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 (delta(20)2Hg(II)-delta Hg-202(0)) 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 d202Hg values of both Hg(0) and Hg(II) shifted substantially toward equilibrium within minutes and evolved to constant delta Hg-202 differences between the Hg(II) and Hg(0) pools. Mixing experiments conducted in water and 10 mM NaCl yielded delta Hg-202(II)-delta Hg-202(0) differences of 2.63 +/- 0.37 and 2.77 +/- 0.70%, respectively. The Hg-199/Hg-198 and Hg-201/Hg-198 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.

Automated summary

Experimental results showed that Hg stable isotope fractionation can be facilitated by rapid Hg isotopic exchange in Hg(II)-Hg(0) redox systems, and the fractionation is influenced by nuclear volume effects.