Nanosilver is the show horse in
the nanotechnology stable – not only does it hold great promise for the future,
it is already contained in hundreds of consumer products today. Cosmetics, food
packaging, disinfectants and cleaning agents are but some examples. Nanosilver
is also commonly used in antibacterial socks and functional clothing. An
estimated 300 tonnes or more of nanosilver are used each year the world over –
and a substantial part of it enters the water cycle via wastewater. Within the
scope of the National Research Programme "Opportunities and Risks of
Nanomaterials" (NRP 64), a team led by Ralf Kägi from Eawag in Dübendorf
has for the first time examined more closely (*) just what happens to
nanosilver on its journey from the drainpipe to the wastewater treatment plant,
and in what form it is eventually released into the environment.
They discovered that nanosilver
does not remain in its metallic form for very long: it is efficiently
transformed into a silver sulfide salt. "We presume that sulfidation
already largely takes place in the sewer channel," Kägi says. That's good
news, because "these salt crystals cause much fewer problems, the silver
is much less soluble in this form". Dissolved ions are the main reason why
silver can be harmful to the environment and can stop bacteria from getting to
work in the sewage sludge.
The Eawag researchers have for the first time clearly shown
that nanosilver, too, is quickly transformed into silver sulfide – regardless
of how the particles are coated. Until now this effect was only known from
wastewater produced by the photo industry. Whether as metallic nanoparticles,
as dissolved silver ions or as an insoluble silver saline deposit, the original
form of the silver apparently does not play a crucial role in sulfidation. However, the salination speed
depends heavily on the size of the particles: small nanosilver (10 nanometres)
is very rapidly transformed, while larger particles may never fully sulfidise
and may continue to release silver ions into the environment.
The researchers were also able to show that
approximately 95% of the nanoparticles are bound in the sewage sludge. Only
5% of the silver remains in the treated water. This percentage could be further
reduced by using better particle filters. Venturing into the nano dimension
would not be necessary, though: the sulfidised nanosilver aggregates almost
entirely on large particles in the wastewater. With a reasonable effort, they
could be removed more efficiently from the wastewater than is presently the
case.
The study did not examine what
happens to nanosilver in the sewage sludge thereafter. In Switzerland, it is
not permissible to use sewage sludge on farmland, and most of the sludge is
therefore burned. The heavy metals separated in this process should not be
released into the environment in large quantities.
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