(520c) Nanoporous Atomically Thin Graphene Membranes for Selective Rare Earth Separations

Criticality of rare earth elements (REEs) is well established. They are important to emerging technologies, specifically clean energy technologies and are associated with supply chain risks.[1] Neodymium and dysprosium are two such REEs. These are currently extracted from geothermal brines with concentrations as low as ppm using solvent extraction.[2]

Graphene membranes are demonstrated as an alternative way for separating REEs. Graphene membranes are transferred onto polyimide track etched membranes and pores are created using focused ion beam + chemical etching. They are tested in both diffusion-driven and pressure-driven regime using a feed solution with eight metal chlorides in a background of acid. These include alkali, alkaline and rare earth metals. It is observed that nanoporous graphene membranes allow for selective transport of alkalis over rare earth ions. The transport rates follow trends with hydration radii which indicates that these ions move through the pores with their hydration shells. A size-selective sieving is observed in these membranes.

The selectivity of alkali and rare earth ions is further enhanced by using defect sealing mechanisms like atomic layer deposition (ALD). ALD of hafnium oxide on graphene membranes before pore creation is demonstrated to seal defects in graphene membranes. It also increases K⁺/Nd³⁺ selectivity four times. The effect of number of cycles of ALD is studied on the transport of ions through these membranes. These membranes are also tested in an in-house developed acid-resistant dead-end-volume pressure driven cell. Concentration of rare earth solutions is observed with a pressure of 2 bar.

Graphene membranes are promising alternatives for initial concentration of rare-earth elements to reduce the energy requirement during solvent extraction. They allow for size-selective separation of the rare earth ions as compared to alkali metal ions.

[1] Dolf Gielen and Martina Lyons, Critical Materials for the Energy Transition: Rare Earth Elements, IRENA - International Renewable Energy Agency, Abu Dhabi, 2022.

[2] York R. Smith, Pankaj Kumar, and John D. McLennan, “On the Extraction of Rare Earth Elements from Geothermal Brines,” Resources 6, no. 3 (2017): 1–16, https://doi.org/10.3390/resources6030039.