Fixation et reduction de l'uranium par les matieres organiques naturelles: mecanismes et aspects cinetiques.

Abstract

The reactivity of lignite towards soluble uranyl species in an aqueous medium has been investigated experimentally as a function of T (20-400oC). The fixation process starts near 45oC, with reduction beginning at approx 120oC, and leads to the formation of chemically and thermally stable organo-uranyl species. The reduction of free uranyl species is accompanied by a stoichiometric (2:1) liberation of protons into the medium; these protons originate from the organic matter which thus undergoes dehydrogenation, explaining the low hydrogen content of natural organic materials associated with U deposits. Alcoholic and aliphatic hydrocarbon groups are responsible for the reduction; the kinetics have been studied at 180, 190 and 200oC and, when extrapolated to 20oC, show that reduction can be a crucial process in the geochemical behaviour of U especially in the thermal conditions in which sedimentary basins evolve. -R.A.H.