Uranium(VI) uptake by geological materials, characterisation by luminescence spectroscopy

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Abstract

Many of the wastes associated with the nuclear fuel cycle are toxic to the biosphere; advancing the use of high resolution spectroscopy applied to these materials will provide the chemical speciation of the interaction between nuclear waste and geological material, improving confidence in a permanent disposal method and informing clean-up operations. Luminescence spectroscopy of uranyl(VI) is a well-established technique for the molecular speciation of uranium-mineral interactions. This work explores the use of both micro- and macroscopic luminescence spectroscopy to expose uranyl(VI) speciative heterogeneity in a range of minerals which have been exposed to uranyl(VI) salt solutions. A comprehensive review of the available literature on the interaction of uranyl(VI) with a range of geological media is assessed and compared. The review finds considerable ambiguity in the speciation of uranyl(VI) at the mineral water interface. A database reporting the multi parametric luminescence properties of uranyl(VI) with silica gel, quartz, bayerite, boehmite, muscovite, kaolinite and montmorillonite (SWy-2 and STx-1b) is presented and discussed. Although some of the results are consistent with previously reported values, many newly identified species are reported and their identification speculated. Parallel factor analysis is used to deconvolute the excitationemission matrix of uranyl(VI) sorbed to silica gel between pH 3 and pH 10. The results are used to identify the spectroscopic properties of complexes >(SiO)2UO2 and >(SiO)2UO2OH and thus new complexation coefficients (log(K)) for their formation with the silica gel surface are determined, log(K1) = 9.22 ± 0.02 and log(K2) = 3.45 ± 0.01, respectively. The investigation also provides insight into the fundamental properties of uranyl(VI) excitation pathways, which are not yet fully understood. Confocal microscopy and phosphorescent lifetime image mapping (PLIM) is used to expose the sub-micron heterogeneity of uranyl(VI) sorption complexation across mineral surfaces of silica gel, bayerite and montmorillonite (STx-1b). The results suggest that changes in the uranyl(VI) lifetime can be used to observe and understand submicron changes in uranyl(VI) complexation at hitherto unknown temporal resolution.

Keyword(s)

FLIM; PARAFAC; PLIM; clay; deconvolution; luminescence; montmorillonite; silica; uranium; uranyl; uranyl(VI)

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