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 (AgNPs) and sulfidized (AgS-NPs) silver nanoparticles under hydroponic conditions. After the formation of iron plaque at the root surface upon exposure to Fe at 0-100 μg mL, rice (Oryza sativa L.) seedlings were transferred to AgNP suspensions. Silver uptake depended on the amount of iron plaque and AgNP species (AgNPs vs. AgS-NPs): AgS-NP exposure had lower or comparable Ag uptake to that of AgNP exposure at low levels of Fe (0-80 μg mL), but significantly higher Ag uptake at 100 μg Fe mL. Such contrasting effects of iron plaque on the bioavailability of AgNPs and AgS-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 AgNPs and AgS-NPs, and raise concerns about the exposure of wetland plants to AgS-NPs in Fe-rich environments, where high Fe levels may facilitate AgS-NP bioavailability.