In situ self-induced electrical stimulation to plants: Modulates morphogenesis, photosynthesis and gene expression in Vigna radiata and Cicer arietinum

Specific stimuli to plants influence intracellular and intercellular communications, activation of ion channels, gene expression, growth and development. The functional role of self-induced in situ electrical stimuli at the rhizosphere of the plant by placing electrode assembly in a defined circuit mode was studied on the growth and development of Vigna radiata and Cicer arietinum plants. Experiments were designed with three-circuit mode configurational variations (CC-P, OC-P and SC-P) and compared with the relative performance of control system (non-potential). The plants cultivated under the in situ electrical stimuli (low-current) showed a marked influence on growth and photosynthetic performance of the plants. CC-P operation showed improved vegetative growth, characterized by increased roots, shoots and biomass along with accelerated plant growth from seed germination to vegetation, flowering and pod formation leading towards earlier and more robust flowering compared to control system. Plants also showed higher aquaporin gene expression levels in CC-P operation. The control operation showed 10 days additional maturation time compared to CC-P operation. The strategy can be beneficially applied to augment the bioremediation capacity of complex pollutants with reference to phytoremediation or constructed wetland systems where the plant and its roots are the main enabler apart from agriculture applications specific to nursery-raised or transplanted plants.

Performance of nitrification-denitrification and denitrifying phosphorus removal driven by in-situ generated biogenic manganese oxides in a moving bed biofilm reactor

Simultaneous removal of NH₄⁺-N, NO₃^--N, COD, and P by manganese redox cycling in nutrient wastewater was established with two moving bed biofilm reactors (MBBRs) with in-situ generated biogenic manganese oxides (BioMnOx) and non-BioMnOx. In-situ generated BioMnOx preferentially promoted the denitrification, and the average removal of NO₃^--N, NH₄⁺-N, and TN in the experimental MBBR with BioMnOx increased to 89.00%, 70.64%, and 76.06% compared with the control MBBR with non-BioMnOx. The relevant enzymes activity, extracellular polymeric substance (EPS), electron transport system activity (ETSA), and reactive oxygen species (ROS) were investigated. The element valence and morphology of purified BioMnOx were characterized by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM), as well as the effect of BioMnOx on nitrogen and phosphorus removal. The results suggested that BioMnOx could improve nitrogen conversion. Electrochemical characteristic and microbial community were detected. This study provided a new strategy for nutrients removal in BioMnOx-mediated wastewater treatment.

Exploring necessity to pre-treat organic fraction of waste prior to use in an earthen MFC modified with bentonite

In this study, the addition of bentonite at different proportions as clay minerals and various thicknesses (4, 5, and 6 mm) of ceramic membranes were evaluated for proton and oxygen mass transfer coefficients. Bentonite (20% and 4 mm) was found to be optimum and was then employed to assess earthen microbial fuel cell (EMFC) performance for different substrates (kitchen waste (KW) slurry and leachate) under batch mode. Both substrates were added in different concentrations of chemical oxygen demand (COD), i.e., 18, 15.2, 12.5, 9.7, and 6.9 g/L to EMFCs. The EMFC achieved superior organic removals for leachate (>96%). Intriguingly, the volatile fatty acids (VFAs) generation and consumption were different for each substrate. Each system expressed affinity towards acetic acid, but limited VFAs (acetic and propionic) were generated by KW while leachate generated acetic, propionic, and butyric. The leachate concentration having COD of 15.2 g/L produced the highest power density of 586.5 ± 38.8 mW/m³, while for KW, only 41.5 mW/m³ (6.9 g/L of COD for KW) was obtained. The study consolidates the need for an intermediate step to pre-treat the organic fraction of waste before its use for resource recovery. Bentonite was found as an effective clay mineral for manufacturing ceramic membranes.

Let's chat: Communication between electroactive microorganisms

Electroactive microorganisms can exchange electrons with other cells or conductive interfaces in their extracellular environment. This property opens the way to a broad range of practical biotechnological applications, from manufacturing sustainable chemicals via electrosynthesis, to bioenergy, bioelectronics or improved, low-energy demanding wastewater treatments. Besides, electroactive microorganisms play key roles in environmental bioremediation, significantly impacting process efficiencies. This review highlights our present knowledge on microbial interactions promoting the communication between electroactive microorganisms in a biofilm on an electrode in bioelectrochemical systems (BES). Furthermore, the immediate knowledge gaps that must be closed to develop novel technologies will also be acknowledged.

Self-sustained recovery of silver with stainless-steel based Cobalt/Molybdenum/Manganese polycrystalline catalytic electrode in bio-electroreduction microbial fuel cell (BEMFC)

In this study, a novel bio-electroreduction microbial fuel cell (BEMFC) assisted by stainless-steel based Cobalt/Molybdenum/Manganese (Co/Mo/Mn-SS) polycrystalline catalytic electrode was used to achieve high recovery to silver. The exoelectrogens (Shewanella sp. etc.) using organic wastewater (the inflow was controlled at 1.2 L d^-1) as nutrient matrix in the anode chamber generated electrons, while silver ions were simultaneously electroreduced and electrodeposited on the surface of the catalytic electrode as electron acceptors. Silver nanoplates could be observed directly. The products of electroreduction on the cathode were analyzed by Scanning Electron Microscopy (SEM), Transmission Electron Microscope (TEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffractometer (XRD), and the results of electrochemical characterization confirmed the existence of silver in the products. In the operation, the silver ions were in-situ recovered and enriched from the initial concentration of 20-300 mg L^-1 to almost complete recovery (8-18 h), with the maximum power density of 1008.2 mW m^-2 and 5.5 A m^-2 current density. The recovery efficiency of silver in the BEMFC using the Co/Mo/Mn-SS electrode was up to 9.60 kg m^-2h^-1, and the energy efficiency was 27.8 kg kWh^-1. Under the continuous flow operation mode, the BEMFC still achieved 90.2% recovery efficiency of the silver.

Performance of Yeast Microbial Fuel Cell Integrated with Sugarcane Bagasse Fermentation for COD Reduction and Electricity Generation

The purpose of this analysis is to evaluate the efficiency of the Microbial Fuel Cell (MFC) system incorporated with the fermentation process, with the aim of reducing COD and generating electricity, using sugarcane bagasse extract as a substrate, in the presence and absence of sugarcane fibers. There is a possibility of turning bagasse extract into renewable bioenergy to promote the sustainability of the environment and energy. As a result, the integration of liquid fermentation (LF) with MFC has improved efficiency compared to semi-solid state fermentation (S-SSF). The maximum power generated was 14.88 mW/m2, with an average COD removal of 39.68% per cycle. The variation margin of the liquid fermentation pH readings remained slightly decrease, with a slight deflection of +0.14 occurring from 4.33. With the absence of bagasse fibers, biofilm can grow freely on the anode surface so that the transfer of electrons is fast and produces a relatively high current. Experimental data showed a positive potential after an effective integration of the LF and MFC systems in the handling of waste. The product is then simultaneously converted into electrical energy. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Emerging trends in microbial fuel cell diversification-Critical analysis

Global need for transformation from fossil-based to bio-based economy is constantly emerging for the production of low-carbon/renewable energy/products. Microbial fuel cell (MFC) catalysed by bio-electrochemical process gained significant attention initially for its unique potential to generate energy. Diversification of MFC is an emerging trend in the context of prioritising/enhancing product output while exploring the mechanism specificity of individual processes. Bioelectrochemical treatment system (BET), microbial electrosynthesis system (MES), bioelectrochemical system (BES), electro-fermentation (EF), microbial desalination cell (MDC), microbial electrolysis cell (MEC) and electro-methanogenesis (EM) are the diversified MFC systems that are being researched actively. Owing to its broad diversification, MFC domain is increasing its potential credibility as a platform technology. Microbial catalyzed electrochemical reactions are the key which directly/indirectly are proportionally linked to electrometabolic activity of microorganisms towards final anticipated output. This review intends to holistically document the mechanisms, applications and current trends of MFC diversifications towards multi-faced applications.

Blending of microbial inocula: An effective strategy for performance enhancement of clayware Biophotovoltaics microbial fuel cells

Performance of clayware Biophotovoltaics (BPVs) with three variants of inocula namely anoxygenic photosynthetic bacteria (APB) rich Effective microbes (EM), Up-flow anaerobic sludge blanket reactor (UASB) sludge, SUPER-MIX the blend of EM and UASB inoculum were evaluated on the basis of electrical output and pollutant removal. SUPER-MIX inocula with microbial community comprising of 28.42% APB and 71.58% of other microbes resulted in peak power density of 275 mW/m², 69.3 ± 1.74% Coulombic efficiency and 91 ± 3.96% organic matter removal. The higher performance of the SUPER-MIX than EM and UASB inocula was due to the syntrophic associations of the various APBs and other heterogenous microorganisms in perfect blend which improved biocatalytic electron transfer, electro-kinetic activities with higher redox current and bio-capacitance. The promising performance of clayware BPVs with SUPER-MIX inocula indicate the possibility of BPVs to move towards the scale-up process to minimize the investment towards pure culture by effective blending strategies of inocula.

Electrotrophy of biocathodes regulates microbial-electro-catalyzation of CO2 to fatty acids in single chambered system

Microbial electrochemical conversion of CO₂ to value-added products needs effectual biocathodes. In this study, three different working electrodes (biocathode) namely carbon cloth (CC, MES1), stainless steel mesh (SS, MES2) and hybrid electrode (CC + SS, MES3) were evaluated in membrane-less single-chambered Microbial electrosynthesis systems (MESs). Performance of MES was assessed by total volatile fatty acids (VFA) productivity and, reductive current generations upon continuous poised potential (-0.4 V vs. Ag/AgCl (3.5 M KCl)). MES3 showed higher VFA synthesis (CC + SS; 1.4 g VFA/L), followed by MES1 (CC; 1.1 g VFA/L) and MES2 (SS; 0.8 g VFA/L) with corresponding reductive current generation of -1.13 mA, -2.74 mA and -0.39 mA. Electro-kinetics revealed the biocathode efficacy towards enhanced electrotrophy with confined electron losses by regulating electron flux in the system. The study infers the potential of hybrid electrode as an efficient biocathode for the reduction of CO₂ to VFA synthesis.

Cathodic selenium recovery in bioelectrochemical system: Regulatory influence on anodic electrogenic activity

Metal(loid)s are used in various industrial activities and widely spread across the environmental settings in various forms and concentrations. Extended releases of metal(loid)s above the regulatory levels cause environmental and health hazards disturbing the ecological balance. Innovative processes for treating the metal(loid)-contaminated sites and recovery of metal(loid)s from disposed waste streams employing biotechnological routes provide a sustainable way forward. Conventional metal recovery technologies demand high energy and/or resource inputs, which are either uneconomic or unsustainable. Microbial electrochemical systems are promising for removal and recovery of metal(loid)s from metal(loid)-laden wastewaters. In this communication, a bioelectrochemical system (BES) was designed and operated with selenium (Se) oxyanion at varied concentrations as terminal electron acceptor (TEA) for reduction of selenite (Se⁴⁺) to elemental selenium (Se⁰) in the abiotic cathode chamber. The influence of varied concentrations of Se⁴⁺ towards Se⁰ recovery at the cathode was also evaluated for its regulatory role on the electrometabolism of anode-respiring bacteria. This study observed 26.4% Se⁰ recovery (cathode; selenite removal efficiency: 73.6%) along with organic substrate degradation of 74% (anode). With increase in the initial selenite concentration, there was a proportional increase in the dehydrogenase activity. Bioelectrochemical characterization depicted increased anodic electrogenic performance with the influence of varied Se⁴⁺ concentrations as TEA and resulted in a maximum power density of 0.034 W/m². The selenite reduction (cathode) was evaluated through spectroscopic, compositional and structural analysis. X-ray diffraction and Raman spectroscopy showed the amorphous nature, while Energy Dispersive X-ray spectroscopy confirmed precipitates of the deposited Se⁰ recovered from the cathode chamber. Scanning electron microscopic images clearly depicted the Se⁰ depositions (spherical shaped; sized approximately 200 nm in diameter) on the electrode and cathode chamber. This study showed the potential of BES in converting soluble Se⁴⁺ to insoluble Se⁰ at the abiotic cathode for metal recovery.

Synergy of anoxic microenvironment and facultative anaerobes on acidogenic metabolism in a self-induced electrofermentation system

Metabolic potential of two different cultures, facultative (FB) and strict anaerobes (AB) under two microenvironments [anoxic (ANOX) and anaerobic (ANA)] was evaluated to understand acidogenic fermentation in a self-induced electrofermentation (EF) system for the production of short-chain fatty acids (SCFA: C2-C4) and biogas. ANA condition positively influenced FB and AB metabolism towards higher acetic (C2:2390 mg/L) and propionic acid (C3: 717 mg/L) production, while butyric acid (C4:1481 mg/L) favored ANOX microenvironment (AB). ANOX microenvironment showed a better self-induced potential compared to ANA metabolism (0.46 V (FB_ANOX); 0.45 V (AB_ANOX)). An improved H₂ (>30%) fraction was noticed with FB while CH₄ production was found favourable with AB. The study illustrated the role of system microenvironment in association with metabolic function towards regulating electrofermentation towards specific products synthesis.

Anodic metabolic activity regulates the desalination efficiency in microbial catalysed electrochemical system

Anodic metabolic rate showed regulatory influence on the desalination performance of microbial desalination cell (MDC) under open (OC) and closed circuit (CC) operations. In this study, three MDCs were tested for desalination with three different organic substrate loads 1500 ± 55 mg/L in MDC-A; 3500 ± 10 g/L in MDC-B; 4500 ± 12 g/L in MDC-C. Higher desalination and substrate removal rates were observed in CC than OC. Average desalination was MDC-C (51.4%-CC) > MDC-B (47.3%-CC) > MDC-A (45.3%-CC) and COD removal efficiencies were MDC-C (68.4%-CC) > MDC-B (64.4%-CC) > MDC-A (51.9%-CC). Increase in organic load resulted in higher desalination efficiency which was due to higher electrochemical and ionic gradient apart from anodic metabolic activity. The voltage and current density were observed to be maximum in MDC-C (685 mV; 2.16 mA/m²) followed by MDC-B (598 mV; 1.98 mA/m²) and MDC-A (501 mV; 1.76 mA/m²). This study demonstrated that the MDCs performance can be regulated by varying organic load and circuitry modes.

Bio-electrochemical power generation in petrochemical wastewater fed microbial fuel cell

The petrochemical wastewater (PCW) from acrylic acid plants possesses a very high chemical oxygen demand (COD) due to the presence of acrylic acid along with other organic acids. The treatment of PCW by conventional aerobic and anaerobic methods is energy intensive. Therefore, the treatment of PCW with concurrent power generation by employing microbial fuel cell (MFC) could be a potential alternative to solve the energy and environmental issues. This study demonstrates the potentiality of PCW from acrylic acid plant with an initial COD of 45,000 mg L^-1 generating maximum power density of 850 mW m^-2 at a current density of 1500 mA m^-2 using acclimatized anaerobic sludge (AS) as biocatalyst. The predominant microbes present in acclimatized AS were identified using Biolog GEN III analysis, which include the electrogenic genera namely Pseudomonas spp. and Bacillus spp. along with methanogenic archea Methanobacterium spp. The mechanism of electron transfer was elucidated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) which clearly demonstrated the natural metabolite-based electron transfer across the electrode/biofilm/solution interface. The abundance of the electron shuttle metabolites was increased with the microbial growth in the bulk solution as well as in the biofilm leading to a high power generation. The COD removal efficiency and the coulombic efficiency (CE) were found to be 40% and 21%, respectively after 11 days of operation using initial COD of 45,000 mg L^-1. The low COD removal efficiency could drastically be increased to 82% when the initial COD of PCW was 5000 mg L^-1 generating a power density of 150 mW m^-2. The current work proves the feasibility of the MFC for the treatment of acrylic acid plant PCW using acclimatized anaerobic sludge (AS) as a biocatalyst.

Strategies for improving the electroactivity and specific metabolic functionality of microorganisms for various microbial electrochemical technologies

Electroactive microorganisms, which possess extracellular electron transfer (EET) capabilities, are the basis of microbial electrochemical technologies (METs) such as microbial fuel and electrolysis cells. These are considered for several applications ranging from the energy-efficient treatment of waste streams to the production of value-added chemicals and fuels, bioremediation, and biosensing. Various aspects related to the microorganisms, electrodes, separators, reactor design, and operational or process parameters influence the overall functioning of METs. The most fundamental and critical performance-determining factor is, however, the microorganism-electrode interactions. Modification of the electrode surfaces and microorganisms for optimizing their interactions has therefore been the major MET research focus area over the last decade. In the case of microorganisms, primarily their EET mechanisms and efficiencies along with the biofilm formation capabilities, collectively considered as microbial electroactivity, affect their interactions with the electrodes. In addition to electroactivity, the specific metabolic or biochemical functionality of microorganisms is equally crucial to the target MET application. In this article, we present the major strategies that are used to enhance the electroactivity and specific functionality of microorganisms pertaining to both anodic and cathodic processes of METs. These include simple physical methods based on the use of heat and magnetic field along with chemical, electrochemical, and growth media amendment approaches to the complex procedure-based microbial bioaugmentation, co-culture, and cell immobilization or entrapment, and advanced toolkit-based biofilm engineering, genetic modifications, and synthetic biology strategies. We further discuss the applicability and limitations of these strategies and possible future research directions for advancing the highly promising microbial electrochemistry-driven biotechnology.

Purification and Characterization of NDH-2 Protein and Elucidating Its Role in Extracellular Electron Transport and Bioelectrogenic Activity

In microbial electrochemical systems, transport of electrons from bacteria to an electrode is the key to its functioning. However, the roles of several electron transport proteins, especially the membrane-bound dehydrogenases which link cellular metabolism to EET pathway are yet to be identified. NDH-2 is a non-proton pumping NADH dehydrogenase located in the inner membrane of several bacteria like Bacillus subtilis, Escherichia coli, etc. Unlike NADH dehydrogenase I, NDH-2 is not impeded by a high proton motive force thus helping in the increase of metabolic flux and carbon utilization. In the current study, NADH dehydrogenase II protein (NDH-2) was heterologously expressed from B. subtilis into E. coli BL21 (DE3) for enhancing electron flux through EET pathway and to understand its role in bioelectrogenesis. We found that E. coli expressing NDH-2 has increased the electron flux through EET and has shown a ninefold increase in current (4.7 μA) production when compared to wild strain with empty vector (0.52 μA). Furthermore, expression of NDH-2 also resulted in increased biofilm formation which can be corroborated with the decrease in charge transfer resistance of NDH-2 strain and increased NADH oxidation. It was also found that NDH-2 strain can reduce ferric citrate at a higher rate than wild type strain suggesting increased electron flux through electron transport chain due to NADH dehydrogenase II activity. Purified NDH-2 was found to be ∼42 kDa and has FAD as a cofactor. This work demonstrates that the primary dehydrogenases like NADH dehydrogenases can be overexpressed to increase the electron flux in EET pathway which can further enhance the microbial fuel cells performance.

Suppressing methanogens and enriching electrogens in bioelectrochemical systems

Suppression of methanogens is considered as one of the main challenges in achieving the practical application of several types of bioelectrochemical system (BES). Feasibility of mixed culture as an inoculum in BES is mainly restricted by methanogenic population. Methanogens compete with electrogens (in bioanodes) or acetogens (in biocathodes) for substrate which results in diminishing Coulombic efficiency. Selection of particular inoculum pretreatment method affects the microbial diversity in anodic/cathodic microenvironments and hence the performance of BES. This review discusses various physical, chemical and biological pretreatment methods for suppressing the growth of methanogens. Selective microbial enrichment in anodic/cathodic biofilm can be promoted with bioaugmentation and/or applied external potential approach to harvest maximum Coulombs from the substrate. For field application of BES, physical pretreatment methods can be proposed with intermittent addition of chemical inhibitors and conversion of methane to electricity in order to make the process inexpensive along with recovering the maximum energy.

Comparative analysis of microbial fuel cell based biosensors developed with a mixed culture and Shewanella loihica PV-4 and underlying biological mechanism

Microbial fuel cell based biosensors (MFC-biosensors) utilize anode biofilms as biological recognition elements to monitor biochemical oxygen demand (BOD) and biotoxicity. However, the relatively poor sensitivity constrains the application of MFC-biosensors. To address this limitation, this study provided a systematic comparison of sensitivity between the MFC-biosensors constructed with two inocula. Higher biomass density and viability were both observed in the anode biofilm of the mixed culture MFC, which resulted in better sensitivity for BOD assessment. Compared with using mixed culture as inoculum, the anode biofilm developed with Shewanella loihica PV-4 presented lower content of extracellular polymeric substances and poorer ability to secrete protein under toxic shocks. Moreover, the looser structure in the S. loihica PV-4 biofilm further facilitated its susceptibilities to toxic agents. Therefore, the MFC-biosensor with a pure culture of S. loihica PV-4 delivered higher sensitivity for biotoxicity monitoring. This study proposed a new perspective to enhance sensor performance.

Steamed cake-derived 3D carbon foam with surface anchored carbon nanoparticles as freestanding anodes for high-performance microbial fuel cells

Anode design is highly significant for microbial fuel cells, since it simultaneously serves as the scaffold for electroactive microorganisms and as a medium for electron migration. In this study, a stiff 3D carbon foam with surface anchored nitrogen-containing carbon nanoparticles was facilely constructed via in-situ polyaniline coating of carbonized steamed cake prior to the carbonization process. The resultant product was determined to be an excellent freestanding anode that enabled the microbial fuel cell to deliver a maximum power density of up to 1307 mW/m², which significantly outperformed its non-coated counterpart, the widely used commercial carbon felt. Further investigations revealed that the overall performance enhancement was associated with the open porosity, enlarged electroactive surface, increased biocompatibility, and decreased electric resistance of the anode scaffold. This promising anode material would offer a green and economical option for fabricating high-performance microbial fuel cell-based devices towards various ends.

Microbial mediated desalination for ground water softening with simultaneous power generation

A novel three-chambered microbial desalination cell (MDC) was designed for evaluating desalination of synthetic ground water with simultaneous energy generation and resource recovery. The specific design enabled efficient interelectrode communication by reducing the distance of separation and also maintained an appropriate surface area to volume ratio. MDC were evaluated in different circuitry modes (open and closed) to assess the desalination efficiency, bioelectricity generation, resource recovery, substrate utilization and bioelectrokinetics. The closed circuit operation has showed efficient desalination efficiency (51.5%) and substrate utilization (70%). Owing to the effective electron transfer kinetics, closed circuit mode of operation showed effective desalination of the synthetic ground water with simultaneous power production (0.35W/m²). Circuitry specific biocatalyst activity was observed with higher peak currents (10.1mA; -5.98mA) in closed circuit mode. MDC can function as sustainable and alternative solution for ground and surface water treatment with power productivity and resource recovery.

A monolithic three-dimensional macroporous graphene anode with low cost for high performance microbial fuel cells

Monolithic 3D-G which is inflexible and has a macroporous structure, crumpled matrix, good conductivity and low cost enhanced the electrogenesis of a MFC.

Bioelectrochemical reduction of volatile fatty acids in anaerobic digestion effluent for the production of biofuels

This study proves for the first time the feasibility of biofuel production from anaerobic digestion effluent via bioelectrochemical cell operation at various applied cell voltages (1.0, 1.5 and 2.0 V). An increase in cell voltage from 1 to 2 V resulted in more reduction current generation (-0.48 to -0.78 mA) at a lowered cathode potential (-0.45 to -0.84 mV vs Ag/AgCl). Various alcohols were produced depending on applied cell voltages, and the main products were butanol, ethanol, and propanol. Hydrogen and methane production were also observed in the headspace of the cell. A large amount of lactic acid was unexpectedly formed at all conditions, which might be the primary cause of the limited biofuel production. The addition of neutral red (NR) to the system could increase the cathodic reduction current, and thus more biofuels were produced with an enhanced alcohol formation compared to without a mediator.

Development of exoelectrogenic bioanode and study on feasibility of hydrogen production using abiotic VITO-CoRE™ and VITO-CASE™ electrodes in a single chamber microbial electrolysis cell (MEC) at low current densities

Single chamber membrane-free microbial electrolysis cell (MEC) was operated for the assessment of exoelectrogenic bacteria (EB) growth at carbon felt anode and resultant hydrogen (H2) production at abiotic cathodes, made using cold rolling (VITO-CoRE™) and casting (VITO-CASE™) methods. Progressive enrichment of EB was observed on anode during 70 days of operation at an applied potential of +0.2V vs Ag/AgCl, and a maximum current density (CD) of 330.59 mA/m(2) (1.38 mA) was recorded. H2 production at selected abiotic cathodes was observed, when the enriched bioanode was coupled to them in galvanostat mode between 0.1 and 1.0 mA current range for 10 min each. Higher H2 production of 114.46±3.75 mL/m(2) was documented with VITO-CoRE™ at 0.6 mA, while 102.76±3.75 mL/m(2) was recorded with VITO-CASE™ at 0.8 mA of current application. This study demonstrates the feasibility of H2 production on abiotic cathodes using enriched bioanode at low current densities.

Closed circuitry operation influence on microbial electrofermentation: Proton/electron effluxes on electro-fuels productivity

A novel biocatalyzed electrofermentor (BEF) was designed which uncovers the intricate role of biocatalyst involved in cogeneration of electro-fuels (hydrogen and electricity). The specific role of external resistance (Rext, electrical load) on the performance of BEF was evaluated. Four BEFs were operated separately with different resistances (25, 50, 100 and 200 Ω) at an organic load of 5 g/L. Among the tested conditions, external resistance (R3) with 100 Ω revealed maximum power and cumulative H2 production (148 mW and 450 mL, respectively). The competence of closed circuitry comparatively excelled because it facilitates congenial ambiance for the enriched EAB (electroactive bacteria) resulting high rate of metabolic activity that paves way for higher substrate degradation and electro-fuel productivity. Probing of electron kinetics was studied using voltammetric analyses wherein electron transfer by redox proteins was noticed. The designed BEF is found to be sustainable system for harnessing renewable energy through wastewater treatment.

Implication of composite electrode on the functioning of photo-bioelectrocatalytic fuel cell operated with heterotrophic-anoxygenic condition

Electrode materials play a vital role in biofilm formation and electron conduction for efficient functioning of fuel cells. In the present study, graphite polymer composite electrode (GPF) was evaluated as anode for photo-bioelectrocatalytic fuel cell (PhFC; biophotovoltaic system) and compared with much studied graphite electrode (Gc) with photosynthetic bacteria as biocatalyst under anoxygenic condition. The electrogenic activity noticed in GPF (584mV; 2.67mA) was slightly lower than Gc (604mV; 2.92mA; OL2/HRT2). Consequently, COD removal observed by GPF (87.3%) was lower than Gc (91.8%). The increase in bacterial chlorophyll pigment showed a positive influence on electrogenic activity for both the electrodes. The polarization resistance (OL2 and HRT2 condition) was significantly higher for GPF (330Ω) as compared to Gc (110Ω). It is interesting to note that the performance of GPF is slightly lower than Gc based PhFC. The findings have opened avenues for composite materials for PhFC.

Biomineralization of azo dye bearing wastewater in periodic discontinuous batch reactor: Effect of microaerophilic conditions on treatment efficiency

The present study illustrates the influence of microaerophilic condition on periodic discontinuous batch reactor (PDBR) operation in treating azo dye containing wastewater. The process performance was evaluated with the function of various dye load operations (50-750 mg/l) by keeping the organic load (1.6 kg COD/m(3)-day) constant. Initially, lower dye operation (50mg dye/l) resulted in higher dye [45 mg dye/l (90%)] and COD [SDR: 1.29 kg COD/m(3)-day (92%)] removal efficiencies. Higher dye load operation (750 mg dye/l) also showed non-inhibitory performance with respect to dye [600 mg dye/l (80%)] and COD [1.25 kg COD/m(3)-day (80%)] removal efficiencies. Increment in dye load showed increment in azo reductase and dehydrogenase activities (39.6 U; 4.96 μg/ml; 750 mg/l). UV-Vis spectroscopy (200-800 nm), FTIR and (1)H NMR studies revealed the disappearance of azo bond (-NN-). First derivative cyclic voltammogram supported the involvement of various membrane bound redox shuttlers, viz., cytochrome-C, cytochrome-bc1 and flavoproteins (FAD (H)).

Bioelectrochemical treatment of paper and pulp wastewater in comparison with anaerobic process: integrating chemical coagulation with simultaneous power production

The efficiency of a bioelectrochemical treatment system (BET) to treat complex paper and pulp wastewater at two different pH conditions (6 and 7) in comparison with conventional anaerobic treatment process (AnT) was evaluated. Among the operating conditions, BET showed good treatment efficiency at pH 7 in terms of COD (BET/AnT: 55%/51%), nitrates (33.5%/19.1%), phosphates (33%/19%) and sulfates (58%/41%) in removal. The effluent obtained from BET system was subjected to coagulation for further treatment which showed good COD removal (BET/AnT, 95%/69%) and color (100%/68%). Bioelectrochemical analysis revealed higher catalytic currents in BET than AnT specific to oxidation and reduction. Besides, derivative of cyclic voltammetric scans (DCV) also supported the involvement of various membrane bound electron transferring complexes like FAD(H) bound enzymes, ubiquinone, NADH(+)/H(+) bound enzymes, etc. Experimental results demonstrated that BET system can be a viable platform to treat complex wastewaters with simultaneous energy recovery in integrated approach.

Azo dye load-shock on relative behavior of biofilm and suspended growth configured periodic discontinuous batch mode operations: critical evaluation with enzymatic and bio-electrocatalytic analysis

Effect of dye (C.I.Acid Black 10B) load-shock was comparatively evaluated in biofilm (self-immobilized) and suspended growth systems operated in periodic discontinuous batch mode (PDBR, anoxic-aerobic-anoxic) was investigated. At higher dye load (1250 mg dye/l), biofilm system showed relatively higher dye (74.5%) and COD (46%) removal efficiencies than the corresponding suspended mode operation (dye/COD removal efficiency, 42%/65%). Increment in dye load showed increment in azo reductase and dehydrogenase enzyme activities. Voltammograms (cyclic) showed higher reduction currents (RC) with increment in dye load specifically in biofilm system. Derivative cyclic voltammograms analysis depicted the involvement of mediators (NAD (+), FAD(+), etc.) which presumably played a major role in electron transport chain and dye degradation. Disappearance of peak (1612 cm(-1)) specific to azo group in FTIR spectrum, at higher loading rate in both the systems indicates the non-inhibitory and robust nature of PDBR operation.

Influence of headspace composition on product diversity by sulphate reducing bacteria biocathode

Mixed culture of sulphate reducing bacteria named TERI-MS-003 was used for development of biocathode on activated carbon fabric fastened to stainless steel mesh for conversion of volatile fatty acids to reduced organic compounds under chronoamperometric conditions of -0.85V vs. Ag/AgCl (3.5M KCl). A range of chemicals were bioelectrosynthesized, however the gases present in headspace environment of the bioelectrochemical reactor governed the product profile. Succinate, ethanol, hydrogen, glycerol and propionate were observed to be the predominant products when the reactor was hermetically sealed. On the other hand, acetone, propionate, isopropanol, propanol, isobutyrate, isovalerate and heptanoate were the predominant products when the reactor was continuously sparged with nitrogen. This study highlights the importance of head space composition in order to manoeuvre the final product profile desired during a microbial electro-synthesis operation and the need for simultaneously developing effective separation and recovery strategies from an economical and practical standpoint.

Microbial catalyzed electrochemical systems: a bio-factory with multi-facet applications

Microbial catalyzed electrochemical systems (MCES) have been intensively pursued in both basic and applied research as a futuristic and sustainable platform specifically in harnessing energy and generating value added bio-products. MCES have documented multiple/diverse applications which include microbial fuel cell (for harnessing bioelectricity), bioelectrochemical treatment system (waste remediation), bioelectrochemical system (bio-electrosynthesis of various value added products) and microbial electrolytic cell (H2 production at lower applied potential). Microorganisms function as biocatalyst in these fuel cell systems and the resulting electron flux from metabolism plays pivotal role in bio-electrogenesis. Exo-electron transfer machineries and strategies that regulate metabolic flux towards exo-electron transport were delineated. This review addresses the contemporary progress and advances made in MCES, focusing on its application towards value addition and waste remediation.

Induced catabolic bio-electrohydrolysis of complex food waste by regulating external resistance for enhancing acidogenic biohydrogen production

A novel bio-electrohydrolysis system (BEH) based on self-inducing electrogenic activity was designed as pretreatment device to enhance biohydrogen (H2) production efficiency from food waste. Two-stage hybrid operation with hydrolysis in the initial stage and acidogenic fermentation of the resulting hydrolysate (after hydrolysis) for H2 production in the second stage was evaluated. Application of variable external resistances viz., 10Ω, 100Ω, 1000Ω and closed circuit (CC) influenced the hydrolysis of substrate in BEH system and hydrogen production in acidogenic reactor compared to control. Pretreated substrate at 100Ω documented higher H2 production (1.05l) than 10Ω (0.93l), CC (0.91l), 1000Ω (0.88l) and control operation (0.68l). Comparatively, 10Ω documented higher substrate degradation (53.4%) followed by CC (52.42%), 100Ω (49.51%), 1000Ω (47.57%) and control (43.68%). Voltammetric profiles were in agreement with the observed bio-electrohydrolysis and H2 production efficiency.

Bioaugmentation of potent acidogenic isolates: a strategy for enhancing biohydrogen production at elevated organic load

The efficiency of bioaugmentation strategy for enhancing biohydrogenesis at elevated organic load was successfully evaluated by augmenting native acidogenic microflora with three acidogenic bacterial isolates viz., Bacillus subtilis, Pseudomonas stutzeri and Lysinibacillus fusiformis related to phyla Firmicutes and Proteobacteria separately. Hydrogen production ceased at 50g COD/l operation due to feed-back inhibition. B. subtilis augmented system showed higher H2 production followed by L. fusiformis, P. stutzeri and control operations, indicating the efficacy of Firmicutes as bioaugmentation biocatalyst. Higher VFA production with acetic acid as a major fraction was specifically observed with B. subtilis augmented system. Shift in metabolic pathway towards acidogenesis favoured higher H2 production. FISH analysis confirmed survivability and persistence of augmented strains apart from improvement in process performance. Bio-electrochemical analysis depicted specific changes in the metabolic activity after augmentation which also facilitated enhanced electron transfer. P. stutzeri augmented system documented relatively higher COD removal.

Bio-electrocatalyzed electron efflux in Gram positive and Gram negative bacteria: an insight into disparity in electron transfer kinetics

Electron transfer (ET) behavior of bacteria varies significantly in a bio-electrocatalyzed environment based on the cell membrane.

Relative effect of different inorganic acids on selective enrichment of acidogenic biocatalyst for fermentative biohydrogen production from wastewater

The effect of different inorganic acids viz., HNO3, HCl, H2SO4 and H3PO4 on inoculum pretreatment to selectively enrich hydrogen (H2) producing acidogenic bacteria was evaluated in anaerobic sequencing batch bioreactors. Relative positive efficiency of HNO3 pretreated consortia in enhancing H2 production (11.85 mol H2/kg CODR) was noticed compared to other acids (HCl, 5.64 mol H2/kg CODR; H2SO4, 7.65 mol H2/kg CODR; H3PO4, 6.90 mol H2/kg CODR) and untreated-parent consortia (control, 6.80 mol H2/kg CODR). On the contrary, substrate degradation (COD removal) was higher with the control operation (ξCOD, 66.3%; substrate degradation rate (SDR), 1.42 kg CODR/m(3)-day) compared to pre-treated culture. HNO3 pre-treatment resulted in a shift in the fermentation pathway towards more acetic acid production, while other acid pretreatment and untreated culture showed mixed type fermentation (acetic, butyric, propionic acids). The bio-electrochemical analysis and dehydrogenase activity supported the biocatalyst performance after HNO3 pretreatment with specific enrichment of Firmicutes and Bacillus.

Relative effect of bioaugmentation with electrochemically active and non-active bacteria on bioelectrogenesis in microbial fuel cell

Bioelectrogenic activity of microbial fuel cells (MFC) augmented with electrochemically active bacteria (EAB, Pseudomonas aeruginosa) and non-EAB (Escherichia coli) as biocatalysts was investigated. Anodic microflora augmented with P. aeruginosa (AMFCP) yielded higher electrogenic activity (418 mV; 3.87 mA) than E. coli (AMFCE; 254 mV; 1.67 mA) and non-augmented native microflora (MFCC; 235 mV; 1.37 mA). Higher redox currents along with lower Tafel-slopes were observed with AMFCP operation compared to AMFCE and MFCC due to manifestation of bioaugmentation thereby minimizing the losses. A fourfold and twofold increase in capacitance and exchange current was observed with AMFCP and AMFCE operation respectively, when compared to MFCC. Tracking of augmented biocatalyst by fluorescent in situ hybridization (FISH) with defined probes documented the survivability of Pseudomonas sp. in higher numbers than Enterobacteriaceae. Study corroborated enhanced electron transfer capability of mixed consortia owing to the synergistic interaction with EAB due to augmentation.

Prolonged applied potential to anode facilitate selective enrichment of bio-electrochemically active Proteobacteria for mediating electron transfer: microbial dynamics and bio-catalytic analysis

Prolonged application of poised potential to anode was evaluated to understand the influence of applied potentials [500 mV (E500); 1000 mV (E1000); 2000 mV (E2000)] on bio-electrogenic activity of microbial fuel cell (MFC) and the resulting dynamics in microbial community in comparison to control operation. E1000 system documented higher electrogenic activity (309 mW/m(2)) followed by E500 (143 mW/m(2)), E2000 (112 mW/m(2)) and control (65 mW/m(2)) operations. The improved power output at optimum applied potential (1000mV) might be attributed to the enrichment of electrochemically active bacteria majorly belonging to the phylum Proteobacteria with less extent of Firmicutes which helped in effective electron (mediated) transfer through release of exogenous shuttlers. Improved bio-electrogenic activity due to enrichment at 1000mV applied potential also correlated well with the observed cyctochrome-c peaks on the voltamatogram, lower ion ohmic losses and bio-electro kinetic analysis. Electric-shock at higher applied potential (E2000) resulted in the survival of less number of microbial species leading to lower electrogenesis.

Bio-electrolytic conversion of acidogenic effluents to biohydrogen: an integration strategy for higher substrate conversion and product recovery

Feasibility of integrating Microbial electrolysis cell (MEC) process with dark-fermentation process for additional hydrogen recovery as well as substrate degradation was demonstrated in the present study. MEC was employed in order to utilize the residual organic fraction present in the acidogenic effluents of dark fermentation process as substrate for hydrogen production with input of small electric current. MEC was operated at volatile fatty acids (VFA) concentration of 3000 mg/l under different poised potentials (0.2, 0.5, 0.6, 0.8 and 1.0 V) using anaerobic consortia as biocatalyst. Maximum hydrogen production rate (HPR), cumulative hydrogen production (CHP) (0.53 mmol/h and 3.6 mmol), dehydrogenase activity (1.65 μg/ml) and VFA utilization (49.8%) was recorded at 0.6 V. Bio-electrochemical behavior of mixed consortia was evaluated using cyclic voltammetry and by Tafel slope analysis. Microbial diversity analysis using denaturing gradient gel electrophoresis confirmed the presence of γ-proteobacteria (50%), Bacilli (25%) and Clostridia (25%).

Rhizosphere mediated electrogenesis with the function of anode placement for harnessing bioenergy through CO2 sequestration

The feasibility of power generation by non-destructive usage of rhizodeposits of Pennisetum setaceum plant formed mainly due to photosynthesis-carbon sequestration mechanism was studied in rhizosphere based microbial fuel-cell (R-MFC). Four fuel-cell assemblies (non-catalyzed graphite-plates; membrane-less operation; air-cathode) were evaluated for their electrogenic activity by varying anode distances from root in rhizosphere [A1 - 0; A2 - 8; A3 - 12 and A4 - 16 cm] at 2 cm depth from soil-layer and analyzed their electrogenic potential. The fuel-cell assembly near to the root zone showed maximum electrogenic-activity (R1, 1007 mV/4.52 mA) followed by R2 (780 mV/4.11 mA), R3 (720 mV/3.4 mA) and R4 (220 mV/1.2 mA). The observed maximum electrogenesis with R1 and minimum with R4 electrode-assemblies enumerated the critical role of root-exudates as substrates. All fuel-cell assemblies showed 10% higher electrogenic activity during day-time operation which can be directly attributed to plant's photosynthetic activity. The study enumerated the potential of plant to harness power in a sustainable way by optimum placement of fuel-cell setup in their rhizosphere.

Influence of terminal electron acceptor availability to the anodic oxidation on the electrogenic activity of microbial fuel cell (MFC)

The electrogenic activity of microbial fuel cell (MFC) with the function of anode placement from the terminal electron acceptor (TEA) was evaluated. Shorter anode distances from TEA showed higher electrogenesis due to the feasibility of higher electron acceptance as well as their discharge towards TEA. Substrate degradation was also higher at shorter anode placements from TEA due to the optimum substrate availability to the anodic biofilm. Bio-electro kinetics showed significant variation in the catalytic currents and exchange current densities with the function of anode placement indicating its role in electron acceptance and their transfer to the cathode. Anode placement of 3cm showed higher electrogenesis (406.38mW/m(2)) and substrate degradation (63.12%) along with significantly reduced polarization (6.72Ω) and charge transfer resistances compared to other anodic placements. The spacing between electrodes is crucial in accepting electrons as well as their discharge towards TEA which ultimately governs the power generation efficacy.

Change in electrogenic activity of the microbial fuel cell (MFC) with the function of biocathode microenvironment as terminal electron accepting condition: influence on overpotentials and bio-electro kinetics

Influence of biocathode microenvironment as terminal electron accepting process (TEAP) on the electrogenic activity of the microbial fuel cell (MFC)/bio-electrochemical system (BES) was evaluated in concurrence with the internal losses and bio-electro kinetics. Aerobic metabolism as TEAP showed power output (37.5 ± 2.7 mW/m(2)) for extended time (240 h) over abiotic (42.5 ± 1.5 mW/m(2)) electron accepting process. On the contrary, anaerobic metabolism as TEAP showed negligible power output in spite of increased retention time due to the absence of electron acceptor. Presence of strong electron acceptor conditions in aerobic metabolism facilitated gradual and stable reduction of electrons which helped to overcome the activation over potential and other potential losses. Voltammetric and amperometric analysis witnessed higher and sustainable electron discharge against the aerobic metabolism at cathode. Bio-electro kinetic analysis also showed lower Tafel slope and electron transfer co-efficient indicating the positive impact of aerobic metabolism at cathode in decreasing the internal losses.

Acid azo dye remediation in anoxic-aerobic-anoxic microenvironment under periodic discontinuous batch operation: bio-electro kinetics and microbial inventory

Functional behavior of anoxic-aerobic-anoxic microenvironment on azo dye (C.I. Acid black 10B) degradation was evaluated in a periodic discontinuous batch mode operation for 26 cycles. Dye removal efficiency and azo-reductase activity (30.50 ± 1 U) increased with each feeding event until 13th cycle and further stabilized. Dehydrogenase activity also increased gradually and stabilized (2.0 ± 0.2 μg/ml) indicating the stable proton shuttling between metabolic intermediates providing higher number of reducing equivalents towards dye degradation. Voltammetric profiles showed drop in redox catalytic currents during stabilized phase also supports the consumption of reducing equivalents towards dye removal. Change in Tafel slopes, polarization resistance and other bioprocess parameters correlated well with the observed dye removal and biocatalyst behavior. Microbial community analysis documented the involvement of specific organism pertaining to aerobic and facultative functions with heterotrophic and autotrophic metabolism. Integrating anoxic microenvironment with aerobic operation might have facilitated effective dye mineralization due to the possibility of combining redox functions.

Microbially-reduced graphene scaffolds to facilitate extracellular electron transfer in microbial fuel cells

A one-pot method is exploited by adding graphene oxide (GO) and acetate into an microbial fuel cell (MFC) in which GO is microbially reduced, leading to in situ construction of a bacteria/graphene network in the anode. The obtained microbially reduced graphene (MRG) exhibits comparable conductivity and physical characteristics to the chemically reduced graphene. Electrochemical measurements reveal that the number of exoelectrogens involved in extracellular electron transfer (EET) to the solid electrode, increases due to the presence of graphene scaffolds, and the EET is facilitated in terms of electron transfer kinetics. As a result, the maximum power density of the MFC is enhanced by 32% (from 1440 to 1905 mW m(-2)) and the coulombic efficiency is improved by 80% (from 30 to 54%). The results demonstrate that the construction of the bacteria/graphene network is an effective alternative to improve the MFC performance.