Surface Charge Densities of Two Actinide(IV) Oxides: UO2 and ThO2

Olsson, Mikael; Jakobsson, Anna‐Maria; Albinsson, Yngve

doi:10.1006/jcis.2002.8586

Cited by 25 publications

(16 citation statements)

References 17 publications

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“…These values are consistent with the previously published pH pzc value (5.5) for stoichiometric UO 2 , which had no oxidized surface layer (Olsson et al, 2002). The presence of an oxidized surface layer on UO 2 has been shown to increase the observed pH pzc by 0.5 pH units, depending on the extent of oxidation and layer thickness (Olsson et al, 2002). The observed pH pzc of uraninite-1 was approximately the same as that of uraninite-2; however, the magnitude of the surface charge was different as a function of pH.…”

Section: Surface and Bulk Biogenic Uo 2 Characterizationsupporting

confidence: 93%

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

Singer

Farges

Brown

2009

Geochimica et Cosmochimica Acta

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Section: Surface and Bulk Biogenic Uo 2 Characterizationsupporting

confidence: 93%

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

Singer

Farges

Brown

2009

Geochimica et Cosmochimica Acta

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“…The value of the saturated adsorbed U(VI) concentration is $0.5 mole adsorbed U(VI) per mole of exchangeable U(IV). This value is reasonable given the point of zero charge (pH 5-5.5) for U(IV) dioxide (Olsson et al, 2002;Singer, 2008). Table 3).…”

Section: Results For the U(iv) S -U(vi) Aq Exchange Experimentsmentioning

confidence: 93%

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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“…In this work, the inevitable sorption of humic acids to UO 2 was not taken into account. The UO 2 surface zero point of charge has been determined as being between 5-5.5 [64]. The sorption was thus likely to be higher at pH 6.5-8 than for SiO 2 [28], but no data are available.…”

Section: Compilation Of Datamentioning

confidence: 99%

Complexation parameters for the actinides(IV)-humic acid system: a search for consistency and application to laboratory and field observations

Reiller¹,

Evans²,

Szabó³

2008

Radiochimica Acta

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SummaryThe coherence of actinide(IV) complexation by humic substances (HS) is reviewed and new data are proposed. In a first attempt, the values of independent data from literature on Th(IV), U(IV), and Pu(IV) are collected, selected, and compiled. The data obtained follow the “classical” trend of increasing conditional formation “constants” with pH, led both by the increasing ionisation of HS and by the extensive hydrolysis of the tetravalent actinides. Even though a fair agreement is evident, the experimental uncertainties do not permit a full analogy between the actinides(IV) to be ascertained. In a second attempt, the experiments from which the original data are available were reinterpreted using only one hydrolysis constant set for U(IV) as an example, considering that all actinides(IV) have analogous humic complexation behaviour. Hence, the obtained evolution of conditional formation “constants” is much more coherent and the uncertainties do not permit to distinguish an actinide(IV) from one another. The obtained data are then applied to independent laboratory and

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Surface Charge Densities of Two Actinide(IV) Oxides: UO2 and ThO2

Cited by 25 publications

References 17 publications

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

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

Complexation parameters for the actinides(IV)-humic acid system: a search for consistency and application to laboratory and field observations

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