Oxidative dissolution of silver nanoparticles by synthetic manganese dioxide investigated by synchrotron X-ray absorption spectroscopy

Silver nanoparticles (AgNPs) are widely used in a variety of industrial and consumer applications and the disposal AgNP-containing materials is a potential source of environmental contamination. This study investigated the reaction of AgNPs with synthetic birnessite (δ-MnO2), a naturally-occurring MnO2 soil mineral shown in previous studies to oxidize both organic and inorganic dissolved species. The AgNPs used in this study ranged in size from 5 to 25 nm with an average particle diameter of 15.6 nm. Batch and kinetic reactions of MnO2-treated AgNP suspensions were studied by detecting AgNP oxidation to Ag+ using a combination of UV-Vis and microwave plasma atomic emission (MP-AES) spectrometries. Synchrotron K-edge X-ray absorption spectroscopy (XANES and EXAFS) was used to investigate the Ag oxidation state and structural characteristics of the reaction products. Oxidation of AgNP by MnO2 was detected in batch reactions showing an initial fast oxidation of AgNP to Ag+ (0–10 min) followed by a slower reaction (> 10 min) where Ag+ was removed by adsorption on MnO2 surfaces. XANES results confirmed that total AgNP oxidation by MnO2 occurred after 48 h when the Mn:Ag mole ratio treatment exceeded 5:1. The final AgNP oxidation product determined by EXAFS was Ag+ ion bound as a AgO4 tetrahedral structure in MnO2 interlayer cation exchange sites with Ag-O and Ag-Mn inter-atomic distances of 2.28 (± 0.02) and 3.88 (± 0.09) A, respectively. This structure is in agreement with previous EXAFS studies of naturally-occurring Ag-bearing MnO2 mineral samples and represents one of many possible Ag+ binding sites on soil mineral surfaces.