Ion-release kinetics and ecotoxicity effects of silver nanoparticles

The environmental toxicity associated with silver nanoparticles (AgNPs) has been a major focus in nanotoxicology. The Ag ⁺ released from AgNPs may affect ecotoxicity, although whether the major toxic effect is governed by Ag ⁺ ions or by AgNPs themselves is unclear. In the present study, we have examined the ecotoxicity of AgNPs in aquatic organisms, silver ion-release kinetics of AgNPs, and their relationship. The 48-h median effective concentration (EC50) values for Daphnia magna of powder-type AgNP suspensions were 0.75μg/L (95% confidence interval [CI]=0.71-0.78) total Ag and 0.37μg/L (95% CI=0.36-0.38) dissolved Ag. For sol-type AgNP suspension, the 48-h EC50 values for D. magna were 7.98μg/L (95% CI=7.04-9.03) total Ag and 0.88μg/L (95% CI=0.80-0.97) dissolved Ag. The EC50 values for the dissolved Ag of powder-type and sol-type AgNPs for D. magna showed similar results (0.37μg/L and 0.88μg/L) despite their differences of EC50 values in total Ag. We observed that the first-order rate constant (k) of Ag ⁺ ions released from AgNPs was 0.0734/h at 0.05mg/L total Ag at 22°C within 6h. The kinetic experiments and the toxicity test showed that 36% and 11% of sol-type AgNPs were converted to the Ag ⁺ ion form under oxidation conditions, respectively. Powder-type AgNPs showed 49% conversion rate of Ag ⁺ ion from AgNPs. We also confirmed that Ag ⁺ ion concentration in AgNP suspension reaches an equilibrium concentration after 48h, which is an exposure time of the acute aquatic toxicity test.