Hydrolysis of Np(IV) and Pu(IV) and their complexation by aqueous Si(OH)4

Formation of plutonium(IV) silicate species in very alkaline reactive media

Studying the speciation of Pu(IV) in very alkaline and silicate ion rich reactive media allowed identification of the formation of plutonium(IV)-silicate colloidal suspensions which were stable for months. These colloids were stabilized in aqueous solution for pH > 13 and for concentrations around 10^-2 mol L^-1. Successive filtration processes allowed evaluation of their size, which was found to be smaller than 6 nm. Their structural characterization by XAS evidenced that their structure was similar to those identified for the other tetravalent actinide-silicate colloidal systems like thorium, uranium and neptunium. Their formation could explain the increase of plutonium solubility usually observed in alkaline silicate-rich solutions and could affect the plutonium mobility as a result in contaminated sites or in other environmental permeable media.

Plutonium complexes in water: new approach to ab initio modeling

Geometry optimization and the electronic structure calculations of Pu Z+ complexes (Z = 3–6) in water solution have been performed, within the framework of the DMol3 and Relativistic Discrete-Variational (RDV) methods. For the simulation of Pu Z+ molecular environment in aqueous solution we used 22 and 32 water molecules randomly distributed around cation. To model the effect of bulk solvent environment we used COSMO (Conductor-like Screening Model) potential for water (ε = 78.54). The obtained results showed that this approach allows the modeling of water dissociation and the formation of hydrolysis products. Our previously suggested scheme for the calculation of interaction energies between selected fragments of multi-molecular systems provides the quantitative estimation of the interaction strengths between plutonium in various oxidation states and each ligand in the first and second coordination shells in water solution.

The formation of PuSiO4 under hydrothermal conditions

Attempts to synthesize plutonium(iv) silicate, PuSiO4, have been made on the basis of results recently reported in the literature for CeSiO4, ThSiO4, and USiO4 under hydrothermal conditions. Although it was not possible to prepare PuSiO4via applying the conditions reported for thorium and uranium, an efficient method of PuSiO4 synthesis was established by applying the conditions optimized for the CeSiO4 system. This method was based on the slow oxidation of plutonium(iii) silicate reactants under hydrothermal conditions at 150 °C in hydrochloric acid (pH = 3-4). These results shed new light on the potential behavior of plutonium in reductive environments, highlighting the representative nature of cerium surrogates when studying plutonium under such conditions and providing some important pieces of information regarding plutonium chemistry in silicate solutions.

Hydration Structure and Hydrolysis of U(IV) and Np(IV) Ions: A Comparative Density Functional Study Using a Modified Continuum Solvation Approach

We studied the hydration and the first hydrolysis reaction of U(IV) and Np(IV) ions in an aqueous environment, applying a relativistic density functional method together with a recently proposed variant of a continuum solvation model where the solute cavities are constructed with effective atomic radii, based on charge-dependent scaling factors. In this way, one obtains improved solvation energies of charged species. We demonstrate that solute cavities, constructed with scaled atomic radii as described, permit one to calculate hydrolysis constants of acceptable accuracy. As a consequence, one is also able to estimate free hydration energies of U(IV) and Np(IV) in adequate agreement with empirical data. According to the model calculations, U(IV) is coordinated by eight to nine water molecules, while the preferred coordination number of Np(IV) is 8. For the highly charged ions under study, the modified solvation model simultaneously yields improved geometries, hydration energies, and hydrolysis constants.

Neptunium(iv)-hydroxamate complexes: their speciation, and kinetics and mechanism of hydrolysis

First time determination of rate parameters for hydrolysis of mono- and bis-acetohydroxamatoneptunium(iv) complexes under conditions relevant to nuclear reprocessing.

Coordination and hydrolysis of plutonium ions in aqueous solution using Car-Parrinello molecular dynamics free energy simulations

Car-Parrinello molecular dynamics (CPMD) simulations have been used to examine the hydration structures, coordination energetics, and the first hydrolysis constants of Pu(3+), Pu(4+), PuO2(+), and PuO2(2+) ions in aqueous solution at 300 K. The coordination numbers and structural properties of the first shell of these ions are in good agreement with available experimental estimates. The hexavalent PuO2(2+) species is coordinated to five aquo ligands while the pentavalent PuO2(+) complex is coordinated to four aquo ligands. The Pu(3+) and Pu(4+) ions are both coordinated to eight water molecules. The first hydrolysis constants obtained for Pu(3+) and PuO2(2+) are 6.65 and 5.70, respectively, all within 0.3 pH unit of the experimental values (6.90 and 5.50, respectively). The hydrolysis constant of Pu(4+), 0.17, disagrees with the value of -0.60 in the most recent update of the Nuclear Energy Agency Thermochemical Database (NEA-TDB) but supports recent experimental findings. The hydrolysis constant of PuO2(+), 9.51, supports the experimental results of Bennett et al. [Radiochim. Acta 1992, 56, 15]. A correlation between the pKa of the first hydrolysis reaction and the effective charge of the plutonium center was found.

Thermodynamic study of metal silicate complexation in perchlorate media

The thermodynamic parameters of formation of theo-silicate complexes of UO22+, Cm3+and Eu3+have been measured in the temperature range 5−45 °C in an aqueous medium of 0.20 M NaClO4) ionic strength. Solvent extraction with bis(2-ethylhexyl) phosphoric acid (HDEHP) was used to measure the stability constant (log β1) values of the silicate complexes as a function of temperature to obtain the enthalpies and entropies of complexation. The complexation reactions are entropy driven and opposed by the enthalpy. These thermodynamic values are consistent with the formation of an inner sphere complex.

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 10-5to 4×10-4M and at pHc0.3–2.1. 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. Thepevalues calculated from the plutonium redox couples Pu(V)/Pu(VI) and PuO2(am,hyd)/Pu(V) were found to be consistent with the measuredpevalues 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-natI=0.5 M and 23 ± 2 °C are calculated and corrected to zero ionic strength using the specific ion interaction theory (SIT): logβ°11= 14.0 ± 0.2, logβ°12= 26.8 ± 0.6, and logβ°13= 38.9 ± 0.9.

Sorption of europium(III) on polymeric silica in perchlorate media

Sorption of europium, Eu(III) with polymeric silicic acid has been studied under varying experimental conditions such as pH, ionic strength (I), concentration of silicic acid and of Eu(III), the presence of carbonate ions, and aging time. Sorption studies of Eu(III) at pH 7.50 ± 0.50) andI= 0.20 M NaClO4indicate 1.32 ± 0.06 × 10-5M Eu as the Critical Coagulation Concentration, CCC. Sorption of Eu(III) on colloidal silica increases with increased ionic strength from 80% (0.20 M NaClO4) to 93% (0.89 M NaClO4). The coagulation process is favored by an increase in silica concentration. The influence of CO32-(1.00 × 10-3M) on the reaction of Eu(III) with polysilicic acid was negligible in the pH range 7.0 to 9.0. Dissolved Eu3+binds more strongly to the silicate polymers than do Cs+and Cu2+.

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 10−5M to 10-2M (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 pHcand 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.