Redox chemistry of Tc(VII)/Tc(IV) in dilute to concentrated NaCl and MgCl2 solutions
Redox chemistry of Tc(VII)/Tc(IV) in dilute to concentrated NaCl and MgCl2 solutions
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稀至浓 NaCl 和 MgCl2 溶液中 Tc(VII)/Tc(IV) 的氧化还原化学
DOI：
10.1515/ract-2014-2272
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发表时间：
2015
影响因子：
1.8
通讯作者：
H. Geckeis
中科院分区：
文献类型：
--
作者：
E. Yalcintas;X. Gaona;Andreas C Scheinost;Taishi Kobayashi;M. Altmaier;H. Geckeis
本文研究了Tc（VII）/Tc（IV）体系在pH 2-14.6范围内的氧化还原行为。（0.5 M和5.0 M）NaCl和（0.25 M、2.0 M和4.5 M）MgCl 2溶液，存在不同还原剂（Na 2S 2 O 4、Sn（II）、Fe（II）/Fe（III）、Fe粉）和宏观量的Fe矿物（磁铁矿、镁钠闪石、菱铁矿：S/L = 20-30 g L-1）。在第一组样品中，初始Tc浓度（1 · 10-5 M，作为Tc（VII））的降低表明根据化学反应TcO 4-+4H ++3e-ParticleTcO 2· 1.6H2O（s）+0.4H2O还原为Tc（IV）。氧化还原形态的Tc在水相进一步证实了溶剂萃取。实验确定的Tc氧化还原分布和热力学计算的基础上NEA-TDB（核能署，热化学数据库）和离子强度修正SIT或Pitzer方法之间获得了良好的协议。这些观察结果表明，在缓冲系统中的实验pH值和Eh值可以被认为是可靠的参数来预测Tc在稀释到高浓度的NaCl和MgCl 2溶液中的氧化还原行为。体系的Eh和与TcO 2· 1.6H2O（s）平衡的Tc（IV）的水溶液浓度强烈地受升高的离子强度的影响，特别是在4.5M MgCl 2溶液的情况下。在这样的浓盐水和碱性条件下（pH c = pH max 1.9），动力学起相关作用，并且在本研究的时间范围内（395天），在过饱和条件下没有达到系统Tc（IV）（aq）ParticleTc（IV）（s）的热力学平衡。在pH = 8 - 9的浓NaCl和MgCl_2溶液中，用磁铁矿、镁钠石和菱铁矿悬浮液还原Tc（VII）为Tc（IV）。在所有情况下，吸附都非常高（Rd ≥ 103 L kg-1），尽管在4.5 M MgCl 2溶液中Rd值显著较低。这些样品的XANES（X射线吸收近边光谱）评价证实，Tc（VII）减少到Tc（IV）的Fe（II）矿物也在浓NaCl和MgCl 2盐水。
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.