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[OT-1.2 Quantification of Mercury Methylation and Bioconcentration and Analysis of Influencing Environmental Factors]

[Fellow]
Jean Remy Davee Guimaraes
Associate Professor at the Federal University of Rio de Janeiro,
Brazil (EFF Felloeship 095-0TA-11).

[Contact Person]

Jean Remy Davee Guimaraes
Hirokatsu Akagi
National Institute for Minamata Disease
Epidemiology Department
Hama 4058-18, Minamata, Kunramoto, 867 Japan
Tel: +81-966-63-3111, Fax +81-966-61-1145

[Abstract]

　Methylmercury(MeHg) is the most neurotoxic mercury compound and it is produced from inorganic mercury, through biological and chemical reactions bottom sediments being the main site of this transformation. The objective of this work is to develop model systems to study the speciation of mercury in the Amazon aquatic systems using a radiochemical approach. For this, sediments from the Minamata river, Japan, were collected 8km upstream Minamata city, spiked with 0.1, 0.5 and 10ppm of divalent ²⁰³Hg and incubated in glass cores with an overlying water column. The water column was periodically sampled and analysed for total ²⁰³Hg, inorganic and Me²⁰³Hg. At the end of the 3 week incubation period, sediment were submitted to the same analysis. Total ²⁰³Hg was measured directly gamma spectrometry. Inorganic ²⁰³Hg and Me²⁰³Hg in water, sediment and fish were separated by thin layer chromatography and quantified by gamma spectrometry or liquid scintillation counting depending on the activity level. For this 1g sediment and fish samples were digested in 8ml of KOH in ethanol, slightly acidified and extracted in 0.1% dithizone in benzene. After centrifugation the organic layer was passed through a Florisil column, the eluate was washed in diluted ammonia and evaporated to dryness. The residue was then dissolved acetone and applied to silica-gel coated chromatography paper. The plates were developed in hexane-benzene and the stripes corresponding to mercuric dethizonate and methylmercury dithizonate were cut and counted for ²⁰³Hg. For water samples KOH digestion and Florisil filtration were not necessary.
　MeHg production in sediment was proportional to time and Hg²⁺ addition but MeHg %(0.5-5.5) was inversely proportional to sediment Hg²⁺concentration. The same applied to water, where the MeHg percentage was 10 for 10ppm Hg²⁺ addition and 55 for 0.1ppm. The sediment: water partition coefficient for MeHg was in the 60-330 range. It was found, however, that MeHg % in water was overestimated because of Hg²⁺ adsorption by the rubber stoppers and the PVC baskets containing the sediments.
　The results show that the radiochemical technique applied here allow a fast and reliable evaluation of MeHg production and distribution among water, sediment and biota. Further experiments are in progress to optimize the MeHg extraction and measurement procedures to allow the use of lower Hg addition levels., The technique will be used in the near future in the fellow's home Institute in studies on Hg methylation in sediments of areas of the Amazon watershed affected by goldmining activities.

[Key Words]

mercury, sediment, methylation methylmercury