Interaction of Ag+ with soil organic matter: Elucidating the formation of silver nanoparticles.

Abstract

Naturally silver nanoparticles (AgNPs) have been widely observed in ore deposits, coal, natural water and soil environment. Identifying the source of these naturally AgNPs could be helpful for the elucidation of the geochemical cycle of Ag+ and AgNPs. This paper presents the formation of AgNPs by reducing Ag+ in the presence of soil organic matter (SOM) under various environmentally relevant conditions. The formation of AgNPs associated with various SOM (peat humic acid (PHA), peat fulvic acid (PFA), and commercial humic acids (HA-1 and HA-2)) was determined and compared. The physicochemical properties of the tested SOM were studied by electron paramagnetic resonance (EPR) and attenuated total reflection-infrared (ATR-FTIR) techniques. The formation of AgNPs depended on reductive reactions mediated by SOM. Other influential parameters that influenced the formation of AgNPs included concentrations of Ag+ and SOM and the reaction temperature on AgNPs. The produced AgNPs were characterized by transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The mean hydrodynamic diameters of AgNPs associated with PHA and PFA were in range from 2.5 to 15nm, which were smaller than that produced from HA-1 and HA-2 in the range from 20 to 120nm. Two different Ag states, i.e., Ag2O and Ag0 species, were observed by XPS technique. The results indicated that the formation of AgNPs depends largely on the types and the properties of natural organic matter. These findings have important implications for the fate of AgNPs under the soil environment.