Mechanistic Investigation of Dimethylmercury Formation Mediated by a Sulfide Mineral Surface

Mercury (Hg) pollution is a global
environmental problem. The abiotic
formation of dimethylmercury (DMeHg) from monomethylmercury (MMeHg)
may account for a large portion of DMeHg in oceans. Previous experimental
work has shown that abiotic formation of DMeHg from MMeHg can be facilitated
by reduced sulfur groups on sulfide mineral surfaces. In that work,
a mechanism was proposed in which neighboring MMeHg moieties bound
to sulfide sites on a mineral surface react through an SN2-type mechanism to form DMeHg and incorporate the remaining Hg atoms
into the mineral surface. Here, we perform density functional theory
calculations to explore the mechanisms of DMeHg formation on the 110
surface of a CdS(s) (hawleyite) nanoparticle. We show that coordination
of MMeHg substituents to adjacent reduced sulfur groups protruding
from the surface indeed facilitates DMeHg formation and that the reaction
proceeds through direct transmethylation from one MMeHg substituent
to another. Coordination of Hg by multiple S atoms provides a transition-state
stabilization and activates a C–Hg bond for methyl transfer.
In addition, solvation effects play an important role in the surface
reconstruction of the nanoparticle and in decreasing the energetic
barrier for DMeHg formation relative to the corresponding reaction
in vacuo.