# New Trends in Separation Techniques of Lithium Isotopes: A Review of Chemical Separation Methods
## 1 Introduction
### Challenges in Development of Separation Methods for Lithium Isotopes Using Electrochemical
#### 2.3.2 Ion-Exchange Methods Using Resin  
Another type of displacement chromatography method is the separation of lithium isotopes using ion-exchange resin. Many studies on lithium isotope separation have been conducted using different types of cation-exchange resins. Nevertheless, due to natural abundance of sodium ions, a preconcentration step is needed to separate lithium from others alkaline ions prior to separation of lithium isotopes [32]. For example, [PERSON] et al. used an AG 50W-X8 cation-exchange resin (200-400 mesh, protonated form) packed in a 20 cm \(\times\) 2 cm I.D. glass column to separate lithium from sodium and potassium ions [33]. Using 0.1 M HCl solution as eluent, the separation and recovery efficiency for lithium ions was recorded to 99.24%, which indicates a very good separation [33]. [PERSON] and [PERSON] were the first to use and report on the cation-exchange method for Li isotope separation in 1937 and since then several other studies were performed using cation-exchange methods for lithium isotope separation [34]. For example, lithium isotope isolation of lithium lactate in dimethyl sulfoxide (DMSO) and water-acetone mixed-solvent media with ion-exchange displacement chromatography was also investigated by [PERSON] et al. [35] and a convex function of the solvent mixing ratio was observed in the single-stage separation factor. The single-stage separation factor had its highest value of 1.00254 at water:DMSO = 25.75 \(v/v\) and 1.00182 at water:acetone = 75:25 \(v/v\)[35]. In order to improve the separation of lithium isotopes, a sulfonated-type cationic exchange resin with higher cross-linkage of 50% (obtained by adding styrene and divinylbenzene in a proportion 1:1) higher than those commercially available was synthesised by [PERSON] et al. [36]. The fractionation experiment was performed in a 1 m length and 8 mm inner diameter and 23.8 g dry high cross-linkage resin using lithium acetate and latter potassium acetate as eluent, while the isotope ratio of lithium in the eluted samples was analysed by [PERSON] et al. [36]. Upon the experiment, a maximum value of isotope separation factor of 1.0066 was found, while the separation coefficient (\(\varepsilon=\alpha-1\)) was 0.0066 [36], values which show the potential of high cross-linkage resins in lithium isotope separation. In another study, [PERSON] et al. [37] investigated the lithium isotope separation in cation exchange resins characterized by 90% high cross-linkage degree obtained by adjusting the ratio between divinylbenzene and styrene to 9:1 during synthesis with suspension polymerization. The separation coefficient (\(\varepsilon\)) obtained in this study [37] was just 0.0069, slightly higher than the separation coefficient previously reported by [PERSON] et al. [36] for a cross-linkage degree of 50%. Therefore, a correlation between the separation of lithium isotopes and cross-linkage degree was also investigated by [PERSON] et al. [37] and the study concluded that saturation of the separation coefficient is attributable to the increase in hydrophobicity of the resin with increasing cross-linkage degree which may induce the so-called \"dehydration effect\", the removal of lithium ions and hydration water molecules from the resin structure. More recently sulfonated pyridine-styrene-divinyl-benzene resin with a cross-linkage degree of 50% was first synthesized by [PERSON] et al. by embedding the polymer into porous silica beads (thus abbreviated as sulfonated Pyr-Styr-DVB/SiO\({}_{2}\)), since the embedding can improve the stability (reduce the shrinking, as well the swelling, of the resin) and mechanical properties of the resin (see Figure 3) [38]. The values separation coefficient during lithium isotope fractionation of the above-mentioned resin compared with sulfonated styrene-divinyl-benzene (sulfonated Styr-DVB resin) had the same cross-linkage degree of 50% in batch experiments spanning a wide temperature range (278 to 333 K).