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.