Following an announcement two years ago that “Nanotube membranes offer [the] possibility of cheaper desalination“, scientists at Lawrence Livermore National Laboratory have continued progress toward the goal of nanotech membranes for water purification that will greatly decrease the energy cost for desalination. In one recent study they showed that carbon nanotubes reject ions that make up common salts while they rush through at great speeds chains of water molecules held together by hydrogen bonds. From LLNL “Livermore researchers use carbon nanotubes for molecular transport“:
Molecular transport across cellular membranes is essential to many of life’s processes, for example electrical signaling in nerves, muscles and synapses.
In biological systems, the membranes often contain a slippery inner surface with selective filter regions made up of specialized protein channels of sub-nanometer size. These pores regulate cellular traffic, allowing some of the smallest molecules in the world to traverse the membrane extremely quickly, while at the same time rejecting other small molecules and ions.
Researchers at Lawrence Livermore National Laboratory are mimicking that process with manmade carbon nanotube membranes, which have pores that are 100,000 times smaller than a human hair, and were able to determine the rejection mechanism within the pores.
“Hydrophobic, narrow diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform,” said Olgica Bakajin, who led the LLNL team whose study appeared in the June 6 online edition of the journal Proceedings of the National Academy of Sciences [abstract].
In the initial discovery, reported in the May 19, 2006 issue of the journal Science [abstract], 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.
…One of the most promising applications for carbon nanotube membranes is sea water desalination. These membranes will some day be able to replace conventional membranes and greatly reduce energy use for desalination.
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.
“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
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.
This press release includes an animation comparing movement of water molecules through an ordinary rough pipe and through a carbon nanotube. A second LLNL press release (via PhysOrg.com) announced a new tool to better understand how water is structured as it moves through the carbon nanotubes “LLNL researchers peer into water in carbon nanotubes“:
Researchers have identified a signature for water inside single-walled carbon nanotubes, helping them understand how water is structured and how it moves within these tiny channels.
This is the first time researchers were able to get a snapshot of the water inside the carbon nanotubes.
…As described in an article appearing in the July edition of Nano Letters [abstract], [LLNL and University of North Carolina researchers] used a technique called Nuclear Magnetic Resonance (NMR) to get a glimpse of the water confined inside one-nanometer diameter SWCNTs.
…Earlier Livermore studies have suggested that carbon nanotubes may be used for desalination and demineralization because of their small pore size and enhanced flow properties. Conventional desalination membranes are typically much less permeable and require large pressures, entailing high energy costs.
However, these more permeable nanotube membranes could reduce the energy costs of desalination significantly.
While the technology offers great promise, there still are important unanswered scientific questions.
“There have been many predictions about how water behaves within carbon nanotubes,” said [Jason] Holt, the principal investigator of the project… “With experiments like these, we can directly probe that water and determine how close those predictions were.”
—Jim
