Pu Oxidation State Research Articles

Sorption of trivalent plutonium onto UO2and the effect of the solid phase on the Pu oxidation state

SummaryA problem with plutonium in sorption studies is its tendency to occur in a mix of oxidation states. This work was a study of the sorption of plutonium on the solid phase UO2— in a perchlorate medium as a function of pH — where plutonium is kept in its trivalent oxidation state. Experiments also showed that uranium(IV) oxide can reduce Pu(IV) to Pu(III) in an acidic solution and maintain it in that state. It was observed at the same time that the chemically inert solid phases TiO2and ThO2possibly increase the rate of disproportionation of Pu(IV) at a pH of about 0.5. In accordance with previous studies MnO2was found to have an oxidizing effect, converting Pu(IV) into Pu(VI).A comparison is made between the sorption of Th(IV), Pu(III) and Co(II) on UO2and TiO2, and Pu(VI) on TiO2.

Pu(V)O2+ Adsorption and Reduction by Synthetic Hematite and Goethite

Changes in aqueous- and solid-phase plutonium oxidation state were monitored over time in hematite (alpha-Fe2O3) and goethite (alpha-FeOOH) suspensions containing 239Pu(V)-amended 0.01 M NaCl. Solid-phase oxidation state distribution was quantified by leaching plutonium into the aqueous phase and applying an ultrafiltration/solvent extraction technique. The technique was verified using oxidation state analogues of plutonium and sediment-free controls of known Pu oxidation state. Batch kinetic experiments were conducted at hematite and goethite concentrations between 10 and 500 m2 L(-1) in the pH range of 3-8. Surface-mediated reduction of Pu(V) was observed for both minerals at pH values of 4.5 and greater. At pH 3 no adsorption of Pu(V) was observed on either goethite or hematite; consequently, no reduction was observed. For hematite, adsorption of Pu(V) was the rate-limiting step in the adsorption/reduction process. In the pH range of 5-8, the overall removal of Pu(V) from the system (solid and aqueous phases) was found to be approximately second order with respect to hematite concentration and of order -0.39 with respect to the hydrogen ion concentration. The overall reaction rate constant (k(rxn)), including both adsorption and reduction of Pu(V), was 1.75+/-2.05 x 10(-10) (m(-2) L)(-2.08) (mol(-1) L)(-0.39) (s(-1)). In contrast to hematite, Pu(V) adsorption to goethite occurred rapidly relative to reduction. At a given pH,the reduction rate was approximately independent of the goethite concentration, although the hydrogen ion concentration (pH) had only a slight effect on the overall reaction rate. For goethite, the overall reaction rates at pH 5 and pH 8 were 6.0 x 10(-5) and 1.5 x 10(-4) s(-1), respectively. For hematite, the reaction rate increased by 3 orders of magnitude across the same pH range.

Pu(V)O2+ Adsorption and Reduction by Synthetic Magnetite (Fe3O4)

Changes in aqueous- and solid-phase Pu oxidation state were monitored over time in magnetite (Fe3O4) suspensions containing 239Pu(V)-amended 0.01 M NaCl. Oxidation state distribution was determined by leaching of Pu into an aqueous phase followed by an ultrafiltration/solvent extraction technique. The capability of the technique to measure Pu oxidation state distribution was verified using 230Th(IV), 237Np(V), and 233U(VI) as oxidation state analogues. Reduction of Pu(V) was observed at all pH values (pH 3 to 8) and magnetite concentrations (10 to 100 m2 L(-1)). In the pH range 5 to 8, adsorption was a rate-limiting step, and reduction was mediated by the solid phase; at pH 3 reduction occurred in the aqueous phase. The overall reaction (describing both adsorption and reduction of Pu(V)) was found to be approximately first order with respect to the magnetite concentration and of order -0.34+/-0.02 with respect to the hydrogen ion concentration. Assuming first order dependence with respect to Pu, the overall reaction rate constant was calculated as k(rxn) = 4.79+/-0.62 x 10(-8) (m(-2) L)0.99(mol(-1) L)-0.34(s(-1)). The Pu(IV) solid-phase species became more stable over time.

First-principles calculations of PuO(2+/-x).

The electronic structure of PuO(2+/-x) was studied using first-principles quantum mechanics, realized with the self-interaction corrected local spin density method. In the stoichiometric PuO2 compound, Pu occurs in the Pu(IV) oxidation state, corresponding to a localized f4 shell. If oxygen is introduced onto the octahedral interstitial site, the nearby Pu atoms turn into Pu(V) (f3) by transferring electrons to the oxygen. Oxygen vacancies cause Pu(III) (f5) to form by taking up electrons released by oxygen. At T = 0, the PuO2 compound is stable with respect to free oxygen, but the delicate energy balance suggests the possible deterioration of the material during long-term storage.

Sources and migration of plutonium in groundwater at the Savannah River site.

The isotopic composition, size distribution, and redox speciation of plutonium (Pu) in the groundwater in the vicinity of the F-area seepage basins at the U.S. Department of Energy Savannah River Site (SRS) were examined. A low 240Pu/239Pu ratio in the upstream control well signifies a Pu source otherthan global fallout and indicates reactor-produced Pu. Elevated 240Pu/239Pu atom ratios downstream from the seepage basins are due to the decay of transplutonium isotopes, mainly 244Cm to 240Pu, which were generated at the SRS. Evidence suggests that the migration of basin-released Pu isotopes is minor. Rather, it is the transplutonium isotopes that migrate preferentially downstream and in the process decay to yield progeny Pu isotopes. Size fractionation studies with cross-flow ultrafiltration show that <4% of the 239Pu or 240Pu is found in the colloidal fraction, a finding that is consistent with the higher Pu oxidation states observed in the SRS groundwater. The observation of a low abundance of colloid-associated Pu in SRS groundwater cannot be extrapolated to all sites, but is in contrast to the conclusions of prior groundwater Pu studies at the SRS and elsewhere. This work is unique in its application of a novel combination of sampling and processing protocols as well as its use of thermal ionization mass spectrometry for the detection of Pu isotopes. This allows quantification of the Pu source terms and better determination of the ambient Pu size and redox speciation representative of in situ conditions.

Solubility and sorption of redox-sensitive radionuclides (Np, Pu) in J-13 water from the Yucca Mountain site: comparison between experiment and theory

This study presents the characterization of Pu-bearing precipitates and the results from uptake studies of Np and Pu on inorganic colloidal particulates in J-13 water from the Yucca Mountain site. Plutonium solubilities determined experimentally at pH values of 6, 7, and 8.5 are about two orders of magnitude higher than those calculated using the existing thermodynamic database indicating the influence of colloidal Pu(IV) species. Solid phase characterization using X-ray diffraction revealed primarily Pu(IV) in all precipitates formed at pH 6, 7, and 8.5. The solubility controlling Pu-bearing solids precipitated at ambient temperature consisted of amorphous Pu(OH) 4(s) with several Pu–O distances between 2.3 and 2.7 Å that are characteristic for Pu(IV) colloids. High temperature (90 °C) increased solid phase crystallinity and produced Pu(IV) solids that contained Pu oxidation state impurities. X-ray absorption spectroscopic studies revealed diminished Pu–O and Pu–Pu distances that were slightly different from those in crystalline PuO 2(s). A Pu–O bond of 1.86 Å was identified that is consistent with the plutonyl(V) distance of 1.81 Å in PuO 2 +(aq). Hematite, montmorillonite, and silica colloids were used for uptake experiments with 239Pu(V) and 237Np(V). The capacity of hematite to sorb Pu significantly exceeded that of montmorillonite and silica. A low desorption rate was indicative of highly stable Pu-hematite colloids, which may facilitate Pu transport to the accessible environment. Neptunium uptake on all mineral phases was far less than Pu(V) uptake suggesting that a potential Pu(V)–Pu(IV) reductive sorption process was involved. The temperature effect on Pu solubility and pseudocolloid formation is also discussed.

Synthesis and structural characterization of the first quaternary plutonium thiophosphates: K(3)Pu(PS(4))(2) and APuP(2)S(7) (A = K, Rb, Cs).

The first quaternary plutonium metal thiophosphates have been synthesized by the reactive flux method and characterized by single-crystal X-ray diffraction: K(3)Pu(PS(4))(2) (I), KPuP(2)S(7) (II), RbPuP(2)S(7) (III), and CsPuP(2)S(7) (IV). All four compounds crystallize in the monoclinic space group P2(1)/c with Z = 4. Compound I has cell parameters of a = 9.157(1) A, b = 16.866(2) A, c = 9.538(1), and beta = 90.610(3)degrees. Compound II has cell parameters of a = 9.641(1) A, b = 12.255(1) A, c = 9.015(1) A, and beta = 90.218(1)degrees. Compound III has cell parameters of a = 9.8011(6) A, b = 12.3977(7) A, c = 9.0263(5) A, and beta = 90.564(1)degrees. Compound IV has cell parameters of a = 10.1034(7) A, b = 12.5412(9) A, c = 9.0306(6) A, and beta = 91.007(1)degrees. Compound I is isostructural to a family of rare-earth metal thiophosphates and comprises bicapped trigonal prismatic PuS(8) polyhedra linked in chains through edge-sharing interactions and through thiophosphate tetrahedra. Compounds II-IV crystallize in a known structure type not related to any previously observed actinide thiophosphates and contain the (P(2)S(7))(4-) corner-shared bitetrahedral ligand as a structural building block. A summary of important bond distances and angles for these new plutonium thiophosphate materials is compared to the limited literature on plutonium solid-state compounds. Diffuse reflectance spectra confirm the Pu(III) oxidation state and Raman spectroscopy confirms the tetrahedral PS(4)(3-) building block in all structures.

Thermodynamic model for the solubility of PuO2(am) in the aqueous Na+-H+-OH--Cl--H2O-ethylenediaminetetraacetate system

The solubility of PuO2(am) was determined in the aqueous Na+H+-OH--Cl--H2O-ethylenediaminetetraacetate (EDTA) system as a function of pH (1 to 13.5) and EDTA concentrations (0.00001 to 0.093 M) to determine PuEDTA complexes important in a wide range of pH values and EDTA concentrations. The solubility studies were carried out as a function of time and show, as expected, that steady-state/equilibrium in this system is reached rapidly (&lt;7 days). The aqueous Pu oxidation states were determined by X-ray absorption near edge spectroscopy and solvent extraction. The aqueous Pu(IV) concentrations in equilibrium with PuO2(am) at fixed concentrations of EDTA (0.0001 or 0.001;M) as a function of pH, and at fixed pH (∼9), and as a function of EDTA were interpreted using the Pitzer ion-interaction model. The model was developed based on a subset of the experimental data and was validated against the remaining, more extensive data not used in model development. The log K0values for the formation of Pu(OH)xEDTAx-according to the generalized reaction (Pu4++ xOH+EDTA4-↔ Pu(OH)xEDTA-x) were determined to be 36.44±0.35, 44.78±0.35, and 49.80±0.35 for x=1, 2, and 3, respectively, when log;K0of -56.85 for (PuO2(am)+2H2O harr; Pu4++4OH-) is used.

Toward the End of PuO 2 's Supremacy?

Plutonium dioxide (PuO) has long been thought to be the most stable plutonium oxide under oxidizing conditions. Its stability has been a key factor in safety considerations for military and civilian uses of plutonium and for long-term storage of nuclear materials. A report by Haschke et al. (page 285) shows that PuOis metastable and can be oxidized to PuO, with more than 25% of plutonium ions oxidized to the more mobile Pu(VI) oxidation state. The results have implications for both military and civilian applications, for the long term storage of plutonium, and for the geological disposal of nuclear wastes.

Characterization of a Plutonium-Bearing Zirconolite-Rich Synroc

ABSTRACTA titanate-based ceramic waste form, rich in phases structurally related to zirconolite (CaZrTi2O7), is being developed as a possible method for immobilizing excess plutonium from dismantled nuclear weapons. As part of this program, Lawrence Livermore National Laboratory (LLNL) produced several ceramics that were then characterized at Argonne National Laboratory (ANL). The plutonium-loaded ceramic was found to contain a Pu-Gd zirconolite phase but also contained plutonium titanates, Gd-polymignyte, and a series of other phases. In addition, much of the Pu was remained as PuO2-x. The Pu oxidation state in the zirconolite was determined to be mainly Pu4+, although some Pu3+was believed to be present.

Behavior of Np, Pu, Am, Tc Upon Glass Corrosion in a Concentrated Mg(Ca)Cl2Solution

AbstractThe objective of this investigation is to describe the extent to which Np, Pu, Am and Tc are mobilized from vitrified high-level radioactive waste into the near field of an HLW repository in a salt formation, when a hot and concentrated salt solution comes into contact with the glass. Waste form corrosion studies are conducted with a salt solution representing the composition of a fluid phase encountered in drill holes in the Gorleben salt dome. Test temperatures are determined by the designed maximum surface temperature of 200°C for the vitrified waste in the Gorleben salt. The following results were obtained: 1. pH changes of the radio-active leachate are the same as in inactive leachates. 2. The time and temperature dependence of the reaction for the radioactive glass are in excellent agreement with that of the inactive glass. 3. Np, Pu, Am, and Tc have not been reimmobilized in secondary minerals. Hence, mobilization of these radionuclides is governed by the kinetics of glass dissolution. Pu oxidation states were analyzed and related to Pu concentrations.

Characterization of the plutonium-water reaction II: Formation of a binary oxide containing Pu(VI)

Characterization of products formed by reacting unalloyed plutonium with water vapor at 15 Torr pressure shows that Pu 2O 3, PuO 2 and a previously unreported binary oxide containing Pu(VI) form on the metal between 200 and 350 °C. Although the reaction produces a uniformly adherent product layer with a thickness of about 5 μm, localized pits of plutonium hydride cover 15–20% of the oxide-metal interface beneath the layer. Analytical results show that the non-equilibrium product contains layers of three oxide phases at all temperatures. Cubic α-Pu 2O 3 appearing at the metal interface is covered by a layer of dioxide. A third oxide formed at the gas-solid interface is identified by X-ray diffraction and X-ray photoelectron spectroscopy as the mixed-valence phase Pu(IV) 3− x Pu(VI) x O 6+ x . Diffraction data suggest that this oxide crystallizes in a fluorite-related NpH 2+δ or Sm 3H 7(Ca 2YF 7)-type structure. Formation of the higher oxide is supported by X-ray photoelectron spectra, indicating the presence of the Pu(VI) oxidation state. Evaluation of the results yields an oxide composition corresponding to a value of x near 0.5. Estimated thermodynamic data suggests that the oxide is a stable phase in the Pu-O system; experimental observations show that the product is preserved during exposure to air and ultrahigh vacuum. Although formation of PuO 2.2 is expected to be a slow process at low temperatures, reaction of PuO 2 with moisture adsorbed on its surface may generate large quantities of hydrogen if dioxide is stored for an extended period.

The coprecipitation of Pu and other radionuclides with CaCO 3

The record of fallout plutonium concentrations in annual bands of corals is strikingly similar to the record of atmospheric deposition of 90Sr. This similarity implies that corals may incorporate Pu from seawater with a constant partition coefficient (constant discrimination). To investigate physicochemical aspects of Pu incorporation, the following have been coprecipitated with CaCO 3 (calcite and aragonite): oxidized and reduced Pu; americium, thorium and uranium as analogs to Pu oxidation states (III, IV, VI), respectively; and 210Pb as a particle-reactive nuclide which may be incorporated by corals with constant discrimination. Americium, thorium and lead adsorb onto both calcite and aragonite, with more than 99% of the recovered activity found associated with the solids. Uranium exhibits a behavior consistent with lattice substitution. Partition coefficients for U in aragonite range from 1.8 to 9.8 and vary inversely with pH and/or rate of precipitation. The partition coefficient for U in calcite is less than 0.2 and may be as low as 0.046. Reduced Pu sorbs with 3 to 4% remaining in solution. Oxidized Pu may both sorb and coprecipitate. The coral record for Pb and U results primarily from biological, rather than physicochemical, effects; it is likely that the Pu coral record also reflects biological discrimination.

Plutonium oxidation states in the southwestern Black Sea: Evidence regarding the origin of the cold intermediate layer

Water column profiles of reduced Pu (III + IV) and oxidized Pu (V + VI) species at two stations in the southwestern Black Sea reveal that oxidized Pu predominates even at the suboxic zone, between 50 and 100 m. Reduction of these oxidized species occurs at the base of the suboxic zone, where we noted a shift to the predominance of reduced Pu forms. At anoxic depths (below 150 m), we found only reduced Pu forms, at concentrations significantly less than the total Pu levels we found in the upper 150 m samples. Our data show that Pu oxidation states are useful in interpreting lateral advective processes that occur within the upper 150 m of the Black Sea water column. An additional interesting result of our measurements is that water at the cold intermediate layer (CIL) of the Black Sea is labeled with predominantly oxidized Pu. We believe this feature can be used to trace the origin of CIL water.

Plutonium distribution and oxidation states in a reactor leaching ponds system.

Concentrations of 239,240Pu and 238Pu in water, net plankton (algal material), suspended particulates and sediment, as well as Pu oxidation states in filtered water, were determined in a test reactor leaching ponds system in southeastern Idaho. The highest Pu concentration in the ponds system was found in net plankton, and concentrations varied significantly between sampling dates. Plutonium Concentration Ratios (CR) for plankton ranged from 3 X 10(4) to 4 X 10(5). The lowest Pu concentration was found in filtered water, primarily because of the absence of complexing agents. The majority of Pu in filtered water was in true solution (60-87%) or present in colloidal particles smaller than 0.22 micron. Plutonium association with sediment was inversely related to particle size. The "environmental" distribution coefficients (Kd) for Pu ranged from 1.6 X 10(4) to 1.2 X 10(5) reflecting the importance of sediments as the main reservoir for Pu in the ponds system. No significant differences were noted between CR or Kd values for 239,240Pu and 238Pu. The reduced Pu oxidation states (III and IV) fractions ranged from 57% to 71% of the total dissolved Pu in water. This is in contrast with oxidation states distribution from other large aquatic systems (Great Lakes and the Irish Sea) where Pu is predominately in oxidized (V and VI) forms.

Determination of aqueous plutonium oxidation states by solvent extraction

The reliability of two solvent extraction techniques for the determination of Pu oxidation states in solution was tested with low-ionic-strength solutions and with high-Na and high-Mg brines that contained Pu concentrations sufficient for spectrophotometric analysis. One procedure only differentiates between reduced Pu [Pu(III) and Pu(IV)] and oxidized Pu [Pu(V) and Pu(VI)], whereas the second procedure was designed to differentiate between Pu(IV), Pu(V), and Pu(VI) in solution. Both procedures successfully differentiated between oxidized and reduced Pu in both dilute solutions and brines when tested with samples that contained only the Pu(IV), Pu(V), or Pu(VI) oxidation states. However, when the second solvent extraction procedure, which differentiates between Pu(V) and Pu(VI), was employed for solutions that did not contain a strong oxidant to maintain the Pu(VI) oxidation state, significant quantities of Pu(VI) were reduced to Pu(V) during extraction, indicating that accurate quantification of Pu(V) and Pu(VI) is not possible with this procedure.

Determination of plutonium oxidation states at trace levels pertinent to nuclear waste disposal

A scheme was developed for the determination of oxidation states of plutonium in environmental samples. The method involves a combination of solvent extractions and coprecipitation. It was tested on solutions with both high-level and trace-level concentrations. The scheme was used to determine Pu oxidation states in solutions from solubility experiments in groundwater from a potential nuclear waste disposal site. At steady-state conditions, Pu was found to be soluble predominantly as Pu(V) and Pu(VI).

Natural and anthropogenic radionuclide distributions in the northwest Atlantic Ocean

We have used in-situ pumps which filter large volumes of sea water through a 1 μm cartridge prefilter and two MnO 2-coated cartridges to obtain information on dissolved and particulate radionuclide distributions in the oceans. Two sites in the northwest Atlantic show subsurface maxima of the fallout radionuclides 137Cs, 239,240Pu and 241Am. Although the processes of scavenging onto sinking particles and release at depth may contribute to the tracer distributions, comparison of predicted and measured water column inventories suggests that at least 35–50% of the Pu and 241Am are supplied to the deep water by advection. The depth distributions of the naturally occurring radionuclides 232Th, 228Th and 230Th reflect their sources to the oceans. 232Th shows high dissolved concentrations in surface waters, presumably as a result of atmospheric or riverine supply. Activities of 232Th decrease with depth to values ⩽ 0.01 dpm/1000 l. 228Th shows high activities in near surface and near bottom water, due to the distribution of its parent, 228Ra. Dissolved 230Th, produced throughout the water column from 234U decay, increases with depth to ∼ 3000 m. Values in the deep water (> 3000 m) are nearly constant (∼ 0.6–0.7 dpm/1000 l), and the distribution of this tracer (and perhaps other long-lived particle-reactive tracers as well) may be affected by the advection inferred from Pu and 241Am data. The ratio of particulate to dissolved activity for both 230Th and 228Th is ∼ 0.15–0.20. This similarity precludes the calculation of sorption rate constants using a simple model of reversible sorption equilibrium. Moreover, in mid-depths 228Th tends to have a higher particulate/dissolved ratio than 230Th, suggesting uptake and release of 230Th and 228Th by different processes. This could occur if 228Th, produced in surface water, were incorporated into biogenic particles formed there and released as those particles dissolved or decomposed during sinking. 230Th, produced throughout the water column, may more closely approach a sorption equilibrium at all depths. 230Th, 241Am and 239,240Pu are partitioned onto particles in the sequence Th > Am > Pu with ∼ 15% of the 230Th on particles compared with ∼ 7% for Am and ∼ 1% for Pu. Distribution coefficients (K d) are 1.3–1.6 × 10 7 for Th, 5–6 × 10 6 for Am and 7–10 × 10 5 for Pu. The lower reactivity for Pu is consistent with analyses of Pu oxidation states which show ∼ 85% oxidized (V + VI) Pu. However, theK d value for Pu may be an upper limit because Pu, like 228Th, may be incorporated into particles in surface waters and released at depth only by destruction of the carrier phase.