The Phenomenon of Electrokinetic Exclusion of Co-Ions Upon Disruption of the Donnan Barrier in Ion Exchangers

Abstract

The acid retardation method is an effective method for separating an acid and its salt from concentrated solutions. The method is carried out dynamically by passing the mixture through an anion exchanger in the form of a common anion of the acid and salt, eliminating ion exchange. Previously, various non-exchangeable sorption approaches, a process utilizing the difference in acid and salt dissociation in the polymer phase, accompanied by Donnan exclusion of ionic forms, sieve effects, and other mechanisms have been considered as possible mechanisms leading to the separation effect. In addition to such interpretations based on equilibrium factors, in the present study, the authors develop an approach that takes kinetic effects into account. This new approach describes the phenomenon of co-ion exclusion upon disruption of the Donnan barrier in the ion exchanger and relies on temporarily occurring differences in the solution composition and the electrokinetic flow of the mixture of its components in the ion exchanger phase. Such nonequilibrium effects are particularly strong in the acid-salt system due to the large difference in the mobility of hydrogen ions and metal cations. In kinetic processes, the starting point is crucial, which in the system described corresponds to the moment the Donnan barrier, which blocks the ion exchanger phase from co-ions, is overcome. The Donnan barrier can be overcome upon contact between the ion exchanger and a solution of a sufficiently high concentration. Thus, a concentration front moving along the sorption column can synchronize the process of electrokinetic exclusion of co-ions. The quantitative parameters of the phenomenon depend on the characteristics of the ion exchanger and the dynamic mode of phase contact. To enable quantitative assessments, an ion exchanger model is considered, which is used to construct an isotherm of electrolyte distribution between the solution and ion exchanger phases, reflecting the effect of breaking the Donnan barrier. The theoretical model also includes the kinetic equations required to describe the effects under study. Special experiments are described that confirm and refine theoretical estimates of equilibrium and kinetic parameters, based on a new approach to acid retention. Model calculations are used to predict the system's behavior under dynamic process conditions.