Seasonal variability in the Mercury speciation of Onondaga Lake (New York)

Dissolved and particulate Hg speciation was determined on four occasions in the Spring to Fall interval of 1989, at three depths of the water column of Onondaga Lake, New York; an urban system in which the sediments and fish flesh are contaminated with H_g. Species determined included total Hg (Hg_t), reactive (‘ionic’) Hg (Hg_i), monomethylmercury (CH₃HgX), elemental Hg (Hg°) and dimethylmercury (CH₃)₂Hg). Onondaga Lake was found to contain very high levels of Hg_t (2 to 25 ng L^?1 Hg), Hg_j (0.5 to 10 ng L^?1 Hg), and CH₃HgX (0.3 to 7 ng L^?1 Hg), which generally increased with depth in the lake. These concentrations represent a significant level of contamination, based upon comparisons with other polluted and pristine sites. Elemental Hg levels were typically about 0.05 ng L^?1 and (CH₃)₂Hg was near the limits of detection (?0.001 ng) L^?1 in most samples. The greatest CH₃HgX concentrations in the hypolimnion, as well as the largest gradients of both CH₃HgX and (Hg_t), were observed upon the first onset of stratification, in early summer. These concentrations did not become more pronounced, however, as stratification and H₂S levels in the hypolimnion increased throughout the summer. The very low concentrations of (CH₃)₂Hg in these MeHg and sulfide-rich waters calls into question the belief that CH₃HgX and H₂S will react to yield volatile dimethyl-mercury, which can then escape to the atmosphere by diffusion. Mercury speciation was highly dynamic, indicating active cycling within the lake, and an apparent sensitivity to changes in attendant Iimnological conditions that track the stratification cycle.     

The role of microorganisms in elemental mercury formation in natural waters

Gas evasion of elemental Hg (Hg°) from the open ocean plays a prominent role in the global mercury cycle. Elemental Hg is formed primarily by reduction of ionic Hg in the mixed layer of aquatic systems. By culturing phytoplankton in defined media, and by incubating natural seawater and freshwater samples, we have demonstrated that Hg° is produced by microorganisms, with formation rates (0.5 to 10% d^?1) similar to those estimated from mass balance studies. Our results also suggest that <3μm microorganisms are the primary Hg reducers in natural waters. Eucaryotic phytoplankton are capable of reducing ionic Hg to Hg° but the rate of reduction is insufficient to account for the observed reduction rates found in incubated field samples. Bacteria are thus the more likely Hg reducers. In seawater, cyanobacteria such asSynecococcus may account for much of the mercury reduction, while in the eutrophic, polluted Upper Mystic Lake north of Boston other procaryotic microorganisms are contributing to the overall Hg reductive capacity of the medium. By reducing ionic Hg, microorganisms play a pivotal role in the aquatic biogeochemistry of Hg, not only by enabling evasion to the atmosphere, but by directly decreasing the amount of ionic Hg available for methylation.     

Mercury fallout in Hawaii

Mercury fallout was measured 10 m from the selected emission site, the Sulfur Banks fumarole area, Hawaii Volcanoes National Park; at stations on Maui and Oahu, respectively 200 km and 380 km distant from the Sulfur Banks; and at the Hawaii Geothermal Project drill site, only 40 km from source. Sulfur Banks and Oahu measurements were carried out on six occasions between 1972 and 1976, each time within the same 24 h period. Gold foil was used for collection of elemental mercury (Hg°) and copper foil for both oxidized (Hg_ox) and elemental forms. The rate of deposition at the Sulfur Banks was high ? 800 μm m^?2 day^?1, or 300 kgkm^?2 annually. The same figure applied to the relatively nearby geothermal project site. At both remote stations the fallout rate was approximately 10 fold lower. At four measurement times out of six the ratio Hg^O/Hg_ox ranged from 0.195 to 0.463 at the Sulfur Banks source and from 1.80 to 5.15 at the remote stations. On two occasions, heavy rains selectively reduced Hg_ox at the emission site. Model calculations compared Sulfur Banks fallout with rates of re-emission of Hg by vegetation, and the importance of the biotic factor in determination of mass balances and fluxes was emphasized. Aspects of the geochemistry and toxicology of Hg were considered briefly in relation to emission and deposition, and to the occurrence of Hg°.     

Foliar exchange of mercury vapor: Evidence for a compensation point

Historical studies for crop and weed species documented elemental Hg vapor (Hg°) deposition to foliage, but they used Hg° concentrations that were orders of magnitude higher than levels now known to occur under background conditions, possibly creating artificially high gradients between the atmosphere and landscape surfaces. Measurements of Hg° exchange with white oak (Quercus alba L.), red maple (Acer rubrum L.), Norway spruce (Picea abies L.), and yellow-poplar (Liriodendron tulipifera L.) foliage were conducted in an open gas exchange system that allows for simultaneous measurements of CO₂, H₂O and Hg° exchange under controlled environmental conditions. When Hg° concentrations were held at 0.5 to 1.5 ng m^?3, red maple (Acer rubrum L.), Norway spruce (Picea abies L.), yellow-poplar (Liriodendron tulipifera L.), and white oak (Quercus alba L.) foliage exhibited mean Hg° emissions of 5.5, 1.7, 2.7, and 5.3 ng m^?2 h^?1, respectively. At Hg° concentrations between 9 and 20 ng m^?3 little net exchange of Hg° was observed. However at concentrations between 50 and 70 ng m^?3 the Hg° was deposited to foliage at rates between 22 and 38 ng m^?2 h^?1. These data suggest that dry foliar surfaces in terrestrial forest landscapes may be a dynamic exchange surface that can function as a source or sink dependent on the magnitude of current Hg° concentrations. These data provide evidence of species-specific compensation concentrations (or compensation points) for Hg° deposition to seedling foliage in the 10–25 ng m^?3 range.     

Catalytic effect of various metal ions on the methylation of mercury in the presence of humic substances

Methylation of Hg²⁺ (Hg(NO₃)₂) in the presence of fulvic acid (FA) and various metal ions has been studied. The concentrations of Hg²⁺ and FA ranged from 5 to 20 mg L^?1 and 171 to 285 mg L^?1 DOC, respectively. The pH range was 3 to 6.5. FA was isolated from an acid brown-water lake by XAD-8 polymeric adsorbent. Methylmercury production in the dark during 2 to 4 days incubation at 30 °C increased with increasing concentrations of Hg²⁺ ion and FA as well as with additions of metal ions (5 to 10 × 10^?5 mole L^?1 The observed catalytic activity of metal ions followed the order Fe³⁺ (Fe²⁺) > Cu²⁺ ≈ Mn²⁺, > Al³⁺. The production of methylmercury had a pH-optimum around 4 to 4.5 at the conditions tested.     

Major controlling factors and prediction models for mercury transfer from soil to carrot

Purpose

Soil-plant transfer models are needed to predict levels of mercury (Hg) in vegetables when evaluating food chain risks of Hg contamination in agricultural soils.

Materials and methods

A total of 21 soils covering a wide range of soil properties were spiked with HgCl₂ to investigate the transfer characteristics of Hg from soil to carrot in a greenhouse experiment. The major controlling factors and prediction models were identified and developed using path analysis and stepwise multiple linear regression analysis.

Results and discussion

Carrot Hg concentration was positively correlated with soil total Hg concentration (R ²?=?0.54, P?<?0.001), and the log-transformation greatly improved the correlation (R ²?=?0.76, P?<?0.001). Acidic soil exhibited the highest bioconcentration factor (BCF) (ratio of Hg concentration in carrot to that in soil), while calcareous soil showed the lowest BCF among the 21 soil types. The significant direct effects of soil total Hg (Hg_soil), pH, and free Al oxide (Al_OX) on the carrot Hg concentration (Hg_carrot) as revealed by path analysis were consistent with the result from stepwise multiple linear regression that yielded a three-term regression model: log [Hg_carrot]?=?0.52log [Hg_soil]???0.06pH???0.64log [Al_OX]???1.05 (R ²?=?0.81, P?<?0.001).

Conclusions

Soil Hg concentration, pH, and Al_OX content were the three most important variables associated with carrot Hg concentration. The extended Freundlich-type function could well describe Hg transfer from soil to carrot.     

Modelization of the mercury fluxes at the air-sea interface

Aqueous and atmospheric Hg° concentrations for remote marine areas such as the equatorial Pacific Ocean and for coastal seas such as the North Sea and the Scheldt Estuary are discussed. Biological processes seem to be at the origin of the supersaturated Hg° concentrations in the water. On the other hand, transfer velocities across the air-sea interface were calculated with a classical shear turbulence model and with a wave breaking model. With these data, Hg° fluxes from the sea to the atmosphere were calculated: in the Pacific Ocean they range from 0.43 to 6.5 μg g Hg.m^?2. yr^?1 at a wind speed of 2.8 m.s^?1 and from 10.3 to 156 μg Hg.m^?2 yr^?1 at a wind speed of 54 m.s^?1, but they are still higher when wave breaking is considered (from 11 to 168 μg Hg.m^?2.yr^?1). These transfer fluxes are an order of magnitude higher in the Scheldt Estuary.     

The gas phase oxidation of elemental mercury by ozone

The gas phase reaction between elemental mercury (Hg⁰) and ozone (0₃) has been studied in sunlight, in darkness, at different temperatures, and different surface-to-volume (s/v) ratios. At 0₃ concentrations above 20 ppm, a loss of Hg⁰ and a simultaneous formation of oxidized mercury (Hg(II)) was observed. The results suggest a partly heterogeneous reaction, with a gas phase rate constant of 3±2×10^?20 cm³ molec.^?1 s^?1 at 20 °C. This corresponds to an atmospheric Hg half-life of about one year at a mean global 0₃ concentration of 30 ppb.     

Methylated and elemental mercury cycling in surface and deep ocean waters of the North Atlantic

Biogeochemical cycling of mercury (Hg) in the ocean and air-sea exchange are integral parts of the global Hg cycle. Ionic Hg (i.e. reactive Hg-Hg°) is converted in ocean surface waters to elemental Hg (Hg°) with the subsequent loss, via gas evasion, of the Hg° to the atmosphere. During a recent cruise in the North Atlantic Ocean, Hg° in surface waters was a substantial fraction of the reactive Hg (85%, on average) and there was a relationship between photosynthetic pigment concentration and Hg°. In addition, there was evidence of Hg bound to “collodial” material (of greater than 1,000 molecular weight). Ionic Hg concentrations were around 0.15 pM, similar to the average colloidal Hg concentration of 0.2 pM. Methylated Hg compounds, both dimethylHg (DMHg) and monomethylHg (MMHg), were found in the deeper waters with DMHg being the predominant methylated species. This contrasts with freshwater lakes where MMHg is the principal species and no DMHg has been found. Preliminary modelling, using estimated rate constants for the formation and decomposition of DMHg and MMHg, predicts an enhanced stability of DMHg in ocean waters relative to fresh water. Deep ocean waters, formed by sinking of surface waters, can preserve DMHg that was produced in the more productive surface regime.     

Cycling of volatile mercury in temperate lakes

The cycling of dissolved gaseous Hg (DGM) has been examined in our studies of the troposphericHg cycle, air-water exchange and their importance to the biogeochemical behavior and fate of Hg in temperate lakes. Five seepage lakes in northcentral Wisconsin, ranging in pH from 4.7 to 7.2, have been studied under a variety of limnological conditions which included the following seasonal periods: summer (peak stratification), fall (following turnover) and late winter (under ice). Analytically, DGM was determined after purging lake water with argon and collecting the volatile Hg fraction on gold coated sand. The Hg collections were analysed by pyrolysis in a two-stage Au amalgamation gas train with detection by atomic fluorescence spectroscopy (AFS). In addition, chemical speciation of the volatile fraction has been achieved by trapping on a nondestructive substrate followed by gas chromatographic separation and AFS detection. The DGM consists principally of elemental Hg (Hg^o) under all sampling conditions, with no significant contribution from volatile organic Hg species (detection limit of 3 femtomolar). Atmospheric gaseous Hg, which also consists principally of Hg^o, was measured and the air-water partitioning determined. In general, the lake waters have been supersaturated with Hg^o relative to the atmosphere. Supersaturation was greater in the summer, ranging from ca. 1.4 to 12 times (x) the saturation concentration. During the other sampling periods, Hg^o ranged from saturation to ca. 7x the equilibrium concentration. The flux of Hg from the lakes due to gas evasion is significant and is estimated at approximately 10% of the annual atmospheric input of Hg to the lakes. An apparent relationship between Hg^o and pH has been observed with lakes of lower pH having smaller Hg^o concentrations.     

Interrelationships between mercury levels in yearling yellow perch,fish condition and water quality

Yearling yellow perch were collected from sixteen Muskoka-Haliburton lakes to determine interrelationships between water quality, Hg residues in fish and fish condition. The lakes studied were Precambrian shield lakes with a pH range of 5.6 to 7.3 and total inflection point alkalinities of 0.4 to 16.0 mg L^?1. Mercury residues in yellow perch ranged from 31 to 233 ng g^?1 and were inversely correlated (p < 0.001; r = 0.84) with lakewater pH. Stepwise linear regression analyses selected lakewater pH as the only significant parameter associated with Hg accumulations. Alkalinities, sulphate, Ca and dissolved organic carbon (DOC) were not selected as significant. Likewise, lakewater pH and Hg residues in yellow perch were inversely (p < 0.001) correlated with fish condition. Lakewater pH, accounted for 74% and Hg in fish a further 11% of the variability in fish condition. Terrestrial drainage size/lake volume ratios were also correlated (p < 0.05; r = 0.78) with Hg accumulations in perch from a subset of nine headwater lakes. No temporal trends in Hg residues were evident in yellow perch over a 9 yr interval (1978–1987).     

Salinity,chloride, and density relationships in ion enriched Onondaga Lake,NY

The relationship between salinity (S), chlorinity (Cl) and density (d) for the ion polluted (S = 3 to 4.5‰) and stratified Onondaga Lake are presented. The data base includes 220 determinations of the major ionic components collected from ten equally spaced depths. The salinity (S) and ionic strength (I) in the lake can be estimated from S = 1.85 Cl ? 0.28, where Cl is in g kg^?1 and I = 1.456 [Cl^?] ? 0.039, where [Cl] is in mmol L^?1. Due to the high concentration of Ca²⁺ in the lake, the seawater equation of state (Chen and Millero, 1977a) could not be used to estimate reliable densities for the lake (Δd ～ 100 × 10^?6 g cm^?3). A reliable equation of state for Onondaga Lake was derived from the composition data $$d = d_0 + A_v {\text{ }}CL + B_v {\text{ }}Cl^{3/2} $$ where d ₀ is the density of water (g cm^?3), A _v is a temperature dependent parameter related to the infinite dilution apparent molal volumes and B _v is related to the ion-ion interactions of the lake salts. The high ionic content of the lake depresses the temperature of maximum density to 3.18°C and alters the stratification of the lake. The salinity component of the stratification represents 40% of the total density stratification.     

High Carbon Dioxide Evasion from an Alpine Peatland Lake: The Central Role of Terrestrial Dissolved Organic Carbon Input

We measured carbon dioxide (CO₂) fluxes across air?Cwater interface with floating chambers in Lake Medo (a small, shallow lake in peatland) on the eastern Tibetan Plateau in the warm season of 2009. During the study period, mean CO₂ fluxes was 488.63?±?1,036.17?mg?CO₂?m^?2?h^?1. The flux rate was high compared to those of lakes in other regions, and represented a ??hotspot?? of CO₂ evasion. Temporal variation of CO₂ flux was significant, with the peak value in the beginning and lowest point in the end of warm season. High concentration of dissolved organic carbon (DOC) in lake water (WDOC) was found to highly correlated to CO₂ flux (R?=?0.47, P?<?0.01, n?=?54). Besides, fluorescence index of WDOC showed its terrestrial origin character. In accordance with lakes in northern and boreal regions, terrestrial DOC concentration in water column was the most important regulator of CO₂ flux from this lake. We suggest that large area of peatlands in catchments support high concentration of DOC in this lake, and consequently high CO₂ evasion.     

A preliminary mercury budget for Narragansett Bay (Rhode Island,USA)

The distribution of total Hg (Hg_t) and reactive (Hg_R) in Narragansett Bay, fresh water tributaries and point source discharges was determined during a synoptic survey, carried out in April, 1986. A Hg budget which includes fluvial inputs and atmospheric Hg deposition was constructed and the estuarine behavior of Hg assessed.     

Mercury Methylation Capacity and Removal of Hg Species from Aqueous Medium by Cyanobacteria

Most technologies used for decontamination presents good results for high concentrations, but limitations for lower ones. The desirable Hg concentration in the water is extremely low because of its toxicity. The aims of this study were to evaluate inorganic mercury (Hg²⁺) and methylmercury (CH₃Hg⁺) toxicity in Nostoc paludosum, to assess the potential of this cyanobacteria strain to remove these Hg species from aqueous medium and also to investigate Hg methylation by the cyanobacteria. CH₃Hg⁺ determination was performed by gas chromatography-pyrolysis-atomic fluorescence spectrometry in cultures exposed to a concentration of 20 μg L^?1 for 30 days. Both Hg species were removed from the supernatant, ranging from 73 to 96% of Hg²⁺ and from 73 to 95% of CH₃Hg⁺. Ultrastructural Hg²⁺ effects in the cyanobacteria cells investigated by transmission electron microscopy revealed higher production of glycogen, cyanophycin, and intrathylacoidal spaces than the control group. When Hg was added to the culture in the form of CH₃Hg⁺, a decrease corresponding to approximately 60% of the initial concentration due to Hg volatilization was observed. The production of CH₃Hg⁺ by the cyanobacteria was detected in concentrations near the limit of detection (0.0025%) of the bioaccumulated THg. This is an advantage for biotechnological decontamination applications, as CH₃Hg⁺ is a very toxic specie and can be bioaccumulated and biomagnified. The results demonstrated that cyanobacteria cells are an efficient alternative to retain and/or remove Hg at low concentrations and they constitute a potential tool for a “final cleaning” of contaminated waste water.     

Development of a fugacity/aquivalence model of mercury dynamics in lakes

A simple, mechanistic model of mercury (Hg) dynamics in a lake has been developed, based on the fugacity/aquivalence approach of Mackay (1991) and Mackay and Diamond (1989) and its extension to treat several interconverting chemical species (Diamond et al., 1992). The model considers the distribution of inorganic (HgII), elemental (Hg°) and methyl (MeHg) mercury species between dissolved and particle-sorbed phases, and fate and transport in a system consisting of a well-mixed water column and an active sediment layer. Hg can enter the lake from watershed runoff and by atmospheric deposition directly to the lake surface. Once in the lake, Hg exchanges between water and air, and water and sediments, and exits by sediment burial, advective flow and volatilization. The model was applied to a hypothetical drainage lake on the Canadian Shield. Model estimates of water and sediment concentrations compare well with measured values. The results suggest that the three Hg species experience significantly different fates and persistence, with overall Hg dynamics dominated by the fate of HgII (the predominant species). A sensitivity analysis illustrates the importance of physical/chemical properties and lake characteristics on the total amount and behavior of Hg in the lake.     

The aqueous reduction of divalent mercury by sulfite

Divalent Hg is reduced by sulfite in aqueous solutions. The proposed mechanism involves the formation of an instable intermediate, HgSO₃, which decomposes to produce Hg⁺ which in turn is rapidly reduced to Hg⁰. The overall rate of the reaction is inversely dependent on the concentration of sulfite. This reaction may influence the concentration of Hg in cloud- and rain-water by reducing water soluble Hg²⁺ to volatile Hg⁰. At low concentrations of SO₂(g) (5 μg m⁻³, 25 °C), the rate of the conversion of Hg(SO₃)₂ ²⁻ to Hg⁰ becomes significant (> 1 % h⁻¹) at pH < 5.5. At higher S02 concentrations (500 pg m⁻³), the same rate is expected at pH < 4.5.     

Solubility of cinnabar (red HgS) and implications for mercury speciation in sulfidic waters

New experiments have been conducted to determine the speciation of dissolved mercury (Hg) over wide pH (1–12) and sulfide concentration ranges (0.5–30 mM) and in the presence of elemental sulfur (S⁰) or Hg⁰, conditions that encompass those of near-bottom and pore waters of sediments. Samples containing synthetic red mercuric sulfide (HgS, cinnabar), buffer solution, aliquots of bisulfide (HS^?1) solution, and, in special cases, S⁰ or Hg⁰ were prepared anaerobically and allowed to equilibrate for several months. Filtered samples were analyzed for pH, total sulfide (ΣS^2?), and total mercury [Hg]_tot. Plots of [Hg]_tot values vs. pH at varying ΣS^2? verified the formation of three previously known mercury-sulfide complexes (HgS₂H_n ^n?2) and revealed that a new Hg₂SOH⁺ complex is important at low pH and low ΣS^2?. Our constants for ionic strength (I) 0.7 and 25⁰ C are as follows: K₁=10^{?5.76(+0.71, ?1.02)} for HgS_cinn+H₂S ? HgS₂H₂ ⁰; K₂=10^{?4.82(+0.72, ?1.10)} for HgS_cinn+HS^? ? HgS₂H^?; K₃=10^{?13.41(+0.76, ?0.93)} for HgS_cinn+HS^? ? HgS₂ ^2?+H⁺; K₄=10^{?8.36(+0.71, ?0.93)} for 2HgS_cinn+H⁺+H₂O ? Hg₂SOH⁺+H₂S. With decreasing pH, below 1, Hg solubility decreased sharply, indicating the formation of a new solid phase, inferred to be corderoite (Hg₃S₂Cl₂). From our solubility data, we calculated the free energy of formation (ΔG_f ^o) of Hg₃S₂Cl₂ to be ?396 (+3, ?11) kJ/mol. In experiments where excess S⁰(s) was present, a new mercury-polysulfide dimer was identified; its formation constant is K₅=10^{?1.99(+0.69, ?1.27)} for 2HgS_cinn+2HS^? + nS⁰ ? Hg₃S₄ ^IIS_n ^oH₂ ^2?. Data from experiments where Hg⁰(aq) was added confirmed the reversibility of HgS dissolution. An application of our mercury-sulfide speciation model to a natural anoxic basin, Saanich Inlet, British Columbia, is discussed.     

Mercury reactions in the human mouth with dental amalgams

This is a preliminary study of the reactions of mercury (Hg) in the human mouth with dental amalgams. It was conducted by analysing saliva samples from subjects with amalgam fillings and control subjects with no amalgams. Samples were collected both prior to and after cleaning the mouth. These samples were analyzed for elemental mercury (Hg⁰), inorganic mercury (Hg²⁺) and methylmercury (MeHg). We concluded that the concentrations after cleaning represented the systemic concentrations. Hg²⁺ and MeHg were found in all systemic samples from both subjects and controls, while Hg⁰ was found only in the samples from subjects with amalgams. In the control group, the concentrations found before and after cleaning the mouth were equivalent. In the amalgam group, concentrations of Hg²⁺ found before cleaning the mouth were 10 to 40 times higher than those found after cleaning, suggesting that the oxidation reaction of Hg° into Hg²⁺ takes place. For MeHg, a similar but less pronounced pattern as Hg²⁺ was found, supporting methylation in the mouth.     

Temperature determinants of plant-soil-air mercury relationships

Using the Arrhenius thermodynamic equation, which relates rates of processes to temperature through the quantity E _a , the ‘Heat (or Energy) of Activation’, we have evaluated the thermal relationships for several parameters of Hg cycling. It is shown that release from isolated leaves (shoots) of Hg⁰ is a two-step process with a higher E _a value below 21 °C than above (28 vs 14 kcal mol^?1). Open field air Hg measurements over a mixed stand of grasses and other plants in volcanic soil show strikingly similar behavior to detached organs. Mercury release from volcanic soil was uniform over a wide temperature range, resembling plant and open field emissions above 18 to 21°C with anE _a value of 13 kcal · mol^?1. We conclude that Hg release below 18 to 21 °C is limited by a physiological process, whereas above that range, release is controlled by the physical process of vaporization itself. Plant concentration of total Hg from 5 to 33 °C (air temperature), is a more complex function involving direct accumulation and re-release of Hg⁰ from the atmosphere, release from tissue storage, and root uptake with reduction.