Water—air gas exchange of elemental mercury: An experimentally determined mercury diffusion coefficient for Hg0 water—air flux calculations

Abstract

The evasion of elemental mercury (Hg0) from water surfaces is a key process in biogeochemical mercury cycling. Knowledge of the Hg0 diffusion coefficient (DHG0) in water is essential for Hg0 water—air flux calculations, but no measured value has been available. In this study, DHG0 was measured in pure water and in water of oceanic salinity within an environmental temperature range between 5°C and 30°C. A diffusion cell was constructed consisting of two chambers separated by an aqueous gel membrane allowing molecular diffusion only. The corresponding parameterizations were developed on the basis of the Eyring equation, which defines an activation energy (Ea) for the diffusion process. The temperature dependences of DHG0 (in cm2 s−1) for freshwater, DfreshHG0 = 0.0335e−(18.63 kJ mol−1)/RT, and for seawater, DseaHG0 = 0.0011e−(11.06 kJ mol−1)/RT, with R the gas constant and T the temperature in Kelvin, were thus obtained with an error of ±15%. Whereas the measured DfreshHG0 was in good agreement with the theoretical proposals of a molecular dynamics (MD) simulation, DseaHG0 was clearly lower, probably because of the unaccounted effect of the polarization of mercury atoms in the salt solution, which hampers diffusion. In geochemistry applications, use of the newly determined DseaHG0 instead of the DHG0 from MD simulations would have differential effects on determinations of mercury emissions from the world's oceans. The effect on the tropical ocean would be the largest, decreasing the Hg0 water—air flux estimate by 20%. Toward higher latitudes (∼50°), the calculated emission would drop by about 10%. On the basis of a recent large data set, the estimated amount of mercury released by the Atlantic Ocean would decrease by approximately 17%.