Adsorption of silver(I) ion on synthetic manganese dioxide following oxidative dissolution of silver nanoparticles

By: Jon Gallardo and Edgar Guzman          

Department: Chemistry

Faculty Advisor: Dr. Bruce Manning

Silver nanoparticles (AgNPs) are extensively used in industry and consumer products, and their disposal presents a potential source of environmental contamination. The overall goal of this study was to determine the most likely fate of AgNPs after reacting with soil, particularly soil components known to be natural oxidants. We investigated the reaction of both laboratory derived AgNPs and Ag+ ion on synthetic manganese dioxide (MnO2) powder, a naturally occurring soil mineral shown in previous studies to oxidize dissolved constituents in soil. Batch reactions containing MnO2 suspensions in centrifuge tubes were mixed with either AgNPs or Ag+ ion and allowed to react for varying times including short-term studies (hours) and long-term studies (days). At the completion of the reactions, the tubes were centrifuged followed by analysis of the supernatant solutions by UV-VIS and microwave plasma atomic emission (MP-AES) spectrometry. 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. The strong adsorption (surface binding) reaction was confirmed in Ag+-treated MnO2 suspensions at varying concentrations (0-200 mg/L Ag+) and pH (2-8). The results of this work confirm that AgNPs will oxidize in most soils with natural background levels of MnO2 (200-500 mg/kg Mn) followed by strong adsorption of the Ag+ on MnO2 and other soil mineral surfaces.