Organic free radicals and micropores in solid graphitic carbonaceous matter at the Oklo natural fission reactors, Gabon

Abstract

The presence, concentration, and distribution of organic free radicals as well as their association with specific surface areas and microporosities help characterize the evolution and behavior of the Oklo carbonaceous matter. Such information is necessary in order to evaluate uranium mineralization, liquid bitumen solidification, and radionuclide containment at Oklo. In the Oklo ore deposits and natural fission reactors carbonaceous matter is often referred to as solid graphitic bitumen. The carbonaceous parts of the natural reactors may contain as much as 65.9% organic C by weight in heterogeneous distribution within the clay-rich matrix. The solid carbonaceous matter immobilized small uraninite crystals and some fission products enclosed in this uraninite and thereby facilitated radionuclide containment in the reactors. Hence, the Oklo natural fission reactors are currently the subjects of detailed studies because they may be useful analogues to support performance assessment of radionuclide containment at anthropogenic radioactive waste repository sites. Seven carbonaceous matter rich samples from the 1968 ± 50 Ma old natural fission reactors and the associated Oklo uranium ore deposit were studied by electron spin resonance (ESR) spectroscopy and by measurements of specific surface areas (BET method). Humic acid, fulvic acid, and fully crystalline graphite standards were also examined by ESR spectroscopy for comparison with the Oklo solid graphitic bitumens. With one exception, the ancient Oklo bitumens have higher organic free radical concentrations than the modern humic and fulvic acid samples. The presence of carbon free radicals in the graphite standard could not be determined due to the conductivity of this material. Oklo solid bitumen samples were subjected to various pressures of O 2, a paramagnetic gas. O 2-organic free radical interactions, as revealed by ESR spectral line broadening, indicate that the organic free radicals of the Oklo solid bitumens are, with one exception, accessible to O 2. This indicates that the organic free radicals are located at or very near the surfaces of these organic solids, including the internal surfaces of pores. The presence of pores was deduced from the BET analyses. The presence of porosity and related permeability has important implications for fluid flow through the uraniferous graphitic bitumen affecting radionuclide containment at Oklo. However, the very small average pore sizes (18.5–38.1Å) together with the non-wettability by aqueous solutions of the uraniferous graphitic bitumen indicate that the pores did not serve as effective conduits for the transport of radionuclides by aqueous solutions through and out of the solid bitumen matrix. The ESR data obtained from the O 2 experiments is comparable to the BET N 2 adsorption data, indicating that the ESR O 2 experiment reflects physical adsorption, and not chemisorption, of O 2 on the Oklo bitumen surfaces.