ELUCIDATING THE EFFECT OF AFFINITY AND ASSOCIATION ON ION TRANSPORT IN POLYMERIC DESALINATION MEMBRANES USING ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

A widespread approach used in current desalination models is to link equilibrium partitioning at the two membrane interfaces (or at the pore entry and exit) to the transport within the homogeneous pores matrix, described by the so-called extended Nernst–Planck equation. Partitioning relations are presently described by a combination of Steric, Donnan, and diElectric exclusion (SDE model). However, filtration data and other independent measurements of ion partitioning indicate that SDE model is not consistent with experiment and fail to predict membrane performance for different salts and concentrations.

In order to understand the deviation form SDE exclusion model, we carry out a systematic quantitative examination of ion transport in the active layer of different polymeric membranes using electrochemical impedance spectroscopy (EIS). By varying the salt type, concentration pH, and membrane type the observed trends may be compared to the SDE predictions and other available data to gain new insights into the nature of ion permeation in desalination membranes.

EIS experiments with single salt solutions clearly show deviations from the standard model. For instance, the fixed charged in reverse osmosis membranes does have a strong impact on the ion permeability, but the impact is significantly lower than the expected from the standard model. These and other results may be explained by the non-homogeneity of reverse osmosis membranes, acting as randomly connected regions of polyamide with a wide distribution of density and charge. Results point also to an exceptionally high affinity of polyamide to protons, which exceeds that of common inorganic ions by several orders of magnitude. These and other data will be discussed and rationalized using a new picture of ion exclusion and transport in desalination polymers.