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.
Published Version
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