Magnetic Nanoparticles Absorb Crude Oil
by Jenna Bilbrey
Materials Research Society | Published: 25 September 2013
Crude oil from the 2010 Deepwater Horizon spill still lingers in the Gulf of Mexico. It is hiding under the surface, dispersed in the water and mixed in with the sand-along with the, now considered toxic, dispersant Corexit. To combat the contamination, researchers from Texas A&M University have developed a non-toxic sequestering agent-iron oxide nanoparticles coated in a polymer mesh-that can hold up to 10 times their weight in crude oil.
Karen Wooley, the principal investigator of the work, says the project is still in the early stages, "but the fact that [our nanoparticle system] can capture 10 times its weight in crude oil is such a promising first result that I think they have significant potential."
As reported in ACS Nano, the nanoparticles consist of an iron oxide core surrounded by a shell of polymeric material-a simple poly(acrylic acid)-block-polystyrene that possesses both hydrophilic (poly(acrylic acid)) and hydrophobic (polystyrene) groups. This amphiphilic copolymer interacts with both the aliphatic hydrocarbons and aromatic components present in crude oil. Crosslinking the polymer makes the shell more stable in aqueous environments while maintaining the crude oil loading potential.
Adriana Pavia-Sanders, the Ph.D. student who developed these unique materials, synthesized magnetic iron oxide nanoparticles with 8 nm diameters. She then dispersed the particles in a solution of premade copolymer. The two parts self-assembled into micelles about 70 nm in diameter. Each micelle holds multiple nanoparticles within the polymer mesh to increase the magnetic response of the material.
The idea is to use the particles to soak up the left-over oil. "There are already mechanisms in place to capture much of the [bulk oil] through mechanical means. The residual sheen is much more difficult to sequester," says Wooley. In fact, "the greatest proportion of birds that were found dead had no visible crude oil coverage."
To simulate an actual oil spill, they weathered a sample of crude oil to match the conditions of the Deepwater Horizon spill. When the nanoparticles were dropped in the oil-water mixture, they immediately changed color from light tan to black as they soaked up the oil. Even more interesting, when the nanoparticles were full they floated to the top. This makes for easy recovery by a conventional magnet, not only from a test vial but also in the ocean. A magnet was held to the side of the vial, nanoparticles collected at the glass, and the water was poured off, leaving the crude oil behind inside of the particles. Each nanoparticle held 10 times its weight in crude oil.
Sonication in ethanol released the oil from the swollen polymer matrix. The nanoparticles returned to a light tan. Although spectroscopic changes were observed after washing, the nanoparticles absorbed the same amount of oil during a second trial. The system is completely reusable. It's cost-effective too. "Jonathan Sanders, one of the students who's working on this project, did a cost analysis and found the particles' price is comparable to current technology," says Wooley.
"I think it's going to attract a lot of attention from nanoparticle scientists. It's quite a clever design," says Joseph Pignatello of the Connecticut Agricultural Experiment Station and Associate Professor of Chemical and Environmental Engineering at Yale, who was not involved in this work. "My main concern is that this is a high tech solution to a low tech problem. I added up the number of chemicals, solvents and individual steps and it's pretty daunting." Polymerization of the block copolymer is a multistep process, and both the polymerization and the nanoparticle synthesis require air-free environments. This doesn't mean the technology won't make it to real work applications. Pignatello was quick to say, "in reality the concept gets put out there and if industry is interested they will refine it to come up with a cheaper and more straightforward [approach]."
The nanoparticles are stable for long periods, making for easy storage. If an oil spill occurs, a stockpile can be ready and waiting. But their longevity may pose a problem. If the recovery phase left any particles behind, they would bob around, collecting with other ocean trash. Wooley has a plan for that. Her lab is working with biodegradable polymers made from natural products. This way any leftover nanoparticles would dissolve over time and not add to the current overflow of ocean pollution.
Read the abstract from ACS Nano here.