Rare Earth Tailings Research Articles

Risk Assessment and Source Appointment of Heavy Metal and Metalloid Pollution in Soil from a Typical Ion-Adsorption Rare Earth Tailing Abandoned for 15 Years

Risk Assessment and Source Appointment of Heavy Metal and Metalloid Pollution in Soil from a Typical Ion-Adsorption Rare Earth Tailing Abandoned for 15 Years

Synergistic leaching process for ion-exchange ammonium from weathered crust elution deposited rare earth tailings with potassium magnesium compound eluent.

The ion-exchangeable ammonium (IE-A) that accounts for 60-90% of the total residual ammonium in rare earth tailings has great potential to pollute the surrounding environment, and much research has been done to seek an effective elution method. However, the current study mainly focused on the single salt solution, which made it hard to reach the desired elution efficiency. In this study, the efficient binary compound eluent was prepared, and the response surface experiments and dynamic elution were performed to optimize the elution condition and evaluate the practical application prospect. Batch experimental results showed that the best IE-A elution efficiency could be achieved at the K:Mg molar ratio of 8:2, the liquid-solid ratio of 26:1, and the concentration of 0.1mol/L at the natural solution pH. Dynamic experimental results indicated that a higher concentration, flow rate, and elution temperature could all accelerate the elution process, and the highest elution efficiency could reach 99%. The fitting results by shrinking core models show that the apparent activation energy of IE-A was 4.24kJ/mol in the temperature range of 288-328K, and the reaction order was 0.16. XPS and FTIR revealed that IE-A was effectively eluted by a potassium and magnesium compound leaching agent via an ion-exchange reaction. Overall, the developed compound solution with potassium and magnesium is a candidate for an elution agent that could be used to remove residual ammonium in a closed field of rare earth ores.

Numerical simulation and safety distance analysis of slope instability of ionic rare earth tailings in different rainy seasons

A significant number of ionic rare earth tailings slopes were generated during the process of rare earth mining and the factor of safety of these slopes continued to decrease under the action of rainfall. This study focussed on a specific tailings slope located in Dayu County, Ganzhou City. Through the utilization of MatDEM numerical simulation software, the variations in infiltration distribution and destabilization evolution of the tailings slope between different seasons were comparatively examined. The results indicated that during the rainy season (May to June), increased rainfall led to increased rate of water infiltration, consequently reducing the stability of the slope and exacerbating landslide damage. Furthermore, the study provided analyses of safety distances under different seasons of rainfall, taking into account the geographical location of the tailings slope in Dayu County, Ganzhou City, and the results of the analyses provided theoretical foundation into ensuring the safety of buildings and residents downstream of the slope.

Dynamic elution of residual ion-exchangeable ammonium from weathered crust elution-deposited rare earth tailings

Ion-exchangeable ammonium (IEA), as one of the major existing forms of the residual ammonium in weathered crust elution-deposited rare earth tailings, has great potential to be released under natural conditions, which may lead to serious environmental pollution. Herein IEA distribution feature was determined, and five common ions (Al3+, Ca2+, Mg2+, Na+, and K+) were used as eluent to remove it from the tailings. Elution process and mechanism were studied in details. The results showed that IEA was mainly concentrated on illite which accounts for the largest proportion of the rare earth clay minerals (>65%). During the elution process, the metal ions initial concentration, flow rate, and particle size affected IEA removal efficiency while pH exhibited only a small effect, and the elution rate was controlled by internal diffusion. Among the selected eluents, the elution efficiency decreased in the order of K+>Mg2+>Al3+>Ca2+>Na+. The highest elution efficiency of IEA can reach 95.9% at an initial K+ concentration of 0.1 mol/L. Characterization results demonstrated that IEA could be effectively eluted by the metal ions through ion-exchange reaction. The results are valuable for choosing an efficient eluent and controlling the environmental pollution caused by the release of IEA.

Phytoremediation with application of anaerobic fermentation residues regulate the assembly of ecological clusters within co-occurrence network in ionic rare earth tailings soil: A pot experiment

The cultivation of energy plants (Pennisetum hybrid) with anaerobic fermentation residues has become an important phytoremediation approach in ionic rare earth elements (REEs) tailings because of its advantages in low cost and sustainability recently. In this study, a comparative pot experiment was carried out to determine the interaction pattern and key ecological clusters in microbial community respond to phytoremediation. Results showed that the application of biogas residues or slurry could effectively mitigate soil acidification, increase soil nutrients, alter REEs bioavailability and promote plant growth. Without fertilization, plant growth was restricted and soil acidification and nutrient-deficiency would be further aggravated. This difference in phytoremediation effect was associated with the assembly of seven key ecological clusters in co-occurrence network of rhizosphere soil. And such assembly pattern of cluster, determined by the environmental preference (e.g. pH, REEs), nutrient demand and interaction among clusters, could alter the microbial communities in response to the changes in soil context rapidly and exert corresponding ecological function during phytoremediation, such as participating in soil nutrient cycling, affecting plant biomass and altering REEs bioavailability. These findings provided new insights for anaerobic fermentation residues application, and can be beneficial to support for studying microbe-plant combined remediation in the future.

Evaluation of growth adaptation of Cinnamomum camphora seedlings in ionic rare earth tailings environment

The root system is an important organ for nutrient uptake and biomass accumulation in plants, while biomass allocation directly affects essential oils content, which plays an essential role in plant growth and development and resistance to adverse environmental conditions. This study was undertaken to investigate the differences and correlation of biomass allocation, root traits and essential oil content (EOC), as well as the adaptations of camphor tree with different chemical types to the ionic rare earth tailing sand habitats. Data from 1-year old cutting seedlings of C. camphora showed that the biomass of C. camphora cuttings was mainly distributed in root system, with the ratio of root biomass 49.9–72.13% and the ratio of root to canopy 1.00–2.64. The total biomass was significantly positively correlated with root length (RL), root surface area (RSA) and dry weight of fine roots (diameter ≤ 2 mm) (P < 0.05). Root biomass and leaf biomass were negatively and positively with specific root length (SRL) and specific root surface area (SRSA), respectively. Leaf biomass presented a positive effect on EOC (P < 0.05), with the correlation coefficient of 0.808. The suitability sort of these camphor trees was as follows: C. camphora β-linalool, C. camphora α-linaloolII, C. camphora α-linaloolI being better adapted to the ionic rare earth tailings substrate, C. camphora citral being the next, and C. porrectum β-linalool and C. camphora borneol being the least adaptive. EOC played a positive role in the adaptation of C. camphora (R2 = 0.6099, P < 0.05). Therefore camphor tree with linalool type is the appropriate choice in the ecological restoration of ionic rare earth tailings. The study could provide scientific recommendations for the ecological restoration of ionic rare earth tailings area combined with industrial development.

The Process Mineralogical Characterization of Bayan Obo Rare-Earth Tailings and Density Functional Theory Study of the Occurrence State of Sc

As Bayan Obo rare-earth tailings, which are generated after the production of mineral products with the raw ore from different mining areas, are considered secondary resources rich in valuable elements such as F, Fe, REE, and Nb, an effective method is urgently needed to recover such valuable elements for resource recycling and environmental conservation. A mineralogical analysis can enable process diagnosis, design, and optimization and is the key to comprehensively utilizing valuable elements. Hence, detailed mineralogical characterization is necessary as a starting point to develop a feasible processing flowsheet. In this study, various detection methods, namely inductively coupled plasma-optical emission spectrometry (ICP), X-ray Fluorescence Spectrometer (XRF), X-ray powder diffractometer (XRD), scanning electron microscopy system with an energy dispersive spectrometer (SEM-EDS), mineral liberation analysis (MLA), and electron probe microanalysis (EPMA), were applied to conduct detailed mineralogical characterization of Bayan Obo rare-earth tailings, and the occurrence state of Sc in the main Sc-bearing minerals was studied using density functional theory (DFT). The results showed that Fe mainly occurs in hematite, riebeckite, ankerite, siderite, and pyrite, with contents of 50.15 wt%, 27.94 wt%, 8.34 wt%, 4.92 wt%, and 5.59 wt%, respectively. Nearly all F occurs in 26.8 wt% fluorite. The main rare-earth minerals are bastnasite, apatite, and monazite (La), with contents of 5.0%, 5.0%, and 1.6% in Bayan Obo rare-earth tailings, respectively. Notably, 48.47%, 21.70%, 10.34%, and 10.28% of niobium element occurs in nioboaeschynite, pyrochlore, dingdaohengite, and ilmenorutile, respectively. Scandium was detected in five minerals, namely aegirine, riebeckite, monazite, ilmenorutile, and niobite, with average contents of 0.04 wt%, 0.22 wt%, 0.06 wt%, 0.06 wt%, and 1.58 wt%, respectively. According to the DFT analysis, the state of Sc in aegirine is different from that in riebeckite. Scandium in aegirine mainly substitutes Fe or enters the interstitial lattice site, while Sc in riebeckite tends to replace Fe. Based on these results, a process for recovering valuable elements from tailings is proposed.

Effects of different ecological restoration methods on the soil bacterial community structure of a light rare earth tailings pond

Effects of different ecological restoration methods on the soil bacterial community structure of a light rare earth tailings pond

Selective recovery of Th(IV) from radioactive rare earth waste residue by utilizing MoS2-modified ion-absorbed type rare earth tailings

In the process of extracting ion-absorbed rare earth ore (IREO), the production of radioactive waste is a major environmental concern. To address this issue, MoS2 was used to modify ion-absorbed rare earth tailings (RET) to synthesize a novel MoS2@RET composite material for the effective handling of radioactive waste generated in IREO separation industry. The composite material was thoroughly characterized using various analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), Fourier-transform infrared (FTIR), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) and energy dispersive spectroscopy (EDS). By optimizing the operating parameters, the optimal experimental conditions were determined to be pH = 3, contact time = 60 min, liquid-solid ratio = 6 g/L, and initial concentration = 150 mg/L. The adsorption data fitted well with the pseudo second-order rate model. The thermodynamic parameters concerning the adsorption of Th(IV) were analyzed and computed. Langmuir isotherm model is a more fitting choice for the adsorption process compared to the Freundlich isotherm model. MoS2@RET was used in the acid leachate of IREO waste residue, achieving the separation of Th and rare earth successfully. The mechanism of Th(IV) adsorption by MoS2@RET was investigated, revealing that the adsorption process involves electrostatic interactions, chemical bonding, and redox reactions. The above research results indicate that MoS2@RET composite materials have application potential in the sustainable treatment of IREO radioactive waste.

Preparation and properties of Portland cement from secondary recycle waste of rare earth tailing

Secondary recycle waste of rare earth tailing (RET) was used as raw material to prepare Portland cement. The phase composition and microstructure of clinker, mechanical strength and hydration characteristic of cement were investigated. The results indicated that the RET recycle waste increased the practical content of early-strength and high-durability mineral phases (C3S and C4AF) in clinker, which respectively rose from 45.3% to 67.5% and 6.1% to 10.6%, and significantly reduced the ratio of f-CaO from 0.4% to 0.23%, finally causing the intensity reduction of first exothermic peak of hydration, prolonging the appearing time of the second and third exothermic peak, and rising initial and final setting time. This RET recycle waste-fabricated cement generated high strength, and the improving rate of 1-day and 3-day compressive strength achieved to 81.2% and 60.3%, due to dense microstructure caused by fast hydration of rich C3S.

Leaching characteristics and kinetics of scandium from Sc-concentrate of Bayan Obo rare earth tailings in sulfuric acid solution

Bayan Obo tailings are rich in valuable elements such as Fe, F, REEs, and Nb, which are regarded as critical secondary resources and have been studied by researchers in recent years on how to utilize them comprehensively. Among them, Sc has the highest value, but its low grade and unclear occurrence state limit its application and development. In this work, the leaching process of Sc-magnetic concentrate of Bayan Obo tailings with the grade above 500 ppm was introduced, and the effects of different leaching conditions, which consisted of stirring speed (300–500 rpm), particle size (25–75 µm), initial CH2SO4 (3–5 mol L−1) and temperature (353.15–383.15 K), on the extraction mechanism of Sc were reported. The results indicated that the leaching temperature, and initial acid concentration were positively correlated with the extraction efficiency of Sc, the mineral particle size was negatively correlated with the extraction efficiency of Sc, and the influence of stirring speed on the leaching efficiency was ignored. The dissolution order of silicate in sulfuric acid solution was biotite ˃ richterite ˃ aegirine. After 6 h of reaction, there was still a large amount of aegirine undissolved, and aegirine is one of the main Sc-riched minerals, which was the reason for the low leaching efficiency of Sc. The kinetics of the acid leaching process of Sc-magnetic concentrate under atmospheric pressure was also studied. The shrinking core model (SCM)-diffusion control-spherical particle was chosen to describe the leaching behavior of Sc, and the parameters of the kinetic equation were determined that the reaction order was 3.26 with respect to sulfuric acid with an activation energy of 54.39 kJ/mol.

Solidification treatment of rare earth tailings by a renewable biological cementation method

Rare earth mineral is a kind of polymetallic co-associated mineral, in addition to containing iron, rare earth, niobium, and a certain amount of thorium and uranium. The tailings after wet leaching of rare earth are both a kind of hazardous waste and a kind of potential strategic resource. Therefore, the treatment and disposal of rare earth tailings should not only achieve the purpose of risk control, but also cannot affect the subsequent reuse. Microbially induced calcium carbonate precipitation (MICP) as an environmentally friendly biological cementation technique, it can solidify the tailings and stabilize the polluting elements. The valuable elements in the tailings can be further recovered after the sample solidified by MICP is decomposed by acid. In this study, a strain of urease-producing bacteria (K-1) with excellent performance was screened by dilution culture method from rare earth tailings. Through physiological and biochemical analysis and 16 s rDNA analysis, the strain K-1 belonged to the genus Lysinibacillus. In the mold, strain K-1 was used to induce the formation of calcium carbonate for cementing rare earth tailings, the strength of the cured body after cementation can reach 1.73 MPa after 28 d. There was a significant reduction in the leaching of heavy metals and radionuclides in the cured body. The results obtained from the scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicated that strain K-1 induced a large amount of calcium carbonate, whose crystal forms are vaterite and calcite. The calcium carbonates cemented the rare earth tailings together for tighter texture and reduced porosity. The X-ray fluorescence (XRF) analysis of the rare earth tailings before and after solidification and after acid decomposition showed that the loss of rare earth elements in rare earth tailings after MICP treatment was small. Moreover, the cured body was relatively easy to dissociate, which facilitated the recovery and reuse of potential valuable elements in the tailings. This study provides technical support for the long-term storage of rare earth tailings and the sustainable development of the rare earth industry.

Characteristics and Dominant Influencing Factors of the Fungal Community Structure in Soils Co-contaminated with Rare Earth Elements and Heavy Metals

Rare earth elements (REEs) have been listed as emerging pollutants and are often enriched together in soils with heavy metals (HMs), which results in ecological crises. The ecological effects caused by REEs have been attracting increasing amounts of attention, but most studies neglect the synergistic effect of REEs and HMs. The soil fungal community plays an important role in maintaining ecosystem functions, and understanding the fungal community structure and its dominant influencing factors in the co-contaminated soils will help to develop soil remediation strategies that could reduce or remedy the impacts of human production activities on the environment. Currently, the effects of long-term contamination of REEs and HMs on the soil fungal communities remain unclear. The Baotou rare earth tailings dam (Inner Mongolia, China) was used as the area of study, and soil samples co-contaminated with REEs and HMs were collected. Illumina high-throughput sequencing with ITS1 gene amplicons was used to analyze the fungal community diversity and structural characteristics. The results showed that the heterogeneity of soil environmental variables determined the distribution of fungal communities in a small area and constituted its own unique ecological niche in the co-contaminated environment. The fungal community richness and diversity in the co-contaminated soils were significantly lower than those in the uncontaminated soils, and the composition of the fungal community was significantly different. The results of a random forest (RF) analysis showed that TN was the most important factor that affected the fungal community richness and diversity, followed by REEs, Zn, and AK. The results of a canonical correspondence analysis (CCA) showed that Zn was the most important factor that affected the fungal community structure. A variation partitioning analysis (VPA) was performed to quantify the relative contributions of different environmental variables on the changes in fungal community structure, and the analytical results showed that all the detected environmental variables could explain 93.3% of the variation in soil fungal community. The combined effect of soil physicochemical properties and pollution factors (REEs and HMs) accounted for 58.5% of the total variation, and their contribution alone accounted for 13.5% and 21%, respectively. The effects of these pollution factors on the fungal communities were slightly higher than those of the soil physicochemical properties. The synergistic contributions of REEs and HMs were 40.1%, and their individual effects were 21.8% and 17.9%, respectively. Therefore, the soil physicochemical properties, REEs, and HMs regulated the fungal community structure and composition in concert. The synergistic contributions of REEs and HMs were greater than their individual effects, and these results suggest that it is necessary to further strengthen the risk control of the co-contamination of REEs and HMs in the soil environment.

Transport and distribution of residual nitrogen in ion-adsorption rare earth tailings

A large amount of nitrogen remains in ion-absorption rare earth tailings with in-situ leaching technology, and it continually ends up in groundwater sources. However, the distribution and transport of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3−-N) across tailings with both depth and hill slopes is still unknown. In this study, the amount of NH4+-N and nitrate nitrogen (NO3−-N) was determined in tailings, and a soil column leaching experiment, served to assess the transport and distribution following mine closure. Firstly, a high concentration of NH4+-N in the leachate at the initial leaching stage was detected, up to 2000 mg L−1, and the concentration of NH4+-N clearly diminished as time passed. Meanwhile, the NH4+-N contents remained relatively high in soil. Secondly, both the content of NH4+-N and NO3−-N varied greatly according to vertical distribution after leaching lasting several years. The amounts of NH4+-N and NO3−-N in surface soil were much smaller than those in deep soil, with 3–4 orders of magnitude variation with depth. Thirdly, when disturbed by NH4+-N, the pH not only diminished but also changed irregularly as depth increased. Fourthly, although the amount of NO3−-N was smaller than that of NH4+-N, both their distribution trend was similar with depth. In fact, NH4+-N and NO3−-N were significantly correlated but this declined from the knap to the piedmont. Based on these results, it is suggested that mining activity could cause nitrogen to be dominated by NH4+-N and acidification in a tailing even if leaching occurs over several years. NO3−-N derived from NH4+-N transports easily and it becomes the main nitrogen pollutant with the potential to be a long-lasting threat to the environment around a mine.

Influence of Reagents on Qualitative Indicators of Artificial Anti-Deflationary Phytocenosis on Waste from a Rare Earth Tailing Facility.

This paper presents an assessment of the effect of various reagents on the qualitative indicators of anti-deflationary single-species sowing phytocenosis on enrichment waste from rare earth ores. It has been established that tailings of loparite ores are not suitable for biological reclamation due to low values of hygroscopic moisture (0.54-2.85%) and clay particles (17.6 ± 0.6%) and high content of bioavailable forms of aluminum (504 ± 14 mg/kg). Seeds of red fescue (Festuca rubra L.) were grown on the tailings of loparite ore enrichment with the addition of opoka (O), brucite (B), and vermiculite (V). The quality of the seed cenosis was assessed by the dry biomass of the above-ground parts of the plants and the plant height. A positive effect (one-way ANOVA followed by Tukey's HSD test (p < 0.05 and p < 0.01)) of the considered combinations of reagents on the growth of above-ground biomass from 31.5% (V) to 70.3 (V + O), 82.4% (V + B), and 81.8% (V + O+B) and on plant height from 53.8% (V) up to 78.6 (V + O), 83.8% (V + B), and 75.4% (V + O+B) was revealed. The use of a combination of V + O and V + B reagents made it possible to significantly reduce the content of Al (by 19.0% and 52.8%), Sr (by 16.5% and 12.9%), La (by 65.2% and 40.6%), and Ce (by 66.8% and 41.9%) in the aerial part of the sowing phytocenosis compared to control. The results obtained here can become the basis for development of a combined sorption technology for the reclamation of technogenically disturbed lands.

Performance changes in the anammox process under the stress of rare-earth element Ce(III) and the evolution of microbial community and functional genes

The high Ce(III) content in ionic rare-earth tailings wastewater has hindered the application of anammox process in this field. Here, the effect of Ce(III) on the performance of anammox processes was investigated, and the evolution of microbial communities and functional genes was explored using metagenomic sequencing. The results showed that the reactor nitrogen removal rate decreased when the Ce(III) concentration reached 25 mg/L, although ammonia nitrogen removal (92.31%) and nitrogen removal efficiency (81.33%) remained at a high level; however, both showed a significant decreasing trend. The relative abundance of anammox bacteria increased continuously from P1–P5, reaching 48.81%, whereas the relative abundance of Candidatus jettenia reached 33.71% at P5, which surpassed that of Candidatus brocadia as the most abundant anammox bacteria, and further analysis of functional genes and metabolic pathways revealed that Candidatus brocadia was richer in biochemical metabolic genes, whereas Candidatus jettenia had richer efflux genes.

Changes of nutrients and microbial communities in recovery process of abandoned rare earth tailings

Rare earth element (REE) mining has wrought severe ecosystem destruction, particularly in the loss of carbon (C), nitrogen (N), and phosphorus (P). Furthermore, its effects on nutrient recovery characteristics and associated microbial abundance are still poorly understood. In this study, an investigation was conducted to determine C, N, and P recovery patterns and microbial communities in abandoned REE mining tailings with different abandoned durations of 1.5 (1.5-Y), 7 (7-Y), 10 (10-Y), and 14 (14-Y) years and at an unmined site (the control group) in Jiangxi Province, China. Longitudinal tailing delamination at each site was investigated in 0–15 (layer 1) and 15–30 (layer 2) cm tailing layers. During the abandoned duration of REE tailings, C and P in layer 1 continued to recover from 1.62 to 8.51 g kg-1 and from 0.71 to 1.94 g kg-1, respectively; N losses of 0.60 and 0.22 g kg-1 occurred at the 1.5-Y and 10-Y sites, while N in layer 1 slightly increased at the 14-Y site. Mesorhizobium, Methylosinus, Bradyrhizobium, Pseudomonas, and Azospirillum were dominant bacteria at the 14-Y site and in the control. The relative abundances of N-fixing bacteria Mesorhizobium (32.94%), Sinorhizobium (0.24%), Frankia (0.71%), and Burkholderia (2.38%) at the 14-Y site were 1.33–3.97 times those in the control (24.19%, 0.18%, 0.32% and 0.60%, respectively), which was helpful for tailing N recovery. At the 14-Y site, Tylospora (50.24%), Luellia (11.02%), Tomentella (6.94%), and Chaetomium (4.34%) were the dominant fungal genera, while Tylospora (56.93%), Suillus (11.45%), and Penicillium (6.78%) predominated in the control. The relative abundance of the P-dissolving fungus Aspergillus at the 14-Y site (0.42%) was 4.2 times that in the control (0.1%), which may lead to the improved tailing P solubility. At the 14-Y site, woody plants such as Pinus massoniana and native pioneering herbs and ferns such as Dicranopteris dichotoma and Nephrolepis auriculata probably invaded from adjacent areas. This study unraveled the natural restoration of plant and microbial communities along with soil nutrient changes in abandoned REE tailings, thus providing a novel insight into ecological recovery and restoration after mining.

Synergistic effect of double Schottky potential well and oxygen vacancy for enhanced plasmonic photocatalytic U(VI) reduction

Plasmonic photocatalysis is an effective strategy to solve radioactive uranium hazards in wastewater. A plasmonic photocatalyst Bi/Bi2O3−x@COFs was synthesized by in-situ growth of covalent organic frameworks (COFs) on Bi/Bi2O3−x surface for the U(VI) adsorption and plasmonic photoreduction in rare earth tailings wastewater. The presence of oxygen vacancy in Bi/Bi2O3−x and Schottky potential well formed by Bi and Bi2O3−x interface increased the number of free electrons, which induced localized surface plasmon resonance (LSPR) and enhanced the light absorption performance of composites. In addition, oxygen vacancy improved the Fermi level of Bi/Bi2O3−x, leading to another potential well between Bi2O3−x and COFs interface. The electron transport direction was reversed, thus increasing the electron density of COFs layer. COFs was an N-type semiconductor with specific binding U(VI) groups and suitable band structure, which could be used as an active reaction site. Bi/Bi2O3−x@COFs had 1411.5 mg g−1 removal capacity and high separation coefficient for U(VI) due to the synergistic action of photogenerated electrons and hot electrons. Moreover, the removal rate of uranium from rare earth tailings wastewater by regenerated Bi/Bi2O3−x@COFs was over 93.9%. The scheme of introducing LSPR and Schottky potential well provides another way to improve the photocatalytic effect.

Soil acidification in a tailing area of ionic rare earth in Southeast China

Ionic rare earth ores are now commonly mined using the ammonium sulfate in situ leaching method, causing soil acidification in tailings. To evaluate the degree of soil acidification in tailings and the influence of mining activities on acidification, we selected an ionic rare earth tailing and a nearby unmined area in Southeast China. This tailing had been closed for 12 years. We sampled the soil from the surface to the bedrock in layers and determined soil properties related to soil acidification. The results showed that the average soil pH was 5.0 in the unmined area and 4.5 in the mined area (tailing area). Rare earth mining led to a decrease in soil pH of 0.47 units per 10 years, which was 2–5 times higher than that of other land uses. The shallow soil acidification in the mined area is not affected by mining. Deep soils were significantly acidified and the H+ concentration in the soil solution was approximately nine times that of the unmined area soil. Deep soil acidification is influenced very little by natural factors. The average soil ammonium‑nitrogen (NH4+-N) and nitrate‑nitrogen contents in the mined area were 58.34 mg kg−1 and 8.19 mg kg−1, respectively, 84 times and 21 times that of the unmined area. There were large amounts of NH4+, NO3−, and H+ in the soil of the mined area, indicating that soil acidification is closely related to exogenous NH4+-N input and nitrogen transformation. Nitrification is the most important driver of soil acidification in mining areas. Continued nitrification of excess NH4+-N will continue to produce H+ and migrate with water, which will cause long-term harm to the soil and surrounding environment in the mining area. Therefore, it is necessary to remove the enriched NH4+-N in tailings soil to avoid further soil acidification.

Elution of the residual water-soluble ammonium from the weathered crust elution-deposited rare earth tailings: Kinetics and mechanism

The water-soluble ammonium (WS-A) in the weathered crust elution-deposited rare earth tailings is adsorbed on the clay mineral surface through weak forces (van der Waals forces and hydrogen bonds), and it is easy to migrate with rainwater, causing serious environmental pollution. Therefore, the study of the elution process of water-soluble ammonium is critical for the remedy of rare earth tailings leached with ammonium. This work aims to investigate the elution process and mechanism of the WS-A from the tailings and determine the optimum elution conditions. Effects of the factors on the removal efficiencies of WS-A were studied under batch and dynamic conditions. Results illustrated that the elution of WS-A is particle size, liquid–solid ratio, pH, ion strength, and temperature dependent, and it was easy to be eluted at weak acid conditions, high liquid–solid ratio, and high temperature from the tailings with small particle size. Moreover, WS-A elution is a fast process controlled by both external and internal diffusion, and a high elution efficiency of 99.98% could be reached under the optimum dynamic conditions. FTIR and XPS showed that nitrogen content was decreased obviously after elution by water, and the concentration gradient diffusion was the main driving force for the elution of WS-A absorbed on illite, halloysite, and kaolinite at different interaction sites. This study had great significance in reducing the environmental contamination caused by residual water-soluble ammonium after the in-situ elution of weathered crust elution-deposited rare earth ore.