Re-evaluation of stability and toxicity of silver sulfide nanoparticle in environmental water: Oxidative dissolution by manganese oxide

Abstract Stability of silver sulfide nanoparticle (Ag 2 S-NP) in the environment has recently drawn considerable attention since it is associated with environmental risk. Although the overestimated stability of Ag 2 S-NP in aqueous solution has already been recognized, studies on transformation of Ag 2 S-NP in environmental water are still very scarce. Here we reported that Ag 2 S-NP could undergo dissolution by manganese(IV) oxide (MnO 2 ), an important naturally occurring oxidant in the environment, even in environmental water, although the dissolved silver would probably be adsorbed onto the particles (>0.45 μm) in environmental water, mitigating the measurable levels of dissolved silver. The extent and rate of Ag 2 S-NP dissolution rose with the increasing concentration of MnO 2 . In addition, environmental factors including natural organic matter, inorganic salts and organic acids could accelerate the Ag 2 S-NP dissolution by MnO 2 , wherein an increase in dissolution extent was also observed. We further documented that Ag 2 S-NP dissolution by MnO 2 was highly dependent on O 2 and it was an oxidative dissolution, with the production of SO 4 2− . Finally, dissolution of Ag 2 S-NP by MnO 2 affected zebra fish ( Danio rerio ) embryo viability, showing significant reduction in embryo survival and hatching rates, compared to embryos exposed to Ag 2 S-NP, MnO 2 or dissolved manganese alone. These findings would further shed light on the stability of Ag 2 S-NP in the natural environment - essential for comprehensive nano risk assessment.