Np(V) complexation with propionate in 0.5–4 M NaCl solutions at 20–85 °C

International audienceLow molecular weight organics (LMWO; e.g. acetate, propionate, lactate) can significantly impact the speciation and mobility of radionuclides in aqueous media. Natural clay rock formations, considered as a potential host rock for nuclear waste disposal, can contain significant amount of organic matter. There is few thermodynamic data reported for the complexation of pentavalent actinides with LMWO, especially at 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 by spectroscopic techniques in 0.5-10 4 M NaCl solutions under systematic variation of ligand concentration and temperature. Slope analysis shows the formation of the 1:1 NpO 2-propionate complex (NpO 2 Prop). The local structure of the NpO 2-propionate complex is determined by Extended X-ray Absorption Fine Structure spectroscopy and suggests 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 20-85 °C. The log β 0 values are linearly correlated to the reciprocal 15 temperature, indicating Δ r H 0 m = const. and Δ r C 0 p,m = 0, allowing the calculation of Δ r H 0 m and Δ r S 0 m for the formation of the NpO 2-propionate complex from the integrated van't Hoff equation. The thermodynamic evaluations indicate that the reaction is endothermic and entropy driven