Electrical Desalination

28 March 2010
Posted by Chris Smith.

Water is one of our most fundamental needs, and many natural disasters can remove our source of clean fresh water. One inexhaustable source of water is of course the sea, but the salt makes it no use for drinking. There are various ways of desalinating this water, but most of them don't scale down very well, even reverse osmosis - essentially filtering out the salt with a special membrane - requires large pressures, which requires a pump and the membranes tend to get gummed up by contaminants.

Electrical desalination system
A new energy-efficient desalination system utilizing ion concentration polarization as the core mechanism. Both salts and any charged colloids in seawater are diverted into the brine channel, leaving the other channel relatively salt- and particle-free. The system could be made portable and operated by current battery technolgy. © Credit: Sung Jae Kim/Jongyoon Han - Nature Nanotechnology

Sung Jae Kim from MIT and colleagues have come up with an alternative. Instead of using a membrane to separate the salt from the water they are using electricity. They have made a y shape of channels about half a mm wide. The dirty water comes in along the stem of the Y and on one of the two arms is a nafion nanojunction. This is a material which will let positively charged ions flow through it but not negative ones. This is made negatively charged and a current flows along the arm in the wrong direction. This leaves lots of negatively charged ions piling up and flow up the other arm. These take any other charged species with it, including the ions in salt and bacteria and other contaminants leaving water with 99% of the salt removed.

It isn't as efficient as reverse osmosis, but it is intrinsically a much smaller scale process so you could build a machine consisting of maybe 1600 of these junctions to purify maybe 15l of water a day and because the contaminats are pushed away from the sensitive parts of the system it doesn't clog up like a membrane.

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