ABSTRACT. Density Functional Theory and small-core, relativistic pseudopotentials were used to look for symmetric and asymmetric transitions states of the gas-phase hydrolysis reaction of uranium hexafluoride, UF 6 , with water. At the B3LYP/6-31G(d,p)/SDD level, an asymmetric transition state leading to the formation of a uranium hydroxyl fluoride, U(OH)F 5 , and hydrogen fluoride was found with an energy barrier of +77.3 kJ/mol and an enthalpy of reaction of +63.0 kJ/mol (both including zero-point energy corrections). Addition of diffuse functions to all atoms except uranium led to only minor changes in the structure and relative energies of the reacting complex and transition state. However, a significant change in the product complex structure was found, significantly reducing the enthalpy of reaction to +31.9 kJ/mol. Similar structures and values were found for PBE0 and MP2 calculations with this larger basis set, supporting the B3LYP results. No symmetric transition state leading to the direct formation of uranium oxide tetrafluoride, UOF 4 , was found, indicating that the reaction under ambient conditions likely includes several more steps than the mechanisms commonly mentioned. The transition state presented here appears to be the first published transition state for the important gas-phase reaction of UF 6 with water.

Highly contaminated (with PAHs) topsoils were extracted with supercritical CO2 to determine the feasibility and mechanism of supercritical fluid extraction (SFE). Effects of supercritical fluid (SCF) density, temperature, and cosolvent type and amount and of slurrying the soil with water were investigated. Experiments were conducted at 308−343 K and 450−850 kg/m3 CO2 density. Water and methanol were used as cosolvents in some experiments, at 0.3−6 wt %. For extractions at an ∼1000 phase ratio (wt of CO2/wt of soil), near-complete contaminant removal was achieved. For an ∼200 phase ratio at 323 K and 13.8 MPa, >95% of two-ring PAHs and >80% of three-ring and heavier PAHs were extracted. Using soil slurries with water had little effect on the total recoveries. Changes in SCF density also had little influence on SFE, while temperature increases improved both initial rates of extraction and total recoveries. Addition of cosolvents generally hindered extraction of the contaminants. These results are used to show that, at typical conditions for CO2-based soil extractions, the solubility of the contaminant in the SCF does not limit SFE. Other internal resistances are important, such as desorption from and diffusion within the polymeric soil organic matter.

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The hydrolysis reaction of uranium hexafluoride (UF 6 ) is a key step in the synthesis of uranium dioxide (UO 2 ) powder for nuclear fuels. Mechanisms for the hydrolysis reactions are studied here with density f unctional theory a nd t he Stuttgart s mall-core scalar relativistic pseudopotential and associated basis set for uranium. The reaction of a single UF 6 molecule with a water molecule in the gas phase has been previously predicted to proceed over a relatively sizeable barrier of 78.2 kJ·mol -1 , indicating this reaction is only feasible at elevated temperatures.Given the observed formation of a second morphology for the UO 2 product coupled with the observations of rapid, spontaneous hydrolysis at ambient conditions, an alternate reaction pathway must exist. In the present work, two trimolecular hydrolysis mechanisms are studied with density functional theory: (1) the reaction between two UF 6 molecules and one water molecule, and (2) the reaction of two water molecules with a single UF 6 molecule. The predicted reaction of two UF 6 molecules with one water molecule displays an interesting "fluorine-shuttle" mechanism, a significant energy barrier of 69.0 kJ·mol -1 to the formation of UF 5 OH, and an enthalpy of reaction (ΔH 298 ) of +17.9 kJ·mol -1 . The reaction of a single UF 6 molecule with two water molecules displays a "proton-shuttle" mechanism, and is more favorable, having a slightly lower computed energy barrier of 58.9 kJ·mol -1 and an exothermic enthalpy of reaction (ΔH 298 ) of -13.9 kJ·mol -1 . The exothermic nature of the overall UF 6 + 2·H 2 O trimolecular reaction and the lowering of the barrier height with respect to the bimolecular reaction are encouraging; however, the sizable energy barrier indicates further study of the UF 6 hydrolysis reaction mechanism is warranted to resolve the remaining discrepancies between the predicted mechanisms and experimental observations. Lind et al.,

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The diffusion coefficient of methyl orange in dense carbon dioxide (CO2) has been measured from (87.6 to 226) bar and from (20 to 40) °C. A hydrodynamic model and the hard sphere diffusivity model have been fit to the measured data. The effect of the micelle-forming, nonionic, nonfluorous surfactant Dehypon Ls-54 was also investigated. Effective diffusivity of methyl orange in the CO2 + Ls-54 + H2O system was measured at 35 °C and 220 bar at Ls-54 concentrations up to 0.165 M. A simple volumetric-based model based on the binary diffusivity data fits the effective diffusivity well, given realistic property estimates.

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