Bonding of ppb levels of methyl mercury to reduced sulfur groups in soil organic matter

Dissolved organic matter thiol concentrations determine methylmercury bioavailability across the terrestrial-marine aquatic continuum

The most critical step for methylmercury (MeHg) bioaccumulation in aquatic food webs is phytoplankton uptake of dissolved MeHg. Dissolved organic matter (DOM) has been known to influence MeHg uptake, but the mechanisms have remained unclear. Here we show that the concentration of DOM-associated thiol functional groups (DOM-RSH) varies substantially across contrasting aquatic systems and dictates MeHg speciation and bioavailability to phytoplankton. Across our 20 study sites, DOM-RSH concentrations decrease 40-fold from terrestrial to marine environments whereas dissolved organic carbon (DOC), the typical proxy for MeHg binding sites in DOM, only has a 5-fold decrease. MeHg accumulation into phytoplankton is shown to be directly linked to the concentration of specific MeHg binding sites (DOM-RSH), rather than DOC. Therefore, MeHg bioavailability increases systematically across the terrestrial-marine aquatic continuum as the DOM-RSH concentration decreases. Our results strongly suggest that measuring DOM-RSH concentrations will improve empirical models in phytoplankton uptake studies and will form a refined basis for modeling MeHg incorporation in aquatic food webs under various environmental conditions.

Mechanisms and biological effects of organic amendments on mercury speciation in soil-rice systems: A review

Mercury (Hg) pollution is a well-recognized global environmental and health issue and exhibits distinctive persistence, neurotoxicity, bioaccumulation, and biomagnification effects. As the largest global Hg reservoir, the Hg cumulatively stored in soils has reached as high as 250-1000 Gg. Even more concerning is that global soil-rice systems distributed in many countries have become central to the global Hg cycle because they are both a major food source for more than 3 billion people worldwide and the central bridge linking atmospheric and soil Hg circulation. In this review, we discuss the form distribution, transformation, and bioavailability of Hg in soil-rice systems by focusing on the Hg methylation and demethylation pathways and distribution, uptake, and accumulation in rice plants and the effects of Hg on the community structure and ecological functions of microorganisms in soil-rice systems. In addition, we clarify the mechanisms through which commonly used humus and biochar organic amendments influence Hg and its environmental effects in soil-rice systems. The review also elaborates on the advantages of sulfur-modified biochars and their critical role in controlling Hg migration and bioavailability in soils. Finally, we provide key information about Hg pollution in soil-rice systems, which is of great significance for developing appropriate strategies and mitigation planning to limit Hg bioconcentration in rice crops and achieving key global sustainable development goals, such as the guarantee of food security and the promotion of sustainable agriculture.

Binding of methylmercury to humic acids (HA): Influence of solar radiation and sulfide addition reaction of HA

Methylmercury (MeHg) is a neurotoxin that bioaccumulates in organisms and it forms strong complexes with reduced sulfur-containing ligands in dissolved organic matter (DOM). In the present study, the influences of solar radiation and sulfide addition reaction of humic acids (HA) on MeHg binding to HA were investigated using synchronous fluorescence and FT-IR two-dimensional correlation spectroscopic (2DCOS) analysis. Results showed that the complexation of fluorescent fractions of HA and sulfur-reacted HA (S-HA) with MeHg was not significantly affected by photoreaction treatments and the affinity of fluorescent fractions followed the order of protein-like fractions > humic-like fractions > fulvic-like fractions for both HA and S-HA. FT-IR 2DCOS analysis showed that the affinity of various binding sites in DOM for MeHg changed under different photoreaction treatments. Under dark treatment, small molecular compounds with low humification degree such as aromatic amino acids may be the site with the strongest binding ability to MeHg in HA, whereas aliphatic amino acids and sulfur-containing groups from sulfide addition reactions play a role in complexing of S-HA and MeHg. Under BS treatment (irradiation of DOM before MeHg binding), aliphatic compounds in HA preferentially bind to MeHg and aliphatic amino acids are the components with the strongest complexing ability; but for S-HA binding to MeHg, unsaturated functional groups and aromatic groups are more sensitive (alkenes > alkanes, phenols > alcohols). Under AS treatment (irradiation of DOM after MeHg binding), unsaturated bonds and aromatic compounds in HA preferentially bind to MeHg and aromatic amino acids show the strongest complexing ability; but for S-HA binding to MeHg, aliphatic groups show the strongest complexing ability (alkanes, alkenes > aromatics). These findings help us to better understand the complexation mechanisms between MeHg and DOM.

Mercury Accumulation in a Stream Ecosystem: Linking Labile Mercury in Sediment Porewaters to Bioaccumulative Mercury in Trophic Webs

Mercury (Hg) deposition and accumulation in the abiotic and biotic environments of a stream ecosystem were studied. This study aimed to link labile Hg in porewater to bioaccumulative Hg in biota. Sediment cores, porewaters, and biota were sampled from four sites along the Fourmile Branch (SC, USA) and measured for total Hg (THg) and methyl-Hg (MHg) concentrations. Water quality parameters were also measured at the sediment–water interface (SWI) to model the Hg speciation. In general, Hg concentrations in porewaters and bulk sediment were relatively high, and most of the sediment Hg was in the solid phase as non-labile species. Surface sediment presented higher Hg concentrations than the medium and bottom layers. Mercury methylation and MHg production in the sediment was primarily influenced by sulfate levels, since positive correlations were observed between sulfate and Hg in the porewaters. The majority of Hg species at the SWI were in non-labile form, and the dominant labile Hg species was complexed with dissolved organic carbon. MHg concentrations in the aquatic food web biomagnified with trophic levels (biofilm, invertebrates, and fish), increasing by 3.31 times per trophic level. Based on the derived data, a modified MHg magnification model was established to estimate the Hg bioaccumulation at any trophic level using Hg concentrations in the abiotic environment (i.e., porewater).

Effective removal of Hg(II) and MeHg from aqueous environment by ball milling aided thiol-modification of biochars: Effect of different pyrolysis temperatures

In order to synthesize biochar with the enhanced adsorption of inorganic mercury (Hg(II)) and organic mercury (Methylmercury, MeHg), biochars pyrolyzed at different pyrolysis temperatures (300, 500, and 700 °C) were ball milled with 3-trimethoxysilylpropanethiol (3-MPTS). Characterization results showed that 3-MPTS acted as an activator which further enlarged pores in biochars and ball-milling increased surface area of biochars. During ball milling, oxygen-containing functional groups increased which facilitated the loading of -SH group. The maximum adsorption capacities for Hg(II) adsorption in ball mill sulfhydryl modified biochars of 300, 500, and 700 °C were 401.8, 379.6 and 270.6 mg/g, respectively; simultaneously, the maximum adsorption capacity of MeHg was 108.16, 85.27 and 39.14 mg/g, respectively, which showed preferential increasing of 5.54 times on MeHg compared to the non-thiol modified biochar at low pyrolysis temperature of 300 °C. Results of kinetic adsorption experiments suggested that sorption data fitted well with Pseudo-second-order kinetic model, which proved that the mainly rate-limiting adsorption step was surface diffusion. Langmuir isotherm model fitting result showed that ligand exchange, surface complexation, surface adsorption and electrostatic attraction are dominant removal mechanisms. The 3-MPTS content and ball milling time are crucial during ball milling, and 2% 3-MPTS and 30 h of ball milling were found to be the most suitable conditions for both Hg(II) and MeHg adsorption. The result suggests that ball milling aided thiol-modification has great potential in synthesis of -SH modified biochar with prioritized MeHg adsorption.

Thiol-functionalized montmorillonite prepared by one-step mechanochemical grafting and its adsorption performance for mercury and methylmercury

The preparation of low-cost and highly efficient functional materials for the cleanup of mercury-contaminated water by adsorption in an environmentally friendly way is of great significance. In this study, thiol-functionalized montmorillonite (BSH-MMT) was prepared by a novel one-step mechanochemical grafting method and applied to aqueous Hg²⁺ and CH₃Hg⁺ adsorption. Characterization results showed that thiol groups were successfully grafted by chemical bonding with Si-OH or broken SiO bonds. The maximum adsorption capacities of BSH-MMT for Hg²⁺ and CH₃Hg⁺ were 104.79 mg g^-1 and 39.27 mg g^-1, which were approximately seven- and nine-fold that of pristine MMT, respectively. Adsorption kinetics and isotherm fitting indicated that Hg²⁺ adsorbs heterogeneously, while CH₃Hg⁺ proceeds through monolayer adsorption, both with chemical adsorption as the rate-limiting step. BSH-MMT maintained high adsorption performance over a wide pH range and in the presence of humic acid because of the high affinity of thiol groups toward mercury. The primary adsorption mechanism of thiol-ligand complexation was confirmed by the results of X-ray photoelectron spectroscopy and X-ray absorption spectra, in which a complex structure of bis-coordinated S-Hg-S (2.30 Å distance) was observed. These results demonstrated that mechanochemical grafting is a promising one-step method to prepare thiol-functionalized montmorillonite for effective cleanup of Hg²⁺/CH₃Hg⁺ contamination in water.

Effects of sulfur-rich biochar amendment on microbial methylation of mercury in rhizosphere paddy soil and methylmercury accumulation in rice

Biochar amendment has the potential to reduce methylmercury (MeHg) uptake by rice grains in soil-rice ecosystem. Considering that sulfur can strongly bind Hg and thus reduce its bioavailability, S-modified biochar has been used to immobilize Hg in soils. However, whether natural S-enriched biochar can further reduce Hg and MeHg phytoavailability remains unknown. Moreover, the rhizosphere is one of the most important microbial hotspots regulating the pollutant dynamics in terrestrial ecosystems. Therefore, it is of greater practical significance to examine the impact of biochar amendment on MeHg production and phytoavailability in the rhizosphere versus nonrhizosphere. Here, by conducting a pot experiment, we evaluated the efficacy of biochar derived from sulfur-enriched oilseed rape straw to reduce MeHg accumulation in rice. The results demonstrated that: (1) biochar-induced enhancement of chloride ion and sulfate levels in the overlying water and pore water facilitate microbial methylation of Hg and thus MeHg production in rhizosphere soil. (2) biochar amendment increased rhizosphere soil sulfur content and humic acid-like substances, strengthening MeHg binding to soil, and thus reducing grain MeHg levels by 47%-75%. Our results highlight the necessity to applying natural sulfur-rich biochar accompanied with exogenous sulfur to further reduce MeHg phytoavailability.

Emerging investigator series: mercury mobility and methylmercury formation in a contaminated agricultural flood plain: influence of flooding and manure addition

The fate and the methylation of mercury (Hg) in the terrestrial environment are still poorly understood and although the main drivers of release and methylation of mercury in soils are known (low redox potential and microbial carbon availability) their interactions are not well understood. This is of concern since many agriculturally used floodplains, where the recurring flooding and agricultural practices (e.g. manure amendments) may have an impact on the fate and the biomethylation of Hg, are at the same time Hg-contaminated. In this study, we modified and validated existing methods to extract and analyze methylmercury (MeHg) by HPLC-ICP-MS in soils and we assessed the Hg and MeHg concentrations in three fields situated in a Hg polluted agricultural floodplain. Further, we incubated the top soil from the three studied fields for 11 days under flooded conditions in presence or absence of 2 mass% of cow manure, a common agricultural amendment in the area. Total Hg and MeHg concentrations ranged from <limit of detection (LOD, 0.012 mg kg-1) to 28.2 mg kg-1 and from 1.2 to 7.8 μg kg-1 respectively. Hg was released to the soil solution after 12 hours with a maximum between day 2 and day 7. MeHg levels in the soil solution were <LOD although it was found in the soil before and after the incubation. The addition of cow manure to saturated soils led to an increase in the MeHg concentrations of the soil solid phase by up to fivefold to a maximum of 26.4 ± 1.1 μg kg-1 (n = 3). Our study demonstrates that the combination of low redox potential because of flooding with common agricultural practices such as the amendment of manures enhances the formation of toxic MeHg.

Effect of different rotation systems on mercury methylation in paddy fields

Rice (Oryza sativa) paddy is the hotspot of mercury (Hg) methylation. Given distinct influences of rotation systems on the physicochemical properties of paddy soils, we hypothesized different rotation systems in rice paddies inducing a large difference in Hg methylation. Here, we investigated Hg species distribution, dissolved organic matter (DOM) features, and Hg methylation in five rotation systems (Other farmland newly reclaim into paddy field, i.e., NR-R; Drain the water in winter, i.e., DW-R; Flooding in winter, i.e., FW-R; Rape-Rice rotation, i.e., Ra-R; Wheat-Rice rotation, i.e., Wh-R) of paddy fields to identify such hypothesis. Results shown that FW-R had the strongest Hg methylation, followed by Ra-R and Wh-R, then DW-R, and finally NR-R. We further found that much higher soil organic matter (SOM) and organo-chelated Hg (Hg-o) from straw residues and root exudates were the main cause for the greater Hg methylation in FW-R, Ra-R and Wh-R. This was because the protein-like fraction of SOM facilitated the net production of methyl Hg (MeHg), meanwhile the humin-like fraction had a strong affinity to MeHg in paddy soils. Therefore, it can be concluded that paddy soil under DW-R was the optimum pattern in order to reduce the occurrence of Hg methylation. However, paddy soils under Ra-R and Wh-R were the recommendable patterns if the productivity of paddy fields was considered.

Effective removal of inorganic mercury and methylmercury from aqueous solution using novel thiol-functionalized graphene oxide/Fe-Mn composite

A novel thiol-functionalized graphene oxide/Fe-Mn (SGO/Fe-Mn) was investigated for aqueous Hg²⁺ and CH₃Hg⁺ removal. Mercury were removed mainly through ligand exchange and surface complexation with surface active sites (i.e., -SH, OH, OCO, CC, SiO, and ππ bond). SH had the strongest binding ability with mercury, forming sulfur-containing organic matter or polymers with Hg²⁺, and sulfur-containing organometallic compounds or thiolate-like species with CH₃Hg⁺. The BET sorption isotherm model well simulated the sorption isotherm data of Hg²⁺ (R²=0.995, q_m=233.17 mg/g) and CH₃Hg⁺ (R²=0.997, q_m=36.69 mg/g), indicating a multilayer adsorption process. The mercury uptake was promoted with the increase of 3-MPTS content, adsorbent dosage, and pH (<5.5), whereas the uptake was inhibited by high pH (>5.5) and high concentrations of humic acid and electrolytes. SGO/Fe-Mn demonstrated high mercury uptake in simulated surface water/groundwater and in the presence of Pb, Cu, Ni, Sb, Cd and Zn. The mercury-laden SGO/Fe-Mn can be successfully regenerated and reused for three times with 98.1% and 67.0% of original Hg²⁺ and CH₃Hg⁺ sorption capacity when 5% thiourea + 2 M KI was used as the desorbing agent. This study demonstrates potential and viability of SGO/Fe-Mn for mercury remediation.

Adsorption of Methylmercury onto Geobacter bemidijensis Bem

The anaerobic bacterium Geobacter bemidijensis Bem has the unique ability to both produce and degrade methylmercury (MeHg). While the adsorption of MeHg onto bacterial surfaces can affect the release of MeHg into aquatic environments as well as the uptake of MeHg for demethylation, the binding of MeHg to the bacterial envelope remains poorly understood. In this study, we quantified the adsorption of MeHg onto G. bemidijensis and applied X-ray absorption spectroscopy (XAS) to elucidate the mechanism of MeHg binding. The results showed MeHg adsorption onto G. bemidijensis cell surfaces was rapid and occurred via complexation to sulfhydryl functional groups. Titration experiments yielded cell surface sulfhydryl concentrations of 3.8 ± 0.2 μmol/g (wet cells). A one-site adsorption model with MeHg binding onto sulfhydryl sites provided excellent fits to adsorption isotherms conducted at different cell densities. The log K binding constant of MeHg onto the sulfhydryl sites was determined to be 10.5 ± 0.4. These findings provide a quantitative framework to describe MeHg binding onto bacterial cell surfaces and elucidate the importance of bacterial cells as possible carriers of adsorbed MeHg in natural aquatic systems.

Probing the DOM-mediated photodegradation of methylmercury by using organic ligands with different molecular structures as the DOM model

Photodegradation is the main depletion pathway for methylmercury (MeHg) in surface water. The formation of MeHg-dissolved organic matter (DOM) complexes has been found to be a key step in MeHg photodegradation. However, the major functional groups involved in the DOM-mediated process have yet to be clearly resolved. In this work, we systematically investigated the effects of DOM molecular structures on MeHg photodegradation by using a variety of organic ligands with different functional groups (e.g., thiosalicylate, thiophenol, and thioaniline). The results showed that thiol and phenyl groups may be the major functional groups governing DOM-mediated MeHg photodegradation, with photodegradation rates also dependent on the type (carboxyl, hydroxyl, and amino group) and position (ortho-, meta-, and para-) of other chemical substituents. The addition of "non-photochemically active" thiol ligands (e.g., mercaptoethanol and dithiothreitol) and high concentrations of Cl^- can significantly inhibit the o-thiosalicylate-induced MeHg photodegradation, indicating that complexation of MeHg with these ligands is necessary for MeHg photodegradation. Sparging with O₂ had a negligible effect on MeHg photodegradation, while sparging with N₂ significantly enhanced MeHg photodegradation. This finding suggests that MeHg photodegradation may be a reductive process, which was further supported by identification of the degradation products of MeHg. A possible protonolysis mechanism of MeHg photodegradation in the presence of o-thiosalicylate was then proposed based on the findings of this study.

Study on the simultaneous reduction of methylmercury by SnCl2 when analyzing inorganic Hg in aqueous samples

Mercury (Hg) is among the most concerned contaminants in the world. It has three major chemical forms in the environment, including Hg⁰, Hg²⁺, and methylmercury (MeHg). Due to their differences in toxicity, mobility, and bioavailability, speciation analysis is critical for understanding Hg cycling and fate in the environment. SnCl₂ reduction-atomic fluorescence spectrometry detection is the most commonly used method for analyzing inorganic Hg. However, it should be noted that MeHg may also be reduced by SnCl₂, which would result in the overestimation of inorganic Hg. In this study, the reduction of MeHg by SnCl₂ in both de-ionized (DI) water and four natural waters was investigated. The results showed that MeHg could be reduced by SnCl₂ in DI water whereas this reaction was hard to occur in tested natural waters. By investigating the effects of water chemical characteristics (dissolved organic matter, pH and common anions and cations) on this reaction, SO₄^2- was identified to be the dominant factor prohibiting SnCl₂ induced MeHg reduction in natural waters. SO₄^2- in natural waters was evidenced to be reduced to S^2- by SnCl₂ and the generated S^2- can complex with MeHg to form MeHgS^- which is hard to be reduced by SnCl₂. Findings of this study indicate that the effect of MeHg reduction by SnCl₂ on inorganic Hg analysis is negligible in natural waters; however, at simulated experimental systems without SO₄^2-, SO₄^2- should be added as protecting agents to prevent MeHg reduction when analyzing inorganic Hg if it would not cause any other unwanted effects.

Methylmercury promotes breast cancer cell proliferation

CONTEXT

Metalloestrogens are small ionic metals that activate the estrogen receptor (ER). Studies have shown that when metalloestrogens bind to the ER, there is an increase in transcription and expression of estrogen-regulated genes, which induces proliferation of estrogen-dependent breast cancer. Methylmercury (MeHg), a metalloestrogen, is present in the environment and is toxic at moderate to high concentrations. However, at lower concentrations MeHg may promote the proliferation of ER-positive breast cancers and protect cells against pro-apoptotic signals.

OBJECTIVE

To investigate the effects of MeHg treatment on breast cancer cells in vitro.

MATERIALS AND METHODS

MCF7 breast cancer cells were treated with concentrations of MeHg ranging from 1 nM to 100 mM. Hg analysis was used to quantify intracellular mercury concentrations. Cell proliferation and apoptosis were determined by cell counting and Annexin-V staining, respectively.

RESULTS

We defined a protocol that maximizes cellular exposure to mercury. Treatment of human ER-positive breast cancer cells with 1 nM MeHg promoted proliferation, while treatment with a concentration of 100 nM induced apoptosis.

DISCUSSION AND CONCLUSIONS

Clarifying the effects of MeHg on breast cancer will improve our understanding of how environmental toxins affect tumor progression and may lead to the development of future therapeutic strategies.

Thermodynamic Modeling of the Solubility and Chemical Speciation of Mercury and Methylmercury Driven by Organic Thiols and Micromolar Sulfide Concentrations in Boreal Wetland Soils

Boreal wetlands have been identified as environments in which inorganic divalent mercury (Hg^II) is transformed to methylmercury (MeHg) by anaerobic microbes. In order to understand this transformation and the mobility and transport of Hg^II and MeHg, factors and conditions in control of the solubility and chemical speciation of Hg^II and MeHg need to be clarified. Here we explore the ability of thermodynamic models to simulate measured solubility of Hg^II and MeHg in different types of boreal wetland soils. With the input of measured concentrations of MeHg, sulfide, eight low molecular mass thiols and thiol groups associated with natural organic matter (NOM), as determined by sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy and Hg L_III-edge extended X-ray absorption fine structure spectroscopy (EXAFS), the model could accurately predict porewater concentrations of MeHg in the wetlands. A similar model for Hg^II successfully predicted the average level of its concentration in the porewaters, but the variability among samples, driven mainly by the concentration of aqueous inorganic sulfide, was predicted to be larger than measurements. The smaller than predicted variability in Hg^II solubility is discussed in light of possible formation of colloidal HgS(s) passing the 0.22 μm filters used to define the aqueous phase. The chemical speciation of the solid/adsorbed and aqueous phases were dominated by NOM associated thiol complexes for MeHg and by an equal contribution from NOM associated thiols and HgS(s) for Hg^II.

Critical role of natural organic matter in photodegradation of methylmercury in water: Molecular weight and interactive effects with other environmental factors

Photodegradation is the main depletion pathway of methylmercury (MeHg) in surface water. However, the underlying mechanism of MeHg photodegradation is still not well understood. In this study, the critical role of natural organic matter (NOM) from Suwannee River natural organic matter of the International Humic Substance Society, especially its molecular weight, and the impacts of other related environmental factors in MeHg photodegradation were investigated. We observed that MeHg cannot photo-degrade in de-ionized water, excluding the direct photodegradation of MeHg. While either NOM or Fe³⁺ alone induced MeHg photodegradation, co-existing NOM significantly inhibited the Fe³⁺-induced degradation, highlighting the critical and complex role of NOM in MeHg photodegradation. Additionally, MeHg exhibited different photodegradation rates in the presence of molecular weight fractionated natural organic matter (M_f-NOM). More importantly, high concentration of NOM caused light attenuation significantly inhibited the photodegradation of MeHg, which was more significant for high molecular weight M_f-NOM. In the presence of M_f-NOM, MeHg photodegradation was also affected by light quality, pH and co-existing Cl^- and NO₃^-. The study is helpful for a better understanding of the critical role of NOM and other environmental factors on MeHg photodegradation in surface water.

Aeshnid dragonfly larvae as bioindicators of methylmercury contamination in aquatic systems impacted by elevated sulfate loading

Methylmercury (MeHg) levels in dragonfly larvae and water were measured over two years in aquatic systems impacted to varying degrees by sulfate releases related to iron mining activity. This study examined the impact of elevated sulfate loads on MeHg concentrations and tested the use of MeHg in dragonfly larvae as an indicator of MeHg levels in a range of aquatic systems including 16 river/stream sites and two lakes. MeHg concentrations in aeshnid dragonfly larvae were positively correlated (R(2) = 0.46, p < 0.01) to peak MeHg concentrations in the dissolved phase for the combined years of 2012 and 2013. This relation was strong in 2012 (R(2) = 0.85, p < 0.01), but showed no correlation in 2013 (R(2) = 0.02, p > 0.05). MeHg in dragonfly larvae were not elevated at the highest sulfate sites, but rather the reverse was generally observed. Record rainfall events in 2012 and above average rainfall in 2013 likely delivered the majority of Hg and MeHg to these systems via interflow and activated groundwater flow through reduced sediments. As a result, the impacts of elevated sulfate releases due to mining activities were not apparent in these systems where little of the sulfate is reduced. Lower bioaccumulation factors for MeHg in aeshnid dragonfly larvae were observed with increasing dissolved organic carbon (DOC) concentrations. This finding is consistent with previous studies showing that MeHg in high DOC systems is less bioavailable; an equilibrium model shows that more MeHg being associated with DOC rather than algae at the base of the food chain readily explains the lower bioaccumulation factors.

Photoreduction of Hg(ii) and photodemethylation of methylmercury: the key role of thiol sites on dissolved organic matter

This study examined the kinetics of photoreduction of Hg(ii) and photodemethylation of methylmercury (MeHg(+)) attached to, or in the presence of, dissolved organic matter (DOM). Both Hg(ii) and MeHg(+) are principally bound to reduced sulfur groups associated with DOM in many freshwater systems. We propose that a direct photolysis mechanism is plausible for reduction of Hg(ii) bound to reduced sulfur groups on DOM while an indirect mechanism is supported for photodemethylation of MeHg(+) bound to DOM. UV spectra of Hg(ii) and MeHg(+) bound to thiol containing molecules demonstrate that the Hg(ii)-S bond is capable of absorbing UV-light in the solar spectrum to a much greater extent than MeHg(+)-S bonds. Experiments with chemically distinct DOM isolates suggest that concentration of DOM matters little in the photochemistry if there are enough reduced S sites present to strongly bind MeHg(+) and Hg(ii); DOM concentration does not play a prominent role in photodemethylation other than to screen light, which was demonstrated in a field experiment in the highly colored St. Louis River where photodemethylation was not observed at depths ≥ 10 cm. Experiments with thiol ligands yielded slower photodegradation rates for MeHg(+) than in experiments with DOM and thiols; rates in the presence of DOM alone were the fastest supporting an intra-DOM mechanism. Hg(ii) photoreduction rates, however, were similar in experiments with only DOM, thiols plus DOM, or only thiols suggesting a direct photolysis mechanism. Quenching experiments also support the existence of an intra-DOM photodemethylation mechanism for MeHg(+). Utilizing the difference in photodemethylation rates measured for MeHg(+) attached to DOM or thiol ligands, the binding constant for MeHg(+) attached to thiol groups on DOM was estimated to be 10(16.7).

Sources and remediation techniques for mercury contaminated soil

Mercury (Hg) in soils has increased by a factor of 3 to 10 in recent times mainly due to combustion of fossil fuels combined with long-range atmospheric transport processes. Other sources as chlor-alkali plants, gold mining and cement production can also be significant, at least locally. This paper summarizes the natural and anthropogenic sources that have contributed to the increase of Hg concentration in soil and reviews major remediation techniques and their applications to control soil Hg contamination. The focus is on soil washing, stabilisation/solidification, thermal treatment and biological techniques; but also the factors that influence Hg mobilisation in soil and therefore are crucial for evaluating and optimizing remediation techniques are discussed. Further research on bioremediation is encouraged and future study should focus on the implementation of different remediation techniques under field conditions.

Sulfide and mercury species profiles in two Ontario boreal shield lakes

The cycling of sulfur in freshwater environments plays an important role in the cycling of metals. In this study, acid volatile sulfides were measured at nanomolar levels using a purge-and-trap preconcentration, followed by methylene blue derivatization with HPLC separation and UV-Vis detection. The limit of detection using the preconcentration step was 7.5ngL(-1) or 0.23nM sulfide. Profiles of sulfide and methylmercury were generated for two Ontario lakes. Sulfide concentrations were inversely related to dissolved oxygen concentrations and significant levels of anoxia had developed in both lakes. In both Plastic Lake and Lake 658, mercury concentrations also increased below the oxycline. Lake 658 showed a strong positive correlation between sulfide and methylmercury (CMeHg=2×10(-6)⋅Csulfide+0.198; r=0.96, p=1.2×10(-5)), at the time of sampling.

Effect of inorganic and organic ligands on the bioavailability of methylmercury as determined by using a mer-lux bioreporter

A mer-lux bioreporter was constructed to assess the bioavailability of methylmercury [CH(3)Hg(II)] in Escherichia coli. The bioreporter was shown to be sensitive, with a detection limit of 2.5 nM CH(3)Hg(II), and was used to investigate the effects of chlorides, humic acids, and thiols on the bioavailability of CH(3)Hg(II) in E. coli. It was found that increasing the concentration of chlorides resulted in an increase in CH(3)Hg(II) bioavailability, suggesting that there was passive diffusion of the neutral complex (CH(3)HgCl(0)). Humic acids were found to reduce the bioavailability of CH(3)Hg(II) in varying degrees. Complexation with cysteine resulted in increased bioavailability of CH(3)Hg(II), while assays with equivalent concentrations of methionine and leucine had little or no effect on bioavailability. The mechanism of uptake of the mercurial-cysteine complexes is likely not passive diffusion but could result from the activities of a cysteine transport system. The bioavailability of CH(3)Hg(II) decreased with increasing glutathione concentrations.

Binding strength of methylmercury to aquatic NOM

A competitive-ligand, equilibrium-dialysis technique using bromide measured methylmercury (MeHg(+)) binding to Suwannee River fulvic acid (SRFA) and NOM from a lake and a bog in Minnesota. Distribution coefficients (K(OC)) and stability constants (K') varied only slightly over a range of [Br(-)] and ratios of MeHg(+) to reduced sulfur, S(re), the putative NOM binding site. For SRFA at pH 3.0, K(OC) ranged from 10(7.7) to 10(8.2) and K' ranged from 10(15.5) to 10(16.0) over MeHg(+):S(re) ratios from 1:1220 to 1:12 200 (well below S(re) saturation). The importance of pH depends on the calculation model for binding constants. Over pH 2.98-7.62, K(OC) had little pH dependence (slope = 0.2; r(2) = 0.4; range 10(7.7)-10(9.1)), but K' calculated using thiol ligands with pK(a) = 9.96 had an inverse relationship (slope = -0.8; r(2) = 0.9; range 10(15.6)-10(12.3)). A pH-independent model was obtained only with thiol pK(a) < or = approximately 4. The mean K'(4) for SRFA (K' with thiol pK(a) = 4.2) was 10(9.8) (range 10(9.11)-10(10.27)) and small slope (0.02). Similar values were found for Spring Lake NOM; bog S2 NOM had values one-tenth as large. These constants are generally similar to published values; differences reflect variations in methods, pH, types of NOM, and calculation models.

Photolytic degradation of methylmercury enhanced by binding to natural organic ligands

Monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs. Sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters. Rates of photodecomposition, however, can vary drastically between surface waters2-5 for reasons that are largely unknown. Here, we show that photodegradation occurs through singlet oxygen, a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter. The kinetics of degradation, however, depended on water constituents that bind methylmercury cations. Relatively fast degradation rates (similar to observations in freshwater lakes) applied only to methylmercury species bound to organic sulfur-containing thiol ligands such as glutathione, mercaptoacetate, and humics. In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive. Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition. Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

Substantial emission of gaseous monomethylmercury from contaminated water-sediment microcosms

Emission rates of gaseous monomethylmercury (CH(3)Hg(II)), as well as elemental mercury (Hg(0)) and dimethylmercury [(CH(3))(2)Hg(II)], were determined in Hg-contaminated water-sediment microcosms (duplicates of three treatments) by gaseous species-specific isotope dilution analysis (SSIDA). Incubation of approximately 500 g (wet mass) of sediments containing 30 mumol of ambient Hg with an addition of 2.6 mumol of (201)Hg(II) tracer resulted in average (n = 6) gaseous emissions of 84 +/- 26, 100 +/- 37, and 830 +/- 380 pmol of ambient CH(3)Hg(II), CH(3)(201)Hg(II), and (201)Hg(0), respectively, during 108 days of incubation. In contrast to Hg(0), a transient temporal pattern was observed for measured CH(3)Hg(II) emission rates, which peaked at day 12 and decreased to much lower levels by the end of the experiments. At day 12, CH(3)Hg(II) constituted 30-50% of the total emitted gaseous Hg, emphasizing the significance of this species to total Hg emissions from anoxic sediment-water systems. Emission rates of gaseous CH(3)Hg(II) did not reflect the accumulated CH(3)Hg(II) content in the sediment, suggesting that emissions mainly originated from newly methylated Hg(II). Speciation modeling of the pore water suggests that CH(3)Hg(II) was emitted as CH(3)HgSH(0)(g).

Size distribution of methylmercury associated with particulate and dissolved organic matter in freshwaters

Water samples were collected from 20 wetland, river and lake sites across Eastern Ontario and Western Quebec to investigate the distribution of methylmercury (MeHg) associated with various size fractions of dissolved organic matter (DOM). Tangential Flow UltraFiltration (TUF) was used to fractionate DOM by nominal molecular size (<0.2 microm, <300 kDa, <30 kDa, <5 kDa and <1 kDa). DOM fluorescence (DOM FL) and absorbance (DOC Abs) were used to quantify DOM photoreactivity and aromaticity in each sample. Significant differences in the size-associated distribution of MeHg, Dissolved Organic Carbon (DOC), DOM FL, and DOM Abs were observed between wetlands, rivers, and lakes. The low molecular weight (LMW) fraction (<5 kDa) in wetlands contained the majority of MeHg (70.0+/-13.8%), DOC (56.1+/-9.4%), and DOM FL (77.4+/-7.5%). DOM FL was also high in the LMW fraction for rivers (60.6+/-25%) and lakes (75.2+/-16.9%). Mean MeHg concentrations in the LMW fraction of lakes (41+/-26 pg L(-1)) and rivers (32+/-19 pg L(-1)) were substantial but much lower than wetlands. Rivers had the highest percentage of methylmercury (38.0+/-23.5%) in the particulate (>0.2 microm) fraction. This research highlights the importance of low molecular weight dissolved organic matter in methylmercury fate. For example, a large proportion of MeHg was found in the LMW weight fractions (mean=47.3+/-25.4%) of the wetlands, rivers, and lakes in this study.

Low-level mercury speciation in freshwaters by isotope dilution GC-ICP-MS

Atmospheric deposition of anthropogenic Hg has led to increased Hg concentrations in many ecosystems. Modeling is an effective method for predicting the complex dynamics of Hg transport in watersheds. Such models require accurate concentrations of water column methylmercury, CH3Hg+, for input parameters, yet these concentrations are very difficult to measure precisely because they are so low. We developed a method for aqueous CH3Hg+ quantification in Lake Champlain, VT, where ambient CH3Hg+ concentrations are < 0.04 ng L(-1). The analysis utilized species-specific isotope dilution, purge and trap gas chromatography inductively coupled plasma mass spectrometry and provided instrument detection limits of about 0.3 fM (0.06 pg L(-1)) and method detection limits of 15 fM (0.003 ng L(-1)) for CH3Hg+, which are among the lowest reported. Artifactual methylation of inorganic Hg(2+) was shown to be minor, and the precision of the isotope dilution method was generally <5% relative standard deviation, which is much lower than would have been the case for an external calibration approach. The method is accurate even at low concentrations of 0.025 ng L(-1). This combination of precision, accuracy, and low detection allows for quantification of significant differences in CH3Hg+ concentration between bays and over time within bays of Lake Champlain, where mean CH3Hg+ concentrations differ by only 0.006 ng L(-1) at concentrations as low as 0.014 ng L(-1).

Automated simultaneous analysis of monomethyl and mercuric Hg in biotic samples by Hg-thiourea complex liquid chromatography following acidic thiourea leaching

A simple leaching procedure has been validated for quantitative isolation of both monomethyl (CH3Hg+) and inorganic (Hg(II)) mercuryfrom fresh or dried biotic tissue for simultaneous analysis via separation and quantification with Hg-thiourea complex liquid chromatography cold vapor atomic fluorescence spectrometry (HgTU/LC-CVAFS). The leaching solution comprises thiourea, hydrochloric acid, and glacial acetic acid and works by protonating thiol binding sites and forming water-soluble cationic CH3HgS=C(NH2)2+ and Hg[S=C(NH2)2]2(2+) complexes, which are easily separated from the solid matrix. The isolated complexes are preconcentrated online by either thiol resin trapping or a new iodide-complex polydivinylbenzene resin trapping (I-PDVB). The I-PDVB trapping involves only one reagent addition, requires no pH adjustments, and is quantitative over a large range of volumes and flow rates. The chromatography system can use either ion chromatography or a new ion-pairing reversed phase separation coupled to cold vapor generation and atomic fluorescence detection. The system allows quantitative sample introduction and yields absolute detection limits of 0.4 pg and 0.7 pg, for CH3Hg+ and Hg(II) respectively, enabling relative detection limits as low as 4 and 7 pg g(-1) with 100 mg samples, and yields % CV routinely less than 5% with well homogenized samples. Accuracy for both forms of mercury has been validated with multiple biotic reference materials and by comparison of the sum of CH3Hg+ and Hg(II) with total Hg on a variety of different biotic sample types (n = 49). The system can be calibrated with either aqueous standards or leached reference materials.

Factors affecting methylmercury distribution in surficial, acidic, base-metal mine tailings

The most toxic form of Hg commonly of concern in the environment is methylmercury (MeHg), as it accumulates in living tissues and bioconcentrates in food webs. Sulfide-rich metal ores are often enriched in Hg, but little is known regarding the potential for Hg methylation in acidic tailings produced from these ores. This study examined acidic tailings from four mines in northern Ontario, Canada, to determine whether they could be an important source of MeHg to downstream environments. Where sulfate reducing bacteria (SRB) were abundant and active in pH-circumneutral, unoxidized layers (Potter mine), negligible MeHg was detected. By contrast, a zone of active sulfate reduction found in the acidic, oxidizing, surficial layers of tailings from the Kidd Metsite contained the highest concentrations of MeHg in bulk tailings (12.1 nmol kg(-1) dry wt. of sediment) and porewaters (88 pM) measured in this study. Cell count estimates of SRB by the "most-probable-number" (MPN) method were low in these surficial tailings, suggesting that sulfate reducers from this environment were acidophilic and did not thrive under the pH-neutral conditions of the MPN incubations. A later study of bacterial DNA from these tailings produced evidence of a novel Deltaproteobacterium which has only previously been detected in acid mine drainage environments. Further research will be necessary to determine whether this Deltaproteobacterium is a sulfate reducer and/or an efficient Hg methylator. Surface water concentrations of MeHg did not exceed Canadian water quality guidelines at any of the sites sampled, but one site (Broulan) featured total Hg (HgT) concentrations of 838 pM in filtered samples, far in excess of recommended levels. Trends in surface water MeHg and HgT reflected corresponding values in porewaters from the same sites, indicating that concentrations of these substances in tailings influence surface water concentrations.

Mercury speciation in sediments at a municipal sewage sludge marine disposal site

Mercury speciation was performed in excess activated sewage sludge (ASS) and in marine sediments collected at the AAS disposal site off the Mediterranean coast of Israel in order to characterize the spatial and vertical distribution of different mercury species and assess their environmental impact. Total Hg (HgT) concentrations ranged between 0.19 and 1003ng/g at the polluted stations and 5.7 and 72.8ng/g at the background station, while the average concentration in ASS was 1181+/-273ng/g. Only at the polluted stations did HgT concentrations decrease exponentially with sediment depth, reaching background values at 16-20cm, the vertical distribution resulting from mixing of natural sediment with ASS solids and bioturbation by large populations of polycheates. Average Methyl Hg (MeHg) concentration in ASS was 39.7+/-7.1ng/g, ca. 3% of the HgT concentration, while the background concentrations ranged between 0.1 and 0.61ng/g. MeHg concentrations in surficial polluted sediments were 0.7-5.9ng/g (ca. 0.5% of the HgT) and decreased vertically, similar to HgT. A positive correlation between MeHg and Hg only at the polluted stations, higher MeHg concentrations at the surface of the sediment and not below the redoxline, and no seasonality in the concentrations suggest that the MeHg originated from the ASS and not from in situ methylation. By doing selective extractions, we found that ca. 80% of the total Hg in ASS and polluted sediments was strongly bound to amorphous organo-sulfur and to inorganic sulfide species that are not bioavailable. The fractions with potential bioaccessible Hg had maximal concentrations in the range in which biotic effects should be expected. Therefore, although no bioaccumulation was found in the biota in the area, the concentration in the polluted sediments are not negligible and should be carefully monitored.

Recent advances in mercury speciation analysis with focus on spectrometric methods and enriched stable isotope applications

This paper discusses some recent advances in spectrometric methods and approaches for mercury speciation analysis of environmental samples with focus on isotope dilution techniques for determination of mercury species' concentrations in gaseous samples and reaction rates in soils and sediments. Such analytical data is important inter alia in fundamental research on mercury biogeochemistry and for risk assessments of mercury-contaminated soils and sediments and for designing effective remedial actions. The paper describes how the use of enriched stable isotope tracers in mercury speciation analysis can improve the traceability and accuracy of results, facilitate rational method developments, and be useful for studying biogeochemical processes, i.e. rate of reactions and fluxes, of mercury species. In particular the possibilities to study and correct for unwanted species transformation reactions during sample treatment and to study "natural" transformations of species in environmental samples, or micro- and mesocosm ecosystems, during incubations are highlighted. Important considerations to generate relevant data in isotope tracer experiments as well as reliability and quality assurance of mercury speciation analysis in general are also discussed.

Analysis of sorption and bioavailability of different species of mercury on model soil components using XAS techniques and sensor bacteria

The present work studies the adsorption behaviour of mercury species on different soil components (montmorillonite, kaolinite and humic acid) spiked with CH3HgCl and CH3HgOH at different pH values, by using XAS techniques and bacterial mercury sensors in order to evaluate the availability of methyl mercury on soil components. The study details and discusses different aspects of the adsorption process, including sample preparation (with analysis of adsorbed methyl mercury by ICP-OES), the various adsorption conditions, and the characterization of spiked samples by XAS techniques performed at two synchrotron facilities (ESRF in Grenoble, France and HASYLAB in Hamburg, Germany), as well as bioavailability studies using mercury-specific sensor bacteria. Results show that XAS is a valuable qualitative technique that can be used to identify the bonding character of the Hg in mercury environment. The amount of methyl in mercury adsorbed to montmorillonite was pH-dependent while for all soil components studied, the bond character was not affected by pH. On the other hand, clays exhibited more ionic bonding character than humic acids did with methyl mercury. This interaction has a higher covalent character and so it is more stable for CH3HgOH than for CH3HgCl, due to the higher reactivity of the hydroxyl group arising from the possible formation of hydrogen bonds. The bioavailability of methyl mercury adsorbed to montmorillonite, kaolinite and humic acids was measured using recombinant luminescent sensor bacterium Escherichia coli MC1061 (pmerBR(BS)luc). In case of contact exposure (suspension assays), the results showed that the bioavailability was higher than it was for exposure to particle-free extracts prepared from these suspensions. The highest bioavailability of methyl mercury was found in suspensions of montmorillonite (about 50% of the total amount), while the bioavailabilities of kaolinite and humic acids were five times lower (about 10%). The behaviour of methyl mercury in the presence of montmorillonite could be explained by the more ionic bonding character of this system, in contrast to the more covalent bonding character observed for humic acids. Thus, XAS techniques seem to provide promising tools for investigating the mechanisms behind the observed bioavailabilities of metals in various environmental matrices, an important topic in environmental toxicology.

Effects of dissolved organic carbon and salinity on bioavailability of mercury

Hypotheses that dissolved organic carbon (DOC) and electrochemical charge affect the rate of methylmercury [CH3Hg(I)] synthesis by modulating the availability of ionic mercury [Hg(II)] to bacteria were tested by using a mer-lux bioindicator (O. Selifonova, R. Burlage, and T. Barkay, Appl. Environ. Microbiol. 59:3083-3090, 1993). A decline in Hg(II)-dependent light production was observed in the presence of increasing concentrations of DOC, and this decline was more pronounced at pH 7 than at pH 5, suggesting that DOC is a factor controlling the bioavailability of Hg(II). A thermodynamic model (MINTEQA2) was used to select assay conditions that clearly distinguished among various Hg(II) species. By using this approach, it was shown that negatively charged forms of mercuric chloride (HgCl3-/HgCl(4)2-) induced less light production than the electrochemically neutral form (HgCl2), and no difference was observed between the two neutral forms, HgCl2 and Hg(OH)2. These results suggest that the negative charge of Hg(II) species reduces their availability to bacteria and may be one reason why accumulation of CH3Hg(I) is more often reported to occur in freshwater than in estuarine and marine biota.