Kinetic aspects of salinity stability of layer-by-layer polyelectrolyte nanofiltration membranes: Impact of soaking time, types of ions and crosslinking

Recent discovery of very high selectivity of Layer-by-Layer (LBL) nanofiltration (NF) membranes for mono/bivalent ions has attracted significant attention, especially for extraction of lithium from salt lake brines. Besides the exceptionally high selectivity, the salinity stability in highly saline solutions (HSS) that often occurred during NF has been largely neglected. Intuitively, because the LBL membranes are assembled in a saline background solution, the assembly disassembles in highly saline environments. To verify above argument, we reported here a comprehensive study on the performance of polystyrene sulfonate (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) and PSS/poly(allylamine) hydrochloride (PAH) LBL NF membranes in HSS by short/long term immersion and dynamic filtration. Greater binding strength of (PSS/PAH)_2.5 corresponded to better salinity stability than (PSS/PDADMAC)_2.5. Adsorption bridging of divalent Mg²⁺ with PSS altered PSS conformation and narrowing the pore size distribution. Compared to static immersion, dynamic filtration accelerated the loss of PEs from (PSS/PAH)_2.5 at c_LiCl = 2 mol/L, but membrane stability was improved by chemical crosslinking. This research highlighted the kinetic aspects in salinity stability and the chemical-physical behavior of polyelectrolyte complexes (PECs) in HSS. The results offer valuable insights for development of LBL NF membranes for ion separation and resource recovery.