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

Determination of Sub-Nanomolar Levels of Low Molecular Mass Thiols in Natural Waters by Liquid Chromatography Tandem Mass Spectrometry after Derivatization with <i>p</i>‑(Hydroxymercuri) Benzoate and Online Preconcentration

Low molecular mass (LMM) thiols is a diverse group of compounds, which play several important roles in aquatic ecosystems, even though they typically occur at low concentrations. Comprehensive studies of LMM thiols in natural waters have so far been hampered by selectivity and limit of detection constraints of previous analytical methods. Here, we describe a selective and robust method for the quantification of 16 LMM thiols in natural waters. Thiols were derivatized with 4-(hydroxymercuri)­benzoate (PHMB) and preconcentrated online by solid-phase extraction (SPE) before separation by liquid chromatography and determination by electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). Their quantification was performed by selective reaction monitoring (SRM), while the presence of a product ion at <i>m</i>/<i>z</i> 355, specific for thiols and common for the investigated compounds, also allows to screen samples for unknown thiols by a precursor ion scan approach. The robustness of the method was validated for aqueous matrices with different pH, sulfide, and dissolved organic carbon (DOC) concentrations. The limits of detection for the thiols were in the sub-nanomolar range (0.06–0.5 nM) and the methodology allowed determination of both reduced and total thiol concentrations (using tris­(2-carboxyethyl)­phosphine (TCEP) as reducing agent). Six thiols (mercaptoacetic acid, cysteine, homocysteine, <i>N</i>-acetyl-cysteine, mercaptoethane-sulfonate, and glutathione) were detected with total concentrations of 7–153 nM in boreal lake or wetland pore waters while four thiols (mercaptoacetic acid, cysteine, homocysteine, and <i>N</i>-acetyl-cysteine) were detected in their reduced form at concentrations of 5–80 nM

Eight Boreal Wetlands as Sources and Sinks for Methyl Mercury in Relation to Soil Acidity, C/N Ratio, and Small-Scale Flooding

Four years of catchment export and wetland input–output mass balances are reported for inorganic Hg (Hg_inorg), methyl mercury (MeHg), dissolved organic carbon (DOC), and sulfate in eight Swedish boreal wetlands. All wetlands had a history of artificial drainage and seven were subjected to small-scale flooding during the complete study period (two sites) or the two last years (five sites). We used an approach in which specific runoff data determined at hydrological stations situated at a distance from the studied sites were used in the calculation of water and element budgets. All wetlands except one were significant sinks for Hg_inorg. Seven wetlands were consistent sources of MeHg and one (an <i>Alnus glutinosa</i> swamp) was a significant sink. The pattern of MeHg yields was in good agreement with previously determined methylation and demethylation rates in the wetland soils of this study, with a maximum MeHg yield obtained in wetlands with an intermediate soil acidity (pH ∼5.0) and C/N ratio (∼20). We hypothesize that an increased nutrient status from poor to intermediate conditions promotes methylation over demethylation, whereas a further increase in nutrient status and trophy to meso- and eutrophic conditions promotes demethylation over methylation. Small-scale flooding showed no or moderate changes in MeHg yield, maintaining differences among wetlands related to nutrient status

Mechanisms of Methyl Mercury Net Degradation in Alder Swamps: The Role of Methanogens and Abiotic Processes

Wetlands are common net producers of the neurotoxin monomethylmercury (MeHg) and are largely responsible for MeHg bioaccumulation in aquatic food-webs. However, not all wetlands net produce MeHg; notable exceptions are black alder (Alnus glutinosa) swamps, which net degrade MeHg. Here we report the mechanisms of MeHg demethylation in one such swamp (EHT), shown to be a sink for MeHg during four consecutive years. The potential demethylation rate constant (<i>k</i>_d) in soil incubations was ∼3 times higher in the downstream (EHT-D: <i>k</i>_d ∼ 0.14 d^–1) as compared to the upstream part of the swamp (EHT-U: <i>k</i>_d ∼ 0.05 d^–1). This difference concurred with increased stream and soil pH, and a change in plant community composition. Electron acceptor and inhibitor addition experiments revealed that abiotic demethylation dominated at EHT-U while an additional and equally large contribution from biotic degradation was observed at EHT-D, explaining the increase in MeHg degradation. Biotic demethylation (EHT-D) was primarily due to methanogens, inferred by a decrease in <i>k</i>_d to autoclaved levels following selective inhibition of methanogens. Though methanogen-specific transcripts (<i>mcrA</i>) were found throughout the wetland, transcripts clustering with <i>Methanosaetaceae</i> were exclusive to EHT-D, suggesting a possible role for these acetoclastic methanogens in the degradation of MeHg

Net Degradation of Methyl Mercury in Alder Swamps

Wetlands are generally considered to be sources of methyl mercury (MeHg) in northern temperate landscapes. However, a recent input-output mass balance study during 2007–2010 revealed a black alder (<i>Alnus glutinosa</i>) swamp in southern Sweden to be a consistent and significant MeHg sink, with a 30–60% loss of MeHg. The soil pool of MeHg varied substantially between years, but it always decreased with distance from the stream inlet to the swamp. The soil MeHg pool was significantly lower in the downstream as compared to the upstream half of the swamp (0.66 and 1.34 ng MeHg g^–1 SOC^–1 annual average^–1, respectively, one-way ANOVA, <i>p</i> = 0.0006). In 2008 a significant decrease of %MeHg in soil was paralleled by a significant increase in potential demethylation rate constant (<i>k</i>_d, <i>p</i> < 0.02 and <i>p</i> < 0.004, respectively). In contrast, the potential methylation rate constant (<i>k</i>_m) was unrelated to distance (<i>p</i> = 0.3). Our results suggest that MeHg was net degraded in the <i>Alnus</i> swamp, and that it had a rapid and dynamic internal turnover of MeHg. Snapshot stream input-output measurements at eight additional <i>Alnus glutinosa</i> swamps in southern Sweden indicate that <i>Alnus</i> swamps in general are sinks for MeHg. Our findings have implications for forestry practices and landscape planning, and suggest that restored or preserved <i>Alnus</i> swamps may be used to mitigate MeHg produced in northern temperate landscapes

Corrections to Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbes

Corrections to Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbe

Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbes

Final harvest (clear-cutting) of coniferous boreal forests has been shown to increase streamwater concentrations and export of the neurotoxin methyl mercury (MeHg) to freshwater ecosystems. Here, the spatial distribution of inorganic Hg and MeHg in soil as a consequence of clear-cutting is reported. A comparison of soils at similar positions along hillslopes in four 80 years old Norway spruce (Picea abies) stands (REFs) with those in four similar stands subjected to clear-cutting (CCs) revealed significantly (<i>p</i> < 0.05) enhanced MeHg concentrations (ng g^–1), MeHg areal masses (g ha^–1), and percent MeHg of Hg_TOT in O horizons of CCs located between 1 and 41 m from streams. Inorganic Hg measures did not differ between REFs and CCs at any position. The O horizon thickness did not differ between CCs and REFs, but the groundwater table and soil water content were significantly higher at CCs than at REFs. The largest difference in percent MeHg of Hg_TOT (12 times higher at CCs compared to REFs, <i>p</i> = 0.003) was observed in concert with a significant enhancement in soil water content (<i>p</i> = 0.0003) at intermediate hillslope positions (20−38 m from stream), outside the stream riparian zone. Incubation experiments demonstrated that soils having significantly enhanced soil pools of MeHg after clear-cutting also showed significantly enhanced methylation potential as compared with similarly positioned soils in mature reference stands. The addition of inhibitors demonstrated that sulfate-reducing bacteria (SRB) and methanogens were key methylators. Rates of demethylation did not differ between CCs and REFs. Our results suggest that enhanced water saturation of organic soils providing readily available electron donors stimulate Hg-methylating microbes to net formation and buildup of MeHg in O horizons after forest harvest

Thermodynamics of Hg(II) Bonding to Thiol Groups in Suwannee River Natural Organic Matter Resolved by Competitive Ligand Exchange, Hg L_III-Edge EXAFS and ¹H NMR Spectroscopy

A molecular level understanding of the thermodynamics and kinetics of the chemical bonding between mercury, Hg­(II), and natural organic matter (NOM) associated thiol functional groups (NOM-RSH) is required if bioavailability and transformation processes of Hg in the environment are to be fully understood. This study provides the thermodynamic stability of the Hg­(NOM-RS)₂ structure using a robust method in which cysteine (Cys) served as a competing ligand to NOM (Suwannee River 2R101N sample) associated RSH groups. The concentration of the latter was quantified to be 7.5 ± 0.4 μmol g^–1 NOM by Hg L_III-edge EXAFS spectroscopy. The Hg­(Cys)₂ molecule concentration in chemical equilibrium with the Hg­(II)-NOM complexes was directly determined by HPLC-ICPMS and losses of free Cys due to secondary reactions with NOM was accounted for in experiments using ¹H NMR spectroscopy and ¹³C isotope labeled Cys. The log <i>K</i> ± SD for the formation of the Hg­(NOM-RS)₂ molecular structure, Hg²⁺ + 2NOM-RS^– = Hg­(NOM-RS)₂, and for the Hg­(Cys)­(NOM-RS) mixed complex, Hg²⁺ + Cys^– + NOM-RS^– = Hg­(Cys)­(NOM-RS), were determined to be 40.0 ± 0.2 and 38.5 ± 0.2, respectively, at pH 3.0. The magnitude of these constants was further confirmed by ¹H NMR spectroscopy and the Hg­(NOM-RS)₂ structure was verified by Hg L_III-edge EXAFS spectroscopy. An important finding is that the thermodynamic stabilities of the complexes Hg­(NOM-RS)₂, Hg­(Cys)­(NOM-RS) and Hg­(Cys)₂ are very similar in magnitude at pH values <7, when all thiol groups are protonated. Together with data on 15 low molecular mass (LMM) thiols, as determined by the same method (Liem-Ngyuen et al. Thermodynamic stability of mercury­(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory. Environ. Chem. 2017, 14, (4), 243−253.), the constants for Hg­(NOM-RS)₂ and Hg­(Cys)­(NOM-RS) represent an internally consistent thermodynamic data set that we recommend is used in studies where the chemical speciation of Hg­(II) is determined in the presence of NOM and LMM thiols

Kinetics of Hg(II) Exchange between Organic Ligands, Goethite, and Natural Organic Matter Studied with an Enriched Stable Isotope Approach

The mobility and bioavailability of toxic Hg­(II) in the environment strongly depends on its interactions with natural organic matter (NOM) and mineral surfaces. Using an enriched stable isotope approach, we investigated the exchange of Hg­(II) between dissolved species (inorganically complexed or cysteine-, EDTA-, or NOM-bound) and solid-bound Hg­(II) (carboxyl-/thiol-resin or goethite) over 30 days under constant conditions (pH, Hg and ligand concentrations). The Hg­(II)-exchange was initially fast, followed by a slower phase, and depended on the properties of the dissolved ligands and sorbents. The results were described by a kinetic model allowing the simultaneous determination of adsorption and desorption rate coefficients. The time scales required to reach equilibrium with the carboxyl-resin varied greatly from 1.2 days for Hg­(OH)₂ to 16 days for Hg­(II)–cysteine complexes and approximately 250 days for EDTA-bound Hg­(II). Other experiments could not be described by an equilibrium model, suggesting that a significant fraction of total-bound Hg was present in a non-exchangeable form (thiol-resin and NOM: 53–58%; goethite: 22–29%). Based on the slow and incomplete exchange of Hg­(II) described in this study, we suggest that kinetic effects must be considered to a greater extent in the assessment of the fate of Hg in the environment and the design of experimental studies, for example, for stability constant determination or metal isotope fractionation during sorption

Effects of Nutrient Loading and Mercury Chemical Speciation on the Formation and Degradation of Methylmercury in Estuarine Sediment

Net formation of methylmercury (MeHg) in sediments is known to be affected by the availability of inorganic divalent mercury (Hg^II) and by the activities of Hg^II methylating and MeHg demethylating bacteria. Enhanced autochthonous organic matter deposition to the benthic zone, following increased loading of nutrients to the pelagic zone, has been suggested to increase the activity of Hg^II methylating bacteria and thus the rate of net methylation. However, the impact of increased nutrient loading on the biogeochemistry of mercury (Hg) is challenging to predict as different geochemical pools of Hg may respond differently to enhanced bacterial activities. Here, we investigate the combined effects of nutrient (N and P) supply to the pelagic zone and the chemical speciation of Hg^II and of MeHg on MeHg formation and degradation in a brackish sediment-water mesocosm model ecosystem. By use of Hg isotope tracers added <i>in situ</i> to the mesocosms or <i>ex situ</i> in incubation experiments, we show that the MeHg formation rate increased with nutrient loading only for Hg^II tracers with a high availability for methylation. Tracers with low availability did not respond significantly to nutrient loading. Thus, both microbial activity (stimulated indirectly through plankton biomass production by nutrient loading) and Hg^II chemical speciation were found to control the MeHg formation rate in marine sediments