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Characterization of Th Carbonate Solutions Using XAS andImplications for Thermodynamic Modeling

Published online by Cambridge University Press:  03 September 2012

N. J. Hess,
A. R. Felmy,
D. Rai and
S. D. Conradson
N. J. Hess
Affiliation:
Pacific Northwest National Laboratory, Richland WA 99352
A. R. Felmy
Affiliation:
Pacific Northwest National Laboratory, Richland WA 99352
D. Rai
Affiliation:
Pacific Northwest National Laboratory, Richland WA 99352
S. D. Conradson
Affiliation:
Los Alamos National Laboratory, Los Alamos NM 89545
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Abstract

The chemical behavior of actinide elements in tank solutions, in soil, andin groundwater is dependent upon the chemical species that form when aqueoussolutions come in contact with the actinide compounds. In particular thechemical speciation of the reduced actinide oxidation states (III and IV)are important, for example, to DOE waste tank processing and, moregenerally, to nuclear waste disposal issues. Predicting the solubility ofthe actinides in these solutions requires identification of the strongaqueous complexes, such as carbonates and organic chelating agents, that canform in aqueous solution.

Previous speciation work has often relied on indirect techniques such aspotentiometric titrations or solubility measurements. Recent XAS experimentsdetermine directly the speciation of the Th carbonato species of sevensolutions under a range of carbonate concentrations and pH conditions. Thepresence of the pentacarbonato complex is confirmed and the complex'sstability at low carbonate concentrations is determined. These experimentalresults support a proposed thermodynamic model that describes the solubilityof Th(IV) hydrous oxide in the aqueous Na+-HCO3--CO32--OH--ClO4--H2O system extending to high concentrations at 25°C.This model is relatively simple in that only two aqueous species areincluded Th(OH)3CO3- and Th(CO3)56-.

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References

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