Volker Neck

Solubility and hydrolysis of tetravalent actinides

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)n4-n are estimated by applying a semi-empirical electrostatic model and an empirical correlation with the known constants of other actinide ions. Based on the known and estimated hydrolysis constants, the solubility products of An(OH)4(am) or AnO2 × xH2O(am) are calculated from experimental solubility data available in the literature. The SIT is used for ionic strength corrections. 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)4(aq) are the predominant aqueous species, the solubilities of AnO2(cr) become equal to those of the amorphous solids. This indicates that the crystalline dioxides are covered by amorphous hydroxide layers.

Aquatic chemistry and solubility phenomena of actinide oxides/hydroxides

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.

Solubility and redox reactions of Pu(IV) hydrous oxide : Evidence for the formation of PuO2+x(s, hyd)

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 PuO2+x (s,hyd), mixed valent (PuV)2x (PuIV)1-2x O2+x(s,hyd) or (PuO2.5)2x (PuO2)1-2x (s,hyd) with x = 0.003 (present study) and x = 0.05–0.06 (literature studies under air), is the solubility controlling solid phase. It can be formed by the oxidation of PuO2(s,hyd) with the oxygen in the system and by co-precipitation of Pu(V) and Pu(IV). The Pu4+ concentration is given by the known solubility product of Pu(IV) hydrous oxide and the PuO2+ concentration is described by the solubility product (Ksp = [PuO2+][OH-]) for the fraction of Pu(V) in PuO2+x(s,hyd): logK°sp = −14.0±0.8 at zero ionic strength. Small Pu(IV) colloids/polymers present in neutral to alkaline solutions at a constant level of log[Pu(IV)]coll = −8.3±1.0 play an important role for the redox potentials in these systems. Similar Pu(V) concentrations and redox potentials were reached from oversaturation in initially Pu(VI) solutions.

Solubility of amorphous Th(IV) hydroxide - application of LIBD to determine the solubility product and EXAFS for aqueous speciation

Summary The 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 1.2×10−2−1.0×10−5 M thorium solutions in the pH range of 2.7–4.5. The accurate solubility limit determined by this method is comparable with data measured from undersaturation with an X-ray amorphous solid precipitated at higher pH and dried at room temperature. Based on hydrolysis constants selected from the literature, the solubility product of Th(OH)4(am) in 0.5 M NaCl is calculated to be log K′sp = −44.48 ± 0.24 and log K°sp = −47.8 ± 0.3 (converted to I=0 with the SIT coefficients of the NEA-TDB). In 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+ and thorium concentrations are prepared by careful coulometric titration and examined by ultrafiltration, LIBD and X-ray absorption fine structure (XAFS) spectroscopy. These measurements demonstrate the presence of a large amount of small Th(IV) colloids. The ThL3 edge EXAFS spectra of these colloidal suspensions are similar to that of the amorphous solid.

Solubility, Hydrolysis and Colloid Formation of Plutonium(IV)

The solubility and hydrolysis behaviour of Pu(IV) is critically reviewed taking into account the effect of colloid formation. Literature data on mononuclear Pu(IV) hydrolysis constants, determined from a solvent extraction study with Pu(IV) trace concentrations, are used to calculate the solubility product of amorphous Pu(OH)4(am) from the available experimental solubility data. Applying the SIT approach, the thermodynamic constant is calculated to be log K°5p = -58.7 ±0.9. The evaluated solubility product agrees well with the literature value determined recently by an indirect method, independent of Pu(IV) hydrolysis reactions. The generation of Pu(IV) colloids is investigated by chemical reduction of Pu(VI) as a function of the Pu concentration in 0.1 M HCIO4. Ultrafiltration and Laser induced breakdown detection (LIBD) measurements demonstrate that colloid formation is the predominant reaction, when the solutions are oversaturated with respect to the thermodynamic solubility of Pu(0H)4(am). In undersaturated solutions, the contribution of Pu(IV) colloids is negligible. The thermodynamic calculation of the solubility is consistent with the present results on colloid formation.

Solid-liquid equilibria of Mg(OH)2(cr) and Mg2(OH)3Cl.4H2O(cr) in the system Mg-Na-H-OH-Cl-H2O at 25°C

Abstract The solubility of crystalline Mg(OH)2(cr) was determined by measuring the equilibrium H+ concentration in water, 0.01–2.7 m MgCl2, 0.1–5.6 m NaCl, and in mixtures of 0.5 and 5.0 m NaCl containing 0.01–0.05 m MgCl2. In MgCl2 solutions above 2 molal, magnesium hydroxide converted into hydrated magnesium oxychloride. The solid-liquid equilibrium of Mg2(OH)3Cl·4H2O(cr) was studied in 2.1–5.2 m MgCl2. Using known ion interaction Pitzer coefficients for the system Mg-Na-H-OH-Cl-H2O (25°C), the following equilibrium constants at I = 0 are calculated: Mg(OH) 2 (cr) + 2 H + ⇔ Mg 2+ + 2 H 2 O log K° s = 17.1 ± 0.2 Mg 2 (OH) 3 Cl·4H 2 O(cr) + 3 H + ⇔ 2 Mg 2+ + Cl − + 7 H 2 O log K° s = 26.0 ± 0.2 The experimental results are discussed with regard to discrepancies in frequently used databases and computer codes for geochemical modeling, such as EQ3/6, Geochemist’s Workbench and CHESS.

Oxidation State and Local Structure of Plutonium Reacted with Magnetite, Mackinawite, and Chukanovite

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.

Application of LIBD to the determination of the solubility product of thorium(IV)-colloids

A new experimental method is presented for the determination of solubility data, which is based on the laser-induced breakdown detection (LIBD). The method is capable of monitoring the initial colloid generation when the metal ion concentration reaches or just exceeds the solubility at given pH. The application is made to determine the solubility of Th(IV) in acidic solutions at I = 0.5 M (NaCl) and 25 °C. The initial colloid formation is determined as a function the H+ concentration in a series of 2.8 × 10-2 - 8.9 × 10-5 M thorium solutions. The conditional solubility product (log K´sp = -49.54 ± 0.22) obtained in this study corresponds to an equilibrium between solution and colloidal thorium dioxide particles. The solubility product at I = 0 (log K°sp = -52.8 ± 0.3) is calculated with the SIT coefficients of the NEA-TDB. It corresponds to the known value for crystalline ThO2(cr), in particular if the small particle size of about 20 nm is taken into account. The present results indicate that the high thorium solubilities measured in the previous studies for amorphous Th(IV) hydroxide or hydrous oxide are primarily caused by the inclusion of polynuclear species or Th(IV) colloids of very small size.

Solubility of crystalline thorium dioxide

Summary The solubility of thorium oxides of different crystallinity is investigated at 25°C by different experimental approaches. The dissolution of bulk crystalline ThO2(cr) is a very slow process and the Th(IV) concentrations measured after one year at pH 1-3 in 0.1 and 0.5M HCl-NaCl solutions do not represent equilibrium data. Coulometric titration of thorium nitrate solutions in the low pH range of 1.5-2.5 leads to the formation of microcrystalline ThO2·xH2O(mcr) particles which subsequently agglomerate to a precipitate. The solubility of this solid, in equilibrium with Th4+(aq), is measured from the oversaturation direction. The solubility product is determined to be log K´sp=-49.9±0.4 in 0.5M NaCl corresponding to log K°sp=-53.2±0.4 (converted to I=0 with the SIT). It is close to the thermochemical value for ThO2(cr) and about 6 orders of magnitude lower than that of X-ray amorphous Th(IV) hydroxide or hydrous oxide. The differences in the solubility products are discussed with regard to the particle size and compared with analogous data for U(IV), Np(IV) and Pu(IV). Above the threshold of hydrolysis of Th4+ at pH>2.5, the dissolution of microcrystalline ThO2·xH2O(mcr) is found to be irreversible. In near-neutral to alkaline solutions, the measured thorium concentrations approach those of amorphous Th(OH)4(am). Similar results are obtained with crystalline ThO2(cr) in 0.5M NaCl-NaOH solutions. The solubility is not controlled by the bulk crystalline solid but by amorphous fractions on the surface.

Hydrolysis of Plutonium(IV) in Acidic Solutions - No Effect of Hydrolysis on Absorption-spectra of Mononuclear Hydroxide Complexes

Tetravalent plutonium readily undergoes hydrolysis even in highly acidic aqueous solutions. In the past, many attempts were made to quantify hydrolysis species by means of optical absorption spectroscopy. In the present work solutions ranging from 10−5 M to 10-2 M (total Pu) concentration in 0.5 M HCl/NaCl (0.3 < pHc < 2.1) are carefully investigated by combining absorption-spectroscopy (UV-Vis, liquid core waveguide capillary) and laser-induced breakdown detection, with special emphasis on the limited solubility of Pu(IV). The results clearly indicate that all changes in the absorption spectra originate from the formation of Pu-polyspecies and colloids. The molar absorptivity of mononuclear Pu(IV) hydroxide complexes does not vary with increasing pHc and ongoing hydrolysis. The normalized absorption spectra of at least the first and the second hydroxide complex (Pu(OH)n4- nn = 1, 2) do not differ from those of the hydrated Pu4+ ion.