New Method can Restore Fertility of Oil-Soaked Soil

While large-volume marine spills get most of the attention, 98 per cent of oil spills occur on land, with more than 25,000 spills a year reported to the Environmental Protection Agency.
New Method can Restore Fertility of Oil-Soaked Soil

Scientists have found a way to clean soil contaminated by heavy oil can and make it fertile again.

Researchers from Rice University in the US fine-tuned their method to remove petroleum contaminants from the soil through the age-old process of pyrolysis.

The technique, described in the journal Environmental Science and Technology, gently heats soil while keeping oxygen out, which avoids the damage usually done to fertile soil when burning hydrocarbons cause temperature spikes.

While large-volume marine spills get most of the attention, 98 percent of oil spills occur on land, with more than 25,000 spills a year reported to the Environmental Protection Agency.

Rice University engineers have figured out how soil contaminated by heavy oil can not only be cleaned but made fertile again.

Rice engineers Kyriacos Zygourakis and Pedro Alvarez and their colleagues have fine-tuned their method to remove petroleum contaminants from the soil through the age-old process of pyrolysis. The technique gently heats soil while keeping oxygen out, which avoids the damage usually done to fertile soil when burning hydrocarbons because of temperature spikes.

While large-volume marine spills get most of the attention, 98 percent of oil spills occur on land, Alvarez points out, with more than 25,000 spills a year reported to the Environmental Protection Agency. That makes the need for cost-effective remediation clear, he said.

"We saw an opportunity to convert a liability, contaminated soil, into a commodity, fertile soil," Alvarez said.

Heating the soil to about 420 degree Celsius represents the sweet spot for treatment, he said.

Heating it to 470 degree Celsius did a marginally better job in removing contaminants, but used more energy and, more importantly, decreased the soil's fertility to the degree that it could not be reused.

"Between 200 and 300 degree Celsius, the light volatile compounds evaporate," Zygourakis said.

"When you get to 350 to 400 degree Celsius, you start breaking first the heteroatom bonds, and then carbon-carbon and carbon-hydrogen bonds triggering a sequence of radical reactions that convert heavier hydrocarbons to stable, low-reactivity char," he said.

The true test of the pilot programme came when the researchers grew Simpson black-seeded lettuce, a variety for which petroleum is highly toxic, on the original clean soil, some contaminated soil and several pyrolyzed soils.

While plants in the treated soils were a bit slower to start, they found that after 21 days, plants grown in pyrolyzed soil with fertilizer or simply water showed the same germination rates and had the same weight as those grown in clean soil.

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