Freezing-Driven Aggregation Versus Stabilization of Ag Nanoparticles in Water Mediated by Plant-Derived Dissolved Organic Matter: Effects of Coexisting Ions and Size Fractionation

Abstract

The ability of dissolved organic matter (DOM) to mediate the reduction of ionic Ag to silver nanoparticles (AgNPs) in sunlit water has been validated, however, there remains a paucity of knowledge regarding the environmental fates of both naturally occurring and engineered AgNPs. This study systematically investigates the aggregation and stabilization mechanisms of AgNPs synthesized by plant-derived DOM under critical environmental stressors. The results indicate that both freezing and the presence of coexisting ions significantly enhance the aggregation of AgNPs. Specifically, anions such as Cl^– and SO₄^2– facilitate aggregation through electrostatic interactions, while divalent cations like Ca²⁺ and Mg²⁺ further promote aggregation via bridging effects and accelerate the reduction of DOM, which indirectly compromises the stability of AgNPs. Notably, AgNPs synthesized from Eriobotrya japonica demonstrate remarkable colloidal stability under various environmental stressors, a phenomenon attributed to specific components within macromolecular DOM (>30 kDa). These components may provide multifunctional protection through mechanisms such as π-Ag coordination, steric hindrance, and the formation of hydration shells. Furthermore, sucrose-6-acetic ester appears to enhance medium viscosity, thereby reducing diffusion during freeze–thaw cycles. These findings are significant for understanding of the diverse roles of plant-derived DOM in controlling fates of AgNPs in DOM-rich surface water.