Rice University scientists have identified an energy efficient method to clean soil contaminated by oil spills and reclaim its fertility.
To achieve this, the researchers use a process called pyrolysis that involves heating contaminated soils in the absence of oxygen. Rice environmental engineer Pedro Alvarez intoned that the approach is much better for the environment as compared to standard incineration techniques used for fast remediation. The team’s original goal was to speed the response to oil spills.
According to Alvarez, off-shore oil spills tend to get the most attention. However, 98 percent of spills – more than 25,000 per year – occur on land. Industry and governments worldwide spend more than $10 billion annually to clean up oil spills.
The Rice team found that pyrolyzing contaminated soil for three hours reduced the amount of residual petroleum hydrocarbons to well below regulatory standards (typically less than 0.1 percent by weight). Furthermore, it also enhanced the soil’s fertility by turning the remaining carbon into beneficial char that resembles coke.
The team removed toxic pollutants and hydrophobicity, and retained some of the carbon and perhaps some of the nutrients. As a result they were able to enhance plant growth, which they demonstrated by successfully growing lettuce in the reclaimed soil. If you are wondering why only lettuce, it has been accepted by the community as very sensitive to toxins, especially petroleum.
Apart from growing food, the reclaimed soil could also be used for re-greening: planting grass to minimize erosion and to restore vegetation.
Rice chemical engineer Kyriacos Zygourakis explained that the process is part thermal desorption, and takes advantage of petroleum chemistry.
The contaminated soil is heated to about 420 degrees Celsius in the absence of oxygen to remove lighter hydrocarbons. Meanwhile, as the temperature increases above 350 degrees, the high-molecular-weight hydrocarbons, the resins and asphaltenes, undergo a series of cracking and condensation reactions to form solid char. Some of the hydrocarbons are retained in a solid, more benign form.
If everything including char is to be removed, the temperature can be raised even higher and oxygen is introduced. However, this will destroy the soil and use 40 to 60 percent more energy.
Rice graduate student Julia Vidonish said that the process is scalable and should work with existing remediation equipment. “We expect companies can take a mobile, field-scale thermal desorption unit and make a couple of modifications to do pyrolysis.”
The team now intends to understand how pyrolysis time and temperature affect the quality of char in the soil.
Image courtesy of Julia Vidonish/Rice University
