Comparison of nanosilver and ionic silver toxicity in Daphnia magna and Pimephales promelas

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 using two test organisms (Daphnia magna and Pimephales promelas). The 48-h D. magna median lethal concentration (LC50) of multiple sizes (10, 20, 30, and 50nm) of commercially prepared nanosilver (nanoComposix) ranged from 4.31 to 30.36μg total Ag L^-1 with increasing toxicity associated with decreasing particle size. A strong relationship between estimated specific particle surface area and acute toxicity was observed. Nanosilver suspensions (10nm) treated with cation exchange resin to reduce the concentration of Ag⁺ associated with it were approximately equally toxic to D. magna compared to untreated nanosilver (48-h LC50s were 2.15 and 2.79μg total Ag L^-1, respectively). The 96-h LC50 and 7-d sublethal 20% effective concentrations (EC20s) for P. promelas were 89.4 and 46.1μg total Ag L^-1, respectively, for 10nm nanosilver and 4.70 and 1.37μg total Ag L^-1, respectively, for Ag⁺; the resulting ratios of 96-h LC50 to 7-d EC20 were not significantly different for nanosilver and ionic silver. Overall, these studies did not provide strong evidence that nanosilver either acts by a different mechanism of toxicity than ionic silver, or is likely to cause acute or lethal toxicity beyond that which would be predicted by mass concentration of total silver. This in turn suggests that regulatory approaches based on the toxicity of ionic silver to aquatic life would not be underprotective for environmental releases of nanosilver.