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

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