Complexation of Borate with Cross-Linked Polysaccharide Anion Exchanger: 11B NMR and Adsorption Properties Studies

Abstract

Boron adsorption behavior on cross-linked polysaccharide anion exchangers, QAE-Sephadex and QA cellulose, was examined in detail by 11B NMR measurements and distribution studies. For the QAE-Sephadex system, two signals of 1:1 borate complexes with α,β- and α,γ-diol moieties of the glucopyranoside residues were observed, showing identical chemical shift values with those for the Sephadex gel system. Only the 1:1 complex of borate with α,β-diol moiety of the gel matrix was formed for QA cellulose. The formation constants of the borate complexes, estimated by the signal intensities of 11B NMR, were almost the same in the anion exchangers and Sephadex gel. The adsorbability of boric acid/borate was not enhanced as much by the presence of charged functional groups, because both the ion-exchange selectivity toward borate and formation constants of borate complexes with the gel matrix are not so high. The pH in the anion exchanger phase is higher than that in the external equilibrated solution, however, is slightly lower than the value expected from the Donnan relation, as a result of the fixation of anionic borate onto the gel matrix by the complexation: −log = − a log[H+] + b (a ≤ 1 and b > 0). Boron adsorption behaviors on the polysaccharide anion exchangers, such as pH dependence, boron concentration dependence, and background electrolyte effect, can be explained satisfactorily on the basis of such properties.