Baumann A, Yalçıntaş E, Gaona X, Polly R, Dardenne K, Prüßmann T, Rothe J, Altmaier M, Geckeis H
Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany.
Dalton Trans. 2018 Mar 28;47(12):4377-4392. doi: 10.1039/c8dt00250a. Epub 2018 Mar 2.
The solubility of Tc(iv) was investigated in dilute to concentrated carbonate solutions (0.01 M ≤ C≤ 1.0 M, with C = [HCO] + [CO]) under systematic variation of ionic strength (I = 0.3-5.0 M NaHCO-NaCO-NaCl-NaOH) and pH (-log[H] = 8.5-14.5). Strongly reducing conditions (pe + pH≈ 2) were set with Sn(ii). Carbonate enhances the solubility of Tc(iv) in alkaline conditions by up to 3.5 log-units compared to carbonate-free systems. Solvent extraction and XANES confirmed that Tc was kept as +IV during the timeframe of the experiments (≤ 650 days). Solid phase characterization performed by XAFS, XRD, SEM-EDS, chemical analysis and TG-DTA confirmed that TcO·0.6HO(am) controls the solubility of Tc(iv) under the conditions investigated. Slope analysis of the solubility data in combination with solid/aqueous phase characterization and DFT calculations indicate the predominance of the species Tc(OH)CO at pH≤ 11 and C≥ 0.01 M, for which thermodynamic and activity models are derived. Solubility data obtained above pH≈ 11 indicates the formation of previously unreported Tc(iv)-carbonate species, possibly Tc(OH)CO, although the likely formation of additional complexes prevents deriving a thermodynamic model valid for this pH-region. This work provides the most comprehensive thermodynamic dataset available for the system Tc-Na-Cl-OH-HCO-CO-HO(l) valid under a range of conditions relevant for nuclear waste disposal.
在离子强度（I = 0.3 - 5.0 M NaHCO₃ - Na₂CO₃ - NaCl - NaOH）和pH值（-log[H⁺] = 8.5 - 14.5）系统变化的情况下，研究了Tc(IV)在稀至浓碳酸盐溶液（0.01 M ≤ C ≤ 1.0 M，其中C = [HCO₃⁻] + [CO₃²⁻]）中的溶解度。用Sn(II)设定强还原条件（pe + pH ≈ 2）。与无碳酸盐体系相比，碳酸盐在碱性条件下可将Tc(IV)的溶解度提高多达3.5个对数单位。溶剂萃取和XANES证实，在实验时间范围内（≤ 650天），Tc保持为+IV价态。通过XAFS、XRD、SEM - EDS、化学分析和TG - DTA进行的固相表征证实，TcO₂·0.6H₂O(am)在所研究的条件下控制着Tc(IV)的溶解度。溶解度数据的斜率分析结合固/水相表征和DFT计算表明，在pH ≤ 11且C ≥ 0.01 M时，物种Tc(OH)CO₃占主导地位，并据此推导了热力学和活度模型。在pH ≈ 11以上获得的溶解度数据表明形成了以前未报道的Tc(IV) - 碳酸盐物种，可能是Tc(OH)₂CO₃，尽管可能形成的其他络合物妨碍了推导适用于该pH范围的热力学模型。这项工作提供了在一系列与核废料处置相关的条件下有效的、关于Tc - Na - Cl - OH - HCO₃ - CO₃ - H₂O(l)体系的最全面的热力学数据集。
