Removal and Recovery of Uranium from Groundwater Using Direct Electrochemical Reduction Method: Performance and Implications

Secondary metal ion-induced electrochemical reduction of U(VI) to U(IV) solids

Recent studies have shown that aqueous U(VI) ions can be transformed into U(VI) precipitates through electrocatalytic redox reactions for uranium recovery. However, there have been no reports of U(IV) solids, such as UO2, using electrochemical methods under ambient conditions since low-valence states of uranium are typically oxidized to U(VI) by O2 or H2O2. Here we developed a secondary metal ion-induced strategy for electrocatalytic production of U(IV) solids from U(VI) solutions using a catalyst consisting of atomically dispersed gallium on hollow nitrogen-doped carbon capsules (Ga-Nx-C). This method relies on the presence of secondary metal ions, e.g., alkaline earth metals, transition metals, lanthanide metals, and actinide metals, which promote the generation of UO2 or bimetallic U(IV)-containing oxides through a two-electron transfer process. No U(IV) solid products were generated in the presence of alkali metal ions. Mechanistic studies revealed that the strong binding affinity between U(IV) and alkaline earth metals (Ca2+/Mg2+/Sr2+/Ba2+), transition metals (Ni2+/Zn2+/Pb2+/Fe3+, etc.) and lanthanide/actinide metals (Ce4+/Eu3+/Th4+/La3+) suppressed re-oxidation of U(IV) to U(VI), leading to the generation of U(IV)O2 and Mx(M = Ce, Eu, Th, La)U(IV)yO2. This work provides fundamental insights into the electrochemical behavior of uranium in aqueous media, whilst guiding uranyl capture from nuclear waste and contaminated water.

A synergistic bioelectrochemical-photocatalytic system for efficient uranium removal and recovery

Bioelectrochemical system (BES) is a promising technology for uranium recovery, which also enables simultaneous electricity generation. However, the bioelectrochemical recovery of uranium is hindered by its slow process due to the low reduction potential provided by microorganisms. Herein, we developed an innovative bioelectrochemical-photocatalytic system (BEPS) that combines the advantages of BES and photocatalysis, achieving enhanced uranium removal and recovery. The photogenerated electrons in BEPS possess a more negative reduction potential and stronger reduction capability than microbial electrons in BES, significantly accelerating uranium reduction and deposition on the electrode surface. Moreover, the electrons from the bioanode combine with photogenerated holes through the external circuit, effectively inhibiting the recombination of charge carriers. The BEPS significantly enhances uranium removal efficiency, kinetic, and electricity generation through a synergistic coupling mechanism between the bioanode and photocathode. Notably, the UO2 deposited on the electrode surface exhibited a recovery efficiency of 98.21 ± 1.37%, and the regenerated electrode sustained its photoelectric response and uranium removal capabilities. Our findings highlight the potential of the BEPS as an effective technology for uranium recovery and electricity generation.

Geospatial analysis of toxic metal contamination in groundwater and associated health risks in the lower Himalayan industrial region

Once known for its clean and natural environment, the lower Himalayan region is now no exception to human-induced disturbances. Rapid industrial growth in Baddi-Barotiwala (BB) industrial region has led to degradation of groundwater resources in the area. Groundwater samples were collected from 37 locations to study the groundwater chemistry, geospatial variation of 15 toxic metals in groundwater, source apportionment, metals of concern and associated health risks in the region. The results showed rock dominated hydrogeology with decreasing order of anion and cation abundance as HCO3- > Cl- > SO42- > NO3- > Br- > F- and Ca+ > Na+ > Mg2+ > K+ > Li+ respectively. Concentrations of Iron (BDL-3.6 mg/l), Nickel (BDL-0.023 mg/l), Barium (0.22-0.89 mg/l), Lead (0.0001-0.085 mg/l) and Zinc (0.006-21.4 mg/l) were found above the permissible limits at few locations. Principal component analysis (PCA) and coefficient of variance (CV) showed both geogenic and anthropogenic origin of metals in groundwater of the BB industrial region. A consistent concentration of Uranium was detected at all the sampling locations with an average value of 0.0039 mg/l and poor spatial variation indicating its natural presence. Overall, non-carcinogenic (N-CR) risk in the study area via oral pathway was high for adults and children (Hazard Index > 1) with geogenic Uranium as the major contributor (Hazard Quotient > 1) followed by Zinc, Lead and Cobalt. Carcinogenic (CR) risk in the region was high for adults having mean value above the threshold (1E-04) with Nickel and Chromium as the metals of major concern. Spatial variation of health risks was overlayed on village boundaries of the region to identify the potential industrial sources of the metals of major concern. The results highlight the need for immediate remediation of groundwater resources in order to achieve a harmonious coexistence between industrialization and human well-being.

Four-in-one multifunctional self-driven photoelectrocatalytic system for water purification: Organics degradation, U(VI) reduction, electricity generation and disinfection against bacteria

This study addresses the energy-intensive nature of conventional wastewater treatment processes and proposes a solution through the development of a green, low-energy, and multifunctional wastewater treatment technology. The research focuses on a multifunctional self-driven photoelectrocatalytic (PEC) system, exploring its four-in-one applications in eliminating organic pollutants, reducing U(VI), generating electrical energy, and disinfecting pathogenic microorganisms. A TiO2-decorated carbon felt (CF@TiO2) cathode is synthesized to enhance interfacial charge transfer, with TiO2 coating improving surface binding sites (edge TiO and adsorbed -OH) for UO22+ adsorption and reduction. The self-driven PEC system, illuminated solely with simulated sunlight, exhibits remarkable efficiency in removing nearly 100 % of uranium within 0.5 h and simultaneously degrading 99.9 % of sulfamethoxazole (SMX) within 1.5 h, all while generating a maximum power output density (Pmax) of approximately 1065 μW·cm-2. The system demonstrates significant anti-interference properties across a wide pH range and coexisting ions. Moreover, 49.4 % of the fixed uranium on the cathode is reduced into U(IV) species, limiting its migration. The self-driven PEC system also excels in detoxifying various toxic organic compounds, including tetracycline, chlortetracycline, and oxytetracycline, and exhibits exceptional sterilization ability by disinfecting nearly 100 % of Escherichia coli within 0.5 h. This work presents an energy-saving, sustainable, and easily recyclable wastewater purification system with four-in-one capabilities, relying solely on sunlight for operation.

Ion pair sites for efficient electrochemical extraction of uranium in real nuclear wastewater

Electrochemical uranium extraction from nuclear wastewater represents an emerging strategy for recycling uranium resources. However, in nuclear fuel production which generates the majority of uranium-containing nuclear wastewater, fluoride ion (F-) co-exists with uranyl (UO22+), resulting in the complex species of UO2Fx and thus decreasing extraction efficiency. Herein, we construct Tiδ+-PO43- ion pair extraction sites in Ti(OH)PO4 for efficient electrochemical uranium extraction in wastewater from nuclear fuel production. These sites selectively bind with UO2Fx through the combined Ti-F and multiple O-U-O bonds. In the uranium extraction, the uranium species undergo a crystalline transition from U3O7 to K3UO2F5. In real nuclear wastewater, the uranium is electrochemically extracted with a high efficiency of 99.6% and finally purified as uranium oxide powder, corresponding to an extraction capacity of 6829 mg g-1 without saturation. This work paves an efficient way for electrochemical uranium recycling in real wastewater of nuclear production.

Global research trends of uranium-containing wastewater treatment based on bibliometric review

The generation of uranium-containing wastewater (UCW) during different stages of uranium mining, processing, and utilization presents a significant ecological and biospheric threat. Consequently, it is crucial for both sustainable development and the protection of human health to adopt appropriate methods for the treatment of UCW as well as the separation and enrichment of uranium. This study conducted a comprehensive search of the Web of Science Core Collection (WOSCC) database for publications related to UCW treatment between 1990 and 2022 to gain insight into current trends in the field. Subsequently, the annual publications, WOSCC categories, geographical distribution, major collaborations, prolific authors, influential journals, and highly cited publications were the subjects of a biliometric analysis that was subsequently carried out. The study findings indicate a significant rise in the overall number of publications in the research field between 1990 and 2022. China, India, and the USA emerged as the primary contributors in terms of publication count. The Chinese Academy of Sciences, the East China University of Technology, and the University of South China were identified as the key research institutions in this field. Furthermore, a majority of the publications in this field were distributed through prestigious journals with high impact factors, such as the Journal of Radioanalytical and Nuclear Chemistry. The top 3 journals were Radioanalytical and Nuclear Chemistry, Chemical Engineering Journal, and Journal of Hazardous Materials. The keyword co-occurrence and burst analysis revealed that the current research on UCW treatment mainly focuses on adsorption-based treatment methods, environmentally functional materials, uranium recovery, etc. Furthermore, the study of the adsorption efficiency of different adsorbent materials, as well as the strengthening and improvement of adsorbent material selectivity and capacity for the recovery of uranium, represents a research hotspot in the field of UCW treatment in the future. This study conducts a comprehensive overview of the current status and prospects of the UCW treatment, which can provide a valuable reference for gaining insights into the development trajectory of the UCW treatment.

An electrochemical technique for sensing uranium adsorption and desorption

Uranium is a toxic, heavy metal that can pose elevated health risks if leached into the environment. Uranium mobility is dependent on many factors, including speciation, solution pH, ligands in solution, and presence of surfaces. Surface adsorption is one phenomenon that inhibits uranium mobility in the environment and is studied as a naturally occurring phenomenon as well an intentional tool for environmental remediation. This work presents and validates a potentiometric, electrochemical technique for sensing uranium adsorption on and desorption from an electrochemically active surface solely through changes of the electrode potential. This novel electrochemical technique presents a new lens to study adsorption that complements external techniques (e.g., spectroscopy). Indication of adsorption and desorption via the electrochemical technique are gravimetrically validated using an electrochemical quartz crystal microbalance. This work contributes a unique, complementary technique that corroborates the adsorption of uranium on an electrode surface.

Electrochemical reduction of wastewater by non-noble metal cathodes: From terminal purification to upcycling recovery

A shift beyond conventional environmental remediation to a sustainable pollutant upgrading conversion is extremely desirable due to the rising demand for resources and widespread chemical contamination. Electrochemical reduction processes (ERPs) have drawn considerable attention in recent years in the fields of oxyanion reduction, metal recovery, detoxification and high-value conversion of halogenated organics and benzenes. ERPs also have the potential to address the inherent limitations of conventional chemical reduction technologies in terms of hydrogen and noble metal requirements. Fundamentally, mechanisms of ERPs can be categorized into three main pathways: direct electron transfer, atomic hydrogen mediation, and electrode redox pairs. Furthermore, this review consolidates state-of-the-art non-noble metal cathodes and their performance comparable to noble metals (e.g., Pd, Pt) in electrochemical reduction of inorganic/organic pollutants. To overview the research trends of ERPs, we innovatively sort out the relationship between the electrochemical reduction rate, the charge of the pollutant, and the number of electron transfers based on the statistical analysis. And we propose potential countermeasures of pulsed electrocatalysis and flow mode enhancement for the bottlenecks in electron injection and mass transfer for electronegative pollutant reduction. We conclude by discussing the gaps in the scientific and engineering level of ERPs, and envisage that ERPs can be a low-carbon pathway for industrial wastewater detoxification and valorization.

Advances in the management of radioactive wastes and radionuclide contamination in environmental compartments: a review

Several anthropogenic activities produce radioactive materials into the environment. According to reports, exposure to high concentrations of radioactive elements such as potassium (⁴⁰K), uranium (²³⁸U and ²³⁵U), and thorium (²³²Th) poses serious health concerns. The scarcity of reviews addressing the occurrence/sources, distribution, and remedial solutions of radioactive contamination in the ecosystems has fueled data collection for this bibliometric survey. In rivers and potable water, reports show that several parts of Europe and Asia have recorded radionuclide concentrations much higher than the permissible level of 1 Bq/L. According to various investigations, activity concentrations of gamma-emitting radioactive elements discovered in soils are higher than the global average crustal values, especially around mining activities. Adsorption technique is the most prevalent remedial method for decontaminating radiochemically polluted sites. However, there is a need to investigate integrated approaches/combination techniques. Although complete radionuclide decontamination utilizing the various technologies is feasible, future research should focus on cost-effectiveness, waste minimization, sustainability, and rapid radionuclide decontamination. Radioactive materials can be harnessed as fuel for nuclear power generation to meet worldwide energy demand. However, proper infrastructure must be put in place to prevent catastrophic disasters.

Electrocatalytic and green system coupling strategy for simultaneous recovery and purification of uranium from uranium-containing wastewater

The recycling of uranium in wastewater is not only beneficial to the protection of ecological safety but also has great significance for the sustainable development of nuclear energy. However, there is no satisfactory method to recover and reuse uranium efficiently up to now. Here, we have developed an efficient and economical strategy that can achieve uranium recovery and direct reuse in wastewater. The feasibility analysis verified that the strategy still had good separation and recovery ability in acidic, alkaline, and high-salinity environments. The purity of uranium recovered from the separated liquid phase after electrochemical purification was up to about 99.95%. Ultrasonication could greatly increase the efficiency of this strategy, and 99.00% of high-purity uranium could be recovered within 2 h. We further improved the overall recovery rate by recovering the residual solid-phase uranium, and the overall recovery of uranium was increased to 99.40%. Moreover, the concentration of impurity ions in the recovered solution met the World Health Organization guidelines. In summary, the development of this strategy is of great importance for the sustainable use of uranium resources and environmental protection.

Enhanced mechanism of calcium towards uranium incorporation and stability in magnetite during electromineralization

Doping uranium into a room-temperature stable Fe₃O₄ lattice structure effectively reduces its migration. However, the synergistic or competitive effects of coexisting ions in an aqueous solution directly affect the uranium mineralization efficiency and the structural stability of uranium-bearing Fe₃O₄. The effects of calcium, carbonate, and phosphate on uranium electromineralization were investigated via batch experiments and theoretical calculations. Calcium incorporated into the Fe₃O₄ lattice increased the level and stability of doped uranium in Fe₃O₄. Uranium and calcium occupied the octahedral and tetrahedral sites of Fe₃O₄, respectively; the formation energy was only -10.23 eV due to strong hybridization effects between Fe1s, U4f, O2p, and Ca3d orbitals. Compared to the uranium-doped Fe₃O₄, uranium leaching ratios decreased by 19.2 % and 48.9 % under strongly acidic and alkaline conditions after 120 days. However, high concentrations of phosphate inhibited Fe₃O₄ crystallization. These results should provide new avenues for the development of multi-metal co-doping technologies and mineralization optimization to treat uranium-containing complex wastewater.

Bi-Functional Co/Al Modified 1T-MoS2 /rGO Catalyst for Enhanced Uranium Extraction and Hydrogen Evolution Reaction in Seawater

Uranium is a key element in the preparation of nuclear fuel. An electrochemical uranium extraction technique is proposed to achieve high efficiency uranium extraction performance through HER catalyst. However, it is still a challenge to design and develop a high-performance hydrogen evolution reaction (HER) catalyst for rapid extraction and recovery of uranium from seawater. Herein, a bi-functional Co, Al modified 1T-MoS₂ /reduced graphene oxide (CA-1T-MoS₂ /rGO) catalyst, showing a good HER performance with a HER overpotential of 466 mV at 10 mA cm^-2 in simulated seawater, is first developed. Benefiting from the high HER performance of CA-1T-MoS₂ /rGO, efficient uranium extraction is achieved with a uranium extraction capacity of 1990 mg g^-1 in simulated seawater without post-treatment, exhibiting a good reusability. The results of experiments and density functional theory (DFT) show that a high uranium extraction and recovery capability is attributed to the synergy effect of the improved HER performance and the strong adsorption capacity between U and OH*. This work provides a new strategy for the design and preparation of bi-functional catalysts with high HER performance and uranium extraction and recovery capabilities in seawater.

Uranium capture by a layered 2D/2D niobium phosphate/holey graphene architecture via an electro-adsorption and electrocatalytic reduction coupling process

As an energy-efficient and eco-friendly technique, capacitive deionization (CDI) has shown great potential for uranium (U(VI)) capture recently. However, extracting U(VI) with high kinetics, capacity and selectivity remains a major challenge due to the current surface active sites-based material and co-existing ions in aqueous solution. Here we rationally designed a layered 2D/2D niobium phosphate/holey graphene (HGNbP) electrode material, and originally demonstrated its efficient U(VI) capture ability via an electro-adsorption and electrocatalytic reduction coupling process. The less-accumulative loose layered architecture, open polycrystalline construction of niobium phosphate with active phosphate sites, and rich in-plane nano-pores on conductive graphene nanosheets endowed HGNbP with fast charge/ion transport, high electroconductivity and superior pseudocapacitance, which enabled U(VI) ions first to be electro-adsorbed, then physico-chemical adsorbed, and finally electrocatalysis reduced/deposited onto electrode surface without the limitation of active sites under a low potential of 1.2 V. Based on these virtues, the HGNbP exhibited a fast adsorption kinetics, with a high removal rate of 99.9% within 30 min in 50 mg L^-1 U(VI) solution, and a high adsorption capacity up to 1340 mg g^-1 in 1000 mg L^-1 U(VI) solution. Furthermore, the good recyclability and selectivity towards U(VI) were also realized.

Weak electro-stimulation promotes microbial uranium removal: Efficacy and mechanisms

Removal and recovery of uranium from uranium-mine wastewater is beneficial to environmental protection and resource preservation. Reduction of soluble hexavalent U (U(VI)) to insoluble tetravalent uranium (U(IV)) by microbes is a plausible approach for this purpose, but its practical implementation has long been restricted by its intrinsic drawbacks. The electro-stimulated microbial process offers promise in overcoming these drawbacks. However, its applicability in real wastewater has not been evaluated yet, and its U(VI) removal mechanisms remain poorly understood. Herein, we report that introducing a weak electro-stimulation considerably boosted microbial U(VI) removal activities in both synthetic and real wastewater. The U(VI) removal has proceeded via U(VI)-to-U(IV) reduction in the biocathode, and the electrochemical characterization demonstrates the crucial role of the electroactive biofilm. Microbial community analysis shows that the broad biodiversity of the cathode biofilm is capable of U(VI) reduction, and the molecular ecological network indicates that synthetic metabolisms among electroactive and metal-reducing bacteria play major roles in electro-microbial-mediated uranium removal. Metagenomic sequencing elucidates that the electro-stimulated U(VI) bioreduction may proceed via e-pili, extracellular electron shuttles, periplasmic and outer membrane cytochrome, and thioredoxin pathways. These findings reveal the potential and mechanism of the electro-stimulated U(VI) bioreduction system for the treatment of U-bearing wastewater.

Electrocatalytic Removal of Low-Concentration Uranium Using TiO2 Nanotube Arrays/Ti Mesh Electrodes

Groundwater containing naturally occurring uranium is a conventional drinking water source in many countries. Removal of low concentrations of uranium complexes in groundwater is a challenging task. Here, we demonstrated that the TiO₂ nanotube arrays/Ti (TNTAs/Ti) mesh electrode could break through the concentration limit and efficiently remove low concentrations of uranium complexes from both simulated and real groundwater. U(VI) complexes in groundwater were electro-reduced to UO₂ and deposited on the TNTAs/Ti mesh electrode surface. The adsorption rate and electron transfer rate of the anatase TNTAs/Ti mesh electrode were twice that of the rutile TNTAs/Ti mesh electrode. Therefore, the anatase TNTAs/Ti mesh electrode exhibited excellent electrocatalytic activity toward the electrochemical removal of U(VI), which could work at a higher potential and significantly reduce the energy consumption of U(VI) removal. The U(VI) adsorption capacity on the anatase TNTAs/Ti mesh electrode was limited due to the low U(VI) concentration. However, the anatase TNTAs/Ti mesh electrode displayed a huge U(VI) removal capacity using the electroreduction method, where adsorption and reduction of U(VI) were mutually promoted and induced continuous accumulation of UO₂ on the electrode. The accumulated UO₂ can be easily recovered in dilute HNO₃, and the electrode can be used repeatedly.

A novel solar-powered electrochemical mineralization system for persistent remediation of uranium-contaminated groundwater

Reduction of the migratory ability of uranium via reduction, co-precipitation or immobilization is a widely used technology for remediation of uranium contaminated groundwater (UCG). However, the re-released uranium due to environmental alterations such as oxidation, acid dissolution, or microbial decomposition limits the long-term effect of UCG remediation. Here, we developed a novel solar-powered electrochemical mineralization (SPEM) system for persistent remediation of UCG under laboratory conditions. The SPEM system incorporates uranium into magnetite crystal to achieve long-term stability of uranium. The effects of photoelectric conversion, subsurface void fraction, groundwater seepage velocity, and electrode configuration on uranium removal were systematically analyzed. The results showed that the remediation system had excellent adaptability to complex water quality and geological conditions, and could remediate large-area contamination. After 12 h of persistent treatment, the system with newly hexagonal two-dimensional electrode configuration (1A6C) reduced uranium concentration by more than 85% in simulated subsurface environment. The mineralized uranium was not re-released within continuous rinsing of treated regions using an acid solution (pH = 3.0), for 370 h. The developed method solely requires metallic iron as a raw material, which has high and long-term efficiency, is eco-friendly, simple, and widely applicable, thus reliable for the remediation of deep UCG.

Electrochemical removal and recovery of uranium: Effects of operation conditions, mechanisms, and implications

Removing and recovering uranium (U) from U-mining wastewater would be appealing, which simultaneously reduces the adverse environmental impact of U mining activities and mitigates the depletion of conventional U resources. In this study, we demonstrate the application of a constant-voltage electrochemical (CVE) method for the removal and recovery of U from U-mining wastewater, in an ambient atmosphere. The effects of operation conditions were elucidated in synthetic U-bearing water experiments, and the cell voltage and the ionic strength were found to play important roles in both the U extraction kinetics and the operation cost. The mechanistic studies show that, in synthetic U-bearing water, the CVE U extraction proceeds exclusively via a single-step one-electron reduction mechanism, where pentavalent U is the end product. In real U-mining wastewater, the interference of water matrices led to the disproportionation of the pentavalent U, resulting in the formation of tetravalent and hexavalent U in the extraction products. The U extraction efficacy of the CVE method was evaluated in real U-mining wastewater, and results show that the CVE U extraction method can be efficient with operation costs ranging from $0.55/kgU ~ $64.65/kgU, with varying cell voltages from 1.0 V to 4.0 V, implying its feasibility from the economic perspective.

A comparative investigation of uranium and thorium adsorption behavior on amidoximated copolymeric hydrogel

This work focuses on investigating the feasibility of using a crosslinked amidoximated copolymeric hydrogel as a potential adsorbent to recover uranium and thorium ions from aqueous media. The hydrogel was synthesized via gamma-irradiation copolymerization and characterized through FTIR, TGA, and SEM. The medium acidity notably affected the adsorption capacity of both ions. The adsorption data was in line with the pseudo-1st-order equation and the Freundlich isothermal model. The thermodynamics analysis showed that the temperature rise promoted the adsorption capacity. The reusability studies highlighted the good performance of the hydrogel up to five regeneration rounds.

Recovery of resources from industrial wastewater employing electrochemical technologies: status, advancements and perspectives

In the last two decades, water use has increased at twice the rate of population growth. The freshwater resources are getting polluted by contaminants like heavy metals, pesticides, hydrocarbons, organic waste, pathogens, fertilizers, and emerging pollutants. Globally more than 80% of the wastewater is released into the environment without proper treatment. Rapid industrialization has a dramatic effect on developing countries leading to significant losses to economic and health well-being in terms of toxicological impacts on humans and the environment through air, water, and soil pollution. This article provides an overview of physical, chemical, and biological processes to remove wastewater contaminants. A physical and/or chemical technique alone appears ineffective for recovering useful resources from wastewater containing complex components. There is a requirement for more processes or processes combined with membrane and biological processes to enhance operational efficiency and quality. More processes or those that are combined with biological and membrane-based processes are required to enhance operational efficiencies and quality. This paper intends to provide an exhaustive review of electrochemical technologies including microbial electrochemical technologies. It provides comprehensive information for the recovery of metals, nutrients, sulfur, hydrogen, and heat from industrial effluents. This article aims to give detailed information into the advancements in electrochemical processes to energy use, improve restoration performance, and achieve commercialization. It also covers bottlenecks and perspectives of this research area.

Efficient and durable uranium extraction from uranium mine tailings seepage water via a photoelectrochemical method

Current photocatalytic uranium (U) extraction methods have intrinsic obstacles, such as the recombination of charge carriers, and the deactivation of catalysts by extracted U. Here we show that, by applying a bias potential on the photocatalyst, the photoelectrochemical (PEC) method can address these limitations. We demonstrate that, owing to efficient spatial charge-carriers separation driven by the applied bias, the PEC method enables efficient and durable U extraction. The effects of multiple operation conditions are investigated. The U extraction proceeds via single-step one-electron reduction, resulting in the formation of pentavalent U, which can facilitate future studies on this often-overlooked U species. In real seepage water the PEC method achieves an extraction capacity of 0.67 gU m-3·h-1 without deactivation for 156 h continuous operation, which is 17 times faster than the photocatalytic method. This work provides an alternative tool for U resource recovery and facilitates future studies on U(V) chemistry.

An anion exchange pretreatment method for the determination of low-level uranium in the environmental water samples

Uranium in environmental water is usually at trace or ultra-trace levels with high concentrations of background ions so that the detection of uranium often couples with pretreatment processes to lower the detection limit, and improve the selectivity and accuracy of instruments. A simple, green, effective and efficient anion exchange pretreatment method was proposed to favor the determination of low-level uranium in natural environmental water samples. To determine the applicability and obtain the optimum operating parameters, the effects of coprecipitation, pH, contact time, uranium concentration, background ions, eluent and the flow speed on the uranium recovery were investigated. The experimental results showed that the proper addition of saturated Na₂CO₃ solution for pH adjustment did not lead to uranium loss in natural water samples, and the optimum pH value for adsorption was determined from 6 to 8. The adsorption speed was improved a lot with the employment of a novel silica-supported anion exchange resin, which also showed good linear dependence in the concentration range from <0.5 μg/L to 1000 μg/L with high tolerance limits towards common background ions. The optimum eluent was determined as 1 M HNO₃, and the optimum flow speeds for adsorption and desorption were about 4.0 and 1.0 mL/min, respectively. Based on these results, a pretreatment process was finally established, which realized the quantitative recovery of uranium from six different natural water samples with the chemical yields exceeding 95% and the enrichment factors about 100 times.

A non-polluting method for rapidly purifying uranium-containing wastewater and efficiently recovering uranium through electrochemical mineralization and oxidative roasting

Iron-based materials have been widely used for treating uranium-containing wastewater. However, the iron-uranium solids originating by treating radioactive water through pollutant transfer methods has become a new uncontrolled source of persistent radioactive pollution. The safe disposal of such hazardous waste is not yet well-resolved. The electrochemical mineralization method was developed to rapidly purify uranium-containing wastewater through lattice doping in magnetite and recover uranium without generating any pollutants. An unexpected isolation of U₃O₈ from uranium-doped magnetite was discovered through in-situ XRD with a temperature variation from 300 °C to 700 °C. Through HRTEM and DFT calculation, it was confirmed that the destruction of the inverse spinel crystal structure during the gradual transformation of magnetite into γ-Fe₂O₃ and α-Fe₂O₃ promoted the migration, aggregation, and isolation of uranium atoms. Uniquely generated U₃O₈ and Fe₂O₃ were easily separated and over 80% uranium and 99.5% iron could be recovered. These results demonstrate a new strategy for uranium utilization and the environmentally friendly treatment of uranium-containing wastewater.

Electrochemical recovery and high value-added reutilization of heavy metal ions from wastewater: Recent advances and future trends

Wastewater treatment for heavy metals is currently transitioning from pollution remediation towards resource recovery. As a controllable and environment-friendly method, electrochemical technologies have recently gained significant attention. However, there is a lack of systematic and goal oriented summarize of electrochemical metal recovery techniques, which has inhibited the optimized application of these methods. This review aims at recent advances in electrochemical metal recovery techniques, by comparing different electrochemical recovery methods, attempts to target recycling heavy metal resources with minimize energy consumption, boost recovery efficiency and realize the commercial application. In this review, different electrochemical recovery methods (including E-adsorption recovery, E-oxidation recovery, E-reduction recovery, and E-precipitation recovery) for recovering heavy metals are introduced, followed an analysis of their corresponding mechanisms, influencing factors, and recovery efficiencies. In addition, the mass transfer efficiency can be promoted further through optimizing electrodes and reactors, and multiple technologies (photo-electrochemical and sono-electrochemical) could to be used synergistically improve recovery efficiencies. Finally, the most promising directions for electrochemical recovery of heavy metals are discussed along with the challenges and future opportunities of electrochemical technology in recycling heavy metals from wastewater.

Capacitive Removal of Heavy Metal Ions from Wastewater via an Electro-Adsorption and Electro-Reaction Coupling Process

Heavy metals widely exist in wastewater, which is a serious threat to human health or water environment. Highly efficient removal of heavy metal ions from wastewater is a major challenge to wastewater treatment. In this work, capacitive removal of heavy metal ions from wastewater via an electro-adsorption and electro-reaction coupling process was originally demonstrated. The removal efficiency of heavy metal ions in the binary-component solutions containing metal nitrate (10 mg/L) and NaCl (100 mg/L) can reach 99%. Even the removal efficiency of heavy metal ions can be close to 99% in the multi-component solution containing all the seven metal nitrates (10 mg/L for each) and 100 mg/L NaCl. Meanwhile, the electro-adsorption and electro-reaction coupling process maintained excellent regeneration ability even after 20 cycles. Furthermore, the heavy metal ions removal mechanism was proven to be the pseudocapacitive intercalation of heavy metal ions into the layered structure of the employed W₁₈O₄₉/graphene in the electro-adsorption and electro-reaction coupling process. This work demonstrates great potential for general applicability to wastewater treatment.

Overview of recent developments of resource recovery from wastewater via electrochemistry-based technologies

As the rapid increase of the worldwide population, recovering valuable resources from wastewater have attracted more and more attention by governments and academia. Electrochemical technologies have been extensively investigated over the past three decades to purify wastewater. However, the application of these technologies for resource recovery from wastewater has just attracted limited attention. In this review, the recent (2010-2020) electrochemical technologies for resource recovery from wastewater are summarized and discussed for the first time. Fundamentals of typical electrochemical technologies are firstly summarized and analyzed, followed by the specific examples of electrochemical resource recovery technologies for different purposes. Based on the fundamentals of electrochemical reactions and without the addition of chemical agents, metallic ions, nutrients, sulfur, hydrogen and chemical compounds can be effectively recovered by means of electrochemical reduction, electrochemical induced precipitation, electrochemical stripping, electrochemical oxidation and membrane-based electrochemical processes, etc. Pros and cons of each electrochemical technology in practical applications are discussed and analyzed. Single-step electrochemical process seems ineffectively to recover valuable resources from the wastewater with complicated constituents. Multiple-step processes or integrated with biological and membrane-based technologies are essential to improve the performance and purity of products. Consequently, this review attempts to offer in-depth insights into the developments of next-generation of electrochemical technologies to minimize energy consumption, boost recovery efficiency and realize the commercial application.