Zapping salt out of seawater
The method, which creates a small electrical field that removes salts from seawater, consumes less energy and is dramatically simpler than conventional techniques.
The technique, described in the journal Angewandte Chemie, requires so little energy that it can run on a store-bought battery.
The process, called electrochemically mediated seawater desalination, evades the problems confronting current desalination methods by eliminating the need for a membrane and by separating salt from water at a microscale.
“The availability of water for drinking and crop irrigation is one of the most basic requirements for maintaining and improving human health,” says Richard Crooks, a chemistry professor at the University of Texas at Austin.
“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.”
This new method holds particular promise for the water-stressed areas in which about a third of the planet’s inhabitants live. Many of these regions have access to abundant seawater but not to the energy infrastructure or money necessary to desalt water using conventional technology. As a result, millions of deaths per year in these regions are attributed to water-related causes.
‘Troll’ diverts fresh water
To achieve desalination, the researchers apply a small voltage (3.0 volts) to a plastic chip filled with seawater. The chip contains a microchannel with two branches.
At the junction of the channel an embedded electrode neutralizes 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 neutralization reaction occurring at the electrode is key to removing the salts in seawater,” says Kyle Knust, a graduate student in Crooks’ lab and first author on the paper. Like a troll at the foot of the bridge, the ion depletion zone prevents salt from passing through, resulting in the production of freshwater.
Thus far Crooks and 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,” says 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 microchannels, about the size of a human hair, produce about 40 nanoliters of desalted water per minute. To make this technique practical for individual or communal use, a device would have to produce liters 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.
Ulrich Tallarek of the University of Marburg helped develop the patent-pending technology, which is in commercial development by Okeanos Technologies.