Formation of Supramolecular Clusters at the Interface of Zeolite X Following the Adsorption of Rare-Earth Cations and Their Impact on the Macroscopic Properties of the Zeolite

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.

An insight into REEs recovery from spent fluorescent lamps: Evaluation of the affinity of an NH4-13X zeolite towards Ce, La, Eu and Y

The constantly increasing demand of Rare Earth Elements (REEs) made them to be part of the so-called "critical elements" indispensable for the energy transition. The monopoly of only a few countries, the so-called balance problem between demand and natural abundance, and the need to limit the environmental costs of their mining, stress the necessity of a recycling policy of these elements. Different methods have been tested for REEs recovery. Despite the well-known ion-exchange properties of zeolites, just few preliminary works investigated their application for REEs separation and recycle. In this work we present a double ion exchange experiment on a NH4-13X zeolite, aimed at the recovery of different REEs from solutions mimicking the composition of liquors obtained from the leaching of spent fluorescent lamps. The results showed that the zeolite was able to exchange all the REEs tested, but the exchange capacity was different: despite Y being the more concentrated REE in the solutions, the cation exchange was lower than less concentrated ones (16 atoms p.u.c. vs 21 atoms for Ce and La solutions), suggesting a possible selectivity. In order to recover REEs from the zeolite, a second exchange with an ammonium solution was performed. The analyses of the zeolites show that almost all of Ce and Eu remain in the zeolite, while nearly half of La and Y are released. This, once again, suggests a possible selective release of REEs and open the possibility for a recovery process in which Rare Earths can be effectively separated.

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.

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.

Evaluation for the Removal Efficiency of VOCs and Heavy Metals by Zeolites-Based Materials in the Wastewater: A Case Study in the Tito Scalo Industrial Area

The current study was designed to demonstrate the efficiency of selected zeolites in the immobilization of heavy metals and volatile organic compounds from water in the industrial area of Tito Scalo (Basilicata Region in Southern Italy). The efficiency of zeolite materials has been evaluated by analyzing real water samples, by a multi-technique approach. Gas chromatography (GC) and inductively coupled plasma optical emission spectrometry (ICP-OES) were selected for the detection of volatile organic compounds (VOCs) and heavy metals respectively, and then by thermal analysis (TG, DTA) and X-ray powder diffraction (XRD) to verify the presence of contaminants in the structural channels of the adsorbents. ZSM-5 zeolite (MFI topology) was suitable for volatile organic compounds, showing removal efficiencies 87%. 13X (FAU topology) was more selective for in situ abatements of heavy metals, with efficiencies up to 100%. After VOCs and heavy metals removal, structure refinements of loaded zeolites highlighted variations of both lattice parameters and extraframework content confirming the pollutants immobilization in the framework microporosities. The occurrence of these species was also confirmed by DTA curves showing different phenomena explained on the basis of the nature and number of extraframework species hosted in the zeolite micropores.

Entropy in multiple equilibria, compounds with different sites

The influence of entropy in multiple chemical equilibria is investigated for systems with different types of sites for the condition that the binding enthalpy of the species is the same within each type of sites and independent of those species that are already bonded. This allows splitting of the free reaction enthalpy into the particle distribution term and all other contributions for each type of sites separately and, hence, to evaluate this entropy contribution to the free reaction enthalpy. The situations for which this applies can be chemically very different, e.g. acid base, ligand exchange, isomerisation, conformational change, rearrangement of a ligand, ion exchange, adsorption of a species on the surface of a particle or a dendrimer, insertion of charged or neutral species into the cavities of a microporous or mesoporous host. We provide physical insight by discussing Xrc1{n1ABn2}Xrc2 systems. The number of coordination sites A and B are n1 and n2, respectively. The indices rc1 = 1, 2,…,n1 and rc2 = 1, 2,…,n2 count the number of X bonded to sites A and sites B, respectively. An important result is that the large number of equilibrium constants needed to describe those situations can be expressed as a function of two constants only. This allows studying systems quantitatively by experimental and theoretical means which otherwise might be difficult to handle. It has also implication for theoretical studies in the sense that it is sufficient to model only two reactions instead of many in order to describe a system. The results remain valid for systems with more than two types of different sites. The description of the entropy driven development of the fractional equilibrium coverage of the sites provides a new tool for understanding adsorption and ion exchange isotherms. The fractional equilibrium coverage of the sites can be described as a linear combination of individual Langmuir isotherms despite of the fact that such a linear combination has never the shape of the original Langmuir isotherm. This is remarkable and very useful. It provides us with new tools for describing and testing isotherms based on well defined, transparent physical ideas. Explicit solution for systems with 2, 3, 4, 5, 6, and 12 coordination sites are reported. Applications to a system with 12 coordination sites serve to illustrate information that can be obtained for complex situations.