Low Rare Earth Element Contents Research Articles

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).

Multistage dolomitization of deeply buried dolomite in the Lower Cambrian Canglangpu Formation, central and northern Sichuan Basin

The dolostone units in the Cambrian Changlangpu Formation of the Sichuan Basin have demonstrated substantial potential for natural gas exploration. A systematic understanding of the various dolomite (and non-carbonate) phases that build these rocks and the diagenetic pathways of the Canglangpu Formation carbonates still needs to be developed. This study combines the petrographic properties of dolomite (and non-carbonate) phases with their geochemical evidence (carbon, oxygen and 87Sr/86Sr isotope data). Moreover, fluid inclusion evidence and in situ rare-earth element (REE) data are presented. The samples studied were taken from hand specimens collected in the field and plugs from cores to explore the properties and evolutions of dolomite in the Cambrian Canglangpu Formation in the central-northern Sichuan Basin. Six dolomite phases are identified: silty crystalline dolomite (D1), fabric-retentive dolomite (D2), fabric-destructive dolomite (D3), fine crystalline dolomite (D4), medium to coarse crystalline dolomite cement (C1a and C1b) and saddle dolomite cement (C2). Silty crystalline dolomite represents the earliest dolomitization and shows δ18O values comparable to those of Cambrian seawater. Fabric-retentive dolomite, fabric-destructive dolomite and fine crystalline dolomite represent reflux dolomitization during shallow burial. Specifically, Fine crystalline dolomite with a turbid-core and clean-rim may have recrystallized from secondary dolomitization of marine-derived porewater. Fabric-destructive dolomite shows the depleted δ13C values, resulting from the secondary dolomitization involving organics degradation in medium to deep burial environment. The fluid inclusion data demonstrates that the medium to coarse crystalline dolomite cement (C1a) formed in relatively elevated temperature and salinity brines. Intrusion of sulfur- and metal-rich (Zn and Fe) medium-temperature hydrothermal fluids gave rise to the formation of medium to coarse crystalline dolomite cement (C1b) and sphalerite mineralization. Cementation of saddle dolomite was promoted by sulfate-rich reducing hydrothermal fluids. δ13C depletion, extremely low REE content and positive δEu in the saddle dolomite cement suggest the presence of thermal sulfate reduction.

Geochemistry of color-zoned apatite from the Chadormalu iron oxide-apatite deposit, Central Iran: Insights into REE mobility during hydrothermal alteration

Apatite in the ‘Kiruna-type’ iron deposits is known as potential resource of rare earth elements (REEs). The present study describes large color-zoned apatite crystals in the Chadormalu magnetite-apatite deposit, Central Iran, to obtain insights into the behavior of REEs and trace elements during apatite mineralization and subsequent hydrothermal alteration. The zoned apatite consists of a beige-colored core and surrounding reddish and white polycrystalline domains. The beige-colored core is optically single crystals, but shows a replacement texture of the early-stage apatite (Ap-I) by the later-stage apatite (Ap-II) with abundant monazite inclusions. Ap-I is characterized by higher REEs, Y, Na, Si, S and Cl contents than Ap II. Both Ap II and reddish/white polycrystalline domains show similar compositions with very low contents of REEs and trace elements. These textural and chemical relations suggest that initial apatite mineralization (Ap-I) was followed by an intensive hydrothermal event, which has resulted in dissolution of Ap-I and precipitation of Ap-II and surrounding polycrystalline domains. REEs in Ap-I were leached into the fluid and precipitated as monazite inclusions in Ap-II and interstitial monazite in the reddish domain of polycrystalline apatite. Parts of REEs leached in the fluid are also incorporated into cores of the polycrystalline apatite. It is concluded that, although the later hydrothermal event remobilized REEs, it did not significantly affect the total REE budget of the investigated apatite-magnetite ore deposit.

Investigating the potential applications of Mg-Zn-Y-Gd-Mn alloy biodegradable implants in the field of orthopedics

The purpose of this work was to validate the osteogenic characteristics and biosafety of Mg-Zn-Y-Gd-Mn alloys with low rare earth content for orthopedic applications. The as-extruded Mg-1.96Zn-0.97Y-0.47Gd-0.38Mn alloy exhibited excellent corrosion resistance and mechanical properties. The tissue surrounding the SD rats femoral implants was studied using micro-CT and H&E staining techniques. Around the Mg-1.96Zn-0.97Y-0.47Gd-0.38Mn alloy implants, there was evidently significant bone development with increased thickness and quantity of trabeculae. In addition, no partial or systemic adverse effects were observed after surgery.

Adsorption of La3+ onto trifluoroacetic acid modified UiO-66-COOH: Adsorption mechanism and application

The application of metal organic frameworks (MOFs) for the recovery of low concentrations of rare-earth elements (REEs) has attracted increasing attention due to the precious value of REEs resources. In this study, UiO-66-COOH was modified by trifluoroacetic acid (TFA) to not only promote material crystallization and the pore structure verified by XRD and SEM characterization, but successfully enhance its adsorption capacity and achieve perfect selectivity for Lanthanum ion (La3+). The adsorption capacity for UiO-66-COOH-24TFA was increased by 58% compared to unmodified material. The adsorption mechanism and application were comprehensively investigated. By adsorption kinetics, isotherm and thermodynamic studies, it was found that the adsorption process mainly exhibited monolayer adsorption with spontaneous endothermic-chemisorption characteristics, and –COOH of the modified materials (UiO-66-COOH-24TFA) played an important role, which was confirmed by XPS. Moreover, UiO-66-COOH-24TFA was applied to REEs actual wastewater by successive six-stage adsorption process, where the overall recovery rate on La3+ reached 94.1% under the coexisting conditions of multiple ions. The application of this acid modified material in the field of the separation and recovery of REEs is of great promise.

Petrogenesis and Geochronology of A1-Type Rhyolites in the Late Late Triassic of the East Kunlun Orogenic Belt: Constraints on the End of the Paleo-Tethys Orogenic Event

The rhyolites which are widely exposed to the northern margin of the East Kunlun orogenic belt were chosen as a research object to discern the post-orogenic tectonic evolution of the East Kunlun orogenic belt and reconstruct the post-collision orogenic processes of the Buqingshan- A’nyemaqen Ocean. We researched zircon U-Pb ages and geochemistry characteristics of the Late Triassic rhyolites in the eastern segment of the East Kunlun Orogenic Belt in the northern Tibetan Plateau. Zircon U-Pb dating yields coeval ages of 200.4 ± 1.4 Ma and 202.8 ± 1.2 Ma for the Keri rhyolites of the East Kunlun Orogenic Belt, indicating that the volcanic rocks were formed in the Late Triassic Rhaetian–Early Jurassic Hettangian. The Keri rhyolite is a product of the late magmatism of the Elashan Formation volcanic rocks. The rhyolites include rhyolitic brecciated tuff lavas and rhyolitic tuff lavas. The rhyolites are peraluminous and are high-K calc-alkaline, with high contents of SiO2, K2O, TFe2O3, and low P2O5 contents. The A/CNK ratios range from 0.97 to 1.09, indicating that the rhyolites are metaluminous to weakly peraluminous. The chondrite-normalized rare earth element (REE) distribution shows a significant negative Eu anomaly and low total REE concentrations. All samples are depleted in high field strength elements (HFSEs, e.g., Eu, Sr, Ti, and P), heavy rare earth elements (HREEs), and enriched in large ion lithophile elements (LILEs, e.g., Rb, Zr, Nd, Th, and U) and light rare earth elements (LREEs). The Keri rhyolite has the characteristics of A1-type magmatic rock, formed in an anorogenic environment after the closure of the Paleo-Tethys Ocean, and was the product of late magmatism in the Elashan Formation volcanic rocks.

The Microstructure and Magnetic Properties of the Strip-Cast (Sm,Zr)(Fe,Co)10.3Ti0.7 Alloy

Sm(Fe,Co,Ti)12-based alloys with low contents of rare-earth elements are promising materials for manufactoring high-energy permanent magnets. The (Sm,Zr)(Fe,Co)10.3Ti0.7 alloy has been produced by strip casting with low quenching rates. The structure and magnetic properties of the alloy were studied by scanning electron microscopy, as well as X-ray and thermomagnetic analysis. The initial inhomogeneous alloy was subjected to solid-solution treatment at 1150°С. The alloy retained a high-anisotropy state typical of the Sm(Fe,Co,Ti)12 phase.

The geochronology, geochemistry and petrogenesis of ca. 2.65 Ga tonalites and ca. 2.61 Ga amphibolites in the Jiefangyingzi area: Implications for the northern boundary of the North China craton

The Bayan Obo–Duolun–Chifeng–Kaiyuan (BDCK) Fault is considered the boundary between the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB). However, the site of this boundary from Duolun to Chifeng and Kaiyuan remains highly controversial. Here, we present an integrated study of petrology, whole‐rock geochemistry, and zircon U–Pb and Sr–Nd–Hf isotope compositions for tonalites and amphibolites in the Bainaimiao arc belt (BAB) to better constrain the northern NCC margin. The tonalites are similar to NCC tonalite–trondhjemite–granodiorites (TTGs), characterized by relatively high Na2O/K2O and Sr/Y ratios and fractionated rare earth element (REE) patterns, with high light REE (LREE) but low heavy REE (HREE) concentrations. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb dating yields formation ages of 2599–2674 Ma, with a weighted average of 2649.6 ± 3.7 Ma. The εHf(t) values (3.5–7.5), two‐stage model ages (2673–2793 Ma), whole‐rock positive εNd(t) values (1.3–1.9), and two‐stage model ages (2841–2880 Ma) suggest that the Neoarchean tonalites likely originated from remelting of thickened Meso‐Neoarchean mafic crust. LA–ICP–MS U–Pb isotope data reveal that the amphibolites formed at ~2610 Ma. They have high‐Fe tholeiitic compositions and are close to normal mid‐ocean ridge basalt (N‐MORB)‐like rocks, indicating that they were generated from partial melting of the depleted mantle. The existence of Neoarchean basement rocks, combined with seismic and isotope data, suggests that the Jiefangyingzi area is part of the Trans‐North China Orogen (TNCO) in the NCC rather than part of the CAOB and that the northern boundary of the NCC in Chifeng is located north to the Jiefangyingzi area.

The role of nanofiltration modelling tools in the design of sustainable valorisation of metal-influenced acidic mine waters: The Aznalcóllar open-pit case

Acid Mine Waters (AMWs) are considered the most challenging environmental problem in the Iberian Pyrite Belt (South of Spain) so far, being characterised by an acidic pH (H2SO4-based), high content of heavy metals (e.g. Co, Pb, rare earth elements (REEs)) and non-metals (e.g. As). Nowadays, AMWs are being contemplated as a source of valuable metals (e.g. Zn, Cu, REEs), and different recovery routes have been studied (e.g. ion-exchange, membrane technologies). Among them, nanofiltration (NF) provides the possibility of water (or H2SO4) recovery and concentration of metals, which can be recovered in a subsequent stage. This work evaluates the performance of the Desal DK NF membrane to treat AMW from Aznalcóllar open-pit, which presents a low pH (2.6) and high contents of Zn (920 mg/L), Mg (2540 mg/L) and REEs (17 mg/L). The effect of solution pH on membrane performance was evaluated. Results showed that the equilibrium SO42-/HSO4- controlled the transport of species. Rejections of sulphate were around 84 % at pH 2.1 (prevalence of HSO4-) and increased to 92 % at pH 3.1 (prevalence of SO42-). Metal rejections remained higher than 74 % (except for monovalent species). Additionally, membrane permeances were determined using the Solution-Electro-Diffusion model coupled with reactive transport, and later used to assess the design of the process treatment of the AMW. Results demonstrated the possibility of concentrating metals and recovering H2SO4 as permeate, but also showed that gypsum and anhydrite precipitation can occur. Additionally, CAPEX and OPEX were estimated for a NF plant treating 2 Hm3/year.

Regulatory actions of rare earth elements (La and Gd) on the cell cycle of root tips in rice seedlings (Oryza sativa L.)

The continuous expansion of the application of rare earth elements (REEs) in various fields has attracted attention to their biosafety. At present, the molecular mechanisms underlying the biological effects of REEs are unclear. In this study, the effects of lanthanum (La) and gadolinium (Gd) on cell cycle progression in the root tips of rice seedlings were investigated. Low concentrations of REEs (0.1 mg L−1) induced an increase in the number of cells in the prophase and metaphase, while high concentrations of REEs (10 mg L−1) induced an increase in the number of cells in the late and terminal stages of the cell cycle, and apoptosis or necrosis. Additionally, low concentrations of REEs induced a significant increase in the expression of the cell cycle factors WEE1, CDKA;1, and CYCB1;1, and promoted the G2/M phase and accelerated root tip growth. However, at high REEs concentrations, the DNA damage response sensitized by BRCA1, MRE11, and TP53 could that prevent root tip growth by inhibiting the transcription factor E2F, resulting in obvious G1/S phase transition block and delayed G2/M phase conversion. Furthermore, by comparing the biological effect mechanisms of La and Gd, we found that these two REEs share regulatory actions on the cell cycle of root tips in rice seedlings.

The selective adsorption of rare earth elements by modified coal fly ash based SBA-15

The modification of fly ash based SBA-15 enables the selective adsorption of REE(III) with a high capacity. • The species distribution of rare earth elements ions was simulated. • DTPADA-SBA-15 owned high adsorption efficiency and selectivity for REE (III). • DTPADA-SBA-15 owned preferable reusability in acidic solution. Rare earth elements (REE) are strategic resources and the recycling of REE in alternative resources is urgent and gets increasingly attention. However, the separation of REE in these alternative resources is still a challenge due to the low concentration of REE and multi coexisted ions in acidic system. In this study, the species distribution of REE within the pH 0–8.0 was calculated. The SBA-15 originated from coal fly ash was modified by two steps with (3-aminopropyl) triethoxysilane (APTES) and diethylenetriaminepentaacetic dianhydride (DTPADA) to obtain DTPADA-SBA-15 adsorbent, which was applied to the selective adsorption of REE. The results showed that DTPADA-SBA-15 possessed excellent adsorption performance on the selective adsorption of REE, including Eu, Gd, Tb, Nd and Sm, in acidic solution (pH 2) with multi competing ions. The FT-IR and Zeta potential characterization verified that the chemical adsorption through the coordination of O in DTPADA-SBA-15 with REE was dominant at lower pH value. The study of adsorption kinetics indicated that the adsorption of rare earth metal ions followed pseudo-second-order kinetic, of which the adsorption process followed the Langmuir isotherm model.

Geochemical characteristics and genesis of ferromanganese nodules and crusts from the Central Rift Seamounts Group of the West Philippine Sea

There is a lack of information on the geochemical characteristics and genesis of ferromanganese nodules and crusts from the Central Rift Seamounts Group of the West Philippine Sea (CRSGWPS). This study presents the major, trace, and rare earth contents and 10Be isotopic data for ferromanganese nodules and crusts from the CRSGWPS to determine their genetic origin, classification, and growth rate. The Mn/Fe ratio of the crust is 0.96–3.97, and the Co + Ni + Cu content is 4499–6324 ppm. The crust is rich in rare earth elements (REE), with significant positive Ce and negative Y anomalies. The Mn/Fe ratio of the nodule is 0.90–1.52, and the Co + Ni + Cu is 1617–9549 ppm. Samples near the core of the nodule have a relatively low REE content, with significant positive Ce and negative Y anomalies. 10Be specific activity and thickness of the nodules and crusts show that growth rates were 2.26 mm/Ma and 2.5 mm/Ma, respectively, indicating that proliferation of nodules and crusts in this area began in the mid-Miocene (9.32–12.03 Ma). A comprehensive study of the growth rate, mineralogy, structure, and chemical characteristics of nodules/crusts in the study area indicates that their formation was mainly hydrogenic, and the early growth of nodules is affected by hydrothermal action.

In Vitro Corrosion Performance of As-Extruded Mg–Gd–Dy–Zr Alloys for Potential Orthopedic Applications

In this study, different contents of rare earth elements with high solid solubility (Gd and Dy) were added into Mg and fabricated through homogenization and hot extrusion processes that enable few second phase formation to efficaciously inhibit the galvanic corrosion. The microstructure and phase characterization of the as-extruded Mg–Gd–Dy–Zr alloys were analyzed by scanning electron microscopy, electron backscattered diffraction, and X-ray diffraction. The in vitro biodegradation behavior of the as-extruded Mg–Gd–Dy–Zr alloys was investigated via the electrochemical measurement and immersion test. The results revealed that all the as-extruded alloys with different RE additions exerted fully recrystallized microstructures. The average grain size was appropriately 20 μm to 30 μm for all alloys and gradually increased by adding more RE. Only a few tiny second-phase particles less than 5 μm dispersed for all the samples and the volume fraction of particles increased slightly with the increase in RE content. The as-extruded Mg–Gd–Dy–Zr alloys with low RE content (GD0.6) allowed for a satisfactory corrosion resistance in Hank’s solution with a controlled corrosion rate less than 0.5 mm/year, which is considered as the tolerance limit for the corrosion rate of orthopedic implants. This study provides a cost-effective choice for promoting biodegradable magnesium alloys for potential orthopedic applications with low rare earth content in Mg alloys.

Uranyl Minerals from Abandoned Podgórze Mine (Sudetes Mountains, SW Poland) and Their REE Content

The Podgórze uranium deposit is located near Kowary in the Sudetes Mountains, SW Poland. The mine is located in the Karkonosze-Izera block, largely comprising Cambrian to Devonian metamorphic rocks intruded by the Variscan Karkonosze granite. Uranyl minerals from the Podgórze mine can be divided into three assemblages. The first one is associated with heavily ventilated mining galleries. The next assemblage is related to the outflow of water from fissures in the walls of the mine galleries. The last assemblage appears in the mine dump, where there is increased activity of other weathering products. The main purpose of this paper is to determine the mineralogical characteristics of uranyl minerals from the abandoned Podgórze uranium mine and reconstruct the physicochemical crystallization conditions based on the concentrations of rare earth elements (REEs) in these minerals. The results of thermodynamic modeling show that the aqueous species of uranyl ion in the mine water are represented by UO2HAsO4 (aq), UO2CO3(OH)3−, UO2CO3 (aq), and UO2OH+. The content of REEs and their distribution patterns were used to determine the crystallization conditions of uranyl minerals. Uranyl carbonates and arsenates have generally low concentrations of REEs compared to uranyl silicates, phosphates, and hydroxides, which have higher concentrations. The differences in REE concentration patterns may be related with the oxidizing nature of water circulating in the subsurface part of the deposit.

Parametric study and speciation analysis of rare earth precipitation using oxalic acid in a chloride solution system

Oxalic acid precipitation is a common step in the purification of rare earth elements (REE) from a concentrated pregnant leach solution (PLS). However, the presence of contaminants such as Al, Fe, and Ca in given amounts decreases the REE precipitation efficiency and product purity while also increasing the amount of oxalic acid needed to maximize recovery. As such, a statistically designed test program was performed to identify the optimal conditions necessary for a relatively low REE content PLS containing elevated concentrations of contaminant ions. The performance objective was maximization of REE precipitation efficiency while minimizing the oxalic acid dosage. A central composite design was utilized to quantify performance impacts and identify the ultimate set of parameter values for oxalic acid dosage, Fe(III) contamination concentration, solution pH, and reaction temperature. The resultant model suggested that oxalic acid dosage and reaction pH are the most significant factors for the REE precipitation efficiency, followed by the interaction of oxalic dosage and Fe concentration. Test results indicate that increasing the oxalic acid concentration from 0 g/L to 80 g/L improved the REE precipitation efficiency from approximately 4.2% to 95.0%. Furthermore, raising the solution pH from 0.5 to 2.5 considerably enhanced the precipitation efficiency from 0.0% to 98.9%. A solution temperature elevation decreased REE recovery, which indicated an exothermic reaction between REEs and oxalate anions. Finally, a high level of Fe contamination adversely impacted REE precipitation efficiency. To further the understanding of the REE-oxalate system, a fundamental solution chemistry study was performed using the equilibrium constants of the reactions. The study resulted in the development of oxalate speciation diagrams and provided an analysis of the REE precipitation characteristics at various oxalate anion concentrations and Fe(III) contamination levels using MINTEQ software. The dominant Fe(III) species in the solution system were found to be Fe-(C2O4)33-, Fe-(C2O4)2-, and Fe-(C2O4)+, which consume the majority of the oxalate anions. The simulated model was found to be in agreement with the experimental findings and helped to explain the adverse impact of increased iron concentrations on REE precipitation efficiency.

Origin of metasomatic fluids in the Bayan Obo rare-earth-element deposit

The Bayan Obo REE–Fe–Nb deposit is the largest light rare earth element (REE) deposit in the world, and it has undergone serious hydrothermal reworking processes. However, the origin and nature of metasomatic fluids in this deposit remain controversial. To determine the origin of metasomatic fluids, we conducted a detailed mineralogical and geochemical investigation on the apatite from the ∼ 1.8-km drill core in the Bayan Obo deposit. The textural and geochemical variations of the apatite could trace the hydrothermal metasomatism during REE mineralization. Four types of apatites were identified according to their cathodoluminescence characteristics, including turbid euhedral apatite (Ap-0), rounded pinkish–violet apatite veinlets (Ap-1), yellowish–green apatite aggregates (Ap-2), and apatite with a pinkish–violet core and a yellowish–green rim (Ap-3). According to the in-situ Sr isotopic compositions, these apatites were classified into two groups. Ap-0, Ap-1, and Ap-2 display constant and low 87Sr/86Sr ratios (0.7030–0.7035), whereas Ap-3 displays various Sr isotopic compositions (0.7047–0.7105). The in-situ Th–Pb age of the bastnäsite associated with the Ap-0 ranges from 390 to 425 Ma. The Sr isotopic compositions and geochronological study indicate that metasomatic fluids in the Bayan Obo deposit were obtained from different sources, including the Silurian Bainaimiao arc plutons and sedimentary materials. The four apatite types display various REE distribution patterns, which could account for hydrothermal fluid properties. F-rich and Na-poor fluids metasomatized the precursor Ap-0, releasing REEs from apatites and resulting in low REE contents of Ap-0. Ap-1 and Ap-2 were derived from the same fluid but were characterized by different trace element compositions, which could be attributed to the evolution of hydrothermal fluids with changes in the Na content and F/Cl ratio. The primary Ap-3 was derived from a REE-rich source and was altered by Na-poor and Cl-rich fluids. Metasomatic fluids with different origins and variable chemical compositions result in complex mineral assemblages and geochemical features in the Bayan Obo deposit.