Comparative tests on the efficiency of three methods of methylmercury extraction in environmental samples

Mercury is used in gold mining in the Brazilian Amazon and released in significant amounts to the environment as Hg-0 After its oxidation it may be methylated, mainly in bottom sediments and in the rhizosphere of floating aquatic macrophytes. Methylmercury (MeHg) is highly bioavailable and subject to biomagnification. The objective of this work was to evaluate the efficiency of three methylmercury extraction techniques, applied to replicates of environmental samples (stream sediments from Floresta da Tijuca, Rio de Janeiro; and roots of the floating macrophyte Eichhornia crassipes from Lagoinha, Rio de Janeiro) previously incubated with Hg-203(2+). Method A is based on acid leaching and extraction of (MeHg)-Hg-203 in toluene. Method B uses alkaline digestion, extraction in dithizone-benzene and separation of organic and inorganic Hg-203 dithizonates by thin-layer chromatography (TLC). Method C consists in separating the (MeHg)-Hg-203 from the sample matrix by distillation. Total Hg-203 and (MeHg)-Hg-203 were detected by gamma spectrometry or liquid scintillation. For both matrices, (MeHg)-Hg-203 extraction efficiency was better for smaller samples (range: 0.08-0.5 g for sediment, 0.1-0.5 g for E. crassipes roots). For the sediment samples, the three selected methods presented similar (MeHg)-Hg-203 extraction efficiencies: 7-13, 5-14 and 4-17% of total added Hg-203 was found as MeHg for procedures A, B and C, respectively. For E. crassipes roots, on the other hand, a lower extraction efficiency was obtained for the procedure C (11-28% of total Hg-203 present as (MeHg)-Hg-203) than for methods B (22-36%) and A (20-44%). In all the root samples prepared with procedure B, a strong and durable chemiluminescence effect was observed, which required measuring the final (MeHg)-Hg-203 extracts by gamma spectrometry only. In the specific conditions we used, extraction via distillation required reextraction of (MeHg)-Hg-203 in the distillate, due to the presence of traces of inorganic Hg-203 in the latter. Copyright (C) 1999 John Wiley & Sons, Ltd.