Phosphate Barriers for Immobilization of Uranium Plumes

Uranium contamination of the subsurface remains a persistent problem plaguing remedial design at sites across the U.S. that were involved with production, handling, storage, milling, and reprocessing of uranium for both civilian and defense related purposes. Remediation efforts to date have relied upon excavation, pump-and-treat, or passive remediation barriers (PRB?s) to remove or attenuate uranium mobility. Documented cases convincingly demonstrate that excavation and pump-and-treat methods are ineffective for a number of highly contaminated sites. There is growing concern that use of conventional PRB's, such as zero-valent iron, may be a temporary solution to a problem that will persist for thousands of years. Alternatives to the standard treatment methods are therefore warranted. The core objective of our research is to demonstrate that a phosphorous amendment strategy will result in a reduction of dissolved uranium to below the proposed drinking water standard. Our hypothesis is that long-chain sodium polyphosphate compounds forestall precipitation of sparingly soluble uranyl phosphate compounds, which is paramount to preventing fouling of wells at the point of injection

The Geochemistry of Technetium: A Summary of the Behavior of an Artificial Element in the Natural Environment

Interest in the chemistry of technetium has only increased since its discovery in 1937, mainly because of the large and growing inventory of 99Tc generated during fission of 235U, its environmental mobility in oxidizing conditions, and its potential radiotoxicity. For every ton of enriched uranium fuel (3% 235U) that is consumed at a typical burn-up rate, nearly 1 kg of 99Tc is generated. Thus, the mass of 99Tc produced since 1993 has nearly quadrupled, and will likely to continue to increase if more emphasis is placed on nuclear power to slow the accumulation of atmospheric greenhouse gases. In order to gain a comprehensive understanding of the interaction of 99Tc and the natural environment, we review the sources of 99Tc in the nuclear fuel cycle, its chemical properties, radiochemistry, and biogeochemical behavior. We include an evaluation of the use of Re as a chemical analog of Tc, as well as a summary of the redox potential, thermodynamics, sorption, colloidal behavior, and interaction of humic substances with Tc, and the potential for re-oxidation and remobilization of Tc(IV). What emerges is a more complicated picture of Tc behavior than that of an easily tractable transition of Tc(VII) to Tc(IV) with consequent immobilization. Reducing conditions (+200 to +100 mV Eh) are generally thought necessary to cause reduction of Tc(VII) to Tc(IV), but far more important are the presence of reducing agents, such as Fe(II) sorbed onto mineral grains. Catalysis of Tc(VII) by surface-mediated Fe(II) will bring the mobile Tc(VII) species to a lower oxidation state and will form the relatively insoluble Tc(IV)O2∙nH2O, but even as a solid, equilibrium concentrations of aqueous Tc are nearly a factor of 20× above the EPA set drinking water standards. However, sequestration of Tc(IV) into Fe(III)-bearing phases, such as goethite or other hydrous oxyhydroxides of iron, may ameliorate concerns over the mobility of Tc. Further, the outcome of many studies on terrestrial and marine sediments that are oxidizing overall indicate that Tc is relatively immobile, due to formation of oxygen-depleted microenvironments that develop in response to bacteriological activities. The rate of re-mobilization of Tc from these microenvironments is just beginning to be assessed, but with no firm consensus. Reassessment of the simple models in which Tc is mobilized and immobilized is therefore urged

Waste Form Release Data Package for the 2001 Immobilized Low-Activity Waste Performance Assessment

This data package documents the experimentally derived input data on the representative waste glasses LAWABP1 and HLP-31 that will be used for simulations of the immobilized lowactivity waste disposal system with the Subsurface Transport Over Reactive Multiphases (STORM) code. The STORM code will be used to provide the near-field radionuclide release source term for a performance assessment to be issued in March of 2001. Documented in this data package are data related to 1) kinetic rate law parameters for glass dissolution, 2) alkali-H ion exchange rate, 3) chemical reaction network of secondary phases that form in accelerated weathering tests, and 4) thermodynamic equilibrium constants assigned to these secondary phases. The kinetic rate law and Na+-H+ ion exchange rate were determined from single-pass flow-through experiments. Pressurized unsaturated flow and vapor hydration experiments were used for accelerated weathering or aging of the glasses. The majority of the thermodynamic data were extracted from the thermodynamic database package shipped with the geochemical code EQ3/6. However, several secondary reaction products identified from laboratory tests with prototypical LAW glasses were not included in this database, nor are the thermodynamic data available in the open literature. One of these phases, herschelite, was determined to have a potentially significant impact on the release calculations and so a solubility product was estimated using a polymer structure model developed for zeolites. Although this data package is relatively complete, final selection of ILAW glass compositions has not been done by the waste treatment plant contractor. Consequently, revisions to this data package to address new ILAW glass formulations are to be regularly expected

The Status of Radiation Damage Experiments

Experiments have been on-going for about two years to determine the effects that radiation damage have on the physical and chemical properties of candidate titanate ceramics for the immobilization of plutonium. We summarize the results of these experiments in this document

Immobilization and Limited Reoxidation of Technetium-99 by Fe(II)-Goethite

This report summarizes the methodology used to test the sequestration of technetium-99 present in both deionized water and simulated Hanford Tank Waste Treatment and Immobilization Plant waste solutions

Characterization of Direct-Push Vadose Zone Sediments from the 241-B and 241-BX Tank Farms

Geochemical tests provide evidence for the transit of a plume of caustic waste solution through the sediment column at the Hanford 241-B and -BX Tank Farms. Direct-push samples recovered from boreholes surrounding Tanks 241-B-110 and 241-BX-102 and related waste transfer lines and diversion boxes included sediments typical of those previously recovered from other localities on the Hanford Site. The Hanford formation sediments are dominantly quartzo-feldspathic sands strewn with lithic fragments, displaying a range of particle size distributions and sorting characteristics. Some moderately well-sorted, fine-grained lithologies are interpreted as lenticular bodies irregularly dispersed in coarser-grained, more poorly sorted sediments. Tier I tests conducted on the vadose zone sediments revealed an inverse correlation between moisture content and sediment size fraction (i.e., there is greater moisture content in finer-grained sediments). The Tier I tests also showed that the pore water solutions were likely sodium-rich, moderately saline, and possessed higher pH values than background (untainted) sediments. These data are characteristic of sediments that have encountered sodium-rich, saline, caustic waste solution, as documented in other reports at other suspect contamination sites around Hanford. Analyses of solutions from 1:1 water extracts reveal relatively balanced cation and anion concentrations, indicating that most of the geochemical species have been accounted for. The water extract data for affected sediments also indicate unusually high concentrations of aluminum, iron, and phosphorus. The relatively high concentrations of aluminum and iron may be the result of dissolution of secondary amorphous phases that precipitated after a reactive plume partially dissolved aluminum- and iron-bearing phases as it migrated through the sediment column. On the other hand, the presence of elevated concentrations of phosphorous may be the tell-tale signature of wastes derived from the bismuth phosphate separation process. Elements typically mobile in the geosphere, such as technetium-99, are present at either low concentrations or are below the analytical detection limit. However, we expect that the mobile elements would be present mainly along a narrow plume front, and if this front had passed deeper into the sediment profile than depths sampled, the retention of these elements would be minor. On the other hand for the direct push sediments from around BX Tank Farm, uranium-238 was detected in nearly all sediment specimens (by acid extract experiments) at concentrations above the natural crustal average (0.763 pCi/g), and we also detected the presence of several anthropogenic radioisotopes, such as cobalt-60, cesium-137, europium-154, and europium-155 (by gamma energy analysis). These data are direct confirmation of contamination of the sediments

Quantifying silica reactivity in subsurface environments: Controls of reaction affinity and solute matrix. 1998 annual progress report

'The authors goal is to develop a quantitative and mechanistic understanding of amorphous silica, SiO{sub 2} (am), dissolution kinetics in aqueous solutions. A knowledge of fundamental controls on the reactivity of simple SiO{sub 2} bonded phases is the compositional baseline for understanding highly complex silica phases. In the Earth, silicate minerals comprise >70% of the crust and dominate virtually every subsurface system. More importantly for the objectives of this EMSP project, silicate minerals and materials are significant because compositionally complex silicate glasses will become the front line of defense in containing radioactive wastes in the nation''s long term and interim storage strategies (Dove and Icenhower, 1997). To date, the behavior of SiO{sub 2} (am) is largely inferred from studies of the better known crystalline polymorphs (e.g. a-quartz). In the first step towards constructing a general model for amorphous silica reactivity in the complex fluid compositions of natural waters, the authors are determining the dissolution behavior as a function of temperature, solution pH and NaCl concentration. With these data they are determining relationships between SiO{sub 2} glass structure and dissolution rates in aqueous solutions, as described below. This report outlines the first year''s progress and the resulting publications to date. In this experimental investigation, the dissolution kinetics of SiO{sub 2} (am) (fused and flame pyrolysis silica) were measured in solutions over the pH range of 4 to 10 containing 0.0 (deionized water, DIW) to 0.15 M NaCl at 40 to 275 C. Dissolution rates were determined in low temperature (40 to 80 C) and hydrothermal (120 to 275 C) reactor systems, using flow-through reactors that are broadly similar in design. Rate data collected from these two reactor designs are consistent with each other and yield the first comprehensive model of amorphous silica reactivity in deionized water and electrolyte solutions (Icenhower and Dove, 1998). Measurements of rates show important similarities and differences between the corrosion behavior of SiO{sub 2} (am) and a-quartz. They find that the experimental energy of activation, E a,xp , for the dissolution of SiO{sub 2} (am) is 75 \261 5 kJ mol -1 in DIW. The introduction of up to 0.05 M NaCl yields similar E a,xp values of 80 \261 5 kJ mol -1 . These values are \30510 kJ mol -1 higher than previous estimates of E a,xp for SiO{sub 2} (am) but are consistent with reported values of E a,xp for a-quartz. Dissolution rates measured at 200 C in DIW show that SiO{sub 2} (am) dissolves \3053 to 30X faster than a-quartz. A possible explanation for this difference is that SiO 2 (am) has a fraction of Si-O-Si bonds (angles up to 180\260) that have a greater ionic character, and are therefore more reactive than a-quartz constituents (mean angle of 152\260 ) (Icenhower and Dove, in prep.). Measurements of SiO{sub 2} (am) dissolution rates versus NaCl concentrations at 200 C show that sodium enhances rates by a factor of \30510 to 30X compared to rates measured in DIW, which are less than rates for a-quartz under identical experimental conditions. In addition, they find that the dissolution rates of the two forms of SiO{sub 2} (am) (fused and flame pyrolysis silica) are similar within the experimental error of the early experiments. Results of this study suggest that the role of physical (structural) properties (e.g., Si-O-Si bonds) in governing reactivities of crystalline versus amorphous SiO{sub 2} polymorphs is significant.

Origins of Deviations from Transition-State Theory: Formulating a New Kinetic Rate Law for Dissolution of Silicates

Present models for dissolution of silicate minerals and glasses, based on transition-state theory (TST), overestimate the reaction rate as solution compositions approach saturation with respect to the rate-governing solid. Therefore, the reactivity of key materials in the environment, such as feldspar, mica, and borosilicate glass, is uncertain, and any prediction of future aqueous durability is suspect. The core objective of this investigation is to determine the origin of these discrepancies and to fashion a quantitative model that reliably predicts the reactivity of silicate materials in realistic environmental conditions. This is being accomplished using newly developed experimental techniques checked against computer simulations based upon first principle theory

The Geochemistry of Technetium: A Summary of the Behavior of an Artificial Element in the Natural Environment

Interest in the chemistry of technetium has only increased since its discovery in 1937, mainly because of the large and growing inventory of 99Tc generated during fission of 235U, its environmental mobility in oxidizing conditions, and its potential radiotoxicity. For every ton of enriched uranium fuel (3% 235U) that is consumed at a typical burn-up rate, nearly 1 kg of 99Tc is generated. Thus, the mass of 99Tc produced since 1993 has nearly quadrupled, and will likely to continue to increase if more emphasis is placed on nuclear power to slow the accumulation of atmospheric greenhouse gases. In order to gain a comprehensive understanding of the interaction of 99Tc and the natural environment, we review the sources of 99Tc in the nuclear fuel cycle, its chemical properties, radiochemistry, and biogeochemical behavior. We include an evaluation of the use of Re as a chemical analog of Tc, as well as a summary of the redox potential, thermodynamics, sorption, colloidal behavior, and interaction of humic substances with Tc, and the potential for re-oxidation and remobilization of Tc(IV). What emerges is a more complicated picture of Tc behavior than that of an easily tractable transition of Tc(VII) to Tc(IV) with consequent immobilization. Reducing conditions (+200 to +100 mV Eh) are generally thought necessary to cause reduction of Tc(VII) to Tc(IV), but far more important are the presence of reducing agents, such as Fe(II) sorbed onto mineral grains. Catalysis of Tc(VII) by surface-mediated Fe(II) will bring the mobile Tc(VII) species to a lower oxidation state and will form the relatively insoluble Tc(IV)O2∙nH2O, but even as a solid, equilibrium concentrations of aqueous Tc are nearly a factor of 20× above the EPA set drinking water standards. However, sequestration of Tc(IV) into Fe(III)-bearing phases, such as goethite or other hydrous oxyhydroxides of iron, may ameliorate concerns over the mobility of Tc. Further, the outcome of many studies on terrestrial and marine sediments that are oxidizing overall indicate that Tc is relatively immobile, due to formation of oxygen-depleted microenvironments that develop in response to bacteriological activities. The rate of re-mobilization of Tc from these microenvironments is just beginning to be assessed, but with no firm consensus. Reassessment of the simple models in which Tc is mobilized and immobilized is therefore urged