Experimental study of mechanisms of fixation and reduction of uranium by sedimentary organic matter under diagenetic or hydrothermal conditions

Interactions between lignite and soluble uranyl species have been investigated experimentally at different temperatures from 20° to 400°C. Fixation of uranyl species by lignite (45° to 250°C) and their reduction to uraninite (120° to 400°C) were observed. The fixation of uranyl species by lignite results in the formation of stable organo-uranyl compounds. The reduction of uranyl species by lignite results in a stoichiometric liberation of H⁺ in the solution medium and in a dehydrogenation of lignite. This dehydrogenation can be attributed to two different processes. The first is an oxidation of alcohol functional groups into aldehyde or ketone functions accompanied by a simultaneous reduction of uranyl species. The second is a dehydrogenation of hydrocarbonaceous aliphatic moieties induced by the uranium species. The molecular hydrogen produced during this process is subsequently used for an additional reduction of uranyl species.