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The engineers at Rice University have understood how soils contaminated with heavy oil can not only be cleaned up, but become fertile again.
How do they know it works? They grew lettuce.
Rice engineers Kyriacos Zygourakis and Pedro Alvarez and their colleagues refined their method of removing petroleum contaminants from the soil during the secular process of pyrolysis. The technique gently heats the soil while preventing oxygen from entering, which avoids the damage usually done to fertile soils when burning hydrocarbons cause temperature spikes.
Alvarez points out that 98% of oil spills occur on Earth, but that more than 25,000 spills a year are reported to the Environmental Protection Agency. This clearly shows the need for cost-effective remedies, he said.
"We saw an opportunity to convert a liability, contaminated soil, into a commodity, a fertile soil," said Alvarez.
To preserve fertility, it is essential to preserve the essential clays from the soil, said Zygourakis. "The clays retain the water and if you raise the temperature too much, you destroy them," he said. "If you exceed 500 degrees Celsius (900 degrees Fahrenheit), dehydration is irreversible."
The researchers put soil samples from Hearne, Texas, contaminated in the laboratory with heavy crude, in an oven to determine the temperature that best eliminated the oil and its processing time.
Their results showed that heating the samples in the rotating drum at 420 ° C (788 ° F) for 15 minutes eliminated 99.9% of the total petroleum hydrocarbons (TPH) and 94.5% of the polycyclic aromatic hydrocarbons (PAHs), leaving treated soils at about the same levels of pollution. found in natural, uncontaminated soil.
The document appears in the American Chemical Society Environmental Science and Technology. It follows several papers from the same group that have detailed the mechanism by which pyrolysis removes contaminants and converts some of the unwanted hydrocarbons into charcoal, leaving a soil almost as fertile as the original. "Although heating the soil to clean it is not a new process," said Zygourakis, "we proved that we could do it quickly in a continuous reactor to remove TPH, and we learned how to optimize pyrolysis conditions. to maximize contaminants. " elimination while minimizing soil damage and loss of fertility.
"We also learned that we could do it with less energy than other methods, and we have detoxified the soil so we can safely put it back on," he said.
Heating the soil to about 420 ° C is the ideal area for treatment, Zygourakis said. Heating at 470 ° C (878 ° F) slightly improved the work by removing contaminants, but used more energy and, more importantly, reduced soil fertility to the point where it could not be reused. .
"Between 200 and 300 C (392-572 F), light volatile compounds evaporate," he said. "When you reach a temperature between 350 and 400 C (662 and 752 F), you first break the heteroatom bonds, then the carbon-carbon and carbon-hydrogen bonds, triggering a sequence of radical reactions that convert heavier hydrocarbons. in stable and unreactive reagents, carbonize. "
The pilot program was truly tested when the researchers cultivated Simpson's black seeded lettuce, a variety for which the oil is highly toxic, on the initially clean soil, on some contaminated soils and on several pyrolyzed soils. Although the starting of the plants in the treated soils was a little slower, they found that after 21 days, the plants grown in a soil pyrolyzed with fertilizer or just water had the same germination rates and the same weight as those grown in clean soil.
"We knew we had a process that effectively cleans hydrocarbon contaminated soils and restores their fertility," said Zygourakis. "But did we really detoxify the soil?"
To answer this last question, the Rice team turned to Bhagavatula Moorthy, professor of neonatology at Baylor College of Medicine, who studies the effects of airborne contaminants on neonatal development. . Moorthy and his lab discovered that extracted extracts of oil-contaminated soils were toxic to human lung cells, while exposure of the same cell lines to treated soil extracts had no undesirable effect. The study dispelled fears that pyrolyzed soils could release airborne dust particles contaminated with highly toxic pollutants such as PAHs.
& # 39; & # 39; One of the important lessons we have learned is that different treatment goals for regulatory compliance, detoxification and soil fertility restoration need not be mutually exclusive and can be achieved simultaneously, "said Alvarez.
Wen Song, a visiting scholar at Rice and a student at Jinan University and Shandong University in China, is the main author of the article. The co-authors are Julia Vidonish, former wife of Rice, Arcadis U.S., Seattle; Pingfeng Yu, postdoctoral researcher in rice; Roopa Kamath, Environmental Consultant at Chevron; Chun Chu, Research Associate, Baylor College of Medicine; and Baoyu Gao, professor of environmental engineering at Shandong University. Alvarez holds the George R. Brown Chair in Materials Science and Nanotechnology and Professor of Civil and Environmental Engineering at Rice. Zygourakis is the A.J. Professor of Chemical and Biomolecular Engineering at Hartsook and Professor of Bioengineering.
The research was funded by Chevron USA, the National Natural Science Foundation of China, Shanghai Tongji Gao Tingyao Environmental Science and Technology Foundation for Development, Taishan Scholarship Program and National Institutes of Health.
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