Mitigation of membrane scaling in electrodialysis by electroconvection enhancement, pH adjustment and pulsed electric field application

Scale formation on the surface of a heterogeneous cation-exchange MK-40 membrane and its modification MK-40(MOD) was studied during electrodialysis of a solution whose mineral composition models a thrice concentrated milk. The modification was obtained by casting a homogeneous cation-conducting Nafion (R) film on the surface of MK-40 membrane used as a substrate. A 0.04 M NaCl solution circulated through the concentrate compartment of a laboratory flow-through cell. Constant current and pulsed electric field modes were applied; the current densities in overlimiting current range were used. The amount of scale was characterized by the potential drop across the membrane and using the scanning electronic microscopy. Scale was found on the membrane surfaces of both cation-exchange membranes and the auxiliary anion-exchange MA-41 membrane facing the diluate compartment of the cell. The MK-40 membrane surface was scaled with CaCO3, Ca(OH)(2) and Mg(OH)(2) compounds. The scale amount on the MK-40(MOD) was essentially lower and it contained only CaCO3. A small amount of CaCO3 was detected on the auxiliary MA-41 membrane, when it was used together with the MK-40 membrane. Negligible precipitation was found on the MA-41 membrane paired with the MK-40(MOD) membrane. At currents 1.5-2 times higher than the limiting current, higher electroconvection and higher contribution of water splitting at the MA-41 membrane, which adjusted the pH of the depleted solution in a slightly acid range, act together to prevent scaling. Lower scaling of the MK-40(MOD) membrane is explained by its more appropriate surface properties: the thin Nafion (R) layer forming the surface is smooth, homogeneous and relatively hydrophobic. These three properties together with a heterogeneous substrate aid an earlier onset of electroconvective instability; enhanced electroconvection partially suppresses water splitting at the cation-exchange membrane, which allows slight acidification of the diluate solution due to a more pronounced water splitting at the anion-exchange membrane. The use of pulsed electric fields with sufficiently high relaxation time allows essential reduction of membrane scaling.