Modification of a heterogeneous cation-exchange membrane by Ti-Si based particles to enhance electroconvection and mitigate scaling during electrodialysis

Abstract

Relatively low counterion fluxes and scaling on the ion-exchange membranes are the major factors limiting commercial applications of electrodialysis. A strategy for simultaneous mass transfer increase and scaling mitigation is realized by adjusting the membrane surface charge. The strategy involves modification of a heterogeneous cation-exchange membrane МК-40 by coating its surface with a thin film of a perfluorosulfonic acid polymer containing metal oxide particles. A series of samples with the particles, which differ in the composition and functional groups grafted to their surface, is studied. The best performance is demonstrated by the sample containing TiO2 and the sample containing Ti-Si based particles grafted with –(CH2)3Cl functional groups. These samples possess an optimal combination of surface properties with respect to the development of electroconvection, namely, a high surface charge while maintaining a sufficiently low degree of its hydrophilicity along with the “better” distribution of current lines at the membrane surface. This results in an increase in mass transfer (an increase in the limiting current density reached 50 % in comparison with the initial membrane) and a decrease in water splitting. These samples are characterized by a significant decrease in scaling during electrodialysis treatment of a model solution containing ions capable of forming sparingly soluble and insoluble compounds at elevated pH. Thus, the enhancement of electroconvection of the depleted solution at the surface of the modified membranes increases mass transfer rate and reduces scaling due to the suppression of water splitting (pH shift) and inducing hydrodynamic factor that can flush away scale deposits from the membrane surface.