# New Trends in Separation Techniques of Lithium Isotopes: A Review of Chemical Separation Methods
## 1 Introduction  
Lithium-6 (\({}^{6}\)Li) and lithium-7 (\({}^{7}\)Li), with the latter being far more abundant (95.15%), are the two stable isotopes of lithium occurring in Earth's lithosphere. A fractionation of lithium isotopes appears during a variety of geological processes [1; 2]. Such processes include mineral formation (by chemical processes like precipitation and ion exchange). Lithium 6 isotope (\({}^{6}\)Li) is sometimes preferred over \({}^{7}\)Li, as the Li\({}^{+}\) ions exchange magnesium or iron in octahedral locations of the clays, which leads to an enrichment of \({}^{6}\)Li in geological processes [3; 4]. In laboratory and technological processes, \({}^{6}\)Li is an important isotope for tritium production via bombardment with neutrons, therefore several techniques for its separation have been developed in the past. Historically, COLEX processes have been used in the past (in the 1950s and 1960s) for lithium isotope separations based on greater affinity of \({}^{6}\)Li over \({}^{7}\)Li for the element mercury. The working principle of the COLEX technology is based on the fractionation of \({}^{6}\)Li between a lithium amalgam and a lithium hydroxide solution, as the amalgam and lithium hydroxide solution come into contact. At industrial scale, theCOLEX technology uses this principle in sequential stages, by passing an aqueous lithium hydroxide flowing up and a counter-flow of lithium-mercury amalgam flowing down. At the top of the column, the lithium hydroxide solution is electrolyzed to liberate the depleted \({}^{7}\)Li fraction, while at bottom of the column, the \({}^{6}\)Li-enriched fraction is separated from the amalgam, and the metallic Hg is recovered and reused in the process. The main advantage of COLEX technology is its minimal production costs, which favours the industrial scale production of enriched lithium. Nevertheless, nowadays its technical (decomposition of the amalgam, high energy consumption) and environmental (generation of toxic Hg vapours, large quantities of Hg-containing waste) drawbacks make this technology obsolete [5]. Additionally, other methods like vacuum distillation were tested in the 1950s [6], but this technology was not applied extensively, possibly due to high energy consumption. More recently, a variety of methods have been developed for lithium isotope separations [7; 8; 9; 10]. Based on their working principles, these methods can be classified into three main groups: (1) chemical exchange methods in two liquid phases, (2) electrochemical exchange methods, and (3) displacement chromatography methods (see Table 1). Chemical exchange methods in two liquid phases employ crown ethers as extractants that can be also used in combination with ion liquids as extraction solvents [11; 12]. The crown ethers have the capacity to separate lithium from other alkaline ions [13] as well to differentiate between \({}^{6}\)Li and \({}^{7}\)Li isotopes. The ion liquids can separate lithium from other alkaline ions and complex metal matrices like lithium battery waste [14], but they cannot solely differentiate between \({}^{6}\)Li and \({}^{7}\)Li, and they must be used in combination with crown ethers [11]. The main advantage of chemical exchange methods in two liquid phases is the separation factor of lithium isotopes (comparable with COLEX technology), but this comes with the drawbacks of very high costs for these reagents, as well their relative toxicity. Electrochemical exchange methods like electromigration and electrodialysis are more eco-friendly [15], but they result in relatively low separation factors of lithium isotope and they require complex experimental methodologies. Displacement chromatography methods have the advantages of involving reusable ion exchange columns (and thus are eco-friendly), but the obtained separation factors are relatively low. The objective of this paper is to highlight the current trends in separation techniques for lithium isotopes, emphasizing the chemical separation methods and spectrometric analysis methods.