Overlimiting behavior of surface-modified heterogeneous anion-exchange membranes

Electrochemical and electrokinetic behavior of ion-exchange systems as a result of electrostatic interactions between the fixed charge and the ionic species present in the electrolyte can significantly vary with the type of electrolyte and the membrane composition. In this paper, we studied experimentally the effect of surface modification of a heterogeneous anion-exchange membrane on its properties and overlimiting behavior. The surface modification of the anion-exchange membrane included carboxylation of its surface through benzophenone-based photochemistry followed by covalent linkage of single stranded DNA molecules. Such surface chemistry produced molecular layers of negative charge on the positively charged membrane surface. We investigated the membrane samples with different two levels of surface modification after their exposure either to KCl solutions or solutions containing a biopolymer serving as a model membrane foulant. The surface modification resulted in larger system resistances that were especially pronounced in the limiting and over -limiting region. Water splitting reaction on both the pristine and the modified membranes occurred approxi-mately to the same extent. Unlike water splitting, electroconvection, as another mechanism driving overlimiting current, tracked by the PIV method was largely suppressed upon surface modification especially with DNA molecules. The membranes with the chemically modified surfaces showed smaller susceptibility towards fouling.