Bioremediation of hydrocarbon contaminated-oil field drill-cuttings with bacterial isolates

Abstract

The effectiveness of 2 bacterial isolates (Bacillus subtilis and Pseudomonas aeruginosa) in the restoration of oil-field drill-cuttings contaminated with polycyclic aromatic hydrocarbons (PAHs) was studied. A mixture of 4 kg of the drill-cuttings and 0.67 kg of top-soil were charged into triplicate plastic reactors labeled A1 to A3, B1 to B3, C1 to C3 and O1 to O3. These were left quiescent for 7 days under ambient conditions before adding to reactors A1 - A3 and B1 - B3 respectively, 20 ml working solution of pure cultures of Bacillus sp and Pseudomonas sp each of cell density 7.6 x 10¹¹ cfu/ml. Another 20 ml working solution containing the both cultures at cell density 1.5 x 10¹² cfu/ml was added to reactors C1 - C3. The working solution was added to each reactor (excluding the controls, O1 - O3) every 2 weeks mixing and watering of the set-ups was done at 3 days interval under ambient temperature of 30^oC over a period of 6 weeks. After 6 weeks of treatment, results showed that the predominant 3-ring PAHs, which made up 90% w/w, of the total PAHs concentration of 223.52 mg/kg, were degraded below detection and the 4-ring PAHs were reduced from 4 to 0.6% by the Pseudomonas while the Bacillus reduced the 3 and 4-ring PAHs respectively to 0.2 and 0.8%. This showed that the Pseudomonas degraded the 3 and 4-ring PAHs relatively better than the Bacillus. Both strains of bacteria degraded the 5 and 6-ring PAHs below detection limits. Furthermore, within the 3-ring PAHs each of the strains of bacteria reduced phenanthrene to approximately 0.2%, whereas both degraded the homologues acenaphthylene, acenaphthene and fluorene as well as anthracene below detection limits. For the 4-ring PAHs, the Pseudomonas degraded fluoranthene and benzo[a]anthracene while the Bacillus also degraded benzo[a]anthracene below detection limits. The Pseudomonas was able to reduce pyrene and chrysene to 0.3 and 0.2% respectively; whereas the Bacillus reduced fluoranthene, pyrene and chrysene to 0.1, 0.01 and 0.4% respectively. However, treatment with the mixed culture resulted in the limited degradation of the 5-ring PAHs particularly in the fourth week, which may be due to the phenomena of cometabolism and inhibition. The pseudo-first-order degradation rate constant of persistent PAHs ranged from 1.9 x 10^-4 to 9.3 x 10^-2 day^-1. Statistical analyses of results, using the 2- factor analysis-of-variance, showed that the treatments applied resulted in significant (p < 0.05) differences in the biodegradation of the PAHs of the drill cuttings after the 6 weeks of treatment.

Key words: Polycyclic aromatic hydrocarbons, petroleum waste, Bacillus, Pseudomonas, bioremediation.