Insights into the Selective Transformation of Ceria Sulfation and Iron/Manganese Mineralization for Enhancing the Selective Recovery of Rare Earth Elements.

To cope with the urgent and unprecedented demands for rare earth elements (REEs) in sophisticated industries, increased attention has been paid to REE recovery from recycled streams. However, the similar geochemical behaviors of REEs and transition metals often result in poor separation performance due to nonselectivity. Here, a unique approach based on the selective transformation between ceria sulfation and iron/manganese mineralization was proposed, leading to the enhancement of the selective separation of REEs. The mechanism of the selective transformation of minerals could be ascribed to the distinct geochemical and metallurgical properties of ions, resulting in different combinations of cations and anions. According to hard-soft acid-base (HSAB) theory, the strong Lewis acid of Ce(III) was inclined to combine with the hard base of sulfates (SO42-), while the borderline acid of Fe(II)/Mn(II) prefers to interact with oxygen ions (O2-). Both in situ characterization and density functional theory (DFT) calculation further revealed that such selective transformation might trigger by the generation of an oxygen vacancy on the surface of CeO2, leading to the formation of Ce2(SO4)3 and Fe/Mn spinel. Although the electron density difference of the configurations (CeO2-x-SO4, Fe2O3-x-SO4, and MnO2-x-SO4) shared a similar direction of the electron transfer from the metals to the sulfate-based oxygen, the higher electron depletion of Ce (QCe = -1.91 e) than Fe (QFe = -1.66 e) and Mn (QMn = -1.64 e) indicated the higher stability in the Ce-O-S complex, resulting in the larger adsorption energy of CeO2-x-SO4 (-6.88 eV) compared with Fe2O3-x-SO4 (-3.10 eV) and MnO2-x-SO4 (-2.49 eV). This research provided new insights into the selective transformation of REEs and transition metals in pyrometallurgy and thus offered a new approach for the selective recovery of REEs from secondary resources.

New Insights into the Role of Natural Organic Matter in Fe-Cr Coprecipitation: Importance of Molecular Selectivity.

Coprecipitation of Fe/Cr hydroxides with natural organic matter (NOM) is an important pathway for Cr immobilization. However, the role of NOM in coprecipitation is still controversial due to its molecular heterogeneity and diversity. This study focused on the molecular selectivity of NOM toward Fe/Cr coprecipitates to uncover the fate of Cr via Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). The results showed that the significant effects of Suwannee River NOM (SRNOM) on Cr immobilization and stability of the Fe/Cr coprecipitates did not merely depend on the adsorption of SRNOM on Fe/Cr hydroxides. FT-ICR-MS spectra suggested that two pathways of molecular selectivity of SRNOM in the coprecipitation affected Cr immobilization. Polycyclic aromatics and polyphenolic compounds in SRNOM preferentially adsorbed on the Fe/Cr hydroxide nanoparticles, which provided extra binding sites and promoted the aggregation. Notably, some specific compounds (i.e., polyphenolic compounds and highly unsaturated phenolic compounds), less unsaturated and more oxygenated than those adsorbed on Fe/Cr hydroxide nanoparticles, were preferentially incorporated into the insoluble Cr-organic complexes in the coprecipitates. Kendrick mass defect analysis revealed that the insoluble Cr-organic complexes contained fewer carbonylated homologous compounds. More importantly, the spatial distribution of insoluble Cr-organic complexes was strongly related to Cr immobilization and stability of the Fe/Cr-NOM coprecipitates. The molecular information of the Fe/Cr-NOM coprecipitates would be beneficial for a better understanding of the transport and fate of Cr and exploration of the related remediation strategy.

Plasma-Membrane-Localized Transporter NREET1 is Responsible for Rare Earth Element Uptake in Hyperaccumulator Dicranopteris linearis.

Rare earth elements (REEs) are critical for numerous modern technologies, and demand is increasing globally; however, production steps are resource-intensive and environmentally damaging. Some plant species are able to hyperaccumulate REEs, and understanding the biology behind this phenomenon could play a pivotal role in developing more environmentally friendly REE recovery technologies. Here, we identified a REE transporter NRAMP REE Transporter 1 (NREET1) from the REE hyperaccumulator fern Dicranopteris linearis. Although NREET1 belongs to the natural resistance-associated macrophage protein (NRAMP) family, it shares a low similarity with other NRAMP members. When expressed in yeast, NREET1 exhibited REE transport capacity, but it could not transport divalent metals, such as zinc, nickel, manganese, or iron. NREET1 is mainly expressed in D. linearis roots and predominantly localized in the plasma membrane. Expression studies in Arabidopsis thaliana revealed that NREET1 functions as a transporter mediating REE uptake and transfer from root cell walls into the cytoplasm. Moreover, NREET1 has a higher affinity for transporting light REEs compared to heavy REEs, which is consistent to the preferential enrichment of light REEs in field-grown D. linearis. We therefore conclude that NREET1 may play an important role in the uptake and consequently hyperaccumulation of REEs in D. linearis. These findings lay the foundation for the use of synthetic biology techniques to design and produce sustainable, plant-based REE recovery systems.

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.

Discrimination and Quantification of Soil Nanoparticles by Dual-Analyte Single Particle ICP-QMS.

This study presents the new application of dual-analyte single particle inductively coupled plasma quadrupole mass spectrometry (spICP-QMS) to the discrimination and quantification of two typical soil nanoparticles (kaolinite and goethite nanoparticles, abbr. KNPs and GNPs) in three samples (SA, SB, and SC) with three detection events (Al unpaired event, Fe unpaired event, and paired event). SA was mainly composed of KNPs with a concentration of 28 443 ± 817 particle mL-1 and a mean particle size of 140.7 ± 0.2 nm. SB was mainly composed of GNPs with a concentration of 39 283 ± 702 particle mL-1 and a mean particle size of 141.8 ± 2.9. In SC, the concentrations of KNPs and GNPs were 22 4541 ± 1401 and 70 604 ± 1623 particle mL-1, respectively, and the mean particle sizes of KNPs and GNPs were 140.7 ± 0.2 and 60.2 ± 0.3 nm, respectively. The accuracy of dual-analyte spICP-QMS was determined by spiking experiments, comparing these results with the measurements of other techniques, analyzing the samples in different SA and SB proportions and in different SC concentrations. Our results demonstrated that the dual-analyte spICP-QMS is a promising approach to distinguishing different kinds of natural NPs in soils.

Characterization of Neodymium Speciation in the Presence of Fulvic Acid by Ion Exchange Technique and Single Particle ICP-MS.

It has been well known that the free ion concentration of metals plays a vital role in metal bioavailability. However, measurement of this fraction is still not easy over years of development. Nowadays, rare earth elements (REEs) are drawing more attentions as an emerging contaminant due to their wide applications in our daily life. To analyze the free ion concentration of neodymium (Nd), we adopted ion-exchange technique (IET) to investigate the changes on Nd free ion concentration in the presence of fulvic acid (FA). With the dynamic mode of IET analysis, the concentrations of Nd free ion were in the range of 0.85-36.8 × 10-8 M at the total Nd concentration of 5 × 10-7 M when FA varied from 0.4 to 10 M. However, these concentrations were 3-58 times higher than the one calculated by WHAM 7.0, which may be due to the particulate Nd spontaneously formed in solution. With single particle ICP-MS analysis, we found 0.25%-2.36% of Nd was in the form of colloids when the total Nd concentrations varied from 8.5 × 10-9 to 4.7 × 10-7 M, with the average particle sizes in the range of 26.5-39.2 nm. The presence of FA significantly decreased the number of Nd colloids, but increased the average particle size. Under the TEM, we found that Nd colloids were amorphous, with the size less than 200 nm. The present study provided a relatively new perspective on REE speciation in water. The natural organic matters not only affect the free ion concentration of Nd, but also influenced the size and numbers of Nd colloids in solution.

Roles of soluble minerals in Cd sorption onto rice straw biochar.

Transforming to biochar provides an environmentally friendly approach for crop residue reutilization, which are usually applied as sorbent for heavy metal removal. As typical silicon-rich material, the specific sorptive mechanisms of rice straw derived biochar (RSBC) are concerned, especially at the low concentration range which is more environmentally relevant. In the present study, Cd sorption onto RSBCs at the concentration of ≤ 5 mg/L was investigated. The sorptive capacity was positively correlated with the pyrolytic temperature of the biochar and the environmental pH value. Water soluble minerals of the RSBCs played the dominant roles in Cd sorption, contributing 29.2%, 62.5% and 82.9% of the total sorption for RSBCs derived under 300°C, 500°C and 700°C, respectively. Increased number of cations, dominantly K+, were exchanged during the sorption. Coprecipitation with cations and carbonates may also be contributive to the sorption. The dissolution of silicon-containing minerals was found to be declined during sorption, suggesting its involvement in the sorption process, possibly through precipitation. Whilst, the sparingly soluble silicate crystals may impose ignorable role in the sorption. Complexation with organic groups is only a minor mechanism in Cd sorption, compared to the much more dominant roles of the inorganic ashes.

Rare earth elements, aluminium and silicon distribution in the fern Dicranopteris linearis revealed by µPIXE Maia analysis.

BACKGROUND: The fern Dicranopteris linearis is a hyperaccumulator of rare earth elements (REEs), aluminium (Al) and silicon (Si). However, the physiological mechanisms of tissue-level tolerance of high concentrations of REE and Al, and possible interactions with Si, are currently incompletely known. METHODS: A particle-induced X-ray emission (µPIXE) microprobe with the Maia detector, scanning electron microscopy with energy-dispersive spectroscopy and chemical speciation modelling were used to decipher the localization and biochemistry of REEs, Al and Si in D. linearis during uptake, translocation and sequestration processes. RESULTS: In the roots >80 % of REEs and Al were in apoplastic fractions, among which the REEs were most significantly co-localized with Si and phosphorus (P) in the epidermis. In the xylem sap, REEs were nearly 100 % present as REEH3SiO42+, without significant differences between the REEs, while 24-45 % of Al was present as Al-citrate and only 1.7-16 % Al was present as AlH3SiO42+. In the pinnules, REEs were mainly concentrated in necrotic lesions and in the epidermis, and REEs and Al were possibly co-deposited within phytoliths (SiO2). Different REEs had similar spatial localizations in the epidermis and exodermis of roots, the necrosis, veins and epidermis of pinnae of D. linearis. CONCLUSIONS: We posit that Si plays a critical role in REE and Al tolerance within the root apoplast, transport within the vascular bundle and sequestration within the blade of D. linearis.

Spatially Resolved Localization of Lanthanum and Cerium in the Rare Earth Element Hyperaccumulator Fern Dicranopteris linearis from China.

The fern Dicranopteris linearis (Gleicheniaceae) from China is a hyperaccumulator of rare earth element (REE), but little is known about the ecophysiology of REE in this species. This study aimed to clarify tissue-level and organ-level distribution of REEs via synchrotron-based X-ray fluorescence microscopy (XFM). The results show that REEs (La + Ce) are mainly colocalized with Mn in the pinnae and pinnules, with the highest concentrations in necrotic lesions and lower concentrations in veins. In the cross sections of the pinnules, midveins, rachis, and stolons, La + Ce and Mn are enriched in the epidermis, vascular bundles, and pericycle (midvein). In these tissues, Mn is localized mainly in the cortex and mesophyll. We hypothesize that the movement of REEs in the transpiration flow in the veins is initially restricted in the veins by the pericycle between vascular bundle and cortex, while excess REEs are transported by evaporation and cocompartmentalized with Mn in the necrotic lesions and epidermis in an immobile form, possibly a Si-coprecipitate. The results presented here provide insights on how D. linearis regulates high concentrations of REEs in vivo, and this knowledge is useful for developing phytotechnological applications (such as REE agromining) using this fern in REE-contaminated sites in China.

Ecological Risk Assessment of Neodymium and Yttrium on Rare Earth Element Mine Sites in Ganzhou, China.

Nowadays rare earth elements (REEs) are widely applied in high-technology and clean energy products, but their environmental risks are still largely unknown. To estimate the ecological risk of REEs, soil samples were collected from REE mine tailings with and without phytoremediation. The results showed that the tailings had rather low organic matter and high total REE concentrations, up to 808.5 mg/kg. The 10% effective concentration (EC10) of neodymium (Nd) and yttrium (Y) were calculated based on the toxicity tests of seed germination and root growth. For both wheat and mung bean, the EC10 of Nd and Y in soils were in the range of 1053.1-1300.1 mg/kg. The average hazard quotient of mine tailing soil without phytoremediation was higher than that with phytoremediation. All the hazard quotient of Nd and Y were less than 1, indicating that Nd or Y alone was unlikely to cause adverse ecological effects. Given to the coexistence of REEs on mine sites, the ecological risk of REE mixture could be potentially high towards local soil environments, even for soils with phytoremdiation.

Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge.

Hyperaccumulator plants are the material basis for phytoextraction research and for practical applications in decontaminating polluted soils and industrial wastes. China's high biodiversity and substantial mineral resources make it a global hotspot for hyperaccumulator plant species. Intensive screening efforts over the past 20 years by researchers working in China have led to the discovery of many different hyperaccumulators for a range of elements. In this review, we present the state of knowledge on all currently reported hyperaccumulator species from China, including Cardamine hupingshanensis (selenium, Se), Dicranopteris dichotoma (rare earth elements, REEs), Elsholtzia splendens (copper, Cu), Phytolacca americana (manganese, Mn), Pteris vittata (arsenic, As), Sedum alfredii, and Sedum plumbizincicola (cadmium/zinc, Cd/Zn). This review covers aspects of the ecophysiology and molecular biology of tolerance and hyperaccumulation for each element. The major scientific advances resulting from the study of hyperaccumulator plants in China are summarized and synthesized.

Accumulation and fractionation of rare earth elements (REEs) in the naturally grown Phytolacca americana L. in southern China.

The widespread use of rare earth elements (REEs) has resulted in problems for soil and human health. Phytolacca americana L. is a herbaceous plant widely distributed in Dingnan county of Jiangxi province, China, which is a REE mining region (ion absorption rare earth mine) and the soil has high levels of REEs. An investigation of REE content of P. americana growing naturally in Dingnan county was conducted. REE concentrations in the roots, stems, and leaves of P. americana and in their rhizospheric soils were determined. Results showed that plant REEs concentrations varied among the sampling sites and can reach 1040 mg/kg in the leaves. Plant REEs concentrations decreased in the order of leaf > root > stem and all tissues were characterized by a light REE enrichment and a heavy REE depletion. However, P. americana exhibited preferential accumulation of light REEs during the absorption process (from soil to root) and preferential accumulation of heavy REEs during the translocation process (from stem to leaf). The ability of P. americana to accumulate high REEs in the shoot makes it a potential candidate for understanding the absorption mechanisms of REEs and for the phytoremediation of REEs contaminated soil.

Zinc Isotope Fractionation in the Hyperaccumulator Noccaea caerulescens and the Nonaccumulating Plant Thlaspi arvense at Low and High Zn Supply.

On the basis of our previous field survey, we postulate that the pattern and degree of zinc (Zn) isotope fractionation in the Zn hyperaccumulator Noccaea caerulescens (J. & C. Presl) F. K. Mey may reflect a relationship between Zn bioavailability and plant uptake strategies. Here, we investigated Zn isotope discrimination during Zn uptake and translocation in N. caerulescens and in a nonaccumulator Thlaspi arvense L. with a contrasting Zn accumulation ability in response to low (Zn-L) and high (Zn-H) Zn supplies. The average isotope fractionations of the N. caerulescens plant as a whole, relative to solution (Δ(66)Znplant-solution), were -0.06 and -0.12‰ at Zn-L-C and Zn-H-C, respectively, indicative of the predominance of a high-affinity (e.g., ZIP transporter proteins) transport across the root cell membrane. For T. arvense, plants were more enriched in light isotopes under Zn-H-A (Δ(66)Znplant-solution = -0.26‰) than under Zn-L-A and N. caerulescens plants, implying that a low-affinity (e.g., ion channel) transport might begin to function in the nonaccumulating plants when external Zn supply increases. Within the root tissues of both species, the apoplast fractions retained up to 30% of Zn mass under Zn-H. Moreover, the highest δ(66)Zn (0.75‰-0.86‰) was found in tightly bound apoplastic Zn, pointing to the strong sequestration in roots (e.g., binding to high-affinity ligands/precipitation with phosphate) when plants suffer from high Zn stress. During translocation, the magnitude of isotope fractionation was significantly greater at Zn-H (Δ(66)Znroot-shoot = 0.79‰) than at Zn-L, indicating that fractionation mechanisms associated with root-shoot translocation might be identical to the two plant species. Hence, we clearly demonstrated that Zn isotope fractionation could provide insight into the internal sequestration mechanisms of roots when plants respond to low and high Zn supplies.

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.

Nickel and zinc isotope fractionation in hyperaccumulating and nonaccumulating plants.

Until now, there has been little data on the isotope fractionation of nickel (Ni) in higher plants and how this can be affected by plant Ni and zinc (Zn) homeostasis. A hydroponic cultivation was conducted to investigate the isotope fractionation of Ni and Zn during plant uptake and translocation processes. The nonaccumulator Thlaspi arvense, the Ni hyperaccumulator Alyssum murale and the Ni and Zn hyperaccumulator Noccaea caerulescens were grown in low (2 µM) and high (50 µM) Ni and Zn solutions. Results showed that plants were inclined to absorb light Ni isotopes, presumably due to the functioning of low-affinity transport systems across root cell membrane. The Ni isotope fractionation between plant and solution was greater in the hyperaccumulators grown in low Zn treatments (Δ(60)Ni(plant-solution) = -0.90 to -0.63‰) than that in the nonaccumulator T. arvense (Δ(60)Ni(plant-solution) = -0.21‰), thus indicating a greater permeability of the low-affinity transport system in hyperaccumulators. Light isotope enrichment of Zn was observed in most of the plants (Δ(66)Zn(plant-solution) = -0.23 to -0.10‰), but to a lesser extent than for Ni. The rapid uptake of Zn on the root surfaces caused concentration gradients, which induced ion diffusion in the rhizosphere and could result in light Zn isotope enrichment in the hyperaccumulator N. caerulescens. In high Zn treatment, Zn could compete with Ni during the uptake process, which reduced Ni concentration in plants and decreased the extent of Ni isotope fractionation (Δ(60)Ni(plant-solution) = -0.11 to -0.07‰), indicating that plants might take up Ni through a low-affinity transport system of Zn. We propose that isotope composition analysis for transition elements could become an empirical tool to study plant physiological processes.

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.

Characterizing the remobilization flux of cadmium from pre-anthesis vegetative pools in rice during grain filling using an improved stable isotope labeling method.

A clear understanding of the allocation of Cd to grains is essential to manage the level of Cd in cereal diets effectively. Yet, debate remains over whether and how the pre-anthesis pools contribute to grain Cd accumulation, resulting in uncertainty regarding the need to control plant Cd uptake during vegetative growth. To this end, rice seedlings were exposed to 111Cd labeled solution until tillering, transplanted to unlabeled soils, and grown under open-air conditions. The remobilization of Cd derived from pre-anthesis vegetative pools was studied through the fluxes of 111Cd-enriched label among organs during grain filling. The 111Cd label was continuously allocated to the grain after anthesis. The lower leaves remobilized the Cd label during the earlier stage of grain development, which was allocated almost equally to the grains and husks + rachis. During the final stage, the Cd label was strongly remobilized from the roots and, less importantly, the internodes, which was strongly allocated to the nodes and, to a less extent, the grains. The results show that the pre-anthesis vegetative pools are an important source of Cd in rice grains. The lower leaves, internodes, and roots are the source organs, whereas the husks + rachis and nodes are the sinks competing with the grain for the remobilized Cd. This study provides insight into understanding the ecophysiological mechanism of Cd remobilization and setting agronomic measures for lowering grain Cd levels.

Heavy metal contamination affects the core microbiome and assembly processes in metal mine soils across Eastern China.

Mining activities in metal mine areas cause serious environmental pollution, thereby imposing stresses to soil ecosystems. Investigating the ecological pattern underlying contaminated soil microbial diversity is essential to understand ecosystem responses to environment changes. Here we collected 624 soil samples from 49 representative metal mines across eastern China and analyzed their soil microbial diversity and biogeographic patterns by using 16 S rRNA gene amplicons. The results showed that deterministic factors dominated in regulating the microbial community in non-contaminated and contaminated soils. Soil pH played a key role in climatic influences on the heavy metal-contaminated soil microbial community. A core microbiome consisting of 25 taxa, which could be employed for the restoration of contaminated soils, was identified. Unlike the non-contaminated soil, stochastic processes were important in shaping the heavy metal-contaminated soil microbial community. The largest source of variations in the soil microbial community was land use type. This result suggests that varied specific ecological remediation strategy ought to be developed for differed land use types. These findings will enhance our understanding of the microbial responses to anthropogenically induced environmental changes and will further help to improve the practices of soil heavy metal contamination remediation.

Selective recovery of rare earth elements and value-added chemicals from the Dicranopteris linearis bio-ore produced by agromining using green fractionation.

The increasing demand for Rare Earth Elements (REEs) and the depletion of mineral resources motivate sustainable strategies for REE recovery from alternative unconventional sources, such as REE hyperaccumulator. The greatest impediment to REE agromining is the difficulty in the separation of REEs and other elements from the harvested biomass (bio-ore). Here, we develop a sulfuric acid assisted ethanol fractionation method for processing D. linearis bio-ore to produce the pure REE compounds and value-added chemicals. The results show that 94.5% of REEs and 87.4% of Ca remained in the solid phase, and most of the impurities (Al, Fe, Mg, and Mn) transferred to the liquid phase. Density functional theory calculations show that the water-cation bonds of REEs and Ca cations were broken more easily than the bonds of the cations of key impurities, causing lower solubility of REEs and Ca compounds. Subsequent separation and purification led to a REE-oxide (REO) product with a purity of 97.1% and a final recovery of 88.9%. In addition, lignin and phenols were obtained during organosolv fractionation coupled with a fast pyrolysis process. This new approach opens up the possibility for simultaneous selective recovery of REEs and to produce value-added chemicals from REE bio-ore refining.

Rare earth elements detoxification mechanism in the hyperaccumulator Dicranopteris linearis: [silicon-pectin] matrix fixation.

Dicranopteris linearis is the best-known hyperaccumulator species of rare earth elements (REEs) and silicon (Si), capable of dealing with toxic level of REEs. Hence, this study aimed to clarify how D. linearis leaves cope with excessive REE stress, and whether Si plays a role in REE detoxification. The results show that lanthanum (La - as a representative of the REEs) stress led to decreased biomass and an increase of metabolism related to leaf cell wall synthesis and modification. However, the La stress-induced responses, especially the increase of pectin-related gene expression level, pectin polysaccharides concentration, and methylesterase activity, could be mitigated by Si supply. Approximately 70% of the Si in D. linearis leaves interacted with the cell walls to form organosilicon Si-O-C linkages. The Si-modified cell walls contained more hydroxyl groups, leading to a more efficient REE retention compared to the Si-free ones. Moreover, this [Si-cell wall] matrix increased the pectin-La accumulation capacity by 64%, with no effect on hemicellulose-La and cellulose-La accumulation capacity. These results suggest that [Si-pectin] matrix fixation is key in REE detoxification in D. linearis, laying the foundation for the development of phytotechnological applications (e.g., REE phytomining) using this species in REE-contaminated sites.