Thermal Reduction of Hg(II) in the Gaseous Atmosphere

Abstract:

Mercury emissions to the atmosphere mostly occur as gaseous elemental mercury (Hg(0), but deposition of mercury to ecosystems mostly occurs as Hg(II) compounds. In the absence of reduction of Hg(II), the rate of oxidation of Hg(0) limits the residence time of mercury in the atmosphere. Our knowledge of the oxidation and reduction chemistry of mercury is growing rapidly, although almost exclusively from computational studies.

As of this writing, we believe that atomic bromine dominates the first step in oxidation of Hg(0):

Br + Hg + M = BrHg• + M (1)

Reactions of BrHg• with radicals generate Hg(II) compounds (gaseous oxidized mercury, GOM). As these reactions are barrierless, they tend to have high rate constants that seem not to depend much on the identity of the radical. NO₂, being the most abundant radical in the atmosphere, probably dominates the fate of BrHg•:

BrHg• + NO₂ + M → BrHgONO + M (2)

Photolysis of Hg(II) compounds can lead to photoreduction, but in the case of BrHgONO it mostly keeps mercury as Hg(II):

BrHgONO + hν → BrHgO• + NO (3)

Reaction (3) is analogous to photolysis of HONO. Previous computational studies showed that BrHgO• behaves a lot like OH, in that it readily abstracts hydrogen atoms from sp³-hybridized carbon atoms (and aldehydes), adds to sp²-hybridized carbon atoms, and adds NO and NO₂. This led us to consider whether BrHgO• could react with CO to reduce Hg(II) to Hg(I):

BrHgO• + CO → BrHg• + CO₂ (4)

The answer is yes! We determined this using Stuttgart-Cologne effective core potentials to account for scalar relativistic effects. Geometries were optimized with the MP2 method and energies refined at CCSD(T) with extrapolation to the basis set limit. We made additional corrections to the energies, including for spin-orbit effects. The reactants form a pre-reactive complex, which can react over a submerged barrier to form trans- and cis- BrHgOCO, which immediately forms BrHg• + CO₂. We are still investigating the kinetics of this reaction, but it appears to compete with, or out-compete, all other reactions of BrHgO• in much of the troposphere.

Reaction (4) reduces Hg(II) to Hg(I), which can re-form Hg(II) or fall apart to Hg(0). We are working to incorporate this reaction into the GEOS-Chem global model of atmospheric chemistry.