Effects of in situ leaching on the origin and migration of rare earth elements in aqueous systems of South China: Insights based on REE patterns, and Ce and Eu anomalies

Rare earth elements in the upstream of Yangtze River Delta: Spatio-temporal distributions, sources and speciations

With the development of high technology, the environmental pollution caused by rare earth elements (REEs) has gradually aroused social concern, especially the water environment. As a upstream tributary in Xin'an river basin, Fengle river has a decisive impact on the downstream Yangtze River Delta. In this study, the spatial and temporal distributions, sources and the speciations of REEs in surface water of wood, agriculture and industry regions of Fengle River in the two seasons were studied. This study provides scientific basis for local government management, give new method in REEs sources identification, and provide data for the global databases.

Fate and potential ecological risk of rare earth elements in 3000-year reclaimed soil chronosequences

The introduction of anthropogenic inputs into natural systems may lead to enduring alterations in the innate characteristics of Rare Earth Elements (REEs). Against this backdrop, the evolutionary processes and environmental drivers of REEs in soil remain uncertain. A 3000-year soil chronosequence with uniform parent material was established in reclaimed farmland along the Yangtze River, reconstructing, for the first time, the dynamic processes of REE accumulation and fractionation over a long-time scale. Analysis of 122 soil samples showed REE concentrations ranging from 146.00 to 216.56 μg/g. Based on reclamation duration, three significant stages of REE evolution were identified: natural leaching, rapid accumulation, and stable accumulation with differentiation. Reclaimed soil after 3000 years exhibited a 14.1 % increase in REE concentrations compared to fresh sediments, attributed to anthro -pedogenic processes. Moreover, Heavy Rare Earth Elements (HREEs) accumulated faster than Light Rare Earth Elements (LREEs), particularly in deeper soils (60-100 cm), where HREE concentrations rose by 34.3 %, mainly due to acidic environments promoting HREE fixation. Additionally, the potential ecological risk posed by REEs heightened with reclamation duration, with HREEs exhibiting a sensitivity of 83 % to 94 %. Our findings stress the urgency of carefully monitoring exogenous REEs introduced through anthropogenic activities, particularly HREEs.

Natural Attenuation of Groundwater Uranium in Post-Neutral-Mining Sites Evidenced from Multiple Isotopes and Dissolved Organic Matter

Although natural attenuation is an economic remediation strategy for uranium (U) contamination, the role of organic molecules in driving U natural attenuation in postmining aquifers is not well-understood. Groundwaters were sampled to investigate the chemical, isotopic, and dissolved organic matter (DOM) compositions and their relationships to U natural attenuation from production wells and postmining wells in a typical U deposit (the Qianjiadian U deposit) mined by neutral in situ leaching. Results showed that Fe(II) concentrations and δ34SSO4 and δ18OSO4 values increased, but U concentrations decreased significantly from production wells to postmining wells, indicating that Fe(III) reduction and sulfate reduction were the predominant processes contributing to U natural attenuation. Microbial humic-like and protein-like components mediated the reduction of Fe(III) and sulfate, respectively. Organic molecules with H/C > 1.5 were conducive to microbe-mediated reduction of Fe(III) and sulfate and facilitated the natural attenuation of dissolved U. The average U attenuation rate was -1.07 mg/L/yr, with which the U-contaminated groundwater would be naturally attenuated in approximately 11.2 years. The study highlights the specific organic molecules regulating the natural attenuation of groundwater U via the reduction of Fe(III) and sulfate.

Exploring Rare Earth Element behavior in the Mount Etna volcanic aquifers (Sicily)

This study presents the first data on REY (Rare Earth Elements plus Yttrium) in the aquifer of Mount Etna (Sicily, Italy). Patterns normalized to chondrites indicate strong water-rock interaction, facilitated by a slightly acidic pH resulting from the dissolution of magma-derived CO2. REY patterns provide insights into the processes of both mineral dissolution and the formation of secondary phases. The relative abundance of light to heavy rare earth elements is compatible with the prevailing dissolution of ferromagnesian minerals (e.g., olivine or clinopyroxenes), reinforced by its strong correlation with other proxies of mineral dissolution (e.g., Mg contents). Pronounced negative Ce anomalies and positive Y anomalies demonstrate an oxidizing environment with continuous formation of secondary iron and/or manganese oxides and hydroxides. The Y/Ho fractionation is strongly influenced by metal complexation with bicarbonate complexes, a common process in C-rich waters. In the studied system, the measured REY contents are always below the limits proposed by Sneller et al. (2000, RIVM report, Issue 601,501, p. 66) for surface water and ensure a very low daily intake from drinking water.

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.

Rare earth element behaviors of groundwater in overlying aquifers under the influence of coal mining in northern Ordos Basin, China

Rare earth elements (REEs) have been used as tracers to reveal the hydrochemical sources and processes in groundwater systems that are usually modified by anthropogenic inputs. However, the REE behaviors in groundwater affected by mining activities have yet to be fully understood. In combination of REE geochemistry with general hydrochemical and isotopic (δ2H and δ18O) methods, this study investigated the concentration and fractionation of REEs in alkaline groundwater from two coal mines with similar aquifer lithology but different mining histories in the Northern Ordos Basin. One of the coal mines started mining in March 2009 (Ningtiaota coal mine, NTT), while the other started mining in December 2018 (Caojiatan coal mine, CJT). Results show that the primary hydrochemical type is HCO3-Ca in NTT groundwater with pH value ranging between 7.68 and 8.60, while CJT groundwater was dominated by the HCO3-Na type with higher pH of 9.09-10.00. The average values of ΣREEs were lower, and the NASC-normalized pattern reflected more intense fractionation in NTT groundwater than those in CJT groundwater. The evident differences are caused by the distinctions in water-rock interaction, complexation of inorganic species, and adsorption of REEs in NTT and CJT groundwater. Furthermore, these processes were closely related to the pH of groundwater that was different in two coal mines, which is likely linked to the different durations of coal mining activities that led to differences in development of rock fractures and pyrite oxidation. It is expected that REEs, combined with other indicators such as pH, can be used to trace and help better understand the hydrochemical changes in groundwater caused by mining.

Insights from distribution and fractionation of the rare earth elements into As enrichment in the Singe Tsangpo River Basin

The geogenic As enrichment occurs extensively in the major river basin from the Tibetan Plateau, while the knowledge involved with the underlying mechanisms is far from completion. The present study utilized the geochemical behaviors of rare earth elements (REE) to study the hydrogeochemical evolution and As enrichment in the Singe Tsnagpo River basin, a typical As-rich river basin in the Tibetan Plateau. The river water was characterized by significant positive Eu anomalies and slight negative Ce anomalies, indicating the hydrogeochemical control of oxidative weathering of sourcing rocks and the contribution of felsic rocks. The PHREEQC modeling results suggested that the carbonate weathering contributed to the complexation of REE in the river water, where REE(CO3)+ and REE(CO3)2- were the predominant complex species. Besides, the reversing scenarios of HREE/LREE enrichment in the river water/sediments suggested a critical control of iron (hydr)oxides on the REE fractionation due to the preferential adsorption of LREE compared with HREE. Interestingly, the variations in Y/Ni and Cr/V ratios from the river sediments suggested a different contribution of sourcing rock weathering along the river flow path, where ultramafic rock showed a substantial contribution to the river sediments in the lower reaches and granite source is predominant in the upper reaches. It was also notable that the concurrent enrichment of REE and As in the river waters showed a response to the substantial enhancement of chemical weathering in the upper reaches of the STR basin, which was evidenced by the corresponding increases in the electrical conductivity and the δ18O values in the river waters. The present study thus provides new insights into utilizing REE as environmental tracers for studying hydrogeochemical evolution and As enrichment in the STR basin, which could also apply to similar alpine arid and cold river basins.

Geochemical behavior and source analysis of rare earth elements in intensive agriculture soils through high-resolution sampling

Rare Earth Elements (REEs) are crucial for a low-carbon economy but overuse pollutes ecosystems. Studying their behavior and source in agricultural soils provides vital insights for soil management. To address this, we collected an extensive dataset comprising 2901 topsoil and 733 subsoil samples from an agriculturally dominant region in eastern China, characterized by uniform parent material. This comprehensive dataset enabled a thorough examination of the geochemical behavior and sources of REEs in the region. Our findings revealed the feasibility of utilizing La, Ce, and Y to predict ΣREE, LREE, HREE, and LREE/HREE ratios, thereby enhancing the REEs database for eastern China. The average ΣREE concentration in the study area measured 193.0 μg/g, with LREEs constituting 89 % of this total, representing a slight enrichment relative to the Upper Continental Crust. Notably, the electron-donating ability of Ce exhibited a correlation with Mn, indicating its potential as an indicator of redox reactions. The results obtained through structural equation modeling and random forest analysis identified Al, Ti, and K as the most influential environmental factors affecting ΣREE concentrations. Additionally, the indirect impact of K on ΣREE through pH was established. The strong correlation observed between major elements and Heavy Rare Earth Elements (HREEs) shed light on the mechanisms responsible for the notable fractionation of REEs in topsoil. Furthermore, our investigation underscored the substantial influence of pedogenetic processes on the enrichment and leaching of REEs. Source analysis pinpointed significant contributors to REEs, including fertilizers, pesticides, domestic wastewater, coal fly ash, and the native soil parent material. In conclusion, our study emphasizes the pressing need for judicious control of anthropogenic REE inputs. This step is pivotal not only for ensuring the sustainable utilization of soil but also for ultimately safeguarding ecosystems and environmental quality.

Co-transport of biochar nanoparticles (BC NPs) and rare earth elements (REEs) in water-saturated porous media: New insights into REE fractionation

The present study investigated the co-transport behavior of three REEs³⁺ (La³⁺, Gd³⁺, and Yb³⁺) with and without biochar nanoparticles (BC NPs) in water-saturated porous media. The presence of REEs³⁺ enhanced the retention of BC NPs in quartz sand (QS) due to decreased electrostatic repulsion between BC NPs and QS, enhanced aggregation of BC NPs, and the contribution of straining. The distribution coefficients (K_D) in packed columns in the co-transport of BC NPs and three REEs³⁺ were much smaller than in batch experiments due to the different hydrodynamic conditions. In addition, we, for the first time, found that REE fractionation in the solid-liquid phase occurred during the co-transport of REEs³⁺ in the presence and absence of BC NPs. Note that the REE fractionation during the co-transport, which is helpful for the tracing application during earth surface processes, was driven by the interaction of REEs³⁺ with QS and BC NPs. This study elucidates novel insights into the fate of BC NPs and REEs³⁺ in porous media and indicates that (i) mutual effects between BC NPs and REE³⁺ should be considered when BC was applied to REE contaminated aquatic and soil systems; and (ii) REE fractionation provides a useful tool for identifying the sources of coexisting substances.