12 research outputs found
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Coupling Sorption to Soil Weathering During Reactive Transport: Impacts of Mineral Transformation and Sorbate Aging on Contaminant Speciation and Mobility
Our work is aimed at developing a predictive-mechanistic understanding of the coupling between mineral weathering from caustic waste release and contaminant (Cs, Sr, I) fate and transport in waste-impacted sediments across space, time and geochemical gradients that encompass the process-level heterogeneity observed at the Hanford DOE site. Our specific objectives are: (1) to assess the molecular-scale mechanisms responsible for time-dependent sequestration of contaminants (Cs, Sr and I) during penetration of waste-induced weathering fronts through sedimentary media; (2) to determine the rate and extent of contaminant release from the sorbed state; (3) to develop a reactive transport model based on molecular mechanisms and macroscopic flow experiments (from (1) and (2)) that accurately simulates adsorption, aging, and desorption at the bench-scale, and that can be applied to--and validated at--field sites such as Hanford
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Caustic Waste-Soil Weathering Reactions and Their Impacts on Trace Contaminant Migration and Sequestration
The principal goal of this project was to assess the molecular nature and stability of radionuclide immobilization during weathering reactions in bulk Hanford sediments and their high surface area clay mineral constitutents
Strontium and Cesium Release Mechanisms during Unsaturated Flow through Waste-Weathered Hanford Sediments
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Proceedings of the Task 4 Waste Isolation Safety Assessment Program second contractor information meeting
Volume II contains the following papers: Laboratory Studies of Radionuclide Distributions between Selected Groundwaters and Geologic Media; Applicability of Microautoradiography to Sorption Studies; The Kinetics and Reversibility of Radionuclide Sorption Reactions with Rocks; Mobility and Sorption Processes of Radioactive Waste Materials in Subsurface Migration; Batch K/sub d/ Experiments with Common Minerals and Representative Groundwaters; Cesium and Strontium Migration in Unconsolidated Geologic Material; Statistical Investigation of the Mechanics Controlling Radionuclide Sorption; The ARDISC Model - A Computer Program to Calculate the Distribution of Trace Element Migration in Partially-Equilibrating Media; and Some Geochemical Aspects of the Canadian Nuclear Waste Disposal System. Individual papers were processed
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Reactivity of Primary Soil Minerals and Secondary Precipitates beneath Leaking Hanford Waste Tanks
Since the late 1950s, leaks from 67 single-shell tanks at the Hanford Site have released about 1 million curies to the underlying sediments. At issue is the distribution of contaminants beneath the tanks, and the processes that led to their current disposition and will control their future mobility. The high ionic strength, high pH, and high aluminum concentrations in the tank liquids can significantly alter the vadose zone sediments through dissolution of primary minerals and precipitation of secondary minerals. Dissolution and precipitation directly influence (1) the flow paths that control contaminant transport and (2) the reactivity of the solid matrix that controls contaminant mobility. The impact of these processes, however, depends on mineral reaction kinetics and the dynamic interaction of the reactions with the flow field and contaminant sorption, neither of which are well-known for this extreme chemical system. Data obtained will be directly useful to other EMSP projects addressing contaminant mobility in the vadose zone. We are addressing three specific issues: (1) Recognized factors that control the kinetics of dissolution and precipitation must be quantified for the unnatural system of tank solutions mixing with soils, including effects of high pH, high ionic strength (especially NaNO3 solutions), temperature, and saturation state. (2) A clear understanding of the roles of nucleation mechanism, nucleation sites on soils minerals, and the role of reactive surface area in simultaneous dissolution and precipitation reactions are the key unknown components in comprehending this contaminated soil system. (3) Results obtained will help build a mechanistic understanding of how tank fluids migrate through the vadose zone. Local changes in porosity and permeability will dictate preferential flow paths which directly regulate the transport of later arriving chemical species. Changes in mineral surface area affect sorption site distribution. A comprehensive model is needed that integrates these feedback mechanisms with all the critically available data
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Caustic Waste-Soil Weathering Reactions and Their Impacts on Trace Contaminant Migration and Sequestration
High pH waste solutions are in gross chemical disequilibrium with the subsurface environment and react with sediment minerals, promoting dissolution and precipitation processes that impact 137Cs, 90Sr, and 129I speciation and migration behavior in the vadose zone. We have conducted long-term kinetic studies, with reaction times ranging from 1 to 369 d, to examine relationships between aluminosilicate weathering in the presence of synthetic tank waste leachate (STWL) and contaminant uptake and release. Our experiments employ a sequence of specimen clay minerals including illite, vermiculite, montmorillonite and kaolinite, which are also important reactive solids in the Hanford sediments. These studies have shown direct coupling between mineral transformation reactions and contaminant sequestration/stabilization
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Vadose zone infiltration rate at Hanford, Washington, inferred from Sr isotope measurements
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Vadose zone infiltration rate at Hanford, Washington, inferred from Sr isotope measurements
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Fixation Mechanisms and Desorption Rates of Sorbed Cs in High-Level Waste Contaminated Subsurface Sediments: Implications to Future Behavior and In-Ground Stability
Research is investigating mineralogic and geochemical factors controlling the desorption rate of 137Cs+ from subsurface sediments on the Hanford Site contaminated with different types of high level waste. The project will develop kinetic data and models that describe the release rates of 137Cs+ from contaminated sediments over a range of potential geochemical conditions that may evolve during waste retrieval from overlying tanks, or in response to meteoric water infiltration. Scientific understanding and computational techniques will be established to predict the future behavior of sorbed, in-ground 137Cs+
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Interfacial Soil Chemistry of Radionuclides in the Unsaturated Zone
This is the final year (in a one-year extension) of a project with the principal goal of investigating the impact of clay surface alteration, resulting from hydroxy-aluminum (HyA) and hydroxyaluminosilicate (HAS) species intercalation on reactivity of soils towards Cs and Sr. Special emphasis has been accorded to the unique geochemical conditions that are representative of the Hanford site vadose zone (high ionic strength, high pH, high Al concentrations). Specific objectives of the research include: (1) Quantify the rate and extent of contaminant sorption to prevalent soil minerals as a function of system composition and contaminant concentration. (2) Determine the effects of intercalation of clays by HyA and HAS on the sorption/ion exchange process. (3) Determine the role of NOM (dissolved and mineral bound) on the particle retention of Cs and Sr. (4) Investigate the nature of mineral transformations induced by high pH and ionic strength conditions characteristic of waste impacted environments as it affects concurrent/subsequent retention of Cs and Sr. (5) Determine the coordination chemistry of contaminants bound into clay surfaces [as measured in objectives (1)-(4)] using NMR and X-ray absorption spectroscopy (XAS). Significant progress has been made in achieving these objectives. We have conducted long-term kinetic studies--reaction times ranging from 1 d to 2 yr--to examine relationships between aluminosilicate weathering in the presence of synthetic tank waste leachate (STWL) and Cs/Sr uptake and release. Our experiments employ a sequence of specimen clay minerals including illite, vermiculite, smectite and kaolinite, which are also important reactive solids in the Hanford sediments (Serne et al., 2001)