Ion Transport and Specificity in Polyamide Membranes Studied by Conductivity and its Activation Energy

Polyamide membranes has long been the benchmark in desalination, yet ion transport in these membranes is still incompletely understood. Here, we report measurements of polyamide film conductance in solutions of different salts and concentrations using electrochemical impedance spectroscopy, with emphasis on activation energies (temperature dependence) and the effects of salt type and concentration. After thoroughly addressing the possible artifacts related to membrane relaxation, the data show that the mechanism that controls membrane conductivity and charge may differ significantly and is not necessarily or only partly affected by the salt involved. Specifically, in concentrations up to about 1 mM, the polyamide conductivity is nearly independent of the salt and is apparently dominated by protons inherently present within polyamide. At intermediate salt concentrations, conductivity is controlled by coupled transport of protons and salt anions, with salts cations excluded, which extends to maximal examined concentration for divalent cations. Only for monovalent cations above about 10 mM, this regime eventually transitions to the conductance fully controlled by the salt uptake. The results also demonstrate subtle difference in conductance and its activation energy between different anions and mono- and divalent cations that follow the trend consistent with the dielectric exclusion mechanism. The results have important implications for modeling ion transport and for design of novel membranes and processes and may help explain sensitivity of performance to various membrane- and process-related parameters.