Carbon nanotubes (CNTs) offer a large specific area and extraordinary mechanical and chemical properties, which are attractive for wastewater clean-up. However, the fine CNT powders can be difficult to handle and hard to retrieve following treatment.

Now, reporting their results in the journal Nanotechnology, Luca Camilli and colleagues at the University of Rome, University of Nantes and the University of L'Aquila have shown that nanosponges - randomly interconnected CNT frameworks - could be an excellent candidate for solving these issues. The structure is both easier to handle than regular carbon nanotubes and has an absorption capability three times the value of previous materials.

In their paper, the authors attribute the superior uptake efficiency of their carbon nanotube sponges to the framework of highly defective carbon nanostructures, which are formed by adding sulphur during the carbon nanotube growth process. "Oils or solvents can easily be absorbed in the empty spaces amongst the CNTs, which is made easier by the rough surfaces," explained Camilli.

The resulting sponge had an average length of 20 mm.

Easy to remove and reuse

During testing, the team observed how their CNT sponges could successfully remove a toxic organic solvent—dichlorobenzene—from water, demonstrating that the porous structure could absorb a mass that was 3.5 times higher than earlier work. The CNT sponges were also shown to absorb vegetable oil up to 150 times its initial weight and soaked up engine oil to a slightly higher capacity than previously reported. Once saturated with contaminants, nanotube sponges then float on the water where they can be easily removed. Simply squeezing or burning them removes the oil so that they can be reused.

Remote control

The defects caused by the addition of sulphur also provide preferential sites for immobilization and subsequent nucleation of iron (Fe) atoms from the ferrocene catalyst, which was added during growth. What’s more, the presence of iron nanoparticles in the carbon nanotube walls allows the sponges to be magnetically controlled and driven remotely, which could also benefit targeted therapeutic applications.

"The next stage of our research is to improve the synthesis process so that the sponges can be produced on a commercial scale," added Camilli. "We must also study the toxicity of the sponges before any real-world applications can be realised."

The researchers published their work in the journal Nanotechnology 25 065701

Further reading

Nanotubes purify water
Deformations of single-wall carbon nanotubes
Growing chiral carbon nanotubes by design