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

Stability of silver sulfide nanoparticle (Ag₂S-NP) in the environment has recently drawn considerable attention since it is associated with environmental risk. Although the overestimated stability of Ag₂S-NP in aqueous solution has already been recognized, studies on transformation of Ag₂S-NP in environmental water are still very scarce. Here we reported that Ag₂S-NP could undergo dissolution by manganese(IV) oxide (MnO₂), 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 Ag₂S-NP dissolution rose with the increasing concentration of MnO₂. In addition, environmental factors including natural organic matter, inorganic salts and organic acids could accelerate the Ag₂S-NP dissolution by MnO₂, wherein an increase in dissolution extent was also observed. We further documented that Ag₂S-NP dissolution by MnO₂ was highly dependent on O₂ and it was an oxidative dissolution, with the production of SO₄^2-. Finally, dissolution of Ag₂S-NP by MnO₂ affected zebra fish (Danio rerio) embryo viability, showing significant reduction in embryo survival and hatching rates, compared to embryos exposed to Ag₂S-NP, MnO₂ or dissolved manganese alone. These findings would further shed light on the stability of Ag₂S-NP in the natural environment - essential for comprehensive nano risk assessment.