Marcus Altmaier

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.

Recent advances in aqueous actinide chemistry and thermodynamics.

Thermodynamics Marcus Altmaier,*,† Xavier Gaona,† and Thomas Fanghan̈el‡,§ †Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany ‡Institute for Transuranium Elements, Joint Research Center, European Commission, P.O. Box 2340, 76125 Karlsruhe, Germany Institute of Physical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

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.

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.

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.

Solubility and colloid formation of Th(IV) in concentrated NaCl and MgCl2 solution

Summary The solubility of crystalline ThO2(cr) and amorphous hydrated Th(IV) oxyhydroxide ThOn(OH)4-2n·xH2O(am) has been measured in dilute to concentrated NaCl and MgCl2 solutions equilibrated with magnesium hydroxide or hydroxychloride at 22±2 °C. The contributions of colloids to the total thorium concentrations observed in both over- and undersaturation experiments with amorphous Th(IV) precipitates have been analysed by ultracentrifugation. The solubility increasing effect of long-time stable Th(IV) eigencolloids, previously investigated in 0.5 M NaCl solutions, is also observed in concentrated 5 M NaCl. Ionic strength and chloride concentration have no effect on the stability of these hydrophilic Th(IV) oxyhydroxide eigencolloids, which are the predominant species in solution. They cause relatively high total thorium concentration in neutral to alkaline steady state solutions, independent of ionic strength: log[Th]tot≈log[Th]coll=-6.3±0.5. In concentrated MgCl2 solutions saturated with magnesium hydroxychloride colloids, the formation of pseudocolloids, i.e., Th(IV) sorbed onto Mg2(OH)3Cl·4H2O(coll), leads to a further increase of the total thorium concentration up to 10-5 M. The present results are discussed with regard to maximum Th(IV) and Pu(IV) concentrations in performance assessment calculations.

New Insights in the Formation Processes of Pu(IV) Colloids

Abstract The high tendency of tetravalent plutonium to form polymeric complexes and colloids is well known but the exact processes underlying their formation are still controversially discussed. In the present work, the nucleation of small polynuclear hydroxide complexes, i.e. , ionic species containing more than one Pu ion, their aggregation and formation of larger colloids (polymers exceeding some 5 nm in size) and finally ripening processes of freshly formed amorphous Pu(IV) colloids towards more crystalline particles are investigated by use of a combination of various spectroscopic techniques. By electrospray mass-spectrometry small polymers such as dimers, trimers and tetramers containing mixed oxidation states of Pu were observed. These polymers might be responsible for the equilibration between the Pu(III)/Pu(IV) and the plutonyl species Pu(V)/Pu(VI) even in dilute solutions in the absence of colloids or precipitates.

Solubility of ThO2 . xH2O(am) in carbonate solution and the formation of ternary Th(IV) hydroxide-carbonate complexes

Abstract The solubility of X-ray amorphous Th(IV) hydroxide or hydrous oxide was determined in carbonate solution at I=0.5 M (NaHCO3-Na2CO3-NaOH-NaCl) and 22 °C. Two series of open system experiments were performed under CO2 partial pressures of 1.0 and 0.1 bar at -log [H+]=4.5– 7.5. In three series of closed system experiments at constant total carbonate concentrations of Ctot=[HCO3-]+[CO32-]=0.1, 0.04 and 0.015 M, the H+ concentration was varied in the range -log [H+] = 8.5–13.5. Some additional solubility data were determined in 0.25–2 M Na2CO3 containing 0.1 M NaOH. There was no indication for the formation of a thorium carbonate solid. The simultaneous evaluation of the different sets of experimental data at I=0.5 M shows that Th(OH)(CO3)45- and Th(OH)2(CO3)22- are the most important ternary complexes. Further contributions to the Th(IV) solubility are coming from Th(OH)2(CO3)(aq), Th(OH)3(CO3)- and Th(OH)4(CO3)2-. Their formation constants, extrapolated to I=0 with the SIT and combined with the solubility product of log K°sp=-47.8±0.3, are calculated to be logβ°114=35.8±0.3, logβ°122=37.0±0.4, logβ°121=30.7±0.4, logβ°131=38.5±0.6 and logβ°141=40.6±0.5. Pure carbonate complexes and other ternary complexes have no significant contributions to the solubility in the present studies at I=0.5 M. Upper limits are derived for their formation constants. Using the SIT for ionic strength corrections, the evaluated set of equilibrium constants is also consistent with the solubility in Na2CO3-NaOH mixtures of higher ionic strength.

Solubility of Zr(IV), Th(IV) and Pu(IV) Hydrous Oxides in CaCl2 Solutions and the Formation of Ternary Ca-M(IV)-OH Complexes

Abstract The solubility of Zr(IV), Th(IV) and Pu(IV) hydrous oxides is investigated at 22±2 °C in alkaline 0.1−4.5 M CaCl2 solutions. Further studies are performed with Zr(IV) over the entire pH range in NaCl and CaCl2 media, and with Zr(IV) and Th(IV) in alkaline Ca(ClO4)2 solutions. The comparison of Zr(IV) data in different ionic media (NaCl, NaClO4, CaCl2 and Ca(ClO4)2) of similar ionic strength shows that the solubility in the acidic and neutral pH range is not affected by strong interactions between the aqueous M(IV) species and the medium ions. However, in alkaline CaCl2 and Ca(ClO4)2 solutions the formation of ternary Ca-M(IV)-OH complexes causes unexpectedly high solubilities of Zr(IV) at pHc=10−12 and [Ca2+]>0.05 M and of Th(IV) at pHc=11−12 and [Ca2+]>0.5 M. The dependence of the Zr(IV) and Th(IV) solubilities on the H+ and CaCl2 concentrations shows that the complexes Zr(OH)62− and Th(OH)84− with an unusual large number of OH− ligands are stabilized by the formation of associates or ion pairs with Ca2+ ions. The SIT is used to derive equilibrium constants at zero ionic strength for the complexes Zr(OH)62− (in calcium-free solutions), Ca2[Zr(OH)6]2+, Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+. In analogous studies with Pu(IV) hydrous oxide, the solubility increasing effect of ternary complex formation with Ca2+ ions is only observed at CaCl2 concentrations above 2 M.