Uranyl Incorporation in Natural Calcite

Kelly, S. D.; Newville, M. G.; Cheng, L.; Kemner, K. M.; Sutton, S. R.; Fenter, P.; Sturchio, N. C.; Spötl, C.

doi:10.1021/es025962f

Cited by 166 publications

(146 citation statements)

References 19 publications

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“…We excluded the formation of tetravalent U in calcite because the transdioxo structure was identified in XAS analyses. Our U-C distance of ∼2.9 Å is significantly shorter than the monodentate coordination distance (∼3.5 Å) reported by Kelly et al (27), but it is reasonably close to the distance (∼2.92 Å) reported by Reeder et al (25,26). However, U-Ca distances at 3.78-4.01 Å (25, 26) could not be included in the fit.…”

Section: Resultssupporting

confidence: 68%

Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site

Arai

Marcus

Tamura

et al. 2007

Environ. Sci. Technol.

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Uranium (U) solid-state speciation in vadose zone sediments collected beneath the former North Process Pond (NPP) in the 300 Area of the Hanford site (Washington) was investigated using multi-scale techniques. In 30 day batch experiments, only a small fraction of total U (∼7.4%) was released to artificial groundwater solutions equilibrated with 1% pCO 2 . Synchrotron-based micro-X-ray fluorescence spectroscopy analyses showed that U was distributed among at least two types of species: (i) U discrete grains associated with Cu and (ii) O) at some U discrete grains, and muscovite at U intermediate concentration areas, were identified in synchrotron-based micro-X-ray diffraction. Scanning electron microscopy/energy dispersive X-ray analyses revealed 8-10 µm size metatorbernite particles that were embedded in C-, Al-, and Si-rich coatings on quartz and albite grains. In µ-and bulk-X-ray absorption structure (µ-XAS and XAS) spectroscopy analyses, the structure of metatorbernite with additional U-C and U-U coordination environments was consistently observed at U discrete grains with high U concentrations. The consistency of the µ-and bulk-XAS analyses suggests that metatorbernite may comprise a significant fraction of the total U in the sample. The entrapped, micrometer-sized metatorbernite particles in C-, Al-, and Si-rich coatings, along with the more soluble precipitated uranyl carbonates and uranophane, likely control the long-term release of U to water associated with the vadose zone sediments.

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Section: Resultssupporting

confidence: 68%

Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site

Arai

Marcus

Tamura

et al. 2007

Environ. Sci. Technol.

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“…Traditionally, optimized geometries and charge densities, rather than energies, have been evaluated for incorporated systems (e.g., [9,21,22]). Quantum-mechanically optimized geometries are compared with experimental measurements, such as extended X-ray absorption fine structure (EXAFS) measurements, to identify coordination environments and distances to nearest neighbors.…”

Section: Previous Approachesmentioning

confidence: 99%

Quantum-Mechanical Methods for Quantifying Incorporation of Contaminants in Proximal Minerals

et al. 2014

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Abstract:Incorporation reactions play an important role in dictating immobilization and release pathways for chemical species in low-temperature geologic environments. Quantum-mechanical investigations of incorporation seek to characterize the stability and geometry of incorporated structures, as well as the thermodynamics and kinetics of the reactions themselves. For a thermodynamic treatment of incorporation reactions, a source of the incorporated ion and a sink for the released ion is necessary. These sources/sinks in a real geochemical system can be solids, but more commonly, they are charged aqueous species. In this contribution, we review the current methods for ab initio calculations of incorporation reactions, many of which do not consider incorporation from aqueous species. We detail a recently-developed approach for the calculation of incorporation reactions and expand on the part that is modeling the interaction of periodic solids with aqueous source and sink phases and present new research using this approach. To model these interactions, a systematic series of calculations must be done to transform periodic solid source and sink phases to aqueous-phase clusters. Examples of this process are provided for three case studies: (1) neptunyl incorporation into studtite and boltwoodite: for the layered boltwoodite, the incorporation energies are smaller (more favorable) for reactions using environmentally relevant source and sink phases (i.e., ΔE rxn (oxides) > ΔE rxn (silicates) > ΔE rxn (aqueous)). Estimates of the solid-solution behavior of Np to predict the limit of Np-incorporation into boltwoodite (172 and 768 ppm at 300 °C, respectively); (2) uranyl and neptunyl incorporation into carbonates and sulfates: for both carbonates and sulfates, it was found that actinyl incorporation into a defect site is more favorable than incorporation into defect-free periodic structures. In addition, actinyl incorporation into carbonates with aragonite structure is more favorable than into carbonates with calcite structure; and (3)

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“…U concentrations and isotope compositions in carbonate rocks could be a very useful paleoredox proxy because oxidized U(VI) is readily incorporated into calcium carbonate mineral structures during carbonate precipitation(i.e., "coprecipitation") in relatively high concentrations of 0.1 to 10 ppm (Chung and Swart, 1990;Kelly et al, 2003Kelly et al, , 2006Reeder et al, 2000). Additionally, the long residence time of U (>10 5 yr; Dunk et al, 2002) suggests that, unlike Ce, U might provide globally integrated paleoredox information.…”

mentioning

confidence: 99%

Uranium isotope fractionation during coprecipitation with aragonite and calcite

Chen

Romaniello

Herrmann

et al. 2016

Geochimica et Cosmochimica Acta

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Natural variations in 238 U/ 235 U of marine carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~ 8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~ 8.5, with heavier U in the solid phase. 238 U/ 235 U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007+0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca 2 UO 2 (CO 3 ) 3 (aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca 2 UO 2 (CO 3 ) 3 (aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.ii These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, P CO2 , [Ca], or [Mg] concentrations. In general, these effects are likely to be small (<0.13 ‰), but are nevertheless potentially significant because of the small natural range of variation of 238 U/ 235 U.

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Uranyl Incorporation in Natural Calcite

Cited by 166 publications

References 19 publications

Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site

Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site

Quantum-Mechanical Methods for Quantifying Incorporation of Contaminants in Proximal Minerals

Uranium isotope fractionation during coprecipitation with aragonite and calcite

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