BIOREMEDIATION OF SELECTED PETROLEUM HYDROCARBONS IN SOIL MICROCOSMS

Abstract

Bioremediation, which uses natural (indigenous) or inoculated microbial activities for the detoxification and removal of environmental pollutants, has become an effective remedial measure for soils contaminated with oil and petroleum hydrocarbons. To date, there appears to be no accurate quantitative method for estimating the biodegradation rate of the hydrocarbons. This has been attributed to the complexity of the multi-phase problem. Besides, a multitude of other factors also affect the rate and extent of biodegradation. This study is thus focused on the investigation of the effects of various important parameters on the biodegradation rate of eight selected petroleum hydrocarbons in seven soil microcosms. It was found at the onset that the total viable count of the pristine soil was of the order usually found in soil and that the hydrocarbon-degrading bacterial count was low. The addition of hydrocarbons highly stimulated the growth of both total viable and hydrocarbon-degrading bacteria, with the effect being greater for the hydrocarbon degraders. In addition, pH control was found critical for the hydrocarbon-degraders. Tilling, air venting and nutrient addition can enhance the growth of both viable and hydrocarbon-degrading bacteria. The plot supplemented with indigenous bacteria recorded the highest microbial levels of both total viable bacteria and hydrocarbon degraders. All alkanes and aromatics used in this experiment were biodegradable. Hydrocarbons with low molecular weight such as ethylbenzene and decane disappeared most quickly. Their removals were primarily caused by evaporation. Intermediate molecular weight hydrocarbons dodecane and naphthalene removals were attributed to both biodegradation and evaporation, and the losses of higher molecular weight hydrocarbons hexadecane, heptadecane, phenanthrene and pristane were mainly due to the biodegradation. Amongst all the chemicals, pristane was the most recalcitrant compound and degraded at the slowest rate. The degradation rate of different hydrocarbons was found to be in the order of ethylbenzene > decane > naphthalene > dodecane > hexadecane > heptadecane > phenanthrene > pristane. Oxygen, temperature, pH and the level of nutrient concentrations were found to be the predominant factors in determining biodegradation rates in soil. Introduction of oxygen into contaminated soils by tilling and air venting was shown to increase the biodegradation rate. Temperature in this study (27°C to 32°C) was within the ideal range of 20°C to 40°C. Higher degradation rate was found in the plot with the pH controlled at about 7. The addition of inorganic nutrients was very effective in accelerating the biodegradation rate of hydrocarbons in soil. However, the supplementation of indigenous hydrocarbon degraders, along with the necessary nutrients was proven to be the most effective in increasing the rate of biodegradation. The concentration of hydrocarbons approached the background level of uncontaminated soil after 100 days of bioremediation. The natural degradation in the untreated plot was found to occur at a relatively slower rate as compared to the other treated plots.