Monitoring silver nanoparticles in a wastewater treatment plant

About the author: Ron Kent is a PhD student in Civil and Environmental Engineering at Virginia Tech. Check out his profile on the VTSuN student page.

In a magnificently illustrated post on the VTSuN blog, Jacob Metch recently introduced the important question of “what happens to nanomaterials that get into our wastewater?” As the number of everyday products containing nanomaterials increases, so too will the quantity of nanomaterials that get washed down the drain and into the wastewater treatment plant. Will wastewater treatment act as a sentinel to mitigate the potential harm of these materials before they are released into lakes and rivers, or will antimicrobial nanomaterials disrupt the wastewater treatment process to the detriment of downstream water quality?

Activated sludge tank of the Christiansburg, Virginia wastewater treatment plant. Photo courtesy of Ryan Hendrix.
Activated sludge tank of the Christiansburg, Virginia wastewater treatment plant. Photo courtesy of Ryan Hendrix.

Answering these questions is difficult. Wastewater is complex and, well, just plain yucky. Even with the rising concentrations of nanomaterials in the environment, studying such tiny particles in such a complicated mixture brings us back to the age-old problem of finding a needle in a haystack. Environmental scientists often approach the haystack problem by performing carefully controlled laboratory experiments in which they can use as many needles and as little hay as they please. They happily study a needlestack to avoid all the messiness of the haystack. As useful as these simplified systems can be, they create for us a needlestack problem—scientific conclusions based on experiments that fail to accurately represent the system that we truly want to know about. Extrapolating such conclusions to real-world environments can be questionable.

VTSuN researchers recently did away with the needlestack and faced the haystack head on in a controlled experiment to find out what really happens to silver nanoparticles in a full-scale wastewater treatment plant. We accomplished this daunting task by using a technique called nanosphere lithography to fabricate patterned arrays of immobilized silver nanoparticles directly on surfaces that could be deployed in a wastewater treatment plant and retrieved at a later date. These field-deployable samples were placed in different regions of the Christiansburg, Virginia wastewater treatment plant to investigate what happens to the silver nanoparticles throughout the wastewater treatment process.

A VTSuN researcher retrieves field-deployable silver nanoparticle samples from the secondary clarifier of a wastewater treatment plant. Photo courtesy of Ryan Hendrix.
A VTSuN researcher retrieves field-deployable silver nanoparticle samples from the secondary clarifier of a wastewater treatment plant. Photo courtesy of Ryan Hendrix.

Powerful analytical techniques, including atomic force microscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy, were used to examine the nanoparticles before and after exposure to wastewater. The regular pattern of the nanoparticle arrays made it easy to recognize them with the microscopic techniques used in this study, even after a week or more in wastewater. Changes in the size, shape, chemical composition, and crystal structure of the silver nanoparticles were observed. The study showed that silver nanoparticles are converted to silver sulfide nanoparticles in the anoxic regions of the wastewater treatment plant, and silver dissolution is of minor importance. Because silver sulfide is less toxic than silver metal, these results suggest that wastewater treatment will likely withstand the threat of silver nanomaterials and diminish their potential harm in the aquatic environment.

Transmission electron microscope image of silver nanoparticles after the particles spent ten days in the primary influent of a wastewater treatment plant. Copyright American Chemical Society, 2014
Transmission electron microscope image of silver nanoparticles after the particles spent ten days in the primary influent of a wastewater treatment plant. Copyright American Chemical Society, 2014

The complete results of this study can be found in our recently accepted paper.

Reference:

Kent, R. D.; Oser, J. G.; Vikesland, P. J. Controlled Evaluation of Silver Nanoparticle Sulfidation in a Full-Scale Wastewater Treatment Plant. Environmental Science and Technology, 2014. DOI: 10.1021/es404989t

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