Nanoparticles in Sewage Sludge May End Up in the Food Chain
By Dave Mosher
Plants and microbes can absorb nano-sized synthetic particles that magnify in concentration within predators up the food chain, according to two new studies.
Nanoparticles can be made of countless different materials, and their safety isn’t well-understood. Yet the minuscule specks are infused into hundreds of consumer products ranging from transparent suncreens to odor-eating socks.
From there, they can wash down drains, ultimately ending up in the sewage sludge of wastewater treatment plants. About 3 million tons of dried-out sludge is subsequently mixed into agricultural soil each year.
“We wanted to look into the possibility of nanoparticles getting into the food chain in this way,” said environmental toxicologist Paul Bertsch of the University of Kentucky. “What we found really surprised us.”
Synthetic nanoparticles are about 1 to 100 nanometers in size (as small as some viruses) and made of silver, titanium dioxide, zinc oxide and other substances. By virtue of their small size and stability, they can nullify odors, prevent food spoilage and absorb harmful ultraviolet radiation, among other feats.
But knowledge about their impacts to the environment is still in a state of infancy, Bertsch said.
To explore nanoparticle absorption in the food chain, Bertsch’s team raised tobacco plants in a hydroponic greenhouse. While the plants grew, the team added super-stable gold nanoparticles to the water to mimic consumer nanoparticles in wastewater sludge.
Gold nanoparticles built up in tobacco leaf tissue, and tobacco hornworms that ate the plants accumulated concentrations of the nanomaterials about 6 to 12 times higher than in the plant.
“We expected [nanoparticles] to accumulate, but not biomagnify like that,” said Bertsch, co-author of the Dec. 3 study in Environmental Science & Technology.
Predatory microbes in a separate study, published Dec. 19 in Nature Nanotechnology, also built up concentrated levels of cadmium selenide nanoparticles after eating smaller microbes that ingested them.
“For me, it’s really interesting to see two different models using two different nanoparticles arrive at conclusions reinforcing each other,” said Patricia Holden, an environmental microbiologist at University of California, Santa Barbara who co-authored the microbe-based study.
At least five government agencies (EPA, FDA, NIH, NIOSH and NIST) host efforts to investigate nanotechnology’s risks to health and the environment, and their funding is increasing each year. And while heavy metals and other toxins in sludge are federally regulated, manmade nanoparticles are not. That may be cause concern as farms increasingly mix sludge into their soils, where nanoparticles may build up over time.
“At this point, the science right now is not saying ’stop using nanoparticles,’” said David Holbrook, a chemical engineer at NIST who wasn’t involved in either study. Holbrook said the new research is important and creates new avenues for nanotechnology safety research. “We’ve got to continue this kind of work,” he said.
There’s some evidence that nanoparticles are toxic under lab-controlled conditions, Bertsch said, but realistically assessing risks to health and the environment demands more advanced models. He and other scientists are already collaborating on an experiment at Cranfield Univeristy in England that will use the institution’s wastewater stream to gauge nanoparticle effects on earthworms and nematodes.
“I expect the results may not be as dramatic,” Bertsch said. “But so far, the jury is still out on safety.”
Image: A tobacco hornworm. Credit: Flickr/cbede
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Dave is an infinitely curious Wired Science contributor who's obsessed with space, physics, biology and technology. He lives in New York City.
Follow @davemosher and @wiredscience on Twitter.
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