Silver nanoparticles can potentially interfere with wastewater treatment

Transmission electron microscopy (TEM) image of silver nanoparticles formed from silver ions in solution with humic acid. Photo credit: National Institute of Standards and Technology, via Flickr.

Research at Oregon State University has shed new light on how increasingly common silver nanoparticles in consumer products can potentially interfere with the treatment of wastewater.

The findings suggest conventional toxicity testing methods for silver concentrations at treatment plants may produce results that yield a false sense of security.

According to researchers, studying this material is important because if silver, which has broad-spectrum antibacterial properties, thwarts the work of the wastewater plants’ beneficial bacteria, then too many nutrients may end up in waterways.

“Silver nanoparticles are being incorporated into a range of products including wound dressings, clothing, water filters, toothpaste and even children’s toys,” said author Tyler Radniecki, an environmental engineering assistant professor at Oregon State University (OSU). “The nanoparticles can end up in wastewater streams through washing or just regular use of the product.”

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The work by Radniecki and collaborators in OSU’s College of Engineering looked at silver nanoparticles, the ionic silver they release and an ammonia-oxidizing bacterium, Nitrosomonas europaea.

Ammonia-oxidizing bacteria, or AOB, are crucial because they convert ammonia to nitrite. The study looked at both free-floating, or planktonic, N. europaea and also the biofilms they create.

The OSU research confirmed earlier observations that biofilms are more resistant to silver than planktonic bacteria.

“Biofilms showed higher resistance for multiple factors,” Radniecki said. “One was simply more mass of cells, and the top layer of cells acted like a sacrificial shield that allowed the bacteria below not to be inhibited. Slow growth rates were also a protection from silver toxicity because the enzymes that silver prevents from turning over aren’t turning over as frequently.”

More importantly said the researchers, the study unveiled that the inhibition of AOB’s ammonia-conversion ability is more a function of silver exposure time than the level of silver concentration.

“Most of the studies investigating the inhibition of wastewater biofilms by nanoparticles have been conducted in short-term exposure scenarios, less than 12 hours,” Radniecki said. “Also, they’ve used an equal amount of time for hydraulic residence and sludge retention.”

The problem with that, Radniecki explains, is that in a treatment plant that uses biofilms, the sludge retention time, i.e., the time the bacteria are in the plant, will be much greater than the hydraulic residence time, i.e., the time the wastewater is in the plant.

“That allows, over time, for the accumulation and concentration of metal contaminants, including ionic silver and silver nanoparticles,” said Radniecki, whose worked involved exposure times of 48 hours. “The immobilized biofilm cells are exposed to a much greater volume of water and mass of contaminants than the planktonic cell systems. What that means is, the results of short-term exposure studies may fail to incorporate the expected accumulation of silver within the biofilm and wastewater plant monitors might be underestimating the potential toxicity of long-term, low-concentration exposure situations.”

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