Portable filter paper laced with silver nanoparticles cleans dirty water.

May 10, 2011

Dankovich, TA and DG Gray. 2011. Bactericidal paper impregnated with silver nanoparticles for point-of-use water treatment. Environmental Science and Technology http://dx.doi.org/10.1021/es103302t.



Synopsis by Audrey Moores and Wendy Hessler

2011-0420bacteriafilter
Audrey Moores

 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.

 


Context

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.

People have used silver  for centuries to preserve potable water. Even in trace quantities, silver is a potent antibacterial and antifungal agent. 

Silver is now being packaged in nanoparticles – very small materials ten thousandth of the width of a human hair. They are composed of hundreds to thousands of silver atoms, either pure or with a some type of coating. Because they are so small, they expose a large surface to the contaminants, and are thus very effective at killing microorganisms.

Many consumer products – including socks, shirts and countertops – contain silver nanoparticles to take advantage of their antimicrobial properties. The products are promoted as having anti-odor or bacteria killing properties.

However, studies show that silver nanoparticles in garments can detach in the washing machine and travel to the sewage treatment plant where they may kill beneficial water-treating bacteria. They also end up in rivers and lakes where they impact naturally occurring bacteria. Thus, new applications using silver in any form should verify how much of it is leaching.

For applications such as water treatment, it is possible to capture nanoparticles of silver inside a porous material to prevent their release in the treated water. Researchers already proposed such a strategy using polyurethane foams or ceramic filters as porous material. These solutions, however, may be too expensive for point-of-use applications.

What did they do?

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 higher the concentration of silver at the beginning, the larger the nanoparticles in the resulting filter paper. The most concentrated filter contained 5.9 milligrams (mg) of silver per gram of paper.

The researchers also produced silver nanoparticles in water and put them onto a fifth sample of blotting paper. In this example, the nanoparticles were not well embedded. A sixth filter was made by exposing a blotting paper sheet to the soluble silver ions without making particles. 

What did they find?

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.

The filter with silver nanoparticles made prior to deposition were less efficient. About 10 million bacteria survived the filtering. However, the filter containing soluble silver salts was very active. Fewer than 10 bacteria remained after filtering the water. This filter, though, did leach significantly more silver – 18 times the EPA limit – into the water.

In all cases, most of the bacteria are killed as they pass through the silver enhanced filter paper. This means that the filter will not be clogged or fouled during the process.

What does it mean?

The researchers showed that a simple water filtering system – using very inexpensive paper and a little bit of silver nanoparticles – makes an efficient tool to kill diarrhea-causing bacteria.

Bringing potable water to all populations is a daunting problem. The cholera outbreak in Haiti after the 2010 earthquake is a recent example of how tainted drinking water can create a public health crisis. Many died because human fecal material polluted the drinking water sources.

In the study, almost all of the two types of bacteria tested – E. coli and E. faecalis – were killed as the water passed through the filters. They effectively lowered the bacterial count by 10 million. This means that in principle, most water sources contaminated with bacteria could be treated with this filter to reach government drinking water standards.

The silver content that leached into the effluent met the EPA's standards for silver exposure. As well, the filtration was fairly quick: it took 10 minutes to filter 100 milliliters of the water.

The researchers are not sure how the filters kill the bacteria. They hypothesize that soluble silver ions may leach from the nanoparticles. Two observations support this idea. First, they observed leaching, which means soluble silver is present. Second, the filters tested with only soluble silver – but no nanoparticles – were more efficient at killing bacteria.

The authors suggest more tests are needed using a more realistic mix of bacteria and possibly particles of dirt. If further tests prove out and the researchers pursue production, the system could be used in emergency situations, where rapid set up of small-scale, portable, point-of-use devises are needed.


Resources

Davies, R and S Etris. 1997. The development and functions of silver in water purification and disease control. Catalysis Today 36(1):107-114.

Jain, P and T Pradeep. 2005. Potential of silver nanoparticle-coated polyurethane foam as an antibacterial water filter. Biotechnology and Bioengineering 90(1):59–63.

Morones, JR, JL Elechiguerra, A Camacho-Bragado, K Holt, JB Kouri, JT Ramírez and MJ Yacaman. 2005. The bactericidal effect of silver nanoparticles. Nanotechnology 16(10):2346-2353.

Oyanedel-Craver, V and J Smith. 2008. Sustainable colloidal-silver impregnated ceramic filter for point-of-use water treatment. Environmental Science and Technology. 42(3):927–933.

World Health Organization. 2005. Water for Life: Making it Happen, World Health Organization. Geneva. p 4.

World Health Organization. 2006. Guidelines for drinking water quality, 3rd ed. IWA Publishing. London. p 121-144.

 

 

 

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Based on a work at www.environmentalhealthnews.org.

 

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