Nanoparticles can be found in the environment due to natural processes and also from man-made sources. Metal nanoparticles occur in the environment due to the discharge of engineered nanoparticles and the natural transformation of metal ions into metal nanoparticles. The transformation mechanism, fates, behaviors, and effects of these nanoparticles in the environment are not clearly known.
Finding
Researchers at Chinese Academy of Sciences have found that sunlight induces reduction of Ionic Ag and Au to metallic nanoparticles and it is due to dissolved organic matter present in the aqueous environment. In rivers and other bodies of water, sunlight can help dissolved organic matter reduce silver and gold ions to form nanoparticles of the metals, according to researchers.
Mechanism
The dissolved organic matter (DOM) in environmental waters can mediate the reduction of ionic Ag and Au to their metallic nanoparticles under natural sunlight with the reduction mediated by super oxide from photo irradiation of the phenol group in DOM, and the dissolved O2 significantly enhancing the formation of Ag nanoparticles.
For example, mercury ions in the environment can get reduced by dissolved organic matter and sunlight and in turn might form nanoparticles. Silver from natural and anthropogenic sources exists as ions in the environment.
Researchers demonstrated the mechanism in a more-controlled system with gold and silver ions, using humic acid as a proxy for nature’s dissolved organic matter. They found that dissolved oxygen plays an important role. When sunlight hits the system, the humic acid donates an electron to the dissolved oxygen, which then reduces the ions to their metallic form as nanoparticles.
According to the researchers it may be difficult to differentiate between natural and engineered nanoparticles, but it shows the extent to which silver and gold can change chemically in the environment which should be considered in the future regulation of the uses of nanotechnology.