Uranium thermodynamic parameters

Table 7-3. Thermodynamic Parameters of Atmospheric-Pressure Gas-Phase Reactions Characterizing Step-by-Step Hydrogen Reduction of Uranium Hexafluoride to Uranium... 

Thermodynamic Parameters Thermodynamic parameters for the hydrolysis species of uranium(IV) and its oxide phases can be determined from the stability... 

Thermodynamic Parameters There are a number of uranium(VI) oxide/hydroxide solid phases. The phases include anhydrous uranium oxide (a-UOjfs), -UOjfs), y-U03(s), 5-U03(s) and e-U03(s)) and hydrated uranium oxide (U03-a H20, where x=2.25 (schoepite (U02)40(0H)5-6H20), x = 2 (metaschoepite (U02)40(OH)6 5H20), x=0.75-1 (dehydrated schoepite and when a =l being equivalent to uranium hydroxide (a-U02(0H)2, -U02(0H)2 and y-1102(014)2)), x =0.648 and. = 0.393). Two uranium(VI) peroxide phases have also been identified U04-4H20 and U04-2H20. 

Thermodynamic parameters for the hydrolysis species of uranium(VI) and its oxide and hydroxide phases can be determined from the stability and solubility constants accepted in the present review together with the thermodynamic data for the oxide or hydroxide phases selected by either Grenthe et al. (1992) or GuUlaumont et al. (2003). The obtained thermodynamic values are listed in Table 9.7. 

The process we have followed Is Identical with the one we used previously for the uranium/oxygen (U/0) system (1-2) and Is summarized by the procedure that Is shown In Figure 1. Thermodynamic functions for the gas-phase molecules were obtained previously (3) from experimental spectroscopic data and estimates of molecular parameters. The functions for the condensed phase have been calculated from an assessment of the available data, Including the heat capacity as a function of temperature (4). The oxygen potential Is found from extension Into the liquid phase of a model that was derived for the solid phase. Thus, we have all the Information needed to apply the procedure outlined In Figure 1. 

Samples from the site contained considerable amounts of freshly precipitated iron hydroxides. Their transformation into thermodynamically more stable minerals such as goethite or hematite has a very slow kinetics, thus ferrihydrite was chosen as the major adsorbing surface. The Diffuse Double Layer model (Dzombak and Morel, 1990) was selected to describe surface complexation. The respective intrinsic surface parameters and the reaction constants for the ions competing with uranium(VI) for sorption sites were taken from a database mainly based on Dzombak and Morel, 1990, with the urani-um(Vl) sorption parameters as determined by Dicke and Smith, 1996. The results, based on runs with 1000 varied parameter sets, are summarized in Table 5.2. 

Metadynamics was used to probe thermodynamics of hydrolysis by calculating the first acidity constant, pKa, for all three oxidation states of aqueous uranium (U (aq), UO faq), and U02 (aq)). The collective variable employed to describe the deprotonation reactions is the coordination number of an arbitrary first-shell water oxygen atom with respect to all protons, no-H- The coordination number parameters were chosen to be k =10A and rau = 1.38A. Starting with well-equilibrated NVT AIMD simulations, standard metadynamics simulations were carried out at 300 K with H = 0.063 kcal/mol and The time between the addition of Gaussians was r = 100(it for U (aq) and t = 20dt for U02(aq) and U02 (aq), where 6t is the simulation time step. Once the free energy difference, AF, for the reaction was known, the first acid dissociation constant, pKa, was computed as... 

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