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 5084 - Chimie Nucléaire Analytique et Bioenviron., Le Haut Vigneau, Gradignan, France

Two typical lakes of different trophic types:

Moscow region map Eutrophic Lake“Beloe” (Kosino)

systematicallycharacterized

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 oC Temp. surface water = 11 oC Temp. of sediment = 8 oC

Sampling the lake sediment and bottom water

Samplinginstrumentation

designed by MolchanoffPermits 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, H2S odoriferous. Dystrophic lake sediment was 98% organic peat and humics, 2% sand.

Samples transported to the IPC RAS for characterization and

model experiments

CommentsMicroorganism

Cells’numberN/ml Ratio in batch: Vsediment : Vliquid = 1:3; Msolid : Mliquid = 1:30

Microalgae 106

Bacteria:Sulphate reducing

MethanogenicNitrifying

Denitrifying

104

104 – 105

103 - 104

105

Total = 2*109 bacteria/g, pH = 7.0 (stable); Eh = -0.20VNHE (inner sediment part); water phase: [HCO3

- ] = 1 –4 mg-C/l; [HSO4

-] = 0 – 50 mg/l, [Cl-] = 35 mg/l.summer - autumn: sediment rich in organic easy forassimilation by microorganismswinter-spring : sediment rich in fermentated organic

Model experiments of radionuclides uptake by eutrophic lake bottom sediment,

[Tc]0 , [Np]0 , [Pu]0 ~ 10-5M

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 - 30,000 NMWL Polysul-phone (RAININ Instr. Co., Inc) were used for microphase separation (colloids and microbes).

-2

-1

0

1

2

0 20 40 60

Time, days

Lg[([

Me]

0-[M

e])/[

Me]

]

TcNpPu

Hypolimnionic uptake of Tc, Np and Pu

More than 95% of Tc 99 were truly dissolved species of TcO4- 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 =

20oC; [Tc]0 = 10-5M)

0

20

40

60

80

100

0 10 20 30 40 50 60 70Time, days

[Tc]

in w

ater

pha

se, %

.

Natural water (NW)

NW + Sulfate 80 mg/l

NW + Nitrate 80 mg/l

Some microorganisms being able of anaerobic respiration had

competition relationships between SO42- or NO3

- and TcO4-. Addition

of SO42- or NO3

- 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.

Np accumulation by dystrophyc lake bottom sedimens

0

2000

4000

6000

8000

10000

0 20 40 60 80 100 120 140

Time, hour

Ac

tiv

ity

No1

No2

No3

No4

No5

No6

No7

No1& No2 - natural (silt+water), t=15oCNo3 & No4 - natural (silt+water), t=6oCNo 5 - natural (silt+water), t=15oC + 0.02 M H2CONo 6 - humine water from the pond; t=15oCNo 7 - humine water from the pond + 0.02 M H2CO; t=15oC

Temperature effect

0

25

50

75

100

0 10 20 30 40 50

Time, days

[Tc]

in w

ater

pha

se, %

22 C

18 C

15 C

6 C

Very small increase of uptake rate for contact time 10-20 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

NaClO4

(8%)HCl

(5%)H2O (5%)

H2O2

(100%)

0

30

60

90

Desorption agent

[Tc]

in w

ater

pha

se, %

of t

otal

acc

umul

ated

ac

tivity

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 H2O, 1M HCl, 1M NaClO4 or 15% H2O2 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 TcO4- and S2- 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 TcO4