Vanadate reducing bacteria and archaea may use different mechanisms to reduce vanadate in vanadium contaminated riverine ecosystems as revealed by the combination of DNA-SIP and metagenomic-binning

Vanadium (V) is a transitional metal that poses health risks to exposed humans. Microorganisms play an important role in remediating V contamination by reducing more toxic and mobile vanadate (V(V)) to less toxic and mobile V(IV). In this study, DNA-stable isotope probing (SIP) coupled with metagenomic-binning was used to identify microorganisms responsible for V(V) reduction and determine potential metabolic mechanisms in cultures inoculated with a V-contaminated river sediment. Anaeromyxobacter and Geobacter spp. were identified as putative V(V)-reducing bacteria, while Methanosarcina spp. were identified as putative V(V)-reducing archaea. The bacteria may use the two nitrate reductases NarG and NapA for respiratory V(V) reduction, as has been demonstrated previously for other species. It is proposed that Methanosarcina spp. may reduce V(V) via anaerobic methane oxidation pathways (AOM-V) rather than via respiratory V(V) reduction performed by their bacterial counterparts, as indicated by the presence of genes associated with anaerobic methane oxidation coupled with metal reduction in the metagenome assembled genome (MAG) of Methanosarcina. Briefly, methane may be oxidized through the "reverse methanogenesis" pathway to produce electrons, which may be further captured by V(V) to promote V(V) reduction. More specially, V(V) reduction by members of Methanosarcina may be driven by electron transport (CoMS-SCoB heterodisulfide reductase (HdrDE), F₄₂₀H₂ dehydrogenases (Fpo), and multi-heme c-type cytochrome (MHC)). The identification of putative V(V)-reducing bacteria and archaea and the prediction of their different pathways for V(V) reduction expand current knowledge regarding the potential fate of V(V) in contaminated sites.

Human health risk of vanadium in farmland soils near various vanadium ore mining areas and bioremediation assessment

Various kinds of vanadium (V) ore mining areas produced serious contamination have been widely recognized, while less relevant research was about the associated health risk and V distribution level for farmland soils around. This study assessed the contamination characteristics and associated human health risk of V in the surface farmland soils near various V ore mining areas. The bioremediation of V contamination by indigenous microbes from them was also evaluated. The farmland soils near stone coal area (Hunan province, China) showed the highest mean concentration of V (543.91 mg/kg), posing high non-carcinogenic risks, with high hazard quotient (HQ) value of 1.29 for children. While, V values of sampled soils near V titanomagnetite, petroleum associated minerals and uvanite areas were lower than that near stone coal area, also with lower HQ values (<1.00). Within 60 h, the removal efficiency of V(V) reached 98.4% with farmland soils near uvanite area, suggesting feasibility of V bioremediation via indigenous microbes. Bacterial communities after long-term cultivation (240 d) with V(V) were dominated by native microbes able to tolerate or reduce the toxicity of V(V), such as Ruminococcaceae_incertae_sedis, Trichococcus and Comamonas. This work is helpful for calling attention to V pollution of farmland near various V ore mining areas and formulating effective strategies for V(V) contamination bioremediation.

Recovery of spent VOSO4 using an organic ligand for vanadium redox flow battery applications

To recover the spent vanadium compound, Rhodamine-B-based Schiff's base ligand (L₁) was synthesized via ultrasonication process and was evaluated with vanadyl sulfate (VOSO₄), which has shown considerable selectivity towards V(IV). The change of the solution color from colorless to pink is attributed to L₁ after the reaction with vanadium ion owing to the successful formation of the vanadium complex and the opening of the spirolactam ring in the L₁ structure. In FT-IR spectra, the vanadyl peaks are co-existed with the L₁ structure, which confirmed the complex formation of the L₁ with vanadium. Similarly, the binding energy of V(IV) was identified at 516.2 eV for V2p_3/2 in XPS spectra. The new strategy for VOSO₄ recovery was established through solvent extraction and acid leaching. After recovery process, the absence of vanadium peak in the XPS confirmed the complete removal of V(IV) from the complex. The recovered VOSO₄ solution used as an electrolyte in vanadium redox flow battery (VRFB) systems, where the unit cell performance is comparable with the conventional electrolyte solution. The advantage of study is reuse of VOSO₄ as a resource for energy storage applications.

Enhanced removal of vanadium(V) from groundwater by layered double hydroxide-supported nanoscale zerovalent iron

To reduce the toxicity of vanadium(V) [V(V)] and inhibit the desorption of adsorbed vanadium in groundwater, we synthesized nanoscale zerovalent iron (nZVI) dispersed on layered double hydroxide (LDH) composites (nZVI@LDH) to remove V(V) from simulated groundwater. We found that nZVI@LDH could reduce high-valence vanadium to low-valence vanadium, then forming vanadium-containing precipitation to reduce the toxicity and inhibiting vanadium from returning to groundwater. SEM and XRD characterizations exhibited the uniform dispersal of nZVI on the surface of LDH. nZVI@LDH with nZVI/LDH at a mass ratio of 1:2 provided the maximum adsorption capacity of 93.7 mg g^-1 at pH 3.0. Coexisting anions and dissolved oxygen in groundwater have little effect on V(V) removal. nZVI@LDH performed well across a wide pH range (3.0-8.0). The surface characterizations and XPS analysis revealed that LDH as supporting materials inhibited the aggregation and passivation of nZVI. The adsorbed V(V) was reduced to V(IV) and V(III) by nZVI and spontaneously transformed into insoluble VO₂ and V₂O₃. The DFT calculations indicated the strong complexation and better stability of the V(IV) and V(III) species with nZVI@LDH than V(V). This work suggests that nZVI@LDH has the potential to serve as an efficient material for the immobilization of V(V) in groundwater.