The solubility and hydrolysis of Th(IV), U(IV), Np(IV), and Pu(IV) are critically reviewed and a comprehensive set of thermodynamic constants at I = 0 and 25°C is presented. The hydrolysis constants are selected preferentially from experimental studies at actinide trace concentrations, where the interference of colloid formation can be excluded. Unknown formation constants of mononuclear complexes An(OH)The solubilities of U(IV), Np(IV), and Pu(IV) hydroxides or hydrous oxides can be calculated by accounting only for mononuclear hydrolysis species. The considerably higher solubilities of amorphous Th(IV) precipitates at pH <5 include major contributions of polynuclear species. The solubility data in acidic solutions depend strongly on the preparation and crystallinity of the solid phase. In neutral and alkaline solutions, where An(OH)

The solubility and redox reactions of Pu(IV) hydrous oxide were analyzed by comparing total Pu concentrations, oxidation state distributions and simultaneously measured redox potentials under air and under Ar containing only traces of oxygen. At pH>3 the aqueous Pu concentration is dominated by Pu(V) for both solubility studies under air and argon. Combining all information strongly indicates that PuO

The hydrolysis behaviour of the NpOi ion has been investigated at 25 by solubility experiments in the pH ränge 7 to 14 in 0.1 M. 1.0 M and 3.0 M NaC104 solutions under pure Ar atmosphere. By measuring the time dependent equiiibrium concentration of Np(V), the chemical State of Np(V) hydroxide precipitatesin each Solution isconfirmed. In 0.1 M NaClO« the precipitate remains amorphous over several months, while in 1.0 M NaClOi the precipitate changes from an amorphous to a more stable aged State within a relatively short time. In 3.0 M NaCiOt the aged modification of NPO2OH is formed from the beginning. Applying specific ion interaction theory (SIT), the thermodynamic constants at 1 = 0 are found to be: log/r,p= -8.76±0.05 for amorphous Np020H(s). logA:,p=-9.44±0.10 for aged Np020H(s), log/?i=2.7±0.2 for the hydrolysis species NpO^OH and iog^2 = 4.35±0.15 for Np02(0H)2".

SummaryThe solubility of amorphous Th(IV) hydroxide at pH 3.0–13.5 and the aqueous speciation at pH < 4 are investigated in 0.5 M NaCl and 25 °C. The laser-induced breakdown detection (LIBD) is used to monitor the initial formation of thorium hydroxide colloids during the coulometric titration of 1.2×10In other solubility studies with amorphous Th(IV) hydroxide or hydrous oxide, considerably higher thorium concentrations are measured at pH 3.5–5. Therefore, solutions of comparable H

Due to their redox reactivity, surface sorption characteristics, and ubiquity as corrosion products or as minerals in natural sediments, iron(II)-bearing minerals control to a large extent the environmental fate of actinides. Pu-L(III)-edge XANES and EXAFS spectra were used to investigate reaction products of aqueous (242)Pu(III) and (242)Pu(V) reacted with magnetite, mackinawite, and chukanovite under anoxic conditions. As Pu concentrations in the liquid phase were rapidly below detection limit, oxidation state and local structure of Pu were determined for Pu associated with the solid mineral phase. Pu(V) was reduced in the presence of all three minerals. A newly identified, highly specific Pu(III)-sorption complex formed with magnetite. Solid PuO(2) phases formed in the presence of mackinawite and chukanovite; in the case of chukanovite, up to one-third of plutonium was also present as Pu(III). This highlights the necessity to consider, under reducing anoxic conditions, Pu(III) species in addition to tetravalent PuO(2) for environmental risk assessment. Our results also demonstrate the necessity to support thermodynamic calculations with spectroscopic data.

X-ray absorption fine structure (XAFS) spectroscopy at the Th L3 edge is applied for the characterization of crystalline, anhydrous ThO(2)(cr), microcrystalline ThO(2).xH(2)O(s), amorphous ThO(n)(OH)(4-2n).xH(2)O(am), aqueous Th(IV) solutions, and colloidal suspensions up to p(c)H 3.7. The microcrystalline, possibly hydrated thorium dioxide, is formed at p(c)H 1.5-2.5 by precipitation from suspensions of 16-23 nm thorium dioxide colloids. The solubility data determined for this solid is several orders of magnitude lower than the values for amorphous Th(IV) hydroxide or hydrous oxide. The EXAFS spectrum of the isolated microcrystalline particles shows that their structure is different from that of anhydrous crystalline ThO(2)(cr) and amorphous ThO(n)(OH)(4-2n).xH(2)O(am) precipitated at higher pH and dried at room temperature. The solubility measured for the amorphous Th(IV) precipitate is comparable to that previously reported for a solid prepared in a similar manner. In other solubility studies with amorphous Th(IV) hydroxide or hydrous oxide, considerably higher thorium concentrations are measured at p(c)H 3.5-5. The aqueous speciation is made by EXAFS for solutions prepared by careful coulometric titration under comparable conditions (p(c)H and thorium concentration). The spectra of these solutions demonstrate the presence of a large amount of Th(IV) polynuclear species or colloids of small size, having a highly asymmetric Th-O coordination. The EXAFS spectrum of these colloids is similar to that of the amorphous solid.

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The present paper gives an overview of the solubility behavior of actinide oxides/hydroxides, taking into account hydrolysis reactions up to colloid formation. The analogies, systematic trends, and differences in the hydrolysis and solubility constants of actinides in the oxidation states An(III) to An(VI) correlate with the charge and size of the actinide ions. The formation of amorphous and crystalline solids and the discrepancies between the corresponding experimental solubility data may be explained as an effect of particle size. However, using thermodynamic data for the stable crystalline solids, the predicted solubilities are often significantly lower than experimental data (even in long-time experiments), indicating that the solubility is controlled by the surface properties. Typical examples are the known U(VI) solids schoepite and sodium diuranate. The most striking example is provided by the tetravalent actinides. Above the threshold of hydrolysis, the dissolution of microcrystalline or crystalline An(IV) oxides is found to be irreversible. The measured concentrations approach those of the amorphous hydroxides or hydrous oxides.