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Nanotubes and desalination

Is there anything that carbon nanotubes can’t do?

I know I’ve asked that question before — but I can’t stop being amazed at the fantastic properties of the tiny tubes.

This morning I heard Olgica Bakajin of Lawrence Livermore National Lab describe how she made a water filter using carbon nanotubes.

She did this by growing a forest of nanotubes on a silicon substrate and then filling in the gaps between the tubes with a nitride material.

After removing the silicon substrate, her team were left with a thin film that is permeated by nanotubes with an average diameter of about 1.6 nm — each of which turns out to be an excellent conductor of water.

Indeed, experiments showed that water flows through the nanotubes about four times faster than what would be expected from simple pipes.

The reason, according to Bakajin, is two fold. Firstly the walls of the nanotubes are hydrophobic — water molecules avoid the walls — which reduces drag. Also, the nanotubes are exceptionally smooth, again reducing drag.

And if that wasn’t good enough, the team found that the nanotube filters are very good at removing ions from water as it passes through. Bakajin thinks that the broken bonds at either end of the tubes attract the ions.

As a result, the filters could play an important role in the desalination of seawater — Bakajin’s filters were able to remove 40% of the chloride ions at a relatively high flow rate. This means that they already outperform commercial nanofilters.

The filters could be improved significantly by increasing the density of nanotubes in the filter; and optimizing the ends of the nanotubes for removing salt.

All of could mean highly-permeable filters that would reduce the amount of energy required in a desalination facility — perhaps making it economically viable.

This entry was posted in APS March Meeting 2009. Bookmark the permalink.
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