Dominant Hg Species Research Articles

"Trojan Horse" Type Internalization Increases the Bioavailability of Mercury Sulfide Nanoparticles and Methylation after Intracellular Dissolution.

Mercury sulfide nanoparticles (HgSNP), as natural metal-containing nanoparticles, are the dominant Hg species in anoxic zones. Although the microbial Hg methylation of HgSNP has been previously reported, the importance of this process in Hg methylation has yet to be clarified due to the lack of knowledge on the internalization and transformation of HgSNP. Here, we investigated the internalization and transformation of HgSNP in microbial methylator Geobacter sulfurreducens PCA through total Hg analysis and different Hg species quantification in medium and cytoplasm. We found that the microbial uptake of HgSNP, via a passive diffusion pathway, was significantly higher than that of the Hg2+-dissolved organic matter (Hg2+-DOM) complex. Internalized HgSNP were dissolved to Hg2+ in cytoplasm with a maximal dissolution of 41%, suggesting a "Trojan horse" mechanism. The intracellular Hg2+ from HgSNP exposure at the initial stage (8 h) was higher than that in Hg2+-DOM group, which led to higher methylation of HgSNP. Furthermore, no differences in methylmercury (MeHg) production from HgSNP were observed between the hgcAB gene knockout (ΔhgcAB) and wild-type strains, suggesting that HgSNP methylation may occur through HgcAB-independent pathways. Considering the possibility of a broad range of hgcAB-lacking microbes serving as methylators for HgSNP and the ubiquity of HgSNP in anoxic environments, this study highlights the importance of HgSNP internalization and methylation in MeHg production and demonstrates the necessity of understanding the assimilation and transformation of nutrient and toxic metal nanoparticles in general.

Chemical Forms of Mercury in Pilot Whales Determined from Species-Averaged Mercury Isotope Signatures

Marine mammals detoxify organic methylmercury (MeHg) as inorganic mercury selenide (HgSe), yet the nature of the reaction intermediate species and the tissue-specific redistribution of Hg species in the body are unknown. We report that the identity and proportion of the dominant Hg species in long-finned pilot whale (Globicephala melas) tissues can be obtained from the bulk variation of isotopic values of δ 202 Hg against the extent of demethylation (percentage of total Hg as MeHg, %MeHg) using an alternating regularized inversion method. Our analysis of isotope data from two previous studies supports that MeHg is demethylated as a tetraselenolate species (Hg(Sec)4), which further transforms into HgSe. Hg(Sec)4 occurs in the liver, kidneys, muscle, heart, and brain, whereas HgSe biomineralization occurs only in the liver and kidneys. This study provides a mathematical approach that facilitates probing the molecular-level chemistry of mercury in biological tissues using bulk isotopic data.

Elemental mercury capture from industrial gas emissions using sulfides and selenides: a review

Mercury emission from industrial activities has become a major environmental health concern, because mercury is one of the most toxic metals encountered in the environment. Elemental mercury is the dominant Hg species in gas streams. Adsorption is regarded as a practical technique for Hg0 removal, for which developing efficient and economic adsorbents are needed. Chalcogenides have recently gained increasing research interest owing to their high binding affinity for Hg0. Chalcogen-based adsorbents include S-modified adsorbents, H2S- and SO2-activated adsorbents, polysulfide chalcogels, mineral sulfides and metal selenides. This article reviews chalcogen-based adsorbents for Hg0 capture from gas streams, with focus on removal performances, mechanisms, advantages and disadvantages. Results show that S modification, and H2S and SO2 activation generate a great variety of active sulfur species on the adsorbent surface, resulting in enhanced Hg0 removal activity. Sulfur species include elemental sulfur, sulfide, thiophene and sulfate. Nonetheless, removal efficiency decreases to some extent due to the presence of acidic flue gas components and steam. On the other hand, mineral sulfides and metal selenides exhibit outstanding Hg0 removal performances with fast adsorption rate, high mercury capacity and excellent resistance to SO2 and H2O. Yet most mineral sulfides and metal selenides perform well below 100 °C, which might limit industrial applications.

Characteristics, speciation, and bioavailability of mercury and methylmercury impacted by an abandoned coal gangue in southwestern China.

During coal mining activities, a lot of coal gangue is produced, which usually contains high mercury (Hg) concentrations as well as the acid mine drainage (AMD) generator of pyrite. In the present study, the total mercury (THg) and methylmercury (MeHg) in gangue, water, sediment, paddy soil, and rice samples, collected from abandoned coal mining areas, were analyzed. Results showed that the THg concentrations ranged from 0.37 to 35mg/kg (11 ± 8.4mg/kg) and 0.15 to 19mg/kg (2.0 ± 3.9mg/kg) in gangue and sediments, respectively. For paddy soils, the THg concentrations and MeHg varied from 0.16 to 0.91mg/kg and 0.71 to 11ng/g, respectively. Rice samples exhibited wide concentration ranges of THg (3.0-22ng/g) and MeHg (0.71-8.9ng/g). Sequential extraction of Hg revealed that the nitric acid-extractable state Hg (F4) was the dominant Hg species in gangue and sediment, while humic acids state Hg (F3) was the dominant form in paddy soil. Compared with gangue, higher percentages of F3 and the residual state Hg (F5) in both sediment and soil samples implied the transformation of F4 to F3 and F5 during transportation. Soil n-HAs (the difference between the total organic carbon and humic acids) were positively correlated with both THg and MeHg in soil and rice, indicating that n-HAs enhance Hg bioavailability under acidic conditions. Further studies should be conducted to reveal the factors influencing the transformation of different Hg fractions, providing ideas on decreasing the bioavailability of Hg in coal mining areas.

Experimental rainwater divalent mercury speciation and photoreduction rates in the presence of halides and organic carbon

Mercury (Hg) photochemical redox reactions control atmospheric Hg lifetime and therefore play an important role in global Hg cycling. Oxidation of Hg(0) to Hg(II) is currently thought to be a Br-initiated two-stage reaction with end-products HgBr2, HgBrOH, HgBrONO, HgBrOHO. Atmospheric photoreduction of these Hg(II) compounds can take place in both the gas and aqueous phase. Here we present new experimental observations on aqueous Hg(II) photoreduction rates in the presence of dissolved organic carbon and halides and compare the findings to rainfall Hg(II) photoreduction rates. The pseudo first-order, gross photoreduction rate constant, kred, for 0.5 μM Hg(II) in the presence of 0.5 mg/ L of dissolved organic carbon (DOC) is 0.23 h−1, which is similar to the mean kred (0.15 ± 0.01 h−1(σ, n = 3)) in high altitude rainfall and at the lower end of the median kred (0.41 h−1, n = 24) in continental and marine waters. Addition of bromide (Br−) to experimental Hg(II)-DOC solutions progressively inhibits Hg(II) photoreduction to reach 0.001 h−1 at total Br− of 10 mM. Halide substitution experiments give Hg(II)Xn(n-2) photoreduction rate constants of 0.016, 0.004 h−1, and < detection limit for X = Cl−, Br−, and I− respectively and reflect increasing stability of the Hg(II)-halide complex. We calculate equilibrium Hg(II) speciation in urban and high-altitude rainfall using Visual Minteq, which indicates Hg(II)-DOC to be the dominant Hg species. The ensemble of observations suggests that atmospheric gaseous HgBr2, HgCl2, HgBrNO2, HgBrHO2 forms, scavenged by aqueous aerosols and cloud droplets, are converted to Hg(II)-DOC forms in rainfall due to abundant organic carbon in aerosols and cloud water. Eventual photoreduction of Hg(II)-DOC in aqueous aerosols and clouds is, however, too slow to be relevant in global atmospheric Hg cycling.

Understanding Enhanced Microbial MeHg Production in Mining-Contaminated Paddy Soils under Sulfate Amendment: Changes in Hg Mobility or Microbial Methylators?

Elevated methylmercury (MeHg) production in mining-contaminated paddy soils, despite the high fraction of refractory HgS(s), has been frequently reported, while the underlying mechanisms are not fully understood. Here, we hypothesized that sulfate input, via fertilization, rainfall, and irrigation, is critical in mobilizing refractory HgS(s) and thus enhancing Hg methylation in mining-contaminated paddy soils. To test this hypothesis, the effects of sulfate amendment on Hg methylation and MeHg bioaccumulation in mining-contaminated soil-rice systems were examined. The results indicated 28-61% higher net MeHg production in soils under sulfate amendment (50-1000 mg kg-1), which in turn increased grain MeHg levels by 22-55%. The enhancement of Hg methylation by Hg mobilization in sulfate-amended soils was supported by two observations: (1) the increased Hg(aq) release from HgS(s), the dominant Hg species in the paddy soils, in the presence of sulfide produced following sulfate reduction and (2) the decreases of refractory HgS(s) in soils under sulfate amendment. By contrast, changes in the abundances/activities of potential microbial Hg methylators in different Hg-contaminated soils were not significant following sulfate amendment. Our results highlight the importance to consider enhanced Hg mobility and thus methylation in soils under sulfate amendment.

Influence of sulfur on the accumulation of mercury in rice plant (Oryza sativa L.) growing in mercury contaminated soils

Sulfur (S) is an essential element for plant growth and its biogeochemical cycling is strongly linked to the species of heavy metals in soil. In this work, the effects of S (sulfate and elemental sulfur) treatment on the accumulation, distribution and chemical forms of Hg in rice growing in Hg contaminated soil were investigated. It was found that S could promote the formation of iron plaque on the root surface and decrease total mercury (T-Hg) and methylmercury (MeHg) accumulation in rice grains, straw, and roots. Hg in the root was dominated in the form of RS-Hg-SR. Sulfate treatment increased the percentage of RS-Hg-SR to T-Hg in the rice root and changed the Hg species in soil. The dominant Hg species (70%) in soil was organic substance bound fractions. Sulfur treatment decreased Hg motility in the rhizosphere soils by promoting the conversion of RS-Hg-SR to HgS. This study is significant since it suggests that low dose sulfur treatment in Hg-containing water irrigated soil can decrease both T-Hg and MeHg accumulation in rice via inactivating Hg in the soil and promoting the formation of iron plaque in rice root, which may reduce health risk for people consuming those crops.

Grain-size dependence of mercury speciation in river suspended matter, sediments and soils in a mercury mining area at varying hydrological conditions

Suspended matter (SM) plays an important role in the transport of mercury (Hg) in aquatic systems. Information about Hg-species in this material is crucial to understand risk potential, especially for Hg methylation and bioavailability. In the Idrija Hg mine (Slovenia) cinnabar (α-HgS) was mined and processed for centuries. These activities caused contamination of the Idrijca river system by dumping of ore roasting residues, as well as atmospheric Hg deposition in soils attributed to excessive Hg emissions from the roasting plant. Cinnabar is the dominant Hg-species in the coarse-grained sediments of the Idrijca river where Hg methylation is generally low, whereas natural-organic-matter-bound Hg (NOM-bound) has caused intense Hg methylation in the Gulf of Trieste (GT), the final sink for Hg released from the Idrijca catchment. Hydrology of the Idrijca river is characterized by high discharge events during heavy rains and snowmelt, which transports large amounts of SM towards the GT. However, the dominant Hg-species transported in SM and their specific source under varying hydrological conditions is largely unknown, yet crucial to predict future transport of bioavailable Hg forms from the mining area to the GT.We analyzed Hg concentrations and Hg-species (Hg-thermo-desorption) in SM and different grain size fractions of soils from the Idrijca catchment to evaluate changes in solid phase Hg-species under low and high flow conditions. Concentrations of dissolved Hg did not change significantly during low and high flow (median: 21.3–28.1 ng L−1, respectively). Hg concentrations in SM in tributaries decreased with distance (∼30 km) from the mine from 32.7 to 0.47 mg kg−1 related to Hg concentrations in fine particulate soil fractions (0.45–20 μm) which ranged from 56.2 to 0.53 mg kg−1. Hg speciation in SM was dominated by NOM-bound Hg forms at low flow (median: 95.5%) which decrease during high flow conditions (median: 53.8%) attributed to increased mobilization of cinnabar from riverbanks and mine residues. Our results show that aqueous Hg transport in the Idrija mining area is dominated by heavy rain events and mobilization of NOM-bound Hg from soils indicating that the frequency of heavy rain events will control transport of bioavailable Hg to the marine environment.

Characterizations of wet mercury deposition on a remote high-elevation site in the southeastern Tibetan Plateau

Accurate measurements of wet mercury (Hg) deposition are critically important for the assessment of ecological responses to pollutant loading. The Hg in wet deposition was measured over a 3-year period in the southeastern Tibetan Plateau. The volume-weighted mean (VWM) total Hg (HgT) concentration was somewhat lower than those reported in other regions of the Tibetan Plateau, but the VWM methyl-Hg concentration and deposition flux were among the highest globally reported values. The VWM HgT concentration was higher in non-monsoon season than in monsoon season, and wet HgT deposition was dominated by the precipitation amount rather than the scavenging of atmospheric Hg by precipitation. The dominant Hg species in precipitation was mainly in the form of dissolved Hg, which indicates the pivotal role of reactive gaseous Hg within-cloud scavenging to wet Hg deposition. Moreover, an increasing trend in precipitation Hg concentrations was synchronous with the recent economic development in South Asia.

Mercury speciation in wet deposition samples collected from a coastal area of Minamata Bay

To better understand the methylation and demethylation of mercury (Hg) in the atmosphere, monomethyl mercury (MMHg) concentrations in wet deposition samples collected in the Minamata Bay area from September, 2009 to August, 2010 were determined. The concentrations of total Hg (dissolved Hg + particulate Hg) and dissolved reactive Hg were also measured. The volume-weighted mean concentrations of dissolved MMHg and total Hg were 0.061 and 5.9 ng L−1, respectively. Almost 90% of total Hg was in the dissolved phase and dissolved reactive Hg was the dominant Hg species in the wet deposition. The wet deposition fluxes of total Hg and reactive Hg increased in the rainy season (summer and fall), while the concentrations and wet deposition fluxes of MMHg were higher in winter and spring than in summer. Factors affecting this seasonal variation of MMHg were also considered. The MMHg in wet depositions may be decomposed by the increased levels of UV radiation in summer months, although MMHg can also be emitted from anthropogenic and biogenic sources and/or produced by photochemical reactions. Long-range transport from the Asian continent may also contribute to the seasonal variation of the wet MMHg deposition flux.

Mercury speciation and mercury isotope fractionation during ore roasting process and their implication to source identification of downstream sediment in the Wanshan mercury mining area, SW China

The environment of Guizhou province of SW China is in part significantly impacted by mercury (Hg) mining activities. The exploitation and processing of Hg-bearing ore in the past have led to multiple sources of Hg contamination, including unprocessed ores and Hg waste calcines and liquid elemental Hg, making source control strategies difficult and expensive to implement. In this study, initially extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the Hg species and to estimate the relative proportions of these species present in Hg-bearing wastes from the Wanshan Hg mine (WSMM) of the eastern Guizhou province. The results showed that cinnabar is the dominant Hg species in the unroasted ore samples, while the most prevalent Hg compounds in mine waste calcine is in the following order: meta-cinnabar, cinnabar and mercuric chloride. Our study demonstrated that mass dependent fractionation of Hg isotopes may occur during transformation of cinnabar to by-products (such as meta-cinnabar and mercuric chloride) by the roasting process. Hg stable isotope analysis of unroasted Hg ores and Hg waste calcines showed that Hg waste calcines (0.08±0.20‰, 2ó, n=11) were enriched by ~0.80‰ in δ202Hg values compared to the unroasted Hg ores (−0.74±0.11‰, 2ó, n=14). Finally, using a combined triple mixing model, the source attribution of the downstream sediment in WSMM was estimated. Our study suggested that the Hg isotope could be a useful tool to trace and quantify the source of Hg in the environment.

Mercury distribution, speciation and bioavailability in sediments from the Pearl River Estuary, Southern China

Surface sediments and sediment cores collected from the Pearl River Estuary (PRE) were analyzed for total mercury (THg) concentrations and speciation using a sequential extraction method. The mobility of Hg in sediments was also assessed using a series of single extraction methods. The surface sediments from the PRE showed slightly elevated levels of Hg, with concentrations ranging from 109 to 453ng/g. The vertical profile of THg in sediment cores indicated an accelerated input of Hg over the past decades. The organo-chelated and strong-complexed Hg species were the dominant Hg species in the sediments, while the more mobile phases of Hg made up less than 0.5% of THg. Less than 10% of the Hg in the sediments was extracted by single extraction, depending on the extractant employed. Significant relationships were found between the total organic carbon and THg, geochemical speciation, and extractability, indicating the important role of organic matter in controlling the distribution, mobility, and bioavailability of Hg in sediments.

Over three millennia of mercury pollution in the Peruvian Andes

We present unambiguous records of preindustrial atmospheric mercury (Hg) pollution, derived from lake-sediment cores collected near Huancavelica, Peru, the largest Hg deposit in the New World. Intensive Hg mining first began ca. 1400 BC, predating the emergence of complex Andean societies, and signifying that the region served as a locus for early Hg extraction. The earliest mining targeted cinnabar (HgS) for the production of vermillion. Pre-Colonial Hg burdens peak ca. 500 BC and ca. 1450 AD, corresponding to the heights of the Chavín and Inca states, respectively. During the Inca, Colonial, and industrial intervals, Hg pollution became regional, as evidenced by a third lake record approximately 225 km distant from Huancavelica. Measurements of sediment-Hg speciation reveal that cinnabar dust was initially the dominant Hg species deposited, and significant increases in deposition were limited to the local environment. After conquest by the Inca (ca. 1450 AD), smelting was adopted at the mine and Hg pollution became more widely circulated, with the deposition of matrix-bound phases of Hg predominating over cinnabar dust. Our results demonstrate the existence of a major Hg mining industry at Huancavelica spanning the past 3,500 years, and place recent Hg enrichment in the Andes in a broader historical context.

Mercury Methylation by Planktonic and Biofilm Cultures of Desulfovibrio desulfuricans

While biofilms are now known to be the predominant form of microbial growth in nature, very little is yet known about their role in environmental mercury (Hg) methylation. Findings of Hg methylation in periphyton communities have indicated the importance of investigating how environmental biofilms affect Hg methylation, as periphyton can be the base of the food webs in aquatic ecosystems. Chemical speciation influences the microbial uptake and methylation of inorganic Hg by planktonic cultures of sulfate-reducing bacteria; however, the effect of speciation on Hg methylation by biofilm cultures of these organisms has previously not been studied. In the present study, Hg methylation rates in biofilm and planktonic cultures of two isolates of Desulfovibrio desulfuricans from a coastal wetland were compared. Notably, the specific Hg methylation rate found was approximately an order of magnitude higher (0.0018 vs. 0.0002 attomol cell(-1) day(-1)) in biofilm cells than in planktonic cells, suggesting an important role for environmental biofilms in Hg methylation. To investigate the role of chemical speciation of Hg, experiments were conducted at two levels of sulfide. Both biofilm and planktonic cultures produced methylmercury at roughly twice the rate at low sulfide, when HgS(0)(aq), rather than HgHS2-, was the dominant Hg species. This indicates that the presence of a biofilm does not alter the relative availability of the dominant Hg species in sulfidic medium, in accordance with our previous studies of Hg uptake by Escherichia coli along a chloride gradient.

Sonolytic Desorption of Mercury from Aluminum Oxide: Effects of pH, Chloride, and Organic Matter

The effects of pH, Cl-, and humic acid (HA) on sonolytic desorption of Hg(II) from aluminum oxide were examined. Results showed that Hg(II) desorption was achieved by lowering the pH from 7.0 to 4.0. Ultrasound enhanced Hg(II) release at short times compared to both hydrodynamic mixing and that expected on the basis of the pH-dependent Hg(II) adsorption curve. However, prolonged sonication led to decreases in Hg(II) desorption due to occlusion by aluminum hydroxide precipitation induced by ultrasound. The presence of Cl- greatly improved Hg(II) desorption at pH 4.0 due to the formation of stable nonadsorbing HgCl2(0) complexes at low pH, reducing free Hg(II) ion in solution. However, Cl- did not affect Hg(ll) desorption at pH 8.0, where Hg(OH)2(0) is the dominant Hg species rather than HgCl2(0). Hg(ll) desorption from HA-laden Al2O3 was dominated by HA. The greater the desorption of HA, the greater the desorption of Hg(II). Ultrasound enhanced the initial Hg(II) release by facilitating HA desorption. However, decreases in Hg(II) desorption were observed over longer sonication times due to the sonochemically induced reassociation of desorbed HA back onto Al2O3. Information obtained in this study provides insight into understanding sonolytic release of Hg from Hg-contaminated particles and sediments.

Mercury and monomethylmercury in fluids from Sea Cliff submarine hydrothermal field, Gorda Ridge

Submarine hydrothermal systems are hypothesized to be a potentially important source of monomethylmercury (MMHg) to the ocean, yet the amount of MMHg in vent fluids is unknown. Here, we report total Hg and MMHg concentrations in hydrothermal vent fluids sampled from the Sea Cliff site on the Gorda Ridge. MMHg is the dominant Hg species, and levels of total Hg are enhanced slightly compared to seawater. Hg is enriched in deposits surrounding the site, suggesting near‐field deposition from fluid plumes, with rapid MMHg demethylation and scavenging of Hg(II) complexes. Assuming the flux of MMHg from Sea Cliff is representative of global submarine hydrothermal inputs, we estimate a flux of 0.1–0.4 Mmoles y−1, which may be attenuated by scavenging near the vents. However, deep waters are not typically known to be elevated in Hg, and thus we suggest that hydrothermal systems are not significant sources of MMHg to commercial fisheries.