A Review of Research Progress on Separation and Purification Methods of Lithium Isotopes Using Cation Exchange Resin and Their Applications

SONG Yilong; LIU Min; HOU Kejun

doi:10.15898/j.ykcs.202409050181

Rock and Mineral Analysis > 2024 > 43(5): 677-692. > DOI: 10.15898/j.ykcs.202409050181

SONG Yilong，LIU Min，HOU Kejun. A Review of Research Progress on Separation and Purification Methods of Lithium Isotopes Using Cation Exchange Resin and Their Applications[J]. Rock and Mineral Analysis，2024，43（5）：677−692. DOI: 10.15898/j.ykcs.202409050181

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A Review of Research Progress on Separation and Purification Methods of Lithium Isotopes Using Cation Exchange Resin and Their Applications

1.
Key Laboratory of Metallogeny and Mineral Assessment, Ministry of Natural Resourecs; Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
2.
School of Earth Sciences and Resources, China University of Geosciences (Beijing）, Beijing 100083, China

More Information

Received Date: September 04, 2024
Revised Date: September 24, 2024
Accepted Date: September 26, 2024
Available Online: October 08, 2024

Abstract

HIGHLIGHTS
(1) The separation and purification method of lithium isotope via cation exchange resin especially focuses on the separation of sodium in the sample matrix under the premise of 100% recovery of lithium in the sample.

(2) The optimization of separation and purification methods of lithium isotope via cation exchange resin mainly involves three aspects: the ratio of height-diameter of the resin column, the type and dosage of resin and the type and dosage of eluent.

(3) There are significant differences in lithium content, Li isotope and its matrix composition in different natural reservoirs, so it is important to continue to optimize and develop the existing separation and purification process and expand the applicability of natural samples to improve the efficiency and accuracy of Li isotope analysis and promote the application research of Li isotopes.

ABSTRACT

Lithium (Li) isotopes serve as effective geochemical tracers in mantle-crust cycling, planetary evolution, climate change, continental weathering, mineralization mechanisms, and environmental pollution studies. Efficient separation of lithium from natural samples is essential due to potential interference from isobaric isotopes during analysis. Over the past decades, cation exchange resin methods have been developed to enhance lithium separation for TIMS and MC-ICP-MS analysis. Since then, these methods have evolved to reduce blank contamination, simplify procedures, improve efficiency, and expand applicability to various natural samples. This review examines recent advances in Li isotope separation using single-column, double-column, multi-column and in-series column methods. Key factors like resin type, eluent volume, and method efficiency for various samples are discussed. Single- and double-column methods dominate current research, of which some methods just use minimal resin and eluent while controlling process blanks to below 0.1‰. However, separation efficiency remains dependent on lithium content and matrix ions in the sample. Further optimization is needed to balance efficiency, cost, and applicability across sample types. As analytical techniques advance, automated elution systems are likely to become central to Li isotope analysis. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202409050181.
- lithium isotope,
- separation and purification,
- chromatographic column,
- cation exchange resin,
- recovery of lithium,
- sample applicability
BRIEF REPORT
Lithium (Li) isotopes are effective geochemical tracers and have been widely applied in studies of mantle-crust cycling^[1], planetary evolution^[2], continent weathering^[3], paleoclimate changes^[4], mineralization^[6], and environmental pollution^[7]. As Li isotope research advances, accurate and efficient analysis techniques have become crucial. Since the late 20th century, methods have evolved from NAA^[8] and AAS^[9] to ICP-MS^[10] and TIMS^[12-13], achieving precision below 1‰. Recently, in situ techniques like SIMS^[17] and LA-MC-ICP-MS^[18] have progressed. However, due to the lack of solid standards, MC-ICP-MS^[14-16] remains the most widely used technique. Li isotope separation and purification are essential for high-precision analysis, with cation exchange resin methods being mainstream. Various separation methods have been developed based on different resins, eluents, and chromatographic configurations. This review summarizes Li isotope separation techniques over the past thirty years, evaluates their applicability, and provides an outlook on future developments.

　　There are distinct Li contents and isotopic compositions in different natural reservoirs. Li exists in two forms in nature: solid, primarily as silicates and Li-bearing minerals, and liquid, widely distributed in natural waters, oceans, and brines. Although Li is a trace element, its content varies significantly across different reservoirs. Its two stable isotopes exhibit significant isotope fractionation during geological processes. Li concentration varies greatly across different reservoirs, ranging from about 1.56μg/L in rivers to 180μg/L in seawater^[4], while reaching up to 227000μg/L in brines^[25]. In solid reservoirs, Li is concentrated in silicate rocks, with content ranging from 1.49μg/g to 46μg/g^[20,26]. Li isotope compositions also vary widely, with seawater at approximately +31‰ and lower in solid reservoirs^[4,29]. Due to the similar partitioning behaviors of Li and some other elements, the separation and purification of Li present challenges. Optimized elution processes are required for different sample matrices to ensure efficient Li separation and purification.

　　The cation exchange resin method is a common technique for extracting Li from complex matrices, often used in the pretreatment of Li isotope analysis. The process involves dissolving the sample, passing it through a stationary phase (resin) and a mobile phase (acid solution)^[37], and using an eluent to separate pure Li. The separation process is based on plate theory, where solute ions maintain dynamic equilibrium between the resin and solution, and are separated by different partition coefficients^[37]. Three key factors influence Li isotope separation: the resin's capacity, the type and concentration of eluent, and the amount of resin and eluent used. Additionally, factors such as the type of chromatography column, resin properties, and elution frequency should be considered to avoid contamination and optimize the purification process^[32,39-40].

　　1. Different separation and purification methods of Li via cation exchange resin and their technical characteristics

　　In the separation and purification methods of Li via cation exchange resin, researchers select appropriate resins, eluents, and chromatography columns based on the composition of the samples. Focusing on elution column numbers as the key variable, this comprehensive review discusses four commonly used methods, single-column, double-column, multi-column, and in-series column, and also analyzes their technical features and applications.

　　1.1 Single-column method

　　The single-column method achieves complete elution of Li using a single column and a one-time elution process. Typically, higher columns are used to increase the plate number and improve separation efficiency. In recent years, this method has become the primary approach for Li isotope separation and purification due to its simplified operation and fewer sample transfer steps^[52-54]. However, over time, significant differences in the choice of columns and eluents have emerged. Early methods often employed acid-alcohol mixed eluents^[14-15], which enhanced the separation between Li and Na but introduced challenges such as large eluent volumes, longer processing times, and higher procedural blanks. Additionally, alcohol-based solutions could degrade the resin, releasing impurities like sodium and further affecting purification^[55]. Recently, Zhu et al^[45] improved this method by using AGMP-50 resin and replacing alcohol-based eluents with simpler hydrochloric or nitric acid solutions, greatly reducing elution time and eluent volume while minimizing the procedural blanks. This refined method is not only applicable to various natural samples, including seawater and rocks, but also ensures a high Li recovery rate of up to 99%, ensuring both efficiency and accuracy in separation and purification.

　　1.2 Double-column method

　　The dual-column method was originally designed to address the challenge of separating Li from Na due to their similar partition coefficients without using alcohol-based eluents. The process is divided into two steps: the first separates most cations from Li; the second step ensures the complete separation of Li from Na. Since this method uses columns with a small height-to-diameter ratio, the elution time is relatively short, making it a widely used approach alongside the single-column method.

　　James et al^[46] firstly developed the application of the dual-column method, using 2.7mL of AG50W-X12 resin and 0.2mol/L HCl as the eluent. Through two repeated elution, they achieved complete purification of Li, providing a foundation for subsequent improvements. Zhang et al^[31] optimized this method by switching to HNO₃ instead of HCl to avoid the formation of complexes with trivalent iron, which could interfere with Li separation. They also reduced the amount of resin, making the process more efficient, with significant reductions in both elution time and eluent volume. In recent years, Li et al^[48], Zhang et al^[44], and Zhu et al^[45] have further refined the dual-column method, developing procedures suitable for different types of natural samples.

　　1.3 Multi-column method

　　The multi-column method was initially developed to meet the high-purity requirements of Li during isotope analysis in TIMS while minimizing the use of resin and eluent to avoid Li blank contamination. This method employs multiple columns or repeated elution steps, using small amounts of resin to gradually remove matrix ions, ultimately achieving 100% purification of Li. In the late 20th century, Moriguti et al^[22] developed a four-step separation method using different concentrations of hydrochloric acid and hydrochloric acid-ethanol mixtures for multiple elution to successfully separate Li. Later, Rudnick et al^[58] simplified this process by retaining only the first three steps and reducing the acid concentration to improve efficiency. Su et al^[39] and Zhao et al^[50] further optimized the method by reducing the eluent volume and increasing separation efficiency. Consequently, research on improving this method has been limited in recent decades, and its application is less widespread compared to the single-column and dual-column methods.

　　1.4 In-series column method

　　The in-series column method achieves Li separation and purification in a single elution by connecting micro-column and long column in series. The micro-column efficiently adsorbs cations, while the long column further separates Li from Na, ensuring high recovery (>96%). Compared to the single-column and dual-column methods, the in-series method has been applied later, and its technique still requires optimization. Zhu et al^[51] found that traditional single-column methods are prone to interference from matrix elements, affecting Li purification efficiency and isotope accuracy. To address this, Zhu et al^[51] optimized elution conditions using 0.5mol/L HCl and increased resin capacity, significantly reducing interference and shortening the elution time to around 2h. This method improves elution efficiency, avoids the negative effects of organic solvents, and reduces costs. After validation, the method demonstrated excellent accuracy across various sample types, especially for low-Li and high-matrix samples.

　　2. The applicability of different separation and purification methods for natural samples

　　The widespread application of Li isotopes in environmental, geological, and geochemical studies relies on accurate and efficient analysis^[59], which requires sample separation and purification based on varying Li content and matrix composition. The Li content and matrix composition differ significantly across reservoirs. For example, Li content is higher in silicate rocks, while carbonate rocks contain higher levels of Ca, Mg and lower content of Li. Among existing Li separation methods, single-column and dual-column methods are the most widely used. Early single-column methods based on nitric acid-methanol^[14-15,41] have been replaced by HCl elution^[42,56], which is more suitable for high-Li samples. Dual-column methods show advantages in handling samples with high Mg/Fe ratios but low Li content^[47]. The in-series column method, as an emerging approach, is applicable to a variety of natural samples and holds promising development prospects.

　　3. Concluding remarks

　　The cation exchange resin method has been sufficiently developed to make significant progress in the separation and purification of Li isotopes. The eluent has evolved from mixed inorganic acids and alcohols to purely inorganic acids, reducing the volume from nearly 300mL to less than 10mL. Today, the purification process for natural samples takes only a few hours, with a precision better than 1‰ and process blanks below 1‰. The single-column method is simple and widely used for high-Li, low-matrix samples; the dual-column method excels with complex matrices, especially high-Na samples; and the in-series column method is highly effective for low-Li, high-matrix samples, being both environmentally friendly and economical. Multi-column methods have been phased out due to complexity.

　　4. Future perspectives

　　Future research should focus on two areas: first, optimizing and innovating separation and purification methods to improve resin selectivity, streamline operations, reduce procedural blanks, and enhance Li recovery and analytical accuracy, particularly for extreme environmental samples and trace Li analysis. Second, as testing technologies advance, automated elution systems are expected to enable more efficient Li separation and purification, becoming a major direction for isotope analysis.

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