Highlights
- Researchers from NREL propose using light-driven techniques to selectively separate rare earth elements with unique optoelectronic properties.
- Photochemical methods leverage luminescence and photoactive ligands to potentially revolutionize REE mining and recycling processes.
- Emerging technologies like metal-organic frameworks and covalent organic frameworks show promise in developing more sustainable REE separation techniques.
Andrew Ferguson, PhD (opens in a new tab) and Melissa Gish, PhD (opens in a new tab) both affiliated with the National Renewable Energy Laboratory (NREL), (opens in a new tab) Golden, Colorado report that current methods for separations of critical rare-earth elements (REEs) require multi-step, waste-generating lacking the ability to selectively separate similarly sized ions, onerous process, or not. With unique optoelectronic properties, REEs are often exploited for photomagnetic or photoluminescent applications but could be harnessed to drive element selective separations. The two authors delve into the latest research involving photochemical reactions of REE complexes suggesting promise for investigating alternative separations based on photoactive molecules and macromolecular frameworks. Do the authors highlight a potential pathway towards realizing practical REE separations to increase the sustainability and longevity of mining and recycling these elements.
This would be transformative technological innovation, or disruption of REE processing markets.
Published in Trends in Chemistry (opens in a new tab), the authors share highlights such as the “sustainable separation of rare earth elements (REEs) comprised of the lanthanides (La–Lu), and Sc and Y is crucial for the continued supply of these critical materials used in many ubiquitous technologies.”
Ferguson and Gish continue that “light-driven REE separations promise elemental selectivity by exploiting unique optoelectronic properties, such as _f_-f transitions; however, direct excitation of metal-centered states is difficult due to weak absorptions.”
They further continue:
“Light-harvesting antenna ligands or frameworks have been used in similar applications forCO2, small molecule, and ion uptake, often including photoswitchable molecules that can alter properties, such as, structure, redox properties, and dipole moments.”
The principles of light-driven separations of REEs have been demonstrated, though a practical separation however not yet realized. But “design principles from other separations provide a path forward.”
General Background on Broader Topic
The use of photochemical reactions involving REE complexes in alternative separations is an exciting field that shows potential for improving the sustainability and efficiency of REE mining and recycling.
This technology leverages the unique optical properties of REEs, such as their sharp electronic transitions and luminescence, to facilitate selective separations.
Rare Earth Exchanges reviews the state of the technology and potential pathways for practical applications.
What about the use of photochemical REE complexesin separation science?
Recent studies have shown that REE complexes with photoactive ligands (e.g., chromophores that absorb specific wavelengths) can be used to selectively bind or release REEs under light irradiation. The selective binding can enable photochemical separations where specific wavelengths of light induce bond changes in the ligand-REE complex, leading to targeted release or capture of specific REEs.
Now, many REEs exhibit distinct luminescent properties, which can be harnessed for separation by tuning specific excitation wavelengths. For example, complexes with europium or terbium might be designed to selectively fluoresce, making it possible to distinguish and separate these from other elements in a mixed sample.
What about macromolecular frameworks for REE capture and separation?
What follows are a couple of approaches including A) metal-organic frameworks (MOFs) and B) covalent organic frameworks (COFs)
MOFs, especially those incorporating photoresponsive ligands, are being explored for REE capture. Photo-switchable MOFs can respond to light by expanding or contracting pores, affecting their affinity for certain ions. These frameworks can be functionalized with ligands selective for REEs, allowing light-triggered changes to selectively adsorb or desorb specific REEs from solutions.
Meanwhile COFs are also promising in this area due to their tunable structure and porosity. Some COFs with photoactive linkers are being researched for their ability to perform light-induced capture and release of metal ions, which can be tailored for REE separation applications.
What kinds of challenges?
While some REE-ligand systems show promising selectivity, developing ligands with high photoactivity and selectivity across a wide range of REEs remains challenging. Advances in ligand design, including fine-tuning ligand structure and electronic properties, may improve the performance of photochemical separations.
Moving from laboratory setups to industrial-scale processes presents challenges, particularly regarding energy costs and the stability of photoactive ligands over repeated cycles. Integrating photochemical separations with existing hydrometallurgical or pyrometallurgical processes could offer pathways for scaling up.
What about hybrid photochemical and chemical separation approaches? Coupling photochemical methods with traditional chemical separation techniques may enhance selectivity while maintaining cost efficiency. For instance, photochemical pretreatment could concentrate specific REEs before more conventional separation methods are applied, reducing reagent use and waste.
What about sustainability and longer-term impacts?
Developing efficient photochemical separations for REEs would help reduce dependence on environmentally damaging mining and extraction processes. By increasing the selectivity and recovery rates of REEs from recycled materials and ores, this technology could extend the lifetime of REE reserves and decrease waste.
And applications in recycling could be promising. Photochemical REE separations have strong potential in recycling, particularly in the treatment of electronic waste and other REE-rich waste streams. This could create a closed-loop system, where REEs are recovered and reused more efficiently.
In summary, while still largely in the research phase, photochemical reactions of REE complexes are promising for developing more sustainable and selective REE separation technologies. There is potential for practical application, especially in recycling, if ongoing efforts to enhance the selectivity, stability, and scalability of these systems are successful.
About National Renewable Energy Laboratory (NREL)
At the National Renewable Energy Laboratory (opens in a new tab) (NREL), we focus on creative answers to today's energy challenges.
From breakthroughs in fundamental science to new clean technologies to integrated energy systems that power our lives, NREL researchers are transforming the way the nation and the world use energy.
Daniel
You Might Also Like…