Low-voltage ‘troll’ zaps salt out of seawater

U. TEXAS-AUSTIN (US) — Chemists are hopeful their new energy-efficient method to desalinate water can be scaled up for personal or even municipal uses.

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.

Drinking water

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.

Source: University of Texas at Austin

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  1. Lorraine Cherry

    With groundwater being depleted throughout the country at an alarming rate, desalinization should be at the highest priority for government funding. Another avenue of research to address this that seems to have disappeared or gone underground is the development of saline-tolerant hybrids of common food plants. There was a lot of research in this area back in the early 70s, based on the observation that several common food plants have relatives that normally grow in coastal areas with brackish waters. In this era of genetic manipulation, it should be possible to bypass the laborious hybridization processes conceptualized back in the 70s.

  2. MD Brown

    Fascinating; I wish to be added to mailing list, if there is one. Thanks.

  3. Robert Reed

    With all due respect, desalinization is a waste of time and money.
    Even illiterate and unconnected peoples can understand that if there is not enough water, don’t have children. Why promote continued overpopulation with gizmos that do not hold much hope for real world situations? The solution to water scarcity is fewer people.
    Kyle, you’re obviously very smart – keep looking!

  4. Lorraine Cherry

    Since Southern California has been living off of borrowed water from the Colorado River for the last 50 years, perhaps they could lead the way in not having any more children?

  5. andarb

    We have inherited biological needs for reproduction, those of us who forgo reproduction “for the sake of the environment” are simply selecting ourselves out of the gene pool.

    It’s certainly not the worst attribute we’re selecting for.

    I’m looking forward to seeing more on the development of desalination processes. Perhaps, with such technology, we may someday be able to begin reducing or even reversing the desertification we’re seeing in arid parts of the globe.

  6. David Carter

    25% is not much, and these precisely engineered microchannels will be hard to scale up, but its a worthy goal. The odd thing is that every power station has a problem dumping excess heat, much of which could be used for evaporative desalination which gets you 100% pure water. Even solar panels work better when cooled, so those desert solar mega-projects could be making irrigation or drinking water as well as power for homes.

  7. Benson Sundheim

    The desalinization of water requires at the least a definite amount of of so-called “free energy”, in this case electrical work. At 90% this is RT Ln[0.01] per 65 grams of NaClThis requirement, operative at any scale, sets a lower limit on the energy required for such a process – i.e.; there really is no free lunch.

  8. Don

    Forget about the battery and use a solar cell to power the unit. Free drinking water using the sun.

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