Silver nanoparticles, commonly added to a number of consumer products for their reported antibacterial properties, may be hampering wastewater treatment efforts, according to research from Oregon State University.
"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 OSU. "The nanoparticles can end up in wastewater streams through washing or just regular use of the product."
As such, the antibacterial components of the metal are likely thwarting plants’ beneficial bacteria, leading to eutrophication -- the presence of excess nutrients in bodies of water -- that in turn can causealgae blooms that can suffocate animal life.
The study analyzed silver nanoparticles and the ionic silver that they release, as well as an ammonia-oxidizing bacterium (AOB) called Nitrosomonas europaea.
AOBs, according to the research, are critical because they convert ammonia to nitrite to initiate the removal of nitrogen from the wastewater.
Looking at the N. europaea and the biofilms they make, researchers concluded that the biofilms better ward off the effects of silver than the 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."
Additionally, the research exposed that inhibiting the AOB’s ammonia conversion was more the result of the time associated with the exposure to silver versus the concentration of silver.
"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 issue, according to Radniecki, is that in treatment plants using biofilms, the sludge retention time — the amount of time the bacteria spend in the plant — will be much greater than the time the wastewater spends in the plant (called hydraulic residence).
“That allows, over time, for the accumulation and concentration of metal contaminants, including ionic silver and silver nanoparticles," said Radniecki, whose work included 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; wastewater plant monitors might be underestimating the potential toxicity of long-term, low-concentration exposure situations."
The study is published in the journal Chemosphere.