Estimation of nuclear volume dependent fractionation of mercury isotopes

Mass-independent fractionation (MIF) of Hg isotopes is currently thought to arise either from the non-linear scaling of nuclear volume with mass for heavy isotopes (Nuclear Volume Effect, NVE) or from the magnetic isotope effect (MIE), which is due to the non-zero nuclear spin and nuclear magnetic moments of odd-mass isotopes. Distinguishing NVE and MIE is difficult because both predict MIF for only the odd isotopes. However since MIE is a kinetic phenomenon only, MIF observed in equilibrium reactions should be attributable to the NVE only. Thus it should be possible to isolate NVE driven MIF from MIE driven MIF. The goal of the current study was to design equilibrium experiments that would express the NVE in order to confirm theoretical predictions of the isotopic signature of the NVE for Hg. The first set of experiments were based on equilibrium octanol-water partitioning of different Hg chloride species. Octanol-water partitioning of Hg depends on the hydrophobicity of the Hg species, so non polar lipophilic species partition into the octanol phase while polar species remain in aqueous phase. At a Cl- concentration of 1 molar and pH<2, significant nuclear volume MIF was theoretically predicted between hydrophilic HgCl42- and non polar HgCl2 at equilibrium due to their contrasting bond characters (Schauble 2007). In addition, a milli molar Cl- concentration experiment was conducted to determine any NVE fractionation between HgCl2 in the aqueous phase and HgCl2 in the octanol phase. Hg isotope data obtained from octanol water partitioning experiments show negligible MIF (<0.03‰, 2SE, in Δ199Hg) in any of the mercury chloride species. The absence of NVE in the HgCl42- ↔ HgCl2 equilibrium reaction is not what was theoretically predicted. The second experiment investigated mercury liquid-vapor evaporation under equilibrium conditions in the dark at room temperature (similar to Estrade et al., 2008). Liquid-vapor evaporation experiments result in mass independent fractionation of the odd isotopes 199Hg and 201Hg in the vapor phase. The ratio of MIF of the two odd isotopes (Δ199Hg/Δ201Hg) in the vapor phase was 1.64±0.10 (2SE). This result is significantly different from the theoretically estimated NVE ratio (Δ199Hg/Δ201Hg between 2 and 2.5). More laboratory experiments are needed to confirm the NVE Δ199Hg/Δ201Hg ratio in order to use differing Δ199Hg/Δ201Hg Hg ratios to identify different mechanisms of MIF in nature. In addition, more experimental and theoretical work is needed to better understand the controls and expression of MIF along with the underlying mechanisms of MIF.