The increasing use of nanosilver in consumer products and the likelihood of
environmental exposure warrant investigation into the toxicity of nanosilver to aquatic organisms.
A series of studies were conducted comparing the potency of nanosilver to ionic silver (Ag+) at
acute and sublethal levels. The results of these tests were examined for evidence that nanosilver
acts by a different mechanism of toxicity than Ag+, with the goal of estimating the adequacy of
current water quality regulations based on the toxicity of Ag+ to protect against environmental
effects of nanosilver.
A variety of simple methods to separate Ag+ from nanosilver by physical exclusion or
charge selectivity were assessed in preliminary studies for the ability to provide insight into the
mechanism of nanosilver toxicity. In a definitive study, ion exchange resin was used to remove
Ag+ from nanosilver (confirmed by the complete removal of silver from AgNO3 solutions) in
order to determine the importance of Ag+ to acute toxicity of nanosilver to Daphnia magna. The
acute toxicity of nanosilver to D. magna after ion exchange was shown to be similar to that of
untreated nanosilver, suggesting that Ag+ did not contribute significantly to the toxicity of the
suspensions, or that ion release occurred rapidly after ion exchange.
D. magna juveniles were exposed to four sizes of nanosilver (10, 20, 30 and 50 nm) and
Ag+ and 48-h LC50s were calculated for each material. Based on mass concentrations, all
nanosilver sizes were less acutely toxic than Ag+, and a trend of increasing toxicity with
decreasing average diameter of nanosilver was observed, with LC50s ranging from 19-42 times
higher than that of Ag+. Calculations of nanosilver specific surface area and theoretical surface
atoms revealed little to no difference in LC50s among the four sizes, suggesting that toxicity may
be dependent on the surface properties of nanosilver. Equivalent calculations for an ionic Ag
exposure series resulted in the finding that, in terms of total surface Ag atoms, all sizes of
nanosilver were more acutely toxic than equivalent exposures of pure Ag+. This implies either
that a second mechanism of toxicity exists for nanosilver which increases its overall potency, or
that the calculation of surface atoms was an underestimate due to the continuous release of Ag+
from nanosilver into the matrix.
Acute-to-chronic ratios (ACRs) were obtained for Pimephales promelas (<24 hours post
hatch) exposed to both Ag+ and nanosilver, to test the hypothesis that a difference in these ratios
would indicate different mechanisms of toxicity. The results of 96-h acute and 7-day sublethal
toxicity tests produced ACRs for Ag+ and nanosilver that were not significantly different based
on their overlapping confidence intervals. Furthermore, the observation that the nanosilver ACR
was smaller than that of Ag+, suggest that if there is a separate toxicity mechanism in nanosilver,
it is unlikely to result in environmental effects beyond those that would be expected from an Ag+
Further studies are needed to determine the degree to which the results of the ion
exchange and size-dependent toxicity tests can be attributed to nanosilver dissolution. Overall,
the results of these tests do not provide unambiguous evidence for a mechanism of nanosilver
toxicity other than Ag+. The U.S. EPA maximum allowable silver concentration for natural
waters is based on dissolved silver, defined as that which passes through a 0.45 μm filter, which
is considerably larger than the average size of nanosilver aggregates in the exposure media.
Therefore, the presence of nanosilver in the environment will increase the apparent dissolved Ag
concentration, resulting in increased protectiveness of this criterion.