On the use of speciation techniques and ab initio modelling to understand tetravalent actinide behavior in a biological medium: An(IV)DTPA case.

In the case of an accidental nuclear event, contamination of human bodies by actinide elements may occur. Such elements have the particularity to exhibit both radiological and chemical toxicities that may induce severe damages at several levels, depending on the biokinetics of the element. In order to eliminate the actinide elements before they are stored in target organs (liver, kidneys, or bone, depending on the element), sequestering agents must be quickly injected. However, to date, there is still no ideal sequestering agent, despite the recent interest in this topic due to contamination concerns. DTPA (diethylene triamine pentaacetic acid) is currently generating interest for the development of oral or alternative self-administrable forms. Although biokinetics data are mostly available, molecular scale characterization of actinide-DTPA complexes is still scarce. Nevertheless, strong interest is growing in the characterization of An(IV)DTPA(-) complexes at the molecular level because this opens the way for predicting the stability constants of unknown systems or even for developing new analytical strategies aimed at better and more selective decorporation. For this purpose, Extended X-ray Absorption Fine Structure (EXAFS) and Ab Initio Molecular Dynamics (AIMD) investigations were undertaken and compared with capillary electrophoresis (CE) used in a very unusual way. Indeed, it is commonly believed that CE is incapable of extracting structural information. In capillary electrophoresis, the electrophoretic mobility of an ion is a function of its charge and size. Despite very similar ratios, partial separations between An(IV)DTPA(-) species (An(IV) = Th, U, Np, Pu) were obtained. A linear relationship between the electrophoretic mobility and the actinide--oxygen distance calculated by AIMD was evidenced. As an example, the interpolated U-O distances in U(IV)DTPA(-) from CE-ICPMS experiments, EXAFS, AIMD, and the relationship between the stability constants and the ratio z/dAn-O, are all in agreement. This results in the capability to evaluate the stability constants for the formation of Pa(IV)DTPA(-), Am(IV)DTPA(-) or Bk(IV)DTPA(-).

Determination of weight distribution ratios of Pa(V) and Np(V) with some extraction chromatography resins and the AG1-X8 resin.

Literature data on distribution ratios (Dw) of Np(V) and Pa(V) for the AG1-X8 resin are scarce whereas those related on resin capacity factors (k') values for TEVA, TRU and U/TEVA resins are absent. Therefore, batch extraction experiments for Pa(V) and Np(V) from HCl and HNO3 media were realized, at tracer scale, with AG1-X8 and EIChroM resins (TEVA, TRU and U/TEVA). Based on the new Dw and k' values obtained in this study, a new protocol for Pa/Np separation has been developed leading to a better separation factor of 10(5) and a chemical yield of 97±3% and 99±1% for Pa and Np, respectively. A separation of (231)Pa from uranium matrix was successfully tested.

Determination of the absolute mobility and the equivalent ionic conductivity of NpO2+ at 25 degrees C and at infinite dilution by capillary electrophoresis-inductively coupled plasma-mass spectrometry.

The absolute mobility of NpO2+ and its equivalent ionic conductivity were extrapolated at 25 degrees C and at infinite dilution using a set of experimental data obtained at various ionic strengths. The separation was carried out by capillary electrophoresis (CE) at various concentrations of creatinine at a pH of 5. The detection of NpO2+ was performed by inductively coupled plasma mass spectrometry coupled on-line with CE. The following values have been found: mu0NpO2+ (25 degrees C) = (2.94 +/- 0.07) x 10(-4) cm2 V(-1) s(-1) and lambda0NpO2+ (x 10(4), 25 degrees C) = 28.3 +/- 0.7 m2 S mol(-1).

Comparison and improvement of the determinations of actinide low activities using several alpha liquid scintillation spectrometers

We applied three procedures using two a liquid scintillation spectrometers (PERALS and TRI-CARB) and two scintillation cocktails (Alphaex and Ultima Gold LLT) for the determination of alpha-emitter low activities. For each procedure, the limit of detection, the resolution, the separation factor, and the Fischer coefficient were determined in order to perform 232U-234U-238U isotopic measurements. The deconvolution usually performed is clean when the PERALS spectrometer is used. This is not possible for the TRI-CARB spectrometer using the Ultima Gold LLT scintillation cocktail. This problem was solved by combining the advantages of both techniques using the Alphaex scintillation cocktail in the TRI-CARB spectrometer. Under these conditions, the limit of detection was improved, the resolution decreased from 500-800 to 420-590 keV, and the separation factor increased from 0.9 to 1.1-1.2. This third procedure was applied with success for 232U-234U-238U isotopic experiments.

Determination of 226Ra in mineral drinking waters by alpha liquid scintillation with rejection of beta-gamma emitters.

The radiotoxicity of radium isotopes (especially the long-half-life 226Ra) requires their monitoring in drinking waters or nuclear wastes. We studied the applicability of the PERALS method of detection (photon electron rejecting alpha liquid scintillation) for radium measurement. This method combines alpha liquid scintillation with pulse shape analysis for beta rejection and specific chemical extractants included in the scintillating cocktail. Radium is separated by an extractive-scintillator cocktail called RADAEX containing 2-methyl-2-heptylnonanoic acid (HMHN) and dicyclohexano-21-crown-7 (Cy(2)21C7) as extractant molecules. The variation of the radium extraction has been studied relative to pH, salt concentrations, anion and cation effects, and the volume ratio between aqueous and organic phases. The main parameter affecting the radium extraction in mineral drinking water is its complexation by inorganic anions, especially sulfate. Due to the lack of thermodynamic data, some complexation constants had to be determined. For instance, the value reported in this paper for radium sulfate (log beta = 2.58 +/- 0.22) is in good agreement with that from the literature. The knowledge of complexation constants allows the determination of radium extraction recovery for any solution when the inorganic anion concentrations had been measured by capillary zone electrophoresis. The detection limit for this technique is found to be equal to 0.006 Bq.L-1 using only 6 mL of sample solution for analysis. Several French mineral waters have been studied and the results compared with determinations of uranium and thorium concentrations by ICPMS and time-resolved laser induced fluorescence (TRLIF).

Determination of Uranium, Thorium, Plutonium, Americium, and Curium Ultratraces by Photon Electron Rejecting α Liquid Scintillation.

The determination of activities of thorium, uranium, plutonium, americium, and curium at very low levels has been performed by a new α liquid scintillation system (PERALS, name registered to Ordela, Inc.). The limit of detection has been determined for these nuclides with calculated values often lower than those obtained by other methods, like ICPMS/HP/Mistral, time-resolved laser-induced spectrofluorometry, and α spectrometry. All the results obtained show that the PERALS system is a promising method for the determination of these activities at very low levels. However, its energy resolution is inferior in comparison to that obtained by α spectrometry. For this reason, we have developed a process for separation of the five actinides as quickly and easily as possible. For each actinide, the conditions required to obtain optimal extraction yields and a complete separation have been determined. It is possible to perform the separation in only six extraction steps and to measure activities as low as a few millibecquerels per liter independently. This process has been applied with success to French granitic mineral or doped water and to complex media (biological samples like urines). In this latter case, the extraction recoveries are not quantitative, and it is necessary to determine the recovery yields by labeling with spikes like (230)Th, (232)U, (236)Pu, (248)Cm, and (148)Gd.