Uranium stability constants

Considering the anion concentration ranges in natural waters (Table II) and the magnitude of the corresponding plutonium stability constants (Table III), the chemistry of plutonium, as well as that of uranium and neptunium, is almost entirely dominated by hydroxide and carbonate complexation, considering inorganic complexes only (41, 48, 49). ... 

Direct comparisons of the stability constants for formation of the dmpp complexes of molybdenum(VI) with those for uranium(VI) are not possible. The mono-ligand complexes have different stoichiometries, MoOaldmpp) versus U02(dmpp), while although log P2 is available for Mo02(dmpp)2 (175) the value of 40.2 refers to formation from Mo04 rather than from MoOg laq), and U02(dmpp)2 is too sparingly soluble for its formation constant to be determined (231). 

From Table 4, it is apparent that almost all uranium in seawater exists in the form of a highly stable tricaibonate complex U02(C03) (its stability constant is approximately 10 ). This fact and its very low concentration in seawater dictates the choice of sorbent most suitable for uranium recovery. The sorbent must be available on a large scale and at low... 

Equilibrium distributions of aqueous and solid-phase uranium at various pH and CO2 conditions were calculated by the computer code MD TEQA2 [7]. The version 3.11 of MD TEQA2 contains 63 kinds of complexation reactions of U with ligands and the stability constants of each reaction. The U species, considered in these complexation reactions are U02 and U. The ligands such as hydroxide, chloride, carbonate, fluoride, sul te, phosphate, and silicate are included. In this study, 20 kinds of complexation reactions of U were added to the code to increase the reliability of the model calculations. The stability constants in the code were also updated. New complexation reactions and stability constants were referred to by the studies of Grenthe and Bond [8,9]. In this model calculation, complexation reactions of U02 with hydroxide and carbonate ions were considered. The species of and ligands such as chloride, sul te and phosphate were not included considering our experimental conditions. 

Such experiments show that oxalate, tartrate, and citrate give fairly strong complexes, and indeed these mixtures do not suffer quite such rapid oxidation as the other systems (57, 70). Stability constants for the complexing of U(III) by acetate, 2-hydroxy-2-methylpropionate, nitriloacetate, trans-cyclohexyl-1,2-diaminotetraacetate, ethylenedi-amine tetraacetate, and diethylenetriamine pentaacetate have been reported, but no pure compounds have been isolated (71). Thiocyanate also accelerates oxidation of the uranium, but the blue complex that is formed can be extracted with triethyl phosphate, tributyl phosphate, or better, trioctyl phosphine oxide the organic extract decomposes only slowly (45, 72). 

TABLE A13.3 Stability constants and AH° values for some reactions involving uranium aqueous species and solids of geochemical interest at 25°C and 1 bar total pressure... 

The donor action of [NOs]" as an anionic ligand towards thorium(iv) and uranium(iv) in the presence of trimethylphosphine and tris(dimethylamino)phosphine oxide in aqueous media has been found to be very similar. The larger nitrate ion was observed to form more stable species with thorium than the chloride ion whereas in the uranium(iv) case both complexes formed equally readily. A study of the sulphate complexes of uranium(iv), neptunium(vi) and plutonium(vi) in HCIO4-H2SO4 solution showed the stability constants to follow the order U[Pg.453]

Hydrolysis studies of the tetravalent cations are limited to Ce and the light (Pa, Th, U, and Pu) actinides (tables 8B and 9). Only in strong acid media do the tetravalent ions exist free of hydrolysis. Similarly, only at very low concentrations are mononuclear hydroxides of significance. For Ce(I V), stability constants have been determined for bi-, ter-, tetra-, and dodeca-cerium units and for species with molecular weights as large as 40,000 (Hardwick and Robertson 1951, Danes 1967, Louwrier and Steemers 1976). X-ray measurements of thorium and uranium solutions show polynuelear complexes built from... 

Stary 2 has determined the stability constants of uranyl acetate, oxalate, tartrate, and EDTA complexes. The effect of these ions was observed on the extraction of uranium (Vl) from O.IM NaClO.2 solutions by 0.1 benzoylacetone in... 

There are no data reported in the literature that give reliable stability or solubility constants for the hydrolytic species of uranium(III) derived from experimental studies. Savenko (1998) estimated a stability constant for U(OH)3(aq) from a solubility constant provided in the literature for U(OH)3(s) (Lure, 1989). The reported solubility constant was log = 3.0, which led to a stability constant for U(OH)j(aq) of log = -32.6. There is no confirmatory evidence for these constants, and, as such, they are not retained. Moreover, given the smaller ionic radius of uranium(III) compared to that of actinium(III) (Shannon, 1976), the former ion should have a more positive log 3 and a more negative log which is not the case, providing support for the rejection of the data for uranium(III). 

Neck and Kim (2001) calculated a stability constant for U(OH)4(aq) by considering solubility constants for U02- H20 available in the literature and combining them with the limiting uranium concentration at pH values above 5. The concentrations show little variation at these pH values and represent the concentration of U(OH)4(aq) in equilibrium with the solid phase. The concentrations can be used to determine log When such data are combined with the values of log the stability constant log 4 can be derived. A similar methodology has been used in the present study, except that the datum from each study has been used separately to derive a value of log 4 and then the weighted average value of these derived values has been retained in this study. The calculated value is... 

There have been only two studies that have postulated polymeric hydrolysis species for uranium(lV). Hietanen (1956b) indicated the formation of a species with the formula U 4.i(OH)3 " " , from earlier work (Hietanen, 1956a) in 3.0 mol 1 NaClO and at 25 C, with a stability constant equal to log =-1.22-3.4m. Baes and Mesmer (1976) recalculated these data... 

Rai, Felmy and Ryan, 1990). None of these studies provide conclusive evidence of an increase in solubility of uranium dioxide in alkaline solutions. Gayer and Leider (1957) and Tremaine et al. (1981) derived stability constants for U(OH)5", but the solutions of Gayer and Leider had likely been oxidised to uranium(VI) (Neck and Kim, 2001), thereby leading to an increase in solubility. The solutions in the Tremaine et al. (1981) study were near the uranium analytical detection limit of the method utilised (Grenthe et al, 1992). Rai, Felmy and Ryan (1990) demonstrated that there was no increase in solubility of an amorphous uranium dioxide sample at least up to pH 14. As such, no stability constants are retained for U(0H)5-. 

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