Fractioning electrodialysis: a current induced ion exchange process

In desalination often multi ionic compositions are encountered. A preferential removal of multivalent ions over monovalent ions can be of interest to prevent scaling in the desalination process. Recently, a novel fractionating electrodialysis stack is described by Zhang et al., 2012 (in Sep. purify. Technol. 88). In the present work a small modification to such a stack was made, to create a current induced ion exchange process, in which no longer desalination occurs. This was done by building a membrane stack in which monovalent-selective ion-exchange membranes and standard grade ion-exchange membranes, with similar charge sign (so either anion or cation exchange groups), were placed alternatingly between an anode and a cathode to form a membrane stack. A proof of principle of the fractioning electrodialysis technology is given. Ternary mixtures, with a divalent-monovalent ion ratio similar to seawater, were used as feed water. For a cation and an anion fractioning stack, maximum fractionations of divalent ions were obtained of approximately 90 and 60%. At higher applied current density, ions can be fractionated to a larger extent than at lower applied current density. For both stacks the water recovery was ∼50%. Coulombic efficiency of both processes decrease rapidly after the start of the experiment. This leads to relatively large volumetric energy consumptions.