Low Rare Earth Element Contents Research Articles

Beneficiation of rare earth elements contained in phosphogypsum using sequenced treatment process

Concerns regarding the availability of rare earth elements (REEs) and the depletion of natural resource reserves have made it necessary to look for a significant breakthrough that may pave the way for the mining of several low-grade deposits and secondary resources. Phosphogypsum (PG) is one of the promising alternatives to preserve a steady flow of REEs and mitigate the loss of natural reserves. Nonetheless, the low concentration of REEs in PG implies the necessity of an enrichment step prior to any extraction attempt. To tackle the difficult challenges of REEs concentration from diluted phosphate-derived by-products, this study concentrated on determining the likely speciation of REEs and their beneficiation, as well as conducting an economic assessment of the potential industrial value of REEs present in PG. It was found that the alkaline solubilization of the gypsum phase of PG using Na4EDTA favored the beneficiation of different qualities of PG. Moreover, it was found that the abatement of residual undesirable phases by mineral acid (HF) for quartz (SiO2) elimination, or pyrometallurgical process, improved the enrichment of REEs in the residual concentrate. A REEs concentration of 4390.23 ppm and 4317.55 ppm were achieved after sequenced alkaline, neutral, and acidic treatment of low-grade PG (238.22 ppm); and sequenced alkaline, neutral, and pyrometallurgical treatment of high-grade PG (389.75 ppm), respectively. Furthermore, an elementary assessment of REEs speciation revealed that REEs could potentially be present as insoluble rare earth fluorides (REF3) and/or rare earth phosphates (REPO4).

Electro-assisted sorption behavior and mechanism of low-concentration rare earth elements on carbon based materials

With increasing demands for rare earth elements (REEs) in high-tech and futuristic applications, developing efficient methods for recovering low-concentration REEs is necessary. Here, electro-assisted sorption of low-concentration REEs, La(III), Gd(III) and Y(III) (as symbol of light, medium, and heavy REEs) on carbon-based materials, activated carbon (AC), graphene, and graphite by a facile method was reported. Recovery of La(III), Gd(III) and Y(III) could reach ∼100 % under low-concentration condition where the maximum adsorption capacity was up to 342.38 mg/(g·dm2). Variants including applied voltage, electrodes-distance, pH, impurities, and concentration were considered to study the effects on REEs recovery. Interestingly, adsorption capacity and recovery under low voltage were respectively almost 20 and 14 folds than that without voltage, because voltage promoted REEs transfer and strengthened electrostatic interaction between electrodes and REEs. The mechanism for enhancing REEs recovery on carbon-based electrodes was resulted from high electric double layer (EDL) capacitance and low charge transfer resistance. Total desorption of REEs from electrodes could be quickly realized using NH4Cl or HCl solution. Overall, the study gave a new insight of low-concentration REEs recovery using carbon-based materials, and provided an efficient route using electric method with porous materials for REEs recovery.

Nephrite from Xinjiang Qiemo Margou Deposit: Gemological and Geochemical Insights

The nephrite belt in the Altun Mountain–Western Kunlun Mountain region, which extends about 1300 km in Xinjiang, NW China, is the largest nephrite deposit in the world. The Qiemo region in the Altun Mountains is a crucial nephrite-producing area in China, with demonstrated substantial prospects for future exploration. While existing research has extensively investigated secondary nephrite deposits in the Karakash River and native black nephrite deposits in Guangxi Dahua, a comprehensive investigation of black nephrite from original deposits in Xinjiang is lacking. Margou black-toned nephrite was recently found in primary deposits in Qiemo County, Xinjiang; this makes in-depth research on the characteristics of this mine necessary. A number of technical analytical methods such as polarizing microscopy, Ultra-Deep Three-Dimensional Microscope, electron microprobe, back-scattered electron image analysis, X-ray fluorescence, and inductively coupled plasma mass spectrometry were employed for this research. An experimental test was conducted to elucidate the chemical and mineralogical composition, further clarifying the genetic types of the black and black cyan nephrite from the Margou deposit in Qiemo, Xinjiang. The results reveal that the nephrite is mainly composed of tremolite–actinolite, characterized by Mg/(Mg + Fe2+) ratios ranging from 0.86 to 1.0. Minor minerals include diopside, epidote, pargasite, apatite, zircon, pyrite, and magnetite. Bulk-rock rare earth element (REE) patterns exhibit distinctive features, such as negative Eu anomalies (δEu = 0.00–0.17), decreasing light REEs, a relatively flat distribution of heavy REEs, and low total REE concentrations (1.6–38.9 μg/g); furthermore, the Cr (6–21 μg/g) and Ni (2.5–4.5 μg/g) contents are remarkably low. The magmatic influence of granite appears to be a fundamental factor in the genesis of the magnesian skarn hosting Margou nephrite. The distinctive black and black cyan colors are attributed to heightened iron content, mainly associated with FeO (0.08~6.29 wt.%). Analyses of the chemical composition allow Margou nephrite to be classified as typical of magnesian skarn deposits.

Phosphorylated porous phenolic resin for efficient extraction of low-concentration rare earth elements from tailing wastewater

Efficient extraction and recycling of rare earth elements (REEs) from low-concentration complex tailing wastewater is critical to meeting the growing industrial demands and optimizing resource utilization. In this work, three phosphorylated porous phenolic resins (PO-PPRs) were prepared via the phosphorylation of phenolic hydroxyl groups with POCl3, and employed to extract REE3+ from both simulated and practical low-concentration REE3+-containing aqueous solution. The PO-PPRs, characterized by hierarchical pore structures, exhibit high adsorption capacities for both light and heavy REE3+ ions over a broad pH working range from 3.0 to 7.0, along with fast adsorption kinetics (<30 min). And the maximum adsorption capacities of PO-PPRs for Nd3+, Dy3+ and Y3+ were found to be 104.1, 118.0 and 63.9 mg/g, respectively. Furthermore, the introduction of phosphonic groups significantly improves the REE3+ adsorption selectivity against various interfering metal ions such as Na+, K+, Ca2+, Mg2+, and Al3+. For instance, the Nd3+/Al3+ separation factor (S.F.) of 24 exceeded that of most commercial resins and reported materials. In a simulated mixed multi-element aqueous solution system containing Nd3+, Dy3+, Y3+, Al3+, Ca2+, Mg2+, Mn2+, K+ and Na+, PO-PPR-1 extracted 82.5% of total REE3+ while maintaining Al3+ adsorption efficiency of less than 13%. Moreover, the extraction efficiency of total REE3+ for PO-PPR-1 in pretreated practical wastewater from ion-adsorbed ores tailing reached 99.6%. Additionally, PO-PPRs demonstrated easy regeneration and excellent recycling performance after 5 consecutive adsorption–desorption cycles, confirming their promising application potential in low-concentration REEs extraction.

Rare earth element systematics of chimney anhydrite from seafloor hydrothermal vents

Comparative studies of sulphide chemistry have been conducted over the past few decades to characterise seafloor hydrothermal mineralisation, but few studies have been carried out on sulphate minerals. Here we report on a systematic study of the S isotopic composition and rare earth element (REE) chemistry of chimney anhydrite from various types of seafloor hydrothermal vent fields in the Central Indian Ridge, North Fiji Basin, and Tonga intra-oceanic arc. Our results show that sulphate δ34S values of anhydrite cluster around that of seawater sulphate, indicating a significant role for fluid–seawater mixing during anhydrite precipitation. However, some anhydrite has δ34S values lower than that of seawater. This likely reflects sulphate derived from magmatic SO2 disproportionation. Chondrite-normalised REE patterns of the anhydrite are not uniform on the scale of individual grains and/or between each type of hydrothermal vent field. The REE compositions of the anhydrite do not have a clear relationship with the REE compositions of the primary host rocks with which the hydrothermal fluids interacted. This indicates that the REEs are not sensitive indicators of the host rock compositions, even though the host rocks are the dominant source of REEs in the seafloor hydrothermal fluids. In contrast, anhydrite REE concentrations and Eu anomalies are correlated with δ34S values and the redox state of the hydrothermal fluids, respectively. We suggest that relatively REE-enriched anhydrite is formed by efficient leaching of REEs from host rocks by acidic magmatic fluids (i.e., a low pH), whereas a low redox potential of the hydrothermal fluids is the most important factor controlling Eu mobility. The original concentrations of REEs supplied to the hydrothermal fluids can also be modified by fluid boiling and fluid–seawater mixing. The former produces light REE-depleted anhydrite because the availability of chloride ligands decreases in low-density vapor-dominated boiled fluids. In situ geochemical analysis of anhydrite on the intra-grain scale shows that high degrees of fluid–seawater mixing contribute to the relatively low REE concentrations and positive Eu anomalies. As such, our results indicate that the REE systematics of anhydrite are mainly controlled by the combined effects of fluid conditions (i.e., redox state, pH, and Cl concentrations) and sub-seafloor geochemical processes (i.e., magmatic input, fluid boiling, and seawater mixing) rather than the nature of the source materials. In situ analysis of anhydrite grains is a useful tool for constraining variable hydrothermal mineralisation conditions during chimney growth, which cannot be solely inferred from geochemical studies of sulphide minerals, particularly in inactive and relict systems where the hydrothermal fluids cannot be sampled.

Effect of mixed rare earth cations on structure, optical, mechanical and radiation shielding parameters of Na2O–B2O3 glass

The value of rare earth-containing glasses continues to increase, resulting in a higher cost for producing such glasses. The reduction in rare earth content reduces the cost of the production process. Investigating whether it is possible to prepare glasses with low rare earth (such as Dy and Sm) contents is valuable. In this work, the impacts of Dy2O3/ Sm2O3 substitutions on borate glass's structural, optical, and mechanical features were investigated. The optical studies reflected the structural modifications. For example, Urbach tail increased from 0.39 to 0.69 eV with further Dy2O3/ Sm2O3 substitutions. This increasing behavior refers to a less stable glass matrix. This outcome was substantiated by the diminished values of all elastic parameters in the current glasses upon additional Dy2O3/ Sm2O3 substitutions. Nonlinear optical parameters were examined across various wavelengths and compositions, revealing negligible variations within the visible and near-infrared (Vis-NIR) spans. However, a notable decrease was observed in the UV region, exhibiting a rapid decline with increasing substitutions. Furthermore, induced defects within the internal structure of the glassy network, notably non-bridging oxygens with additional Dy2O3/ Sm2O3 substitution, resulted in heightened nonlinear optical parameters. Simultaneously, all elasticity moduli decreased with further Dy2O3 additions in the current glassy system, attributed to increased disorder caused by defect production, dangling bonds, and non-bridging oxygens in the glassy matrix.In addition, the half-value layers of all glass samples were compared to those of typical and widely used shielding materials. Through this analysis, certain glass samples demonstrated superior qualities compared to conventional shielding materials. Notably, the glass sample with the highest Sm2O3 level (and correspondingly the lowest Dy2O3 value) exhibited particularly favorable attributes. Consequently, our glass samples show potential as effective shields in nuclear radiation environments.

Phase evolution, microstructure, and improved magnetic properties for off-stoichiometric SmFe12-based alloys

Low rare earth content and high intrinsic magnetic properties make SmFe12-based permanent magnets a potential candidate for NdFeB-based magnets in medium to high temperature environments. However, the easy formation of multiple ferromagnetic intergranular phases limits the preparation of high coercivity magnets. Composition optimization is an indispensable measure for the realization of high-performance magnets. In this work, off-stoichiometric composition design is employed to prepare SmFe12-based alloys. Ti is first introduced to inhibit the precipitation of α-Fe phase, obtaining a high coercivity of 484 kA/m, while the ribbon undergoes a reduction of remanence significantly. After regulating Fe–Co content to 11.5, the remanence recoveries from 0.51 to 0.65 T with a slight increase in coercivity (484–493 kA/m). Finally, Fe content is optimized to further improve magnetic properties, with both coercivity and remanence are improved simultaneously, reaching 509 kA/m and 0.70 T, respectively. Phase evolution, microstructure and their relationship with magnetic properties are investigated in detail. Micromagnetic simulation shows that the increased domain wall pinning effect induced by the refinement of grain size is the main reason for high coercivity. This work provides a feasible compositional design method for the preparation of high performance SmFe12-based alloys.

Long-time aging of La(Fe,Si,Mn)13Hz microparticles using different fluids for magnetic refrigeration systems

La(Fe,Si,Mn)13Hz alloys are promising solid-state refrigerant candidates for room-temperature magnetocaloric refrigeration due to their giant magnetocaloric effect, adjustable Curie temperature (TC) and low rare-earth content. For enhanced efficiency, the magnetocaloric material (MCM) is configured into an active magnetic regenerator (AMR), a porous, layered component with a chemically-tuned TC gradient. The porous structure facilitates heat exchange with a working fluid. It is crucial to preserve the thermomagnetic properties, such as TC and the magnetocaloric effect intensity, during refrigeration cycles. This study conducts a systematic investigation of the stability of three batches of La(Fe,Si,Mn)13Hz magnetocaloric microparticles, each with different values of TC. The commercially available microparticles (average size about 620 µm), which simulate AMR applications, underwent a 1-year immersion in deionized water and three water mixtures with corrosion inhibitors: Entek FNE (2 % vol.), ME-1 (5 %. vol.) and ME-3 (5 % vol.). Optical and scanning electron microscopy revealed surface changes within the first week of immersion in water and later in two fluids. The TC shifted towards higher temperatures (up to 13 K) after immersion, especially in water and in the ME-3-containing medium. After 40 weeks, the Entek FNE mixture caused some damage to the magnetocaloric materials. The lattice parameter of the La(Fe,Si,Mn)13Hz magnetocaloric phase changed, and magnetic entropy decreased for most aqueous media. Only ME-1 proved effective in preserving magnetocaloric microparticle properties for all TCs over a year, making it a potential candidate for long-term prototypes and commercial products.

Preparation of phosphorylated conjugated microporous organic polymers and extraction of low-concentration rare earth elements

Aiming at the extraction and recovery of rare earth elements (REEs) in low-concentration rare earth leaching tailings, a kind of phosphorylated conjugated microporous organic polymer (P-CMP) was prepared and applied to adsorb rare earth ions (Nd3+, Dy3+, Y3+). The structure of P-CMP was characterized by FT-IR. SEM and EDS were utilized to illustrate the surface morphology and element distribution of P-CMP, respectively.The adsorption mechanism of P-CMP was determined by XPS. The effects of pH, ion concentration and adsorption time on adsorption capacity were investigated. The isotherm of the rare earth adsorption process was fitted by Langmuir and Freundlich models, and the kinetics was fitted by Pseudo-firstorder and Pseudo-second-order. The results show that P-CMP is an amorphous microporous material, which has strong interaction with Nd3+ (30.18 mg/g), Dy3+ (37.84 mg/g) and Y3+ (22.01 mg/g), and the synergistic effect of amino and phosphate groups provides good selectivity (S.F.(Nd3+/Men+) = 5*103~5*104). The PCMP selectively adsorbs rare earth ions in the actual system and cycles 8 times.

Melting processes, cooling rates, and tectonic settings of the Neo-Tethyan mantle: the in-situ mineral chemical record

Peridotites in the Yarlung Zangbo ophiolite (YZO) have the potential to elaborate the mantle dynamics that operated below the Neo-Tethyan oceanic crust. Here, we measured major and trace element concentrations of minerals (e.g., clinopyroxene, orthopyroxene, olivine, and spinel) in YZO harzburgites and lherzolites to trace the origin and evolution of the YZO. The rare earth element (REE) patterns of clinopyroxenes in lherzolites and harzburgites plot within the field of abyssal and forearc peridotites, respectively, indicating that the latter experienced a higher degree of partial melting. Based on the relative extents of light (LREE) and heavy (HREE) REE depletions in clinopyroxene, we classified the lherzolites into two groups (I and II). The group I clinopyroxenes are depleted in HREEs and enriched in LREEs and MREEs compared to those of group II. Meanwhile, the REE patterns of harzburgite clinopyroxenes are identical to that of group II, although they display lower REE concentrations. Models based on REE abundances in clinopyroxene suggest that group I lherzolites experienced ~8–11% partial melting in the spinel domain, whereas group II lherzolites formed after 5% melting in the garnet domain followed by 6–9% and 9–16% partial melting in the spinel domain, respectively. These high degrees of melting may suggest that the harzburgites formed in a supra-subduction zone (SSZ) setting. Surprisingly, the REE-in-two-pyroxenes thermometer (TREE) shows that the YZO peridotites record high-temperature cooling at 991–1218 °C which is lower than the conditions below mid-oceanic ridges (MORs), but compatible with recent reports of high closure temperatures in forearc environments. Furthermore, our TREE results constrain the cooling rate of the YZO peridotites to ~0.01–0.5 °C/yr, overlapping with rates reported for SSZ, MOR, and abyssal peridotites. By integrating the petrological, geochemical, and thermal features, we tentatively interpret the group I lherzolites accreted in an Early Cretaceous forearc during subduction initiation, whereas the group II lherzolites and the harzburgites formed by further melting of the earlier lherzolites in a SSZ setting.

In situ U-Pb dating of carbonate veins in Cambrian shales constrains fluid flow and hydrocarbon evolution at the southeastern margin of the Upper Yangtze platform, southwestern China

Fluid flow in sedimentary basins not only impacts redistribution of the geothermal cycle and precipitation of ore deposits, but also exerts control on hydrocarbon migration and accumulation. However, reconstructing the history of fluid flow in basins that have experienced multiple tectonic deformation events is exceedingly difficult. Here, we examined petrography, in situ U-Pb geochronology, and rare earth element (REE) and C-O isotope geochemistry, as well as fluid inclusion microthermometry of fracture fillings within the Cambrian Niutitang Formation shales at the southeastern margin of the Upper Yangtze platform, southwestern China. The results show that four main fluid flow pulses are identified based on cathodoluminescence images, U-Pb ages, and geochemical data, namely, 446−428 Ma (fibrous calcite and barytocalcite), 343−329 Ma (calcite I), 113 Ma (calcite II), and 63 Ma (calcite III). The fibrous calcite (ca. 446 Ma) and barytocalcite (ca. 428 Ma) veins, corresponding to the late Caledonian Orogeny, show significantly positive Eu-Y anomalies, negative Ce anomalies, and enrichment in heavy REE, similar to their host rocks, suggesting that the mineral-forming fluids were derived mainly from dissolution of the host rocks. An abundance of bitumen inclusions with homogenization temperatures (Th) of 93.1−137.4 °C and high salinities (5−8 wt%) indicate that the first fluid flow pulse occurred during the oil generation stage in a closed fluid system. Calcite I (ca. 343−329 Ma) exhibits REE depletion and high Y/Ho ratios, a low fluid inclusion salinity (2−10 wt%) with Th = 78.4−125.8 °C, and C-O isotopic compositions similar to the underlying marine carbonates. This suggests that calcite I formed in an open fluid system, which was related to the transition from compression to extension during the Hercynian Orogeny. The pre-existing faults were reactivated and opened, resulting in the leakage and reconstruction of hydrocarbon reservoirs. Calcite II (ca. 113.4 Ma) has similar REE+Y patterns and C-O isotopic compositions to the host rocks. It contains abundant single-phase hydrocarbon gas (CH4) inclusions with high Th (164.1−211.1 °C) and salinity (6−14 wt%) values, indicating that the third phase fluid was derived largely from the host rocks and migrated during the early Yanshanian Orogeny. Lastly, calcite III (ca. 62.7 Ma) exhibits extremely low REE concentrations, low δ13CPDB [Peedee belemnite] values (−6.74‰), and low fluid inclusion salinities (0.3−7.0 wt%) with Th = 61.9−97.1 °C, suggesting that the fourth fluid flow pulse was affected by meteoric water to some extent. This can be interpreted to represent an open fluid system, which caused gas dispersion in the Niutitang Formation shales. Our findings provide important references for reconstructing the history of fluid flow in tectonically complex basins worldwide.

U–Pb Dating of Fibrous Dolomite in the Hydrothermal Dolostone of the Dengying Formation, Central Sichuan Basin, and Its Response to Supercontinent Breakup

Super-deep drilling in the central Sichuan Basin encountered volcanic rocks of the Suxiong Formation, which are overlain by multiple hydrothermal alterations within the upper section of the Ediacaran Dengying Formation. This provides an excellent research opportunity to understand the pre-Cambrian hydrothermal activity and geological evolution of the western margin of the Yangtze Craton. Observations revealed the development of a series of hydrothermal dolomite aggregates characterized by the presence of brown sphalerite within fractures and pores of the carbonate rock. Microscopically, the dolomite exhibited fibrous columnar crystallization, forming radial bands with a gradual decrease in crystallization intensity from the center to the periphery. Cathodoluminescence analysis revealed the presence of approximately eight dolomite bands within the aggregate. U–Pb dating from the inner to outer bands yielded isotopic ages of 781 ± 12 Ma for the second band, 683 ± 12 Ma for the fifth, 562 ± 12 Ma for the sixth, and 545.4 ± 6.9 Ma for the seventh. The in situ rare earth element (REE) distribution patterns of the 781 and 683 Ma dolomite bands exhibited similarities. They both showed low total REE content (∑REE), with significant fractionation between light and heavy REEs. Additionally, they exhibited negative anomalies in cerium (Ce) and europium (Eu), while heavy REEs were relatively enriched. The dolomite bands at 562 and 545 Ma also exhibited similar REE characteristics, with low ∑REE and weak fractionation between light and heavy REEs. They also displayed distinct negative anomalies in Ce and Eu, indicating similar distribution patterns. These findings suggested that formation of the banded hydrothermal dolostone occurred during different tectonic events, and the presence of heavy REE-enriched hydrothermal fluids suggested a deep-seated origin. This study has provided preliminary evidence that the Dengying Formation, previously considered to be of the Ediacaran age, has undergone multiple episodes of deep-seated fluid infiltration and alteration since the Mesoproterozoic Era. Importantly, these events coincide with the rifting of the Rodinia and Pannotia supercontinents, aligning with their respective timeframes. This finding raises questions regarding the stratigraphic division and correlation of the formations in the deeply buried core area of the basin.

Occurrence of rare earth elements in water, sediment, and freshwater fish of diverse trophic levels and feeding ecology: Insights from the Po river (northwest Italy)

To date, the occurrence of rare earth elements (REEs) in freshwater ecosystems has garnered limited attention in the scientific literature. Furthermore, a dearth of data exists regarding their potential bioaccumulation in freshwater fish. To fill this knowledge gap, we studied REEs concentrations in water, sediment, and fish specimens collected along the Po River (northwest Italy) and calculated biota-sediment accumulation (BSAF) and bioconcentration (BCF) factors, while taking into account fish feeding behavior and trophic level effects on the overall content of total REEs (ƩREEs). The fish communities were composed of native and non-native species. Remarkably low concentrations of REEs (<0.0003 mg/L) were detected in the water samples, indicating REEs insolubility. In contrast, sediment samples were found to be a good sink for REEs, with a higher mean ƩREEs recorded for the samples from the Moncalieri station (70.93 mg/kg). Notably, no significant differences in ƩREEs concentration were observed in the muscle tissue of fish samples from the three stations. The highest mean ƩREEs was recorded in the samples from the Murazzi station (0.027 mg/kg). The BSAF was very low, consistently below the unit, indicating an absence of bioaccumulation in fish muscle from sediment. In contrast, the BCF was high for several REEs, mainly for Sc and Y. While feeding ecology did not appear to affect REEs accumulation in muscle, there was a significant negative relationship between the trophic level and ΣREEs, indicating a trophic dilution of REEs from predator (Silurus glanis) to planktivorous (Alburnus arborella) fish. This study provides baseline concentrations, trophic transfers, and patterns of REEs in a river system. Further studies are needed to understand the transfer of REEs to other biotic components of lotic ecosystems.

Accumulation, translocation, and fractionation of rare earth elements (REEs) in fern species of hyperaccumulators and non-hyperaccumulators growing in urban areas

The issue of ion-adsorption type rare earth deposits (IADs) in urban areas of South China has garnered significant attention due to its environmental implications. Hyperaccumulator-based phytoremediation is a potentially effective solution for reducing the environmental impact of IADs in urban areas, particularly using ferns as they are known to be REE hyperaccumulators. However, the ability of different fern species to accumulate REEs in urban areas remains unknown. In this study, four fern species, including known hyperaccumulators (Dicranopteris linearis and Blechnum orientale) and other ferns (Pteris ensiformis and Cibotium barometz), were studied to investigate their REE accumulation abilities in the Guangzhou urban area. The aboveground parts of Dicranopteris linearis (848.7 μg g−1) and Blechum orientale (1046.8 μg g−1) have been found to accumulate high concentrations of REEs, demonstrating they probably can be applied for phytoremediation in the natural environments. Despite having lower REE concentrations than REE hyperaccumulators, Pteris ensiformis and Cibotium barometz still probably have the function as phytostabilizers in urban areas, as REEs can be enriched in their roots beyond the normal levels of plants. The enrichment of REEs in ferns is influenced by the availability of various nutrients (K, Ca, Fe, and P), which probably can be associated with different growth processes. The four fern species show LREE enrichment, moderate Eu anomalies and different Ce anomalies. It is difficult to absorb and transfer Ce to the aboveground parts of Blechnum orientale and Cibotium barometz. The study also identified selective enrichment of Ce in Pteris ensiformis, which has potential for comprehensive extraction of REEs when combined with other REE hyperaccumulators. REE fractionations are probably determined by the specific characteristics of different fern parts. Overall, these findings provide insights for addressing potential environmental problems related to IADs and offer guidelines for phytoremediation technology in addressing high REE levels in urban areas.

Bioleaching of Rare Earth Elements: Perspectives from Mineral Characteristics and Microbial Species

Bioleaching exhibits high potential for the processing of low-grade complex mineral resources. With the development of the economy and an increase in demand, rare earth elements (REEs) in secondary resources, such as phosphogypsum, red mud and coal-related resources, are gaining more and more attention. In this review, the bioleaching performance of diverse microorganisms is summarized and compared for primary (mainly monazite) and secondary REE resources, based on publications from the past decade. The mineral characteristics of these REE resources are different, as they can be found in phosphate, sulfate, or silicate forms. Correspondingly, microbial species suitable for use in bioleaching differ. The most efficient bioleaching microbe for monazite is Paecilomyces sp., while Acidianus manzaensis is effective in processing red mud. Acidophilic sulfur oxidizers are suitable for processing acidic phosphogypsum. Acidithiobacillus thiooxidans could recover a significant amount of REEs from coal fly ash. In particular, monazite has a high REE content but extremely low bioleaching efficiency compared to that of secondary resources, supporting the understanding that bioleaching approaches are more competitive for minerals with low REE contents. Overall, great progress has been made over the last decade, as considerable REE recovery rates have been achieved, and the main metabolites of microbes were identified. However, numerous challenges still exist. Future efforts should focus on improving biorecovery efficiency, reducing the cost of cell-culture media, and exploring the interaction mechanism between cells and minerals, with an emphasis on mineralogical phase transformations and the molecular regulation mechanisms inside cells during the bioleaching process.

The role of root carboxylate release on rare earth element (hyper)accumulation in plants – a biogeochemical perspective on rhizosphere chemistry

AbstractThe paper of van der Ent et al. (Plant Soil 485:247–257, 2023), published in the previous issue, reports the hyperaccumulation of rare earth elements (REE) in plant species from the Proteaceae for the first time. Indeed, the high REE accumulation in Proteaceae is not completely unexpected, given that the plants release large amounts of carboxylates to acquire phosphorus and micronutrients. However, it is somewhat questionable that the efficiency of element mobilization alone sufficiently explains the large variability in REE accumulation among different taxa of Proteaceae or other P-efficient species that typically show low concentrations of REE. Given that REE3+ share chemical similarities to Ca2+ but form stable complexes with ligands similar to Al3+, it is reasonable that uptake and accumulation of REE depend not solely on element mobility but also on the dynamics of element speciation governed by the formation, stability, and fate of carboxylate-REE-complexes in the rhizosheaths. The rationale behind this contention is that for elements with low mobility in soil, changes in chemical speciation may increase the availability only if the complex stabilities that depend on rhizosphere pH allow a breakdown during uptake. In this commentary, we explore the idea that REE accumulation depends on rhizosphere processes related to nutrient acquisition and element exclusion that overlap in time, space, and function depending on the composition of metal-chelating ligands released by plant roots in concert with rhizosphere pH. Based on data from greenhouse and field experiments, we propose a model where plants with a P-mining strategy (hyper)accumulate REE when rhizosphere pH is below a critical value shifting the REE speciation to available forms.

Pilot Scale Testing of Lignite Adsorption Capability and the Benefits for the Recovery of Rare Earth Elements from Dilute Leach Solutions

Naturally occurring organic materials containing humic acids show a strong affinity towards rare earth elements (REE) and other critical elements. Leaching experiments on lignite coal waste produced from construction sand production revealed that the contained REEs were associated with the organic matter. Furthermore, adsorption studies revealed that the lignite waste was capable of extracting REEs from a model solution and increased the REE content of the lignite waste by more than 100%. As such, this study aimed to utilize the lignite waste to adsorb REEs from pregnant leach solutions and acid mine drainage sources having low REE concentrations and subsequently leach the lignite material to produce pregnant leach solutions containing relatively high amounts of REEs, which benefits the performance and economic viability of downstream separation and purification processes. An integrated flowsheet was developed based on this concept and tested at a pilot scale. The pregnant leachate solution (PLS) was generated from a heap leach pad containing 2000 tons of Baker seam coarse refuse. The pilot scale circuit was comprised of aluminum precipitation, adsorption using the waste lignite, and rare earth-critical metal (RE-CM) precipitation stages in succession. The results indicated that the aluminum precipitation stage removed over 88% and 99% of the Al and Fe, respectively. The adsorption stage increased the REE content associated with the waste lignite from 457 ppm to 1065 ppm on a whole mass basis. Furthermore, the heavy REE (HREE) content in the feedstock increased by approximately 250%, which raised the percentage of HREE in the REE distribution by 19 absolute percentage points. In addition to the REEs, concentrations of other critical elements such as Mn, Ni, and Zn also improved by 75%, 37%, and 250%, respectively. Bench-scale tests revealed that increasing the solids concentration in the waste lignite and PLS mix from 1% to 20% by weight enhanced the adsorption efficiency from 32.0% to 99.5%, respectively. As such, a new flowsheet was proposed which provides significantly higher REE concentrations in the PLS that can be fed directly to solvent extraction and/or oxalic acid precipitation and, thereby, enhancing process efficiency and economics.

A novel ion exchange-electrodialysis hybrid system to treat rare-earth oxalic precipitation mother liquid: Contamination reduction, efficient Y3+ recovery, and acid separation

A novel ion exchange process combined with selective electrodialysis (SED) process has been demonstrated to not only efficiently recover low concentration rare-earth elements (REEs) and separate hydrochloric acid, but also significantly reduce membrane fouling. Two resins with different functional groups were selected for pretreatment and recovery of Y3+ from Y rare-earth oxalic precipitation (Y REOP) mother liquor. CH-93 resin showed a Y recovery rate of over 94 %, and the Y recovery efficiency remained above 90 % after 5 cycles. The mother liquor treated with resin (R-T) and the untreated mother liquor (W-R-T) were subjected to acid separation via SED, and membrane fouling was systematically evaluated. Compared with W-R-T group, the hydrochloric acid recovery efficiency of R-T group increased by 21.01 % (12 V), and the unit energy consumption decreased by 54.30 % (12 V). The results of 3D fluorescence excitation-emission matrix (3D-EEM) and fluorescent region integration (FRI) showed the dissolved organic matter (DOM) on AEM and CEM in R-T group both decreased by >45 %. The AEM of W-R-T group showed the highest fluorescence intensity ratio in V region (36 %), indicating humic acid-like substances were the main cause of AEM fouling. Generally, this study provides a new approach for the resource utilization of REOP mother liquor.

The sinks of rare earth elements in peloids from hydrothermal, estuarine, coastal, and saline formation environments from Cuba

This study investigated the main inorganic sinks of rare earth elements (REEs) in six peloid samples from four geological environments (hydrothermal, estuarine, coastal, and saline). We collected samples from each environment and determined pH, electrical conductivity, redox potential, temperature, elemental concentrations (major, minor, and REEs), mineralogy, and REE speciation. Additionally, principal component, hierarchical cluster, and multivariate analyses were used to investigate any relationship between REEs and phyllosilicates, iron, manganese, aluminum, and inorganic and organic carbon. Results showed that peloids from hydrothermal and estuarine environments presented the highest concentrations of minor elements and REEs (up to 109.78 mg/kg) with mineralogy dominated by quartz (31.6 to 92.4%) and phyllosilicates (49.3%). In contrast, coastal and saline peloids presented low concentrations of minor elements and REEs (up to 6.47 mg/kg) with mineralogy dominated by calcite (up to 74.6%), aragonite (up to 25.8%), and evaporitic minerals. Positive relationships were found between REE concentrations and the percentages of phyllosilicates (r = 0.978) and iron concentration (r = 0.986), which were supported by the operational speciation that showed that REEs were mainly associated with the residual and reducible fractions in hydrothermal and estuarine peloids. In coastal and saline peloids, the lower concentrations of REEs were attributed to their distribution on the reducible and oxidable fractions, from which they can mobilize. The results from this study revealed the inorganic sinks of REEs in peloids from hydrothermal, estuarine, coastal, and saline environments.

Petrogenesis and Tectonic Significance of Late Triassic A1-Type Granite from the West Section of North Qinling Orogenic Belt: Constraints from Geochronology and Geochemistry

The North China Block and the South China Block collided in the Middle Triassic, but there is still a lack of consensus regarding the end of collisional orogeny and the closure time of the Paleo-Tethys. In this paper, we report zircon U–Pb ages and geochemistry for the Shimen pluton in the northern margin of the West Qinling Orogenic Belt to investigate its genesis and tectonic environment. The new findings allow to constrain the end time of the Triassic orogeny in the Qinling Orogenic Belt and the closure time of the Paleo-Tethys. The weighted average 206Pb/238U ages of the Shimen pluton are 218.6 ± 1.5 Ma and 221.0 ± 1.7 Ma. Thus, we suggest that the Shimen pluton crystallized at the 218.6 Ma and 221.0 Ma and was formed during the Late Triassic (Norian). The Shimen pluton is mainly syenogranite and has alkaline dark minerals aegirine–augite. It is composed of 73.45 to 77.80 wt.% SiO2, 8.28 to 9.76 wt.% alkali, and 11.35 to 13.58 wt.% Al2O3, with A/CNK ranging from 0.91 to 1.02 and 10,000 Ga/Al ranging from 2.39 to 3.15. These findings indicate that the Shimen pluton is typical A-type granite. The plutons have low rare earth element contents, ranging from 73.92 to 203.58 ppm, with a moderate negative Eu anomaly. All the samples are enriched in large-ion lithophile elements, such as Rb, Nd, Th and U, and light rare earth elements, and are depleted in high field strength elements, such as Nb, P, Zr, Ba, and Sr. The depletion of Ba, Sr, and Zr may be related to the fractionation and evolution of the granite. According to the petrological and geochemical characteristics, the Shimen pluton is an A1-type granite formed in an anorogenic extensional environment. Combined with its tectonic characteristics and petrogenesis, the Shimen pluton was probably formed by the partial melting of the crust under high temperature and low pressure in the intraplate environment after the subduction of the South China Block beneath the North China Block. This observation indicates that the Triassic orogeny in the Qinling Orogenic Belt had ended and the Paleo-Tethys-Mianlve Ocean had also closed by the Late Triassic (Norian).