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Mononuclear U(IV) complexes and ningyoite as major uranium species in lake sediments



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​Geochemical Perspectives Letters / Volume 2, pages 95-105, février 2016

Type de document > *Article de revue

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Unité de recherche > IRSN/PRP-DGE/SRTG/LT2S

Auteurs > MORIN Guillaume, [et al.]

Date de publication > 16/02/2016


​Natural attenuation of uranium in subsurface environments is generally assigned to immobilisation processes due to microbial reduction of U(VI). Recent laboratory studies have established that the end products of such a process include both low solubility biogenic uraninite and more labile non-crystalline U(IV) species. Indeed, biogenic uraninite formation may be inhibited in the presence of organic or inorganic phosphoryl ligands, leading to the formation of non-crystalline U(IV)-phosphate complexes or nanoscale U(IV)-phosphate solids. Such species have been observed in shallow contaminated alluvial aquifers and can thus be suspected to form in other important environments, among which lacustrine sediments have a global environmental significance since they may represent major uranium accumulation reservoirs in riverine watersheds. Here, on the basis of microscopic, spectroscopic and chemical extraction analyses, we report the occurrence of mononuclear U(IV)-phosphate/silicate complexes, accompanied by nano-crystalline ningyoite-like U(IV)-phosphate minerals, as major scavengers for uranium in lacustrine sediments downstream from a former uranium mine in France. This observation reveals that uranium trapping mechanisms during early diagenesis of lacustrine sediments can virtually exclude uraninite formation, which has important implications for better modelling uranium cycling in natural and contaminated freshwaters. Moreover, our results raise issues concerning the long term fate of mononuclear U(IV) complexes and U(IV) phosphate nano-minerals, especially with respect to re-oxidation events.

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Laboratoire IRSN impliqué

Laboratoire d'étude des transferts en sub-surface (LT2S)