Formation of Nanosilver from Silver Sulfide Nanoparticles in Natural Waters by Photoinduced Fe(II, III) Redox Cycling.

Nanosilver (nAg) has been repeatedly demonstrated to end up as silver sulfide nanoparticles (Ag2SNPs), but little is known about the potential transformations of Ag2SNPs in natural environments that are very important for comprehensive assessments of nAg risks to human and environmental health. Here we show that Ag2SNPs can release tiny amounts of silver ion via cation exchange reactions between Ag(I) and Fe(III) in the dark, while in the light dramatic dissolution of Ag2SNP occurs, which is mainly attributed to the Ag2SNP oxidation by the hydroxyl radical formed during the reduction of Fe(III) to Fe(II) in water under sunlit conditions. However, silver ions are subsequently reduced to nAg in the light due to the strong reducing power of Fe(II). Thus, the formation of nAg from Ag2SNPs in the presence of Fe(III) under light conditions proceeds through a two-step reaction mechanism, the photoinduced and Fe(III)-dependent dissolution of Ag2SNPs, followed by the reduction of silver ions to nAg by Fe(II). The formation of nAg from Ag2SNPs is also validated in environmental waters under light conditions. It is thus concluded that photoinduced Fe(III)/Fe(II) redox cycling can drive the formation of nAg from Ag2SNPs in natural waters. These findings suggest that the previous consensus about the stability of Ag2SNPs in aquatic environments should be reconsidered.