Mechanism for glutamic acid adsorption on a weak-base ion exchanger

Abstract

Although numerous studies have investigated adsorption equilibrium for the case of a strong-electrolyte ion-exchange adsorbent with a fixed ion capacity, there is relatively little work addressing weak-electrolyte ion-exchange adsorbents, particularly when the adsorbates are also weak electrolytes. Recently, Yoshida et al. (1994, 1995), Kishimoto and Yoshida (1995), and Yoshida and Kishimoto (1995) investigated glutamic acid adsorption on various weak-electrolyte ion exchangers, and proposed that the zwitterion form of glutamic acid effectively becomes chemisorbed onto the adsorbents studied. Furthermore, Langmuir isotherms consistent with this mechanism were proposed for the commercial weak-base ion-exchanger DIAION WA30 and found to fit experimental data. In this study, an ion-exchange formalism where the adsorbed phase is represented as a charged polyelectrolyte containing exchangeable counterions, and where the chemical potential of electrically neutral ion combinations are equated in the liquid and adsorbed phases, is used to obtain adsorption isotherms for a weak-electrolyte ion exchanger. Since this approach does not lead to a Langmuir isotherm, comparisons of theoretical predictions with experimental data for glutamic acid adsorption on DIAION WA30 can be used to determine whether these data discriminate between the two adsorption mechanisms just described.