Elsevier

Bioresource Technology

Volume 165, August 2014, Pages 355-364
Bioresource Technology

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

https://doi.org/10.1016/j.biortech.2014.03.048Get rights and content

Highlights

  • Review advances and application of the microbial catalyzed electrochemical systems.

  • Bioelectrochemical system application for waste remediation was elaborated.

  • Exo-electron transfer machinery of electroactive bacteria was discussed.

Abstract

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.

Introduction

Microbial catalyzed electrochemical systems (MCES) have emerged as a potential alternative to conventional energy sources for sustainable production of energy and value added products by using microorganisms as biocatalyst. The concept of remodelling microbial metabolism to recover energy and value added products by degradation of organic matter is a flexible and sustainable platform (Venkata Mohan et al., 2013a, Venkata Mohan et al., 2013b, Wang and Ren, 2013). Microbes generally carry out their metabolic activities by utilizing available substrates and generate reducing equivalents [protons (H+) and electrons (e)]. These e and H+ move via a series of redox components/carriers (NAD+, FAD, FMN, etc.,) towards an available terminal electron acceptor (TEA), thus generating proton motive force which inturn facilitates generation of energy rich phosphate bonds that are useful for the microbial growth and subsequent metabolic activities. Electrons (e) in reduced substrate (e.g., carbohydrates, organic acids, etc.) initiate an “electric circuit” that closes the loop with an e sink furnished by a final electron acceptor (Venkata Mohan et al., 2013b). The function of a MCES is based on harnessing the available electrons by introducing electrodes, as intermediary electron acceptors. Anode chamber, resembles a biofactory that facilitates the generation of reducing equivalents through a series of bio-electrochemical redox reactions under anaerobic microenvironment (Logan, 2010, Zhou et al., 2012, Alister et al., 2012, Venkata Mohan et al., 2007, Venkata Mohan et al., 2008a, Venkata Mohan et al., 2008b, Venkata Mohan et al., 2013a, Venkata Mohan et al., 2013b). MCES are being used to harvest bioelectricity, to remediate complex pollutants in wastewater, to produce various forms of value added products, etc., either in the anode chamber, cathode chamber or in combination (Venkata Mohan et al., 2013a, Venkata Mohan et al., 2013b, Wang and Ren, 2013, Li et al., 2013, Huang et al., 2011). MCES can be customized to achieve multiple-tasks as required due to its feasibility to integrate biodegradation, electrolytic dissociation and electrochemical oxidation/reduction processes in the presence of the solid electron acceptor (anode and cathode) (Venkata Mohan et al., 2009). Essential function of all MCES depends on the electron transfer mechanism and cascade of redox reactions.

Microbial catalyzed electrochemical systems (MCES) based on their functional utility can be broadly classified into microbial fuel cell (MFC), bioelectrochemical treatment system (BET), bioelectrochemical system (BES) and microbial electrolysis cell (MEC) (Sfig. 1). MFC has garnered significant interest in both basic and applied research due to its potential to harness clean energy (Cheng and Logan, 2007, Venkata Mohan et al., 2007, Venkata Mohan et al., 2008a, Venkata Mohan et al., 2008b). MFC has documented its potential to harness electric current from a wide range of soluble or dissolved complex organic wastes/wastewater and renewable biomass as substrate and therefore helps to offset the operational cost of effluent treatment (Venkata Mohan et al., 2013a, Venkata Mohan et al., 2013b, Mohanakrishna et al., 2010a, Mohanakrishna et al., 2010b). MFCs are being viewed as viable and effective treatment units in addition to bioelectricity generation and therefore can be termed as bioelectrochemical treatment (BET) system when the primary focus is towards treatment (Venkata Mohan et al., 2009; Huang et al., 2013, Li et al., 2013, Liu et al., 2014, Velvizhi and Venkata Mohan, 2011). Product recovery is another alternative, where applied potential is used to produce various value added products through microbial electrolysis cell (MEC) and microbial electrosynthesis (MES) process (Rabaey and Rozendal, 2010, Lenin Babu et al., 2013, Venkata Mohan and Lenin Babu, 2011). In the case of MEC, the H+ produced in the anode chamber migrates to the cathode and gets reduced to form H2 in the presence of an e arrived from the anode, under the influence of a small applied voltage. This facilitates the crossing of endothermic activation energy barrier to form H2 gas. In addition to hydrogen, various other value added products like ethanol, butanol, succinate, etc., can be synthesized at the cathode by applying a small input of voltage (Gong et al., 2013, Marshall et al., 2013, Rabaey and Rozendal, 2010).

In this perspective, the present review delineates the function of Microbial catalyzed electrochemical systems (MCES) pertaining to product recovery and wastewater treatment. Significant development and sustainable research has been reported on BES till date (Sfig. 2). As per the ISI web of knowledge, reports on MCES were sparce from 1998 to 2010, however considerable impact has been noticed henceforth. The first investigation on BET was reported in 1994, but a consistent progress was only observed from 2007 onwards. Microbial electrolysis cells (MEC) which debuted in the year 2006 have been emerging as a potential and useful process for effective biohydrogen production from different substrates. MCES represents a new and promising biological approach for solving the environmental pollution problem and energy crisis with a unified approach. The present review also discusses about exo-electron transfer machinery to understand the electron transfer mechanism in MCES.

Section snippets

Extracellular electron transporting machinery

Transfer of electrons from bacteria to electrode is the primary process that occurs in a MCES. There are three possible ways viz., direct contact, electron shuttling via mediators and through electrically conductive appendages (nanowires/pilin) by which a bacterium can communicate with external electron acceptors (Coursolle et al., 2010; Jiang et al., 2013; Boesen and Nielsen, 2013). Bacteria may exploit one or more of the above mechanisms to transfer electrons extracellularly. In direct

Microbial fuel cell (MFC)

In MFC, an electrical circuit will be established between e source (bacterial metabolism of substrate) and the e sink (oxygen)/TEA by placing electrode as an intermediary e acceptor. It facilities harnessing of bioenergy in the form of bioelectricity by the development of a potential due to the flow of electrons through the circuits. This mechanism facilitates harnessing of energy in the form of bioelectricity during the MFC operation. MFCs facilitate direct conversion of substrate to

Bioelectrochemical treatment system (BET)

MFCs have been conceived and intensively studied as a promising technology for treatment of wastewater which is termed as bioelectrochemical treatment (BET). BET is gaining prominence because it facilitates sustainable wastewater treatment with simultaneous energy generation (Venkata Mohan et al., 2009, Mohanakrishna et al., 2010b, Huang et al., 2011, Aulenta et al., 2013, Velvizhi et al., 2014, Liu et al., 2014). During BET operation, there exists a possibility of integrating diverse

Microbial electrolysis cell (MEC)

Microbial electrolysis cells (MECs) emerged as an alternative route to H2 production from renewable resources (Venkata Mohan and Lenin Babu, 2011, Lenin Babu et al., 2013, Venkata Mohan et al., 2013a). MEC is similar to MFC, but an external potential is applied to facilitate e and H+ to cross the endothermic barrier to form H2 gas. The H+ migrate to the cathode and get reduced to form H2 in presence of e travelling from the anode under applied voltage. Standard redox potential for the

Engineering efficient exo-electrogenic strains

Performance of the microbial catalyzed electrochemical systems like MFC, BET, MES, etc., is dependent on the efficiency of the electron transfer machinery. Power output (in MFC), quantity of pollutant treated (in BET) and nature of value added product synthesized (in MES) depends on the electron flux manifested by the bacteria (Venkata Mohan et al., 2013a). Thus it is very important to study the extra cellular electron transfer mechanisms so that losses can be minimized. Generally there are

Current challenges and future perspectives

Research on MCES has been intensified over the the past decade and still requires considerable research inputs from various fields like engineering, microbiology, biochemistry material sciences, electrochemistry, etc. for achieving its full potential for optimizing various parameters affecting its performance. Genetic manipulation of specific proteins involved in electron transfer can help increase the electron transfer by the biocatalyst. Synergistic interaction between the MCES components and

Acknowledgements

The authors wish to thank The Director, CSIR-IICT for encouragement. S.V.M. acknowledges Department of Biotechnology (DBT), Government of India for providing research grant in the framework of National Bioscience Award 2012 (BT/HRD/NBA/34/01/2012(VI)). G.V. and K.V.K. duly acknowledge CSIR for providing research fellowships.

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