Behavior Of Rare Earth Elements Research Articles

Exotic REE behaviors of zircon in the Koktokay No. 3 granitic pegmatite, Xinjiang, northwest China

Rare earth elements (REEs) can be incorporated into zircon at significant concentrations that are controlled by the temperature, pressure, and composition of their growth environment. However, the distribution of REEs between zircon and melt does not always conform to the currently available partitioning data. This study presents evidence of exotic REE behaviors in zircon from the marginal zone of the Koktokay No. 3 granitic pegmatite in Xinjiang, northwest China. The behavior of the Koktokay REEs represents by large negative Ce anomalies, flat heavy REE patterns, and lanthanide tetrad effects. Two generations of zircon (Zrn-I and Zrn-II) are identified, which were formed by dissolution and re-precipitation processes. Zrn-I represents relic zircon domains with dissolution textures that formed from pegmatite melt during the late stage of the magmatic–hydrothermal transition. Zrn-II occurs as overgrowths and infilling fractures around or within Zrn-I, which formed by hydrothermal re-precipitation. From Zrn-I to Zrn-II, the zircon exhibits increasingly negative Ce anomalies and greater enrichment in light REEs and other non-stochiometric elements (e.g., Ca, Al, Fe, and Mg). Given the lack of Ce depletion in the host granite, the negative Ce anomalies in zircon can be attributed to Ce being sequestered into hydrothermal minerals (i.e., cerianite and/or Fe–Mn oxyhydroxides) as tetravalent Ce. In addition, the flat heavy REE patterns of both Zrn-I and Zrn-II likely resulted from the crystallization of heavy REE-enriched minerals, such as zircon and garnet, and were further enhanced by hydrothermal alteration. The two generations of zircon display identical M-type lanthanide tetrad effects in REE patterns, with similar TE3 values. This ubiquitous M-type tetrad effect of the REEs in garnet, zircon, and the host granite can be attributed to the exsolution and subsequent interaction with magmatically derived fluids containing F–aquo-complexes. Therefore, we conclude that such anomalous REE incorporation into zircon from a hydrous pegmatitic melt is controlled by the evolution of the hydrous melt and dissolution–re-precipitation processes. Our results suggest that the reliability of zircon REE partitioning data should be revaluated, and that further experimental studies are needed to fully understand the behavior of REEs in hydrothermal systems.

Segregation of rare earth elements in the melting process for injection casting of U–Zr metallic fuels

We investigated the segregation behavior of rare earth elements (RE = La, Ce, Pr, and Nd) in the injection casting process of U–Zr metallic fuel using the characterization methods such as density measurement, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and inductively coupled plasma−atomic emission spectroscopy. Injection casting of the metallic fuels with nominal compositions of U–10Zr−xRE (in wt%, x = 5, 7, and 10) resulted in the fuel slugs exhibiting similar compositions regardless of the RE content charged in the melting crucible. The microstructure observations revealed that RE-segregated layer occupied the upper region of the melt-residue in case of injection casting of U–10Zr−7RE and U–10Zr−10RE while the melt-residue of U–10Zr−5RE was relatively homogenous in terms of RE distribution. The segregation was caused by the immiscibility of RE in U–Zr alloys. When the RE content was higher than 5 wt%, the thickness of the segregation layer increased with the amount of the charged RE. RE inclusions migrated toward the upper region of the molten alloy owing to the different densities and specific gravities as compared to that of the U–Zr alloy, which led to the non-uniformity in the composition of the melt-residue after injection casting of U–10Zr−7RE and U–10Zr−10RE. The migration behavior of RE during melting was suggested to be induced by the fractional crystallization owing to the immiscibility and the difference in solidification temperature.

Sources and geochemical behaviors of rare earth elements in suspended particulate matter in a wet-dry tropical river

The processes of rock weathering and soil erosion, and hydrochemical characteristics are significantly affected by the climate in a basin. However, the sources of rare earth elements (REEs) in suspended particulate matter (SPM) under soil erosion, as well as the geochemical behaviors of REEs with changes in hydrochemical properties between seasons, have received little attention in the tropical monsoon zone. In this study, the temporal and spatial characteristics of the REEs in SPM were investigated in the Mun River (a wet-dry tropical river), Northeast Thailand. During the dry season, the compositions of the major elements and REEs in SPM were very similar to those in local soils. However, there was a clear difference between the compositions of these major elements and REEs in SPM and those in local soils during the rainy season. This suggests that the SPM and its REEs during the dry season were primarily derived from soil materials, while those during the rainy season were primarily derived from soil materials and products of rock weathering. The ∑REE contents in SPM decreased from 191.2 mg kg−1 to 170.6 mg kg−1 along the flow direction during the dry season, while they increased from 100.7 mg kg−1 to 135.3 mg kg−1 during the rainy season. The δEu (mean 1.26) and δGd (mean 1.58) values in SPM during the rainy season were higher than those (mean δEu 1.21 and mean δGd 1.12) during the dry season, and both of them were mainly controlled by the relative contributions of rock weathering products and soil materials to SPM. The results suggest that the temporal differences of REE geochemical characteristics in SPM were closely associated with SPM sources, while their spatial variations were mainly affected by the water-particle interaction in the tropical monsoon zone.

Origin and distribution of rare earth elements (REEs) in the soils of Meizhou City, southern China with high abundance of regolith-hosted REEs

Currently, regolith-hosted rare earth element (REE) deposits in South China are the major source of global REEs, particularly heavy REEs (HREEs). This type of REE resource contains abundant movable, bioavailable, and ion-exchangeable REEs and may act as an environmental hazard; however, little is known regarding REEs in soils from areas with abundant deposits. Here, we investigated the REEs in the soil, surface water and crops, collected from Meizhou City, Guangdong Province, South China, as a representative region with abundant regolith-hosted REEs. Mean REE concentration (ΣREE) in the soil (318 μg g−1) exceeded the local soil background (169 μg g−1). The LREE/HREE ratio ranged from 2.04 to 69.4 in soil samples, suggesting the enrichment of light REEs (LREEs) compared with HREEs. Most soil samples exhibited positive Ce anomalies and negative Eu anomalies. The highest ΣREE (1162 μg g−1) was detected in forest land, followed by garden land (793 μg g−1) and paddy field (519 μg g−1). According to the correlation analysis, pH as well as soil organic matter (SOM), Fe, and Mn content contributed to REE enrichment, while SOM and Mn content affected REE fractionation. REE enrichment in soil may be related to natural sources (e.g., parent materials) and anthropogenic activities (e.g., fertilizer application and coal combustion). The REE contamination of surface water was not serious. However, ΣREE in crops (7.08 μg g−1·dw) indicated that these elements migrate from soils to plants, producing potential adverse effects on crops and humans. These findings shed new light on the geochemical behavior of REEs in the environments with abundant regolith-hosted REEs and can serve as a reference to prevent and control REE contamination.

Mineralogy and Geochemistry of Clay in Sokor and Jeli, Kelantan.

Clay is considered as fine-grained, natural and earthy argillaceous material which are widely distributed in sediments as the products of sedimentation and diagenesis. The sample of rocks and clay soil from Sokor and several areas in Jeli, Kelantan have been analysed for geochemical and mineralogical properties. For geochemical analysis, the concentration of major elements was analysed using X-Ray Diffraction Fluorescene (XRF) and X-Ray Diffraction (XRD) Spectroscopy in order to validate the clay mineral existence. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) also was used for trace element determination while the Scanning Electron Microscopy (SEM) were used to study the mineralogy of clay as well as petrography. From mineralogy and petrography studies using optical microscope and Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS) analysis show that the mineral distribution in the sample from both Sokor and Jeli area dominantly by quartz, biotite and K-feldspar mineral. It is supported by XRD analysis which showing the prominent peaks of quartz on 27Å and kaolinite on 12Å from the graph is easily identified. Chondrite normalise pattern is generated from ICP-MS result to constrain rare earth element (REE) behaviour and geochemical information. The normalised fractionation trends recorded below than usual REE deposition. This is because there is no mineral association of REE such as apatite and titanite found from the sample.

Competitive extraction and stripping behaviors of rare earth elements from industrial wet-process phosphoric acid using di-(2-ethyl-hexyl) phosphoric acid solvent: Optimization and thermodynamic studies

Rare earth elements (REEs) have gained a great amount of attention in the last decades, as they become indispensable and ubiquitous owing to their versatile applications in modern world and high technologies. The main aim of this work is to study the extraction behavior of REEs from the wet process phosphoric acid (WPPA) using Di-(2-ethylhexyl) phosphoric acid (D2EHPA) with a focus on yttrium (Y) and scandium (Sc) as the major REEs. The extraction and stripping operating conditions of REEs from WPPA were optimized, including the extractant concentration, the O:A ratio (organic phase / aqueous phase), the contact time, the residence time and stirring speed. Single-stage extraction results revealed an extraction efficiency of 58 and 56 % for ΣREEs and Sc, respectively.Besides, it was demonstrated that D2EHPA shows a high affinity towards yttrium (Y) with high extraction of 91 % under the optimized conditions of 2 M of D2EHPA, O:A = 1, 600 rpm during 20 min. The McCabe–Thiele diagrams were established to specify the required theoretical number of extraction stages of ΣREEs. It was revealed that four stages are required for ΣREEs extraction, two stages for Sc and a single extraction stage for Y at an O:A ratio of 1. The best extraction efficiencies were obtained for a total percentage of 97, 87 and 90 % for ΣREEs, Sc and Y, respectively. The stripping was successfully achieved using H2SO4 solution with efficiencies of 73 and 90 % for ΣREEs and Y, respectively. Finally, an integrated process for the recovery of REEs from WPPA was proposed.

Effects of Heating on the Binding of Rare Earth Elements to Humic Acids

In deep underground environments, temperature is one of the key factors affecting the geochemistry behaviors of rare earth elements (REE) in organic-rich fluid. However, the influence of temperature on the interaction between humic acids (HA) and REE is not well known. In the present study, the influence of temperature on the HA–REE-binding behavior was evaluated based on heating experiments of REE-doped HA solution. Lignite-extracted HA and REE-binding experiments were conducted over a temperature range of 20 to 200 °C to quantify HA–REE complexation and the influence of temperature on HA binding sites. Results showed that increasing temperature and decreasing [REE]/[HA] ratio cause an increase of Kd value (the partition coefficient of REE between HA and aqueous solution). During heating KdREE KdREE patterns gradually change from middle REE-enriched-type (M-type) at 20 °C to light and middle REE-enriched-type (L-M-type) at 50 and 100 °C, and to light REE-enriched-type (L-type) at 150 °C and 200 °C. The increase of REE bonded with HA and modifications of KdREE patterns during the thermal treatment may be attributed to the increase of REE-binding sites, especially carboxylic sites, as a consequent of HA decomposition. This study provides a glimpse into the HA–REE-binding behaviors in the deep underground environment, which may shed light on the geochemical characteristics of REE in some organic-bearing rocks, and their changes during the coalification process.

Rare Earth Elements behaviour in different types of mineral waters within Baikal Lake

The current study contains a review of the REE abundance and distribution in thermal and cold mineral waters which are widely spread around Baikal Lake. Studied mineral waters were divided into CH4, N2 and CO2-rich waters by the associated gas composition. We found that the highest REE concentrations were connected with thermal waters that were accumulated in swamps (slow-moving waters). Another core process for accumulating of REE in waters is the rock weathering in CO2 presence. This study shows that there are no correlations between REE and main ions, Eh, TDS and temperature. But dependence of REE on pH and positive correlations of REE with Al, P, Mn, V, Cu, Pb and As, which indicate common processes of microelements migration, were established. Within the study waters the light REE concentrations dominate over heavy REE. The presence of CO2 gas increases heavy REE concentrations, and concentrations of several light elements (La, Ce, Nd). The observed differences in REE contents in studied waters can be provenance dependent and possibly relate to more complicated evolutionary processes as water circulation time, depth and mixing. This is the first investigation of REE distribution and abundance in mineral waters of the Baikal Lake.

Distribution, source and behavior of rare earth elements in surface water and sediments in a subtropical freshwater lake influenced by human activities

As tracers, rare earth elements (REEs) can reflect the influence of human activities on the environmental changes in aquatic systems. To reveal the geochemical behavior of REEs in a water–sediment system influenced by human activities, the contents of REEs in the surface water and sediment in the Chaohu Lake Basin were measured by inductively coupled plasma mass spectrometry (ICP–MS). The results show that the ΣREE contents in the surface water are 0.10–0.850 μg L−1, the ΣREE contents in the sediments are 71.14–210.01 μg g−1, and the average contents are 0.24 μg L−1 and 126.72 μg g−1, respectively. Almost all water and sediment samples have obvious light REE (LREE) enrichment, which is the result of the input of LREE-rich substances released by natural processes and human activities (industrial and agricultural production). Under the alkaline water quality conditions of Chaohu Lake, REEs (especially LREEs) are easily removed from water by adsorption/coprecipitation reactions with suspended colloidal particles, which leads to the enrichment of LREEs in sediments. The Ce anomaly of the water–sediment system is related to the oxidation environment, while the Eu anomaly is related to the plagioclase crystallization. Significant Gd anomalies was observed in the downstream of rivers flowing through urban areas, which was related to the anthropogenic Gd wastewater discharged by hospitals. The ∑REE–δEu and provenance index (PI) discrimination results are consistent, indicating that the sediments in Chaohu Lake mainly come from rivers flowing through the southwest farmland. Furthermore, the spatial distribution of REEs shows that these tributaries are significantly affected by agricultural activities. The distribution and accumulation of REEs in Chaohu Lake are the result of the interaction of natural and human processes. The results can provide a scientific reference for the distribution and environmental behavior of REEs in aquatic environments disturbed by human beings.

Weathering stage and topographic control on rare earth element (REE) behavior: New constraints from a deeply weathered granite hill

Rare earth elements (REE) are sensitive tracers of rock-water interactions and soil formation processes. In this study, we investigated the REE concentration and distribution patterns for both bulk samples and acid leachable phase for a series of regolith and soil profiles developed on a deeply weathered granite hillslope in a subtropical climate. The exogenous input had little impact on the REE distributions on the studied hillslope. The REE distribution in the profiles was controlled by changes in the physical and chemical environmental parameters and mineral transformation during weathering. However, the specific mechanisms differed among the different weathering stages and profiles located at different positions along the slope. Among the environmental parameters, the profile redox state and acidity were the dominant factors controlling the REE behavior, with organic materials as the main regulators. A strong positive relationship between chemical index of alteration (CIA) and La/Yb was observed for the regolith samples, but the La/Yb ratio decreased upward from a depth of 120 cm (the bottom depth of the soil horizon) as the CIA value increased, suggesting a preference for light REE (LREE) fixation in kaolinite formed during the dissolution of feldspar; however, a conversion occurred during the subsequent pedogenesis period. The hilltop soil profile was characterized by the strongest LREE depletion and the highest positive Ce anomaly (~7.31–14.28), compared with other profiles down the hillslope (~0.84–2.78). The strongest LREE depletion on the hilltop profile JLN-S4 suggested a preference for LREE release resulting from the transformation of kaolinite to gibbsite, and the highest Ce anomaly indicates that Ce was fixed in a more oxidized environment within a well-drained profile. The hill foot profile JLN-S1 had the highest LREE leaching rate, but also the lowest heavy REE (HREE) leaching rate, which was ascribed to its lowest erosion rate, lowest pH value, and reductive conditions in a more hydromorphic zone down the hill. The releasing rates of REE calculated by a mass balance model at different positions along the catena ranged from 1395 to 14,379 μg m−2y−1. All the observed REE fractionation regimes and weathering rate variations along the hillslope illustrated that topography-regulated hydrologic condition played a critical role in the release and migration of REE during granite weathering.

Review of the concentration, bioaccumulation, and effects of lanthanides in marine systems

Rare earth elements (REEs) or lanthanides are often found together in nature, and they are used in multiple anthropogenic activities from green energy and medical technologies to telecommunications and defense systems. However, the current understanding on the concentration and behavior of REEs in oceans and marine organisms is limited, and no regulatory information or limits have been settled. Here, we present a review of the concentrations of lanthanides in marine waters and biota. REEs reach aquatic ecosystems mainly by continental contributions, and the maximum reported concentrations of REEs are found on the platform surface near the coast due to their continental origin. For coastal waters, we find maximum REE levels in the surface water that decrease with depth until a certain stability. Their concentrations diminish as they move toward the open ocean, where concentrations tend to increase vertically with depth in the water column. Only cerium (Ce) showed different patterns from other REEs caused by Ce different redox states: III and IV, reflecting the oxidation of dissolved Ce (III) to particulate Ce (IV) when reacting with the O2 to form CeO2. In seawater, heavy REEs tend to remain in solution forming complexes usually unavailable for organisms, while light REEs are most likely to be assimilated by them, posing potential biological implications. Bioaccumulation of REEs decreases as marine trophic level increases, showing a trophic dilution pattern. Generally, higher concentrations are found in organisms such as phytoplankton, zooplankton, and algae species, while the lowest concentrations are found in mollusks, corals, and fish species. According to the current trend in the REE industry, the increasing anthropogenic emissions are a fact; therefore, more studies will be needed regarding their fractionation, the transformation processes with which they become bioavailable, and their pathways in marine systems.

Rare earth element behaviour in seawater under the influence of organic matter cycling during a phytoplankton spring bloom – A mesocosm study

Rare earth elements (REEs) are used as powerful proxies for a variety of oceanic processes. The understanding of their biogeochemical behaviour in the marine environment is therefore essential. While the influence of OM-cycling on REE patterns in seawater is considered as insignificant, it has been shown that algae and bacteria provide good sorption surfaces for REEs and that components of the dissolved OM pool are able to complex REEs, thus potentially altering their behaviour. To investigate the impact of bio-associated processes on REEs in the bio-productive marine environment, we conducted an indoor mesocosm experiment that mimicked a phytoplankton spring bloom in the neritic coastal North Sea. The incubation period of 38 days covered two distinct phytoplankton bloom phases (diatoms followed by Phaeocystis sp.) and an interjacent bacterioplankton maximum. All dissolved REEs (dREEs) except samarium showed similar temporal concentration patterns, which were closely connected to the bloom succession. The concentration patterns were shaped by the ‘phytoplankton-shuttle’, which summarizes adsorption processes on phytoplankton-derived particulate OM (POM) and resulted in decreasing dREE concentrations alongside chlorophyll-a and POM maxima. The ‘heterotrophic-shuttle’ resulted in increasing dREE concentrations likely linked to heterotrophically mediated regeneration of POM and associated desorption processes. The effect of these processes on dREEs resulted in enhanced fractionation of light REEs (LREEs) relative to heavy REEs (HREEs) during adsorption processes and decreased fractionation as a result of desorption. At times of high dissolved organic carbon (DOC) concentrations, we observed a stabilization of especially dHREEs likely in organic complexes. To test the potential influence of DOC on dREEs, we used a PHREEQC model approach that revealed dREE complexation with components of the DOC pool and an increase in complexation with atomic mass of the REEs. That is, at high DOC concentrations OM-dREE complexation leads to an effective and preferential buffering of dHREE against adsorption. Our findings reveal that OM-cycling influences concentration patterns of dREEs via ad- and desorption processes as well as organic complexation with parts of the OM pool, suggesting these processes can have a significant impact on dREE concentrations in the natural marine environment under high OM conditions.

A Reconnaissance Study of Rare Earth Elements Systematic in Phosphate Coprolites: Comparison between Continental Shelf and Intracontinental Basin Deposits

The behavior of rare earth elements (REE) in phosphate coprolites; deposited in marine upwelling shelf system (Iraq) and a condensed section of the intracontinental basin (Czech Republic) is compared and discussed in the present study. The eight coprolite samples show that the REE incorporated in the apatite of these phosphatized faecal trace fossils have significantly different behavior in the two sedimentary regimes and hold the REE source’s signature in the local sedimentary conditions. The behavior of heavy REE, light REE, Ce-anomaly and shale-normalized ratios of Ce/La, Sm/Yb and Pr/Ce can be mainly attributed to REE fractionation at the source. In contrast, middle REE enrichment may be attributed to local fractionation below the sediment-water interface. Nd/La and REE/P2O5 ratios indicate local sedimentation rate and residence time in contact with seawater. Elevated La/Yb ratios indicate shallowing and submarine reworking events. Most of the REE patterns in the studied coprolites were acquired in the early stages of diagenetic phosphatization and crystallization of the apatite, and minor modifications took place later. The REE behavior in phosphate coprolites proved useful indicators of marine sedimentary environments in the present reconnaissance study and encourage further work on phosphate coprolite samples from other world deposits.

Unsupervised pattern-recognition and radiological risk assessment applied to the evaluation of behavior of rare earth elements, Th, and U in monazite sand.

The Brazilian coast is rich in monazite which is found in beach sand deposits. In this study, the composition of the monazite sands from beaches of State of Espírito Santo, Brazil, was investigated. The concentrations of rare earth elements (REEs), Th, and U were determined by inductively coupled plasma mass spectrometry (ICP-MS). In the studied region, the mean concentration of investigated elements increased in the following order: Tm < Yb < Ho < Lu < Eu < Er < Tb < Dy < U < Y < Th < Gd < Sm < Pr < Nd < La < Ce. The sampling sites were classified into three clusters and discriminated by the concentrations of REEs, Th, and U found. In general, the radiological risk indices were higher than the established limits, and the risk of developing cancer was estimated to be higher than the world average.

Controls on the behaviors of rare earth elements in acidic and alkaline thermal springs

Understanding the rare earth elements (REE) behaviors in aquatic environment is important to reveal the complex hydrochemical processes and hydrological circulation, such as water-rock interactions, mixing, redox reactions. Based on the samples collected from the Rehai geothermal field, southwestern China and the data from published literature, this study revealed that there are apparently differences in general hydrochemistry and REE geochemistry characteristics between the acidic and alkaline springs. The content of REE in acidic springs is one to three orders of magnitude higher than that of alkaline springs and river water. The NASC-normalized REE patterns of acidic springs are more similar to river water and volcanic rocks, but heavy rare earth elements (HREE) in all samples are enriched compared with volcanic rocks. Moreover, the most remarkable characteristic is that the total concentration of REE decreases significantly with the increase in pH of the springs, but rises slightly after water becomes neutral. Through speciation calculations, major REE species are Ln3+ and LnSO4+ in acidic springs (Ln represents REE), and REE hydroxide complexes are predominant in alkaline springs. The behaviors of REE in thermal springs are controlled by three processes: (i) the dissolution and alteration of primary minerals containing REE vary with the pH of water. The co-alteration of the whole-rock minerals contribute to the REE in the acidic springs, whereas the dissolution of feldspar is main source of REE in the alkaline springs; (ii) the content and species of Fe and Mn affects the adsorption, desorption and co-precipitation of REE, especially under weakly alkaline conditions; and (iii) inorganic complexation and adsorption are responsible for the fractionation between LREE and HREE. The positive Eu anomaly is mainly owing to preferential dissolution of feldspar in alkaline springs. Combined with the general hydrochemistry of thermal springs, isotopic data and inverse geochemical modeling, acidic springs in the Rehai geothermal field generate from oxidized shallow water heated by steam containing H2S, and alkaline springs are derived from deep geothermal water.

Mobility and Retention of Rare Earth Elements in Porous Media.

There is growing concern that rare earth elements (REEs) will become emerging soil–water contaminants because of their increased use in new technologies and products, which may lead to unavoidable release to the environment. To better understand the environmental behavior of REEs, a comprehensive set of adsorption and column transport experiments was conducted in quartz sand media. The retention and mobility of three representative REEs (La, Gd, and Er) were studied in the presence and absence of humic acid (HA; 5, 20, and 50 mg L–1) and under a range of pH conditions (5–8). Results show that REE mobility and retention are controlled by the amount of REE–HA complexes formed in a solution, which increases with increasing HA concentrations and solution pH. Gadolinium is the most mobile among the representative REEs, followed by Er and La, corresponding to the amount of (calculated) REE–HA complexes. Increasing HA concentrations in the REE solution inhibits REE retention in both the batch adsorption and column experiments. The same retardation trend was observed for lower HA concentrations (Gd > Er > La). In a fixed HA concentration, HA and REE adsorption decrease simultaneously as the solution pH increases, indicating the co-adsorption of REEs and HA on the sand. Scanning electron microscopy detection of elongated regions attached to the sand, where high REE and carbon (HA) concentrations were measured, further suggests the co-adsorption of REE–HA complexes. Modeling the column experiments shows that the time-dependent attachment is dominant at high HA concentrations, while at lower HA concentrations, both the time-dependent and spontaneous attachments play equal roles. These results provide a quantitative characterization of REE retention and mobility in sand media.

Partitioning behavior and mechanisms of rare earth elements during precipitation in acid mine drainage

Rare earth elements (REEs) are frequently found concentrated in acid mine drainage (AMD). The recovery of REEs from AMD has been successfully achieved using selective chemical precipitation. However, a portion of the REEs is often lost to the precipitates of the dominant metal contaminant ions. To better understand the REE partitioning behavior and mechanisms during the precipitation process, a systematic study was performed on both natural and synthetic AMD solutions. Precipitation test results show that REE removal was noticeably elevated at pH 4.0 after adding H2O2 to convert ferrous to ferric ions, causing nearly complete precipitation of iron. Solution equilibrium calculations suggested that the REE removal increase was realized through adsorption onto the surfaces of the ferric precipitates. The presence of aluminum species in the solutions reduced the adsorption of REEs on the ferric precipitates. Based on electro-kinetic test results, it was concluded that aluminum species neutralize the negative surface charge of the ferric precipitates and compete with REEs for the adsorption active sites. The presence of ferrous ions in the solutions reduced REE adsorption on the aluminum precipitates at lower pH values (e.g., 5.0) due to competitive adsorption. However, at higher pH values (e.g., 6.0), REE removal to the precipitate product increased due to the precipitation of ferrous ions. In addition to the electro-kinetic tests and solution equilibrium calculations, mineralogy characterization, specific surface area measurement, particle size analysis, and morphology analysis were also conducted to investigate and identify the partitioning mechanisms.

No direct effect of F, Cl and P on REE partitioning between carbonate and alkaline silicate melts

This study presents new insights into the effects of halogens (F and Cl) and phosphorous (P) on rare earth element (REE) partitioning between carbonatite and alkaline silicate melts. F, Cl and P are elements that are abundant in carbonatites and alkaline magmatic systems and they are considered to play an important role on the REE behaviour. Nonetheless, their effect on REE partitioning between carbonate and alkaline silicate melts has not yet been constrained.

Rare earth element resource evaluation of coal byproducts: A case study from the Powder River Basin, Wyoming

Domestic Rare Earth Element sources and production are limited in the United States and currently rely on final processing overseas. Increasing demand and resource security domestically has led to significant investigation into rare earth element domestic resources. Much of this work focuses on unconventional potential ore stocks, including coal and coal byproducts. This investigation focuses on coal byproducts generated as ash from coal burning power stations. Wyoming's Powder River Basin hosts the largest U.S. coal stocks for energy production, providing approximately 40% of all thermal coal mined in the U.S. In this effort, in section I, we have studied coal byproducts for rare earth element concentrations and compare these data to current alternative resource knowledge. We find that coal byproducts in this investigation are consistently high enough in rare earth element concentration (above the current Department of Energy 300 ppm cutoff grade) to warrant consideration as a promising potential resource. Rare earth element behavior within the host coal seams is also considered in an effort to better understand resource prospecting and ore body description. In Section II, we evaluate the economic feasibility of rare earth extraction from Powder River Basin coal byproducts using net present value analysis and the rare earth concentrations data from Section I. We calculate the break-even ash-to-oxide output and input unit costs for four coal stations in the Powder River Basin. All four stations have break-even unit costs that are higher than the mine-to-oxide operating cost reported for a traditional rare earth element mine. This is a promising result, especially given that it is more costly to refine rare earths from mined material than from ash. The results are highly sensitive to rare earth prices: given low long-term prices, none of the four stations can feasibly break even. Section III summarizes federal policy considerations in rare earth element resource development. The history of policy development, most recent focus on rare earth element specific funding legislation, paired with section I and II herein, suggest a robust opportunity for development of Wyoming based coal byproducts as a partial solution to current domestic rare earth element short falls and strategic needs.

A molecular dynamics investigation of La3+ and Lu3+-ligand speciation in aqueous solution

Ion-pair association of rare earth elements (REE) with ligands is important for its mobilization, enrichment, and fractionation during hydrothermal transport process, which is of great significance to study REE ore genesis, weathering process, the origin and evolution of earth and other related geoscience issues. However, reliable experimental data are limited to a specific system or ambient conditions, which hampered our understanding of the geochemical behavior of REE. In the present study, classical molecular dynamics simulations were carried out to calculate the potential of mean force (PMF) between La3+/Lu3+ and ligands of Cl−, F−, OH−, HCO3−, CO32− and SO42− in dilute aqueous solutions from ambient to supercritical thermodynamic conditions. Stability constants were calculated from the PMFs and compared with literature reported and thermodynamic calculated data. The analyses of the PMFs and stability constants suggest smaller or highly charged ions are easier to form stable complexes. For both La3+ and Lu3+ containing systems, the stability of ion pair decreases in the order of CO32− > OH− > SO42− > F− > HCO3− > Cl−. The smaller Lu3+ is to form more stable aqueous complexes at all thermodynamic conditions and these stability difference may lead to significant fractionation between La (light) and Lu (heavy). The trends of PMFs calculated stability constants are consistent with thermodynamic calculated values and provide association constants of ligands for which no experimental data exist.