Major oil spills are dramatic and deadly, and most occur offshore, hitting hard and understandably grabbing big headlines.But 98 percent of all spills – that’s more than 25,000 a year – are on land. Clean-up costs exceed $10 billion annually and the environmental impact of all that contaminated soil is enormous. What if we could reclaim the poisoned land?
Scientists at Rice University have developed a process known as “pyrolysis” to turn the black oily dirt into good fertile soil. The technique, in which contaminated soil is heated in the absence of oxygen, is fast, energy efficient, and much cheaper than current methods.
The dream becomes reality
The new approach is also much better for the environment than standard incineration techniques for fast remediation, says Pedro Alvarez, professor and chair of Rice’s civil and environmental engineering department.
“Our original goal was to speed the response to oil spills, but our aspiration was to turn contaminated soil into fertile soil,” says Alvarez. The professor and his team turned that dream into reality.
“Pyrolyzing” the contaminated soil for three hours reduced the amount of petroleum hydrocarbon pollutants to well below regulatory standards. But, as an unexpected bonus, pyrolysis also enhanced the soil’s fertility by turning the remaining carbon into beneficial “char”.
“We initially thought we could turn the hydrocarbons into biochar,” Alvarez says. “We turned out to be partly wrong: We didn’t get biochar, but a carbonaceous material that we call char and resembles coke.”
“Biochar is a particle that is separate from the soil’s mineral grains,” says biogeochemist and co-author Caroline Masiello, an associate professor of Earth science.
While biochar is itself a particle, the coke-like char appears to coat existing soil particles. “It has an internal physical structure that allows it to hold water and nutrients and provides a home for microbes, but here, we’re not making any of those things. We’re making an organic film that coats the minerals,” explained Masiello.
Growing lettuce from the dead land
By removing toxic pollutants and the hydrophobicity that repels water, as well as retaining some of the carbon and nutrients, Alvarez hoped the reclaimed soil would enhance plant growth.
So the researchers went on to test their discovery by successfully growing lettuce in reclaimed soil. “There’s no one plant officially accepted as the standard for testing petroleum toxicity, but lettuce has been accepted by the community as very sensitive to toxins, especially petroleum,” said graduate student Julia Vidonish, the paper’s lead author.
Not just desert sand
“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.
The process takes advantage of existing petroleum chemistry. But the end product is clean. “The Environmental Protection Agency does not classify petroleum coke as hazardous waste,” said chemical engineer and co-author Kyriacos Zygourakis, professor of chemical and biomolecular engineering.
“We proved we can remove all the bad actors and all the contaminants and at the same time have a final product with agricultural value. We don’t just turn it into desert sand.”
The new paper is published in the journal Environmental Science and Technology.
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