Spontaneous accumulation of cesium ions based on the membrane potential using a selectively-permeable-polyvinyl chloride capsule containing concentrated potassium ions and zeolites

Abstract

Moderate amounts of potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB) and 2-nitrophenyl octyl ether (NPOE) were added in a polyvinyl chloride (PVC) capsule membrane, and a concentrated KCl solution was kept inside the capsule as an inner solution. When the PVC capsule was put into an aqueous solution containing low concentrations of K+, the membrane potential between the capsule outside and inside was generated according to the K+ concentration ratio of the capsule outside to the inside. Since the transport of Cs+ from the aqueous phase to the NPOE phase is much easier than that of K+, Cs+ is always taken from the outside into the membrane phase. Accordingly, it was turned out that Cs+ was spontaneously accumulated from the outside to the inside until the Cs+ concentration ratio of the capsule outside to the inside was equivalent to the K+ concentration ratio. In this case, TFPB− was served as not only a counter anion but also a carrier of K+ and Cs+ within the membrane, and some amounts of K+ was replaced by Cs+. Furthermore, the addition of mordenite to the capsule inside facilitated the accumulation of Cs+ because the activity of Cs+ within the capsule was lowered by the adsorption of Cs+ in mordenite.