State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences , Beijing 100085, China.
Institute of Environment and Health, Jianghan University , Wuhan 430056, China.
Environ Sci Technol. 2016 Dec 20;50(24):13342-13350. doi: 10.1021/acs.est.6b04042. Epub 2016 Nov 28.
Nanosilver (nAg) has been repeatedly demonstrated to end up as silver sulfide nanoparticles (AgSNPs), but little is known about the potential transformations of AgSNPs in natural environments that are very important for comprehensive assessments of nAg risks to human and environmental health. Here we show that AgSNPs 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 AgSNP occurs, which is mainly attributed to the AgSNP 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 AgSNPs in the presence of Fe(III) under light conditions proceeds through a two-step reaction mechanism, the photoinduced and Fe(III)-dependent dissolution of AgSNPs, followed by the reduction of silver ions to nAg by Fe(II). The formation of nAg from AgSNPs 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 AgSNPs in natural waters. These findings suggest that the previous consensus about the stability of AgSNPs in aquatic environments should be reconsidered.
纳米银（nAg）已被反复证明会最终转化为硫化银纳米颗粒（AgSNPs），但对于 AgSNPs 在自然环境中的潜在转化过程，人们知之甚少，而这对于全面评估 nAg 对人类和环境健康的风险非常重要。在这里，我们表明，AgSNPs 可以通过黑暗中 Ag(I)和 Fe(III)之间的阳离子交换反应，释放出少量的银离子，而在光照下，AgSNP 会发生剧烈的溶解，这主要归因于 AgSNP 在阳光照射下水中 Fe(III)还原为 Fe(II)过程中形成的羟基自由基对 AgSNP 的氧化。然而，由于 Fe(II)具有很强的还原能力，在光照下，银离子随后会被还原为 nAg。因此，在有 Fe(III)存在的情况下，光照条件下从 AgSNPs 形成 nAg 的过程是通过两步反应机制进行的，即 AgSNPs 的光诱导和 Fe(III)依赖性溶解，随后是 Fe(II)将银离子还原为 nAg。在光照条件下的环境水中也验证了从 AgSNPs 形成 nAg 的过程。因此可以得出结论，光诱导的 Fe(III)/Fe(II)氧化还原循环可以驱动天然水中 AgSNPs 形成 nAg。这些发现表明，先前关于 AgSNPs 在水生环境中稳定性的共识应该重新考虑。
