Contrasting effects of iron plaque on the bioavailability of metallic and sulfidized silver nanoparticles to rice
Interaction between silver nanoparticles (AgNPs) and iron plaque, which forms at the root surface of wetland plants under waterlogging conditions, is a critical process that controls the bioavailability of AgNPs. In this study, we comparatively evaluated how and to what extent iron plaque affected silver uptake sourced from metallic (Ag0NPs) and sulfidized (Ag2S-NPs) silver nanoparticles under hydroponic conditions. After the formation of iron plaque at the root surface upon exposure to Fe2+ at 0–100 ?g mL?1, rice (Oryza sativa L.) seedlings were transferred to AgNP suspensions. Silver uptake depended on the amount of iron plaque and AgNP species (Ag0NPs vs. Ag2S-NPs): Ag2S-NP exposure had lower or comparable Ag uptake to that of Ag0NP exposure at low levels of Fe2+ (0–80 ?g mL?1), but significantly higher Ag uptake at 100 ?g Fe2+ mL?1. Such contrasting effects of iron plaque on the bioavailability of Ag0NPs and Ag2S-NPs were attributed to their influences on AgNP dissolution. However, the translocation factors (TFs) and particle size distribution of NPs in planta (as determined by single-particle inductively coupled plasma-mass spectrometry analysis) were not affected by the amount of iron plaque. These results reveal contrasting effects of iron plaque on the bioavailability of Ag0NPs and Ag2S-NPs, and raise concerns about the exposure of wetland plants to Ag2S-NPs in Fe-rich environments, where high Fe levels may facilitate Ag2S-NP bioavailability.