Ion transport in the polyamide layer of RO membranes: Composite membranes and free-standing films

The permeability of the composite ESPA1 membrane to ferro- and ferricyanide ions without and with an excess of supporting buffer was analyzed by three different methods: filtration followed by phenomenological analysis, and electrochemical impedance spectroscopy and chronoamperometry using free polyamide films isolated from the membrane. Appropriate relations were developed that relate the observed permeability to actual permeability of the trace redox ions and correct for concentration polarization of the trace ion in presence of a more concentrated (dominant) electrolyte. Within the uncertainties associated with polarization correction, coupling of ionic fluxes, and ion association in solution and within the membrane, consistent values of permeabilities, of about 0.03–0.05 μm/s, were obtained by all methods. The results validate that (a) the contribution of convection to salt permeation through the active layer of thin-film composite membranes is small; (b) the procedures used to isolate the active polyamide layer from commercial membranes has a minimal impact on the transport properties of the polyamide films; the isolated free films may then adequately represent the properties of the original active layer in characterization studies. This information may have important implications for studying transport, modeling and characterization of thin-film composite membranes.