Toluene anaerobic biodegradation

Fig. 4. Proposed initial steps in the anaerobic biodegradation of toluene in different organisms.

Ahad JME, BS Foliar, EA Edwards, GF Slater, BE Sleep (2000) Carbon isotope fractionation during anaerobic biodegradation of toluene implications for intrinsic bioremediation. Environ Sci Technol 34 892-896. 

The application of deuterated toluene in assessing anaerobic biodegradation (Fischer et al. 2006) has already been noted in Chapter 6, Part 1. A review (Lovley 1997) has summarized the various strategies and suggests that uncertainties, particularly in the bioremediation of benzene, can only be resolved by greater emphasis on field-oriented studies, and a better understanding of the reactions involved and the factors that limit the rates of degradation. 

Table 3.2. Summary of results from reports on the anaerobic biodegradation of toluene... 

As with nitrate, the anaerobic biodegradation of m- and /-xylene could also be linked with the reduction of sulfate at a rate of about 1 /M/day. Edwards et al. (1992) demonstrated the sulfate-dependent degradation of all three xylene isomers. Interestingly, the xylenes were not degraded until toluene was almost depleted. 

In contrast, Meckenstock et al. [280] reported larger isotopic enrichments in residual toluene, 3-6%o and up to 10%o during anaerobic and aerobic biodegradation experiments, respectively. These results indicated that isotopic fractionation effects may be different for different compounds, terminal electron-accepting processes (TEAP), degradative metabolic pathways, or microbial populations. 

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... 

Toluene, or methylbenzene, is the second most water-soluble hydrocarbon found in refined gasoline and is on the EPA s regulatory list of pollutant chemicals. Contrary to belief of only a very few years ago, toluene is now known to be biodegradable under a wide variety of anaerobic conditions (Table 3.2). Research on this topic has progressed rapidly and reached the point where several isolates have been obtained and different metabolic pathways have been proposed. 

Biodegradation. Nitrosubstituted compounds are subject to a variety of degradative processes. Under anaerobic conditions TNT is readily reduced to the corresponding aromatic amines and subsequently deaminated to toluene. As shown in the section on hydrocarbons, Llie latter can be mineralized under anaerobic conditions, leading to the potentially complete mineralization of TNT in the absence of oxygen. 

The biodegradation of trichloroethylene is the most studied since this is probably the most widespread halogenated solvent contaminant. Several substrates drive ttichlorethylene co-oxidation, including methane, propane, propylene, toluene, isopropylbenzene, and ammonia (25). The enzymes that metabolize these substrates have subtly different selectivities with regard to the halogenated solvents, and to date none are capable of co-oxidizing carbon tetrachloride or tetrachloroethylene. Complete mineralization of these compounds can, however, be achieved by sequential anaerobic and aerobic process. Biorem edia tion. 

---