Improvements of reliability for methylmercury determination in environmental samples

Dimethylmercury Degradation by Dissolved Sulfide and Mackinawite

Potential degradation pathways of dimethylmercury (DMHg) remain as one of the critical knowledge gaps in the marine biogeochemical cycle of mercury (Hg). Although Hg is known to be highly reactive with reduced sulfur, demethylation of DMHg in the presence of sulfide has until now remained experimentally untested. Here, we provide the first experimental support for demethylation of DMHg to monomethylmercury (MMHg) in the presence of both dissolved sulfide and mackinawite (FeS(s)_m). The degradation of DMHg was shown to be pH dependent, with higher demethylation rates at pH 9 than pH 5. At room temperature and environmentally relevant DMHg to sulfide molar ratios, we observed demethylation rates up to 0.05 d^-1. When comparing the number of active sites available, FeS(s)_m was found to have a higher capacity to demethylate DMHg, in comparison with dissolved sulfide. Our study suggests that dissolved sulfide and FeS(s)_m mediated demethylation of DMHg may act as a sink for DMHg, and a potential source of MMHg, in aquatic systems.

Determination of methylmercury in marine biota samples with advanced mercury analyzer: method validation

In this paper, we present a simple, fast and cost-effective method for determination of methyl mercury (MeHg) in marine samples. All important parameters influencing the sample preparation process were investigated and optimized. Full validation of the method was performed in accordance to the ISO-17025 (ISO/IEC, 2005) and Eurachem guidelines. Blanks, selectivity, working range (0.09-3.0ng), recovery (92-108%), intermediate precision (1.7-4.5%), traceability, limit of detection (0.009ng), limit of quantification (0.045ng) and expanded uncertainty (15%, k=2) were assessed. Estimation of the uncertainty contribution of each parameter and the demonstration of traceability of measurement results was provided as well. Furthermore, the selectivity of the method was studied by analyzing the same sample extracts by advanced mercury analyzer (AMA) and gas chromatography-atomic fluorescence spectrometry (GC-AFS). Additional validation of the proposed procedure was effectuated by participation in the IAEA-461 worldwide inter-laboratory comparison exercises.

Determination of methylmercury in marine biota samples: method validation

Regulatory authorities are expected to measure concentration of contaminants in foodstuffs, but the simple determination of total amount cannot be sufficient for fully judging its impact on the human health. In particular, the methylation of metals generally increases their toxicity; therefore validated analytical methods producing reliable results for the assessment of methylated species are highly needed. Nowadays, there is no legal limit for methylmercury (MeHg) in food matrices. Hence, no standardized method for the determination of MeHg exists within the international jurisdiction. Contemplating the possibility of a future legislative limit, a method for low level determination of MeHg in marine biota matrixes, based on aqueous-phase ethylation followed by purge and trap and gas chromatography (GC) coupled to pyrolysis-atomic fluorescence spectrometry (Py-AFS) detection, has been developed and validated. Five different extraction procedures, namely acid and alkaline leaching assisted by microwave and conventional oven heating, as well as enzymatic digestion, were evaluated in terms of their efficiency to extract MeHg from Scallop soft tissue IAEA-452 Certified Reference Material. Alkaline extraction with 25% (w/w) KOH in methanol, microwave-assisted extraction (MAE) with 5M HCl and enzymatic digestion with protease XIV yielded the highest extraction recoveries. Standard addition or the introduction of a dilution step were successfully applied to overcome the matrix effects observed when microwave-assisted extraction using 25% (w/w) KOH in methanol or 25% (w/v) aqueous TMAH were used. ISO 17025 and Eurachem guidelines were followed to perform the validation of the methodology. Accordingly, blanks, selectivity, calibration curve, linearity (0.9995), working range (1-800pg), recovery (97%), precision, traceability, limit of detection (0.45pg), limit of quantification (0.85pg) and expanded uncertainty (15.86%, k=2) were assessed with Fish protein Dorm-3 Certified Reference Material. The major contributions to the expanded uncertainty, i.e. 86.1%, arose from the uncertainty associated with recovery, followed by the contribution from fluorescence signal. Additional validation of the methodology developed was effectuated by the comparison with the values reported for MeHg in the IAEA-452 inter-laboratory comparison exercise.

Effect of sulfide, selenite and mercuric mercury on the growth and methylation capacity of the sulfate reducing bacterium Desulfovibrio desulfuricans

Cultures of the sulfate reducing bacteria Desulfovibrio desulfuricans were grown under anoxic conditions to study the effect of added sulfide, selenite and mercuric ions. A chemical trap consisting in a CuSO4 solution was used to control the poisoning effect induced by the bacterial production of hydrogen sulfide via the precipitation of CuS. Following the addition of Hg(2+), the formation of methylmercury (MeHg) was correlated to bacterial proliferation with most of MeHg found in the culture medium. A large fraction (50-80%) of added Hg(2+) to a culture ended up in a solid phase (Hg(0) and likely HgS) limiting its bioavailability to cells with elemental Hg representing ~40% of the solid. Following the addition of selenite, a small fraction was converted into Se(0) inside the cells and, even though the conversion to this selenium species increased with the increase of added selenite, it never reached more than 49% of the added amount. The formation of volatile dimethylselenide is suggested as another detoxification mechanism. In cultures containing both added selenite and mercuric ions, elemental forms of the two compounds were still produced and the increase of selenium in the residual fraction of the culture suggests the formation of mercuric selenite limiting the bioavailability of both elements to cells.

Inverse relationships between selenium and mercury in tissues of young walleye (Stizosedion vitreum) from Canadian boreal lakes

The concentrations of total mercury (Hg), methylmercury (MeHg) and total selenium (Se) were determined in muscle, liver and brain tissues of young-of-the-year walleye (Stizosedion vitreum) specimens collected from 8 boreal lakes that are located within 107km around the Sudbury smelters in Ontario, Canada. Dry weight basis concentrations of Hg were highest in muscle and lowest in brain (p<0.05), those of MeHg were higher in muscle than in liver and brain but there was no significant difference between liver and brain (p<0.05). The highest Se concentrations were found in liver and the lowest in brain (p<0.05). Considering the biomass of the studied tissues, muscle was the part of the body where most of Hg, MeHg and Se were accumulated. In fish muscle, the percentage of MeHg over Hg was the highest and this percentage was the lowest in liver. The concentrations of Hg, MeHg and Se in the studied tissues were closely related to the concentrations of total dissolved Se in lake waters which vary with the distance of the lakes from the smelters. Thresholds of Se concentrations in tissues were revealed (6.2, 12.0 and 3.5mgkg(-1) dry wt., for muscle, liver and brain, respectively), above which a significant reduction of MeHg concentrations was observed in all studied tissues compared to lower Se levels in the same tissues. Based on the collected information and data analysis, possible mechanisms for the biological processes behind the observed inverse relationships between Se and Hg in fish tissues are discussed.