Even low concentrations of silver can foil wastewater treatment

May 15, 2018 by Steve Lundeberg, Oregon State University
Credit: CC0 Public Domain

Research at Oregon State University has shed new light how an increasingly common consumer product component—silver nanoparticles—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.

The research is important because if silver, which has broad-spectrum antibacterial properties, thwarts the work of the plants' beneficial bacteria, then too many nutrients end up in waterways.

That in turn can lead to eutrophication: An overabundance of nutrients in a body of water that results in an explosion of vegetation, such as an algae bloom, and a squeezing out of animal life due to a lack of oxygen.

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

The work by Radniecki and collaborators in the College of Engineering looked at , 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 to begin the process of getting one of those nutrients, nitrogen, out of the wastewater. 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 better able than planktonic bacteria to ward off silver's effects.

"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, the work unveiled a new wrinkle: 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, he explains, is that in a treatment plant that uses biofilms, the sludge retention time—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 within the ; wastewater plant monitors might be underestimating the potential toxicity of long-term, low-concentration exposure situations."

Explore further: Silver nanoparticle concentration too low to be harmful in water supply, paper finds

More information: L.K. Barker et al, Effects of short and long-term exposure of silver nanoparticles and silver ions to Nitrosomonas europaea biofilms and planktonic cells, Chemosphere (2018). DOI: 10.1016/j.chemosphere.2018.05.017

Related Stories

Panning for silver in laundry wastewater

December 20, 2017

Silver nanoparticles are being used in clothing for their anti-odor abilities but some of this silver comes off when the clothes are laundered. The wastewater from this process could end up in the environment, possibly harming ...

Too much nanotechnology may be killing beneficial bacteria

April 29, 2008

Too much of a good thing could be harmful to the environment. For years, scientists have known about silver’s ability to kill harmful bacteria and, recently, have used this knowledge to create consumer products containing ...

The impact of anti-odor clothing on the environment

March 30, 2016

Anti-odor athletic clothes containing silver nanoparticles have gained a foothold among exercise buffs, but questions have arisen over how safe and effective they are. Now scientists report in ACS' journal Environmental Science ...

Recommended for you

Observing cellular activity, one molecule at a time

May 21, 2018

Proteins and molecules assemble and disassemble naturally as part of many essential biological processes. It is very difficult to observe these mechanisms, which are often complex and take place at the nanometer scale, far ...

A soft solution to the hard problem of energy storage

May 18, 2018

It's great in the lab, but will it actually work? That's the million-dollar question perpetually leveled at engineering researchers. For a family of layered nanomaterials, developed and studied at Drexel University—and ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.