Mechanism for sulfidation of silver nanoparticles by copper sulfide in water under aerobic conditions

Silver nanoparticles (Ag-NP) can readily react with dissolved sulfides in anoxic waters to form silver sulfide nanoparticles (Ag2S-NP) via a Ag-Ag2S core-shell pathway; however, the questions remain as to whether and how Ag-NPs are transformed into Ag2S-NP by metal sulfides in water under aerobic conditions. We thus adopted copper sulfide nanoparticles (CuS-NP) with different surface charges as a model metal sulfide to coexist with Ag-NP in water under aerobic conditions. The Ag-NP underwent sulfidation by CuS-NP, where Ag2S-NP were observed, along with measurable levels of dissolved copper but negligible SO42- in water. The sulfidation of Ag-NP by CuS-NP was dependent on the molar Ag/S ratio and natural organic matter and inorganic salts depressed Ag-NP sulfidation. Moreover, the oxygen-dependent dissolution of Ag-NP was demonstrated to be very important for Ag-NP sulfidation. On the basis of experimental data, we propose that Ag-NP sulfidation by CuS-NP in water under aerobic conditions proceeds through the following reactions: oxygen-dependent dissolution releases silver ions, followed by cation exchange reactions with CuS-NP to form Ag2S-NP. This study illustrates the potential mechanism for Ag-NP sulfidation by CuS-NP in water under aerobic conditions and sheds light on potential transformations of Ag-NP in the environment.