Effect of homogenization and hydrophobization of a cation-exchange membrane surface on its scaling in the presence of calcium and magnesium chlorides during electrodialysis

In this paper, two cation-exchange membranes, i.e. MK-40 and MK-40MOD, were studied by chronopotentiometry and voltammetry, MK-40MOD being obtained by covering the heterogeneous surface of a commercial MK-40 membrane with a homogeneous 20 µm thick Nafion® film. Electrodialysis process was realized in an electrodialysis flow-through laboratory cell, in which the cation-exchange membrane under study formed a desalination chamber with an auxiliary anion-exchange (Neosepta AMX-SB) membrane. 0.02 M and 0.04 M solutions of CaCl2, MgCl2 and NaCl were used. The current densities were changed in the range from 0.25 ilimth to 2.5 ilimth, where the theoretical limiting current density, ilimth, was calculated using the Leveque equation. The potential drop over the modified MK-40MOD membrane and the water splitting at this membrane turned out to be lower in all studied cases. Formation of scaling was observed only in the case of the MK-40/0.04 M MgCl2 system at current densities in the range from 1.1 ilimth to 1.4 ilimth. For these current densities, the (quasi)steady state value of potential drop slowly increased with time and the crystals of Mg(OH)2 were found on the ion-exchange particles embedded onto the MK-40 membrane surface facing the desalination chamber. At higher currents, stronger electroconvection at this membrane and higher water splitting at the AMX-SB membrane (the latter providing lower pH in the desalination chamber) prevented scaling. No scaling was found on the modified membrane at any current. It is due to the Nafion® film, which is relatively more hydrophobic than pristine MK-40 and which provides a “better” distribution of current lines near the surface, thus enhancing electroconvection and decreasing water splitting.