Influence of in situ biofilm coverage on the radionuclide adsorption capacity of subsurface granite.
Any migration of radionuclides from nuclear waste repositories is expected to be mitigated by adsorption to the host rocks surrounding hydraulically conductive fractures. Fluid rock interfaces are considered to be important barriers for nuclear waste disposal schemes but their adsorptive capacity can be affected by the growth of microbial biofilms. This study indicates that biofilms growing on fracture surfaces decrease the rocks adsorption capacity for migrating radionuclides except for trivalent species. Potential suppression of adsorption by biofilms should, therefore, be accounted for in performance safety assessment models. In this study, the adsorptive capacity of in situ anaerobic biofilms grown 450 m underground on either glass or granite slides was compared to the capacity of the same surfaces without biofilms. Surfaces were exposed to the radiotracers 60Co(II), 147Pm(III), 241Am(III), 234Th(IV), and 237Np(V) for a period of 660 h in a pH neutral anaerobic synthetic groundwater. Adsorption was investigated at multiple time points over the 660 h using liquid scintillation and ICP-MS. Results indicate that these surfaces adsorb between 0 and 85% of the added tracers under the conditions of the specific experiments. After 660 h, the distribution coefficients, R (ratio between what is sorbed and what is left in the aqueous phase), approached 3 x 10(4) m for 60Co, 3 x 10(5) m for 147Pm and 241Am, 1 x 10(6)m for 234Th, and 1 x 10(3) m for 237Np. The highest rate of adsorption was during the first 200 h of the adsorption experiments and started to approach equilibrium after 500 h. Adsorption to colloids and precipitates contributed to decreases of up to 20% in the available 60Co, 147Pm, 241Am, and 237Np in the adsorption systems. In the 234Th system 95% of the aqueous 234Th was removed by adsorbing to colloids. Although the range of Rvalues for each surface tested generally overlapped, the biofilms consistently demonstrated lower R values except for the trivalant 147Pm and 241Am adsorption systems.
Anderson
Craig
C
Deep Biosphere Laboratory, Department of Cell and Molecular Biology, University of Göteborg, PO Box 462, SE-405 30, Göteborg, Sweden. andersoc@ebs.ogi.edu
Jakobsson
Anna-Maria
AM
Pedersen
Karsten
K
D000327:Adsorption; D000693:Anaerobiosis; D018441:Biofilms; D011232:Chemical Precipitation; D003102:Colloids / Q000737:chemistry; D004787:Environmental Pollution / Q000517:prevention & control*; D006863:Hydrogen-Ion Concentration; D013058:Mass Spectrometry; D011868:Radioisotopes / Q000737:chemistry*; D012588:Scintillation Counting; D012822:Silicon Dioxide / Q000737:chemistry*; D013997:Time Factors; D014865:Waste Disposal, Fluid / Q000379:methods*
D003102:Colloids; D011868:Radioisotopes; C007886:granite; D012822:Silicon Dioxide