Redox potential, microbiologically

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

Environmental conditions determine in large part the chemical reactions that will occur when waste is injected. For example, precipitation-dissolution reactions are strongly controlled by pH. Thus, iron oxides, which may be dissolved in acidic wastes, may precipitate when injection-zone mixing increases the pH of the waste. Similarly, redox potential (Eh) exerts a strong control on the type of microbiological degradation of wastes. 

TABLE 2.1 Redox Potentials Important for Selected Microbiological Processes (Madigan et a ., 2000). The Potentials are given under Standard Conditions, i.e., at pH = 7, 25°C, 1 atm and 1 Molar Concentration of Relevant Components... 

Virtually all microbiological reaction pathways change the redox potential, which can also be measured. Of those listed below, there is no method that is inherently superior to any other method better correlating to machine productivity and paper quality. 

Several variables are known to have an influence on corrosion rates in soil these include water, degree of aeration, pH, redox potential, resistivity, soluble ionic species (salts), and microbiological activity. The complex nature of selected variables is presented graphically in Fig. 10.2 [2]. 

The redox potential is essentially a measure of the degree of aeration in a soil. A high redox potential indicates a high oxygen level. Low redox values may provide an indication that conditions are conducive to anaerobic microbiological activity. Sampling of soil will obviously lead to oxygen exposure, and unstable redox potentials are thus likely to be measured in disturbed soil. 

Salvarezza, R.C., de Mele, M.F.L., and Videla, H.A. (1981). Redox potential and the microbiological corrosion of alximinum and its alloys in fuel/water systems. Brit. Corros. J., 16, 162-168. 

Dolfing J, Harrison BK (1993) Redox and Reduction Potentials as Parameters to Predict the Degradation Pathway of Chlorinated Benzenes in Anaerobic Environments. REMS Microbiology Ecology 13 23-30... 

Fig. 4.6 Cyclic voltammograms of a mediator-producing bacterial suspension fed glucose at time 0 (black line), and after 30 min (red line) and 120 min (green line). Arrows indicate peaks while dashed lines that cross between oxidation and reduction peaks intersect the x-axis at the formal potential of the redox couple. The peak at -170 mV disappeared when bacteria were washed by centrifugation, but the peak at 180 mV never disappeared. There were no peaks observed from sterile medium. [From Rabaey et al. (2004), reprinted with permission of the American Society for Microbiology.]...

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