Direct Desalination of Seawater with Nanofluidic PDMS Chips

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Access to clean water may pose the next great challenge to humanity in the coming decades.  Over 2.5 billion people live without sanitation, nearly 800 million of whom do not have access to clean drinking water.  Most of those who do not have access to clean water and sanitation reside in the poorest nations of the world.  Further complicating these issues are the fact that watersheds do not prescribe to defined national boundaries, often spanning socioculturally diverse regions.  In theory, desalination offers promise of solving the world’s water woes, but in reality, desalinating salt and brackish water is energy and resource intensive.  Conventional desalination methods include distillation, the lowest tech, but most resource and energy intensive method as well as reverse osmosis and electrodialysis.  All three of these methods are expensive, energy intensive and only efficient in large systems.  These factors make traditional desalination methods ill-suited for cash or resource deficient nations or disaster situations. 

Researchers recently published proof of concept illustrating the effectiveness of Ion Concentration Polarization (ICP) at desalting ocean water.  ICP is an electrochemical phenomenon that separates ions in solution when a current is passed through an ion selective membrane.  While this desalination method uses ion-selective membranes, it is considered a membraneless filtration method because it uses repulsive forces to direct ions away from the membrane, only allowing deionized water through.  This allows for the filtration system to avoid challenges such as membrane fouling and salt build-up that plague other desalination techniques.  This method also uses less pressure and less electricity than reverse osmosis and electrodialysis, reducing the resource and infrastructure costs of installing a system based on this technology.  Finally, since this technology does not rely upon filtration and it separates out things like blood, viruses, bacteria and salt (all ionized species), the feed water needs little treatment other than filtering out sand, large organic particles and seaweed.

The proof of concept system used Polydimethylsiloxane (PDMS) microfluidic chips comprised of microchannels separated by nanochannels.  The nanochannels—perm selective nanojunctions—were created by infiltrating nafion polymer solution between mechanically fabricated microchannels on the PDMS microfluidic chips.  The resultant chip was then bonded to a glass plate via plasma treatment methods.  The electrodes in the chip are comprised of gold and titanium bonded to the metal buffer channel system via a nafion polymer coating.

The system was shown to produce desalinated water recovery of about 50% with approximately 99% ion removal and a pH of 7-7.5.  The energy efficiency of this is between 3.75 to 5 watt hours per liter, making it at least equivalent to the most efficient and largest reverse osmosis plants that boast total system efficiency rates around 5 watt hours per liter.  This system is also more efficient at removing ions that electrodialysis systems because ICP systems do not need to match the current to the amount of ions being removed.  Rather, these systems use a consistent current to deflect the ions in solution.

Although the early results of this technology are promising, much more research and engineering is necessary to create systems that could potentially be scalable.  Engineers will need to refine manufacturing techniques to ensure the chips can be made cost effectively.  Engineers must also work to create components that can be aggregated into stationary and portable ICP systems for commercial desalination.

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Desalination of water through ICP is a potential low energy, low cost solution for providing access to clean water in developing nations and during disaster response. This technology can also be integrated with renewable technologies, and is efficient and practical in smaller scales than conventional desalination techniques.

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There is little risk associated with ICP as it is simply the separation of ions in water through the use of nanostructured Polydimethylsiloxane (PDMS) nanofluidic chips and an electric current. The materials are made of safe polymers, conductors and glass, using electrochemical forces to separate salt, ions and biological charged particles from water. There are no additional constituents in the return water beyond the concentrated ions and particles already present. Any risks associated with this technology are purely ecological and are dependent on the individual system. Ecological risks associated with siting depend on the topography, bathymetry and ecology of the surrounding environment. Returning large amounts of concentrated water to a poorly circulated area can upset the ecological balance in these sensitive coastal habitats. Additionally, other potential risks exist depending on the power source used to operate the system. Systems powered via solar arrays or battery systems would pose fewer environmental risks than systems powered by conventional generation methods such as coal, natural gas or diesel combustion.

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