Provenance of uranium in a sediment core from a natural reservoir, South China: Application of Pb stable isotope analysis

Distribution and migration of rare earth elements in sediment profile near a decommissioned uranium hydrometallurgical site in South China: Environmental implications

Rare earth elements have garnered increasing attention due to their strategic properties and chronic toxicity to humans. To better understand the content, migration, and ecological risk of rare earth elements in a 180 cm depth sediment profile downstream of a decommissioned uranium hydrometallurgical site in South China, X-ray powder diffraction (XRD) and High-resolution transmission electron microscope (HRTEM) were additionally used to quantify and clarify the mineral composition features. The results showed a high enrichment level of total rare earth elements in the sediment depth profile (range: 129.6-1264.3 mg/kg); the concentration variation of light rare earth elements was more dependent on depth than heavy rare earth elements. Overall, there was an obvious enrichment trend of light rare earth elements relative to heavy rare earth elements and negative anomalies of Ce and Eu. The fractionation and anomaly of rare earth elements in sediments were closely related to the formation and weathering of iron-bearing minerals and clay minerals, as confirmed by the correlation analysis of rare earth elements with Fe (r2 = 0.77-0.90) and Al (r2 = 0.50-0.71). The mineralogical composition of sediments mainly consisted of quartz, feldspar, magnetite, goethite, and hematite. Pollution assessment based on the potential ecological risk index, pollution load index (PLI), enrichment factor, and geological accumulation index (Igeo) showed that almost all the sediments had varying degrees of pollution and a high level of ecological risk. This study implied that continued environmental supervision and management are needed to secure the ecological health in terms of rare earth elements enrichment around a decommissioned uranium hydrometallurgical site. The findings may provide valuable insights for other uranium mining and hydrometallurgical areas globally.

Unveiling the origins and transport processes of radioactive pollutants downstream from a former U-mine site using isotopic tracers and U-238 series disequilibrium

High U concentrations (reaching up to 14,850 mg ⋅ kg-1), were determined in soils and sediments of a wetland downstream of a former U mine in France. This study aims to identify the origin of radioactive contaminants in the wetland by employing Pb isotope fingerprinting, (234U/238U) disequilibrium, SEM, and SIMS observations. Additionally, information about U and 226Ra transport processes was studied using U-238 series disequilibrium. The results of Pb fingerprinting highlighted inherited material inputs of different U-mines with mainly two types of U-ores: i) pitchblende (UO2), and ii) parsonsite (Pb2(UO2)(PO4)2). Moreover, significant disequilibrium of (230Th/238U) and (226Ra/230Th) activity ratios highlighted the mobility of 238U and 226Ra in the wetland, primarily driven by the water table fluctuations. Finally, this work uncovered a limitation of Pb isotope fingerprinting in the case of parsonsite materials, as the high natural Pb content of this mineral may hide the uranogenic Pb signature in the samples.

Pb isotopic fingerprinting of uranium pollution: New insight on uranium transport in stream-river sediments

Uranium mill tailings (UMT) present a significant environmental concern due to high levels of radioactive and toxic elements, including uranium (U), thorium (Th), and lead (Pb), which can pose serious health risks to aquatic ecosystems. While Pb isotopic tracers have been widely utilized in environmental studies to identify elemental sources and geological processes, their application in U geochemistry remains relatively limited. In this study, we investigate the distribution and migration of U in stream-river sediments surrounding a decommissioned U hydrometallurgical area, employing Pb isotopes as tracers. Our findings reveal significant enrichment and ecological risk of U, Pb, and Th in the sediments. Uranium predominantly associates with quartz and silicate minerals, and its dispersion process is influenced by continuous leaching and precipitation cycles of typical U-bearing minerals. Furthermore, we establish a compelling positive relationship (r2 = 0.97) between 208Pb/207Pb and 206Pb/207Pb in the stream-river sediments and sediment derived from UMT. Application of a binary Pb mixing model indicates that anthropogenic hydrometallurgical activities contribute to 2.5-62.7% of the stream-river sediments. Notably, these values are lower than the 6.6-89.6% recorded about 10 years ago, prior to the decommissioning of the U hydrometallurgical activity. Our results underscore the continued risk of U pollution dispersion even after decommission, highlighting the long-term environmental impact of UMT.

Occurrence characteristics of uranium mineral-related substances in various environmental media in China: A critical review

The high demand and extensive exploitation of uranium resources resulted in the ubiquity and high detection levels of uranium mineral-related substances in various environment media in China. The potential adverse effects of uranium mineral-related substances on environment and human health have received extensive attention. Therefore, we reviewed the occurrence and spatial distribution of uranium mineral-related substances in various basins and environmental media in China to obtain an overall understanding. We collected information from over 70 papers reporting the occurrence and distribution of uranium mineral-related substances in multiple environments and 183 articles on the genesis of uranium deposits in China from 2001 to 2021. Then the occurrence of uranium mineral-related substances and corresponding correlation in different basins, environmental media and depth ranges were compared in detail. And this review assessed the uranium mineral-related pollution in China based on various environmental quality standards of China, EPA and WHO, and proposed the priority uranium mineral-related heavy metals and radioactive substances based on cluster analysis. This review showed that there were obvious differences in the occurrence characteristics of various uranium mineral-related substances in different environmental media, especially in the surrounding environment of sandstone type and hard rock type uranium deposits. These results will guide us to tackle the challenge of uranium mineral-related pollution in China. The correlation analysis of uranium mineral-related pollutants in different environmental media and the identification of priority pollutants will also provide instructions for us to control uranium mineral-related pollution. Finally, we put forward a series of urgent and practical suggestions on risk management and control of uranium mining according to the current situation of uranium mining environment in China, which is of guiding significance for the realization of "green uranium mining".

Deciphering sources of U contamination using isotope ratio signatures in the Loire River sediments: Exploring the relevance of 233U/236U and stable Pb isotope ratios

A broad range of contaminants has been recorded in sediments of the Loire River over the last century. Among a variety of anthropogenic activities of this nuclearized watershed, extraction of uranium and associated activities during more than 50 years as well as operation of several nuclear power plants led to industrial discharges, which could persist for decades in sedimentary archives of the Loire River. Highlighting and identifying the origin of radionuclides that transited during the last decades and were recorded in the sediments is challenging due to i) the low concentrations which are often close or below the detection limits of routine environmental surveys and ii) the mixing of different sources. The determination of the sources of anthropogenic radioactivity was performed using multi-isotopic fingerprints (²³⁶U/²³⁸U, ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb) and the newly developed ²³³U/²³⁶U tracer. For the first time ²³³U/²³⁶U data in a well-dated river sediment core in the French river Loire are reported here. Results highlight potential sources of contamination among which a clear signature of anthropogenic inputs related to two accidents of a former NUGG NPP that occurred in 1969 and 1980. The ²³³U and ²³⁶U isotopes were measured by recent high performance analytical methods due to their ultra-trace levels in the samples and show a negligible radiological impact on health and on the environment. The determination of mining activities by the use of stable Pb isotopes is still challenging probably owing to the limited dissemination of the Pb-bearing material marked by the U-ore signature downstream to the former U mines.

Combined U-Pb isotopic signatures of U mill tailings from France and Gabon: A new potential tracer to assess their fingerprint on the environment

Uranium milling activities have produced high volumes of long-lived radioactive processed wastes stored worldwide in near surface environment. The aim of this study is to highlight relevant tracers that can be used for environmental impact assessment studies involving U mill tailings. A multi-tracer study involving elemental content, ²³⁸U decay products disequilibria and stable Pb isotopes was performed in different types of U mill tailings (alkaline, acid, neutralized acid) collected from five Tailings Management Facilities in France (Le Bosc, L'Ecarpière, Le Bernardan, and Bellezane) and Gabon (Mounana). Our results showed that U and Pb concentrations range between 30 and 594 ppm and 66-805 ppm, respectively. These tailings have a strong disequilibrium of (²³⁴U/²³⁸U) and (²³⁰Th/²³⁸U) activity ratios (1.27-1.87 and 6-65, respectively), as well as higher ²⁰⁶Pb/²⁰⁷Pb (1.86-7.15) and lower ²⁰⁸Pb/²⁰⁷Pb (0.22-2.39) compared to geochemical background ((²³⁴U/²³⁸U) and (²³⁰Th/²³⁸U) equal to unity; ²⁰⁶Pb/²⁰⁷Pb = 1.20; ²⁰⁸Pb/²⁰⁷Pb = 2.47). In situ analyzes (SEM, SIMS) showed that Pb-bearing phases with high ²⁰⁶Pb/²⁰⁷Pb are related to remaining U-rich phases, S-rich phases and potentially clay minerals or oxyhydroxides. We suggest that the combination of the ²⁰⁶Pb/²⁰⁷Pb with the (²³⁴U/²³⁸U) ratio is a relevant tool for the fingerprinting of the impact of U milling activities on the environment.

Potentially Toxic Element Contaminations and Lead Isotopic Fingerprinting in Soils and Sediments from a Historical Gold Mining Site

Lead (Pb) isotopes have been widely used to identify and quantify Pb contamination in the environment. Here, the Pb isotopes, as well as the current contamination levels of Cu, Pb, Zn, Cr, Ni, Cd, As, and Hg, were investigated in soil and sediment from the historical gold mining area upstream of Miyun Reservoir, Beijing, China. The sediment had higher ²⁰⁶Pb/²⁰⁷Pb ratios (1.137 ± 0.0111) than unpolluted soil did (1.167 ± 0.0029), while the soil samples inside the mining area were much more variable (1.121 ± 0.0175). The mean concentrations (soil/sediment in mg·kg⁻¹) of Pb (2470/42.5), Zn (181/113), Cu (199/36.7), Cr (117/68.8), Ni (40.4/28.9), Cd (0.791/0.336), As (8.52/5.10), and Hg (0.168/0.000343) characterized the soil/sediment of the studied area with mean I_geo values of the potentially toxic element (PTE) ranging from −4.71 to 9.59 for soil and from −3.39 to 2.43 for sediment. Meanwhile, principal component analysis (PCA) and hierarchical cluster analysis (HCA) coupled with Pearson’s correlation coefficient among PTEs indicated that the major source of the Cu, Zn, Pb, and Cd contamination was likely the mining activities. Evidence from Pb isotopic fingerprinting and a binary mixing model further confirmed that Pb contamination in soil and sediment came from mixed sources that are dominated by mining activity. These results highlight the persistence of PTE contamination in the historical mining site and the usefulness of Pb isotopes combined with multivariate statistical analysis to quantify contamination from mining activities.

Factorial kriging for estimating and mapping the geochemical background from in situ gamma dose rate measurements downstream of a former uranium mine

A geostatistical approach is applied for extracting the geochemical background from gamma dose rate data acquired downstream of a former French uranium mining area. The exploratory data analysis shows that the spatial structure of the gamma dose rate consists of two components: a short isotropic component (10 m-range) that corresponds to the geochemical background; and a long anisotropic component (30-60 m-range) that corresponds to the drainage network features previously fed by the mine discharge water. The gamma dose rate on the whole area of interest was estimated and simulated (to deal with uncertainties) through a kriging of the measured values. The spatial component related to the geochemical background was then extracted through factorial kriging. The proportion of the gamma dose rate explained by the geochemical background according to factorial kriging is consistent within uncertainties with geochemical analyses performed on soil and sediment samples. This study thus highlights the potential of such geostatistical approaches for better exploiting radiometric data.

Uranium re-adsorption on uranium mill tailings and environmental implications

Uranium mill tailings (UMTs) are one critical source of environmental U pollution. Leaching test has been extensively used to reveal U release capacity and mechanism from UMTs, while little attention has been paid to the effects of re-adsorption process on U release. In this study, the role of U re-adsorption behaviors during leaching test with UMTs was comprehensively investigated. Through paired data on mineralogical composition and aqueous U speciation, the influence of environmentally relevant factors on U re-absorption capacity and mechanism on UMTs with different particle sizes was revealed. Significant amounts of U re-adsorption were observed and primarily attributed to the adsorption on chlorite, albite and muscovite as well as combined reduction-sequestration by muscovite. Uranium re-adsorption predominantly occurred via inner-sphere complexation and surface precipitation depending on leachant pH. Coexisting sulfate or phosphate could further enhance U re-adsorption. The enhanced re-adsorption from sulfate occurred when inner-sphere complexation governed the re-adsorption process. These findings suggest that the environmental hazards and ecological risks of the U containing (waste) solids might have been underestimated due to the ignorance of the re-adsorption process, since the re-adsorbed U could be easily re-mobilized. The insights from this study are also helpful in developing effective in-situ remediation strategies.

Geochemical and U-Th isotopic insights on uranium enrichment in reservoir sediments

Uranium (U) geochemistry and its isotopic compositions of reservoir sediments in U mine area were poorly understood. Herein, U and Th isotopic compositions were employed to investigate source apportionment and geochemical behavior of U in 41 reservoir sediments from a U mining area, Guangdong, China. The remarkably high contents of both total U (207.3-1117.7 mg/kg) and acid-leachable U (90.3-638.5 mg/kg) in the sediments exhibit a severe U contamination and mobilization-release risk. The U/Th activity ratios (ARs) indicate that all sediments have been contaminated apparently by U as a result of discharge of U containing wastewater, especially uranium mill tailings (UMT) leachate, while the variations of U/Th ARs are dominated by U geochemical behaviors (mainly redox process and adsorption). The U isotopic compositions (δ²³⁸U) showed a large variance through the sediment profile, varying from - 0.62 to - 0.04‰. The relation between δ²³⁸U and acid-leachable U fraction demonstrates that the U isotopic fractionation in sediments can be controlled by bedrock weathering (natural activity), UMT leachate (anthropogenic activity) and subsequent biogeochemical processes. The findings suggest that U-Th isotopes are a powerful tool to better understand U geochemical processes and enrichment mechanism in sediments that were affected by combined sources and driving forces.

Emergent thallium exposure from uranium mill tailings

Thallium (Tl) pollution caused by the exploitation of uranium (U) mines has long been neglected due to its low crustal abundance. However, Tl may be enriched in minerals of U ore because Tl has both sulfurophile and lithophile properties. Herein, a semi-dynamic leaching experiment combined with statistical analysis, geochemical speciation and multi-characterization provided novel insight into the distinct features and mechanisms of Tl release from uranium mill tailings (UMT). The results showed that particle size effects prevail over the pH on Tl release, and surface dissolution is the pivotal mechanism controlling Tl release based on Fick's diffusion model. The study revealed that long-term leaching and weathering can lead to the increased acid-extractable and oxidizable fractions of Tl in UMT, and that the exposure and dissolution of Tl-containing sulfides would largely enhance the flux of Tl release. The findings indicate that UMT containing (abundant) pyrite should be paid particular attention due to Tl exposure. Besides, critical concern over the potential Tl pollution in universal U mining and hydrometallurgical areas likewise may need to be seriously reconsidered.

Emerging risks of toxic metal(loid)s in soil-vegetables influenced by steel-making activities and isotopic source apportionment

Industrial activities tend to deteriorate adjacent agricultural lands due to accumulation of potentially toxic elements in soils and crops. However, better understanding of their distinctive source partitions and transfer process remains insufficient in steel-making area. The paper focuses on the pollution levels, health risks, and provenance identification of Tl, As, Pb, Cu, Ni, Co, Sb, Cd, Zn, Be, Cr, Fe, Mn, Mo, Sn, and V in common vegetables from different farmlands near a steel-making plant. The results showed that the Tl, As, Pb, Cd, Cr, Cu and Mn were of high-level contamination in soils and generally above the maximum permissible level (MPL). Calculation using hazard quotients (HQ) exhibited that consumption of the studied vegetables may entail significant health risks to residents, especially for children, resulting from the elevated contents of Tl, As and associated toxic elements. Calculation by binary mixing model using Pb isotopic compositions suggested that steel-making activities contributed to 35-80% of the contamination of Pb and As in vegetables. It is necessary to adopt appropriate remediation measures to mitigate the farmland contamination and ensure the food safety of the agricultural products.

Facile modification of graphene oxide and its application for the aqueous uranyl ion sequestration: Insights on the mechanism

Graphene oxide (GO) has been proved with favorable affinity to U(VI), while some drawbacks such as poor dispersity and low adsorption performance limit its application. Herein, cetyltrimethylammonium bromide (CTAB) modified graphene oxide (MGO) composites were successfully fabricated, characterized and compared with graphene oxide (GO) in the sequestration of U(VI) in aqueous solutions. The results showed that maximum adsorption rate of MGO (99.21%) was obviously higher than that of GO (66.51%) under the same initial condition. Simultaneous introduction of C-H and NO coupled with the enhanced dispersity of GO after modification were mainly responsible for the updated performance verified with multiple characterization techniques. Based on the results of kinetics and isotherms investigations, the experimental data were best described by Pseudo-first-order kinetic model and Redlich-Peterson isotherm model. The results of ΔH, ΔS and ΔG show that adsorptive behaviors of uranyl ion on MGO are endothermic and spontaneous. The study provides a feasible alternative to the chemical modification of GO and enhancing the performance towards uranyl ion removal from solution.

Critical insight and indication on particle size effects towards uranium release from uranium mill tailings: Geochemical and mineralogical aspects

Uranium (U) is both chemically toxic and radioactive. Uranium mill tailings (UMTs) are one of the most important sources of U contamination in the environment, wherein the mechanisms that control U release from UMTs with different granularities have not yet been well understood. Herein, the release behaviours and underlying release mechanisms of U from UMTs with five different particle size fractions (<0.45, 0.45-0.9, 0.9-2, 2-6 and 6-10 mm) were studied with a well-defined leaching test (ANS 16.1) combined with geochemical and mineralogical characterizations. The results showed that the most remarkable U release unexpectedly emerged from UMT_{2-6 mm}; in contrast, the smallest particle size UMT_{<0.45 mm} contributed to the least U release. The predominant mechanism of U release from UMT_{2-6 mm} was the oxidative dissolution of U-bearing sulfides, while abundant gypsum present in UMT_{<0.45 mm} inhibited U release. The study highlights the importance of combined geochemical and mineralogical investigation when performing leaching tests of mineral-containing hazardous materials such as UMTs with consideration of particle size effects. The findings also indicate that elevating the content of gypsum and avoiding the oxidation of sulfides can effectively help immobilize and minimize the residual U release from the UMTs.

Radionuclides in the environment around the uranium mines in Guangxi, China

Uranium and plutonium are both poisonous radioactive elements, which are very harmful to human health and environment. Therefore, it is of great significance to study the distribution of ²³⁸U concentration and ^239+240Pu activity in the uranium mine surrounding soils. We have collected some surface soil sediments within 2 km of two uranium mines and a solid waste management center in Guangxi Province. The ²³⁸U concentration in these study areas is in the range of 1.44-83.91 mg/g, and the ²³⁸U concentration in the A uranium mine surrounding surface soils is higher than that in the B uranium mine and the solid waste management center. While the B uranium mine and the solid waster management center don't pollute the surrounding soils because the ²³⁸U concentrations in their surrounding soils are similar to the average ²³⁸U concentration in the soil. The ^239+240Pu activities in soil samples collected around the two uranium mines and the solid waste management center are close ranged from 0.06 mBq/g to 0.51 mBq/g. Moreover, the ²⁴⁰Pu/²³⁹Pu atom ratios in our study samples are ranged from 0.15 to 0.23, which indicate the Pu may come from the global fallout. In addition, we study heavy metals in our collected samples, only heavy metal Tl has weak positive correlations with ²³⁸U concentrations and ^239+240Pu activities. And there is a weak positive correlation between ²³⁸U concentrations and ^239+240Pu activities.

Temporal sedimentary record of thallium pollution in an urban lake: An emerging thallium pollution source from copper metallurgy

Thallium (Tl) is an extremely toxic metal with high risk. Sediment samples from a well dated sediment core retrieved from a heavily polluted urban lake were measured to assess the magnitude of Tl pollution and reveal its transfer mechanisms within the lake. The results show that the lake has experienced serious Tl pollution during the past 60 years, owing to a large influx of Tl-bearing wastes mostly from a neighboring copper (Cu) smelter. The Tl contents in the sediment core at different depths generally correlated positively with the production scale of Cu smelting activities in the past. Further mineralogical analysis on the electrostatic precipitator dust from the Cu smelter and highly-polluted samples from the sediment core by electronic microscopy and X-ray diffraction suggested that Tl was primarily bound to quartz and aluminosilicates. Large quantities of Tl were also observed in diatom frustules collected from the sediments, suggesting a potentially significant role of biogenic silicates in the uptake and transfer of Tl in the sediment-water system. The findings indicate a new and emerging pollution source of thallium arising from copper metallurgy activities. The results also highlight the necessity of strengthened monitoring and regulations towards Tl pollution in environmental systems impacted by Cu smelting activities in China and other nations.

Thallium contamination, health risk assessment and source apportionment in common vegetables

As an element with well-known toxicity, excessive thallium (Tl) in farmland soils, may threaten food security and induce extreme risks to human health. Identification of key contamination sources is prerequisite for remediation technologies. This study aims to examine the contamination level, health risks and source apportionment of Tl in common vegetables from typical farmlands distributed over a densely populated residential area in a pyrite mine city, which has been exploiting Tl-bearing pyrite minerals over 50 years. Results showed excessive Tl levels were exhibited in most of the vegetables (0.16-20.33 mg/kg) and alarming health risks may induce from the vegetables via the food chain. Source apportionment of Tl contamination in vegetables was then evaluated by using Pb isotope fingerprinting technique. Both vegetables and soils were characterized with overall low ²⁰⁶Pb/²⁰⁷Pb. This indicated that a significant contribution may be ascribed to the anthropogenic activities involving pyrite deposit exploitation, whose raw material and salgs were featured with lower ²⁰⁶Pb/²⁰⁷Pb. Further calculation by binary mixing model suggested that pyrite mining and smelting activities contributed 54-88% to the thallium contamination in vegetables. The results highlighted that Pb isotope tracing is a suitable technique for source apportionment of Tl contamination in vegetables and prime contamination from pyrite mining/smelting activities urges authorities to initiate proper practices of remediation.

New Insights into Pb Isotope Fingerprinting of U-Mine Material Dissemination in the Environment: Pb Isotopes as a Memory Dissemination Tracer

Stable Pb isotope ratios were measured and compared to U distributions in three soil cores located in a wetland highly impacted by water discharge of a former U-mine. Pb isotope ratios showed notable alignments in binary mixing plots, demonstrating the dissemination of radioactive-enriched material from the U-mine. Thanks to these alignments and to the measurement of the ²⁰⁴Pb isotope, a precise characterization of the Pb isotope composition of the U-ore was performed without the use of U-ore samples. The well-defined end-members with the help of a reevaluated isotope mixing model allowed the accurate determination of the radiogenic Pb percentages in the cores that were overall found to be >50%. Noncorrelated distributions of radiogenic ²⁰⁶Pb and U are observed in several of the wetland soil samples. They reveal postdepositional U redistribution explained by major U speciation changes due to redox cycling in the wetland. On the contrary, the radiogenic ²⁰⁶Pb showed no or little postdepositional mobility and thus can be considered to be a memory tracer of the dissemination of U-rich radioactive material: even after an important U loss, the radiogenic ²⁰⁶Pb is able to reveal past contamination by U-rich materials.

Uranium and thorium contents in soils and bottom sediments of lake Bolshoye Yarovoye, western Siberia

The uranium and thorium contents in the soils and bottom sediments of the Lake Bolshoye Yarovoye natural system correspond to the data for the steppe climate zone and are noticeably lower than the background values for the Altai Territory as a whole. Factors that reduce the content of uranium and thorium are: 1 - lighter grain size distribution of the substrate, and, accordingly, the soils in this area; 2 - low content of organic matter (high ash content at 600 °C); 3 - the presence of significant quantities of quartz and calcite; 4 - general salinization of soils and the development of the solonchak process; 5 - remoteness from the region of material removal from the Altai mountains with its uranium-thorium-bearing granites and various mineraliszation. The distribution of uranium and thorium in the soils of the catchment area is heterogeneous. Minimum contents are observed in the soils of the boggy eastern coast and in its southern part in connection with the development of the solonchak process. This part of the catchment is characterized by high salinity (HCO₃^-- Na⁺) of water extracts in soils. Under subalkaline and alkaline conditions (pH 7.1-8.4) in soil waters, an increase in the content of these ions facilitates the transition of uranium to the liquid phase and its migration to the lake. In this part of the water area, the maximum uranium content in bottom sediments is observed. The results of cluster analysis indicate a change in the correlation of uranium and thorium during their redistribution from soils to bottom sediments. In soils, there is no correlation between uranium and thorium; in the bottom sediments, a strong positive correlation is observed between them (correlation coefficient 0.9). Uranium in soils has only a bond with Cr and possibly with Mn. The absence of a correlation with the elements of the mineral component of the soil confirms it finding, mainly in soluble form. In bottom sediments, both elements are associated with the mineral component. The established features of the distribution of uranium and thorium in the soils and bottom sediments of Lake Bolshoye Yarovoye indicate the need for detailed geochemical studies in lakes of a similar type. This will help to avoid ecological risks when choosing such lakes as anthropogenic objects.

Mechanisms of U(VI) removal by biochar derived from Ficus microcarpa aerial root: A comparison between raw and modified biochar

Uranium (U) is a toxic and radioactive element. Excessive amounts of aqueous U(VI) generated from U mining, processing and nuclear industry may result in severe and irreversible damage to the environment. Herein, Ficus microcarpa aerial root (FMAR), a biowaste material, was used to adsorb U(VI) from aqueous solutions for the first time. Potassium permanganate (KMnO₄)-modified FMAR biochar was synthesised, characterised and compared with raw (unmodified) biochar with respect to U(VI) adsorption. The results showed that the adsorption capability of the modified FMAR biochar was evidently higher than that of the raw biochar. Multiple characterisation techniques confirmed that the discrepancy was mainly due to the increased content of O-H and formation of irregular sheet-like nanostructure with the ultrafine MnO₂ nanoparticles on the biochar surfaces after KMnO₄ modification. The abundance of O-H and nanoscale MnO₂ notably enhanced the adsorption of U(VI) by means of coordination and Lewis acid-base interaction. The results indicate that KMnO₄-modified FMAR biochar has a good potential to serve as an environment-friendly adsorbent for the removal of U(VI) from solution.

Understanding the solid phase chemical fractionation of uranium in soil profile near a hydrometallurgical factory

Uranium (U) contamination of soil has become a major concern with respect to its toxicity, accumulation in the food chain, and persistence in the environment. Anthropogenic activities like mining and processing of U ores has become pressing issues throughout the world. The aim of the work is to understand the chemical fractionation of U in polluted soil and the mechanism involved. U-free soils samples of eluvial (E), illuvial (B), and parent-material (C) horizons from a hydrometallurgical factory area were used. The experimental results showed that the U adsorption capacity decreased with depth, and its mobility in the upper soil is better than the lower. It was closely related to distribution coefficient (K_d), pH, organic-matter (OM), and carbonate content of soil horizons. The chemical fractionation of U was studied using the BCR sequential extraction scheme for soils after saturated adsorption. It was noted that the U reducible and oxidizable fraction in the E and B horizons can vertically transfer to the C horizon and occurs a significant rearrangement of U in different horizons. BET, SEM, XRD, and FT-IR analyses showed that different U distribution and migration in soil profile is mainly affected by specific surface area, soil particle size, mineral composition, and active groups. The XPS data further indicated that U (VI) is gradually converted to U (IV) with decreased depth and fixed in deeper soil becoming insoluble and immobile. It is the first step to investigate potential migration and plan U mining and milling area long-term management.

Enrichment effect of Hypnum plumaeforme on 210Po and 210Pb

The mosses have been widely used as bioindicators to investigate pollution and changes of heavy metals in different countries and regions. In this research, the field surveys were carried out for understanding the moss community and enrichment effects of the dominant species of mosses around the uranium mill tailings impoundment in South China, especially for the enriched contentions of ²¹⁰Pb and ²¹⁰Po in soil and Hypnum plumaeforme. The results showed that the maximum concentrations of U and ²²⁶Ra in the study sites were 93 mg kg^-1 and 1130.8 Bq kg^-1, respectively. The exhalation rate of ²²²Rn (ER_Rn), soil U, and ²²⁶Ra contents in SS₁, SS₂, and SS₃ were higher than CKS. With the increase of the distance from the central well, the contents of nuclides (U and ²²⁶Ra) in soils and H. plumaeforme were both decreased. And, the bioconcentration factors of H. plumaeforme for ²¹⁰Pb and ²¹⁰Po ranged from 1.05 to 1.49, and 1.25 to 1.40, respectively, indicating an accumulation of ²¹⁰Po and ²¹⁰Pb from soil by H. plumaeforme. These results indicated that H. plumaeforme is hopeful to become an accumulator plant for remediation of radon pollution, and also can be used as a potential indicator plant for radon pollution monitoring.

A simple method for preparing ultra-light graphene aerogel for rapid removal of U(VI) from aqueous solution

In this study, graphene aerogel (GA) was successfully prepared through a simple hydrothermal method. The resulting GA exhibited a porous network structure with a large specific surface area (350.8 m²/g), ultra-light mass and easy separation from water. The pH_IEP value of the GA was estimated to be 3.5. The adsorption process and the factors that affect adsorption capacity were studied. The adsorption could be conducted in a wide pH range from 2.0 to 7.0. The maximum adsorption capacity of GA towards U(VI) at pH 4.0 and T = 298 K was 238.67 mg/g calculated from the Langmuir model. The GA had greatly rapid adsorption property for the removal of U(VI) at pH 4.0. Kinetic data showed good correlation with pseudo-second-order equation. Fourier transform infrared spectroscopy and X-ray photoelectron spectrometry characterizations showed that GA adsorbed U(VI) through chemical interaction by oxygen-containing and nitrogen-containing groups functional groups. The results show that GA has excellent application potential as an adsorbent material for removing U(VI) from aqueous solution.

Occurrence and distribution of antibiotics and antibiotic resistant genes in water and sediments of urban rivers with black-odor water in Guangzhou, South China

Urban rivers in some countries have been heavily polluted and the water became black and odor. Nevertheless, only few studies reported the occurrence of antibiotics and their corresponding antibiotic resistant genes (ARGs) in urban rivers with black-odor water with and without remediation. In this study, nine antibiotics (belonging to sulfonamides, tetracyclines, quinolones, and macrolides) and their corresponding ARGs in water and sediments of six urban rivers in Guangzhou, South China were analyzed to investigate their spatial distribution and the influence of water remediation. The concentrations of individual antibiotics varied from ND (not detectable) to 2702 ng/L and ND to 449 μg/kg in surface water and sediments, respectively. Norfloxacin displayed the highest average concentrations, followed by ciprofloxacin. The relative abundance of quinolone-resistance gene qnrA (~10³ ARGs/16S rRNA) was the highest, followed by tetracyclines-resistance genes tetC (~10^-2 ARGs/16S rRNA). The antibiotics and ARGs in sediments from various rivers exhibited distinct spatial distribution with large variation from upstream to downstream. Generally, levels of antibiotics and tetracyclines-resistance genes (tetA, tetC and tetM) in urban rivers with black-odor water (affected by industrial and domestic sewage) were higher than those in remediated urban rivers. Significant positive correlations were observed only between the relative abundances of tetA (or tetC) with the concentrations of some antibiotics (e.g., ciprofloxacin and norfloxacin). TetA was also significantly positively correlated with the concentrations of Ni, Cr, and As in sediments. This study found that urban rivers remediated with dredging might lower antibiotic levels in sediment, but high relative abundance of certain ARGs (e.g., tetB, qnrA) may still exist.

Selenium translocation in the soil-rice system in the Enshi seleniferous area, Central China

Rice is an important source of selenium (Se) exposure; however, the transformation and translocation of Se in the soil-rice system remain poorly understood. Here, we investigated the speciation of Se in Se-rich soils from Enshi, Central China and assessed which Se species is bioavailable for rice grown in Enshi. Extremely high Se concentrations (0.85 to 11.46 mg/kg) were observed in the soils. The soil Se fractions, which include water-soluble Se (0.2 to 3.4%), ligand-exchangeable Se (4.5 to 15.0%), organically bound Se (57.8 to 80.0%) and residual Se (6.1 to 32.9%), are largely controlled by soil organic matter (SOM) levels. Decomposition of SOM promotes the transformation of organically bound Se to water-soluble Se and ligand-exchangeable Se, thereby increasing the bioavailability of Se. The bioaccumulation factors (BAFs) of Se decrease in the following order: roots (0.84 ± 0.30) > bran (0.33 ± 0.17) > leaves (0.18 ± 0.09) > polished rice (0.14 ± 0.07) > stems (0.12 ± 0.07) > husks (0.11 ± 0.07). Selenium levels in rice plants are affected by multiple soil Se fractions in the soil. Water-soluble, ligand-exchangeable and organically bound Se fractions are the major sources of Se in rice tissues.

Response of microbial communities and interactions to thallium in contaminated sediments near a pyrite mining area

Thallium (Tl) is a well-recognized hazardous heavy metal with very high toxicity. It is usually concentrated in sulfide minerals, such as pyrite (FeS₂), sphalerite (ZnS), chalcopyrite (CuS) and galena (PbS). Here, this study was carried out to investigate the indigenous microbial communities via 16S rRNA gene sequence analysis in typical surface sediments with various levels of Tl pollution (1.8-16.1 mg/kg) due to acid mine drainage from an active Tl-containing pyrite mining site in South China. It was found with more than 50 phyla from the domain Bacteria and 1 phyla from the domain Archaea. Sequences assigned to the genera Ferroplasma, Leptospirillum, Ferrovum, Metallibacterium, Acidithiobacillus, and Sulfuriferula manifested high relative abundances in all sequencing libraries from the relatively high Tl contamination. Canonical correspondence analysis further uncovered that the overall microbial community in this area was dominantly structured by the geochemical fractionation of Tl and geochemical parameters such as pH and Eh. Spearman's rank correlation analysis indicated a strong positive correlation between acidophilic Fe-metabolizing species and Tl_total, Tl_oxi, and Tl_res. The findings clarify potential roles of such phylotypes in the biogeochemical cycling of Tl, which may facilitate the development of in-situ bioremediation technology for Tl-contaminated sediments.

Thallium contamination in farmlands and common vegetables in a pyrite mining city and potential health risks

Thallium (Tl) is a trace metal of severe toxicity. Its health concerns via consumption of contaminated vegetables have often been overlooked or underestimated. This study was designed to gain insight into the actual level and distribution characteristics of Tl and metal (loid)s (Pb, Cd, Cr, Sb, Mn, Cu, Zn, Ni, and Co) in agricultural soils and common vegetables cultivated in different zones (upstream, midstream, and downstream) of a densely populated residential area in a typical mine city, which has been open-pit exploiting Tl-bearing pyrite minerals since 1960s. The results show that most of the agricultural soils exhibit contaminated levels of Tl, with Tl contents (upstream: 1.35-4.31 mg/kg, midstream: 2.43-5.19 mg/kg, and downstream: 0.65-2.33 mg/kg) mostly exceeding the maximum permissible level (MPL) for agricultural land use (1 mg/kg). Sequential extraction procedure indicates that even Tl is predominantly retained in the residual fraction, significant levels of Tl are still present in the geochemically mobile fractions. Besides, metals like Cu, Cd, Mn, and Co are mostly distributed in the labile fractions. Almost all metal (loid)s in edible parts of the vegetables exceed their corresponding MPL for consumption. The chronic daily intake (CDI) and hazard quotient (HQ) values calculated for inhabitants at different ages indicate non-negligible Tl risks via consumption of local vegetables, especially for children. Therefore, it is critical to establish effective measures for hazardous waste management and enforceable regulations in Tl-polluted area to mitigate potential severe impacts of Tl on human health through food chain.

Mechanism of uranium release from uranium mill tailings under long-term exposure to simulated acid rain: Geochemical evidence and environmental implication

To date, there is not sufficient knowledge to fully understand the occurrence, transport and fate of residual uranium (U) from uranium mill tailings (UMT). Herein this study investigated different U release behaviors from natural UMT (without grinding) under four simulated acid rain (pH = 2.0-5.0) compared with controlled scenario (pH = 6.0) for 25 weeks. The results showed that the most notable U release was observed from UMT_pH2.0, followed by UMT_pH3.0 whereas a nonlinear relationship between pH and U release was observed from UMT_pH4.0-6.0. The divergence of U release behaviors was attributed to the presence of minerals such as calcite and clinochlore. Autunite, a secondary mineral formed after leaching, might regulate U release in UMT_pH3.0-6.0. Fick theory model revealed the shift of U release mechanism from surface dissolution to diffusion transport for UMT_pH2.0, UMT_pH3.0 and UMT_pH5.0 at varied stage, whereas UMT_pH4.0 and UMT_pH6.0 displayed univocal dissolution and diffusion mechanism, respectively. This study highlights the necessity of performing long-term leaching tests to detect the "shift event" of leaching kinetics and to better understand the mechanism of U release influenced by mineralogy of the natural UMT under simulated acid rain conditions, which is conducive to developing UMT management strategies to minimize the risk of U release and exposure.

Targeted nanoparticle binding & detection in petroleum hydrocarbon impacted porous media

Targeted nanoparticle binding has become a core feature of experimental pharmaceutical product design which enables more efficient payload delivery and enhances medical imaging by accumulating nanoparticles in specific tissues. Environmental remediation and geophysical monitoring encounter similar challenges which may be addressed in part by the adoption of targeted nanoparticle binding strategies. This study illustrates that engineered nanoparticles can bind to crude oil-impacted silica sand, a selective adsorption driven by active targeting based on an amphiphilic polymer coating. This coating strategy resulted in 2 mg/kg attachment to clean silica sand compared to 8 mg/kg attachment to oil-impacted silica sand. It was also shown that modifying the surface coating influenced the binding behaviour of the engineered nanoparticles - more hydrophobic polymers resulted in increased binding. Successful targeting of Pluronic-coated iron oxide nanoparticles to a crude oil and silica sand mixture was demonstrated through a combined quantitative Orbital Emission Spectroscopy mass analysis supported by Vibrating Scanning Magnetometer magnetometry, and a qualitative X-ray micro-computed tomography (CT) visualization approach. These non-destructive characterization techniques facilitated efficient analysis of nanoparticles in porous medium samples with minimal sample preparation, and in the case of X-Ray CT, illustrated how targeted nanoparticle binding may be used to produce 3-D images of contaminated porous media. This work demonstrated successful implementation of nanoparticle targeted binding toward viscous LNAPL such as crude oil in the presence of a porous medium, a step which opens the door to successful application of targeted delivery technology in environmental remediation and monitoring.