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Highly Sensitive and Selective Amperometric Microbial Biosensor for Direct Determination of p-Nitrophenyl-Substituted Organophosphate Nerve Agents 2

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Highly Sensitive and Selective Amperometric Microbial Biosensor for Direct Determination of p-Nitrophenyl-Substituted Organophosphate Nerve Agents 2 Powered By Docstoc
					Journal of Scientific & Industrial Research RECENT DEVELOPMENTS IN MICROBIAL FUEL CELLS: A REVIEW
                    DAS & MANGWANI :                                                                                                     727
Vol. 69, October 2010, pp. 727-731




                        Recent developments in microbial fuel cells: a review
                                                   Surajit Das* and Neelam Mangwani
                          Department of Life Science, National Institute of Technology, Rourkela 769 008, India

                                  Received 26 March 2010; revised 23 July 2010; accepted 26 July 2010

           This review presents microbial fuel cells (MFCs) that convert biochemical metabolic energy into electrical energy.
      Microbes can be fed with waste products rich in organic matter (domestic wastewater, lignocellulosic biomass, brewery
      wastewater, starch processing wastewater, landfill leachates etc.) to generate electricity. MFCs can also be used for wastewater
      treatment, as biosensors and production of secondary fuels like hydrogen.

      Keywords: Biosensors, ETC, MFCs, Wastewater treatment



Introduction                                                            MFCs: Setup and Working Principle for Electricity
     Microbial fuel cells (MFCs) employ microbes to                     Generation
generate electricity from biochemical energy produced                   Basic Machinery
during metabolism of organic substrates. MFC consists                       An ideal MFC apparatus (Fig. 1) consists of two
of anode and cathode connected by an external circuit                   chambers (anodic and cathodic) made up of glass,
and separated by proton exchange membrane (PEM). In                     polycarbonate or Plexiglas, with respective electrode of
anode chamber, decomposition of organic substrates by                   graphite, graphite felt, carbon paper, carbon-cloth, Pt,
microbes generates electrons (e-) and protons (H+) that                 Pt black or reticulated vitreous carbon (RVC). These
are transferred to cathode through circuit and membrane                 chambers are separated by PEM (Nafion or Ultrex5).
respectively1. Organic substrates are utilized by microbes              Anodic chamber is filled with organic substrates that are
as their energy sources, outcome of this process is in                  metabolized by microbes for growth and energy
release of high-energy electrons that are transferred to                production while generating electron and proton.
electron acceptors (molecular oxygen) but in absence of                 Cathode is filled with a high potential electron acceptor
                                                                        to complete circuit. An ideal electron acceptor should
such electron acceptor in a MFC, microorganisms shuttle
                                                                        not interfere with microbes in any way and must be a
electron onto anode surface that results in generation of
                                                                        sustainable compound with no toxic effect. Oxygen
electricity2. Bacteria are most preferred microbes that
                                                                        serves as an ideal electron acceptor due to its non-toxic
can be used in MFCs to generate electricity while                       effect and preferred as oxidizing reagent as it simplifies
accomplishing biodegradation of organic matters or                      operation of an MFC, otherwise standard media with
wastes3,4. Biodegradable organic rich waters (municipal                 suitable electron acceptor such as ferricyanid can also
solid waste, industrial and agriculture wastewaters) are                be used to increase power density 1,6 . Based upon
ideal candidates of sustainable energy sources for                      assembly of anode and cathode chambers, a simple MFC
electricity production. MFCs can also be used as                        prototype can either be a double chambered or single
biosensors and in secondary fuel production.                            chambered. Besides these two common designs, several
     This paper reviews recent developments in                          adaptations have been made in prototype of MFC
MFC technology highlighting working principle and                       design and structure7.
applications of MFC technology.
                                                                        Double Compartment MFC System
*Author for correspondence                                                  In general, this type of MFCs has an anodic and a
E-mail: surajit@myself.com, surajit@nitrkl.ac.in                        cathodic chamber connected by a PEM that mediates
728                                      J SCI IND RES VOL 69 OCTOBER 2010




                                      Fig. 1—A simple setup of microbial fuel cell (MFC)

proton transfer from anode to cathode while blocking            either by a spontaneous (direct) or by means of some
diffusion of oxygen into anode. This type of system is          electron shuttling mediators9. Direct electron transfer to
generally used for waste treatment with simultaneous            anode by bacteria requires some physical contact with
power generation. Scaling up of two compartment MFCs            electrode for current generation. Plunge line up between
to industrial size is quite tough. Moreover periodic            bacteria and anode surface involves outer membrane
aeration of cathodic chambers also limits application           bound cytochromes or putative conductive pili called
spectrum of double compartment MFCs6.                           nanowires2. These mediator-less MFCs often utilize
                                                                anodophiles to form a biofilm on anode surface to make
Single Compartment MFC System                                   easy use of anode as their end terminal electron acceptor
     In a single compartment MFC, an anodic chamber             in anaerobic respiration.
is linked to a porous air exposed cathode separated by a            In mediated electron transfer machinery, microbes
gas diffusion layer or a PEM. Electrons are transferred         produce / acquire indigenous soluble redox compounds
to porous cathode to complete circuit. Limited                  (quinones and flavin) or synthetic exogenous mediators
requirement of periodic recharging with an oxidative            (dye or metalloorganics) to shuttle electron between
media and aeration makes single compartment                     terminal respiratory enzyme and anode surface3. These
microbial fuel cell system more versatile. Among                mediators can divert electrons from respiratory chain by
different advantages, single compartment MFC includes           entering outer cell membrane, becoming reduced, and
its reduced setup costs (due to absence of expensive            then leaving in a reduced state to shuttle electron to
membranes and cathodic chambers) that make flexible             electrode10. Numbers of electron and proton fabricated
application in wastewater treatment and power                   depends upon substrate utilized by microbes.
                                                                Mediator-less MFCs have more commercial potential as
generation8.
                                                                mediators are expensive and are sometimes toxic to
Working Principle                                               microorganisms. Electrode reactions in a MFC
    MFC explores metabolic potential of microbes for            compartments are as follows:
conversion of organic substrate into electricity by              i) If acetate is used as substrate
transferring electrons from cell to circuit. In anodic                                         microbes
chamber, oxidation of substrate in the absence of
                                                                Anodic reaction: CH3COO- + H2O → 2CO2+2H++8e-
oxygen by respiratory bacteria produce electron and
                                                                Cathodic reaction: O2 +4e- +4H+ → 2H2O
proton that are passed onto terminal e - acceptor
[O2, nitrate or Fe (III)] through electron transport chain      ii) If sucrose is used as substrate
(ETC) (Fig. 2). However, in absence of e- acceptor in a                                         microbes
MFC, some microorganisms pass electron onto anode2.             Anodic reaction: C12H22O11+13H2O → 12CO2+48H++48e-
An efficient electron shuttle to anode can be achieved          Cathodic reaction: O2 +4e- +4H+  → 2H2O
                   DAS & MANGWANI : RECENT DEVELOPMENTS IN MICROBIAL FUEL CELLS: A REVIEW                                          729




                                               Fig. 2—Electron shuttling mechanism in a MFC9

                                                                           (from industrial and municipal wastewaters)14, synthetic
            Table 1—Commonly used bacteria in MFCs
                                                                           or chemical wastewater, dye wastewater and landfill
Bacteria                      Mode of operation                            leachates are some unconventional substrates used for
Actinobacillus succinogenes   Requires exogenous mediators3
Erwinia dissolven             Mediator based MFC16
                                                                           electricity production via MFCs7.
Proteus mirabilis             Mediator based17
Pseudomonas aeruginosa        With exogenous mediators18                   Commonly used Microbes in Microbial Fuel Cells (MFCs)
Shewanella oneidensis         With exogenous mediators19                       Usually mixed culture of microbes is used for
Streptococcus lactis          With mediators16                             anaerobic digestion of substrate as complex mixed
Aeromonas hydrophila          Mediator-less MFC20
Geobacter metallireducens     Mediator-less MFC21
                                                                           culture permits broad substrate utilization. But there are
G. sulfurreducens             Mediator-less MFC22,23                       some regular MFCs designs which explore metabolic
Rhodoferax ferrireducens      Mediator-less MFC24,25                       tendency of single microbial species to generate
Shewanella putrefacien        Mediators-less MFC26, Exogenous              electricity 9 . Organic component rich sources
                              mediators improve electricity production27
Klebsiella pneumoniae         Mediator based28                             (marine sediment, soil, wastewater, fresh water sediment
                                                                           and activated sludge) are rich source of microbes that
Substrate used for Electricity Generation                                  can be used in MFCs catalytic unit15. Bacteria used in
     Substrate is a key factor for efficient production of                 MFCs with mediator or without mediators have been
electricity from a MFC. Substrate spectrum used for                        extensively studied and reviewed (Table 1). Metal
electricity generation ranges from simple to complex                       reducing and anodophilic microorganisms show better
mixture of organic matter present in wastewater.                           opportunities for mediator-less operation of a MFC.
Although substrate rich in complex organic content helps
in growth of diverse active microbes but simple substrates                 Other Applications
considered to be good for immediate productive output.                     Wastewater Treatment and Electricity Generation
Acetate and glucose are most preferred substrate for basic                      Due to unique metabolic assets of microbes, variety
MFC operations and electricity generation.                                 of microorganisms are used in MFCs either single
Lignocellulosic biomass from agriculture residues as                       species or consortia. Some substrates (sanitary wastes,
hydrolysis products (monosaccharides) are a good source                    food processing wastewater, swine wastewater and corn
for electricity production in MFCs11. Another promising                    stovers) are exceptionally loaded with organic matter that
and most preferred unusual substrate used in MFCs                          itself feed wide range of microbes used in MFCs. MFCs
operations for power generation is brewery wastewater                      using certain microbes have a special ability to remove
as it is supplemented with growth promoting organic                        sulfides as required in wastewater treatment 29 .
matter and devoid of inhibitory substances12. Starch                       MFC substrates have huge content of growth promoters
processing water can be used to develop microbial                          that can enhance growth of bio-electrochemically active
consortium in MFCs 13 . Cellulose and chitin                               microbes during wastewater treatment. This
730                                     J SCI IND RES VOL 69 OCTOBER 2010


simultaneous operation not only reduces energy demand       reviewed focusing on recent improvement5, practical
on treatment plant but also reduces amount of               implementation 39, anode performance 40, cathodic
unfeasible sludge produce by existing anaerobic             limitations41, different substrates used in MFCs7 etc.
production30. MFCs connected in series have high level      MFCs have been explored as a new source of electricity
of removal efficiency to treat leachate with                generation during operational waste treatment4. In
supplementary benefit of generating electricity31.          addition, some of the recent modification in MFCs
                                                            technology includes its use as microbial electrolysis cell
Biosensors                                                  (MEC), in which anoxic cathode is used with increased
    MFCs with replaceable anaerobic consortium could        external potential at cathode37. Phototrophic MFCs42 and
be used as a biosensor for on-line monitoring of organic    solar powered MFC43 also represent an exceptional
matter32. Though diverse conventional methods are used      attempt in the progress of MFCs technology for
to calculate organic content in term of biological          electricity production.
oxygen demand (BOD) in wastewater, most of them are
unsuitable for on-line monitoring and control of            Conclusions
biological wastewater treatment processes. A linear              MFC is an ideal way of generating electricity since
correlation between Coulombic yield of MFC and
                                                            it not only as a renewable source but also it can be used
strength of organic matter in wastewater makes MFC a
                                                            to treat waste. It can also be used for production of
possible BOD sensor33-35. Coulombic yield of MFC
                                                            secondary fuel as well as in bioremediation of toxic
provides an idea about BOD of liquid stream that proves
                                                            compounds. However, this technology is only in research
to be an accurate method to measure BOD value at quite
                                                            stage and more research is required before domestic
wide concentration range of organic matter in waste
                                                            MFCs can be made available for commercialization.
water.

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