By Velda Addison, Hart Energy

Scientists at Rice University have figured out how to turn oil-contaminated soil into fertile ground by using heat in the absence of oxygen.

The process called pyrolysis heats the soil to about 420 degrees C, transforming it into a petroleum coke-like material the scientists called “char,” not to be confused with biochar. As described in a research paper abstract, published online by American Chemical Society journal Environmental Science and Technology, the process reduced the total petroleum hydrocarbon content of the contaminated soil to below regulatory standards within three hours.

A bonus was that the team of scientists accomplished this feat using only 40 to 60% of the energy required for incineration at 600-1,200 degrees C.

“Our original goal was to speed the response to oil spills, but our aspiration was to turn contaminated soil into fertile soil,” Pedro Alvarez, an environmental engineer, professor and chair of Rice’s Civil and Environmental Engineering Department, said in a news release.

Alvarez explained how the group of scientists thought the hydrocarbons would turn into biochar, which is charcoal. Instead, pyrolysis resulted in a carbonaceous material that resembled petroleum coke, or char as the scientists call it.

Unlike biochar, a particle, the coke-like char coats existing soil particles, according to the scientists.

“We were correct in thinking that by removing toxic pollutants and the hydrophobicity that repels water that plants need, and by retaining some of the carbon and perhaps some of the nutrients, we would enhance plant growth,” he added.

Rice chemical engineer Kyriacos Zygourakis, one of the paper’s co-authors, provided more details about the process in the news release. The scientists were able to remove the lighter hydrocarbons at 420 degrees. But when the temperature drops to about 350 degrees, heavier hydrocarbons turn into solid char after undergoing cracking and condensation reactions, Zygourakis explained, noting the process is similar to how petroleum coke is produced.

To completely eliminate hydrocarbons from the soil, oxygen must be introduced and temperatures must be raised higher than 420 degrees. But that destroys the soil and uses more energy.

Their work could prove beneficial for the oil industry and its oil spill remediation efforts.

According to the U.S. Environmental Protection Agency, thousands of emergencies that involve oil spills or the release of hazardous substances are reported annually in the U.S. Response methods, which vary depending on the spill, include removing the contaminated soil, bioremediation, in-situ burning and using sorbents.

The pyrolysis process could be another tool in the oil spill cleanup toolbox.

And it could enhance plant growth. But that doesn’t mean that it’s OK to start growing food in such areas, although the scientists said they successfully grew lettuce in the reclaimed soil in a lab.

“Reclaimed soil may not necessarily be used to grow food, but it certainly could be used for re-greening: planting grass to minimize erosion and to restore vegetation," Alvarez said in the release.

In addition to Alvarez and Zygourakis, Julia Vidonish, Caroline Masiello, Xiaodong Gao and Jacques Mathieu contributed to the report.

The scientists don’t plan to stop work here and move on to the next major project. Plans are to study how changes in the pyrolysis time and temperature impact char quality. That could mean more good news for the oil industry and others charged with cleanup and restoring soil quality should such accidents happen.

Velda Addison can be reached at or via Twitter @veldaaddison.