Purifying the sea one drop at a time
Microfluidic channels offer promise of cheap, portable desalination.
Disaster-struck areas desperate for fresh water could benefit from an ion-repelling device that cleans up contaminated salt water.
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 polarization. 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 try 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 actually touch the nanojunction. The microchannel splits into two at the junction, with fresh water flowing straight on to be collected while the repelled salty water is pushed away through the second microchannel.
Han and his co-workers used sea water from Crane Beach in Ipswich, Massachusetts, 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 centimetres 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. "If you hit that kind of flow rate it's going to be really useful."
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. For large-scale desalination, reverse osmosis — whereby water is forced through holes small enough to exclude many molecules — is still the way to go, he says.
Han says that his device isn't intended to go head-to-head with large-scale purification plants: "this technology's not going to be in competition and it doesn't need to be," he says.
He is working with MIT to scale up the technology by making an array of the little water purifiers. In their paper, published online in Nature Nanotechnology1, Han and his team say that litre for litre the energy used by the device is comparable to that of large-scale reverse osmosis plants, and less than that for small-scale reverse osmosis devices.
But Lawler has his doubts. "Whether this thing can be manufactured in a way that's cheap enough so it can be used instantly, we don't know," he says.
The device doesn't remove neutral molecules, however, which could be a limitation, Lawler says. But he adds that, in a disaster zone, perfect purity is not necessarily what is required. "It depends on how desperate people are," he says. "In many situations a disaster might completely disrupt a water purification system. Having a technology that can be deployed quickly will be useful."
- Kim, S, J., Ko, S. H., Kang, K. H. & Han, J. Nature Nanotechnol. doi:10.1038/nnano.2010.34 (2010).