasma polymerization to enhance rejection and minimize fouling.

The surface of a thin-film composite membrane featuring a polyamide active layer was modified via cold plasma polymerization using either allylamine or maleic anhydride. Membrane characterization included surface charge analysis through zeta potential measurements and determination of mean pore size by fitting neutral solute rejection data with a steric hindrance model. Additionally, tangential filtration experiments involving various cationic and anionic pollutants and a model foulant were performed to compare the performance of pristine and modified membranes.

Zeta potential measurements reveal that electrical properties are significantly altered by both grafting. In the case of poly(allylamine) deposit, the isoelectric point is shifted towards basic pH (Fig. 1). This results in a charge reversal from negative to positive at 2.5 < pH < 4 after just 5 minutes of deposition, due to the protonated amine groups introduced to the surface. In contrast, in the case of maleic anhydride, the opposite is observed: the pie shifts to acidic pH values due to the deprotonation of grafted carboxyl groups. Structural properties are also affected: the mean pore radius is roughly halved from 2.45 nm to 1.32 nm for poly(allylamine) and reduced to [...] nm after maleic anhydride grafting. In both cases, membrane permeability decreases by approximately [...] % due to the deposit of the polymer layer.

Filtration experiments conducted with the membrane modified with poly(allylamine) deposit and multi-ionic solutions reveal a marked increase in the rejection of multivalent cations (Ba²⁺, Ni²⁺, Co²⁺, and Cr³⁺), reaching up to 95% (Fig. 2). This behaviour results from the charge reversal from negative to positive at the pH of filtration experiments. In the case of the membrane with a maleic anhydride deposition, an improvement in the rejection of multivalent cations is also observed, which is certainly due to steric effects induced by pore narrowing (since the sign of the membrane charge has not changed). In contrast, Li⁺ and Pb²⁺ ions exhibit low rejection rates for the membrane modified with poly(allylamine), and even negative rejection rates for the membrane modified with maleic anhydride. While the limited impact on lithium rejection was anticipated [...]