Tuesday, May 31, 2011

Silver Nanoparticles Filter Water

"According to the World Health Organization, over one billion people do not have access to clean water. Bacterial contamination of water is a major cause of life threatening disease outbreaks, such as cholera or gastroenteritis. There is a pressing need for small-scale filter systems that can purify water of bacteria and other harmful microorganisms. They must also be cheap, safe, portable and easy to use."

A simple and inexpensive filtering system could provide safe drinking water for millions of people who are in short supply of clean water, especially following natural disasters and other emergencies, report McGill University researchers who devised the first-of-its-kind system. People without a reliable source of potable water could use the paper filter, which is impregnated with small particles of pure silver that kill disease-causing bacteria as they pass through it. Very little silver escapes the portable filter, which could provide an efficient source of potable water to alleviate diseases – such as cholera and giardiasis – that are associated with drinking foul water.

The authors passed water contaminated with E. coli and E. faecalis through six different paper filters containing silver. E. coli and E. faecalis are bacteria commonly associated with fecal contamination and responsible for diarrhea outbreaks. After being filtered, the researchers collected, grew and counted the live bacteria.

Simple, absorbant blotting paper – 0.5 millimeter thick – was used in all experiments. Chemists soaked the paper for 30 minutes in water solutions containing silver ions. Four different silver concentrations were tested. After rinsing, the filters were exposed to a chemical that transformed the soluble silver into nanoparticles right inside the blotting paper – much like small balls of silver trapped in a web of paper fibers.

The filter containing the highest content in silver nanoparticles – 5.9 mg per gram of paper – was the most effective and efficient filter tested. Almost all of the bacteria were killed. One billion E. coli bacteria at the beginning fell to less than 100 after filtering. This beats U.S. Environmental Protection Agency (EPA) antibacterial efficiency standards by 100 times.

Silver did leach out of the filter, but at low levels. The researchers measured amounts that were about two times lower than the EPA limit for silver content in drinking water.

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