Trends in the Rare Earth Element Content of U.S.-Based Coal Combustion Fly Ashes

Organic Ligand-Mediated Dissolution and Fractionation of Rare-Earth Elements (REEs) from Carbonate and Phosphate Minerals

Global efforts to build a net-zero economy and the irreplaceable roles of rare-earth elements (REEs) in low-carbon technologies urge the understanding of REE occurrence in natural deposits, discovery of alternative REE resources, and development of green extraction technologies. Advancement in these directions requires comprehensive knowledge on geochemical behaviors of REEs in the presence of naturally prevalent organic ligands, yet much remains unknown about organic ligand-mediated REE mobilization/fractionation and related mechanisms. Herein, we investigated REE mobilization from representative host minerals induced by three representative organic ligands: oxalate, citrate, and the siderophore desferrioxamine B (DFOB). Reaction pH conditions were selected to isolate the ligand-complexation effect versus proton dissolution. The presence of these organic ligands displayed varied impacts, with REE dissolution remarkably enhanced by citrate, mildly promoted by DFOB, and showing divergent effects in the presence of oxalate, depending on the mineral type and reaction pH. Thermodynamic modeling indicates the dominant presence of REE-ligand complexes under studied conditions and suggests ligand-promoted REE dissolution to be the dominant mechanism, consistent with experimental data. In addition, REE dissolution mediated by these ligands exhibited a distinct fractionation toward heavy REE (HREE) enrichment in the solution phase, which can be mainly attributed to the formation of thermodynamically predicted more stable HREE-ligand complexes. The combined thermodynamic modeling and experimental approach provides a framework for the systematic investigation of REE mobilization, distribution, and fractionation in the presence of organic ligands in natural systems and for the design of green extraction technologies.

Recovery of rare earth elements from low-grade coal fly ash using a recyclable protein biosorbent

Rare earth elements (REEs), including those in the lanthanide series, are crucial components essential for clean energy transitions, but they originate from geographically limited regions. Exploiting new and diverse supply sources is vital to facilitating a clean energy future. Hence, we explored the recovery of REEs from coal fly ash (FA), a complex, low-grade industrial feedstock that is currently underutilized (leachate concentrations of REEs in FA are < 0.003 mol%). Herein, we demonstrated the thermo-responsive genetically encoded REE-selective elastin-like polypeptides (RELPs) as a recyclable bioengineered protein adsorbent for the selective retrieval of REEs from coal fly ash over multiple cycles. The results showed that RELPs could be efficiently separated using temperature cycling and reused with high stability, as they retained ∼95% of their initial REE binding capacity even after four cycles. Moreover, RELPs selectively recovered high-purity REEs from the simulated solution containing one representative REE in the range of 0.0001-0.005 mol%, resulting in up to a 100,000-fold increase in REE purity. This study offers a sustainable approach to diversifying REE supplies by recovering REEs from low-grade coal fly ash in industrial wastes and provides a scientific basis for the extraction of high-purity REEs for industrial purposes.

Engineering biomaterials for the recovery of rare earth elements

The escalating global demand for rare earth elements (REEs) and the overabundance of REE-containing waste require innovative technologies for REE recovery from waste to achieve a sustainable supply of REEs while reducing the environmental burden. Biosorption mediated by peptides or proteins has emerged as a promising approach for selective REE recovery. To date, multiple peptides and proteins with high REE-binding affinity and selectivity have been discovered, and various strategies are being exploited to engineer robust and reusable biosorptive materials for selective REE recovery. This review highlights recent advances in discovering and engineering peptides and proteins for REE recovery. Future research prospects and challenges are also discussed.

Recovery of Rare Earth Elements from Acid Mine Drainage with Supported Liquid Membranes: Impacts of Feedstock Composition for Extraction Performance

Acid mine drainage (AMD) from inactive coal mines can be enriched in rare earth elements (REEs) and has gained much attention as an alternative source for these technology-critical metals. However, AMD is a relatively low-grade REE resource in which the abundance of impurities and the composition variability of the feedstock create major uncertainties for the performance of REE extraction technologies. This study sought to identify AMD feedstock variables that influence the extraction efficiency of REEs by supported liquid membranes (SLMs). SLM separation is a process involving a hydrophobic membrane embedded with an extracting solvent that facilitates the selective extraction of REE ions. The major aims were to (1) assess the effectiveness of SLM-based REE separation from several AMD samples representing a spectrum of aqueous composition, (2) determine the effects of AMD storage and holding time on extraction performance, and (3) assess the impact of AMD pretreatment (e.g., filtration and pH adjustment) on REE recovery. The results showed that relative extraction fluxes of REE correlated with AMD characteristics such as pH and major ions such as Fe, Ca, and Mn. The purity of the acid strippant product, expressed as the REE dry weight content, depended on the initial REE concentrations in the AMD source rather than the flux of individual REEs across the membrane. For AMD samples stored for 3 months prior to extraction, REE recovery by SLM separations was substantially decreased if oxidation of Fe(II) to Fe(III) was observed during sample storage. Pretreatment of AMD feedstocks by pH adjustment did not substantially improve the separation performance. Overall, this study establishes primary water quality parameters of AMD that influence the SLM separation flux and product purity. Such insights contribute to a mechanistic understanding of critical metals extractions by SLM for complex and nontraditional feedstocks such as AMD wastes.

Ecological and Health Risks Attributed to Rare Earth Elements in Coal Fly Ash

The occurrence and distribution of yttrium and rare earth elements (REYs), along with major elements and heavy metal(loid)s (HMs) in coal fly ash (CFA) from five coal-fired power plants (CFPPs), were analyzed, and the REY-associated ecological and health risks were assessed. The individual REYs in CFA were abundant in the following order: Ce > La > Nd > Y > Pr > Gd > Sm > Dy > Er > Yb > Eu > Ho > Tb > Tm > Lu. The total REY content ranged from 135 to 362 mg/kg, averaging 302 mg/kg. The mean light-to-heavy REY ratio was 4.1, indicating prevalent light REY enrichment in CFA. Significantly positive correlations between the REYs suggested that they coexist and share similar origins in CFA. REYs were estimated to pose low to moderate ecological risks, with risk index (RI) values ranging from 66 to 245. The hazard index (HI) and target cancer risk (TCR) of REYs from CFA, estimated to be higher for children (HIc = 0.15, TCRc = 8.4 × 10-16) than for adults (HIa = 0.017, TCRa = 3.6 × 10-16), were well below the safety limits (HI = 1, TCR = 1.0 × 10-6). However, the danger to human health posed by HMs in the same CFA samples (HIc = 5.74, TCRc = 2.6 × 10-4, TCRa = 1.1 × 10-4) exceeded the safe thresholds (excl. HIa = 0.63). The mean RI and HI attributed to REYs in CFA were 14% and 2.6%, respectively, of the total risks that include HMs.

Analysis of enrichment, correlation, and leaching patterns of rare earth elements in coal fly ash assisted by statistical measures

Coal fly ash (CFA) is a typical industrial solid waste, which has recently been reported to contain rare earth elements (REEs). REEs are important materials in many industrial fields. Therefore, extracting REEs from CFA becomes a win-win strategy to both make full use of CFA and reclaim REEs. However, the stable crystalline structure of CFA is hard to break, which limits the extraction of REEs. The inter-correlation and the leaching patterns of the REEs in CFA also remain unclear. In this work, REEs were enriched by desilication, and the correlation and the influences of multiple acids of the leached REEs were investigated. It was found that desilication could increase the leachable amount of REEs from 137.37 ppm to 346.12 ppm. The light rare earth elements (LREEs) were less inter-correlated than heavy rare earth elements (HREEs) and desilication enhanced the leaching of LREEs more than that of HREEs. The ratio and type of the leaching acids both influenced the extraction of REEs from CFA: HCl and HF played important roles in the extraction from the untreated CFA while HNO3 and HF were more decisive for the desilicated CFA. In addition, we used statistical analysis to quantificationally confirm that desilication and acids both significantly influenced the extraction of REEs. This work provides evidence for the enrichment of REEs in CFA and acid choosing when leaching REEs from CFA.

An assessment of the strategies for the energy-critical elements necessary for the development of sustainable energy sources

There have been several strategies developed to increase the diversified supply of energy so that it can meet all of the future demands for energy. As a result, to ensure a healthy and sustainable energy future, it is imperative to warrant reliable and diverse energy supply sources if the "green energy economy" is to be realized. The purpose of developing and deploying clean energy technologies is to improve our overall energy security, reduce our carbon footprint, and ensure that the generation of energy is secure and reliable in the future, making sure that we can spur economic growth in the future. In this paper, advancements in alternative sources of energy sustainability and strategies will be examined to ensure there will be enough fuel to supply all the future demands for energy. Several emerging clean energy technologies rely heavily on the availability of materials that exhibit unique properties that are necessary for their development. This paper examines the roles that rare earth and other energy-critical materials play in securing a clean energy economy and the development of clean energy economies in general. For the development of these technologies to be successful and sustainable, a number of these energy-critical materials are at risk of becoming unavailable. This is due to their limited availability, disruptions in supply, and a lack of suitable resources for their development. An action plan focusing on producing energy-critical materials in energy-efficient ways is discussed as part of an initiative to advance the development of clean and sustainable energy.

Nanoscale heterogeneity of arsenic and selenium species in coal fly ash particles: analysis using enhanced spectroscopic imaging and speciation techniques

Coal combustion byproducts are known to be enriched in arsenic (As) and selenium (Se). This enrichment is a concern during the handling, disposal, and reuse of the ash as both elements can be harmful to wildlife and humans if mobilized into water and soils. The leaching potential and bioaccessibility of As and Se in coal fly ash depends on the chemical forms of these elements and their association with the large variety of particles that comprise coal fly ash. The overall goal of this research was to determine nanoscale and microscale solid phase mineral associations and oxidation states of As and Se in fly ash. We utilized nanoscale 2D imaging (30-50 nm spot size) with the Hard X-ray Nanoprobe (HXN) in combination with microprobe X-ray capabilities (∼5 μm resolution) to determine the As and Se elemental associations. Speciation of As and Se was also measured at the nano- to microscale with X-ray absorption spectroscopy. The enhanced resolution of HXN showed As and Se as either diffusely located around or comingled with Ca- and Fe-rich particles. The results also showed nanoparticles of Se attached to the surface of fly ash grains. Overall, a comparison of As and Se species across scales highlights the heterogeneity and complexity of chemical associations for these trace elements of concern in coal fly ash.

Reprocessed construction and demolition waste as an adsorbent: An appraisal

Construction and Demolition (C&D) waste is solid wastes generated from the construction, demolition, and renovation activities that constitute almost 30-40 % of globally generated solid wastes. Improper disposal and management of these materials can cause negative impacts on the environment, economy, and human health. Most research on C&D waste is limited to reduction, recycling, and reuse of the wastes. However, there is no systematic review dedicated entirely to the applicability of C&D wastes as adsorbent for waste management. This review presents the utilization of C&D wastes-based adsorbents for removing contaminants from environmental matrices covering triple edge benefits in the viewpoints of waste treatment, solid waste management, and disposal. The properties, the capability of C&D waste adsorbents on contaminant removal, and the influence of various factors on the adsorptive removal is detailed. Further, the mechanisms involved in contaminant removal by C&D waste are summarized. The review revealed that, chemisorption is the prominent mechanism of contaminant removal by most C&D wastes. Among the three types of C&D waste reviewed; concrete-based adsorbents were the most efficient for contaminant removal. Limited studies are avaiable in the literature on binary and multiple contaminant systems, reusability studies, and high dependence on solution pH, therefore further studies are warrated. As C&D waste contain trace concentration of heavy metals and contaminants, its leaching potential at different pH levels and adsorbate concentration need to be conducted, which has been hitherto neglected. Finally, the approaches, obstacles, and potential solutions to build an industrially and economically efficient C&D adsorbent are discussed.

Clickable polymer scaffolds enable Ce recovery with peptide ligands

Rare earth elements (REEs) are a vital part of many technologies with particular importance to the renewable energy sector and there is a pressing need for environmentally friendly and sustainable processes to recover and recycle them from waste streams. Functionalized polymer scaffolds are a promising means to recover REEs due to the ability to engineer both transport properties of the porous material and specificity for target ions. In this work, REE adsorbing polymer scaffolds were synthesized by first introducing poly(glycidyl methacrylate) (GMA) brushes onto porous polyvinylidene fluoride (PVDF) surface through activator generated electron transfer atom transfer radical polymerization (AGET ATRP). Azide moieties were then introduced through a ring opening reaction of GMA. Subsequently, REE-binding peptides were conjugated to the polymer surface through copper catalyzed azide alkyne cycloaddition (CuAAC) click chemistry. The presence of GMA, azide, and peptide was confirmed through Fourier transform infrared spectroscopy. Polymer scaffolds functionalized with the REE-binding peptide bound cerium, while polymer scaffolds functionalized with a scrambled control peptide bound significantly less cerium. Importantly, this study shows that the REE binding peptide retains its functionality when bound to a polymer surface. The conjugation strategy employed in this work can be used to introduce peptides onto other polymeric surfaces and tailor surface specificity for a wide variety of ions and small molecules.

Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash

Due to the growing demands of rare earth elements (REEs) and the vulnerability of REEs to potential supply disruption, there have been increasing interests in recovering REEs from waste streams such as coal fly ash (CFA). Meanwhile, CFA as a large industrial waste stream in the United States (U.S.) poses significant environmental and economic burdens. Recovery of REEs from CFA is a promising solution to the REE scarcity issue and also brings opportunities for CFA management. This study demonstrates a green system for REE recovery from Class F and C CFA that consists of three modules: REE leaching using citrate, REE separation and concentration using oxalate, and zeolite synthesis using secondary wastes from Modules I and II. In Module I, ∼10 and 60% REEs were leached from the Class F and C CFA samples, respectively, using citrate at pH 4. In Module II, the addition of oxalate selectively precipitated and concentrated REEs from the leachate via the formation of weddellite (CaC₂O₄·2H₂O), while other trace metals remained in solution. In Module III, zeolite was synthesized using wastes from Modules I and II. This study is characterized by the successful recovery of REEs and upcycling of secondary wastes, which addresses both REE recovery and CFA management challenges.

Environmental Impacts of Coal Combustion Residuals: Current Understanding and Future Perspectives

On-site solid-waste impoundments, landfills, and receiving water bodies have served as long-term disposal sites for coal combustion residuals (CCRs) across the United States for decades and collectively contain billions of tons of CCR material. CCR components include fine particulate material, minerals, and trace elements such as mercury, arsenic, selenium, lead, etc., which can have deleterious effects on ecosystem functioning and public health. Effects on communities can occur through consumption of drinking water, fish, and other aquatic organisms. The structural failure of impoundments, water infiltration, leakage from impoundments due to poor construction and monitoring, and CCR effluent discharges to water bodies have in the past resulted in harmful environmental impacts. Moreover, the risks posed by CCRs are present to this day, as coal continues to account for 11% of the energy production in the United States. In this Critical Review, the legacy of CCR disposal and the concomitant risks posed to public health and ecosystems are assessed. The resiliency of CCR disposal sites in the context of increased frequency and intensity of storm events and other hazards, such as floods and earthquakes, is also evaluated. We discuss the current state of knowledge on the environmental fate of CCR-derived elements, as well as advances in and limitations of analytical tools, which can improve the current understanding of CCR environmental impacts in order to mitigate the associated risks. An assessment of the 2015 Coal Ash Final Rule is also presented, along with needs to improve monitoring of CCR disposal sites and regulatory enforcement.

Rare-Earth Elements Extraction from Low-Alkali Desilicated Coal Fly Ash by (NH4)2SO4 + H2SO4

Coal fly ash (CFA) obtained from pulverized coal furnaces is a highly refractory waste that can be used for alumina and rare-earth elements (REEs) extraction. The REEs in this type of CFA are associated with a mullite and amorphous glassy mass that forms a core-shell structure. In this research, it was shown that complete dissolution of amorphous aluminosilicates from the mullite surface with the formation of the low-alkali mullite concentrate prior to sulfuric acid leaching with the addition of (NH₄)₂SO₄ helps to accelerate the extraction of REEs. The extraction degree of Sc and other REEs reaches 70-80% after 5 h of leaching at 110 °C and acid concentration of 5 M versus less than 20% for the raw CFA at the same conditions. To study the leaching kinetics of the process, the effects of temperature (90-110 °C), liquid-to-solid ratio (5-10), and leaching time (15-120 min) on the degrees of Al and rare-earth elements (REEs) extraction were evaluated. After 120 min of leaching at 110 °C and L/S ratio = 10, the extraction of Al was found to be lower than 30%. At the same time, total REEs (TREE) and Fe extraction were greater than 60%, which indicates that a part of the TREE was transferred into the acid soluble phase. After leaching, the residues were studied by laser diffraction (LD), X-ray diffraction (XRD), X-ray fluorescence (XRF), and scanning electron microscopy (SEM-EDS) to evaluate the leaching mechanism and the solubility of Al- and Fe-containing minerals, such as mullite, hematite, and amorphous aluminosilicate.

From Ashes to Riches: Microscale Phenomena Controlling Rare Earths Recovery from Coal Fly Ash

Coal fly ash is an alternative source of rare earth elements (REEs), which are critical in modern energy and electronic technologies. Current hydrometallurgical processes, however, yield variable recovery rates because of the limited understanding of the microscale phenomena controlling the extraction of REEs from fly ash. This work investigates the microscale processes that dictate the recovery of REEs from ash particulates via a spatiochemical analysis. We find that REE-bearing minerals are hosted in three modes with distinct recovery mechanisms: (i) REEs encapsulated in dense particles are recovered via the cation exchange between matrix metals (Al, Ca, Mg, etc.) and solution cations, (ii) REEs within permeable particles are recovered via intraparticle pore-scale fluid flow, and (iii) discrete and surface-bound REE-bearing minerals are recovered via direct exposure to reagents. The role of metal content and the limiting transport mechanisms are further probed for dense particles, the predominant mode of occurrence. This study highlights, for the first time, how the morphology and the elemental makeup of the ash matrix play a critical role in the accessibility of REEs, furthering the knowledge base required for the design of cost-effective and environmentally benign REEs recovery techniques.

Highly-efficient and sequential recovery of rare earth elements, alumina and silica from coal fly ash via a novel recyclable ZnO sinter method

A novel sinter method using ZnO as the activator instead of the conventional Na₂CO₃/CaCO₃, (NH₄)₂SO₄, and K₂S₂O₇ was developed to achieve efficient sequential extraction of rare earth elements (REEs), alumina (Al), and silica (Si) from coal fly ash (CFA). Up to 93.3% Si, 87.1% REEs (70.7% Ce, 82.5% La, 83.2% Gd, 87.1% Nd, 62.3% Dy, and 81.7% Y), and 92.9% Al were extracted from CFA, respectively. Moreover, 93.1% of the ZnO activator was efficiently recycled, and the yield of red mud was only 14.9%. X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) results showed that the speciation transformation of Al/Si during CFA/ZnO roasting was as follows: mullite, quartz, amorphous Al₂O₃, and SiO₂ → Zn_0.75Al_1.5Si_1.5O₆, kyanite and willemite → gahnite and quartz/cristobalite solid solutions. The change in the REEs occurrence mode hinted at the migration of most REEs in aluminosilicates forms with Si during roasting, and disassociation with Si into the acid-soluble form after alkali leaching. These results indicate that the coupling of Al-Si-REE in CF was broken by this ZnO sinter method, promoting the sequential and efficient extraction of REEs, Al, and Si from CFA. This study provides a green and efficient strategy for element recovery from CFA, substantially reducing residues and favoring REEs concentration.

Environmental damage caused by coal combustion residue disposal: A critical review of risk assessment methodologies

Coal combustion generates almost 40% of world's electricity. However, it also produces 1.1 billion tons of coal combustion residues (CCR) annually, half of which end up in landfills. Although current regulations require proper lining and monitoring programs, the ubiquitous old, abandoned landfills are often not lined nor included in these programs. In addition, the total number of coal ash disposal sites and their status in the world is unknown. Therefore, this article reviews the environmental damage caused by CCR and three commonly used risk assessment methodologies: leaching assessment, groundwater assessment, and toxicity testing. Leaching methods are usually the first step in coal ash risk assessment, however, a large number of methods with different parameters make a comparison of data difficult. Groundwater pollution is commonly detected near coal ash disposal sites, but other anthropogenic activities may also exist nearby. Therefore, multivariate statistical methods and isotope traces should be used to differentiate between different sources of pollution. So far, both stable (δ¹⁸O, δD, δ¹¹B, δ³⁴S, δ⁷Li) and radiogenic (⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁷Pb) isotopes have been successfully used as coal ash pollution tracers. Coal ash also negatively affects biota, reduces the diversity of organisms, affects children's health, and increases the risk for developing various diseases. Toxicity studies are great for early screening of coal ash safety; however, they provide no insights into mechanisms causing the adverse effects. Future directions are also proposed, such as the development of new 'low-level' detection methods for coal ash pollution and sustainable and selective method for recovery of critical elements.

Residual levels and health risk assessment of rare earth elements in Chinese resident diet: A market-based investigation

The widespread use of rare earth elements (REEs) in agriculture and high-tech industry resulted in significant release of REEs into the environment. However, there is a scarcity of studies focusing on the presence of REEs in the food worldwide. The present study investigated the residual levels of REEs in 14 representative food categories collected from 33 major cities of China. The measured total REEs (ΣREE) levels in the foods of aquatic origin were 174.97 μg kg^-1 wet weight (ww), which was 6.35 times higher than those of terrestrial origin. It is interesting to observe a trophic dilution effect for REEs in both terrestrial and aquatic food samples. REEs in food samples at low trophic levels exhibited relatively high REEs levels; while for high trophic level food, relatively low REEs levels were observed. The distribution patterns of REEs varied across the different food categories and regions, with Ce being the most abundant REEs in all food samples, followed by La, Nd and Sm. High levels of ΣREE in food samples were observed in Midland, while low levels were found in the Northeast. Cereals was the dominant contributor to the estimated daily intake of REEs. The health risk of REEs by daily food consumption in China was acceptable.

Characterisation of rare earth elements and toxic heavy metals in coal and coal fly ash

Due to increasing demand for rare earth elements (REE), growing concerns over their sustainability, and domination of their supply by China, coal fly ash has recently emerged as a viable target for REE recovery. With billions of tonnes in repositories and still more being generated across the globe, it is necessary to develop environmentally friendly and economical extraction technologies for the recovery of the REEs from coal fly ash, and to consider the environmental implications of such a recovery process. This study reports characterisation of Nigerian simulant coal fly ash, and investigates the distribution and leaching of the REEs and U, Th, As, Cr, Cd and Pb from these materials using ethanoic acid. Significant amounts (14% to 31%) of the REEs were recovered in the acid-soluble fraction of a sequential extraction procedure using ethanoic acid. While the greatest amounts of U (53% to 62%) and Th (89% to 96%) were recovered in the stable residual fraction, significant amounts (3% to 13%) of U were recovered in the acid-soluble fraction. As was the most enriched element in the mobile acid-soluble fraction (46% to 60%), followed by Cd (15% to 34%). These results demonstrate that REEs contained within coal fly ash - especially those sourced from coal-fired power plants burning coal at temperatures between 700 °C and 1100 °C - can be recovered through an environmentally friendly procedure using the cost-effective heap leaching method, with ethanoic acid or the more cheaply-available vinegar as lixiviant. These results are also valuable for cost evaluation of rare earths recovery from coal fly ash generated by fluidised bed combustion coal fired power plants, and the development of methodologies for coal fly ash management.

Phytomining of rare earth elements - A review

The increasing demand for rare earth elements (REEs) for modern industry has led to a surge in mining activities and consequently has released these metals into the environment. Intensifying REEs in a habitat has impacts on its ecosystem, but on the other side, it also provides the opportunity to recover REEs from low-grade minerals. Phytomining has emerged as an ecologically sound technique to extract these valuable elements from contaminated soils where traditional mining is not competitive. This paper presents and reviews the concept of REE phytomining from three scientific areas. The accumulation of rare earth metals in plants is the first stage, referred to as the phytoextraction process. This is followed by elevating REE concentrations into bio-ores via the enrichment phase. Ultimately, extraction is the final step to complete the phytomining pathway for reclaiming REEs in brownfield land.

Optimization of Iron Removal in the Recovery of Rare-Earth Elements from Coal Fly Ash Using a Recyclable Ionic Liquid

Rare-earth elements (REEs) are essential for modern technologies, and the United States currently lacks a secure domestic supply. Coal combustion residuals, specifically coal fly ash (CFA), can be a potential source. Our previous work demonstrated that REEs could be preferentially extracted from CFA using the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf₂N]), and the process yielded a mildly acidic REE-rich solution with coextracted Fe and regenerated IL. In this study, we investigated three strategies to limit Fe coextraction: magnetic separation, complexing salts, and ascorbic acid (AA) reduction. Magnetic separation of CFA was ineffective in significantly lowering the Fe content in the IL phase. When NaCl was used instead of NaNO₃ during extraction, chloride complexation lowered iron distribution to the IL phase over the aqueous phase (D_Fe) by five folds, from ∼75 to ∼14, while REE leaching (L_REEs) and recovery (R_REEs) both increased. Using AA for iron reduction lowered the overall amount of Fe extracted and further decreased D_Fe to ∼0.16, effectively shifting Fe preference from the IL phase to the aqueous phase. Combining the strategies of NaCl, AA, and supplemental betaine addition, leaching and extraction of REEs from CFA by [Hbet][Tf₂N] were achieved in higher efficiency for REE recovery with minimized Fe concentration.

Separation of rare earth elements from mixed-metal feedstocks by micelle enhanced ultrafiltration with sodium dodecyl sulfate

Micelle enhanced ultrafiltration (MEUF) is a surfactant-based membrane separation process that may be used to separate target ions from mixed metal aqueous solutions, such as leachates of coal ash and other geological wastewaters. The ability of MEUF to separate rare earth elements (REEs) was evaluated using sodium dodecyl sulfate (SDS) as the sorbent in surfactant micelle phase, which was subsequently separated using ultrafiltration, acidification, and ferricyanide precipitation. Separation experiments were performed with a synthetic coal ash leachate feedstock as an example mixed-metal feedstock. Experiments tested the influence of surfactant concentration, pH, and co-existing competitive ions on REE recoveries, and also tested methods for SDS recovery and reuse. Membrane rejection efficiencies of REEs were 97% and 71% respectively for synthetic and real leachate under optimized operating conditions. A two-step process of precipitation with CaCl₂ and Na₂CO₃ following membrane separation was the best for recovering SDS with a yield of 99.7%.

Electronic, Structural, and Magnetic Upgrading of Coal-Based Products through Laser Annealing

Most coal-to-product routes require complex thermal treatment to carbonize the raw materials. However, the lack of unified comparison of products made from different kinds of coals downplays the role of initial coal chemistry in high-temperature reactions. Here, we used a CO₂ laser to investigate the roles that aromatic content and maturity play in the structural evolution and doping of coals during annealing. Results show that a bituminous coal (DECS 19) with aromatic content and maturity in between higher rank, more mature anthracite (DECS 21) and lower rank, lower maturity lignite (DECS 25) leads to more graphite-like structure observed from the highest 2D peak on the Raman spectrum and conductivity (sheet resistance ∼30 ohm sq^-1) after lasing. When nitrogen dopants are incorporated with saturated urea dopants into coals through laser ablation, nitrogen preferentially incorporates at the edge sites of graphitic grains. Furthermore, oxide nanoparticles can be incorporated into the graphitic backbone of coal to modify their electronic and magnetic properties through laser annealing. Leveraging tunable magnetic behavior, we demonstrate a soft actuator using both conductive and magnetic coal-Fe/Co oxide. Through laser annealing, we propose a paradigm to understand and control coal chemistry toward flexible and tunable doping and magnetism.

Rare earth elements from waste

Rare earth elements (REEs) are critical materials in electronics and clean technologies. With the diminishing of easily accessible minerals for mining, the REE recovery from waste is an alternative toward a circular economy. Present methods for REE recovery suffer from lengthy purifications, low extractability, and high wastewater streams. Here, we report an ultrafast electrothermal process (~3000°C, ~1 s) based on flash Joule heating (FJH) for activating wastes to improve REE extractability. FJH thermally degrades or reduces the hard-to-dissolve REE species to components with high thermodynamic solubility, leading to ~2× increase in leachability and high recovery yields using diluted acid (e.g., 0.1 M HCl). The activation strategy is feasible for various wastes including coal fly ash, bauxite residue, and electronic waste. The rapid FJH process is energy-efficient with a low electrical energy consumption of 600 kWh ton^-1. The potential for this route to be rapidly scaled is outlined.

Integrative Study Assessing Space and Time Variations with Emphasis on Rare Earth Element (REE) Distribution and Their Potential on Ashes from Commercial (Colombian) Coal

The increasing demand for rare earth elements (REEs), which is associated with their economic importance and the supply risk, has motivated the research for alternative secondary sources of these elements. Coal and coal combustion ash have been pointed out as promising REE raw materials. This research seeks to understand REE fractionation, from feed coals to ashes, considering seasonal variations, and to assess the trends within the ash fractions that can be used for further beneficiation processes. Colombian commercial feed coals, combustion ashes, and their respective fractions were sampled from a Portuguese power plant and were characterized via petrographical, mineralogical, and chemical analyses. The total REE concentrations in the feed coals studied range between 6.97 and 23.15 ppm, while, in the ashes, they vary from 159.9 to 266.6 ppm. Fly ash (FA) from electrostatic precipitator (ESP) presented higher concentrations than the bottom (BA) and economizer (ECO) ashes. Furthermore, REEs and the LREE/HREE ratio increased slightly towards the back rows of the ESP. In the feed coals, the REEs are significantly correlated with ash, and they occur in micrometric phosphate minerals intermixed with clays. In the ashes, the REEs were mostly detected in micrometric particles, with P and Al-Si as the major components. The results from the fractioned samples show that the REEs were enriched in the fine (<25 µm) and nonmagnetic fractions of the ESP FA. A single trial combining sieving and magnetic separation enabled the attainment of a REE recovery of 53%, and a final enrichment factor of 1.25. Coal combustion ashes and their respective size fractions are promising REE raw materials; however, the REE oxide concentrations are below the economical cutoff of 1000 ppm.

A database and framework for carbon ore resources and associated supply chain data

The Carbon Ore Resources Database (CORD) is a working collection of 399 data files associated with carbon ore resources in the United States. The collection includes spatial/non-spatial, filtered, processed, and secondary data files with original data acquisition efforts focused on domestic coal resources. All data were acquired via open-source, online sources from a combination of 18 national, state, and university entities. Datasets are categorized to represent aspects of carbon ore resources, to include: Geochemistry, Geology, Infrastructure, and Samples. Geospatial datasets are summarized and analyzed by record and dataset density or the number of records or datasets per 400 square kilometer grid cells. Additionally, the “CORD Platform,” an ArcGIS Online geospatial dashboard web application, enables users to interact and query with CORD datasets. The CORD provides a single database and location for data-driven analytical needs associated with the utilization of carbon ore resources.

A review on mineralogical speciation, global occurrence and distribution of rare earths and Yttrium (REY) in coal ash

Abstract

Rare earths and Yttrium (REY) are a group of critical metals essential for this electronic and digital era. China is the leading producer of REY with more than 90% of global export. Mines of REY are limited and the need for green and efficient energies have augmented the demand of REY and it is putting enormous pressure on global production. REY market is predicted to grow from USD 5.3 billion in 2021 to 9.6 billion by 2026, at a CAGR (Compound Annual Growth Rate) of 12.3%. The need for permanent magnets is propelling the demand of the critical group REY and is expected to rise gradually in the coming years. In the present review, we have summarized the minable REY resources and their applications. The requirement for alternative resource is pivotal to meet our future needs. We have extensively reviewed the studies of REY in coal fly ash (CFA). A comprehensive analysis has been done for the REY resources worldwide for the last several decades in coal ash (CFA and bottom ash) and divulged into the application, speciation and distribution for major coal-consuming countries like China, India, USA, Russia, UK, Poland, etc., individually. We have also made a comparative global study and inferred potential extractable coal ash resources using various parameters such as global average, critical percentage (C_p), outlook coefficient (C_out), etc., for a better understanding of economic exploitation.

Research highlights

We have put up enormous effort to synthesize rare earth elemental data of coal ash from different coal-consuming countries. Following are the major highlights of this review article.

We have compiled data on occurrence of Rare Earths and Yttrium (REY) in coal ash from 13 countries such as China, India, USA, UK, Poland, etc.

Up-to-date global data of mined REY resources and reserves have been compiled.

Broad characterization and classifications of REY have been introduced.

Comprehensive analysis of application, speciation and environmental impact of REY in coal ash have also been compiled.

Comparative study has been done using parameters such as global average, critical percentage, outlook coefficient, etc. These parameters would help in determining ideal candidates for beneficial extraction of REY.

This study would serve as a knowledge resource centre for new research related to REY.

Fast Screening of Coal Fly Ash with Potential for Rare Earth Element Recovery by Electron Paramagnetic Resonance Spectroscopy

Rare earth elements (REYs) are in increasing global demand, but their mining is costly and environmentally destructive. Coal fly ash (CFA) is a promising alternative source of REYs, but it is necessary to identify CFA with sufficiently high REY concentrations. This study proposes the use of electron paramagnetic resonance (EPR) spectroscopy as part of a simple method to identify CFAs with adequate REY concentrations. The EPR spectra of CFA samples taken from 186 Chinese commercial coal-fired power plants were analyzed. The results suggest that CFAs without evident 6-fold resonances are worth recycling (REY concentrations of 416 ± 108 mg/kg), while those with conspicuous 6-fold resonances are not worth recycling (REY concentrations of 55 ± 26 mg/kg). This is probably due to isomorphic substitution of Ca(II) for Mn(II) and REY(III), resulting in low concentrations of Mn(II) and REY(III) in Ca-rich CFAs. This EPR evaluation method does not require specialized sample preparation, professional skills, or secondary data analysis and has potential global significance in the fast screening of CFAs with REY-recycling potential.

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.

Effects of Rare Earth Elements on Blood Pressure and Their Exposure Biomarkers: Evidence from Animal Experiments

Solid fuel combustion is an important source of the release of rare earth elements (REEs) into the ambient environment, resulting in potential adverse effects on human cardiovascular health. Our study aimed to identify reliable exposure biomarkers of REE intake and their potential role in blood pressure change. A total of 24 rats were administered with 14 REE chlorides at four doses (six rats per group). Fur samples were collected both before and after administration. Blood samples were collected after 12 weeks of REE intake. The REE concentrations in rat fur and blood samples were measured by inductively coupled plasma mass spectrometry. For each week, blood pressure, as well as heart rate and pulse pressure, were measured. The linear mixed-effect model was used to analyze the relationship between REE administration dose and blood pressure change. We found that the REE concentration in fur, but not blood, samples exhibited significant dose–response relationships with administration dose. It suggested that hair samples are a more efficient matrix for indicating the exposure level of a population to REEs than blood samples. However, there was no dose–response relationships between the administration dose and blood pressure change of rats, or with heart rate and pulse pressure for the 14 REEs. We also did not find a dose–response relationship between REE administration levels and plasma concentration of 8-hydroxy-2’-deoxyguanosine, as an important DNA oxidative stress damage biomarker. In conclusion, hair samples are more suitable as a sample type to reliably assess exposure to REEs than blood samples, and REEs did not have a direct adverse effect on blood pressure in our rat model.

Geochemical Occurrence of Rare Earth Elements in Mining Waste and Mine Water: A Review

Μining waste, processing by-products and mine water discharges pose a serious threat to the environment as in many cases they contain high concentrations of toxic substances. However, they may also be valuable resources. The main target of the current review is the comparative study of the occurrence of rare earth elements (REE) in mining waste and mine water discharges produced from the exploitation of coal, bauxite, phosphate rock and other ore deposits. Coal combustion ashes, bauxite residue and phosphogypsum present high percentages of critical REEs (up to 41% of the total REE content) with ΣREY content ranging from 77 to 1957.7 ppm. The total REE concentrations in mine discharges from different coal and ore mining areas around the globe are also characterised by a high range of concentrations from 0.25 to 9.8 ppm and from 1.6 to 24.8 ppm, respectively. Acid mine discharges and their associated natural and treatment precipitates seem to be also promising sources of REE if their extraction is coupled with the simultaneous removal of toxic pollutants.

Preferential Recovery of Rare-Earth Elements from Coal Fly Ash Using a Recyclable Ionic Liquid

Recent global geopolitical tensions have exacerbated the scarcity of rare-earth elements (REEs), which are critical across many industries. REE-rich coal fly ash (CFA), a coal combustion residual, has been proposed as a potential source. Conventional REE-CFA recovery methods are energy- and material-intensive and leach elements indiscriminately. This study has developed a new valorization process based on the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf₂N]) for preferential extraction of REEs from different CFAs. Efficient extraction relies on [Hbet][Tf₂N]'s thermomorphic behavior with water: upon heating, water and the IL form a single liquid phase, and REEs are leached from CFA via a proton-exchange mechanism. Upon cooling, the water and IL separate, and leached elements partition between the two phases. REEs were preferentially extracted over bulk elements from CFAs into the IL phase and then recovered in a subsequent mild-acid stripping step, regenerating the IL. Alkaline pretreatment significantly improved REE leaching efficiency from recalcitrant Class-F CFAs, and additional betaine improved REE and bulk element separation. Weathered CFA showed slightly higher REE leaching efficiency than unweathered CFA, and Class-C CFA demonstrated higher leaching efficiency but less selective partitioning than Class-F CFAs. Significantly, this method consistently exhibits a particularly high extraction efficiency for scandium across different CFAs.

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.

Characterization of rare earth elements present in coal ash by sequential extraction

Although it has recently been reported that notable amounts of rare earth elements (REEs) are present in the residual coal ash, little is currently known regarding the association of these elements with the coal ash matrix, thereby limiting the potential for extraction of REEs from coal ash. In this study, we analyzed the binding characteristics of REEs within coal ash via sequential extraction and examined REE recovery during a coal ash recycling process. Major components of coal ash were found to be mineral oxides, mainly composed of Si, Fe, Al, and Ca, and residual carbons. Bottom and fly ashes were found to contain 185.8 mg/kg and 179.2 mg/kg of REEs, respectively. Tessier sequential extraction confirmed that 85 % of REEs are included in the residual fraction of both bottom and fly ashes. Furthermore, BCR sequential extraction revealed that 60-70 % of REEs are contained within the residual fraction, thereby indicating that REEs are strongly bound in both bottom and fly ashes and the use of very strong acids is required for the thorough extraction of REEs from coal ash. Additionally, it was found that 46.3 % of REEs can be recovered from the wastewaters produced during the process of coal ash-derived zeolite synthesis.

Selective and Efficient Biomacromolecular Extraction of Rare-Earth Elements using Lanmodulin

Lanmodulin (LanM) is a recently discovered protein that undergoes a large conformational change in response to rare-earth elements (REEs). Here, we use multiple physicochemical methods to demonstrate that LanM is the most selective macromolecule for REEs characterized to date and even outperforms many synthetic chelators. Moreover, LanM exhibits metal-binding properties and structural stability unseen in most other metalloproteins. LanM retains REE binding down to pH ≈ 2.5, and LanM-REE complexes withstand high temperature (up to 95 °C), repeated acid treatments, and up to molar amounts of competing non-REE metal ions (including Mg, Ca, Zn, and Cu), allowing the protein's use in harsh chemical processes. LanM's unrivaled properties were applied to metal extraction from two distinct REE-containing industrial feedstocks covering a broad range of REE and non-REE concentrations, namely, precombustion coal and electronic waste leachates. After only a single all-aqueous step, quantitative and selective recovery of the REEs from all non-REEs initially present (Li, Na, Mg, Ca, Sr, Al, Si, Mn, Fe, Co, Ni, Cu, Zn, and U) was achieved, demonstrating the universal selectivity of LanM for REEs against non-REEs and its potential application even for industrial low-grade sources, which are currently underutilized. Our work indicates that biosourced macromolecules such as LanM may offer a new paradigm for extractive metallurgy and other applications involving f-elements.

A Lysine-Modified Polyethersulfone (PES) Membrane for the Recovery of Lanthanides

Rare-earth elements (which include all lanthanides, scandium, and yttrium) play a key role in many fields including oil refining, metallurgy, electronics manufacturing, and other high-technology applications. Although the available lanthanide resources are enough for current levels of manufacturing, increased future demand for lanthanides will require new, efficient recovery methods to provide a sustainable supply. Membrane adsorbers are promising separation materials to recover lanthanides from high volumes of wastewater due to their tailorable surface chemistry, high binding capacity and high throughput. In this work, membrane adsorbers were synthesized by first using ultraviolet-initiated free radical polymerization to graft a poly(glycidyl methacrylate) (p-GMA) layer to the surface of polyethersulfone membranes. Then, the reactive epoxy groups of the grafted p-GMA were used for the covalent attachment of lysine molecules via a zinc perchlorate-catalyzed, epoxide ring-opening reaction at 35°C. Changes in membrane surface chemistry throughout the functionalization process were monitored with Fourier Transform Infrared Spectroscopy. The degree of grafting for the p-GMA film was quantified gravimetrically and increased with increasing polymerization time. Equilibrium adsorption experiments were performed for single specie solutions of La³⁺, Ce³⁺, Nd³⁺, Na⁺, Ca²⁺, and Mg²⁺ at pH 5.25 ± 0.25. Lysine-modified membranes showed negligible uptake of Na⁺, Ca²⁺, and Mg²⁺. The maximum capacities modeled by the Langmuir isotherm for La³⁺ and Ce³⁺ were 6.11 ± 0.58 and 6.45 ± 1.29 mg/g adsorbent, respectively. Nd³⁺ adsorbed to the membrane; however, the fit of the Langmuir model was not significant and it adsorbed to a lower extent than La³⁺ and Ce³⁺. Lower adsorption of the higher charge density species indicates that the primary binding mode is through the amine moieties of lysine and not the carboxylic acid. Dynamic adsorption experiments were conducted with 1 ppm La³⁺ feed solutions at different flow rates using either a single membrane or three membranes in series. The dynamic binding capacity at 50% breakthrough was independent of flowrate within the tested range. The low-temperature membrane functionalization methodology presented in this work can be used to immobilize biomolecules with even higher specificity, like engineered peptides or proteins, on membrane surfaces.

Recovery of rare earth elements from coal fly ash by integrated physical separation and acid leaching

Coal fly ash (CFA) is one of the most promising secondary sources of rare earth elements and yttrium (REY). This research first studied the modes of occurrence of REY in CFA collected from a China's power generation plant which utilizes a coal feedstock with an elevated REY content. The fact that rare earth minerals remain in CFA and REY associate with metal oxides was proved by emission-scanning electron microscope with an energy-dispersive X-ray spectrometer. The technical feasibility of recovery of REY from CFA was then studied through conducting various physical separation methods followed by acid leaching. It was found that REY are concentrated in fine particle size, non-magnetic and middle density fractions. Using combined physical separation processes, the REY of CFA was enriched from 782 μg·g^-1to 1025 μg g^-1. The acid leaching process was optimized for various parameters via the Taguchi three-level experimental design. Upon optimization, the physical separation product was leached at the optimum condition and 79.85% leaching efficiency was obtained. Based on the obtained results, a conceptual process flowsheet was developed for recovery of REY from CFA. Such recovery maximizes REY resources utilization and enhances sustainability of CFA disposal.

A Comprehensive Review of Rare Earth Elements Recovery from Coal-Related Materials

Many studies have been published in recent years focusing on the recovery of rare earth elements (REEs) from coal-related materials, including coal, coal refuse, coal mine drainage, and coal combustion byproducts particularly fly ash. The scientific basis and technology development have been supported by coal geologists and extractive metallurgists, and through these efforts, the concept has progressed from feasibility assessment to pilot-scale production over the last five years. Physical beneficiation, acid leaching, ion-exchange leaching, bio-leaching, thermal treatment, alkali treatment, solvent extraction, and other recovery technologies have been evaluated with varying degrees of success depending on the feedstock properties. In general, physical beneficiation can be a suitable low-cost option for preliminary upgrading; however, most studies showed exceedingly low recovery values unless ultrafine grinding was first performed. This finding is largely attributed to the combination of small RE-bearing mineral particle size and complex REE mineralogy in coal-based resources. Alternatively, direct chemical extraction by acid was able to produce moderate recovery values, and the inclusion of leaching additives, alkaline pretreatment, and/or thermal pretreatment considerably improved the process performance. The studies reviewed in this article revealed two major pilot plants where these processes have been successfully deployed along with suitable solution purification technologies to continuously produce high-grade mixed rare earth products (as high as +95%) from coal-based resources. This article presents a systematic review of the recovery methods, testing outcomes, and separation mechanisms that are involved in REE extraction from coal-related materials. The most recent findings regarding the modes of occurrence of REEs in coal-related materials are also included.

Mineralogical and Chemical Characteristics of Coal Ashes from Two High-Sulfur Coal-Fired Power Plants in Wuhai, Inner Mongolia, China

The mineralogical and chemical characteristics of the feed coals and coal combustion products (CCPs) from two power plants (Xilaifeng and Damo) that consume coals from the Wuda Coalfield, Inner Mongolia, were investigated, using XRD, SEM–EDS, XRF, and ICP-MS. The feed coals from Xilaifeng and Damo are both of high ash yield (52.93% and 48.36%, respectively), and medium and high total sulfur content (2.22% and 3.32%, respectively). The minerals in the feed coals are primarily composed of kaolinite, quartz, illite, pyrite, and, to a lesser extent, gypsum and anatase. In addition to the elevated incompatible elements (Nb, Ta, Zr, Hf and Th), Li and Hg are enriched in the feed coals from the Xilaifeng and Damo power plants, respectively. Rare earth elements and yttrium (REY) are more enriched in the feed coals from Xilaifeng (194 μg/g) than those of Damo (93.9 μg/g). The inorganic phases of CCPs from both power plants are mainly composed of amorphous phase, quartz, hematite, illite, and anhydrite. Compared with the feed coals, concentrations of most trace elements in the CCPs are elevated, and they are preferentially enriched in the fly ashes relative to the bottom ashes (*f/b > 1), especially F, As, Sr, Mo, Se, and Hg (*f/b > 2.5). Furthermore, most trace elements (Xilaifeng: excluding Li, Cr, Co, Ni, Rb, Nb and Cs; Damo: excluding Li, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Cs and Ba) are more enriched in the (fine) fly ashes relative to the laboratory high-temperature coal ashes (HTAs). The REY barely differentiate in either the fly ash or bottom ash from Xilaifeng. In contrast, the REY in the fine and coarse fly ashes from Damo have very similar H-type distribution patterns with negative Ce and slightly positive Y anomalies. Attention should be paid to the enriched toxic elements (including F, As and Hg) in the fly ashes from both power plants due to possible adverse environmental effect.

Potentially toxic elements in solid waste streams: Fate and management approaches

Solid wastes containing potentially toxic elements (PTEs) are widely generated around the globe. Critical concerns have been raised over their impacts on human health and the environment, especially for the exposure to PTEs during the transfer and disposal of the wastes. It is important to devise highly-efficient and cost-effective treatment technologies for the removal or immobilisation of PTEs in solid wastes. However, there is an inadequate overview of the global flow of PTEs-contaminated solid wastes in terms of geographical distribution patterns, which is vital information for decision making in sustainable waste management. Moreover, in view of the scarcity of resources and the call for a circular economy, there is a pressing need to recover materials (e.g., precious metals and rare earth elements) from waste streams and this is a more sustainable and environmentally friendly practice compared with ore mining. Therefore, this article aims to give a thorough overview to the global flow of PTEs and the recovery of waste materials. This review first summarises PTEs content in various types of solid wastes; then, toxic metal(loid)s, radioactive elements, and rare earth elements are critically reviewed, with respect to their patterns of transport transformation and risks in the changing environment. Different treatments for the management of these contaminated solid wastes are discussed. Based on an improved understanding of the dynamics of metal(loid) fates and a review of existing management options, new scientific insights are provided for future research in the development of high-performance and sustainable treatment technologies for PTEs in solid wastes.

Adsorption kinetics, thermodynamics, and isotherm studies for functionalized lanthanide-chelating resins

Conventional ion exchange resins are widely utilized to remove metals from aqueous solutions, but their limited selectivity precludes dilute ion extraction. This research investigated the adsorption performance of ligand-functionalized resins towards rare earth elements (REE). Functionalized resin particles were synthesized by grafting different ligands (diethylenetriaminepentaacetic dianhydride (DTPADA), phosphonoacetic acid (PAA), or N,N-bis(phosphonomethyl)glycine (BPG)) onto pre-aminated polymeric adsorbents (diameter ∼ 0.6 mm). Lanthanide uptake trends were evaluated for the functionalized resins using batch adsorption experiments with a mixture of three REEs (Nd, Gd, and Ho at 0.1-1000 mg/L each). Resin physical-chemical properties were determined by measuring their surface area, ligand concentrations, and acidity constants. The aminated supports contained 4.0 mmol/g primary amines, and ligand densities for the functionalized resins were 0.33 mmol/g (PAA), 0.22 mmol/g (BPG), and 0.42 mmol/g (DTPADA). Kinetic studies revealed that the functionalized resins followed pseudo-second order binding kinetics with rates limited by intraparticle diffusion. Capacity estimates for total REE adsorption based on Langmuir q_Max were 0.12 mg/g (amine; ≈ 0.77 µmol/g), 5.0 mg/g (PAA; ≈ 32.16 µmol/g), 3.0 mg/g (BPG; ≈ 19.30 µmol/g), and 2.9 mg/g (DTPADA; ≈ 18.65 µmol/g). Attaching ligands to the aminated resins greatly improved their REE binding strength and adsorption efficiency.

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.