Tiny tubes desalinate water faster, cheaper
Currently, desalinisation is too expensive and energy intensive for large-scale feasibility. The study, published in the journal ‘Science’, showed that carbon nano tubes are better at desalinisation than any other existing method.
A carbon nanotube is like an impossibly small rolled
up sheet of paper, about a nanometer in diameter. For comparison, the diameter of a human hair is 50 to 70 micrometers — 50,000 times wider. The tube’s miniscule size, exactly 0.8 nm, only allows one water molecule to pass through at a time. This single-file lineup disrupts the hydrogen bonds, so water can be pushed through the tubes at an accelerated pace, with no bulking, the study said.
To conduct the research, Meni Wanunu, Associate Professor of Physics at Northeastern University in Boston and post doctoral student Robert Henley collaborated with scientists at the Lawrence Livermore National Laboratory in California. In addition to being precisely the right size for passing single water molecules, carbon nanotubes have a negative electric charge. This causes them to reject anything with the same charge, like the negative ions in salt, as well as other unwanted particles. The finding offers a novel system that could have major implications for the future of water security.
A similar study which was done at Lawrence Livermore National Laboratory, California, by Ramya H. Tunuguntla and colleagues revealed that at just the right size, carbon nanotubes can filter water with better efficiency than biological proteins. They found that carbon nano tubes with a width of 0.8 nanometers outperformed aquaporins, a class of biological proteins, in filtering efficiency by a factor of six.
These narrow carbon nanotube porins (nCNTPs) were still slim enough to force the water molecules into a single-file chain. The researchers attribute the differences between aquaporins and nCNTPS to differences in hydrogen bonding - whereas pore-lining residues in aquaporins can donate or accept H bonds to incoming water molecules, the walls of CNTPs cannot form H bonds, permitting unimpeded water flow.
The nCNTPs in this study maintained permeability exceeding that of typical saltwater, only diminishing at very high salt concentrations. Lastly, the team found that by changing the charges at the mouth of the nanotube, they can alter the ion selectivity.