1.4　Microbial fuel cell technique

Microbial fuel cell(MFC)has been widely applied in bioelectricity generation from organic matters in the presence of electrochemically active bacteria(EAB)under mild reaction conditions(Du et al.,2007; Venkata Mohan et al.,2008).The MFC is divided into an anodic compartment(anaerobic environment)and a cathodic compartment(aerobic environment)based on the function of proton exchange membrane(PEM).Under the working condition,the organic matter was taken part in the metabolism and growth of microorganisms under an anaerobic environment.During the process of microbial metabolism,the organic matter could be oxidized by electrochemically active bacteria to provide electrons and protons.Since the cathode chamber and the anode chamber are respectively in aerobic and anaerobic environments,they form an oxidation-reduction potential difference.The electrons are transferred to the cathode chamber through an external circuit under the potential gradient,and the protons are transmitted through the PEM or directly transfer into the cathode chamber.Protons and electrons combine with oxygen to form water under the action of microbial catalyst.In this process,some nitrates and nitrites can participate in the electron reduction process as electron acceptors.And the reaction process of MFC using glucose as organic matter is listed as follows:

Anode reaction：

(1.19)

Cathode reaction：

(1.20)

With the constant replacement of MFC substrates,the newly added organic matter is used as a “renewable energy source” to participate in the bioelectricity production process by microorganisms,thereby the MFC could continuously deliver biological current.The working principle of MFC was showed in Figure 1.2.

1.4.1　The electron transfer mechanism of electrochemically active bacteria

Electrochemically active bacteria are important component of MFC,and the electrons generated during their metabolism can be directly or indirectly transmitted into extracellular electron acceptors(Xie et al.,2013).The process contained three main pathways:①cell direct contact transmission: the periplasmic space and cell membrane surface of this type microorganisms(such as Geobacter sulfurreducens)distributed some electronic carriers such as Cytochrome c(Meitl et al.,2009),the electrons could be transferred to the extracellular space after contact with these carriers.②“Nano wire” transfer: some bacteria,such as Shewanella oneidensis and Geobacter sulfurreducens,form specific structures such as “nanowires” on the cell surface of microorganisms attached on the minerals,metal oxides and electrode material(Figure 1.3)(Logan et al.,2006),which could be used to deliver electrons to the outside of the cell through “nanowires”.In this process,the electron-conductive ability of the “nanowire”of the microorganism is closely related to Cytochrome c.It has reported that a destruction of gene in this part would directly damage the electron-conductive ability of the “nanowire”(Gorby et al.,2006).③Intermediary transmission: the dissimilatory metal reduction bacteria can absorb the humus and other intermediates from natural environments,and get electrons from the cellular respiratory metabolism.Then the electrons would be transferred out of microbial cell(Gralnick and Newman,2007).Eventually,these electrons,which are conducted to the surface of the anode chamber electrode material,are directly transmitted to the electron acceptor through an external circuit.

1.4.2　The main types of electrochemically active bacteria

The electrochemically active bacteria produce extracellular electrons under the action of respiratory metabolism,which is the fundamental reason for the bioelectricity output from MFC.The main process of bioenergy generation is to transfer the electrons generated by microorganisms to the outside of the cell and electrode to be used as the electron acceptor.With the further research on MFC,more and more species of electrochemically active bacteria were discovered,mainly distributed in Proteobacteria,Firmicutes,Acidobacteria and Acitinobacteria.Most of these bacteria are facultative anaerobic bacteria and using carbohydrates and organic acids as electron donors.The common electrochemically active microorganisms were presented in Table 1.1.

1.4.3　Factors effected on bioelectricity output in microbial fuel cell

1.4.3.1　Anode chamber(anaerobic zone)

Organic matter is mainly oxidatively degraded in the anode compartment,where the active-producing bacteria participate in the oxidation reaction to produce electrons and protons.The anode chamber area forms an environment which is suitable for the growth of anaerobic microorganisms.The anode chamber mainly includes an anode electrode material,an electrode solution,and microorganisms.Anodic materials,microbial growth,community formation and bioelectric output are closely related,which are also the center of MFC components.To obtain materials with high electro-catalytic activity and improve the power density of MFC.Up to now,many scholars have developed materials modified with conductive polymers,metals,graphene,and carbon nanotubes,as well as those materials obtained by modifying traditional carbon materials.The specific surface area of the new material(Antolini,2015),making bioelectric power output of MFC improve greatly,but the cost of electrode material is relatively high.Some scholars also proposed to increase the volume ratio of the electrode material into the MFC.Besides,the configuration of the electrode material is also progressed to from a single two-dimensional electrode with a multi-electrode and a three-dimensional electrode(Choi,2015).However,it is found that common anode materials mainly include carbon felt,stainless steel,copper wire,carbon cloth,carbon paper,graphite rod,biocokes,graphite felt and glassy carbon in the CW-MFC research.These materials have advantages of corrosion-good conductivity,large specific surface area,low cost and high biocompatibility(Logan and Regan,2006).The results are also based on comprehensive consideration of CW-MFC cost,operability,and maintenance.

The anolyte mainly refers to the nutrients(organic matter)that can be bio-oxidatively degraded by microorganisms.It mainly intends for the rapid growth and reproduction of microorganisms to provide appropriate growth environment and electron transfer media,while the fuel is not only essential for the growth of microbial energy substances,but also bioelectric output of electronic sources.Fuels mainly consist of organic compounds such as glucose,cellulose and acids produced in production and life,and some inorganic compounds such as sulfides,ammonia and complex azo dyes(Yadav et al.,2012).The ability of MFCs exploiting multiple fuel matrices can offer the possibility of their use in waste water treatment and bioelectrical output at the same time.Meanwhile,some scholars have confirmed the advantages of increasing electrolyte ionic strength and conductivity of the solution,such as adding NaCl solution,MFC output current increased by 15%.

1.4.3.2　Proton exchange membrane

Proton exchange membrane(PEM)is an important part of MFC.Its main function is to isolate the fuel and cathode electron acceptor.In addition,proton(or other cations and anions)can permeate the cathode chamber and the anode chamber to achieve charge balance.It should have the characteristics of low corrosion resistance and internal resistance.At present,CW-MFC commonly used PEM mainly glass fiber,nylon mesh,potassium-based and sodium bentonite,but these PEM added to CW-MFC will affect the long-term operation of the device may cause local blockage of wetlands,causing to hydraulic “dead zone” increasing with water treatment efficiency decreasing.Therefore,in the design of a CW-MFC,an air-cathode,PEM-free MFC device should be simulated.

1.4.3.3　Cathode chamber(aerobic zone)

The cathode compartment is the collection of electrons and electron acceptor collection area.The cathode compartment contains the cathode electrode material,the cathode solution and MFC reduction reaction place.The reaction speed in this area is significantly higher than the oxidation reaction catalyzed by the microorganisms in the anode compartment and the extracellular electron transfer process of the EAB.Therefore,common carbon materials can be used for the cathode electrode material of CW-MFC.

Cathode fluid mainly includes electron acceptors and buffer substances.Cathode electron acceptor types can significantly affect the nature and rate of cathodic reduction reactions.They are also important factors which can affect MFC performance.At present,the common electron acceptors contained in water are mainly ,CO2.The lower potential electron acceptor is,the higher voltage and power output it can obtain.Some researchers,of course,have charged and modified precious metals.For instance,Pt is put on the cathode material to improve the reduction rate of oxygen.However,this precious metal is not suitable for CW-MFC with low consumption.The ideal electron acceptor should have high redox potential,high reactivity and large mass transfer co-efficiency.Some researchers believe that the biological cathode chamber is a widely used and widely used cathodic material,and its main advantages are as follows:①It replaces the noble metal catalyst with the microorganism to catalyze the catalytic oxygen reduction to reduce the MFC cost；②In the cathode,MnO2 or is first electrochemically reduced and then oxidized by the microorganism,enabling the recycling of electron acceptors;③Cathodic microorganisms such as nitrate,perchlorate,heavy metal ions,dyes molecules and other pollutants can be used as electron acceptors,which significantly expand the scope of application of MFC in the field of the environment.Among them,the CW-MFC coupled system,plant rhizosphere and oxygen can be domesticated with the aerobic flora dominated microbial structure.These microorganisms derive from rhizospheric secretions or the organic oxidation matter of short-chain fatty acids electron acceptor in the water.The process can also enhance the bioelectrical output of CW-MFC system.

1.4.3.4　External circuit

The main function of the external circuit is to complete the current loop of the internal circuit group.Thus,the electrons generated by the anode EAB can reach the cathode to form a complete loop under the effect of the potential difference resulting in that the current flow into the CW-MFC.The composition of the external circuit includes the wire,load(external resistance)and circuit monitoring device.

1.4.4　Electricity performance evaluation

The characterization parameters(volume power density,internal resistance,coulombic efficiency,polarization curve and power density curve)are commonly used to evaluate the bioelectricity production performance of MFC systems.The calculation of volumetric power density Formula(1.21),the internal resistance is usually plotted by voltage and current,and the slope of the resulted line is the internal resistance(Rint).Coulomb’s efficiency Formula(1.22),where the polarization curve is used to represent the relationship between current and voltage current density,the current is usually divided by the electrode area(usually anode electrode material surface area).EMFC,V,Rex,tb,I,F,VAn,and ΔCOD represent electric power(W),voltage(V),external resistance(Ω),single cycle time(s),current(A),Faraday’s constant[96485 C/(mol·e)],anode compartment volume(m3),and COD removal(mg/L),respectively.

(1.21)

(1.22)