By coating a simple polymer sponge with graphene nanoribbons, researchers have made an absorbent that, when heated, can quickly pull highly viscous crude oil from water.

In order to clean up oil spills in water, emergency responders typically use floating booms or skimmers. But these are slow and labor-intensive to use. So researchers have been developing advanced porous materials to quickly and cheaply remove oil from water. These systems, such as carbon-based aerogels and nanowire membranes, are typically porous absorbents modified with functional nanomaterials that are made to repel water and attract oil.

But the materials made so far only work efficiently with light and medium crude oil. Thicker heavy crude oil with viscosity ranging from 1,000–100,000 mPa s seeps into and through the pores at much slower speeds.

Effective technologies for viscous oil remediation are critical because large heavy crude reserves have been discovered in Canada, California, and Venezuela, and specialized techniques for its extraction are increasing production. And since heavy crude is difficult to process and transport, this could increase the chances of accidental marine oil spills. Besides, “even light crude oil can experience a remarkable increase of viscosity within several hours of spill due to the evaporation of its light components and weathering effect,” says Shu-Hong Yu, a professor of chemistry at the University of Science and Technology of China in Hefei.

To make an efficient remediation system for heavy oil, Yu and his colleagues soaked a commercial melamine sponge in a graphene oxide suspension. They spun the sponge in a centrifuge to get rid of excess graphene oxide, leaving behind only graphene oxide nanosheets plastered on nearly all the surfaces of the sponge, including its tiny pores. Then they treated it with a hydrogen iodide solution to reduce the graphene oxide to graphene.

The graphene coating makes the sponge water-repelling and conductive. In an experimental heavy oil-spill setup, the graphene-coated sponge floats on the oil surface. Because it is hydrophobic, it selectively draws oil into the pores, but the viscous oil flows in very slowly. When a few tens of volts are passed through the sponge, though, the graphene heats up and heats the oil around it. This reduces the oil’s viscosity, letting it quickly enter and diffuse through the sponge’s pores. The technique works across the typical viscosity range of heavy crudes; most nanotechnology-based remediation technologies have been tested up to viscosities of 500 mPa s.

The heated sponge soaks up oil over 94% faster than a non-heated sponge. The warm oil can also be removed seven times faster than non-heated oil by squeezing the sponge, which can then be reused several times.

The researchers also demonstrated that they could directly pump viscous crude oil through the sponge. They calculated that each kg of the heated sponge could remove 2,320 kg of oil per hour, about 30 times faster than the non-heated material. The results are presented in the journal Nature Nanotechnology.

The sponge is now about the size of a one-inch thick stack of #10 envelopes, but it should be possible to make very large sponges with industrial centrifuges, Yu says. To keep costs reasonable for large-scale practical application, only the bottom part of the sponge could be coated and heated. The researchers have designed a multi-electrode sheet that could heat the sponge from the bottom close to the oil surface, reducing energy consumption. “The oil-soaked sponge would then be peeled off from the electrode and compressed to remove the oil,” he says. “At the same time, a new sponge can be fixed on the electrode.”

“This approach takes advantage of graphene’s joule heating properties in order to transform a simple sponge into an advanced [absorption] system,” says Despina Fragouli, a smart materials researcher at the Italian Institute of Technology. “It makes possible the remediation of very viscous oils by absorption, something not possible by methods presented so far.” To make the process more efficient and low-cost, the researchers should reduce overall energy consumption and better isolate the system so that all the generated heat goes to the viscous oil and not partly to the air.

Read the abstract in Nature Nanotechnology.