The interactions between sulfate reducing anaerobic bacteria and plutonium,with or without bentonite present, were investigated using distributioncoefficients {Kd (ml/g)} as an index of the radionuclide behaviour.Plutonium Kds for living bacteria varied within a large range, from 1,804 to112,952, depending on the pH, while the Kds ranged from 1,180 to 5,931 fordead bacteria. In general, living bacteria had higher plutonium Kds thandead bacteria. Furthermore, the higher Kd values of 39,677 to 106,915 forliving bacteria were obtained for a pH range between 6.83 and 8.25, while novisible pH effect was observed for dead bacteria. These Kd values wereobtained using tracers for both 236Pu and 239Pu, whichcan check the experimental procedures and mass balance.
Another comparison was conducted for plutonium Kd values of mixtures ofliving bacteria with bentonite and sterilized bacteria with bentonite. Therange of Kd values for the non-sterilized bacteria with bentonite were 1,194to 83,648 while Kd values for the sterilized bacteria with bentonite werefrom 624 to 17,236. Again, the Kd values for the living bacteria withbentonite were higher than those of sterilized bacteria with bentonite. Inother words, the presence of living anaerobic bacteria with bentoniteincreased, by roughly 50 times, the Kd values of 239Pu whencompared to the mixture of dead bacteria with bentonite. The plutonium Kdvalues for bentonite alone, both non-sterilized and sterilized, were withina constant range of around 10,000 even though some of the data are not yetavailable.
The bentonite used for this experiment was a product of Japan and thesulfate reducing anaerobic bacteria was previously used for the treatment ofa pulp and paper wastewater. The results indicate that the effects ofanaerobic bacteria within the engineered barrier system (in this casebentonite) will play a significant role in the behaviour of plutonium ingeologic repositories.