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[en] Protonation and complexation of α-isosaccharinic acid with U(VI) and Fe(III) have been studied in acidic solutions at t=25 deg C and j=1.0 mol dm-3 NaCIO4. From the potentiometric titrations, the protonation constant of the carboxylate group is calculated to be 3.65 +- 0.05 and the data are consistent with the presence of three and four succesive mononuclear complexes for U(VI) and Fe(III), respectively. The formation constants of the complexes, log #Beta#j for the reactions of M+L=MLj where j=1-3 for U(VI), j=1-4 for Fe(III) and L stands for isosaccharinate, are determined to be 2.91 +- 0.15 (UO2L), 5.37 +- ).07 (UO2L2), 7.25 +- 0.18 (UO2L3), 5.06 +- 0.17 (FeL), 8.51 +- 0.15 (FeL2), 11.00 +- 0.16 (FeL3) anda 12.99 +- 0.17 (FeL4). From the calorimetric titrations, the enthalpy of protonation of the carboxylate group is determined to be (-7.94 +- 0.03)kJ mol -1, similar to that of other α-hydroxycaroxylates. The enthalpies of complexation betwen U(VI) and isosaccharinate are quite small: ΔH1 = -(1.0 +- 1.0)kJ mol-1, ΔH2 =1.4 +- 1.8 kJ mol-1 and ΔH3=-(6.2 +- 3.0)kJ mol-1, typical of the interactions between carboxylates and hard-acid cations. The complexation between U(VI) and isosaccharinate is mainly entropy-driven. In comparison, the enthalpies of complexation for FeL3 and FeL4 are large and exothermic, contributing significantly to the stability of the complexes
[en] Obtaining reliable thermodynamic data for Pu(V) is difficult because of its redox and/or disproportionation reactions in most aqueous systems. The known stability of Pu(V) in PuO2(am) suspensions in slightly acidic to near neutral conditions was used to study the solubility of PuO2(am) in 0.4 and 4.0 M NaCl or NaClO4 solutions ranging in pcH#sup +# values from 4 to 9 as a function of time. The close agreement between the observed solubility and the predicted solubility using Pitzer ion-interaction parameters of Np(V) with Cl- or ClO4- for Pu(V) indicates that Pu(V), as expected, behaves in an analogous fashion to Np(V) and confirms the value of using Np(V) data to model Pu(V) behavior
[en] Isosaccharinate (ISA-) is expected to be one of the important ligands in low-level nuclear wastes. Comprehensive thermodynamic data for complexation reactions of ISA- with any of the tetravalent actinides have not been available
[en] The sorption of selenite, SeO32−, by carbonate substituted hydroxylapatite was investigated using batch kinetic and equilibrium experiments. The carbonate substituted hydroxylapatite was prepared by a precipitation method and characterized by SEM, XRD, FT-IR, TGA, BET and solubility measurements. The material is poorly crystalline, contains approximately 9.4% carbonate by weight and has a surface area of 210.2 m2/g. Uptake of selenite by the carbonated hydroxylapatite was approximately an order of magnitude higher than the uptake by uncarbonated hydroxylapatite reported in the literature. Distribution coefficients, Kd, determined for the carbonated apatite in this work ranged from approximately 4200 to over 14,000 L/kg. A comparison of the results from kinetic experiments performed in this work and literature kinetic data indicates the carbonated apatite synthesized in this study sorbed selenite 23 times faster than uncarbonated hydroxylapatite based on values normalized to the surface area of each material. The results indicate carbonated apatite is a potential candidate for use as a sorbent for pump-and-treat technologies, soil amendments or for use in permeable reactive barriers for the remediation of selenium contaminated sediments and groundwaters. - The sorption of selenite by carbonated apatite including distribution coefficeints and kinetic data are reported.
[en] In the S-I cycle, iodine and sulfur dioxide are combined with water to create two immiscible acid phases. The phases are separated in the presence of excess iodine. The sulfuric acid phase is decomposed at temperatures near 850degC, and the resulting sulfur dioxide is recycled back into the process. The hydriodic acid in the lower phase is separated from water and iodine, and is then decomposed into the product hydrogen and iodine. The water and iodine from these steps are recycled. In an International Nuclear Energy Research Initiative (INERI) project supported by the US DOE Office of Nuclear Energy, Sandia National Labs (SNL) has teamed with the Commissariat a l'Energie Atomique (CEA) in France, and industrial partner General Atomics (GA) to construct and operate a closed-loop device for demonstration of hydrogen production by the S-I process. Previous work in Japan has demonstrated continuous closed-loop operation of the S-I cycle for up to one week using glass components at atmospheric pressure. This work aims for operation under process conditions expected at the pilot plant-level and beyond - pressures up to 20 bar using engineering materials of construction. Staff at CEA is responsible for the acid-generation step known as the Bunsen reaction. They have built a transportable skid-based apparatus with equipment for generating and separating the two acid phases. SNL is handling the sulfuric acid decomposition section, and likewise have prepared a portable device for execution of this process step. GA is providing equipment for the separation of hydriodic acid from iodine and water and decomposing it into the product hydrogen. The completed integrated device is scaled to produce a minimum of 100 standard liters per hour of hydrogen. This paper will summarize project goals, work done to date, current status, and scheduled future work on the INERI S-I Integrated-Loop Experiment. (author)
[en] We have performed an initial evaluation and testing program to assess the effectiveness of a hydroxyapatite (Ca10(PO4)6(OH)2) permeable reactive barrier and source area treatment to decrease uranium mobility at the Department of Energy (DOE) former Old Rifle uranium mill processing site in Rifle, western Colorado. Uranium ore was processed at the site from the 1940s to the 1970s. The mill facilities at the site as well as the uranium mill tailings previously stored there have all been removed. Groundwater in the alluvial aquifer beneath the site still contains elevated concentrations of uranium, and is currently used for field tests to study uranium behavior in groundwater and investigate potential uranium remediation technologies. The technology investigated in this work is based on in situ formation of apatite in sediment to create a subsurface apatite PRB and also for source area treatment. The process is based on injecting a solution containing calcium citrate and sodium into the subsurface for constructing the PRB within the uranium plume. As the indigenous sediment micro-organisms biodegrade the injected citrate, the calcium is released and reacts with the phosphate to form hydroxyapatite (precipitate). This paper reports on proof-of-principle column tests with Old Rifle sediment and synthetic groundwater.
[en] The Old Rifle Site is a former vanadium and uranium ore-processing facility located adjacent to the Colorado River and approximately 0.3 miles east of the city of Rifle, CO. The former processing facilities have been removed and the site uranium mill tailings are interned at a disposal cell north of the city of Rifle. However, some low level remnant uranium contamination still exists at the Old Rifle site. In 2002, the United States Nuclear Regulatory Commission (US NRC) concurred with United States Department of Energy (US DOE) on a groundwater compliance strategy of natural flushing with institutional controls to decrease contaminant concentrations in the aquifer. In addition to active monitoring of contaminant concentrations, the site is also used for DOE Legacy Management (LM) and other DOE-funded small-scale field tests of remediation technologies. The purpose of this laboratory scale study was to evaluate the effectiveness of a hydroxyapatite (Ca10(PO4)6(OH)2) permeable reactive barrier and source area treatment in Old Rifle sediments. Phosphate treatment impact was evaluated by comparing uranium leaching and surface phase changes in untreated to PO4-treated sediments. The impact of the amount of phosphate precipitation in the sediment on uranium mobility was evaluated with three different phosphate loadings. A range of flow velocity and uranium concentration conditions (i.e., uranium flux through the phosphate-treated sediment) was also evaluated to quantify the uranium uptake mass and rate by the phosphate precipitate.