Assessing the suitability of the OECD 29 guidance document to investigate the transformation and dissolution of silver nanoparticles in aqueous media

The OECD guidance document No. 29 was designed to determine the rate and extend to which metals can produce soluble available ionic metal species. This transformation/dissolution protocol was applied to silver nanomaterials. The results prove that concentrations of released Ag+ at pH 8 were nearly similar at all three different loadings. At pH 6, the concentration of Ag+ was almost the same at loadings of 10 and 100 mg L-1 AgNPs. However, the study showed changes in concentrations of nanoparticles and aggregates (operationally defined as the fraction passing a 0.2 µm filter). At the higher pH both the concentrations in the test medium of Ag+ and of AgNPs (fraction < 0.2 µm) decreased. After 7 days of test duration, 71 µg L-1 of Ag+ was found in pH 6 medium (initial loading of 100 mg L-1). In pH 8 medium a maximum concentration of 29 µg L-1 Ag+ was measured (initial loading of 10 mg L-1). The maximum transformation from AgNPs to Ag+ was 2.7% (27 µg L-1) in pH 8 medium (loading of 1 mg L-1) after 7 days. At an initial loading of 100 mg L-1 AgNPs in medium at pH 8, only 0.03% (30 µg L-1) were transformed to Ag+ after 7 days. At the loading of 1 mg L-1 AgNPs all silver concentrations remain relatively constant for the duration of the test after 7 until 28 days. The results reveal that only low concentrations of Ag+ are released from AgNPs under the applied conditions.