Micro-scale desalination method is highly energy efficient
28 June 2013
A new method for the desalination of seawater consumes less energy and is dramatically simpler than conventional techniques.
A prototype 'water chip' developed by researchers at the University of Texas at Austin in collaboration with Okeanos Technologies (photo courtesy of Okeanos Technologies)
By creating a small electrical field that removes salts from seawater, chemists at The University of Texas at Austin and the University of Marburg in Germany have introduced a new method for the desalination of seawater that consumes less energy and is dramatically simpler than conventional techniques. The new method requires so little energy that it can run on a store-bought battery.
The process evades the problems confronting current desalination methods by eliminating the need for a membrane and by separating salt from water at a micro-scale.
The research team was led by Richard Crooks of The University of Texas at Austin and Ulrich Tallarek of the University of Marburg. The technique, called electrochemically mediated seawater desalination, is patent-pending and is in commercial development by startup company Okeanos Technologies.
"The availability of water for drinking and crop irrigation is one of the most basic requirements for maintaining and improving human health," said Crooks. "Seawater desalination is one way to address this need, but most current methods for desalinating water rely on expensive and easily contaminated membranes.
"The membrane-free method we've developed still needs to be refined and scaled up, but if we can succeed at that, then one day it might be possible to provide fresh water on a massive scale using a simple, even portable, system."
To achieve desalination, the researchers apply a small voltage (3.0V) to a plastic chip filled with seawater. The chip contains a micro-channel with two branches. At the junction of the channel an embedded electrode neutralises some of the chloride ions in seawater to create an 'ion depletion zone' that increases the local electric field compared with the rest of the channel. This change in the electric field is sufficient to redirect salts into one branch, allowing desalinated water to pass through the other branch.
"The neutralisation reaction occurring at the electrode is key to removing the salts in seawater," said Kyle Knust, a graduate student in Crooks' lab. The ion depletion zone prevents salt from passing through, resulting in the production of freshwater.
Thus far Crooks and his colleagues have achieved 25 percent desalination. Although drinking water requires 99 percent desalination, they are confident that goal can be achieved.
"This was a proof of principle," said Knust. "We've made comparable performance improvements while developing other applications based on the formation of an ion depletion zone. That suggests that 99 percent desalination is not beyond our reach."
The other major challenge is to scale up the process. Right now the micro-channels, about the size of a human hair, produce about 40 nanolitres of desalted water per minute. To make this technique practical for individual or communal use, a device would have to produce litres of water per day. The authors are confident that this can be achieved as well.
If these engineering challenges are surmounted, they foresee a future in which the technology is deployed at different scales to meet different needs.