Sorption of histidine by different types ion exchangers

Abstract

In order to compare the sorption of polyfunctional substances on ion exchangers of different nature, the absorption of histidine hydrochloride from aqueous solutions by an AN-251 anion exchanger in chloride ionic form, KB-4P-2 cation exchanger in sodium ionic form, and ANKB-2 polyampholite in NaCl salt form was studied. It was established that in the first case, non-exchange sorption occurs due to ion-dipole interactions between the sorption centre "functional group –counterion" and the charged group of an amino acid with the participation of water molecules. Ion exchange occurs on the cation exchanger and polyampholite, since the system contains two types of cations Na⁺ and His⁺, the values of which, however, are small. In these systems, non-exchange absorption occurs due to ion-dipole interactions of oppositely charged structural elements with the participation of solvent molecules, which can occur with a counterion of any nature arising in the system due to ion exchange.
It was found that regardless of the nature of the sorbent, the amount of absorbed histidine is approximately the same due to additional interactions occurring in the system with the involvement of sorption centres (in KB-4P-2 and ANKB-2) and a small swelling of ion exchanger (AN-251).
The thermodynamic description of the systems using approaches based on the idea of stoichiometry of substance fixation in the sorbent was carried out. The values of the equilibrium coefficients, corrected equilibrium coefficients, ac-tivity coefficients of the sorbent phase components, and thermodynamic constants of ion-exchange and non-exchange equi-librium, as well as Gibbs differential energies related to an ion exchanger of a certain composition, were calculated. The state of equilibrium of the ion exchanger in the monoionic form with water was chosen as the comparison state for both cases.
The deviation of systems from the comparison state during sorption was insignificant due to the small amount of absorbed compound in comparison with the number of sorption centres. The thermodynamic equilibrium constants were close to one. The joint ion exchange and non-exchange sorption had a greater influence on the state of the sorbent compared to non-exchange sorption due to the change in the ionic form of the sorbent from mineral to organic.