Distribution of naturally occurring radionuclides (U, Th) in Timahdit's black shale (Morocco)

Attention has been recently focused on the use of Moroccan's black shale as the raw material for production of a new type of adsorbents. The purpose of the present work was to characterize a black shale specimen, collected in the region of Timahdit, in terms of the total uranium and thorium contents, measurements of some geochemically important elements (Al, Fe, Si, K, Mn, P, Ca), and XRD/SEM analysis. Selective leaching procedure, followed by radiochemical purification and alpha-counting, was also performed to assess the distribution of 238U, 234U, 235U, 232Th, 228Th, 230Th in the main structures. It was found that calcite, dolomite, quartz, clays constitute the main bulk composition of inorganic matrix. Organic matter counts for at least 15 wt. % of the sample. As in most other organic rich rocks, uranium is highly enriched in the black shale. It was interpreted to have been concentrated over a long period of time under anaerobic environment. This actinide is associated predominantly with humic acids, the precursor of kerogen. An integrated isotopic approach points out its mobilization from these humic acids to carbonates and apatite phases. The radionuclide that is the less mobile in this environment is 232Th, as was expected from its chemical properties, and in agreement with the most common view in the literature. It is partitioned between silicate minerals (49%), pyrite and kerogen (51%). Speciation, chemical behaviour of uranium and thorium and alpha decay related processes are widely responsible for disequilibria in the uranium decay series. (author)