Stanford University is testing an inexpensive water filter, suitable for developing countries, that removes bacteria quickly and without clogging. The filter uses a piece of cotton treated with nanomaterial inks, that kills bacteria with electrical fields but uses just 20 percent of the power required by pressure-driven filters.
The Stanford filter, which is driven by gravity, has pores large enough to allow for a high flow rate–about 100,000 liters per hour. It uses electrical pulses to inactivate bacteria by poking holes in their cell walls. The research was led by Stanford materials science and engineering professors Yi Cui and Sarah Heilshorn.
To make the filter, researchers dip a piece of cotton batting in a water-based carbon-nanotube ink, let it dry, then dip it in an alcohol-based silver-nanowire ink and let it dry again. Cui and others have used similar dipping methods to make paper-nanotube battery electrodes and nanotube textiles. The long, narrow nanotubes and nanowires get enmeshed in the fibers.
Speedy sterilization: A Stanford researcher pours water through a funnel fitted with a cotton-nanotube filter that rapidly kills bacteria. The red cords supply electricity to the device, which uses electrical fields to poke holes in the bacteria. Photo: Technology Review
In preliminary tests, described online in the journal Nano Letters, the filter inactivated about 98 percent of E. coli bacteria.
In the tests, a wire, connected to an electrical power source, which could be car batteries or solar panels, is immersed in the water filter. The researchers think that the strong local electrical fields, which are formed at the tip of the silver nanowires, piercing the cell walls. When the electricity is off, the silver (which has antimicrobial properties) prevents bacteria from fouling the surface, a common problem with filters.
There have been no definitive studies of the effects of water-borne carbon nanotubes and silver nanowires on people and lower organisms; experiments with airborne carbon nanotubes have shown that their effect on mice lungs is similar to the effect of asbestos. But early tests on thousands of gallons of water suggest that the nanomaterials are not leaching into the water. The researchers will perform further tests to determine whether the nanomaterials remain enmeshed in the filter or are dislodged into the water over time.
The next step required is to improve the filter’s efficacy and show that it can work with a broad range of water-borne pathogens, including viruses and protozoa.
Watch the related video “Making a Nano-Water Filter for the Developing World”
Source: Katherine Bourzac, Technology Review, 08 Sep 2010