Rate of formation and dissolution of mercury sulfide nanoparticles: The dual role of natural organic matter

Differentiated availability of geochemical mercury pools controls methylmercury levels in estuarine sediment and biota

“…We synthesized solid/adsorbed phase Hg tracers, and added them to the sediment, as defined thermodynamically favoured chemical forms (that is, b-HgS(s), Hg(SR-NOM) 2 and MeHgSR-NOM), whereas the dissolved tracers were added to the water phase as labile complexes (dominated by Hg(OH) 2 0 and MeHgOH 0 , respectively). The rationale for this approach is that the time required for Hg II and MeHg to reach equilibrium with components in the aqueous phase (including organic and inorganic particles) is relatively short (hours to maximum a few days) 18,19 as compared with the slower rate (days to weeks) [42][43][44] to form b-HgS(s) and adsorbed phases of Hg(SR-NOM) 2 in the sediment. As described above, the Hg II tracers added to the sediment were taken to represent sources for in situ MeHg formation and Hg II and MeHg added to the water phase were used as proxies for both terrestrial and atmospheric inputs.…”

Chemical Speciation of Mercury in Soil and Sediment

“…In addition to this process, nanoparticulate HgS(s) physicochemically stabilized by DOM has been shown to contribute to an apparent dissolution of HgS(s) (Ravichandran et al, 1999;Deonarine and Hsu-Kim, 2009). Thus, in experiments where HgS(s) has been allowed to precipitate out from solution, DOM has been demonstrated to have a dual role of decreasing the growth rate and aggregation of HgS polymers and at the same time stabilizing HgS colloids (Deonarine and Hsu-Kim, 2009;Slowey, 2010). Results from solubility experiments with cinnabar and DOM may suggest that hydrophobic fractions of DOM enhance the apparent solubility more than hydrophilic fractions of DOM (Ravichandran et al, 1998;Waples et al, 2005), independent of the concentration of thiol groups in these fractions.…”

“…In apparent contrast, LMM amino acids with thiol groups have been shown to be much more efficient than molecules with amino and carboxyl groups alone to inhibit aggregation and to stabilize HgS nanoparticulates in experiments where metacinnabar has been allowed to precipitate after reaction between HS − and Hg 2+ (Deonarine and Hsu-Kim, 2009). In the latter type of experiment, it has been shown that reactions between DOM, Hg 2+ , and HS − and the formation of metacinnabar are dynamic over time and do not seem to reach equilibrium even after weeks (Slowey, 2010).…”

In Situ Biotransformation of Contaminants in Sediments

“…Conditions which promote the formation of nanoparticulate metacinnabar, and thus promote methylation rates, include DOM with high aromaticity (Gerbig et al, 2011) and higher DOM to Hg ratios (Graham et al, 2012), tend to increase methylation rates. The kinetically controlled process of nanoparticulate cinnabar formation suggests that equilibrium-based models of methylation may underestimate MeHg production in some ecosystems (Slowey, 2010), especially where high levels of both organic matter and sulfide coexist (e.g., wetlands). Alternatively, DOM can act to limit methylation in low sulfide settings by binding with Hg(II) to prevent inorganic mercury uptake due to molecular size restrictions (see Ravichandran, 2004 for a review) or by limiting the formation of neutral, soluble mercuric sulfide complexes (Miller et al, 2007).…”