Environmental context. The production of nuclear materials has generated a very large amount of highly radioactive wastes that need to be disposed of in a manner that will keep them from posing a danger for millions of years until the radioactivity decays. The process being considered for this daunting task is to contain the wastes in glass. Although studies with ancient and natural glass suggest the weathering of glass is slow, experiments are being conducted to determine the impact of this material on the natural environment and attempt to predict its long-term behaviour. The present paper briefly discusses three models that are being considered for implementing this process and the one that appears to hold the most promise. Abstract. Single-pass flow-through experiments were conducted with aluminoborosilicate waste glasses to evaluate how changes in solution composition affect the dissolution rate (r) at 40°C and pH (23°C) = 9.0. The three prototypic low-activity waste (LAW) glasses, LAWE-1A, -95A and -290A, used in these experiments span a wide range covering the expected processing composition of candidate immobilised low-activity waste (ILAW) glasses. Results suggest incongruent release of Al, B, Na, and Si at low flow-rate (q) to sample surface area (S), in units of (m s–1), (log10(q/S) < –8.9) whereas congruent release is observed at high q/S (log10(q/S) > –7.9). Dissolution rates increase from log10(q/S) ≈ –9.3 to –8.0 and then become constant at log10(q/S) > –7.9. Forward (maximum) dissolution rates, based on B release, are the same irrespective of glass composition, evident by the dissolution rates being within the experimental error of one another (r1A = 0.0301 ± 0.0153 g m–2 day–1, r95A = 0.0248 ± 0.0125 g m–2 day–1, and r290A = 0.0389 ± 0.0197 g m–2 day–1). The results also illustrate that as the activity of SiO2(aq) increases, the rate of glass dissolution decreases to a residual rate. The pseudo-equilibrium constant, Kg, (log10(Kg) = –3.7) predicted with these results is slightly lower than the K for chalcedony (log10(K) = –3.48) at 40°C. Finally, these results support the use of a chemical affinity-based rate law to describe glass dissolution as a function of solution composition.
Environmental context. Contamination of surface and subsurface geologic media by heavy metals and radionuclides is a significant problem within the United State Department of Energy complex as a result of past nuclear operations. Numerous phosphate-based remediation strategies have been proposed to introduce hydroxylapatite directly or indirectly (i.e. through in situ precipitation) into subsurface regimes to act as an efficient sorbent for sequestration of metals and radionuclides such as uranium. Results presented here illustrate the importance of variable geochemical conditions on the mechanism of sequestration and long-term retention of uranium in the presence of hydroxylapatite. Abstract. Numerous solid- and aqueous-phase phosphate-based technologies for remediating heavy metals and radionuclides have the common premise of sequestration by hydroxylapatite. Complexation reactions and hydrolysis generally preclude actinides from incorporation into intracrystalline sites; rather, surface sorption and precipitation are significant mechanisms for the sequestration of actinides. The effect of pH, aqueous speciation, and the availability of reactive surface sites on minerals such as hydroxylapatite have a significant impact on the mechanism and degree of sequestration and retention of variably charged contaminants such as uranium. Yet, little attention has been given to the sequestration and retention of uranium by hydroxylapatite under dynamic geochemical conditions that may be encountered during remediation activities. We present the results of an investigation evaluating the removal of uranium by hydroxylapatite in systems near equilibrium with respect to hydroxylapatite, and the effect of dynamic aqueous geochemical conditions, such as those encountered during and subsequent to remediation activities, on the retention of uranium. Results presented here support previous investigations demonstrating the efficiency of hydroxylapatite for sequestration of uranium and illustrate the importance of geochemical conditions, including changes to surface properties and aqueous speciation, on the sequestration and retention of uranium.
SummaryFor fiscal year 2006, the United States Congress authorized $10 million dollars to Hanford for "…analyzing contaminant migration to the Columbia River, and for the introduction of new technology approaches to solving contamination migration issues." These funds are administered through the U.S. Department of Energy Office of Environmental Management (specifically, . After a peer review and selection process, nine projects were selected to meet the objectives of the appropriation. As part of this effort, Pacific Northwest National Laboratory (PNNL) is performing bench-and field-scale treatability testing designed to evaluate the efficacy of using polyphosphate injections to reduce uranium concentrations in the groundwater to meet drinking water standards (30 μg/L) in situ. This technology works by forming phosphate minerals (autunite and apatite) in the aquifer, which directly sequesters the existing aqueous uranium in autunite minerals and precipitates apatite minerals for sorption and long-term treatment of uranium migrating into the treatment zone, thus reducing current and future aqueous uranium concentrations. Polyphosphate injection was selected for testing based on technology screening as part of the 300-FF-5 Phase III Feasibility Study for treatment of uranium in the 300 Area.The overall objectives of the treatability test include the following:• Optimize the use of multi-length polyphosphate amendment formulations, quantify the hydrolysis rates of polyphosphate, quantify the kinetics of autunite and apatite formation, and determine the long-term immobilization of uranium by apatite and longevity for polyphosphate injections to remediate uranium such that costs for full-scale application can be estimated effectively.• Inject polyphosphate to evaluate reduction of aqueous uranium concentrations and to determine the longevity of treatment of the process at full scale.• Demonstrate field-scale application of polyphosphate injections to evaluate whether a full-scale process can be implemented.This report presents results from bench-scale treatability studies conducted under site-specific conditions to optimize the polyphosphate amendment for implementation of a field-scale technology demonstration to treat aqueous uranium within the 300 Area aquifer of the Hanford Site. The general treatability testing approach consisted of conducting studies with site sediment and under site conditions, to develop an effective chemical formulation for the polyphosphate amendments and evaluate the transport properties of these amendments under site conditions. Phosphorus-31 nuclear magnetic resonance was used to determine the effects of Hanford groundwater and sediment on the degradation of inorganic phosphates. Static batch tests were conducted to optimize the composition of the polyphosphate formulation for the precipitation of apatite and autunite, and to quantify the kinetics, loading, and stability of apatite as a long-term sorbent for uranium. Dynamic column tests were used to further optimize the polyphosphate f...
Uranium in soluble form is of concern for chemical toxicity and for radiological exposure. Despite the cessation of uranium releases and the removal of shallow vadose zone source materials, groundwater beneath the 300 Area at the Hanford Site in Southeastern Washington State continues to contain uranium at concentrations that exceed US Environmental Protection Agency (EPA) maximum contaminant level (MCL). Polyphosphate remediation technology was optimized through a site-specific treatability test for enhanced monitored natural attenuation of the uranium plume within the 300 Area aquifer. The objective was to demonstrate the efficacy of polyphosphate to: 1) reduce the inventory of available uranium that contributes to the groundwater plume through direct precipitation of uranylphosphate (autunite) solids, and 2) provide secondary containment to influent uranium through the precipitation of apatite that can serve as a long-term sorbent for uranium. The field-scale demonstration test site contained 15 monitoring wells and an injection well near the process trenches that had previously received uranium-bearing effluents. The results indicated that direct formation of autunite appears to have been successful; however, the formation of apatite during the test was limited. On the basis of this study, we can conclude that two separate overarching issues impact the efficacy of apatite remediation for uranium sequestration within the 300 Area: 1) efficacy of apatite for sequestering uranium under the present geochemical and hydrodynamic conditions, and 2) formation and emplacement of apatite via polyphosphate technology.
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