G2 (I) Mercury stable isotope biogeochemistry

Soils are the most important terrestrial sink for atmospherically deposited mercury. Iron (oxyhydr)oxides are common soil minerals and can play an important role in the immobilisation of mercury, especially in organic-poor soils. Environmental changes like acidification, salinization, or land use change can lead to a leaching of mineral-bound mercury. Stable mercury isotopes are a promising tracer for the biogeochemical Hg cycle in soils. To interpret natural Hg isotope ratios, a fundamental understanding of the processes causing Hg isotope fractionation in soils is needed. Therefore we investigated the sorption of Hg(II) to goethite (a-FeOOH), an important Fe (oxyhydr)oxide mineral in most soils, and the corresponding isotope fractionation mechanisms.
We performed laboratory scale sorption experiments with Hg(II) solution (dissolved Hg(II)-nitrate) and goethite (synthesized and characterized) under different pH conditions (buffered at pH 7 with 2.5 mM MOPS, or unbuffered at pH 2-6) and different Cl^- concentrations (0 and 0.5 mM). Equilibration experiments from 12 to 72 h and kinetic desorption experiments (acidification to pH 3) between 0.25 and 24 h were performed. The dissolved fraction was separated from the goethite by centrifugation and filtration, and the sorbed fraction was collected by dissolving the goethite in 6M HCl. Concentrations were measured by CV-AFS and isotope ratios by CV-MC-ICPMS (Nu Plasma) yielding a standard reproducibility of ±0.07 ‰ (2SD for d²⁰²Hg, 18 measurements over 6 months) using Tl mass bias correction and bracketing standards.

The experimental data revealed systematic mass dependent fractionation (MDF) during the equilibration experiments (72 h) with an enrichment of light Hg isotopes at the goethite surface (d²⁰²Hg_diss - d²⁰²Hg_sorb = 0.4 ‰). Low pH and increased Cl^- concentration reduced the sorbed fraction of Hg(II), but did not affect the isotope fractionation factor compared with pH 7. No mass independent fractionation (MIF), which might be caused by nuclear volume effects in equilibrium systems, was measured in all experiments. Our study provides: (i) kinetic and equilibrium isotope fractionation factors of Hg(II) sorption to goethite and (ii) new insights into the sorption and desorption kinetics and mechanisms. In combination with the previous findings on Hg(II) sorption to thiol groups (Wiederhold et al., 2010, ES&T, 44, 4191–4197), a model study for sorption to organic matter, our data suggest that light Hg isotopes are preferentially sequestered in soils and one could expect an enrichment of heavy Hg isotopes in the mobile fraction which is leached from soils into surrounding ecosystems.

Alkylation of inorganic mercury (IHg) to organic Hg species (i.e. MeHg) is one of the most important transformations occurring in the biogeochemical cycle of Hg because MeHg accumulate in food webs and is a highly toxic form in the environment. Hg is predominantly methylated by sulphate-reducing bacteria in aquatic environment at the water-sediment interface, but it can be also methylated via abiotic pathways, notably by methyl donors such as methylcobalamin (MeCo), a vitamin B12 derivative. Stable isotopes of Hg have recently been used to assess the extent of Hg fractionation during either biotic and/or methylation, demethylation and reduction pathways.

The aim of this study was to compare and clear up the isotopic fractionation of each Hg species during laboratory kinetic studies of either biotic or abiotic Hg methylation. Biotic methylation experiments have been carried out incubating inorganic Hg²⁺ (NIST 3133) with Desulfovibrio sp. (BerOc1) under fermentative and for the first time under sulphate-reducing conditions. Abiotic methylation experiments have been carried out incubating Hg²⁺ (NIST 3133) with MeCo under different conditions. Both experiments have been done during 48h with sampling of aliquots at different time. Hg isotopic signatures of species were determined simultaneously by coupling Gas chromatography with a multi-collector ICP-MS after a derivatization step. Quantification of the species has been done by isotopic dilution GC-Q-ICP-MS.

Quantification of the species at each sampling time has shown good recovery of initial Hg concentration and has allowed to determine rate constants of methylation/demethylation reactions. Biotic methylation, both under fermentative and sulphate-reducing conditions, produces MeHg which is enriched in lighter isotopes (d²⁰²MeHg between -0.88 and -0.45 ‰) in comparison with residual IHg (d²⁰²IHg between +0.32 to +0.62‰). Abiotic methylation experiments also show an enrichment of the products in lighter isotopes relative to the reagents (d²⁰²MeHg of -0.56 to -0.35‰), while the inhibition of the methylation extent by the addition of NaCl yields higher isotope fractionation (d²⁰²MeHg from -2.11 to -1.07‰).

This study demonstrates that biotic and abiotic methylation of Hg provides kinetic mass-dependent fractionation of Hg isotopes which may help to unravel Hg isotopic signature in environmental samples. For example, Hg methylation by anaerobic bacteria seems to be independent from the bacteria physiology (i.e. sulphate-reductoin vs fermentation) and closely controlled by an intracellular methylation step.

The recent discovery of both large mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes has made the promise of tracing Hg in the environment and in food webs very exciting. In order to utilize Hg isotopes, it is necessary to understand mechanisms of fractionation along with the environmental factors that affect the expression of isotopic fractionation. Currently it is thought that Hg isotope fractionation is driven by three mechanisms: (1) classical MDF, (2) the magnetic isotope effect (MIE), and (3) the nuclear volume effect (NVE). The MIF observed for Hg isotopes is thought to arise either from the MIE, which is due to the non-zero nuclear spin and nuclear magnetic moments of odd-mass isotopes, or the NVE, which arises from the non-linear scaling of nuclear volume with mass for heavy isotopes. Distinguishing MIE and NVE is difficult because both predict MIF for only the odd isotopes. However it may be possible to distinguish between these mechanisms and different biogeochemical transformations of Hg using both the magnitude of MIF observed and the ∆¹⁹⁹Hg/∆²⁰¹Hg ratio. Results of recent experiments to characterize MIE and NVE for Hg reactions will be discussed.

So far, the only processes demonstrated experimentally to produce large MIF for Hg (>1‰, similar in magnitude to the MIF observed in natural samples such as fish) are photochemical processes. The first experiments that demonstrated MIF for Hg isotopes were for aqueous photo-reduction, and it was suggested that the cause of MIF was the MIE. We have continued to investigate photo-reduction and the factors that affect photo-reduction. Experiments were performed investigating the frequency and intensity of light available and the amount and type of dissolved organic matter. We observe a relationship between the magnitude of MIF and the energy of light available, and the type and amount of DOC also affect MIF. We have also investigated aqueous photo-oxidation and observe large MIF for this process. Additionally, MIF is observed dark kinetic reactions. With large MIF, it is likely that the cause is MIE. However with smaller MIF (<0.5‰), the NVE could also be responsible for MIF. To confirm theoretical predictions of the isotopic signature of the NVE for Hg, we investigated mercury liquid-vapor evaporation under equilibrium conditions in the dark at room temperature (similar to Estrade et al., 2008). These Isotopic results and their ∆¹⁹⁹Hg/∆²⁰¹Hg ratio Hg ratios will be discussed.

Preliminary studies have demonstrated both mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes in the environment and the potential for their application in biochemistry and geochemistry (1). Though atmospheric deposition is a primary pathway by which Hg enters earth surface ecosystem, little has been reported on Hg isotopes in precipitation. Laboratory experiments and measurements of Hg accumulated in lichens predicted negative MIF of odd Hg isotopes (¹⁹⁹Hg, ²⁰¹Hg) in the atmosphere. However, a recent study showed positive MIF of odd Hg isotopes in both precipitation and ambient air and reported, for the first time, positive MIF of even-mass Hg isotope (²⁰⁰Hg) in precipitation (2).

In order to examine Hg isotopic composition in precipitation, 19 rainwater and 4 snow samples were collected in Peterborough (Ontario, Canada) in 2010. Hg isotopic compositions were determined after Hg pre-concentration using a method developed by Chen et al. (2010). The results displayed evident MDF and confirmed MIF of both odd and even Hg isotopes. d²⁰²Hg in samples varied from -1.59‰ to -0.02‰. An urban impact was evident from single event samples when considering also metrological data. Low-temperature samples (snow and several rain events) showed distinctive patterns, pointing to a different source. Our results imply that the (atmospheric) process introducing the MIF for even Hg isotopes may be different from that producing MIF of odd isotopes in the aqueous environment. More research is required to fully understand the behavior of Hg isotopes in the atmosphere.

1) Bergquist, B. A.; Blum, J. D. Sci. 2007, 318, 417-420. 2) Gratz et al., EST 2010, 44, 7764-7770. 3) Chen et al., JAAS,2010, 25, 1402-1409.

The transport and fate of atmospheric mercury emissions from anthropogenic and natural sources of mercury (Hg) is uncertain. Due to the long atmospheric residence time of elemental Hg (Hg⁰), it is difficult to distinguish local and regional sources of atmospheric Hg from the global background. Recent evidence suggests that differences in the isotopic composition of Hg in atmospheric samples may reflect differences in the source of Hg and atmospheric processes that may cause isotopic fractionation after emission. If these potential sources of variation in the Hg isotope composition can be discerned from one another then the Hg isotope signature of a variety of atmospheric samples representing different Hg species could serve as a powerful tool for understanding the transport and fate of emitted Hg. Here we report the Hg isotope composition of particulate Hg (Hg_P) collected daily during a two-week field campaign in August 2010 at Pensacola, FL and Grand Bay, MS. Both sites were characterized by negative d²⁰²Hg (d²⁰²Hg = -2.09‰ to -0.50‰). Significant positive mass-independent fractionation (MIF) was observed in all samples with the exception of two. The positive MIF values range from ∆¹⁹⁹Hg = 0.24‰ to 1.78‰ and ∆²⁰¹Hg = 0.06‰ to 1.59‰. Two samples, one collected at each site on adjacent days, displayed negative MIF (∆¹⁹⁹Hg = -2.36‰ at Pensacola; ∆¹⁹⁹Hg = -1.21‰ at Grand Bay). The large variation in the isotopic composition of Hg_P observed suggests that the homogeneity of atmospheric Hg is affected by local and regional sources of atmospheric Hg or by local and regional atmospheric processes or a combination of both. Furthermore, analysis of the Hg isotope composition of total gaseous mercury (TGM) samples, large volume precipitation samples, and samples of reactive gaseous mercury (RGM) collected on KCl-coated Whatman QM-A quartz fiber filters from both sites may help elucidate the processes that fractionate Hg in the atmosphere.

Forests play a key role in the cycling of Hg between the atmosphere and terrestrial ecosystems, yet the sources and fate of Hg stored and exchanged within forests are somewhat elusive. In particular, it is difficult to identify the source(s) of Hg accumulated in foliage or evaded from the forest floor. Here, we use natural abundance Hg isotope measurements to trace the biogeochemical cycling of Hg in a forested ecosystem. We quantified the stable Hg isotopic composition of precipitation, foliage, soil, and total gaseous mercury (THg_gaseous = GEM + RGM) in the atmosphere, as well as in evasion from soil, in 10-year-old aspen stands at the FACE experiment in Rhinelander, WI, USA. Precipitation samples were characterized by d²⁰²Hg values ranging from -0.74 to 0.06 ‰, and ∆¹⁹⁹Hg values ranging from 0.16 to 0.82 ‰. The isotopic composition of THg_gaseous collected above the forest canopy was characterized by d²⁰²Hg values ranging from 0.48 to 0.93 ‰ and ∆¹⁹⁹Hg values ranging from -0.21 to -0.15 ‰. Uptake of gaseous mercury by foliage resulted in a large (-3.0 ‰) shift in d²⁰²Hg values, with the foliage displaying d²⁰²Hg values ranging from -2.53 to -1.89 ‰. The ∆¹⁹⁹Hg values of foliage ranged from -0.37 to -0.23 ‰. Forest floor soil samples were characterized by d²⁰²Hg values ranging from -1.88 to -1.22 ‰ and ∆¹⁹⁹Hg values ranging from -0.22 to -0.14 ‰. On a d²⁰²Hg vs ∆¹⁹⁹Hg plot, soils fall along a mixing line between foliage and precipitation, which represent sources of wet and dry deposition to the forest floor. The isotopic composition of Hg evading from the forest floor was similar to that of THg_gaseous collected above the forest canopy. Because there are no mechanisms that we would expect to produce a large shift in d²⁰²Hg during evasion of mercury from forest soils, we conclude that mercury evading from the forest floor originates from the volatilization of recent wet deposition of mercury (i.e., precipitation), and does not represent the emission of legacy Hg from the soil. Furthermore, we calculate that the large mass dependent isotope fractionation of mercury upon uptake by vegetation may be important to understanding the isotopic balance of the global mercury cycle.

Mercury (Hg) levels in lichens and mosses have been used as surrogates for atmospheric Hg deposition to continental surfaces. Recently the stable isotopic composition of Hg in epiphytic (tree-located) lichens was studied in both urban and remote locations (Carignan et al., 2009). These authors observed substantially negative mass independent Hg isotope fractionation (MIF) with ∆¹⁹⁹Hg down to -1 ‰ in North-America. Recent direct measurements of Hg MIF in North–American atmospheric precipitation found exclusively positive ∆¹⁹⁹Hg up to +0.5 ‰ (Gratz et al., 2010). Unpublished co-located lichen and precipitation samples appear to confirm a similar contradiction between directly measured atmospheric Hg in precipitation and Hg recovered from epiphytes (Ghosh and Odom., 2010).

In this study we collected and analyzed Hg concentrations in 160 samples of epiphytic lichens and terricolous (growing on the ground) lichens and mosses from remote locations across the Russian Arctic and sub-Arctic (50 to 72^oN, 30 to 160^oE). Sixty sub-samples were analyzed for their Hg stable isotope signatures by MC-ICPMS after micro-wave assisted acid digestion following Estrade et al., 2010. Hg concentrations ranged from 14 to 5987 ng/g (median 49 ng/g). Epiphytic lichens had significantly higher Hg levels (255 ng/g) than terricolous lichens (40 ng/g) and mosses (79 ng/g). Both mass dependent and mass independent Hg isotope fractionation was observed across all samples: d²⁰²Hg, -5.7 to +0.1 ‰; ∆¹⁹⁹Hg, -0.5 to +0.3 ‰. The relationship between Hg MIF signatures, with a linear regression slope of ∆¹⁹⁹Hg over ∆²⁰¹Hg of 1.1 suggests a dominant photochemical origin for Hg MIF. The data will be interpreted in the context of the above outlined contradiction between direct and indirect atmospheric Hg deposition. In particular the possibility that MIF signatures originate from post-emission MIF will be discussed.

Carignan, J., Estrade, N., Sonke, J. E., and Donard, O. F. X., 2009. ES&T 43, 5560-5564.
Estrade, N., Carignan, J., Sonke, J. E., and Donard, O. F. X., 2010. GGR 34, 79-93.
Ghosh, S. and Odom, A. L., 2010. In vivo Hg MIF effect in epiphytes? GCA 74, A328.
Gratz, L., Keeler, G., Blum, J., and Sherman, L. S., 2010. ES&T 44, 7764–7770.

A single species of epiphytic tree lichen (Hypogymnia physodes) was sampled at remote sites in a grid pattern 0.5 to 200 km from the Alberta oil sands industrial complex north of Ft McMurray, Alberta, Canada. Mercury concentrations in lichens have been used in many previous studies as a measure of spatial variations in atmospheric Hg deposition rates. The stable Hg isotopic composition has also recently been used as a monitor of additions of anthropogenic Hg to its regional background deposition. Hg concentrations ranged from 0.07 to 0.27 µg/g, which is similar to that measured by others for remote sites >800 km from Hudson Bay (beyond the area of influence of atmospheric mercury depletion events), in southern Ontario and Quebec. Concentrations were not correlated with distance from the sources, and based on concentration alone we would conclude that there is no evidence for anomalously high atmospheric Hg deposition near the industrial complex. However, the stable Hg isotope composition of the lichens does change systematically with distance from these sources. Most notably the lichens display varying levels of mass independent fractionation (MIF) with both ∆201Hg and ∆199Hg ranging from near 0.0‰ close to the sources and systematically falling to -0.4‰ about 10 km farther away. The ∆199Hg/∆201Hg slope is close to one, suggesting that the MIF results from photochemical reduction of Hg(II) to Hg(0). Mass dependent fractionation (MDF) does not change systematically with distance from the oil sands development sources, and d202Hg averages -1.9‰. The spatial trends show that the industrial sources affect the MIF isotope values without significantly affecting the MDF values or the Hg concentrations of the lichens. It is possible that other atmospheric pollutants, such as SO2 (which is known to affect the vitality of lichens), are influencing Hg deposition rates to lichens and also possibly affecting the degree of photochemical reduction of Hg on lichen surfaces, which may be the cause of the observed MIF. Investigations of the Hg concentration and isotopic composition of emissions from the oils sands development may provide further insight into the interpretation of Hg isotope trends in the lichens.