Silver socks have cloudy lining

A first assessment of socks containing silver nanoparticles, meant to cut down on foot odors, shows that the fabrics release most of the tiny particles in the wash—and possibly into the environment via solid waste from water treatment plants.

Tiny particles of silver meant to keep socks microbe-free may not work for long. New research shows that the nanomaterials tend to wash out of the textiles they are meant to treat, and once the silver goes down the drain, it may end up in the biosolids left over from wastewater treatment processes. Researchers from Arizona State University report on the fate of nanosilver from treated socks in ES&T (DOI: 10.1021/es7032718).

The research is "the first hard data" on the release of nanosilver from textiles in a domestic setting, says Bernd Nowack, a researcher at the Swiss Federal Laboratories for Materials Testing and Research (Empa). Nowack found a dearth of data when modeling releases of nanoparticles, including nanosilver, carbon nanotubes, and titanium dioxide, into the environment (to be published in ES&T, DOI: 10.1021/es7029637).

Graduate student Troy Benn and his thesis adviser Paul Westerhoff tested six brands of nanosilver-treated socks on the market. The researchers measured the nanosilver content of the socks in several ways, including digesting the materials in acid to examine the remaining silver and measuring the socks' silver content directly with scanning and transmission electron microscopy.

After determining the nanosilver content of the textiles, the researchers put the socks through the wash. The socks soaked in 1- or 24-hour-long wash cycles in distilled water without detergent to limit variables in the tests. One batch soaked in Tempe, Ariz., tap water for a more realistic washing.

The nanosilver particles clumped together in the cloth itself as well as in the wash, which was not surprising, Westerhoff says. Some of the socks had tiny corkscrew-shaped nanosilver particles that stuck like burrs to the fabric, clinging more tightly than some simpler nanosilver forms. Nevertheless, some of the socks lost the bulk of their nanosilver after two to four washings, the team found. The new research is "very simple, but it had to be done," Nowack says. "It's difficult to know what exactly is in these commercial products."

The researchers also tested activated sludge from a local wastewater treatment plant and found that it contained most of the silver washed out of the socks. Wastewater discharge may be an important environmental source of nanosilver, Westerhoff says, but biosolids might capture most of the particles. "It really changes the route of nanomaterials into the environment," he adds. "How much of that silver would be land-applied?"

Although Benn and Westerhoff cannot pinpoint exact amounts, they estimate that more than half of the nanoparticles dissolve into ionic silver. Westerhoff adds that the ionic silver could react with sulfur to eventually form silver sulfides in the environment. Past research shows minimal damage from silver nanoparticles to fish, but the effects remain unknown for other creatures. Silver sulfide is less toxic than silver alone but more persistent and may be more biologically available.

In a real-world wastewater treatment plant, says Martin Scheringer of the Swiss Federal Institute of Technology (ETH-Zurich), silver nanoparticles probably would dissolve and bind to sulfur or sulfide. Some of that silver precipitate would then wash out of a treatment plant without attaching to biosolids—and enter freshwater systems. Recent modeling by Scheringer and co-workers (Sci. Total Environ. 2008, 390, 396–409) resulted in estimates that nanotreated commercial products would emit about 15% of all silver in EU rivers and streams. —NAOMI LUBICK