Mercury (Hg) is a ubiquitous, toxic and bioaccumulative environmental contaminant that undergoes reaction to change speciation and environmental fate. Some areas, like the Oakridge National Laboratory Y-12 National Security Complex (TN, USA), have experienced historical contamination of surrounding soils with Hg, and remediation of such sites has been complicated by poor mechanistic understanding of Hg reactions. Specifically, non-volatile Hg species in soil can undergo reduction to form volatile Hg(0), which can then be lost to the atmosphere; however, the mechanism of this reaction is not known. This work used a computational study, coupled with laboratory experiments to propose a mechanism for reduction of one environmentally relevant Hg species, mercury (II) chloride (HgCl2), in soil-like environments. Computational modelling using Gaussian software suggests that HgCl2 reduction is a unimolecular dissociation driven by scissoring of the chlorine bonds to form Hg(0). Based on the energy of excitation suggested in these computational results, we hypothesized that UVB radiation would provide the energy necessary to reduce HgCl2 to Hg(0) via photolysis, without requiring a secondary electron donor. To test this, clean silica sand was spiked with an aqueous solution of HgCl2, and exposed to UVB radiation, while the flux of Hg(0) from this sand was quantified over time. Additional experiments were performed to determine the influence of temperature on HgCl2 photoreduction and flux of Hg(0) from sand.

Mechanistic investigation of the reduction and volatilisation of mercury in soil

251st National Meeting (2016)

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