Nanosilver is the nanoparticle that has worried people the most, when it comes to human health. Consequently, it's also been the one most scrutinized.
In fact, the seemingly endless parade of research studies examining the risk of nanosilver led two Danish researchers to publish an exasperated article in the prestigious journal Nature last year entitled “When Enough is Enough.”
One of the researchers, Steffen Foss Hansen, remarked back then in an interview with Nanowerk, “Most of these questions—and possibly all of them—have already been addressed by no less than 18 review articles in scientific journals, the oldest dating back to 2008, plus at least seven more reviews and reports commissioned and/or funded by governments and other organizations" he said. "Many of these reviews and reports go through the same literature, cover the same ground and identify many of the same data gaps and research needs."
Not only had a great deal of research gone into the toxicology of nanosilver, but there appeared to be an entire regulatory framework—at least in the US—that could control the use of nanosilver in products. As I later pointed out, nanosilver algaecides have been regulated under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) as a pesticide since 1954. FIFRA was originally administered by the U.S. Department of Agriculture but that task was transferred to EPA when it was founded by the Nixon Administration in 1970. So nanosilver has been regulated under FIFRA for nearly 60 years, and for around forty of those years silver has been regulated under FIFRA by EPA.
Despite a large, thorough, and still-growing body of research addressing the toxicology issue, and a regulatory framework from which to control the substance, there has been lingering concern about the lifecycle of nanosilver in our water supply through wastewater.
Now researchers at the Swiss Federal Institute of Aquatic Science and Technology (Eawag) have looked at what happens to nanosilver when it goes down our drainpipe after it has been washed off products (primarily clothes, but also the washing machines) containing them. They followed it to wastewater treatment plants and then into the environment. It turns out there is barely any nanosilver by the time it returns to the water supply.
In addition to taking samples from the Swiss wastewater system, the researchers demonstrated through lab experiments what actually happens to nanosilver in wastewater and wastewater treatment plants. They discovered that the nanosilver does not remain in its metallic form for very long. Instead it is transformed into a silver sulfide salt, which is far less soluble than the metallic form. The concern had long been that silver ions would dissolve into the water, which—beyond its inherent toxicity—would have prevented bacteria from breaking down the sewage in the wastewater treatment.
“We presume that sulfidation already largely takes place in the sewer channel," says Ralf Kägi, one of the Eawag researchers, in a press release.
The Eawag researchers also determined that the smaller the size of the nanosilver nanoparticles the more quickly it transforms into the more benign form of silver sulfide salt.
The researchers demonstrated that 95 percent of the nanosilver binds itself to the sewer sludge and that only 5 percent of the silver remains in the treated water. Since the silver sulfide salt collects onto larger particles, these can easily filtered out.
Once again, there is a new addition to the body of research that indicates that nanosilver does not present the risk that some have believed. But much to the irritation of Steffen Foss Hansen and other researchers, there will surely be many more studies like this.
Photo: Christoph Ort/Eawag