Sorption of aromatic amino acids on weak base anion exchangers in unprotonated form

Abstract

It is known that weak base ion exchangers which contain nitrogen of different basicity in their functional groups are capable of ion-exchange sorption only in an acidic medium. The use of such media significantly narrows the range of solutions that can be demineralised by such anion exchangers. The sorbents under consideration have high capacity, and the functional groups can serve as sorption centres for the absorption of substances through mechanisms that do not involve ion exchange. However, the sorption of amino acids by unprotonated weak base anion exchangers has been unreasonably poorly studied. Therefore, the aim of this work was to study the sorption of aromatic amino acids and the mechanism of their attachment in the sorbent phase when using unprotonated anion exchangers. We also sought to compare the results with those obtained when using sorbents in salt form. The sorption of aromatic amino acids of phenylalanine, tyrosine, and histidine from aqueous solutions was studied under static conditions using the method of variable concentrations on anion exchangers AN-221, AN-251, and AN-31. The amino acids in the equilibrium solutions were determined spectrophotometrically. It was shown that the type of isotherms, and hence the attachment mechanism, was determined by the nature of the amino acid, and the amount of absorbed substance was determined by the type of ion exchanger. We found a fundamental difference in the mechanism of amino acid attachment during sorption on the anion-exchangers in unprotonated form as compared to the Сl^- form. The non-exchange attachment of amino acids in the phase of protonated anion exchangers occurs due to ion-dipole interactions between oppositely charged functional groups (ions) of the sorbate and sorbent and hydrogen bonds formed between their hydrate shells. If we consider the unprotonated form, the attachment occurs mainly due to the proton transfer from the amino group of the amino acid to the unprotonated nitrogen from the functional group of the anion exchanger. In the case of the unprotonated form, the absorption of amino acids increases with an increase in the basicity of the functional groups of the sorbent, which is due to their ability to be protonated. Overall, for sorbents in unprotonated form, like the salt form, the absorption mechanism is the same for each amino acid, and the amount of absorbed substance is determined by the type of ion-exchanger.