Rapid Attainment of Isotopic Equilibrium after Mercury Reduction by Ferrous Iron Minerals and Isotopic Exchange between Hg(II) and Hg(0)

We examined Hg stable isotope fractionation after the partial reduction of Hg(II) to Hg(0) by the siderite and green rust of ferrous iron minerals. The fractionation of Hg isotopes in closed-system experiments followed an equilibrium fractionation model, with Hg(II) enriched in heavier isotopes. The results indicated isotopic fractionation (δ202HgII-δ202Hg0) of 2.43 ± 0.38 and 2.28 ± 0.40‰ for the siderite and green rust experiments, respectively. Experiments were also performed to determine if the rapid attainment of isotopic equilibrium was attributed to isotopic exchange between Hg(II) and Hg(0). In the absence of other redox-active species, we observed that the δ202Hg values of both Hg(0) and Hg(II) shifted substantially toward equilibrium within minutes and evolved to constant δ202Hg differences between the Hg(II) and Hg(0) pools. Mixing experiments conducted in water and 10 mM NaCl yielded δ202HgII-δ202Hg0 differences of 2.63 ± 0.37 and 2.77 ± 0.70‰, respectively. The 199Hg/198Hg and 201Hg/198Hg results were consistent with previously published experimental and computational studies indicating the involvement of nuclear volume effects in the observed fractionations between the mercury species. Together, these findings suggest that rapid Hg isotopic exchange can facilitate Hg stable isotope fractionation in Hg(II)-Hg(0) redox systems and overprint isotopic fractionation caused by kinetic processes.