PNNL

The Science

Separating critical materials, specifically rare earth elements, from unconventional liquid feedstocks is essential to establishing and maintaining robust and secure domestic supply chains. Graphene oxide (GO) membranes represent a promising separation approach that leverages confinement effects and interfacial interactions in nanoscale transport channels. Researchers studied multilayer adsorbents and membranes made from different-sized GO sheets. The adsorbents and membranes exhibit distinct transport mechanisms and surface interactions with rare-earth elements, influencing the efficiency of critical material separations. The effect of even relatively small differences in GO sheet size is amplified in multilayer laminate structures, resulting in pronounced changes in the ion permeation rate and adsorption capacity.

The Impact

Extracting critical materials, such as rare earth elements, is increasingly necessary for advanced technologies ranging from lighting to nuclear reactors. These materials are currently separated using energy- and chemical-intensive processes that consume large amounts of water and generate significant waste. GO adsorbents and membranes present a promising atom- and energy-efficient alternative for purifying these materials. Understanding how the size of GO sheets used to prepare adsorbents and membranes affects the transport properties, ion permeation rate, and adsorption capacity of rare earth ions offers an additional way to predictively design material properties for more effective separations. 

Summary

Developing efficient, cost-effective ways to isolate rare earth elements is essential to securing stable supply chains for a range of critical materials. GO-based membranes have been shown to separate different cations based on physical and chemical properties, but new work explored the size effects of GO sheets in layered materials. While GO sheets of varying sizes display small differences in their properties, the ion adsorption capacities and permeation rates of adsorbents and membranes made from these GO sheets depend strongly on size. Smaller GO sheets demonstrate greater europium (Eu³⁺) adsorption capacities but lower permeation rates than those constructed from larger GO sheets. This behavior results from three contributing factors: 1) a transition in the primary Eu³⁺ diffusion pathway from horizontal interlayer transport channels among larger vertically stacked GO sheets to vertical pores that are more abundant between smaller adjacent GO sheets in nearby planes, 2) Coulombic effects caused by strong electrostatic interactions between negatively charged carboxylate groups (–COO^-) at the edges of smaller GO sheets and positively charged Eu³⁺ cations, and 3) varying binding energies between other oxygen functional groups on GO and Eu³⁺. Understanding the role of the dimensions and chemical functionality of GO sheets in ion adsorption and transport offers valuable insight into the rational design of enhanced adsorbents and membranes, creating new opportunities for the separation of critical materials, such as rare-earth elements.

Contact

Grant Johnson, Pacific Northwest National Laboratory, grant.johnson@pnnl.gov

Funding

This work was supported by the Department of Energy (DOE), Office of Science (SC), Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, project 81462 (Harnessing Confinement Effects, Stimuli, and Reactive Intermediates in Separations). This work was supported in part by the DOE-SC, Office of Workforce Development for Teachers and Scientists under the Science Undergraduate Laboratory Internships Program. PNNL is a multiprogram national laboratory operated by Battelle for the DOE under Contract DE-AC05-76RL01830. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a DOE-SC user facility using NERSC award BES-ERCAP0027218.