Sequential anaerobic and electro-Fenton processes mediated by W and Mo oxides for degradation/mineralization of azo dye methyl orange in photo assisted microbial fuel cells

The intensification of the degradation and mineralization of the azo dye methyl orange (MO) in contaminated water with simultaneous production of renewable electrical energy was achieved in photo-assisted microbial fuel cells (MFCs) operated sequentially under anaerobic - aerobic processes, in the presence of Fe(III) and W and Mo oxides catalytic species. In this novel process, the W and Mo oxides deposited on the graphite felt cathodes accelerated electron transfer and the reductive decolorization of MO. Simultaneously, the mineralization of MO and intermediate products was intensified by the production of hydroxyl radicals (HO[rad]) produced by (i) the photoreduction of Fe(III) to Fe(II), and by (ii) the reaction of the photochemically and electrochemically produced Fe(II) with hydrogen peroxide, which was produced in-situ during the aerobic stage. Under anaerobic conditions, the reductive decolorization of MO was driven by cathodic electrons, while the partial oxidation of the intermediates proceeded through holes oxidation, producing N,N-dimethyl-p-phenylenediamine. In contrast, under aerobic conditions superoxide radicals (O2[rad]−) were predominant to HO[rad], forming 4-hydroxy-N,N-dimethylaniline. In the presence of Fe(III) and under aerobic conditions, the oxidation of the intermediate products driven by HO[rad] superseded that of O2[rad]−, yielding phenol and amines, via the oxidation of 4-hydroxy-N,N-dimethylaniline and N,N-dimethyl-p-phenylenediamine. These sequential anaerobic and electro-Fenton processes led to the production of benzene and significantly faster oxidation reactions, compared to either the anaerobic or the aerobic operation in the presence of Fe(III). Complete degradation and mineralization (96.8 ± 3.5%) of MO (20 mg/L) with simultaneous electricity production (0.0002 kW h/kg MO) was therefore achieved with sequential anaerobic (20 min) - aerobic (100 min) operation in the presence of Fe(III) (10 mg/L). This study demonstrates an alternative and environmentally benign approach for efficient remediation of azo dye contaminated water with simultaneous production of renewable energy

Electricity generation and bivalent copper reduction as a function of operation time and cathode electrode material in microbial fuel cells

The performance of carbon rod (CR), titanium sheet (TS), stainless steel woven mesh (SSM) and copper sheet (CS) cathode materials are investigated in microbial fuel cells (MFCs) for simultaneous electricity generation and Cu(II) reduction, in multiple batch cycle operations. After 12 cycles, the MFC with CR exhibits 55% reduction in the maximum power density and 76% increase in Cu(II) removal. In contrast, the TS and SSM cathodes at cycle 12 show maximum power densities of 1.7 (TS) and 3.4 (SSM) times, and Cu(II) removal of 1.2 (TS) and 1.3 (SSM) times higher than those observed during the first cycle. Diffusional resistance in the TS and SSM cathodes is found to appreciably decrease over time due to the copper deposition. In contrast to CR, TS and SSM, the cathode made with CS is heavily corroded in the first cycle, exhibiting significant reduction in both the maximum power density and Cu(II) removal at cycle 2, after which the performance stabilizes. These results demonstrate that the initial deposition of copper on the cathodes of MFCs is crucial for efficient and continuous Cu(II) reduction and electricity generation over prolonged time. This effect is closely associated with the nature of the cathode material. Among the materials examined, the SSM is the most effective and inexpensive cathode for practical use in MFCs

Impact of Fe(III) as an effective electron-shuttle mediator for enhanced Cr(VI) reduction in microbial fuel cells: Reduction of diffusional resistances and cathode overpotentials

© 2016 Elsevier B.V.The role of Fe(III) was investigated as an electron-shuttle mediator to enhance the reduction rate of the toxic heavy metal hexavalent chromium (Cr(VI)) in wastewaters, using microbial fuel cells (MFCs). The direct reduction of chromate (CrO4−) and dichromate (Cr2O72−) anions in MFCs was hampered by the electrical repulsion between the negatively charged cathode and Cr(VI) functional groups. In contrast, in the presence of Fe(III), the conversion of Cr(VI) and the cathodic coulombic efficiency in the MFCs were 65.6% and 81.7%, respectively, 1.6 times and 1.4 folds as those recorded in the absence of Fe(III). Multiple analytical approaches, including linear sweep voltammetry, Tafel plot, cyclic voltammetry, electrochemical impedance spectroscopy and kinetic calculations demonstrated that the complete reduction of Cr(VI) occurred through an indirect mechanism mediated by Fe(III). The direct reduction of Cr(VI) with cathode electrons in the presence of Fe(III) was insignificant. Fe(III) played a critical role in decreasing both the diffusional resistance of Cr(VI) species and the overpotential for Cr(VI) reduction. This study demonstrated that the reduction of Cr(VI) in MFCs was effective in the presence of Fe(III), providing an alternative and environmentally benign approach for efficient remediation of Cr(VI) contaminated sites with simultaneous production of renewable energy

Correlation between circuital current, Cu(II) reduction and cellular electron transfer in EAB isolated from Cu(II)-reduced biocathodes of microbial fuel cells

The performance of four indigenous electrochemically active bacteria (EAB) (Stenotrophomonas maltophilia JY1, Citrobacter sp. JY3, Pseudomonas aeruginosa JY5 and Stenotrophomonas sp. JY6) was evaluated for Cu(II) reduction on the cathodes of microbial fuel cells (MFCs). These EAB were isolated from well adapted mixed cultures on the MFC cathodes operated for Cu(II) reduction. The relationship between circuital current, Cu(II) reduction rate, and cellular electron transfer processes was investigated from a mechanistic point of view using X-ray photoelectron spectroscopy, scanning electronic microscopy coupled with energy dispersive X-ray spectrometry, linear sweep voltammetry and cyclic voltammetry. JY1 and JY5 exhibited a weak correlation between circuital current and Cu(II) reduction. A much stronger correlation was observed for JY3 followed by JY6, demonstrating the relationship between circuital current and Cu(II) reduction for these species. In the presence of electron transfer inhibitors (2,4-dinitrophenol or rotenone), significant inhibition on JY6 activity and a weak effect on JY1, JY3 and JY5 was observed, confirming a strong correlation between cellular electron transfer processes and either Cu(II) reduction or circuital current. This study provides evidence of the diverse functions played by these EAB, and adds to a deeper understanding of the capabilities exerted by diverse EAB associated with Cu(II) reduction

Electrosynthesis of acetate from inorganic carbon (HCO₃⁻) with simultaneous hydrogen production and Cd(II) removal in multifunctional microbial electrosynthesis systems (MES)

The simultaneous production of acetate from bicarbonate (from CO2 sequestration) and hydrogen gas, with concomitant removal of Cd(II) heavy metal in water is demonstrated in multifunctional metallurgical microbial electrosynthesis systems (MES) incorporating Cd(II) tolerant electrochemically active bacteria (EAB) (Ochrobactrum sp. X1, Pseudomonas sp. X3, Pseudomonas delhiensis X5, and Ochrobactrum anthropi X7). Strain X5 favored the production of acetate, while X7 preferred the production of hydrogen. The rate of Cd(II) removal by all EAB (1.20–1.32 mg/L/h), and the rates of acetate production by X5 (29.4 mg/L/d) and hydrogen evolution by X7 (0.0187 m3/m3/d) increased in the presence of a circuital current. The production of acetate and hydrogen was regulated by the release of extracellular polymeric substances (EPS), which also exhibited invariable catalytic activity toward the reduction of Cd(II) to Cd(0). The intracellular activities of glutathione (GSH), catalase (CAT), superoxide dismutase (SOD) and dehydrogenase were altered by the circuital current and Cd(II) concentration, and these regulated the products distribution. Such understanding enables the targeted manipulation of the MES operational conditions that favor the production of acetate from CO2 sequestration with simultaneous hydrogen production and removal/recovery of Cd(II) from metal-contaminated and organics-barren waters

Intensified degradation and mineralization of antibiotic metronidazole in photo-assisted microbial fuel cells with Mo-W catalytic cathodes under anaerobic or aerobic conditions in the presence of Fe(III)

A novel strategy to intensify the degradation and mineralization of the antibiotic drug metronidazole (MNZ) in water with simultaneous production of renewable electrical energy was achieved in photo-assisted microbial fuel cells (MFCs). In this system Mo and W catalytic species immobilized onto a graphite felt cathode intensified the cathodic reduction of MNZ under anaerobic conditions and the oxidation of MNZ under aerobic conditions. The aerobic oxidation process was further accelerated in the presence of Fe(III), realizing a combined photo-assisted MFCs and Fenton-MFCs process. The highest rates of MNZ degradation (94.5 ± 1.4%; 75.6 ± 1.1 mg/L/h) and mineralization (89.5 ± 1.1%; 71.6 ± 0.9 mg/L/h), and power production (251 mW/m2; 0.015 kWh/m3; 0.22 kWh/kg COD) were achieved at a Mo/W loading of 0.18 mg/cm2with a Mo/W ratio of 0.17:1.0, in the presence of 10 mg/L of Fe(III) and at an incident photon flux of 23.3 mW/cm2. Photo-generated holes were directly involved into the oxidation of MNZ under anaerobic conditions. Conversely, under aerobic conditions, the photo-generated electrons favored the production of O2[rad]−over [rad]OH, while in the presence of Fe(III), [rad]OH was predominant over O2[rad]−, explaining the intensification of the MNZ mineralization observed. This study demonstrates an alternative and environmentally benign approach for the intensification of the removal of the antibiotic MNZ in water and possibly other contaminants of emerging concern by combining photo-assisted MFCs and Fenton-MFCs in a single process with simultaneous production of renewable electrical energy

Physiological response of electroactive bacteria via secretion of extracellular polymeric substances in microbial electrochemical processes: a review

Microbial electrochemical processes involving microbial catalyzed electrochemical reactions are directly/indirectly related to the electroactive bacteria (EAB) physiological secretion of extracellular polymeric substances (EPS) with favorable biofilm dispersion. EPS harbor cytochrome-like substances, and thus accelerate extracellular electron transfer (EET) processes resulting in the simultaneous removal of environmental contaminants or in the conversion of dissolved CO2 to key-block chemicals. This review holistically documents case-studies in the last five years focusing on EAB and their physiological release of EPS in response to external stimuli. The important role played by EPS in the performance of microbial electrochemical systems and the relationship of the EPS compositional diversity with the process parameters is reviewed. Targeting the physiological response of electroactive bacteria towards the release of EPS with precise compositional diversity provides tremendous opportunities for optimizing microbial electrochemical processes. This review provides kernels, quantitative approaches and promising advanced techniques guiding further research directions in this exciting field.</p

Mixotrophic bacteria for environmental detoxification of contaminated waste and wastewater

Mixotrophic bacteria provide a desirable alternative to the use of classical heterotrophic or chemolithoautotrophic bacteria in environmental technology, particularly under limiting nutrients conditions. Their bi-modal ability of adapting to inorganic or organic carbon feed and sulfur, nitrogen, or even heavy metal stress conditions are attractive features to achieve efficient bacterial activity and favorable operation conditions for the environmental detoxification or remediation of contaminated waste and wastewater. This review provides an overview on the state of the art and summarizes the metabolic traits of the most promising and emerging non-model mixotrophic bacteria for the environmental detoxification of contaminated wastewater and waste containing excess amounts of limiting nutrients. Although mixotrophic bacteria usually function with low organic carbon sources, the unusual capabilities of mixotrophic electroactive exoelectrogens and electrotrophs in bioelectrochemical systems and in microbial electrosynthesis for accelerating simultaneous metabolism of inorganic or organic C and N, S or heavy metals are reviewed. The identification of the mixotrophic properties of electroactive bacteria and their capability to drive mono- or bidirectional electron transfer processes are highly exciting and promising aspects. These aspects provide an appealing potential for unearthing new mixotrophic exoelectrogens and electrotrophs, and thus inspire the next generation of microbial electrochemical technology and mixotrophic bacterial metabolic engineering. Key points: • Mixotrophic bacteria efficiently and simultaneously remove C and N, S or heavy metals. • Exoelectrogens and electrotrophs accelerate metabolism of C and N, S or heavy metals. • New mixotrophic exoelectrogens and electrotrophs should be discovered and exploited. Graphical abstract: [Figure not available: see fulltext.]

An Organic Molecule Modulated Chemoselective Cyclization of Alkynyl Nitriles Tethered to 2‑Alkyl Substituted Chromones with Multireactive Sites

A small organic molecule phenylacetonitrile promoted chemoselective cyclization of (chromen-3-yl)alkynylnitriles to generate a novel tetracyclic or tricyclic chromone scaffold has been discovered. Importantly, the phenylacetonitrile serves as an anion transfer reagent changing the normal reaction of the substrate

Phase Transfer Reagent Promoted Tandem Ring-Opening and Ring-Closing Reactions of Unique 3‑(1-Alkynyl) Chromones

A phase transfer reagent promoted tandem ring-opening and ring-closing reaction of 3-(1-alkynyl) chromones has been developed. This process remarkably generates functionalized 3-acyl-2-substituted chromones. Interestingly, when 3-(hepta-1,6-diyn-1-yl)­chromone derivatives are applied, a novel tetracyclic chromone scaffold can be obtained by a further intramolecular 4 + 2 cyclization