Water desalination plants can effectively turn seawater into drinking water, but they're hardly portable. Now a team has created a salt-removing gadget so small that hundreds of them could fit onto a penny. If researchers can scale up this invention into a working device, it could generate up to a glass of fresh water per minute using about the same energy as a table lamp does.
The most common desalination technology, known as reverse osmosis, involves applying pressure to seawater to force salt ions through a membrane. Reverse osmosis is used in large facilities that generate drinking water for entire cities. Although portable reverse-osmosis devices are available, most work slowly and have trouble filtering out water pollutants.
So engineer Jongyoon Han of the Massachusetts Institute of Technology in Cambridge and his team took a look at another technology: ion channel polarization . ICP has been around for a while, says Han, but it's never been used to desalinate water. In ICP, a liquid with charged and neutral ions, such as seawater, is run through a channel. Along the channel is an electrical potential that repels charged particles. This causes the liquid to split, creating one stream with charged particles and another with neutral particles. "We thought, if we apply this technique to the seawater, then we can eliminate [salt] ions," says Sung Jae Kim, head of the project and a postdoc in Han's lab.
To test whether ICP could remove salt and other charged contaminants in water, such as bacteria and certain pollutants, Kim added blood cells to seawater and tagged them with fluorescent dye. Kim then put the saltwater through a channel the width of a few hairs and had it pass through the ion-repulsion area. On one side was a very salty, fluorescent mixture, and on the other side was freshwater-though that water would still need to be put through a charcoal filter to eliminate neutral materials, such as hydrocarbons, from industrial pollution. So you could shower with water produced by ICP alone, says Kim, but you still wouldn't want to drink it.
The researchers, who report their findings today online in Nature Nanotechnology, haven't yet figured out how to string thousands of these units together. Their dream is to create a portable device that can run on a solar cell battery and provide emergency relief to people living in drought-ridden or disaster-struck countries. Kim says his calculations suggest that he will need to integrate some 1600 nano-units onto a 20-centimeter wafer to generate about 300 milliliters of water per minute.
An obstacle, says Olgica Bakajin, chief technology officer and a nanotechnologist at Porifera, a company that designs water-filtration membranes in Hayward, California, is for the researchers to figure out how to take this nano-unit and turn it into a portable gizmo. It won't be enough to link all those units without creating some sort of device, she says. "I just can't see that a wafer's going to be something you put in your backpack."
Bruce Logan, an environmental engineer at Pennsylvania State University, University Park, says that ICP seems to offer one clear advantage over reverse osmosis: it does a better job of sifting out charged pollutants and could be more efficient at removing bacteria and some industrial waste compounds.
But many reverse-osmosis facilities and devices remove even more salt ions, says Tom Pankratz, director of the International Desalination Association in Topsfield, Massachusetts. Just because something meets accepted ion-level standards, he says, doesn't make it taste salt-free. "My guess is that [ICP desalination] will never replace reverse osmosis, but it will find some little market," he says.