Complexation of plutonium(IV) with sulfate at variable temperatures

The complexation of plutonium(IV) with sulfate at variable temperatures has been investigated by solvent extraction method. A NaBrO₃ solution was used as holding oxidant to maintain the plutonium(IV) oxidation state throughout the experiments. The distribution ratio of Pu(IV) between the organic and aqueous phases was found to decrease as the concentrations of sulfate were increased. Stability constants of the 1:1 and 1:2 Pu(IV)-HSO₄ ⁻ complexes, dominant in the aqueous phase, were calculated from the effect of [HSO₄ ⁻] on the distribution ratio. The enthalpy and entropy of complexation were calculated from the stability constants at different temperatures using the Van’t Hoff equation.     

Actinide speciation in the environment

Nuclear test explosions and nuclear reactor wastes and accidents have released large amounts of radioactivity into the environment. Actinideions in waters often are not in a state of thermodynamic equilibrium and their solubility and migration behavior is related to the form in which the nuclides are introduced into the aquatic system. Chemical speciation, oxidation state, redox reactions, and sorption characteristics are necessary in predicting solubility of the different actinides, their migration behaviors and their potential effects on marine biota. The most significant of these variables is the oxidation state of the metal ion as the simultaneous presence of more than one oxidation state for some actinides in a solution complicates actinide environmental behavior. Both Np(V)O₂ ⁺ and Pu(V)O₂ ⁺, the most significant soluble states in natural oxic waters, are relatively noncomplexing and resistant to hydrolysis and subsequent precipitation. The solubility of NpO₂ ⁺ can be as high as 10⁻⁴M while that of PuO₂ ⁺ is much more limited by reduction to the insoluble tetravalent species, Pu(OH)₄, (pK_sp≥56) but which can be present in the pentavalent form in aqautic phases as colloidal material. The solubility of hexavalent UO₂ ²⁺ in sea water is relatively high due to formation of carbonate complexes. The insoluble trivalent americium hydroxocarbonate, Am(OH)(CO₃) is the limiting species for the solubility of Am(III) in sea water. Thorium(IV) is present as Th(OH)₄, in colloidal form. The chemistry of actinide ions in the environment is reviewed to show the spectrum of reactions that can occur in natural waters which must be considered in assessing the environmental behavior of actinides. Much is understood about sorption of actinides on surfaces, the mode of migration of actinides in such waters and the potential effects of these radioactive species on marine biota, but much more understanding of the behavior of the actinides in the environment is needed to allow proper and reliable modeling needed for disposition of nuclear waste over many thousands of years. This paper has been presented as a Hevesy Medal Award Lecture by the medalist author at the 1st International Nuclear Chemistry Congress (1st-INCC), 22–29 May 2005, Kusadasi, Turkey     

Environmental Mobility of Pu(IV) in the Presence of Ethylenediaminetetraacetic Acid: Myth or Reality?

Ethylenediaminetetracetic acid (EDTA), which was co-disposed with Pu at several US Department of Energy sites, has been reported to enhance the solubility and transport of Pu. It is generally assumed that this enhanced transport of Pu in geologic environments is a result of complexation of Pu(IV) with EDTA. However, the fundamental basis for this assumption has never been fully explored. Whether EDTA can mobilize Pu(IV) in geologic environments is dependent on many factors, chief among them are not only the complexation constants of Pu with EDTA and dominant oxidation state and the nature of Pu solids, but also (1) the complexation constants of environmentally important metal ions (e.g., Fe, Al, Ca, Mg) that compete with Pu for EDTA and (2) EDTA interactions with the geomedia (e.g., adsorption, biodegradation) that reduce effective EDTA concentrations available for complexation. Extensive studies over a large range of pH values (1 to 14) and EDTA concentrations (0.0001 to 0.01 mol⋅L⁻¹) as a function of time were conducted on the solubility of 2-line ferrihydrite (Fe(OH)₃(s)), PuO₂(am) in the presence of different concentrations of Ca ions, and mixtures of PuO₂(am) and Fe(OH)₃(s). The solubility data were interpreted using Pitzer’s ion-interaction approach to determine/validate the solubility product of Fe(OH)₃(s), the complexation constants of Pu(IV)-EDTA and Fe(III)-EDTA, and to determine the effect of EDTA in solubilizing Pu(IV) from PuO₂(am) in the presence of Fe(III) compounds and aqueous Ca concentrations. Predictions based on these extensive fundamental data show that environmental mobility of Pu as a result of Pu(IV)-EDTA complexation as reported/implied in the literature is a myth rather than the reality. The data also show that in geologic environments where Pu(III) and Pu(V) are stable, the EDTA complexes of these oxidation states may play an important role in Pu mobility.     

Redox speciation of plutonium in natural waters

Data on the stability of Pu(V) as the dominant oxidation state of tracer concentrations of plutonium in natural waters is reviewed. Laboratory experiments for solutions of 0.1 and 1.0M (NaCl) ionic strength and pH 3–10 confirm the dominance of Pu(V) as the state in solution. Humics in the waters can cause reduction to Pu(IV).     

Modeling Speciation and Solubility in Aqueous Systems Containing U(IV,VI), Np(IV,V, VI), Pu(III,IV, V,VI), Am(III), and Cm(III)

A comprehensive thermodynamic model, referred to as the Mixed-Solvent Electrolyte model, has been applied to calculate phase equilibria and chemical speciation in selected aqueous actinide systems. The solution chemistry of U(IV, VI), Np(IV, V, VI), Pu(III, IV, V, VI), Am(III), and Cm(III) has been analyzed to develop the parameters of the model. These parameters include the standard-state thermochemical properties of aqueous and solid actinide species as well as the ion interaction parameters that reflect the solution’s nonideality. The model reproduces the solubility behavior and accurately predicts the formation of competing solid phases as a function of pH (from 0 to 14 and higher), temperature (up to 573 K), partial pressure of CO₂ (up to \( p_{{{\text{CO}}_{2} }} \)  = 1 bar), and concentrations of acids (to 127 mol·kg^?1), bases (to 18 mol·kg^?1), carbonates (to 6 mol·kg^?1) and other ionic components (i.e., Na⁺, Ca²⁺, Mg²⁺, OH^?, Cl^?, \( {\text{ClO}}_{4}^{ - } \), and \( {\text{NO}}_{3}^{ - } \)). Redox effects on solubility and speciation have been incorporated into the model, as exemplified by the reductive and oxidative dissolution of Np(VI) and Pu(IV) solids, respectively. Thus, the model can be used to elucidate the phase and chemical equilibria for radionuclides in natural aquatic systems or in nuclear waste repository environments as a function of environmental conditions. Additionally, the model has been applied to systems relevant to nuclear fuel processing, in which nitric acid and nitrate salts of plutonium and uranium are present at high concentrations. The model reproduces speciation and solubility in the U(VI) + HNO₃ + H₂O and Pu(IV, VI) + HNO₃ + H₂O systems up to very high nitric acid concentrations (\( x_{{{\text{HNO}}_{3} }} \approx 0.70 \)). Furthermore, the similarities and differences in the solubility behavior of the actinides have been analyzed in terms of aqueous speciation.     

Solid-phase extraction of plutonium in various oxidation states from simulated groundwater using N-benzoylphenylhydroxylamine

Solid-phase extraction of plutonium in different individual and mixed oxidation states from simulated groundwater (pH 8.5) was studied. The extraction of plutonium species was carried out in a dynamic mode using DIAPAK C16 cartridges modified by N-benzoylphenylhydroxylamine (BPHA). It was shown that the extent of recovery depends on the oxidation state of plutonium. The extraction of Pu(IV) was at the level of 98–99% regardless of the volume and flow-rate of the sample solution. Pu(V) was extracted by 90–95% and 75–80% from 10- and 100-mL aliquots of the samples, respectively, whereas the extraction of Pu(VI) did not exceed 45–50%. An equimolar mixture of Pu(IV), Pu(V), and Pu(VI) was extracted by 74%. The distribution coefficients (K _d) and kinetic exchange capacities (S) of plutonium in various oxidation states were measured. It was found that during the sorption process, Pu(V) was reduced to Pu(IV) by 80–90% after an hour-long contact with the solid phase. Pu(VI) is reduced to Pu(V) by 34% and to Pu(IV) by 55%. In the case of mixed-valent solution of plutonium, only Pu(V) and Pu(IV) were found in the effluents.     

Determination of trace amounts of plutonium in low-active liquid wastes from spent nuclear-fuel reprocessing plants by flow injection-based solid-phase extraction/electrochemical detection system

A flow injection-based electrochemical detection system coupled to a solid-phase extraction column was developed for the determination of trace amounts of plutonium in low-active liquid wastes from spent nuclear-fuel reprocessing plants. The oxidation state of plutonium in a sample solution was adjusted to Pu(VI) by the addition of silver(II) oxide. A sample solution was made up in 3 mol L^?1 HNO₃ and loaded onto a column packed with UTEVA^® with 3 mol L^?1 HNO₃ as the carrier. Plutonium(VI) was adsorbed onto the resin, and interfering elements were removed by rinsing the column with 3 mol L^?1 HNO₃. Subsequently, the adsorbed Pu(VI) was eluted with 0.01 mol L^?1 HNO₃, and then introduced directly into the flow-through electrolysis cell with boron-doped diamond electrode. The eluted Pu(VI) was detected by an electrochemical amperometric method at a working potential of 0.1 V (vs. Ag/AgCl). The current produced on reduction of Pu(VI) was continuously monitored and recorded. The plutonium concentration was calculated from the relationship between the peak area and concentration of plutonium. The relative standard deviation of ten analyses was 1.1% for a plutonium solution of 25 μg L^?1 containing 50 ng of Pu. The detection limit calculated from three-times the standard deviation was 0.82 μg L^?1 (1.6 ng of Pu).     

Redox behaviors of Fe(II/III) and U(IV/VI) studied in synthetic water and KURT groundwater by potentiometry and spectroscopy

The redox behaviors of Fe(II/III) and U(IV/VI) in both synthetic samples and natural groundwater were investigated with potentiometry, UV/VIS absorption spectroscopy, and time-resolved laser fluorescence spectroscopy. Total dissolved Fe(II/III) concentration along with presence of mixed redox couples of Fe²⁺/Fe³⁺ and Fe²⁺/Fe₂O₃(s) were revealed to be the major factors influencing on the redox potentials. Considerable discrepancies between redox potentials obtained with quantitative analysis and chemical speciation of Fe(II/III) and U(IV/VI) ions were identified in the KAERI Underground Research Tunnel groundwater. Chemical speciation of U(IV) in natural groundwater without considering relevant complexation reaction might cause relatively large uncertainties in redox potential calculations.     

Analysis of plutonium in biological and environmental materials

A procedure for separation of plutonium from some biological and environmental materials has been tested in model and real conditions. The procedure involves a commonly used way of conversion of plutonium to oxidation state (IV) in nitric acid medium and sorption of Pu(IV) on a strongly basic anion exchanger from hydrochloric acid medium thus eliminating interference of²²⁸Th with the²³⁸Pu analysis.     

Sorption of Pu in various oxidation states onto multiwalled carbon nanotubes

The sorption of Pu(IV), polymeric Pu(IV), Pu(V) and Pu(VI) from the 0.1 M NaClO₄ solution onto multiwalled carbon nanotubes was investigated. The kinetic study of the sorption process have shown that the polymeric Pu(IV) has the highest sorption rate, while decrease of sorption rate for plutonium aqua-ions in the order Pu(VI) > Pu(IV) > Pu(V) was found. Strong dependence of sorption kinetics of ionic plutonium species on pH was shown, in contrast to polymeric species, that were shown to quantitatively sorb (99%) in the wide pH range (pH 2–10). Two different sorption mechanisms for ionic and polymeric plutonium species were proposed: on the bases of sorption isotherms chemisorptions of plutonium aqua-ions onto carbon nanotubes and through intermolecular interaction for the polymeric plutonium species was defined. Distribution coefficients of plutonium in various oxidation states were found to increase with pH, showing the highest values for polymeric plutonium sorption (K _d  = 2.4 × 10⁵ mL g⁻¹ at pH = 6).     

Pu and Am sorption to the Baltic Sea bottom sediments

Sorption of Am and Pu isotopes to bottom sediments of the Baltic Sea has been studied under natural and laboratory conditions. Data obtained from sequential extraction, sorption of Am(III), Pu(IV) and Pu(V) as well as oxidation state distribution experiments have shown that Pu(V) sorption mechanism includes a very fast Pu(V) reduction (reaction rate ≤ 2.33 × 10^?3 s^?1) to Pu(IV) by humic substances and/or by Fe(II) to Pu(IV) and partly to Pu(III). Following reduction Pu isotopes were bound to various components of bottom sediments via ion exchange and surface complexation reactions and a slow incorporation into the crystalline structure of Fe minerals. Kinetics experiments showed that the sorption of Pu(V), Pu(IV) and Am(III) to bottom sediments from natural seawater was controlled by the inert layer diffusion process.     

Plutonium Activity Ratios in Plankton: New Evidence of Hold-Up Time in Irish Sea Sediments

The determination of activity ratios for radioisotopes of different half-lives can be used to estimate transit times from a point source to locations further away. For conservative elements, this time is approximately equivalent to the net hydrological transport. However, for non-conservative elements such as plutonium, the additional influence of biogeochemical processes decreases the net transport time. In this study, ²⁴¹Pu and ^239,240Pu concentrations in Irish Sea plankton samples, collected in May 1994, were determined and the ²⁴¹Pu/^239,240Pu ratios calculated. Plutonium-239,240 was measured using a standard method by ion exchange chromatography and alpha counting, and ²⁴¹Pu was determined by liquid scintillation counting using the disk-supported technique. The latter showed some methodological problems, which are briefly discussed. The ²⁴¹Pu/^239,240Pu ratios gave an estimate of the "transit time" from Sellafield to the different sampling points. In fact, this time represents the age of plutonium in plankton, i.e., the time lag between release from Sellafield and detection at the different sampling stations. The mean plutonium age was 17±2 years (n = 10) and 18.6±0.8 years (n = 13) in phytoplankton and zooplankton, respectively. The spatial distribution was reasonably homogeneous over the Irish Sea. The assimilation-elimination processes of plutonium in plankton are rather rapid. Therefore, it may be assumed that, in this time scale, the plutonium concentrations were in equilibrium with surrounding waters. Thus, it is concluded that plutonium was rather old because resuspension-sedimentation processes had occurred that delayed its transport within the Irish Sea. Therefore, the age of plutonium in plankton represented the hold-up time of plutonium in the sediments from the Irish Sea.     

Biogeochemical behaviour of plutonium and americium and geochemical modelling of the soil solution

Field observations suggest that plutonium and americium in the environment are present in very different chemical forms in the interstitial waters of an intertidal sediment. Themodynamic modelling using the PHREEQE code predicts that plutonium is present entirely in oxidation state (V) as the PuO₂CO ₃ ^– ion, whereas americium is present entirely in oxidation state (III), largely as the uncharged Am(OH)CO₃ species, but with significant concentrations of the Am³⁺ and the AmSO ₄ ⁺ ions. There are, however, differences between these predictions and others published for a very similar system which apparently arise from uncertainties in the thermodynamic data. Field data cannot resolve these differences unambiguously.     

Determining the distribution of Pu, Np, and U oxidation states in dilute NaCl and synthetic brine solutions

The effect of iron powder (Fe⁰) on the reduction of Pu(VI),Np(V), and U(VI) was investigated in dilute NaCl and synthetic brines. Thetotal concentrations and oxidation states of the actinides in these solutionswere monitored as functions of pC H +, Eh, and time using techniques includingVis/Near IR absorption spectrophotometry, solvent extraction, activity counting,and inductively coupled plasma spectroscopy-mass spectrometry (ICP-MS). Whenconcentrations were too low and the oxidation states could not be directlydetermined by spectrophotometry or solvent extraction, comparing the measuredconcentrations with the solubility of reference systems helped to define thefinal oxidation states. In general, the reduction was more rapid, and couldproceed further, in the dilute NaCl solution than in the brine solutions.The experimental observations can be summarized as follows: (1) in the diluteNaCl solutions (pC H + 7 to 12), all three actinides, Pu(VI), Np(V) and U(VI),were reduced to lower oxidation states (most likely the tetravalent state)within a few days to a few months in the presence of Fe⁰; (2) insynthetic brines containing Fe⁰ (pC H + 8 to 13), the reductionof Pu(VI) was much slower than in the dilute NaCl solution. The dominant oxidationstate of Pu in the brine solution was Pu(V), the concentration of which wascontrolled by the electrochemical potential and could probably be representedby a heterogeneous redox reaction PuO₂ . xH₂O(s) PuO₂ + +e ^—; (3) in synthetic brines containing Fe⁰ (pC H + 8 to 13), Np(V) was probably reduced to Np(IV) and precipitatedfrom the solution; (4) in synthetic brines containing Fe⁰ (pC H+ 8 to 13), no significant reduction of U(VI) was observed within 55 days.     

Distribution and identification of Plutonium(IV) species in tri-n-butyl phosphate/HNO3 extraction system containing acetohydroxamic acid

There was a significant research progress achieved with the aim to modify conventional PUREX process by stripping of plutonium from the tri-n-butyl phosphate (TBP) extraction product in the form of non-extractable complexes upon addition of back-hold complexation agents. The present paper reports effects of such salt-free complexant, acetohydroxamic acid (HAHA), on distribution ratio of Pu(IV) under wide concentration of nitric acid and additional nitrate. General formula of plutonium species present in the organic phase can be described as Pu(OH)_x(AHA)_y(NO3)_4−x−y·2TBP·wHNO₃.     

Uptake of plutonium from nuclear waste water by natural and chemically modified sorbents

The uptake of plutonium from model solution of boric acid labelled with²³⁹Pu by natural sorbents was studied. The range of pH of solution was from 5.1 to 8. For the uptake of Pu were used different natural and chemically modified natural sorbents of different mineralogical composition and from different deposits. The distribution coefficients for plutonium uptake were calculated and the best conditions for uptake were evaluated.     

Oxidation State of Neptunium and Plutonium and Their Leaching from Sodium–Aluminum–(Iron) Phosphate Glasses

The oxidation states of neptunium and plutonium in doped sodium–aluminum–(iron) phosphate glasses have been determined by X-ray photoelectron spectroscopy. Neptunium is present in the form of Np(IV) as Np⁴⁺ and, in smaller amounts, in the form of Np(V) as neptunyl ions. Plutonium is present mainly in the form of Pu(IV) and additionally Pu(III). The most easily water-leachable element is neptunium, which is attributed to its existence in the mobile form of neptunyl (NpO₂)⁺. The leaching rates of plutonium and impurity americium are approximately two orders of magnitude lower.     

Influence of manganese dioxide on the oxidation states of plutonium

When manganese dioxide impregnated filters have been used to concentrate plutonium from water solutions some anomalies were detected, which were ascribed to the probable existence of plutonium in two different oxidation states. The results of this paper seem to confirm this assumption that the Pu in the solution is a mixture of Pu(III) and Pu(IV). After contact with MnO₂, plutonium in the solution consists of only Pu(IV).     

The application of <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethylhydroxylamine as reductant for the separation of plutonium from uranium

Both single stage and multi-stages experiments on stripping plutonium with N,N-dimethylhydroxylamine (DMHAN) as reductant with methylhydrozine (MMH) as supporting reductant were carried out. The effect of contact time, temperature, acidity, concentration of DMHAN on back-extraction rate of plutonium was investigated in the single stage experiment. The results demonstrated that the reaction of stripping Pu(IV) in the organic phase (30% TBP–kerosene) 1BF solutions by DMHAN exhibits excellent stripping efficiency. Under the given conditions, the back-extraction rate of plutonium reaches 90% within 2 min. Higher temperature, lower acidity and the increased concentration of DMHAN benifit the stripping reaction. The concentration profile of HNO₃, uranium and plutonium were determined in a multi-stages mixer-settler after the steady state of the back-extraction, and the multi-stages results show that the plutonium can be separated effectively from uranium. The recovery of plutonium and uranium reach 99.995% or over 99.99% respectively. The separation factor of U from Pu (SF_Pu/U) is about 2 × 10⁴.     

Titrimetric determination of uranium in the presence of plutonium in sulphuric acid medium

Present work summairzes a method for the estimation of uranium in the presence of plutonium involving the reduction of uranium to U/IV/ and plutonium to Pu/III/ by Zn/Hg/ followed by the selective oxidation of Pu/III/to Pu/IV/with HNO₃ catalyzed by molybdate in the presence of large sulphate concenration [5M H₂SO₄+1.5M /NH₄/₂SO₄]. The oxidation of U/IV/ by K₂Cr₂O₇ is then carried out in the presence of excess of Fe/III/ and Al/NO₃/₃ to a sharp potentiometric end point. R.S.D. obtained for 20 determinations of uranium /3–6 mg/ was 0.3% in the presence of 0.35 mg of plutonium. Larger quantity for plutonium was found to interfere.