Efficiency of solvent extraction methods for the determination of methyl mercury in forest soils

Distribution and Homogenization of Multiple Mercury Species Inputs to Freshwater Wetland Mesocosms

Mercury (Hg)-impaired aquatic ecosystems often receive multiple inputs of different Hg species with varying potentials for transformation and bioaccumulation. Over time, these distinct input pools of Hg homogenize in their relative distributions and bioaccumulation potentials as a result of biogeochemical processes and other aging processes within the ecosystem. This study sought to evaluate the relative time scale for homogenization of multiple Hg inputs to wetlands, information that is relevant for ecosystem management strategies that consider Hg source apportionment. We performed experiments in simulated freshwater wetland mesocosms that were dosed with four isotopically labeled mercury forms: two dissolved forms (Hg2+ and Hg-humic acid) and two particulate forms (nano-HgS and Hg adsorbed to FeS). Over the course of one year, we monitored the four Hg isotope endmembers for their relative distribution between surface water, sediment, and fish in the mesocosms, partitioning between soluble and particulate forms, and conversion to methylated mercury (MeHg). We also evaluated the reactivity and mobility of Hg through sequential selective extractions of sediment and the uptake flux of aqueous Hg in a diffusive gradient in thin-film (DGT) passive samplers. We observed that the four isotope spikes were relatively similar in surface water concentration (ca. 3000 ng/L) immediately after spike addition. At 1-3 months after dosing, Hg concentrations were 1-50 ng/L and were greater for the initially dissolved isotope endmembers than the initially particulate endmembers. In contrast, the Hg isotope endmembers in surface sediments were similar in relative concentration within 2 months after spike addition. However, the uptake fluxes of Hg in DGT samplers, deployed in both the water column and surface sediment, were generally greater for initially dissolved Hg endmembers and lower for initially particulate endmembers. At one year postdosing, the DGT-uptake fluxes were converging toward similar values between the Hg isotope endmembers. However, the relative distribution of isotope endmembers was still significantly different in both the water column and sediment (p < 0.01 according to one-way ANOVA analysis). In contrast, selective sequential extractions resulted in a homogeneous distribution, with >90% of each endmember extracted in the KOH fraction, suggesting that Hg species were associated with sediment organic matter. For MeHg concentrations in surface sediment and fish, the relative contributions from each endmember were significantly different at all sampling time points. Altogether, these results provide insights into the time scales of distribution for different Hg species that enter a wetland ecosystem. While these inputs attain homogeneity in concentration in primary storage compartments (i.e., sediments) within weeks after addition, these input pools remain differentiated for more than one year in terms of reactivity for passive samplers, MeHg concentration, and bioaccumulation.

Utility of Diffusive Gradient in Thin-Film Passive Samplers for Predicting Mercury Methylation Potential and Bioaccumulation in Freshwater Wetlands

Mercury is a risk in aquatic ecosystems when the metal is converted to methylmercury (MeHg) and subsequently bioaccumulates in aquatic food webs. This risk can be difficult to manage because of the complexity of biogeochemical processes for mercury and the need for accessible techniques to navigate this complexity. Here, we explored the use of diffusive gradient in thin-film (DGT) passive samplers as a tool to simultaneously quantify the methylation potential of inorganic Hg (IHg) and the bioaccumulation potential of MeHg in freshwater wetlands. Outdoor freshwater wetland mesocosms were amended with four isotopically labeled and geochemically relevant IHg forms that represent a range of methylation potentials (²⁰²Hg²⁺, ²⁰¹Hg-humic acid, ¹⁹⁹Hg-sorbed to FeS, and ²⁰⁰HgS nanoparticles). Six weeks after the spikes, we deployed DGT samplers in the mesocosm water and sediments, evaluated DGT-uptake rates of total Hg, MeHg, and IHg (calculated by difference) for the Hg isotope spikes, and examined correlations with total Hg, MeHg, and IHg concentrations in sediment, water, and micro and macrofauna in the ecosystem. In the sediments, we observed greater relative MeHg concentrations from the initially dissolved IHg isotope spikes and lower MeHg levels from the initially particulate IHg spikes. These trends were consistent with uptake flux of IHg into DGTs deployed in surface sediments. Moreover, we observed correlations between total Hg-DGT uptake flux and MeHg levels in periphyton biofilms, submergent plant stems, snails, and mosquitofish in the ecosystem. These correlations were better for DGTs deployed in the water column compared to DGTs in the sediments, suggesting the importance of vertical distribution of bioavailable MeHg in relation to food sources for macrofauna. Overall, these results demonstrate that DGT passive samplers are a relatively simple and efficient tool for predicting IHg methylation and MeHg bioaccumulation potentials without the need to explicitly delineate IHg and MeHg speciation and partitioning in complex ecosystems.

Critical evaluation of distillation procedure for the determination of methylmercury in soil samples

In the present work, the efficiency of distillation process for extracting monomethylmercury (MMHg) from soil samples was studied and optimized using an experimental design methodology. The influence of soil composition on MMHg extraction was evaluated by testing of four soil samples with different geochemical characteristics. Optimization suggested that the acid concentration and the duration of the distillation process were most significant and the most favorable conditions, established as a compromise for the studied soils, were determined to be a 70 min distillation using an 0.2 M acid. Corresponding limits of detection (LOD) and quantification (LOQ) were 0.21 and 0.7 pg absolute, respectively. The optimized methodology was applied with satisfactory results to soil samples and was compared to a reference methodology based on isotopic dilution analysis followed by gas chromatography-inductively coupled plasma mass spectrometry (IDA-GC-ICP-MS). Using the optimized conditions, recoveries ranged from 82 to 98%, which is an increase of 9-34% relative to the previously used standard operating procedure. Finally, the validated methodology was applied to quantify MMHg in soils collected from different sites impacted by coal fired power plants in the north-central zone of Chile, measuring MMHg concentrations ranging from 0.091 to 2.8 ng g^-1. These data are to the best of our knowledge the first MMHg measurements reported for Chile.

The influence of sample drying and storage conditions on methylmercury determination in soils and sediments

The separate influences of drying and storage conditions on methylmercury (MeHg) concentrations in soil and sediment samples were investigated. Concentrations of MeHg and total Hg were determined in various soil and sediment samples that had been stored or dried under differing conditions. The influence of drying conditions (oven-drying (40 °C) versus freeze-drying) on MeHg concentrations in marine sediments, river sediments, soils, and paddy field soils was investigated (n = 43). The ratio of the MeHg concentration in oven-dried sub-samples divided by the concentration in freeze-dried sub-samples ranged from 0 to 336%. In order to confirm the production of MeHg during storage in some samples, Hg²⁺ was added at 15 mg kg^-1 to a paddy soil, and the sample was then stored at 30 °C. The concentrations of MeHg at 1-h, 1-day, 4-days and 7-days after Hg²⁺ spiking were 2.0 ± 0.1, 13.8 ± 1.0, 36.0 ± 5.0, and 24.9 ± 1.6 μg kg^-1 (n = 3), respectively. The concentration of MeHg at 4-days after Hg spiking and sterilizing (121 °C, 30 min) was 1.8 μg kg^-1, similar to the original value. These results indicate that bacterial Hg methylation and MeHg demethylation occurred within days in the soil. In addition, tests of the stability of MeHg in wet and dry samples during storage were also performed. Overall, our results indicate that the best way to preserve MeHg in soil and sediment samples is to freeze the samples immediately after collection, followed subsequently by freeze-drying, grinding, homogenization, and storage of the dry material in cool, dark conditions until analysis.

Determination of low methylmercury concentrations in peat soil samples by isotope dilution GC-ICP-MS using distillation and solvent extraction methods

Most often, only total mercury concentrations in soil samples are determined in environmental studies. However, the determination of extremely toxic methylmercury (MeHg) in addition to the total mercury is critical to understand the biogeochemistry of mercury in the environment. In this study, N2-assisted distillation and acidic KBr/CuSO4 solvent extraction methods were applied to isolate MeHg from wet peat soil samples collected from boreal forest catchments. Determination of MeHg was performed using a purge and trap GC-ICP-MS technique with a species-specific isotope dilution quantification. Distillation is known to be more prone to artificial MeHg formation compared to solvent extraction which may result in the erroneous MeHg results, especially with samples containing high amounts of inorganic mercury. However, methylation of inorganic mercury during the distillation step had no effect on the reliability of the final MeHg results when natural peat soil samples were distilled. MeHg concentrations determined in peat soil samples after distillation were compared to those determined after the solvent extraction method. MeHg concentrations in peat soil samples varied from 0.8 to 18 μg kg(-1) (dry weight) and the results obtained with the two different methods did not differ significantly (p=0.05). The distillation method with an isotope dilution GC-ICP-MS was shown to be a reliable method for the determination of low MeHg concentrations in unpolluted soil samples. Furthermore, the distillation method is solvent-free and less time-consuming and labor-intensive when compared to the solvent extraction method.

Simple and accessible analytical methods for the determination of mercury in soil and coal samples

Simple and accessible analytical methods compared to conventional methods such as US EPA Method 7471B and ASTM-D6414 for the determination of mercury (Hg) in soil and coal samples are proposed. The new methods are consisted of fewer steps without the Hg oxidizing step consequently eliminating a step necessary to reduce excess oxidant. In the proposed methods, a Hg extraction is an inexpensive and accessible step utilizing a disposable test tube and a heating block instead of an expensive autoclave vessel and a specially-designed microwave. Also, a common laboratory vacuum filtration was used for the extracts instead of centrifugation. As for the optimal conditions, first, best acids for extracting Hg from soil and coal samples was investigated using certified reference materials (CRMs). Among common laboratory acids (HCl, HNO3, H2SO4, and aqua regia), aqua regia was most effective for the soil CRM whereas HNO3 was for the coal CRM. Next, the optimal heating temperature and time for Hg extraction were evaluated. The most effective Hg extraction was obtained at 120°C for 30min for soil CRM and at 70°C for 90min for coal CRM. Further tests using selected CRMs showed that all the measured values were within the allowable certification range. Finally, actual soil and coal samples were analyzed using the new methods and the US EPA Method 7473. The relative standard deviation values of 1.71-6.55% for soil and 0.97-12.11% for coal samples were obtained proving that the proposed methods were not only simple and accessible but also accurate.

The influence of sulphate deposition on the seasonal variation of peat pore water methyl Hg in a boreal mire

In this paper we investigate the hypothesis that long-term sulphate (SO₄²⁻) deposition has made peatlands a larger source of methyl mercury (MeHg) to remote boreal lakes. This was done on experimental plots at a boreal, low sedge mire where the effect of long-term addition of SO₄²⁻ on peat pore water MeHg concentrations was observed weekly throughout the snow-free portion of 1999. The additions of SO₄²⁻ started in 1995. The seasonal mean of the pore water MeHg concentrations on the plots with 17 kg ha⁻¹ yr⁻¹ of sulphur (S) addition (1.3±0.08 ng L⁻¹, SE; n = 44) was significantly (p<0.0001) higher than the mean MeHg concentration on the plots with 3 kg ha⁻¹ yr⁻¹ of ambient S deposition (0.6±0.02 ng L⁻¹, SE; n = 44). The temporal variation in pore water MeHg concentrations during the snow free season was larger in the S-addition plots, with an amplitude of >2 ng L⁻¹ compared to +/−0.5 ng L⁻¹ in the ambient S deposition plots. The concentrations of pore water MeHg in the S-addition plots were positively correlated (r² = 0.21; p = 0.001) to the groundwater level, with the lowest concentrations of MeHg during the period with the lowest groundwater levels. The pore water MeHg concentrations were not correlated to total Hg, DOC concentration or pH. The results from this study indicate that the persistently higher pore water concentrations of MeHg in the S-addition plots are caused by the long-term additions of SO₄²⁻ to the mire surface. Since these waters are an important source of runoff, the results support the hypothesis that SO₄²⁻ deposition has increased the contribution of peatlands to MeHg in downstream aquatic systems. This would mean that the increased deposition of SO₄²⁻ in acid rain has contributed to the modern increase in the MeHg burdens of remote lakes hydrologically connected to peatlands.

Distribution of total and methylmercury in different ecosystem compartments in the Everglades: implications for mercury bioaccumulation

We analyzed Hg species distribution patterns among ecosystem compartments in the Everglades at the landscape level in order to explore the implications of Hg distribution for Hg bioaccumulation and to investigate major biogeochemical processes that are pertinent to the observed Hg distribution patterns. At an Everglade-wide scale, THg concentrations were significantly increased in the following order: periphyton<flocculent material (floc)<soil, while relatively high MeHg concentrations were observed in floc and periphyton. Differences in the methylation potential, THg concentration, and MeHg retention capacity could explain the relatively high MeHg concentrations in floc and periphyton. The MeHg/THg ratio was higher for water than for soil, floc, or periphyton probably due to high dissolved organic carbon (DOC) concentrations present in the Everglades. Mosquitofish THg positively correlated with periphyton MeHg and DOC-normalized water MeHg. The relative THg and MeHg distribution patterns among ecosystem compartments favor Hg bioaccumulation in the Everglades.

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.

Determination of methylmercury fluxes across the air–water and air–soil interfaces by gas chromatography with electron capture detection

A method for the determination of methylmercury (MeHg) fluxes across the air-water and air-soil interfaces was developed using an in situ chamber. The MeHg in the air coming out of the chamber was captured by a column containing sulfhydryl cotton fiber adsorbent. MeHg was then desorbed from the column by using 2 mol L(-1) HCl. The MeHg in the effluent was extracted with benzene, and determined by gas chromatography with electron capture detection. Finally, the MeHg flux was calculated using the chamber. The method was applied to simulated experiments, and the results showed that the MeHg fluxes in the air-water system were higher than those in the air-soil-water system. The method was also successfully applied to the field measurements of an environment polluted by a chemical factory, and the results showed that the MeHg fluxes across the air-soil and air-water interfaces were 0.21-3.09 and 0.14-0.79 ng m(-2) h(-1), respectively. The method will be a useful tool in the environmental study of MeHg.

Application of speciated isotope dilution mass spectrometry to evaluate extraction methods for determining mercury speciation in soils and sediments

Extraction techniques commonly used to extract methylmercury or mercury species from various matrixes have been evaluated regarding their potential to transform inorganic mercury to methylmercury, or vice versa, during sample preparation steps by applying speciated isotope dilution mass spectrometry. Two of the five tested methods were highly prone to form inorganic mercury from methylmercury. Some published methods converted methylmercury to inorganic mercury approximately 100% (including the spiked CH(3)(201)Hg(+)). In other methods, as much as 45% of methylmercury was converted to inorganic mercury during extraction. The methods evaluated included cold acid extraction and sonication. Other methods, such as the proposed EPA RCRA Draft Method 3200, microwave-assisted extraction, and another sonication-based methods induced very little or no methylmercury transformation to inorganic mercury. Among these three methods, the proposed Draft EPA Method 3200 was found to be the most efficient.

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

The strong binding of CH3Hg+ to natural organic matter (NOM) in soils and waters determines the speciation of CH3Hg under aerobic conditions and indirectly its bioavailability and rates of demethylation. In lab experiments, halides (Cl, Br, I) were used as competing ligands to determine the strength of CH3Hg+ binding to solid-phase soil organic carbon (SOC) and to dissolved soil organic carbon (DOC) as a function of time, pH, and concentration of halide. Experiments were conducted with native concentrations of CH3Hg (1.7-9.8 ng g(-1)) in organic soils, and equilibrium concentrations of CH3Hg were determined by species-specific-isotope-dilution (SSID) gas-chromatography-induced-coupled-plasma-mass-spectrometry (GC-ICP-MS). A simple model (RS- + CH3Hg+ = CH3HgSR; log KCH3HgSR) was used to simulate the binding to SOC and DOC, in which the binding sites (RSH) were independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. The pKa values of RSH groups were fixed at 8.50 and 9.95, reflecting the two major thiol groups in proteins. Log KCH3HgSR values determined for SOC and DOC were similar, showing a range of 15.6-17.1 for all experiments covering a pH range of 2.0-5.1. Despite large differences in affinities between Cl, Br, and I for CH3Hg+, determined constants were independent of type and concentration of halide used in the experiments (log KCH3HgSR = 16.1-16.7 at pH 3.5-3.6). Even if our log KCH3HgSR values were conditional in that they decreased with pH above 3.5, they were in fair agreement with stability constants determined for the association between CH3Hg+ and thiol groups in well-defined organic molecules (log K1 = 15.7-17.5). Speciation calculations based on our results show that, in absence of substantial concentrations of inorganic sulfides, neutral chloro-complexes (CH3HgCl) and free CH3Hg+ reach concentrations on the order of 10(-17)-10(-18) M at pH 5 in soil solutions with 3 x 10(-5) M of chloride.

Introduction of high carbon content solvents into inductively coupled plasma mass spectrometry by a direct injection high efficiency nebuliser

A preliminary investigation on the introduction of high carbon content solvents into inductively coupled plasma mass spectrometry (ICP-MS) by a direct injection high efficiency nebuliser (DIHEN) is presented. Ethanol, hexane, toluene and a natural gas condensate were introduced using a flow injection system. The performance for determinations of total concentrations of mercury present in organic solvents as different species was evaluated. The most critical operating parameters were the nebuliser gas flow rate and amount of oxygen added to the plasma. For the DIHEN a nebuliser gas flow rate of 0.3 L min(-1) and 50 mL min(-1) of oxygen added to the plasma auxiliary gas flow gave stable conditions and high analyte sensitivity. Species recoveries for HgCl(2), CH(3)HgCl, (CH(3))(2)Hg and Hg(0) in hexane were 99+/-4, 101+/-4, 95+/-4 and 104+/-7%, respectively. Detection limit for mercury in hexane was 85 pg mL(-1) based on 3 sigma of a (201)Hg-spiked blank. A gradual deposition was observed at the nebuliser tip, partly blocking the gas annulus. The rate of deposition seemed to be related to the amount of carbon introduced through the nebuliser. With the optimised conditions used in this work, the nebuliser could be used for approximately 50 hexane samples before cleaning was necessary.