Deposition and Reduction of Oxidized Mercury on the Ice Surface: Quantum-Chemical Study and Implication of Mercury Activities in the Arctic

Abstract

Mercury is released into the atmosphere as atomic Hg(0) where it is oxidized under ultraviolet light to form oxidized mercury molecules such as XHgY, BrHgOX, BrHgXO, XHgOH, XHgO2H, and XHgNO2, where X and Y represent Cl, Br, and I atoms. These gaseous oxidized mercury species then deposit onto the surface, including on the Arctic ice and snow. The deposition and reduction mechanisms of oxidized mercury on the ice surface are investigated using first-principles density functional theory. Deposition of oxidized mercury molecules is characterized by adsorption on the (0001) ice surface. Calculated adsorption energies between −2.33 and −4.33 eV confirm the strong interaction between mercury molecules and the ice surface. Further, including the thermal corrections to the total electronic energy and entropy for each mercury molecule, the Gibbs free energy of adsorption is calculated at 0 °C. The calculated Gibbs free energy of adsorption is negative (−1.60 to −3.60 eV), which confirms the exoergic nature of adsorption processes. Further, strong interactions between the ice surface and mercury molecules indicate the retention of mercury molecules on the surface and validate the previous studies on the high concentration of Hg during springtime in the Arctic. Other than BrHgOBr and BrHgOI, all molecules are dissociatively adsorbed on the surface. The dissociation of mercury molecules leads to the formation of a reduced Hg atom on the surface. As the elemental mercury has low vapor pressure, low water solubility, and is weakly adsorbed on the surface, the surface reduction of mercury provides a new path for mercury reduction and re-emission into the atmosphere.