Panda Water Tech Pvt. Ltd.™

ISO 9001 CERTIFIED

EDI Unit Electro dialysis is a process in which charged species are separated from an electrolyte solution by an applied electrical current. This is achieved inside a module equipped with ion-selective membranes, placed between two electrodes. The process streams are circulated through this module. A Cation-exchange membrane (CEM) possesses fixed, negative charges, surrounded by pore electrolyte containing (mobile) positively charged ‘counter-ions’. The fixed charges can be (among others) sulfonic acid or carboxylic acid groups. The sulfonic group is dissociated in the entire pH range, the carboxylic group is undissociated below a pH value of 3. In most anion-exchange membranes (AEM) the fixed charges are ammonium groups . They can range from quaternary ammonium groups which are dissociated in the entire pH range, to primary –NH3 + groups, which are positively charged only in acid surroundings. Here the counter-ions are the negative ions. The backbone for most types of membrane is formed by a cross-linked polystyrene (hydrocarbon) structure. In the case of the Nafion Cation-exchange membrane, this is atetrafluoroethylene backbone. The ionic groups introduce hydrophilic properties into the otherwise hydrophobic structure . This allows water to fill up the free space, to form an internal ‘solid polymer’ electrolyte. In both types of membrane, the concentration of fixed charges, is in the order of 1-2 M. In dilute electrolytes, the counter-ions tend to move out of the membrane, driven by a concentration gradient. This leads to a (very small) charge of the membrane, which counteracts further counter-ion diffusion, and limits the amount of oppositely charged ‘co-ions’ entering the membrane. When an electrical current is imposed on the membrane, both types of mobile ions will carry the charge, but as the counter-ions outnumber the co-ions, these are separated from the solution.