Freezing Facilitates Formation of Silver Nanoparticles under Natural and Simulated Sunlight Conditions

Freezing is essential in the light-mediated transformation of organic pollutants. However, the effects of the freezing process on the reduction of Ag⁺ by natural organic matter (NOM) remains unclear, causing significant uncertainties in the natural formation of silver nanoparticles (AgNPs). This study investigated the sunlight-induced reduction of Ag⁺ by NOM under natural or controlled freezing processes. Natural (outdoor) freezing experiments demonstrated intense aggregation and precipitation of AgNPs in three aqueous media, including a NOM solution and two river water samples, under natural sunlight irradiation. Indoor experiments under simulated sunlight irradiation and controlled freezing processes showed that freezing at -20 °C and repeated freeze-thaw cycles (-20 to 4 °C) drastically accelerated the formation and growth of AgNPs compared to maintenance at 4 °C. Finally, under the natural freezing process, commercial AgNPs were found to influence the redox reduction of Ag⁺ probably through a reduction in dissolution rates and homoaggregation with AgNPs newly formed in the river water samples. Additionally, the enhancement effect of freezing on AgNP formation was confirmed in the presence of Ag⁺ and AgNPs both at environmentally relevant concentration levels, especially upon light irradiation. This work emphasizes the importance of freezing processes on the natural formation of AgNPs.