Photoreduction and Stabilization Capability of Molecular Weight Fractionated Natural Organic Matter in Transformation of Silver Ion to Metallic Nanoparticle

Photoinduced reduction of silver ion (Ag+) to silver nanoparticles (AgNPs) by dissolved organic matter (DOM) plays a crucial role in the transformation and transport of engineered AgNPs and Ag+ in aquatic environments. DOM is a mixture of natural polymers with wide molecular weight (MW) distribution, and the roles of specific components of DOM in the photoreduction of Ag+ to AgNPs are still not understood. In this study, MW fractionated natural organic matter (M-f-NOM) were investigated for their roles on the photoreduction process and stabilization of the formed AgNPs. This photoinduced reduction process depends highly on pH, concentration of Ag+ and NOM, light quality, and the MW of M-f-NOM. Monochromatic radiation and light attenuation correction suggested that the difference of M-f-NOM on reduction was mainly ascribed to the differential light attenuation of M-f-NOM rather than the "real" reductive ability. More importantly, compared with low MW fractions, the high MW M-f-NOMs exhibit drastically higher capability in stabilizing the photosynthesized AgNPs against Ca2+-induced aggregation. This finding is important for a better understanding of the differential roles of M-f-NOM in the transformation and transport of Ag+ and engineered AgNPs in DOM-rich surface water.