Rare Earth Element Characteristics Research Articles

Probing the geochemical characteristics of rare earth elements in the weathering profile of yttrium-rich heavy rare earth mine

Understanding the complex geochemical characteristics of rare earth elements (REEs) in the weathering profile of ion-adsorbed rare earth ore is a crucial issue for establishing the best leaching agent dosage during in-situ leaching processes. This study focuses on soil samples collected from nine drill holes located at three hillslopes of a mining area in southwest Fujian. Analyzing the geochemical features of REEs revealed that the ore predominantly comprises Y, La, Ce, and Nd, with Y being the most abundant, constituting 20.24 %–33.64 % of total rare earth elements (TREEs) in each weathering profile. This categorizes the ore as an yttrium-rich heavy rare earth ion-adsorbed mine. Notably, REEs exhibit a concentration in the middle layer of the weathering profile, with content increasing first and then declining with deeper depth from the surface to the bottom. The ratio of light rare earth elements (LREEs) to heavy rare earth elements (HREEs) diminishes noticeably from shallow soil to middle soil, while deep soil reveals a slightly higher ratio than middle soil. These findings offer valuable insights into the scientific mining of this area and similar ion-adsorbed rare earth mines concerning their economic potential.

Geochemical characteristics of rare earth elements in late Palaeozoic coals from North China

Introduction: China is a large country of REE resources and production, supplying more than 90% of the world’s rare earth consumption market, China’s traditional REE resources, although rich, but reserves are also declining, in recent years, scholars have shown that the REEs in the specific geological conditions in the coals can be enriched, and even reach the industrial grade, the search for more REE mineral resources will be of great strategic significance.Methods: The article has selected the Zibo, Taozao, Huainan, Datong, Huozhou, Jungar, and Weibei coalfields in North China as the research objects. Based on inductively coupled plasma mass spectrometry (ICP-MS) analyses, the study provides a detailed analysis of the enrichment rules and distribution patterns of rare earth elements (REEs) in coal under different geological conditions.Results: The REE distribution patterns in Late Palaeozoic coals in North China are characterized by enrichment of LREEs, deficit of HREEs with gentle changes, and obvious negative anomalies of Eu, the enrichment of REE in coal is controlled by provenance and volcanism, and is related to distance of transport terrigenous material and the sedimentary environment. The REEs concentrations in North China indicates that Late Palaeozoic coals in the mining areas of the north, northwest, and western parts of North China are significantly enriched in REEs. In addition, REEs in Late Palaeozoic coals in the southeastern mining areas are relatively enriched, and the central and eastern regions are significantly depleted.Discussion: Large coal-type rare earth deposits are expected to be developed in North China, particularly the Datong coalfield, Jungar coal mine and the Weibei coal mine in the eastern margin of the Ordos Basin, and the Zibo coalfield.

Sedimentary characteristics of ion-adsorption rare earth elements and assessment of their resource reuse potential due to soil and water erosion in western Fujian.

With severe soil and water erosion, the crucial ion-adsorption rare earth elements (REEs) have attracted much global attention. REEs play a vital role in tracing material sources and exploring sedimentary characteristics due to their unique and stable geochemistry properties. In the present work, three representational possible redeposition areas in western Fujian were selected as the study areas. The geochemical characteristics of REEs in the sediments of the study areas were evaluated to elucidate that REEs are the products of soil and water erosion and to assess their redeposition characteristics. In the research results, the properties of the parent rocks shown in the samples, together with the negative correlation between the content of REEs in the samples and altitude as well as the relief degree on the land surface (RDLS), fully indicate that the sediments in the study areas are the products of migration caused by soil erosion and redeposition in the downstream areas. At the same time, according to the widely applicable standard of rare earth resources exploitation, that is the boundary grade of ion-adsorption rare earth ore in southern China (∑REE = 500mg·kg-1), we found that the content of REEs in the study areas was close to or exceeded this standard, and the maximum ∑REE of Guozhai Reservoir (869.11mg·kg-1) was much larger than this standard. Therefore, the redeposited rare earth in Changting Country has high reuse potential under the current scarce resources.

Mineral Chemistry and Chronology Investigation of Uraninite in the Jinguanchong Uranium Deposit in Eastern Hunan Province and the Implications for Geological Significance.

Jinguanchong deposit, a part of the Mingyuefeng ore field in eastern Hunan Province, China, is a typical perigranitic uranium deposit (a subtype of granite-related deposit) discovered recently with considerable uranium mineralization. Herein, uraninite, the primary ore mineral in the deposit, was investigated via scanning electron microscopy and electron probe microanalysis. Additionally, laser ablation-inductively coupled plasma-mass spectrometry was used for the first time to determine the in situ U-Pb age and the rare-earth element characteristics of uraninite. Uraninite mainly comprises UO2, CaO, and PbO with a low ThO2 content. Uraninite exhibits a low total content of rare-earth elements with a distinct fractionation between light and heavy rare-earth elements while displaying a negative Eu anomaly. The presence of major elements and rare-earth elements in uraninite suggests its formation within a hydrothermal environment at moderate to low temperatures below 350 °C, thereby classifying the Jinguanchong deposit as a typical hydrothermal vein-type uranium deposit. The uranium metallogenic age is determined to be 93.8 ± 1.4 Ma, falling within the midlate Cretaceous period. This age corresponds to the Mesozoic lithospheric extension and thinning events (approximately 85-95 Ma) in South China, suggesting that the formation of the Jinguanchong uranium deposit might be associated with the tectonic dynamics of lithospheric extension and thinning.

Effect of Calcination Atmosphere on the Acid Leaching Recovery of Rare Earth Elements in Coal-Byproducts

ABSTRACT Thermal treatment of coal byproducts has been shown to improve leaching characteristics of rare earth elements (REEs) in bituminous coal sources due to its influence on the associated minerals. This study investigated the impact of the calcination atmosphere on REE leaching behavior in different density fractions of Baker and Fire Clay seam coarse refuse sources using a thermogravimetric analyzer to create an oxidizing or reducing atmosphere at a desired temperature for 2 h. The effect on leaching characteristics was evaluated using 1.2 M sulfuric acid at 75°C for 5 h at 1% w/v solid concentration. Rare earth element (REE) recovery for the 2.2 specific gravity (SG) sink fraction improved by approximately 15 absolute percentage points at 1000°C relative to the 600°C calcined sample in an inert atmosphere and equilibrated within 10 min from the start of the test. Contrarily, thermal treatment at 1000°C under oxidizing atmosphere significantly reduced the REE recovery due to sintering of the clays, which limited access to the RE minerals for leaching. In general, thermal treatment of heavier fractions in both atmospheres significantly improved the light REE (LREE) recovery while the increase in heavy REE (HREE) recovery was relatively less. As common clay minerals in the coal sources, kaolinite and illite samples were tested under the same conditions and using a digital scanning calorimeter. The findings confirmed that the improvement observed in REE leaching through thermal treatment at 600°C was due to decarbonization for the lighter density fractions and dehydroxylation of clays.

Study on the Hydrothermal Superposition Period: Mineralization–Alteration Zoning Model and Zoning Mechanism of the Dahongshan Fe-Cu Deposit in Yunnan Province

The Dahongshan large-scale iron (Fe)–copper (Cu) polymetallic deposit is in the Proterozoic metallogenic domain on the western margin of the Yangtze Block. It is a typical representative of Fe-Cu polymetallic composite mineralization in the Kangdian area. The deposit comprises a group of layered orebodies formed by volcanic exhalation sedimentation and metamorphism, and a group of vein-like orebodies formed by hydrothermal superposition. The large-scale mapping of altered lithofacies in the deposit has resolved issues of weak links and unclear mineralization and alteration zoning of hydrothermal superimposed deposits within the study area. The mineralization type, hydrothermal alteration type and intensity, mineral assemblage, and mineral structure of the vein-type Cu polymetallic deposits during the hydrothermal superposition period are meticulously analyzed and studied. Finally, the zoning relationships of vein orebodies (mineralization) are summarized. On the basis of the results of the study of the distribution pattern of this mineral body, a mineralization alteration zoning model of the hydrothermal superposition period is constructed. The results show that the alteration is primarily silicification, carbonation, and chloritization, and the mineralization is chalcopyrite, bornite, chalcocite, and pyrite. The Dibadu anticline and the cutting layer faults and fractures strictly control the hydrothermal alteration zoning. The mineralization alteration zoning from the core to the flank is divided into coarse vein zone (I) → stockwork zone (II) → veinlet zone (III). The corresponding mineral assemblages are quartz–calcite–chalcocite–bornite–(native copper) (I) → calcite–dolomite–quartz–bornite–chalcopyrite–chlorite (II) → dolomite–quartz–chalcopyrite–(pyrite) (III), where the stockwork zone has the most substantial mineralization. The mineral assemblages of each alteration zone, the characteristics of rare earth elements of typical samples, and the test results on the fluid inclusions confirm that pH and Eh primarily control the zoning mechanism. This study has significance for deepening the understanding of the composite metallogenic system, guiding deep and peripheral prospecting, and providing significant enlightenment for the study of this type of deposit.

The distribution and enrichment characteristics of rare earth elements between coals and coal ashes from four coal-fired power plants.

Coal and coal ash (slag and fly ash) have emerged as important potential sources for rare earth elements (REY). The distribution and enrichment of REY during coal combustion from coal-fired power plants are of great significance for the recovery of REY. In this study, the concentration of REY in coal and coal ash from four coal-fired power plants in China was determined using microwave digestion-inductively coupled plasma mass spectrometry. The distribution and enrichment characteristics of REY in coal and coal ash were studied. The coal ash contains 310-683 μg g-1 of REY, and the proportion of critical REY was greater than 30%, which is higher than the extraction concentration (300 μg g-1) recommended by the United States Department of Energy, indicating significant REY recovery potential. The concentration of REY in coal was positively correlated with ash yield, which could be explained by the fact that REY are mainly distributed in minerals. Coal and coal ash show similar REY distribution patterns, indicating that REY were not fractionated during the coal combustion. In addition, the enrichment coefficient of REY in fly ash was greater than that in slag, which revealed that REY were mainly enriched in fly ash. Therefore, the REY in fly ash has more recycling potential.

Rare Earth Element Characteristics of Shales from Wufeng-Longmaxi Formations in Deep-Buried Areas of the Northern Sichuan Basin, Southern China: Implications for Provenance, Depositional Conditions, and Paleoclimate.

To explore the sedimentary environment and the background of the source area of organic-rich shales in the Wufeng-Longmaxi Formations in the northern Sichuan Basin, samples from Well XX1 in the area were subjected to geochemical testing and analysis of organic carbon content, trace elements, and rare earth elements (REEs). The results show that the total content of REE (ΣREE) of the shale in the Wufeng-Longmaxi Formations varied from 183.08 to 234.66 μg/g with an average of 212.59 μg/g, which is significantly higher than the content of the North American shale composite. The fluctuations in the total amount of REEs in the shale of the Wufeng-Longmaxi Formations reflect certain differences in the geochemical conditions of the Upper Ordovician-Lower Silurian shale. The ratios of LREE/HREE, LaN/YbN, LaN/SmN, and GdN/YbN and the distribution of normalized REE patterns indicate that the source supply or sedimentary structural background may have changed during the shale deposition period of the Wufeng Formation, while the shale deposition period of the Longmaxi Formation may be in a relatively stable source supply and sedimentary structural background. There is no significant correlation between δCe and ΣREE, and the obviously negative Eu abnormity and the weak Ce abnormity indicated that the diagenesis had a limited impact on REEs. Geochemical parameters such as values of ∑REE, δEu, δCe, Ceanom, and LaN/YbN indicate that the climate during the Wufeng-Longmaxi Formation shale deposition period was warm and humid, and the shale was deposited mainly in the suboxic-anoxic water environment. The deposition rate was stable and slow, providing good conditions for the production and preservation of organic matter. At the same time, this shows that the water environment of Wufeng Formation is more anoxic and reductive than that of Longmaxi Formation, which is more conducive to the preservation of organic matter. The correlation between ΣREE and the content of Sc, Ti, Cr, Co, Zr, Nb, Th, Hf, Ta, and other elements indicates that the sources of REEs in the shale of Wufeng and Longmaxi Formations in the study area are similar, mainly terrestrial clasts, and some may come from the sea. The REE distribution pattern shows that the shale provenance of the Wufeng-Longmaxi Formations mainly comes from the upper crust. The La/Yb-∑REE diagram shows that the sediment-parent rocks are mainly early sedimentary rocks and these sediment-parent rocks have granite provenance characteristics. Compared to La/Yb, LREE/HREE, LaN/YbN, and other REE characteristic parameters, it is inferred that the tectonic background of the study area is dominated by passive continental margin.

Associations of maternal urinary rare earth elements individually and in mixtures with neonatal size at birth

Prenatal rare earth elements (REEs) exposure is linked to unfavorable health consequences. Epidemiologic research on repeated measurements of REEs during gestation correlated with fetal growth is exiguous. Until now, few studies have characterized exposure characteristics of REEs in pregnant women. We aimed to ascertain the characteristics and predictors of REEs exposure over three trimesters among pregnant women and examine the possible effects of prenatal REEs exposure on size at birth. Urinary REEs concentrations exhibited considerable within-subject variation with intraclass correlation coefficients ranging from 0.16 to 0.58. Maternal age, household income, gestational weight gain, passive smoking during pregnancy, parity, and neonatal gender were associated with maternal urinary REEs concentrations. Elevated maternal urinary holmium and thulium concentrations in the 3rd trimester were significantly related to reductions in birth weight. Weighted quantile sum (WQS) regression model identified that urinary REEs mixture in the 3rd trimester were negatively related to birth weight (WQSREEs β = −26.22; 95% confidence interval [CI]: −47.62, −4.82), with holmium (40%) and thulium (24%) receiving the highest weights. Male infants received the most weight (>50%) related to decreased birth weight. This study revealed a significant association between individual and mixture REE exposure in late pregnancy with a reduction in birth weight.

Geochemistry of dissolved rare earth elements in the Lancang River with cascade dams and implications for anthropogenic impacts

Anthropogenic effects like the construction of cascade dams are increasingly affecting the biogeochemical cycles of rare earth elements (REE) in rivers. However, our understanding on the REE compositions of dissolved loads in rivers under the effect of cascade dams remains limited. In this study, forty-four water samples in the Lancang River were analyzed for REE concentrations to assess the impact of cascade dams. The results showed that total dissolved REE concentrations ranged from 4.0 to 65.7 ng/L, with an average value of 21.8 ng/L. The concentrations in the middle (519.6 ng/L) and lower (331.5 ng/L) reaches were higher than those in the upper reaches (108.1 ng/L). All river samples showed (La/Yb)N values < 1, and the PAAS-normalized REE patterns indicated a substantial enrichment of heavy REE, characterized by negative Ce (0.1–0.9) and positive Eu (1.5–32.1) anomalies. The enrichment of heavy REE can be attributed to their preferential release from minerals during the weathering process and their interaction with inorganic complex anions. The negative Ce anomaly observed is likely the result of redox conditions, while the positive Eu anomaly was inherited from the REE characteristics of the source rocks. Moreover, the Ce anomaly decreased to 0.29 in the Huangdeng reservoir and 0.12 in the Dachaoshan reservoir. The value of Ce anomaly displayed a sharp decline in the middle and lower reaches affected by the cascade reservoirs. The effect of the dams played an important role in Ce anomaly by decreasing the DOC concentrations, and rivers with low DOC concentrations are commonly associated with negative Ce anomaly. This study revealed that cascade dams paly an essential role in the variation of REE in dissolved loads and REE anomaly may have the potential to track anthropogenic activities.

Geochemical and Mineralogical Characteristics of Ion-Adsorption Type REE Mineralization in the Mosuoying Granite, Panxi Area, Southwest China

The ion-adsorption rare earth deposit developed on the Mosuoying granite in the Panxi area of southwestern China represents a significant advancement in the exploration of ion-adsorption rare earth deposits in Sichuan. Being the first and currently the sole ion-adsorption rare earth deposit in Sichuan, studying its rare earth mineralization characteristics holds great importance. This paper aims to investigate the geochemical properties of the Mosuoying granite and its overlying weathered crust using rock geochemical methods based on field geological investigations. The findings reveal that the deposit belongs to the light rare earth type, with the ore-forming parent rock attributed to the high-potassium calc-alkaline series. It exhibits a high rock REE content ranging from 419 to 578 ppm, indicating favorable mineralization potential. Hydrothermal alteration reduces the REE content of the parent rock, leading to a notable increase in the LREE/HREE ratio, thus impacting the partitioning of rare earth elements and subsequent ore formation. The distribution characteristics of rare earth elements in each layer of the weathered crust are controlled by the parent rock and exhibit a light rare earth distribution pattern. The completely weathered layer is the main enrichment zone for rare earth elements, and the migration and enrichment patterns of rare earth elements in the weathered crust are evident. From the semi-weathered layer to the completely weathered layer, all REEs were gained, with a higher degree of migration for LREE. From the completely weathered layer to the clay layer, all REEs were lost, and the vertical distribution of rare earth content shows a “low-high-low” pattern.

The characteristics of rare earth elements in the main water filling sources of Sunan mining area

This study employs rare earth elements to investigate the features and procedures of water-rock interactions within aquifers of loose strata, sandstone, and limestone in the Sunan mining region of North Anhui, China. Through a comparison of the differential characteristics of rare earth elements across primary water sources, we are able to uncover the influence of water-rock interactions on the transfer and conversion of rare earth elements within various water-sediment systems. Indeed, the normalization of water compositions against host rock concentrations revealed a distinct pattern of rare-earth elements for each group of waters. Not only did the absolute concentrations of the elements in each sampling site change, but also the normalized patterns. In contrast to sediment samples, all water samples exhibit a clear enrichment of MREEs, but they also reveal a trend of HREEs enrichment. This is attributed to the differential adsorption effect of oxides on rare earth elements, which is influenced by pH, and the complexation reactions between rare earth elements and carbonate ions. The significant Ho and Ce anomalies in the water-sediment system are associated with robust water rock interactions, whereas the high Ba content significantly impacts the Eu anomaly. The enrichment of Eu is attributed to the interaction between the water environment and the dissolution of minerals rich in Eu. Furthermore, we utilize decision trees and recursive feature elimination techniques for selecting indices, and create a model that incorporates triangular fuzzy numbers (TFN), CRITIC, D-S evidence theory (DSET), factor space (FS), and set pair cloud model (SPCM) to differentiate, describe, and evaluate three distinct groups of waters with varying hydrofacies. The conclusion drawn is that sandstone and limestone aquifers are the main sources of water inrush, with a mixing ratio exceeding 50%. Moreover, this marks the inaugural attempt to utilize patterns of rare-earth elements for establishing a comparison between various water groups, thereby enabling us to distinguish and differentiate depletion patterns resulting from dilution processes from those resulting from precipitation processes, with the aim of determining the contribution rates of mine water. The findings offer valuable insights for predicting water disasters in the Sunan mining area.

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

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

Influence of multistage hydrothermal fluids on dolomite reservoirs: A case study from the Lower Ordovician Yeli-Liangjiashan Formation in the Chengdao-Zhuanghai area, Jiyang subbasin, Bohai Bay Basin, China

It remains controversial whether the influence of hydrothermal fluids on dolomite reservoirs is dominated by dissolution or precipitation. In this study, the influence of multistage hydrothermal fluids on the dolomite reservoirs of the Lower Ordovician Yeli-Liangjiashan Formation in the Chengdao-Zhuanghai area was investigated based on petrographic observations and geochemical analyses, with an emphasis on the temporal relationships among dolomitization, pore formation, and multistage hydrothermal fluid activities determined by in situ U-Pb dating. The δ18O, δ13C, and 87Sr/86Sr data indicate that the genesis of pre-hydrothermal dolomites and the earliest-formed dolomite cement (Cd1) closest to the edges of pores were related to coeval seawater. The U-Pb age of pre-hydrothermal dolomites is 427 ± 11 Ma, representing the time when early dolomitization occurred. The original pore formation predates the precipitation of Cd1 with a U-Pb age of 391 ± 12 Ma. Stage I and stage II hydrothermal fluids were derived from the Precambrian basement and the mantle, respectively, as evidenced by differences in δ18Ofluid values and rare earth element characteristics. Stage I and II hydrothermal fluids occurred at 161 ± 12 Ma and 81 ± 16 Ma, respectively. Therefore, early dolomitization and pore formation are not correlated with the two stages of hydrothermal fluids. Stage II hydrothermal fluids contained a relatively higher amount of H2S than stage I hydrothermal fluids. H2S-related dissolution caused by stage II hydrothermal fluids mainly occurred in the dolomite reservoirs adjacent to major faults, and H2S was gradually consumed as the distance from the major faults increased. The two stages of hydrothermal minerals successively filled most of the spaces in pores and fractures. The influence of multistage hydrothermal fluids on dolomite reservoirs was found to be dominated by precipitation.

Geochemical and mineral composition characteristics of hydrothermal-related clay-sized surface sediments from southern Mid-Atlantic Ridge: Implications for hydrothermal depositional environment

Numerous hydrothermal activities were observed along the southern Mid-Atlantic Ridge (SMAR) in recent years. In this study, six clay-sized surface sediments collected from three hydrothermal fields (Caifan, Xunmei and Taiji) and one sample from Kaifeng seamount were analyzed for mineralogical and geochemical compositions. X-ray diffraction (XRD) results reveal that samples are mainly composed of fine-grained goethite, chlorite, and occasionally with jarosite and smectite. Interestingly, smectite not observed under XRD patterns are determined with the application of transmission electron microscope (TEM). Smectite is probably formed by interaction between Fe-oxyhydroxides (goethite) and detrital/hydrothermal silicates. Major and minor elements geochemical data indicate samples from Caifan and Xunmei are highly influenced by hydrothermal activities, and alteration of sulfides was prominent. However, it is not true for the sample from Kaifeng seamount and Taiji hydrothermal field, where are dominant by Mg-rich chlorite of volcanic rocks origin rather than aeolian. Rare earth element (REE) concentrations of both clay-sized and bulk sediment were compared to illustrate the REE characteristics and the degree of hydrothermal contribution. In the Taiji hydrothermal field and Kaifeng seamount, REE were mainly present in clay-sized fraction and chlorite may act as an efficient carrier of REE in these samples. On the other hand, in the Caifan and Xunmei hydrothermal fields, only goethite (or Fe-oxyhydroxides) plays an important role on carry of REE rather than smectite due to low abundance and poor crystallinity.

Geochemical behaviors of rare earth elements in granite-hosted geothermal systems in SE China

Rare earth elements (REEs) are effective indicators in tracking water-rock interactions in the hydrothermal systems. Subject to multiple environmental effects, none a unified conclusion has been achieved to generalize the mechanism of diversifying the REEs in various thermal systems. Low-medium temperature hydrothermal systems occurring in fractured-granite exist widely in South China. However, geochemical behavior of REEs in geothermal water is not well studied though there are lots of REEs deposits. This study focuses on hydrogeochemical characteristics of REEs and constrains from salinity. Results show that the TDS (total dissolved solids) mutation due to transfer of sedimentary environments from coastal to relatively inland areas caused the great differentiation of REEs and their species. When TDS is greater than 1 g/L, Ln3+, LnOH2+, LnSO4+, and LnF2+ (Ln represents REEs) prevail and fractions of them except LnOH2+ are positively correlated with TDS. In contrast, when TDS is less than 1 g/L, LnCO3+ and Ln (CO3)2− constitute the most important part and mutually compete in fluids. A positive Eu anomaly is proved to originate primarily from the preferential dissolution of Eu-rich minerals and the seawater intrusion somewhat contributed to the negative Ce anomaly. This work first proposed the salinity effect on REEs by analysis of the in-situ sampling thermal waters that may have been neglected before and provided a new perspective to comprehensively understand the occurrence and migration mechanisms of REEs in different hydrothermal systems.

Geochemical characteristics and migration patterns of rare earth elements in coal mining subsidence lakes under the influence of multiple factors

Mining activities cause surface subsidence and the formation of subsidence lakes, which dynamically change with the continuous coal mining activities. Under the combined influence of various human activities such as agriculture, aquaculture, and floating photovoltaic (FPV), the lake environment undergoes continuous changes, thereby altering the geochemical characteristics of rare earth elements (REEs) in the sediment. This study focused on the subsidence lakes in the Huainan coalfield in eastern China to examine the REEs content in the sediment, elucidated the temporal variations and geochemical characteristics of REEs distribution, explored the main controlling factors of REEs in the sediment, and revealed the migration and transformation behavior of REEs during dynamic subsidence processes. The study revealed that the migration pattern of REEs in the sediment was closely related to the duration of subsidence. The average content of REEs in lake sediments with subsidence duration <5 years increased from 219 μg·g−1 to 248 μg·g−1 compared to the soil, showing an enrichment model primarily driven by rainwater runoff, groundwater input retention, and mineral dissolution. With further subsidence, the processes of reduction dissolution of Fe–Mn oxides/hydroxides, organic colloid adsorption, and hydraulic disturbance gradually replaced the aforementioned enrichment behavior as the main migration pathways, resulting in a decrease in the average REEs content in the sediment to 179 μg·g−1 for subsidence durations exceeding 10 years. There was no strong correlation between REEs fractionation and subsidence duration. Artificial activities, such as FPV, are important factors causing Cerium and Erbium anomalies in some subsidence lake sediments. This study was not only of significant importance for understanding the migration, distribution, and environmental behavior of pollutants in aquatic environments under the interference of human activities but also provided a solid theoretical foundation for the future management of coal mining subsidence lakes.

Distribution, source and contamination of rare earth elements in sediments from lower reaches of the Xiangjiang River, China

Rare earth elements (REEs) are emerging micropollutants in aquatic environments. In this study, concentrations of REEs and major elements, and mineralogical compositions of sediments from lower reaches of the Xiangjiang River (China) were analyzed using ICP-MS technique. The results suggested that sediments were characterized by terrigenous compositions TiO2, SiO2, Al2O3, K2O, Na2O and P2O, and contained high concentrations of REEs with mean total REE concentrations (∑REE) of 318.7 mg/kg. REEs were moderately enriched in upper river sediments, and slightly or less enriched in downriver sediments. The normalized REE distribution pattern for sediments was characterized by flat shalelike and Eu depleted V-shape REE patterns, which indicated REEs in sediments were lithologically contributed from sedimentary rocks and granites distributed in the watershed respectively. REEs in sediments were hosted mainly in Fe–Mn oxides, and sulfide and organic matters that were characterized by middle REEs (MREE) enrichments relative to light REEs (LREE) and heavy REEs (HREE), and the distribution and differentiation of REEs in sediments were controlled by clays, Fe–Mn oxides, organic matters and finer grains; and also by accessory minerals (e.g., zircon) from granite. The distribution features of REEs in sediments and BCR extraction results suggested that the sediment REE enrichment resulted from additional REE input from anthropogenic sources, including those in discharges from sulfide-ore smelting industries at Zhuzhou city and from phosphate fertilizer plants at Xiangtan city along the river. Thus, sediments were contaminated with REEs in moderate degree in upper river area, and REE contamination was then formed by superimposing anthropogenic REEs on lithological residues. Finally, concentrations of Ce > 100 mg/kg, Gd > 8.12 mg/kg, ∑REE >274.9 mg/kg, ∑LREE >252.3 mg/kg and ∑HREE >28.8 mg/kg here were recommended as the REE contamination levels that represented as REE indices for identifying and rating REE contamination in this mining impacted river.

Optimizing calcination of coal by-products for maximizing REE leaching recovery and minimizing Al, Ca, and Fe contamination

High-temperature calcination is effective in improving the leaching characteristics of rare earth elements (REEs) from conventional REE-bearing minerals by transforming them into more leachable forms. However, the recovery of contaminants such as Al, Fe, and Ca also increases significantly. The objective of the study was to maximize the REE extraction while minimizing the concentration of contaminant ions (Al, Ca, and Fe) in the leach solution. Representative density fractions of the coarse refuse from the Baker and Fire Clay seam coal refuse were pulverized to a top size of 180 μm and used as a feedstock for the study. All the samples generated from thermal treatment were leached at 1% (w/v) using 1.2 mol/L sulfuric acid at 75 °C for 2 h. Test results from a parametric program based on a Box-Behnken design were used for modeling and optimization of three operating parameters associated with the lab-scale calcination process. The parameters include temperature (400–800 °C), ramp rate (2–10 °C/min) and holding time (0–120 min). It is determined that the calcination temperature is the most significant parameter influencing REE recovery. The majority of the heavy rare earth elements (HREEs) is found to be present in an insoluble form in the calcination products and is marginally affected by thermal treatment. A strong correlation between light rare earth elements recovery (LREEs) and Al suggests a possible association with clays. The optimum operating conditions for high REE recovery with comparatively lower contaminant concentration for 1.6 specific gravity float and 2.2 sink fractions of Baker and Fire Clay seam material were determined to be at 776, 800 °C and 407, 800 °C, calcining temperatures, respectively. A higher optimum temperature of 1.6 float fraction of Baker seam relative to Fire Clay seam material is due to elevated pyrite concentration in Baker seam material. Thermal treatment at 400 °C converts pyrite into a soluble intermedium iron oxide, resulting in a high Fe content in the leachate. Calcining at 776 °C converts most of the soluble iron oxide into insoluble α-hematite, considerably reducing the Fe concentration in the solution.

Trace and rare earth elements as the source and transport indicators of different topsoil end-members in the desert peripheral regions of China

Solis in grassland regions, located downwind of and adjacent to deserts, display grain-size sorting and weathering features that cause the mineral and chemical elements of their particulate matter to have some characteristics different from those in the source area. This makes it an issue to be explored whether source tracers previously obtained from deserts can be well applied in grassland areas. We collected 40 topsoil samples in the western Inner Mongolia grassland near the Badain Jaran Desert and the Mongolian Gobi and then studied the three grain-size end-members of their <300 μm components. End-member 1 is matched with samples' <30 μm grain-size fractions, end-member 2 and end-member 3 are matched with some samples' 30–300 μm grain-size fractions. By obtaining the trace element and rare earth element characteristics of the two different grain-size fractions, that of the three end- members are obtained. Therefore, the impact of sorting and weathering on different element ratio proxies is discussed from the perspective of end-members. Then element ratio proxies that are more applicable to grasslands on the periphery of deserts are established, where Sm/Gd, Tb/Dy, Nb/Ta, Er/Tm, La/Ce, Sc/V, Yb/Lu, and Ce/Ce* are least affected by sorting and weathering, and they effectively distinguish and identify the sources of each grain-size end-member. Using these proxies, it can be inferred that end-member 1 (mode grain size is 5 μm) in the grassland topsoil comes from a vast area of deserts and Gobi in Mongolian Plateau. End-member 2 (70 μm) and end-member 3 (120 μm) have stronger source affinity with the Badain Jaran Desert and Altay Mountains in the western Mongolian Plateau; and end-member 2 also contains a small amount of material from the local Yinshan Mountains. This result can be further applied to the sediment sequences of the grassland region in the future, which will help to explain the sources of their end-members and the environmental change processes at different time scales implied by end-members.