Ground contamination from crude oil and tar is notoriously difficult to clean up because these substances cling tenaciously to the soil. Remediation of such pollution at some of the hundreds of hazardous-waste sites around the United States has proved inefficient and costly.
This could be one place where nanotechnology can make a difference, say researchers at Cornell University who have designed polymer vesicles that, when pumped through soil, trap contaminants and carry them to the surface.
Civil and environmental engineer Leonard Lion and his colleagues created the particles from long polymer molecules. Each molecule consists of a hydrophilic (water-attracting) and a hydrophobic (water-repelling) segment. In water, the molecules self-assemble into soluble particles with their hydrophobic chains oriented toward the center of the particle and their hydrophilic chains facing the outside.
With an average diameter of only 20 nanometers, the particles can travel swiftly through water in the soil without getting stuck.
The Cornell team worked with sand contaminated with phenanthrene, a polycyclic aromatic hydrocarbon (PAH) typically found in coal tar. The researchers placed the sand in a stainless steel column and then added a solution of the particles, pumped in from the bottom.
As the particles moved up through the column, their hydrophobic cores drew the phenanthrene compounds off the sand grains and into the interiors of the particles. “The polymer acts like an alternative refuge for the contaminants,” says Lion.
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The Cornell team describes the particles in an upcoming issue of Environmental Science and Technology.
Lion envisions using the particles to improve upon what’s called pump-and-treat remediation. At its most basic, this method pumps contaminated groundwater to the surface, removes the contaminants, and then reinjects the cleaned water underground.
To make the method more efficient, engineers can inject surfactants into the polluted soil. These detergentlike chemicals cluster around the contaminants, making them easier to pump to the surface. However, the surfactants have a tendency to break down and stick to soil grains.
The Cornell researchers hope that replacing the surfactants with their group’s polymer particles will greatly enhance the efficiency of this remediation process. After the particles carry their cargo to the surface, they can be relieved of their toxic loads and used again, Lion points out.
“The concept is very simple and very attractive,” says Wei-xian Zhang of Lehigh University in Bethlehem, Pa. Making it ready for field testing, however, will take some doing, he says.
For one thing, the same mobility in the soil that could enable the particles to capture large quantities of PAHs could also make the particles and their cargo difficult to retrieve. “These highly mobile particles could spread in every direction in the soil,” says Zhang.
To circumvent that potential pitfall, Zhang and his colleagues are developing iron nanoparticles that can convert contaminants such as polychlorinated biphenyls (PCBs) into harmless chemicals on contact. With this strategy, there would be no need to retrieve the particles.
Both approaches reflect the growing interest among remediation scientists and engineers in using nanoscale materials for cleaning up the environment.