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

ABSTRACT

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 SO42-. 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.