Researchers identify silver nanoparticles in sewage sludge of wastewater treatment plants

(Nanowerk Spotlight) Silver nanoparticles are one of the most extensively used type of nanoparticles in consumer products due to the unique antibacterial activity of silver. There have been raising environmental concerns over their adverse ecological effects, along with ionic silver potentially released from the particles (see for instance: "Toxicity of silver nanoparticles increases during storage").
"To predict the environmental impact of engineered silver nanoparticles, their characterization from environmental matrices should be pursued, yet no field-scale studies are available to date" Bojeong Kim, a research associate at The Center for NanoBioEarth at Virginia Tech's Department of Geosciences, tells Nanowerk. "In addition, analyses examining the sizes, morphologies, elemental compositions, degrees of crystallinity and atomic structures, coatings, and aggregation states of nanosized silver particles in the environment are rare, limiting our ability to conduct a sound risk assessment. Absence of such information may be due to technical difficulties of retrieving trace levels of the silver nanoparticles from very complex heterogeneous systems."

Kim is first author of a recent research paper in Environmental Science & Technology ("Discovery and Characterization of Silver Sulfide Nanoparticles in Final Sewage Sludge Products") that was motivated by the fact that silver nanoparticles in consumer products are likely being released during and/or after the product's lifetime. The silver nanoparticles will likely get into wastewater streams and subsequently enter wastewater treatment plants. During wastewater treatment processes, silver nanoparticles may be incorporated into the sewage sludge matrix and concentrated over time.

"Therefore, we looked for the presence of silver nanoparticles in sewage sludge materials that were collected from a full-scale municipal wastewater treatment plant located in a metropolitan area of the Midwest region of the US" explains Kim. "We successfully identified and characterized the silver nanoparticles that were present in the sewage sludge materials using analytical high-resolution TEM."

This analytical high-resolution transmission electron microscopy (TEM) study by the Virginia Tech researchers, led by Michael Hochella, provides for the first time field-scale nanoparticle-level information of silver sulfide (Ag2S) that was present in the final stage sewage sludge materials. They also developed and refined a sampling and TEM-sample preparation protocol for this type of very complex, heterogeneous environmental material.

"Certain types of metal-based nanoparticles can transform once they are released into different environments, possessing completely different properties than those of the mother – or original – material" says Kim. "This is certainly true for the case of silver. The type and source of silver that enter the wastewater plant can vary, but they are likely to form silver sulfide in the presence of reduced sulfur under anaerobic conditions in the plant. Therefore, the potential transformation processes and their products need to take into consideration to predict the environmental fate and entire life cycle of engineered nanoparticles."
In recent risk assessment studies, not only for silver but also other engineered nanomaterials, sewage treatment plants are considered to be key intermediate stations that control the most prominent flows of nanoparticle between anthropogenic and natural environments (see for instance: "Discovery may help manage nanoparticle wastes from consumer products").
The Virginia Tech team points out that the speciation of silver nanoparticles collected in settled sewage sludges is also valuable information to wastewater treatment plant managers for operation planning and control. This is because the inhibitory action of engineered silver nanoparticles on bacterial communities and biofilms has been well-documented, but once they undergo transformation processes, like forming silver sulfide complexes/precipitates with sulfide, their toxicity will differ.
While this current study provides for the first time nanoparticle-level information of the silver sulfide present in sewage sludge products, and further suggests the role of wastewater treatment processes on transformation of silver nanoparticles and ionic silver potentially released from them, future studies of size-dependent reactivity, particularly solubility, of silver sulfide nanoparticles will be useful in understanding the environmental fate, influence, and entire life cycle of engineered silver nanoparticles.
As Kim points out, "first of all, at a field scale, we still need detailed information regarding silver nanoparticles levels and speciation in wastewater streams, treated plant effluent, and receiving streams from the plant. Working with very complex environmental samples is always challenging, but such studies will be very useful in understanding the environmental fate, influence, and entire life cycle of engineered silver nanoparticles."
"Secondly, the presence of silver sulfide nanocrystals in sewage sludge materials is now identified, and it is necessary to investigate the time-dependent changes in their chemical and physical properties when they enter different environments later in their life. For example, this would be the case for using the sewage sludge materials containing silver sulfide on agricultural lands as a soil amendment."
"Finally, size-dependent reactivity, particularly solubility of silver sulfide, needs to be studied. silver sulfide is known to be one of the most insoluble minerals with extremely low water solubility. The size of silver sulfide shown in our study ranged from 5 to 20 nm. At the nanosize regime, its solubility may differ from that of bulk silver sulfide, but no one has studied this systematically, yet."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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