Rapid Radiochemical Separations of Americium from Complex Matrices

Abstract

The rapid purification of americium from complex matrices representative of those found in spent nuclear fuel raffinate, environmental samples and nuclear forensics samples is a challenging task. The greatest challenge is the separation of americium from the chemically and physically similar lanthanide elements which are ubiquitous in these sample types. Typically americium separations rely upon time consuming multi-stage flowsheets which see americium removed only at the last stage of the separations rather than selectively removing americium at the beginning of the schemes. In this work several novel extraction chromatography resins (EXC) have been synthesised by solvent impregnation of the triazine ligands 6,6'-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo[1,2,4]triazin-3-yl)-2,2'-bipyridine (CyMe4BTBP) and 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-benzo[1,2,4]triazin-3-yl)-1,10-phenanthroline (CyMe4BTPhen) into Amberlite XAD7 and Amberchrom CG300 polymer supports. The resins have been physically characterised by a suite of spectroscopic, analytical and imaging techniques. The resins have also been evaluated in terms of their ability to selectively extract americium from complex matrices. The resins have been compared with previously reported attempts to generate EXC resins based on CyMe4BTBP and CyMe4BTPhen. Previously reported resins all rely on complex synthesis for the formation of a covalent bond between extractant and support in contrast with the simpler solvent impregnation method reported here. The Amberchrom supported CyMe4BTBP resin achieved a decontamination factor (DF) of >1000 for americium over lanthanides by column chromatography. The extraction performance and mechanism of 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-benzo[1,2,4]triazin-3-yl)- 1,10-phenanthroline (CyMe4BTPhen) in the room temperature ionic liquid 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]) have been studied and are herein reported. Americium is extracted from nitric acid solutions by cation exchange and is most efficient at low (< 1 M) nitric acid concentrations as opposed to the high (4 M) concentration when using most traditional molecular diluents. The kinetics are slow, taking 24 hours to achieve quantitative extraction of americium but are greatly improved, achieving the same within 8 hours, by dilution with 1-octanol. A high SFAm/Eu >3000 was achieved, although decontamination factors for americium from more complex matrices were non-competitive with extraction using traditional molecular diluents. Modest improvements in decontamination factor were attained by addition of the hydrophilic reagent N,N,N'N'-tetraethyl diglycolamide (TEDGA). An effective method for the rapid separation of americium from complex matrices by the combined use of two commercially available extraction chromatography (EXC) resins has been demonstrated. The EXC resin TRU was shown to be capable of purifying americium and the lanthanides together as a group from Group 1, Group 2 and transition metals. The TRU column also tolerated high loadings (50 mg) of iron, aluminium, calcium sodium and potassium. The EXC resin TEVA was then able to effectively purify americium from the lanthanide contaminants by elution with ammonium thiocyanate. Decontamination factors (DF) for americium from the lanthanides of >20000 were achieved within one working day. The affinity of TEVA resin for americium, curium and several lanthanides as a function of ammonium thiocyanate concentration has been reported. The possibility of americium separation from lanthanides on LN resin using low concentrations of nitric acid has also been explored.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Scott Heath (Supervisor)