Membrane free desalination chip – a “troll at the foot of the bridge”

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A patent pending technology using a mediated seawater desalination process could produce desalinated water using a small electrical field.

Researched by chemists at The University of Texas at Austin and the University of Marburg in Germany, the process works by applying a small voltage (3.0 volts) to a plastic chip filled with seawater.

Although the research teams have only achieved a 25% desalination rate, they believe that a 99% desalination rate is “not beyond our reach”.

The other major challenge is to scale up the process. Currently the microchannels, about the size of a human hair, produce about 40 nanoliters of desalted water per minute.

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 said to be sufficient enough to redirect salts into one branch, allowing desalinated water to pass through the other branch.

In layman’s terms, the process has been described like a “troll at the foot of the bridge”, with the ion depletion zone preventing salt from passing through, resulting in the production of freshwater.

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 start-up company, Okeanos Technologies.

Robert Gerard from membrane consultancy Aqualogy told WWi magazine: "It is a very interesting new process that has the potential to produce desalinated water at lower energy consumption than reverse osmosis for low flow rates. I do not expect the technology to be able to compete with large scale desalination technologies in the near future due to the limited salt removal rates."

Tony Frudakis, founder and CEO of Okeanos Technologies, said the technology could eventually be used in disaster relief.

“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,” he said.

A similar "nanofluidic desalination" research project by Dr Sung Jae Kim, research scientist in the Massachusetts Institute of Technology's (MIT) Department of Electrical Engineering & Computer Science, was presented in Nature Nanotechnology in March 2010. The process uses nano-fluidics and ion concentration polarization to desalinate water. Also membrane-less, the process creates a repulsion zone to separate charged particles from flowing seawater.

In July 2012 researchers from MIT also announced a new form of desalination filtration material using sheets of graphene, a one-atom-thick form of the element carbon, which was claimed could purify water at a far lower cost than traditional methods.

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