Asphaltene biodegradation using microorganisms isolated from oil samples
Highlights
► Twenty-five species isolated from oil and polluted soil samples of Dorood oilfield. ► The bacterial mixed culture growth, in a bioreactor was evaluated by CHN and FTIR. ► pH, salinity and asphaltene concentration by B. lentus were optimized using RSM. ► Biodegradation of asphaltene by the mixed culture of five bacteria was 48%. ► Kinetic study showed that the data of asphaltene biodegradation fit to Tessier model.
Introduction
Asphaltenes are petroleum hydrocarbons with extremely complex molecular structure containing sulfur (0.3–10.3%), oxygen (0.3–4.8%), nitrogen (0.6–3.3%), and metal elements, such as Fe, Ni, and V in a small amount. From several studies on asphaltenes [1], [2], [3] there seems to be enough evidence to indicate that the average weight of an individual asphaltene molecule may hardly exceed 1000. However, in crude oil at ambient and high-pressure conditions, as well as in vacuum residuum and toluene solutions, asphaltenes may form aggregates of several molecules, however their molecular weight is in the range of 600 to 2,000,000 [4], [5]. It has been shown that components of asphaltene precipitation are varied, considering precipitator factor and reservoir [4].
One of the most important problems involved in oil industries is the precipitation of heavy components of crude oil such as asphaltenes. The precipitation of these compounds causes different problems like the blockage of crude oil extraction and transport pipes and pollution of ecosystems [6], [7], [8], [9]. Moreover, the complex molecular structure of asphaltenes makes them to resist biodegradation, causing their accumulation in ecosystems where petroleum and its refining by-products are spilled in either accidental or purposeful ways [10], [11]. The complex molecular structure does not allow an easy degradation by the microorganism populations of the ecosystems, which produces an accumulation of asphaltenes in places where they get in touch with both petroleum and the environment [12].
To solve these problems, mechanical, chemical, thermal and electromagnetical methods or combinations of them have been applied. Current methods for removing or preventing heavy hydrocarbon precipitates are expensive and troublesome.
In one hand, environmental restriction increase, high costs and dangers caused by conventional chemical methods, and in the other hand, healthiness, simplicity and probable profitability of biological processes caused increase in number of researchers and oil companies, who are interested in these methods [13], [14], [15]. Until now, few studies have been done for removing asphaltene precipitates in a biological way and not more than 35% biodegradation has been reported [16], [17]. Pineda and coworkers also studied the mineralization of asphaltene precipitates from Maya crude oil by a microbial mixed culture included strains of Bacillus, Brevibacillus, Staphylococcus and Corynebacterium. The isolated strains degraded and used asphaltene as their only carbon and energy source [18].
The main objective of this study was to isolate the microbial species with greater capability for asphaltene biodegradation from an Iranian oilfield collected samples, and to optimize the culture conditions for the asphaltene biodegradation by selected isolates through response surface methodology (RSM). In this work the kinetic behavior of the isolates during the asphaltene biodegradation was also investigated.
Section snippets
Isolation and screening of the microorganisms
Microorganisms were isolated from crude oil and polluted soil samples obtained from Dorood oilfield located in the southern part of Iran. To isolate microorganisms, 2 mL crude oil sample was added to 98 mL of the sterile mineral medium, ISO 9439, and 2 g of the polluted soil to 100 mL of the medium, each separately [18]. This medium consisted of (g L−1): KH2PO4, 0.085; K2HPO4, 0.21; Na2HPO4·2H2O, 0.33; NH4Cl, 0.005; MgSO4·7H2O, 0.0225; CaCl2, 0.0275; FeCl3·6H2O, 0.025; supplied with asphaltene
Isolation and screening of microorganisms
Twenty-five strains isolated from crude oil and polluted soil samples of the Dorood oilfield, 5 strains, TMU2-5, TMU1-1, TMU5-2, TMU8-2 and TMU6-2, were selected based on their degradability of asphaltene as a sole carbon and energy source for further studies. The amount of biodegradations using each of these five strains, in the screening medium and 28 °C after 60 days, were 43%, 42%, 46%, 40% and 43%, respectively. The highest previous biodegradation report of asphaltene in the literature is
Conclusions
Five asphaltene degrading bacterial strains were obtained from crude oil and polluted soil samples of Dorood oilfield in the south of Iran. Maximum degradation ability of 46% and 48% were observed by B. Lentus and the mixed culture of five selected isolates at 28 °C, respectively. Statistical optimization of asphaltene biodegradation was successfully carried out using RSM based on central composite design. The optimum values of pH, salinity and asphaltene concentration for asphaltene
Acknowledgements
This work was financially supported by the Iranian Offshore Oil Company. The authors are grateful to Stat-Ease, Minneapolis, MN, USA, for the provision of the Design-Expert package. The first author also would like to thank Dr. Amir Maghsoodi for his assistance during the course of this research.
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