Mechanism of the fractionation of 7Li ions into aqueous solutions: ion-exchange with the six-membered oxygen ring of zeolite-A

Abstract

Lithium-isotope fractionation by ion exchange was investigated using an aqueous solution (LiOH + HNO₃) fed to an NH₄⁺-zeolite-A column at 293 K. The Li⁺ ion concentration [Li] and isotopic ratio [⁷Li]/[⁶Li] of the feed solution and the effluent fractions were measured, and from their differences, the accumulation of Li⁺ ions and the isotopic ratio ([⁷Li]/[⁶Li])_zl in the zeolite were determined. The accumulation rate changed significantly at 60% Li filling. The accumulation below 60% Li filling was attributed to the Li⁺ ion adsorption on the six-membered oxygen rings of zeolite-A. The isotopic ratio ([⁷Li]/[⁶Li])_ef for the effluent passing through the six-membered oxygen rings showed a wide plateau as a function of the effluent volume. The observed ratio ([⁷Li]/[⁶Li])_ef/([⁷Li]/[⁶Li])_feed = 1.03, corresponded to three stages of separation. A single-stage factor of 1.01 was evaluated from the ratio ([⁷Li]/[⁶Li])_feed/([⁷Li]/[⁶Li])_zl. The basis of the wide plateau ([⁷Li]/[⁶Li])_ef was attributable to the thermodynamic coexistence of the two solid-phases of zeolite. The fractionation mechanism of ⁷Li is discussed using ab initio molecular orbital calculations. The excess of ⁷Li in the Li⁺ tetrahydrate and the deficit of ⁷Li in the six-membered oxygen rings were theoretically deduced. The calculations proved that the small number of Li oscillatory modes, where the O atoms are almost at rest, served for ⁷Li enrichment. The mode frequencies accounted for the magnitude of the separation factor.