LLNL Researchers use carbon nanotubes for molecular transport
The last time I wrote about the researchers at LLNL was back in December 2006. Their carbon nanotube research is the most promising imho of a half dozen interesting lines of research that I’ve seen. That is, the goal of membrane research is a to have a pipe that ends in in a covered mushroom shape that rises above the ocean floor in 50-100 feet of salt water–someplace where there is a strong coastal current. Fresh water filters through a membrane without extra energy and falls through some kind gas that’s hostile to aerobic and anaerobic bacteria–like maybe chlorine. This research provides a path to that goal.
In the initial discovery, reported in the May 19, 2006 issue of the journal Science, the LLNL team found that water molecules in a carbon nanotube move fast and do not stick to the nanotube’s super smooth surface, much like water moves through biological channels. The water molecules travel in chains – because they interact with each other strongly via hydrogen bonds.
Of course one of the most promising applications for this process is seawater desalination.
These membranes will some day be able to replace conventional membranes and greatly reduce energy use for desalination.
The current study looked at the process in more detail.
In the recent study, the researchers wanted to find out if the membranes with 1.6 nanometer (nm) pores reject ions that make up common salts. In fact, the pores did reject the ions and the team was able to understand the rejection mechanism.
What was the rejection mechanism?
Fast flow through carbon nanotube pores makes nanotube membranes more permeable than other membranes with the same pore sizes. Yet, just like conventional membranes, nanotube membranes exclude ions and other particles due to a combination of small pore size and pore charge effects.
But it was principally charge that did the deed.
“Our study showed that pores with a diameter of 1.6nm on the average, the salts get rejected due to the charge at the ends of the carbon nanotubes,” said Francesco Fornasiero, an LLNL postdoctoral researcher, team member and the study’s first author.
The salinity of the water studied was much lower than brackish water. So work will need to be done to figure out how to increase the charge at the tip of the nanotubes. Might be good to highly charge the filler material. Or put imperfections in the carbon nanotubes to increase their charge. In this blog i mention that charge might be related to something else. Here’s still another take on charge. Might be good work for simulations. Earlier work last fall showed a nice congruence between experimental work and computer models.
Finally Siemans recently announced that they had developed a process that would cut energy use in half. Their method involved removing salt using an electric field. So an interesting way to “artificially” introduce a larger charge for higher salt concentrations would be to create a small electric field along the surface of the carbon nanotube. this of course, costs energy. But it would make an interesting interim step.
As well, its helpful to mention that the study just announced by LLNL was not about how water flowed through the membrane but rather the experiment was designed to more precisely peg the mechanism by which salt rejection took place at the carbon nanotube’s tip. So the animation in the press release is a bit misleading
Some further study of the process by which water flowed through the nanotubes was done by Jason Holt.
A while back I asked a member of the LLNL team what the best investment of dollars would be for research in this field. He said that the best investment currently would be “in coming up with scalable (economical) processes for producing membranes that use nanotubes or other useful nanomaterials for desalination.”
Here is a link to the LLNL press release.
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