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Lettuce shows you how to restore soil soaked in oil

Lettuce growing on a soil once contaminated with oil, revived by a process developed by engineers at Rice University. Rice's team determined that pyrolyzing soil soaked in oil for 15 minutes at 420 degrees Celsius is sufficient to remove contaminants and preserve soil fertility. The lettuce plants shown here, in treated and fertilized soils, showed robust growth for 14 days. Credit: Wen Song / Rice University

Engineers at Rice University have discovered how soil contaminated with heavy oil can not only be cleaned but also becomes fertile.

How do you know it works? They cultivated lettuce.

Rice engineers Kyriacos Zygourakis and Pedro Alvarez and their colleagues have perfected their method to remove oil contaminants from the soil through the old pyrolysis process. The technique gently heats the soil while keeping oxygen out, which prevents the damage that usually occurs in fertile soil when the burning of hydrocarbons causes temperature peaks.

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

"We saw the opportunity to turn a passive, contaminated soil into a product, fertile soil," said Alvarez.

The key to conserving fertility is preserving the essential clays of the soil, said Zygourakis. "Clays hold water, and if you raise the temperature too much, you basically destroy it," he said. "If you exceed 500 degrees Celsius (900 degrees Fahrenheit), dehydration is irreversible."

The researchers placed soil samples from Hearne, Texas, contaminated in the laboratory with heavy crude oil, in an oven to see what temperature best eliminated the largest amount of oil and how long it took.

Their results showed that heating samples in the rotating drum at 420 C (788 F) for 15 minutes eliminated 99.9 percent of total petroleum hydrocarbons (TPH) and 94.5 percent of polycyclic aromatic hydrocarbons (PAHs), leaving treated soils with approximately the same levels of contaminants. It is found in natural, uncontaminated soil.

The article appears in the American Chemical Society magazine. Environmental science and technology. Follow several articles from the same group that details the mechanism by which pyrolysis removes contaminants and converts some of the unwanted hydrocarbons into coal, while leaving the soil almost as fertile as the original. "While heating the soil to clean it is not a new process," said Zygourakis, "we have shown that we can do it quickly in a continuous reactor to eliminate TPH, and we have learned how to optimize the pyrolysis conditions to maximize the contaminant. At the same time minimize soil damage and loss of fertility.

"We also learned that we can do it with less energy than with other methods, and we have detoxified the soil so we can put it back safely," he said.

Heating the soil to approximately 420 C represents the sweet spot for the treatment, said Zygourakis. Heating it to 470 C (878 F) did a better marginal job in removing contaminants, but it used more energy and, more importantly, decreased soil fertility to the extent that it could not be reused.

"Between 200 and 300 C (392-572 F), the light volatile compounds evaporate," he said. "When you get to 350-400 C (662-752 F), you start breaking the heteroatom bonds first, and then the carbon-carbon and carbon-hydrogen bonds that trigger a sequence of radical reactions that convert the heavier hydrocarbons into a stable and low reactivity, carbonize. "

The true test of the pilot program occurred when the researchers cultivated Simpson black seed lettuce, a variety for which oil is highly toxic, in the original clean soil, a contaminated soil and several pyrolysed soils. While the plants in the treated soils took a little longer to start, they found that after 21 days, plants grown in pyrolized soil with fertilizer or simply water showed the same germination rates and had the same weight as those grown in clean soil. .

"We knew we had a process that effectively cleans the soil contaminated with oil and restores its fertility," Zygourakis said. "But, have we really detoxified the soil?"

To answer this final question, Rice's team addressed Bhagavatula Moorthy, professor of neonatology at Baylor College of Medicine, who studies the effects of air pollutants on neonatal development. Moorthy and his laboratory found that extracts taken from soil contaminated with oil were toxic to human lung cells, while exposure of the same cell lines to extracts from treated soils had no adverse effects. The study alleviated concerns that pyrolyzed soil could release dust particles in the air with highly toxic pollutants such as PAHs.

& # 39; & # 39; An important lesson we learned is that the different treatment goals for regulatory compliance, detoxification and restoration of soil fertility do not have to be mutually exclusive and can be achieved simultaneously, "Alvarez said. .

Wen Song, Rice's visiting scholar and a student at Jinan University and Shandong University, China, is the lead author of the article. The co-authors are the student of Rice Julia Vidonish of Arcadis U.S., Seattle; Postdoctoral researcher of rice Pingfeng Yu; Roopa Kamath, an environmental consultant for Chevron; Chun Chu, research associate at Baylor College of Medicine; and Baoyu Gao, professor of environmental engineering at Shandong University. Alvarez is Professor George R. Brown of Materials Science and Nanoengineering and Professor of Civil and Environmental Engineering at Rice. Zygourakis is the A.J. Hartsook Professor of Chemical and Biomolecular Engineering and professor of bioengineering.

Explore further:
Scientists turn oily soil into fertile soil.

More information:
Wen Song et al. Pilot-scale pyrolytic remediation of soil contaminated with crude oil in a continuous feed reactor: treatment intensity offsets, Environmental Science and Technology (2019). DOI: 10.1021 / acs.est.8b05825

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