An elemental mercury diffusion coefficient for natural waters determined by molecular dynamics simulation.

Mercury pollution affects ecosystems globally, especially through its transfer between water and air. While the movement of mercury across water surfaces is influenced by its diffusion coefficient, D(Hg), this value has remained uncertain. Prior studies have not provided a reliable estimate for D(Hg) in natural waters. Understanding mercury diffusion is essential for predicting its atmospheric emissions. Existing data for noble gases like xenon suggest molecular dynamics (MD) simulations can yield accurate diffusion coefficients. However, no prior work had resolved D(Hg) specifically for elemental mercury in freshwater and seawater. This gap motivated the use of MD simulations to model mercury diffusion. The study aimed to determine a precise D(Hg) to improve mercury budget models. By comparing simulated and measured values for xenon, the reliability of the simulation approach was validated. This work addresses a critical uncertainty in mercury transport modeling.

The goal of this study was to determine the diffusion coefficient of elemental mercury in water using molecular dynamics simulation. Accurate D(Hg) values are needed to model mercury transport between water and air. The authors aimed to simulate mercury diffusion in both freshwater and seawater systems. By comparing simulation results with known values for xenon, the method's accuracy was tested. The study sought to provide a new, reliable D(Hg) for use in global mercury budget models. This would allow for more accurate predictions of mercury emissions into the atmosphere. The researchers also aimed to calculate activation energies for mercury diffusion in different water types. Their findings could improve environmental risk assessments related to mercury pollution.

The researchers used molecular dynamics (MD) simulation to model mercury diffusion in water. They simulated the movement of elemental mercury (Hg0) and xenon in water systems containing 1000 TIP4P-Ew water molecules. Simulations ran for 50 nanoseconds to track atomic motion. The same simulation setup was used for both freshwater and seawater environments. Diffusion coefficients were calculated based on atomic displacement over time. The MD approach was validated by comparing simulated xenon diffusion with experimental data. Once validated, the method was applied to determine D(Hg) for mercury. Activation energies for diffusion were derived using the Eyring equation.

The simulated diffusion coefficient for xenon matched experimental data closely, confirming the simulation's accuracy. The calculated D(Hg) for elemental mercury in freshwater was 0.63 × 10⁻⁹ m²/s. In seawater, the D(Hg) was slightly lower at 0.59 × 10⁻⁹ m²/s. Activation energy for mercury diffusion in freshwater was 17.0 kJ mol⁻¹. In seawater, the activation energy was 17.8 kJ mol⁻¹. These values are significantly lower than previously assumed D(Hg) values. The new D(Hg) suggests lower mercury emissions from oceans than prior models predicted. The Eyring equation accurately described the diffusion behavior in both water types. The results indicate that mercury diffusion is slower than previously estimated.

The study provides a new, simulation-based estimate for the mercury diffusion coefficient in water. The calculated D(Hg) is lower than previously assumed values in mercury budget models. This suggests that global ocean mercury emissions may be overestimated. The use of molecular dynamics simulation was validated through xenon comparison. The activation energy values support the reliability of the simulation approach. The findings should improve mercury transport models by incorporating the new D(Hg). The lower diffusion coefficient implies slower mercury transfer between water and air. The results highlight the importance of accurate D(Hg) values in environmental modeling.