Vanadium in the Environment: Biogeochemistry and Bioremediation

Vanadium(V) is a highly toxic multivalent, redox-sensitive element. It is widely distributed in the environment and employed in various industrial applications. Interactions between V and (micro)organisms have recently garnered considerable attention. This Review discusses the biogeochemical cycling of V and its corresponding bioremediation strategies. Anthropogenic activities have resulted in elevated environmental V concentrations compared to natural emissions. The global distributions of V in the atmosphere, soils, water bodies, and sediments are outlined here, with notable prevalence in Europe. Soluble V(V) predominantly exists in the environment and exhibits high mobility and chemical reactivity. The transport of V within environmental media and across food chains is also discussed. Microbially mediated V transformation is evaluated to shed light on the primary mechanisms underlying microbial V(V) reduction, namely electron transfer and enzymatic catalysis. Additionally, this Review highlights bioremediation strategies by exploring their geochemical influences and technical implementation methods. The identified knowledge gaps include the particulate speciation of V and its associated environmental behaviors as well as the biogeochemical processes of V in marine environments. Finally, challenges for future research are reported, including the screening of V hyperaccumulators and V(V)-reducing microbes and field tests for bioremediation approaches.

Risk assessment and microbial community structure in agricultural soils contaminated by vanadium from stone coal mining

High health risks of vanadium (V) released by the mining of vanadium titanomagnetite (VTM) have been widely recognized, but little is known about the risks and microbial community responses of V pollution as a consequence of the stone coal mining (SCM), another important resource for V mining. In this study, the topsoils and the profile soils were collected from the agricultural soils around a typical SCM in Hunan Province, China, with the investigation of ecological, health risks and microbial community structures. The results showed that ∼97.6% of sampling sites had levels of total V exceeding the Chinese National standard (i.e., 130 mg/kg), and up to 41.1% of V speciation in the topsoils was pentavalent vanadium (V(V)). Meanwhile, the proportions of HQ > 1 and 0.6-1 in the topsoils were ∼8.3% and ∼31.0% respectively, indicating that V might pose a non-carcinogenic risk to children. In addition, the microbial community varied between the topsoils and the profile soils. Both sulfur-oxidizing bacteria (e.g. Thiobacillus, MND1, Ignavibacterium) and sulfate-reducing bacteria (e.g. Desulfatiglans, GOUTB8, GOUTA6) might have been involved in V(V) reductive detoxification. This study helps better understand the pollution and associated risks of V in the soils of SCM and provides a potential strategy for bioremediation of the V-contaminated environment.

Heterotrophic Bioleaching of Vanadium from Low-Grade Stone Coal by Aerobic Microbial Consortium

Bioleaching is a viable method that assists in increasing the vanadium output in an economical and environmentally friendly manner. Most bioleaching is conducted by pure cultures under autotrophic conditions, which frequently require strong acidity and produce acid wastewater. However, little is known about heterotrophic bioleaching of vanadium by mixed culture. This study investigated the bioleaching of vanadium from low-grade stone coal by heterotrophic microbial consortium. According to the results, vanadium was efficiently extracted by the employed culture, with the vanadium recovery percentage in the biosystem being 7.24 times greater than that in the control group without inoculum. The average vanadium leaching concentration reached 680.7 μg/L in the first three cycles. The kinetic equation indicated that the main leaching process of vanadium was modulated by a diffusion process. Scanning electron microscopy revealed traces of bacterial erosion with fluffy structures on the surface of the treated stone coal. X-ray photoelectron spectroscopy confirmed the reduction of the vanadium content in the stone coal after leaching. Analysis of high-throughput 16S rRNA gene sequencing revealed that the metal-oxidizing bacteria, Acidovorax and Delftia, and the heterotrophic-metal-resistant Pseudomonas, were significantly enriched in the bioleaching system. Our findings advance the understanding of bioleaching by aerobic heterotrophic microbial consortium and offer a promising technique for vanadium extraction from low-grade stone coals.

Synchronous microbial V(V) reduction and denitrification using corn straw as the sole carbon source

The coexistence of nitrate and V(V) in groundwater aquifers poses potential threats to ecological environment and public health. However, much remains to be elucidated about how the complex microbial community coupled nitrate and V(V) simultaneous bio-reduction with carbon source oxidation. For the first time, it was demonstrated that denitrification and V(V) bio-reduction occur by using corn straw as the sole carbon and energy source. Corn straw was proved to have efficient denitrification and V(V) bio-reduction performance in various environments, especially at V(V) concentrations of 100 mg/L for optimal V(V) reduction rate (19.25 mg/L·d) and at pH of 11 with the best nitrate reduction rate (3.12 d^-1). In addition, an interesting phenomenon was found that the release of V(V) occurred when the carbon source was insufficient and the competitive electron acceptor (NO₃^--N) existed. Metagenomic analysis showed that the addition of corn straw increased the abundance of genes related to metal resistance, cytochrome and dimethyl sulfoxide, and increased the abundance of glycolytic process, which may play a vital role in facilitating the reduction of V(V). These findings can provide basic suggestions for improving the mechanism of V(V) reduction pathway and provide guidance for the remediation of groundwater polluted by nitrate and V(V).

Soil indigenous microorganisms alleviate soluble vanadium release from industrial dusts

Vanadium-bearing dusts from industrial processes release abundant toxic vanadium, posing imminent ecological and human health concerns. Although the precipitation of these dusts has been recognized as the main source of soil vanadium pollution, little is known regarding the interrelationships between industrial dusts and soil inherent compositions. In this study, the interactions between dusts from vanadium smelting and soil indigenous microorganisms were investigated. Soluble vanadium (V) [V(V)] released from industrial dusts was reduced by 41.5 ± 0.39% with soil addition, compared to water leaching. Reducible fraction accounted for the highest proportion (55.1 ± 1.73%) of vanadium speciation in the resultant soils, while residual vanadium fraction increased to 83.7 ± 3.22% in the leached dusts. Functional genera (e.g., Aliihoeflea, Actinotalea) that transformed V(V) to insoluble vanadium (IV) alleviated dissolved vanadium release. Nitrate/nitrite reduction and glutathione metabolisms contributed to V(V) immobilization primarily. Structural equation model analysis indicated that V(V) reducers had significant negative impacts on soluble V(V) in the leachate. This first-attempt study highlights the importance of soil microorganisms in immobilizing vanadium from industrial dusts, which is helpful to develop novel strategies to reduce their environmental risks associated to vanadium smelting process.

Concurrent anaerobic chromate bio-reduction and pentachlorophenol bio-degradation in a synthetic aquifer

Chromate [Cr(VI)] and pentachlorophenol (PCP) coexist widely in the environment and are highly toxic to public health. However, whether Cr(VI) bio-reduction is accompanied by PCP bio-degradation and how microbial communities can keep long-term stability to mediate these bioprocesses in aquifer remain elusive. Herein, we conducted a 365-day continuous column experiment, during which the concurrent removals of Cr(VI) and PCP were realized under anaerobic condition. This process allowed for complete Cr(VI) bio-reduction and PCP bio-degradation at an efficiency of 92.8 ± 4.2% using ethanol as a co-metabolic substrate. More specifically, Cr(VI) was reduced to insoluble chromium (III) and PCP was efficiently dechlorinated with chloride ion release. Collectively, Acinetobacter and Spirochaeta regulated Cr(VI) bio-reduction heterotrophically, while Pseudomonas mediated not only Cr(VI) bio-reduction but also PCP bio-dechlorination. The bio-dechlorinated products were further mineralized by Azospira and Longilinea. Genes encoding proteins for Cr(VI) bio-reduction (chrA and yieF) and PCP bio-degradation (pceA) were upregulated. Cytochrome c and intracellular nicotinamide adenine dinucleotide were involved in Cr(VI) and PCP detoxification by promoting electron transfer. Taken together, our findings provide a promising bioremediation strategy for concurrent removal of Cr(VI) and PCP in aquifers through bio-stimulation with supplementation of appropriate substrates.

Dynamics of vertical vanadium migration in soil and interactions with indigenous microorganisms adjacent to tailing reservoir

Severe vanadium pollution in deep soil through surface infiltration during mining activities has been particularly concerned, but little is known about vanadium migration dynamics in vertical soil profile. Indigenous microorganisms widely exist in soil, however, their functions and suffered impacts during vertical vanadium migration have rarely been investigated. In this study, 100 cm height columns were constructed with undisturbed soil around vanadium tailing reservoir were constructed to describe vertical vanadium transport process and corresponding interactions between vanadium and indigenous microorganisms. 91 d continuous leaching with pentavalent vanadium [V(V)] showed that V(V) gradually downward migrated. Soil microorganisms slowed down vertical V(V) migration rate by transferring V(V) to insoluble tetravalent vanadium. Enriched Gemmatimonadaceae and Actinobacteria were identified to contribute to microbial V(V) transformation. Co-existing nitrate weakened the soil's ability to intercept V(V) via electron competition. Microbial communities were reshaped by vanadium during leaching, while enzyme activities increased slightly due to vanadium stimulation. This work advances the understanding of vertical vanadium migration characteristics in soil, which is essential to risk management and effective remediation of vanadium-polluted sites.

Optimizing the added ratio of mixed auxiliary packings for enhancing the biological vanadium (V) removal

Developing sustainable and low-cost bio-reduction technologies is essential for vanadium (V) bioremediation in groundwater. With both agricultural waste (wheat stalk) being a solid carbon source and ceramsite and medical stone being auxiliary packings, V(V) removal was confirmed and optimized in this study. The ratio of ceramsite to medical stone was maintained at 1:3 in Group I, which accomplished a V(V) removal efficiency up to 97.5% within 120 h and an average removal rate was around 0.305 mg/(L·h). The dissolution and utilization of carbon and trace elements (Mg, Fe, Mo and Ni) by microbes also contributed to the V(V) bio-reduction enhancement. The main components of DOM (tryptophan and humic acid-like substances) were vital in the V(V) binding and electron transfer processes. This study could promote the current knowledge on the sustainable V(V) bioremediation by using agricultural waste and auxiliary packings.

Synchronous bio-reduction of Uranium(VI) and Vanadium(V) in aquifer: Performance and mechanisms

Uranium and vanadium commonly co-exist in groundwater aquifer where uranium was smelted from vanadium tailings. However, little is known about interrelationships of U(VI) and V(V) during their bio-reduction processes. In this work, 92.7 ± 1.52% U(VI) and 100% V(V) were simultaneously removed with sodium acetate as the sole exogenous electron donor and carbon source under anaerobic condition. Various conditions (i.e., increased uranium, reduced hydraulic retention time and acetate) were observed to affect removal efficiencies. Characterization of column fillings indicated that U(VI) was precipitated to U(IV) and V(V) was reduced to insoluble V(IV). Microbial community structure was observed to change, where Aquabacterium and Hydrogenophaga promoted bioreductions of U(VI) and V(V). Enriched Novosphingobium and Rhodobacter also played a vital role in reducing U(VI) and V(V). These findings could be used to study the biogeochemical fates of U(VI) and V(V) in the aquifer and to remediate groundwater co-contaminated by U(VI) and V(V).

Recent advances in removal techniques of vanadium from water: A comprehensive review

In recent years, with the development of economy and industry, water contaminated with heavy metal has become a global environmental problem. Vanadium (V) is an emerging contaminant reported in wastewater along with the increasing mining, smelting and recovering of vanadium ores and application in many fields as a significant national strategy resource. The increasing attention has been paid to the separations of V from water due to its potential toxic to animals and human beings. In the present study, the most common V removal techniques including adsorption, microbiological treatment, chemical precipitation, solvent extraction, electrokinetic remediation, photocatalysis, coagulation and membrane filtration are presented with discussion of their advantages, limitations and the recent achievements. Several major influencing factors and mechanisms of various processes have been briefly analyzed. Some research perspectives are proposed for improving the capacities to remove V from water. The core objective of this review is to provide comprehensive information or database for the superior approach for V removal.

The role of available phosphorous in vanadate decontamination by soil indigenous microbial consortia

Indigenous microbial consortia are closely associated with soil inherent components including nutrients and minerals. Although indigenous microbial consortia present great prospects for bioremediation of vanadate [V(V)] contaminated soil, influences of some key components, such as available phosphorus (AP), on V(V) biodetoxification are poorly understood. In this study, surface soils sampled from five representative vanadium smelter sites were employed as inocula without pretreatment. V(V) removal efficiency ranged from 81.7 ± 1.4% to 99.5 ± 0.2% in batch experiment, and the maximum V(V) removal rates were positively correlated with AP contents. Long-term V(V) removal was achieved under fluctuant hydrodynamic and hydrochemical conditions in column experiment. Geobacter and Bacillus, which were found in both original soils and bioreactors, catalytically reduced V(V) to insoluble tetravalent vanadium. Phosphate-solubilizing bacterium affiliated to Gemmatimonadaceae were also identified abundantly. Microbial functional characterization indicated the enrichment of phosphate ABC transporter, which could accelerate V(V) transfer into intercellular space for efficient reduction due to the structural similarity of V(V) and phosphate. This study reveals the critical role of AP in microbial V(V) decontamination and provides promising strategy for in situ bioremediation of V(V) polluted soil.

Synergy between pyridine anaerobic mineralization and vanadium (V) oxyanion bio-reduction for aquifer remediation

The co-occurrence of toxic pyridine (Pyr) and vanadium (V) oxyanion [V(V)] in aquifer has been of emerging concern. However, interactions between their biogeochemical fates remain poorly characterized, with absence of efficient route to decontamination of this combined pollution. In this work, microbial-driven Pyr degradation coupled to V(V) reduction was demonstrated for the first time. Removal efficiencies of Pyr and V(V) reached 94.8 ± 1.55% and 51.2 ± 0.20% in 72 h operation. The supplementation of co-substrate (glucose) deteriorated Pyr degradation slightly, but significantly promoted V(V) reduction efficiency to 84.5 ± 0.635%. Pyr was mineralized with NH₄⁺-N accumulation, while insoluble vanadium (IV) was the major product from V(V) bio-reduction. It was observed that Bacillus and Pseudomonas realized synchronous Pyr and V(V) removals independently. Interspecific synergy between Pyr degraders and V(V) reducers also functioned with addition of co-substrate. V(V) was bio-reduced through alternative electron acceptor pathway conducted by gene nirS encoded nitrite reductase, which was evidenced by gene abundance and enzyme activity. Cytochrome c, nicotinamide adenine dinucleotide and extracellular polymeric substances also contributed to the coupled bioprocess. This work provides new insights into biogeochemical activities of Pyr and V(V), and proposes novel strategy for remediation of their co-contaminated aquifer.

Hydrodynamics- and hydrochemistry-affected microbial selenate reduction in aquifer: Performance and mechanisms

Selenate [Se(VI)] with higher content in groundwater will be harmful for human beings. Hence, effective treatment for Se(VI) in aquifer should be conducted reasonably. Microbial reduction of Se(VI) to elemental selenium with weak movability and toxicity has attracted significant attention due to its high efficiency and no secondary contamination. However, hydrodynamic and hydrochemical influences with corresponding mechanisms during Se(VI) bioreduction are still not clear. In this study, influences of flow rate, initial Se(VI) and organic concentrations, coexisting nitrate were evaluated. Se(VI) removal efficiency and capacity reached 96.42 ± 6.82% and 41.28 ± 3.41 (g/m³·d) with flow rate of 0.56 mL/min, initial Se(VI) and chemical organic demand concentrations of 10 mg/L and 400 mg/L. Dechloromonas and Pseudomonas were presumably contributed to Se(VI) reduction, with upregulated serA and tatC genes. Solid Se⁰ was identified as the final product from Se(VI) reduction. These results will be beneficial for the further comprehending of Se(VI) remediation in aquifer.

Vanadium in soil-plant system: Source, fate, toxicity, and bioremediation

Vanadium(V) is an important component of industrial activities, while it may pose toxic hazards to plants, animals, and humans at high levels. Owing to its various uses in numerous industrial processes, high amount of V is released into the soil environment. Previous literature has focused on the biogeochemistry and ecotoxicity of V in soil-plant system. Consequently, this overview presents its source, fate, phyto-uptake, phyto-toxicity, detoxification, and bioremediation based on available data, especially published from 2015 to 2020. Vanadium occurs as various chemical forms (primarily as V(V) and V(IV)) in the soil environment, and its biogeochemical behaviour is easily influenced by soil conditions including redox potential, soil pH, organic matter, and microorganisms. Vanadium mainly accumulates in plant roots with very limited translocation to shoots. However, plants such as dog's tail grass and green bean are reported to accumulate high levels of V in aboveground tissues. An insight into the processes and mechanisms that allow plants to absorb and translocate V in soil-plant system is also stressed in this overview. In plants, low levels of V have beneficial effects on plant growth and development. Nevertheless, excessive V provokes numerous deleterious effects including reducing seed germination, inhibiting root and shoot growth, depressing photosynthesis, interfering with nutrients uptake, inducing overgeneration of ROS, and leading to lipid peroxidation. Mechanisms related to detoxification strategies like sequestration in root system, compartmentation in vacuoles and cell wall, and antioxidant defence systems to endure V-induced toxicity in plants are discussed as well. The detailed knowledge of bioremediation involved in the cleanup of V-contaminated soils would immensely help understand and improve the remediation process. Furthermore, this overview outlines several research gaps requiring further investigation in order to advance our understanding of the biogeochemical roles of V in soil-plant systems.

Sulfur-based Mixotrophic Vanadium (V) Bio-reduction towards Lower Organic Requirement and Sulfate Accumulation

Although remediation of toxic vanadium (V) [V(V)] pollution can be achieved through either heterotrophic or sulfur-based autotrophic microbial reduction, these processes would require a large amount of organic carbons or generate excessive sulfate. This study reported that by using mixotrophic V(V) bio-reduction with acetate and elemental sulfur [S(0)] as joint electron donors, V(V) removal performance was enhanced due to cooccurrence of heterotrophic and autotrophic activities. Deposited vanadium (IV) was identified as the main reduction product by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Based on 16S rRNA gene amplicon sequencing, qPCR and genus-specific reverse transcription qPCR, it was observed that V(V) was likely detoxified by heterotrophic V(V) reducers (e.g., Syntrophobacter, Spirochaeta and Geobacter). Cytochrome c, intracellular nicotinamide adenine dinucleotide and extracellular polymeric substances were involved in V(V) reduction and binding. Organic metabolites synthesized by autotrophs (e.g., Thioclava) with energy from S(0) oxidation might compensate electron donors for heterotrophic V(V) and sulfate reducers. Less sulfate was accumulated presumably due to activities of sulfur-respiring genera (e.g., Desulfurella). This study demonstrates mixotrophic microbial V(V) reduction can save organic dosage and avoid excessive sulfate accumulation, which will be beneficial to bioremediation of V(V) contamination.

A newly discovered function of nitrate reductase in chemoautotrophic vanadate transformation by natural mackinawite in aquifer

Mackinawite (FeS), a widely-distributed natural reducing mineral, can donate electron for various (bio)processes. However, little is known about mackinawite-driven chemoautotrophic bioreduction of toxic vanadate [V(V)] in aquifer. This study demonstrates that V(V) is successfully bioreduced by mackinawite under anaerobic condition via 150-d operation of constructed aquifer. Complete V(V) removal was achieved at the initial concentration of 10 mg/L and flow rate of 0.125 mL/min. Fluctuant hydrochemistry and hydrodynamics affected V(V) removal performance. Biotic activity was identified as the major contribution to V(V) transformation (76.4 ± 1.01%). Chemoautotrophic genera (e.g., Thiobacillus) could oxidize FeS coupled to direct V(V) reduction independently. Heterotrophic V(V) reducers (e.g., Pseudomonas and Spirochaeta) could also achieve V(V) detoxification by utilizing metabolic intermediates synthesized by autotrophic Fe(II) oxidizers (e.g., Thiobacillus) and S(-II) oxidizing genera (e.g., Sulfuricurvum). Gene abundance and enzymatic activity tests confirmed that nitrate reductase gene napA functioned crucially in chemoautotrophic V(V) reduction by Fe(II) and S(-II) donating electron. V(V) was reduced to insoluble V(IV) while elements in mackinawite were oxidized to Fe(III) and SO₄^2-. This study reveals the coupling of iron, sulfur and vanadium in biogeochemical cycling, and offers a promising strategy for remediation of V(V)-polluted aquifer.

Microbial removal of vanadium (V) from groundwater by sawdust used as a sole carbon source

Bioremediation of vanadium (V) (V(V)) for polluted groundwater is an emerging topic globally. With this study, microbial removal of V(V) was investigated by sawdust of pine used as a sole carbon source. The removal efficiency of V(V) reached up to 90.3% with anaerobic sludge as inocula and sawdust as the carbon source in nutrient solution. Microbial removal of V(V) could be enhanced by adding medical stone and phosphate rock, from 53.2% up to 82.6% in real groundwater. Microbiological analysis revealed such microbes as Thauera accumulated, which could contribute to V(V) reduction. Such functional species as Bacteroidetes vadinHA17 norank and Anaerolineaceae norank helped degradation of sawdust. In column experiments with domesticated sludge or indigenous microbes from soils, microbial V(V) removal efficiencies (on 26 d) with sawdust were around 58.7% (BS), 54.8% (BP) and 38.4% (BU), respectively. The study can offer a potential approach to microbially removing V(V) for contaminated groundwater and even for disposal of agricultural and forestry wastes.

Microbial vanadate reduction coupled to co-metabolic phenanthrene biodegradation in groundwater

Vanadate [V(V)] and phenanthrene (PHE) commonly coexist in groundwater aquifer, posing potential threats to ecological environment and public health. However, little is known about the complicated biogeochemical processes involving microbial V(V) reduction coupled with co-metabolic PHE biodegradation. Herein we demonstrated that synchronous removal of V(V) and PHE could be realized under anaerobic condition. Complete V(V) removal and PHE degradation efficiency of 82.0 ± 0.8% were achieved in 7-d operation in batch experiment. 250-d continuous column experiment implied that hydrochemical condition affected V(V) and PHE removals. V(V) was reduced to insoluble vanadium (IV) and PHE was degraded into small molecule organics (e.g. salicylic acid). Geobacter and Acetobacterium used methanol and intermediates from PHE degradation as electron donors for V(V) reduction. PHE was decomposed by Mycobacterium and Clostridium with methanol as co-metabolic substrate and V(V) as electron acceptor. Genes encoding proteins for V(V) reduction (omcA, omcB and mtrC) and PHE degradation (phnAc) were upregulated. Cytochrome c and nicotinamide adenine dinucleotide promoted electron transfer for V(V) and PHE detoxification. Extracellular polymeric substances could bind V(V) and improve the bioavailability of PHE. Our findings provide a robust strategy for remediation of V(V) and PHE co-contaminated groundwater.

The role of natural Fe(II)-bearing minerals in chemoautotrophic chromium (VI) bio-reduction in groundwater

To date, comparatively little is known about the role of natural Fe(II)-bearing minerals in bioremediation of chromium (VI) contaminated aquifers subject to chemoautotrophic conditions. This work employed four kinds of Fe(II)-bearing minerals (pyrite, mackinawite, wustite, and magnetite) as inorganic electron donors to support Cr(VI) bio-reduction. In batch experiments, mackinawite (FeS) performed best, with Cr(VI) removal efficiency of 98.1 ± 1.21 % in 96 h. Continuous column experiments lasting 180 d implied that groundwater chemistry and hydrodynamics influenced the Cr(VI) removal process. A breakthrough study suggested that biotic and abiotic contributions to Cr(VI) reduction were 76.0 ± 1.12 % and 24.1 ± 1.43 %, respectively. Cr(VI) was reduced to insoluble Cr(III), whereas Fe(II) and S(-II) in mackinawite were finally oxidized to Fe(III) and sulfate. Mackinawite evolved progressively into pyrrhotite. High-throughput 16S rRNA gene sequencing indicated that mackinawite-driven Cr(VI) reduction was mediated through synergistic interactions of microbial consortia; i.e. autotrophs as Acidovorax synthesized volatile fatty acids as metabolic intermediates, which were consumed by Cr(VI) reducers as Geobacter. Genes encoding enzymes for S oxidation (soxB) and Cr(VI) reduction (chrA, yieF) were upregulated. Cytochrome c participating in Fe(II) oxidation increased significantly. This work advances the development of sustainable techniques for Cr(VI) polluted groundwater remediation.

Effects of biogenic nanopalladium precipitation on the performance and microbial community structure of anaerobic granular sludge

Biogenic nanopalladium (BioPd) catalysts have drawn increasing attentions recently as a combination of metal catalyst over the support of biomass. Anaerobic granular sludge (AGS), as a special microbial granule, demonstrates a strong potential to reduce Pd(II) and precipitate Bio-Pd in the sludge body. The problem how the Bio-Pd precipitates would influence the function and microbial community of the Pd hosting AGS (Pd-AGS) remains unknown. In this study, Pd-AGS with different Bio-Pd loadings (1.7, 3.0, 4.4 and 8.0 wt% of Pd) was obtained through bio-reduction at different Na₂PdCl₄ concentrations. Effects of Bio-Pd precipitates on acidogenesis and methanogenesis of AGS were assayed. Response of bacterial and archaeal community of AGS towards Bio-Pd precipitation were also revealed based on high-throughput sequencing data on Illumina Miseq platform. Results showed that Bio-Pd precipitates affected the acidogenesis and methanogenesis process of the Pd-AGS, as the produced total volatile fatty acids (VFA) and methane were reduced by 25.8-53.0% and 33.9-87.7%, respectively, comparing to the native AGS. Bio-Pd precipitation resulted in microbial community shift and a decrease in the microbial diversity. The bacterial community suffered more influence than the archaeal community. Hydrogenotrophic methanogens were more sensitive to the toxicity of Pd(II)/Bio-Pd than acetotrophic methanogens. Overall, when the heterogeneous Pd-AGS catalyst is designed to possess both the function of microbial metabolism and Pd catalysis, it is necessary to control a suitable Pd(II) concentration during reduction process and the final Bio-Pd loading in AGS (<4.4 wt% of Pd).

Microbial vanadate and nitrate reductions coupled with anaerobic methane oxidation in groundwater

Vanadate contaminant in groundwater receives increasing attentions, but little is known on its biogeochemical transformation with gaseous electron donors. This study investigated bio-reduction of vanadate coupled with anaerobic methane oxidation and its relationship with nitrate reduction. Results showed 95.8 ± 3.1% of 1 mM vanadate was removed within 7 days using methane as the sole electron donor. Tetravalent vanadium compounds were the main reduction products, which precipitated naturally in groundwater environment. The introduction of nitrate inhibited vanadate reduction, though both were reduced in parallel. Accumulations of volatile fatty acids (VFAs) were observed from methane oxidation. Preliminary microbial community structure and metabolite analyses indicated that vanadate was likely reduced via Methylomonas coupled with methane oxidation or through synergistic relationships between methane oxidizing bacteria and heterotrophic vanadate reducers with VFAs served as the intermediates.

Enhancement of synchronous bio-reductions of vanadium (V) and chromium (VI) by mixed anaerobic culture

The co-occurrence of toxic vanadium (V) and chromium (VI) in groundwater receives incremental attention while knowledge on their interactions in biogeochemical processes is limited, with lack of efficient removal means. This study is the first to realize synchronous bio-reductions of V(V) and Cr(VI) with high efficiency by mixed anaerobic culture. After 72-h operation, 97.0 ± 1.0% of V(V) and 99.1 ± 0.7% of Cr(VI) were removed, respectively, with initial concentration of 1 mM for both V(V) and Cr(VI). Cr(VI) bio-reduction took priority while V(V) detoxification was inhibited. V(IV) and Cr(III) were the identified reduction products, both of which could precipitate naturally. Initial Cr(VI) and acetate concentrations as well as pH affected this process significantly. High-throughput 16S rRNA gene sequencing analysis indicated the accumulation of Anaerolineaceae, Spirochaeta and Spirochaetaceae, which could contribute to V(V) and Cr(VI) bio-reductions. The new knowledge obtained in this study will facilitate understanding the biogeochemical fate of co-existing V(V) and Cr(VI) in groundwater and development of bioremediation strategy for their induced combined pollution.

Synchronous microbial vanadium (V) reduction and denitrification in groundwater using hydrogen as the sole electron donor

Groundwater co-contaminated by vanadium (V) (V(V)) and nitrate requires efficient remediation to prevent adverse environmental impacts. However, little is known about simultaneous bio-reductions of V(V) and nitrate supported by gaseous electron donors in aquifers. This study is among the first to examine microbial V(V) reduction and denitrification with hydrogen as the sole electron donor. V(V) removal efficiency of 91.0 ± 3.2% was achieved in test bioreactors within 7 d, with synchronous, complete removal of nitrate. V(V) was reduced to V(IV), which precipitated naturally under near-neutral conditions, and nitrate tended to be converted to nitrogen, both of which processes helped to purify the groundwater. Volatile fatty acids (VFAs) were produced from hydrogen oxidation. High-throughput 16S rRNA gene sequencing and metagenomic analyses revealed the evolutionary behavior of microbial communities and functional genes. The genera Dechloromonas and Hydrogenophaga promoted bio-reductions of V(V) and nitrate directly coupled to hydrogen oxidation. Enriched Geobacter and denitrifiers also indicated synergistic mechanism, with VFAs acting as organic carbon sources for heterotrophically functional bacteria while reducing V(V) and nitrate. These findings are likely to be useful in revealing biogeochemical fates of V(V) and nitrate in aquifer and developing technology for removing them simultaneously from groundwater.

Autotrophic Vanadium(V) Bioreduction in Groundwater by Elemental Sulfur and Zerovalent Iron

Vanadium (V) is an emerging contaminant in groundwater that can adversely affect human health. Although bioremediation has been shown effective, little is known on autotrophic V(V) bioreduction in the context of oligotrophic characteristics of groundwater. In this study, we demonstrate that efficient V(V) bioreductions can be coupled with bio-oxidation of elemental sulfur (S(0)) or zerovalent iron (Fe(0)), and the V(V) removal efficiencies reached 97.5 ± 1.2% and 86.6 ± 2.5% within 120 h using S(0) and Fe(0), respectively. V(IV) is the main reduction product and precipitates naturally in near-neutral conditions. Microbial community, functional gene, and metabolites analyses reveal that synthetic metabolisms among autotrophs and heterotrophs played major roles in V(V) reduction using S(0) and Fe(0). These results demonstrate a new approach for V(V) contaminated groundwater remediation.

Biosynthesis of copper nanoparticles using Shewanella loihica PV-4 with antibacterial activity: Novel approach and mechanisms investigation

Metallic nanoparticle based disinfection represents a promising approach for microbial pollution control in drinking water and thus, biosynthesis of non precious metal nanoparticles is of considerable interest. Herein, an original and efficient route for directly microbial synthesis of copper nanoparticles (Cu-NPs) by Shewanella loihica PV-4 is described and their satisfactorily antimicrobial activities are established. Cu-NPs were successfully synthesized and most of them attaching on the bacterial cell surfaces suggested extracellular Cu(II) bioreduction mainly contributed to this biosynthesis. Using a suite of characterization methods, polycrystalline nature and face centered cubic lattice of Cu-NPs were revealed, with size in the range of 10-16 nm. With Cu-NPs dosage of 100 μg/mL and 10⁵ CFU/mL fresh Escherichia coli suspension, the obtained antibacterial efficiency reached as high as 86.3 ± 0.2% within 12 h. Cell damages were primarily caused by the generated reactive oxygen species with H₂O₂ playing significant roles. Both cell membrane and cytoplasm components were destroyed, while the key inactivation mechanisms were lipid peroxidation and DNA damage as concluded through correlation analysis. The cost-effective and eco-friendly biosynthesis of Cu-NPs with high antibacterial activities make them particularly attractive for drinking water disinfection.

Microbial reduction of vanadium (V) in groundwater: Interactions with coexisting common electron acceptors and analysis of microbial community

Vanadium (V) pollution in groundwater has posed serious risks to the environment and public health. Anaerobic microbial reduction can achieve efficient and cost-effective remediation of V(V) pollution, but its interactions with coexisting common electron acceptors such as NO₃^-, Fe³⁺, SO₄^2- and CO₂ in groundwater remain unknown. In this study, the interactions between V(V) reduction and reduction of common electron acceptors were examined with revealing relevant microbial community and identifying dominant species. The results showed that the presence of NO₃^- slowed down the removal of V(V) in the early stage of the reaction but eventually led to a similar reduction efficiency (90.0% ± 0.4% in 72-h operation) to that in the reactor without NO₃^-. The addition of Fe³⁺, SO₄^2-, or CO₂ decreased the efficiency of V(V) reduction. Furthermore, the microbial reduction of these coexisting electron acceptors was also adversely affected by the presence of V(V). The addition of V(V) as well as the extra dose of Fe³⁺, SO₄^2- and CO₂ decreased microbial diversity and evenness, whereas the reactor supplied with NO₃^- showed the increased diversity. High-throughput 16S rRNA gene pyrosequencing analysis indicated the accumulation of Geobacter, Longilinea, Syntrophobacter, Spirochaeta and Anaerolinea, which might be responsible for the reduction of multiple electron acceptors. The findings of this study have demonstrated the feasibility of anaerobic bioremediation of V(V) and the possible influence of coexisting electron acceptors commonly found in groundwater.