Low Cost Filter Uses Electrified Nanostructures to Purify Water

A textile based one-dimensional nanostructures device allows high-speed electrical sterilization of water using silver nanowires, carbon nanotubes, and cotton.

Researchers at Stanford University (CA, USA) developed the new filter, which instead of physically trapping bacteria by size exclusion (as most existing filters do), kills bacteria as they flow through the device with the aid of an electrical field that runs through highly conductive "nanocoated" cotton. The very high surface area of the device, coupled with large electric field concentrations near silver nanowire tips, allows for the highly effective bacterial inactivation. Since the new filter does not need to trap the bacteria, it can have much larger pores, allowing water to speed through about 80,000 times faster than filters that trap bacteria. The larger pore spaces also keep the filter from being clogged, which is a problem with filters that physically pull bacteria out of the water.

To make the filters as inexpensively as possible, the researchers dipped ordinary woven cotton fabric into a solution of carbon nanotubes. The nanotubes create a smooth, continuous surface on the cotton fibers, with the longer nanowires generally attached to the cotton fiber at one end, while the other end branches off into the void space between fibers. The researchers then waited for the fabric to dry, and dipped it again into a silver nanowire solution. This helped form a continuous structure along the length of the fabric, which allowed the use of a minimal electrical current--only a few milliamperes strong--to kill the bacteria. In fact, the electricity needed to run current through the filter is 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, a common problem in filtration filters. The study describing the new filter was published in the September 2010 issue of Nano Letters.

"This really provides a new water treatment method to kill pathogens. With one filter, we can kill 98% of the bacteria,” said lead author Yi Cui, Ph.D., an associate professor of materials science and engineering. "For drinking water, you don't want any live bacteria in the water, so we will have to use multiple filter stages. It can easily be used in remote areas where people don't have access to chemical treatments such as chlorine."

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