With freshwater declining throughout the globe, desalination looks increasingly attractive, but current technologies are expensive, demand far too much energy and are prone to contamination. Now researchers from the University of Texas at Austin and the University of Marburg in Germany have developed a “water chip” that creates a small electrical field that separates salt from seawater. The technology, which is still under development and works at the nano scale, uses so little energy it can run off a store-bought battery!
Our limited supply of fresh water is in high demand, and as we try to cope with “peak water”, one of the strategies is to work toward making desalination technology more efficient, cheaper, and less energy-intensive. Having an affordable, simple, and effective way to turn seawater or other briny water sources into clean drinking water could mean the difference between life and death in many parts of the world.
Desalination methods are available, but getting them up and running takes a lot of money and energy, so for places with access to abundant seawater, yet without the necessary funds or energy infrastructure to build or run desalination plants, new methods of desalination are called for.
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.
Although various alternative technologies are being developed, the large-scale desalination of seawater typically involves forcing it through a membrane that allows the water to pass through, but that traps the salt. These membranes can be costly, they can get fouled, and powerful pumps are required to push the water through. Now, however, scientists from the University of Texas at Austin and Germany’s University of Marburg are taking another approach. They’ve developed a chip that separates salt from water.
The researchers apply a 3.0 volt electrical charge to the plastic water chip, which has a microchannel with two branches. By creating an “ion depletion zone” with an embedded electrode that neutralizes chloride ions, they are able to redirect the salts in the water down one channel, while the fresh water goes down another.
“Like a troll at the foot of the bridge, the ion depletion zone prevents salt from passing through, resulting in the production of freshwater,” the team wrote in a recent press release.
Less energy-intensive than current desalination plants, the water chip doesn’t rely on a membrane, and can be made portable so that just about anybody living near the sea can purify their own water at home.
“The availability of water for drinking and crop irrigation is one of the most basic requirements for maintaining and improving human health,” said Crooks, the Robert A. Welch Chair in Chemistry in the College of Natural Sciences. “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.”
The technique, called electrochemically mediated seawater desalination, was described last week in the journal Angewandte Chemie. The research team was led by Richard Crooks of The University of Texas at Austin and Ulrich Tallarek of the University of Marburg. It’s patent-pending and is in commercial development by startup company Okeanos Technologies.
The prototype plastic “water chip” contains a microchannel that branches in two, and utilizes a process known as electrochemically mediated seawater desalination.
That process begins with seawater being run into the microchannel, and a 3-volt electrical current being applied. This causes an electrode embedded at the branching point of the channel to neutralize some of the chloride ions in the water, which in turn increases the electrical field at that point in the channel. That area of increased current, called an ion depletion zone, diverts the salt to one branch in the channel while allowing the water to continue down another.
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In its present form, the system can run on so little energy that a store-bought battery is all that’s required as a power source. Two challenges still need to be overcome, however.
First of all, the chip currently only removes 25 percent of the salt from the water – 99 percent must be removed in order for seawater to be considered drinkable. Secondly, the system must be scaled up in order to be practical. It presently produces about 40 nanoliters of desalted water per minute. That said, the scientists are confident that with further research, they can rectify both issues.
“The neutralization reaction occurring at the electrode is key to removing the salts in seawater,” said 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 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.”
Currently the technology purifies just one nanoliter at a time and only has a 25% efficiency rate, but the team is confident that their proof of concept can be first improved and then scaled up.
A small startup called Okeanos Technologies has been created to continue developing the technology. Its head, Tony Frudakis, said in the statement that people are dying for want of freshwater, and they will persevere to prevent that from happening.
“You could build a disaster relief array or a municipal-scale unit,” said Frudakis.
“Okeanos has even contemplated building a small system that would look like a Coke machine and would operate in a standalone fashion to produce enough water for a small village.”
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.
“You could build a disaster relief array or a municipal-scale unit,” said Frudakis. “Okeanos has even contemplated building a small system that would look like a Coke machine and would operate in a standalone fashion to produce enough water for a small village.”
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