Boron-nitride nanotubes speed desalination, says Australian research

Researchers from The Australian National University (ANU) claim to have discovered a way to speed the desalination of seawater by up to five times using nanotubes made from boron and nitrogen atoms.

In a paper published in the journal Small, researchers Dr Tamsyn Hilder, Dr Dan Gordon and group leader Professor Shin-Ho Chung from the Computational Biophysics Group at the Research School of Biology at ANU say that boron-nitride nanotubes have shown superior water flow properties compared with carbon nanotubes, and are thus expected to provide a more efficient water purification device.

“Using boron nitride nanotubes, and the same operating pressure as current desalination methods, we can achieve 100% salt rejection for concentrations twice that of seawater with water flowing four times faster, which means a much faster and more efficient desalination process,” says Dr Hilder.

Hilder, Gordon and Chung use computational tools to simulate the water and salt moving through the nanotube. They found that the boron nitride nanotubes not only eliminate salt but also allow water to flow through extraordinarily fast, comparable to biological water channels naturally found in the body.

“Our research also suggests the possibility of engineering simple nanotubes that mimic some of the functions of complex biological nanotubes or nanochannels,” said Professor Chung, and work is continuing to investigate these possibilities further. These devices, once successfully manufactured, may be used for antibiotics, ultra-sensitive detectors or anti-cancer drugs

Using molecular dynamics simulations, the paper shows that a (5, 5) boron-nitride nanotube embedded in a silicon-nitride membrane can, in principle, obtain 100% salt rejection at high concentrations owing to a high energy barrier, while still allowing water molecules to flow at a rate as high as 10.7 water molecules per nanosecond (or 0.9268 L/m² h).

Furthermore, ions continue to be rejected under the influence of high hydrostatic pressures up to 612 MPa. When the nanotube radius is increased to 4.14 Å, the tube becomes cation-selective, and at 5.52 Å, the tube becomes anion-selective.

On the health dangers of nanotubes, Dr Hilder told D&WR, “Our research investigates the use of nanotubes made from boron nitride nanotubes, which a have improved biocompatibility as compared to carbon nanotubes. Recent studies have shown that boron nitride nanotubes are non-cytotoxic.”