Effects of Silver Nitrate are a Conservative Estimate for the Effects of Silver Nanoparticles on Algae Growth and Daphnia magna Reproduction.

Silver (Ag) salts have been shown to be highly toxic to freshwater organisms. There is nevertheless still a high level of uncertainty as to the aquatic effects of Ag nanoparticles (AgNPs), and how these relate to the effects of soluble Ag salts. As part of the substance evaluation for Ag of the European Union Registration, Evaluation, Authorisation, and Restriction of Chemicals regulation, we have generated new data to justify read-across from soluble Ag salts to AgNPs. The aquatic toxicity to algae growth and Daphnia magna reproduction, fate, and behavior of AgNO3 versus AgNPs were tested and compared. Chloride salts in the test media were replaced with equimolar concentrations of nitrate salts. Total Ag, "conventionally" dissolved Ag (0.45 µm), and "truly" dissolved Ag (3 kDa) were determined. Algae were the most sensitive test species to AgNO3 (10% effect concentration [EC10] 0.10 µg Ag/L) when expressed as conventionally dissolved Ag. The corresponding value for AgNPs was 0.26 µg/L. For D. magna reproduction, the lowest EC10 values were 3.49 µg Ag/L for AgNO3 and 33.4 µg Ag/L for AgNPs. Using measured Ag concentrations, AgNO3 was experimentally shown to be more toxic than AgNPs for all Ag fractions. We explain these observations by a different dissolution behavior of AgNO3 versus AgNPs. The results provide experimental confirmation that AgNO3 can be used as a conservative estimate for the aquatic effects of AgNPs at comparable Ag concentrations. Environ Toxicol Chem 2019;38:1701-1713.