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

Abstract

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 205Tl/ 203Tl and 202Hg/ 198Hg 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 (Tl 3+ and Hg 2+) relative to reduced phases (Tl + and Hg 0). Among Hg 2+-bearing species, inorganic molecules and complexes like HgCl 2, HgCl 4 2 - and Hg(H 2 O) 6 2 + will have higher 202Hg/ 198Hg 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 205Tl/ 203Tl fractionation between Tl(H 2 O) 6 3 + and Tl(H 2 O) 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., Hg 2+—[Xe]4f 145d 106s 0 vs. Hg 0—[Xe]4f 145d 106s 2) or enhanced s-electron screening by extra p, d, or f electrons (e.g., Tl 0—[Xe]4f 145d 106s 26p 1 vs. Tl +—[Xe]4f 145d 106s 26p 0). Nuclear-volume fractionations become much smaller for lighter elements, declining from ∼1‰/amu for thallium and mercury to ∼0.2‰/amu for ruthenium and ∼0.02‰/amu for sulfur.