**1. Introduction**

232 Mass Transfer in Chemical Engineering Processes

Suchdeo, S. & Schultz, J. (1974). The permeability of gases through reacting solutions: the

Ward, W. & Robb, W. (1967). Carbon dioxide-oxigen separation: facilitated transport of

carbon dioxide across a liquid film. *Science* Vol.156, pp. 1481-1484

23

carbon dioxide-bicarbonate membrane system. *Chem. Eng. Sci.,* Vol.29, No.1, pp. 13-

As the energy sources decrease and the climate conditions change, demand for new and clean sources of energy has increased (Hong et al., 2009; Li et al., 2010a). Fuel cells , as a high efficiency energy converting device, have attracted more and more attention recently with low/zero emission (Liu et al., 2006). Moreover, conventional sewage treatment requires high energy and capital cost so there is great interest for finding clean and sustainable energy with very low or zero emission and cost effective that is an alternative for treatment technology (Appleby, 1988; Min et al., 2005).

Microbial fuel cells (MFCs) are one kind of fuel cell and also new source of energy. In MFCs, electrons are supplied from chemical bonds with the aids of microorganisms. Then the produced electrons are transported to anode surfaces and protons are moved through proton exchange membrane or salt bridge toward cathode (Wen et al., 2009). The electron flows through an electrical external circuit while anode is connected to cathode. The flow of electron has a current (I) and power (P) is resulted. The reduction of organic substances in anode was catalyzed by the living organism in anode chamber (Chen et al., 2008; Rahimnejad et al., 2009)

Traditional MFC is consist of two separated chambers named cathode and anode ones. Oxidation of substrate by microorganisms leads to generation of electrons and protons in anaerobic anode compartment. (Rahimnejad et al., 2009). A typical biological fuel cell is shown schematically in Fig.1.

Several parameters affect on the performance of MFC, namely microbial inoculums, chemical substrates, mass transfer area, absence or existence of proton exchange materials, mechanism of electron transfer to the anode surface ,cell internal and external resistance, solution ionic strength, electrode materials and the electrode spacing (Park and Zeikus, 2000; Gil et al., 2003; Rosenbaum et al., 2007; Zhang et al., 2007; Li et al., 2010b)

Direct electron transfers from anaerobic anode chamber to anode surface had shown to take place only at very low efficiency (Park et al., 2000; Lovley, 2006) . Electron transfer efficiencies in MFCs would be improved with the use of suitable electron mediators. Most MFCs use electron mediator component to improve the output of the cells. It has been reported in the literature that mediators are artificially added to anode chamber, such as Methylen blue (MB), Neutral red (NR), Thionin, Ferricyanide, Humic acid or Methyl viologen (Kim and Lee). The presence of artificial electron mediators are essential in some of MFCs to improve the performance of MFCs (Park and Zeikus, 1999; 2000) . But recently,

Effect of Mass Transfer on Performance of Microbial Fuel Cell 235

Commonly oxygen as terminal electron acceptor was used in cathode compartment. Consumption of electrons and protons that are combined with oxygen, forms water at last, and end this transfer cycle. Oxidized mediators, can also accelerate reaction of forming

The objective of this chapter was to demonstrate the power production from glucose as sole electron donors in MFC. But the main purpose of this present research was to investigated

*Saccharomyces cerevisiae* PTCC 5269 was supplied by Iranian Research Organization for Science and Technology (Tehran, Iran). The microorganisms were grown at anaerobic condition in an anaerobic jar vessel. The prepared medium for seed culture consisted of glucose, yeast extract,

The medium pH was initially adjusted to 6.5 and the inoculums were introduced into the media at ambient temperature. The inoculated cultures were incubated at 30°C. The bacteria were fully grown in a 100ml flask without any agitation for the duration of 24 hours. Substrate consumption was calculated based on determination of the remaining sugars in the culture. Growth was monitored by measuring the optical density (OD at 620nm). Substrate consumption was calculated based on determination of the remained sugars in the

All chemicals and reagents used for the experiments were analytical grades and supplied by Merck (Darmstadt, Germany). The pH meter, HANA 211(Romania) model glass-electrode was employed to measure pH values of the aqueous phase. The initial pH of the working solution was adjusted by addition of diluted HNO3 or 0.1M NaOH solutions. Dinitrosalicylic acid [3, 5(NO2)2C6H2-2OH-COONa.H2O] (DNS) method was developed to detect and measure substrate consumption using colorimetric method. Before analysis,

Scan Electron Microscope (SEM): The anode electrode before and at the end of the experiment was examined by a Scanning Electronic Microscope (SEM) (Phillips XL30, Holland). Finally, images of the samples were taken under SEM at magnifications of 5000. SEM images were used to demonstrate the physical characteristics of the electrode surface

Different kinds of MFCs were made up to investigation of mass transfer area on performance of MFC. All MFCs fabricated from Plexiglas material were used as MFCs in laboratory scale. The volume of each chamber (anode and cathode chambers) was 800 ml with a working volume of 615 ml. The sample port was provided for the anode chamber, wire point input and inlet port. The selected electrodes in MFC were graphite plates, size of 40×70×1.2mm. Proton exchange membrane (PEM; NAFION 117, Sigma–Aldrich) was used to separate two compartments. Proton exchange membrane, nafion, was subjected to a course of pretreatment to take off any impurities that was boiling for 1h in 3% H2O2, washed with deionized water, 0.5 M H2SO4, and finally washed with deionized water. In order to maintain membrane for good conductivity, the anode and cathode compartments were filled with deionized water

NH4Cl, NaH2PO4, MgSO4 and MnSO4: 10, 3, 0.2, 0.6, 0.2 and 0.05 g.l-1, respectively.

culture according to Sadasivam and Manickam(Sadasivam and Manickam, 2005).

liquid samples were filtered by a 0.45 μm syringe membrane (Sartorius Minisart).

and to examine the growth of yeast on the anode surface.

water in cathode chamber (Heitner-Wirguin, 1996).

the effect of mass transfer area on MFCs performance.

**2. Materials and methods** 

**2.2 Chemical and analysis** 

**2.3 MFC** 

**2.1 Microorganism and cultivation** 

Fig. 1. A typical MFC representing current generation with the help of microorganisms (Shukla et al., 2004)

mediators less MFCs became an interesting issue for many researchers (Kim et al., 2002; Chaudhuri and Lovley, 2003; Venkata Mohan et al., 2007; Huang et al., 2008; Venkata Mohan et al., 2008) . Table 1 shows a list of MFCs were examined with or without mediators used as component along with substrate.


Table 1. Microorganisms used in MFC

Commonly oxygen as terminal electron acceptor was used in cathode compartment. Consumption of electrons and protons that are combined with oxygen, forms water at last, and end this transfer cycle. Oxidized mediators, can also accelerate reaction of forming water in cathode chamber (Heitner-Wirguin, 1996).

The objective of this chapter was to demonstrate the power production from glucose as sole electron donors in MFC. But the main purpose of this present research was to investigated the effect of mass transfer area on MFCs performance.
