Artifact formation of methyl mercury during aqueous distillation and alternative techniques for the extraction of methyl mercury from environmental samples

Abstract

 Aqueous distillation as a pre-extraction step for monomethyl mercury (MMHg) in waters and sediments is the most common method because it eliminates negative interferences in the aqueous ethylation procedure. However, the distillation procedure generates a positive MMHg artifact as a result of the action of naturally occurring organic substances on inorganic Hg in the sample. Methylation of Hg(II) does not occur in deionized water samples, indicating that the artifact is not due to the distillation itself or the reagents added (ammonium pyrrolidine dithiocarbamate, HCl), but rather to naturally occurring organic matter. Methylation of Hg(II) spikes ranged from 0.001% for rainwater and oligotrophic lake water to 0.08% for brown, humic-rich water. Methylation of Hg(II) spikes in sediments ranged from 0.005 to 0.1%, with the highest conversions in wetland peat. In most water samples, the artifact is not significant, as the ambient concentration of MMHg is 1–10% of the total, well above the contribution due to the artifact. The artifact may be significant in sediments and in Hg-contaminated water samples, where the measured fraction of MMHg is less than 1% of the total. The best of six alternative techniques involves leaching with KBr/H₂SO₄/CuSO₄ and extraction into CH₂Cl₂, followed by back extraction into water, and subsequent ethylation.