Silver nanoparticles in seawater: A dynamic mass balance at part per trillion silver concentrations

This study investigates the dynamic processes affecting silver (Ag) nanoparticles that have been spiked into seawater at environmentally relevant concentrations (200 and 2000ngAgL^-1). Seawater samples were taken at regular time intervals from multiple microcosm tanks and analysed rapidly, without any sample preparation, using a recently developed flow injection on-line dilution single particle inductively coupled plasma mass spectrometry method. Dissolution was found to be the predominant process of Ag nanoparticle transformation, with its rate being influenced by the type and thickness of the nanoparticle organic coating. More specifically the branched poly(ethyleneimine) coating provided additional stability to the 40 and 60nmAg nanoparticles that were tested, compared to the poly(vinylpyrrolidone) coated ones. At high Ag nanoparticle spiking levels and after 24h of exposure an extra Ag-containing nanoparticle peak appeared at the low range of the NP size distribution histogram. This peak corresponds to Ag-containing particles that contain Ag mass equivalent to 25-30nm Ag nanoparticles (assuming spherical shape). However, the composition and the "real" size of these particles remains unknown as the particles may have formed from the in-situ reduction of dissolved silver or they originate from other processes involving nanocrystal formation, as has been shown to occur in sewage sludge, or interaction with natural organic matter. Overall, this study provides additional insight into the physicochemical mechanisms behind Ag nanoparticle behavior in marine media.