Comparative studies on performance of interpolymer and heterogeneous ion-exchange membranes for water desalination by electrodialysis

As the preparation of interpolyme- type anion-exchange membranes involves several steps and the use of hazardous chemicals such as chloromethyl ether and trimethyl amine, it was envisaged to prepare heterogeneous anion-exchange membranes through a simple technique involving casting of a solution containing a binder and a polyelectrolyte. Electrochemical characterization of interpolymer and heterogeneous membranes was carried out by measurement of electrical resistance, counter-ion transport number, ion-exchange capacity and volume fraction of water. Interpolymer ion-exchange membranes exhibited better electrochemical and electrodialytic properties in comparison to a heterogeneous membrane. In order to develop alternate anion-exchange membranes, a heterogeneous type of anion-exchange membrane was prepared, and its electrodialytic performance in combination with an interpolymer cation-exchange membrane was studied. Electrodialysis experiments in recirculation mode of operation were performed under varied feed concentrations and applied potential gradients in two configurations: (1) interpolymer cation- and anion-exchange membranes and (2) interpolymer cation- and heterogeneous anion-exchange membranes, packed together, in order to assess their comparative efficiencies for water desalination by electrodialysis. Under both configurations energy consumption and current efficiency were similar under comparable experimental conditions, but the rate of desalination from configuration (1) was much faster.

Interpolymer and heterogeneous types of ionexchange membranes are both suitable for water desalination by ED with good current efficiency and reasonable energy consumption. The rate of desalination with IPC/IPA was faster in comparison to IPC/HGA because of low resistance and high counter-ion transport number for the IPA membrane compared to the HGA membrane. For a given dialytic rate, one needs more membrane area of HGA compared to IPA under similar experimental conditions. Preparation procedures of the HGA membrane are quite easy and do not involve handling of chloromethylether (which is a hazardous chemical) as for the IPA membrane. Thus, one can use the IPC/HGA configuration for efficient water desalination by ED to avoid a multi-step method and hazardous chemicals for anion-exchange membrane preparation. Furthermore, since HGA membranes are reinforced on fabric, they are thus less flexible and are associated with leakage problems when packed in the ED unit. This problem may be solved by variation of membrane gasket thickness or by other alterations.