Microbial Chromate Reduction Coupled to Anaerobic Oxidation of Elemental Sulfur or Zerovalent Iron

Bioreduction performances and mechanisms of Cr(VI) by Sporosarcina saromensis W5, a novel Cr(VI)-reducing facultative anaerobic bacteria

This study reported a novel facultative anaerobic Cr(VI)-reducing bacteria (Sporosarcina saromensis W5) and investigated its Cr(VI) removal performances and removal mechanisms. The strain W5 was able to grow and reduce Cr(VI) under aerobic and anaerobic environment, and exhibited considerable Cr(VI) reduction capabilities under a wide range of pH (8.0-13.0), temperature (20-40 °C) and initial Cr(VI) concentration (50-800 mg/L). The addition of Cd²⁺ severely inhibited its growth and Cr(VI) removal, while Cu²⁺ and Fe³⁺ significantly enhanced the removal efficiencies. The strain W5 could utilize various electron donors and mediators to accelerate Cr(VI) reduction. Aerobic Cr(VI) reduction mainly occurred in cytoplasm and the final products were soluble organo-Cr(III) complexes. Anaerobic Cr(VI) reduction was located in both cytoplasm and membrane, and the reduction products were soluble organo-Cr(III) complexes and Cr(III) precipitates. The functional groups of hydroxyl, carboxyl and phosphoryl on cell surface participated in the combination with Cr(III). Due to its facultative anaerobic property, S. saromensis W5 offers itself as a promising engineering strain for the bioremediation of Cr(VI)-contaminated areas, especially in hypoxia environments.

Fulvic Acid-Mediated Interfacial Reactions on Exposed Hematite Facets during Dissimilatory Iron Reduction

Although the dual role of natural organic matter (NOM) as an electron shuttle and an electron donor for dissimilatory iron (Fe) reduction has been extensively investigated, the underlying interfacial interactions between various exposed facets and NOM are poorly understood. In this study, fulvic acid (FA), as typical NOM, was used and its effect on the dissimilatory reduction of hematite {001} and {100} by Shewanella putrefaciens CN-32 was investigated. FA accelerates the bioreduction rates of hematite {001} and {100}, where the rate of hematite {100} is lower than that of hematite {001}. Secondary Fe minerals were not observed, but the HR-TEM images reveal significant defects. The ATR-FTIR results demonstrate that facet-dependent binding mainly occurs via surface complexation between the surface iron atoms and carboxyl groups of NOM. The spectroscopic and mass spectrometry analyses suggest that organic compounds with large molecular weight, highly aromatic and unsaturated structures, and lower H/C ratios are easily adsorbed on Fe oxides or decomposed by bacteria in FA-hematite {001} treatment after iron reduction. Due to the metabolic processes of cells, a significant number of compounds with higher H/C and medium O/C ratios appear. The Tafel curves show that hematite {100} possessed higher resistance (4.1-2.6 Ω) than hematite {001} (3.5-2.2 Ω) at FA concentrations ranging from 0 to 500 mg L^-1, indicating that hematite {100} is less conductive during the electron transfer from reduced FA or cells to Fe oxides than hematite {001}. Overall, the discrepancy in the iron bioreduction of two exposed facets is attributed to both the different electrochemical activities of the Fe oxides and the different impacts on the properties and composition of OM. Our findings shed light on the molecular mechanisms of mutual interactions between FA and Fe oxides with various facets.

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.

Disentangling Microbial Syntrophic Mechanisms for Hexavalent Chromium Reduction in Autotrophic Biosystems

Hexavalent chromium [Cr(VI)] is one of the common heavy-metal contaminants in groundwater, and the availability of electron donors is considered to be a key parameter for Cr(VI) biotransformation. During the autotrophic remediation process, however, much remains to be illuminated about how complex syntrophic microbial communities couple Cr(VI) reduction with other elemental cycles. Two series of Cr(VI)-reducing groundwater bioreactors were independently amended by elemental sulfur and iron and inoculated with the same inoculum. After 160 days of incubation, both bioreactors showed similar archaea-dominating microbiota compositions, whereas a higher Cr(VI)-reducing rate and more methane production were detected in the Fe⁰-driven one. Metabolic reconstruction of 23 retrieved genomes revealed complex symbiotic relationships driving distinct elemental cycles coupled with Cr(VI) reduction in bioreactors. In both bioreactors, these Cr(VI) reducers were assumed to live in syntrophy with oxidizers of sulfur, iron, hydrogen, and volatile fatty acids and methane produced by carbon fixers and multitrophic methanogens, respectively. The significant difference in methane production was mainly due to the fact that the yielded sulfate greatly retarded acetoclastic methanogenesis in the S-bioreactor. These findings provide insights into mutualistic symbioses of carbon, sulfur, iron, and chromium metabolisms in groundwater systems and have implications for bioremediation of Cr(VI)-contaminated groundwater.

Column study of enhanced Cr(Ⅵ) removal and removal mechanisms by Sporosarcina saromensis W5 assisted bio-permeable reactive barrier

In this study, the performances of Sporosarcina saromensis W5 assisted bio-permeable reactive barrier, containing activated carbon (AC) or zero-valent iron (ZVI), were investigated by column experiments in removal of Cr(Ⅵ) from simulated groundwater. The enhanced Cr(Ⅵ) removal performances were observed in biotic columns. Cr(Ⅵ) was first detected in effluent on day 24 and day 85 in Bio-AC and Bio-ZVI columns, respectively whereas it breakthrough only on day 4 and day 15 in AC and ZVI columns. Additionally, Cr(Ⅵ) removal performances induced by biofilm in Bio-QZ columns were promoted with the increase of influent Cr(Ⅵ) concentrations. According to fluorescent images, activated carbon was found to be the best biofilm carrier. Fe⁰ may not be suitable for microbial colonization because biofilm depolymerization occurred on Fe⁰ surface. Moreover, high concentration of Cr(Ⅵ) would lag the evolution of biofilm. Magnetite generating was found on the Fe⁰ surface. X-ray photoelectron spectroscopy (XPS) analysis indicated that the removal mechanism of Cr(Ⅵ) in biotic columns was biotransformation of Cr(Ⅵ) to Cr(Ш) species. Our results may provide a new insight in Cr(Ⅵ) in-situ remediation from groundwater by Bio-PRB system.

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.

Simultaneous arsenite and nitrate removal from simulated groundwater based on pyrrhotite autotrophic denitrification

In this study, pyrrhotite is applied to remove arsenite (As(III)) and NO₃^- from groundwater simultaneously. Batch experiments find that sulfur autotrophic denitrifiers are not inhibited by As(III) with concentration up to 70 mg·L^-1, and pyrrhotite autotrophic denitrification (PAD) can effectively remove As(III), NO₃^- and PO₄^3- simultaneously. Treating water with As(III) 874.50±32.76 µg·L^-1, NO₃^--N 30 mg·L^-1, and PO₄^3--P 0.5 mg·L^-1, the pyrrhotite-sulfur-limestone autotrophic denitrification (PSLAD) biofilter can achieve effluent with total Arsenic (As) 7.84±7.29 µg·L^-1, NO₃^--N 3.78±1.14 mg·L^-1, and PO₄^3--P below detection limit at hydraulic retention time 6 h. In the PSLAD biofilter, Thiobacillus is the most abundant bacterium, and it uses pyrrhotite and sulfur as electron donor to reduce NO₃^-, and basically Fe²⁺ and As(III) are oxidized to Fe³⁺ and arsenate, respectively. As and PO₄^3- were mainly removed through precipitates FeAsO₄ and FePO₄, respectively. Technology based on the PAD is a simple, cost-effective and efficient way for remediation of As(III) and NO₃^- co-contaminated groundwater, and avoiding contaminants transference between groundwater and surface water.

Chemical-Assisted Microbially Mediated Chromium (Cr) (VI) Reduction Under the Influence of Various Electron Donors, Redox Mediators, and Other Additives: An Outlook on Enhanced Cr(VI) Removal

Chromium (Cr) (VI) is a well-known toxin to all types of biological organisms. Over the past few decades, many investigators have employed numerous bioprocesses to neutralize the toxic effects of Cr(VI). One of the main process for its treatment is bioreduction into Cr(III). Key to this process is the ability of microbial enzymes, which facilitate the transfer of electrons into the high valence state of the metal that acts as an electron acceptor. Many underlying previous efforts have stressed on the use of different external organic and inorganic substances as electron donors to promote Cr(VI) reduction process by different microorganisms. The use of various redox mediators enabled electron transport facility for extracellular Cr(VI) reduction and accelerated the reaction. Also, many chemicals have employed diverse roles to improve the Cr(VI) reduction process in different microorganisms. The application of aforementioned materials at the contaminated systems has offered a variety of influence on Cr(VI) bioremediation by altering microbial community structures and functions and redox environment. The collective insights suggest that the knowledge of appropriate implementation of suitable nutrients can strongly inspire the Cr(VI) reduction rate and efficiency. However, a comprehensive information on such substances and their roles and biochemical pathways in different microorganisms remains elusive. In this regard, our review sheds light on the contributions of various chemicals as electron donors, redox mediators, cofactors, etc., on microbial Cr(VI) reduction for enhanced treatment practices.

Conversion mechanisms between organic sulfur and inorganic sulfur in surface sediments in coastal rivers

Geochemical processes of sulfur (S) in river aquatic systems play a crucial role in environmental evolution. In this study, the distributions and sources of reduced inorganic sulfur (RIS) and organic sulfur (OS) in coastal river surface sediments were investigated. The results indicated that OS dominated total S (80%), and OS (i.e., humic acid sulfur, HAS; fulvic acid sulfur, FAS) correlated with the availability of labile organic matter (OM) and reactive iron (Fe). Terrigenous inputs and sulfurization contributed to the enrichment of FAS through the S reduction. Autochthonous biological inputs were potential sources of HAS from S oxidization. The X-ray photoelectron spectroscopy showed that the main sources of S in surface sediments were deposited as the form of organic ester-sulfate. Aquatic life could break S down further, producing reduced S compounds accumulated as thiols and RIS in anoxic sediments. RIS was dominated by acid volatile sulfur (AVS) and chromium (II)-reducible sulfur (CRS). Reactive Fe oxides were major control factors for the conversation from hydrogen sulfide (H₂S) to AVS, whereas elemental sulfide (ES) controlled the conversion from AVS into CRS in coastal rivers.

Autotrophic induced heterotrophic bioreduction of bromate in use of elemental sulfur or zerovalent iron as electron donor

Bioreduction of bromate using elementary sulfur (S(0)) or zerovalent iron (Fe(0)) as electron donor was studied. After 60-day cultivation, the microbial consortium achieved high bromate conversion of 91.9 ± 2.1% and 90.0 ± 4.0% in the S(0) and Fe(0) amended tests. A coupling mechanism involving autotrophic oxidation of S(0) or Fe(0) to convert bicarbonate to volatile fatty acids followed by bromate reduction at the oxidation of the volatile fatty acids to CO₂ was proposed to interpret the noted reactor performances. The key functional strains including S(0) or Fe(0) oxidizing bacteria (Thiomonas and Ferrovibrio) and bromate reducing bacteria (Pseudoxanthomonas and Clostridium sp.) are identified. The studied system can provide an efficient BrO₃^- conversion way with no external organic carbon sources.

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.

Temporal dynamics of heavy metal distribution and associated microbial community in ambient aerosols from vanadium smelter

Heavy metals (HMs) such as vanadium (V), zinc (Zn), arsenic (As), chromium (Cr), copper (Cu) and nickel (Ni) are released into atmosphere during V smelting activities, resulting in their co-existence with airborne microbes. However, little is known about HMs distributions and associated microbes in aerosols from such industrial districts. This study reveals seasonal dynamics of HMs and microbes in ambient aerosols from V smelter in Panzhihua, China. Multiple HMs were detected, while V concentration was the highest, maximizing at 228.0 ± 10.3 ng/m³ in Spring. Health risks displayed similar trends to HMs distributions, and children were posed much higher risks than adults due to their more sensitivity to HMs. V and As contributed dramatically to total health risks among all examined HMs. High-throughput 16S rRNA gene sequencing analysis revealed microbes tolerant to V, Zn, As, Cr, Cu and Ni. Acinetobacter widely existed with function of detoxifying V(V) and more species as Bacillus, Gobacter and Thauera tolerating V, Zn, As, Cr, Cu and Ni appeared in Summer. These findings shed light on understandings of HMs dynamics and associated microbial community in aerosols from smelting regions.

Insight into the nitrogen accumulation in urban center river from functional genes and bacterial community

Along with urbanization, the intensified nitrogen pollution in urban rivers and the form of black-odor rivers has become one of the biggest concerns. Better understanding of the nitrogen transformations and microbial mechanisms occurring within urban rivers could help to manage their water quality. In this study, pollution characteristics, potential nitrogen removal rate, composition and function of bacterial community, and abundance of functional genes associated with nitrogen transformation were comparatively investigated in a typical urban river (FC) and a suburban river (LH). Compared with LH, FC was characterized by higher content of nutrients, lower potential nitrogen removal rate and lower abundance of functional genes associated with nitrogen transformation in both overlying water and sediment, especially in summer. Sediment dissolved organic matter characterized by excitation−emission matrix (EEM) showed that FC was more severely polluted by high nitrogen organic matter. Our results revealed that anammox was the main nitrogen removal pathway in both rivers and potential nitrogen removal rates decreased significantly in summer. Bacterial community analysis showed that the benthic communities were more severely influenced by the pollutant than aquatic ones in both rivers. Furthermore, the FC benthic community was dominated by anaerobic respiring, fermentative, sulfate reduction bacteria. Quantitatively, the denitrification rate showed a significant positive correlation with the abundance of denitrification genes, whilst the anammox rate was significantly negatively correlated with bacterial diversity. Meanwhile, NH₄⁺-N had a significant negative correlation to both denitrification and anammox in sediment. Taken together, the results indicated that the increased nitrogen pollutants in an urban river altered nitrogen removal pathways and bacterial communities, which could in turn exacerbate the nitrogen pollution to this river.

Enhanced Microbial Chromate Reduction Using Hydrogen and Methane as Joint Electron Donors

Hydrogen and methane commonly co-exist in aquifer. Either hydrogen or methane has been individually utilized as electron donor for bio-reducing chromate. However, little is known whether microbial chromate reduction would be suppressed or promoted when both hydrogen and methane are simultaneously supplied as joint electron donors. This study for the first time demonstrated microbial chromate reduction rate could be accelerated by both hydrogen and methane donating electrons. The maximum chromate reduction rate (4.70 ± 0.03 mg/L·d) with a volume ratio of hydrogen to methane at 1:1 was significantly higher than that with pure hydrogen (2.53 ± 0.02 mg/L·d) or pure methane (2.01 ± 0.02 mg/L·d) as the sole electron donor (p < 0.01). High-throughput 16S rRNA gene amplicon sequencing detected potential chromate reducers (e.g., Spirochaetaceae, Delftia and Azonexus) and hydrogenotrophic bacteria (e.g., Acetoanaerobium) and methane-metabolizing microorganisms (e.g., Methanobacterium), indicating that these microorganisms might play important roles on microbial chromate reduction using both hydrogen and methane as electron donors. Abundant hupL and mcrA genes responsible for hydrogen oxidation and methane conversion were harbored, together with chrA gene for chromate reduction. More abundant extracellular cytochrome c and intracellular NADH were detected with joint electron donors, suggesting more active electron transfers.

Prospecting bacterial consortia from a geothermal site for metals biotransformation

Biomats that flourished in a fumarole located on the geothermal site Los Azufres (Mexico) were used as inocula to select aerobic and sulfate-reducing bacteria consortia for studying their capacity to reduce hexavalent chromium [Cr(VI)], aiming to use these consortia in biotransformation technologies. The sample site is characterized by slightly warm (nearly 27 [Formula: see text]C), acid (pH 3) and about hypoxic (1.8 mg L[Formula: see text] of dissolved oxygen) conditions. Four culture systems (2 aerobic and 2 anaerobic) were investigated, including their enzymatic activity, capacity to produce biofilms, and an analysis of the total bacterial populations. For the anaerobic condition (using sulfate and sulfur as electron acceptors), four pH values (from 2 to 8) and four carbon sources (pyruvate, glycerol, Na-lactate and Na-acetate) were probed. Significant biological Cr(VI) removal was observed for all the pH values probed, particularly during the first 12 h, being more effective at the most acid conditions. At a pH value of 4 and using pyruvate as carbon source, 100 mg L[Formula: see text] of Cr(VI) were completely depleted in less than 12 h, while the use of Na-lactate was less effective but still reasonable. These results indicate that sulfate-reducing bacteria consortia from geothermal sites like the one studied here are capable of biotransforming Cr(VI) and have the potential to provide metal bioremediation technologies.

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.

Phytoassessment of in situ weed diversity for their chromium distribution pattern and accumulation indices of abundant weeds at South Kaliapani chromite mining area with their phytoremediation prospective

Release of huge quantities of toxic hexavalent chromium (Cr⁶⁺) owing to its widespread use in several industrial and mining activities is a major environmental concern in 21st century. The present in situ vegetation analysis at three study sites gives insight on Cr hyperaccumulation potentiality which evaluates the phytoremediation ability of abundant native weeds growing in and around the Cr contaminated effluent discharge site at South Kaliapani chromite mine area, Odisha, India. Moreover, the study measure the relative bioconcentration of chromium in different plant parts with analysis of uptake related phytoremediation indices viz. Bioconcentration factor (BCF), Transportation index (Ti), Translocation factor (TF) and Metal Extraction Ratio (MER) to assess hyperaccumulation potentiality. Vegetation study near mine effluent discharge site (Site-1) reported maximum abundance for Diectomis fastigiata (8.25) followed by Vernonia cinerea (7.6) with Ti values 56 and 657 respectively. In site-2 (uncultivated barren land near site-1), Croton sparsiflorus showed maximum abundance (6.7) followed by Tephrosia purpurea (5.8) with Ti values 95.2 and 87.8 respectively whereas Kyllinga monocephala reported maximum abundance (6.1) followed by Fern sps. (5.9) with Ti values 62.4 and 81.1 respectively in site-3 (a swampy land flooded with chemically treated mine waste effluent). The highest Cr concentrations was recorded in roots of Diectomis fastigiata (2371 mg/kg dry matter) and shoot of Vernonia cinerea (5500 mg/kg dry matter) indicating their Cr phytoremediation potential. Moreover, the study provides the early indicative tools for detecting native Cr hyperaccumulators growing in an in situ environment with an in situ phytoremediation perspective.

Exploration of the reduction mechanism of Cr(VI) in anaerobic hydrogen fermenter

The bio-reduction of hexavalent chromium (Cr(VI)) by anaerobic fermentation is considered as a promising, low-cost and environment-friendly way. However, it is unclear for the reduction mechanisms of Cr(VI) in an anaerobic hydrogen fermenter, such as reduction kinetics, related electron donors, migration and transformation, reduction site and key components, and related microorganisms. To clarify these issues, a hydrogen fermenter was designed to reduce Cr(VI) at 55 °C with glucose as initial substrate. Results show that 100 mg/L Cr(VI) can be completely reduced (99.5%) to trivalent chromium (Cr(III) through chemical and biological reactions. Bio-reduction dominates Cr(VI) removal in a first-order exponential decay mode with both glucose and its metabolites (volatile fatty acids) as electron donors. Moreover, volatile fatty acids are more suitable as electron donors for Cr(VI) bio-reduction than glucose. Bacilli, Clostridia and Thermotogae in the fermenter dominated the reduction of Cr(VI) by regulating the production and composition of extracellular polymers (EPSs), in which carboxyl and hydroxyl groups play an important role for Cr(VI) reduction by coordination. The results can guide us to regulate the bio-reduction of Cr(VI), and provide reference for the development of bio-reduction technology of Cr(VI).

Enhanced removal of chromium(vi) by Fe(iii)-reducing bacterium coated ZVI for wastewater treatment: batch and column experiments

In order to effectively destroy the structure of the passive oxidation film that covers zero-valent iron (ZVI), an Fe(iii)-reducing strain, namely Morganella sp., was isolated from anaerobic activated sludge and coated on ZVI, which was distributed in porous ceramsite made of iron dust, kaolin and straw, with a ratio of 7 : 3 : 1. Batch experiments showed that under the optimized conditions, the maximum removal amount of Cr(vi) by ZVI increased from 7.33 mg g⁻¹ to 26.87 mg g⁻¹ in the presence of the Fe(iii)-reducing bacterium. The column experiment was performed with the addition of the agar globules to supply nutrients to the strain. Compared with ZVI, the column penetration time and maximum capture amount of RB-ZVI increased to 17 h and 112.5 mg g⁻¹, respectively, on the 15^th day. Furthermore, the service life of RB-ZVI was prolonged in the existence of the strain. Based on X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy analyses, the key mechanisms for the removal of Cr(vi) by ZVI coated with Fe(iii)-reducing bacterium were determined to be adsorption, reduction, coprecipitation and biomineralization.

To effectively destroy the structure of the passive oxidation film covering zero-valent iron (ZVI), an Fe(iii)-reducing strain, Morganella sp., was isolated from anaerobic activated sludge and coated on the ZVI.