A. Seibert

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.

Humic colloid-borne Np migration: influence of the oxidation state

The migration behavior of Np(IV/V) is investigated by column experiments using a sandy sediment with groundwater rich in humic substances. NIR spectroscopy, redox speciation by TTA extraction and ultrafiltration are used for the Np speciation in the groundwater prior to its introduction into the column and after elution. Np is found to be transported as humic colloid-bound species which are eluted slightly faster than the water flow velocity. The fraction of humic colloid-borne Np increases with the reduction progress of Np(V) to Np(IV), because the interaction of Np(IV) with humic substances is much stronger than Np(V). The results demonstrate the importance of the speciation of redox sensitive actinides in natural aquifers for the assessment of the humic colloid facilitated migration.

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.

The INE-Beamline for actinide science at ANKA

Since its inauguration in 2005, the INE-Beamline for actinide research at the synchrotron source ANKA (KIT North Campus) provides dedicated instrumentation for x-ray spectroscopic characterization of actinide samples and other radioactive materials. R&D work at the beamline focuses on various aspects of nuclear waste disposal within INEs mission to provide the scientific basis for assessing long-term safety of a final nuclear waste repository. The INE-Beamline is accessible for the actinide and radiochemistry community through the ANKA proposal system and the European Union Integrated Infrastructure Initiative ACTINET-I3. Experiments with activities up to 1 × 10(+6) times the European exemption limit are feasible within a safe but flexible containment concept. Measurements with monochromatic radiation are performed at photon energies varying between ~2.1 keV (P K-edge) and ~25 keV (Pd K-edge), including the lanthanide L-edges and the actinide M- and L3-edges up to Cf. The close proximity of the INE-Beamline to INE controlled area labs offers infrastructure unique in Europe for the spectroscopic and microscopic characterization of actinide samples. The modular beamline design enables sufficient flexibility to adapt sample environments and detection systems to many scientific questions. The well-established bulk techniques x-ray absorption fine structure (XAFS) spectroscopy in transmission and fluorescence mode have been augmented by advanced methods using a microfocused beam, including (confocal) XAFS/x-ray fluorescence detection and a combination of (micro-)XAFS and (micro-)x-ray diffraction. Additional instrumentation for high energy-resolution x-ray emission spectroscopy has been successfully developed and tested.

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.

The redox behaviour of plutonium in humic rich groundwater

Summary Experiments with different oxidation states of Pu in GoHy-532 groundwater under reducing conditions reveal that Pu(VI) and Pu(V) are reduced rapidly to Pu(IV) at pH 7. The half-life of the redox reactions Pu(VI)/(V) and Pu(V)/(IV) are in the range of minutes. The rates of both reduction reactions decrease with decreasing pH values. A portion of the Pu(IV) is not stable in the same groundwater and is reduced slowly to Pu(III) in the range of weeks. Ultrafiltration experiments show Pu to be totally bound to the humic substances in the groundwater. At Pu concentrations of 10-5 to 10-4 M, most of the Pu occurs as a Pu-colloid and/or Pu-colloid bound to humic substances (Pu-colloid-HS) in solution, which is indicated by EXAFS/XANES and XPS measurements.

Complexation behaviour of neptunium with humic acid

The complexation of Np(V) with humic acid (HA) was studied under environmental conditions (pH≥6, concentration of HA 1-150 mg/L) over a wide range of metal ion concentrations (10-14 to 10-4 mol/L). The experimental methods used for the determination of the complexation constants were continuous electrophoretic ion focusing, anion exchange, and ultrafiltration in combination with radiometric measurements, and, as a direct speciation method, UV/Vis-spectroscopy. The interaction between Np(V) and humic acid has been described with the charge neutralization model including a correction for different loading capacities (LC) which makes possible the comparison of results obtained at various experimental conditions. Furthermore, the degree of deprotonation has also been used to calculate the complexation constants. The complexation constants calculated for Np(V) with the LC-concept vary with the metal ion concentration in the range between 10-14 and 10-4 mol/L. At metal concentrations below 10-11 mol/L, a complexation constant logβLC=5.0±0.3 was obtained. In the concentration range 10-7 to 5×10-4 mol/L, the logβLC values decrease continuously to 3.7±0.4 (10-5-5×10-4 mol/L). The latter is in agreement with direct measurements (UV/Vis-spectroscopy) at this concentration and data from the literature.

Complexation studies of UO22+ with humic acid at low metal ion concentrations by indirect speciation methods

The complexation of uranium(VI) with humic acid was studied at pH 4 and 5 in 0.1 mol/l and 4.10-3 mol/l NaClO4 by indirect speciation methods, i.e., electrophoretic ion focusing (EIF) and anion exchange (AE). The concentration range investigated was from 10-8 mol/l to 5 · 10-6 mol/l for the uranyl ion and from 1 to 2.5 mg/l for the humic acid. The experimental data obtained by AE are well described by the complexation reaction involving the charge neutralization of the uranyl ion with two exchanging sites of humic acid. The complexation constant calculated by taking the loading capacity values into account is found to be log β = 6.08 ± 0.15. Concerning EIF, an interfering interaction between the humic acid and U(VI), occurring under the effect of the electric field, is evident and makes this separation technique not well suitable for the investigation of the U(VI):humic acid system.

Investigation of the hydrolysis of plutonium(IV) by a combination of spectroscopy and redox potential measurements

The hydrolysis of tetravalent plutonium in 0.5 M HCl/NaCl has been investigated by a combination of spectroscopy and redox potential measurements in the concentration range from 10-5 to 4×10-4 M and at pHc 0.3–2.1. Redox potentials were both measured and calculated from the plutonium oxidation state distributions using well-known equilibrium constants for the redox reactions of plutonium. The plutonium oxidation states in solution were quantified by spectroscopy. The pe values calculated from the plutonium redox couples Pu(V)/Pu(VI) and PuO2(am,hyd)/Pu(V) were found to be consistent with the measured pe values whereas those calculated from the Pu(III)/Pu(IV) couple were found to deviate, suggesting that the formation constants selected in recent reviews for the mononuclear hydroxide complexes of Pu(IV) are overestimated. Combining the redox potentials and the spectroscopically determined Pu(III) and Pu(IV) concentrations, the formation constants of the first, second, and third mononuclear hydroxide complexes Pu(OH)n4- n at I=0.5 M and 23 ± 2 °C are calculated and corrected to zero ionic strength using the specific ion interaction theory (SIT): log β°11 = 14.0 ± 0.2, logβ°12 = 26.8 ± 0.6, and log β°13 = 38.9 ± 0.9.

Plutonium(III,IV,VI) speciation in Gorleben groundwater using XAFS

Abstract Pu speciation is investigated by X-ray absorption fine structure (XAFS) at the PuL3-edge in Gorleben (GoHy) groundwater for Pu added to the groundwater in three different oxidation states (III, IV, VI). The GoHy groundwater stems from an anaerobic aquifer rich in humic substances (HS) (GoHy-532 with ∼60−70 mg/L DOC) and has a neutral pH. Analysis of XANES spectra shows that the samples contain Pu(IV), regardless if Pu(III), Pu(IV) or Pu(VI) is added to the groundwater. The GoHy-532 sample with Pu(VI) added has some Pu(V) and in addition Pu(IV). EXAFS analysis shows Pu-O bond distances to be split into a short and a longer distance. The short Pu-O distance is from bound hydroxy ligands resulting from Pu hydrolysis and/or from bonding to HS phenol groups. A Pu-Pu distance is also found at 3.82−3.85 Å, indicative of hydrolysis and polymeric species formation. Pu(IV) species formed in GoHy-532 are compared to Pu(IV) complexed with fulvic acid (FA) isolated from GoHy-532 at pHc 1.0 and 1.8. Results indicate that the hydrolysis is less pronounced in the natural groundwater samples than in the Pu(IV)-fulvates prepared from a colloid containing Pu(IV) solution. We observe the Gorleben GoHy and FA apparently stabilize small Pu(IV) hydrolysis species and inhibits precipitation of polynuclear hydrolysis products.