The three most significant groups of bacteria that may mineralize hazardous organic compounds are as follows:
1. Denitrifiers, which reduce nitrate to nitrogen
2. Sulfate reducers, which reduce sulfate to hydrogen sulfide
3. Methanogens, which reduce carbon dioxide to methane
Biodegradation of organic compounds under denitrifying conditions has been the least-studied of the three groups. Ehrlich and colleagues89 inferred that acrylonitrile injected into a carbonate aquifer was completely degraded because the waste was not found in samples taken from a monitoring well where the waste arrived about 260 d after injection began, or in any subsequent samples. Bouwer and McCarty90 observed partial to almost complete degradation of carbon tetrachloride (>95%), bromodichloromethane (>55%), dibromochloromethane (>85%), and bromoform (>90%) in laboratory batch experiments simulating denitrifying conditions. Compounds studied that did not show significant degradation under these conditions include chlorinated benzenes, ethylbenzene, naphthalene, chloroform, 1,1,1-trichloroethane, and 1,2-dibromomethane. Phthalic acids, phenol, tri-sodium nitrilotriacetate, and o- and m-xylene3 are other compounds for which degradation has been observed under denitrifying conditions.
Degradation of organic compounds by sulfate-reducing bacteria has been studied mostly in the context of petroleum deposits.91,92 These microbes are good scavengers of organic waste products regardless of the source of the waste. Novelli and ZoBell91 reported finding some strains of sulfate-reducing bacteria that use hydrocarbons, beginning with decane and higher forms, paraffin oil and paraffin wax. In this study, the aromatic hydrocarbons—benzene, xylene, anthracene, andnaphthalene— are not degraded, nor are aliphatic hydrocarbons, hydrocarbons with molecular weight lower than that of decane, or hydrocarbons of the naphthene series (cyclohexane). Rosenfeld92 reported that high-molecular-weight aliphatic hydrocarbons are quickly decomposed by sulfate-reducing bacteria. However, the thinking is that molecular oxygen is required to degrade saturated hydrocarbons and that the experiments in the above-cited papers did not fully simulate anoxic conditions.
Degradation of organic compounds by methanogens has been the most extensively studied of the three groups. Methanogenic bacteria can readily degrade a number of monocyclic aromatics, phenol and some chlorophenols, benzene, ethyl benzene and a number of C1 and C2 halogenated aliphatic compounds.3 However, the amount of degradation depends on the specific compound and conditions favorable for bacteria that can adapt to degrade the compound.
Biodegradation in groundwater systems may involve complex interactions among many types of bacteria, including denitrifying, sulfate-reducing, methanogenic, and others. Whether complete mineralization occurs depends on the compound, environmental conditions at the site, and the microorganisms that are best adapted to those conditions.
Iron- and manganese-reducing and ammonia-producing bacteria may also be significant in biochemical reactions that occur in the subsurface environment. Iron and manganese oxides are usually broken down through microbial reduction. Consequently, the possibility of this process should be considered when evaluating chemical reactions of iron and manganese species in the deep-well environment. Lovley93 reviews the literature on biomineralization of organic matter with the reduction of ferric iron, and Ehrlich94 reviews the literature on manganese oxide reduction through anaerobic respiration.
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