Eco-Friendly Method can Help Remove Oil from Water Surfaces

Aquatic life is under significant threat due to oil. Now, inspired by nature, scientists at the University of Bonn, the RWTH Aachen University, and the Heimbach-GmbH have created a new, eco-friendly technology to remove such kinds of contamination.

For instance, textiles with unique surface characteristics can passively remove the oil and shift it to a floating container. Surfaces from the plant kingdom were used by the researchers as a model. The study was recently published in the journal Philosophical Transactions A.

The video clip is short but remarkable: The sequence, which runs for 18 seconds, demonstrates dark-colored crude oil flowing from a pipette and falling slowly into a glass of water. When a scientist holds a green leaf against the spot, the leaf absorbs the oil from the water surface within seconds, without leaving behind any trace of the oil.

The tiny green leaf, which is the star of the video clip, belongs to Salvinia, a kind of floating fern. The unique abilities of Salvinia’s leaves make it fascinating for researchers because of its excellent hydrophobic properties; when the leaves are immersed they enclose themselves in an air jacket and continue to remain fully dry.

This behavior is referred to as “superhydrophobic”—which can be translated as “extremely water repellent”—by investigators. But the surface of Salvinia has an affinity for oil, which is, to some extent, a drawback of super-hydrophobia.

This allows the leaves to transport an oil film on their surface. And we have also been able to transfer this property to technically producible surfaces, such as textiles.

Dr Wilhelm Barthlott, Professor Emeritus, University of Bonn

Barthlott was also the former director of the university’s botanic gardens.

Functional Textiles as “Suction Tubes”

Super-hydrophobic substances like these can be used, for example, to efficiently remove the oil films from the surface of water without using any chemicals.

But super-hydrophobic substances are different from other types of materials that have been utilized to date for this purpose and do not have the ability to absorb oil.

Instead, it travels along the surface of the fabric, moved forward solely by its adhesive forces. For example, in the laboratory we hung such fabric tapes over the edge of a container floating on the water. Within a short time they had almost completely removed the oil from the water surface and transported it into the container.

Dr Wilhelm Barthlott, Professor Emeritus, University of Bonn

Power is provided by gravity; therefore, the base of the container should be below the surface of water that includes the oil film.

“The oil is then completely skimmed off—as if using an automatic skimming spoon for meat stock,” added Barthlott.

This phenomenon also makes super-hydrophobic textiles fascinating for environmental technology as they provide a novel method to resolve the significant environmental issues of oil spills in bodies of water.

Several problems are caused by oil films that float on water. Such oil films not only prevent the exchange of gases via the surface but also pose a risk to many animals and plants upon contact. Moreover, since oil films spread rapidly across huge surfaces, they can pose a hazard to entire ecosystems.

Cleaning Without Chemicals

The novel process developed by the researchers does not involve the use of chemicals. Traditional binding agents just soak up the oil and are typically burned afterward. The super-hydrophobia technique is different from the traditional one: “The oil skimmed into the floating container is so clean that it can be reused,” explained Professor Barthlott.

The process is not meant for widespread oil disasters like those that take place following a tanker accident. However, specifically minor contaminations, like heating oil or leaks, or engine oil from ships or cars, represent a more urgent issue.

“Even minor quantities become a danger to the ecosystem, especially in stagnant or slow-flowing waters,” the biologist pointed out. According to Barthlott, this is one area where the technique offers a major application potential. The University of Bonn has filed a patent for this method.

In a general sense, several surfaces display super-hydrophobic behavior, although to different levels. Most importantly, the fundamental prerequisite is that the material itself is resistant to water, for instance, because of a wax coating. But that aspect alone is not adequate.

Superhydrophobia is always based on certain structures on the surface, such as small hairs or warts—often on a nanotechnological scale.

Dr Wilhelm Barthlott, Professor Emeritus, University of Bonn

Thanks to Barthlott, science has learned much more about these associations than it did several years ago.

“This now helps us to develop oil-absorbing materials with particularly good transport properties, in cooperation with RWTH Aachen University,” concluded Barthlott.

The study was supported by the Deutsche Bundesstiftung Umwelt DBU.

Video Credit: © W. Barthlott, M. Mail/Uni Bonn.