High-level, long-lived nuclear waste arising from spent fuel reprocessing is vitrified in silicate glasses for final disposal in deep geologic formations. In order to better understand the mechanisms driving glass dissolution, glass alteration studies, based on silicon isotope ratio monitoring of Si-doped aqueous solutions, were carried out in laboratories. This work explores the capabilities of the new type of quadrupole-based ICP-MS, the Agilent 8800 tandem quadrupole ICP-MS/MS, for accurate silicon isotope ratio determination for alteration studies of nuclear waste glasses. In order to avoid silicon polyatomic interferences, a new analytical method was developed using O as the reaction gas in the Octopole Reaction System (ORS), and silicon isotopes were measured in mass-shift mode. A careful analysis of the potential polyatomic interferences on SiO and SiO ion species was performed, and we found that SiO ion species suffer from important polyatomic interferences coming from the matrix of sample and standard solutions (0.5M HNO). For SiO, no interferences were detected, and thus, these ion species were chosen for silicon isotope ratio determination. A number of key settings for accurate isotope ratio analysis like, detector dead time, integration time, number of sweeps, wait time offset, memory blank and instrumental mass fractionation, were considered and optimized. Particular attention was paid to the optimization of abundance sensitivity of the quadrupole mass filter before the ORS. We showed that poor abundance sensitivity leads to a significant shift of the data away from the Exponential Mass Fractionation Law (EMFL) due to the spectral overlaps of silicon isotopes combined with different oxygen isotopes (i.e. SiOO, SiOO). The developed method was validated by measuring a series of reference solutions with different Si enrichment. Isotope ratio trueness, uncertainty and repeatability were found to be<0.2%, <0.5% and <0.6%, respectively. These performances meet the requirements of the studies of nuclear glasses alteration and open up possibilities to use this method for precise determination of silicon content in natural samples by Isotope Dilution.
Iron oxyhydroxides play a significant role in radium behavior in uranium mill tailings. Although they are present at trace level (less than 1% in mass), iron oxyhydroxides are responsible of the retention of significant amounts of radium in these residues. A part of these oxyhydroxides is present as amorphous or ill-crystallized forms, the other part corresponds to crystalline iron oxyhydroxides like goethite.In this paper, we present data concerning radium sorption on a well-characterized goethite (α-FeOOH). Batch experiments have been performed and the influence of different parameters such as contact time, pH and electrolyte concentration of the solution has been examined. The sorption equilibrium was obtained after a contact time of 1 hour. In sodium perchlorate medium, the amount of sorbed radium reaches a maximum for pH higher than 8 and no significant influence of sodium concentration was observed. However, the calcium concentration greatly influences the radium sorption: the greater the calcium concentration, the less radium is sorbed. The kinetics of the radium desorption are slower than those of sorption: more than 30% of radium remains sorbed on goethite after 1 week of contact time.Using the characteristics of the goethite surface (total number of sites, intrinsic acidity constants, intrinsic constants of calcium), modelling of the experimental data with inner sphere complexes gave satisfactory results. Two surface species involving radium can be formed (SORa + and SORaOH). These sorption constants determined allow prediction of the radium sorption onto goethite in the chemical conditions of uranium mill tailings.
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