Concomitant Leaching and Electrochemical Extraction of Rare Earth Elements from Monazite

Adsorption of rare earth elements on a magnetic geopolymer derived from rice husk: studies in batch, column, and application in real phosphogypsum leachate sample

There is a growing need to develop new strategies for rare earth element (REE) recovery from secondary resources. Herein, a novel approach to utilize biogenic silica (from rice husk) and metakaolin was employed to fabricate magnetic geopolymer (MGP) by incorporating metallic iron. The fabricated MGP adsorbent material was used to uptake Ce3+, La3+, and Nd3+ from synthetic solutions and real phosphogypsum leachate in batch and column modes. The MGP offers a negatively charged surface at pH above 2.7, and the uptake of REEs rises from pH 3 to 6. The kinetic study validated that the kinetics was much faster for Nd3+, followed by La3+ and Ce3+. A thermodynamic investigation validated the exothermic nature of the adsorption process for all selected REEs. The desorption experiment using 2 mol L-1 H2SO4 as the eluent demonstrated approximately 100% desorption of REEs from the adsorbent. After six adsorption-desorption cycles, the MGP maintained a high adsorption performance up to cycle five before suffering a significant decrease in performance in cycle six. The effectiveness of MGP was also assessed for its applicability in recovering numerous REEs (La3+, Ce3+, Pr3+, Sm3+, and Nd3+) from real leachate from phosphogypsum wastes, and the highest recovery was achieved for Nd3+ (95.03%) followed by Ce3+ (86.33%). The operation was also feasible in the column presenting suitable values of the length of the mass transfer zone. The findings of this investigation indicate that MGP adsorbent prepared via a simple route has the potential for the recovery of REEs from synthetic and real samples in both batch and continuous operations modes.

Scalable and Consolidated Microbial Platform for Rare Earth Element Leaching and Recovery from Waste Sources

Chemical methods for the extraction and refinement of technologically critical rare earth elements (REEs) are energy-intensive, hazardous, and environmentally destructive. Current biobased extraction systems rely on extremophilic organisms and generate many of the same detrimental effects as chemical methodologies. The mesophilic methylotrophic bacterium Methylobacterium extorquens AM1 was previously shown to grow using electronic waste by naturally acquiring REEs to power methanol metabolism. Here we show that growth using electronic waste as a sole REE source is scalable up to 10 L with consistent metal yields without the use of harsh acids or high temperatures. The addition of organic acids increases REE leaching in a nonspecific manner. REE-specific bioleaching can be engineered through the overproduction of REE-binding ligands (called lanthanophores) and pyrroloquinoline quinone. REE bioaccumulation increases with the leachate concentration and is highly specific. REEs are stored intracellularly in polyphosphate granules, and genetic engineering to eliminate exopolyphosphatase activity increases metal accumulation, confirming the link between phosphate metabolism and biological REE use. Finally, we report the innate ability of M. extorquens to grow using other complex REE sources, including pulverized smartphones, demonstrating the flexibility and potential for use as a recovery platform for these critical metals.

The importance of the pretreatment of samples in Nd quantification from NdFeB magnets through inductively coupled plasma atomic emission spectroscopy (ICP-OES)-a rapid and streamlined methodology

In this study, we emphasize the critical role of sample pretreatment. We report on the behavior of NdFeB magnet samples exposed to four different acid media for digestion. NdFeB magnets are becoming a significant source of neodymium, a rare-earth element critical to many technologies and a potential substitute for traditional mining of the element. To address this, we meticulously tested nitric acid, hydrochloric acid, acetic acid, and citric acid, all at a concentration of 1.6 M, as economical and environmentally friendly alternatives to the concentrated mineral acids commonly used in the leaching of these materials. The pivotal stage involves the initial characterization of samples in the solid state using SEM-EDX and XPS analysis to obtain their initial composition. Subsequently, the samples are dissolved in the four aforementioned acids. Finally, neodymium is quantified using ICP-OES. Throughout our investigation, we evaluated some analytical parameters to determine the best candidate for performing the digestion, including time, limits of detection and quantification, accuracy, recovery of spike samples, and robustness. After careful consideration, we unequivocally conclude that 1.6 M nitric acid stands out as the optimal choice for dissolving NdFeB magnet samples, with the pretreatment of the samples being the critical aspect of this report.

Tailoring the pore size of expanded porphyrinoids for lanthanide selectivity

Despite increase in demand, capacity for the recycling of rare earth elements remains limited, partly due to the inefficiencies with processes currently utilised in the separation of lanthanides. This study highlights the potential use of expanded porphyrinoids in lanthanide separation through selective binding, dependent on the tailored pore size of the macrocycle. Each emerging trend is subjected to multi-factored analysis to decompose the underlying source. Results promote the viability of size-based separation with preferential binding of larger lanthanum(iii) ions to amethyrin and isoamethyrin macrocycles, while smaller macrocycles such as pentaphyrin(0.0.0.0.0) present a preferential binding of lutetium(iii) ions. Additionally, the porphyrin(2.2.2.2) macrocycle shows a selectivity for gadolinium(iii) ions over both larger and smaller ions. An upper limit of applicable pore size is shown to be ≈2.8 Å, beyond which the formed complexes are predicted to be less stable than the corresponding nitrate complexes.

Electrochemical Wastewater Refining: A Vision for Circular Chemical Manufacturing

Wastewater is an underleveraged resource; it contains pollutants that can be transformed into valuable high-purity products. Innovations in chemistry and chemical engineering will play critical roles in valorizing wastewater to remediate environmental pollution, provide equitable access to chemical resources and services, and secure critical materials from diminishing feedstock availability. This perspective envisions electrochemical wastewater refining─the use of electrochemical processes to tune and recover specific products from wastewaters─as the necessary framework to accelerate wastewater-based electrochemistry to widespread practice. We define and prescribe a use-informed approach that simultaneously serves specific wastewater-pollutant-product triads and uncovers a mechanistic understanding generalizable to broad use cases. We use this approach to evaluate research needs in specific case studies of electrocatalysis, stoichiometric electrochemical conversions, and electrochemical separations. Finally, we provide rationale and guidance for intentionally expanding the electrochemical wastewater refining product portfolio. Wastewater refining will require a coordinated effort from multiple expertise areas to meet the urgent need of extracting maximal value from complex, variable, diverse, and abundant wastewater resources.

Efficient and selective capture of thorium ions by a covalent organic framework

The selective separation of thorium from rare earth elements and uranium is a critical part of the development and application of thorium nuclear energy in the future. To better understand the role of different N sites on the selective capture of Th(IV), we design an ionic COF named Py-TFImI-25 COF and its deionization analog named Py-TFIm-25 COF, both of which exhibit record-high separation factors ranging from 102 to 105. Py-TFIm-25 COF exhibits a significantly higher Th(IV) uptake capacity and adsorption rate than Py-TFImI-25 COF, which also outperforms the majority of previously reported adsorbents. The selective capture of Py-TFImI-25 COF and Py-TFIm-25 COF on thorium is via Th-N coordination interaction. The prioritization of Th(IV) binding at different N sites and the mechanism of selective coordination are then investigated. This work provides an in-depth insight into the relationship between structure and performance, which can provide positive feedback on the design of novel adsorbents for this field.

γ-Resistant Microporous CAU-1 MOF for Selective Remediation of Thorium

A simple solvothermal method was used to synthesize a metal-organic framework (MOF) with an Al metal entity, viz., CAU-1 NH₂. The synthesized MOF was characterized using different techniques like X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy (SEM), field emission SEM (FE-SEM), transmission electron microscopy, small-angle X-ray scattering, positron annihilation lifetime spectroscopy, and X-ray photoelectron spectroscopy. The radiation stability was evaluated by irradiating the material up to a cumulative dose of 2 MGy using ⁶⁰Co for the first time. The studies showed a remarkable gamma irradiation stability of the material up to 1 MGy. The porosity and surface area of the synthesized MOF were determined by Brunauer-Emmett-Teller, which showed a high specific surface area of 550 m²/g. The pH dependence study of Th uptake from an aqueous solution was performed from pH 2-8, followed by adsorption isotherm and adsorption kinetics studies. These results revealed that the Langmuir and pseudo-second-order kinetic models can be well adapted for understanding the Th uptake and kinetics, respectively. The synthesized MOF exhibited an ∼404 mg/g thorium adsorption capacity. Selectivity studies of adsorption of Th w.r.t. to U and different metal ions such as Cu, Co, Ni, and Fe showed that Th gets adsorbed preferentially as compared to other metal ions. In addition, the MOF could be used multiple times without much deterioration.

Highly Tunable 4-Phosphoryl Pyrazolone Receptors for Selective Rare-Earth Separation

Highly selective rare-earth separation has become increasingly important due to the indispensable role of these elements in various cutting-edge technologies including clean energy. However, the similar physicochemical properties of rare-earth elements (REEs) render their separation very challenging, and the development of new selective receptors for these elements is potentially of very considerable economic and environmental importance. Herein, we report the development of a series of 4-phosphoryl pyrazolone receptors for the selective separation of trivalent lanthanum, europium, and ytterbium as the representatives of light, middle, and heavy REEs, respectively. X-ray crystallography studies were employed to obtain solid-state structures across 11 of the resulting complexes, allowing comparative structure-function relationships to be probed, including the effect of lanthanide contraction that occurs along the series from lanthanum to europium to ytterbium and which potentially provides a basis for REE ion separation. In addition, the influence of ligand structure and lipophilicity on lanthanide binding and selectivity was systematically investigated via n-octanol/water distribution and liquid-liquid extraction (LLE) studies. Corresponding stoichiometry relationships between solid and solution states were well established using slope analyses. The results provide new insights into some fundamental lanthanide coordination chemistry from a separation perspective and establish 4-phosphoryl pyrazolone derivatives as potential practical extraction reagents for the selective separation of REEs in the future.

Intelligent modelling for the elimination of lanthanides (La3+, Ce3+, Nd3+ and Eu3+) from aqueous solution by magnetic CoFe2O4 and CoFe2O4-GO spinel ferrite nanocomposites

In this research, a novel CoFe₂O₄-GO (Graphen Oxide) resulting from the combination of high applicable magnetic and organic base materials and synthesized with a simple and fast co-precipitation route was synthesized for the REEs (Rare Earth Elements) extraction. This adsorbent could remove the La³⁺, Ce³⁺, Nd³⁺ and Eu³⁺ by maximum adsorption capacity of 625, 626, 714.2, 1111.2 mg/g at optimized pH = 6, respectively. A data-driven model was obtained using Group Method of Data Handling (GMDH)-based Neural Network to estimate the adsorption capacity of these LREEs as a function of time, pH, temperature, adsorbent ζ (zeta)- potential, initial concentration of lanthanides ions, and ε which is defined by the physico-chemical properties of lanthanides. The results clearly indicated that the model estimate the experimental values with good deviation (mostly less than 10%) and it can be used for the prediction of the results from other similar researches with less than 25% deviation. The results of sensitivity analysis indicated that the adsorption capacity is more sensitive to pH of the solution, temperature, and ε. Finally, the desorption studies showed an excellent removal efficiency (97%) at least for three adsorption-desorption cycles. These results claimed that the CoFe₂O₄-GO is a highly efficient adsorbent for the REEs extraction.

Study on Economic Significance of Rare Earth Mineral Resources Development Based on Goal Programming and Few-Shot Learning

Rare earth is one of the most important strategic minerals in the world today. The wide application of new products and technologies in the global market has made the world's demand for rare earth resources grow rapidly. As an important basic resource of high technology, rare earth plays a significant role in national security and strategy. As rare earth mineral resources are irreplaceable in civil, military, and nuclear industries, they have become national strategic resources of various countries in the world. Although China's rare earth industry has occupied a leading position in the world, with the continuous expansion of the scale of the rare earth industry, the pollution problem of “three wastes” produced in the recovery of rare earth mineral resources is becoming more and more serious. From the perspective of resource endowment, China is rich in rare earth resources, but the declining trend of resource reserves is obvious, and the advantages and disadvantages of resources are more prominent. Based on the goal planning and few-shot learning, this paper studies the economic significance of rare earth resource development, aiming at solving some problems in the development of rare earth mineral resources in China, and thus promoting the scientific and healthy development of the rare earth industry.

Multi-step Phase Transformation from Metal-Organic Frameworks to Inorganic Compounds for High-Purity Th(IV) Generation

The generation of high-purity thorium is the precondition for next-generation nuclear energy; however, this remains a challenging task. To this end, we present herein an ultrasimple technique with the combination of crystallization plus phase transformation. Crystallization into ECUT-68 is found to show almost 100% selective uptake of Th(IV) over rare earth and UO₂²⁺ ions, while multistep phase transformation from metal-organic frameworks (MOFs) to inorganic compounds is found to directly generate inorganic Th(IV) compound and then Th(IV) solution, suggesting its superior application in the generation of high-purity thorium.

Electrified bioreactors: the next power-up for biometallurgical wastewater treatment

Over the past decades, biological treatment of metallurgical wastewaters has become commonplace. Passive systems require intensive land use due to their slow treatment rates, do not recover embedded resources and are poorly controllable. Active systems however require the addition of chemicals, increasing operational costs and possibly negatively affecting safety and the environment. Electrification of biological systems can reduce the use of chemicals, operational costs, surface footprint and environmental impact when compared to passive and active technologies whilst increasing the recovery of resources and the extraction of products. Electrification of low rate applications has resulted in the development of bioelectrochemical systems (BES), but electrification of high rate systems has been lagging behind due to the limited mass transfer, electron transfer and biomass density in BES. We postulate that for high rate applications, the electrification of bioreactors, for example, through the use of electrolyzers, may herald a new generation of electrified biological systems (EBS). In this review, we evaluate the latest trends in the field of biometallurgical and microbial-electrochemical wastewater treatment and discuss the advantages and challenges of these existing treatment technologies. We advocate for future research to focus on the development of electrified bioreactors, exploring the boundaries and limitations of these systems, and their validity upon treating industrial wastewaters.

Highly efficient recovery of heavy rare earth elements by using an amino-functionalized magnetic graphene oxide with acid and base resistance

In the application of various magnetic materials for water treatment, control of surface resistance to acid and alkali corrosion remains largely overlooked, which could greatly extend their service life. We herein prepare amino grafted magnetic graphene oxide composites using a simple one-step cross-link reaction between the graphene oxide and magnetic Fe₃O₄/C nanoparticles. The as-prepared magnetic graphene oxide composites have long-term stability under acid and alkali solutions and shows an excellent performance in removing Ho(III), a representative rare earth element (REE) from water. The observed adsorption capacity of 72.1 mg Ho(III)/g exceeded that of most magnetic materials previously reported. Regeneration of the magnetic composites was realized in acid and alkali solutions but their structural integrity and physicochemical properties retained even after 18 adsorption-desorption cycles. The current adsorbent also shows excellent adsorption performance for other heavy REEs, such as Er(III), Eu(III), Lu(III), Tm(III), Y(III) and Yb(III). This work can provide a new strategy for constructing an acid and base resistance magnetic graphene oxide for the high-efficient recovery of heavy REEs from aqueous solution.

Thorium inhibits human respiratory chain complex IV (cytochrome c oxidase)

Thorium is a radioactive heavy metal and an emerging environmental pollutant. Ecological and human health risks from thorium exposure are growing with the excavation of rare earth metals and implementation of thorium-based nuclear reactors. Thorium poisoning is associated with carcinogenesis, liver impairments, and congenital anomalies. To date, the biomolecular targets that underlie thorium-induced toxicity remain unknown. Here, we used in vitro enzymatic activity assays to comprehensively evaluate the effects of thorium on the mitochondrial respiration process. Thorium was found to inhibit respiratory chain complex IV (cytochrome c oxidase) at sub-micromolar concentrations (IC₅₀ ~ 0.4 μM, 90 μg/L). This is lower than the thorium level limit (246 μg/L) in drinking water specified by the World Health Organization. The inhibitory effects were further verified in mitochondria from human bone and liver cells (thorium mainly deposits in these organs). The inhibition of cytochrome c oxidase can readily rationalize well-documented cellular toxicities of thorium, such as alteration of mitochondrial membrane potential and production of reactive oxygen species. Therefore, cytochrome c oxidase is potentially a key molecular target underlying thorium-induced toxicological effect.

Interaction-determined extraction capacity between rare earth ions and extractants: taking lanthanum and lutetium as models through theoretical calculations

Extractant plays an important role in the separation and purification of rare earth elements (REEs), whereas, extraction performance is the most effective tactic to evaluate whether an extractant is complete...

Rational assembly of GO-based heterocyclic sulfur- and nitrogen-containing aerogels and their adsorption properties toward rare earth elementals

An aromatic heterocyclic compound, 2-aminobenzothiazole (ABT), was used to decorate graphene oxide (GO) by a facile hydrothermal self-assembly procedure. The developed three-dimensional (3D) GO-ABT composite aerogels could be utilized as high-powered and sustainable adsorbents for the enrichment and recovery of low concentration rare earth elements (REEs) from aqueous solutions. The composition and microstructure of GO-ABT composites were explored various characterization methods. The enrichment properties of GO-ABT composites for REEs were investigated in detail, revealing the existence of S-, N- and -NH₂ in ABT, as well as the carboxyl and hydroxyl groups of GO which might act as the major REE binding sites. The adsorption of GO-ABT composites for low concentration REEs could reach equilibrium in 30 min. Our investigations confirmed that the optimal pH value of GO-ABT composites for REEs was pH 4.0-5.0. For the adsorbent regeneration study, 50.0 mg of GO-ABT_{15:1/120 °C/6 h} composite was used toward 20.0 mL of Er³⁺ solutions. After ten regeneration cycles, the adsorption rates of GO-ABT composites for Er³⁺ remained around 100%, and the desorption rates maintained over 90%. The long-term storage of the adsorbent did not affect its adsorption ability, while desorption rates increased, indicating it possessed relatively higher stability.

Selective Leaching of Rare Earth Elements from Ion-Adsorption Rare Earth Tailings: A Synergy between CeO2 Reduction and Fe/Mn Stabilization

The increasing demand for rare earth elements (REEs) motivates the development of novel strategies for cost-effective REE recovery from secondary sources, especially rare earth tailings. The biggest challenges in recovering REEs from ion-adsorption rare earth tailings are incomplete extraction of cerium (Ce) and the coleaching of iron (Fe) and manganese (Mn). Here, a synergistic process between reduction and stabilization was proposed by innovatively using elemental sulfur (S) as reductant for converting insoluble CeO2 into soluble Ce2(SO4)3 and transforming Fe and Mn oxides into inert FeFe2O4 and MnFe2O4 spinel minerals. After the calcination at 400 °C, 97.0% of Ce can be dissolved using a diluted sulfuric acid, along with only 3.67% of Fe and 23.3% of Mn leached out. Thermodynamic analysis reveals that CeO2 was indirectly reduced by the intermediates MnSO4 and FeS in the system. Density functional theory calculations indicated that Fe(II) and Mn(II) shared similar outer electron arrangements and coordination environments, favoring Mn(II) over Ce(III) as a replacement for Fe(II) in the FeO6 octahedral structure of FeFe2O4. Further investigation on the leaching process suggested that 0.5 mol L-1 H2SO4 is sufficient for the recovery of REEs (97.0%). This research provides a promising strategy to selectively recover REEs from mining tailings or secondary sources via controlling the mineral phase transformation.

Release behavior of iodine during leaching and calcination of phosphate rock

A series of experiments of column leaching under different pHs (pH 1.8, 3.8, 6.5, and 8.5) and calcination at different temperatures (200-1100 °C) were carried out for evaluation of release behavior of iodine in phosphate rock. The modes of occurrence of iodine in the phosphate rock and its leaching and calcination residues were extracted with sequential chemical extraction. Iodine in solution and solid samples was measured with ion chromatography (IC) and pyrohydrolysis combined ion chromatography (PIC), respectively. Mineralogical compositions of phosphate rock and the leached and calcined residues were determined by XRD (X-ray diffraction) and FT-IR (Fourier infrared spectrum). The results show that iodine in phosphate rock occurred in a descending order of significance, as forms of residual, carbonate bound, ion-exchange, organic bound, Fe-Mn oxide bound, and water soluble. Under pH 1.8, 3.8, 6.5, and 8.5, the release iodine may almost reach the maximum at the leaching time of 65, 93, 90, and 165 h, with leaching rates of 5.28%, 1.24%, 0.550%, and 1.08% and the average iodine concentrations in the leachates of 2300 μg/L, 378 μg/L, 164 μg/L, and 189 μg/L, respectively. The forms of the leached iodine were mostly ion-exchange and carbonate-bound iodine under pH 1.8 and water soluble and ion-exchange iodine under pH 3.8, 6.5, and 8.5. By calcination, the total iodine was released rapidly in 200-300 °C and 700-1000 °C, and almost released completely at 1000 °C, with a leaching rate of 96.6%. The ion-exchange and organic-bound iodine were, respectively, released at 200-1000 °C and at less than 300 °C; the carbonate-bound and residual iodine were mainly released at more than 700 °C. The release iodine in phosphate rock leached by natural water and calcined at a high temperature may lead to the increase of iodine concentration of water body and atmosphere.

Selective leaching of copper and zinc from primary ores and secondary mineral residues using biogenic ammonia

With the number of easily accessible ores depleting, alternate primary and secondary sources are required to meet the increasing demand of economically important metals. Whilst highly abundant, these materials are of lower grade with respect to traditional ores, thus highly selective and sustainable metal extraction technologies are needed to reduce processing costs. Here, we investigated the metal leaching potential of biogenic ammonia produced by a ureolytic strain of Lysinibacillus sphaericus on eight primary and secondary materials, comprised of mining and metallurgical residues, sludges and automotive shredder residues (ASR). For the majority of materials, moderate to high yields (30-70%) and very high selectivity (>97% against iron) of copper and zinc were obtained with 1 mol L^-1 total ammonia. Optimal leaching was achieved and further refined for the ASR in a two-step indirect leaching system with biogenic ammonia. Copper leaching was the result of local corrosion and differences in leaching against the synthetic (NH₄)₂CO₃ control could be accounted for by pH shifts from microbial metabolism, subsequently altering free NH₃ required for coordination. These results provide important findings for future sustainable metal recovery technologies from secondary materials.

Recovering rare earth elements from contaminated soils: Critical overview of current remediation technologies

Rare earth elements (REE) are essential for sustainable energies such as solar and wind power, with rising demand due to the ambitious goal for a circular society. REE are currently mined from virgin ores while REE-rich contaminated soil is left untreated in the environment. Soil remediation strategies are needed that concomitantly cleanup soil and harvest metals that contribute to process circular economy. In this review we aim to (i) define REE concentrations in contaminated soils as well as (ii) identify soil remediation techniques used in remediating REE from soils, emphasizing the ones that extract REE. Current literature lists REE polluted soils in the vicinities of REE mines, coal mines, high traffic roads and agricultural soils (due to REE association with phosphate fertilizers). We first list the conventional separation methods used in the mining industry and their main strategies in extracting/precipitating REE. Solvent extraction is the most commonly conventional method used followed by electrodeposition of REE at high temperatures. We then highlight soil remediation techniques that are used to treat REE. These techniques can be separated into two types: the ones that (a) stabilize REE in soils, and the ones that (b) extract REE from soils. Bioremediation, soil amendments and others offer stabilization of REE, eventually creating a legacy problem since REE keep accumulating in the soil. Soil remediation techniques that achieve REE extraction are a step closer to resource recovery, contributing to the circularity of REE. Techniques such as phytoremediation, soil washing and electrokinetic treatment show promising extraction results.

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.

Emerging technologies for the recovery of rare earth elements (REEs) from the end-of-life electronic wastes: a review on progress, challenges, and perspectives

The demand for rare earth elements (REEs) has significantly increased due to their indispensable uses in integrated circuits of modern technology. However, due to the extensive use of high-tech applications in our daily life and the depletion of their primary ores, REE's recovery from secondary sources is today needed. REEs have now attracted attention to policymakers and scientists to develop novel recovery technologies for materials' supply sustainability. This paper summarizes the recent progress for the recovery of REEs using various emerging technologies such as bioleaching, biosorption, cryo-milling, electrochemical processes and nanomaterials, siderophores, hydrometallurgy, pyrometallurgy, and supercritical CO₂. The challenges facing this recovery are discussed comprehensively and some possible improvements are presented. This work also highlights the economic and engineering aspects of the recovery of REE from waste electrical and electronic equipment (WEEE). Finally, this review suggests that greener and low chemical consuming technologies, such as siderophores and electrochemical processes, are promising for the recovery of REEs present in small quantities. These technologies present also a potential for large-scale application.

Mineralogy and Geochemistry of the Kinnikinic Quartzite at the Arco Hills Silica and Gold Project in Butte County, Idaho: Results of an Ore Quality Spot Check and Implications for Potential Plasma Furnace Processing

Plasma furnace processing has the potential to transform solar cell production. If informed decisions regarding silicon ore and mineral exploration can be made such that waste streams are also of high economic value, then production is ultimately more environmentally integrated. This study presents results from a spot check of the Kinnikinic Quartzite, ~4.5 km east of Arco, Butte County, Idaho (43.639091°, −113.243295°), for ore quality. The mineralogical and geochemical characteristics are explored within the context of a planned plasma furnace project at the sampled site and are compared to previous consulting reports. X-ray diffraction analysis detected only quartz, while scanning electron microscopy identified quartz grains, secondary quartz cement, trace amounts of potassium feldspar, minor iron oxides, and secondary illite. The bulk chemical characterization of 20 samples (including repeats) reports several wt. % variation in SiO2 from 96.47 to 99.66. Other notable chemical components include Al2O3, K2O, CaO, and Rb, all consistent with the presence of potassium feldspar (and illite). Gold concentrations vary from below detection (n = 12 out of 20) to a maximum concentration of 0.086 ppm. Total sum REE concentrations vary from 13 to 143 ppm. Conservatively and optimistically, assuming ideal extraction and recovery in plasma furnace operation, a resulting waste stream would have approximately 15.2 ppm (0.488 oz./metric ton) gold and 3400 ppm REE in the average waste. Gold (and REE extraction) may, however, be complicated by the presence of Fe and Cu if cyanide approaches were implemented. Gold concentrations are significantly lower than reported in previous work, warranting further characterization of this unit locally and regionally in order to characterize ore potential. This study works to demonstrate the possibility of evaluating other potential silicon ore units, such as the St. Peter Sandstone in Illinois and Missouri, for the co-production of materials in support of an emerging green economy.

Recent advances in the recovery of metals from waste through biological processes

Wastes containing critical metals are generated in various fields, such as energy and computer manufacturing. Metal-bearing wastes are considered as secondary sources of critical metals. The conventional physicochemical methods of metals recovery are energy-intensive and cause further pollution. Low-cost and eco-friendly technologies including biosorbents, bioelectrochemical systems (BESs), bioleaching, and biomineralization, have become alternatives in the recovery of critical metals. However, a relatively low recovery rate and selectivity severely hinder their large-scale applications. Researchers have expanded their focus to exploit novel strain resources and strategies to improve the biorecovery efficiency. The mechanisms and potential applicability of modified biological techniques for improving the recovery of critical metals need more attention. Hence, this review summarize and compare the strategies that have been developed for critical metals recovery, and provides useful insights for energy-efficient recovery of critical metals in future industrial applications.

Selective Recovery of Europium and Yttrium Ions with Cyanex 272-Polyacrylonitrile Nanofibers

Rare earth elements (REEs), which include lanthanides as yttrium and europium became crucial in the last decade in many sectors like automotive, energy, and defense. They contribute to the increment efficiency and performance of different products. In this paper nanofiber membranes have been successfully applied for the selective recovery of Eu(III) and Y(III) from aqueous solutions. Polyacrylonitrile (PAN) electrospun nanofibers were impregnated with a commercial organic extractant, Cyanex 272, in order to increase their affinity to rare earth metals ions. The coated nanofibers were characterized by SEM, ATR-FTIR, and TGA. Firstly, the adsorption of Eu(III) and Y(III) were evaluated in batch mode. Experimental data showed that the adsorption of Y(III) and Eu(III) corresponds to pseudo-second order model, with Langmuir sorption model being the best fit for both target ions. The results demonstrated that the adsorption capacity was high, showing a maximum capacity of 200 and 400 mg/g for Y(III) and Eu(III), respectively. Additionally, the presence of interfering ions does not show significative effects in the adsorption process. Finally, experiments in continuous mode indicated that the adsorption of the target elements is close to 100%, showing that PAN-272 is a promising material for the recovery of earth metal ions.

Effective removal of calcium and magnesium sulfates from wastewater in the rare earth industry

The wastewater discharged from the rare earth (RE) industry generally contains a high level of calcium and magnesium sulfates, which confers permanent hardness and causes difficulties in recycling this wastewater. In this study, the alkyl phenoxy acetic acid derivatives including 4-methyl phenoxy acetic acid (M-POAA), 4-tert-butyl phenoxy acetic acid (B-POAA) and 4-tert-octyl phenoxy acetic acid (O-POAA), were synthesized via the Williamson reaction and characterized by nuclear magnetic resonance (NMR), infrared (IR), and ultra-violet (UV) spectroscopy, as well as elemental analysis and X-ray diffraction (XRD). Synthesis of the POAAs were simple and green, and the raw materials used for their production are widely available and low-cost. The potential for removal of Ca and Mg sulfates from industrial wastewater using POAAs as the organic precipitants was assessed. The total precipitation efficiencies of Ca and Mg from wastewater with the use of POAAs increased with the following order: M-POAA < B-POAA < O-POAA. The residual concentrations of Ca and Mg using O-POAA as the precipitant were lower than 0.099 and 0.089 g L-1, respectively. The O-POAA could be regenerated five times without any significant change in its structure and precipitation performance. Thus, the use of the novel precipitants is a prospective alternative to the conventional processes for softening wastewater.

Design and optimization of a fused-silica microfluidic device for separation of trivalent lanthanides by isotachophoresis

Elemental analysis of rare earth elements is essential in a variety of fields including environmental monitoring and nuclear safeguards; however, current techniques are often labor intensive, time consuming, and/or costly to perform. The difficulty arises in preparing samples, which requires separating the chemically and physically similar lanthanides. However, by transitioning these separations to the microscale, the speed, cost, and simplicity of sample preparation can be drastically improved. Here, all fourteen non-radioactive lanthanides (lanthanum through lutetium minus promethium) are separated by ITP for the first time in a serpentine fused-silica microchannel (70 µm wide × 70 µm tall × 33 cm long) in <10 min at voltages ≤8 kV with limits of detection on the order of picomoles. This time includes the 2 min electrokinetic injection time at 2 kV to load sample into the microchannel. The final leading electrolyte consisted of 10 mM ammonium acetate, 7 mM α-hydroxyisobutyric acid, 1% polyvinylpyrrolidone, and the final terminating electrolyte consisted of 10 mM acetic acid, 7 mM α-hydroxyisobutyric acid, and 1% polyvinylpyrrolidone. Electrophoretic electrodes are embedded in the microchip reservoirs so that voltages can be quickly applied and switched during operation. The limits of detection are quantified using a commercial capacitively coupled contactless conductivity detector (C⁴ D) to calculate ITP zone lengths in combination with ITP theory. Optimization of experimental procedures and reproducibility based on statistical analysis of subsequent experimental results are addressed. Percent error values in band length and conductivity are ≤8.1 and 0.37%, respectively.

Comprehensive Understandings of Rare Earth Element (REE) Speciation in Coal Fly Ashes and Implication for REE Extractability

In recent years, recovery of rare earth elements (REEs) from coal fly ashes (CFAs) has been considered as a promising resource recovery option. Yet, quantitative information on REE speciation in CFAs and its correlation with REE extractability are not well established. This study systematically investigated the REE speciation-extractability relationship in four representative CFA samples by employing multiple analytical and spectroscopic techniques across the micro to bulk scale and in combination with thermodynamic calculations. A range of REE-bearing phases are identified, such as REE oxides, REE phosphates, apatite, zircon, and REE-bearing glass phase. REEs can occur as discrete particles, as particles encapsulated in the glass phase, or distribute throughout the glass phase. Although certain discrepancies exist on the REE speciation quantified by X-ray adsorption spectroscopy and acid leaching due to intrinsic limitations of each method, both approaches show significant fractions of REE oxides, REE phosphates, apatite, and REE-bearing Fe oxides. This study contributes to an in-depth understanding of the REE speciation-distribution-extractability relationship in CFAs and can help identify uncertainties associated with the quantification of REE speciation. It also provides a general methodology for future studies on REE speciation in complex environmental samples and a knowledge basis for the development of effective REE recovery techniques.

Acrylic Acid-Functionalized Metal-Organic Frameworks for Sc(III) Selective Adsorption

The increasing demand for rare-earth elements (REEs) due to their extensive high-tech applications has encouraged the development of new sustainable approaches for REE recovery and separation. In this work, a series of acrylic acid-functionalized metal-organic framework materials (named as y-AA- x@MIL-101s) were prepared and used for selective adsorption of Sc(III). The adsorbent was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, nitrogen adsorption, X-ray photoelectron spectroscopy, and zeta potential and surface functional-group titration. The adsorption isotherm and kinetics data were accurately described by the Langmuir and pseudo-second-order models. The adsorption capacity of the material for Sc(III), Nd(III), Gd(III), and Er(III) was 90.21, 104.59, 58.29, and 74.94 mg g^-1, respectively. Importantly, the adsorbent was better for selective recovery of Sc(III) not only from the 16 REE mixed system but also the Cu(II), Zn(II), Mn(II), Co(II), and Al(III) coexistence solution. Except for Sc(III), the material displayed high affinity for Nd(III) in the light rare-earth mixture and for Gd(III) in the middle rare-earth mixture. All in all, this study provides a new method for separation and recovery of REEs, which makes this work highly significant in separation and enrichment.

Porous Covalent Organic Polymers Comprising a Phosphite Skeleton for Aqueous Nd(III) Capture

In order to meet the ever-increasing industrial demand for rare-earth elements (REEs), it is desirable to separate and recycle them at low concentrations from various sources including industrial and urban wastes. Here, we introduced phosphorus binding sites on the hydrophobic surface of a robust and high-surface area porous polymer backbone for environmentally benign and selective recovery of REEs via adsorption. For this purpose, two porous covalent organic polymer (COP) materials incorporated with in-built phosphite functionality (P-COP-1 and P-COP-2) were synthesized and applied for the adsorptive separation of Nd(III) ions from aqueous solution. A strategy to develop a series of P-COPs via a simple Friedel-Crafts reaction was introduced, and their application to the selective adsorption of REEs was explored for the first time. The newly synthesized P-COPs were amorphous and/or weakly crystalline and showed excellent chemical stability and large specific surface area with sufficient mesoporosity for enhanced diffusion of REE ions. P-COP-1 exhibited an exceptionally high Nd(III) adsorption capacity of 321.0 mg/g, corresponding to the stoichiometric ratio of P/Nd(III) = 1:0.7 and high selectivity of >86% over other competing transition and alkaline earth metal ions, whereas P-COP-2 gave a Nd(III) adsorption capacity of 175.6 mg/g at 25 °C and pH 5. Moreover, P-COP-1 showed a distribution coefficient value of 5.45 × 10⁵ mL/g, which is superior to other benchmark adsorbent materials reported so far. Finally, the P-COPs were reusable for a minimum of 10 cycles without deterioration in adsorption capacities.

Bioreductive Dissolution as a Pretreatment for Recalcitrant Rare-Earth Phosphate Minerals Associated with Lateritic Ores

Recent research has demonstrated the applicability of a biotechnological approach for extracting base metals using acidophilic bacteria that catalyze the reductive dissolution of ferric iron oxides from oxidized ores, using elemental sulfur as an electron donor. In Brazil, lateritic deposits are frequently associated with phosphate minerals such as monazite, which is one of the most abundant rare-earth phosphate minerals. Given the fact that monazite is highly refractory, rare earth elements (REE) extraction is very difficult to achieve and conventionally involves digesting with concentrated sodium hydroxide and/or sulfuric acid at high temperatures; therefore, it has not been considered as a potential resource. This study aimed to determine the effect of the bioreductive dissolution of ferric iron minerals associated with monazite using Acidithiobacillus (A.) species in pH- and temperature-controlled stirred reactors. Under aerobic conditions, using A. thiooxidans at extremely low pH greatly enhanced the solubilization of iron from ferric iron minerals, as well that of phosphate (about 35%), which can be used as an indicator of the dissolution of monazite. The results from this study have demonstrated the potential of using bioreductive mineral dissolution, which can be applied as pretreatment to remove coverings of ferric iron minerals in a process analogous to the bio-oxidation of refractory golds and expand the range of minerals that could be processed using this approach.

Ratiometric Monitoring of Thorium Contamination in Natural Water Using a Dual-Emission Luminescent Europium Organic Framework

Traditional analytical methods of thorium contamination suffer from several disadvantages such as time-consuming procedures and an equipment-intensive nature, leading to substantial challenges in rapid and on-site monitoring of thorium concentrations in complex natural water systems. We report here the first case of a luminescent metal organic framework based probe (ThP-1) for highly sensitive and selective self-calibrated sensing of Th⁴⁺ contamination in natural fresh water media with a notably facilitated detection procedure. The detection limit of ThP-1 was determined to be 24.2 μg/L, much lower than the thorium contamination standard of 246 μg/L in drinking water defined by the World Health Organization. Importantly, the detection procedure based on the rarely reported self-calibration manner is greatly beneficial in improving the detection accuracy. The self-calibrated luminescence evolution process shows a great anti-interference ability capable of detecting thorium contamination in a wide concentration range from 24.2 μg/L to 300 mg/L, which can not be achieved directly by the traditional methods. The Th⁴⁺-selective luminescence response originates from the selective uptake and efficient enrichment of Th⁴⁺ by the host framework of ThP-1 through inner-sphere coordination, which is further confirmed by batch experiments, X-ray absorption spectroscopic study, and DFT calculations.

Size-dependent selective crystallization using an inorganic mixed-oxoanion system for lanthanide separation

A unique selective crystallization approach for simple and efficient lanthanide separation has been developed by employing an iodate–sulfate mixed-anion system.

Screening and selection of technologically applicable microorganisms for recovery of rare earth elements from fluorescent powder

Rare Earth Elements (REE) are essential elements in many new technology products. Up to now, recycling is poorly established and no environmentally friendly strategies are applied. Modern biotechnologies like bioleaching can contribute to overcome the current limitations. In this study, we investigated bioleaching approaches exemplary for fluorescent phosphor (FP), which is accumulated during the recycling of fluorescent tubes and energy saving bulbs. A broad spectrum of different microorganisms were tested regarding their potential to leach REE from FP. Among them were classical acidophilic microorganisms, as well as various heterotrophic ones, producing organic acids or metal complexing metabolites, or having a high metal tolerance. Larger amounts of REE were leached with the strains Komagataeibacter xylinus, Lactobacillus casei, and Yarrowia lipolytica. Besides the COOH-functionality, also other biotic processes contribute to metal leaching, as comparison with indirect leaching approaches showed. Among the different REE components of the FP preferably the oxidic red dye yttrium europium oxide (YOE) that contain the critical REE yttrium and europium was leached. The results provide the basis for the development of an environmentally friendly recycling process for REE from waste materials.

Selective Adsorption of Rare Earth Elements over Functionalized Cr-MIL-101

Efficient rare earth elements (REEs) separation and recovery are crucial to meet the ever-increasing demand for REEs extensively used in various high technology devices. Herein, we synthesized a highly stable chromium-based metal-organic framework (MOF) structure, Cr-MIL-101, and its derivatives with different organic functional groups (MIL-101-NH₂, MIL-101-ED (ED: ethylenediamine), MIL-101-DETA (DETA: diethylenetriamine), and MIL-101-PMIDA (PMIDA: N-(phosphonomethyl)iminodiacetic acid)) and explored their effectiveness in the separation and recovery of La³⁺, Ce³⁺, Nd³⁺, Sm³⁺, and Gd³⁺ in aqueous solutions. The prepared materials were characterized using various analytical instrumentation. These MOFs showed increasing REE adsorption capacities in the sequence MIL-101 < MIL-101-NH₂ < MIL-101-ED < MIL-101-DETA < MIL-101-PMIDA. MIL-101-PMIDA showed superior REE adsorption capacities compared to other MOFs, with Gd³⁺ being the element most efficiently adsorbed by the material. The adsorption of Gd³⁺ onto MIL-101-PMIDA was examined in detail as a function of the solution pH, initial REE concentration, and contact time. The obtained adsorption equilibrium data were well represented by the Langmuir model, and the kinetics were treated with a pseudo-second-order model. A plausible mechanism for the adsorption of Gd³⁺ on MIL-101-PMIDA was proposed by considering the surface complexation and electrostatic interaction between the functional groups and Gd³⁺ ions under different pH conditions. Finally, recycling tests were carried out and demonstrated the higher structural stability of MIL-101-PMIDA during the five adsorption-regeneration runs.

Mesoporous SiO2 Nanoparticles: A Unique Platform Enabling Sensitive Detection of Rare Earth Ions with Smartphone Camera

Fast and sensitive detection of dilute rare earth species still represents a challenge for an on-site survey of new resources and evaluation of the economic value. In this work, a robust and low-cost protocol has been developed to analyze the concentration of rare earth ions using a smartphone camera. The success of this protocol relies on mesoporous silica nanoparticles (MSNs) with large-area negatively charged surfaces, on which the rare earth cations (e.g., Eu³⁺) are efficiently adsorbed through electrostatic attraction to enable a "concentrating effect". The initial adsorption rate is as fast as 4025 mg (g min)^-1, and the adsorption capacity of Eu³⁺ ions in the MSNs is as high as 4730 mg g^-1 (equivalent to ~ 41.2 M) at 70 °C. The concentrated Eu³⁺ ions in the MSNs can form a complex with a light sensitizer of 1,10-phenanthroline to significantly enhance the characteristic red emission of Eu³⁺ ions due to an "antenna effect" that relies on the efficient energy transfer from the light sensitizer to the Eu³⁺ ions. The positive synergy of "concentrating effect" and "antenna effect" in the MSNs enables the analysis of rare earth ions in a wide dynamic range and with a detection limit down to ~ 80 nM even using a smartphone camera. Our results highlight the promise of the protocol in fieldwork for exploring valuable rare earth resources.

Syntrophic effect of indigenous and inoculated microorganisms in the leaching of rare earth elements from Western Australian monazite

The unique physiochemical properties exhibited by rare earth elements (REEs) and their increasing application in high-tech industries has created a demand for secure supply lines with established recovery procedures that create minimal environmental damage. Bioleaching experiments conducted on a non-sterile monazite concentrate with a known phosphate solubilising microorganism (PSM) resulted in greater mobilisation of REEs into solution in comparison to experiments conducted on sterile monazite. By combining the native consortia with an introduced PSM, a syntrophic effect between the populations effectively leached a greater amount of REEs than either a single PSM or the indigenous population alone. With sterile monazite, Penicillium sp.CF1 inoculated experiments released a total REE concentration of 12.32 mg L^-1 after incubation for 8 days, whereas on non-sterile ore, double the soluble REE concentration was recorded (23.7 mg L^-1). Comparable effects were recorded with Enterobacter aerogenes, Pantoea agglomerans and Pseudomonas putida. Alterations in the microbial populations during bioleaching of the monazite ore were determined by diversity profiling and demonstrated noticeable changes in community inhabitants over 14 days. The presence of native Firmicutes on the monazite appears to greatly contribute to the increased leaching recorded when using non-sterile monazite for REE recovery.

Fabrication of novel metal ion imprinted xanthan gum-layered double hydroxide nanocomposite for adsorption of rare earth elements

The work focus to enhance the properties of xanthan gum (XG) by anchoring metal ions (Fe, Zr) and encapsulating inorganic matrix (M@XG-ZA). The fabricated nanocomposite was characterized by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Fourier Transform Infrared Spectroscopy (FTIR), surface area (BET) and zeta potential analysis. The adsorption of Sc, Nd, Tm and Yb was investigated after screening of synthesized materials in detail to understand the influence of pH, contact time, temperature and initial REE (rare earth element) concentration both in single and multicomponent system via batch adsorption. The adsorption mechanism was verified by FTIR, SEM and elemental mapping. The SEM images of Zr@XG-ZA demonstrate scutes structure, which disappeared after adsorption of REEs. The maximum adsorption capacities were 132.30, 14.01, 18.15 and 25.73 mg/g for Sc, Nd, Tm and Yb, respectively. The adsorption efficiency over Zr@XG-ZA in multicomponent system was higher than single system and the REEs followed the order: Sc > Yb > Tm > Nd. The Zr@XG-ZA demonstrate good adsorption behavior for REEs up to five cycles and then it can be used as photocatalyst for the degradation of tetracycline. Thus, the work adds a new insight to design and preparation of efficient bifunctional adsorbents from sustainable materials for water purification.

A novel bioelectrochemical system for chemical-free permanent treatment of acid mine drainage

The mining sector is currently under unprecedented pressure due to stringent environmental regulations. As a consequence, a permanent acid mine drainage (AMD) treatment is increasingly being regarded as a desirable target with direct benefits for the environment and the operational and economic viability of the resources sector. In this study we demonstrate that a novel bioelectrochemical system (BES) can deliver permanent treatment of acid mine drainage without chemical dosing. The technology consists of a two-cell bioelectrochemical setup to enable the removal of sulfate from the ongoing reduction-oxidation sulfur cycle to less than 550 mg L^-1 (85 ± 2% removal from a real AMD of an abandoned silver mine), thereby also reducing salinity at an electrical energy requirement of 10 ± 0.3 kWh kg^-1 of SO₄^2--S removed. In addition, the BES operation drove the removal and recovery of the main cations Al, Fe, Mg, Zn at rates of 151 ± 0 g Al m^-3 d^-1, 179 ± 1 g Fe m^-3 d^-1, 172 ± 1 g Mg m^-3 d^-1 and 46 ± 0 g Zn m^-3 d^-1 into a concentrate stream containing 263 ± 2 mg Al, 279 ± 2 mg Fe, 152 ± 0 mg Mg and 90 ± 0 mg Zn per gram of solid precipitated after BES fed-rate control treatment. The solid metal-sludge was twice less voluminous and 9 times more readily settleable than metal-sludge precipitated using NaOH. The continuous BES treatment also demonstrated the concomitant precipitation of rare earth elements together with yttrium (REY), with up to 498 ± 70 μg Y, 166 ± 27 μg Nd, 155 ± 14 μg Gd per gram of solid, among other high-value metals. The high-REY precipitates could be used to offset the treatment costs.