Environ. Sci. Technol. 2010, 44, 278–283
Substantial Emission of Gaseous Monomethylmercury from Contaminated Water-Sediment Microcosms S O F I J O N S S O N , †,‡ U L F S K Y L L B E R G , § A N D ¨ R N * ,† ERIK BJO Department of Chemistry, Umeå University, SE-901 87 Umeå, Sweden, Umeå Marine Sciences Centre, Umeå University, SE-910 20 Ho¨rnefors, Sweden, and Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83 Umeå, Sweden
Received July 8, 2009. Revised manuscript received October 31, 2009. Accepted November 11, 2009.
Emission rates of gaseous monomethylmercury (CH3HgII), as well as elemental mercury (Hg0) and dimethylmercury [(CH3)2HgII], were determined in Hg-contaminated water-sediment microcosms (duplicates of three treatments) by gaseous speciesspecific isotope dilution analysis (SSIDA). Incubation of ∼500 g (wet mass) of sediments containing 30 µmol of ambient Hg with an addition of 2.6 µmol of 201HgII tracer resulted in average (n ) 6) gaseous emissions of 84 ( 26, 100 ( 37, and 830 ( 380 pmol of ambient CH3HgII, CH3201HgII, and 201Hg0, respectively, during 108 days of incubation. In contrast to Hg0, a transient temporal pattern was observed for measured CH3HgII emission rates, which peaked at day 12 and decreased to much lower levels by the end of the experiments. At day 12, CH3HgII constituted 30-50% of the total emitted gaseous Hg, emphasizing the significance of this species to total Hg emissions from anoxic sediment-water systems. Emission rates of gaseous CH3HgII did not reflect the accumulated CH3HgII content in the sediment, suggesting that emissions mainly originated from newly methylated HgII. Speciation modeling of the pore water suggests that CH3HgII was emitted as CH3HgSH0(g).
Introduction Formation and evasion of gaseous mercury (Hg) species from aquatic ecosystems is important for the biogeochemical cycle of Hg (1). Studies of such processes have been focused on emissions of elemental Hg (Hg0), indirectly by detection of dissolved gaseous mercury (DGM). Significant contributions from volatile dimethylmercury [(CH3)2HgII] to DGM have been reported (2), whereas monomethylmercury (CH3HgII), which can form semivolatile complexes, usually has been considered to give insignificant contributions. The production of DGM in aquatic systems has primarily been localized to surface waters (3-7) but has also been observed at greater water depths, as well as at the water-sediment interface (3, 6, 8). For surface water, the net effect of photoinduced Hg reduction and oxidation is generally believed to control the concentration of DGM. Suggested reduction mechanisms are photo* Corresponding author phone: +46(0)90-786 51 89; e-mail:
[email protected]. † Department of Chemistry, Umeå University. ‡ Umeå Marine Sciences Centre, Umeå University. § Swedish University of Agricultural Sciences. 278
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 1, 2010
induced electron transfer to HgII in dissolved organic carbon (DOC)-HgII complexes or via photoactivated DOC compounds to other dissolved HgII complexes (6, 9). Mechanisms of photooxidation are less studied, but in laboratory experiments, chloride and semiquinones have been shown to be involved (10, 11). Processes of DGM formation at greater water depths and at the sediment-water interface include reduction of CH3HgII and HgII to Hg0 by Hg-resistant bacteria via the mer operon (12, 13) and abiotic reduction of HgII by humic substances at low oxygen and Cl concentrations (14) or via oxidation of FeII to FeIII under strictly anaerobic conditions (15). Formation, degradation, and transportation of CH3HgII in aquatic systems are of great concern due to this compound’s bioaccumulative and toxic properties. In brackish and saline water systems, CH3HgII is formed mainly under suboxic and anoxic redox conditions mediated by sulfatereducing bacteria (SRB). These bacteria are believed to take up neutral HgII-sulfide complexes via diffusion and then, accidentally, to methylate HgII (16). The formation of CH3HgII is thus governed by the bioavailability of HgII and the activity of the SRB. Methylation of HgII via Fe-reducing bacteria (FeRB) activity and by abiotic processes have also been reported (17, 18). The sediment-water interface, including anoxic bottom water, has been shown to be important for CH3HgII formation in freshwater and estuarine ecosystems (19-21). To what extent volatilization of CH3HgII from this zone also occurs is, however, uncertain. The only semivolatile CH3HgII complexes with a known Henry’s law constant are complexes with hydroxide and halides (Table S6, Supporting Information). However, in suboxic and anoxic sediment pore waters, the concentrations of these complexes are several orders of magnitude lower than complexes with reduced sulfur ligands for which the Henry’s law constant is not established. Evasion of gaseous CH3HgII is potentially important for transporting CH3HgII from the sediment-water interface, but knowledge about this process is scanty, likely because of the lack of suitable analytical methods to quantify gaseous CH3HgII. Larsson et al. (22, 23) recently developed a methodology based on species-specific isotope dilution analysis (SSIDA) for the determination of gaseous Hg0, HgII, CH3HgII, and (CH3)2HgII, where species-specific Hg isotopically enriched internal standards are continuously added to the gaseous sample during sampling. This approach proved to efficiently address many of the problems present in other methods for gaseous Hg speciation and fractionation, including incomplete derivatization, trapping, and desorption of Hg species, as well as Hg species transformation reactions. In the present study, emission rates were determined for gaseous CH3HgII, as well as Hg0 and (CH3)2HgII, formed from ambient Hg and an added 201HgII tracer, in Hg-contaminated sediment-water microcosm systems. The SSIDA methodology was used and three different microcosm treatment schemes were investigated in duplicate: (i) exposure to indoor light, (ii) addition of acetate to dark microcosms, and (iii) reference systems with no exposure to light or addition of acetate.
Experimental Section Sediment Sampling and Characterization. The top 5 cm of the sediment and near-bottom water were collected in September 2007 at Ko¨pmanholmen, Sweden (63°10′ N, 18° 36′ E), at a water depth of 6.5 m by use of a core sampler. The site has been subjected to local Hg contamination from chlor-alkali industry and pulp fiber from pulp industrial activities [for details see Drott et al. (24)]. Contact with air 10.1021/es9020348
2010 American Chemical Society
Published on Web 12/01/2009
TABLE 1. Concentrations, Log KD, and Species Distribution of Ambient HgII and CH3HgII in Pore Water for Original Sediment and Duplicates of Microcosmsa
sediment ref I ref II L-T I L-T II Ac-T I Ac-T II
HgII, nmol L-1
log KD, L kg-1
HgS22-, %
HgSH-, %
Hg(SH)20, %
Hg(SR)2, %
CH3HgII, pmol L-1
log KD, L kg-1
CH3HgS-, %
CH3HgSH0, %
CH3HgSR, %
0.61 2.1 1.9 1.3 2.1 1.1 1.1
5.2 4.8 4.9 5.2 4.6 5.0 5.2
1.1 9.7 10 5.0 7.0 9.6 14
67 88 88 90 89 88 85
31 2.5 2.4 5.1 3.6 2.6 1.6
