The Fate of Silver Nanoparticle Waste

Manufacturers have incorporated silver nanoparticles into more than 200 consumer products, including clothing and cosmetics, because of their antibacterial properties. Now researchers analyzing sewer sludge provide the first evidence that silver leaching from these consumer products transforms into silver sulfide nanoparticles in wastewater treatment plants (Environ. Sci. Technol., DOI: 10.1021/es101565j). The findings provide scientists with important new information about the life cycle of these nanomaterials.

Despite their widespread use, scientists know little about how nanomaterials move from manufactured products into the environment and what their impact might be. Based on a 2009 Environmental Protection Agency study of publicly-owned wastewater treatment facilities, Michael Hochella and his colleagues at Virginia Polytechnic Institute & State University knew that all these facilities had silver in their sludge. However, identifying and characterizing tiny particles among myriad organic compounds presented a daunting challenge.

So the Virginia Tech researchers decided to use x-ray transmission electron microscopy, an extremely sensitive technique that can identify both composition and structure. With micrographs of sludge from a Midwest treatment plant, they identified nanoparticles 5 to 20 nm in diameter and determined that the particles had a 2-to-1 silver-to-sulfur ratio. The scientists also obtained a crystal structure to confirm that the particles were Ag₂S.

The nanomaterials likely entered the treatment plant in the form of silver nanoparticles, Hochella says, and then transformed into silver sulfide, because wastewater plants contain high concentrations of sulfide and silver readily binds to sulfur. Hochella adds that this work underscores some of the complexity in studying environmental effects of nanoparticles: "What we start with is not what ends up in nature."

The finding is remarkable, says Samuel Luoma of the University of California, Davis and an emeritus scientist with the U.S. Geological Survey. "It's one of the first times we've found a way to detect silver nanoparticles in the environment," he says.

The observations also point out the growing use of these nanomaterials, Luoma adds: "We're using enough of these silver nanoparticles in commercial products that they're already starting to show up in waste treatment facilities."

But their environmental impact is still unclear. The conversion of silver to silver sulfide within waste treatment plants could be good news for the environment, Luoma says. Bulk silver sulfide is highly insoluble in water, particularly when compared with silver ions. But even if silver sulfide nanoparticles are not traveling through water, he adds, these materials could still enter the ecosystem via animals who consume particles in the sludge.

Researchers also don't know how much incoming silver ends up as silver sulfide. The Virginia Tech researchers next plan to analyze samples from each stage of the treatment process at the same wastewater plant.