BIOACCUMULATION OF Tc-99, Pu-239 AND Np-237 BY BOTTOM SEDIMENTS OF FRESH WATER LAKES OF MOSCOW REGION German K.E., Firsova E.V., Peretrukhin V.F., Khijnyak T.V., Simonoff M. * - Institute of Physical Chemistry of Russian Academy of Sciences, Moscow, Russia * - UMR Chimie Nucléaire Analytique et Bioenviron., Le Haut Vigneau, Gradignan, France

Two typical lakes of different trophic types: Moscow region map Eutrophic Lake “Beloe” (Kosino) systematically characterized since 1896 Dystrophic Lake “Peaty” (Shatura) the typical of peatbogs

After the treatment of literature data on the lakes of Moscow reg. we have chosen the sampling sites at l. Beloe, Kosino (depth 9 m): l. Beloe, Kosino

Expedition to lake Beloe (Assembling the boat and the research team)

Expedition to lake Beloe (autumn ) Temperature of air (day) = 12 o C Temp. surface water = 11 o C Temp. of sediment = 8 o C

Sampling the lake sediment and bottom water Sampling instrumentation designed by Molchanoff Permits to control - Temperature & Depth - No air contact - Min. layer mixing Solid : Liquid = 1:3 by volume Solid : Liquid = 1:33 by dry weight Eutrophic lake sediment was mainly formless organic detritus, gray colored, H 2 S odoriferous. Dystrophic lake sediment was 98% organic peat and humics, 2% sand.

Samples transported to the IPC RAS for characterization and model experiments

Model experiments of radionuclides uptake by eutrophic lake bottom sediment, [Tc] 0, [Np] 0, [Pu] 0 ~ M Two equilibration modes were modeled - overhead agitation - hypolimnionic Tc transfer to sediment (diffusion+microbial) - Centrifuge MPW-210 (1500 g, 15 min), and microfilterfuge tubes 10, ,000 NMWL Polysul- phone (RAININ Instr. Co., Inc) were used for microphase separation (colloids and microbes). Hypolimnionic uptake of Tc, Np and Pu More than 95% of Tc 99 were truly dissolved species of TcO 4 - in water phase all time through the experiment. The kinetics’ analysis of data has shown that the half-time values of the initial content removal from the water-phase form 400h for Tc, 10h for Np and less than 1h for Pu. Two different uptake rates were characteristic for plutonium and neptunium sorption: fast uptake during the first hour when 60% and 50% of initial input was sorbed by sediment of eutrophic and dystrophic lakes, and a slow bioaccumulation period when neptunium uptake was completed in 1 and 2 months for eutrophic and dystrophic lakes respectively

Substrates (additions of sulfate and nitrate) effect on the kinetics of Tc-99 uptake by eutrophic lake sediment (t = 20 o C; [Tc] 0 = M) Some microorganisms being able of anaerobic respiration had competition relationships between SO 4 2- or NO 3 - and TcO 4 -. Addition of SO 4 2- or NO 3 - decreases the rate of Tc accumulation by sediment. Concentration factors were 1700 ml/g (eutrophic) and 56 ml/g (dystrophic) after 1550 hours of equilibration of the lake sediment with spiked water phase. The microflora of lake played important role in the accumulation and reduction of radionuclides.

No1& No2 - natural (silt+water), t=15 o C No3 & No4 - natural (silt+water), t=6 o C No 5 - natural (silt+water), t=15 o C M H 2 CO No 6 - humine water from the pond; t=15 o C No 7 - humine water from the pond M H 2 CO; t=15 o C

Temperature effect Very small increase of uptake rate for contact time days was found for cold weather. This observation correlates with the hypothesis that sulfate-reducing bacteria (dominating at this temperature if no artificial aeration was applied) are the most active microbial species toward Tc reduction among these present in the lake sediment.

Desorption tests  The sediments from freshwater lakes have a considerable sorption capacity.  4 consecutive sorption runs have demonstrated only small decrease of Tc uptake rate. The desorption of Tc was carried out with H 2 O, 1M HCl, 1M NaClO 4 or 15% H 2 O 2 and gave the desorption factors of 0.05, 0.05, 0.08 and more 0.99 thus indicating the reduction of technetium to be the main mechanism of its uptake.

Mechanism of radionuclide uptake by fresh water lake sediment At the actual TcO 4 - and S 2- concentrations the reduction of pertechnetate with hydrosulfide is inhibited. Tc(VII) reduction and further uptake by the lake sediment is due to microbial activity of the sediment components, most probably to sulfate-reducing bacteria Reduction of Np, Pu and Tc to Me(IV) oxidation state and sorption of hydrolyzed species are the main mechanisms for Np, Pu and Tc accumulation in this biosystem The difference in the uptake rate is associated with the higher chemical stability of TcO 4