An Elemental Mercury Diffusion Coefficient for Natural Waters Determined by Molecular Dynamics Simulation

Abstract

Mercury is a priority pollutant as its mobility between the hydrosphere and the atmosphere threatens the biosphere globally. The air-water gas transfer of elemental mercury (Hg0) is controlled by its diffusion through the water-side boundary layer and thus by its diffusion coefficient, D(Hg), the value of which, however, has not been established. Here, the diffusion of Hg0 in water was modeled by molecular dynamics (MD) simulation and the diffusion coefficient subsequently determined. Therefore the movement of either Hg(0) or xenon and 1000 model water molecules (TIP4P-Ew) were traced for time spans of 50 ns. The modeled D(Xe) of the monatomic noble gas agreed well with measured data; thus, MD simulation was assumed to be a reliable approach to determine D(Hg) for monatomic Hg(0) as well. Accordingly, Hg(0) diffusion was then simulated for freshwater and seawater, and the data were well-described by the equation of Eyring. The activation energies for the diffusion of Hg0 in freshwater was 17.0 kJ mol(-1) and in seawater 17.8 kJ mol(-1). The newly determined D(Hg) is clearly lower than the one previously used for an oceanic mercury budget. Thus, its incorporation into the model should lead to lower estimates of global ocean mercury emissions.

Global observations and modeling of atmosphere–surface exchange of elemental mercury: a critical review

“…Bulk methods is the most widely utilized approach for the oceanic surface (> 80 % of the field data, Table 1). Sommar et al (2013a) summarized the methodologies of the bulk method, which is generally controlled by both kinetic (overall mass transfer coefficient, k) and thermodynamic (partial pressure related concentration gradients) forcing (Wanninkhof, 1992;Wanninkhof et al, 2009;Kuss et al, 2009;Kuss, 2014):…”

Elemental mercury (Hg(0)) in air and surface waters of the Yellow Sea during late spring and late fall 2012: Concentration, spatial-temporal distribution and air/sea flux

“…Sc Hg is the Schmidt number of Hg (0) for seawater: Sc Hg ¼ v=D, where v is the kinematic viscosity (cm 2 s À1 ) of sea water and D is the aqueous diffusion coefficient (cm 2 s À1 ) of Hg(0). The v and D is calculated according to the method described by Wanninkhof (1992) and Kuss et al (2009), respectively.…”

High-resolution measurements of elemental mercury in surface water for an improved quantitative understanding of the Baltic Sea as a source of atmospheric mercury

“…However, previous calculations of marine Hg 0 emission 15 fluxes were subject to several limitations; these have been partly overcome in recent years. Data coverage for emission estimates before about the year 2000 was sparse and the parameters determining Hg 0 flux were not well known and were thus roughly approximated (Kuss et al, 2009). Recently, the solubility of Hg 0 in water was newly determined (Andersson et al, 2008) and the diffusion coefficient of Hg 0 (DHg0) in water and seawater was measured (Kuss, 2014).…”