Sulfide-Driven Precipitation of Mercury Sulfide Nanoparticles In the Presence of Natural Organic Matter

The methylation of mercury to CH₃Hg⁺ depends on the speciation of inorganic mercury. We examined the speciation of Hg(II) in the presence of dissolved organic carbon (DOC) and bisulfide (HS^-) in synthetic aqueous media. Our focus is on the formation and properties of mercuric sulfide nanoparticles. Bicarbonate-buffered solutions containing 10^-9 to 10^-5 M of Hg(NO₃)₂ and 10 mg C/l (from one of three different DOC isolates) were allowed to react for 24 hours at pH 6-7, after which time varying amounts of boric acid-buffered (pH 9) HS^- was added (S:C molar ratios varied from 0.1 to 1, pH 7 to 8). Both the solid and liquid phases were monitored over time. Dissolved (less than 3 nm) Hg(II) was quantified using CVAFS following ultracentrifugation. The molecular speciation of Hg in quickly (one minute) frozen, freeze-dried samples was determined using Hg L_III-edge X-ray absorption spectroscopy (XAS). The nucleation, growth, and aggregation of nanophase-Hg was monitored in situ using small- and wide-angle X-ray scattering (SAXS/WAXS), UV-visible spectroscopy, and dynamic light scattering (DLS). XAS, UV-vis, and DLS measurements suggest the formation of metacinnabar (b-HgS) nanoparticles on the order of 10 nm over timescales of hours. Preliminary XAS results further suggest that during the first 30 hours of reaction time DOC competitively binds Hg(II) to both oxygen and sulfur functional groups in the presence of a 40-fold molar excess of HS^- to Hg(II) and a S:C ratio of 0.5. If aqueous thermodynamic speciation models are to be used to better understand the impact of inorganic mercury loading on habitat ecology, we recommend that these models include the possible existence of nanoparticulate HgS(s). We infer that the kinetic efficacy of DOC for binding Hg(II) in the presence of bisulfide undermines the use of thermodynamic modeling over short timescales.