Reduction and amalgamation of mercury in silver nanoparticle suspensions under dark conditions

Mercury (Hg) is a global pollutant of concern, and its transport and transformation are controlled by various environmental factors, with aquatic particles being an important driver. Understanding the interactions between silver nanoparticles (AgNPs) and Hg under dark condition is a prerequisite for studying the extent of AgNPs interaction with light and its participation in Hg biogeochemical cycling. Herein, under laboratory experimental setting, it was found that the reduction of divalent Hg (Hg(II)) to gaseous elemental Hg (Hg⁰) by AgNPs readily occurred. Within 30 min, less than 20% of Hg(II) were reduced to Hg⁰, with more than 80% remaining in solution. The Hg⁰ generated distributed between AgNPs and aqueous phase with very small amount (<1%) released as purgeable Hg⁰ and a substantial amount (6.28-15.7%) amalgamated to AgNPs to form Ag-Hg amalgam. The exposure of AgNPs to Hg(II) resulted in blueshift in maximum wavelength of absorption of AgNPs due to Ag-Hg amalgam formation. Also, we found a decrease in the visible light absorption and an increase in the size of AgNPs due to redox interaction between AgNPs and Hg(II) and the increase in suspension ionic strength, respectively. Overall, our findings provide essential insights into the behavior of AgNPs on exposure to Hg(II) and suggest that AgNPs could significantly impact the biogeochemical cycling of Hg.