Microbes Quickly Degrade A Popular Biofuel

Readily made from vegetable oil, biodiesel has become a popular fuel, with worldwide production now exceeding 10 million tons per year. Yet like all energy sources, biodiesel has its share of drawbacks. New research shows that it can accelerate corrosion of the carbon steel used to manufacture pipelines, storage tanks, and other components of the fuel infrastructure (Energy Fuels, 2010, 24, 2924).

Co-author and microbiologist Joseph Suflita, of the University of Oklahoma, Norman, says that steel weakened from biodiesel interactions could leak fuel and other hazardous materials to the environment.

"What we do in the guise of being environmentally green might not really be that green after all," he says.

Biodiesel, a mix of fatty acid methyl esters, doesn't start out corrosive. Instead, bacteria and other microbes hydrolyze it in reactions that ultimately generate hydrogen sulfide and organic acids, Suflita's research shows. These compounds eat into steel and degrade it, he explains. Manufacturers already have to demonstrate biodiesel's chemical compatibility with the fuel infrastructure. But the fuel's biological stability during transport and storage generally isn't considered, he adds.

To investigate its biological degradation, Suflita's research team exposed soy-based biodiesel to anaerobic microbes from five marine and freshwater locations including natural seawater from Key West, Fla., and a contaminated aquifer overlying a natural gas field near Ft. Lupton, Colo. Because pipelines and other components of the fuel infrastructure quickly become anaerobic when microbes deplete available oxygen, Suflita's team focused on how anaerobic species might degrade biodiesel. The researchers found that organisms from each environment degraded the fuel in less than a month.

"Biodiesel is at least seven to eight times more biodegradable than traditional petrodiesel," Suflita says.

What's more, metabolite profiling using GC/MS revealed a complex suite of fatty acids. Derived from biodiesel's ester backbone, fatty acids are further metabolized along with sulfates in the environment to generate hydrogen sulfide, Suflita says. The scientists observed the corrosive effect of these reactions when they immersed carbon steel samples in the Key West seawater mixed with biodiesel. The samples emerged blackened and pitted.

Suflita's research raises difficult issues, says microbiologist Gill Geesey, of Montana State University, Boseman. Some diesel fuels today contain up to 20% biodiesel, added in part to decrease fuel's friction, Suflita says. Corrosion is already a widespread problem and the amount of biodiesel in the fuel infrastructure is growing. "I suspect we will see an increase in biocorrosion in the materials used in biodiesel production and processing," Geesey says.

Geesey proposes that plastic pipes and non-reactive polymer linings might minimize microbial degradation. But Suflita speculates that polymers may find only niche applications. Broader use, he says, would mean replacing "hundreds of thousands of miles of pipeline, storage tanks, ballast tanks, oil-water separators and more."

Instead, Suflita proposes that researchers should strive to chemically alter fuels to resist biodegradation. "Of course," he adds, "we don't want to go overboard and create a different environmental problem, i.e. biofuels that cause problems when released to the environment. As a society we need to strike a balance and I think that we can."

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