X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations

Kelly, Shelly D.; Kemner, Kenneth M.; Brooks, Scott C.

doi:10.1016/j.gca.2006.10.013

Cited by 94 publications

(82 citation statements)

References 51 publications

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“…Direct verification of the uranyl carbonate dominance in vadose zone pore waters from borehole 299-E33-45 is presented in Appendix D of RPP (2002). Recent studies Kalmykov and Choppin 2000;Kelly et al 2007) also indicate that dissolved calcium uranyl carbonate complexes, such as Ca 2 UO 2 (CO 3 ) 3 0 (aq), have an important effect on the geochemical behavior of U(VI) in oxic, calcium-rich aqueous systems at near-neutral to basic pH conditions such as those found at the Hanford Site. Complexes with phosphate, sulfate, fluoride, and possibly chloride are potentially important U(VI) species where concentrations of these anions are high.…”

Section: Uranium-235 238mentioning

confidence: 97%

“…Recent studies Kalmykov and Choppin 2000;Kelly et al 2007) indicate that dissolved calcium uranyl carbonate complexes also have an important effect on the geochemical behavior of U(VI) in oxic, calcium-rich aqueous systems at nearneutral to basic pH conditions such as those at the Hanford site. The results of their series of studies provide evidence for the formation of a strong, uncharged aqueous complex, Ca 2 UO 2 (CO 3 ) 3 0 (aq).…”

Section: A13 Uranium-235238mentioning

confidence: 99%

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Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

Cantrell¹,

Zachara²,

Dresel³

et al. 2007

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Companion reports that review other subject matter areas relevant to contaminant transport within the vadose zone beneath the SST WMAs, the IDF, and groundwater have been recently published. The specific subject areas include the geology of the SST WMAs iv contaminants released to the vadose zone exhibit a wide range of mobility behaviors. Many tank waste contaminants have been strongly retarded by adsorption and precipitation reactions (e.g., cesium, plutonium, americium, and europium), while others have remained mobile (e.g., technetium, nitrate, and selenium). Still others show variable, waste-specific behaviors (e.g., cesium [where sodium concentrations are very high], strontium, chromium, and uranium) that are closely tied to evolving composition of pore water in the sediments, and for chromium, a change in oxidation state. The temperature of in-ground tank waste has moderated by heat exchange with the subsurface sediment, and high concentrations of base (OH -) have been neutralized through reactions with sediment minerals and secondary mineral precipitation. Major geochemical features that may potentially affect the mobility of key contaminants of interest in the vadose zone include oxidation state, aqueous speciation, solubility, and adsorption reactions.Processes suspected of facilitating the far-field migration of immobile radionuclides, such as formation of stable aqueous complex formation and mobile colloids, were found to be potentially operative, but unlikely to occur in the field. An exception is the enhanced migration of cobalt-60 facilitated by the formation of highly stable aqueous-cyanide complexes. Certain fission-product oxyanions (e. • Adsorption is suppressed by large concentrations of tank waste anions (e.g., NO 3 -, OH -, and CO 3 2-)• Surface charge of clay-size minerals is negative and will therefore tend to repel anions• Unlike chromium, their less-soluble, less-mobile reduced forms are unstable in oxidizing environments.Laboratory studies conducted through PNNL's Vadose Zone Characterization Project and basic-science geochemical studies are expected to continue in partnership to provide information to further refine the conceptual model used for risk assessment modeling. Results of these studies can be used to help ascertain uncertainties associated with the refinement of the conceptual models for contaminant migration in the vadose zone beneath the single-shell tanks. Results can also provide improved parameterization, such as distribution coefficients (K d ), and mathematical constructs used by modelers to predict contaminant mobility under these conditions, and to decrease the degree of conservatism incorporated in these models.For various reasons, risk assessment models typically use various approximations to represent the conceptual model developed for the site. Adsorption is frequently approximated using empirical distribution coefficients (K d s). Although the use of empirical distribution coefficients can potentially lead to erroneous results under certain conditions where the...

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Section: Uranium-235 238mentioning

confidence: 97%

Section: A13 Uranium-235238mentioning

confidence: 99%

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

Cantrell¹,

Zachara²,

Dresel³

et al. 2007

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show abstract

“…At circumneutral and more basic pH values and calcite-buffered alkalinity, the principal aqueous U(VI) species involve the uranyl ion (UO 2 2+ ) complexed with calcium and carbonate, such as Ca 2 UO 2 (CO 3 ) 3°( aq) and CaUO 2 (CO 3 ) 3 2- Kelly et al 2007). At lower concentrations of dissolved calcium, speciation of dissolved U(VI) at circumneutral to basic pH values is dominated by a series of strong neutral and anionic uranyl carbonate complexes [e.g., UO 2 CO 3°( aq), UO 2 (CO 3 ) 2 2-, and UO 2 (CO 3 ) 3 4-].…”

Section: Uranium Reductionmentioning

confidence: 99%

Technical Basis for Assessing Uranium Bioremediation Performance

Long¹,

Yabusaki²,

Meyer³

et al. 2008

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“…Further studies of the calcium uranyl carbonate complexes introduced the CaUO 2 (CO 3 ) 3 2-complex and showed that they play an important role in the aqueous chemistry of U(VI) at neutral to alkaline pH range (Kalmykov and Choppin 2000;Bernhard et al 2001;Zheng et al 2003). Kelly et al (2007) noticed the limited and slow bioreduction of U(VI) in the presence of calcium and the formation of calcium uranyl carbonate complexes. In circumneutral pH range, U(VI) sorption in calcareous soils is strongly influenced by calcium uranyl carbonate complexes (Zheng et al 2003).…”

mentioning

confidence: 99%

Impact of Alkaline Earth Metals on Aqueous Speciation of Uranium(VI) and Sorption on Quartz

Nair

Merkel

2011

Aquat Geochem

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The effect of Mg-, Ca-, and Sr-Uranyl-Carbonato complexes with respect to sorption on quartz was studied by means of batch experiments with U(VI) concentration of 0.126 9 10 -6 M in the presence and absence of Mg, Ca, and Sr (each 1 mM) at pH from 6.5 to 9. In the absence of alkaline earth elements, 90% of the U(VI) sorbed on the quartz surface at all pH. In the presence of Mg, Ca, and Sr, the sorption of U(VI) on quartz decreased to 50, 10, and 30%, respectively. Sorption kinetics of U(VI) on quartz is faster in the absence of alkaline earth elements and reached equilibrium after 12 h, whereas in the presence of Mg, Ca and Sr, the kinetics of U(VI) sorption on quartz is pH dependent and attained equilibrium after 24 h. Aqueous speciation calculations for alkaline earth uranyl carbonates were carried out by using PHREEQC with the Nuclear Energy Agency thermodynamic database (NEA_2007) by adding constants for MUO 2 (CO 3 ) 3 2-and M 2 UO 2 (CO 3 ) 3 0 (M = Ca, Mg, Sr). This study reveals that alkaline earth elements can have a significant effect on the aqueous speciation of U(VI) under neutral to alkaline pH conditions and subsequently sorption behavior and mobility of U(VI) in aqueous environments.

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X-ray absorption spectroscopy identifies calcium-uranyl-carbonate complexes at environmental concentrations

Cited by 94 publications

References 51 publications

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

Technical Basis for Assessing Uranium Bioremediation Performance

Impact of Alkaline Earth Metals on Aqueous Speciation of Uranium(VI) and Sorption on Quartz

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