Sulfide-mediated azo dye degradation and microbial community analysis in a single-chamber air cathode microbial fuel cell

CeO2@Co3O4-decorated carbon felt anode with mesoporous nanoparticles for enhanced POM wastewater treatment and electricity generation in microbial fuel cells

Microbial fuel cells (MFCs) have been widely used in the treatment of organic wastewater because of its advantages of high efficiency, environmental friendliness, and no secondary pollution. In this study, we developed a composite anode, CeO2@Co3O4-PEDOT/CF (PEDOT: poly(3,4-ethylenedioxythiophene), CF: carbon felt), by incorporating bimesoporous CeO2@Co3O4 nanoparticles and PEDOT layer. The MFCs equipped with this novel anode had demonstrated exceptional catalytic performance in the treatment of Polyoxymethylene (POM) wastewater. Specifically, it achieved a 74.9 % increase in output voltage (586 ± 23 mV), a 2.5-fold increase in power density (3564 ± 27 mW/m2), and enhancements of 41.8 %, 40.1 %, and 47.0 % in the degradation rates of formaldehyde (HCHO, 97.2 %), 1,3,5-trioxane (TOX, 93.6 %), and chemical oxygen demand (COD) removal efficiency (84.4 %), respectively, when compared to a bare CF anode. Furthermore, the MFC with the composite anode demonstrated high degradation efficiency and COD removal efficiency across various industrial water environments. Additionally, the wheat seed germination experiments indicated a significant reduction in the toxicity of the degraded solution. This enhanced performance could be attributed to the synergistic effects of the bimesoporous CeO2@Co3O4 nanoparticles and the PEDOT layer. This study presents a promising strategy for the degradation of organic pollutants and the generation of bioelectricity in practical applications.

Alternating current-powered bioelectrode for enhanced sulfide removal from sulfide-rich organic effluent

Conventional biological wastewater treatments often struggle with efficient sulfide removal. This study presents an innovative and sustainable approach using an alternating current (AC)-powered bioelectrode to facilitate periodic microbial oxidation and reduction for sulfide removal from sulfide-rich organic effluents. The AC bioelectrode addressed the drawbacks of conventional direct current (DC) systems, demonstrating 93.2 % sulfide removal, 64.5 % elemental sulfur production and 75.8 % chemical oxygen demand removal, coupled with a 25 % decrease in energy consumption. AC stimulation mitigated elemental sulfur accumulation on the electrode surface, promoted biofilm colonization, and enhanced bidirectional electrocatalytic activity, thereby elevating active biomass and electron utilization efficiency. Additionally, AC bioelectrode enriched functional microbiomes capable of sulfur-oxidizing, sulfur-reducing, and electron-transport capacities, as confirmed by enzyme gene expression profiling. This study offers an efficient, resource-efficient, and sustainable solution to treating sulfide-rich organic effluents.

Constructed wetland-microbial fuel cell (CW-MFC) mediated bio-electrodegradation of azo dyes from textile wastewater

Azo dyes constitute 60%-70% of commercially used dyes and are complex, carcinogenic, and mutagenic pollutants that negatively impact soil composition, water bodies, flora, and fauna. Conventional azo dye degradation techniques have drawbacks such as high production and maintenance costs, use of hazardous chemicals, membrane clogging, and sludge generation. Constructed wetland-microbial fuel cells (CW-MFCs) offer a promising sustainable approach for the bio-electrodegradation of azo dyes from textile wastewater. CW-MFCs harness the phytodegradation capabilities of wetland plants like Azolla, water hyacinth, and Ipomoea, along with microalgae such as Nostoc, Oscillatoria, Chlorella, and Anabaena, to break down azo dyes into aromatic amines. These intermediates are then reduced to CO2 and H2O by microalgae in the fuel cells while simultaneously generating electricity. CW-MFCs offer advantages including low cost, sustainability, and use of renewable energy. The valorization of the resulting algal and plant biomass further enhances the sustainability of this approach, as it can be used for biofuel production, nutraceuticals, pharmaceuticals, and bio-composting. Implementing CW-MFCs as a tertiary treatment step in textile industries aligns with the circular economy concept and contributes to achieving several sustainable development goals.

Unveiling microbial community structure and metabolic pathway over carbon cloth-titanium nitride-polyaniline biocathode for effective dichloromethane transformation

Chlorinated volatile organic compounds (Cl-VOCs) have dramatically biotoxicity and environmental persistence due to the presence of chlorine atoms, seriously jeopardizing ecological security and human health. Dichloromethane (DCM) as a model pollutant, is widely applied in solvents, extractants and cleaning agents in the pharmaceutical, chemical and food industries. In this study, highly biocompatible and conductive carbon cloth-titanium nitride-polyaniline (CC-TiN-PANI) bioelectrodes were obtained for DCM degradation in microbial electrolysis cell (MEC). The good adhesion of TiN and PANI on the electrode surface was demonstrated. The degradation kinetics were fitted by the Haldane model, compared to the CC bioelectrode (0.8 h-1), the proportion of maximum degradation rates to half-saturation concentration (Vmax/Km) of CC-TiN (1.4 h-1) and CC-TiN-PANI (2.2 h-1) bioelectrodes were enhanced by 1.8 and 2.8 times, respectively. Microbial community structure analysis illuminated that the dominant genera on the biofilm were Alicycliphilus and Hyphomicrobium, and the abundance was enhanced significantly with the modification of TiN and PANI. The dechlorination of DCM to formaldehyde could be catalyzed by DCM dehalogenase (DcmA) or by haloalkane dehalogenase (DhlA). And further oxidized to formate: 1) direct catalyzed by formaldehyde dehydrogenase (FdhA); 2) conjugated with glutathione by S-(hydroxymethyl)-glutathione synthase (Gfa), S-(hydroxymethyl)-glutathione dehydrogenase (FrmA) and S-formyl-glutathione hydrolase (FrmB); 3) conjugation with tetrahydrofolate (H4F) and/or tetrahydromethanopterin.

Enhanced performance of acridine degradation and power generation by microbial fuel cell with g-C3N4/PANI-DA/CF anode

Microbial fuel cell (MFC) has attracted much attention in treating organic wastewater due to its double functions of degrading organics and generating electricity with microorganisms as biocatalysts. Unfortunately, some organics with biological toxicity such as acridine could inhibit the growth and activity of the microorganisms on the anode so that the double functions of MFC would recede. Enhancing microbial activity by using new biocompatible materials as anodes is prospective to solve problem. A novel anode was achieved by electrodepositing g-C3N4 sheets to the carbon felt (CF) modified with polyaniline-dopamine composite film, and used to treat wastewater containing acridine for the first time. After the operation of 13 d, MFC loading with the composite anode showed a degradation efficiency of 98.3% in 150 mg L-1 acridine, while that of CF-MFC was 55.8%. Moreover, MFC loading the modified anode obtained a maximum power density of 1976 ± 47 mW m-2, 140.1% higher than that of CF-MFC. Further analysis revealed that the functional microorganisms associated with acridine degradation such as Achromobacter and Alcaligenes were enriched on the g-C3N4/PANI-DA/CF anode. Moreover, the composite anode could improve the activity of microorganisms and elicit them to generate conductive nanowires, which was beneficial to transferring electrons from microbes to anode over long distances, suggesting a promising prospect application in MFC.

Metagenomics revealing biomolecular insights into the enhanced toluene removal and electricity generation in PANI@CNT bioanode

Microbial fuel cells (MFCs) have significant potential for environmental remediation and energy recycling directly from refractory aromatic hydrocarbons. To boost the capacities of toluene removal and the electricity production in MFCs, this study constructed a polyaniline@carbon nanotube (PANI@CNT) bioanode with a three-dimensional framework structure. Compared with the control bioanode based on graphite sheet, the PANI@CNT bioanode increased the output voltage and toluene degradation kinetics by 2.27-fold and 1.40-fold to 0.399 V and 0.60 h-1, respectively. Metagenomic analysis revealed that the PANI@CNT bioanode promoted the selective enrichment of Pseudomonas, with the dual functions of degrading toluene and generating exogenous electrons. Additionally, compelling genomic evidence elucidating the relationship between functional genes and microorganisms was found. It was interesting that the genes derived from Pseudomonas related to extracellular electron transfer, tricarboxylic acid cycle, and toluene degradation were upregulated due to the existence of PANI@CNT. This study provided biomolecular insights into key genes and related microorganisms that effectively facilitated the organic pollutant degradation and energy recovery in MFCs, offering a novel alternative for high-performance bioanode.

The global anaerobic metabolism regulator fnr is necessary for the degradation of food dyes and drugs by Escherichia coli

IMPORTANCE

This work has broad relevance due to the ubiquity of dyes containing azo bonds in food and drugs. We report that azo dyes can be degraded by human gut bacteria through both enzymatic and nonenzymatic mechanisms, even from a single gut bacterial species. Furthermore, we revealed that environmental factors, oxygen, and L-Cysteine control the ability of E. coli to degrade azo dyes due to their impacts on bacterial transcription and metabolism. These results open up new opportunities to manipulate the azoreductase activity of the gut microbiome through the manipulation of host diet, suggest that azoreductase potential may be altered in patients suffering from gastrointestinal disease, and highlight the importance of studying bacterial enzymes for drug metabolism in their natural cellular and ecological context.

Electroactive microorganism-assisted remediation of groundwater contamination: Advances and challenges

Groundwater contamination has become increasingly prominent, therefore, the development of efficient remediation technology is crucial for improving groundwater quality. Bioremediation is cost-effective and environmentally friendly, while coexisting pollutant stress can affect microbial processes, and the heterogeneous character of groundwater medium can induce bioavailability limitations and electron donor/acceptor imbalances. Electroactive microorganisms (EAMs) are advantageous in contaminated groundwater because of their unique bidirectional electron transfer mechanism, which allows them to use solid electrodes as electron donors/acceptors. However, the relatively low-conductivity groundwater environment is unfavorable for electron transfer, which becomes a bottleneck problem that limits the remediation efficiency of EAMs. Therefore, this study reviews the recent advances and challenges of EAMs applied in the groundwater environment with complex coexisting ions, heterogeneity, and low conductivity and proposes corresponding future directions.

Anaerobic Membrane Bioreactor (AnMBR) for the Removal of Dyes from Water and Wastewater: Progress, Challenges, and Future Perspectives

The presence of dyes in aquatic environments can have harmful effects on aquatic life, including inhibiting photosynthesis, decreasing dissolved oxygen levels, and altering the behavior and reproductive patterns of aquatic organisms. In the initial phase of this review study, our aim was to examine the categories and properties of dyes as well as the impact of their toxicity on aquatic environments. Azo, phthalocyanine, and xanthene are among the most frequently utilized dyes, almost 70–80% of used dyes, in industrial processes and have been identified as some of the most commonly occurring dyes in water bodies. Apart from that, the toxicity effects of dyes on aquatic ecosystems were discussed. Toxicity testing relies heavily on two key measures: the LC50 (half-lethal concentration) and EC50 (half-maximal effective concentration). In a recent study, microalgae exposed to Congo Red displayed a minimum EC50 of 4.8 mg/L, while fish exposed to Disperse Yellow 7 exhibited a minimum LC50 of 0.01 mg/L. Anaerobic membrane bioreactors (AnMBRs) are a promising method for removing dyes from water bodies. In the second stage of the study, the effectiveness of different AnMBRs in removing dyes was evaluated. Hybrid AnMBRs and AnMBRs with innovative designs have shown the capacity to eliminate dyes completely, reaching up to 100%. Proteobacteria, Firmicutes, and Bacteroidetes were found to be the dominant bacterial phyla in AnMBRs applied for dye treatment. However, fouling has been identified as a significant drawback of AnMBRs, and innovative designs and techniques are required to address this issue in the future.

Recent progress in microbial fuel cells using substrates from diverse sources

Increasing untreated environmental outputs from industry and the rising human population have increased the burden of wastewater and other waste streams on the environment. The most prevalent wastewater treatment methods include the activated sludge process, which requires aeration and is, therefore, energy and cost-intensive. The current trend towards a circular economy facilitates the recovery of waste materials as a resource. Along with the amount, the complexity of wastewater is increasing day by day. Therefore, wastewater treatment processes must be transformed into cost-effective and sustainable methods. Microbial fuel cells (MFCs) use electroactive microbes to extract chemical energy from waste organic molecules to generate electricity via waste treatment. This review focuses use of MFCs as an energy converter using wastewater from various sources. The different substrate sources that are evaluated include industrial, agricultural, domestic, and pharmaceutical types. The article also highlights the effect of operational parameters such as organic load, pH, current, and concentration on the MFC output. The article also covers MFC functioning with respect to the substrate, and the associated performance parameters, such as power generation and wastewater treatment matrices, are given. The review also illustrates the success stories of various MFC configurations. We emphasize the significant measures required to fill in the gaps related to the effect of substrate type on different MFC configurations, identification of microbes for use as biocatalysts, and development of biocathodes for the further improvement of the system. Finally, we shortlisted the best performing substrates based on the maximum current and power, Coulombic efficiency, and chemical oxygen demand removal upon the treatment of substrates in MFCs. This information will guide industries that wish to use MFC technology to treat generated effluent from various processes.

Study on the catalytic degradation of Acid Orange 7 and the potential mechanism by ferrous-percarbonate

Factors affecting the degradation of Acid Orange 7 (AO7) were evaluated and optimized when ferrous was used to catalyze percarbonate in the present study. The optimized conditions included the initial pH values ranging from 3 to 11 for AO7 solution, the initial level of AO7, sodium percarbonate (SPC), and Fe²⁺ . Some ions and natural organic materials, which commonly exist in natural water, were also tested to evaluate their potential impacts on the degradation of AO7. The degradation efficiency of AO7 was up to 95% under the optimized test conditions, where the ferrous/percarbonate/AO7 molar ratio was 15/10/1 in the 0.285 mmol/l AO7 aqueous solution. The presence of Cl^- , SO₄ ^2- , NO₃ ^- , Na⁺ , and Mg²⁺ did not affect the removal of AO7. The addition of HCO₃ ^- significantly inhibited its removal, even when the concentration of HCO₃ ^- was low to 0.6 mmol/l. A slight inhibition effect was observed when the added concentration of humic acid ranged from 0.5 to 5 mg/l, whereas the residue of AO7 was significantly enhanced when the level of humic acid was continually increased from 50 to 100 mg/l. Hydroxyl radicals (•OH) were the main reactive intermediates controlling the oxidation of AO7 in the present Fe²⁺ /SPC system. The produced intermediates through the degradation of AO7 were identified to include 2-coumaranone, 2-naphthol, phthalic acid, phthalimide, N-methylnaphthylamine, and 2-methylphenol. The proposed degradation pathways are consistent with the radical formation and the identified intermediates. PRACTITIONER POINTS: The ferrous/percarbonate system can remove 95% of AO7 under the optimized conditions. AO7 removal was inhibited by adding HCO₃ ^- and humic acid, but not affected by Cl^- , SO₄ ^2- , NO₃ ^- , Na⁺ , and Mg²⁺ . Hydroxylation, ring opening, and mineralization driven by the generated hydroxyl radicals were derived as the major processes for degrading AO7.

Treasuring industrial sulfur by-products: A review on add-value to reductive sulfide and sulfite for contaminant removal and hydrogen production

Reductive sulfur-containing by-products (S-BPs) released from industrial process mainly exist in the simple form of sulfide and sulfite. In this study, recent advances to remove and make full use of reductive S-BPs to achieve efficient contaminant removal and hydrogen production are critically reviewed. Sulfide, serves as both reductant and nucleophile, can form intermediates with the catalyst surface functional group through chemical interaction, efficiently promoting the catalytic reduction process to remove contaminants. Sulfite assisted catalytic process could be classified to the advanced reduction processes (ARPs) and advanced oxidation processes (AOPs), mainly depending on the presence of dissolved oxygen (DO) in the solution. During ARPs, sulfite could generate reductive active species including hydrated electron (e_aq^-), hydrogen radical (H·), and sulfite radical (SO₃^•-) under the irradiation of UV light, leading to the efficient reduction removal of a variety of contaminants. During AOPs, sulfite could first produce SO₃^•- under the action of the catalyst or energy, initiating a series of reactions to produce oxysulfur radicals. Various contaminants could be effectively removed under the role of these oxidizing active species. Sulfides and sulfites could also be removed along with promoting hydrogen production via photocatalytic and electrocatalytic processes. Besides, the present limitations and the prospects for future practical applications of the process with these S-BPs are proposed. Overall, this review gives a comprehensive summary and aims to provide new insights and thoughts in promoting contaminant removal and hydrogen production through taking full advantage of reductive S-BPs.

Snorkels enhance alkanes respiration at ambient and increased hydrostatic pressure (10 MPa) by either supporting the TCA cycle or limiting alternative routes for acetyl-CoA metabolism

The impact of piezosensitive microorganisms is generally underestimated in the ecology of underwater environments exposed to increasing hydrostatic pressure (HP), including the biodegradation of crude oil components. Yet, no isolated pressure-loving (piezophile) microorganism grows optimally on hydrocarbons, and no isolated piezophile at all has a HP optimum <10 MPa (e.g. 1000 m below sea water level). Piezosensitive heterotrophs are thus largely accountable for oil clean up < 10 MPa, however, they are affected by such a mild HP increase in ways which are not completely clear. In a first study, the application of a bioelectrochemical system (called "oil-spill snorkel") enhanced the alkane oxidation capacity in sediments collected at surface water but tested up to 10 MPa. Here, the fingerprint left on transcript abundance was studied to explore which metabolic routes are 1) supported by snorkels application and 2) negatively impacted by HP increase. Transcript abundance was comparable for beta-oxidation across all treatments (also at a taxonomical level), while the metabolism of acetyl-CoA was highly impacted: at either 0.1 or 10 MPa, snorkels supported acetyl-CoA oxidation within the TCA cycle, while in negative controls using non-conductive rods several alternative routes for acetyl-CoA were stimulated (including those leading to internal carbon reserves e.g. 2,3 butanediol and dihydroxyacetone). In general, increased HP had opposite effects as compared to snorkels, thus indicating that snorkels could enhance hydrocarbons oxidation by alleviating in part the stressing effects imposed by increased HP on the anaerobic, respiratory electron transport chain. 16S rRNA gene analysis of sediments and biofilms on snorkels suggest a crosstalk between oil-degrading, sulfate-reducing microorganisms and sulfur oxidizers. In fact, no sulfur was deposited on snorkels, however, iron, aluminum and phosphorous were found to preferentially deposit on snorkels at 10 MPa. This data indicates that a passive BES such as the oil-spill snorkel can mitigate the stress imposed by increased HP on piezosensitive microorganisms (up to 10 MPa) without being subjected to passivation. An improved setup applying these principles can further support this deep-sea bioremediation strategy.

Microbial fuel cells for mineralization and decolorization of azo dyes: Recent advances in design and materials

Microbial fuel cells (MFCs) exhibit tremendous potential in the sustainable management of dye wastewater via degrading azo dyes while generating electricity. The past decade has witnessed advances in MFC configurations and materials; however, comprehensive analyses of design and material and its association with dye degradation and electricity generation are required for their industrial application. MFC models with high efficiency of dye decolorization (96-100%) and a wide variation in power generation (29.4-940 mW/m²) have been reported. However, only 28 out of 104 studies analyzed dye mineralization - a prerequisite to obviate dye toxicity. Consequently, the current review aims to provide an in-depth analysis of MFCs potential in dye degradation and mineralization and evaluates materials and designs as crucial factors. Also, structural and operation parameters critical to large-scale applicability and complete mineralization of azo dye were evaluated. Choice of materials, i.e., bacteria, anode, cathode, cathode catalyst, membrane, and substrate and their effects on power density and dye decolorization efficiency presented in review will help in economic feasibility and MFCs scalability to develop a self-sustainable solution for treating azo dye wastewater.

Polyaniline on Stainless Steel Fiber Felt as Anodes for Bioelectrodegradation of Acid Blue 29 in Microbial Fuel Cells

This study investigated the advantages of using low-cost polyaniline-fabricated stainless steel fiber felt anode-based microbial fuel cells (PANI-SSFF-MFCs) for azo dye acid blue 29 (AB29) containing wastewater treatment integrated with an aerobic bioreactor. The findings of electrochemical impedance spectroscopy (EIS) and polarization studies showed that the PANI–SSFF anode considerably decreased the MFC internal resistance. The highest power density of 103 ± 3.6 mW m−2 was achieved by PANI-SSFF-MFCs with a decolorization efficiency of 93 ± 3.1% and a start-up time of 13 days. The final chemical oxygen demand (COD) removal efficiencies for integrated PANI–SSFF–MFC–bioreactor and SSFF–MFC–bioreactor set-ups were 92.5 ± 2% and 80 ± 2%, respectively. Based on 16S rRNA gene sequencing, a substantial microbial community change was observed in MFCs. The majority of sequences were from the Proteobacteria phylum, accounting for 72% and 55% in PANI–SSFF–anodic biofilm and suspension, respectively, and 58 and 45% in SSFF–anodic biofilm and suspension, respectively. The relative abundance of the seven most abundant genera (Pseudomonas, Acinetobacter, Stenotrophomonas, Geothrix, Dysgonomonas, Shinella, and Rhizobiales) was higher in PANI–SSFF–MFCs (46.1% in biofilm and 55.4% in suspension) as compared to SSFF–MFC (43% in biofilm and 40.8% in suspension) which predominantly contributed to the decolorization of AB29 and/or electron transfer. We demonstrate in this work that microbial consortia acclimated to the MFC environment and PANI-fabricated anodes are capable of high decolorization rates with enhanced electricity production. A combined single-chamber MFC (SMFC)-aerobic bioreactor operation was also performed in this study for the efficient biodegradation of AB29.

Microbial Degradation of Azo Dyes: Approaches and Prospects for a Hazard-Free Conversion by Microorganisms

Azo dyes have become a staple in various industries, as colors play an important role in consumer choices. However, these dyes pose various health and environmental risks. Although different wastewater treatments are available, the search for more eco-friendly options persists. Bioremediation utilizing microorganisms has been of great interest to researchers and industries, as the transition toward greener solutions has become more in demand through the years. This review tackles the health and environmental repercussions of azo dyes and its metabolites, available biological approaches to eliminate such dyes from the environment with a focus on the use of different microorganisms, enzymes that are involved in the degradation of azo dyes, and recent trends that could be applied for the treatment of azo dyes.

Role of bioelectrochemical systems for the remediation of emerging contaminants from wastewater: A review

Bioelectrochemical systems (BESs) are a unique group of wastewater remediating technology that possesses the added advantage of valuable recovery with concomitant wastewater treatment. Moreover, due to the application of robust microbial biocatalysts in BESs, effective removal of emerging contaminants (ECs) can be accomplished in these BESs. Thus, this review emphasizes the recent demonstrations pertaining to the removal of complex organic pollutants of emerging concern present in wastewater through BES. Owing to the recalcitrant nature of these pollutants, they are not effectively removed through conventional wastewater treatment systems and thereby are discharged into the environment without proper treatment. Application of BES in terms of ECs removal and degradation mechanism along with valuables that can be recovered are discussed. Moreover, the factors affecting the performance of BES, like biocatalyst, substrate, salinity, and applied potential are also summarized. In addition, the present review also elucidates the occurrence and toxic nature of ECs as well as future recommendations pertaining to the commercialization of this BES technology for the removal of ECs from wastewater. Therefore, the present review intends to aid the researchers in developing more efficient BESs for the removal of ECs from wastewater.

Gaseous isopropanol removal in a microbial fuel cell with deoxidizing anode: Performance, anode characteristics and microbial community

A deoxidizing packing material (DPM) with an encapsulated deoxidizing agent (DA) was developed to construct the packed anodes of a trickle-bed microbial fuel cell (TB-MFC) for treating waste gas. The encapsulated DA can consume O₂ in waste gas and increase the voltage output and power density (PD) of the constructed TB-MFC. The DPM effectively enables the circulating water in TB-MFC for maintaining a low level of dissolved oxygen for 80 h. The results revealed that when the concentration of isopropanol (IPA) in waste gas was 0.74 g/m³, the TB-MFC (DPM with DA) exhibited an IPA removal efficiency (RE) of up to 99.7%. When DPM with DA was used as the packing material of the TB-MFC (486.6 mW/m³), the PD was 2.54 times that obtained when using coke as the packing material (191.6 mW/m³). The next-generation sequencing results demonstrated that because the oxygen content of the MFC anode chamber decreased over time in the TB-MFC, the richness of anaerobic electrogens (Pseudoxanthomonas, Flavobacterium, and Ferruginibacter) in the packing materials was increased. These electrogens mainly attached to the DPM, and IPA-degraders appeared in the circulating water of the TB-MFC. This enabled the TB-MFC to simultaneously achieve a high voltage output and IPA RE.

Degradation of Azo Dyes: Bacterial Potential for Bioremediation

The use of dyes dates to ancient times and has increased due to population and industrial growth, leading to the rise of synthetic dyes. These pollutants are of great environmental impact and azo dyes deserve special attention due their widespread use and challenging degradation. Among the biological solutions developed to mitigate this issue, bacteria are highlighted for being versatile organisms, which can be applied as single organism cultures, microbial consortia, in bioreactors, acting in the detoxification of azo dyes breakage by-products and have the potential to combine biodegradation with the production of products of economic interest. These characteristics go hand in hand with the ability of various strains to act under various chemical and physical parameters, such as a wide range of pH, salinity, and temperature, with good performance under industry, and environmental, relevant conditions. This review encompasses studies with promising results related to the use of bacteria in the bioremediation of environments contaminated with azo dyes in the most diverse techniques and parameters, both in environmental and laboratory samples, also addressing their mechanisms and the legislation involving these dyes around the world, showcasing the importance of bacterial bioremediation, specialty in a scenario in an ever-increasing pursuit for sustainable production.

Heterogeneous Structure Regulated by Selection Pressure on Bacterial Adhesion Optimized the Viability Stratification Structure of Electroactive Biofilms

As the core of microbial fuel cells (MFCs), the components and structure of electroactive biofilms (EABs) are essential for MFC performance. Bacterial adhesion plays a vital role in shaping the structure of EABs, but the effect of bacterial adhesion under selection pressure on EABs has not been systematically studied. Here, the response of the composition, structure, and electrochemical performance of EABs to the selective adhesion pressure due to the selective coordination of Fe(III) and Co(II) with thiol and the different affinities for bacteria on hybrid electrodes (Fe₁Co, Fe₄Co, and Fe₁₀Co) were comprehensively investigated. Compared with carbon cloth (CC), the appropriate selective adhesion pressure of Fe₄Co activated the dead inner core of EABs and optimized their viability stratification structure. Both the total viability and the viability of the inner core layer in the Fe₄Co EAB (0.67, 0.70 ± 0.01) were higher than those of the CC (0.46, 0.54 ± 0.01), Fe₁Co (0.50, 0.48 ± 0.03), and Fe₁₀Co (0.51, 0.51 ± 0.03). Moreover, a higher proportion of proteins was detected in the Fe₄Co EAB, enhancing the redox activity of extracellular polymeric substances. Fe₄Co enriched Geobacter and stimulated microbial metabolism. Electrochemical analysis revealed that the Fe₄Co EAB was the most electroactive EAB, with a maximum power density of 2032.4 mW m^-2, which was 1.7, 1.3, and 1.1 times that of the CC (1202.6 mW m^-2), Fe₁Co (1610.3 mW m^-2), and Fe₁₀Co (1824.4 mW m^-2) EABs, respectively. Our findings confirmed that highly active EABs could be formed by imposing selection pressure on bacterial adhesion.

Effects of ferroferric oxide on azo dye degradation in a sulfate-containing anaerobic reactor: From electron transfer capacity and microbial community

Anaerobic decolorization of azo dye in sulfate-containing wastewater has been regarded as an economical and effective method, but it is generally limited by the high concentration of azo dye and accumulation of toxic intermediates. To address this problem, Fe₃O₄ was added to one of the anaerobic reactors to investigate the effects on system performances. Results showed that AO7 removal rate, COD removal rate, and sulfate reduction were enhanced with the addition of Fe₃O₄ under various influent AO7 concentrations (153 mgCOD/L - 1787 mgCOD/L). According to the proposed pathway for the degradation of AO7, more intermediates (2-hydroxy-1,4-naphthoquinone, phthalide, 4-methylphenol) were produced in the presence of Fe₃O₄. The electron transfer capacity of sludge was also increased since Fe₃O₄ could stimulate to secrete humic acid-like organics in EPS. Microbial analysis showed that iron-reducing bacteria like Clostridium and Geobacter were also enriched, which were capable of azo dye and aromatic compounds degradation.

Current advances in microbial fuel cell technology toward removal of organic contaminants - A review

At present, water pollution and demand for clean energy are most pressing global issues. On a daily basis, huge quantity of organic wastes gets released into the water ecosystems, causing health related problems. The need-of-the-hour is to utilize proficient and cheaper techniques for complete removal of harmful organic contaminants from water. In this regard, microbial fuel cell (MFC) has emerged as a promising technique, which can produce useful electrical energy from organic wastes and decontaminate polluted water. Herein, we have systematically reviewed recently published results, observations and progress made on the applications of MFCs in degradation of organic contaminants, including organic synthetic dyes, agro pollutants, health care contaminants and other organics (such as phenols and their derivatives, polyhydrocarbons and caffeine). MFC-based hybrid technologies, including MFC-constructed wetland, MFC-photocatalysis, MFC-catalysis, MFC-Fenton process, etc., developed to obtain high removal efficiency and bioelectricity production simultaneously have been discussed. Further, this review assessed the influence of factors, such as nature of electrode catalysts, organic pollutants, electrolyte, microbes and operational conditions, on the performance of pristine and hybrid MFC reactors in terms of pollutant removal efficiency and power generation simultaneously. Moreover, the limitations and future research directions of MFCs for wastewater treatment have been discussed. Finally, a conclusive summary of the findings has been outlined.

Decoding the roles of extremophilic microbes in the anaerobic environments: Past, Present, and Future

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The inaccessible extreme environments harbor a large majority of anaerobic microbes which remain unknown.

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Anaerobic microbes are used in a variety of industrial applications.

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In the future, metagenomic-assisted techniques can be used to identify novel anaerobic microbes from the unexplored extreme environments.

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Genetic engineering can be used to enhance the efficiency of anaerobic microbes for various processes.

The genome of an organism is directly or indirectly correlated with its environment. Consequently, different microbes have evolved to survive and sustain themselves in a variety of environments, including unusual anaerobic environments. It is believed that their genetic material could have played an important role in the early evolution of their existence in the past. Presently, out of the uncountable number of microbes found in different ecosystems we have been able to discover only one percent of the total communities. A large majority of the microbial populations exists in the most unusual and extreme environments. For instance, many anaerobic bacteria are found in the gastrointestinal tract of humans, soil, and hydrothermal vents. The recent advancements in Metagenomics and Next Generation Sequencing technologies have improved the understanding of their roles in these environments. Presently, anaerobic bacteria are used in various industries associated with biofuels, fermentation, production of enzymes, vaccines, vitamins, and dairy products. This broad applicability brings focus to the significant contribution of their genomes in these functions. Although the anaerobic microbes have become an irreplaceable component of our lives, a major and important section of such anaerobic microbes still remain unexplored. Therefore, it can be said that unlocking the role of the microbial genomes of the anaerobes can be a noteworthy discovery not just for mankind but for the entire biosystem as well.

Biogenic sulfide for azo dye decolorization from textile dyeing wastewater

Azo dye is the most versatile class of dyes used in the textile industry. Although the sulfidogenic process shows superiority in the removal of azo dye, the role of biogenic sulfide produced by sulfate-reducing bacteria (SRB) in the decolorization of azo dye is unclear. This study explored the mechanism of biogenic sulfide for removal of a model azo dye (Direct Red 81 (DR 81)) through biotic and abiotic batch tests with analysis of intermediates of the azo dye degradation. The results showed that biogenic sulfide produced from sulfate reduction directly cleaved two groups of azo bond (-NN-), thereby achieving decolorization. Moreover, the decolorization rate was enhanced by nearly 3-fold (up to 42 ± 1 mg/L-hr; removal efficiency > 99%) by adding an external carbon source or elevating the initial azo dye concentration. This study showed that biogenic sulfide plays an essential role in azo dye decolorization and provides a new avenue for the potential application of biogenic sulfide from the sulfidogenic system for the treatment of azo dye-laden wastewater.

Interrelation between sulphur and conductive materials and its impact on ammonium and organic pollutants removal in electroactive wetlands

This investigation is the first of its kind to evaluate the interrelation of sulphate (SO₄^2-) with conductive materials as well as their individual and synergetic effects on the removal of ammonium and organic pollutants in electroactive wetlands, also known as constructed wetland (CW) - microbial fuel cell (MFC). The role of MFC components in CW was investigated to treat the sulphate containing wastewater under a long-term operation without any toxicity build-up in the system. A comparative study was also performed between CW-MFC and CW, where sulphate containing wastewater (S-replete) and without sulphate wastewater (S-deplete) was assessed. The S-replete showed high NH₄⁺ removal than the S-deplete, and the requesnce of removal was: CW-MFC-replete>CW-MFC-deplete>CW-replete>CW-deplete. The chemical oxygen demand (COD) removal was high in the case of CW-MFC-replete, and the sequence of removal was CW-MFC-replete>CW-MFC-deplete>CW-deplete>CW-replete. X-ray photon spectroscopic study indicates 0.84% sulphur accumulation in CW-MFC-replete and 2.49% in CW-replete, indicating high sulphur precipitation in CW without the MFC component. The high relative abundance of class Deltaproteobacteria (7.3%) in CW-MFC-replete along with increased microbial diversity (Shannon index=3.5) rationalise the symbiosis of sulphate reducing/oxidising microbes and its impact on the treatment performance and electrochemical activity.

Study of membrane fouling mechanism during the phenol degradation in microbial fuel cell and membrane bioreactor coupling system

This study evaluated the feasibility of phenol degradation in microbial fuel cell (MFC) and membrane bioreactor (MBR) coupling system, and explored the mechanism of MBR membrane fouling. Four aspects were researched in open and closed circuit conditions: the degradation capacity of the coupling system, the increase of trans-membrane pressure (TMP), and the adhesion of phenol degradation products and microorganisms on the membrane. The results showed that the degradation of phenol and COD in the closed circuit coupling system was higher than that in the open circuit. The micro-electric field can inhibit the growth of TMP and keep dodecamethylcyclohexasiloxane away from the membrane, meanwhile can also reduce the abundance and species diversity of microorganisms. Nevertheless, the micro-electric field could not completely eliminate the membrane fouling due to the fact that the phenol degradation product of ethanethiol, microorganisms of Proteobacteria and Actinobacteria were more favorable on the membrane.

Recent advancements in azo dye decolorization in bio-electrochemical systems (BESs): Insights into decolorization mechanism and practical application

Recent advances in bio-electrochemical systems (BESs) for azo dye removal are gaining momentum due to having electrode biocarrier and electro-active bacteria that could stimulate decolorization via extracellular electron transfer. Enhanced decolorization performance is observed in most laboratory studies, indicating the great potential of BESs as an alternative to the traditional biological processes or serving as a pre-/post-processing unit to improve the performance of biological processes. It is proven more competitive in environmental friendly than physicochemical methods. While, the successful application of BESs to azo dye-containing wastewater remediation requires a deeper evaluation of its performance, mechanism and typical attributes, and a comprehensive potential evaluation of BESs practical application in terms of economic analysis and technical optimizations. This review is organized to address BESs as a practical option for azo dye removal by analyzing the decolorization mechanisms and involved functional microorganisms, followed by the comparisons of device configurations, operational conditions, and economic evaluation. It further highlights the current hurdles and prospects for the abatement of azo dyes via BES related techniques.

Diversity of Synthetic Dyes from Textile Industries, Discharge Impacts and Treatment Methods

Natural dyes have been used from ancient times for multiple purposes, most importantly in the field of textile dying. The increasing demand and excessive costs of natural dye extraction engendered the discovery of synthetic dyes from petrochemical compounds. Nowadays, they are dominating the textile market, with nearly 8 × 105 tons produced per year due to their wide range of color pigments and consistent coloration. Textile industries consume huge amounts of water in the dyeing processes, making it hard to treat the enormous quantities of this hazardous wastewater. Thus, they have harmful impacts when discharged in non-treated or partially treated forms in the environment (air, soil, plants and water), causing several human diseases. In the present work we focused on synthetic dyes. We started by studying their classification which depended on the nature of the manufactured fiber (cellulose, protein and synthetic fiber dyes). Then, we mentioned the characteristics of synthetic dyes, however, we focused more on their negative impacts on the ecosystem (soil, plants, water and air) and on humans. Lastly, we discussed the applied physical, chemical and biological strategies solely or in combination for textile dye wastewater treatments. Additionally, we described the newly established nanotechnology which achieves complete discharge decontamination.

Generation of electrical energy in a microbial fuel cell coupling acetate oxidation to Fe3+ reduction and isolation of the involved bacteria

An iron reducing enrichment was obtained from sulfate reducing sludge and was evaluated on the capability of reducing Fe³⁺ coupled to acetate oxidation in a microbial fuel cell (MFC). Three molar ratios for acetate/Fe³⁺ were evaluated (2/16, 3.4/27 and 6.9/55 mM). The percentages of Fe³⁺ reduction were in a range of 80-90, 60-70 and 40-50% for the MFCs at closed circuit for the molar ratios of 2/16, 3.4/27 and 6.9/55 mM, respectively. Acetate consumption was in a range of 80-90% in all cases. The results obtained at closed circuit for current density were: 11.37 mA/m², 4.5 mA/m² and 7.37 mA/m² for the molar ratios of 2/16, 3.4/27 and 6.9/55 mM, respectively. Some microorganisms that were isolated and identified in the MFCs were Azospira oryzae, Cupriavidus metallidurans CH34, Enterobacter bugandensis 247BMC, Citrobacter freundii ATCC8090 and Citrobacter murliniae CDC2970-59, these bacteria have been reported as exoelectrogens in MFC and in MFC involving metals removal but not all of them have been reported to utilize acetate as preferred substrate. The results demonstrate that the isolates can utilize acetate as the sole source of carbon and suggest that Fe³⁺ reduction was carried out by a combination of different mechanisms (direct contact and redox mediators) utilized by the bacteria identified in the MFC. Storage of the energy generated from the 2/16 mM MFC system arranged in a series of three demonstrated that it is possible to utilize the energy to charge a battery.

Magnesium ferrite spinels as anode modifier for the treatment of Congo red and energy recovery in a single chambered microbial fuel cell

Magnesium Ferrite (MgFe₂O₄) spinel structures prepared by a solid-state reaction was used as an anode modifier in the microbial fuel cell (MFC) treatment of Congo red dye. The performance of the reactors with unmodified stainless-steel mesh anode (CR-1) and MgFe₂O₄ coated stainless steel mesh anode (CR-2) were tested and compared followed by aerobic treatment. The peak power density was observed to be 295.936 (CR-1) and 430.336 mW/m² (CR-2) revealing increased bioenergy output and better electron transfer in the reactor with the MgFe₂O₄ modified anode. The final decolourisation efficiencies were found to be 92.053% for CR-1 and 98.386% for CR-2. The formation of metabolites (diaminonaphthalene-1-sulfonate, 1-(biphenyl-4-yl)-2-(naphthalene-2-yl) diazene, benzidine and phthalic acid, monoethyl ether) during the anaerobic-aerobic biotreatment of azo dye was confirmed using Gas chromatography coupled Mass spectrometry system. Scanning electron microscopy confirmed a uniform coating of MgFe₂O₄ on the anode surface with evidence of biofilm formation in the system. Electrochemical studies confirmed the superior performance of spinel coated anode with enhanced redox activity. In addition, the charge-discharge studies confirmed the high capacitive nature of the modified electrode improving the electrodes charge holding capacity. The study suggested an effective treatment strategy for the treatment of Congo red dye.

Biochar Derived from Agricultural Wastes as a Means of Facilitating the Degradation of Azo Dyes by Sulfides

Dyes are common contaminants, some of which are teratogenic, carcinogenic, and causative of ecological damage, and dye wastewater often contains toxic sulfides. Biochar has been widely used for the adsorption and catalysis degradation of pollutants, including dyes and sulfides, due to its abundant surface functional groups and large specific surface area. Therefore, the simultaneous treatment of dyes and sulfides with biochar may be a feasible, effective, and novel solution. This study sought to utilize low-cost, environmentally friendly, and widely sourced biochar materials from agricultural wastes such as corn stalk, rice chaff, and bean stalk to promote the reduction of dyes by sulfides. Through the action of different biochars, sulfides can rapidly decompose and transform oxidizing dyes. The RCB800 (rice chaff biochar material prepared at 800 °C) was observed to have the best effect, with a degradation rate of 96.6% in 40 min and 100% in 50 min for methyl orange. This series of materials are highly adaptable to temperature and pH, and the concentration of sulfides has a significant effect on degradation rates. Compared with commercial carbon materials, biochars are similar in terms of their catalytic mechanism and are more economical. Scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption and desorption characterization results indicated that biochar contains more pores, including mesopores, and a sufficient specific surface area, both of which are conducive to the combination of sulfides and dyes with biochar. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy showed that there are oxygen-containing functional groups (examples include quinones and carboxyl groups) on the surface of biochar that promote the reaction of sulfide and dye. The formation of active polysulfides also potentially plays an important role in the degradation reaction. This article outlines a new method for improving the degradation efficiency of azo dyes and sulfides via biochar materials derived from widely sourced agricultural wastes.

Integrated air cathode microbial fuel cell-aerobic bioreactor set-up for enhanced bioelectrodegradation of azo dye Acid Blue 29

In this study, an azo dye (Acid Blue 29 or AB29) was efficiently degraded with acetate as co-substrate into less contaminated biodegraded products using an integrated single chamber microbial fuel cell (SMFC)-aerobic bioreactor set-up. The decolorization efficiencies were varied from 91 ± 2% to 94 ± 1.9% and more than 85% of chemical oxygen demand (COD) removal was achieved for all dye concentrations after different operating time. The highest coulombic efficiency (CE) and cell potential were 3.18 ± 0.45% and 287.2 mV, respectively, for SMFC treating 100 mg L^-1 of AB29. Electrochemical impedance spectroscopy (EIS) revealed that the anode resistance was 0.3 Ω representing an entirely grown biofilm on the anode surface resulted in higher electron transfer rate. Gas chromatography coupled mass spectrometry (GC-MS) investigation demonstrated that initially biodegradation of AB29 started with the cleavage of the azo bond (-N=N-), resulted the biotransformation into aromatic amines. In successive aerobic treatment stage, these amines were biodegraded into lower molecular weight compounds. The 16S rRNA microbial community analysis indicated that at phylum level, both inoculum and dye acclimated cultures were mainly consisting of Proteobacteria which was 27.9, 53.6 and 68.9% in inoculum, suspension and anodic biofilm, respectively. At genus level, both suspension and biofilm contained decolorization as well as electrochemically active bacteria. The outcomes exhibited that the AB29 decolorization would contest with electrogenic bacteria for electrons.

Enhanced performance of microbial fuel cells using Ag nanoparticles modified Co, N co-doped carbon nanosheets as bifunctional cathode catalyst

The slow kinetics of oxygen reduction reaction (ORR) and the formation of biofilm on cathode severely limited the performance of microbial fuel cells (MFCs). An efficient way to enhance the power-generation capacity and long-term stability of MFCs is to develop bifunctional catalyst by incorporating the efficient ORR catalysts with antibacterial ingredient. In this study, the Ag/Co-N-C nanosheets were designed and synthesized by decorating Ag nanoparticles (NPs) onto Co-N-C nanosheets, which were prepared from Zn/Co bimetallic metal-organic framework (ZIF-67/ZIF-8) precursor. The Zn/Co ratio, Ag doping amount and the calcination temperature of the precursor were systematically investigated. The optimum sample Ag/Co-N-C-30 revealed the excellent ORR performance with a half-wave potential of 0.80 V vs. RHE, which was slightly lower than that of Pt/C (0.82 V vs. RHE). The MFCs equipped with Ag/Co-N-C-30 cathode exhibited maximum power density of 548 ± 12.6 mW m^-2 and superior durability even after 1600 h operation. Besides, the selective antimicrobial ability of Ag/Co-N-C-30 was further explored and the aerobic bacteria in cathode biofilm was found to be obviously inhibited by Ag/Co-N-C-30. The results suggested the Ag/Co-N-C nanosheets can serve as a promising cathode catalyst for practical applications of MFCs.

Outlook on the Role of Microbial Fuel Cells in Remediation of Environmental Pollutants with Electricity Generation

A wide variety of pollutants are discharged into water bodies like lakes, rivers, canal, etc. due to the growing world population, industrial development, depletion of water resources, improper disposal of agricultural and native wastes. Water pollution is becoming a severe problem for the whole world from small villages to big cities. The toxic metals and organic dyes pollutants are considered as significant contaminants that cause severe hazards to human beings and aquatic life. The microbial fuel cell (MFC) is the most promising, eco-friendly, and emerging technique. In this technique, microorganisms play an important role in bioremediation of water pollutants simultaneously generating an electric current. In this review, a new approach based on microbial fuel cells for bioremediation of organic dyes and toxic metals has been summarized. This technique offers an alternative with great potential in the field of wastewater treatment. Finally, their applications are discussed to explore the research gaps for future research direction. From a literature survey of more than 170 recent papers, it is evident that MFCs have demonstrated outstanding removal capabilities for various pollutants.

Co-metabolism for enhanced phenol degradation and bioelectricity generation in microbial fuel cell

Co-metabolism is one of the effective approaches to increase the removal of refractory pollutants in microbial fuel cells (MFCs), but studies on the links between the co-substrates and biodegradation remain limited. In this study, four external carbon resources were used as co-substrates for phenol removal and power generation in MFC. The result demonstrated that acetate was the most efficient co-substrate with an initial phenol degradation of 78.8% and the voltage output of 389.0 mV. Polarization curves and cyclic voltammogram analysis indicated that acetate significantly increased the activity of extracellular electron transfer (EET) enzyme of the anodic microorganism, such as cytochrome c OmcA. GC-MS and LC-MS results suggested that phenol was biodegraded via catechol, 2-hydroxymuconic semialdehyde, and pyruvic acid, and these intermediates were reduced apparently in acetate feeding MFC. The microbial community analysis by high-throughput sequencing showed that Acidovorax, Geobacter, and Thauera were predominant species when using acetate as co-substrate. It can be concluded that the efficient removal of phenol was contributed to the positive interactions between electrochemically active bacteria and phenolic degradation bacteria. This study might provide new insight into the positive role of the co-substrate during the treatment of phenolic wastewater by MFC.

Degradation pathways, microbial community and electricity properties analysis of antibiotic sulfamethoxazole by bio-electro-Fenton system

Sulfamethoxazole (SMX) is a general antibiotic that is frequently identified in wastewater and surface water. In this study, the degradation and metabolic pathway of SMX by bio-electro-Fenton systems equipped with a CNT/r-FeOOH cathode were investigated. When initial SMX = 25 mg/L, the removal efficiency of SMX reached 94.66% by the bio-electro-Fenton system. The concentrations of sul1, sul2, sul3, sulA, intI1 and 16S rRNA genes were examined in effluents. Four out of the six ARGs analysed were detected. Among all quantified sul genes, sul1 and sulA were the most abundant. High-throughput sequencing revealed that the microbial communities and relative abundance at the phylum and genus levels were affected by different SMX concentrations. In addition, the intermediates were detected and the possible SMX degradation pathway by the bio-electro-Fenton process in the present system was proposed. Furthermore, the highest power density obtained was 283.32 ± 16.35 mW/m² (SMX = 25 mg/L). This study provides an efficient and cost effective method for degrading antibiotics.