53 research outputs found
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Chemical Speciation of Sr, Am and Cm in high Level Waste: predictive modeling of phase partitioning during tank processing
During this contract period, a number of papers were published. The papers prior to this report have been reported in earlier annual reports. This final report covers the 2005 & 2006 publications which have been published as well as the last few which have been submitted, but are still under review for acceptance for publication. The titles and abstracts of the papers are presented in section A, and the full published papers in Section B
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The Aqueous Thermodynamics and Complexation Reactions of Anionic Silica and Uranium Species to High Concentration
The objective of this research project is to develop the necessary thermodynamic data, including aqueous phase stability constants and Pitzer ion-interaction parameters, to predict the changes in the aqueous phase chemistry that occur when high ionic strength, highly basic tank wastes enter the vadose zone
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Research Program to Determine Redox Properties and Their Effects on Speciation and Mobility of Pu in DOE Wastes
This research is investigating the kinetics of the oxidation of plutonium(IV) by manganese dioxide. Manganese dioxide is a component in some stored radioactive wastes where it may influence the oxidation state distribution of plutonium. Manganese dioxide is also present in many natural waters and may contribute to the oxidation of plutonium(IV) to higher oxidation states in such waters. In this study, the oxidation of Pu(IV) by manganese dioxide is being investigated by solvent extraction at an ionic strength of 1.0 m (NaCl) over a range of pH values
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Research Program to Determine Redox Properties and Their Effects on Speciation and Mobility of Pu in DOE Wastes
Plutonium contamination is a major problem at many DOE sites. This project seeks to provide fundamental data and models for predicting plutonium speciation and environmental behavior that will allow evaluation of the many processes proposed for remediation of plutonium-containing DOE wastes. Basic to this prediction are (1) the complete fundamental data on aqueous complexation, solubility reactions, and geochemical conditions under which different plutonium oxidation states transform from one to another, and (2) a model containing this fundamental data that can predict site specific reactions. We propose to summarize existing data and develop additional fundamental data to measure the oxidation state (or states) in a variety of solid/solution systems in the presence and absence of chemicals likely to be present in DOE wastes and which can promote redox transformations and complexation reactions. Specifically, we will (1) develop methodology to determine complexation constants of Pu(IV) with strongly complexing ligands, (2) determine stability constants of Pu(IV) with strong complexants such as EDTA, (3) determine the geochemical conditions under which different reductants (e.g., Fe(0), Fe2+, H2O2) can reduce Pu(VI) to Pu(III)/Pu(IV) and oxidants (e.g., MnO2 and radiolytic products) that can oxidize Pu(IV) to Pu(V)/Pu(VI), and (4) summarize fundamental Pu data for use in a model for predicting Pu behavior under different geologic conditions
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Thermodynamics and Complexation Reactions of Anionic Silicate Species
Highly basic tank wastes contain several important radionuclides, including {sup 90}Sr, {sup 99}Tc, and {sup 60}Co as well as actinide elements (isotopes of U, Pu and Am). A fraction of these wastes are known to have leaked into the vadose zone at the Hanford Site. Upon entering the sediments in the vadose zone, such highly basic solutions dissolve concentrations of silica from the silica and aluminosilicate minerals present in subsurface. These dissolution reactions alter the chemical composition of the leaking solutions, transforming them from highly basic (as high as 2 M NaOH) solution into a pore solution with dissolved silica and significantly reduced pH. This moderately basic (pH 9 to 10), high-silica solution has the potential to complex radionuclides and promote migration through the subsurface. This path of radionuclide migration currently is not a recognized transport mode in the factors that are modeled for radionuclide transport through the vadose zone beneath leaking tanks. The goal of this project is to ascertain the free monosilicic acid concentration, and the degree of polymerization as a function of pH and total concentration of silicate ions, and to use this data to measure the interaction of radionuclides of Co(II), Sr(II), Nd(III), Eu(III), Am(III), U(VI) and Th(IV) with the ionic silicate
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The Aqueous Thermodynamics and Complexation Reactions of Anionic Silica and Uranium Species to High Concentration.
Highly basic tank wastes contain several important radionuclides, including 90Sr, 99Tc, and 60Co, as well as actinide elements (i.e., isotopes of U, Pu, and Am). These highly basic tank wastes are known to have leaked into the vadose zone at the Hanford Site. In particular, wastes from the bismuth phosphate process contained very high concentrations of U as well as carbonate, phosphate, nitrate, and other components (AEC 1951) and these solutions have leaked into the subsurface at the Hanford site. The tanks containing the bismuth phosphate wastes were frequently saturated with respect to the solid phases of these components [e.g., NaUO2PO4(c) and Na4UO2(CO3)3(c)]. These solids were referred to as ''hard sludge'' (Na4UO2(CO3)3(c)) and ?soft sludge? [NaUO2PO4(c)] because of their different crystal forms. The preliminary studies of the solubility of these solids in tank wastes (AEC 1951) indicate that aqueous U carbonate complexes dominate the solution chemistry of uranium even when the equilibrium solid was NaUO2PO4. Thus there was a need to develop an accurate thermodynamic model for the solubility of potentially important U(VI) phosphate and carbonate phases as well as to develop a model for the uranium carbonate complexes valid to high ionic strength. In this project we are examining the solubility of these important solid phases as well as the aqueous thermodynamics of U(VI) species under strongly basic conditions. Also included is a description of our efforts to include these thermodynamic models in the reactive transport and residual leaching models being used at the Hanford site and elsewhere
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Variation of stability constants of thorium citrate complexes and of thorium hydrolysis constants with ionic strength
Citrate is among the organic anions that are expected to be present in the wastes planned for deposition in the Waste Isolation Pilot Plant repository. In this study, a solvent extraction method has been used to measure the stability constants of Thorium(IV)[Th(IV)] with citrate anions in aqueous solutions with (a) NaClO{sub 4} and (b) NaCl as the background electrolytes. The ionic strengths were varied up to 5 m (NaCl) and 14 m (NaClO{sub 4}). The data from the NaClO{sub 4} solutions at varying pH values were used to calculate the hydrolysis constants for formation of Th(OH){sup 3+} at the different ionic strengths
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Chemical Speciation of Strontium, Americium, and Curium in High Level Waste: Predictive Modeling of Phase Partitioning During Tank Processing
In this research program, Pacific Northwest National Laboratory (PNNL) and Florida State University (FSU) are investigating the speciation of strontium and americium/curium in the presence of selected organic chelating agents (ethylenediaminetetraacetic acid (EDTA), N-(2-hydroxyethyl) aethylenediaminetriacetic acid (HEDTA), nitrilotriacetic acid (NTA), and iminodiacetic acid (IDA)) over ranges of hydroxide, carbonate, ionic strength, and competing metal ion concentrations present in high-level waste tanks. The project is composed of integrated research tasks that approach the problem of chemical speciation using macroscopic thermodynamic measurements of metal-ligand competition reactions, molecular modeling studies to identify structures or complexes of unusual stability, and mass spectrometry measurements of complex charge/mass ratio that can be applied to mixed metal-chelate systems. This fundamental information is then used to develop thermodynamic models, which allow the prediction of changes in chemical speciation and solubility that can occur in response to changes in tank processing conditions. In this way, we can develop new approaches that address fundamental problems in aqueous speciation and at the same time provide useful and practical information needed for tank processing
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Chemical Speciation of Strontium, Americium, and Curium in High Level Waste: Predictive Modeling of Phase Partitioning During Tank Processing
The objective of this research project is to measure the effects of organic chelate complexation on the speciation and solubility of Sr and trivalent actinides under strongly basic, high carbonate conditions, similar to those present in high- level waste tanks at U.S. Department of Energy storage sites. We proposed, (1) extension to important chelates not previously studied; (2) studies of completing metal ions; and (3) specific studies using Am(III)/Cm(III). The chelate complexation studies would extend our previous research on EDTA, HEDTA, NTA, and IDA to citrate and oxalate. In addition, we propose to address the possible formation of mixed ligand- ligand complexes for Eu(III) in EDTA-HEDTA, EDTA-NTA, HEDTA-NTA, and ligand-carbonate solutions. The fundamental data on chemical speciation and solubility will be used to develop accurate thermodynamic models which are valid to high ionic strength
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The Aqueous Thermodynamics and Complexation Reactions of Anionic Silica and Uranium Species to High Concentration
During this contract period, a number of papers were published. The papers prior to this report have been reported in earlier annual reports. This final report covers the 2005 & 2006 publications which have been published as well as the last few which have been submitted, but are still under review for acceptance for publication. The titles and Abstract of the papers are presented in Section A, and the full published papers in Section B
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