Modelling non-stationary ion transfer in neutralization dialysis

A non-steady state theoretical model is developed using the Nernst-Planck equations in order to study ion transport kinetics through Ion-Exchange Membranes (IEMs) during water desalination by Neutralization Dialysis (ND) in batch mode. The ND cell under study involves three compartments (acid, saline, and alkali) separated by two membranes (a cation-exchange and an anion-exchange ones) assumed ideally permselective and homogeneous. The presence of Diffusion Boundary Layers (DBLs) at the membrane-solution interface is taken into account in the saline compartment. The results of numerical simulation are compared with known experimental data. A good agreement is obtained between experimental and theoretical values representing the pH and the conductivity of the saline circulating solution as functions of time. These experimental results are also compared with the calculations made using the quasi-steady state model developed by Denisov et al. [10]. It is shown that the quasi-steady state approach is not applicable at the beginning of the ND process, during a few tens of minutes, where the concentration profiles in the membranes are far from linear. Within this stage, a few pH fluctuations are possible, while only one or two pH fluctuations occurs in the quasi-steady stage. The mechanism of these fluctuations, which are determined by the periodical change of the "leadership" between the cation-exchange and the anion-exchange membranes, delivering the H+ and the OH-ions into the saline solution, respectively, is discussed.