A Michaelis–Menten type equation for describing methylmercury dependence on inorganic mercury in aquatic sediments

Methylation of mercury (Hg) is the crucial process that controls Hg biomagnification along the aquatic food chains. Aquatic sediments are of particular interest because they constitute an essential reservoir where inorganic divalent Hg (Hg^II) is methylated. Methylmercury (MeHg) concentrations in sediments mainly result from the balance between methylation and demethylation reactions, two opposite natural processes primarily mediated by aquatic microorganisms. Thus, Hg availability and the activity of methylating microbial communities control the MeHg abundance in sediments. Consistently, some studies have reported a significant positive correlation between MeHg and Hg^II or total Hg (Hg_T), taken as a proxy for Hg^II, in aquatic sediments using enzyme-catalyzed methylation/demethylation mechanisms. By compiling 1,442 published and unpublished Hg_T–MeHg couples from lacustrine, riverine, estuarine and marine sediments covering various environmental conditions, from deep pristine abyssal to heavily contaminated riverine sediments, we show that a Michaelis–Menten type relationship is an appropriate model to relate the two parameters: MeHg = aHg_T/(K _m  + Hg_T), with a = 0.277 ± 0.011 and K _m  = 188 ± 15 (R ² = 0.70, p < 0.001). From K _m variations, which depend on the various encountered environmental conditions, it appears that MeHg formation and accumulation are favoured in marine sediments compared to freshwater ones, and under oxic/suboxic conditions compared to anoxic ones, with redox potential and organic matter lability being the governing factors.     

Behavior of mercury in the Patuxent River estuary

An overview of a comprehensive study of the behavior and fate of mercury in the estuarine Patuxent River is presented. Total Hg (Hg_T) and methylmercury (MeHg) exhibited weakly non-conservative behavior in the estuary. Total Hg concentrations ranged from 6 ng L^-1 in the upper reaches of the sub-urbanized tidal freshwater river to <0.5 ng L^-1 in the mesohaline lower estuary. Filterable (0.2 µm) Hg_T ranged from 0.2 to 1.5 ng L^-1. On average, MeHg accounted for <5% of unfiltered Hg_T and <2% of filterable Hg_T. Dissolved gaseous section Hg (DGHg) concentrations were highest (up to 150 pg L^-1) in the summer in the mesohaline, but were not well correlated with primary production or chlorophyll a, demonstrating the complex nature of Hg⁰ formation and cycling in an estuarine environment. Organic matter content appeared to control the Hg_T content of sediments, while MeHg in sediments was positively correlated with Hg_T and organic matter, and negatively correlated with sulfide. MeHg in sediments was low (0.1 to 0.5% of Hg_T). Preliminary findings suggest that net MeHg production within sediments exceeds net accumulation. Although Hg_T in pore waters increased with increasing sulfide, bulk MeHg concentrations decreased. The concentration of MeHg in sediments was not related to the concentration of Hg_T in pore waters. These observations support the hypothesis that sulfide affects the speciation and therefore bioavailability of dissolved and/or solid-phase Hg for methylation. Comparison with other ecosystems, and the negative correlation between pore water sulfide and sediment MeHg, suggest that sulfide limits production and accumulation of MeHg in this system.     

Dissolved and suspended mercury. species in the Wabigoon River (Ontario,Canada): seasonal and regional variations

Seasonal and regional variations in the speciation, sediment-water partitioning, and dynamics of mercury (Hg) were studied at selected sites along the Hg-polluted Wabigoon River, and at unpolluted headwater and tributary sites, during April–September, 1979. ‘Dissolved’ and ‘particulate’ forms of Hg in the water were separated by continuous-flow centrifugation in the field. The Hg and other pollutants such as wood chips and salt had been discharged from a chlor-alkali plant and paper mill at Dryden, Ontario. Concentrations and loadings of particulate methyl mercury (CH₃Hg⁺) and total particulate Hg (and loadings of total ‘dissolved’ Hg) were greatest during the spring flood (April-May) owing to accelerated resuspension and transport of sediments. Concentrations of ‘dissolved’ CH₃Hg⁺, however, were highest in the summer (July–September), probably reflecting stimulation of microbial methylating activity by elevated temperatures, together with factors such as reduced levels of metal-scavenging particulates and minimal dilution by runoff. Total dissolved Hg concentrations were relatively high in September at polluted sites only, possibly because of desorption from sediments due to elevated concentrations of Cl⁻ ions. Loadings of dissolved CH₃Hg⁺ tended to be high in the summer but were generally depressed (suggesting sorption by suspended particles) during the major spring-flood episode in May. During July–August dissolved CH₃Hg⁺ was a function of total dissolved Hg, suggesting rapid biomethylation of desorbed inorganic Hg; but in general dissolved and suspended CH₃Hg⁺ levels depended on environmental variables and were unrelated to total Hg concentrations. In the summer only, total dissolved Hg was a function of dissolved Cl⁻. Hg species in particulates were associated with sulfides, hydrated Fe and Mn oxides, organic matter (notably high molecular weight humic and humic-Fe components), and selenium (Se); but CH₃Hg⁺ and total Hg differed in their specific preferences for binding agents, implying that binding sites discriminate between CH₃Hg⁺ and Hg²⁺ ions. CH₃Hg⁺ was associated with sulfide and (in the spring only) with Fe oxides, whereas total Hg was associated with organic matter and Se and with DTPA- and NaOH-extractable Fe in the spring but with Mn oxide and NaOH-extractable organics in the summer. Sulfides were most abundant in May, indicating that they were eroded from bottom sediments, but Fe and Mn oxides were most abundant in the summer, probably owing to activities of filamentous iron bacteria and other micro-organisms. Particulate Hg was 98–100% nonextractable by mild solvents such as Ca acetate, CaCl₂, dilute acetic acid, and (at polluted sites only) DTPA solutions, suggesting that the particulate Hg mobilized in the spring may not be readily available to organisms; association with Se and high molecular weight humic matter also supports this hypothesis. Hg probably becomes more bio-available in the summer, as suggested by the upsurge in dissolved CH₃Hg⁺ and total dissolved Hg levels, and by increases in the solubility of particulate Hg in acetic acid, DTPA, H₂O₂, and NaOH solutions, as well as an increase in the relative importance of lower molecular weight fractions of NaOH-extractable Hg (in September). Regional variations in Hg speciation and partitioning reflected a gradient in sediment composition from wood chips near Dryden to silt-clay mud further downstream. Hg in silt-clay mud relatively far (> 35 km) downstream from the source of pollution or in unpolluted areas appeared to be more readily solubilized by Cl⁻ ions or chelators such as DTPA, more readily methylated (as indicated by downstream increases in dissolved CH₃Hg⁺ levels and CH₃Hg⁺/total Hg ratios), and was to a greater degree organically bound (H₂O₂-extractable), and thus was probably more bio-available, than Hg in wood-chip deposits. Possible explanations include weaker binding of Hg by the mud, the more finely divided state of the mud, and improved microbial growth at lower concentrations of toxic pollutants. Owing to enrichment in sulfides and Fe oxides, resuspended wood-chip sediments were especially efficient scavengers of CH₃Hg⁺. The results indicate that in any pollution abatement plan aimed at lowering the Hg levels in the biota of lakes fed by the Wabigoon River, immobilization, removal, or detoxification of dissolved as well as particulate forms of Hg in the river would probably have to be considered. Possibly, Hg species could be ‘scrubbed’ from the river water by increasing the suspended load and by sedimentation and treatment with Hg-binding agents in special receiving basins.     

Acclimation of aquatic microbial communities to Hg(II) and CH3Hg+ in polluted freshwater ponds

The relationship of mercury resistance to the concentration and chemical speciation of mercurial compounds was evaluated for microbial communities of mercury-polluted and control waters. Methodologies based on the direct viable counting (DVC) method were adapted to enumerate mercury-resistant communities. Elevated tolerance to Hg(II) was observed for the microbial community of one mercury-polluted pond as compared to the community of control waters. These results suggest an in situ acclimation to Hg(II). The results of the methylmercury resistance-DVC assay suggested that minimal acclimation to CH₃Hg⁺ occurred since similar concentrations of CH₃HgCl inhibited growth of 50% of organisms in both the control and polluted communities. Analyses of different mercury species in pond waters suggested that total mercury, but not CH₃Hg⁺ concentrations, approached toxic levels in the polluted ponds. Thus, microbial acclimation was specific to the chemical species of mercury present in the water at concentrations high enough to cause toxic effects to nonacclimated bacterial communities.     

Seasonal changes in phytoplankton community structure in relation to physico-chemical factors in Loch Lomond,Scotland

Surface waters, sediments and the polychaete Nereis diversicolor were sampled in the ScheldtEstuary between 1990 and 1994. In surface watersparticulate Hg (Hg_P) concentrations ranged from350–1610 ng g^-1. They are essentiallycontrolled by physical mixing of polluted fluvialparticulates with relatively unpolluted marineparticulates, but unaffected by seasonal changes.Dissolved Hg species, on the other hand, show largeseasonal variations essentially controlled by theredox conditions in the estuary, as well as bybacterial and phytoplankton activity. Total dissolvedHg (Hg_TD) concentrations ranged from 0.5 to 3.0 ngl^-1 with 10 to 90% as reactive Hg. Highconcentrations of Hg_TD are found in the upperestuary in the winter and decrease rapidly withincreasing salinity. In summer Hg_TDconcentrations are low in the anoxic upper estuary andincrease as oxygen is restored in the estuary.Significant variations were observed in dissolvedMonomethyl Hg (MMHg) concentrations withconcentrations ranging from 0.01 to 0.120 ng l^-1 in the winter and 0.08 to 0.6 ng l^-1 in summerand autumn. Particulate MMHg ranged from 2 to 6 ngg^-1 in winter and from 4 to 10 ng g^-1 insummer and accounted for 20 to 80% of the total MMHg.Hg° concentrations ranged from 0.02 to 0.130 ngl^-1 and are higher in summer than in winter. Inthe lower estuary a positive correlation betweenHg° and phytoplankton pigments was observed. Sediments and the polychaete N. diversicolorwere sampled on the intertidal flat GrootBuitenschoor. Hg_{_T} concentrations in surfacesediments ranged from 144 to 1890 ng g^-1 and MMHgfrom 0.8 to 6 ng g^-1 accounting for 0.4 to 0.8%of the total mercury present. Both total Hg (Hg_T)and MMHg concentrations increased with increasedorganic matter content and anoxic conditions. On theother hand, accumulation of Hg_T and MMHg washigher in N. diversicolor living in coarse grainsandy sediments than in muddy sediments. MMHgconcentrations in N. diversicolor ranged from2.2 to 20.9 ng g^-1 accounting for an average of18% of the Hg_T. Seasonal variationssignificantly affected Hg speciation in sediments andN. diversicolor. Higher Hg_T concentrationswere found in the sediments in autumn and winter,whereas MMHg concentrations increased in spring andsummer. Likewise, higher MMHg concentrations were alsoobserved in N. diversicolor in spring andsummer. This revised version was published online in July 2006 with corrections to the Cover Date.     

A coumarin‐based fluorescent probe for Hg2+ and its application in living cells and zebrafish

Mercury (Hg) is a heavy metal with high toxicity and easy migration; it can be enriched through the food chain, and cause serious threats to the natural environment and human health. So, the development of a method that can be used to detect mercury ions (Hg²⁺) in the environment, in cells, and in organisms is very important. Here, a new 7‐hydroxycoumarin‐derived carbonothioate‐based probe ( CC‐Hg ) was designed and synthesized for detection of Hg²⁺. After addition of Hg²⁺, a large fluorescence enhancement was observed due to the formation of 7‐hydroxyl, which reinforced the intramolecular charge transfer process. The CC‐Hg probe had good water solubility and selectivity. Moreover, the probe was able to detect Hg²⁺ quantitatively over the concentration range 0–2 μM and with a detection limit of 7.9 nM. Importantly, we successfully applied the probe to detect Hg²⁺ in water samples, in living cells, and in zebrafish. The experimental results demonstrated its potential value in practical applications.     

Bioaccumulation and Trophic Transfer of Mercury and Selenium in African Sub-Tropical Fluvial Reservoirs Food Webs (Burkina Faso)

The bioaccumulation and biomagnification of mercury (Hg) and selenium (Se) were investigated in sub-tropical freshwater food webs from Burkina Faso, West Africa, a region where very few ecosystem studies on contaminants have been performed. During the 2010 rainy season, samples of water, sediment, fish, zooplankton, and mollusks were collected from three water reservoirs and analysed for total Hg (THg), methylmercury (MeHg), and total Se (TSe). Ratios of δ¹³C and δ¹⁵N were measured to determine food web structures and patterns of contaminant accumulation and transfer to fish. Food chain lengths (FCLs) were calculated using mean δ¹⁵N of all primary consumer taxa collected as the site-specific baseline. We report relatively low concentrations of THg and TSe in most fish. We also found in all studied reservoirs short food chain lengths, ranging from 3.3 to 3.7, with most fish relying on a mixture of pelagic and littoral sources for their diet. Mercury was biomagnified in fish food webs with an enrichment factor ranging from 2.9 to 6.5 for THg and from 2.9 to 6.6 for MeHg. However, there was no evidence of selenium biomagnification in these food webs. An inverse relationship was observed between adjusted δ¹⁵N and log-transformed Se:Hg ratios, indicating that Se has a lesser protective effect in top predators, which are also the most contaminated animals with respect to MeHg. Trophic position, carbon source, and fish total length were the factors best explaining Hg concentration in fish. In a broader comparison of our study sites with literature data for other African lakes, the THg biomagnification rate was positively correlated with FCL. We conclude that these reservoir systems from tropical Western Africa have low Hg biomagnification associated with short food chains. This finding may partly explain low concentrations of Hg commonly reported in fish from this area.     

Selenium–Mercury Interactions in Man and Animals

Selenium–mercury interactions were most extensively studied in relation to alleviation of Hg toxicity by added selenium. This presentation considers the influence of mercury on endogenous selenium, on its tissue and cellular “status” after lifelong or acute exposure to mercury vapor (Hg^o). Discussed are data obtained from (1) humans living near or working in a mercury mine, and (2) rats experimentally exposed in the mine. Mercury vapor is unique—or similar to methylmercury—because of its ability to penetrate cell membranes and so invade all cells, where it is oxidized in the biologically active form (Hg⁺⁺) by catalase. Such in situ-generated ions can react with endogenously generated highly reactive Se metabolites, like HSe−, and render a part of the selenium unavailable for selenoprotein synthesis. Data on human populations indicate that in moderate Hg exposure combined with an adequate selenium supply through diet, Se bioavailability can be preserved. On the other hand, the results of an acute exposure study emphasize the dual role of selenium in mercury detoxification. Besides the well-known Se coaccumulation through formation of nontoxic Hg–Se complexes, we observed noticeable Se (co)excretion, at least at the beginning of exposure. The higher Hg accumulation rate in the group of animals with lower basal selenium levels can also point to selenium involvement in mercury excretion. In such conditions there is a higher probability for decreased selenoprotein levels (synthesis) in some tissues or organs, depending on the synthesis hierarchy.     

Mercury Methylation and Demethylation in Anoxic Lake Sediments and by Strictly Anaerobic Bacteria

After spiking anoxic sediment slurries of three acidic oligotrophic lakes with either HgCl₂ at 1.0 μg/ml or CH₃HgI at 0.1 μg/ml, both mercury methylation and demethylation rates were measured. High mercury methylation potentials were accompanied by high demethylation potentials in the same sediment. These high potentials correlated positively with the concentrations of organic matter and dissolved sulfate in the sediment and with mercury levels in fish. Adjustment of the acidic sediment pH to neutrality failed to influence either the methylation or the demethylation rate of mercury. The opposing methylation and demethylation processes converged to establish similar Hg²⁺-CH₃Hg⁺ equilibria in all three sediments. Because of their metabolic dominance in anoxic sediments, mercury methylation and demethylation in pure cultures of sulfidogenic, methanogenic, and acetogenic bacteria were also measured. Sulfidogens both methylated and demethylated mercury, but the methanogen tested only catalyzed demethylation and the acetogen neither methylated nor demethylated mercury.     

Automated Speciation of Mercury in the Hair of Breastfed Infants Exposed to Ethylmercury from Thimerosal-Containing Vaccines

A simplified thiourea-based chromatography method, originally developed for methyl and inorganic mercury, was adapted to separate methylmercury (MeHg), ethylmercury (EtHg), and inorganic mercury (Hg^II) in infants' hair. Samples were weighed and leached with an acidic thiourea solution. Leachates were concentrated on a polymeric resin prior to analysis by Hg-thiourea liquid chromatography/cold vapor atomic fluorescence spectrometry. All but one sample showed small amounts of EtHg, and four of the six analyzed samples had proportionally higher Hg^II as a percent of total Hg. Breastfed infants from riverine Amazonian communities are exposed to mercury in breast milk (from high levels of maternal sources that include both fish consumption and dental amalgam) and to EtHg in vaccines (from thimerosal). The method proved sensitive enough to detect and quantify acute EtHg exposure after shots of thimerosal-containing vaccines. Based on work with MeHg and Hg^II, estimated detection limits for this method are 0.050, 0.10, and 0.10 ng g⁻¹ for MeHg, Hg^II, and EtHg, respectively, for a 20-mg sample. Specific limits depend on the amount of sample extracted and the amount of extract injected.     

Mercury Concentration in Sediment Cores from Sundarban Mangrove Wetland,India

The work presents an extended database (n = 123) of total mercury (T_Hg) in fine-grained sediment fraction (<63 μm) of core samples in 10 sampling stations of the Sundarban mangrove wetland, India, formed at the estuarine realm of the Hugli (Ganga) River. Results revealed a wide range of spatial, seasonal, and intertidal flat variations of T_Hg (7.3 to 93.3 ng/g) with a definite enhancement level at the lower stretch of the estuary, which has extreme mangrove vegetation. An overall enrichment of T_Hg in surface/subsurface layers of the core is tentatively explained by remobilization and resuspension of the metal from deeper sediments (36–40 cm). A strong positive correlation was observed between the Hg and clay fraction content of the sediments, while correlations of Hg with organic carbon was poor. Based on the index of geoaccumulation (I_geo), enrichment factor (EF), and anthropogenic factor (AF) values, it is suggested that the sediments of Sundarban were found to be less polluted with respect to total mercury. The data reported are a useful baseline for T_Hg in Sundarban and would be of importance in future sediment quality studies.     

A new proposal regarding the transport mechanism of mercury in biological membranes

The interactions of mercury (Hg²⁺) with biological membranes have been investigated. The experimental results indicate that Hg²⁺ induces a rapid alkalinization in energized Lysosomes from rat liver. The interpretation of the process is that the mercury enters the Lysosomes as a Hg(OH)₂ electroneutral compound, thus inducing alkalinization in the matrix.     

Ultrasensitive green spectrofluorimetric approach for quantification of Hg(II) in environmental samples (water and fish samples) using cysteine@MnO2 dots

Mercury (Hg²⁺) is a natural element present in foods such as fish, water and soil. Exposure to mercury leads to severe toxic effects on the nervous, digestive, and immune systems. Here, a novel, green, and environmentally friendly fluorescent probe decorated with cysteine/MnO₂ quantum dots (Cys@MnO₂ QDs) was synthesized. This synthesis was carried out using a simple ultrasound technique with the aid of cysteine for fabricating Cys@MnO₂ QDs to estimate Hg levels in fish and water samples. In this morphological study, Cys@MnO₂ QDs were fully characterized using high-resolution transmission electron microscopy, zeta potential analysis, fluorescence, ultraviolet–visible and infrared spectroscopy. The fluorescence of the synthesized Cys@MnO₂ QDs was significantly quenched by gradually increasing the Hg(II) concentration. The quenching mechanism based on the Hg–S bonds strengthened the utility of the Cys@MnO₂ QDs as a novel luminescent nanoprobe. The estimation of Hg was linear in the concentration range 0.7–100.0 ng mL⁻¹ with a limit of quantitation equal to 0.30 ng mL⁻¹. The Cys@MnO₂ QDs are fluorescent probes with various benefits such as speed, ease of use, cost- effective, and being environmentally friendly; they are easily applied in food manufacturing and for public health improvement.     

On the structure of Hg2I3+ in dimethylsulfoxide and aqueous solution. An X-ray scattering and raman spectroscopic study

Dimethylsulfoxide and aqueous solutions of mercury(II) in large excess over iodide have been investigated by X-ray scattering techniques supported by Raman spectroscopic measurements. The composition of the solutions has been selected to ensure that the cationic complex Hg₂I³⁺ is the predominant iodide species. The structure parameters of the solvated Hg₂I³⁺ ion have been refined by a least- squares procedure on the scattering data, using known structural parameters for the additional molecular entities present. The Hg₂I³⁺ entity is more or less identical in DMSO and water. The HgI bond distance is 2.613(12) and 2.632(5) Å and the HgHg distance is 3.66(5) and 3.70(1) Å in DMSO and water, respectively. This yields a HgIHg angle of 89° in both solvents. The mercury(II) atom in this complex is most probably solvated in a tetrahedral fashion by three DMSO or H₂O molecules. The structure of Hg₂I³⁺ is discussed in the light of recent results for the Ag₄I³⁺ complex in solution and relevant crystal structures.     

Mercury dynamics in Tivoli South Bay, a freshwater tidal mudflat wetland in the Hudson River

The accumulation of total mercury (Hg_T) andmethylmercury (MeHg) was evaluated in sediments ofTivoli South Bay, a freshwater tidal mudflat wetlandin the Hudson River National Estuarine ResearchReserve system. Hg_T concentrations in sedimentcores were measured to evaluate the spatialvariability of Hg_T deposition, and to establisha chronology of Hg_T accumulation. Cores takenfrom the northern, middle, and southern sections ofthe bay had similar distribution patterns andconcentrations of Hg_T, suggesting a common sourceof Hg_T throughout the bay. Sedimentconcentrations ranged from 190 to 1040 ng Hg g^–1,2 to 10 times greater than concentrations expected insediments from non-anthropogenic sources. Hg_Tdeposition rates were similar in different regions ofthe bay, and increased from 200 ng Hg cm^–2yr^–1in the 1930s to a maximum of 300 ngHg cm^–2 yr^–1 in the 1960s. Deposition rateshave steadily declined since the 1970s and arecurrently at 80 ng Hg cm^–2 yr^–1. Transportof Hg_T by tidal waters from the Hudson River islikely the main source of Hg_T in the bay.Distribution patterns and absolute concentrations ofMeHg in sediment cores were similar throughout thebay, with concentrations ranging from 0.43 to 2.95ng g^–1. Maxima in MeHg concentration profilesoccurred just below the sediment-water interface andat a depth of 30 cm. The maximum at 30 cm wascoincident with maximum Hg_T concentrations. MeHgconcentrations in suspended particulate matter (SPM)from the Hudson River suggest that MeHg in the baycould be derived from riverine SPM rather than formedin situ.     

Diet differences among age classes of Arctic seals: evidence from stable isotope and mercury biomarkers

A basic understanding of current food web dynamics and baseline data from which to measure future change is necessary to understand species re-distribution and altered competition for food with climate change. We use mercury (Hg) and carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios as biomarkers to understand species diet differences and age class differences among ringed (Phoca hispida), bearded (Erignathus barbatus), and harbour (P. vitulina) seals in a subarctic marine ecosystem. Adult bearded seals had significantly lower δ¹⁵N and muscle Hg than bearded seal pups, whereas the opposite was observed in ringed seals where pups had lower δ¹⁵N than adults, suggesting age specific foraging differences in trophic food level for both species. For harbour seals, δ¹⁵N did not differ significantly among ages while Hg and δ¹³C did. The δ¹³C in muscle supports that bearded seals in this study are benthic feeders and are part of a separate food web from ringed seals and harbour seals. Harbour seals had the highest levels of mercury and δ¹⁵N, indicating they feed at a higher trophic level relative to the other two seal species. Carbon and nitrogen isotopic ratios and Hg levels illustrate how resources are partitioned among three seal species and offer evidence for separation based on life stages within species.     

Reduction of Mercury Chloride by Chlorella: Evidence for a Reducing Factor

Evidence is brought for the presence of low molecular weight, heat stable, mercury reducing factors in Chlorella cells. Some of the properties of these factors, which appears to be normal metabolities, are described. These factors are also present in the medium in which the cells grow. The reduced mercury, Hg⁰, is volatilized from the culture medium more rapidly than Hg⁺². The resultant decrease in the Hg concentration appears to be the main reason for the recovery of the cells from Hg stress. No resistance to Hg developed in cells emerging from the stress.     

Review of characteristics of mercury speciation and mobility from areas of mercury mining in semi-arid environments

The speciation of mercury—including most phase minerals, secondary phases, gaseous and aqueous species—is very important for evaluating the environmental impact and mobilization of this contaminant. Mining activities produce mercury mine waste, which includes several types of material (mainly mine waste and calcines) with varying mercury content and speciation depending on the ore deposit and processing technology. The main phase minerals are cinnabar, metacinnabar, metallic Hg⁰, corderoite, livingstonite, calomel and schuetteite. The aqueous mobilization of mercury is controlled by complex formation, pH-Eh conditions, the primary phase mineral of mercury, and organic-matter and iron oxyhydroxide content. The possibility of colloidal transport of mercury from mine waste is influenced by the atmospheric emission of metallic Hg⁰ and the leaching of waste by episodic high-intensity precipitations. In these climatic conditions, mercury can be mobilized to pore water, surface water or groundwater by the dissolution of metallic Hg⁰ and cinnabar in acidic conditions, and by the colloidal transport. The presence of Hg-soluble phases (chlorides and oxychlorides) may enhance the mobilization of mercury. In the semi-arid conditions of the Valle del Azogue (SE Spain) the atmospheric emissions of the Hg⁰ present in calcines and mine waste may be significant and the dissolution of Hg⁰ and metal-sulfate salts during episodic runoff events may explain the mobilization of Hg, Fe, Pb, Zn and other heavy metals.     

Using Earthworms to Assess Hg Distribution and Bioavailability in Gold Mining Soils

Between 1980 and 2000, the municipality of Cachoeira do Piriá, located in Pará State, Brazil, experienced an intense gold rush with approximately 5,000 artisanal miners discharging more than four tonnes of mercury into soils, air and aquatic systems. Mercury is dispersed across an area of approximately 2,100 ha and concentrations in soils and sediments frequently exceed 1,000 μg.kg^?1. The metallic mercury discharged by miners into the environment has the potential to be transformed into a highly toxic form of mercury, methylmercury. A 28-day bioassay with the earthworm Eisenia fetida was used to assess mercury bioavailability in mine tailings, soils, and sediments. Experiments indicated that the highest Hg concentration in earthworms was associated with low-Hg-organic-rich soils collected from densely vegetated areas despite higher mercury concentrations in organic-poor tailings. This indicates that reaction with organic acids is an important pathway for mercury incorporation into food chains. The quick, inexpensive, and simple bioassay also provided a means to evaluate remedial measures (i.e. by capping “hotspots” with local soils). Earthworm experiments indicate that covering “environmental hotspots” (sites with high Hg bioavailability) with local clay-rich sediments is very effective in terms of preventing uptake of mercury from tailings, while organic-rich sediments are relatively ineffective.