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

Abstract

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.

Determination of complex formation constants of neptunium(V) with propionate and lactate in 0.5–2.6 m NaCl solutions at 22–60°C using a solvent extraction technique

“…These values are shown in Table 4 and are compared with literature data. Log β° and Δε values are in excellent agreement with a previous spectroscopic study of Np(V) propionate complexation (log β° = 1.26 ± 0.03) [11]. Solvent extraction is a technique from which log β is calculated by using the total complexed metal species, including outer and inner sphere complexes, whereas…”

“…For both, NpO 2 Prop and NpO 2 Acet, the molar standard reaction entropy is positive indicating that the formation of these complexes is entropy driven. Relatively high values of Δ r S°m (75±6 J•mol -1 •K -1 [32] and 77±9 J•mol -1 •K -1 for acetate and propionate, respectively) suggest a bidentate coordination mode of NpO 2 -OOC binding for acetate and propionate both with replacement of two water molecules from the inner sphere of Np(V), as previously confirmed by X-ray absorption spectroscopy [11]. - [38] *the values were calculated using eq.…”

“…In light of the foregoing, it is necessary to outline the lack of information about the thermodynamic data of neptunium complex formation with ligands present in clay rocks. A recent study [11] investigated Np(V)-propionate complexation by spectroscopy and delivered related thermodynamic parameters. In [11], a solvent extraction method has been applied to determine the thermodynamic parameters for the Np(V)-propionate complexes at various I and T. The complexation of NpO 2 + with lactate has been studied previously at room temperature using solvent extraction technique [13][14][15].…”

“…A recent study [11] investigated Np(V)-propionate complexation by spectroscopy and delivered related thermodynamic parameters. In [11], a solvent extraction method has been applied to determine the thermodynamic parameters for the Np(V)-propionate complexes at various I and T. The complexation of NpO 2 + with lactate has been studied previously at room temperature using solvent extraction technique [13][14][15]. The present paper is aiming to complete these data with complexation data at high temperatures that correspond to the HLW repositories conditions.…”

“…pH c was only determined at room temperature. The apparent proton dissociation constant of propionate/lactate (pK a ) is below 5 within the range of [NaCl] and T investigated [11,13]. For pH c = 7 (i.e.…”