Christian M. Marquardt

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.

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.

X-ray absorption spectroscopic study of trivalent and tetravalent actinides in solution at varying pH values

Abstract We perform X-ray absorption spectroscopy (XAS) investigations to monitor the stabilization of redox sensitive trivalent and tetravalent actinide ions in solution at acidic conditions in a pH range from 0 to 3 after treatment with holding reductants, hydroxylamine hydrochloride (NH2OHHCl) and Rongalite (sodium hydroxymethanesulfinate, CH3NaO3S). X-ray absorption near edge structure (XANES) measurements clearly demonstrate the stability of the actinide species for several hours under the given experimental conditions. Hence, structural parameters can be accurately derived by extended X-ray absorption fine structure (EXAFS) investigations. The coordination structure of oxygen atoms belonging to water ligands surrounding the actinide ions does not change with increasing pH value (approximately 11 O atoms at 2.42 Å in the case of U(IV) at pH 1, 9 O atoms at 2.52 Å for Np(III) at pH 1.5, and 10 O atoms at 2.49 Å for Pu(III) up to pH 3), indicating that hydrolysis reactions are suppressed under the given chemical conditions.

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.

A spectroscopic study of the hydrolysis, colloid formation and solubility of Np(IV)

The hydrolysis, colloid formation and solubility of Np(IV) are investigated in aqueous HClO4-NaClO4 solutions (log [H+] = 0 to -2.5) by absorption spectroscopy in the wavelength range of 680-1000 nm. Applying Laser induced photoacoustic spectroscopy (LPAS) in the range of 680-760 nm, the study is extended to low Np(IV) concentrations of 10-6 mol/l in DClO4-NaClO4-D2O solutions up to log [D+]=-3.3. Laser induced breakdown detection (LIBD) demonstrates the formation of Np(IV) colloids when the Np(IV) concentration exceeds the solubility of Np(OH)4(am) at given pH. The simultaneous decrease of the Np(IV) absorption bands at 723 and 960 nm cannot be ascribed to the formation of the mononuclear complex Np(OH)3+ as assumed in the literature. It is found to be caused by polynucleation. In undersaturated Np(IV) solutions below 10-4 mol/l, the position and intensity of the absorption maximum at 723 nm are practically insensitive to the pH change. In oversaturated solutions the absorption band decreases significantly. The spectroscopically determined pH-dependent equilibrium concentration of mononuclear Np(IV) species above freshly formed solid or colloidal Np(IV) particles indicates that Np(OH)22+ is the predominant species in the pH range of 1.5-3. This finding is in agreement with the Np(IV) hydrolysis constants reported in the literature from a solvent extraction study with 239Np(IV) trace concentrations. The solubility product of freshly formed Np(OH)4(am) particles is determined to be log Ksp = -54.4±0.4 in 0.1 M HClO4-NaClO4 and log K°sp=-56.5±0.4 (converted to I=0 by applying the SIT).

The Reduction of Np(V) in Groundwater Rich in Humic Substances

The reduction of Np(V) to Np(IV) in one of the Gorleben aquifer systems containing humic substances was investigated by absorption spectroscopy and ultrafiltration in the neutral pH range. Batch and column experiments were carried out to estimate the reduction behaviour under different experimental conditions. Natural humic substances in groundwater are found to cause the Np(V) reduction. The tetravalent neptunium thus produced appears to be a humic colloid-borne species that can be separated from water by ultrafiltration, while the original pentavalent neptunium is present as Np02 + ion, Np(V) mono-carbonate and / or Np(V) húmate depending on pH.

Np(IV)/Np(V) valence determinations from Np L3 edge XANES/EXAFS.

X-ray absorption near edge structure (XANES) spectroscopy for in situ metal valence determination has become a powerful analytical tool in heterogeneous systems. This is in part because it is applicable without prior separation procedures. For some systems, however, determining the oxidation state from XANES spectra is not straightforward and caution must be used. We show that the analysis of L3,2 edge EXAFS (extended X-ray absorption fine structure) spectra is better suited to distinguish between Np(IV) and Np(V) than from their XANES spectra. Whereas evidence for the oxidation of Np(IV) in solution samples from their Np L3 XANES is unclear, their EXAFS data unequivocally reveals Np(V) formation in the solutions.