Np(v) complexation with propionate in 0.5-4 M NaCl solutions at 20-85 °C.

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 ß°1,1 = 1.26 ± 0.03. The stability constants increase continuously with increasing temperature between 20 and 85 °C. The log ß0 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.

Characterization of redox sensitive plutonium(III) complexed with alkylated 2,6-ditriazinylpyridine (BTP) in organic solution.

Spectroscopic, solvent extraction methods and computational chemistry are applied for the characterization of redox sensitive trivalent plutonium complexed with the N-donor extracting agent alkylated 2,6-ditriazinylpyridines (n-C3H7-BTP) in organic solution. The redox stabilization and speciation of Pu(III) is discussed. Extraction of Pu(III) with n-C3H7-BTP is compared to that for redox stable Am(III). The speciation of Pu(III) with n-C3H7-BTP in organic solution is studied by UV-Vis/NIR and XAFS spectroscopy. Only the 1:3 complex, [Pu(n-C3H7-BTP)3](3+) is observed to form, which is confirmed by EXAFS and solvent extraction. The [Pu(n-C3H7-BTP)3](3+) complex is compared to the isostructural U(III), Am(III), and Cm(III) complexes studied earlier.