Uranium re-adsorption on uranium mill tailings and environmental implications

Simple synthesis of graphene oxide-supported and phosphorylated chitosan gel bead to uptake uranium from wastewater

With the development of the nuclear industry, the direct discharge of uranium-containing wastewater has become increasingly harmful to the environment. A novel graphene oxide-supported and phosphoric-crosslinked chitosan gel bead (C-PGCB) with excellent uranium uptake capability was successfully fabricated to treat uranium-containing wastewater. The experimental results showed that the introduction of PO and CO bonds through phosphoric acid crosslinking could greatly improve the capturing ability of chitosan-based materials, which could reach 97.1 % and 460.9 mg g-1 within 20 min, respectively. In the presence of multiple coexisting ions, the adsorbent still maintained high uranium adsorption efficiency. After 5 cycles, the uptake efficiency of C-PGCB with good cycle stability remained above 90 %. The fitting both the dynamic model and the isotherm model showed that the uranium uptake of C-PGCB was a uniform single-layer chemisorption process. Furthermore, the excellent dynamic separation properties proved the great potential value in the uranium-containing wastewater treatment. The uptake of C-PGCB for uranium could be owed to synergistic complexation and electrostatic interaction between uranium and multifunctional groups on C-PGCB. This work offered a chitosan-based adsorbent to effectively remove uranium from wastewater and gave a novel and effective way for the construction of chitosan-based adsorbents to enhance capture performance.

The effect of bacteria on uranium sequestration stability by different forms of phosphorus

Immobilisation of uranium (U (VI)) by direct precipitation of uranyl phosphate (U-P) exhibits a great potential application in the remediation of U (VI)-contaminated environments. However, phosphorus, vital element of bacteria's decomposition, absorption and transformationmay affect the stability of U (VI) with ageing time. The main purpose of this work is to study the effect of bacteria on uranium sequestration mechanism and stability by different forms of phosphorus in a water sedimentary system. The results showed that phosphate effectively enhanced the removal of U (VI), with 99.84%. X-Ray Diffraction (XRD), Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS), and X-ray Photoelectron Spectroscopy (XPS) analyses imply that U (VI) and U (IV) co-exist on the surface of the samples. Combined with BCR results, it demonstrated that bacteria and phosphorus have a synergistic effect on the removal of U (VI), realising the immobilisation of U (VI) from a transferable phase to a stable phase. However, from a long-term perspective, the redissolution and release of uranium immobilisation of U (VI) by pure bacteria with ageing time are worthy of attention, especially in uranium mining environments rich in sensitive substances. This observation implies that the stability of the uranium may be impacted by the prevailing environmental conditions. The novel findings could provide theoretical evidence for U (VI) bio-immobilisation in U (VI)-contaminated environments.

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.

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012)

Mitigating uranium transport in groundwater is imperative for ensuring access to clean water across the globe. Here, in situ resonant anomalous X-ray reflectivity is used to investigate the adsorption of uranyl on alumina (012) in acidic aqueous solutions, representing typical UVI concentrations of contaminated water near mining sites. The analyses reveal that UVI adsorbs at two distinct heights of 2.4-3.2 and 5-5.3 Å from the surface terminal oxygens. The former is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter is interpreted as an outer-sphere complex that shares one equatorial H2O with the terminal surface oxygen. With increasing pH, we observe an increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species. The presented work provides a molecular-scale understanding of sorption of uranyl on Al-based-oxide surfaces that has implications for environmental chemistry and materials science.

Dipole Moment and Built-In Polarization Electric Field Induced by Oxygen Vacancies in BiOX for Boosting Piezoelectric-Photocatalytic Removal of Uranium(VI)

Piezoelectric-photocatalysis is distinguished by its piezoelectricity as an external force that induces deformation within the catalyst to engender a polarized electric field compared to conventional photocatalysis. Herein, the piezoelectric photocatalyst BiOBr has been expertly synthesized via a plasma process and applied for piezoelectric-photocatalysis removal of uranium(VI) for the first time. The abundant surface oxygen vacancies (OVs) could induce a dipole moment and built-in electric field, which endows BiOBr with excellent separation and transport efficiency of photogenerated charges to actuate more charges to participate in the piezoelectric-photocatalytic reduction process. Consequently, under visible light and ultrasound (150 W and 40 kHz), the removal rate constant of OVs-BiOBr-30 (0.0306 min-1) was 2.4, 30.6, and 6 times higher than those of BiOBr (0.01273 min-1), ultrasound, or photocatalysis, respectively. The piezoelectric-photocatalytic synergy is also universal for BiOX (X = Cl, Br, or I) to accelerate the reduction rate of uranium(VI). This work highlights the role of piezoelectric-photocatalysis in the treatment of uranium-containing wastewater, which is of great significance for resource conservation and environmental remediation.

Microbial features with uranium pollution in artificial reservoir sediments at different depths under drought stress

The uranium (U) containing leachate from uranium tailings dam into the natural settings, may greatly affect the downstream environment. To reveal such relationship between uranium contamination and microbial communities in the most affected downstream environment under drought stress, a 180 cm downstream artificial reservoir depth sediment profile was collected, and the microbial communities and related genes were analyzed by 16S rDNA and metagenomics. Besides, the sequential extraction scheme was employed to shed light on the distinct role of U geochemical speciations in shaping microbial community structures. The results showed that U content ranged from 28.1 to 70.1 mg/kg, with an average content of 44.9 mg/kg, significantly exceeding the value of background sediments. Further, U in all the studied sediments was related to remarkably high portions of mobile fractions, and U was likely deposited layer by layer depending on the discharge/leachate inputs from uranium-involving anthoropogenic facilities/activities upstream. The nexus between U speciation, physico-chemical indicators and microbial composition showed that Fe, S, and N metabolism played a vital role in microbial adaptation to U-enriched environment; meanwhile, the fraction of Ureducible and the Fe and S contents had the most significant effects on microbial community composition in the sediments under drought stress.

Uranium removal in groundwater by Priestia sp. isolated from uranium-contaminated mining soil

Priestia sp. WW1 was isolated from a uranium-contaminated mining soil and identified. The uranium removal characteristics and mechanism of Priestia sp. WW1 were investigated. The results showed that the removal efficiency of uranium decreased with the increase of initial uranium concentration. When the uranium initial concentration was 5 mg/L, the uranium removal efficiency achieved 92.1%. The increase of temperature could promote the uranium removal. Carbon source could affect the removal rate of uranium, which was the fastest when the methanol was used as carbon source. The solution pH had significant effect on the uranium removal efficiency, which reached the maximum under solution pH 5.0. The experimental results and FTIR as well as XPS demonstrated that Priestia sp. WW1 could remove uranium via both adsorption and reduction. The common chloride ions, sulfate ions, Mn(II) and Cu(II) enhanced the uranium removal, while Fe(III) depressed the uranium removal. The Priestia sp. WW1 could effectively remove the uranium in the actual mining groundwater, and the increase of initial biomass could improve the removal efficiency of uranium in the actual mining groundwater. This study provided a promising bacterium for uranium remediation in the groundwater.

Distribution and migration of uranium, chromium, and accompanying metal(loid)s in soil-plants system around a uranium hydrometallurgical area

The extraction and utilization of uranium (U) ores have led to the release of significant amounts of potentially toxic metal(loid)s (PTMs) into the environment, constituting a grave threat to the ecosystem. However, research on the distribution and migration mechanism of U, chromium (Cr), and their accompanying PTMs in soil-plant system around U hydrometallurgical area remains insufficient and poorly understood. Herein, the distribution, migration, and risk level of PTMs were evaluated in soil and plant samples around U hydrometallurgical area, Northern Guangdong, China. The results demonstrated that the maximum content of U and Cr found in the analyzed soils were up to 84.2 and 238.9 mg/kg, respectively. These values far exceed the soil background values in China and other countries. The highest content of U (53.6 mg/kg) was detected in Colocasia antiquorum Schott, and the highest content of Cr (349.5 mg/kg) was observed in Pteridium aquilinum, both of which were enriched in their roots. The risk assessment of PTMs demonstrated that the study area suffered from severe pollution (PN > 3), especially from U, Cr, Th, and As, suggesting the non-negligible anthropogenic impacts. Hence, in light of the significant ecological hazard posed by the U hydrometallurgical area, it is imperative to implement appropriate restoration measures to ensure the human health and maintain the stability of the ecosystem.

One-step synthesis of phospho-rich, silica-enhanced chitosan aerogel for the efficient adsorption of uranium(VI)

In this study, an amorphous silica reinforced, phosphoric-crosslinked chitosan foam (P-CTS@SixOy) was prepared. The introduction of amorphous silica not only increased the affinity of the adsorbent for uranium, but also improved the stability of the material. The number of active sites of P-CTS@SixOy was increased by the introduction of phosphate groups. The material exhibited excellent uranium adsorption performance with the removal capacity and efficiency of 850.5 mg g-1 and 98.1 %, respectively. After regenerations, the morphology of P-CTS@SixOy still maintained, and the uranium adsorption efficiency remained above 90 %, manifesting the excellent cycle performance of P-CTS@SixOy. In the dynamic adsorption experiment, P-CTS@SixOy successfully concentrated the volume of uranium-containing solution, and exhibited excellent uranium adsorption performance. The analysis of kinetics, isotherms, and thermodynamics manifested that the uranium adsorption behavior of P-CTS@SixOy was a spontaneous, endothermic, monolayer chemical adsorption process. X-ray photoelectron spectroscopy, Scanning Electron Microscope, and Fourier Transform Infrared Spectrometer were used to characterized the P-CTS@SixOy before and after adsorption, which demonstrated that the main interaction mechanism between uranium and P-CTS@SixOy was the complexation. These studies indicated the huge application prospect of P-CTS@SixOy in the treatment of large-scale uranium-containing wastewater.

Synergistically adsorbing and reducing Uranium from water by a novel nano zero-valent copper/MXene 0D/2D nanocomposite

Proper disposal of uranium-containing waste is of utmost importance for safeguarding the environment and human health. In this study, we proposed a novel zero-dimensional (0D)/two-dimensional (2D) nanocomposite material, nZVC/Ti3C2, composed of nano zero-valent copper (nZVC) nanoparticles loaded onto Ti3C2 MXene nanoflakes, which was prepared using a simple in situ chemical reduction method. The uniform dispersion of 0D nZVC nanoparticles, with a size of approximately 5 nm, onto the 2D ultrathin Ti3C2 MXene effectively prevented agglomeration and corrosion of nZVC. This unique configuration provided numerous adsorption sites for UO22+and facilitated a fascinating charge channel for reducing adsorbed UO22+ into low-mobilized UO2 by nZVC. Under the synergistic effect of Ti3C2 MXene and nZVC, remarkable efficiency and selectivity of nZVC/Ti3C2 for U (VI) removal were demonstrated, which exhibited an exceptional adsorption capacity of up to 360 mg/g, coupled with a high removal efficiency of 97.5 % and rapid kinetics. Importantly, the presence of humic acid did not significantly affect the U (VI) removal efficiency of the composite because of the reduction effect of nZVC. The underlying mechanism of U (VI) removal was elucidated, revealing the involvement of reductive immobilization in the form of UO2 (as high as 73.6 %), inner-sphere surface complexation, and hydrolytic precipitation. This mechanism was dependent on the availability of active nZVC and the solution's pH. These findings highlight the potential of nZVC/Ti3C2 composites as efficient decontaminants for radioactive wastewater, thus contributing to advancements in environmental remediation endeavors.

Enhanced uranium separation by charge enabling γ-MnO2 with oxygen vacancies

Numerous efforts have been devoted to understanding the electron transfer process of uranium (UO22+) on adsorbent materials, whereas the potential oxygen vacancies (OVs) in metal oxides have long been overlooked. Once these interactions are taken into account, the emerging molecular orbital effects undoubtedly affect the adsorption process. Here, we synthesized CC/γ-MnO2 by growing MnO2 on carbon cloth (CC), followed by the creation of oxygen vacancies (OVs) through electrochemical methods to form CC/γ-MnO2-OVs. The CC/γ-MnO2-OVs shows significantly enhanced selectivity and durability for UO22+, with the maximum adsorption capacity increasing from 456.8 to 1648.1 mg/g (by a factor of 3.6). Theoretical calculations suggest that the generation of OVs leads to an increase in charge transfer and a decrease in adsorption energy between UO22+ and CC/γ-MnO2, due to the interaction between Mn 3d orbital in CC/γ-MnO2 and O 2p orbital in UO22+. The OVs in CC/γ-MnO2 provide a spatial structure for anchoring the OU=O moiety of UO22+, while the surface van der Waals forces and the formation of chemical bonds between Mn-U contribute to charge interactions. This synergistic effect allows CC/γ-MnO2-OVs to exhibit favorable selectivity, a large adsorption capacity, and rapid adsorption kinetics towards uranyl ions. This work achieves enhanced UO22+ separation by introducing OVs in CC/γ-MnO2 through a facile electrochemical strategy, highlighting the great potential for nuclear waste processing.

Preparation of biochar@chitosan-polyethyleneimine for the efficient removal of uranium from water environment

A novel chitosan-based composite with rich active sites was synthesized by uniformly dispersing biochar into the cross-linked network structure formed by chitosan and polyethyleneimine. Due to the synergistic effect of biochar (minerals) and chitosan-polyethyleneimine interpenetrating network (amino and hydroxyl), the chitosan-based composite possessed an excellent adsorption performance for uranium(VI). It could rapidly (<60 min) achieve a high adsorption efficiency (96.7 %) for uranium(VI) from water and a high static saturated adsorption capacity (633.4 mg/g), which was far superior to other chitosan-based adsorbents. Moreover, the separation for uranium(VI) on the chitosan-based composite was suitable for a variety of actual water environments and the adsorption efficiencies all exceeded 70 % in different water bodies. The soluble uranium(VI) could be completely removed by the chitosan-based composite in the continuous adsorption process, which could meet the permissible limits of the World Health Organization. In sum, the novel chitosan-based composite could overcome the bottleneck of current chitosan-based adsorption materials and become a potential adsorbent for the remediation of actual uranium(VI) contaminated wastewater.

Guest Molecular Assembly Strategy in Covalent Organic Frameworks for Electrochemiluminescence Sensing of Uranyl

The application of covalent organic frameworks (COFs) in electrochemiluminescence (ECL) is promising in environmental monitoring. Developing an emerging design strategy to expand the class of COF-based ECL luminophores is highly desirable. Here, a COF-based host-guest system was constructed through guest molecular assembly to deal with nuclear contamination analysis. The efficient charge transport network was formed by inserting an electron-withdrawing guest tetracyanoquinodimethane (TCNQ) into the open space of the COF host (TP-TBDA; TP = 2,4,6-trihydroxy-1,3,5-benzenetricarbaldehyde and TBDA = 2,5-di(thiophen-2-yl)benzene-1,4-diamine) with an electron-donating property; the construction of the COF-based host-guest system (TP-TBDA@TCNQ) triggered the ECL emission of non-emitting TP-TBDA. Furthermore, the dense active sites in TP-TBDA were utilized to capture the target substance UO₂²⁺. The presence of UO₂²⁺ broke the charge-transfer effect in TP-TBDA@TCNQ, resulting in the weakening of the ECL signal, thus the established ECL system integrating the low detection limit with high selectivity monitors UO₂²⁺. This COF-based host-guest system provides a novel material platform for constructing late-model ECL luminophores and creates an opportunity for the vigorous ECL technology.

Biomimetic Photocatalytic System Designed by Spatially Separated Cocatalysts on Z-scheme Heterojunction with Identified Charge-transfer Processes for Boosting Removal of U(VI)

Designing highly efficient photocatalysts with rapid migration of photogenerated charges and surface reaction kinetics for the photocatalytic removal of uranium (U(VI)) from uranium mine wastewater remains a significant challenge. Inspired by natural photosynthesis, a biomimetic photocatalytic system is assembled by designing a novel hollow nanosphere MnOx @TiO2 @CdS@Au (MTCA) with loading MnOx and Au nano particles (Au NPs) cocatalysts on the inner and outer surfaces of the TiO2 @CdS. The spatially separated cocatalysts efficiently drive the photogenerated charges to migrate in opposite directions, while the Z-scheme heterogeneous shell further separates the interfacial charges. Theoretical calculation identifies multiple consecutive forward charge transfers without charge recombination within MTCA. Thus, MTCA could efficiently remove 99.61% of U(VI) after 15 min of simulated sunlight irradiation within 3 mmol L-1 NaHCO3 with 0.231 min-1 of the reduction rate constant, outperforming most previously reported photocatalysts. MTCA further significantly removes 91.83% of U(VI) from the natural uranium mining wastewater under sunlight irradiation. This study provides a novel approach to designing an ideal biomimetic photocatalyst for remediating environmental pollution.

Study of natural attenuation after acid in situ leaching of uranium mines using isotope fractionation and geochemical data

Acid in situ leaching (AISL) is a subsurface mining approach suitable for low-grade ores which does not generate tailings, and has been adopted widely in uranium mining. However, this technique causes an extremely high concentration of contaminants at post-mining sites and in the surroundings soon after the mining ceases. As a potential AISL remediation strategy, natural attenuation has not been studied in detail. To address this problem, groundwater collected from 26 wells located within, adjacent, upgradient, and downgradient of a post-mining site were chosen to analyze the fate of U(VI), SO₄^2-, δ³⁴S, and δ²³⁸U, to reveal the main mechanisms governing the migration and attenuation of the dominant contaminants and the spatio-temporal evolutions of contaminants in the confined aquifer of the post-mining site. The δ²³⁸U values vary from -0.07 ‰ to 0.09 ‰ in the post-mining site and from -1.43 ‰ to 0.03 ‰ around the post-mining site. The δ³⁴S values were found to vary from 3.3 ‰ to 6.2 ‰ in the post-mining site and from 6.0 ‰ to 11.0 ‰ around the post-mining site. Detailed analysis suggests that there are large differences between the range of isotopic composition variation and the range of pollutants concentration distribution, and the estimated Rayleigh isotope fractionation factor is 0.9994-0.9997 for uranium and 1.0032-1.0061 for sulfur. The isotope ratio of uranium and sulfur can be used to deduce the migration history of the contaminants and the irreversibility of the natural attenuation process in the anoxic confined aquifer. Combining the isotopic fractionation data for U and S with the concentrations of uranium and sulfate improved the accuracy of understanding of reducing conditions along the flow path. The study also indicated that as long as the geological conditions are favorable for redox reactions, natural attenuation could be used as a cost-effective remediation scheme.

In situ synthesis of magnesium-doped hydroxyapatite aerogel for highly efficient U(VI) separation with ultra high adsorption capacity and excellent recyclability

Mg-doped HAP aerogel (MHAPA) was firstly in situ prepared via freeze-drying-calcination technology to capture U(VI). The U(VI) removal capacity by MHAPA even arrived 2685.6 mg g^-1, which was about 2 times over purchased HAP, illustrating that the incorporation of Mg ions could greatly enhance the U(VI) removal capacity. Compared with HAP, MHAPA also showed better anti-ion interference ability and dynamic removal performances. In comparison with other HAP-based adsorbents, MHAPA possessed good recyclability and its desorption rate was up to 93.4% in the first cycle. The excellent U(VI) removal performances of MHAPA might be owing to its low crystallinity and grain size, fast ion exchange rate and partial ionization under acidic conditions, which would accelerate the process of electrostatic attraction, ion-exchange, and complexation to immobilize U(VI). To sum up, the prepared MHAPA was expected to be an environmentally friendly, recyclable and effective adsorbent to immobilize U(VI) in actual wastewater.

Analysis of influencing factors of phenanthrene adsorption by different soils in Guanzhong basin based on response surface method

Adsorption desorption is an important behavior affecting the migration of phenanthrene in soil. In this study, three typical soils of loess, silts and silty sand in Guanzhong Basin, Shaanxi Province, China were used as adsorbents. Batch equilibrium experiments were carried out to study the adsorption desorption kinetics and isotherm of phenanthrene in different soils. Response surface method (RSM) was used to study the effects of temperature, pH, phenanthrene concentration and organic matter content on soil adsorption of phenanthrene. The results showed that after adsorption, the outline of soil particles became more blurred and the degree of cementation increased. The kinetic adsorption of phenanthrene by soil conforms to the quasi second-order kinetic model, and the adsorption desorption isotherm is nonlinear and conforms to the Freundlich model. Due to the difference of soil properties, the adsorption amount of phenanthrene by soil is loess > silty sand > silts. The thermodynamic results show that the adsorption of phenanthrene by soil is spontaneous and endothermic, and the desorption is spontaneous and exothermic. Through RSM, the interaction between phenanthrene concentration and soil organic matter in Loess and silts is significant, and the interaction between temperature and soil organic matter in silty sand is significant. Among the four factors affecting the adsorption rate of loess, silts and silty sand, soil organic matter is the most significant. The theoretical optimum adsorption rates of loess, silts and silty sand are 98.89%, 96.59% and 93.37% respectively.

Nano-scale analysis of uranium release behavior from river sediment in the Ili basin

Due to the limitations of the conventional water sample pretreatment methods, some of the colloidal uranium (U) has long been misidentified as "dissolved" phase. In this work, the U species in river water in the Ili Basin was classified into submicron-colloidal (0.1-1 μm), nano-colloidal (0.1 μm-3 kDa) and dissolved phases (< 3 kDa) by using high-speed centrifugation and ultrafiltration. The U concentration in the river water was 5.39-8.75 μg/L, which was dominated by nano-colloidal phase (55-70%). The nano-colloidal particles were mainly composed of particulate organic matter (POM) and had a very high adsorption capacity for U (accounting for 70 ± 23% of colloidal U). Sediment disturbance, low temperature, and high inorganic carbon greatly improved the release of nano-colloidal U, but high levels of Ca²⁺ inhibited it. The simulated river experiments indicated that the flow regime determined the release of nano-colloidal U, and large amounts of nano-colloidal U might be released during spring floods in the Ili basin. Moreover, global warming increases river flow and inorganic carbon content, which may greatly promote the release and migration of nano-colloidal U.

Ultrahigh efficient and selective adsorption of U(VI) with amino acids-modified magnetic chitosan biosorbents: Performance and mechanism

Exploiting eco-friendly, highly controlled preparation and convenient solid-liquid separation adsorbent to separate uranium from aquatic medium is of importance and in demand. In this study, magnetic ferroferric oxide nanoparticles synthesized through a facile hydrothermal reaction was cross-linked with chitosan. The intermediate product was subsequently chemically grafting with four amino acids such as alanine, serine, glycine or L-cysteine to produce Ala-MCS, Ser-MCS, Gly-MCS and Cys-MCS. The resultants were verified by SEM, EDS, XRD, VSM, FT-IR and XPS. Adsorption of uranium with amino acids-modified magnetic chitosans were carried out. The parameters that affected the adsorption ability, selectivity toward uranium, and reusability have been illustrated. pH 6.5 was the most beneficial for the adsorption. The saturation adsorption capacity of Ala-MCS, Ser-MCS, Gly-MCS, Cys-MCS were found as 658.88 mg/g ± 1.0 %, 616.10 ± 0.3 % mg/g, 646.38 ± 1.8 % mg/g, 653.96 ± 3.4 % mg/g and 409.15 ± 4.6 % mg/g, respectively. The adsorption process was analyzed using kinetics (pseudo-first-order, pseudo-second-order and intraparticle diffusion models) and isotherms models (Langmuir and Freundlich models). The adsorption of uranium on Ala-MCS, Ser-MCS, Gly-MCS and Cys-MCS happened on monolayer and were controlled by chemisorption. The certified high adsorption amount and efficient solid-liquid separation proved amino acids-modified magnetic chitosan are promising adsorbents for removal of uranium from wastewater.

Fabrication and investigation of novel monochloroacetic acid fortified, tripolyphosphate-crosslinked chitosan for highly efficient adsorption of uranyl ions from radioactive effluents

Chitosan crosslinked with potassium tripolyphosphate (CTPP) and monochloroacetic-acid-modified chitosan crosslinked with potassium tripolyphosphate (MCTPP) were synthesized for removing UO₂²⁺ from acidic radioactive effluents. The influential factors, operational requirements, and interactive mechanisms of the adsorption process were systematically investigated. The mesh-structured composites adsorbed UO₂²⁺ most effectively at pH 5.0. The maximum adsorption capacities for pure chitosan, CTPP, and MCTPP were 374.93, 780.89, and 1487.72 mg/g, respectively. Batch experiments indicated that the pH and adsorbent dose strongly influenced UO₂²⁺ adsorption. MCTPP could adsorb most UO₂²⁺ within 15 min, and equilibrium was reached by ~1 h. The adsorption isotherms indicated that UO₂²⁺ adsorption by MCTPP may be an endothermic single-layer adsorption process. Moreover, common metal ions in single-metal systems only slightly affected this process. The results of instrumental characterization and natural water application suggested that the highly developed pore structure and abundant tripolyphosphate groups in synthesized composites were dominant adsorption contributors besides amino and hydroxyl groups. Successful development of the novel material for efficiently adsorbing UO₂²⁺ and identification of the adsorption mechanism will provide valuable guidance to chitosan modification and further remediation practices of radioactive effluents.

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.

Bismuth impregnated biochar for efficient uranium removal from solution: Adsorption behavior and interfacial mechanism

In this work, Bi₂O₃ doped horse manure-derived biochar was obtained by carbonizing the H₂O₂-modified horse manure loaded with bismuth nitrate under nitrogen atmosphere at 500 °C. The results showed that there was a sharp response between the as-prepared bismuth impregnated biochar and uranium(VI) species in solution, which resulted in a short equilibrium time (<80 min), a fast adsorption rate (about 5.0 mg/(g·min)), a high removal efficiency (93.9%) and a large adsorption capacity (516.5 mg/g) (T = 298 K, pH = 4, C_i = 10 mg/L and m/V = 0.1 g/L). Besides, the removal behavior of the bismuth impregnated biochar for uranium(VI) did not depend on the interfering ions and ion strength, except Al³⁺, Ca²⁺, CO₃^2- and PO₄^3-. These results indicated that the modified biochar might possess the potential of remediating the actual uranium(VI)-containing wastewater. Moreover, the interaction mechanism between Bi₂O₃ doped biochar and uranium(VI) species was further explored. The results demonstrated that the enrichment of uranium(VI) on the surface of the as-prepared biochar was controlled by various factors, such as surface complexation, ion exchange, electrostatic attraction, precipitation and reduction, which facilitated the adsorption of uranium(VI) on the bismuth impregnated biochar.

Uptake, organ distribution and health risk assessment of potentially toxic elements in crops in abandoned indigenous smelting region

Inorganic pollution induced by smelting waste has threatened the safety of environment, whereas the impacts on farmlands with regards to potentially toxic elements (PTEs) receive insufficient attention. Herein, the contents, transfer pathways and potential risks of the PTEs in common crops were examined from different farmlands distributed around an indigenous Zn-smelting area in Guizhou, China. The results showed that Tl in cabbage (Brassica oleracea L.) (up to 3.74 mg/kg) and radish (Raphanus sativus L.) (up to 1.16 mg/kg) at some sites exceeded the maximum permissible level (MPL) (0.5 mg/kg) for food, and, under the same pollution condition, cabbage and radish were more likely to enrich PTEs, and the edible portion of maize was not prone to Tl risk. Hazard quotient calculations of Tl, Ba, and U were greater than 1, indicating the edible risk of crops for these PTEs. Further characterization of selected soils revealed that MnFe₂O₄ and Fe₂O₃ controlled the phase transformation of Tl(III) in rhizospheric soils. Furthermore, distinctive mullite was detected in the soil which confirmed the contribution of high temperature smelting to PTEs pollution. The findings indicate an emergent need for soil remediation around historical indigenous metal smelting areas for the sake of food security.

U(VI) adsorption by green and facilely modified Ficus microcarpa aerial roots: Behavior and mechanism investigation

Uranium (U)-containing wastewater poses serious pressure to human health and environmental safety. The treatment of U-bearing wastewater using green and facilely fabricated materials is considered a promising alternative. Herein, the raw and modified aerial roots of Ficus microcarpa (RARF and MARF, respectively) were prepared and applied to the treatment of synthesized U-containing wastewater. The results showed that the adsorption process was spontaneous and chemically controlled, which was in good accordance with the pseudo-second-order kinetic and the Redlich-Peterson isotherm adsorption model. The adsorption mechanisms were proposed to be the complexation between U(VI) and oxygen/phosphorus-containing functional groups on MARF.

Iron phosphonate for highly efficient capture of U(VI) from acidic solution

In this study, a novel, high surface area iron phosphonate (IP) for highly efficient adsorption of uranyl ion in acidic medium was described. The as-prepared IP was amorphous with its specific surface area and total pore volume as high as 268 m²/g and 1.04 cm³/g, respectively. Particularly, the as-prepared IP with ferrous ions and oxygen, nitrogen-bearing functional groups prove excellent U(VI) adsorption capacity (154.6 mg/g) as compared to that of amorphous FePO₄ (67.3 mg/g) and Fe₃(PO₄)₂(H₂O)₈ (33.8 mg/g). Surprising, the saturation adsorption capacity could achieve up to 353.9 mg/g. Besides, the IP also had a fast adsorption rate for attaining adsorption equilibrium within 20 min, and followed pseudo-second-order kinetic and Freundlich models. Moreover, both the Dubinin-Radushkevich isotherm adsorption model and the value of enthalpy indicated a chemisorption process. Otherwise, the Na⁺-independent U(VI) adsorption on IP and the adsorption-desorption isotherm studies revealed that inner-layer surface complexation is the control step for U(VI) adsorption process, and the adsorbent featured an irreversible adsorption process. The structure and functional groups of the adsorbent remained unchanged after capture of U(VI). Further, X-ray photoelectron spectra (XPS) analysis demonstrated that the capture mechanism of U(VI) on IP from acidic aqueous solution was due to not only redox reaction, but also ascribed to the coordinated chemical adsorption.

Transformation and fate of thallium and accompanying metal(loid)s in paddy soils and rice: A case study from a large-scale industrial area in China

Thallium (Tl) is an extremely toxic metal, while its occurrence and fate in paddy soil environment remain understudied. Herein, the enrichment and migration mechanisms and potential health risks of Tl and metal(loid)s were evaluated in paddy soils surrounding an industrial park utilizing Tl-bearing minerals. The results showed that Tl contamination was evident (0.63-3.16 mg/kg) in the paddy soils and Tl was generally enriched in root of rice (Oryza sativa L.) with a mean content of 1.27 mg/kg. A remarkably high level of Tl(III) (30-50%) was observed in the paddy soils. Further analyses by STEM-EDS and XPS indicated that Tl(I) in the paddy soils was jointly controlled by adsorption, oxidation, and precipitation of Fe/Mn(hydr)oxide (e.g. hematite and birnessite), which might act as important stabilization mechanisms for inhibiting potential Tl uptake by rice grains. The health quotient (HQ) values indicated a potentially high Tl risk for inhabitants via consumption of the rice grains. Therefore, it is critical to establish effective measures for controlling the discharge of Tl-containing waste and wastewater from different industrial activities to ensure food safety in the rice paddy soils.

Stable isotope fractionation of thallium as novel evidence for its geochemical transfer during lead‑zinc smelting activities

Thallium (Tl) is a highly toxic trace metal. Lead (Pb)‑zinc (Zn) smelting, which is a pillar industry in various countries, is regarded as one of the dominant anthropogenic sources of Tl contamination in the environment. In this study, thallium isotope data have been evaluated for raw material and a set of industrial wastes produced at different stages of Pb-Zn smelting in a representative large facility located by the North River, South China, in order to capture Tl isotope signatures of such typical anthropogenic origin for laying the foundation of tracking Tl pollution. Large variations in Tl isotopic compositions of raw Pb-Zn ores and solid smelting wastes produced along the process chain were observed. The ε²⁰⁵Tl values of raw Pb-Zn ores and return fines are -0.87 ± 0.26 and -1.0 ± 0.17, respectively, contrasted by increasingly more negative values for electrostatic precipitator dust (ε²⁰⁵Tl = -2.03 ± 0.14), lime neutralizing slag (ε²⁰⁵Tl = -2.36 ± 0.18), and acid sludge (ε²⁰⁵Tl = -4.62 ± 0.76). The heaviest ε²⁰⁵Tl (1.12 ± 0.51) was found in clinker. These results show that isotopic fractionation occurs during the smelting processes. Obviously, the lighter Tl isotope is enriched in the vapor phase (-3.75 ε²⁰⁵Tl units). Further XPS and STEM-EDS analyses show that Tl isotope fractionation conforms to the Rayleigh fractionation model, and adsorption of ²⁰⁵Tl onto hematite (Fe₂O₃) may play an important role in the enrichment of the heavier Tl isotope. The findings demonstrate that Tl isotope analysis is a robust tool to aid our understanding of Tl behavior in smelting processes and to provide a basis for source apportionment of Tl contaminations.

Metal-organic frameworks as superior porous adsorbents for radionuclide sequestration: Current status and perspectives

Efficient separation of hazardous radionuclides from radioactive waste remains a challenge to the global acceptance of nuclear power due to complex nature of the waste, high radiotoxicities and presence of large number of interfering elements. Sorption of radioactive elements from liquid phase, gas phase or their solid particulates on various synthetic organic, inorganic or biological sorbents is looked as one of the options for their remediation. In this context, highly porous materials, termed as metal-organic frameworks (MOFs), have shown promise for efficient capturing of various types of radioactive elements. Major advantages that have been advocated for the application of MOFs in radionuclide sorption are their excellent chemical stability, and their large surface area due to abundant functional groups, and porosity. In this review, recent developments on the application of MOFs for radionuclide sequestration are briefly discussed. Focus has been devoted to address the separation of few crucial radioactive elements such as Th, U, Tc, Re, Se, Sr and Cs from aqueous solutions, which are important for liquid radioactive waste management. Apart from these radioactive metal ions, removal of radionuclide bearing gases such as I₂, Xe, and Kr are also discussed. Aspects related to the interaction of MOFs with the radionuclides are also discussed. Finally, a perspective for comprehensive investigation of MOFs for their applications in radioactive waste management has been outlined.

Response and Dynamic Change of Microbial Community during Bioremediation of Uranium Tailings by Bacillus sp

Bacillus sp. is widely used in the remediation of uranium-contaminated sites. However, little is known about the competitive process of microbial community in the environment during bioremediation. The bioremediation of uranium tailings using Bacillus sp. was explored, and the bacterial community was analyzed by high-throughput sequencing at different stages of remediation. Bacillus sp. reduced the leaching of uranium from uranium tailings. The lowest uranium concentration was 17.25 μg/L. Alpha diversity revealed that the abundance and diversity of microorganisms increased with the extension of the culture time. The microbial abundance and diversity were higher in the treatment group than in the control group. The dominant species at the phyla level were Firmicutes and Proteobacteria in the uranium tailings environment, whereas the phylum of Proteobacteria was significantly increased in the treatment group. Based on the genus level, the proportions of Arthrobacter, Rhodococcus and Paenarthrobacter decreased significantly, whereas those of Clostridium sp., Bacillus and Pseudomonas increased dramatically. Hence, the remediation of uranium contamination in the environment was due to the functional microorganisms, which gradually became the dominant strain in the treatment, such as Desulfotomaculum, Desulfosporporosinus, Anaerocolumna, Ruminiclostridium and Burkholderia. These findings provided a promising outlook of the potential for remediation strategies of soil contaminated by uranium. The dynamic characteristics of the microbial community are likely to provide a foundation for the bioremediation process in practice.