Electrodialysis of moderately concentrated solutions: Experiment and modeling based on a simplified characterization of ion-exchange membranes

Electrodialysis (ED) is a cost-effective and environmentally friendly process; it is considered a key step in Zero Liquid Discharge systems. However, the ED process producing a fairly concentrated solution is not well understood. An ED process where 10 L of 0.5 M NaCl was circulated in diluate stream (DS) and 0.1 L of initially 2.0 M NaCl circulated in concentrate stream (CS) was studied. The concentration of concentrate increased up to 3.5 M. The electrodialyzer ' s design eliminated current leakage. Ion -exchange membranes, MK -40, MA -41 (Shchekinoazot) and CJMA-3 (Chemjoy Polymer Materials), were characterized prior to ED. Based on the concentration dependences of membrane conductivity and diffusion permeability, the " true " ( t i *) ion transport numbers in the membranes were found; the water transport number ( t w ) was found from volumetric measurements. It was shown that the current efficiency, eta , found using t i * is quantitatively consistent with eta found in the ED experiments. To calculate the time dependences of solution concentration and volume in CS, a mathematical model was built. For the first time, t i * and t w determined in independent experiments, were used as parameters. The only fitting parameter was the osmotic permeability coefficient. A good agreement with experimental results is obtained.