Separation of uranium isotopes by uranium(IV)-uranium(VI) chemical exchange

Florence, T.M.; Batley, Graeme E.; Ekstrom, A.; Fardy, J. J.; Farrar, Yvonne J.

doi:10.1016/0022-1902(75)80925-0

Cited by 29 publications

(23 citation statements)

References 14 publications

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“…According to Eq. (3), at 298 K (25°C) in hydrochloric media, the overall isotope fractionation is 1.4 ± 0.3& (Fujii et al, 2006), which is consistent with the 1.3 ± 0.4& (1r) fractionation determined by Florence et al (1975) in sulfuric acid. Our recent experiment with oxidation of dissolved U(IV) by dissolved oxygen in 0.1 N hydrochloric acid obtained an equilibrium-like (but not completely following an equilibrium model) isotope fractionation of 1.1 ± 0.3& (2r), which is generally consistent with previous equilibration experiments involving net redox reactions (Florence et al, 1975;Nomura et al, 1996;Fujii et al, 2006).…”

Section: Equilibrium Fractionation: Comparison With Previous Studiessupporting

confidence: 77%

“…(3), at 298 K (25°C) in hydrochloric media, the overall isotope fractionation is 1.4 ± 0.3& (Fujii et al, 2006), which is consistent with the 1.3 ± 0.4& (1r) fractionation determined by Florence et al (1975) in sulfuric acid. Our recent experiment with oxidation of dissolved U(IV) by dissolved oxygen in 0.1 N hydrochloric acid obtained an equilibrium-like (but not completely following an equilibrium model) isotope fractionation of 1.1 ± 0.3& (2r), which is generally consistent with previous equilibration experiments involving net redox reactions (Florence et al, 1975;Nomura et al, 1996;Fujii et al, 2006). The direct measurement (1.64 ± 0.16&, 2r) of equilibrium isotope fractionation by our experiment at room temperature in hydrochloric acid media is, within the uncertainties, consistent with previous experiments involving multi-stage reduction and oxidation reactions (Florence et al, 1975;Nomura et al, 1996;Fujii et al, 2006).…”

Section: Equilibrium Fractionation: Comparison With Previous Studiessupporting

confidence: 76%

“…U(IV) and U(VI) bearing phases may be able to exchange isotopes with each other through the reaction: 238 UðVIÞ þ 235 UðIVÞ () 238 UðIVÞ þ 235 UðVIÞ ð 1Þ Importantly, the isotope exchange described here, which involves no net redox changes between U(IV) and U(VI), is different from previous isotope equilibration experiments (Florence et al, 1975;Nomura et al, 1996;Fujii et al, 2006), where oxidants and reductants were used to induce U redox cycling for nuclear enrichment purposes. The isotope exchange reaction given in Eq.…”

Section: Isotope Exchange Reactionmentioning

confidence: 94%

“…Measurements of equilibrium isotope fractionation between U(IV) and U(VI) has been attempted previously, mostly for nuclear enrichment purposes (Florence et al, 1975;Nomura et al, 1996;Fujii et al, 2006). However, the experimental conditions (high concentrations in acid solutions, called "U(IV) aq -U(VI) aq exchange experiments" hereafter) were far from natural environmental conditions.…”

Section: Previous Studiesmentioning

confidence: 99%

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Low temperature equilibrium isotope fractionation and isotope exchange kinetics between U(IV) and U(VI)

Wang

Lundstrom

2015

Geochimica et Cosmochimica Acta

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Measurements of the uranium (U) isotope ratio 238 U/ 235 U provide an emerging redox proxy in environmental and paleoredox studies, but many key parameters concerning U isotope fractionation are still poorly constrained. Here we report the equilibrium isotopic fractionation between dissolved U(IV) and dissolved U(VI), and rates of isotope exchange between solid-phase U(IV) and dissolved U(VI). We conducted one experiment at high concentration [35 mM U(IV) and 32 mM U(VI)] and low pH (0.2) in hydrochloric acid media at room temperature to determine the equilibrium isotopic fractionation between dissolved U(IV) and dissolved U(VI). Isotopic equilibrium was reached in about 19 days under such experimental conditions. The equilibrium isotope fractionation was determined to be 1.64 ± 0.16&, with U(IV) being enriched in 238 U relative to U(VI). Applicability of the determined equilibrium fractionation is discussed.We also conducted a set of experiments to determine isotopic exchange rates between dissolved U(VI) and nanouraninite U(IV) under conditions closer to those in natural system, with lower concentrations and neutral pH. The exchange rate was found to conform to the rate law R = k[U(VI)] adsorbed , in which R is the isotopic exchange rate (lM day À1 ); k is the rate constant determined to be 0.21 day À1 ; and [U(VI)] adsorbed is the concentration of U(VI) adsorbed to nanouraninite (lM).Our results, combined with consideration of the variables controlling U(VI)-U(IV) contact in natural settings, indicate that the timescale for significant isotope equilibration varies depending on environmental conditions, mostly uranium concentrations. In natural uncontaminated sediments with low uranium concentrations, equilibration is expected to occur on a timescale of hundreds to thousands of years. In contrast, in U-contaminated aquifers with high U concentrations, significant equilibration could occur on timescales of weeks to years.

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Section: Equilibrium Fractionation: Comparison With Previous Studiessupporting

confidence: 77%

Section: Equilibrium Fractionation: Comparison With Previous Studiessupporting

confidence: 76%

Section: Isotope Exchange Reactionmentioning

confidence: 94%

Section: Previous Studiesmentioning

confidence: 99%

See 2 more Smart Citations

Low temperature equilibrium isotope fractionation and isotope exchange kinetics between U(IV) and U(VI)

Wang

Lundstrom

2015

Geochimica et Cosmochimica Acta

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“…[20] The slope of a plot of En a versus 1/T at high temperature is finite as a consequence of the nuclear field shift. At low temperature, the nuclear field shift does not change the 1/T law in en a.…”

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confidence: 99%

Temperature dependence of the isotope chemistry of the heavy elements.

Bigeleisen

1996

Proc. Natl. Acad. Sci. U.S.A.

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The temperature coefficient of equilibrium isotope fractionation in the heavy elements is shown to be larger at high temperatures than that expected from the well-studied vibrational isotope effects. The difference in the isotopic behavior of the heavy elements as compared with the light elements is due to the large nuclear isotope field shifts in the heavy elements. The field shifts introduce new mechanisms for maxima, minima, crossovers, and large massindependent isotope effects in the isotope chemistry of the heavy elements. The generalizations are illustrated by the temperature dependence of the isotopic fractionation in the redox reaction between U(VI) and U(IV) ions.Stern and colleagues (1, 2) have studied the temperature dependence of equilibrium isotope fractionation for the elements hydrogen, carbon, nitrogen, and oxygen over the temperature range 20-2000 K; they used a computational approach to the problem. The vibrational frequencies of isotopomers of a large number of compounds of the above elements were calculated using the Wilson FG matrix method. Input parameters were valence force constants, F matrices, and the masses, bond distances, and bond angles of the molecules, G matrices. The calculated frequencies were then substituted into the Bigeleisen-Mayer equation (3) for the isotope fractionation factor. In accord with theoretical expectations, Stern and colleagues (1, 2) found, in all cases, that the logarithm of the fractionation factor obeys a 1/T law at low temperature and a 11T2 law at high temperature. The chemical basis for these two limiting temperature dependences can be readily understood through the Bigeleisen-Mayer free energy function.

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