Biotic transformation of inorganic mercury, Hg(II), to mono methyl mercury (MeHg) is proposed to be largely controlled by passive uptake of neutral Hg complexes by sulfate reducing bacteria (SRB). In this study, the chemical speciation of Hg(II) in seven locally contaminated sediments covering environments such as (i) brackish water, (ii) low-productivity freshwater, and, (iii) high-productivity freshwater was related to potential Hg methylation rates, determined by incubation at 23 degrees C for 48 h under N2(g), and to total MeHg concentrations in sediments. Pore water speciation was modeled considering Hg complexes with halides, organic thiols [Hg(SR)2(aq), associated to dissolved organic matter], monosulfides, and bisulfides. The sum of neutral mercury sulfides [Hg(SH)20(aq)] and [HgS0(aq)] was significantly, positively (p < 0.001, n = 20) correlated to the specific methylation rate constant (Km, day(-1)) at depths of 5-100 cm in two brackish water sediments. Total Hg, total mercury sulfides or Hg(SR)2(aq) in pore water gave no significant relationships with Km. In two subsets of freshwater sediments, neutral mercury sulfides were positively correlated to total Hg in pore water, and therefore, total Hg also gave significant relationships with Km. The sum of [Hg(SH)20(aq)] and [HgS0(aq)] was significantly, positively correlated to total sediment MeHg (microg kg-1) in brackish waters (p < 0.001, n = 23), in southern, high-productivity freshwaters (p < 0.001, n = 20), as well as in northern, low-productivity freshwater (p = 0.048, n = 6). The slopes (b, b') of the relationships Km (day-1) = a + b([Hg(SH)20(aq)] + [HgS0(aq)]) and MeHg (microg kg-1) = a' + b'([Hg(SH)20(aq)] + [HgS0(aq)]) showed an inverse relationship with the C/N ratio, supposedly reflecting differences in primary production and energy-rich organic matter availability among sites. We conclude that concentrations of neutral inorganic mercury sulfide species, together with the availability of energy-rich organic matter, largely control Hg methylation rates in contaminated sediments. Furthermore, Hg(SH)20(aq) is suggested to be the dominant species taken up by MeHg producing bacteria in organic-rich sediments without formation of HgS(s).

Knowledge about the chemical speciation of Hg(II) is a prerequisite for a proper understanding of biogeochemical processes in control of the transformation of Hg(II) into toxic and bioaccumulating monomethyl mercury. Of critical importance are structures and the stability of Hg(II)-complexes with inorganic and organic sulfur ligands in aqueous and solid phases of soils and sediments. On the basis of Hg L(III)-edge EXAFS experiments, we report Hg(II) to form a four-coordinated metacinnabar [beta-HgS(s)] phase when reacted with disordered FeS(s) (mackinawite), at pH 9.0 and a Hg(II) to FeS(s) molar ratio of 0.002-0.012. When Hg(II) (1000-20,000 microg Hg g(-1)) was added to mixtures of <5 days of aged FeS(s) (2-20%) and an organic soil at pH 5.7-6.1, a mixture of Hg(II) coordinated with two organic thiols [Hg(SR)(2)] and Hg(II) coordinated with four inorganic sulfides in a metacinnabar-like phase was formed. Surface complex formation between Hg(II) and FeS(s), or substitution of Hg(II) for Fe(II) in FeS(s), was not observed. Quantities of beta-HgS(s) and Hg(SR)(2) formed (as determined by EXAFS) were in fair agreement with theoretical thermodynamic calculations, as described by the reaction: Hg(SR)(2) + FeS(s) = HgS(s) + Fe(2+) + 2RS(-). The calculated stability constant for this reaction (log K = -16.1 - -15.4) supports a strong bonding of Hg(II) to organic thiols, corresponding to a log beta(2) for the formation of Hg(SR)(2) on the order of 42 or greater.

Relationships between the short-term mono-methyl mercury (MeHg) production, determined as the specific, potential methylation rate constant Km (day(-1)) after 48 h of incubation with isotope-enriched 201Hg(II) at 23 degrees C, and the long-term accumulation of ambient MeHg, were investigated in contaminated sediments. The sediments covered a range of environments from small freshwater lakes to large brackish water estuaries and differed with respect to source and concentration of Hg, salinity, primary productivity, quantity and quality of organic matter, and temperature climate. Significant (p < 0.001), positive relationships were observed between Km (day(-1)) and the concentration of MeHg normalized to total Hg (%MeHg) for surface sediments (0-10, 0-15, and in one case 0-20 cm) across all environments, and across subsets of organic and minerogenic freshwaters. This suggests that the methylation process (MeHg production) overruled demethylation and net transport processes in the surface sediments. The lack of a relationship between Km and %MeHg in two brackish water sediment depth profiles (0-100 cm) indicates that demethylation and the net effect of input-output are relatively more important at greater depths. Differences in the primary production and subsequent availability of easily degradable organic matter (serving as electron donor for methylating bacteria) was indicated to be the most important factor behind observed differences in %MeHg and Km among sites. In contrast, concentrations of sulfate were not correlated to Km, %MeHg, or absolute concentrations of MeHg. We conclude that total concentrations of Hg are of importance for the long-term accumulation of MeHg, and that %MeHg in surface sediments can be used as a proxy for the rate of methylation, across a range of sites from different environments.