期刊
GEOCHIMICA ET COSMOCHIMICA ACTA
卷 71, 期 9, 页码 2170-2189出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2007.02.004
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Equilibrium stable isotope fractionations of mercury and thallium are estimated for molecules, atoms and ions using first-principles vibrational frequency and electronic structure calculations. These calculations suggest that isotopic variation in nuclear volume is the dominant cause of equilibrium fractionation, driving Tl-205/Tl-203 and Hg-202/Hg-198 fractionations of up to 3%. at room temperature. Mass-dependent fractionations are smaller, ca. 0.5-1%. for the same isotopes. Both fractionation mechanisms tend to enrich the neutron-rich isotopes in oxidized mercury- and thallium-bearing phases (Tl13+ and Hg2+) relative to reduced phases (Tl+ and Hg-0). Among Hg2+ -bearing species, inorganic molecules and complexes like HgCl2, HgCl42- and Hg(H2O)(6)(2+) will have higher Hg-202/Hg-198 than coexisting methylmercury species, suggesting a possible application of Hg-isotope measurements to understanding mercury methylation and increasing methylmercury concentrations at the top of the food chain. Estimated Tl-205/Tl-203 fractionation between Tl(H2O)(6)(3+) and Tl(H2O)(3)(+) is in reasonable agreement with the fractionations previously observed between seawater and Fe-Mn crusts, supporting an equilibrium-like reduction/oxidation fractionation mechanism. More generally, nuclear-volume isotope fractionation will concentrate larger (heavier) nuclei in species where the electron density at the nucleus is small-due to lack of s-electrons (e.g., Hg2+-[Xe]4f(14) 5d(10)6s(0) vs. Hg-0-[Xe]4f(14)5d(10)6s(2)) or enhanced s-electron screening by extra p, d, or f electrons (e.g., Tl-0-[Xe]4f(14)5d(10)6s(2)6p(1) vs. Tl+-[Xe]4f(14)5d(10)6s(2)6p(0)). Nuclear-volume fractionations become much smaller for lighter elements, declining from similar to 1%(0)/amu for thallium and mercury to similar to 0.2%(0)/amu for ruthenium and similar to 0.02%(0)/amu for sulfur. (C) 2007 Elsevier Ltd. All rights reserved.
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