Ionic transport across bipolar membrane and adjacent Nernst layers

A mathematical model for the ionic transport across bipolar ion-exchange membranes is generalized to take into account the solution content on both sides of a membrane and the concentration polarization of external boundary of the membrane within the solution. The model is based on the assumption of local chemical equilibrium of water dissociation–recombination. The regimes of under- and over-limiting currents are considered. In the overlimiting current regime the partial current due to hydroxyl and hydrogen ions transport is much higher than the current transferred by the salt ions. The later is proportional to the partial current due to the hydrogen and hydroxyl ions. The coefficient of proportionality depends on the selectivity of the layers composing the membrane. The dependency between the salt contamination of the final products (acid and base) versus the total current has a minimum, due to the concentration polarization within the solution. The contamination decreases when the system goes to the over-limiting-current regime and then increases under those high currents which give rise to the substantial changes of the concentrations in the diffusion layers.