Effect of ethanol on aerobic biodegradation of aromatic compounds

Ethanol has been used as a fuel oxygenate to improve fuel combustion efficiency and decrease exhaust emissions. In recent years, the usage of gasohol, which has a high content of ethanol mixed with conventional gasoline, has been increasing. Soil and groundwater contamination with the high-ethanol-content gasohol due to leaking storage tanks and accidental spills is an emerging environmental problem. Ethanol can influence the distribution and biodegradation of aromatic compounds such as BTEX (benzene, toluene, ethylbenzene, and xylene) that are gasoline compounds. In this study, the effect of ethanol on the aerobic biodegradation of B, T and E was investigated using a BTEXdegrading bacterium, Rhodococcus sp. EH831. Strain EH831 precultured in B, T, or E was inoculated in a 600-ml serum bottle and sealed with a butyl rubber. B, T, E, and ethanol were injected to the bottles: The final addition amount of B, T, or E was 25, 25, and 20 μmol, respectively, and then ethanol was added to the 1:0, 1:4, and 4(aromatic compound, mole):1(ethanol, mole) ratios. The bottles were incubated at 30C and 180 rpm, and then the concentrations of B, T, and E in the headspace were monitored by gas chromatography. The degradation rates of benzene in the conditions of 1:1, 1:4, and 4:1 mixtures were 5.00±0.37, 3.8±0.2, and 4.5±0.1 μmole·g-dry cell wight^-1·h^-1, respectively. Although these values were higher or lower than that at the condition without ethanol (4.3±0.1 μmole.g-dry cell wight^-1·h^-1), there was no statistically difference among these values. The degradation rates of ethylbenzene were from 1.0 to 1.6 μmole·g-dry cell wight^-1·h^-1 in the presence or absence of ethanol. However, the degradation rate of toluene was 4.41±0.04 μmole·g-dry cell wight^-1·h^-1 in the 1:4 mixture, and it was lower than that at the condition without ethanol (5.44±0.01 μmole·g-dry cell wight^-1·h^-1) (p<0.05). Furthermore, the degradation rate of toluene was improved to 6.63±0.06 μmole·g-dry cell wight^-1·h^-1 when the mixture ratio of ethylbenzene and ethanol was 4:1 (mole : mole) (p<0.05). In conclusion, the biodegradation of toluene was inhibited or stimulated by the co-existence of ethanol, even though the biodegradation of benzene and ethylbenzene was not significantly influenced by ethanol.