Size-Selective Separation of Rare Earth Elements Using Functionalized Mesoporous Silica Materials

Local Coordination Environment of Lanthanides Adsorbed onto Cr- and Zr-based Metal-Organic Frameworks

Separating individual lanthanide (Ln) elements in aqueous mixtures is challenging. Ion-selective capture by porous materials, such as metal-organic frameworks (MOFs), is a promising approach. To design ion-selective MOFs, molecular details of the Ln adsorption complexes within the MOFs must be understood. We determine the local coordination environment of lanthanides Nd(III), Gd(III), and Lu(III) adsorbed onto Cr(III)-based terephthalate MOF (Cr-MIL-101) and Zr(IV)-based Universitet in Oslo MOFs (UiO-66 and UiO-68) and their derivatives. In the Cr(III)- and Zr(IV)-based MOFs, Ln adsorb as inner-sphere complexes at the metal oxo clusters, regardless of whether the organic linkers are decorated with amino groups. Missing linkers result in favorable Ln binding sites at oxo clusters; however, Ln can coordinate to metal sites even with linkers in place. MOF functionalization with phosphonate groups led to Ln chemisorption onto these groups, which out-compete metal cluster sites. Ln form monodentate and bidentate and mononuclear and binuclear surface complexes. We conclude that MOFs for ion-selective Ln capture can be designed by a combination of (1) maximizing metal-lanthanide interactions via shared O atoms at the metal oxo cluster sites, where mixed oxo clusters can lead to ion-selective Ln adsorption, and (2) functionalizing MOFs with Ln-selective groups capable of out-completing the metal oxo cluster sites.

Efficient Capture and Release of the Rare-Earth Element Neodymium in Aqueous Solution by Recyclable Covalent Organic Frameworks

Rare-earth elements (REEs) are present in a broad range of critical materials. The development of solid adsorbents for REE capture could enable the cost-effective recycling of REE-containing magnets and electronics. In this context, covalent organic frameworks (COFs) are promising candidates for REE adsorption due to their exceptionally high surface area. Despite having attractive physical properties, COFs are heavily underutilized for REE capture applications due to their limited lifecycle in aqueous acidic environments, as well as synthetic challenges associated with the incorporation of ligands suitable for REE capture. Here, we show how the Ugi multicomponent reaction can be leveraged to postsynthetically modify imine-based COFs for the introduction of a diglycolic acid (DGA) moiety, an efficient scaffold for REE capture. The adsorption capacity of the DGA-functionalized COF was found to be more than 40 times higher than that of the pristine imine COF precursor and more than four times higher than that of the next-best reported DGA-functionalized solid support. This rationally designed COF has appealing characteristics of high adsorption capacity, fast and efficient capture and release of the REE ions, and reliable recyclability, making it one of the most promising adsorbents for solid-liquid REE ion extractions reported to date.

Thiol-Functionalized Adsorbents through Atomic Layer Deposition and Vapor-Phase Silanization for Heavy Metal Ion Removal

The removal of toxic heavy metal ions from water resources is crucial for environmental protection and public health. In this study, we address this challenge by developing a surface functionalization technique for the selective adsorption of these contaminants. Our approach involves atomic layer deposition (ALD) followed by vapor-phase silanization of porous substrates. We utilized porous silica gel powder (∼100 μm particles, 89 m2/g surface area, ∼30 nm pores) as an initial substrate. This powder was first coated with ∼0.5 nm ALD Al2O3, followed by vapor-phase grafting of a thiol-functional silane. The modified powder, particularly in acidic conditions (pH = 4), showed high selectivity in adsorbing Cd(II), As(V), Pb(II), Hg(II), and Cu(II) heavy metal ions in mixed ion solutions over common benign ions (e.g., Na, K, Ca, and Mg). Langmuir adsorption isotherms and breakthrough adsorption studies were conducted to assess heavy metal binding affinity and revealed the order of Cd(II) < Pb(II) < Cu(II) < As(V) < Hg(II), with a significantly higher affinity for As(V) and Hg(II) ions. Time-dependent uptake studies demonstrated rapid removal of heavy metal ions from aqueous environments, with Hg(II) exhibiting the fastest adsorption kinetics on thiol-modified surfaces. These findings highlight the potential of ALD and vapor-phase silanization to create effective adsorbents for the targeted removal of hazardous contaminants from water.

Overview of Functionalized Porous Materials for Rare-Earth Element Separation and Recovery

The exceptional photoelectromagnetic characteristics of rare-earth elements contribute significantly to their indispensable position in the high-tech industry. The exponential expansion of the demand for high-purity rare earth and related compounds can be attributed to the swift advancement of contemporary technology. Nevertheless, rare-earth elements are finite and limited resources, and their excessive mining unavoidably results in resource depletion and environmental degradation. Hence, it is crucial to establish a highly effective approach for the extraction and reclamation of rare-earth elements. Adsorption is regarded as a promising technique for the recovery of rare-earth elements owing to its simplicity, environmentally friendly nature, and cost-effectiveness. The efficacy of adsorption is contingent upon the performance characteristics of the adsorbent material. Presently, there is a prevalent utilization of porous adsorbent materials with substantial specific surface areas and plentiful surface functional groups in the realm of selectively separating and recovering rare-earth elements. This paper presents a thorough examination of porous inorganic carbon materials, porous inorganic silicon materials, porous organic polymers, and metal-organic framework materials. The adsorption performance and processes for rare-earth elements are the focal points of discussion about these materials. Furthermore, this work investigates the potential applications of porous materials in the domain of the adsorption of rare-earth elements.

Rationally designed nanotrap structures for efficient separation of rare earth elements over a single step

Extracting rare earth elements (REEs) from wastewater is essential for the growth and an eco-friendly sustainable economy. However, it is a daunting challenge to separate individual rare earth elements by their subtle differences. To overcome this difficulty, we report a unique REE nanotrap that features dense uncoordinated carboxyl groups and triazole N atoms in a two-fold interpenetrated metal-organic framework (named NCU-1). Notably, the synergistic effect of suitable pore sizes and REE nanotraps in NCU-1 is highly responsive to the size variation of rare-earth ions and shows high selectivity toward light REE. As a proof of concept, Pr/Lu and Nd/Er are used as binary models, which give a high separation factor of SFPr/Lu = 796 and SFNd/Er = 273, demonstrating highly efficient separation over a single step. This ability achieves efficient and selective extraction and separation of REEs from mine tailings, establishing this platform as an important advance for sustainable obtaining high-purity REEs.

Capturing Rare-Earth Elements by Synthetic Aluminosilicate MCM-22: Mechanistic Understanding of Yb(III) Capture

We studied the mechanism underlying the solid-phase adsorption of a heavy rare-earth element (HREE, Yb) from acidic solutions employing MCM-22 zeolite, serving as both a layered synthetic clay mimic and a new platform for the mechanistic study of HREE adsorption on aluminosilicate materials. Mechanistic studies revealed that the adsorption of Yb(III) at the surface adsorption site occurs primarily through the electrostatic interaction between the site and Yb(III) species. The dependence of Yb adsorption on the pH of the solution indicated the role of surface charge, and the content of framework Al suggested that the Brønsted acid sites (BAS) are involved in the adsorption of Yb(III) ions, which was further scrutinized by spectroscopic analysis and theoretical calculations. Our findings have illuminated the roles of surface sites in the solid-phase adsorption of HREEs from acidic solutions.

Highly selective extraction of uranium from wastewater using amine-bridged diacetamide-functionalized silica

A major environmental concern related to nuclear energy is wastewater contaminated with uranium, thus necessitating the development of pollutant-reducing materials with efficiency and effectiveness. Herein, highly selective mesoporous silicas functionalized with amine-bridged diacetamide ligands SBA-15-ABDMA were prepared. Different spectroscopy techniques were used to probe the chemical environment and reactivity of the chelating ligands before and after sorption. The results showed that the functionalized SBA-15-ABDMA had a strong affinity for uranium at low pH (pH = 3) with desirable sorption capacity (68.82 mg/g) and good reusability (> 5). It showed excellent separation performance with a high distribution coefficient (K_d,U > 10⁵ mL/g) and separation factors SF_U/Ln > 1000 at a pH of 3.5 in the presence of lanthanide nuclides, alkaline earth metal and transition metal ions. In particular, SiO₂spheres-ABDMA was used as a column material, which achieved excellent recovery of U(VI) (> 98%) and good reusability for samples of simulated mining and nuclear industries wastewater. XPS and crystallography studies clearly illustrated the tridentate coordination mode of U(VI)/PEABDMA and the mechanism and origin behind the high selectivity for U.

A perspective on developing solid-phase extraction technologies for industrial-scale critical materials recovery

Critical materials (CMs) are a group of elements that have been determined to be important for the modern economy, but which may face current or potential supply limitations. Some examples of metals that have received the CM designation include the rare earth elements, indium, gallium, and lithium. The last decade has seen a major push for the development of new and improved technologies for the recovery and purification of CMs from various traditional and non-traditional resources in an effort to diversify supply. Solid-phase extraction (SPE) is one broad category of these experimental extraction technologies. SPE involves the application of a solid material to preferentially retain in the solid phase one or more specific components of an aqueous solution, leaving the other components behind in the aqueous phase. A wide range of different sorbents has been used for SPE, and many offer significant potential advantages, including low cost, low environmental impact, and high customizability. Hierarchically porous silica monoliths are one example of a cutting-edge sorbent that provides a durable, high surface area foundation that can be functionalized with a variety of targeted ligands for the selective extraction of specific CMs. Despite impressive recent advances in SPE, there remain areas for improvement that are common across the discipline. To demonstrate the practical viability of these innovative CM recovery systems, future SPE studies would benefit from devoting additional focus to the scalability of their material, as well as from focusing on real-world feedstocks and conducting techno-economic analyses and environmental impact studies.

Critical review of functionalized silica sorbent strategies for selective extraction of rare earth elements from acid mine drainage

The ubiquitous and growing global reliance on rare earth elements (REEs) for modern technology and the need for reliable domestic sources underscore the rising trend in REE-related research. Adsorption-based methods for REE recovery from liquid waste sources are well-positioned to compete with those of solvent extraction, both because of their expected lower negative environmental impact and simpler process operations. Functionalized silica represents a rising category of low cost and stable sorbents for heavy metal and REE recovery. These materials have collectively achieved high capacity and/or high selective removal of REEs from ideal solutions and synthetic or real coal wastewater and other leachate sources. These sorbents are competitive with conventional materials, such as ion exchange resins, activated carbon; and novel polymeric materials like ion-imprinted particles and metal organic frameworks (MOFs). This critical review first presents a data mining analysis for rare earth element recovery publications indexed in Web of science, highlighting changes in REE recovery research foci and confirming the sharply growing interest in functionalized silica sorbents. A detailed examination of sorbent formulation and operation strategies to selectively separate heavy (HREE), middle (MREE), and light (LREE) REEs from the aqueous sources is presented. Selectivity values for sorbents were largely calculated from available figure data and gauged the success of the associated strategies, primarily: (1) silane-grafted ligands, (2) impregnated ligands, and (3) bottom-up ligand/silica hybrids. These were often accompanied by successful co-strategies, especially bite angle control, site saturation, and selective REE elution. Recognizing the need to remove competing fouling metals to achieve purified REE "baskets," we highlight techniques for eliminating these species from acid mine drainage (AMD) and suggest a novel adsorption-based process for purified REE extraction that could be adapted to different water systems.

Understanding the Adsorption of Rare-Earth Elements in Oligo-Grafted Mesoporous Carbon

Rare-earth elements (REEs) are 17 elements of the periodic table primarily consisting of lanthanides. In modern society, the usage of REEs is ubiquitous in almost all modern gadgets and therefore efficient recovery and separation of REEs are of high importance. Selective adsorption and chelation of REEs in solid sorbents is a unique and sustainable process for their recovery. In this work, single-stranded oligos with 100 units of thymine were grafted onto carboxylated mesoporous carbon to synthesize a sorbent with phosphorus and oxygen functionalities. The sorbent was characterized by X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and scanning electron microscopy-energy-dispersive X-ray spectroscopy. Three different REEs with varying atomic radii and densities, Lu, Dy, and La, were adsorbed onto the carbon from aqueous solutions. It was observed that the adsorbed amounts increased with the increase in the atomic radius or decrease in the atomic density. Calculation of the distribution coefficients for all the equilibrium adsorption amounts suggested that adsorption is more effective in the lower concentration region. The L₃-edge X-ray absorption near-edge structure confirmed a 3+ oxidation state of REEs in the adsorbed phase. Extended X-ray absorption fine structure (EXAFS) confirmed the binding of REEs with oxygen functionalities in the adsorbed phase. The radial distribution functions calculated from the EXAFS data suggest a longer RE-O distance for La compared to those for Lu and Dy. The coordination numbers and Debye-Waller factors have typical values of about 8-9 atoms and 0.01-0.02 Ų, respectively.

Bridging Hydrometallurgy and Biochemistry: A Protein-Based Process for Recovery and Separation of Rare Earth Elements

The extraction and subsequent separation of individual rare earth elements (REEs) from REE-bearing feedstocks represent a challenging yet essential task for the growth and sustainability of renewable energy technologies. As an important step toward overcoming the technical and environmental limitations of current REE processing methods, we demonstrate a biobased, all-aqueous REE extraction and separation scheme using the REE-selective lanmodulin protein. Lanmodulin was conjugated onto porous support materials using thiol-maleimide chemistry to enable tandem REE purification and separation under flow-through conditions. Immobilized lanmodulin maintains the attractive properties of the soluble protein, including remarkable REE selectivity, the ability to bind REEs at low pH, and high stability over numerous low-pH adsorption/desorption cycles. We further demonstrate the ability of immobilized lanmodulin to achieve high-purity separation of the clean-energy-critical REE pair Nd/Dy and to transform a low-grade leachate (0.043 mol % REEs) into separate heavy and light REE fractions (88 mol % purity of total REEs) in a single column run while using ∼90% of the column capacity. This ability to achieve, for the first time, tandem extraction and grouped separation of REEs from very complex aqueous feedstock solutions without requiring organic solvents establishes this lanmodulin-based approach as an important advance for sustainable hydrometallurgy.

Comparison of Usefulness of Four Chelating Agents (EDTA, NTA, ODA and IDA) for the Chromatographic Separation of Micro and Macro Amounts of Rare Earth Elements

Literature on the use of four chelating agents namely: ethylenediaminetetraacetic acid, nitrilotriacetic acid, diglycolic acid and iminodiacetic acid for the chromatographic separation of micro and macro amounts of rare earth elements was critically reviewed and supplemented with some new unpublished data from our Laboratory. Advantages and disadvantages of ion exchange chromatography both in cation and anion mode as well as ion interaction chromatography techniques, which were used for rare earth elements separation, are discussed. The usefulness of some of the chromatographic systems for micro-macro separations was discussed and demonstrated. The importance of resilience of the separation method to column overloading in some analytical and larger scale separations was emphasized. The methods described in this article might suit well for recovering of individual lanthanides and yttrium from e-waste and other industrial wastes which were fast accumulating in recent years.

Critical Rare Earth Element Recovery from Coal Ash Using Microsphere Flower Carbon

There is a need to develop new solid-phase adsorbents to extract elements from the coal ash. High surface area carbon adsorbents are remarkably good at adsorption of rare earth elements and have good stability in acidic media. A high surface area (1162 m²/g), surface-oxidized microsphere flower carbon (MFC-O) has been prepared for the extraction of rare earth elements as well as thorium and uranium. MFC-O exhibits outstanding distribution coefficients up to k_d = 1.2 × 10⁶ for thorium, uranium, and rare earth elements. It was found that thorium and uranium can be separated from the rare earth elements by adjusting the pH. The maximum extraction capacity (71.3 mg/g) was performed up to 88 ppm with 18 competitive elements (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, and U), and element recovery was >85%. A coal ash sample (NIST SRM 1633c) with a known concentration of elements (Na, Ca, Al, Si, Fe, Sc, La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, Lu, Th, and U) was leeched resulting in 45% Ce recovery. The leeched solution from NIST 1633c was then mixed with MFC-O for Ce extraction of 74%, Na (17%), Ca (13%), Al (24%), Si (41%), and Fe (17%). The binding properties of MFC-O show that it is an attractive material for the selective extraction of rare earth elements from coal ash.

Microbe-Encapsulated Silica Gel Biosorbents for Selective Extraction of Scandium from Coal Byproducts

Scandium (Sc) has great potential for use in aerospace and clean energy applications, but its supply is currently limited by a lack of commercially viable deposits and the environmental burden of its production. In this work, a biosorption-based flow-through process was developed for extraction of Sc from low-grade feedstocks. A microbe-encapsulated silica gel (MESG) biosorbent was synthesized through sol-gel encapsulation of Arthrobacter nicotianae, a bacterium that selectively adsorbs Sc. Microscopic imaging revealed a high cell loading and macroporous structure, which enabled rapid mass transport and adsorption/desorption of metal ions. The biosorbent displayed high Sc selectivity against lanthanides and major base metals, with the exception of Fe(III). Following pH adjustment to remove Fe(III) from an acid leachate prepared from lignite coal, a packed-bed column loaded with the MESG biosorbent exhibited near-complete Sc separation from lanthanides; the column eluate had a Sc enrichment factor of 10.9, with Sc constituting 96.4% of the total rare earth elements. The MESG biosorbent exhibited no significant degradation with regard to both adsorption capacity and physical structure after 10 adsorption/desorption cycles. Overall, our results suggest that the MESG biosorbent offers an effective and green alternative to conventional liquid-liquid extraction for Sc recovery.

Mesoporous silica adsorbents modified with amino polycarboxylate ligands - functional characteristics, health and environmental effects

A series of hybrid adsorbents were produced by surface modification with amino polycarboxylate ligands of industrially available microparticles (MP) of Kromasil® mesoporous nanostructured silica beads, bearing grafted amino propyl ligands. Produced materials, bearing covalently bonded functions as EDTA and TTHA, original Kromasil®, bearing amino propyl ligands, and bare particles, obtained by thermal treatment of Kromasil® in air, were characterized by SEM-EDS, AFM, FTIR, TGA and gas sorption techniques. Adsorption kinetics and capacity of surface-modified particles to adsorb Rare Earth Elements (REE), crucial for extraction in recycling processes, were evaluated under dynamic conditions, revealing specificity matching the ligand nature and the size of REE cations. A detailed comparison with earlier reported adsorbents for REE extraction was presented. The cytotoxicity was assessed using four different types of healthy cells, human skeletal muscles derived cells (SKMDC), fibroblast cells, macrophage cells (RAW264.7), and human umbilical vein endothelial cells (HUVECs), indicating lower toxicity of ligand-free MP than MP bearing amino poly-carboxylate functions. Internalization of the MP inside the cells and release of nitric oxide were observed. In addition, zebrafish embryos were exposed to high concentrations of MP and did not show any pronounced toxicity.

Understanding Selectivity of Mesoporous Silica-Grafted Diglycolamide-Type Ligands in the Solid-Phase Extraction of Rare Earths

Rare earth elements (REEs) and their compounds are essential for rapidly developing modern technologies. These materials are especially critical in the area of green/sustainable energy; however, only very high-purity fractions are appropriate for these applications. Yet, achieving efficient REE separation and purification in an economically and environmentally effective way remains a challenge. Moreover, current extraction technologies often generate large amounts of undesirable wastes. In that perspective, the development of selective, reusable, and extremely efficient sorbents is needed. Among numerous ligands used in the liquid-liquid extraction (LLE) process, the diglycolamide-based (DGA) ligands play a leading role. Although these ligands display notable extraction performance in the liquid phase, their extractive chemistry is not widely studied when such ligands are tethered to a solid support. A detailed understanding of the relationship between chemical structure and function (i.e., extraction selectivity) at the molecular level is still missing although it is a key factor for the development of advanced sorbents with tailored selectivity. Herein, a series of functionalized mesoporous silica (KIT-6) solid phases were investigated as sorbents for the selective extraction of REEs. To better understand the extraction behavior of these sorbents, different spectroscopic techniques (solid-state NMR, X-ray photoelectron spectroscopy, XPS, and Fourier transform infrared spectroscopy, FT-IR) were implemented. The obtained spectroscopic results provide useful insights into the chemical environment and reactivity of the chelating ligand anchored on the KIT-6 support. Furthermore, it can be suggested that depending on the extracted metal and/or structure of the ligand and its attachment to KIT-6, different functional groups (i.e., C═O, N-H, or silanols) act as the main adsorption centers and preferentially capture targeted elements, which in turn may be associated with the different selectivity of the synthesized sorbents. Thus, by determining how metals interact with different supports, we aim to better understand the solid-phase extraction process of hybrid (organo)silica sorbents and design better extraction materials.

Biological, biomolecular, and bio-inspired strategies for detection, extraction, and separations of lanthanides and actinides

Lanthanides and actinides are elements of ever-increasing technological importance in the modern world. However, the similar chemical and physical properties within these groups make purification of individual elements a challenge. Current industrial standards for the extraction, separation, and purification of these metals from natural sources, recycled materials, and industrial waste are inefficient, relying upon harsh conditions, repetitive steps, and ligands with only modest selectivity. Biological, biomolecular, and bio-inspired strategies towards improving these separations and making them more environmentally sustainable have been researched for many years; however, these methods often have insufficient selectivity for practical application. Recent developments in the understanding of how lanthanides are selectively acquired and used by certain bacteria offer the opportunity for a newer, more efficient take on these designs, as well as the possibility for fundamentally new designs and strategies. Herein, we review current cell-based and biomolecular (primarily small-molecule and protein-based) methods for detection, extraction, and separations of f-block elements. We discuss how the increasing knowledge regarding the selective recognition, uptake, trafficking, and storage of these elements in biological systems has informed and will continue to promote development of novel approaches to achieve these ends.

Highly Hierarchical Fibrillar Biogenic Silica with Mesoporous Structure Derived from the Perennial Plant Equisetum Fluviatile

A new discovery of highly hierarchical fibrillar biogenic silica with mesoporous structure derived from the perennial plant Equisetum fluviatile was made. By removing the organic compounds through chemical and heat treatment, the biogenic silica skeleton can largely retained the original highly hierarchical structure of the plant stems. Infrared spectra, X-ray diffraction, and small-angle X-ray scattering, as well as nitrogen sorption analysis, were employed to characterize the crystalline phases, nanostructure, and porosity of the resulting material. Scanning electron microscopy and transmission electron microscopy investigation reveal that the biogenic silica are fibers with diameters of about 120-150 μm and lengths of more than a few centimeters. These fibers consist of smaller fasciculus with diameters of about 5-15 μm that are composed of three levels of particles with mass and surface fractal characteristics: primary particles on the order of 3-5 nm, secondary particles on the order of 9-12 nm, and tertiary particles on the order of 90-120 nm in size. It is also shown that the biogenic silica have mesoporous structure with an average pore size of 4-6 nm and a specific surface of 100-300 m²/g. Heat treatment at high temperature and residual K⁺ significantly affects the characteristics of the mesoporous structure of the biogenic silica, although it has little effect on the surface fractal structure of the secondary particles.

Selective Th(iv) capture from a new metal–organic framework with O− groups

New MOF adsorbent with functional N⁺–O⁻ groups was designed. The material shows fast adsorption of Th(iv) and high removal efficiency, and is selective over La(iii), Sm(iii), Ho(iii), Cd(ii), Pb(ii) and K(i) ions.