• Journal of Microbiology and Biotechnology
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• v.19 no.8
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• pp.836-844
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• 2009

A denitrification bacterium was isolated from riverbed soil and identified as Ochrobactrum sp., whose specific enzymes for denitrification metabolism were biochemically assayed or confirmed with specific coding genes. The denitrification activity of strain G3-1 was proportional to glucose/nitrate balance, which was consistent with the theoretical balance (0.5). The modified graphite felt cathode with neutral red, which functions as a solid electron mediator, enhanced the electron transfer from electrode to bacterial cell. The porous carbon anode was coated with a ceramic membrane and cellulose acetate film in order to permit the penetration of water molecules from the catholyte to the outside through anode, which functions as an air anode. A non-compartmented electrochemical bioreactor (NCEB) comprised of a solid electron mediator and an air anode was employed for cultivation of G3-1 cells. The intact G3-1 cells were immobilized in the solid electron mediator, by which denitrification activity was greatly increased at the lower glucose/nitrate balance than the theoretical balance (0.5). Metabolic stability of the intact G3-1 cells immobilized in the solid electron mediator was extended to 20 days, even at a glucose/nitrate balance of 0.1.

• Biotechnology and Bioprocess Engineering:BBE
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• v.10 no.1
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• pp.47-51
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• 2005

The bioelectrochemical reduction of nitrate in the presence of various mediators including methyl viologen and azure A was studied using a 3-electrode voltammetric system. The catalytic potential for the reduction of the mediators was observed in the reactor, which for methyl viologen and azure A were -0.74 V and -0.32 V, respectively, with respect to the potential of Ag/AgCl reference electrode. This potential was then applied to a working electrode to reduce each mediator for enzymatic nitrate reduction. Nitrite, the product of the reaction, was measured to observe the enzymatic nitrate reduction in the reaction media. Methyl viologen was observed as the most efficient mediator among those tested, while azure A showed the highest electron efficiency at the intrinsic reduction potential when the mediated enzyme reactions were carried out with the freely solubilized mediator. The electron transfer of azure A with respect to time was due to the adhesion of azure A to the hydrophilic surface during the reduction. In addition, the use of the adsorbed mediator on conductive activated carbon was proposed to inhibit the change in the electron transfer rate during the reaction by maintaining a constant mediator concentration and active surface area of the electrode. Azure A showed better than nitrite formation than methyl viologen when used with activated carbon.

• Journal of Microbiology and Biotechnology
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• v.17 no.3
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• pp.428-437
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• 2007

The potential for humic substances to serve as terminal electron acceptors in microbial respiration and the effects of humic substances on microbial azoreduction were investigated. The dissimilatory azoreducing microorganism Shewanella decolorationis S12 was able to conserve energy to support growth from electron transport to humics coupled to the oxidation of various organic substances or $H_2$. Batch experiments suggested that when the concentration of anthraquinone-2-sulfonate (AQS), a humics analog, was lower than 3 mmol/l, azoreduction of strain S12 was accelerated under anaerobic condition. However, there was obvious inhibition to azoreduction when the concentration of the AQS was higher than 5 mmol/l. Another humics analog, anthraquinone-2-sulfonate (AQDS), could still prominently accelerate azoreduction, even when the concentration was up to 12 mmol/l, but the rate of acceleration gradually decreased with the increasing concentration of the AQDS. Toxic experiments revealed that AQS can inhibit growth of strain S12 if the concentration past a critical one, but AQDS had no effect on the metabolism and growth of strain S12 although the concentration was up to 20 mmol/l. These results demonstrated that a low concentration of humic substances not only could serve as the terminal electron acceptors for conserving energy for growth, but also act as redox mediator shuttling electrons for the anaerobic azoreduction by S. decolorationis S12. However, a high concentration of humic substances could inhibit the bacterial azoreduction, resulting on the one hand from the toxic effect on cell metabolism and growth, and on the other hand from competion with azo dyes for electrons as electron acceptor.

• Journal of Soil and Groundwater Environment
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• v.19 no.1
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• pp.63-69
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• 2014

This paper presents the feasibility of using different iron oxides (microscale hematite (HT), microscale magnetite (MT), and nanoscale maghemite (NMH)) in enhancing nitrate reduction by zero-valent iron (Fe(0)) under two solution conditions (artificial acidic water and real groundwater). Addition of MT and NMH into Fe(0) system resulted in enhancement of nitrate reduction compared to Fe(0) along reaction, especially in groundwater condition, while HT had little effect on nitrate reduction in both solutions. Field emission scanning electron microscopy (FESEM) analysis showed association of MT and NMH with Fe(0) surface, presumably due to magnetic attraction. The rate enhancement effect of the minerals is presumed to arise from its role as an electron mediator that facilitated electron transport from Fe(0) to nitrate. The greater enhancement of MT and NMH in groundwater was attributed to surface charge neutralization by calcium and magnesium ions in groundwater, which in turn facilitated adsorption of nitrate on Fe(0) surface.

• Journal of the Korean Solar Energy Society
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• v.23 no.4
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• pp.29-35
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• 2003

We studied the water splitting into $H_2$ and $O_2$ using two different semiconductor photo catalysts and redox mediator, mimicking the Z-scheme mechanism of the photosynthesis, $H_2$ evolution took place on a Pt-$SrTiO_2$ (Cr-Ta doped) photocatalyst using $I^-$ electron donor under the visible light irradiation. The Pt-$WO_3$ photocatalyst showed an excellent activity of the $O_2$ evolution using $IO_3^-$ electron acceptor under visible light. $H_2$ and $O_2$ gases evolved in the stoichiometric ratio($H_2/O_2$=2) under visible light using a mixture of the Pt-$WO_3$ and Pt-$SrTiO_3$ (Cr-Ta doped) suspended in NaI aqueous solution. We proposed a two-step photo-excitation mechanism using redox mediator under the visible irradiation.

• Bulletin of the Korean Chemical Society
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• v.35 no.11
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• pp.3143-3155
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• 2014

This tutorial review provides a general account of the electrochemical behavior of quinones and their various applications. Quinone electrochemistry has been investigated for a long time due to its complexity. A simple point of view is developed that considers the relative stability of the reduced quinone species and the values of the first and second reduction potentials. The 9-membered square scheme in buffered aqueous solutions is explained and semiquinone radical stability is discussed in this context. Quinone redox reaction has also been employed in various studies. Diverse examples are presented under three broad categories defined by the roles of quinone: molecular tool for physical chemistry, versatile electron mediator, and charge storage for energy conversion devices.

• Journal of Microbiology and Biotechnology
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• v.15 no.2
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• pp.281-286
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• 2005

We created a new graphite-Cu(II) electrode and found that the electrode could catalyze FADH$_2$ oxidation and FAD reduction coupled to electricity production and consumption, respectively. In a fuel cell with graphite-Cu(II) anode and graphite-Fe(III) cathode, the electricity was produced by coupling to the spontaneous oxidation of FADH$_2$ Fumarate and xylose were not produced from the enzymatic oxidation of succinate and xylitol without FAD, respectively, but produced with FAD. The production of fumarate and xylose in the reactor with FAD electrochemically regenerated was maximally 2- 5 times higher than that in the reactor with FAD. By using this new electrode with catalytic function, a bioelectrocatalysts can be engineered; namely, oxidoreductase (e.g., lactate dehydrogenase) and FAD can function for biotransformation without an electron mediator and second oxidoreductase for cofactors recycling.

• Bulletin of the Korean Chemical Society
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• v.35 no.9
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• pp.2803-2808
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• 2014

High potential and fast electron transfer of a cathode mediator are significant factors for improving the performance of biofuel cells. This paper reports the first synthesis of a cathode redox polymer that is a coordination complex of poly (acrylic acid-vinylpyridine-acryl amide) (PAA-PVP-PAA) and [Os(4,4'-dicarboxylic acid-2,2'-bipyridine)$_2Cl_2]^{/+}$ ($E^{\circ}=0.48V$ versus Ag/AgCl). Bilirubin oxidase can be easily incorporated into this polymer matrix, which carried out the four-electron oxygen under typical physiological conditions (pH 7.2, 0.14 M NaCl, and $37^{\circ}C$). This new polymer showed an approximately 0.1 V higher redox potential than existing cathode mediators such as PAA-PVI-$[Os(dCl-bpy)_2Cl]^{+/2+}$. In addition, we suggest increasing the polymer solubility with two hydrophilic groups present in the polymer skeleton to further improve fast electron transfer within the active sites of the enzyme. The maximum power density achieved was 60% higher than that of PAA-PVI-$[Os(dCl-bpy)_2Cl]^{+/2+}$. Furthermore, high current density and electrode stability were confirmed for this osmium polymer, which makes it a promising candidate for high-efficiency biofuel cells.

• Bulletin of the Korean Chemical Society
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• v.32 no.11
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• pp.3849-3850
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• 2011

• Journal of Microbiology and Biotechnology
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• v.16 no.5
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• pp.678-682
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• 2006

To remove nitrate from wastewater, a novel bioelectrochemical denitrification system is introduced. In this proposed system, biological reactions are coupled with reactions on the electrode, whereby the electrons are transferred to the bacterial enzymes via a mediator as an electron carrier. The denitrification reaction was achieved with permeabilized Ochrobactrum anthropi SY509 containing denitrifying enzymes, such as nitrate reductase, nitrite reductase, and nitrous oxide reductase, and methyl viologen was used as the mediator. The electron transfer from the electrode to the enzymes in the bacterial cells was confirmed using cyclic voltammetry. A high removal efficiency of nitrate was achieved when the bioelectrochemical system was used with the permeabilized cells. Furthermore, when the permeabilized cells were immobilized to a graphite felt electrode using a calcium alginate matrix containing graphite powder, a high removal efficiency was achieved (4.38 nmol/min mg cell) that was comparable to the result when using the free permeabilized cells.