What happens to silver-based nanoparticles if they meet seawater?

Silver based nanoparticles (Ag-b-NPs) in the environment are of current concern as they may pose risks to human and environmental health, even at low concentration levels. It is widely known that Ag-b-NPs, once released from products containing these particles for antimicrobial reasons, can pass through wastewater treatment plants to some extent. These particles are transported via running waterways and eventually reach the sea. However, the fate of environmentally relevant ng L^-1 traces of Ag-b-NPs in seawater has not yet been sufficiently studied. Analytical techniques capable of determining these ultratraces of Ag-b-NPs in seawater are scarce and struggle furthermore with the high chloride content in highly saline matrices, such as seawater. In this study, we extracted Ag-b-NPs from matrices with varying salinity via cloud point extraction (CPE) and determined concentration and size of Ag-b-NPs in extracts with single particle inductively coupled plasma mass spectrometry (sp-ICP-MS). Applying this extraction and measurement technique, we were able to investigate the fate of Ag-b-NPs with different coatings (citrate and the predominant coatings in nature, silver sulfide and silver chloride) in matrices with increasing salinity and real seawater. All types of Ag-b-NPs were dissolved in all matrices almost independently of the chemical composition of the nanoparticles (NPs), whereas dissolution rates increased with increasing salinity due to the formation of soluble Ag(I) species and - in the presence of chloride - AgCl_x^1-x (x > 1) complexes. After an incubation time of not more than 72 h, Ag-b-NPs were dissolved almost completely. During the dissolution process, NP shrinkage could be clearly observed by sp-ICP-MS. Supplementary electron microscopy measurements revealed that the sulfur content in silver sulfide nanoparticles (Ag₂S-NPs) increased during the dissolution process. Finally, we were able to investigate the dissolution process of real Ag-b-NPs in wastewater after increasing the salinity to seawater levels.