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Issue 8, 2015
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Np(V) complexation with propionate in 0.5–4 M NaCl solutions at 20–85 °C

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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 ΔrH0m = const. and ΔrC0p,m = 0, allowing the calculation of ΔrH0m and ΔrS0m 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.

Graphical abstract: Np(v) complexation with propionate in 0.5–4 M NaCl solutions at 20–85 °C

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The article was received on 02 Dec 2014, accepted on 06 Jan 2015 and first published on 22 Jan 2015

Article type: Paper
DOI: 10.1039/C4DT03688C
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Citation: Dalton Trans., 2015,44, 3837-3844
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    Np(V) complexation with propionate in 0.5–4 M NaCl solutions at 20–85 °C

    A. N. Vasiliev, N. L. Banik, R. Marsac, D. R. Froehlich, J. Rothe, S. N. Kalmykov and C. M. Marquardt, Dalton Trans., 2015, 44, 3837
    DOI: 10.1039/C4DT03688C

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