Very interesting. However after some time when the filter gets clogged and you have to wash the cloth, then what happens to the nanotubes?
Anil K Rajvanshi On Thu, Sep 2, 2010 at 12:22 AM, Crispin Pemberton-Pigott < [email protected]> wrote: > Dear Friends > > I know many subscribers to this list are interested in development in > general. Water purification (boiling) is a common task for stoves. With the > looming advent of cheap electrical power from a stove, this article may of > great interest to a few of you. > > It is another (and very interesting) application of a small amount of > electricity for rural improvement. > > Regards > Crispin > > ...from Gizmag. > > > http://www.gizmag.com/water-purifying-nanofilter/16190/?utm_source=Gizmag+Subscribers&utm_campaign=bed812d73b-UA-2235360-4&utm_medium=email > > 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 up > > Filters 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 down > > But 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 power > > The 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. > > > _______________________________________________ > Stoves mailing list > [email protected] > http://listserv.repp.org/mailman/listinfo/stoves_listserv.repp.org > http://stoves.bioenergylists.org > http://info.bioenergylists.org -- Nimbkar Agricultural Research Institute (NARI) Tambmal, Phaltan-Lonand Road P.O.Box 44 Phaltan-415523, Maharashtra, India Ph:91-2166-222396/220945 e-mail:[email protected] <e-mail%[email protected]> [email protected] http://www.nariphaltan.org http://nariphaltan.virtualave.net
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