Tetravalent Actinide Ions Research Articles

Ion interaction (SIT) coefficients for the Th4+ion and trace activity coefficients in NaClO4, NaNO3and NaCl solution determined by solvent extraction with TBP

Trace activity coefficients of the Th4+ion in dilute to concentrated NaClO4, NaNO3and NaCl solutions ([H+]=0.01−0.02 M) have been determined at 22 °C from liquid-liquid phase equilibria with 10−50 vol.% TBP inn-dodecane. Using the specific ion interaction theory (SIT) to describe these equilibria as a function of the medium electrolyte concentration, the following ion interaction coefficients are calculated: ε(Th4+, ClO4-)=0.70±0.06 kg/mol and ε(Th4+, NO3-) = 0.31±0.12 kg/mol. The latter value differs considerably from ε(Th4+, NO3-) = 0.11±0.02 kg/mol used in the NEA-TDB. The low distribution coefficients atmNaCl<3 mol/kg do not allow the linear SIT extrapolation toI=0, but the equilibrium constants in 2.5−5.0 m NaCl are compatible with the NEA-TDB value of ε(Th4+, Cl-)=0.25±0.03 kg/mol.The SIT coefficients determined for the Th4+ion follow the linear correlations between known values of ε(MZ+, NO3-), ε(MZ+, Cl-) and ε(MZ+, ClO4-) for non-complexed cations MZ+withZ=1−4, including ε(Pu4+, ClO4-)=0.82±0.07 kg/mol and ε(Pu4+, Cl-)=0.37±0.05 kg/mol calculated from data accepted in the NEA-TDB. The interaction coefficients in the series of the tetravalent actinide ions show a slight systematic dependence on the ionic radius.

Systematics of Hydrolysis Constants of Tetravalent Actinide Ions

Systematic trends of mononuclear and polynuclear hydrolysis constants of tetravalent actinide ions were analyzed by using an electrostatic hard sphere model. The effective charges of actinide ions were introduced into the model by considering possible contributions of non-electrostatic interactions of actinide ions in addition to those of ordinary electrostatic ones. The systematic trends of hydrolysis constants were well fitted by the present model, and the parameter values such as the effective charges of actinide ions were determined. Some predictions were made and discussed in comparison with the solubility curves of tetravalent actinides.

Systematics of Hydrolysis Constants of Tetravalent Actinide Ions

Systematic trends of mononuclear and polynuclear hydrolysis constants of tetravalent actinide ions were analyzed by using an electrostatic hard sphere model. The effective charges of actinide ions were introduced into the model by considering possible contributions of non-electrostatic interactions of actinide ions in addition to those of ordinary electrostatic ones. The systematic trends of hydrolysis constants were well fitted by the present model, and the parameter values such as the effective charges of actinide ions were determined. Some predictions were made and discussed in comparison with the solubility curves of tetravalent actinides.

Results of the colloid and radionuclide retention experiment (CRR) at the Grimsel Test Site (GTS), Switzerland – impact of reaction kinetics and speciation on radionuclide migration

SummaryThe influence of smectite colloids on the migration behaviour of U(VI), Th(IV), Pu(IV), Am(III), Np(V), Sr(II) and Cs(I) is investigated within the Colloid and Radionuclide Retardation experiment (CRR). Twoin situexperiments in a well-characterized granitic fracture zone are carried out in presence and absence of bentonite colloids. Radionuclide retardation observed in the field studies increases in the sequence Np(V)∼U(VI)<Sr(II)<Cs(I), where a small fraction of colloid borne breakthrough is only stated for Cs(I) in presence of bentonite colloids. Am(III) and Th/Pu(IV) mainly migrate as colloids without retardation in the presence and absence of smectitic colloids. The radionuclide migration behaviour is discussed on the basis of results obtained in laboratory batch sorption experiments and spectroscopic studies. Consistent with the field observation, laboratory derivedKdvalues increase in the order Np(V)∼U(VI)<Sr(II)<Cs(I). Significant kinetic hindrance for the sorption to fault gauge minerals is observed for Sr(II) and Cs(I), but notably for Am(III) and Pu(IV). The slow sorption reaction of tri- and tetravalent actinide ions is explained by their kinetically hindered dissociation from colloidal species. In order to explain the colloidal behaviour of tri- and tetravalent actinides even in absence of bentonite colloids, ultracentrifugation and spectroscopic experiments are performed. It is found that up to 60% of Pu(IV) and Am(III) species can be centrifuged off. Adding Cm(III) (5×10-8mol L-1) into both injection solutions instead of Am(III) allows for a spectroscopic study by using the time resolved laser fluorescence spectroscopy (TRLFS). Peak position and fluorescence lifetimes (λ=604 nm, τ=110-114 μs) together with the fact that Cm(III) can be widely separated by ultracentrifugation, suggest the existence of inner-sphere surface complexes on groundwater and bentonite colloids. CarbonK-edge XANES analysis of the bentonite colloids reveal the presence of natural organic constituents. They are mainly of aliphatic nature containing high fractions of carboxylate groups. A contribution of these organic species towards the bentonite colloid stability and sorption of actinides is assumed to be likely.

Colloid influence on the radionuclide migration from a nuclear waste repository

Abstract Colloid formation is discussed as a possible pathway for the radionuclide release from a nuclear waste repository. An assessment of the colloid relevance on radionuclide migration requires insight into the possible colloid generation mechanisms, their stability and mobility under given groundwater conditions. In various experiments dedicated to the investigation of nuclear waste form behaviour in contact with groundwater, colloidal species are observed mainly for the tri- and tetravalent actinides even in rather high ionic strength solutions where destabilization of colloids is expected. Experimental evidence of laboratory and field studies suggests colloid instability in saline groundwater with time. Groundwater of low ionic strength and high pH enhances colloid stability, as demonstrated in various laboratory and field experiments. The results of an in situ colloid characterization study at the Äspö hard rock laboratory in Sweden are discussed as an example. The mechanism of radionuclide interaction with colloids and notably the reversibility of the radionuclide-colloid binding are other key issues of colloid studies. Kinetic stabilization of the radionuclide binding to colloids may lead to a considerable enhancement of the colloid-mediated radionuclide migration. Substantial kinetic inhibition of actinide dissociation from humic colloids has been established by studying the behaviour of natural humic colloid borne U, Th and rare earth elements. Such behaviour might be explained by the incorporation of these elements into inorganic nanoparticles stabilized by humic coating. Spectroscopic evidence for the occurrence of actinide incorporation into colloidal structures is briefly discussed and the importance of considering the kinetics involved in all kinds of colloidal processes is emphasized. The enhanced migration of tri- and tetravalent actinide ions has been observed recently in various in situ dipole tests at the Grimsel hard rock laboratory in Switzerland. Such experimental findings underline the necessity of further studies on the colloid influence on actinide mobility. Moreover, an improved understanding of colloid-rock interaction mechanisms is required, which is essential for the description and prediction of colloid filtration processes.

Humic colloid mediated transport of tetravalent actinides and technetium

Abstract The humic colloid mediated transport of tetravalent actinide and technetium ions is discussed. Numerous published and new results are used from comparable experimental systems and conditions. The results originate from investigations on two humic rich Gorleben groundwater/sediment systems under near-natural conditions. The conditioning time between groundwater and radionuclides vary from few minutes to four years. The migration time (residence time in the column) is varied from four hours to three days. The concentration of radionuclides added is varied between about 10-12 and 10-4mol/L. Humic colloids in natural groundwater contain trace metal ions in varying concentrations. In addition to studying the behavior of radionuclides added to the groundwater in the laboratory, the dissociation behavior of these natural tetravalent trace metal ions is studied by scavenging dissociating ions with a cation exchanger. The results show a kinetic behavior of the trace metal ion interaction as frequently reported in the literature, i.e. with increasing contact time, slow dissociation modes are progressively populated resulting in an enhanced radionuclide transport. The strong dependency of the radionuclide transport on the radionuclide concentration shows that humic colloid mediated transport may be overestimated if radionuclide concentrations used in laboratory studies are too high. Dissociation of the natural inventory of humic associated tetravalent trace metal ions proceeds with a kinetics comparable to the radionuclides added. This, however, is only observed for a small portion of the inventory and extrapolation beyond the observation range is not permissible. Therefore, the key question for application to predictive modeling of the mobility of tetravalent radionuclides in natural groundwater is if the behavior of the natural trace element inventory is applicable or the behavior of radionuclides added in laboratory studies.

Synthesis and metal ion complexation properties of a novel polyethyleneimine N-methylhydroxamic acid water soluble polymer

A major goal of our research program is to develop water soluble chelating polymers for the selective separation of trivalent cations such as iron(III) and tetravalent actinide ions such as Th(IV) or Pu(IV) from aqueous, radioactive waste streams. As part of this effort, we have synthesized a new water soluble polyethyleneimine polymer that incorporates N-methylhydroxamic acid chelating groups. Both the synthesis of this chelating polymer, PEI-NMH, and its ability to achieve selective separation of target metal ions using ultrafiltration will be discussed. It is interesting to note that the PEI-NMH polymer selectively binds iron(III) over thorium(IV) at pH 1 in contrast to its primary hydroxamate analog PEI-H. The binding properties of these polymers have been evaluated using both potentiometric and spectrophotometric methods. A comparison of the binding properties of PEI-NMH and PEI-H reveals the importance of the nature of the ligand in the complexation process.

Coordination of rare-earth elements in complexes with monovacant Wells-Dawson polyoxoanions.

The alpha-1 and alpha-2 isomers of the monovacant Wells-Dawson heteropolyoxoanion [P(2)W(17)O(61)](10-) are complexants of trivalent rare-earth (RE) ions and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and actinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions with alpha-1-[P(2)W(17)O(61)](10-) and alpha-2-[P(2)W(17)O(61)](10-) were investigated to address issues of complex formation and stability. We present structural insights about the Ln(III) coordination environment and hydration in two types of stoichiometric complexes, [Ln(alpha-1-P(2)W(17)O(61))](7-) and [Ln(alpha-2-X(2)W(17)O(61))(2)](17-) (for Ln identical with Sm, Eu, Lu; X identical with P, As). The crystal and molecular structures of [(H(2)O)(4)Lu(alpha-1-P(2)W(17)O(61))](7-) (1) and [Lu(alpha-2-P(2)W(17)O(61))(2)](17-) (2) were solved and refined through use of single-crystal X-ray diffraction. The crystallographic results are supported with corresponding insights from XAFS (X-ray absorption fine structure) for a series of nine solid-state complexes as well as from optical luminescence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln ions are eight-coordinate with oxygen atoms in a square antiprism arrangement. For the 1:1 stoichiometric Ln/alpha-1-[P(2)W(17)O(61)](10-) complexes, the Ln ions are bound to four O atoms of the lacunary polyoxometalate framework in addition to four O atoms from solvent (water) molecules as [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-). This structure (1) is the first of its kind for any metal complex of alpha-1-[P(2)W(17)O(61)](10-), and the data indicate that the general stoichiometry [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-) is maintained throughout the lanthanide series. For the 1:2 stoichiometric Ln/alpha-2-[X(2)W(17)O(61)](10-) complexes, no water molecules are in the Ln-O(8) coordination sphere. The Ln ions are bound to eight O atoms-four from each of two heteropolyanions-as [Ln(alpha-2-X(2)W(17)O(61))(2)](17-). The average Ln-O interatomic distances decrease across the lanthanide series, consistent with the decreasing Ln ionic radius.

Analysis of the crystal-field spectra of the actinide tetrafluorides. II. AmF4, CmF4, Cm4+:CeF4, and Bk4+:CeF4

We report a systematic analysis of the crystal-field spectra of four fluoride compounds containing tetravalent actinide ions. The first part of this work [J. Chem. Phys. 95, 7194 (1991)] provided interpretation of the absorption spectra of UF4, NpF4, and PuF4. To extend our analysis to heavier elements of the series, low-temperature absorption spectra of AmF4 and CmF4, and site selective laser-induced emission and excitation spectra of Cm4+:CeF4 and Bk4+:CeF4 were obtained. A model energy level calculation was found to be in good agreement with the experimental results. It is shown that the crystal-field interaction in combination with spin–orbital coupling results in significant J mixing in the excited states, but ground state wave functions are still relatively pure in J character for the tetravalent actinide ions Am4+, Cm4+, and Bk4+. Trends in the parameters of the effective operator Hamiltonian are compared with those of a Hartree–Fock free-ion model. Interpretation of the ground-state splitting of the nominal S-state ion Bk4+ in CeF4 and color center formation in AmF4 are also discussed.

Linear and non-linear spectroscopy of tetravalent actinide ions in CeF 4

This paper reports laser spectroscopy studies on the tetravalent actinide ions Bk 4+ and Cf 4+ doped into CeF 4. Fluorescence line narrowing (FLN) and spectral hole burning were applied to unravel the complex 5f state spectra that arise from inhomogeneous line broadening and the presence of two intrinsic metal ion sites. When a persistent spectral hole was burned in an inhomogeneously broadened zero phonon line of 1 at.% Cf 4+ in CeF 4, satellite spectral holes were also observed in other inhomogeneously broadened lines of Cf 4+. In the FLN spectra of 0.1 at.% Bk 4+ in CeF 4, a linear relationship was observed between the excitation-laser photon energy and the energies of the Bk 4+ emission lines. These experimental results can be understood in terms of one limit of a general model of inhomogeneous broadening, namely the limit in which there is a one-to-one correspondence between the optical transition energy and local structure in a disordered system.

Potentiometric studies on aqueous fluoride complexes of actinides: Stability constants of Th(IV)-, U(IV)-, Np(IV)- and Pu(IV)-fluorides

Complex formation of Th(IV), U(IV), Np(IV), and Pu(IV) with fluoride ions in aqueous perchloric acid solution of 1M ionic strength has been studied potentiometrically, using a fluoride ion selective electrode. A highly acidic medium was essential to prevent hydrolysis and disproportionation of the tetravalent actinide ions. A procedure developed earlier in this laboratory for studying the fluoride complexes of hexavalent actinides was successfully extended to this work. The β1 values follow the trend Th(IV) Np(IV)>Pu(IV), and β2 values the trend Th(IV) Np(IV)<Pu(IV), while β3 and β4 values followed an increasing order.

Critical stability constants, enthalpies and entropies for the formation of metal complexes of aminopolycarboxylic acids and carboxylic acids

Formation constants, enthalpies of formation, and entropies of formation were compiled and critically selected for complexes of alkali metal ions, alkaline earth ions, trivalent lanthanide ions, trivalent and tetravalent actinide ions, dioxoactinide ions, and divalent and trivalent ions of the metals of the first transition series with aminopolycarboxylates (CDTA, DTPA, EDDA, EDTA, IDA, MIDA, NTA, TTHA), and carboxylates (RCO 2 −, oxalate, malonate). The trends in the formation constants, enthalpies and entropies are discussed. The formation constants of the aminopolycarboxylate complexes are compared with those of the carboxylate complexes. Several constants not yet measured experimentally were estimated. The long-range objective of these investigations is a complete data base that will include all metal ions and ligands present in the environment. The data base can be used to calculate the distribution of metal ions among various ligands and provide the information needed for the assessment of the toxicities of metal ions, ligands and their coordination compounds.

Optical and magnetic properties of tetravalent actinide ions and compounds

A survey is given of the optical and magnetic data available for tetravalent actinide ions in a variety of single crystals and as pure compounds. Where possible, conclusions are drawn from the values of the “free-ion” parameters in crystals when compared with the values for the free ions in the gaseous phase. Problems with the theoretical model are discussed.

Complex formation between trivalent actinides and pyrocatechol

A continuous research work was done for the development of powerful chelating agents specific for trivalent and tetravalent actinide ions. In the recent years intensive investigations were carried out using some catecholate chelating ligands connected with the strong complexing power of the catecholate group [1–3]. The catecholate ligand is essential for the metabolism in biological systems especially some substituted catechol derivatives. They can be used as specific sequestering agents for removal of actinides from biological systems [4], which is important for environmental research and nuclear medicine as well. It was found also that some substituted pyrocatechols act as effective extractants for both tracer and macroquantities of transplutonium elements as well as rare earths from strong alkaline solutions [5]. While most of the research was performed studying tetravalent actinides, little is known about the complexation between pyrocatechol and actinides in the trivalent state. Thus, we have chosen to investigate the catecholate reactions with trivalent actinides by means of the solvent extraction method. We found out that hydrolysis and sorption of Am 3+ prevail from pH higher than 6 and because pyrocatecholate ligand is not stable in the alkaline region the experiments have to be performed in argon atmosphere. The complex formation between americium and pyrocatechol is obviously stronger than both sorption and hydrolysis at pH higher than 9. There is also experimental evidence that the ligand itself is extracted by the TBP agent which further complicates the system. The mechanism of complex formation and the stability of the complexes formed will be investigated in detail to serve as a base for comparison with the complexation of some biologically active catechols. This work is supported by the International Atomic Energy Agency and the Swedish Natural Science Council.

Synthesis and crystal growth of some thorium and uranium tetrahalides: ThCl 4, ThBr 4, UCl 4 and UBr 4

A kilogram scale production of thorium and uranium tetrachloride and tetrabromide in polycrystalline form has been developed. Crystal growth of single crystals of several cm 3 in size using the Bridgman method is also described. Some physical properties of these tetrahalides published elsewhere, such as crystalline structure, refractive index, luminescence, vibrations modes, and phase transition are briefly mentioned. The optical properties of ThBr 4 and ThCl 4 and the use of these materials as a host matrix for spectroscopic studies of tetravalent actinide ions is also reported.