Role of nuclear volume in driving equilibrium stable isotope fractionation of mercury, thallium, and other very heavy elements

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.