Microbial processes terminal electron acceptors

As a result of the highly reduced state of petroleum hydrocarbons, the preferred and most thermodynamically terminal electron acceptor for microbial processes is oxygen. The inverse relationship between the concentrations of BTEX and dissolved oxygen within a plume is indicative of the extent of microbial metabolism of this class of contaminant. Data from various sites indicate that the natural attenuation of BTEX proceeds at higher rates under oxygenated conditions. The biodegradation of... 

Anaerobic metabolism occnrs nnder conditions in which the diffusion rate is insufficient to meet the microbial demand, and alternative electron acceptors are needed. The type of anaerobic microbial reaction controls the redox potential (Eh), the denitrification process, reduction of Mu and SO , and the transformation of selenium and arsenate. Keeney (1983) emphasized that denitrification is the most significant anaerobic reaction occurring in the subsurface. Denitrification may be defined as the process in which N-oxides serve as terminal electron acceptors for respiratory electron transport (Firestone 1982), because nitrification and NOj" reduction to produce gaseous N-oxides. hi this case, a reduced electron-donating substrate enhances the formation of more N-oxides through numerous elechocarriers. Anaerobic conditions also lead to the transformation of organic toxic compounds (e.g., DDT) in many cases, these transformations are more rapid than under aerobic conditions. 

Another solid-phase treatment involves the same approach to enhancing microbial activity but relies on a different way of providing O2. Here, additional air is provided by vacuum extraction of soil above the water table (i.e., the vadose zone or the unsaturated soil layer), thereby supplying the terminal electron acceptor needed by the aerobic bacteria. This process, designed for hydrocarbon-contaminated sites, is termed bioventing or simply venting. 

Trimble and Ehrlich (1968) have suggested that adapted cultures may use Mn(IV) in preference to oxygen as the terminal electron acceptor, and that Mn(IV) and O2 do not compete with each other in such cultures. They have summarized the process of microbial solubilization of Mn02(s) as follows ... 

A role of microbial processes in release of arsenic into groundwater concomitant with the reductive dissolution of Fe(ni) oxyhydroxides has been suggested based on the observed correlation between dissolved arsenic and bicarbonate concentrations (94,95). Increased bicarbonate concentrations are attributed to the oxidation of organic matter with Fe(III) oxyhydroxides as the terminal electron acceptor. Like oxidative dissolution, reductive dissolution may be kinetically limited. Rates of microbial reduction may be limited by the supply (and nature) of organic carbon. 

Two experiments out of a series of laboratory studies were selected for this purpose. The first was conducted in order to study the microbially mediated degradation of benzene, toluene, m,/ ,o-xylene, and naphthalene with sulfate as terminal electron acceptor (Winter, 1997). No degradation could be observed for benzene, i,/7-xylene and naphthalene and therefore the simulations were carried out for toluene and o-xylene only. This experiment will be referred to as Experiment 1. The second experiment described in this paper (Experiment 2) was performed with special emphasis on the interaction of the toluene and o-xylene degradation processes. 

The methods required to create and maintain conditions that are suitable for growing anaerobic cultures are more difficult than those required for culturing aerobic cultures. Nonetheless, use of methods such as the serum bottle modification of the Hungate technique [24] is now routine in many laboratories. As discussed in the previous section, the formulation of the medium depends on whether the fiber or fabric is to serve as the sole source of carbon, nitrogen or sulfur. In addition, the formulation of the anaerobic culture medium depends upon which terminal electron acceptor is to be considered in the study. As shown in Table 1.2, the list of terminal electron acceptors includes Mn(IV), nitrate, Fe(III), and sulfate. In addition, bicarbonate (carbon dioxide) serves as the terminal electron acceptor for methanogenesis (equation 1.1). Supplementing the medium with an abundant supply of one of the terminal electron acceptors prescribes the microbial process that occurs in the cultures. Fermentation, in which some oxidized organic compound serves as the terminal electron acceptor, also occurs under anaerobic conditions. 

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