Cation-modulated permselectivity regulation of polyelectrolyte nanofiltration membranes for water purification

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Polyelectrolyte multilayer (PEM) membranes fabricated by layer-by-layer (LBL) assembly have been widely adopted for nanofiltration (NF) due to their unique merits of highly tunable surface charge and pore size. However, current PEM membranes generally suffer from cumbersome preparation procedures and unsatisfying water permeance and/or solute retention rates, which stymie their practical applications. This study discloses a facile and effective method to deliberately regulate the interactions between polyelectrolytes and their LBL assembly behaviors by using cations with various valences and sizes as the modulators. Compared with the acquiescent background electrolyte NaCl, the PEMs modulated by divalent cations (i.e., Ca2+) exhibit intensified surface electronegativity with dense packing structures, and the resulting PEM membranes thus show high rejections for both inorganic salts (i.e., 94.0 % for Na2SO4) and negatively charged emerging micropollutants (EMPs) (i.e., 94.9 % and 99.9 % for tetracycline hydrochloride and perfluorododecanoic acid, respectively). In contrast, monovalent cations (i.e., Li+) lead to the formation of PEMs with loose structures and strong surface electronegativity, which endow the PEM membranes with a high water permeance of 15.0 LMH/bar and slightly reduced solute rejections. Nevertheless, the retention rates of Na2SO4 and perfluorooctanoic acid still surpass those obtained by the NaCl-modulated PEM membrane. Interestingly, trivalent cations such as Fe3+ and Al3+ are unable to facilitate the formation of an intact PEM. Characterization data and theoretical models imply that cations play a pivotal role in the stacking structure of PEMs through synergistically modulating polyelectrolyte-counterion interactions and the overcompensation and subsequent site diffusion of polyanions. This work demonstrates an effective approach to finely tune the nanostructure and permselectivity of the PEMs, which provides a valuable platform for the rational design of PEM NF membranes for effective separations of salts and EMPs.