As their name suggests, most existing water purifying filters clean the water by physically trapping or filtering out bacteria. Stanford researchers have now developed a new kind of water purifying filter that isn’t really a filter at all. Instead of trapping bacteria, the new filter actually lets them pass right through. But, by the time they emerge from the filter they have been killed by an electrical field running through it. Not only is the new filter more than 80,000 times faster than existing filters, it is also low-cost, has no moving parts and uses very little power, which should make it particularly attractive for use in the developing world where it is needed most.
The key to the new filter is coating the filter fabric – ordinary cotton – with nanotubes and silver nanowires. When an electric field is passed through the highly conductive “nano-coated” cotton, it kills almost all the bacteria passing through it. In lab tests, over 98 percent of Escherichia coli bacteria that were exposed to 20 volts of electricity in the filter for several seconds were killed. Multiple layers of fabric were used to make the filter 2.5 inches thick.
"This really provides a new water treatment method to kill pathogens," said Yi Cui, an associate professor of materials science and engineering at Stanford whose research team is also responsible for using nanomaterials to build batteries from paper. "It can easily be used in remote areas where people don't have access to chemical treatments such as chlorine."
Speeding things upFilters that physically trap bacteria must have pore spaces small enough to keep the pathogens from slipping through, but that restricts the filters' flow rate. Since the new filter doesn't trap bacteria, it can have much larger pores, allowing water to speed through at a faster rate – about 80,000 times faster. The larger pore spaces in Cui's filter also keep it from getting clogged, which is a problem with filters that physically pull bacteria out of the water.
Cui's research group teamed with that of Sarah Heilshorn, an assistant professor of materials science and engineering, whose group brought its bioengineering expertise to bear on designing the filters.
Silver has long been known to have chemical properties that kill bacteria. "In the days before pasteurization and refrigeration, people would sometimes drop silver dollars into milk bottles to combat bacteria, or even swallow it," Heilshorn said.
Cui's group knew from previous projects that carbon nanotubes were good electrical conductors, so the researchers reasoned the two materials in concert would be effective against bacteria. "This approach really takes silver out of the folk remedy realm and into a high-tech setting, where it is much more effective," Heilshorn said.
Keeping costs downBut the scientists also wanted to design the filters to be as inexpensive as possible. The amount of silver used for the nanowires was so small the cost was negligible, Cui said. Still, they needed a foundation material that was "cheap, widely available and chemically and mechanically robust." So they went with ordinary woven cotton fabric. "We got it at Wal-mart," Cui said.
To turn their discount store cotton into a filter, they dipped it into a solution of carbon nanotubes, let it dry, then dipped it into the silver nanowire solution. They also tried mixing both nanomaterials together and doing a single dunk, which also worked. They let the cotton soak for at least a few minutes, sometimes up to 20, but that was all it took.
The big advantage of the nanomaterials is that their small size makes it easier for them to stick to the cotton, Cui said. The nanowires range from 40 to 100 billionths of a meter in diameter and up to 10 millionths of a meter in length. The nanotubes were only a few millionths of a meter long and as narrow as a single billionth of a meter. Because the nanomaterials stick so well, the nanotubes create a smooth, continuous surface on the cotton fibers. The longer nanowires generally have one end attached with the nanotubes and the other end branching off, poking into the void space between cotton fibers.
"With a continuous structure along the length, you can move the electrons very efficiently and really make the filter very conducting," he said. "That means the filter requires less voltage."
Low powerThe electrical current that helps do the killing is only a few milliamperes strong – barely enough to cause a tingling sensation in a person and easily supplied by a small solar panel or a couple 12-volt car batteries. The electrical current can also be generated from a stationary bicycle or by a hand-cranked device.
The low electricity requirement of the new filter is another advantage over those that physically filter bacteria, which use electric pumps to force water through their tiny pores. Those pumps take a lot of electricity to operate, Cui said. However, the pores in the nano-filter are large enough that no pumping is needed – the force of gravity is enough to send the water speeding through.
In some of the lab tests of the nano-filter, the electricity needed to run current through the filter was only a fifth of what a filtration pump would have needed to filter a comparable amount of water.
Although the new filter is designed to let bacteria pass through, an added advantage of using the silver nanowire is that if any bacteria were to linger, the silver would likely kill it. This avoids biofouling, in which bacteria form a film on a filter. Biofouling is a common problem in filters that use small pores to filter out bacteria.
Cui said the electricity passing through the conducting filter may also be altering the pH of the water near the filter surface, which could add to its lethality toward the bacteria.
Next Cui and his team will try the filter on different types of bacteria and run tests using several successive filters.
"With one filter, we can kill 98 percent of the bacteria," Cui said. "For drinking water, you don't want any live bacteria in the water, so we will have to use multiple filter stages."
Cui is the senior author of a paper describing the research that will be published in an upcoming issue of Nano Letters. The paper is available online now.
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