A thin membranes made from a web of nanowires might become a promising tool for cleaning up oil spills and removing toxic contaminants from groundwater. When dipped into a mixture of water and oil, the 50-micrometer-thick membrane absorbs the oil, swelling to 20 times its weight.
Typically, oil spills are cleaned up using the same basic technology used 20 years ago. This includes using absorbent materials to sop up traces of oil. Natural sorbents such as hay and cellulose can soak up between 3 and 15 times their weight in oil, while synthetic polymer-based sorbents can absorb up to 70 times their weight. But these materials tend to absorb water as well.
The new membrane absorbs oil and solvents and is superhydrophobic, which means it strongly repels water. “If you were to put it in water for a month and take it out it would still be dry,” says Francesco Stellacci, the MIT Materials science and engineering professor who led the work, published online in Nature Nanotechnology. Stellaci says the material should not be too expensive to make in large quantities and can be easily reused many times, although the researchers haven’t measured how many times yet.
Michael Rubner, an MIT materials science and engineering professor who was not involved in the project, says that the membrane’s reusability is its most distinctive feature. Other hydrophobic structures have typically been made from organic materials. The inorganic nanowires can handle temperatures up to 600 degrees Celsius, where organic materials would degrade. “If the membrane becomes foul with oil or you have to remove the oil, … [you] can basically cook it and clean it up and, in principle, use it over and over again,” Rubner says.
The membrane is a mat of potassium manganese oxide nanowires, each about 20 nanometers wide. Stellacci and his colleagues assemble the mats using a method similar to one used to produce paper: they make a suspension of nanowires and dry it on a substrate. They have made membranes that are 27 centimeters on each side, but Stellacci says they could be made in larger mats.
Two important characteristics give the membrane its exceptional oil-absorbing and water-repelling properties. First, the nanowire mesh has tiny pores–10-nanometers wide on averagecapable of wicking water and other liquids up into the membrane. To keep water away, researchers coat the membrane with water-repelling silicone. The result: water rolls off the surface of the membrane while oil travels quickly up the pores. Stellacci and his colleagues tested the membrane with mixtures of different organic solvents and oils, including motor oil, gasoline and toluene.
The researchers also found that the membrane can separate nearly identical solvents; when dipped into a mixture of benzene and toluene, the membrane absorbs only the toluene. “That’s pretty amazing, because they’re quite similar molecules,” says Joerg Lahann, a chemical engineering professor at the University of Michigan. This property could open up other applications, such as purifying or separating chemicals and solvents.
Researchers hope that the nanomembrane could reduce waste and lower the cost of cleaning oil spills from boats and in the petroleum industry. But Doug Helton, a scientist with the National Oceanic and Atmospheric Administration, says that it might be too early to say whether the nanomembrane might be practical in cleaning up large oil spills. The heating technique needed to clean the membrane might prove “a fairly onerous process,” he says.
Plus, the membrane’s oil-sopping capacity might diminish at a real spill. “Oil spills are pretty messy,” Helton says. “There might be a lot of debris. That might reduce the efficiency of the sorbent.” For now, Helton thinks the membrane could be good for removing water contaminants at factories or cleaning up smaller oil spills–in garages and machine shops.