(Nanowerk Spotlight) As high-tech industries continue to expand, the demand for rare earth elements – essential for everything from smartphones to electric vehicles – has skyrocketed, making the efficient extraction of these critical materials more urgent than ever. These 17 metallic elements, with names like neodymium, europium, and scandium, possess unique magnetic, luminescent, and catalytic properties that make them crucial components in high-tech devices.
Yet, their name is somewhat misleading –rare earth elements are not particularly scarce in the Earth’s crust. The term “rare earth” comes from their initial discovery in the late 18th and early 19th centuries, when they were found in minerals that were difficult to extract and thought to be scarce. Despite the name, these elements are relatively abundant, but the challenge lies in separating them from one another due to their similar chemical properties and the complexity of processing them from mineral ores.
Rare earth elements often occur together in mineral deposits, their chemical similarities making them notoriously difficult to separate from one another. This intertwining of elements creates a significant bottleneck in the supply chain, driving up costs and limiting the availability of purified rare earths for advanced applications.
Traditional separation methods, such as solvent extraction and ion exchange, are often energy-intensive, environmentally problematic, and struggle to achieve high levels of purity.
The quest for more efficient separation techniques has led researchers to explore a variety of innovative approaches. Membrane-based separation, a technology that has revolutionized water purification and gas processing, has emerged as a promising candidate. The idea is tantalizing – create a membrane with pores or channels so precisely tailored that they could distinguish between ions of similar size and charge, allowing one type of rare earth to pass through while blocking others.
However, turning this concept into reality has proven to be a formidable challenge. Early attempts at rare earth separation using commercial membranes yielded disappointing results, with poor selectivity and low throughput. The breakthrough would require new materials and novel fabrication techniques capable of controlling structures at the molecular level.
Recent advances in nanotechnology have opened up new possibilities. Two-dimensional materials like graphene oxide (GO) have shown particular promise for creating ultra-thin membranes with highly ordered structures. By stacking sheets of GO, researchers can create channels just a few nanometers wide – a scale small enough to exploit the selective properties of these channels. Yet, even with these advanced materials, achieving the precise control needed for rare earth separation remained elusive.
Enter a team of scientists from Lanzhou University and other Chinese institutions, who have taken a radically different approach to membrane design. Drawing inspiration from the intricate symbiotic relationships found in nature, they’ve developed a method to grow specialized nanostructures within the confined spaces between graphene oxide sheets.
Confined symbiosis synthesis of G/Z/P membranes. A) Comparison of ZIF-8′s 3D open system growth, 2D confined space growth, and 2D confined “bottom-up” symbiotic growth (The yellow balls and orange balls in A) respectively represent two different sized pores existing in 3D ZIF-8). B) Schematic illustration of 2D confined “bottom-up” symbiotic growth of G/Z/P membranes. (Image: Adapted from DOI:10.1002/adfm.202409274 with permission by Wiley-VCH Verlag)
The researchers used a technique they call “confined symbiotic reactions” to synthesize two-dimensional sheets of zeolitic imidazolate framework-8 (ZIF-8) and polydopamine (PDA) within the nanoscale spaces between graphene oxide layers.
The resulting membranes, termed G/Z/P, demonstrated remarkable selectivity for separating scandium from other rare earth elements. Scandium, while classified as a rare earth element, has unique properties that make it valuable for applications in aerospace materials and next-generation catalysts. However, its scarcity and difficulty of extraction have limited widespread use.
This novel approach draws inspiration from symbiotic relationships in nature. They introduced precursor molecules for both ZIF-8 (a metal-organic framework) and PDA (a biomimetic polymer) into the confined space between GO layers. The alkaline environment created by one precursor triggered the polymerization of the other, while the confined space directed the growth of both materials into two-dimensional sheets.
This symbiotic synthesis resulted in a vertically stacked heterojunction structure within the GO membrane. The ZIF-8 component provided selective binding sites for scandium ions, while the PDA enhanced the membrane’s stability and helped control interlayer spacing. The combination allowed for precise control over the membrane’s separation properties.
In separation experiments, the G/Z/P membranes showed exceptional performance. They achieved complete rejection of scandium ions within 12 hours, while allowing other rare earth ions to pass through. Over 24 hours, the average separation factor for other rare earths compared to scandium reached an impressive 68.73. This level of selectivity surpasses previously reported methods for scandium separation.
The team conducted detailed analyses to understand the separation mechanism. They found that the membrane’s performance relies on a two-step process. First, the controlled interlayer spacing provides a size-based screening effect. The larger hydrated scandium ions (with a hydration shell diameter of 7.74 Ångström) are initially blocked, while smaller lanthanide ions, such as lanthanum (with a hydration shell diameter of 5.24 Å), can shed some water molecules and enter the membrane structure.
In the second step, the scandium ions that do enter the membrane become trapped within the pores of the ZIF-8 component, whose pore size ranges from 4.0 to 4.2 Å. Meanwhile, other rare earth ions, particularly lanthanum, interact with the PDA component. This interaction helps maintain the optimal interlayer spacing for selective separation.
Importantly, the G/Z/P membranes also demonstrated excellent stability and mechanical properties. The incorporation of PDA significantly reduced the membrane swelling that often plagues GO-based materials in aqueous environments. The membranes retained over 80% of their separation performance after ten cycles of use, indicating good potential for practical applications.
The researchers’ approach to membrane synthesis offers several advantages over traditional methods. By conducting the material growth within the confined space between GO layers, they achieved precise control over the structure and composition of the separation channels. This bottom-up assembly method allows for the creation of tailored nanoscale environments optimized for specific separation tasks.
The success of this work opens up new possibilities for the design of highly selective separation membranes. While the current study focused on rare earth elements, the principles could potentially be applied to other challenging separations in fields such as water purification, gas separation, and chemical processing.
The ability to efficiently separate scandium from other rare earths could have significant implications for the production and utilization of this valuable element. More broadly, the development of energy-efficient, highly selective membrane separation processes could contribute to more sustainable resource extraction and purification methods across various industries.
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