Organic-mineral colloids regulate the migration and fractionation of rare earth elements in groundwater systems impacted by ion-adsorption deposits mining in South China.

Ion-adsorption rare earth element (REE) deposits distributed in the subtropics provide a rich global source of REEs, but in situ injection of REEs extractant into the mine can result in leachate being leaked into the surrounding groundwater systems. Due to the lack of understanding of REE speciation distribution, particularly colloidal characteristics in a mining area, the risks of REEs migration caused by in situ leaching of ion-adsorption REE deposits has not been concerned. Here, ultrafiltration and asymmetric flow field-flow fractionation coupled with inductively coupled plasma mass spectrometry (AF4-ICP-MS) were integrated to characterize the size and composition of REEs in leachate and groundwater from mining catchments in South China. Results show that REEs were associated with four fractions: 1) the <1 kDa fraction including dissolved REEs; 2) the 1 - 100 kDa nano-colloidal fraction containing organic compounds; 3) the 100 kDa - 220 nm fine colloids including organic-mineral (Fe, Mn and Al (oxy)hydroxides and clay minerals); 4) the >220 nm coarse colloids and acid soluble particles (ASPs) comprising minerals. Influenced by the ion exchange effect of in situ leaching, REEs in leachate were mostly dissolved (79 %). The pH of the groundwater far from the mine site was increased (5.8 - 7.3), the fine organic-mineral colloids (46 % - 80 %) were the main vectors of transport for REEs. Further analysis by AF4 revealed that the fine colloids can be divided into mineral-rich (F1, 100 kDa - 120 nm) and organic matter-rich (F2, 120 - 220 nm) populations. The main colloids associated with REEs shifted from F1 (64 % ∼ 76 %) to F2 (50 % ∼ 52 %) away from the mining area. For F1 and F2, the metal/C molar ratio decreased away from the mining area and middle to heavy REE enrichment was presented. According to the REE fractionation, organic matter was the predominant component capable of binding REEs in fine colloids. Overall, our results indicate that REEs in the groundwater system shifted from the dissolved to the colloidal phase in a catchment affected by in situ leaching, and organic-mineral colloids play an important role in facilitating the migration of REEs.

A human health risk assessment of rare earth elements through daily diet consumption from Bayan Obo Mining Area, China.

Rare earth elements (REEs) have been broad application in a range of industries, including the electronics industry, advanced materials, and medicine. However, health risks associated with REEs received increasing attention. 31 residents (16 males and 15 females) from Bayan Obo mining in Inner Mongolia, China, were enrolled in this study. In total, 677 food samples, the major human exposure matrices (drinking water and duplicate diets), and bio-samples (urine and blood) of 31 participants were obtained. The concentrations of REEs were measured to characterize their external and internal exposures, and the potential health risk of exposure to REE through the ingestion route was analyzed. The results revealed that the detection rate in blood samples (100%) is higher than in urine (32.86%), and only a few REEs were detected in water samples (8.06%), the urine concentrations were considerably lower than in blood. Exposure to REEs through drinking water was considered negligible compared to food intake. Lanthanum and cerium were the most concentrated REEs in food samples. Health risks were calculated based on a dose-response model, the total hazard quotients (THQ) values for all food groups were within normal levels, and the Monte Carlo simulation results show that the 5th, the 50th, and the 95th percentile values of HI were found as 1.45 × 10-2, 3.52 × 10-2, and 9.13 × 10-2, respectively, neither exceeds the threshold, indicating low health risks associated with food intake exposure for this area. The sensitivity results suggest that underweight people are at higher risk, cerium, lanthanum, and yttrium concentrations, and food intake contributes more to health risks. The use of probability distribution methods can improve the accuracy of the results. The cumulative health risk through food intake is negligible, and further attention should be paid to the health risk induced by other routes of exposure to REEs by the local residents.

Untangling microbial diversity and assembly patterns in rare earth element mine drainage in South China.

Ion-adsorption rare earth element (REE) deposits are the main reservoirs of REEs worldwide, and are widely exploited in South China. Microbial diversity is essential for maintaining the performance and function of mining ecosystems. Investigating the ecological patterns underlying the REE mine microbiome is essential to understand ecosystem responses to environmental changes and to improve the bioremediation of mining areas. We applied 16S rRNA and ITS gene sequence analyses to investigate the composition characteristics of prokaryotic (bacteria, archaea) and fungal communities in a river impacted by REE acid mine drainage (REE-AMD). The river formed a unique micro-ecosystem, including the main prokaryotic taxa of Proteobacteria, Acidobacteria, Crenarchaeota, and Euryarchaeota, as well as the main fungal taxa of Ascomycota, Basidiomycota, and Chytridiomycota. Analysis of microbial diversity showed that, unlike prokaryotic communities that responded drastically to pollution disturbances, fungal communities were less affected by REE-AMD, but fluctuated significantly in different seasons. Ecological network analysis revealed that fungal communities have lower connectivity and centrality, and higher modularity than prokaryotic networks, indicating that fungal communities have more stable network structures. The introduction of REE-AMD mainly reduced the complexity of the community network and the number of keystone species, while the proportion of negative prokaryotic-fungal associations in the network increased. Ecological process analysis revealed that, compared to the importance of environmental selection for prokaryotes, stochastic processes might have contributed primarily to fungal communities in REE mining areas. These findings confirm that the different assembly mechanisms of prokaryotic and fungal communities are key to the differences in their responses to environmental perturbations. The findings also provide the first insights into microbiota assembly patterns in REE-AMD and important ecological knowledge for the formation and development of microbial communities in REE mining areas.

Resource recovery: Adsorption and biomineralization of cerium by Bacillus licheniformis.

Cerium is a critical element to modern technologies. Nowadays, its increased applications have led to elevated levels in the environment. Cerium recovery by microorganisms has gained a great deal of attention. Here, our research showed that Bacillus licheniformis could be used to recover Ce3+ from aqueous solution. The adsorption capacity of cerium on this bacterial strain achieved 38.93 mg/g (dry weight) biomass. Adsorption kinetics followed a pseudo-second-order rate model, and adsorption isotherm was fitted well with the Freundlich model. Scanning electron microscope (SEM) observations coupled with X-ray energy dispersive spectroscopy (EDS) analysis revealed a spatial association of Ce with C, N, O, S, and P. Fourier transform infrared spectroscopy (FT-IR) analysis further suggested that the phosphate and carboxyl groups on the cell surface might be responsible for the adsorption of cerium. Furthermore, X-ray diffraction (XRD) and transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) suggested that cerium initially occurred on the bacterial cell surface as Ce(OH)3, which was mainly converted to monazite (CePO4) and a small amount of CeO2 overtime. Hydrothermal treatment was used to accelerate the mineralization process of cerium by B. licheniformis. The hydrothermal treatment is conducted for comparative analysis of mineralization process in extreme geological condition.

Adsorption of Rare Earth Elements onto DNA-Functionalized Mesoporous Carbon.

The recovery and separation of rare earth elements (REEs) are of national importance owing to the specific usages, high demand, and low supply of these elements. In this research, we have investigated the adsorption of rare earth elements onto DNA-functionalized mesoporous carbons with a BET surface area of 605 m2/g and a median mesopore width of 48 Å. Three types of single-stranded DNA, one with 100 base units of thymine, another with 20 units of thymine, and the third, a 2000 unit long DNA from salmon milt were grafted on the carboxylated mesoporous carbon surface. All of the DNA-functionalized mesoporous carbons demonstrated higher adsorption of REEs compared to pristine mesoporous carbon and DNA grafted with 100 units of thymine demonstrated slightly higher adsorbed amounts compared to others. Pure neodymium (Nd(III)) adsorption in the aqueous phase demonstrated an adsorbed amount of 110.4 mg/g with respect to the initial concentration of 500 mg/g. A pH variation study with pure Nd(III) demonstrated that the adsorbed amount is higher at elevated pH compared to that at lower pH, thereby suggesting possible recovery at lower pH. Adsorption of a mixture of 16 REEs, including Sc, Lu, Tm, Yb, Er, Ho, Tb, Dy, Y, Eu, Gd, Sm, Ce, Nd, Pr, and La revealed that the adsorbed amount increased with an increase in the atomic weight and metallic radii of elements within the lanthanides. The calculation of the distribution coefficients for all of the equilibrium adsorption amounts suggested that adsorption is more effective in the lower concentration region. The Nd L3-edge X-ray absorption near edge structure (XANES) confirmed a 3+ oxidation state of Nd in the adsorbed phase. The extended X-ray absorption fine structure (EXAFS) confirmed the local atomic structure relaxation of Nd complexes in the adsorbed phase and shortening of the Nd-O bond distance by about 0.03-0.04 Å, which may be associated with their local complexation at the carbon surface.

Adsorption and mineralization of REE-lanthanum onto bacterial cell surface.

A large number of rare earth element mining and application resulted in a series of problems of soil and water pollution. Environmental remediation of these REE-contaminated sites has become a top priority. This paper explores the use of Bacillus licheniformis to adsorb lanthanum and subsequent mineralization process in contaminated water. The maximum adsorption capacity of lanthanum on bacteria was 113.98 mg/g (dry weight) biomass. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data indicated that adsorbed lanthanum on bacterial cell surface occurred in an amorphous form at the initial stage. Scanning electron microscopy with X-ray energy-dispersive spectroscopy (SEM/EDS) results indicated that lanthanum adsorption was correlated with phosphate. The amorphous material was converted into scorpion-like monazite (LaPO4 nanoparticles) in a month. The above results provide a method of using bacterial surface as adsorption and nucleation sites to treat REE-contaminated water.