Ezgi Yalçıntaş

Application of neodymium isotope ratio measurements for the origin assessment of uranium ore concentrates

A novel procedure has been developed for the measurement of (143)Nd/(144)Nd isotope ratio in various uranium-bearing materials, such as uranium ores and ore concentrates (UOC) in order to evaluate the usefulness and applicability of variations of (143)Nd/(144)Nd isotope ratio for provenance assessment in nuclear forensics. Neodymium was separated and pre-concentrated by extraction chromatography and then the isotope ratios were measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The method was validated by the measurement of standard reference materials (La Jolla, JB-2 and BCR-2) and the applicability of the procedure was demonstrated by the analysis of uranium samples of world-wide origin. The investigated samples show distinct (143)Nd/(144)Nd ratio depending on the ore type, deposit age and Sm/Nd ratio. Together with other characteristics of the material in question, the Nd isotope ratio is a promising signature for nuclear forensics and suggests being indicative of the source material, the uranium ore.

Redox chemistry of Tc(VII)/Tc(IV) in dilute to concentrated NaCl and MgCl2 solutions

Abstract The redox behaviour of Tc(VII)/Tc(IV) was investigated within the pHc range 2–14.6 in (0.5 M and 5.0 M) NaCl and (0.25 M, 2.0 M and 4.5 M) MgCl2 solutions in the presence of different reducing agents (Na2S2O4, Sn(II), Fe(II)/Fe(III), Fe powder) and macroscopic amounts of Fe minerals (magnetite, mackinawite, siderite: S/L = 20–30 g L–1). In the first group of samples, the decrease of the initial Tc concentration (1 · 10–5 M, as Tc(VII)) indicated the reduction to Tc(IV) according to the chemical reaction TcO4– + 4H++ 3e– ↔ TcO2 · 1.6H2O(s) + 0.4H2O. Redox speciation of Tc in the aqueous phase was further confirmed by solvent extraction. A good agreement is obtained between the experimentally determined Tc redox distribution and thermodynamic calculations based on NEA–TDB (Nuclear Energy Agency, Thermochemical Database) and ionic strength corrections by SIT or Pitzer approaches. These observations indicate that experimental pHc and Eh values in buffered systems can be considered as reliable parameters to predict the redox behaviour of Tc in dilute to highly concentrated NaCl and MgCl2 solutions. Eh of the system and aqueous concentration of Tc(IV) in equilibrium with TcO2 · 1.6H2O(s) are strongly affected by elevated ionic strength, especially in the case of 4.5 M MgCl2 solutions. In such concentrated brines and under alkaline conditions (pHc = pHmax ∼ 9), kinetics play a relevant role and thermodynamic equilibrium for the system Tc(IV)(aq) ↔ Tc(IV)(s) was not attained from oversaturation conditions within the timeframe of this study (395 days). Tc(VII) is reduced to Tc(IV) by magnetite, mackinawite and siderite suspensions at pHc = 8 – 9 in concentrated NaCl and MgCl2 solutions. Sorption is very high in all cases (Rd ≥ 103 L kg–1), although Rd values are significantly lower in 4.5 M MgCl2 solutions. XANES (X-ray absorption near edge spectroscopy) evaluation of these samples confirms that Tc(VII) is reduced to Tc(IV) by Fe(II) minerals also in concentrated NaCl and MgCl2 brines.

Thermodynamic description of Tc(iv) solubility and hydrolysis in dilute to concentrated NaCl, MgCl2 and CaCl2 solutions.

We present the first systematic investigation of Tc(iv) solubility, hydrolysis and speciation in dilute to concentrated NaCl, MgCl2 and CaCl2 systems, and comprehensive thermodynamic and activity models for the system Tc(4+)-H(+)-Na(+)-Mg(2+)-Ca(2+)-OH(-)-Cl(-)-H2O using both SIT and Pitzer approaches. The results are advancing the fundamental scientific understanding of Tc(iv) solution chemistry and are highly relevant in the applied context of nuclear waste disposal. The solubility of Tc(iv) was investigated in carbonate-free NaCl-NaOH (0.1-5.0 M), MgCl2 (0.25-4.5 M) and CaCl2 (0.25-4.5 M) solutions within 2 ≤ pHm≤ 14.5. Undersaturation solubility experiments were performed under an Ar atmosphere at T = 22 ± 2 °C. Strongly reducing conditions (pe + pHm≤ 2) were imposed with Na2S2O4, SnCl2 and Fe powder to stabilize technetium in the +IV redox state. The predominance of Tc(iv) in the aqueous phase was confirmed by solvent extraction and XANES/EXAFS spectroscopy. Solid phase characterization was accomplished after attaining thermodynamic equilibrium using XRD, SEM-EDS, XANES/EXAFS, TG-DTA and quantitative chemical analysis, and indicated that TcO2·0.6H2O(s) exerts solubility-control in all evaluated systems. The definition of the polyatomic Tc3O5(2+) species instead of TcO(2+) is favoured under acidic conditions, consistently with slope analysis (mTcvs. pHm) of the solubility data gained in this work and spectroscopic evidence previously reported in the literature. The additional formation of Tc(iv)-OH/O-Cl aqueous species in concentrated chloride media ([Cl(-)] = 9 M) and pHm≤ 4 is suggested by solubility and EXAFS data. The pH-independent behaviour of the solubility observed under weakly acidic to weakly alkaline pHm conditions can be explained with the equilibrium reaction TcO2·0.6H2O(s) + 0.4H2O(l) ⇔ TcO(OH)2(aq). Solubility data determined in dilute NaCl systems with pHm≥ 11 follow a well-defined slope of +1, consistent with the predominance of TcO(OH)3(-) previously selected by NEA-TDB. In concentrated MgCl2 and CaCl2 solutions with pHm≥ 8, the formation of the ternary Mg3[TcO(OH)5](3+) and Ca3[TcO(OH)5](3+) species is proposed based on the slope analysis of the solubility data, model calculations and previous observations for analogous An(iv) and Zr(iv) systems. The formation and stability of these hitherto unknown Tc(iv) species are supported by DFT calculations. Based on the newly generated experimental data and previous spectroscopic observations, new comprehensive chemical, thermodynamic and activity models (SIT, Pitzer) for these systems are derived.

Solubility and hydrolysis of Tc(IV) in dilute to concentrated KCl solutions: an extended thermodynamic model for Tc4+–H+–K+–Na+–Mg2+–Ca2+–OH−–Cl−–H2O(l) mixed systems

The solubility of 99Tc(IV) under strongly reducing conditions (pe + pHm = 2, with pHm = −log[H+]) is investigated in KCl solutions of different ionic strengths (I = 0.1, 0.5, 3.0 and 4.0 M) at 1.5 ≤ pHm ≤ 14.5. Undersaturation solubility experiments were conducted in an Ar-glovebox (O2 < 1 ppm) at T = (22 ± 2) °C. Liquid–liquid extraction confirmed that the redox state of Tc was kept as +IV during the timeframe of the experiments (≤500 days). Solid phase characterization performed by XRD, SEM-EDS, quantitative chemical analysis and TG-DTA confirmed that TcO2·0.6H2O(s) controls the solubility of Tc(IV) under the conditions investigated. Experimental solubility data determined at 1 ≤ pHm ≤ 10 are properly explained assuming the predominance of Tc3O52+ and TcO(OH)2(aq) in the aqueous phase, and considering thermodynamic and activity models recently reported in the literature for these species. Above pHm ≈ 10, the combination of solid phase characterization with slope analysis of solubility data indicates a solubility control by the chemical reaction TcO2·0.6H2O(s) + 1.4H2O(l) ⇔ TcO(OH)3− + H+. The hydrolysis constant and SIT/Pitzer ion interaction coefficients of TcO(OH)3− with K+ have been determined in the present work. Additional solubility experiments with TcO2·0.6H2O(s) in mixed alkaline KCl–NaCl–MgCl2–CaCl2 electrolyte solutions of specific relevance in the context of nuclear waste disposal show excellent agreement with thermodynamic calculations using thermodynamic and activity models derived in our group for the system Tc4+–H+–K+–Na+–Mg2+–Ca2+–OH−–Cl−–H2O(l).