A new biosurfactant for use in the cleanup of oil spills on sea water environment

Transcription

1 A new biosurfactant for use in the cleanup of oil spills on sea water environment F. Crescenzi], M. Buffagni2, E. D Angelil &F. Porcelli lenitecnologie, Monterotondo, Italy. 2ENIlDiv. Agip, S.Donato Mi[anese, Italy. Abstract In this paper we describe a new environmentally friendly surface active compound of biological origin (biosurfactant) which is capable of disrupting oil slicks floating on the water surface, promoting oil dispersion and enhancing its natural biodegradation. Trials with oil slicks on seawater using this product have been carried out, Results show the disaggregation of the biosurfactant treated slick into a stable emulsion whose biological degradation by natural marine microorganisms is greatly enhanced, The structure of the biosurfactant basically consists of a polymeric polysaccharidic chain with hydrophobic ester linked fatty acid substitutions of carbon atoms length. This biosurfactant can be used for both treating slicks floating on water surfaces and for application as beach cleaning agent, Compared to commercially available chemical dispersants the new biosurfactant is totally non toxic, it forms a stable emulsion that actively promotes oil biodegradation, Unlike other oil spill products claiming to increase biodegradation of the oil, the biosurfactant based product does not incorporate mineral nutrients nor living microorganisms aiming to augment their concentration in the treated environment. The enhancement of oil biodegradation as a result of the new biosurfactant is due to the enhanced adhesion action of marine microorganisms onto the oil surface. 1 Introduction Oil spills remain a major threat for the marine environment. The environmental consequences of oil spills are dramatic for marine habitat and sea related biological and human activities. Remediation is difficult, costly and inefficient

2 especially in areas close to coastal regions, New took capable of decreasing the cost and improving the efficiency of clean up operations are strongly sought after. Oil spills dispersants may constitute a valuable and cost efficient tool for responding to oil spills on sea water. The acceptance of dispersants use by operators and the public, however, has been and continues to be quite limited due to the drawbacks of increasing the chemical load to marine environment and concern about the toxicity of the chemical products for the impacted fauna. Biosurfactants are natural surface active compounds produced by biological processes that promise to be more environmentally friendly than chemical products, They have been studied for a variety of applications in many industrial fields from health care to oil production [1]. The use of biosurfactants for responding to oil spills has encountered limited success mainly because of their high cost [2]. The objective of the work was to develop a dispersant for oil spills on water, based on a new biosurfactant named EPS. The biosurfactant was produced in high yield by fermentation process utilizing a natural bacterium isolated from a hydrocarbons polluted soil. 2 Materials and methods The biosurfactant EPS was produced by fermentation of a strain of.4cine/obacter Cakoaceticus (CBS ). Structural characterization of the purified main molecular component of BPS was done using NMR Spectroscopy and Gas Chromatography Mass Spectroscopy. Molecular weight was determined by size exclusion chromatography. Production conditions, details of polymer purification, and sample preparation for analysis have been published elsewhere [3]. Surface activity of the biosurfactant water solution was measured with a tensiometer (LAUDA Mod. TE- lc). For oil dispersion experiments, a paraffinic crude coming tlom an offshore field in the Adriatic Sea was used. Table 1 shows the chemical-physical characteristics of this crude and of the residues obtained topping it at different temperatures to simulate weathering. In dispersion efficiency experiments, total hydrocarbon content in water samples was measured by methylene chloride extraction and spectrophotometry at 580 nm. Oil droplets dimensions of water dispersed oil samples were measured with a granulometer (Coulter Mod. LS200). Bacterial adhesion measurements were conducted following the method described in reference [4]. Table 1. Chemical-physical characteristics of the Adriatic Sea field crude Density Viscosity Pour Wax Asph. Residue (g/ml) 10 s- Point (Wt%) (Wt%) (CP) ( C) Fresh 0, ,8 150 c oc+ 0, ,7 250 C ,2

3 3 Results Biosurfactant simplified structure is shown in Figure 1. The hydrophilic part of the molecule consists of substituted hexose rings. Fatty acid substitutions of carbon atoms length are linked to the rings, giving partially hydrophobic character to the polymeric molecule. Average molecular weight of the molecule was found to be about 2 x 10GDalton. R = CH S(CH2)nCOO- Figure 1: Simplified structure of EPS biosurfactant (n=8-12) 70, Lo 0,1 0,2 0,3 0,4 0,5 0,6 0,7 WMOI I Figure 2: Water/ hexadecane interracial tension of EPS biosurfactant solutions Figure 2 shows the interracial tension between a water solution of EPS biosurfactant and hexadecane, measured with respect to biosurfactant concentration.

4 248 Oil,Im[ Hidrocwbon Spills 1[1 The lowest attainable interracial tension is higher than the values usually reached with chemical surfactants, However, the concentration of biosurfactant needed to attain the lowest interracial tension is very small indicating high affinity of the polymeric biosurfactant for the oil/water interface. The dispersion efficiency of EPS biosurfactant was tested with the apparatus depicted in Figure 3. In a pool tilled with sea water a two square meter surface area was delimited with plastic walls immersed in the water to 1.5 meters below the surface. In the confined area a weighted amount of oil was poured so as to form a 1-2 mm thick layer, After a weathering period of 24 hours either the biosurfactant EPS or a third generation commercial dispersant commonly used for oil spill dispersion was applied at a ratio 1:25 with respect to the remaining volume of weathered oil. Previous trials were used to estimate the fraction evaporated in the 24 hour weathering time, This fraction was in general in the order of 35?+0 of the oil used and reached aplateau after 24 hour. Watar Sampling Ports Oil / d d Water Jet,,.,,,,.,,,. & Sea water Figure 3: Experimental setup for oil dispersion measurements Dispersants were applied by spraying it onto the oil surface with a laboratory sprayer, Soon after dispersant application, that lasted about one minute, mixing energy was applied to the oil surface with a water jet, Water jet exit velocity and flow were cautiously monitored so as to estimate the mixing power applied to the oil floating on the water. Mixing was continued for few minutes leading to a total applied energy of about 1000 joule per square meter. While calm sea state may not provide this mixing power, it can be easily delivered by existing fire monitor systems. After mixing, sea water was sampled at different depths below the surface, In the water samples, total oil concen~ation and oil droplets dimensions were measured.

5 oil md H\drocw+oll Spills III 149 Figure 4 shows the oil concentration in the water ailer dispersion. The oil dispersed with the biosurfactant is compared with oil dispersed with a commercial dispersant, With both products more than 90 YO of the oil was found in the water co&nn below the sur~ace.oil droplets radius was in the range microns indicating that stable dispersions were attained with both products. Blosutfactant Chemical dispersant l.~ L Total hydrocarbon concentration in sea water (ppm) Figure 4: Distribution of dispersed oil below the water surface Differences for the two uroducts were found in the distribution urofile of the dispersed oil along the ~ater column below the surface. These differences are quite small and are probably due to differences in the timing of sampling. Soon afier the end of mixing some of the dispersed oil was seen resurfacing. The amount of oil coalesced back in a continuous phase was estimated by sampling the oil layer laying on the surface after one hour from the end of mixing. The fractions of the original oil found as a coalesced layer floating on the water were found to be 7 Afor the biosurfactant and 210/0for the chemical dispersant. Results indicate that the oil dispersed with the biosurfactant was much more resistant to coalescence than the oil dispersed with the commercial dispersant. The great stability of the emulsion formed with the biosurfactant is probably the result of its polymeric structure and the large number of fictional groups present on each molecule. In previous experiments [4] it has been found that the biological degradation of oil dispersed with EPS biosurfactant is enhanced in natural sea water. The biological degradation rate increase was suggested to be the result of the enhancement of the adhesion of degrading bacteria to the dispersed oil droplets surfaces. In fact, in natural environments very intense microbial activities may

6 take place on and in proximity of oil/water interfaces. Microorganisms came close and adhere to hydrophobic surfaces in order to facilitate metabolic reactions essential to life, In some cases, microbial attachment to oi~water interfaces has enormous ecological significance as in the biodegradation of water insoluble pollutants like hydrocarbons, where attachment of microbial cleaners to hydrophobic surface constitutes the fust step of natural response to pollution. Beside increasing the available surface by dispersion, surfactant are expected to modify the interaction between micro-organisms and hydrophobic substrata by altering the surface tension and the surface affinity to microbes leading to a possible enhancement or inhibition of microbial adhesion to the interface. To verify if this effect was operating with the EPS biosurfactant, the number of bacterial cells adhering to hexadecane/ water interface was measured in presence of both a chemical surfactant and the EPS biosurfactant. Results are shown in Figure 5, I I bioaurfactant.. ~.- --q.-...*!. I 0 o Cells in suspension (106/ml) Figure 5: Adhesion of bacterial cells to an oil/water interface It has been seen that the number of bacterial cells adhering to the interface is roughly proportional to the number of cells present in suspension in the water. The proportionality constant, however, changes if a surfactant is present in the water. It is found that if a chemical surfactant is added to the dispersed-oil water syste~ the proportionality constant is 10wer than in absence of surfactant indicating that adhesion of bacterial cells onto the interface is somewhat inhibited by the presence of the chemical compound. An opposite behavior is found in presence of the EPS biosurfactant, In this case the fraction of the cells suspended in the water that adhere to the interface is greater than in absence of surfactant, indicating that the interface has increased its affinity for the bacterial celis.

7 Adhesion to the oil/water interface is the fust step toward biological degradation of hydrocarbons. Stimulating microbial adhesion may play an important role especially at the start of the process when the number of degrading bacteria are low and there is a lack of naturally produced biosurfactant able to promote successful interaction between oil and microbes. The production process of EPS biosurfactant has been developed up to pilot scale dimension. A 1000 liters fermenter was employed for the production process and both fed batch and continuous mode of operation were used. Fermentation conditions were similar to those used in laboratory fermenters. Preliminary production cost estimate has been calculated on the bases of pilot data. The economical analysis shows that although EPS biosurfactant costs more than chemical dispersants, the application cost of an oil spill dispersant based on EPS will be quite similar to the cost of using chemical dispersants. 4 Conclusions EPS can be mass produced at low cost, if a market will be developed for biosurfactants in the oil spill sector; it will constitute an innovative tool in the arsenal of operators combating oil spills, EPS is a natural product produced by hydrocarbon degrading bacteria that does not have the toxicity of chemical dispersants and does not add to the chemical load of already impacted marine habitats. Oil dispersed with EPS biosurfactant appears to biodegrade faster than oil dispersed chemically. It is hoped that the environmental friendliness of biosurfactants may ease the adoption of oil dispersion as response option so as to contribute to decreasing the high cleanup cost of marine oil spills. Acknowledgments The authors gratefully acknowledge the contribution of Pasquale Sacceddu for performing the bacterial adhesion tests. References [1] Desay, J,D, & Desay, A.J., Production of biosurfactants, Biosurfactants: production, properties, applications, ed, N. Kosaric, Marcel Dekker, Inc.: New York, pp , 1993, [2] Desay, J.D. & Banat, I,M., Microbial production of surfactants and their commercial potential. Microbiology and molecular biology reviews, 61(1), Pp , 1997, [3] Prosperi, G., et al., Lipopolysaccharide biosurfactant. United States Patent #6,063,602, [4] Crescenzi, F.,Camille, M,, Fascetti, E., Porcelli, F., Prosperi, G., & Sacceddu, P., Microbial degradation of biosurfactant dispersed oil. Proc, of 1999 International Oil Spill Conference, American Petroleum Institute: Washington, DC, pp , 1999.