Ion-exchange particles (resins) usually consist of highly cross-linked styrene and divinylbenzene polymers functionalized with functional groups that provide a so called fixed charge when the particles are immersed in an electrolyte. Depending on the type of the functional groups, these ion-exchange particles are divided into anion-exchange and cation-exchange particles. The former bear positive fixed charge and allow exchange and transport of anions, the later possess negative fixed charge and allow exchange or transport of cations. These ion-exchange particles find various applications, e. g. separation, purification or water decontamination. One specific area, where ion exchange particles play an irreplaceable role, is electromembrane separation processes as electrodialysis or electrodeionization used in removal of ions from water by means of an electrical field. They are a functional component of a so called heterogeneous ion-exchange membranes which besides the ion exchange particles contain a polymeric binder and polymeric fibers or they directly fill intermembrane space of the desalination units.
Because the ion exchange particles are the main functional component of these systems, it is of utmost importance to characterize their electrochemical properties as well as to describe their behavior in the electrical field. We developed a simple system that allows to study and electrochemically characterize a single ion-exchange particle by performing standard electrochemical measurements including current-voltage characteristics, chronoamperometry and potentiometry. At the same time the system allows observation of both electrolyte-particle interfaces, e. g. by using fluorescent microscopy, that is an invaluable tool in identification the fundamental phenomena that govern ion exchange, ion concentration polarization and associated electrokinetics. These phenomena can include electrohydrodynamic vortices or water splitting reaction occurring on the depletion side of the ion-exchange systems. Our electrochemical analysis and behavior is completed with carrying out Micro X-ray CT (computed tomography) scans of fully swollen particles being electrochemically characterized. The tomography of the particles adds important information on the structure and geometry of the measured system. In this paper we will present our experimental results for both anion- and cation-exchange particles and discuss the effect of the charge on the behavior of the ion-exchange systems.
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