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

Abstract

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.

Demystifying mercury geochemistry in contaminated soil–groundwater systems with complementary mercury stable isotope, concentration, and speciation analyses

“…Equilibrium isotope fractionation between Hg 0 and Hg 2+ results in more positive δ 202 Hg values in the Hg 2+ pool, 83,85,86 which could play a role in the observed Hg stable isotope data in SCB1. The δ 202 Hg value of the bulk sample will become more positive only if there has been Hg 0 loss from the solid-phase (evaporation unfavourable in subsurface samples).…”

“…This fractionation to heavier isotopes should be enhanced when we consider that Hg 2+ species are also enriched in heavier isotopes by equilibrium isotope fractionation between Hg 0 and Hg 2+ . 85,86 Thus, evaporative losses of Hg 0 from the solid-or liquid-phase to soil airspaces or even groundwater well shas would result in further enrichment of remaining solid and liquid-phase pools in heavy isotopes.…”

“…Small MIF shis in the opposite direction to the MDF shi are expected for any process inuenced by the nuclear volume effect, such as equilibrium isotope fractionation between co-existing Hg(0) and Hg(II). Recent studies revealed that the exchange kinetics of this process are surprisingly fast, 86,114 suggesting that equilibrium Hg isotope fractionation between Hg redox states can be expected to be relevant in all natural systems in which Hg(0) and Hg(II) co-exist and get in contact with each other.…”

Mass-Independent Fractionation of Mercury Isotopes during Photoreduction of Soot Particle Bound Hg(II)

“…26,46 Adsorption and/or desorption processes may therefore have affected MDF by at most 0.62‰ 26,46 but probably had a limited effect on MIF in these experiments. While isotopic exchange due to aqueous-phase Hg(II)−Hg(0) equilibration has been observed in nonpurged, closed-system experiments, 28 such Hg isotope fractionation is unlikely to have exerted a major influence in our purged reactor experiments in which Hg(0) was continuously removed.…”

“…Stable isotopes of Hg exhibit both mass-dependent fractionation (MDF, represented by δ 202 Hg) and mass-independent fractionation (MIF) during Hg transformations under various environmental conditions. − MIF includes odd-mass-number MIF (odd-MIF), represented by Δ 199 Hg and Δ 201 Hg, and even-mass-number MIF (even-MIF), represented by Δ 200 Hg and Δ 204 Hg. Prior studies have shown that MDF is induced by several Hg transformation and transport processes, ,− but large values of odd-MIF result primarily from photochemical reactions under various environmental conditions. ,, …”