Effects of Simulated Rare Earth Recycling Wastewaters on Biological Nitrification

Pollution caused by mining reshaped the structure and function of bacterial communities in China's largest ion-adsorption rare earth mine watershed

Ion-adsorption rare earth mining results in the production of high levels of nitrogen, multiple metals, and strong acidic mine drainage (AMD), the impacts of which on microbial assembly and ecological functions remain unclear. To address this knowledge gap, we collected river sediments from the watershed of China's largest ion-adsorption rare earth mine and analyzed the bacterial community's structure, function, and assembly mechanisms. Results showed that bacterial community assembly was weakly affected by spatial dispersion, and dispersal limitation and homogeneous selection were the dominant ecological processes, with the latter increasing with pollution gradients. Bacterial alpha diversity decreased with pollution, which was mainly influenced by lead (Pb), pH, rare earth elements (REEs), and electrical conductivity (EC). However, bacteria developed survival strategies (i.e., enhanced acid tolerance and interspecific competition) to adapt to extreme environments, sustaining species diversity and community stability. Community structure and function showed a consistent response to the polluted environment (r = 0.662, P = 0.001). Enhanced environmental selection reshaped key microbial-mediated biogeochemical processes in the mining area, in particular weakening the potential for microbial denitrification. These findings provide new insights into the ecological response of microbes to compound pollution and offer theoretical support for proposing effective remediation and management strategies for polluted areas.

Activating soil nitrification by co-application of peanut straw biochar and organic fertilizer in a rare earth mining soil

The intensive mining activities to extract rare earth elements from ion-adsorption rare earth deposits have introduced massive amounts of ammonium into the tailing soils in southern China. Compared to the ubiquitous soil nitrification in cropland, forest, and grassland soils, however, there is no feasible strategy to alleviate the ammonium contamination in tailing soil. Herein, the feasibility to remove ammonium by adding ammonium adsorbents (e.g., biochar, activated carbon, and zeolite), alkaline materials, and organic fertilizer to the rare earth mining soil was explored. The amendment of rice straw biochar, activated carbon, or zeolite in combination with CaCO₃ and organic fertilizer showed no significant effect on ammonium removal due to their limited capacity to elevate soil pH. However, the co-application of peanut straw biochar (PSBC), CaCO₃, and organic fertilizer activated both the ammonia volatilization and soil nitrification processes. Specifically, the three components functioned as follows: organic fertilizer supplied active ammonia-oxidizing bacteria (AOB); PSBC stimulated AOB proliferation by elevating soil pH above 7.75; CaCO₃ ameliorated soil acidity and reduced the lag time for activating soil nitrification. The soil ammonium removal and nitrate accumulation rates were positively correlated to the acid neutralization capacity of PSBC prepared at 400 °C-800 °C. The qPCR and microbial community analysis results indicated that Nitrosomonas europaea was the dominant AOB that was responsible for enhanced soil nitrification. Our findings pave the way for developing cost-effective strategies to remediate ammonium contamination in rare earth mining soils.

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.

Genome- and community-level interaction insights into the ecological role of archaea in rare earth element mine drainage in South China

Microbial communities play crucial roles in mine drainage generation and remediation. Despite the wide distribution of archaea in the mine ecosystem, their diversity and ecological roles remain less understood than bacteria. Here, we retrieved 56 archaeal metagenome-assembled genomes from a river impacted by rare earth element (REE) mining activities in South China. Genomic analysis showed that archaea represented four distinct lineages, including phyla of Thaumarchaeota, Micrarchaeota, Nanoarchaeota and Thermoplasmata. These archaea represented a considerable fraction (up to 40%) of the total prokaryote community, which might contribute to nitrogen and sulfur cycling in the REE mine drainage. Reconstructed metabolic potential among diverse archaea taxa revealed that archaea were involved in the network of ammonia oxidation, denitrification, sulfate redox reaction, and required substrates supplied by other community members. As the dominant driver of ammonia oxidation, Thaumarchaeota might provide substrates to support the survival of two nano-sized archaea belonging to Micrarchaeota and Nanoarchaeota. Despite the absence of biosynthesis pathways for amino acids and nucleotides, the potential capacity for nitrite reduction (nirD) was observed in Micrarchaeota, indicating that these nano-sized archaea encompassed diverse metabolisms. Moreover, Thermoplasmata, as keystone taxa in community, might be the main genetic donor for the other three archaeal phyla, transferring many environmental resistance related genes (e.g., V/A-type ATPase and Vitamin B12-transporting ATPase). The genetic interactions within archaeal community through horizontal gene transfer might be the key to the formation of archaeal resistance and functional partitioning. This study provides putative metabolic and genetic insights into the diverse archaea taxa from community-level perspectives, and highlights the ecological roles of archaea in REE contaminated aquatic environment.

Indicator species drive the key ecological functions of microbiota in a river impacted by acid mine drainage generated by rare earth elements mining in South China

Acid mine drainage (AMD) generated by rare earth elements (REEs) deposits exploration contains high concentrations of REEs, ammonium and sulfates, which is quite different from typical metallic AMD. Currently, microbial responses and ecological functions in REEs-AMD impacted rivers are unknown. Here, 16S rRNA analysis and genome-resolved metagenomics were performed on microbial community collected from a REEs-AMD contaminated river. The results showed that REEs-AMD significantly changed river microbial diversity and shaped unique indicator species (e.g. Thaumarchaeota, Methylophilales, Rhodospirillales and Burkholderiales). The main environmental factors regulating community were pH, ammonium and REEs, among which high concentration of REEs increased REEs-dependent enzyme-encoding genes (XoxF and ExaF/PedH). Additionally, we reconstructed 566 metagenome-assembled genomes covering 70.4% of identifying indicators. Genome-centric analysis revealed that the abundant archaea Thaumarchaeota and Xanthomonadaceae were often involved in nitrification and denitrification, while family Burkholderiaceae were capable of sulfide oxidation coupled with dissimilatory nitrate reduction to ammonium. These indicators play crucial roles in nitrogen and sulfur cycling as well as REEs immobilization in REEs-AMD contaminated rivers. This study confirmed the potential dual effect of REEs on microbial community at the functional gene level. Our investigation on the ecological roles of indicators further provided new insights for the development of REEs-AMD bioremediation.

Impacts of anthropogenic gadolinium on the activity of the ammonia oxidizing bacterium Nitrosomonas europaea

Widespread use of gadolinium-based contrast agents in medical imaging has resulted in increased Gd inputs to municipal wastewater treatment plants. Others have reported that typical wastewater treatment does not attenuate Gd, resulting in discharges to natural waters. However, whether elevated Gd impacts the performance of biological treatment has not been investigated. We examined whether gadolinium chloride or Gd chelated with diethylenetriaminepentaacetic acid (DTPA) affected the activity of the model nitrifying bacterium Nitrosomonas europaea. At nominal GdCl₃ additions ranging from 1 to 500 μM, no impact was observed compared to the control. Most (>98%) of the added Gd precipitated, and extracellular GdPO₄ nanoparticles were observed. When chelated with DTPA, Gd remained soluble, but no statistically significant impact on ammonia oxidation was observed until the highest concentrations tested. At 300 and 500 μM Gd-DTPA, a temporary reduction of nitrite production relative to the control (effect size 1.3 mg l^-1 and 1.5 mg l^-1, respectively, at 24 h) was seen. By itself, DTPA was highly inhibitory. Modeling suggested that DTPA likely chelated other metals, but adjusting the concentrations of the most abundant metals in the medium, calcium and magnesium, indicated that lowering their free ion activities was probably not the cause of inhibition. Complexation of other essential metals was more likely. Our studies indicate that while the low bioavailability of Gd may limit its ecosystem impacts, the role of synthetic ligands used with Gd and other rare earth elements should be considered as the production, use and disposal of these elements increases.

Effective removal of calcium and magnesium sulfates from wastewater in the rare earth industry

The wastewater discharged from the rare earth (RE) industry generally contains a high level of calcium and magnesium sulfates, which confers permanent hardness and causes difficulties in recycling this wastewater. In this study, the alkyl phenoxy acetic acid derivatives including 4-methyl phenoxy acetic acid (M-POAA), 4-tert-butyl phenoxy acetic acid (B-POAA) and 4-tert-octyl phenoxy acetic acid (O-POAA), were synthesized via the Williamson reaction and characterized by nuclear magnetic resonance (NMR), infrared (IR), and ultra-violet (UV) spectroscopy, as well as elemental analysis and X-ray diffraction (XRD). Synthesis of the POAAs were simple and green, and the raw materials used for their production are widely available and low-cost. The potential for removal of Ca and Mg sulfates from industrial wastewater using POAAs as the organic precipitants was assessed. The total precipitation efficiencies of Ca and Mg from wastewater with the use of POAAs increased with the following order: M-POAA < B-POAA < O-POAA. The residual concentrations of Ca and Mg using O-POAA as the precipitant were lower than 0.099 and 0.089 g L^-1, respectively. The O-POAA could be regenerated five times without any significant change in its structure and precipitation performance. Thus, the use of the novel precipitants is a prospective alternative to the conventional processes for softening wastewater.

Fabrication of novel metal ion imprinted xanthan gum-layered double hydroxide nanocomposite for adsorption of rare earth elements

The work focus to enhance the properties of xanthan gum (XG) by anchoring metal ions (Fe, Zr) and encapsulating inorganic matrix (M@XG-ZA). The fabricated nanocomposite was characterized by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Fourier Transform Infrared Spectroscopy (FTIR), surface area (BET) and zeta potential analysis. The adsorption of Sc, Nd, Tm and Yb was investigated after screening of synthesized materials in detail to understand the influence of pH, contact time, temperature and initial REE (rare earth element) concentration both in single and multicomponent system via batch adsorption. The adsorption mechanism was verified by FTIR, SEM and elemental mapping. The SEM images of Zr@XG-ZA demonstrate scutes structure, which disappeared after adsorption of REEs. The maximum adsorption capacities were 132.30, 14.01, 18.15 and 25.73 mg/g for Sc, Nd, Tm and Yb, respectively. The adsorption efficiency over Zr@XG-ZA in multicomponent system was higher than single system and the REEs followed the order: Sc > Yb > Tm > Nd. The Zr@XG-ZA demonstrate good adsorption behavior for REEs up to five cycles and then it can be used as photocatalyst for the degradation of tetracycline. Thus, the work adds a new insight to design and preparation of efficient bifunctional adsorbents from sustainable materials for water purification.

Structure, Variation, and Co-occurrence of Soil Microbial Communities in Abandoned Sites of a Rare Earth Elements Mine

Mining activity for rare earth elements (REEs) has caused serious environmental pollution, particularly for soil ecosystems. However, the effects of REEs on soil microbiota are still poorly understood. In this study, soils were collected from abandoned sites of a REEs mine, and the structure, diversity, and co-occurrence patterns of soil microbiota were evaluated by Illumina high-throughput sequencing targeting 16S rRNA genes. Although microbiota developed significantly along with the natural restoration, the microbial structure on the site abandoned for 10 years still significantly differed from that on the unmined site. Potential plant growth promoting bacteria (PGPB) were identified by comparing 16S sequences against a self-constructed PGPB database via BLAST, and it was found that siderophore-producing and phosphorus-solubilizing bacteria were more abundant in the studied soils than in reference soils. Canonical correspondence analysis indicated that species richness of plant community was the prime factor affecting microbial structure, followed by limiting nutrients (total carbon and total nitrogen) and REEs content. Further co-occurring network analysis revealed nonrandom assembly patterns of microbiota in the studied soils. These results increase our understanding of microbial variation and assembly pattern during natural restoration in REE contaminated soils.

Bioadsorption of Rare Earth Elements through Cell Surface Display of Lanthanide Binding Tags

With the increasing demand for rare earth elements (REEs) in many emerging clean energy technologies, there is an urgent need for the development of new approaches for efficient REE extraction and recovery. As a step toward this goal, we genetically engineered the aerobic bacterium Caulobacter crescentus for REE adsorption through high-density cell surface display of lanthanide binding tags (LBTs) on its S-layer. The LBT-displayed strains exhibited enhanced adsorption of REEs compared to cells lacking LBT, high specificity for REEs, and an adsorption preference for REEs with small atomic radii. Adsorbed Tb(3+) could be effectively recovered using citrate, consistent with thermodynamic speciation calculations that predicted strong complexation of Tb(3+) by citrate. No reduction in Tb(3+) adsorption capacity was observed following citrate elution, enabling consecutive adsorption/desorption cycles. The LBT-displayed strain was effective for extracting REEs from the acid leachate of core samples collected at a prospective rare earth mine. Our collective results demonstrate a rapid, efficient, and reversible process for REE adsorption with potential industrial application for REE enrichment and separation.