Catalytic role of iron in the formation of silver nanoparticles in photo-irradiated Ag(+)-dissolved organic matter solution.

Photo-reduction of Ag(+) to silver nanoparticle (AgNPs) by dissolved organic matter (DOM) is a possible source of naturally occurring AgNPs. However, how this photo-reduction process is influenced by ubiquitous metal ions is still not well understood. In addition, in previous studies, the formation of AgNPs in DOM solution was usually monitored by UV-Vis spectroscopy, and there is still lack of quantitative analysis for the formed AgNPs. In the present study, the role of Fe(2+)/Fe(3+) at environmental concentration level on this photochemical process was investigated, and the enhanced formation of AgNPs by Fe(2+)/Fe(3+) was probed and quantified by using UV-Vis spectroscopy, transmission electron microscopy, and liquid chromatography-inductively coupled plasma mass spectrometry. It was demonstrated that while Fe(3+) can oxidize AgNPs to release Ag(+), Fe(2+) can reduce Ag(+) into AgNPs. However, the DOM-induced reduction of Fe(3+) makes iron an effective electron shuttle between DOM and Ag(+), and both Fe(2+) and Fe(3+) enhanced AgNP formation. The impacts of environmentally relevant factors, including DOM concentration and solution pH, on this process were studied comprehensively, which showed that the catalytic role of iron was more significant at higher DOM concentration and lower pH. This iron-enhanced formation of AgNPs in photo-irradiated Ag(+)-DOM solution have great environmental implications on the formation of natural AgNPs and the transformation of engineered AgNPs in acidic surface water with high iron content.