Estimation of nuclear volume dependent fractionation of mercury isotopes in equilibrium liquid-vapor evaporation experiments

To confirm theoretical predictions of the isotopic signature of the nuclear volume effect (NVE) for Hg, liquid-vapor Hg isotope fractionation was investigated under equilibrium conditions in the dark at room temperature. Since the other mass independent fractionation (MIF) mechanism proposed for Hg is a kinetic phenomenon only, equilibrium experiments are the best approach to isolate the NVE. Because of the small magnitude of NVE MIF, previous equilibrium experimental results have had relatively large error. The equilibrium evaporation experiment resulted both in mass dependent fractionation (represented by delta Hg-202) and MIF (Delta Hg-199 and Delta Hg-201) The average liquid-vapor delta Hg-202 difference was 134 +/- 0.18%. (2SE) and 1.08 +/- 0.07 parts per thousand. (2SE) for two sets of experiments with the vapor phase isotopically lighter than the liquid. A positive MIF of the odd isotopes in the vapor phase was observed with an average Delta Hg-199 of 0.14 +/- 0.01 parts per thousand. (2SE) and an average Delta Hg-201 of 0.09 +/- 0.01 parts per thousand (2SE). For the 6 experiments, the average ratio of NVE MIF of the two odd isotopes (Delta Hg-199/Delta Hg-201) in the vapor phase was 1.59 +/- 0.05 (2SE) confirming a recent estimation of the NVE Delta Hg-199/Delta Hg-201 ratio of similar to 1.65. A conceptual model applying first principles was used to explain the fractionation observed between Hg liquid and Hg vapor under equilibrium conditions. (C) 2012 Elsevier B.V. All rights reserved.