Potentials for use of biosurfactants in oil spills cleanup and oil bioremediation

Transcription

1 Potentials for use of biosurfactants in oil spills cleanup and oil bioremediation I.M. Banat School of Environmental Studies, University of Ulster, Northern Ireland Abstract Biosurfactant are environmental friendly, effective and stable compounds with many advantages over synthetic surfactants. Their use is gaining prominence in several industrial applications due to their broad capabilities which includes emulsification, wetting, foaming, solubilisation and viscosity reduction. Their main types are glycolipids in which carbohydrates are attached to a long-chain aliphatic acids. Other more complex types such as lipopeptides, lipoproteins and heteropolysaccharides also exist. The use of biosurfactants for oil spills cleanup or enhanced oil recovery involves a reduction of the oil-water interfacial tension leading to its emulsification. Stable emulsions are formed because biosurfactants lowers interfacial tension between interfaces and oil. Such an effect can be achieved through the direct addition of active microbial cells to the contaminated environments or augmentation with the biosurfactant compounds. Laboratory studies have shown that the addition of biosurfactant mixtures alone may be useful for stimulating biodegradation of hydrocarbon contaminants in the environment. Biosurfactants are also useful in solubilisation and removal of oil from sand and sludge in oil storage tanks. In ecological terms, the use of biosurfactants is obvious in closed systems but remains speculative in the open environment. The precise mechanism of enhanced oil recovery in situ however, remains unclear and mainly economically unfeasible. Both product characterisation and production process optimisation are expected to facilitate biosurfactants' future applications particularly in oil-related industries and environmental protection and by cleaning-up agencies. This article reviews the state of the art in potential uses in biosurfactants in oil bioremediation.

2 178 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control Introduction Biosurfactants are extracellular amphiphilic compounds produced by microorganisms. They contain both a hydrophobic and a hydrophilic moieties that renders them capable of reducing surface tension and interfacial tensions between molecules at the surfaces and interfaces leading to the formation of micro-emulsions of oil in water or water in oil. The worldwide surfactants markets in 1994 was estimated at around $9.4 billion per annum [1] and is expected to steadily increase [2]. Most of the commercially available surfactants are petroleum-derived chemical. Rapid advances in biotechnology and increased environmental awareness combined with expected new legislation has provided further impetus for consideration of biological surfactants as possible alternatives to the existing products. Microbial biosurfactants' spontaneous release and function are often related to hydrocarbon uptake; therefore, they are predominantly synthesized by hydrocarbon degrading microorganisms. Some, however, have been reported produced on water-soluble compounds such as glucose, sucrose, glycerol or ethanol. In some instances, these compounds have antibiotic properties that may serve to disrupt membranes of microorganisms competing for food. Low toxicity, biodegradable nature and diversity have gained them considerable interest in recent years. The range of potential industrial applications includes enhanced oil recovery (EOR), crude oil drilling, lubricants, surfactant aided bioremediation. Other developing areas of biosurfactants use include health care, cosmetic and foods industries. A large variety of biosurfactants are known; their type, quantity and quality are influenced by the nature of the C, N, P, Mg, Fe and Mn ions available in the medium and their culture conditions, including ph, temperature, agitation and dilution rate. When considering what microorganisms to use for EOR, the varying conditions in which they will be used, such as temperature, pressure, ph and salinity must be assessed. Typically, microorganisms injected into an oil well should be able to endure high temperatures, pressures, salinity and be capable of growth under anaerobic or microaerophilic conditions. Several types of biosurfactants have been isolated and characterised including glycolipids, phospholipids, neutral lipids, fatty acids, peptidolipids, lipopolysaccarides and others not fully characterised [2], Certain microorganisms are likely to be found better adapted to particular environments such as oil reservoirs, soil or the ocean environment. Several techniques have been developed to identify biosurfactant-producing microbes. These technique include: 1- The axisymmetric drop shape analysis by profile (ADSA-P), which simultaneously determines the contact angle and liquid surface tension from the profile of a culture droplet resting on a solid surface. 2- Coloured indicator technique for determining anionogenic bacterial peptidolipid based on the ability of the anionic surfactants to form a coloured complex with the cationic indicator such as methylene blue [3]. 3- Measurements of surface and interfacial tension reductions in culture broth.

3 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control 179 In addition to the above other simpler methods were described [4] such as blood hemolysis (a known characteristic for some biosurfactant compounds), the emulsification index value (E-24) obtained on kerosene or simply by observing drop collapsing characteristics of culture broth suspensions placed on an oil-coated surface. Drops containing biosurfactants were observed to collapse, whereas non-surfactant-containing drops remain stable. Biosurfactant production and recovery Conditions that promote biosurfactants production vary and have been determined for several microorganisms. On the one hand, the most common bacteria reported (f. aeruginosd) has been shown to produce rhamnolipids biosurfactants on C12 n-alkanes [5]. Increased production was noted in some cases in phosphate-limited medium [6], or upon the exhaustion of nitrogen in the medium [7]. On the other hand, a Rhodococcus sp. had maximum growth and biosurfactant production on medium containing 2%(v/v) n-paraffin using nitrate as N source and it's product was found to be a primary metabolite that could be produced in continuous culture [8]. Other substrate such as olive oil mill effluent (Oome), whey and peat pressate have also been used for biosurfactant production. A Pseudomonas strain Pet-1006 required two carbon sources; a readily available one (glucose) and a hydrocarbon (oleic acid) to be utilised upon glucose exhaustion therefore triggering biosurfactant production [9]. Biosurfactants recovery from medium or microorganisms is usually desirable. Several procedures have been reported the most common being methanol or isopropanol precipitation, or acidification of culture media followed by solvent extraction with chloroform / methanol. Biosurfactants applications Oil industry is the largest market expected for biosurfactants use, both in petroleum production and incorporation into oil formulations. Other applications related to the oil industries includes oil spill bioremediation/dispersion, both inland and at sea, removal/ mobilisation of oil sludge from storage tanks and enhanced oil recovery [4,10]. The second largest market for biosurfactants is emulsion polymerisation for paints, paper and industrial coatings. Surfactants are also used in food and cosmetic industries, industrial cleaning of products as well as in agricultural chemicals as pesticides and to dilute and disperse fertilisers and enhance penetration of active compounds into plants. Various potential applications of biosurfactants are shown in Table 1, some of which are discussed below.

4 180 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control Table 1. Areas of possible potential applications for biosurfactants in industry. No Property Emulsifiers and dispersant Solubilizers and microemulsions Wetting and penetrating agents Detergents Foaming agents Thickening agents Metal sequestering agents Vesicle forming materials Microbial growth enhancers Demulsifiers Fungicide Viscosity reducing agents Dispersants Potential fields of application Cosmetics, paints, bioremediation, oil tanks cleaning Toiletries and pharmaceuticals Pharmaceuticals, textile industry and paints Household, agriculture and high tech. Products Toiletries, cosmetics and ore floatation Paints Mining Cosmetics, drug delivery system Sewage sludge treatments for oily wastes Waste treatment and oil recovery/separation Biological control of some plant pathogens Pipeline transportation Coal -oil and coal-water slurry mixing Biosurfactants use in hydrocarbons bioremediation Several oil spill accidents and occasionally deliberate releases have taken place in recent years resulting in significant contamination of oceans and shoreline environments. Such incidents have intensified attempts to develop various products, procedures and techniques for combating oil pollution both at sea and the shorelines. Biosurfactants are one such chemical, which has been applied in parts of the Exxon Valdez oil spill [11]. The ability of biosurfactants to emulsify hydrocarbon-water mixtures enhances the degradation of hydrocarbons in the environment. The presence of hydrocarbon degrading microorganisms in seawater renders biodegradation one of the most efficient methods for removing pollutants. Most biosurfactants have lower possible toxicity and persistence in the environment in comparison to chemical surfactants [12]. The ability of a surfactant to enhance biodegradation of slightly soluble organic compound depends on extent to which it increases the bioavailability of the compound. Harvey et al. [11] tested a biosurfactant from P. aeruginosa for its ability to remove oil from contaminated Alaskan gravel samples under various conditions including concentration of surfactant, time of contact, temperature of the wash and presence or absence of gum. They reported increased oil displacement (about 2-3 folds) in comparison to water alone. Necessary contact time for the maximum effect was also reduced from min. for water to 1 min. In addition, the Environmental Technology Laboratory at University of

5 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control 181 Alaska, Fairbanks reported complete removal of diesel range petroleum hydrocarbons (to the limit of 0.5mg kg"') while semi volatile petroleum hydrocarbons were reduced to 70% level, a removal of 30% [13]. These results demonstrated the capacity of biosurfactants to remove oil from naturally occurring substrate. Interest in biosurfactants applications in treating hydrocarboncontaminated soils has recently intensified [10]. Hydrocarbon degradation by the microbes present in the contaminated soil is the primary method for removal of hydrocarbon pollutant from the soil. Partially purified biosurfactants can be either used in bioreactors or in situ to emulsify and increase the solubility of hydrophobic contaminants. Alternatively, either surfactant producing microorganisms or growth limiting factors may be added to the soil to enhance growth of added or indigenous microorganisms capable of producing biosurfactants. Biodetox (Germany) described a process to decontaminate soils, industrial sludge and waste waters [14]. The procedure involves transport of contaminated materials to a biopit process for microbial degradation. Biodetox also performs in situ bioreclamation for surface, deep ground and ground water contamination. Microorganisms are added by means of "Biodetox foam", which is not harmful to the environment, contains bacteria, nutrients and biosurfactants and can be biodegraded. Jain et al. [15] found that the addition of Pseudomonas biosurfactant enhanced the biodegradation of tetradecane, pristane, and hexadecane in a slit loam with 2.1% organic matter. Similarly Zhang & Miller [16] reported the enhanced octadecane dispersion and biodegradation by a Pseudomonas rhamnolipids surfactant. Falatko & Novak [17] studied biosurfactant-facilitated removal of gasoline overlaid on the top of coarse grain sand packed column. Up to 15-fold increase in the effluent concentration of four gasoline constituents; toluene, m-xylene, 1,2,4-trimethylbenzene, and naphthalene was observed upon the addition of biosurfactant solution (600mg 1"*) Herman et al. [18] investigated the effects of rhamnolipids biosurfactants on in situ biodegradation of hydrocarbon entrapped in porous matrix and reported a mobilisation of hydrocarbon entrapped within the soil matrix at biosurfactants concentration higher than the critical micelle concentration (CMC). At concentrations lower than CMC they detected enhanced in-situ mineralization of entrapped hydrocarbon. One of the methods of removing oil contaminants is to add biosurfactants into soil to increase hydrocarbon mobility. The emulsified hydrocarbon can then be recovered by a production well and degraded above ground in a bioreactor. Bai et al. [19] used an anionic mono rhamnolipid biosurfactant from P. aeruginosa to remove residual hydrocarbons from sand columns. They recovered approximately 84% of residual hydrocarbon (hexadecane) from sand column packed with 20/30 mesh sand and 22% hydrocarbon from 40/50-mesh sand, primarily because of increased mobilisation. They reported the optimal concentration of rhamnolipid of 500 mg 1"' and a range of possible use of mg 1"\

6 182 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control Biosurfactants and PAH & metal bioremediation Applying surfactants as immobilizing agents might be one way to enhance the solubility of PAHs as they may increase their solublization or emulsification, to release hydrocarbons sorbed to soil organic matter and increase the aqueous concentrations of hydrophobic compounds resulting in higher mass transfer rates. In an investigation of the capacity of PAH utilising bacteria to produce biosurfactants using naphthalene and phenanthrene Daziel et al. [20] concluded that biosurfactant production was responsible for an increase in the aqueous concentration of naphthalene. This indicates a potential role for biosurfactant in increasing the solubility of such compounds. Similarly Zhang et al. [21] tested two rhamnolipid biosurfactants' effects on dissolution and bioavailability of phenanthrene and reported increased solubility and degradation rates. Similarly Noordman et al. [22] tested rhamnolipid (500 mg 1"*) biosurfactant solution's ability to enhanced removal of phenanthrene from contaminated soil in a laboratory columns' study and detected significant enhanced removal of phenanthrene compared to controls. Biosurfactant have also been reported to promote heavy metals desorption from soils in two ways [23]. The first is through complexation of the free form of the metal residing in solution which decreases the solution-phase activity of the metal and therefore promotes de-sorption. The second occurs under reduced interfacial tension conditions; the biosurfactants will accumulate at the solid-solution interface that may allow the direct contact between the biosurfactant and the sorbed metal. Other workers [24] reported significant complexation / sorption of some metals from contaminated soils using to a rhamnolipid biosurfactant. Although bioremediation of metal contaminated soils appear to have promise it is important to understand the factors affecting rhamnolipids sorption to the soil to achieve better metals removal and develop this technology. These factors include ionic strength, mineral composition and pore water chemistry within metal contaminated soils. Future success of biosurfactant technology in bioremediation initiatives will require targeting their use to the physical conditions and chemical nature of the polluted sites to maximise efficiency and economical viability. Biosurfactant and microbial enhanced oil recovery An area of considerable potential for biosurfactant application is in the field of microbial enhanced oil recovery (MEOR). Biosurfactants can aid in oil emulsification and assist in the detachment of oil films from rocks [4,10]. In situ removal of oil is due to multiple effects of the microorganisms on environment and oil. These effects include gas and acid production, reduction in oil viscosity, plugging by biomass accumulation, reduction in interfacial tension by biosurfactants and degradation of large organic molecules. These are all factors responsible for decreasing the oil viscosity and making its recovery easier. The strategies involved in the MEOR depend on the oil reservoir prevalent conditions including temperature, pressure, ph, porosity salinity, geologic make

7 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control 183 up of the reservoir, available nutrients and the presence of indigenous microorganisms. These factors should be considered before devising a strategy for use in an oil well. There are three main strategies for use of biosurfactants in Enhanced Oil Recovery (EOR) or mobilisation of heavy oils. a- Production in batch or continuous culture under industrial conditions followed by addition to the reservoir in the conventional way along with the water flood (ex situ MEOR). b- Production of surface active compounds by microorganisms at the cell-oil interface within the reservoir formation implying penetration of metabolically active cells into the reservoir. c- Injection of selected nutrients into a reservoir, thus stimulating the growth of indigenous biosurfactant producing microorganisms. The first strategy is expensive due to capital required for bioreactors operation, product purification and introduction into oil containing rocks. The second and third strategy requires that the reservoir contain bacteria capable of producing sufficient amounts of surfactants. For production of biosurfactants, microorganisms are usually provided with low cost substrates such as molasses and inorganic nutrients, which promote growth and surfactant production. Alternatively surfactant-producing strains may be introduced into the well. The introduced organism faces competition from the indigenous population of microbes for the binding sites on rocks and for the added nutrients. Another application of biosurfactants is oil storage tank cleaning. Surfactants have been studied for use in reducing the viscosity of heavy oils, thereby facilitating recovery, transportation and pipelining. In a full-scale field investigation Banat et al. [9] tested the ability of biosurfactant to clean oil storage tanks and to recover hydrocarbon from the emulsified sludge. Two tones of biosurfactant-containing whole cell culture were used to mobilise and clean 850 m^ oil sludge. Approximately 91% (774 m^) of this sludge was recovered as re-sellable crude oil and 76 nf non-hydrocarbon materials remained as impurities to be manually cleaned. The value of the recovered crude covered the cost of the cleaning operation ($100, per tank). Such a clean up processes is therefore economically rewarding and less hazardous to persons involved in the process compared to conventional process. It is also an environmentally sound technology leading to less disposal of oily sludge in the natural environment. To our knowledge however, further commercial applications of this technology has not been carried out. Conclusion The usefulness of biosurfactants in bioremediation is expected to gain increasing importance in the future. To date, biosurfactants are unable to compete economically with the chemically synthesised compounds in the market mainly due to their high production costs and lack of comprehensive toxicity testing. Their success in bioremediation will require precise targeting to the physical conditions and chemical nature of the pollutant affected areas. Encouraging

8 184 Oil and Hydrocarbon Spills II: Modelling, Analysis and Control results have been obtained for use of biosurfactants in hydrocarbon pollution control in marine biotypes, in closed systems (oil storage tanks) and although many laboratory studies indicate potentials for use in open environment a lot remains undemonstrated in pollution treatment in marine environments or coastal areas. The possible use of biosurfactants in MEOR has many advantages, yet more information about structures and factors such as interaction with soil, structure function analysis of surfactant solubilization, scale up and cost analysis for ex-situ production are required. Acknowledgement We would like to thank the Environment and Heritage Service, DOE, for FRDF financial support under the N. Ireland Single Programme (Ref. WM 47/99). References [1] Shaw, A. Surfactants-94. Soap Cosmetics Chemical Specialities, 70, pp [2] Desai J.D. & Banat I.M. Microbial production of surfactants and their commercial potential. Microbiology & Molecular Biology Reviews, 61, pp , [3] Shulga, A. N., Karpenko, E.V., Eliseev, S.A. & Turovsky, A.A. The method for determination of anionogenic bacterial surface-active peptidolipids. Microbiology Journal, 55, pp , [4] Banat, I.M. Biosurfactants production and possible uses in microbial enhanced oil recovery and oil pollution remediation: A review. Bioresource Technoogy, 51, pp [5] Robert, M., Mercade, M.E., Bosch, M.P., Parra, J. L., Espuny, M.J., Manresa, M.A. & Guinea, J. Effect of the carbon source on biosurfactant production by Pseudomonas aeruginosa 44T. Biotechnology Letters, 11, pp , [6] Mulligan, C.N., Mahmourides, G. & Gibbs, B.F. The influence of phosphate metabolism on biosurfactant production by Pseudomonas aeruginosa. Journal ofbiotechnoogy, 12, pp , [7] Venkata, R. & Karanth, N.G. Factors affecting biosurfactant production using Pseudomonas aeruginosa CFTR-6 under submerged conditions. Journal of Chemical Technology & Biotechnology, 45, pp , [8] Abu-Ruwaida, A.S., Banat, I.M., Haditirto, S. & Khamis, A. Nutritional requirements and growth characteristics of a biosurfactant- producing Rhodococcus bacterium. World Journal Microbiology & Biotechnology, 7, pp ,1991. [9] Banat, I.M., Samarah, N., Murad, M., Home, R. & Benerjee, S. Biosurfactant production and use in oil tank clean-up. World Journal Microbiology & Biotechnology,!,pp , [10] Banat, I.M. Characterisation of biosurfactants and their use in pollution removal, State of the art review. ACTA Biotechologica, 15, pp , 1995.

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