Nicolas S. Bloom

Comparison of distillation with other current isolation methods for the determination of methyl mercury compounds in low level environmental samples: Part II. Water

In the present paper two isolation procedures for the separation of methyl mercury compounds (MeHg) from natural water samples, followed by aqueous phase ethylation, precollection on the Carbotrap, isothermal gas chromatography and cold vapour atomic fluorescence (CV-AFS) detection were compared and evaluated. The first isolation technique is based on extraction of MeHg into methylene chloride and back extraction into water by solvent evaporation. The second is based on the distillation of MeHg compounds. A comparison of these two isolation procedures was performed on 110 water samples of various origin (oxic, anoxic fresh water and sea water) and containing a wide MeHg concentration range (from 0.01 to 25 ng l−1. A relatively good agreement of the results was obtained in the concentration range below 1 ng l−1 as Hg (r2=0.919, n=61). In anoxic, sulphide rich and organic matter rich water samples that usually contain higher MeHg values, significantly higher values (approx. 30%) were obtained by distillation. This is partly due to sulphide interference during the ethylation step, as well as incomplete release of MeHg from bound sites. Distillation was found advantageous to solvent extraction. It gives consistent and high recoveries (80–95%) for various water samples, and achieves lower detection limits (0.006 ng l−1 for 50 ml water sample). It is also less laborious and without the use of organic solvent. Additionally, distillation provides the specific separation of MeHg and dimethylmercury, if present.

Selective extractions to assess the biogeochemically relevant fractionation of inorganic mercury in sediments and soils

Here we present a new method for sequential selective extractions (SSEs) for Hg in geological solids, validated with extensive quality assurance procedures. Mercury was separated into fractions which “make sense” biogeochemically, rather than being identified by specific compounds. Experiments elucidated the effects of extraction time, solids-to-liquid ratio, and alternate solvents in natural samples, reference materials, and pure compounds. Compounds tested included HgS (red and black), HgCl2 ,H g 0 ,H g 2Cl2, HgSe, HgO, Hg(II) adsorbed on goethite, Hg–humate, and gold amalgamated Hg. Based on these findings, a five-step sequence of extractions was established to separate the compounds into biogeochemically distinct categories. The fractions and leaching media were as follows: F1 (deionized water), F2 (0.01 M HCl + 0. 1M CH3COOH), F3 (1 M KOH), F4 (12 M HNO 3), and F5 (aqua regia). Method blanks and method detection limits (MDLs) of 0.1–5 ng/g were obtained for the various analytical fractions, depending on the reagent concentrations used. Precision ranged from 2 to 8% for the major fractions in a sample, but increased to 2–40% for fractions making up <5% of the total. Recovery of total Hg by the sum of species in reference materials showed that the accuracy of the method ranges from 90 to 105%. Methylation potential, determined by anoxic incubation sample aliquots with biologically active sediments, showed that inorganic Hg extracted in the F3 fraction is most strongly correlated with methylation potential. In most natural and sediment incubation samples, the majority of Hg present was found either in the F3 or F5 fractions.

Impact of acidification on the methylmercury cycle of remote seepage lakes

Concentrations of monomethylmercury [CH3Hg] were measured in the water and seston of five nearly pristine Wisconsin lakes, which span a range of pH from about 4.6 to 7.2. Previous studies had established a clear inverse relationship between [CH3Hg] in fish and the pH of lakes in this region. Here, we examined the pH dependency of [CH3Hg] in lake water and explored the partitioning of CH3Hg between water, seston, and fish as a function of pH. Results indicate that [CH3Hg] in lake water tends to increase as pH decreases, but that seasonal and spatial variability of [CH3Hg] in individual lakes confounds a simple analysis of the relationship. The partitioning of CH3Hg was related only weakly, if at all, to pH. Average partitioning coefficients (log kd=log (Cp/Cw)) were higher for yearling yellow perch (6.0 to 6.5) than for seston (5.5 to 6.0) but did not vary significantly between lakes. This suggests that acidification has a stronger effect on the supply of CH3Hg to the ecosystem than on specific rates of uptake by the biota.

Methylated mercury species in municipal waste landfill gas sampled in Florida, USA1

Mercury-bearing material has been placed in municipal landfills from a wide array of sources including fluorescent lights, batteries, electrical switches, thermometers, and general waste. Despite its known volatility, persistence, and toxicity in the environment, the fate of mercury in landfills has not been widely studied. The nature of landfills designed to reduce waste through generation of methane by anaerobic bacteria suggests the possibility that these systems might also serve as bioreactors for the production of methylated mercury compounds. The toxicity of such species mandates the need to determine if they are emitted in municipal landfill gas (LFG). In a previous study, we had measured levels of total gaseous mercury (TGM) in LFG in the μg/m3 range in two Florida landfills, and elevated levels of monomethyl mercury (MMM) were identified in LFG condensate, suggesting the possible existence of gaseous organic Hg compounds in LFG. In the current study, we measured TGM, Hg0, and methylated mercury compounds directly in LFG from another Florida landfill. Again, TGM was in the μg/m3 range, MMM was found in condensate, and this time we positively identified dimethyl mercury (DMM) in the LGF in the ng/m3 range. These results identify landfills as a possible anthropogenic source of DMM emissions to air, and may help explain the reports of MMM in continental rainfall.

Mercury speciation adsorption (MESA) method for combustion flue gas: Methodology, artifacts, intercomparison, and atmospheric implications

Chemical speciation of mercury (Hg) in a wide variety of combustion flue gas matrices has been determined using the mercury speciation adsorption (MESA) method. The MESA sampling system for gas phase Hg species employs a series of heated, solid phase adsorbent traps. Flue gas oxidized Hg species (Hg(II) and MMHg) are adsorbed by a potassium chloride (KC1) impregnated soda lime sorbent. Elemental Hg (Hg°) is collected by an iodated carbon sorbent after passing through the KCl/soda lime sorbent. Total Hg (Hgt) is determined by summation of species. In the laboratory, cold vapor atomic fluorescence spectroscopy (CVAFS) is used for detection of Hg collected on the solid sorbents, after appropriate sample digestion and preparation. The MESA method has been evaluated for species stability, matrix effects, breakthrough, artifacts and precision. Based on eight duplicate samples a mean precision of 6.8% 11% and 4.5% (relative percent difference) has been calculated for Hg°, Hg(II) and Hgt respectively. Intercomparison of the MESA method with other methods shows very good agreement for Hgt. Mass balance calculations at 5 sites range from 75 to 140%, with a mean of 97±25%. Overall mean speciation results from 19 separate determinations suggest that Hg(II) has a 1 sigma range of 40 to 94% in coal combustion flue gas at, the inlet to pollution control devices.

Mercury cycling in a northern wisconsin seepage lake: The role of particulate matter in vertical transport

During summer stratification, total mercury (Hgτ) reached maximum concentrations in the O2 :depleted, hypolimnion of Little Rock Lake, Wl. Initially, the hypolimnetic increase was attributed solely to redox-controlled release of Hg from bottom sediments. However, subsequent depth profiles of Hg indicated that hypolimnetic Hg enrichment could also result from the downward transport and recycling of particulate Hg prior to incorporation in the sediments. Contrasts between Fe and Hg cycles in this lake reinforce this notion. Increases in hypolimnetic Fe were observed during both summer and winter O2 decreases. In contrast, hypolimnetic Hg concentrations declined during winter. In the ice-free season, the distribution of particulate mercury (Hgp) correlated with the distribution of chlorophyllous particulates in this lake, re-emphasizing the importance of biotic processes in controlling Hg cycling in the hypolimnion.

Results of the international aqueous mercury speciation intercomparison exercise

Twenty-seven laboratories from around the world agreed to participate in an intercomparison exercise for total Hg (Hgt) and methyl Hg (MMHg) in pristine lake water. Unfiltered samples from a remote brown water lake in northern Wisconsin (USA) were collected into acid cleaned Teflon® bottles using ultra-clean sample handling techniques. The samples were acidified in the field with 0.4% by volume of pre-analyzed HCl (12N; <5 pg Hg/mL), and sent to the primary reference laboratory (PRL) by overnight mail. Within one week of receipt, the samples were randomized, and 10% analyzed for Hgt and MMHg at the PRL to verify the homogeneity of the set. Each participating laboratory was then sent 3 randomly selected 1 L bottles, while the PRL retained 30, and the secondary reference laboratory (SRL) retained 12 samples. The participating laboratories were asked to analyze each bottle in triplicate for both Hgt and MMHg, reporting all QA data including blanks, spike recoveries, and detection limits. The PRL analyzed samples in triplicate at both the beginning and the end of the analytical window, to provide a controlled estimate of any changes in concentration or speciation over that time. Of the 23 laboratories that returned results, 18 utilized BrCl oxidation, gold trapping, and cold vapor atomic fluorescence (CVAFS) detection for Hgt. Four laboratories reported similar techniques, varying either in detector (cold vapor atomic absorption), or wet chemistry. Only 16 laboratories reported MMHg results, with 15 using a variation of the aqueous phase ethylation, GC separation, and CVAFS detection technique. The results show good convergence between the participating labs for both Hgt and MMHg. For Hgt 18 of 23 labs reported means within 20% of the consensus value and PRL results (1.27±0.18 ng/L and 1.27±0.14 ng/L respectively). For MMHg, 13 of the 16 labs reported results within 20% of either the consensus value (0.420±0.055 ng/L) or the PRL value (0.446±0.041 ng/L).

Chemical correlates of Hg and methyl-Hg in northern Wisconsin lake waters under ice-cover

Total and dissolved concentrations of Hg and methyl-Hg (MeHg) were determined in the surface waters of 19 northern Wisconsin lakes under ice-cover when differences due to temperature, hydrology, productivity, and atmospheric exchange were minimal. Measured concentrations ranged from 0.3 to 5.3 ng/L for HgT and from 0.01 to 2.8 ng/L for MeHgT. Dissolved species comprised 30% to 95% of the HgT and MeHgT. MeHg was strongly correlated with Hg for both total and dissolved fractions. Thirteen ancillary constituents were measured in conjunction with the Hg determinations (pH, DOC, DIC, DO2, conductivity, suspended particulate matter (SPM), Ca, Mg, Mn, Fe, Na, SO4, Cl). Simple linear regressions indicated that DOC explained 87% of the variability in HgT and 79% of the variability in MeHgT. Of the other measured variables, pH, DO2, Fe and Mn showed weak but significant simple correlations with Hg and MeHg (@ p < 0.05). Multiple regression models containing two independent variables, (DOC and pH), explained 92% of the variability in HgT and 83% of the variability in MeHgT. Models containing DOC alone fit the dissolved Hg data well. We conclude that organic carbon concentrations have a strong effect on the concentrations of Hg and MeHg in these lakewaters.

Effect of pH on the bioaccumulation of low level, dissolved methylmercury by rainbow trout (Oncorhynchus mykiss)

An inverse relationship has been observed between pH and McHg concentration in freshwater fish. Many hypotheses exist regarding the mechanisms which lead to elevated levels of organic Hg in fish from low pH lakes. To determine if pH has a direct effect on the rate of McHg bioaccumulation in fish, rainbow trout fingerlings (Oncorhynchus mykiss) were exposed to a low concentration of aqueous methylmercuric chloride (1.38 ± 0.49 ng·L−1) at four pH-levels (8.2, 7.0, 6.3, 5.8) for eight weeks. McHg and total Hg were specifically determined on whole fish homogenates and water samples. The pH was found to have a significant: inverse effect on the rate of McHg bioaccumulation in the fish only in the lowest exposure level. Fish held at pH 5.8 had an uptake rate of 1.11 ± 0.07 ng·g−1·d−1, while those at pH ≥ 6.3 had a MeHg tissue uptake rate of ≤ 0.64 ± 0.07 ng·g−1·d−1. Total body burden of MeHg in the fish held at pH 5.8 also showed an elevated level of MeHg when compared with fish held at higher pH-levels, but the difference was less dramatic. These results suggest that a portion of the Hg burden in fish from low pH systems may be due to the direct effects of low pH on bioaccumulation, but that a threshold may exist above which pH does not play a significant role.

Development and complete validation of a method for the determination of dimethyl mercury in air and other media

The rigorous validation of a previously developed research method for the determination of dimethyl mercury ((CH3)2Hg) in environmental samples is presented. During atmospheric analysis, (CH3)2Hg was trapped on Carbotrap™ and analyzed by thermal desorption, isothermal GC separation, and cold vapor atomic fluorescence spectrometry (CVAFS). Water samples were analyzed after direct purging of 100mL aliquots onto Carbotrap™, while sediment and tissue samples were digested with 10mL of 25% KOH in methanol at 60°C and diluted to 40mL with methanol. An ambient air-spiking manifold, which allowed simultaneous replicate sampling, was constructed in a room controlled for temperature and humidity. (CH3)2Hg was introduced into the feed airflow (0.4m3min-1) from a well-calibrated diffusion cell, to obtain a concentration of approximately 5.5ngm-3 as Hg. Samples were collected onto Carbotrap™ columns, and the total volumes quantified by integrating mass flow meters. Trapping efficiency was investigated over a range of sampler flow rates (0.05-0.25Lmin-1), volumes (2-200L), collection temperatures (15-42°C) and relative humidity levels (10-70%). Method detection limits (MDLs), analytical precision and accuracy were quantified for all media. Carbotrap™ was found to be the best choice as a sampling media, whereas Tenax™ was found to be inadequate due to high breakthrough (>70%). This study verified that the method is sufficiently precise, accurate and robust for field sampling at mercury contaminated sites. No interferences were observed from elevated levels of potential co-contaminants, Hg0 (125ngm-3) and H2S (1.27ppmv).