Hg0 Production Research Articles

Elemental mercury production from contaminated riparian soil suspensions under air and nitrogen bubbling conditions.

The dynamic change of redox conditions is a key factor in emission of elemental mercury (Hg0) from riparian soils. The objective of this study was to elucidate the influences of redox conditions on Hg0 emission from riparian soils. Soil suspension experiments were conducted to measure Hg0 emission from five Hg-contaminated soil samples in two redox conditions (i.e., treated with air or with N2). In four of the five samples, Hg0 emission was higher in air treatment than on N2 treatment. Remaining one soil, which has higher organic matter than other soils, showed no distinct difference in Hg0 production between air and N2 treatment. In soil suspensions subject to N2 treatment, the dissolved organic carbon (DOC) and Fe2+ concentrations were 3.38- to 1.34-fold and 1.44- to 2.28-fold higher than those in air treatment, respectively. Positive correlations were also found between the DOC and Fe2+ (r = 0.911, p < 0.01) and Hg2+ (r = 0.815, p < 0.01) concentrations in soil solutions, suggesting Fe2+ formation led to the release of DOC, which bound to Hg2+ in the soil and, in turn, limited the availability of Hg2+ for reduction to Hg0 in N2 treatment. On the other hand, for remaining one soil, more Hg2+ might be adsorbed onto the DOM in the air treatment, resulted in the inhibition of Hg0 production in air treatment. These results imply that the organic matter is important to prevent Hg0 production by changing redox condition. Further study is needed to prove the role of organic matter in the production of Hg0.

Development and optimization of a triple quadrupole ICP-MS-based system for the quantification of pre-concentrated gaseous elementary mercury.

Understanding the redox reactions and transformation rates of mercury (Hg) species in the environment is important for predicting future gaseous elemental Hg (Hg0) levels and assessing the impacts of anthropogenic Hg0 emissions on human health. Stable Hg isotope tracers are a promising tool for estimating Hg0 production rates; however, traditional analytical approaches for quantifying Hg0, such as atomic fluorescence spectroscopy or atomic absorption spectrometry, cannot differentiate between Hg isotopes, and alternative approaches, such as inductively coupled plasma mass spectrometry (ICP-MS) with a typical aqueous sample introductory system, have relatively higher detection limit of Hg. Here, we developed and evaluated a custom-made thermal desorption unit coupled directly to a triple quadrupole ICP-MS (ICP-QQQ) for the quantification of Hg0 pre-concentrated on Au traps. The performance of the system was validated with measurements of a Hg standard gas and of Hg0 generated from aqueous Hg standards. Using our system, we were able to detect ultra-trace amounts of Hg0 and obtain precise Hg isotope measurements with an analytical error of ≤ 3.5%. Calibration curves with superb linearity (r2 > 0.999) were obtained for the Hg concentration range of 0-300pg. The method detection limit was approximately 0.01-0.03pg of Hg. Using the latest ICP-QQQ instrument (Agilent 8900; Agilent Technologies Ltd.) was far superior to using a previous model (Agilent 8800), with the Agilent 8900 showing approximately five times higher sensitivity than the Agilent 8800 as well as the ability to precisely and simultaneously analyze up to five Hg isotopes by time-resolved analysis.

Real-time monitoring of aqueous Hg2+ reduction dynamics by magnetite/iron metal composite powders synthesized hydrothermally

Abstract An iron-based powder material composed of zerovalent iron (8 wt.%) and magnetite (92 wt.%), has been synthesized hydrothermally at 200 °C from zero-valent iron. Its effect on the reduction of aqueous Hg2+ into gaseous Hg0 has been investigated at ambient conditions for pH comprised between 4 and 8.5. The production of Hg0 was monitored with an online mercury vapor analyzer at the picogram level for concentrations of iron-based composite of a few tenths of mg L−1. Starting from a solution having an Hg2+ concentration of 25 ng L−1 at pH = 4, a succession of two Hg0 production events was recorded. The first event is related to the Hg2+ reduction by ZVI which fully dissolved within the first hours. Upon ZVI consumption, pH drifted towards the pH window where magnetite can efficiently reduce Hg2+ at the hour timescale, resulting in a second Hg0 production peak. The combined use of ZVI and magnetite to remove aqueous Hg2+ by formation of Hg0 (volatile) under mild acidic pH allows (1) to maximize the Hg2+ reduction rate and (2) to take benefit of the longer lifetime of magnetite compared to ZVI.

Dark Reduction Drives Evasion of Mercury From the Ocean

Much of the surface water of the ocean is supersaturated in elemental mercury (Hg0) with respect to the atmosphere, leading to sea-to-air transfer or evasion. This flux is large, and nearly balances inputs from the atmosphere, rivers and hydrothermal vents. While the photochemical production of Hg0 from ionic and methylated mercury is reasonably well-studied and can produce Hg0 at fairly high rates, there is also abundant Hg0 in aphotic waters, indicating that other important formation pathways exist. Here, we present results of gross reduction rate measurements, depth profiles and diel cycling studies to argue that dark reduction of Hg2+ is also capable of sustaining Hg0 concentrations in the open ocean mixed layer. In locations where vertical mixing is deep enough relative to the vertical penetration of UV-B and photosynthetically active radiation (the principal forms of light involved in abiotic and biotic Hg photoreduction), dark reduction will contribute the majority of Hg0 produced in the surface ocean mixed layer. Our measurements and modeling suggest that these conditions are met nearly everywhere except at high latitudes during local summer. Furthermore, the residence time of Hg0 in the mixed layer with respect to evasion is longer than that of redox, a situation that allows dark reduction-oxidation to effectively set the steady-state ratio of Hg0 to Hg2+ in surface waters. The nature of these dark redox reactions in the ocean was not resolved by this study, but our experiments suggest a likely mechanism or mechanisms involving enzymes and/or important redox agents such as reactive oxygen species and manganese (III).

A potential route for photolytic reduction of HgCl2 and HgBr2 in dry air and analysis about the impacts from Ozone

Among various gaseous oxidized mercury species, HgCl2 and HgBr2 are known to be more recalcitrant to direct photolysis under radiations. In this study, we examined a possible route to reduce HgCl2 and HgBr2 in the dry air under UV, visible and solar radiations with different levels of irradiances. Potential impact of O3 on the reductions of HgCl2 and HgBr2 were discussed. The laboratory results showed that if without any radiation, O3 would not influence the chemical conversions between HgCl2/HgBr2 and Hg0. However, if with involvement of radiation, a rapid reduction of HgCl2 and HgBr2 was observed, resulting in a significant production of Hg0 in the air. The magnitudes of reduction reactions heavily depended on the frequency of radiations as well as the irradiances. UV light had the strongest capacity to promote photoreduction of HgCl2 and HgBr2 even at relatively low irradiances. A relationship analysis indicated that the reduction of HgCl2 and HgBr2 was possibly related to the O3 concentrations in the air. We proposed that O3 might be involved in productions of the unstable intermediate HgIX (X = Br, Cl), which has a quicker photolysis rate than the HgIIX2. Our results suggest that the impacts of solar radiation and O3 on reductions of HgII species in the atmosphere are worth of being investigated theoretically and in both laboratory and field studies in the future, since it might be an important but lesser-known part in the atmospheric Hg chemistry.

To what extent can the biogeochemical cycling of mercury modulate the measurement of dissolved mercury in surface freshwaters by passive sampling?

Mercury (Hg) is a pollutant of global concern owing to its great toxicity even at very low concentrations. Its toxicity depends on its chemical forms evidencing the importance to study its speciation. Dissolved Hg (Hg(d)) and methylmercury (MeHg(d)) monitoring in surface freshwaters represents a great challenge because of their very low concentrations and substantial temporal variability at different timescales. The Hg(d) temporal variability depends on the environmental conditions such as the hydrology, water temperature, redox potential (Eh), and solar photo cycle. Passive samplers represent an alternative to improve the assessment of Hg(d) and MeHg(d) concentrations in surface freshwaters by integrating their temporal variability. An original sampling strategy was designed to assess the relevance of 3-mercaptopropyl DGT (Diffusive Gradient in Thin films) to integrate in situ the temporal variations of labile Hg (Hg(DGT)) and MeHg (MeHg(DGT)) concentrations. This strategy was implemented on two rivers to study the dynamics of Hg(d), Hg(DGT), MeHg(d) and MeHg(DGT) at diurnal and annual timescales. We evidenced that Hg(DGT) and MeHg(DGT) concentrations were generally consistent with discrete sampling measurements of Hg(d) and MeHg(d) in dynamic surface freshwaters. However, Hg(DGT) concentrations were overestimated (2–16 times higher) in case of low flow or low water depth, low suspended particulate matter (SPM) concentrations and elevated daily photoperiod. The most probable hypothesis is that such conditions promoted Hg0 production, and resulted in Hg0 uptake by DGT. Thus, attention should be paid when interpreting Hg(DGT) concentrations in surface freshwaters in environmental conditions that could promote Hg0 production.

Reduced sulphur sources favour HgII reduction during anoxygenic photosynthesis by Heliobacteria.

The consumption of rice has become a global food safety issue because rice paddies support the production of high levels of the potent neurotoxin, methylmercury. The production of methylmercury is carried out by chemotrophic anaerobes that rely on a diversity of terminal electron acceptors, namely sulphate. Sulphur can be a limiting nutrient in rice paddies, and sulphate amendments are often used to stimulate crop production, which can increase methylmercury production. Mercury (Hg) redox cycling can affect Hg methylation by controlling the delivery of inorganic Hg substrates to methylators in anoxic habitats. Whereas sulphur is recognized as a key substrate controlling methylmercury production, the controls sulphur exerts on other microbe-mediated Hg transformations remain poorly understood. To explore the potential coupling between sulphur assimilation and anaerobic HgII reduction to Hg0 , we studied Heliobacillus mobilis, a mesophilic anoxygenic phototroph representative from the Heliobacteriacea family originally isolated from a rice paddy. Here, we tested whether the redox state of the sulphur sources available to H.mobilis would affect its ability to reduce HgII . By comparing Hg0 production over a redox gradient of sulphur sources, we demonstrate that phototrophic HgII reduction is favoured in the presence of reduced sulphur sources such as thiosulphate and cysteine. We also show that cysteine exerts dynamic control on Hg cycling by affecting not only Hg's bioavailability but also its abiotic photoreduction under low light conditions. Specifically, in the absence of cells we show that organic matter (as yeast extract) and cysteine are both required for photoreduction to occur. This study offers insights into how one of the most primitive forms of photosynthesis affects Hg redox transformations and frames Heliobacteria as key players in Hg cycling within paddy soils, forming a basis for management strategies to mitigate Hg accumulation in rice.

Microbial generation of elemental mercury from dissolved methylmercury in seawater.

Elemental mercury (Hg0) formation from other mercury species in seawater results from photoreduction and microbial activity, leading to possible evasion from seawater to overlying air. Microbial conversion of monomethylmercury (MeHg) to Hg0 in seawater remains unquantified. A rapid radioassay method was developed using gamma-emitting 203Hg as a tracer to evaluate Hg0 production from Hg(II) and MeHg in the low pM range. Bacterioplankton assemblages in Atlantic surface seawater and Long Island Sound water were found to rapidly produce Hg0, with production rate constants being directly related to bacterial biomass and independent of dissolved Hg(II) and MeHg concentrations. About 32% of Hg(II) and 19% of MeHg were converted to Hg0 in 4 d in Atlantic surface seawater containing low bacterial biomass, and in Long Island Sound water with higher bacterial biomass, 54% of Hg(II) and 8% of MeHg were transformed to Hg0. Decreasing temperatures from 24°C to 4°C reduced Hg0 production rates cell-1 from Hg(II) 3.3 times as much as from a MeHg source. Because Hg0 production rates were linearly related to microbial biomass and temperature, and microbial mercuric reductase was detected in our field samples, we inferred that microbial metabolic activities and enzymatic reactions primarily govern Hg0 formation in subsurface waters where light penetration is diminished.

Heliobacteria Reveal Fermentation As a Key Pathway for Mercury Reduction in Anoxic Environments.

The accumulation of mercury (Hg) in rice, a dietary staple for over half of the world's population, is rapidly becoming a global food safety issue. Rice paddies support the anaerobic production of toxic methylmercury that accumulates in plant tissue, however the microbial controls of Hg cycling in anoxic environments remain poorly understood. In this study, we reveal a novel reductive Hg metabolism in a representative of the family Heliobacteria ( Heliobacterium modesticaldum Ice1) that we confirm in model chemotrophic anaerobes. Heliobacteria served as our initial model because they are a family of spore-forming fermentative photoheterotrophs commonly isolated from terrestrial environments. We observed that H. modesticaldum reduced up to 75% of HgII under phototrophic or fermentative conditions. Fermentative HgII reduction relied on the ability of cells to oxidize pyruvate whereas phototrophic HgII reduction could be supported even in the absence of a carbon source. Inhibiting pyruvate fermentation eliminated HgII reduction in all chemotrophic strains tested, whereas phototrophic cells remained unaffected. Here we propose a non mer-operon dependent mechanism for Hg0 production in anoxic environments devoid of light where external electron acceptors are limited. These mechanistic details provide the foundation for novel bioremediation strategies to limit the negative impacts of Hg pollution.

Mercury in a stream-lake network of Andean Patagonia (Southern Volcanic Zone): Partitioning and interaction with dissolved organic matter

Lake Nahuel Huapi (NH) is a large, ultraoligotrophic deep system located in Nahuel Huapi National Park (NHNP) and collecting a major headwater network of Northwestern Patagonia (Argentina). Brazo Rincón (BR), the westernmost branch of NH, is close to the active volcanic formation Puyehue-Cordón Caulle. In BR, aquatic biota and sediments display high levels of total Hg (THg), ranging in contamination levels although it is an unpolluted region. In this survey, Hg species and fractionation were assessed in association with dissolved organic matter (DOM) in several aquatic systems draining to BR. THg varied between 16.8 and 363 ng L−1, with inorganic Hg (Hg2+) contributing up to 99.8% and methyl mercury (MeHg) up to 2.10%. DOC levels were low (0.31–1.02 mg L−1) resulting in high THg:DOC and reflecting in high Hg2+ availability for binding particles (partitioning coefficient log Kd up to 6.03). In streams, Hg fractionation and speciation related directly with DOM terrestrial prints, indicating coupled Hg-DOM inputs from the catchment. In the lake, DOM quality and photochemical and biological processing drive Hg fractionation, speciation and vertical levels. Dissolved gaseous Hg (Hg0) reached higher values in BR (up to 3.8%), particularly in upper lake layers where solar radiation enhances the photoreduction of Hg2+ and Hg-DOM complexes. The environmental conditions in BR catchment promote Hg2+ binding to abiotic particles and bioaccumulation and the production of Hg0, features enhancing Hg mobilization among ecosystem compartments. Overall, the aquatic network studied can be considered a “natural Hg hotspot” within NHNP.

Methylmercury degradation by Pseudomonas putida V1

Environmental contamination of mercury (Hg) has caused public health concerns with focuses on the neurotoxic substance methylmercury, due to its bioaccumulation and biomagnification in food chains. The goals of the present study were to examine: (i) the transformation of methylmercury, thimerosal, phenylmercuric acetate and mercuric chloride by cultures of Pseudomonas putida V1, (ii) the presence of the genes merA and merB in P. putida V1, and (iii) the degradation pathways of methylmercury by P. putida V1. Strain V1 cultures readily degraded methylmercury, thimerosal, phenylmercury acetate, and reduced mercuric chloride into gaseous Hg(0). However, the Hg transformation in LB broth by P. putida V1 was influenced by the type of Hg compounds. The merA gene was detected in P. putida V1, on the other hand, the merB gene was not detected. The sequencing of this gene, showed high similarity (100%) to the mercuric reductase gene of other Pseudomonas spp. Furthermore, tests using radioactive 14C-methylmercury indicated an uncommon release of 14CO2 concomitant with the production of Hg0. The results of the present work suggest that P. putida V1 has the potential to remove methylmercury from contaminated sites. More studies are warranted to determine the mechanism of removal of methylmercury by P. putida V1.

Effect of Abiotic Factors on the Mercury Reduction Process by Humic Acids in Aqueous Systems

As a global pollutant process, the reduction of mercury (Hg) is especially important. One pathway is through an abiotic reduction with humic acids (HAs), which is controlled by different factors, including initial Hg and HA concentrations, pH, temperature and light. In this study, three humic acids were selected to illustrate the Hg2+ abiotic reduction mechanisms by HAs, and to identify the key limiting factors for reduction rates and amounts. In addition, the initial status of the HAs as a solid or in an aqueous solution were also compared, to help explain why HAs show different dominant characteristics (e.g. complexation or reduction) in the reaction process with Hg. Results indicated that HAs were able to reduce Hg abiotically. Higher initial Hg, higher HA concentrations and either high (8.1) or low (3.6) solution pH decreased the HA reduction capacity. In addition, Hg0 production rates increased with increasing temperature, and the same trend was observed with light exposure. Humic acids added as an aqueous solution resulted in significantly greater Hg0 production than addition as a bulk solid. Finally, the Hg reduction rate and capacity varied significantly (P < 0.05) with HAs from different sources. These findings helped to explain why HAs showed different dominant characteristics (e.g. complexation or reduction) in the reaction process with Hg, and evidentially demonstrated the existence of a possible pathway of Hg2+ reduction, which indicated that humic substances in natural environments, especially in water bodies, could act either as a sink or a source for Hg.

Evidence for microbially mediated production of elemental mercury (Hg0) in subarctic lake sediments

The production and fate of Hg0 in lake sediments remains poorly characterized, although it can potentially influence Hg toxicity and mobility. Using slurry incubations, we assessed the effect of nutrients, pH, ionic strength, and tested the role of microbes, on Hg0 production from pristine lake sediments of the Hudson Bay Lowlands in Ontario, Canada. We showed that Hg0 production from oxygenated sediments was low (<1% of the Hg flux to sediments), biogenic, and did not appear to be dependent on the predicted HgII speciation in the slurry. The addition of biologically labile carbon sources and ionic strength, particularly [Na+], had the greatest impact on Hg0 production, increasing it by over 10-fold. These results suggest that under changing conditions, such as increasing temperature or alteration of water chemistry as observed in numerous locations throughout Arctic and subarctic environments, a fraction of sedimentary Hg may be remobilized and available for microbial reduction.

Atlantic mercury emission determined from continuous analysis of the elemental mercury sea-air concentration difference within transects between 50°N and 50°S

[1] Mercury in the environment deserves serious concern because of the mobility of volatile elemental mercury (Hg0) in the atmosphere, in combination with the harmful effect of Hg compounds on human health and the ecosystem. A major source of global atmospheric mercury is presumed to be oceanic Hg0 emission. However, available Hg0 surface water data to reliably estimate the ocean's mercury emissions are sparse. In this study, high-resolution surface water and air measurements of Hg0 were carried out between Europe and South Africa in November 2008 and between South America and Europe in April–May 2009. On each cruise a strong enrichment of Hg0 in tropical surface water was determined that apparently followed the seasonal shift of the Intertropical Convergence Zone (ITCZ). A combination of a high Hg0 production rate constant and the actual low wind speeds, which prevented emission, probably caused the accumulation of Hg0 in surface waters of the ITCZ. Hg0 emissions in the tropics were significant only if wind speed variability on a monthly scale was considered, in which case the observed significant decline of total Hg in tropical surface waters during the northern winter could be explained. In the midlatitudes, increased autumn Hg0 emissions were calculated for November in the Northern Hemisphere and for May in the Southern Hemisphere; conversely, emissions were low during both the northern and the southern spring. Mercury removal from surface waters by Hg0 emission and sinking particles was comparable to its supply through wet and dry deposition.

Reduction of Oxidized Mercury Species by Dicarboxylic Acids (C2−C4): Kinetic and Product Studies

Mercury is an environmental contaminant of global concern. The reduction of oxidized mercury species (Hg(II)) by organic acids to elemental mercury (Hg0) is significant for understanding the cycling of mercury between the atmosphere and aqueous systems. This study focused on the reduction of Hg(II) by small, semivolatile dicarboxylic acids (C2-C4). The reaction kinetics was studied using cold vapor atomic fluorescence spectroscopy (CVAFS), and the products of the reaction were analyzed using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and nuclear magnetic resonance (NMR) spectrometry. The effects of light, dissolved oxygen and chloride ion on reaction rates were also investigated. The highest reaction rates were observed in systems free of both oxygen and chloride ion with the second-order apparent rate constants of 1.2 x 10(4), 4.9 x 10(3), and 2.8 x 10(3) (L x mol(-1) x s(-1)) for oxalic, malonic, and succinic acids at pH 3.0 and T = 296 +/- 2 K, respectively. The photoreduction of Hg(II) was mediated by the complexes formed between Hg" and dicarboxylic acids, and the identified products were Hg0, hydroxycarboxylic acids and monocarboxylic acids. Our results also indicated that the presence of chloride ion significantly reduced the reduction rate by competing with the complexation of Hg" with dicarboxylic acids, while dissolved oxygen retarded the production of Hg0 by involving in the reoxidation of reduced Hg species to Hg(II). Based on our experimental results, a tentative mechanism is proposed and the potential environmental implications are discussed.

Mercury speciation changes in waters of the sepetiba Bay, SE Brazil during tidal events and different seasons

This work describes changes in Hg speciation in water along the transport from Sepetiba Bay, SE Brazil, to the adjacent continental platform during tide events and in different seasons. The chemical evaluation of Sepetiba Bay waters showed that the bay exports suspended particulate material (SPM) impoverish in organic carbon to adjacent continental platform areas, mainly during the wet season and imports SPM enriched in organic carbon from the ocean, probably of phytoplanktonic origin, during the dry season. The chemical speciation of Hg during the study period showed that Hg0 production can be controlled by primary productivity and that the bay acts as a producer and exporter of organic species of Hg to adjacent areas, since the highest non-reactive Hg concentrations were observed during the ebb tide. No significant correlations were found between non-reactive Hg and dissolved or particulate organic matter concentrations. In general the particulate Hg, non-reactive Hg and reactive Hg fractions represents 55%, 35.5% and 9.5% of the total Hg content in Sepetiba Bay waters, respectively.

Production of gaseous mercury in tropical hydromorphic soils in the presence of ferrous iron: a laboratory study

SummaryIn this paper, we demonstrate that reduction of Hg(II) to Hg0 under anaerobic conditions, followed by volatilization of Hg0 to the atmosphere, might be responsible for the removal of Hg from tropical hydromorphic soils. We conducted a series of kinetic batch experiments in which we added Hg(II) to anoxic suspensions of a soil clay fraction and haematite nanoparticles. The clay fraction came from three depths in a hydromorphic soil by the Leblond River in French Guiana, South America: close to the upper and lower boundaries (30–40 cm and 90–100 cm) and within the central part (60–70 cm) of the frequently water‐saturated clay horizon. We prepared a second set of soil clay fraction suspensions with Fe(III) citrate, whose reduction acted as a source of dissolved Fe(II) to investigate the influence of Fe(II) on the production of Hg0. Reduction of Hg(II) to Hg0 occurred with all samples amended with Fe(III) citrate. Laboratory experiments with haematite suspensions demonstrated that adsorption of Fe(II) to the haematite surface created very reactive sites for the reduction of Hg(II), while in the absence of haematite particles, no production of Hg0 occurred. The greatest production of Hg0 was found for the depth intervals 30–40 cm and 90–100 cm, where the total mercury concentration exhibits a local minimum. The observed pseudo‐first order rate constants for the 90–100 cm depth sample were close to rate constants reported for abiotic reduction of Hg(II) by Fe(II) adsorbed on mineral surfaces. Significant production of Hg0 was found for the 90–100 cm depth interval sample, both with and without Fe(III) citrate. A biotic pathway as well as abiotic reduction by Fe(II) might be involved in the reduction of Hg(II) to Hg0.

Evidence for enhanced mercury reactivity in response to estuarine mixing

Bioaccumulation of methylmercury in coastal U.S. fisheries has led to the issuance of numerous fish consumption advisories, and yet little is known about the processes that make Hg species chemically labile in coastal and estuarine systems. This study examined the role of estuarine mixing in formation of labile Hg complexes (reactive Hg) from relatively refractory Hg‐organic associations in river water and characterized the behavior and distribution of Hg species in the Connecticut River estuary during three distinct collection periods. Results indicate that while total Hg partitioning and concentrations remained fairly constant with increasing salinity, the fraction present as reactive Hg concentrations increased, primarily in the particulate phase. Mixing experiments using both natural and prepared waters indicate that riverine organic ligands rapidly scavenge reactive Hg from natural waters on timescales of minutes to hours, while samples free of riverine influence remained much more “reactive.” Modeling of the estuarine system suggests that elevated concentrations of chloride and dilution of the dominant organic ligand associated with estuarine mixing enhance reactive Hg and predict a bulk log formation constant for the binding ligand of approximately 21. Analysis of Hg0 production from Hg(II)‐spiked, incubated estuarine samples supports the speciation data as higher reactive Hg concentrations and Hg0 production rates were observed in the more saline samples. These results suggest that estuarine mixing may exacerbate Hg methylation, evasion, and bioaccumulation in some systems by promoting the formation of Hg species that are readily labile.

Diurnal cycles of gaseous mercury within the snowpack at Kuujjuarapik/Whapmagoostui, Québec, Canada.

Mercury is a globally dispersed and toxic pollutant that can be transported far from its emission sources. In polar and subpolar regions, recent research activities have demonstrated its ability to be converted and deposited rapidly onto snow surfaces during the so-known Mercury Depletion Events (MDEs). The fate of mercury once deposited onto snow surfaces is still unclear: a part could be re-emitted to the atmosphere, the other part could contaminate water systems at the snowmelt. Its capacity to transform to more toxic form and to bioaccumulate in the food chain has consequently made mercury a threat for Arctic ecosystems. The snowpack is a medium that greatly interacts with a variety of atmospheric gases. Its role in the understanding of the fate of deposited mercury is crucial though it is poorly understood. In April 2002, we studied an environmental component of mercury, which is interstitial gaseous mercury (IGM) present in the air of the snowpack at Kuujjuarapik/Whapmagoostui (55 degrees N, 77 degrees W), Canada on the east shore of the Hudson Bay. We report here for the first time continuous IGM measurements at various depths inside a seasonal snowpack. IGM concentrations exhibit a well-marked diurnal cycle with uninterrupted events of Hg0 depletion and production within the snowpack. A possible explanation of Hg0 depletion within the snowpack may be Hg0 oxidation processes. Additionally, we assume that the notable production of Hg0 during the daytime may be the results of photoreduction and photoinitiated reduction of Hg(II) complexes. These new observations show that the snowpack plays undoubtedly a role in the global mercury cycle.

Speciation and sorption of mercury in two macro-tidal estuaries

Mercury speciation and partition were determined in two macro-tidal estuaries, the Loire and the Seine estuaries (France) during several cruises covering various hydrological conditions. Additional measurements were performed in freshwaters in order to assess temporal variations of mercury inputs to the estuaries during an annual cycle. In freshwater, average total ‘dissolved’ (< 0.8 μm) mercury concentration [(HgT)D] was 4.1 ± 2.1 pM in the Loire and 11.5 ± 9.7 pM in the Seine. Mean particulate mercury concentration [(HgT)p] was 0.95 ± 0.63 nmol g−1 in the Loire and 5.4 ± 2.6 nmol g−1 in the Seine. The seasonal variation in the mercury concentrations in the two rivers are interpreted in terms of changes in the various water sources during flood events and variable contribution of colloidal materials. In the estuarine parts, (HgT)D concentrations generally varied from 2 to 6 pM in the Seine and from 1 to 3 pM in the Loire, while (HgT)P concentrations varied between 0.5 and 5 nmol g−1 in the Loire and between 2.5 and 10 nmol g−1 in the Seine. Because of its high distribution coefficient (log Kd = 5–6) and the high turbidity of these environments, mercury is mainly transported in particulate form in freshwaters as well as in mixing zones. Relationships between Kd and concentrations of suspended particles, dissolved and particulate organic carbon allow to describe the partition of mercury between solid and dissolved phases in terms of colloids, organic complexation in solution and sorption processes. Chlorocomplex formation does not play the major role in the speciation of Hg during the estuarine mixing. Organic associations, through sulphydryl groups, seems to be the dominant processes governing mercury distribution and partition in the two macro-tidal estuaries studied. Primary production appears to play an important role in the mercury dynamic. (HgT)P is correlated with the chlorophyll content of the particles in high flow regime of the Loire river, while elemental mercury (Hg0) was found to be related to the oxygen saturation rate in both estuaries. The production of Hg0 was sufficient to sustain a significant evasion flux to the atmosphere. Dimethylmercury was detected at fentomolar level during one cruise in the Baie de la Seine during an hypoxia episode. Highest monomethylmercury concentrations (up to 4% of the HgT) were associated with warmer and organic rich freshwater. Candidate sources for methylated mercury species are discussed. Both estuaries appear to be affected by local anthropogenic inputs. However, the Seine river is more deeply impacted by human activities. Watershed yields of the total Hg were calculated for the two drainage basins. The Seine yield (41 mmol km−2 a−1) is in the range of what is found in other urban areas, while for the Loire (9 mmol km−2 a−1) it is close to values found for agricultural watersheds. In spite of the anthropogenic inputs within the two estuaries, the riverine dissolved mercury inputs to the adjacent coastal waters (la Baie de la Seine and le Grand estuaire de la Loire) have been estimated to be half of the corresponding atmospheric depositions.