G. Velvizhi

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.

Biocatalyst behavior under self-induced electrogenic microenvironment in comparison with anaerobic treatment: evaluation with pharmaceutical wastewater for multi-pollutant removal.

Biocatalyst behavior was comparatively evaluated under diverse microenvironments viz., self-induced electrogenic (bioelectrochemical treatment, BET) and anaerobic treatment (AnT) microenvironments, with real-field pharmaceutical wastewater. Relatively higher treatment efficiency was observed with BET (COD removal, 78.70%) over AnT (32%) along with the power output. Voltammetric profiles of AnT showed persistent reduction behavior, while BET depicted simultaneous redox behavior. BET operation documented significantly higher bio-electrocatalytic activity (kapp, 245.22 s(-1)) than AnT (kapp, 7.35 s(-1)). The electron accepting conditions due to the presence of electrode in the BET might contributed to higher electrogenesis leading to enhanced substrate degradation along with the removal of multiple pollutants accounting for the effective reduction of toxicity levels of wastewater. Even at higher organic loads, BET operation showed good treatment efficiency without process inhibition. Introduction of electrode-membrane assembly in anaerobic microenvironment showed significant change in the electrocatalytic behavior of biocatalyst resulting in enhanced treatment of complex wastewater.

Bioelectrogenic role of anoxic microbial anode in the treatment of chemical wastewater: Microbial dynamics with bioelectro-characterization

A membrane-less anoxic bioelectrochemical treatment (AxBET) system was evaluated to study the influence of bioelectrogenic activity during the treatment of chemical wastewater (CW). Increment in power generation was observed with increase in substrate loading (61-204 mW/m(2)) indicating the ability of anodic bacteria in BET system to utilize the complex chemicals as the sole carbon source. Derivative analysis of voltammograms depicted by positive and negative peak potentials which relate to the extracellular electron transport sites (EETs) that presumably play a significant role in electron transfer. These self-driven redox mediators varied with respect to the substrate load. The microbial population was dominated by anaerobic microorganisms which are commonly involved in effluent treatment plants during the initial phase of operation. A gradual shift in the microbial community was observed towards enrichment of electrogenically active bacteria belonging to phyla viz., Firmicutes and Proteobacteria after prolonged operation. Shannon Index and principal component analysis correlated with the microbial profile studies. The feasibility of self-driven bioremediation of chemical wastewater in an AxBET system demonstrated bioelectricity production along with multipollutant removal simultaneously.

A Circular Bioeconomy with Biobased Products from CO2 Sequestration.

The unprecedented climate change influenced by elevated concentrations of CO2 has compelled the research world to focus on CO2 sequestration. Although existing natural and anthropogenic CO2 sinks have proven valuable, their ability to further assimilate CO2 is now questioned. Thus, we highlight here the importance of biological sequestration methods as alternate and viable routes for mitigating climate change while simultaneously synthesizing value-added products that could sustainably fuel the circular bioeconomy. Four conceptual models for CO2 biosequestration and the synthesis of biobased products, as well as an integrated CO2 biorefinery model, are proposed. Optimizing and implementing this biorefinery model might overcome the limitations of existing sequestration methods and could help realign the carbon balance.

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.

Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater

The present study evaluates the sequential integration of two advanced biological treatment methods viz., sequencing batch reactor (SBR) and bioelectrochemical treatment systems (BET) for the treatment of real-field petrochemical wastewater (PCW). Initially two SBR reactors were operated in aerobic (SBR(Ae)) and anoxic (SBR(Ax)) microenvironments with an organic loading rate (OLR) of 9.68 kg COD/m(3)-day. Relatively, SBR(Ax) showed higher substrate degradation (3.34 kg COD/m(3)-day) compared to SBR(Ae) (2.9 kg COD/m(3)-day). To further improve treatment efficiency, the effluents from SBR process were fed to BET reactors. BET(Ax) depicted higher SDR (1.92 kg COD/m(3)-day) with simultaneous power generation (17.12 mW/m(2)) followed by BET(Ae) (1.80 kg COD/m(3)-day; 14.25 mW/m(2)). Integrating both the processes documented significant improvement in COD removal efficiency due to the flexibility of combining multiple microenvironments sequentially. Results were supported with GC-MS and FTIR, which confirmed the increment in biodegradability of wastewater.

Microbial Fuel Cells for Sustainable Bioenergy Generation: Principles and Perspective Applications

The energy gain in microbes is driven by oxidizing an electron donor and reducing an electron acceptor. Variation in the electron acceptor conditions creates a feasibility to harness energy. In order to support the microbial respiration, electrons will transfer to the exocellular medium toward the available electron acceptor, especially in the absence of oxygen. The microbes can use a wide range of electron acceptors such as metals, nutrients, minerals, etc., including solid electrodes. When the microbes use the solid electrode as electron acceptors, the setup is called microbial fuel cell (MFC) and the electrons can be harvested and used for different applications. MFC can be defined as a microbially catalyzed electrochemical system which can facilitate the direct conversion of substrate to electricity through a cascade of redox reactions, especially in the absence of oxygen. Linking the microbial metabolism to anode and then transmitting the electrons to cathode generates a net electrical power from the degradation of available electron donor. This concept of MFC operation has expanded considerable interest in the recent research due to its application in the energy recovery from wastewater. Microbes in MFC can also use variety of organic or inorganic electron donors as well as acceptors to produce a surfeit of desirable biofuels or biochemicals which is termed as microbial electrosynthesis. Apart from the electrogensis, the applications of MFC are widespread in different fields including waste/wastewater remediation, toxic pollutants/xenobiotics removal, recovery of commercially viable products, sequestration of CO2, harvesting the energy stored in marine sediments, desalination, etc. In this chapter, an attempt was made to bring out all the existing applications of MFC into one platform to make a comprehensive understanding on the inherent potential of microbial metabolism, when the designated electron acceptor is present.

Anoxic bio-electrochemical system for treatment of complex chemical wastewater with simultaneous bioelectricity generation.

Bioelectrochemical treatment system (BET) with anoxic anodic microenvironment was studied with chemical wastewater (CW) in comparison with anoxic treatment (AxT, sequencing batch reactor (SBR)) with same parent anaerobic consortia. BET system documented relatively higher treatment efficiency at higher organic load (5.0 kg COD/m(3)) accounting for COD removal efficiency of (90%) along with nitrate (48%), phosphate (51%), sulphates (68%), colour (63%) and turbidity (90%) removal, compared to AxT operation (COD, 47%; nitrate, 36%; phosphate, 32%; sulphate, 35%; colour, 45% and turbidity, 54%). The self-induced bio-potential developed due to the electrode assembly in BET resulted in effective treatment with simultaneous bioelectricity generation (631 mA/m(2)). AxT operation showed persistent reduction behaviour, while simultaneous redox behaviour was observed with BET indicating balanced electron transfer. BET operation illustrated higher wastewater toxicity reduction compared to the AxT system which documents the variation in bio-electrocatalytic behaviour of same consortia under different microenvironment.

Evaluation of voltage sag-regain phases to understand the stability of bioelectrochemical system: Electro-kinetic analysis

Voltage sag, regain and their stabilization phases were evaluated with time across load (closed circuit) and absence of load (short circuit) to understand the stability of the bio-electrochemical system (BES) under varying organic loads (OL). Closed circuit operation showed good stability along with the electrogenic activity over short circuit operation during both the sag and regain phases due to the regulated electron flow in the closed circuit. Relative change in voltage with time (dV/dt) was observed to be decreasing with increasing OL in the zone of sag, while it was observed to be increasing with increase in OL during the zone of regain. However, the change in dV/dt was not proportional with increasing OL during both the sag and regain phases indicating the influence of OL on the biocatalytic activity. Bio-electrocatalytic evaluation through Tafel analysis showed a gradually decreasing reductive slope from OL1 (0.62 V/dec) to OL4 (0.516 V/dec) indicating higher electrocatalytic activity towards reduction. While, the oxidative slope increased from OL1 (0.085 V/dec) to OL2 and was almost similar with further increment in the OL (0.099 ± 0.002 V/dec) which indicates marginal change in the electrocatalytic activity during oxidation even with increasing OL. Exchange current density from Tafel analysis was also observed to increase with increase in OL (OL1, 7.86 mA m−2; OL2, 8.33 mA m−2; OL3, 9.59 mA m−2; OL4, 11.77 mA m−2). Polarization resistance showed a decreasing trend with increasing OL (OL1, 12.23 Ω to OL4, 9.8 Ω) resulting in higher electron transfer.

Solid electron acceptor effect on biocatalyst activity in treating azo dye based wastewater

The functional activity of anaerobic bacteria in the presence of an electrode as solid electron acceptor was comprehensively evaluated during the treatment of azo dye based wastewater. The experiments were performed in three different reactor setups, viz., bio-electrochemical treatment (BET; with electrode assembly and anaerobic biocatalyst), anaerobic treatment (AnT; with anaerobic biocatalyst and absence of electrode assembly) and abiotic reactor (control; with electrode assembly and absence of anaerobic biocatalyst) with 50 mg l−1 azo dye concentration. Maximum dye removal was observed with BET (69.9%) followed by AnT (42%) and control (2.4%). The bioelectrogenic performance was observed to be higher in BET (92.1 mW m−2) in comparison to abiotic-control (0.41 mW m−2), which is attributed to the significant influence of bacteria as biocatalyst in concurrence with the function of the electrode as solid electron acceptor in BET. The study also documented electron acceptor dependent respiration, exemplifying the influence of conjunction between electrode and bacteria on dye degradation. Two possible electron transfer mechanisms, viz., direct electron transfer (DET) through the membrane bound cytochromes to the solid electron acceptor and mediated electron transfer (MET) through reduced dye intermediates as electron shuttles, were observed during BET operation. However, AnT and abiotic control operation resulted in less and no dye breakdown, respectively, due to the lack of conjunction between the biocatalyst and electrode. The study provides a new insight into the electron acceptor dependent respiration wherein the electrode serving as a solid electron acceptor enables efficient function of anode respiring bacteria (ARB) in terms of electron flux towards dye degradation and electrogenesis.