Environmentally Relevant Freeze-Thaw Cycles Enhance the Redox-Mediated Morphological Changes of Silver Nanoparticles

Silver nanoparticles (AgNPs) are inevitably released into natural systems, particularly into aquatic environments, where they are oxidized and release Ag⁺, which is reduced back to AgNPs. Environmental freeze-thaw cycles or freezing may accelerate the dynamic transformation between AgNPs and Ag⁺. Herein, the significant morphological changes caused by freezing treatments were assessed by UV-vis spectroscopy and high-resolution transmission electron microscopy, which revealed that reductive regeneration, particle fusion, and coalescence of the AgNPs occurred. In addition, a stable Ag isotope was used to track the AgNP redox reaction, which was found to be accelerated under freezing and freeze-thaw cycles relative to the reaction of particles stored at a normal temperature (4 °C, 25 °C). Furthermore, natural organic matter was found to stabilize the particle morphology. Ca²⁺ and Cl^- intensified the morphological changes and redox reaction through Ca²⁺-induced particle coalescence and Cl^--enhanced reduction of Ag⁺ during the freeze-thaw treatment. These physicochemical changes also occurred for an environmentally relevant concentration of AgNPs (50 ng L^-1) in simulated environmental conditions and natural water samples after freeze-thaw cycles. Since the morphological changes and redox acceleration induced by environmental freezing conditions could dramatically influence the mobility, bioavailability, toxicity, and environmental fate of AgNPs, the freeze-thaw-induced effects should be considered in the environmental risk assessment of AgNPs.