Vasiliev Aleksandr N, Banik Nidhu L, Marsac Rémi, Froehlich Daniel R, Rothe Jörg, Kalmykov Stepan N, Marquardt Christian M
Karlsruher Institut für Technologie (KIT), Institut für Nukleare Entsorgung, P.O. Box 3640, 76021 Karlsruhe, Germany.
Dalton Trans. 2015 Feb 28;44(8):3837-44. doi: 10.1039/c4dt03688c.
Low molecular weight organics (LMWO; e.g. acetate, propionate, lactate) can significantly impact the speciation and mobility of radionuclides in aqueous media. Natural clay rock formation, considered as a potential host rock for nuclear waste disposal, can contain a significant amount of organic matter. There are less thermodynamic data reported for the complexation of pentavalent actinides with LMWO, especially under elevated temperature conditions, relevant for assessing the long-term safety of disposal options for heat-producing high-level nuclear waste. In the present study, the complexation of Np(v) with propionate is studied using spectroscopic techniques in 0.5-4 M NaCl solutions by systematic variation of the ligand concentration and temperature. Slope analysis shows the formation of the 1 : 1 NpO2-propionate complex (NpO2Prop). The local structure of the NpO2-propionate complex is determined by extended X-ray absorption fine structure spectroscopy, the results of which suggest that propionate binds to Np(v) in a bidentate mode. Using the specific ion interaction theory (SIT), the stability constant at zero ionic strength and 25 °C is determined as log β°1,1 = 1.26 ± 0.03. The stability constants increase continuously with increasing temperature between 20 and 85 °C. The log β0 values are linearly correlated with the reciprocal temperature, indicating ΔrH = const. and ΔrC = 0, allowing the calculation of ΔrH and ΔrS for the formation of the NpO2-propionate complex using the integrated van't Hoff equation. The thermodynamic evaluation indicates that the reaction is endothermic and entropy driven.
低分子量有机物（LMWO；例如乙酸盐、丙酸盐、乳酸盐）会显著影响放射性核素在水介质中的形态和迁移率。天然粘土岩层被视为核废料处置的潜在主岩，可能含有大量有机物。关于五价锕系元素与低分子量有机物络合的热力学数据报道较少，尤其是在高温条件下，而高温条件对于评估产热高放核废料处置方案的长期安全性至关重要。在本研究中，通过系统改变配体浓度和温度，利用光谱技术在0.5 - 4 M氯化钠溶液中研究了Np(v)与丙酸盐的络合作用。斜率分析表明形成了1:1的NpO₂ - 丙酸盐络合物（NpO₂Prop）。通过扩展X射线吸收精细结构光谱确定了NpO₂ - 丙酸盐络合物的局部结构，结果表明丙酸盐以双齿模式与Np(v)结合。使用特定离子相互作用理论（SIT），确定零离子强度和25°C时的稳定常数为logβ°1,1 = 1.26 ± 0.03。在20至85°C之间，稳定常数随温度升高而持续增加。logβ0值与倒数温度呈线性相关，表明ΔrH = const.且ΔrC = 0，这使得可以使用积分范特霍夫方程计算形成NpO₂ - 丙酸盐络合物的ΔrH和ΔrS。热力学评估表明该反应是吸热的且由熵驱动。
