Biodegradability electron acceptors

Table 3-1 Electron acceptors that are used in the biodegradation of organic material in marine sediments. More on the chemistry of these processes is presented in Chapters 8 and 16...

Hutchins SR (1991) Biodegradation of monoaromatic hydrocarbons by aquifer microorganisms using oxygen, nitrate or nitrous oxide as the terminal electron acceptors. Appl Environ Microbiol 57 2403-2407. 

Licht D, BK Ahring, E Arvin (1996) Effects of electron acceptors, reducing agents, and toxic metabolites on anaerobic degradation of heterocyclic compounds. Biodegradation 1 83-90. 

Typical Benzene Biodegradation Reactions under Various Electron Acceptor and Redox Conditions... 

Tetrachoroethylene (perchloroethylene, PCE) is the only chlorinated ethene that resists aerobic biodegradation. This compound can be dechlorinated to less- or nonchlorinated ethenes only under anaerobic conditions. This process, known as reductive dehalogenation, was initially thought to be a co-metabolic activity. Recently, however, it was shown that some bacteria species can use PCE as terminal electron acceptor in their basic metabolism i.e., they couple their growth with the reductive dechlorination of PCE.35 Reductive dehalogenation is a promising method for the remediation of PCE-contaminated sites, provided that the process is well controlled to prevent the buildup of even more toxic intermediates, such as the vinyl chloride, a proven carcinogen. 

However, the many pathways by which MTBE and other oxygenates may be biodegraded anaerobically have been the subject of recent research and ongoing studies. Table 24.12 highlights the various electron acceptors that are used in anaerobic bioremediation studies and contrasts the products of complete anaerobic degradation with those for aerobic metabolism. 

Azo dye-containing wastewaters seems to be one of the most polluted wastewaters, which require efficient decolorization and subsequent aromatic amine metabolism. On the basis of the available literature, it can be concluded that anaerobic-aerobic SBR operations are quite convenient for the complete biodegradation of both azo dyes and their breakdown products. Nevertheless, like the other methods used for biological treatment, SBRs treating colored wastewaters have some limitations. Presence of forceful alternative electron acceptors such as nitrate and oxygen, availability of an electron donor, microorganisms, and cycle times of anaerobic and aerobic reaction phases can be evaluated as quite significant. 

While 02 serves as the electron acceptor in aerobic biodegradation processes forming H20 as the final product, degradation in anaerobic systems depends on alternative electron acceptors such as sulfate, nitrate or carbonate, which yield, ultimately, hydrogen sulfide (H2S), molecular nitrogen (N2) and/or ammonia (NH3) and methane (CH4), respectively. 

Bioremediation usually requires a procedure for stimulation of and maintaining the activity of microorganisms. For biodegradation to be successful, it is necessary to provide a continuous supply of a suitable electron acceptor (such as oxygen or nitrate), nutrients (nitrogen, phosphorus), and a carbon source for energy and cell material. The most commonly deficient components in the subsurface are eiectron acceptors and nutrients. 

For biodegradation to occur, everything that bacteria require for growth and reproduction must be available in the microenvironment in the immediate vicinity of the bacterium. The soil-aquifer system must provide water, attachment medium, a source of carbon, gas exchange, electron acceptor compounds, and nutrients. If any of the required items is not available, bacterial functions will be reduced or cease. 

Vinyl chloride is the least-oxidized chlorinated aliphatic hydrocarbon, and may serve as an electron donor. A vinyl chloride molecule consists of more hydrogen atoms relative to chloride atoms (3 to 1) thus, reductive dechlorination is not favorable to biodegradation. However, under aerobic conditions, vinyl chloride can serve as an electron donor with oxygen as an electron acceptor. 

Without appropriate cleanup measures, BTEX often persist in subsurface environments, endangering groundwater resources and public health. Bioremediation, in conjunction with free product recovery, is one of the most cost-effective approaches to clean up BTEX-contaminated sites [326]. However, while all BTEX compounds are biodegradable, there are several factors that can limit the success of BTEX bioremediation, such as pollutant concentration, active biomass concentration, temperature, pH, presence of other substrates or toxicants, availability of nutrients and electron acceptors, mass transfer limitations, and microbial adaptation. These factors have been recognized in various attempts to optimize clean-up operations. Yet, limited attention has been given to the exploitation of favorable substrate interactions to enhance in situ BTEX biodegradation. 

Keywords Micro-organisms, biodegradation, bioprecipitation, biostimulation, bioaugmentation, electron donor, electron acceptor, injection. 

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 conditions often develop in hydrocarbon-contaminated subsurface sites due to rapid aerobic biodegradation rates and limited supply of oxygen. In the absence of O, oxidized forms or natural organic materials, such as humic substances, are used by microorganisms as electron acceptors. Because many sites polluted by petroleum hydrocarbons are depleted of oxygen, alternative degradation pathways under anaerobic conditions tend to develop. Cervantes et al. (2001) tested the possibility of microbially mediated mineralization of toluene by quinones and humus as terminal electron acceptors. Anaerobic microbial oxidation of toluene to CO, coupled to humus respiration, was demonstrated by use of enriched anaerobic sediments (e.g., from the Amsterdam petroleum harbor). Natural humic acids and... 

Enhancement of Biodegradation by Electron Acceptors Other Than Oxygen... 

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