Introduction
The widespread use of crude oil and its products has caused numerous environmental pollutions. One of such admixtures is the sludge found at the bottom of the crude oil storage tanks, i.e. a sticky and relatively solid compound. This sludge is formed by crude oil storage in refinery tanks. The discharge of this oily sludge into the soil poses great risks to the environment and human health; it enters volatile hydrocarbons into the air and leaks pollutants into groundwater and soil. Various biochemical technologies, such as heat treatment, stabilization, and incineration, are used to remove this pollutant from regions with the relatively low area; however, these methods are not economical for eliminating widespread pollution. As a result, more efficient economical methods, like bioremediation, are required. The bioremediation of petroleum-contaminated environments is a modern approach to clean these areas and reduce environmental pollution.
Due to the different resistance of crude oil compounds to biodegradation, the isolation and screening of resistant and high yielding microbial strains are essential. If the selected strain is capable of consuming and decomposing petroleum compounds, petroleum hydrocarbons are used as a source of carbon and energy along with the mineral nutrients required to make their biomass. Thus, as the microbes grow, the contamination is gradually removed from the environment. Microorganisms living in petroleum-contaminated environments are more capable of decomposing hydrocarbons; therefore, to remove oil contamination, selecting microorganisms from natural environments, i.e. more enzymatic than other microorganisms, is highly cost-effective. This study aimed to isolate, identify, and determine the metabolic properties of a native resistant bacteria capable of degrading oil sludge.
Materials and methods
Bushnell-Haas medium was used for the isolation and screening of oil-degrading bacteria. After the initial isolation of 24 bacteria from the oil sludge, Acinetobacter radioresistens strain KA2 with the highest growth and degradation ability was selected. To ensure the concentration and quality of the extracted bacterial DNA, the absorbance ratio of the samples at a wavelength of 260 and 280 nm was measured by a UV spectrophotometer. The 16s rRNA sequences were used for the identification of the molecular form of bacteria; thus, the DNA extracted from this bacterium was applied for Polymerase Chain Reaction (PCR) analysis. Then, the degradation of various concentrations of crude oil at different PHs (5, 6, 7, 8, & 9), bacterial adherence to hydrocarbons (BATH), and the emulsification index of the selected strain were measured.
Results
Isolated strain characteristics for crude oil degradation were determined by 16S rRNA gene sequencing. Sequence-similarity search in NCBI Genbank suggested that the isolated bacterium was Acinetobacter radioresistens strain KA2. The reported number for the isolated bacterium was MK127544. The obtained results revealed that the degradation efficiency of the isolated strain for the crude oil at the concentrations of 1%, 2%, 3%, 4%, and 5%, after 7 days of incubation were 65.24, 76.14, 53.81, 31.84, and 25.21%, respectively; therefore, the decomposition of petroleum hydrocarbons was affected by its initial concentration, where the high concentrations of crude oil decreased the decomposition rate. We also observed that the initial crude oil concentration of 2% was the most appropriate concentration for bacterial growth in consuming petroleum hydrocarbons efficiently. Crude oil degradation at pH values of 5, 6, 7, 8, and 9 was equal to 42.4, 69.16, 65.24, 59.41, and 48.24%, respectively. Crude oil degradation and bacterial growth rates significantly reduced at pH values of 5 and 9, respectively. The emulsification index was calculated to investigate the potential of the strain in biosurfactant production. The obtained emulsification index for the study strain was equal to 59.14%. The biosurfactant reduces the surface tension between the liquid and solid phases and increases the amount of emulsification; therefore, the intracellular absorption and degradation of petroleum hydrocarbons improve. The BATH of the isolated strain was also calculated to evaluate the bacterial affinity to petroleum hydrocarbons. The obtained BATH was equal to 13.69%. Therefore, the bacterial tendency to attach to petroleum compounds improved the decomposition of petroleum hydrocarbons.
Conclusion
Oil sludge in the petrochemical industries could be efficiently treated by biodegradation as an economical and environmental-friendly method; Acinetobacter radioresistens strain KA2 could be used in this regard.
Ethical Considerations
Compliance with ethical guidelines
This study has been approved by the Research Ethics Committee of the Islamic Azad University of Kermanshah Branch (code: 19250587962001 ).
Funding
This study was extracted from a PhD. thesis of Mohammad Saeed Poorsoleiman in Department of Natural Resources and Environment, Kermanshah Branch, Islamic Azad University, Kermanshah, and received financial support from the Islamic Azad University of Kermanshah Branch.
Authors' contributions
Investigation, analysis, and initial draft preparation: Mohammad Saeed Poorsoleiman; conceptualization, methodology, validation, formal analysis, investigation, resources, original draft preparation, writing-review & editing, visualization, supervision: Seyed Ahmad Hosseini, Alireza Etminan, Hamid Abtahi, and Ali Koolivand.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
The authors would like to thank the personnel of Arak University of Medical Sciences and Islamic Azad University of Kermanshah Branch for their valuable cooperation.
References