Autoradiographic Comparisons of Radionuclide Adsorption Between Subsurface Anaerobic Biofilms and Granitic Host Rocks

Abstract

In high level nuclear waste repositories, the host rock is considered to be an important barrier to radionuclide migration by adsorbing metals at fluid rock interfaces. In granitic rock environments the surfaces of hydraulically conductive fractures are covered with mixed community biofilms. Biofilms were grown in situ on glass and rock surfaces in high pressure flow cells using groundwater sourced from a borehole 450 meters below sea level in the Äspö hard rock laboratory, Sweden. Scanning electron microscopy (SEM), epifluorescence microscopy and energy dispersive X-Ray spectroscopy (EDS) revealed monolayer biofilms consisting of up to 2 × 104 bacteria/mm2 surrounded by an extensive extracellular matrix and carbonate precipitates that covered <10% of the total surface area of each glass slide. The total organic carbon (TOC) associated with each slide was 11 μg ± 1.1 μg of which approximately 40% was attributed to bacteria and the rest to the extracellular matrix. The adsorption capacity of these biofilms was compared to the capacity of granite rock from the same environment. Surfaces were immersed for 42 days in an anaerobic synthetic groundwater containing either 60Co (II), 147Pm (III), 241Am (III), 234Th (IV), 237Np (V) or 99Mo (VI) as radioactive tracers. Adsorption and distribution of the tracers was investigated using 2D autoradiography. Compared to the biofilm, the rock adsorbed 88% more 60Co, 87% more 99Mo, 40% more 241Am and 237Np and 12% more 234Th per unit area. The biofilm adsorbed 57% more 147Pm than the rock. The β-emitting daughter products 233Pa and 234Pa also adsorbed to both rock and biofilms. Comparisons between rock surfaces and rock surfaces covered in biofilms indicate that the biofilms form a barrier between the rock and the groundwater and slow down radionuclide diffusion to the rock. These results suggest that the differences in adsorption are related to the surface functional groups available and the chemical properties of the radionuclides. Biological suppression of subsurface radionuclide adsorption should be accounted for in performance safety assessment models.