Elucidating ion transport mechanism in polyelectrolyte-complex membranes

Polyelectrolyte-complex (PEC) nanofiltration (NF) membranes attract much attention, however, the mechanisms governing ion separation in PEC films is not well understood. Here, we elucidate the ion transport in PECs using a recently reported Nafion-polyvinylamine (PVAm) membrane prepared via double-coating approach tuned to “rejection neutrality”, i.e., similar rejection of MgCl₂ and Na₂SO₄ as single salts. New insights are gained by examining ion rejection for single- and mixed-salt solutions of NaCl, MgCl₂ and Na₂SO₄ of varying concentrations and pH. The single salt permeability was found to vary with concentration, obeying a power law with an exponent around 0.4, matching neither the Donnan-dielectric nor a proposed PEC dissociation model. This is explained by progressive dissociation of the complex, which raises membrane swelling and dissociation constants, and weakenis dielectric exclusion, when salt concentration increases. Nevertheless, the membrane remains highly stable in all conditions, which is ascribed to the insolubility of Nafion in water. The results also indicate that “rejection-neutral” PEC still possesses a net negative charge, affecting ion selectivity at low salinities. The insights and physical picture proposed here may help understand and tune separation performance of PEC NF membranes and facilitate their implementation in applications such as purification and reuse of contaminated waters, resource recovery, and ion separations.