Asphaltenes biodegradation under shaking and static conditions
Introduction
Asphaltenes, causing such problems as reduction in oil recovery and environmental contamination, are thought to be recalcitrant to biological transformations [1], [2], [3], [4], [5]. Asphaltenes are high molecular weight solids which are soluble in aromatic solvents such as benzene and toluene and insoluble in paraffinic solvents. The transformation rates for these large molecules are limited by low solubility and mass transfer rates in aqueous media [6]. Despite these facts, there is evidence in the literature for bacterial transformation of these complex, high molecular weight substrates. This is possible because these compounds contain carbon, hydrogen, sulfur, nitrogen and oxygen, which are necessary elements for microbial growth [7], [8], [9], [10], [11].
Some researchers extracted asphaltenes from crude oil and examined their biodegradability as a separated fraction. Pendrys isolated seven gram negative, aerobic asphaltenes degrading bacteria by an enrichment technique. The predominant genera of these isolates were Pseusomonas, Acinetobacter, Alcaligenes and Flavobacterium. A mixed culture of these bacteria could use asphaltenes as a sole source of carbon and energy [12]. Pineda et al. reported the utilization of asphaltenes as a sole source of carbon and energy by a microbial consortium isolated from Maya crude oil. The isolates were identified as Corynebacterium sp., Bacillus sp., Brevibacillus sp., and Staphylococcus sp. [13]. Tavassoli et al. reported 46% biodegradation of asphaltenes after two months, with the initial concentration of 5 g/L. They reported the capability of the microorganisms, identified as Pseudomonas sp, Bacillus licheniformis, Bacillus lentus, Bacillus cereus and Bacillus firmus, to utilize asphaltenes as carbon and energy source. Furthermore, they concluded that asphaltenes biodegradation follows Tessier kinetics model [14]. In another development Lavania et al. isolated an asphaltenes degrading bacterial species identified as Garciaella petrolearia. Growth of this bacterium on asphaltenes resulted in viscosity reduction of up to 37% [15].
The main goal of this study was to investigate the biodegradability of asphaltenes extracted from a crude oil of an Iranian oilfield. Biodegradation studies comprised of isolation of indigenous microorganisms, quantification of biodegradation under shaking and static conditions, and kinetic studies under shaking conditions.
Section snippets
Asphaltenes extraction
Asphaltenes were extracted from a crude oil sample from southern Bangestan oilfield in Iran. The crude oil contained 13% asphaltene according to SARA (Saturated, Aromatic, Resin and Asphaltene content) test. ASTM D3279-90 standard procedure was used to extract asphaltenes from the crude oil. In summary, one liter of the crude oil was dissolved in 40 L n-heptane. Unsolved fraction of the oil was then separated using a filter paper (Watman No. 42), and dried at 40 °C for 24 h.
Isolation of microorganisms
A liquid medium
Identification of microorganisms
From the four soils and sludge samples, four separate microbial consortia were obtained. The members of all consortia were identified. Table 1 shows the results of biochemical tests. The sequences of isolates were submitted to National Center for Biotechnology Information (NCBI) (www.ncbi.nlm.nih.gov) for species identification. Table 2 shows the species obtained from the original samples.
Asphaltenes biodegradation
Fig. 1, Fig. 2 show the degree of biodegradation, after two months of incubation, under shaking and static
Conclusion
From this research work it was concluded that asphaltenes fraction of petroleum is biodegradable, but the rate of biodegradation is slow compared to other oil fractions. Seven bacterial species namely, Pseudomonas aeruginosa, Pseudomonas fluorescens, Citrobacter amalonaticus, Enterobacter cloacae, Staphylococcus hominis, Bacillus cereus, and Lysinibacillus fusiformis were confirmed in this study to be asphaltenes degraders. Shaking enhances the rate of asphaltenes biodegradation, and Tessier
Acknowledgements
The first author would like to thank EOR research center in School of Chemical, Gas and Petroleum Engineering, Shiraz University for laboratory instruments and financial supports. Cooperation and scientific aids provided by the Institute of Biotechnology, Shiraz University is sincerely acknowledged.
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