Vol 14, No 3 (2024)

Concentration polarization (CP) in membrane systems is understood as the phenomenon of the emergence of concentration gradients in a solution near the membrane surface, which is a result of the selective transfer of certain components of the solution through the membrane under the influence of transmembrane driving forces. CP accompanies all types of membrane processes. It affects transfer conditions and reduces the efficiency of separation processes: in most cases, there is a decrease in the overall transfer rate and an increase in energy consumption, as well as a loss of permselectivity. This review examines the general patterns and features of the CP phenomenon in the processes of electrodialysis, reverse osmosis, nanofiltration, ultrafiltration, pervaporation, as well as in membrane sensor systems and fuel cells. The fundamental principles of the CP phenomenon and experimental methods for its study are considered.

In this study, we investigated the electrodialysis process for treating a dilute sodium chloride solution using various anion exchange membranes – specifically, the commercial heterogeneous MA-41 and homogeneous Neosepta AMX, along with the experimental homogeneous membrane MA-1. We observed an increase in the desalting rate and the limiting current for the studied anion-exchange membranes in the series MA-41, MA-1, and AMX. We found that with commercial membranes, the decrease of the solution concnetration leads to the development of conjugated effects of concentration polarization. For the AMX membrane, useful mass transfer due to electroconvection increases, whereas for the MA-41 membrane, the flux of salt ions decreases due to the occurrence of the water dissociation reaction. For the MA-1 membrane, a decrease in the solution concentration leads to a transition of the system to the underlimiting current mode. This behavior may be associated with a significant contribution of equilibrium electroconvection to the process of ion transfer in dilute solutions in electromembrane systems with this membrane. Due to these differences in membrane properties, the mass transfer coefficients for the MA-1 membrane are higher compared to the AMX membrane at potential drops of 1 and 2 V. Our findings suggest that the most optimal operating mode for the MA-1 membrane is at a potential drop of 1 V in the electromembrane system, which results in a specific energy consumption of 0.24 kWh/mol. Contrastingly, under comparable conditions for the AMX membrane, the specific energy consumption is 0.34 kWh/mol.

A comprehensive characterization of heterogeneous anion exchange MA-40 and MA-41 membranes, differing in the nature of functional groups and the exchange capacity (3.32 and 1.41 mmol/g_dry, respectively), was carried out. The MA-40 membrane contains low basic secondary and tertiary amino groups, while the MA-41 membrane contains predominantly quaternary ammonium bases. Concentration dependences of conductivity and diffusion permeability, current-voltage curves were obtained, and the transport and structural parameters of a microheterogeneous model of membrane in solutions of different natures (salts and acids) containing singly and doubly charged cations and anions (sodium and calcium chlorides, sodium sulfate and sulfuric acid). The influence of counter- and co-ions on the electrical transport properties of the studied membranes was revealed and it was shown that changes in their properties are determined not only by the nature of the electrolyte, but also by the value of the exchange capacity of the samples, as well as the nature of their functional groups.