Title: Association of actinides with microorganisms and clay: Implications for radionuclide migration from waste-repository sites

Abstract

We conducted a series of basic studies on the microbial accumulation of actinides to elucidate their migration behavior around backfill materials used in the geological disposal of radioactive wastes. We explored the interactions of U(VI) and Pu(VI) with Bacillus subtilis, kaolinite clay, and within a mixture of the two, directly analyzing their association with the bacterium in the mixture by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The accumulation of U by the mixture rose as the numbers of B. subtilis cells increased. Treating the kaolinite with potassium acetate (CH{sub 3}COOK) removed approximately 80% of the associated uranium while only 65% was removed in the presence of B. subtilis. TEM-EDS analysis confirmed that most of the U taken from solution was associated with B. subtilis. XANES analyses revealed that the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both was U(VI). The amount of Pu sorbed by B. subtilis increased with time, but did not reach equilibrium in 48 h; in kaolinite alone, equilibrium was attained within 8 h. After 48 h, the oxidation state of Pu in the solutions exposed to B. subtilis and to the mixture had changed to Pu(V),more » whereas the oxidation state of the Pu associated with both was Pu(IV). In contrast, there was no change in the oxidation state of Pu in the solution nor on kaolinite after exposure to Pu(VI). SEM-EDS analysis indicated that most of the Pu in the mixture was associated with the bacteria. These results suggest that U(VI) and Pu(VI) preferentially are sorbed to bacterial cells in the presence of kaolinite clay, and that the mechanism of accumulation of U and Pu differs. U(VI) is sorbed directly to the bacterial cells, whereas Pu(VI) first is reduced to Pu(V) and then to Pu(IV), and the latter is associated with the cells. These results have important implications on the migrations of radionuclides around the repository sites of geological disposal. Microbial cells compete with clay colloids for radionuclides accumulation, and because of their higher affinity and larger size, the microbes accumulate radionuclides and migrate much slower than do the clay colloids. Additionally, biofilm coatings formed on the fractured rock surfaces also accumulate radionuclides, thereby retarding radionuclide migration.« less

Authors:

Ohnuki, T; Francis, A; Kozai, N; Sakamoto, F; Ozaki, T; Nankawa, T; Suzuki, Y

Publication Date:

Thu Apr 01 00:00:00 EDT 2010

Research Org.:

Brookhaven National Lab. (BNL), Upton, NY (United States)

Sponsoring Org.:

Doe - Office Of Science

OSTI Identifier:

988762

Report Number(s):

BNL-93736-2010-JA
Journal ID: ISSN 0149-0451; GEJODG; R&D Project: EE-639-EEDA; KP1504010; TRN: US1006953

DOE Contract Number:  

DE-AC02-98CH10886

Resource Type:

Journal Article

Journal Name:

Geomicrobiology Journal

Additional Journal Information:

Journal Volume: 27; Journal Issue: 3; Journal ID: ISSN 0149-0451

Country of Publication:

United States

Language:

English

Subject:

12 MANAGEMENT OF RADIOACTIVE WASTES, AND NON-RADIOACTIVE WASTES FROM NUCLEAR FACILITIES; 59 BASIC BIOLOGICAL SCIENCES; 99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; BIOLOGICAL ACCUMULATION; RADIONUCLIDE MIGRATION; RADIOACTIVE WASTE DISPOSAL; URANIUM; PLUTONIUM; BACILLUS SUBTILIS; KAOLINITE; VALENCE