Biodegradation and environmental behavior of biodiesel mixtures in the sea: An initial study
Introduction
With the rapid rise in the price of crude oil and projected decreases in oil supplies, alternative fuels are receiving considerable attention (Hill et al., 2006). One of the most promising alternatives is biodiesel, which is a mixture of fatty acid methyl esters (FAMEs) derived from the transesterification of animal fats and vegetable oils. Please note that some articles refer to biodiesel as the actual fats or oils prior to these reactions (sometimes called straight fats or oils), but in this manuscript, biodiesel refers to only FAMEs. Proponents of biodiesel in the United States as well as other countries tout its ability to enhance engine lubrication, decrease harmful emissions, and minimize the dependence on foreign oil imports (Kemp, 2006). It is also proposed as a partial “solution” to CO2 emissions contributing to global warming by closing the carbon cycle, i.e., being carbon neutral (Peterson and Hustrulid, 1998).
One of Rudolph Diesel’s interests after developing the diesel engine was for farmers to use vegetable oils as fuels (Pahl, 2005). Initial efforts in the early 1900s to run diesel engines with straight vegetable oil were hindered by the oil’s high viscosity. This inspired others to chemically modify vegetable oils into mixtures that had lower viscosities, with processes such as transesterification to FAMEs, which was first patented in 1937. Although, numerous other methods have since been developed (Demirbas, 2005, Demirbas and Kara, 2006), transesterification is now the most commonly used method (Noureddini and Zhu, 1997). When the transesterification is performed with methanol, these reactions convert glycerol-based fats and oils into glycerin and FAMEs. Methanol and the production of FAMEs are the norm because methanol is the least expensive alcohol, although other esters, such as iso-propyl esters, have been shown to have better fuel properties than methyl esters (Knothe, 2005). The chain length and degree of unsaturation can vary in animal fats and vegetable oils for numerous reasons (Gurr et al., 2002), but transesterification of either yields primarily a mixture of methyl hexadecanoate (C16 FAME), methyl octadecanoate (C18 FAME), and C18 FAME isomers with one, two, or three double bonds referred to as C18:1, C18:2, or C18:3 FAMEs, respectively.
Biodiesel is used to formulate a range of mixtures from B2 (2% biodiesel mixed with 98% fossil diesel) to B100 (100% biodiesel). Almost any diesel engine can be run on B2 though B20 and performance characteristics are comparable to those of burning 100% fossil diesel (Kaplan et al., 2006). One potential concern with biodiesel mixtures has been whether they degrade seals and fittings in fuel systems. However, in a survey of transportation agencies of 48 states in the United States, there were no reports of fuel system leaks resulting from biodiesel blends as high as B20 (Humburg et al., 2006).
Biodiesel is moving past the novelty stage and closer to mainstream usage. It is available in the United States at over 1000 distributors and also formulated by private consumers or user groups. In addition, companies are now beginning to manufacture diesel engines that are designed to run on biodiesel blends. One example is the 2007 6.7 l Dodge Ram Turbo Diesel Engine. Despite this large swell in usage and significant anecdotal information by user groups, there has been little peer-reviewed research on the environmental chemistry of this product. Biodiesel engine emissions have been characterized and some toxicity tests have been performed (Jung et al., 2006, Krahl et al., 2005, McCormick and Alleman, 2005, Peterson and Möller, 2005, Smekens et al., 2005, Turrio-Baldassarri et al., 2004). A recent review stressed the need for more research on the health effects of biodiesel exhaust (Swanson et al., 2007). There have been several studies on the microbial degradation of biodiesels (Donofrio, 1996, Floro, 1996, Follis, 1994, Lapinskienė et al., 2006, Peterson and Möller, 2005, Zhang et al., 1998), which have shown that they degrade. One of these efforts published gas chromatographic traces to monitor this process qualitatively (Zhang et al., 1998).
It is noteworthy that there have been numerous spills of vegetable oils (straight oils) in the sea where microbes are capable of degrading them (Bucas and Saliot, 2002). However, reactions initiated at the double bonds of the fatty acids in these glyercol-based oils have been shown to polymerize making them less available to bacteria even when stimulated with nutrients. In fact, Mudge (1997) hypothesized that some residues of spilled vegetable oils may be more recalcitrant than mineral oils.
To expand our current knowledge on the behavior in the marine environment of biodiesel and mixtures of biodiesel with fossil diesel, we performed a series of experiments and calculations. In particular, we amended seawater and autoclaved seawater with 100% fossil diesel, B8, B25, and B100. Individual samples were harvested over the course of 53 days and analyzed by gas chromatography (GC). Our main goals were: (1) to determine the microbial and environmental fate of fossil diesel in seawater cultures when biodiesel-derived FAMEs (or vice versa) were present; (2) to measure the relative degradation rate of FAMEs compared to components of fossil diesel; (3) to evaluate whether any new indicators or ratios of molecules within these mixtures could be identified so that in future cases they could be used to assess the short and long term fate of a biodiesel spill; (4) to consider other potential environmental processes (e.g., abiotic hydrolysis) that may act on biodiesel in the environment; and (5) to evaluate whether biodiesel-derived FAMEs may affect the environmental fate and transport of petroleum hydrocarbons when there is a spill. One aspect that we did not study in this manuscript was photochemistry or oxidation of the double bonds on the FAMEs.
Section snippets
Obtaining and preparing the fuel mixtures used in this experiment
The 100% fossil diesel was collected from the cargo hold of the oil barge Bouchard 65, at the time that it spilled oil in Buzzards Bay in October, 1974, and has been previously well characterized (Arey et al., 2005, Peacock et al., 2007). The B100 sample was purchased from Loud Fuel (Falmouth, MA). The biodiesel mixtures, B8 and B25, were prepared by weighing and mixing the 100% fossil diesel and B100. To confirm that the biodiesel mixtures would be the same whether mixed by mass or volume, we
Results and discussion
In this study, we investigated the microbial degradation of four materials: 100% fossil diesel, B8, B25, and B100. GC-FID chromatograms of each are shown in Fig. 1. The 100% fossil diesel is quite typical, composed of resolved straight-chain and branched alkanes along with many unresolved saturated and aromatic hydrocarbons that elute within the boiling range of dodecane (n-C12) and tetracosane (n-C24) (Fig. 1a). In comparison, the B100 sample is a simple mixture, composed of C14, C16, C16:1, C
Conclusions
In this study, we conducted a preliminary investigation of the biodegradation and behavior of biodiesel under controlled conditions. Our results provide a baseline for future work. From the biodegradation experiments, we observed that the FAMEs were degraded at a similar rate as the n-alkanes, and certainly more quickly than other fossil diesel components. Hence, in the event of a biodiesel mixture spill, we predict that the FAMEs will be consumed by bacteria, and samples from a contaminated
Acknowledgements
This work was supported by funds from the National Science Foundation (IIS-0430835), the Department of Energy (DE-FG02-06ER15775), and an Office of Naval Research Young Investigator Award (N00014-04-01-0029). We thank Professor James Quinn (University of Rhode Island), Dr. John Farrington (WHOI), Ms. Leah Houghton (WHOI), Mr. Bruce Tripp (WHOI), and Dr. Gerhard Knothe (USDA) for their assistance.
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