Rice University
researchers pyrolyzed oil-contaminated soil to reduce total petroleum
hydrocarbons
(TPH) below
federal standards, while leaving beneficial carbons in the soil. The lab grew
lettuce in
samples of
reclaimed soil to test its viability. (Credit: Julia Vidonish/Rice University)
Rice University discovery uses less energy while reclaiming
soil at oil spills
(August 20, 2015)
Rice University scientists are cleaning soil contaminated by oil spills
in a way that saves energy and reclaims the soil’s fertility.
They use a process known as pyrolysis, which involves
heating contaminated soils in the absence of oxygen. This approach is much
better for the environment than standard incineration techniques for fast remediation,
said Rice environmental engineer Pedro Alvarez.
“Our original goal was to speed the response to oil spills,
but our aspiration was to turn contaminated soil into fertile soil,” said
Alvarez, the George R. Brown Professor and chair of Rice’s Civil and
Environmental Engineering Department.
The new paper by Alvarez and his Rice colleagues in the
American Chemical Society journal Environmental Science and Technology
demonstrates how they’ve done just that.
Off-shore oil spills tend to get the most attention, Alvarez
said, but 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.
Rice University
researchers have discovered that pyrolysis of oil-contaminated soil
reduces
hydrocarbon levels to below regulatory mandates and leaves behind char that
enhances the
soil’s fertility. From left: Graduate student Julia Vidonish and
Professors
Caroline Masiello, Kyriacos Zygourakis and Pedro Alvarez. (Credit: Rice
University)
The Rice team found that pyrolyzing contaminated soil for
three hours not only reduced the amount of petroleum hydrocarbons left to well
below regulatory standards (typically less than 0.1 percent by weight), but
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 said. “We turned out to be partly wrong: We didn’t get
biochar, but [we got] a carbonaceous material that we call char and resembles
coke.
“But we were correct in thinking that by removing toxic pollutants
and the hydrophobicity that repels water that plants need, and by retaining
some of the carbon and perhaps some of the nutrients, we would enhance plant
growth,” he said.
The researchers proved that by successfully growing lettuce
in reclaimed soil in the lab. “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 Rice
graduate student Julia Vidonish, the paper’s lead author.
“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.
“Our process is part thermal desorption, but it takes advantage
of petroleum chemistry,” said Rice chemical engineer and co-author Kyriacos
Zygourakis. “By heating the contaminated soils to about 420 degrees Celsius in
the absence of oxygen, we first drive out the lighter hydrocarbons. That’s the
desorption part. But when the temperature gets above 350 degrees, the
high-molecular-weight hydrocarbons, the resins and asphaltenes, undergo a
series of cracking and condensation reactions to form solid char, similar to
the petroleum coke produced in refineries.