Independent and synergistic bio-reductions of uranium (VI) driven by zerovalent iron in aquifer

Influence of enriched nitrate reducing bacteria communities on bacterial community structure and groundwater condition during in situ bioremediation of nitrate in acidic uranium-contaminated groundwater

In-situ leaching (ISL) is the predominant technology used in uranium mining currently, although it leads to significant environmental challenges. Nitrates, a key component in leaching agents, not only pose a threat to human health but also impede the bioreduction of U(VI) in uranium-contaminated water. In this study, the nitrate reducing bacterial (NRB) communities adapted to acidic uranium-contaminated groundwater from a site in Northwest China were gained by an enrichment micro-model. The effects of the NRB communities on the groundwater parameters and microbial diversity were evaluated using the groundwater-core column leaching system during the in-situ bioremediation of nitrate. The enrichment experiments revealed that NRB communities adapted to acidic uranium-contaminated groundwater were successfully enriched, of which Tumebacillus was the main functional bacterium. The column leaching experiment results showed that adding NRB communities successfully reduced nitrate levels from 100.91 mg/L to 0.7 mg/L in just 8 days, improved groundwater acidity and redox conditions. Additionally, the metagenomic analysis showed that introducing NRB communities increased biomass and indigenous NRB, but decreased microbial diversity. The KEGG enrichment analysis suggested that butanoate metabolism and valine, leucine and isoleucine degradation were promoted by adding enriched NRB communities. This research lays the groundwork for nitrate removal from contaminated groundwater in areas affected by ISL in uranium mines, setting the stage for future in situ bioremediation of U(VI).

Sequestration process and mechanism of U(VI) on montmorillonite-aspergillus niger composite

The existence state and spatiotemporal evolution process of uranium in mineral-microbe complex systems are important factors that constrain its ecotoxicity. This study investigated the sequestration of U(VI) by montmorillonite-Aspergillus niger (MTA) composite using bioassay and spectroscopies approaches. The results demonstrate that the sequestration process and mechanism of U(VI) on MTA differ substantially from those of individual components. Under neutral conditions, the sorption of U(VI) decreased from 92.4 ± 4.6 % on MT to 73.2 ± 2.4 % on MTA4 and 74.9 ± 6.3 % on MTA10, respectively, while the stability of U(VI) species on MTA increased obviously compared to MT. In the case of MTA formed over 4 days (MTA4), the biosorption effect of A. niger hyphae dominated the sequestration of U(VI). In contrast, for MTA formed over 10 days (MTA10), the interactions between MT and A. niger became more pronounced, and the hyphae of Aspergillus niger played a pivotal role in U(VI) sequestration, immobilizing U(VI) through complexation with organic ligands and bioreduction reactions. The high expandability of MT facilitated the penetration of extracellular polymeric substances (EPS) from A. niger into its interlayer of MT, enhancing U(VI) complexation and reduction. These processes significantly contributed to the effective sequestration of U(VI) by the MTA composite.

Extracellular electron transfer-dependent bioremediation of uranium-contaminated groundwater: Advancements and challenges

Efficient and sustainable remediation of uranium-contaminated groundwater is critical for groundwater safety and the sustainable development of nuclear energy, particularly in the context of global carbon neutrality goals. This review explores the potential of microbial reduction processes that utilize extracellular electron transfer (EET) to convert soluble uranium (U(VI)) into its insoluble form (U(IV)), presenting a promising approach to groundwater remediation. The review first outlines the key processes and factors influencing the effectiveness of dissimilatory metal-reducing bacteria (DMRB), such as Geobacter and Shewanella, during uranium bioremediation and recovery. The cutting-edge progress on the molecular mechanism of EET-driven U(VI) reduction mediated by c-type cytochromes, conductive pili, and electron mediators, is critically reviewed. Additionally, advanced strategies such as optimizing electron transfer, leveraging synthetic biology approach, and integration with machine learning are discussed to enhance the efficiency of EET-driven processes. The review also considers the integration of EET processes into practical engineering applications, highlighting the need for optimization and innovation in bioremediation technologies. By providing a comprehensive overview of current progress and challenges, this review aims to inspire novel research and practical advancements in the field of uranium-contaminated groundwater remediation.

Microbial production of methyl-uranium via the Wood-Ljungdahl pathway

The misuse of uranium is a major threat to human health and the environment. In microbial ecosystems, microbes deploy various strategies to cope with uranium-induced stress. However, the exact ecological strategies and mechanisms underlying uranium tolerance in microbes remain unclear. Therefore, this study aimed to investigate the survival strategies and tolerance mechanisms of microbial communities in uranium-contaminated soil and groundwater. Microbial co-occurrence networks and molecular biology techniques were used to analyze the properties of microbes in groundwater and soil samples from various depths of uranium-contaminated areas in Northwest China. Uranium pollution altered microbial ecological strategies. Uranium stress facilitated the formation of microbial community structures, leading to symbiosis. Furthermore, microbes primarily resisted uranium hazards by producing polysaccharides and phosphate groups that chelate uranium, releasing phosphate substances that precipitate uranium, and reducing U(VI) through sulfate- and iron-reducing processes. The relative abundance of metal-methylation genes in soil microorganisms positively correlated with uranium concentration, indicating that soil microorganisms can produce methyl uranium via the Wood-Ljungdahl pathway. Furthermore, soil and groundwater microorganisms demonstrated different responses to uranium stress. This study provides new insights into microbial responses to uranium stress and novel approaches for the bioremediation of uranium-contaminated sites.

New insights on zero-valent iron permeable reactive barrier for Cr(VI) removal: The function of FeS reaction zone downstream in-situ generated by sulfate-reducing bacteria

The biogeochemical behavior downstream of the zero-valent iron permeable reactive barrier (ZVI-PRB) plays an enormous positive role in the remediation of contaminated-groundwater, but has been completely neglected for a long time. Therefore, this study conducted a 240-day SRB-enhanced ZVI-PRB column experiment, focusing on what exactly happens downstream of ZVI-PRB. Results show that biosulfidation of SRB inside ZVI-PRB prolonged the complete Cr(VI) removal longevity of ZVI-PRB from 38 days to at least 240 days. More importantly, unlike previous studies that focused on improving the performance of ZVI-PRB itself, this study found an in-situ generated FeS reduction reaction zone downstream of the ZVI-PRB. When the ZVI-PRB fails, the downstream reaction zone can continue to play a role in Cr(VI) removal. The maximum Cr(VI) removal capacity of the aquifer media from the reaction zone reached 155.1 mg/kg, which was 39.7 % of commercial ZVI capacity. The reduction zone was further confirmed to be predominantly FeS rather than FeS2. Biogeochemistry occurring within and downstream of ZVI-PRB leads to the formation of FeS. Gene sequencing revealed significantly higher SRB abundance downstream of ZVI-PRB than within the ZVI-PRB. The understanding of the downstream FeS reaction zone provides new insights for more effective remediation using ZVI-PRB.

Interplay between vanadium distribution and microbial community in soil-plant system

Soil-plant system play an essential role in distribution and transformation of vanadium (V). V shapes the diversity of soil communities, while soil microorganisms mediate V transformation. Plants also absorb V from surrounding soil. However, the study of microbial response to V stress in different soil-plant compartments is limited, and the metabolic functions driving V transformation across these systems remain elusive. The study investigates the distribution of V in soil-plant systems nearby a V smelter. 16S rRNA sequencing and metagenomics are utilized to reveal the microbial adaptation and V transformation in bulk soil, rhizosphere, and endosphere. Bothriochloa ischaemum (L.) Keng. (BK) exhibits higher phytoextraction potential (TF = 0.74 ± 0.26). Environmental variables, including pH, V, OM, and AP, show significant (p < 0.05) influence in soil community composition, with homogeneous selection governing the assembly processes in bulk soil and rhizosphere, while stochastic process dominates endospheric assembly. Metagenomic investigation revealed a coordinated metabolic pathway between functional taxa in soil and plants, which lead to root uptake and translocation. V stress is mitigated through Nocardioide, Microvirga, and Solirubrobacter, putatively harboring V(V) reduction genes n arG and mtrC in soil. In rhizosphere, citrate synthase gltA and alkaline phosphatase phoD exhibit functional potential to facilitate formation of V-complexation to increase V mobility. In endoshere, endophytic Enterobacter further detoxifies V(V), and likely promotes V translocation through siderophore biosynthesis gene, iucA. These findings enhance our understanding on interplay between V and microbial community in soil-plant systems, which is instrumental in developing mitigation plan for V contaminated sites.

Unlocking soil revival: the role of sulfate-reducing bacteria in mitigating heavy metal contamination

Soil contamination with heavy metals from industrial and mining activities poses significant environmental and public health risks, necessitating effective remediation strategies. This review examines the utilization of sulfate-reducing bacteria (SRB) for bioremediation of heavy metal-contaminated soils. Specifically, it focuses on SRB metabolic pathways for heavy metal immobilization, interactions with other microorganisms, and integration with complementary remediation techniques such as soil amendments and phytoremediation. We explore the mechanisms of SRB action, their synergistic relationships within soil ecosystems, and the effectiveness of combined remediation approaches. Our findings indicate that SRB can effectively immobilize heavy metals by converting sulfate to sulfide, forming stable metal sulfides, thereby reducing the bioavailability and toxicity of heavy metals. Nevertheless, challenges persist, including the need to optimize environmental conditions for SRB activity, address their sensitivity to acidic conditions and high heavy metal concentrations, and mitigate the risk of secondary pollution from excessive carbon sources. This study underscores the necessity for innovative and sustainable SRB-based bioremediation strategies that integrate multiple techniques to address the complex issue of heavy metal soil contamination. Such advancements are crucial for promoting green mining practices and environmental restoration.

Electrochemistry-assisted in-situ regeneration of oxygen vacancies and Ti(III) active sites for persistent uranium recovery at a low potential

Electrochemical uranium extraction (EUE) from seawater is a very promising strategy, but its practical application is hindered by the high potential for electrochemical system, as well as the low selectivity, efficiency, and poor stability of electrode. Herein, we developed creatively a low potential strategy for persistent uranium recovery by electrochemistry-assisted in-situ regeneration of oxygen vacancies and Ti(III) active sites coupled with indirect reduction of uranium, finally achieving high selectivity, efficient and persistent uranium recovery. As-designed titanium dioxide rich in oxygen vacancies (TiO2-VO) electrode displayed an EUE efficiency of ∼99.9 % within 180 min at a low potential of 0.09 V in simulated seawater with uranium of 5∼20 ppm. Moreover, the TiO2-VO electrode also showed high selectivity (89.9 %) to uranium, long-term cycling stability and antifouling activity in natural seawater. The excellent EUE property was attributed to the fact that electrochemistry-assisted in-situ regeneration of oxygen vacancies and Ti(III) active sites enhanced EUE cycling process and achieved persistent uranium recovery. The continuous regeneration of oxygen vacancies not only reduced the adsorption energy of U(VI)O22+ but also serve as a storage and transportation channel for electrons, accelerating electron transfer from Ti(III) to U(VI) at solid-liquid interface and promoting EUE kinetic rate.

Bibliometric insights into the evolution of uranium contamination reduction research topics: Focus on microbial reduction of uranium

Confronting the threat of environment uranium pollution, decades of research have yielded advanced and significant findings in uranium bioremediation, resulting in the accumulation of tremendous amount of high-quality literature. In this study, we analyzed over 10,000 uranium reduction-related papers published from 1990 to the present in the Web of Science based on bibliometrics, and revealed some critical information on knowledge structure, thematic evolution and additional attention. Methods including contribution comparison, co-occurrence and temporal evolution analysis are applied. The results of the distribution and impact analysis of authors, sources, and journals indicated that the United States is a leader in this field of research and China is on the rise. The top keywords remained stable, primarily focused on chemicals (uranium, iron, plutonium, nitrat, carbon), characters (divers, surfac, speciat), and microbiology (microbial commun, cytochrome, extracellular polymeric subst). Keywords related to new strains, reduction mechanisms and product characteristics demonstrated the strongest uptrend, while some keywords related to mechanism and performance were clearly emerging in the past 5 years. Furthermore, the evolution of the thematic progression can be categorized into three stages, commencing with the discovery of the enzymatic reduction of hexavalent uranium to tetravalent uranium, developing in the groundwater remediation process at uranium-contaminated sites, and delving into the research on microbial reduction mechanisms of uranium. For future research, enhancing the understanding of mechanisms, improving uranium removal performance, and exploring practical applications can be considered. This study provides unique insights into microbial uranium reduction research, providing valuable references for related studies in this field.

A review of remediation technologies for uranium-contaminated water

Uranium is a naturally existing radioactive element present in the Earth's crust. It exhibits lithophilic characteristics, indicating its tendency to be located near the surface of the Earth and tightly bound to oxygen. It is ecotoxic, hence the need for its removal from the aqueous environment. This paper focuses on the variety of water treatment processes for the removal of uranium from water and this includes physical (membrane separation, adsorption and electrocoagulation), chemical (ion exchange, photocatalysis and persulfate reduction), and biological (bio-reduction and biosorption) approaches. It was observed that membrane filtration and ion exchange are the most popular and promising processes for this application. Membrane processes have high throughput but with the challenge of high power requirements and fouling. Besides high pH sensitivity, ion exchange does not have any major challenges related to its application. Several other unique observations were derived from this review. Chitosan/Chlorella pyrenoidosa composite adsorbent bearing phosphate ligand, hydroxyapatite aerogel and MXene/graphene oxide composite has shown super-adsorbent performance (>1000 mg/g uptake capacity) for uranium. Ultrafiltration (UF) membranes, reverse osmosis (RO) membranes and electrocoagulation have been observed not to go below 97% uranium removal/conversion efficiency for most cases reported in the literature. Heat persulfate reduction has been explored quite recently and shown to achieve as high as 86% uranium reduction efficiency. We anticipate that future studies would explore hybrid processes (which are any combinations of multiple conventional techniques) to solve various aspects of the process design and performance challenges.

Pyrrhotite-dependent microbial reduction and magnetic separation for efficient vanadium detoxification and recovery in contaminated aquifer

Microbial reduction under anaerobic condition is a promising method for remediating vanadate [V(V)] contamination in aquifers, while V(V) may be re-generated with redox fluctuations. The inability to remove vanadium after remediation has become a key issue limiting bioremediation. In this study, we proposed the use of pyrrhotite, a natural mineral with magnetic properties, to immobilize V(V) to insoluble V(IV) under microbial action and remove vanadium from the aquifer using a magnetic field, which could avoid the problem of V(V) recontamination under redox fluctuating conditions. Up to 49.0 ± 4.7 % of vanadium could be removed from the aquifer by the applied magnetic field, and the vanadium in the aquifer after the reaction was mainly in the acid-extractable and reducible states. pH had a strong effect on the magnetic recovery of V(V), while the influence of initial V(V) concentration was weak. Microbial community structure analysis showed that Thiobacillus, Proteiniphilum, Fermentimonas, and Desulfurivibrio played key roles for V(V) reduction and pyrrhotite oxidation. Structural equation model indicated the positive correlation between these genera with the magnetic recovery of vanadium. Real time-qPCR confirmed the roles of functional genes of V(V) reduction (napA and nirK) and SO42- reduction (dsrA) in such biological processes. This study provides a novel route to sustainable V(V) remediation in aquifers, with synchronous recovery of vanadium resources without rebound.

Insights into the long-term immobilization performances and mechanisms of CMC-Fe0/FeS with different sulfur sources for uranium under anoxic and oxic aging

Nanoscale zerovalent iron (Fe0)-based materials have been demonstrated to be a effective method for the U(VI) removal. However, limited research has been conducted on the long-term immobilization efficiency and mechanism of Fe0-based materials for U(VI), which are essential for achieving safe handling and disposal of U(VI) on a large scale. In this study, the prepared carboxymethyl cellulose (CMC) and sulfurization dual stabilized Fe0 (CMC-Fe0/FeS) exhibited excellent long-term immobilization performances for U(VI) under both anoxic and oxic conditions, with the immobilization efficiencies were respectively reached over 98.0 % and 94.8 % after 180 days of aging. Most importantly, different from the immobilization mechanisms of the fresh CMC-Fe0/FeS for U(VI) (the adsorption effect of -COOH and -OH groups, coordination effect with sulfur species, as well as reduction effect of Fe0), the re-mobilized U(VI) were finally re-immobilized by the formed FeOOH and Fe3O4 on the aged CMC-Fe0/FeS. Under anoxic conditions, more Fe3O4 was produced, which may be the main reason for the long-term immobilization U(VI). Under oxic conditions, the production of Fe3O4 and FeOOH were relatively high, which both played significant roles in re-immobilizing U(VI) through surface complexation, reduction and incorporation effects.

Deciphering Vanadium Speciation in Smelting Ash and Adaptive Responses of Soil Microorganisms

Abundant smelting ash is discharged during pyrometallurgical vanadium (V) production. However, its associated V speciation and resultant ecological impact have remained elusive. In this study, V speciation in smelting ash and its influence on the metabolism of soil microorganisms were investigated. Smelting ashes from V smelters contained abundant V (19.6-115.9 mg/g). V(V) was the dominant species for soluble V, while solid V primarily existed in bioavailable forms. Previously unrevealed V nanoparticles (V-NPs) were prevalently detected, with a peak concentration of 1.3 × 1013 particles/g, a minimal size of 136.0 ± 0.6 nm, and primary constituents comprising FeVO4, VO2, and V2O5. Incubation experiments implied that smelting ash reshaped the soil microbial community. Metagenomic binning, gene transcription, and component quantification revealed that Microbacterium sp. and Tabrizicola sp. secreted extracellular polymeric substances through epsB and yhxB gene regulation for V-NPs aggregation to alleviate toxicity under aerobic operations. The V K-edge X-ray absorption near-edge structure (XANES) spectra suggested that VO2 NPs were oxidized to V2O5 NPs. In the anaerobic case, Comamonas sp. and Achromobacter sp. reduced V(V) to V(IV) for detoxification regulated by the napA gene. This study provides a deep understanding of the V speciation in smelting ash and microbial responses, inspiring promising bioremediation strategies to reduce its negative environmental impacts.

Fe-MMT/WO3 composites for chemical and photocatalysis synergistic reduction of uranium (VI)

The preparation of Fe-MMT/WO3 composites by the hydrothermal method has been explored in this study for the construction of a chemical and photocatalytic catalyst for the reduction of U (VI). This research found that the visible light absorption and reduction potential of the Fe-MMT/WO3 composites were relatively superior compared to Fe-MMT and WO3 alone. Based on an evaluation of the performance of the Fe-MMT/WO3 composites under visible light irradiation, it was discovered that they had greater uranium extraction capacity, where the maximum extraction capacity of U (VI) was determined to be 1862.69 mg g-1, with removal efficiency reaching 93.32%. To investigate the electron transfer and U (VI) to U (IV) reduction mechanisms after the composite, XPS and DFT calculations were conducted. Results showed that Fe (II) is converted to a higher state Fe (III) and WO3 produce photoelectrons which together reduce U (VI) to U (IV). Moreover, the photoelectrons partially transferred to Fe-MMT with low reduction potential to reduce Fe (III) to Fe (II), allowing iron cycling during uranium extraction to be achieved.

Upcycling spent palladium-based catalysts into high value-added catalysts via electronic regulation of Escherichia coli to high-efficiently reduce hexavalent chromium

Upgrading and recycling Palladium (Pd) from spent catalysts may address Pd resource shortages and environmental problems. In this paper, Escherichia coli (E. coli) was used as an electron transfer intermediate to upcycle spent Pd-based catalysts into high-perform hexavalent chromium bio-catalysts. The results showed that Pd (0) nanoparticles (NPs) combined with the bacterial surface changed the electron transfer by enhancing the cell conductivity, thus promoting the removal rate of Pd(II). The recovery efficiency of Pd exceeded 98.6%. Notably, E. coli heightened the adsorption of H• and HCOO• via electron transfer of the Pd NPs electron-rich centre, resulting in a higher catalytic performance of the recycled spent catalysed the reduction of 20 ppm Cr(VI) under mild conditions within 18 min, in which maintained above 98% catalytic activity after recycling five times. This efficiency was found to be higher than that of the reported Pd-based catalysts. Hence, an electron transfer mechanism for E. coli recovery Pd-based catalyst under electron donor adjusting is proposed. These findings provide an important method for recovering Pd NPs from spent catalysts and are crucial to effectively reuse Pd resources.

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.

Coupling of selenate reduction and pyrrhotite oxidation by indigenous microbial consortium in natural aquifer

Pyrrhotite is ubiquitously found in natural environment and involved in diverse (bio)processes. However, the pyrrhotite-driven bioreduction of toxic selenate [Se(VI)] remains largely unknown. This study demonstrates that Se(VI) is successfully bioreduced under anaerobic condition with the participation of pyrrhotite for the first time. Completely removal of Se(VI) was achieved at initial concentration of 10 mg/L Se(VI) and 0.56 mL/min flow rate in continuous column experiment with indigenous microbial consortium and pyrrhotite. Variation in hydrochemistry and hydrodynamics affected Se(VI) removal performance. Se(VI) was reduced to insoluble Se(0) while elements in pyrrhotite were oxidized to Fe(III) and SO₄^2-. Breakthrough study indicated that biotic activity contributed 81.4 ± 1.07% to Se(VI) transformation. Microbial community analysis suggested that chemoautotrophic genera (e.g., Thiobacillus) could realize pyrrhotite oxidation and Se(VI) reduction independently, while heterotrophic genera (e.g., Bacillus, Pseudomonas) contributed to Se(VI) detoxification by utilizing metabolic intermediates generated through Fe(II) and S(-II) oxidation, which were further verified by pure culture tests. Metagenomic and qPCR analyses indicated genes encoding enzymes for Se(VI) reduction (e.g., serA, napA and srdBAC), S oxidation (e.g., soxB) and Fe oxidation (e.g., mtrA) were upregulated. The elevated electron transporters (e.g., nicotinamide adenine dinucleotide, cytochrome c) promoted electron transfer from pyrrhotite to Se(VI). This study gains insights into Se biogeochemistry under the effect of Fe(II)-bearing minerals and provides a sustainable strategy for Se(VI) bioremediation in natural aquifer.