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What a tiny sun-powered water purifier could mean for developing countries

Scientists have developed a quicker, cheaper way to purify water – with a solar-powered device half the size of a postage stamp. 

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    Villagers cross the polluted Ciujung river at Astana Agung village in Serang, Indonesia's Banten province August 14, 2012. According to a villager in Astana Agung, the villagers must walk to another location to get clean water, bathe and wash their clothes as the Ciujung river is polluted from waste being dumped by factories located along the river.
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Clean water may soon be more accessible in developing countries, thanks to a new water purification device powered by sunlight. 

The tiny device, developed by researchers at the SLAC National Accelerator Laboratory and Stanford University, is able to disinfect water quicker and far more cheaply than existing methods, according to a report published Monday in the journal Nature Nanotechnology. 

Using the sun to disinfect water is already common in developing countries. But the process of using UV rays to kill bacteria is relatively lengthy: it takes six hours for the process to begin, and can take up to 48 hours to complete. In contrast, the new device is able to kill 99.999 percent of bacteria in 25 milliliters of water in just 20 minutes, leaving only pure water in its wake. 

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"Our device looks like a little rectangle of black glass. We just dropped it into the water and put everything under the sun, and the sun did all the work," explained Chong Liu, lead author of the report, in an SLAC news release. 

While the device is relatively simple to use, creating it was a slightly more complex process. The device, which is roughly half the size of a postage stamp, is lined with thin, closely spaced films, or "nanoflakes," of molybdenum disulfide.

Typically, molybdenum disulfide is used as an industrial lubricant. However, when it's made into layers just several atoms thick, it becomes a photocatalyst. When light reaches it, many of the electrons shift away from their usual locations, leaving "holes" behind and paving the way for chemical reactions to take place. 

To get the device to absorb as much visible sunlight as possible, the researchers had to make the molybdenum disulfide walls exactly the right thickness. They then topped each wall with a thin layer of copper: this allowed them to use that sunlight to trigger reactions that produce "reactive oxygen species," such as hydrogen peroxide, to kill bacteria in the water. 

Beyond being useful for hikers and other outdoor adventurers, researchers say the cheaply and easily manufactured device also has potential for widespread use in developing countries lacking clean water. 

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Access to better drinking water has improved significantly in the past several decades, as 2.6 billion people have gained access to clean water since 1990. However, that still leaves 783 million people around the world without clean water, most of them in developing countries. 

The device has currently only been tested on three strains of bacteria, but the researchers expect that it will work on other strains and microbes as well. The team plans to conduct additional tests in rivers in developing countries in the near future, as it has so far only been tested in a laboratory setting on less than an ounce of water mixed with bacteria.

Though the device doesn't have the capability to remove chemical pollutants from water, as a filter might, researchers expect that its ability to eliminate bacteria in a quick and cost effective way could greatly improve the quality of life for many people around the world. 

"It's very exciting to see that by just designing a material you can achieve a good performance. It really works," said Ms. Liu. "Our intention is to solve environmental pollution problems so people can live better."

The device likely will not be available for commercial sale for another three to five years, SLAC spokesman Andrew Gordon told The Mercury News. 

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