Vozchikova S.V., Alferov S.V.

Tula State University, Russia

Assessment of oxygen concentration and substrate consumption in a microbial biofuel cell

Biofuel element (BFC) is a device that converts chemical energy of organic compounds into electricity by oxidation of microorganisms. As well as a fuel cell, the microbial fuel cell is theoretically high-performance devices, but in contrast to the fuel cell operating on hydrogen or methanol can use waste water, which makes such systems extremely effective means not only for generating electric energy but also protect the environment from polluting substances contained in these substrates.

As biocatalysts for BFC have been applied acetic acid bacteria Gluconobacter oxydans, oxidizing a wide range of substrates, which includes carbohydrates and alcohols. Moreover, these organisms have specific metabolic system - the membrane localization of key enzymes - dehydrogenases. Using the whole-cell bacteria in BFC eliminates the need for individual enzymes. However, the energy parameters of the enzyme fuel cell is significantly higher than for BFC, based on whole cells. There is an alternative of enzymes application as biocatalysts, is the use of a membrane fraction of G. oxydans bacteria.

For anaerobic bacteria G.oxydans, oxygen is the natural electron acceptor, so in the presence of an artificial electron acceptor in the anode chamber may be a competition between them. Substrate oxidation by bacteria G.oxydans associated with the consumption of oxygen. Earlier work on simultaneous estimation of substrates and oxygen consumption by different biocatalysts, based on bacteria G.oxydans in a BFC model was not carried out.

The aim of this work was to study changes in the concentration of oxygen and substrates in the anode compartment of BFC.

Estimates of the rate of oxygen reduction and oxidation of substrates was carried out in the system: a background electrolyte (30 mM Na-phosphate buffer solution), substrates glucose or ethanol (10 mM), and biocatalyst is presented in the form of suspended or immobilized on the electrode surface of whole cells or membrane fraction of bacteria G.oxydans. Measurements of oxygen concentration was carried out in cases of absence and presence of electron transport mediators – 2,6-dichlorophenolindophenol (DCPIP), phenazinemethosulfate (PMS) and potassium ferrocyanide.
The data obtained on the reduction of oxygen in the anode compartment of BFC has a comparative evaluation (Table 1).
Table 1. The rates of reduction of oxygen in the BFC with catalysts based on G. oxydans.

Typebiocatalyst	The rate of oxygen reduction, normalized to unity biocatalyst mol/s•g
The suspension of bacteria (in the presence of the mediator)	8.4±0.6
The suspension of bacteria (in the absence of a mediator)	6.9±0.9
The suspension of the membrane fraction (in the presence of the mediator)	2.07±0.3
The suspension of the membrane fraction (in the absence of a mediator)	2.10±0.02
The immobilized cells of bacteria on the anode (in the presence of the mediator)	0.22±0.03
The immobilized bacterial cells in the anode (in the absence of a mediator)	0.180±0.004
Immobilized on the anode membrane fraction of bacteria (in the presence of the mediator)	0.024±0.002
Immobilized on the anode membrane fraction bacteria (no mediator)	0.022±0.001

The rate of oxygen reduction by bacterial suspension of G.oxydansin BFC in 10 and 100 times higher than the values obtained for the BFC based on immobilized cells and immobilized membrane fraction on the surface of the anode, respectively. The results obtained for systems in which the presence of artificial electron acceptors are almost indistinguishable from the results for systems without mediators.
Bacteria Gluconobacter oxydans contain various enzymes of carbohydrates and alcohols catabolism; in particular glucose and ethanol. Comparative assessment of substrate consumption in the anode compartment of BFC was carried out obtaining the oxidation rate of glucose and ethanol using different biocatalysts (Table 2).
Table 2. The oxidation rate constants of glucose and ethanol on the basis of various biocatalysts, normalized by the number of the biocatalyst

Type of biocatalyst	The rate constant of the glucose oxidation, mmol/g•min	The rate constant for the oxidation of ethanol, mmol/g•min
The suspension ofG.oxydans	1,10±0,01	1,3±0,2
The immobilized cells of G.oxydans	0,50±0,07	0,5±0,1
The suspension of the membrane fraction of G.oxydans	1,2±0,1	1,63±0,09
Immobilized membrane fraction of G.oxydans	0,4±0,1	0,6±0,1

Based on the rate constants it is seen that catalysts based on suspension of G.oxydans cells and the suspension of the membrane fraction of G.oxydans most intensively oxidized both substrates in the anolyte of biofuel cell than biocatalysts basis on immobilized whole cells of G.oxydans and immobilized membrane fraction of G.oxydans. This is primarily due to the fact that the oxidation of substrates by suspension occurs throughout the volume of the cell and in the case of using the membrane fraction of facilitated diffusion substrate to the enzyme active site. However, it should be noted that only part of electrons released during oxidation of a substrate by suspended biocatalyst fall on the electrode, which leads to energy loss in BFC.
The results obtained for suspended and immobilized biocatalyst significantly differ. This is because the oxidation of the substrate and the associated consumption of oxygen free bacterial cells occurs throughout the volume of the anode cells, whereas immobilized cells are capable of oxidizing a substrate and only consume oxygen in the electrode space. Thus, the rate of oxygen consumption in the volume of the anode compartment by immobilized bacteria is much lower than by the suspension of G.oxydans bacterial cells. Furthermore, the ability to consume oxygen by the membrane fraction is much lower than for whole cells that can be attributed to the functionality of the entire set of enzymes and total respiratory chain in whole cell bacterial composition. Regardless of the type of biocatalyst rate of oxygen reduction in the BFC in the presence and absence of electron transport mediators are virtually identical. This indicates that in this system there is no competition between oxygen and electron transport mediator while accepting electrons from reducing biocatalyst sites.
The work supported by the state task in the field of scientific activities of the Ministry of Education and Science of the Russian Federation.