Munching Microbes Make A Meal Out Of Toxic Substances

May 15, 1997

WEST LAFAYETTE, Ind. -- When it comes to cleaning up the environment, the answer may be right under our feet.

A Purdue University engineer is investigating how and why bacteria in the soil eat certain toxic chemicals, and he is developing procedures for using the hungry bugs in environmental cleanup efforts.

"We've known for decades that the soil contains microbes with the ability to degrade chemicals such as petroleum compounds," says Loring Nies, assistant professor of environmental engineering at Purdue. "But using them for that purpose has not been widely implemented because few on-site engineers have the knowledge or experience."

One of Nies' projects involves studying microbes that eat pentachlorophenol, a chemical used as a wood preservative.

"We're investigating a contaminated site in Indiana that used to be owned by a company that treated wood," Nies says. "For years they would dip the wood into the preservative, then hang it over the soil to drip dry. There are organisms 10 to 12 feet down in the soil that are degrading this chemical, both aerobically -- in the presence of oxygen -- and anaerobically, without oxygen. It's a fascinating case, because they don't degrade it all the time, and we don't really know why yet."

Nies will present a paper on his research on pentachlorophenol at the conference In-Situ and On-Site Bioremediation April 28-May 1 in New Orleans. The paper also will be published in the premier issue of Bioremediation Journal, to be available at the end of April.

Nies says the knowledge gained from his studies may help in understanding how and why microbes eat other types of toxic chemicals in the soil, and how they might be used in bioremediation efforts.

Nies and his students have taken samples from the contaminated site and are trying to determine the behavioral characteristics of microbes in the soil.

"There's some interesting environmental chemistry happening here," he says. "The pentachlorophenol is not very soluble in water, and there's not much water in the soil, so sometimes the chemical is bound to the soil, making it unavailable to the microorganisms.

"We know we can get these organisms to degrade the chemical, we just don't know yet how to get them to do it consistently. Also, we don't know what concentration they will degrade it down to, and that's where understanding the bioavailability issue becomes important."

In another project, Nies is working with the Indiana Department of Transportation to clean up soil contaminated by gasoline that has leaked from underground storage tanks. Newer storage tanks are made to resist corrosion, but the older ones corrode, break open and leak.

"These tanks are everywhere; it's a nationwide problem," he says. "One of the biggest problems is that benzene is a constituent of gasoline, and benzene is known to cause leukemia."

Nies and his students tested several sites in Indiana for the presence of microbes that eat benzoate, a chemical that is commonly found in soil and that is very similar to petroleum compounds. Theoretically, a benzoate-degrading organism could degrade petroleum as well, Nies says.

Nies found that at sites contaminated with petroleum, the number of benzoate-degrading organisms in the soil was higher than normal background levels. "Our conclusion is that there are more of this type of microbe at these sites because they're eating one of their favorite foods -- petroleum," Nies says.

The standard method for cleaning up contaminated soil is to excavate it and truck it to a landfill, which not only costs more than bioremediation, but also is not as environmentally friendly, Nies says.

In situations where there is no immediate health hazard, such as a threat of contaminated drinking water, Nies says the best solution to cleaning up gasoline in the soil may be to just leave the site alone.

"If there are microbes in the soil already eating the contaminants, you could just leave it alone and let them do their thing, or add some commercial nutrients to the soil to help speed the process," he says. "We would then monitor the area over months or years to determine the progress."

This spring Nies and his students will begin using bioremediation to clean up two sites, operated by the Indiana Department of Transportation, that have been contaminated by leaking underground gasoline tanks. They will monitor petroleum concentrations, determine the extent of the contamination, examine the nutrient content of the soil, and determine if microorganisms are already degrading the petroleum. Then they'll develop and implement a bioremediation plan for each site.

Nies says bioremediation, to be most effective, should be an aerobic process, which means getting as much oxygen as possible to the microbes. In Indiana, that can be a problem because the soil is silty and gas doesn't travel well through it. However, Nies says, there are some relatively simple methods for aerating soil, such as using blowers or compressors to pump oxygen into the earth.

Contaminated areas that also affect ground water can be cleaned up using microbes as well, but the process is more complicated, Nies says.

"If water is involved, we have to set up a liquid circulation system, which involves digging wells and adding oxygen, either by dissolving oxygen in the water, pumping air directly into the water, or pumping it into the ground. There are a variety of methods we can use."

One of the problems with bioremediation is that not many professionals are qualified to do it.

"There are companies out there like snake-oil salesmen of the Old West, selling people microbes they already have in their soil, or selling expensive products and services that have no scientific or engineering basis behind them," Nies says.

"For example, out west the soil is very alkaline, and just adding any petroleum degrader to the soil isn't going to be effective because some can't survive in those conditions."

Nies and Purdue research scientist Bobby McCullouch are developing a CD-ROM on bioremediation and how to put it into practice. The goal is to increase the professional awareness of bioremediation as an option and help give engineers the knowledge they need to implement it.

"We have standard practices for other areas of engineering, but not for bioremediation. We're trying to come up with a balance between a responsible environmental cleanup method and protecting public health, while still being cost-conscious," Nies says. Source: Loring F. Nies, (765) 494-8327; e-mail,
Writer: Amanda Siegfried, (765) 494-4709; e-mail,
Purdue News Service: (765) 494-2096; e-mail,

Purdue University

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