Dissolution and transformation of silver nanoparticles in flow systems: Sulfidation and effect of flow rate

The release of engineered nanoparticles (ENPs) into the environment is an emerging concern with significant implications for organism exposure due to the possible toxicity caused by these materials. Understanding the dissolution and transformation of silver nanoparticles (AgNPs) under dynamic flow conditions is critical, as these processes directly govern the mobility, persistence, and bioavailability of this material in natural aquatic systems, ultimately influencing its ecological risk and environmental impact. In this work, a methodology based on continuous- flow systems to assess the dissolution and transformation behavior of nanomaterials (NMs) under environmentally relevant conditions has been applied specifically to AgNPs. In this way, AgNPs nanopowder has been analyzed employing different environmental conditions (presence of oxygen, background electrolyte, types of buffers and NOM,). Results confirmed the expected dependency of AgNPs dissolution on the oxygen availability, corroborating previous batch assays. The evaluation of the AgNPs sulfidation in oxic and anoxic regimes were applied under different flow rates. AgNPs sulfidation is a complex process that takes place in two steps, oxidation followed by the AgS formation. Two different sulfide compounds with different solubilities can be formed: amorphous AgxSy which is related to an elevated Ag ion release, and high ordered Ag_xS_y phase which create a protective layer that reduces the ion release. Overall, the proposed continuous-flow methodology offers a reliable tool to investigate the dissolution and transformation of ENPS under environmentally relevant conditions. It provides valuable insights into the mechanisms that govern nanoparticles behavior, particularity during the initial stages of reaction.