Teodora Retegan

An overview and historical look back at the solvent extraction using nitrogen donor ligands to extract and separate An(III) from Ln(III)

The partitioning of minor trivalent actinides (An) from lanthanides (Ln) is one of the challenges in the chemical treatment of nuclear waste. The optimal ligand to carry out the separation of An(III) and Ln(III) using solvent extraction has to meet several important criteria: high selectivity towards the solute, chemical and radiolytic stability, stripping possibilities and recycling of the organic phase, high separation factors and good distribution ratio, to name just a few of them. A chronological line can be drawn along the development of each extraction ligand family and some milestones are emphasized in this overview. Further developments in organic synthesis of extracting ligands are expected.

A TBP/BTBP-based GANEX Separation Process. Part 1: Feasibility

Abstract A GANEX (Group ActiNide EXtraction) separation system for transmutation has been developed. In this separation process the actinides should be extracted as a group from the lanthanides and the fission and corrosion/activation products. This can be achieved by combining BTBP (bis-triazine-bipyridine) with TBP (tri-butyl phosphate) in cyclohexanone. From 4M nitric acid this organic system extracts the actinides (log(DAm) = 2.19, log(DPu) = 2.31, log(DU) = 1.03, log(DNp) = 0.53) and also separates them from the lanthanides (log(DLa) = −2.0, log(DCe) = −1.72, log(DNd) = −1.05, log(DSm) = −0.18, log(DEu) = −0.02). One problem encountered is that some of the fission and corrosion products are also extracted. The new system however still looks feasible.

Extraction of actinides with different 6,60-bis(5,6-dialkyl-[1,2,4]-triazin-3-yl)[2,20]-bipyridines (BTBPs)

Abstract The extraction of Am(III), Th(IV), Np(V), and U(VI) from nitric acid by 6,6′‐bis(5,6‐dialkyl‐[1,2,4]‐triazin‐3‐yl)‐[2,2′]‐bipyridines (C2‐, C4‐, C5‐, and CyMe4‐BTBP) was studied. Since only americium and neptunium extraction was dependent on the BTBP concentration, computational chemistry was used to explain this behavior. It has been shown that the coordination of the metal played an important role in forming an extractable complex into the organic phase, thus making it possible to extract pentavalent and trivalent elements from tetravalent and hexavalent elements. This is very important, especially because it shows other possible utilizations of a group of molecules meant to separate the actinides from the lanthanides. In addition, the level of extraction at very low or no BTBP concentration was explained by coordination chemistry.

Electrospray Ionization Mass Spectrometry Investigation of BTBP - Lanthanide(III) and Actinide(III) Complexes

In the framework of nuclear waste reprocessing, the separation processes of minor actinides from fission products are developed using liquid‐liquid extraction. To gain an understanding of the mechanism involved in the extraction process, a complex formation of actinides and lanthanides with BTBPs (6,6′‐bis(5,6‐dialkyl‐1,2,4‐triazin‐3‐yl)‐2,2′‐bipyridines) was characterized using the Electrospray Ionization Mass Spectrometry (ESI‐MS) technique. This study was carried out to compare the influence of diluents and side groups of the extractants on complex formation. Three different diluents, nitrobenzene, octanol and cyclohexanone, and two extractants, C5‐BTBP and CyMe4‐BTBP, were selected for this experiment. It was found that the change of the diluent and of the substituent on the BTBP moiety does not modify the stoichiometry of the complexes which is L2M(NO3)3. It is proposed that one nitrate is directly coordinated to the metal ion, the two other anions probably remaining in the outer coordination sphere. The difference observed in extracting properties is probably due to the solvation of the complexes by the diluent. The noncovalent force that holds complexes together are likely to be largely governed by electrostatic interactions even if the hydrophobic exterior of the complexes plays an important role in the complexation/extraction mechanism. The study of the stability of the ions in the gas phase shows that the C5‐BTBP ligand has a labile hydrogen atom, which is a fragility point of C5‐BTBP.

Influence of dose rate on the radiolytic stability of a BTBP solvent for actinide(III)/lanthanide(III) separation

Abstract The recently developed ligand MF2-BTBP dissolved in cyclohexanone is a promising solvent for the group separation of trivalent actinides(III) from the lanthanides(III). Its high stability against nitric acid has been demonstrated recently. Since the solvent is also exposed to a continuously high radiation level in the counter current process, the radiolytic stability of the solvent was examined in this study. Irradiation experiments were carried out up to an absorbed dose of 100 kGy and the effect of the dose rate was investigated. The extraction behaviour for An(III)/Ln(III) separation was studied after radiolysis for evaluation. It was found that during high dose rate irradiation the extraction efficiency for both Am(III) and Eu(III) decreased significantly with increasing absorbed dose, whereas during the low dose rate irradiation the extraction efficiencies remained more or less at the same level.

The Behaviour of Organic Solvents Containing C5-BTBP and CyMe4-BTBP at Low Irradiation Doses

Low doses of gamma radiation were given to four different solvents containing C5-BTBP and CyMe4-BTBP, each molecule dissolved both in cyclohexanone and hexanol. Four corresponding solvents were kept unirradiated and used as references for the extraction experiments. Multiple samples were taken from both the irradiated solutions and the reference solutions at certain time intervals. The samples were used in extraction experiments with the radionuclides 241Am and 152Eu. The protection against radiolysis of the extracting molecules by the diluent used for dissolution without adding a scavenger molecule was checked. The interplay between the diluent and the side group of the extracting molecule for protection against radiolysis was also studied by keeping the same type of core molecule for binding to the metal ions and varying the diluent and side group. The results were unexpected. The presence of a cyclic molecule as both a side group or diluent seems to keep the extraction of europium almost unaffected by radiolysis, while americium behaves differently from solvent to solvent. The diluent alone does not protect the extracting molecule. In some of the studied systems there is a distinct change in the extraction behaviour of Am between the irradiated and reference solutions, an effect that is however only present at the beginning of the experimental series. At later times the difference in distribution ratios between the irradiated and reference solution is constant. This phenomenon is found only when the side group and diluent are structurally dissimilar.

Radiolysis of solvents containing C5-BTBP: identification of degradation products and their dependence on absorbed dose and dose rate

Solvents intended for the separation of trivalent actinides from trivalent lanthanides in spent nuclear fuel have been irradiated with gamma-radiation. The solvents initially contained 0.005 M C5-BTBP dissolved in either hexanol or cyclohexanone and they were exposed to doses up to 20 kGy. Identification of degradation products was done using atmospheric pressure chemical ionization-mass spectrometry (APCI-MS). Structures were suggested for a number of degradation products and the relative intensity of the peaks in the MS spectra was estimated. It was clear that the content of the original molecule, C5-BTBP, decreased with dose, while the content of the various degradation products increased. It was also shown that both the choice of diluent and the dose rate (Gy/h) affect the amount of degradation products formed. A degradation scheme was proposed for the radiolytic degradation of C5-BTBP.

Studies on the Solvent Extraction of Rare Earth Metals from Fluorescent Lamp Waste Using Cyanex 923

The growing need for materials such as rare earth metals (REMs) has focused attention towards their recovery from various end-of-life products. Fluorescent lamps are considered a viable target, and can be a source of up to six REMs: lanthanum, cerium, europium, gadolinium, terbium, and yttrium. In this study a commercial mix of trialkylphosphine oxides (Cyanex 923) was investigated for the extraction of REMs from fluorescent lamp waste leachates. The kinetics of the extraction is addressed, together with the co-extraction of undesired elements (iron and mercury), the influence of temperature, nitric acid concentration in the aqueous phase and ligand concentration in the organic phase. The extraction of REMs was found to be enthalpically driven, with good separation factors between the light and heavier elements. Selective stripping of REMs was possible in a single step using 4 M hydrochloric acid solution. Further recovery of iron and mercury was carried out using nitric and oxalic acid solutions.

Extraction Behavior of Nickel(II) using some of the BTBP‐Class Ligands

Abstract Recently the BTBP‐family of solvating ligands have been studied for their ability to separate trivalent actinides from lanthanides. Five of the BTBPs were evaluated for their ability to extract nickel(II) from aqueous nitrate media into cyclohexanone. It was shown by both solvent extraction and X‐ray diffraction experiments that the BTBPs are capable of forming both 1:1 and 1:2 complexes with nickel(II). When the BTBP concentration is low the nickel distribution ratio is governed by the formation of the nickel/BTBP complex while at higher BTBP concentrations the partitioning of the nickel complex between the two phases dictates the nickel distribution ratio.

Investigations regarding the wet decontamination of fluorescent lamp waste using iodine in potassium iodide solutions

With the rising popularity of fluorescent lighting, simple and efficient methods for the decontamination of discarded lamps are needed. Due to their mercury content end-of-life fluorescent lamps are classified as hazardous waste, requiring special treatment for disposal. A simple wet-based decontamination process is required, especially for streams where thermal desorption, a commonly used but energy demanding method, cannot be applied. In this study the potential of a wet-based process using iodine in potassium iodide solution was studied for the recovery of mercury from fluorescent lamp waste. The influence of the leaching agents concentration and solid/liquid ratio on the decontamination efficiency was investigated. The leaching behaviour of mercury was studied over time, as well as its recovery from the obtained leachates by means of anion exchange, reduction, and solvent extraction. Dissolution of more than 90% of the contained mercury was achieved using 0.025/0.05 M I2/KI solution at 21 °C for two hours. The efficiency of the process increased with an increase in leachant concentration. 97.3 ± 0.6% of the mercury contained was dissolved at 21 °C, in two hours, using a 0.25/0.5M I2/KI solution and a solid to liquid ratio of 10% w/v. Iodine and mercury can be efficiently removed from the leachates using Dowex 1X8 anion exchange resin or reducing agents such as sodium hydrosulphite, allowing the disposal of the obtained solution as non-hazardous industrial wastewater. The extractant CyMe4BTBP showed good removal of mercury, with an extraction efficiency of 97.5 ± 0.7% being achieved in a single stage. Better removal of mercury was achieved in a single stage using the extractants Cyanex 302 and Cyanex 923 in kerosene, respectively.