Atomistic View of Mercury Cycling in Polar Snowpacks: Probing the Role of Hg2+ Adsorption Using Ab Initio Calculations

Abstract

Photochemical oxidation of atmospheric elemental mercury (Hg0) promotes reactive oxidized Hg (HgII) adsorption on particles and deposition to the polar snowpack. The deposited Hg either returns to the atmosphere via photochemical reduction or remains in the snowpack potentially depending on the strength of adsorption. In this study, we performed ab initio calculations to understand the atomic-level cause of the fate of adsorbed Hg by determining the adsorption affinity of Hg2+, the simplest form of HgII, for barite, halite, muscovite, illite and ice-Ih as potential adsorbents. The adsorption affinity was estimated by calculating the energy required to dissociate adsorbed Hg2+ from adsorbents. The results reveal that Hg2+ is stable on the surfaces of the selected adsorbents, except barite, but is liable to be photodissociated under solar ultraviolet radiation. This mild adsorption is expected to contribute to bidirectional exchange of Hg between the atmosphere and the polar snowpack. Thus, this theoretical approach can provide complementary perspectives on polar Hg dynamics beyond the limitations of field and laboratory experiments. Further studies on more complicated and realistic adsorption models with different HgII species and diverse defective structures of the absorbent surfaces are required for better comprehension of air-snow Hg cycling in the polar regions.