Study Finds Biodiesel Easily Susceptible to Anaerobic Biodegradation, With Resulting Accelerated Biocorrosion of Metal
A study by researchers from the University of Oklahoma and the US Naval Research Laboratory has found that biodiesel is easily susceptible to biological degradation by anaerobic bacteria. Although many fuel components are susceptible to anaerobic decay, the results suggest that biodiesel is far more amenable to such biodegradation processes than traditional hydrocarbons, the researchers said.
The organisms hydrolyze biodiesel and convert it to a variety of fatty acid intermediates, which are also metabolized. The acidic nature of these intermediates accelerates the pitting corrosion process of the most common metal alloy (carbon steel) used throughout the fuel infrastructure. A paper on their work was published online 5 May in the ACS journal Energy & Fuels.
In a fuel environment, anaerobic conditions prevail whenever heterotrophic microbial respiration consumes oxygen at a rate that exceeds diffusion. This is typically the case in subsurface environments, the researchers note, including oil reservoirs, oil-contaminated habitats, refineries, storage vessels, pipelines, oil-water separators, and ballast tanks.
The corrosion of pipelines, tanks, storage units, and associated equipment increases the risk of the release of hazardous materials to the environment, with concomitant pollution issues. With the widespread use of biodiesel as an additive to fuel supplies, it is at least prudent to consider how best to avoid the negative consequences associated with the microbial metabolism of these labile fuel components.
—Aktas et al.
Aktas et al. incubated five anaerobic inocula with soy biodiesel. The microorganisms originated from fresh and marine environments with differing histories of exposure to hydrocarbons, biodiesel, and oxygen:
- an inoculum capable of hydrocarbon metabolism under both sulfate-reducing and methanogenic conditions, obtained from the contaminated aquifer sediments that overlie a natural gas field;
- an alkane-degrading methanogenic bacterial consortium enriched from the same aquifer;
- a marine oil-degrading sulfate-reducing inoculum obtained from the sunken remains of the USS Arizona in Pearl Harbor, HI;
- an unenriched sample obtained from a seawater compensated ballast tank aboard the USS Gettysburg; and
- natural Key West, FL, coastal seawater.
Biodegradation of biodiesel was evaluated using gas chromatography-mass spectroscopy (GC-MS), and corrosion of carbon steel was monitored over a 60-day period using electrochemical and imaging techniques.
The common feature of these experiments was that all inocula metabolized biodiesel, regardless of their freshwater or marine origins or prior exposure history to biodiesel, over a very short time period, from weeks to 1 month. Over the past 2 decades, it has been recognized that many fuel components are susceptible to anaerobic decay. Because biodiesel is a modern component of transportation fuels, it too must be held in similar regard. However, our evidence suggests that this biofuel is far more amenable to biodegradation processes than traditional hydrocarbons.
...Prior research suggests that, while the anaerobic metabolism of hydrocarbons in fuels is fairly restricted and requires “specialist organisms”, the ability of bacteria, archaea, and even eukaryotes to hydrolyze the component esters of biodiesel is common.
—Aktas et al.
Deniz F. Aktas, Jason S. Lee, Brenda J. Little, Richard I. Ray, Irene A. Davidova, Christopher N. Lyles and Joseph M. Suflita (2010) Anaerobic Metabolism of Biodiesel and Its Impact on Metal Corrosion. Energy Fuels, Article ASAP doi: 10.1021/ef100084j