Removal of hardness by electrodialysis using homogeneous and heterogeneous ion exchange membranes

The effect of membrane characteristics on removal of hardness ions from its dilute solutions using electrodialysis (ED) system by applying batch recirculation mode has been investigated. Two types of membrane pairs were used. Type I: Ionac MC3470 and MA3475 (Sybron Chem. Co.); Type II: Neosepta CMX and Neosepta AMX (Astom Corp.). These membranes have different ionic permselectivity, electrical resistance and thickness. In the first part of the study, the effects of the feed concentration, dilute flow rate and dilute pH on the hardness removal were investigated using Type I membranes. The optimum values for feed concentration, flow rate and pH were determined as 0.01 M, 2.6 mL/s and 6.8, respectively. In the second part, the experiments were conducted at different voltage values using Type II membranes under the optimum conditions of these variables obtained. The 95% removal of Mg2+ with the energy consumption of 2.07 kWh/mol and 93% removal of Ca2+ with the energy consumption of 2.12 kWh/mol were obtained in 200 min, applying 20 V potential using Type I membranes. On the other hand, the 100% removal of Mg2+ was achieved by the energy consumption of 1.69 kWh/mol in only 60 min. and the removal of Ca2+ was 98.3% with the energy consumption of 3.39 kWh/mol in 170 min. applying 20 V and using Type II membranes. The results obtained from this study showed that the membrane characteristics affect the efficiency of the ED significantly.