Kinetic Fractionation of LITHIUM Isotopes by Diffusion in Water
John N. Christensen, Frank Richter 1 , and Ruslan Mendybaev
Contact: John N. Christensen, 510/486-6735, JNChristensen@lbl.gov
Research Objectives
The mass-dependent fractionation of stable isotopes can occur during a number of physical processes, only some of which are well understood theoretically and well investigated experimentally. In particular, the kinetic fractionation of isotopes during diffusion, though understood for gaseous systems, is not well characterized and understood for condensed systems such as liquid water or silicate melts. In the simple case of diffusion of a monatomic gaseous species in an isothermal gas, the ratio of the diffusivities for two isotopes of an element is proportional to the square root of the inverse ratio of the isotope masses. This arises from the equivalence in kinetic energy for molecules in an isothermal gas. In condensed systems such as silicate melts, this simple relationship does not appear to hold. We undertook experiments in aqueous solutions to better understand the phenomenon of kinetic fractionation during diffusion. Results could provide a signature for diffusion in geochemical systems, as well as add to fundamental understanding of the structure and behavior of ion complexes in aqueous solutions.
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 Plot of F, the fraction of Li remaining in the flask, vs. 7Li, a measure of the Li isotopic composition. Large black squares=data for flasks, blue triangles=data for containers. Model curves are shown for dfferent hydration numbers (# of waters in the hydration sphere) and exponential factors (=b). The inset shows the experimental setup,a flask of Li solution immersed and communicating with a larger volume of pure water.
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