Addressing Ruthenium Speciation in Tri-n-butyl-phosphate Solvent Extraction Process by Fourier Transform Infrared, Extended X-ray Absorption Fine Structure, and Single Crystal X-ray Diffraction

Abstract

In industrial nuclear fuel reprocessing, small amounts of ruthenium are extracted by tri-n-butyl-phosphate (TBP) at the same time as uranium and plutonium. This behavior increases solvent radiolysis and requires secondary extraction cycles to minimize the residual ruthenium content in uranium and plutonium products. However, the solvent ruthenium extraction mechanism remains largely unexplored. This study addresses the speciation of ruthenium in solvent extraction conditions by complementary infrared and X-ray absorption spectroscopy. First, spectroscopic result interpretation is supported by a single crystal X-ray diffraction study on reference compounds to unambiguously demonstrate that the ruthenium extraction mechanism is driven by a weak outer-sphere Ru–TBP interaction. Second, the ruthenium coordination sphere is quantitatively characterized. Ruthenium speciation in the organic phase depends on the initial aqueous phase, and both monomeric ruthenium nitrosyl trinitrate complexes and a hydrolyzed dimeric ruthenium nitrosyl complex are shown. Average coordination numbers for nitrate, hydroxide, and aquo ligands are accurately determined in both phases, by applying a constrained EXAFS fit approach.

Complexation and bonding studies on [Ru(NO)(H2O)5]3+ with nitrate ions by using density functional theory calculation

“…Table 2 shows the calculated IR frequencies and assignment of normal vibrational modes with the experimental values in solid state. 12,16 The calculated frequencies of three isomers show the similar values to the experimental frequencies, especially, those of the abc model were found to well reproduce the O-H stretching vibration compared to the other isomers. The IR frequencies and intensities of normal vibrational modes employed for the estimation are summarized in Table S2.…”

“…Experimental bond lengths of the complex with x ¼ 3, which was tted as the abc model, in solution were reported by using EXAFS measurement as 2.04(3), 2.11(9), and 2.08(1)Å for Ru-O eq (NO 3 ), Ru-O eq (H 2 O), and Ru-O ax (H 2 O) bond lengths, respectively. 16 The corresponding calculated values, 2.06(3), 2.13, and 2.08Å, respectively, were consistent with the experimental values within the standard deviation. We also showed S values, which denote the sum of the deviations from the ideal octahedron of the cis Ru-ligand bond angles in Table 1.…”

“…However, such systematic properties of the rutheniumnitrosyl complexes, even if limited to [Ru(NO)(NO 3 ) x (H 2 O) y ] (3-Àx)+/À system, has not been understood, although the precise modeling of coordination geometries in nitric acid solution is required to understand the separation mechanism in solvent extraction. 16 Density functional theory (DFT) calculations have been employed to interpret experimental results and to understand the coordination character of PGM complexes. 17,18 Recently, the DFT applications to the Rh and Pd complexes with H 2 O and NO 3 À ligands, considering the geometrical isomers for the unidentate system, have been reported by combining them with experimental techniques.…”

Synthesis of the Ruthenium Nitrosyl Complex with Coordinated Ammonia and Pyridine at Room Temperature

“…The pairs of ammine and pyridine ligands are cis ‐coordinated in the equatorial plane. The average Ru–N distance in the equatorial plane is the same as in trans ‐[RuNO(NH 3 ) 4 (NO 3 )] 2+ (2.103 Å), with the average Ru/ammine distance being (2.098 Å) slightly smaller than the average ruthenium‐pyridine distance (2.109 Å). The bonds lengths in the coordinated pyridines are 1.386 and 1.351 for carbon‐carbon and carbon‐nitrogen bonds respectively.…”

“…The ruthenium atom is shifted from the equatorial plane toward the nitrosyl group by 0.08 Å. For the related cationic trans ‐[RuNO(NH 3 ) 4 X] 2+ complexes, the shift is larger (0.12 Å) for X equal to nitrate or hydroxyl,, and is similar to the chloride complex . The RuNO fragment is close to linear with 176.26(8)° angle and the Ru–N and N–O bonds having usual distances of 1.7420(8) and 1.1338(11) Å respectively , , …”