Separation techniques with high selectivity are useful in a multitude of applications. The various nuclear fields and applications have a particular need for highly selective and fast separation techniques, not only for cost savings but also for the short half-lives of the separation targets [[1], [2], [3], [4]]. Many techniques have been developed to effect chemical separations including precipitations, distillations, solvent extraction, and chromatography. Solvent extraction is an excellent method to effect chemical separation, however, it poses some challenges including the generation of organic liquid wastes. Solvent extraction is well suited to large-scale continuous process applications, but when done at a bench scale the technique-dependent variability can be large. Post-detonation nuclear forensics applications are generally done at the bench scale; therefore solvent extraction is not the most reproducible option for separations.

The drawbacks of solvent extraction at the bench scale can be mitigated via the use of extraction chromatography [5,6]. Extraction chromatography effectively reproduces the process of solvent extraction by immobilizing the organic extractant on an inert solid support. This allows the selectivity found in solvent extraction to be translated to a column chromatography context and minimizes the generation of organic solvent wastes [7]. Over the last several decades increasing interest has been applied towards extraction chromatography in the context of radiochemical separations.

Extensive effort into reprocessing, remediating, and analyzing nuclear fuels has produced a large body of research into extractants and extraction systems, largely targeting the removal of actinides and lanthanides from a system for reprocessing [8]. Organophosphates have been widely studied and modified to produce optimal separation conditions, resulting in the use of tributyl phosphate (TPB) in the PUREX (Plutonium Uranium Reduction EXtraction) process and similar phosphates have been used in post-PUREX or alternative solvent extraction schemes as well as in Eichrom's LN resins [[9], [10], [11]]. Diglycolamides have been successfully implemented into actinide-selective processes like Diamex-Sanex or ALSEP (Actinide Lanthanide SEParation), and also have been also adapted to a successful extraction resin in Eichrom's DGA resin [[12], [13], [14], [15]].

The extractant of focus for this study, 2-thenoyltrifluoroacetone (TTA), did not make it into the major uranium or plutonium reprocessing schemes in modern use as it was outcompeted by the PUREX process extractants, largely TBP. Modern-designed systems also tend to target only CHON-containing extractants for environmental safety and sustainability concerns [16]. As TTA contains sulfur it is less favorable for consideration. Extensive study of its extraction ability and use was published and remains a valuable resource and remains a potential candidate in alternative extraction techniques such as supercritical fluid extraction [[17], [18], [19], [20]]. TTA is an acidic β-diketone (pkA ∼ 6.5) allowing it to behave as an extractant of a very large range of analytes through ion-exchange mechanisms while at appropriately high pH values for a given analyte. As the pH decreases TTA becomes a much more selective extractant, preferring to extract tetravalent ions such as Zr4+, Hf4+, Np4+, Pu4+, and Th4+ [21]. This increase in selectivity is driven by the decrease in the α proton lability, with increasing H+ concentration. While most metal ions have greater affinity for TTA with increasing pH, notable exceptions exist with gold, iron, molybdenum, and technetium, which each have higher retention at very low pH and decreases with increasing pH [19].

Gold is an attractive target for low-concentration harvesting due to its high value and low natural abundance. Mining techniques utilizing mercury are highly effective at extracting elemental gold and cyanide processes are effective at extracting gold from ores [22]. Gold harvesting from seawater has been successfully pursued utilizing Mn2O3 nanoparticles [23]. Electronic waste recycling, which can contain much higher concentrations of gold and other precious metals than ores, has been actively pursued for decades with several commercially successful processes developed [[24], [25], [26]]. These processes generally combine a physical separation like grinding or delamination with organic solvents to separate the metallic components, followed by isolation of gold through cyanide leaching or pyrometallurgy [24,27]. Alternative leachate solutions including thiosulfate, thiourea, and Cl2 in HCl have been utilized, however cyanide remains the most efficient [28]. Radiochemical separations of gold have focused on the precipitation via reduction to metal utilizing carrier methods or ion exchange chromatography [22,29]. Recently an effective but low-yield separation of gold from a complex fission and activation product sample was demonstrated utilizing Sr resin [30]. A comparison of the major methods to the method presented in this work is shown in Table 1.

This existing body of knowledge makes TTA an ideal candidate for use in extraction chromatography. One previous publication utilized TTA embedded on a polyurethane support and thoroughly examined the thermodynamics of europium extraction, however the analysis of other analytes was limited to the separation from europium at pH 3.5 [31]. This concept is expanded here where we examine an extraction resin consisting of TTA deposited on Eichrom prefilter resin and its use in and optimization of separation schemes for the isolation of Fe3+, Ga3+, Au3+, and Zr4+ from fission and activation product samples. High selectivity was determined by testing with a suite of stable analytes in nitric and hydrochloric acids. The effectiveness of the separations developed was verified by testing with irradiated fission and activation product samples.