Redox system reduction polarization

Also ascomycetes yeast strains showed decolorizing behaviors due to extracellular reactions on polar dyes. The process occur when an alternative carbon and energy source is available. The involvement of an externally directed plasma membrane redox system was suggested in S. cerevisiae, the plasma membrane ferric reductase system participates in the extracellular reduction of azo dyes [25]. 

Some redox systems which appear reversible in the absence of current manifest a considerable polarization on the passage of current through the electrode in the course of electrolytic reduction. Reduction of pentavalent vanadium ions at a smooth platinum electrode in a solution of vanadic acid in sulphuric acid oan be quoted as an example. This reduotion takes place in two steps, i. e. Vv -> VIV and Vxv -> V111. Similar behavior is manifested by molybdenio acid dissolved either in hydroohlorio or sulphuric aoid in the course... 

Interestingly, the anodic dark current at n-Ge electrodes increases considerably upon addition of the oxidized species of a redox system, for instance Ce" ", to the electrolyte, as shown in Fig. 8.4 [7]. The cathodic current is due to the reduction of Ce. The latter process occurs also via the valence band (see Chapter 7), i.e. since electrons are transferred from the valence band to Ce", holes are injected into the Ge electrode. Under cathodic polarization these holes drift into the bulk of the semiconductor where they recombine with the electrons (majority carriers) and the latter finally carry the cathodic current. In the case of anodic polarization, however, the injected holes remain at the interface and are consumed for the anodic decomposition of germanium, as illustrated in the insert of Fig. 8.4. Accordingly, the cathodic and anodic current should be compensated to zero. Since, however, the anodic current is increased upon addition of the redox system there is obviously a current multiplication involved, similarly to the case of two-step redox processes (see Section 7.6). Thus, in step (e) (Fig. 8.1) electrons are injected into the conduction band. This experimental result is a very nice proof of the analytical result presented by Brattain and Garrett [3]. 

Figure 1-29. Superposition of the current density potential curves of an Me/Me " and a redox electrode, which yields the polarization curve of anodic metal dissolution and cathodic reduction of the redox system Eq.m nd Fq, redox t Nernst potentials, r is the rest potential, i o,m

Figure 1-29 has shown the superposition of metal dissolution and a cathodic process which leads to zero total current density i and a corrosion rate I c at the rest potential Sr. If the Nernst potential of the metal/met-al-ion electrode and the involved redox system are sufficiently separated and the related I-E characteristic is sufficiently steep, the polarization curve only contains the anodic metal dissolution and the cathodic reduction of the redox system. The related opposite reactions are neglected. 

For large cathodic polarizations, the reduction of the redox system equals the total current density. Figure 1-30 presents the logarithm of the partial current densities for large anodic or cathodic n as an example. 

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

To reach the reductive step of the azo bond cleavage, due to the reaction between reduced electron carriers (flavins or hydroquinones) and azo dyes, either the reduced electron carrier or the azo compound should pass the cell plasma membrane barrier. Highly polar azo dyes, such as sulfonated compounds, cannot pass the plasma membrane barrier, as sulfonic acid substitution of the azo dye structure apparently blocks effective dye permeation [28], The removal of the block to the dye permeation by treatment with toluene of Bacillus cereus cells induced a significant increase of the uptake of sulfonated azo dyes and of their reduction rate [29]. Moreover, cell extracts usually show to be more active in anaerobic reduction of azo dyes than whole cells. Therefore, intracellular reductases activities are not the best way to reach sulfonated azo dyes reduction the biological systems in which the transport of redox mediators or of azo dye through the plasma membrane is not required are preferable to achieve their degradation [13]. 

Another way of arranging the intramolecular transmembrane electron transfer is to use the so called molecular wires, i.e. molecules with the electron conduction chain of conjugated bonds, redox active polar terminal groups and the length sufficient to span across the membrane. Such molecules can in principle provide for electron transfer from the externally added or photogenerated reductant across the membrane to the oxidant. This mechanism was suggested [41, 94] to explain the action of carotene-containing System 1 and 38 of Table 1. However, as it was shown later, the transmembrane PET in these systems proceeded also without carotene. 

---