Manipulating overcompensation of poly (allylamine hydrochloride) in layer-by-layer nanofiltration membranes for Mg²⁺/Li⁺ separation

Surface modification of the positive charges in the polyamide thin-film composite nanofiltration (NF) membranes has been explored for separation of Mg²⁺/Li⁺ via Donnan exclusion. However, manipulation of the surface charges in layer-by-layer (LBL) NF membranes presents significant challenges because the overall charge within the entire separation layer, rather than just the top surface, provides effective Donnan effect. In this work, we reported a facile approach to increase the positive charge within poly (styrene sulfonate) (PSS)/poly (allylamine hydrochloride) (PAH) LBL NF membranes. A significant improvement in the permselectivity of (PSS/PAH)_2.5 NF membranes was achieved by varying the PAH concentration from 0.001 M to 0.1 M. The increased PAH concentration led to overcompensation within the separation layer, resulting in a higher charge density and increase in the rejection of MgCl₂, MgSO₄ and LiCl, but decline in the rejection of Na₂SO₄. Although the pore size slightly reduced, the overall improvement in the permselectivty was mainly attributed to the Donnan exclusion effect. The optimal membrane, prepared using a PAH concentration of 0.1 M, showed a high pure water permeance of 10.9 L m⁻² h⁻¹ bar⁻¹ with a MgCl₂ rejection of 96.6 %, and the highest separation factor S_Mg,Li of 19.6 for mixed salt solutions. The assembly process of LBL membranes typically involved two stages during each coating cycle: polyelectrolyte (PE) adsorption and diffusion. PAH concentration primarily affected the adsorption process, while the diffusion process was influenced by NaCl concentration. Higher concentration of PAH and NaCl led to coiled conformation. This collectively led to the overcompensation of PAH. Among these, PAH concentration was identified as a more important factor for overcompensation. This study highlights the versatility of LBL NF membranes, demonstrating their potential for precise ion separation through tailored surface charge manipulation.