Purifying the sea, a drop at a time

 Jongyoon Han at the Massachusetts Institute of Technology (MIT) in Cambridge and his colleagues have come up with a device that could be used as a simple, portable water-desalination system run from a battery or on solar power.

Han and his team were investigating the physics behind a phenomenon called ion-concentration polarisation.

This occurs when a voltage is applied across a membrane, setting up an ion current. Because only positive ions can pass through the membrane, a mismatch is created across it. A higher proportion of positive ions amass on one side of the membrane together with the negative ions that were unable to traverse it. The researchers decided to exploit this effect to scrub salts out of water. Instead of a membrane, however, they used an ion-selective material called Nafion to make a nanojunction. This connects to a larger, micrometre-sized channel that has sea water flowing through it.
When a voltage is turned on across the nanojunction, salts are repelled from the sea water as it flows by, although the sea water doesn’t touch the nanojunction. The microchannel splits into two at the junction, with fresh water to be collected while the repelled salty water is pushed away through the second microchannel.

Promising trickle
Han and his co-workers used sea water from Crane Beach in Ipswich, Mass., to test the device, which worked right away. It repelled more than just salt, eliminating any charged particles, including many proteins and microorganisms. The team tested this by contaminating the water with human blood that had been stained with fluorescent markers, and found that the markers flowed into the same channel as the salts. And because the sea water doesn’t touch the nanojunction, the device is unlikely to get fouled up by microbes sticking to it.

The device is just a few centimeters square, and not enough water passes through one for practical purposes, just 250 nanolitres of fresh water can be collected per minute. But Han says that if it were possible to put many of the devices onto some kind of chip, he could produce something to rival portable household water filters. This would lead to flow rates of about 100 millilitres per minute, he says.
Desmond Lawler, an engineer who works on water desalination at the University of Texas at Austin, says that such a device could be used in disaster zones, where small amounts of pure water are needed quickly and cheaply. Han says that his device isn’t intended to go head-to-head with large-scale purification plants
Katharine Sanderson
NYT News Service