Evidence of Biological Degradation of BTEX Compounds

Early research into in situ bioremediation identified only aerobic degradation of petroleum hydrocarbon groundwater contaminants. Further research, however, indicated that petroleum hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes (the BTEX compounds), are some of the most aerobically biodegradable found in the subsurface environment . Petroleum aromatic compounds have been shown to degrade by cleavage of the aromatic carbon ring as shown here for benzene:

Although natural or artificial recharge may stimulate aerobic biodegradation by reintroducing oxygen to anaerobic regions, the low solubility of oxygen and the rapid reaction rates typical of aerobic environments can severely limit the aerobic biodegradation of petroleum hydrocarbons

Recent research has recognized the importance of anaerobic degradation of aromatic hydrocarbons, as shown for the BTEX compounds in Table 1 below. Research has shown degradation of aromatic hydrocarbons using nitrate, iron (III), and sulfate as electron acceptors (see Baker and Herson, 1994 for summary). Methanogenic contaminant reduction (i.e., using carbon dioxide as an electron acceptor to form methane gas) has also been demonstrated, but at significantly slower rates than the other degradation processes.The most recent discovery of iron (III) reduction is evidenced by reduced contaminant concentrations and increased levels of aqueous phase iron (II) under anaerobic conditions. Additional research shows that iron (III) is preferentially used over sulphate, but also shows that iron (III) utilization is inhibited by the presence of oxygen and nitrate. For the BTEX compounds in particular, research has shown toluene, ethylbenzene, and some of the xylenes to degrade anaerobically, with toluene being the most anaerobically degradable. Anaerobic degradation of ethylbenzene and xylene appear to be most significant when they are cometabolized with toluene. Evidence of anaerobic degradation of benzene, though, has been inconclusive.

Table 1. Research supporting biodegradation of BTEX compounds. (Modified from Bedient et al., 1994).

Source: Brauner (1995)

Anaerobic biodegradation rates for aromatic hydrocarbons are typically an order of magnitude or more less than aerobic rates. However, anaerobic biodegradation may still significantly influence substrate reduction due to longer reaction times. These longer reaction rates can be attributed to the inefficiency of anaerobes relative to aerobes and the inhibitory effects of alternate electron acceptors present in the subsurface, especially oxygen. The most important factor in determining if a contaminant plume can be successfully remediated may be the identification of the terminal electron accepting process.