Inhibition of Precipitation and Aggregation of Metacinnabar (Mercuric Sulfide) by Dissolved Organic Matter Isolated from the Florida Everglades

Abstract

Precipitation and aggregation of metacinnabar (black HgS) was inhibited in the presence of low concentrations (≥3 mg C/L) of humic fractions of dissolved organic matter (DOM) isolated from the Florida Everglades. At low Hg concentrations (≤5 × 10-8 M), DOM prevented the precipitation of metacinnabar. At moderate Hg concentrations (5 × 10-5 M), DOM inhibited the aggregation of colloidal metacinnabar (Hg passed through a 0.1 μm filter but was removed by centrifugation). At Hg concentrations greater than 5 × 10-4 M, mercury formed solid metacinnabar particles that were removed from solution by a 0.1 μm filter. Organic matter rich in aromatic moieties was preferentially removed with the solid. Hydrophobic organic acids (humic and fulvic acids) inhibited aggregation better than hydrophilic organic acids. The presence of chloride, acetate, salicylate, EDTA, and cysteine did not inhibit the precipitation or aggregation of metacinnabar. Calcium enhanced metacinnabar aggregation even in the presence of DOM, but the magnitude of the effect was dependent on the concentrations of DOM, Hg, and Ca. Inhibition of metacinnabar precipitation appears to be a result of strong DOM-Hg binding. Prevention of aggregation of colloidal particles appears to be caused by adsorption of DOM and electrostatic repulsion.

Methylmercury cycling in estuarine sediment pore waters (Penobscot River estuary, Maine, USA)

“…Although high S(-II) concentrations may degrade the quality of labile microbial substrate (Wakeham et al 1995) or limit microbial access to the trace metals (including Co, Ni, and Zn) required to form metabolic enzymes (Patidar and Tare, 2004), there is little consistent evidence for S(-II)-mediated toxicity to SRB at S(-II) field concentrations less than 2-3 mM (e.g., Sundback et al 1990;Reis et al 1992). Moreover, it is questionable whether HgS (s) is stably sequestered and/or microbially unavailable under sulfidic conditions (Morse and Luther 1999;Ravichandran et al 1999;Hintelmann et al 2000). If the concentration of uncharged Hg-S(-II) species indeed affects methylation rate linearly, some fraction of the observed significant rate decline in zone C may result from a decrease in the uncharged Hg-S(-II) species concentration at depth in the sediment ( Fig.…”

Chemical and physical speciation of mercury in Offatts Bayou: A seasonally anoxic bayou in Galveston Bay

“…Enrichment of particulate Hg in the bottom water is possibly due to adsorption of dissolved Hg-sulfide species onto solid CuFeS 2 and FeS. We tested this possibility by modeling the interaction with the following conditions: (1) A neutral surface site abundance for solid FeS of 1.2 mmol g 21 FeS (Wolthers et al 2003); (2) A particulate Fe concentration of 346 nmol L 21 (6.8 m in 2004); (3) The log K for the equilibrium of surface adsorption sites by protons (;FeSH 0 « ;FeS 2 + H + ) is 2 6.5 (Wolthers et al 2003); (4) The log K for the adsorption of inorganic Hg (;FeS 2 + Hg 2+ « ;FeSHg + ) is 36.1 (Miller 2006); (5) The % organic C in SPM ranges from 2% to 10% (Warnken and Santschi 2004); (6) The % reduced sulfur (S) in organic matter is 0.4% (Ravichandran 2004); (7) An SPM concentration of 5.6 mg L 21 ; (8) The pK a for a typical organic thiol (RSH) is 10.0 and (9) The interaction between inorganic Hg and organic thiols is given by RS 2 + Hg 2+ « RSHg + , log K 5 27.3 (Miller 2006 orders of magnitude higher than the concentration ratio observed for the Hg associated with particulate organic matter (the ratio of [Hg-POM] to [Hg 2+ ]). This modeling prediction indicates that solid FeS may be an important sorption site for dissolved Hg in anoxic waters.…”

Effects of ano×ia and sulfide on concentrations of total and methyl mercury in sediment and water in two Hg-polluted lakes

“…Yet another explanation for hypolimnetic Hg enrichment is that Hg(II) and MeHg form soluble complexes with bisulfide and (or) polysulfide or that Hg(II) forms colloidal HgS (Wang and Driscoll 1995;Hintelmann et al 1997;Paquette and Helz 1997). However, precipitation of Hg(II) as HgS(s) or its coprecipitation with FeS may also occur, depending on several factors such as the free sulfide concentration, pH, Fe(II) concentration, and the level and quality of dissolved organic matter (Dyrssen and Wedborg 1991;Ravichandran et al 1999). The fact that sulfide levels were high when [IHg] and [MeHg] peaked in the hypolimnion of the upper lake indicates that the solubility of Hg increased instead of decreasing in the presence of sulfide (Fig.…”