A new type of carbon nanotube (CNT) sponge that contains sulfur and iron has been developed and is proving to be more effective at soaking up water contaminants, such as oil, fertilizers, pesticides and pharmaceuticals, than previously seen. The magnetic properties of these nanosponges also make them easier to retrieve from the environment once the clean-up job is done.

Gizmag first reported on the development of reusable, oil-absorbing nanosponges created by adding boron to carbon back in April 2012. The new approach, being pioneered by researchers at the University of Roma, the University of Nantes and the University of L’Aquila, instead uses sulfur and iron to increase size, porosity and the magnetic properties of the sponges.

According to the Institute of Physics (IOP), the researchers found that by adding sulfur during the CNT production process, the average size of the sponges could be increased to around 2 cm (0.8 in). The nanosponges also gained porosity as beneficial defects occurred in the structure. This porosity has a threefold advantage; it creates buoyancy, makes the material sponge-like and reusable, and allows for the addition of iron in the spaces created. By adding iron, in the form of ferrocene, the structure is easier to control and can be recovered from the environment using magnets.

"It is quite tricky using CNT powders to remove oil spilled in the ocean," says Luca Camilli, lead author of the research from the University of Roma. "They are hard to handle and can eventually get lost or dispersed in the ocean after they are released. However, millimeter- or centimeter-scale CNTs, as we’ve synthesized in this study, are much easier to handle. They float on water because of their porous structure and, once saturated with oil, can be easily removed. By simply squeezing them and releasing the oil, they can then be re-used.”

The researchers say these nanosponges were shown to remove the toxic organic solvent dichlorobenzene from water more effectively than previous methods, absorbing a mass 3.5 times greater than previously seen. The sponges are also capable of absorbing vegetable oil up to 150 times their initial weight and can absorb engine oil more effectively than previously possible.

"The improved absorption properties of the sponge are down to the porous structure and the rough surface of the CNTs," says Camilli. "Oils or solvent can easily be absorbed in the empty spaces amongst the CNTs, which is made easier by the rough surfaces."

The team will now look to improve the synthesis process to make it possible to produce the sponges on a commercial scale. Camilli says the toxicity of the sponges will need to be studied before they can be put to use in real-world applications.

The results of the team's study are published in the journal Nanotechnology.