Mobility Of Rare Earth Elements Research Articles

Distribution, and mobility of rare earth elements in surface sediment of Gomishan Wetland

Rare earth elements (REEs) are emerging environmental contaminants that pose a threat to ecosystem health. Their accumulation in sediments and potential release into the water column can raise concerns. Understanding the factors influencing REE mobility is crucial. Furthermore, our knowledge of the risk and occurrence of REEs in wetland ecosystems is inadequate. This study investigated REE release from sediments under varying redox conditions (Eh). The different redox potential values were adjusted using a photoanode (Ti/TiO2) and a cathode (graphite). Also, to recognize the relationship between REEs and other parameters, Pearson correlation (PC) was employed. Results showed that increasing Eh initially enhanced REE mobility (until around 300 mV), followed by a decrease at higher Eh values. Among the studied REEs, lanthanum (La), cerium (Ce), and gadolinium (Gd) exhibited the highest release potential into water, with measured concentrations reaching 199.4, 93.1, and 31.2 µg/L, respectively. Moreover, according to several eco-geochemical risk assessment indices, the sediment of the study area was classified as minimal contamination. Statistical analyses revealed that Eh indirectly affects REE mobility by influencing linked changes in pH, salinity, and iron-manganese (Fe-Mn) chemistry, which in turn impacts REE solubility and release from sediments.

Experimental constraints on the role of temperature and pyrogenic mineral assemblage in wildfire-induced major and trace element mobilisation

Wildfires impact a large and increasing proportion of the Earth’s surface. With documented soil surface temperatures of up to ∼850 °C, wildfires may fundamentally alter the mineralogy and geochemistry of soils and regolith, more conventionally thought to be dominated by low temperature weathering processes. Here we use an experimental approach to test the effect of temperature on the formation of pyrogenic minerals, and on the distribution and mobility of major, trace and rare earth elements following post-fire chemical weathering. We focus on ferruginous nodules, common Fe-oxide cemented components of soils, which transform from non-magnetic to maghemite-bearing, magnetic nodules under wildfire conditions. These transformations provide a valuable record of fire impacts and facilitate the study of thermal processes and element mobility. Our results show heating produces a typical pyrogenic mineral assemblage of hematite, maghemite, metakaolin and transition alumina. At 900 °C the high temperature Fe2O3 polymorph luogufengite forms, which has never been reported in natural fire-affected substrates and therefore places an upper boundary on palaeowildfire temperatures at the soil-fire interface. Chemical leaching, employed to simulate the impacts of post-fire weathering, demonstrates that formation and subsequent breakdown of these pyrogenic minerals results in increased mobility of several elements including Li, Si, Sc, Cr, Co, Cu, Zn, Rb, Cs, La, Pb and U. Further, we propose that incongruent dissolution of pyrogenic metakaolin may be responsible for the formation of fusic material, an aluminous cement commonly found in soils. We conclude by discussing the significance of these results for the release of potentially toxic metals following a fire, identify trace elements that have the greatest potential to be used as palaeowildfire geochemical proxies (decreased alkali metal concentrations, decreased U/Th ratios, and decreased La compared to other rare earth elements), and the potential impact of wildfire on global geochemical cycles.

Mobility and fractionation of rare earth elements during black shale weathering: Implications from acid rock drainage and sequential extraction study

Black shale is a type of sedimentary rocks that are enriched in rare earth elements (REEs). It is of both economic importance and environmental significance to understand REE mobility during black shale weathering. The present study approaches to this by analysing REEs in acid rock drainage (ARD) from black shale weathering system, fresh and weathered black shales, soils derived from black shales, and sequential extractants from black shales at Dongping town in Hunan province (China). Results showed that REEs had variable high concentrations in ARD as shown by total REE + Y (∑REY) concentrations from 162 to 4074 (μg/L). REEs in ARD displayed hat-shape NASC-normalized patterns with significant enrichments of middle REEs (MREE) relative to light REEs (LREE) and heavy REEs (HREE), and had significant negative Ce (Ce/Ce⁎ = 0.6) and positive Y (Y/Y⁎ = 1.5) anomalies. MREE enrichment in ARD could be evaluated using MREE/MREE⁎ values, which varied from 1.43 to 1.81 with a mean of 1.65, distinctly higher than those of whole rocks (around 1.0). 1 M HCl extraction results suggested that REEs were integratedly mobilized during shale weathering, while six-step extraction studies identified that REEs in ARD resulted from exchangeable and Fe-oxide fractions with MREE and HREE enrichment in shales respectively. MREE in exchangeable and HREE in Fe-oxide fractions were preferentially released during weathering, as illustrated by plots of MREE/MREE⁎ against HREE/LREE ratios of ARD and six-step extractants. Therefore, geochemical processes for REE mobility during black shale weathering included integrated mobilization and preferential release. Integrated REE mobilization resulted from the dissolution of REE-bearing minerals and oxidation of sulfides. Preferential REE release resulted from acid fluids produced by sulfide oxidation during weathering. Thus, a new model was proposed for interpreting geochemical processes of REE mobility during black shale weathering, and for understanding REE distribution in ARD from natural and anthropogenic systems.

Are nano-colloids controlling rare earth elements mobility or is it the opposite? Insight from A4F-UV-QQQ-ICP-MS

Rare earth element (REE) mobility in the environment is expected to be controlled by colloids. Recent research has detailed the structure of iron-organic colloids (Fe-OM colloids), which include both large colloids and smaller nano-colloids. To assess how these nano-colloids affect REE mobility, their interactions with REE and calcium (Ca) were investigated at pH 4 and 6. Using Asymmetric Flow Field Flow Fractionation (A4F) combined with UV and Triple Quadrupole Inductively Coupled Plasma Mass Spectrometry (QQQ-ICP-MS), Fe-OM nano-colloids were separated from bulk Fe-OM colloids and their REE and Ca content were analyzed. Without REE and Ca, nano-colloids had an average diameter of approximately 25 nm. Their structure is pH-dependent, with aggregation increasing as pH decreases. At high REE loadings (REE/Fe ≥ 0.05), REE induced a size increase of nano-colloids, regardless of pH. Heavy REE (HREE), with their high affinity for organic matter, formed strong complexes with Fe-OM colloids, resulting in large aggregates. In contrast, light REE (LREE), which bind less strongly to organic molecules, were associated with the smallest nano-colloids. Low REE loading did not cause noticeable fractionation. Calcium further enhanced the aggregation process at both pH levels by neutralizing the charges on nano-colloids. These findings indicate that REE can act as aggregating agent controlling their own mobility, and regulating colloid transfer.

Experimental evidence reveals the mobilization and mineralization processes of rare earth elements in carbonatites.

Whereas the genesis of carbonatitic rare earth element (REE) deposits has long been a focus of study, the controls on mobilization and mineralization of REEs during magmatic-hydrothermal processes still remain open to debate. Here, we present our investigation of the dissolution and crystallization of REE (fluor)carbonate minerals in alkaline carbonate brine-melts up to 850°C and 11.6 kbar. Our results show that REEs are soluble in Na2CO3 brine-melts, achieving concentrations exceeding 8 weight % at temperatures above 650°C. The addition of calcium and/or fluoride has minimal impact on REE mobilization, whereas introduction of silica suppresses REE solubilities by half, due to britholite formation above 550°C. Upon cooling, sodium and REEs combine to crystallize in burbankite or carbocernaite in sodium-enriched brine-melts, even at fluoride saturation. However, while the brine-melts contain substantial ferro- or aluminosilicate, REE mineralization in fluorcarbonates occurs after sufficient sodium precipitation in alkaline silicate minerals, hence revealing how silicate and sodium carbonate govern REE mineralization.

Mobilization of rare earth elements during allanite-fluid interaction: Insights into formation of regolith-hosted rare earth element deposits in South China

Mobilization of rare earth elements during allanite-fluid interaction: Insights into formation of regolith-hosted rare earth element deposits in South China

The Global Biogeochemical Cycle of the Rare Earth Elements

AbstractTo improve our understanding and guide future studies and applications, we review the biogeochemistry of the rare earth elements (REE). The REEs, which form a chemically uniform group due to their nearly identical physicochemical properties, include the lanthanide series elements plus scandium (Sc) and yttrium (Y). These elements, in conjunction with the neodymium isotopes, are powerful tools for understanding key oceanic, terrestrial, biological and even anthropogenic processes. Furthermore, their unique properties render them essential for various technological processes and products. Here, we delve into the characteristics of REE biogeochemistry and discuss normalization procedures and REE anomalies. We also examine the aqueous speciation of REEs, contributing to a better understanding of their behavior in aquatic settings, including the role of neodymium isotopes. We then focus on their environmental distribution, fractionation, and controlling processes in different environmental systems across the land‐ocean continuum. In addition, we analyze sinks, sources, and the mobility of REEs, providing insights into their behavior in these environments. We further investigate the sources of anthropogenic REEs and their bioavailability, bioaccumulation, and transfer along food webs. We also explore the potential effects of climate change on the cycling, mobility and bioavailability of REEs, underlining the importance of current research in this evolving field. In summary, we provide a comprehensive review of REE behavior in the environment, from their properties and roles to their distribution and anthropogenic impacts, offering valuable insights and pinpointing key knowledge gaps.

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.

Enrichment and fractionation of rare earth elements (REEs) in ion-adsorption-type REE deposits: Constraints of an iron (hydr)oxide–clay mineral composite

Abstract Ion-adsorption-type rare earth element (REE) deposits are the source of more than 90% of global heavy REEs (HREEs). Thus, understanding the ore genesis of REEs, particularly the distribution characteristics and enrichment mechanisms of HREEs, is vital for efficient exploration and mining of ion-adsorption-type REE deposits worldwide. The characteristics and petrogenesis of bedrock, and the aqueous mobility of REEs, are known to be important factors controlling REE accumulation and fractionation in the weathering crust of REE deposits. However, the effect of REE adsorption on secondary minerals, which is a crucial step in deposit formation, remains poorly understood. This problem was addressed by the study described herein, which involved a systematic analysis of the complete weathering profile (78 m) of the Renju ion-adsorption-type REE deposit in South China and a simulated adsorption experiment. Clay minerals and iron (Fe) (hydr)oxides are the dominant REE adsorbents in the weathering crust and most are micro-to-nanosized particles. Thus, the fine particle fraction (< 2 μm) was separated from field samples, to better disclose their effects on the concentration and redistribution of REEs. Phase compositions and morphologies were characterized by X-ray diffraction, Mössbauer spectrometry, and scanning/transmission electron microscopy (SEM/TEM), which revealed that various clay minerals and Fe (hydr)oxides form composites along the profile of the deposit. Composites of ferrihydrite–kaolinite, goethite–kaolinite/halloysite, and hematite–kaolinite/halloysite were found to be distributed in the semi-weathered, completely weathered, and topsoil layers, respectively, with different sizes and shapes. The concentrations and partition patterns of REEs in different occurrence states were distinguished after sequential extraction. Ion-exchangeable-REEs were the major state and enriched in the upper completely weathered layer. These species were found to be adsorbed onto kaolinite and halloysite via electrostatic attraction, without obvious fractionation. Fe (hydr)oxides were determined to comprise ca. 20% of REEs at most depths and over 50% of REEs in the topsoil and semi-weathered layer. It was found that Fe (hydr)oxides scavenge REEs through complexation and oxidation, resulting in HREE enrichment and a positive cerium (Ce) anomaly, respectively. In addition, compared with crystalline Fe (hydr)oxides, amorphous Fe (hydr)oxides immobilize more REEs but exhibit weaker preferential adsorption of HREEs. The above-described findings are consistent with the results of simulated experiments for REE adsorption onto a clay mineral–Fe (hydr)oxide composite (Bt-60), which was obtained from hydrothermal processing of biotite. Furthermore, the distributions and stabilities of LREEs and HREEs were distinguished by TEM–energy–dispersive spectroscopy (EDS) of Bt-60 before and after REE extraction by ammonium sulfate. The phase transformation pathways of clay minerals and Fe (hydr)oxides, and their different enrichment and fractionation characteristics in REEs, were also discussed in terms of the structure and surface properties of minerals, adsorption mechanisms, and variations in chemical properties across the REE group. The results shed new light on how clay minerals and Fe (hydr)oxides affect the enrichment and fractionation of REEs in ion-adsorption-type deposits.

Origin of rare earth elements in acid mine drainage traced by strontium and neodymium isotopes

Acid mine drainage (AMD) generated from coal and sulfide mining has attracted much interest due to the high concentrations of rare earth elements (REE) observed. However, the origin of REE in AMD and particularly the mechanism of enrichment in medium REE remain uncertain. The combined study of Sr and Nd isotopes can be used to trace the processes that control the mobility and fractionation of REE in aquatic systems such as AMD. This work reports for the first time worldwide isotopic data of Sr and Nd in AMDs from sulfide mining (Iberian Pyrite Belt, SW Spain) and compare with those in host rocks. Additionally, leaching experiments have been carried out to simulate the water–rock interactions found in these acidic systems. The results obtained strongly suggest that the origin of REE in AMDs from metal mining may be related to the preferential dissolution of shales and felsic rocks. The suitability of Sr and Nd isotopic signatures to trace the REE source in AMD-affected systems has been proven, it provides a new environmental and geochemical prospecting tool which could be useful in different geological water–rock interaction systems.

The impact of global climate changes on trace and rare earth elements mobilization in emerging periglacial terrains: Insights from western shore of Admiralty Bay (King George Island, Antarctic)

In the rapidly changing climate, the biogeochemical behaviours of trace elements and Rare Earth Elements (REEs) in emerging periglacial environments assumes profound importance. This study provides pivotal insights into this dynamic by investigating the Antarctic's response to global climate change. The bedrock of King George Island is rich in REEs, with the presence of trace metals (TEs), with the highest concentrations of metals found in ornithogenic soil (∑REE 84.01–85.53 mg∙kg−1 dry weight). REEs in the studied soil, found mainly in igneous rocks, as is indicated by the positive correlation of these elements with sodium and calcium. The TEs released as a result of weathering are leached by water flowing down local watercourses to Admiralty Bay, as indicated by the decreasing results of ∑REE = 11.59 μg∙dm−3 in watercourse water, ∑REE = 1.62 μg∙dm−3 in watercourse pools and ∑REE = 0.66 μg∙dm−3 in the water of Admiralty Bay at the outlet of the watercourse. Water originating from the melting of snow on the glacier also carried REEs (∑REE = 0.14 μg∙dm−3), a fact which suggest the further influx of these elements from atmospheric deposition. The Prasiola crispa turned out to be the most susceptible to the accumulation of REEs (∑ 80.73 ± 5.05 μg g−1) and TEs, with the exception of chromium and zinc, whose concentrations were found to be at their highest in Deschampsia antarctica. In Usnea antarctica, Xanthoria candelaria, and Ceratodon purpureus and Politrichastrum alpinum, a dominant role in the accumulation of REEs was played by HREEs. The determined enrichment factor (EF) indicates that the soil cover is a source of REEs (EFAlgae for ∑REE = 5.07; EFLichen for ∑REE = 6.65; EFBryophyta for ∑REE = 5.04; EFVascular for ∑REE = 4.38), while Ni, As and Pb accumulated in plants may originate from other sources than the soil.

Mobility of Rare Earth Elements in Coastal Aquifer Materials under Fresh and Brackish Water Conditions.

The indispensable role of rare earth elements (REEs) in manufacturing high-tech products and developing various technologies has resulted in a surge in REE extraction and processing. The latter, in turn, intensifies the release of anthropogenic REEs into the environment, particularly in the groundwater system. REE contamination in coastal aquifer systems, which serve as drinking and domestic water sources for large populations, demands a thorough understanding of the mechanisms that govern REE transport and retention in these environments. In this study, we conducted batch and column experiments using five representative coastal aquifer materials and an acid-wash sand sample as a benchmark. These experiments were conducted by adding humic acid (HA) to the REE solution under fresh and brackish water conditions using NaCl, representing different groundwater compositions in coastal aquifers. The REEs were shown to be most mobile in the acid-wash sand and natural sand samples, followed by two types of low-carbonate calcareous sandstone and one type of high-calcareous sandstone and the least mobile in red loamy sand. The mobility of REEs, found in solution primarily as REE-HA complexes, was controlled mainly by the retention of HA, which increases with increasing ionic strength and surface area of the aquifer material. Furthermore, it was found that the presence of carbonate and clay minerals reduces the REE mobility due to enhanced surface interactions. The higher recoveries of middle-REE (MREE) in the column experiment effluents observed for the acid-wash sand and natural sand samples were due to the higher stabilization of MREE-HA complexes compared to light-REE (LREE) and heavy-REE (HREE) HA complexes. Higher HREE recoveries were observed for the calcareous sandstones due to the preferred complexation of HREE with carbonate ions and for the red loamy sand due to the preferred retention of LREE and MREE by clay, iron, and manganese minerals.

Can rare earth elements be recovered from abandoned mine tailings by means of electrokinetic-assisted phytoextraction?

Given the high impact of traditional mining, the recovery of rare earth elements (REEs) from hazardous waste materials could become an option for the future in accordance with the principles of the circular economy. In this work, the technical feasibility of REEs recovery from metal mine tailings has been explored using electrokinetic-assisted phytoremediation with ryegrass (Lolium perenne L.). Phytoextraction combined with both AC current and DC current with reversal polarity was applied (1 V cm−1, 8 h day−1) to real mine tailings containing a total concentration of REEs (Sc, Y, La, Ce, Pr, and Nd) of around 146 mg kg−1. Changes in REEs geochemical fractionation and their concentrations in the soil pore water showed the mobilization of REEs caused by plants and electric current; REE availability was increased to a higher extent for combined electrokinetic-assisted phytoextraction treatments showing the relevant role of plants in the process. Our results demonstrated the initial hypothesis that it is feasible to recover REEs from real metal mining waste by phytoextraction and that the performance of this technology can be significantly improved by applying electric current, especially of the AC type, which increased REE accumulation in ryegrass in the range 57–68% as compared to that of the treatment without electric field application.

Early diagenetic control on the enrichment and mobilization of rare earth elements and transition metals in buried ferromanganese crust

Ferromanganese crusts on seamounts represent a major reservoir of various critical metal elements (e.g., rare earth elements and transition metals) and hold substantial economic value. However, current research primarily focuses on crusts that are exposed to seawater, and little is known about the resource potential of buried crusts in sediments and the impact of early diagenesis on buried crusts. Here, we present in-situ high-resolution major and trace element analyses of a buried crust from the western Pacific Ocean to unravel how rare earth elements and transition metals are enriched and mobilized as burial. The elemental profiles of the buried crust indicate that as the crust is buried, dissolved nickel (Ni) and copper (Cu) are gradually released from it into the porewater. Manganese and cobalt (Co) contents of the buried crust remain relatively constant, while rare earth elements are incorporated into the buried crust. Collectively, the early diagenesis leads to the release of Ni and Cu into the porewater, whereas it facilitates the fixation of rare earth elements within the buried crust. Given the widespread distribution of buried crusts in modern oceans, they represent a potential resource for future economic extraction of critical metals, particularly rare earth elements and Co.

Mobilization of rare earth elements during extreme weathering of basalt

To investigate the mobility of elements during extreme weathering of basalts, major and trace elements concentrations along with the mineralogical abundance of weathered products, are reported for lateritic profiles developed over the ~118.1 ± 0.3 Ma old Rajmahal Trap basalts in Eastern India. Special emphasis has been given to studying the re-distribution of rare earth elements during extreme chemical weathering of relatively less explored Rajmahal flood basalt, which results in the development of lateritic profiles. Here, we conduct a systematic attempt to decipher the alteration of the composition of laterite developed over the Rajmahal flood basalt.We demonstrate that the chemical index of alteration (CIA) and the mafic index of alteration (MIA) are less effective in deciphering extreme chemical weathering events, where desilication dominates the weathering process. In the weathered residues, the retention of REEs generally follows the trend of light rare earth elements (LREE) > middle rare earth elements (MREE) > heavy rare earth elements (HREE). Except for La and Ce, all other REEs show a depleting trend with reference to the parent rock composition. The ∑REE did not show any significant relationship with the commonly used weathering indices, which shows that the retention of REEs is not a function of the extent of weathering. The covariation of Ce anomaly with the Fe and Mn distribution in the samples reflects a redox-controlled mechanism. The negative correlation between the Eu anomaly and kaolinite abundance suggests that Eu mobilization from parent rock happens during initial weathering phases when plagioclase weathering dominates. The samples show an interesting positive Gd anomaly, which is explained by enhanced retention of Gd than neighboring Eu and Tb. During the chemical weathering of basalt, the retention of Ho was found to be greater than its geochemical twin Y. The results of this study show that significant differences exist between the REEs' distribution of parent basalt and that of the weathered residue, which has implications for the application of REEs as a provenance indicator and/or weathering proxy. The enrichment of Th and U in the weathered samples is explained through the addition of aeolian dust of upper continental crustal composition.

Enhancing La(III) biosorption and biomineralization with Micromonospora saelicesensis: Involvement of phosphorus and formation of monazite nano-minerals

The release of rare earth elements (REEs) from mining wastes and their applications has significant environmental implications, necessitating the development of effective prevention and reclamation strategies. The mobility of REEs in groundwater due to microorganisms has garnered considerable attention. In this study, a La(III) resistant actinobacterium, Micromonospora saelicesensis KLBMP 9669, was isolated from REE enrichment soil in GuiZhou, China, and evaluated for its ability to adsorb and biomineralize La(III). The findings demonstrated that M. saelicesensis KLBMP 9669 immobilized La(III) through the physical and chemical interactions, with immobilization being influenced by the initial La(III) concentration, biomass, and pH. The adsorption kinetics followed a pseudo-second-order rate model, and the adsorption isotherm conformed to the Langmuir model. La(III) adsorption capacity of this strain was 90 mg/g, and removal rate was 94 %. Scanning electron microscope (SEM) coupled with energy dispersive X-ray spectrometer (EDS) analysis revealed the coexistence of La(III) with C, N, O, and P. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) investigations further indicated that carboxyl, amino, carbonyl, and phosphate groups on the mycelial surface may participate in lanthanum adsorption. Transmission electron microscopy (TEM) revealed that La(III) accumulation throughout the M. saelicesensis KLBMP 9669, with some granular deposits on the mycelial surface. Selected area electron diffraction (SAED) confirmed the presence of LaPO4 crystals on the M. saelicesensis KLBMP 9669 biomass after a prolonged period of La(III) accumulation. This post-sorption nano-crystallization on the M. saelicesensis KLBMP 9669 mycelial surface is expected to play a crucial role in limiting the bioimmobilization of REEs in geological repositories.

Structure and mobility of rare earth ions in interlayer space of montmorillonite: a molecular dynamics study.

Ion adsorption-type deposits (IADs) are the dominant sources of rare earth elements (REEs), in which REEs are mainly enriched in clay minerals. However, the adsorption mechanism of REEs in the interlayer region of clay minerals is still poorly understood. In this study, by using molecular dynamics (MD) simulations, we explored the interlayer structures and dynamics of REEs-intercalated montmorillonite. La3+ and Lu3+ were used as the model cations for light REEs (LREEs) and heavy REEs (HREEs), respectively. It was found that the most thermodynamically stable state for both LREE- and HREE-montmorillonite was the double-hydration state and the corresponding basal spacing was calculated to be ∼16.1 Å. REE ions are located at the middle plane of the interlayer space and adsorbed on the montmorillonite basal surface through hydrogen bonds between its coordination water and the basal oxygens (i.e. as outer-sphere complexes). La3+ was 9-fold coordinated in the interlayer space with a mono-capped square antiprism coordination shell, while Lu3+ was 8-fold coordinated in a square antiprism cage. The mobility of REEs intercalated in the interlayer was significantly reduced compared to the mobility of REEs in aqueous solutions. The microscopic structures, thermodynamic data, and mobility obtained in the present study can help understand the enrichment and mobilization of REEs in IADs, and provide a molecular level basis for developing more efficient extraction techniques.

UV-Vis spectrophotometric determination of rare earth elements (REE) speciation at near-neutral to alkaline pH. Part I: m-cresol purple properties from 25-75 °C and Er hydrolysis.

The speciation and mobility of rare earth elements (REE) strongly depends on pH, which controls the formation of charged aqueous hydroxyl species. The latter potentially play an important role in controlling heavy REE adsorption on clay minerals in near-neutral to alkaline waters such as in regolith-hosted REE mineral deposits. However, accurate REE hydrolysis constants are needed for developing geochemical models that can predict the role of these charged species in natural systems. Here, we develop a robust experimental UV-Vis spectrophotometric method using m-cresol purple to determine in situ pH from 25 to 75 °C. This method is used to derive the average ligand number and hydrolysis constants of erbium (Er) at 25 °C in aqueous solutions with low ionic strength (≤0.001 mol L-1) at pH from ∼7 to 9.5 and in the presence of Er concentrations from 0 to 0.057 mM. The average ligand number ranges between 1 and 3 indicating that Er(OH)2+, Er(OH)2+ and Er(OH)03 control speciation in the experiments. The logarithm of the Er hydrolysis constants (, n = 1 to 3) derived at infinite dilution for the reaction Er3+ + nH2O = Er(OH)n3-n + nH+ are: , , . Implementation of these experimental data into a geochemical model indicates that the Er(OH)2+ and Er(OH)03 species are both stable in a much wider pH range than previously predicted. Consequently, the positively charged REE hydroxyl complexes can potentially control the fractionation of light vs. heavy REE via adsorption as observed in the formation of certain regolith-hosted REE deposits.

Controls on rapid rare earth element enrichment in sediments deposited by a continental-scale river system

Clay-hosted rare earth element (REE) deposits, generally considered products of extended subtropical weathering of REE-rich igneous protoliths, are a critical source of REEs globally.The Koppamurra deposit of southeast South Australia, in contrast, is developed in Pleistocene lacustrine clays overlying a REE-poor marine limestone. Here we examine the REE + Y (REY) sources and enrichment mechanisms of this unusual setting. We find that the greatest REY enrichment (up to 5000 ppm) occurs in lacustrine clays at the base of a ∼1–2 m thick contact zone with the underlying limestone. This zone features an increased proportion of leachable REYs (>50 %), increased smectite abundance and a sharp increase in pH from ∼5.0 to ∼8.3. Co-variation of HREEs with a suite of clay-specific elements demonstrates that HREEs are hosted mainly by Fe-rich smectite, with separate sorbed vs structurally bound HREE pools that are of similar magnitude. LREEs are mainly hosted in secondary LREE carbonates, these are preferentially intergrown with smectite but are only rarely present in kaolinite domains. Cerium occurs mainly as cerianite. The narrow range and overlapping εNd of samples within and above the REY enriched zone suggests that enrichment is related to internal redistribution within the clay unit. The clays are isotopically incompatible with local igneous or metasedimentary rocks, but 87Sr/86Sr and εNd isotopic fingerprinting shows that they were deposited by the Murray River, Australia’s largest river system. Murray sediments are enriched in REY due to the presence of material eroded from the Lachlan Orogen granites of southeast Australia. Deposition of these sediments occurred in a coastal lacustrine setting analogous to the modern-day Coorong, with tectonic uplift and shoreline progradation allowing the onset of subaerial weathering ∼1–2 Mya. We propose that vertical and lateral solute transport allowed for the concentration of REEs in local depressions in the karst-weathering surface which separates the limestone and overlying clay, facilitated by water table oscillation and pore water redox cycling driven by seasonally contrasted rainfall. Poor soil drainage, evaporative concentration of soil waters and alkaline pH adjacent to the limestone immobilized REEs in secondary carbonates and Fe-rich smectite. Our findings illustrate that REE mobilization and enrichment in clay deposits can occur more rapidly, over a larger area and in a broader range of host phases and geologic settings than is currently appreciated, with important implications not only for REE exploration but also for the use of REEs as tracers in the Earth Sciences.

U-Th-Pb dating, trace elements, and Sr-Nd isotopes of monazite and allanite as recorders for multi-stage rare earth element mineralization and remobilization in carbonatite dike systems

Carbonatite hosts the most important rare earth resources in the world, but the precise timing, ore-forming history, and mechanism of rare earth mineralization in carbonatite systems are still in debate. Here, we report a rare corona texture of monazite-allanite-fluorapatite from the Huangjiagou carbonatite in the Lesser Qinling of central China, and demonstrate that the U-Th-Pb dating, trace elements, and Sr-Nd isotopes of these minerals in the corona are useful tools to unravel multiple-stage events for rare earth element (REE) mineralization and mobilization. The first mineralization event took place at ca. 219 Ma as revealed by the monazite U-Pb age, the same as regional carbonatite forming ages, but the Th-Pb age has been disturbed, which shows a negative correlation with Th contents. The second mineralization event occurred at ca. 128 Ma, as revealed by in situ U-Pb dating of allanite, coeval with the intrusions of neighboring I-type granite. The initial Sr-Nd isotope ratios of allanite show a downtrend from the center to the rim of monazite-allanite-apatite coronas to approach the ratios of neighboring granite, indicating an increasing effect by the metasomatism of magmatic-hydrothermal fluids during the growth of these REE-mineral coronas. Therefore, a two-episode REE mineralization was recognized with the replacement of ca. 219 Ma monazite by ca. 128 Ma allanite-apatite coronas on the function of magmatic-hydrothermal fluid metasomatism, and this process accompanies the disturbance of Th/Pb geochronology in monazite. Allanite as the product of monazite dissolution can represent the later-stage REE mineralization tracing the REE reworking processes under the hydrothermal conditions in carbonatite systems. Our study highlights the implication of monazite-allanite-fluorapatite coronas on the REE remobilization and mineralization in carbonatite systems.