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Mercury (Hg) is a toxic trace element of global environmental concern which has been increasingly dispersed into the environment since the industrial revolution. In aquatic and terrestrial systems, Hg can be reduced to elemental Hg (Hg0) and escape to the atmosphere or converted to methylmercury (MeHg), a potent neurotoxin that accumulates in food webs. FeII-bearing minerals such as magnetite, green rusts, siderite, and mackinawite are recognized HgII reducers. Another potentially Hg-reducing mineral, which commonly occurs in Fe- and organic/P-rich sediments and soils, is the ferrous iron phosphate mineral vivianite (FeII3(PO4)2·8H2O), but its reaction with HgII has not been studied to date. Here, nanoparticulate vivianite (particle size ∼ 50 nm; FeII content > 98%) was chemically synthesized and characterized by a combination of chemical, spectroscopic, and microscopic analyses. Its ability to reduce HgII was investigated at circumneutral pH under anoxic conditions over a range of FeII/HgII ratios (0.1–1000). For FeII/HgII ratios ≥1, which are representative of natural environments, HgII was very quickly and efficiently reduced to Hg0. The ability of vivianite to reduce HgII was found to be similar to those of carbonate green rust and siderite, two of the most effective Hg-reducing minerals. Our results suggest that vivianite may be involved in abiotic HgII reduction in Fe and organic/P-rich soils and sediments, potentially contributing to Hg evasion while also limiting MeHg formation in these ecosystems., Environmental Science & Technology, 55 (5), ISSN:0013-936X, ISSN:1520-5851

International audience; Monomethylmercury (MMHg) concentrations in aquatic biota from Lake Titicaca are elevated although the mercury (Hg) contamination level of the lake is low. The contribution of sediments to the lake MMHg pool remained however unclear. In this work, seven cores representative of the contrasted sediments and aquatic ecotopes of Lake Titicaca were sliced and analyzed for Hg and redox-sensitive elements (Mn, Fe, N and S) speciation in pore-water (PW) and sediment to document early diagenetic processes responsible for MMHg production and accumulation in PW during organic matter (OM) oxidation.The highest MMHg concentrations (up to 12.2 ng L−1 and 90% of THg) were found in subsurface PWs of the carbonate-rich sediments which cover 75% of the small basin and 20% of the large one. In other sediment facies, the larger content of OM restricted MMHg production and accumulation in PW by sequestering Hg in the solid phase and potentially also by decreasing its bioavailability in the PW. Diagenetically reduced S and Fe played a dual role either favoring or restricting the availability of Hg for biomethylation.The calculation of theoretical diffusive fluxes suggests that Lake Titicaca bottom sediments are a net source of MMHg, accounting for more than one third of the daily MMHg accumulated in the water column of the Lago Menor. We suggest that in the context of rising anthropogenic pressure, the enhancement of eutrophication in high altitude Altiplano lakes may increase these MMHg effluxes into the water column and favor its accumulation in water and biota.

Mercury (Hg) pollution threatens ecosystems and human health. Wastewater treatment plants (WWTPs) play a key role in limiting Hg discharges from wastewaters to rivers and lakes, but large-scale studies to estimate Hg loads and discharge at national levels are scarce. We assessed the concentration, flux, speciation, and removal of Hg in municipal wastewater throughout Switzerland by investigating 64 WWTPs in a pre-study and a subset of 28 WWTPs in the main study. We also studied the behavior and pathways of Hg along the various treatment steps in a state-of-the-art WWTP. The resulting dataset, representative of industrialized countries, provides an overview of (i) current Hg concentration ranges, (ii) average per capita loads, and (iii) wastewater Hg inputs into surface waters. The results allowed estimation of a total Hg (THg) load in Swiss wastewater of 130 ± 30 kg THg/year (15.7 mg/capita/y), of which 96 ± 4% is retained in sewage sludge. About 4.7 ± 0.5 kg THg/year (0.57 mg/capita/y) is discharged with the treated wastewater into surface waters. This corresponds to only 1.5–3% of the THg load carried by the major Swiss rivers, indicating that >95% of riverine Hg originates from other sources. Extrapolation to the population of Europe would yield a total amount of 11,700 kg THg/year in raw wastewater, with some 480 kg THg/year discharged to surface waters. Monomethyl mercury on average accounted for 0.23% of THg, and its fraction remained constant along the different treatment steps. ISSN:0043-1354 ISSN:1879-2448

International audience; Oxygen-​depleted areas are spreading in coastal and offshore waters worldwide, but the implication for prodn. and bioaccumulation of neurotoxic methylmercury (MeHg) is uncertain. We combined observations from six cruises in the Baltic Sea with speciation modeling and incubation expts. to gain insights into mercury (Hg) dynamics in oxygen depleted systems. We then developed a conceptual model describing the main drivers of Hg speciation, fluxes, and transformations in water columns with steep redox gradients. MeHg concns. were 2-​6 and 30-​55 times higher in hypoxic and anoxic than in normoxic water, resp., while only 1-​3 and 1-​2 times higher for total Hg (THg)​. We systematically detected divalent inorg. Hg (HgII) methylation in anoxic water but rarely in other waters. In anoxic water, high concns. of dissolved sulfide cause formation of dissolved species of HgII: HgS2H-​(aq)​ and Hg (SH)​20(aq)​. This prolongs the lifetime and increases the reservoir of HgII readily available for methylation, driving the high MeHg concns. in anoxic zones. In the hypoxic zone and at the hypoxic-​anoxic interface, Hg concns., partitioning, and speciation are all highly dynamic due to processes linked to the iron and sulfur cycles. This causes a large variability in bioavailability of Hg, and thereby MeHg concns., in these zones. We find that zooplankton in the summertime are exposed to 2-​6 times higher MeHg concns. in hypoxic than in normoxic water. The current spread of hypoxic zones in coastal systems worldwide could thus cause an increase in the MeHg exposure of food webs.

International audience; Volcanism is one of the major natural processes emitting mercury (Hg) to the atmosphere, representing a significant component of the global Hg budget. The importance of volcanic eruptions for local-scale Hg deposition was investigated using analyses of Hg, inorganic elemental tracers, and organic biomarkers in a sediment sequence from Lake Chungará (4520 m a.s.l.). Environmental change and Hg deposition in the immediate vicinity of the Parinacota volcano were reconstructed over the last 2700 years, encompassing the pre-anthropogenic and anthropogenic periods. Twenty eruptions delivering large amounts of Hg (1 to 457 μg Hg m−2 yr−1 deposited at the timescale of the event) were locally recorded. Peaks of Hg concentration recorded after most of the eruptions were attributed to a decrease in sedimentation rate together with the rapid re-oxidation of gaseous elemental Hg and deposition with fine particles and incorporation into lake primary producers. Over the study period, the contribution of volcanic emissions has been estimated as 32% of the total Hg input to the lake. Sharp depletions in primary production occurred at each eruption, likely resulting from massive volcaniclastic inputs and changes in the lake-water physico-chemistry. Excluding the volcanic deposition periods, Hg accumulation rates rose from natural background values (1.9 ± 0.5 μg m−2 yr−1) by a factor of 2.3 during the pre-colonial mining period (1400–900 yr cal. BP), and by a factor of 6 and 7.6, respectively, during the Hispanic colonial epoch (400–150 yr cal. BP) and the industrial era (~140 yr cal. BP to present). Altogether, the dataset indicates that lake primary production has been the main, but not limiting, carrier for Hg to the sediment. Volcanic activity and climate change are only secondary drivers of local Hg deposition relative to the magnitude of regional and global anthropogenic emissions.

In many northern temperate regions, the water color of lakes has increased over the past decades (lake browning), probably caused by an increased export of dissolved organic matter from soils. We i ...

International audience; The current EU legislation lays down Environmental Quality Standards (EQS) for 45 priority substances in surface waters; among them levels for (organo)metallic species of Hg, Sn and Pb are set between ng L−1 (for Hg and Sn) and μg L−1 (for Pb). To date, only a few analytical methods can reach these very restrictive limits and there is thus a need for comprehensive methods able to analyze these species down to these levels in natural waters. The aim of this work was to develop an online automated pre-concentration method using large volume injections with a Programmed Temperature Vaporization (PTV) injector fitted with a sorbent packed liner coupled to GC-ICP-MS to further improve the detection limits associated to this well-established method. The influence of several parameters such as the PTV transfer temperature and time, carrier gas flow rate and amount of packing material was investigated. Finally, the maximum volume injected through single or multiple injection modes was optimized to obtain the best compromise between chromatographic resolution and sensitivity. After optimization, very satisfactory results in terms of absolute and methodological detection limits were achieved, down to the pg L−1 level for all species studied. The potential of the method was exemplified by determining the concentrations of organometallic compounds in unpolluted river waters samples from the Adour river basin (SW France) and results were compared with conventional (splitless) GC-ICP-MS. The strength of this analytical method lies in the low detection limits reached for the simultaneous analysis of a wide group of organometallic compounds, and the potential to transfer this method to other gas chromatographic applications with inherent lower sensitivity.

Because mercury (Hg) undergoes significant biogeochemical processes along the estuarine-coastal continuum, the objective of this work was to investigate the distribution and reactivity of methylmercury (MeHg), inorganic mercury (Hg(II)) and gaseous Hg (DGM) in plume waters of the Adour River estuary (Bay of Biscay). Vertical profiles, spatial and tidal variability of Hg species concentrations were evaluated during two campaigns (April 2007 and May 2010) characterized by significant plume extents over the coastal zone. Incubations with isotopically enriched tracers were performed on bulk and filtered waters under sunlight or dark conditions to investigate processes involved in Hg methylation, demethylation and reduction rates. Total Hg(II) concentrations were more dispersed in April 2007 (5.2 ± 4.9 pM) than in May 2010 (2.5 ± 1.1 pM) while total MeHg concentrations were similar for both seasons and averaged 0.13 ± 0.07 and 0.18 ± 0.11 pM, respectively. DGM concentrations were also similar between the two campaigns, averaging 0.26 ± 0.10 and 0.20 ± 0.09 pM, respectively. Methylation yields remained low within the estuarine plume (0.01-0.4% day(-1)) while MeHg was efficiently demethylated via both biotic and abiotic pathways (2.3-55.3% day(-1)), mainly photo-induced. Hg reduction was also effective in these waters (0.3-43.5% day(-1)) and was occurring in both light and dark conditions. The results suggest that the plume is overall a sink for MeHg with integrated net demethylation rates, ranging from 2.0-3.7 g (Hg) d(-1), in the same range than the estimated MeHg inputs from the estuary (respectively, 0.9 and 3.5 g (Hg) d(-1)). The large evasion of DGM from the plume waters to the atmosphere (8.8-26.9 g (Hg) d(-1)) may also limit HgT inputs to coastal waters (33-69 g (Hg) d(-1)). These processes are thus considered to be most significant in controlling the fate of Hg transferred from the river to the coastal zone.

International audience; Stable isotopic tracer methodologies now allow the evaluation of the reactivity of the endogenous (ambient) and exogenous (added) Hg to further predict the potential effect of Hg inputs in ecosystems. The differential reactivity of endogenous and exogenous Hg was compared in superficial sediments collected in a coastal lagoon (Arcachon Bay) and in an estuary (Adour River) from the Bay of Biscay (SW France). All Hg species (gaseous, aqueous, and solid fraction) and ancillary data were measured during time course slurry experiments under variable redox conditions. The average endogenous methylation yield was higher in the estuarine (1. 2 %) than in the lagoonal sediment (0. 5 %), although both methylation and demethylation rates were higher in the lagoonal sediment in relation with a higher sulfate-reducing activity. Demethylation was overall more consistent than methylation in both sediments. The endogenous and exogenous Hg behaviors were always correlated but the exogenous inorganic Hg (IHg) partitioning into water was 2. 0-4. 3 times higher than the endogenous one. Its methylation was just slightly higher (1. 4) in the estuarine sediment while the difference in the lagoonal sediment was much larger (3. 6). The relative endogenous and exogenous methylation yields were not correlated to IHg partitioning, demonstrating that the bioavailable species distributions were different for the two IHg pools. In both sediments, the exogenous IHg partitioning equaled the endogenous one within a week, while its higher methylation lasted for months. Such results provide an original assessment approach to compare coastal sediment response to Hg inputs.

International audience; As the methylation of inorganic mercury to neurotoxic methylmercury has been attributed to the activity of anaerobic bacteria, the formation of methylmercury in the oxic water column of marine ecosystems has puzzled scientists over the past years. Here we show for the first time that methylmercury can be produced in particles sinking through oxygenated water column of lakes. Total mercury and methylmercury concentrations were measured in the settling particles and in surface sediments of the largest freshwater lake in Western Europe (Lake Geneva). While total mercury concentration differences between sediments and settling particles were not significant, methylmercury concentrations were about ten-fold greater in settling particles. Methylmercury demethylation rate constants (kd) were of similar magnitude in both compartments. In contrast, mercury methylation rate constants (km) were one order of magnitude greater in settling particles. The net potential for methylmercury formation, assessed by the ratio between the two rate constants (km kd -1), was therefore up to ten fold greater in settling particles, denoting that in situ transformations likely contributed to the high methylmercury concentration found in settling particles. Mercury methylation was inhibited (∼80%) in settling particles amended with molybdate, demonstrating the prominent role of biological sulfate-reduction in the process.

Sewage treatment plants (STPs) are important point sources of mercury (Hg) to the environment. STPs are also significant sources of iron when hydrated ferric oxide (HFO) is used as a dephosphatation agent during water purification. In this study, we combined geochemical and microbiological characterization with Hg speciation and sediment amendments to evaluate the impact of STP's effluents on monomethylmercury (MMHg) production. The highest in-situ Hg methylation was found close to the discharge pipe in subsurface sediments enriched with Hg, organic matter, and iron. There, ferruginous conditions were prevailing with high concentrations of dissolved Fe2+ and virtually no free sulfide in the porewater. Sediment incubations demonstrated that the high MMHg production close to the discharge was controlled by low demethylation yields. Inhibition of dissimilatory sulfate reduction with molybdate led to increased iron reduction rates and Hg-methylation, suggesting that sulfate-reducing bacteria (SRB) may not have been the main Hg methylators under these conditions. However, Hg methylation in sediments amended with amorphous Fe(III)-oxides was only slightly higher than control conditions. Thus, in addition to iron-reducing bacteria, other non-SRB most likely contributed to Hg methylation. Overall, this study highlights that sediments impacted by STP discharges can become local hot-spots for Hg methylation due to the combined inputs of i) Hg, ii) organic matter, which fuels bacterial activities and iii) iron, which keeps porewater sulfide concentration low and hence Hg bioavailable.

An important issue in mercury (Hg) biogeochemistry is to explore the influence of aqueous Hg(II) forms on bacterial uptake, and subsequent methyl mercury formation, under iron(III) and sulfate reducing conditions. The success of this is dependent on relevant information on the thermodynamic stability of Hg-sulfides. In the present study, we determined the solubility of a commercially available HgS(s) phase, which was shown by X-ray diffraction to be a mixture of 83% metacinnabar and 17% cinnabar. At aqueous sulfide concentrations between 0.060 and 84 μM, well below levels in previous studies, we report a solubility product (log Ksp ± SE) of -36.8 ± 0.1 (HgS(s) + H(+) = Hg(2+) + HS(-), I = 0, T = 25 °C, pH 6-10, n = 20) for metacinnabar. This value is 0.7 log units higher than previous estimates. Complementing our data with data from Paquette and Helz (1997), we took advantage of a large data set (n = 65) covering a wide range of aqueous sulfide (0.06 μM-140 mM) and pH (1-11). On the basis of this, we report refined formation constants (±SE) for the three aqueous Hg(II)-sulfide species proposed by Schwarzenbach and Widmer (1963): Hg(2+) + 2HS(-) = Hg(SH)2(0); log K = 39.1 ± 0.1, Hg(2+) + 2HS(-) = HgS2H(-) + H(+); log K = 32.5 ± 0.1, Hg(2+) + 2HS(-) = HgS2(2-) + 2H(+); log K = 23.2 ± 0.1. Our refined log K values differ from previous estimates by 0.2-0.6 log units. Furthermore, at the low sulfide concentrations in our study we could rule out the value of -10.0 for the reaction HgS(s) + H2O = HgOHSH(aq) as reported by Dyrssén and Wedborg (1991). By establishing a solubility product for the most environmentally relevant HgS(s) phase, metacinnabar, and extending the range of aqueous sulfide concentrations to sub-micromolar levels, relevant for soils, sediments, and waters, this study decreases the uncertainty in stability constants for Hg-sulfides, thereby improving the basis for understanding the bioavailability and mobility of Hg(II) in the environment.

Mercury (Hg) evasional fluxes from coastal waters are an important feature of the global Hg cycling; however, they remain poorly characterized due to limitations in datasets and a lack of suitable gas exchange models for such an environment. The objective of this study was to improve this evasional assessment by examining the extent and variability of gaseous Hg exchanges, computed with different gas exchange models, over various European estuaries (Gironde, Scheldt, Rhine) and coastal lagoons (Arcachon, Thau). The major outcomes of this work are the comparison of different models used to compute gaseous Hg exchanges and also the estimation of the variability of Hg fluxes at the air–water interface using a large database composed of 425 measurements acquired over a wide range of coastal ecosystems and seasons. The relative variance, expressed as the relative standard deviation of Hg° fluxes, was used to estimate the variability among models, seasons and sites. The inter-model relative variance (101 %) was the lowest compared to inter-site (182 %) or inter-season (172 %) variability. The highest fluxes were found during spring for the Gironde Estuary (73.7 pmol m−2 h−1), during summer for the Scheldt Estuary, the Arcachon and Thau Lagoons with respectively, 53.3, 55.3 and 23.6 pmol m−2 h−1 and during winter for the Rhine Estuary (50.1 pmol m−2 h−1). They were mainly explained by a combination of greater gaseous Hg production in the water column and wind speed. Overall, the results demonstrate that the sites and seasonal variations will generally overcome the uncertainty arising from the model selection. Generic models are therefore suitable for evasional assessments at the regional scale, while site-specific models should be used for local studies, when accurate mass balances are required.

International audience; The fate of mercury (Hg) and tin (Sn) compounds in ecosystems is strongly determined by their alkylation/dealkylation pathways. However, the experimental determination of those transformations is still not straightforward and methodologies need to be refined. The purpose of this work is the development of a comprehensive and adaptable tool for an accurate experimental assessment of specific formation/degradation yields and half-lives of elemental species in different aquatic environments. The methodology combines field incubations of coastal waters and surface sediments with the addition of species-specific isotopically enriched tracers and a mathematical approach based on the deconvolution of isotopic patterns. The method has been applied to the study of the environmental reactivity of Hg and Sn compounds in coastal water and surface sediment samples collected in two different coastal ecosystems of the South French Atlantic Coast (Arcachon Bay and Adour Estuary). Both the level of isotopically enriched species and the spiking solution composition were found to alter dibutyltin and monomethylmercury degradation yields, while no significant changes were measurable for tributyltin and Hg(II). For butyltin species, the presence of light was found to be the main source of degradation and removal of these contaminants from surface coastal environments. In contrast, photomediated processes do not significantly influence either the methylation of mercury or the demethylation of methylmercury. The proposed method constitutes an advancement from the previous element-specific isotopic tracers' approaches, which allows for instance to discriminate the extent of net and oxidative Hg demethylation and to identify which debutylation step is controlling the environmental persistence of butyltin compounds.

Purpose: Chlor-alkali plants are one of the most important point sources of mercury to aquatic environment. The problem of Hg contamination has been studied in a region, Rm Valcea (Romania), impacted by the wastewater discharge of a chlor-alkali plant. The purpose of the present study is to evaluate the current status of mercury pollution in the Babeni reservoir (Olt River) and the exposure of local population via fish consumption to mercury originating from the chlor-alkali plant. Methods: Sediments were collected from Valcea, Govora and Babeni reservoirs. Grain size distribution, organic content and total mercury (THg) concentrations were analysed in sediments. Fish were purchased from local anglers, and the scalp hair was collected from volunteers. THg in sediment, fish and hair samples was determined using an atomic absorption spectrophotometer for Hg determination. Monomethylmercury (MMHg) was analysed in the muscle and liver tissues by species-specific isotope dilution and capillary gas chromatography hyphenated to inductively coupled plasma mass spectrometer. Results: High mercury concentrations were found in the sediments and in fish from Babeni reservoir, with a median of 2.1mg/kg (IQR = 3.2) in sediments and a mean value of 1.8 ± 0.8mg/kg_ww in fish muscle. MMHg concentrations in fish were well above the WHO guidelines for fish consumption. Local population consuming fish from the Babeni reservoir had THg concentrations in hair significantly higher than those consuming fish from upstream reservoirs and/or from the shops and reached a median value of 2.5mg/kg (IQR = 3.6). Conclusions: The remnant pollution in the fish of this reservoir, and probably many other lakes and reservoirs receiving Hg polluted wastewater, represents a considerable health risk for the local fish consumers

This work describes a laboratory experiment designed to unravel mercury species reactivity in superficial coastal sediments oscillating between oxic and anoxic conditions. The experimental set-up has been applied to a sediment slurry from the Arcachon Bay (France) to follow the evolution of both naturally occurring (i.e. endogenous) and isotopically enriched added mercury species (i.e. exogenous, 199IHg and 201MMHg) at environmental levels. The transformation and partition between the different phases (aqueous, solid and gaseous) of the endogenous and exogenous mercury species (inorganic Hg (IHg), monomethyl Hg (MMHg), elemental Hg (Hg°) and dimethyl Hg (DMHg)) have been investigated by isotopic speciation methods throughout the experiment. The results demonstrate that the experimental approach is able to promote sediment redox oscillations and to simultaneously follow the biogeochemical fate of naturally occurring or added mercury species. Experimentally driven redox transition events were found to significantly enhance the aqueous Hg species concentrations, while the MMHg burden is not greatly affected. Indeed, during the anoxic-oxic transition, while aqueous endogenous IHg and MMHg exhibited a two-fold increase, aqueous exogenous IHg and MMHg increased 7 and 4 times, respectively. Transient increases of the net IHg methylation were recorded during the redox transitions with the largest increase of the MMHg contents (factor 1.8) observed during the oxic-anoxic transition. High resolution in situ redox experiments have not been performed up to now, therefore the developed experimental set-up provides novel insights in both the influence of redox conditions on Hg methylation/demethylation and adsorption/desorption processes and kinetics in superficial sediments.

In Lake Geneva, Switzerland, the most Hg-contaminated sediments have been found in the Vidy Bay where high Hg contents largely exceeds the background levels of Lake Geneva sediments. This contamination has been attributed to the discharge of a waste water treatment plant (WWTP). Total Hg (THg) and monomethylmercury (MMHg) were determined in bulk sediment and in three different grain size fractions (i: clay and silt, ii: fine-coarse sand, iii: and very coarse sand and gravel) collected close to the outlet pipe of a WWTP in order to verify whether the standardized procedures of sediment treatment is adequate for this setting and, by extension, for similar contaminated sites. THg was homogeneously distributed in the different grain size fractions and was correlated to organic matter content (R2 = 0.6). MMHg was homogeneously distributed in the two finer grain fractions (Φ < 0.063 mm; 0.063 mm < Φ