Modification of polyamide nanofiltration membrane with ultra-high multivalent cations rejections and mono-/divalent cation selectivity

Due to their inherent negatively charged surface and large intra-membrane pore, the typical polyamide nanofiltration membranes possess the inferior retentions against Ca2+/Mg2+ and undesired mono-/divalent cation selectivity. Here, a novel dual-functional acyl chloride monomer comprising two protected amino groups, 3,5-bis-(sulfinylamino)benzoyl chloride (AB2), is designed and used as organic phase additive that enables simultaneous regulation of the surface charge and pore structure of polyamide (PA) selective layer. Both structure characterizations and simulations confirm that AB2 additive significantly reduces surface negative charges, surface carboxyl density, surface roughness, thickness and mean pore size of the- separation layer, which sharpens pore size distribution, strengthens electrostatic repulsion to cations and enhances water molecule transport. As a result, the optimal PIP-TMC/AB2 membrane shows the remarkably improved CaCl2/MgCl2 rejections (above 99.0%) and mono-/divalent cation selectivity, together with a preferable permeate flux (135.3 ± 3.12 LMH), surpassing most commercial and advanced NF membranes. Moreover, its multivalent cations removal capability and Mg2+/Li+ separation performance retain outstanding in highly concentrated feed solution, suggesting it has enormous potential for water softening and lithium recovery from brine.