Selective Adsorption of Lithium Ions from Geothermal Water using Crown Ether-Functionalized Magnetic Nanoparticles

The increasing demand for lithium has driven a search for alternative extraction sources. Typically, Li⁺ is extracted from salar brines and hard-rock ores, but recent projects suggest that oilfield-produced waters and geothermal waters might be viable sources as well.^[1] One of the challenges associated with extracting Li⁺ from these alternative sources is the presence of competing ions in much higher concentrations, such as Na⁺ and Mg²⁺.^[2] Therefore, extraction techniques must be highly selective. Additionally, to extract meaningful amounts of Li⁺ from these waters, large volumes must be processed.

One approach to the selective adsorption of Li⁺ from water is the use of crown ethers (CEs).^[2] These cyclic structures can exhibit high selectivity for metal ions based on their size, shape, and electrochemical properties. Subsequent desorption of the metal can be achieved by shifting the pH. To make the process more efficient, it would be advantageous to stabilize the crown ethers in a way that makes them easier to extract from the water following the adsorption. Towards this goal, our group successfully synthesized crown ether-functionalized magnetic nanoparticles (MNPs) for the selective adsorption of Li⁺ from geothermal water. Crown ethers, activated with N-Hydroxysuccinimide (NHS) and N,N’-Dicyclohexylcarbodiimide (DCC) to form an NHS ester, were reacted with aminosilanized MNPs, which were synthesized in a coprecipitation reaction.^[3] A stable amide bond was formed, covering the surface of the MNP with crown ethers. The resulting particles were characterized using Raman spectroscopy, dynamic light scattering (DLS) and electron microscopy techniques. In addition, a reactor has been designed which functions to hold the MNP-CEs in place using external magnets as water flows past. This allows for the continual adsorption of Li⁺ and subsequently, simple extraction of the magnetic host-guest complex from water. By using a pH shift, the Li⁺ can be desorbed from the CE, and the MNP-CE compound can be reused for further adsorption of additional Li⁺. First experiments with the prototype reactor have demonstrated that it is possible to hold MNPs in suspension between two external neodymium magnets, with hardly any MNPs being swept out by the flow of the water.

We are currently working to optimize the parameters of the prototype reactor (e.g., magnet placement, water flow rate), and we will perform experiments to determine the adsorption isotherms of the CE-Li⁺ reaction. In addition, the selectivity of several different CEs for Li⁺ versus other metal ions will be tested, and the optimal residence time required for sufficient Li⁺ adsorption will be determined. Further, the stability and reusability of the MNP-CE compound will be assessed. Inductively coupled plasma mass spectrometry (ICP-MS) will be used to determine the concentrations of Li⁺ and other metal ions in geothermal water before, during and after adsorption experiments.

If successful, our optimization of both the MNP-CE properties and the reactor setup will result in an efficient, environmentally friendly, and scalable extraction method for Li⁺.

Citations

[1] S. Yang, Y. et al., Nature 2024, 636, 309–321.

[2] I. Oral, S et al., Sep. Purif. Technol. 2022, 294, 121142.

[3] M. Rieger et al. Anal. Bioanal. Chem. 2012, 403, 2529–2540.