In situ characterization of compaction, ionic barrier and hydrodynamics of polyamide reverse osmosis membranes using electrical impedance spectroscopy

Abstract

The Maxwell–Wagner model facilitates discernment of differing electrical contributions of substructural layers in a membrane system from impedance spectra acquired from the whole system but in the absence of water fluxes. The fluxes manifest phenomenological contributions in the impedance spectra that can mask the substructural contributions. A model is presented that distinguishes between these contributions in spectra acquired from a thin-film-composite reverse osmosis polyamide membrane mounted in a cross-flow chamber that facilitated 4-teminal impedance measurements whilst sustaining pressures and steady state fluxes encountered in desalination plants. The phenomenological contributions were modelled with a simple electrical circuit comprised of inductive, conductive and generative components. The component values were strongly correlated with measurements of the water flux, which halved as the membrane was compacted at constant pressure. The unmasked Maxwell–Wagner contributions reflected realistic dielectric, conductive and geometrical properties of the filtration channel, membrane bulk and ionic barrier that were consistent with independent microscopic and spectroscopy characterizations. Changes in these properties and concomitant measurements of the membrane potential as the membrane performance approached the manufacturer׳s specification were consistent with an ion exclusion mechanism associated with a nanometer thick ionic depletion layer that manifests at the junction of the carboxyl-rich and amine-dominated regions of the polyamide active layer as the membrane compresses and hydrates.