Fouling of a heterogeneous anion-exchange membrane and single anion-exchange resin particle by ssDNA manifests differently

Electrostatic interactions between the charge fixed in ion-exchange membranes and the charge borne by large ions present in the desalted electrolyte can significantly promote fouling of the ion-exchange membranes in the case the two charges are opposite. Here we study the interactions between anion-exchange systems represented by a heterogeneous anion-exchange membrane and homogeneous anion-exchange resin particle both possessing positive fixed charge and short fragments of single stranded DNA as a large molecule with negative charge. We investigate the effect of DNA on both systems by measuring current-voltage curves in the solutions having different concentrations of mobile DNA. Our experimental data show that while there is a profound effect of DNA on the limiting and overlimiting region of current-voltage curves for the heterogeneous anion-exchange membrane, there is very limited influence on current-voltage curves of the anion-exchange resin particle. Direct observation of the depletion side of the pristine heterogeneous anion-exchange membrane showed that the polarization of the membrane leads to formation of intensive chaotic vortex localized to the membrane surface on which two large counter-rotating vortices superimpose later further polarization. The addition of DNA does not affect the formation of two large vortices however our data strongly indicate that there is an effect on the local vortex, which is responsible for the large changes in the current-voltage curves. Surprisingly, the observation of the anion-exchange particle revealed that DNA strongly affects the electrokinetics around the particle although this is not manifested in the measured current-voltage curves. On this system, the DNA changes the electrokinetic picture completely and very likely creates bipolar junction capable of water splitting.