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Socks with Nanosilver Could Present Problems

Photonics Spectra
Jun 2008
Hank Hogan

The saying “It all comes out in the wash” could take on a whole new meaning as nanoparticles become more common in consumer goods. Using scanning and transmission electron microscopy, Troy M. Benn and Paul K. Westerhoff of Arizona State University in Tempe looked at what happens when common socks manufactured with antimicrobial nanoscale silver are washed. The release of silver — which is toxic in its ionic form — is a potential problem, and the Environmental Protection Agency enforces a standard of 100 parts per billion of the metal in drinking water.

The use of nanosilver for antimicrobial applications is relatively new, as are other nanoproducts in consumer goods. However, how much of the silver is released during everyday activities remains unknown. The researchers, therefore, looked at what happens to washed socks, picking that type of clothing because it is readily available and small enough for laboratory experiments.

They selected six sock brands thought to contain silver, based on manufacturers’ claims. They confirmed its presence by digesting the socks with acid, then quantifying the amount of silver present via optical emission spectroscopy with an inductively coupled plasma system made by Thermo Scientific (now Thermo Fisher Scientific Inc.). They simulated washing the socks, using agitation cycles of either 1 or 24 h. The latter ensured that enough silver was freed, while the former was probably closer to what happens in the real world, according to Benn.

NanoSock.jpg

A scanning electron microscope image of ashed sock material shows spherical silver particles ~100 nm in diameter. The silver acts as an antimicrobial agent. Researchers are exploring the possible environmental effects of nanosilver that leaches from washed clothing. Reprinted with permission from the American Chemical Society.


In analyzing both sock material and wastewater samples, they used scanning and electron microscopy. They did the first with a system from FEI Co. of Hillsboro, Ore., that has energy-dispersive x-ray spectrometry capability. For the second, they used a system from Tokyo-based JEOL Ltd. with the same energy-dispersive extension.

They determined that five of the six brands had silver in them; however, only three released detectable silver, with the nanoparticles typically in the 100- to 500-nm range. This release, they discovered, happened after a single hour of simulated washing. Benn said that these results suggest that something in the manufacturing process controls the release, potentially making it possible to prevent the release entirely.

The investigators also modeled the effect of the released silver particles on a wastewater treatment plant, finding that it could handle a high concentration of incoming silver but that there might be an effect on the plant’s ability to dispose of agricultural fertilizer and other biosolids.

They looked at the adsorption of the leached silver by the biomass in a wastewater treatment plant. Their findings indicate that a facility probably could treat a high concentration of incoming silver. Given that plants have been treating silver in wastewater for some time, Benn said he was not surprised by this result. He noted also that nanosilver might be even easier to treat than other forms of the metal. “The nanoparticle form has a high surface area to mass ratio, so we would expect [the silver nanoparticles] to attach to biomass surfaces fairly well.”

One downside is that this abatement might limit the ability of the biomass to be disposed of as agricultural fertilizer, a route currently not regulated for silver. However, the situation may change as nanosilver becomes more common.

The study may have looked only at socks, but the implications could be much greater. “We assume that our results with the socks could be applied to other clothing articles containing silver,” Benn said.

The next steps will be to estimate the number of consumer products that can be used before this effect becomes a problem and to evaluate other consumer goods made with nanoparticles, with an ultimate outcome perhaps being policy and regulatory changes.

Environmental Science & Technology, ASAP Edition, April 9, 2008, doi: 10.1021/es7032718.


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