Carbonaceous nanomaterial-initiated reductive transformation of silver ions in the aqueous environment under sunlight

The aquatic systems are among the major sinks for discharged substances, and these substances will likely associate with each other. The present work, therefore, aims to study the transformation of metal ions to nanoparticles by discharged carbonaceous materials of emerging concern (e.g., carbon nanotubes (CNTs)) coexisting in the aqueous environment. Here we undertook a systematic study of the reduction of silver ions by CNT suspensions under sunlight irradiation. The formation rate of silver nanoparticles (AgNPs) is suppressed by an increasing amount of dissolved oxygen or strong solution acidity, as well as the presence of other cations. The photoreduction of Ag⁺ by CNTs involves a charge transfer process between Ag⁺ and the CNTs. The way in which carbonaceous nanomaterial properties influence the formation kinetics, size, and morphology of the AgNPs was examined. An enhanced sunlight-driven formation of AgNPs with highly monodispersity was observed in CNTs with nitrogen-containing functional groups due to their active electrochemical and stabilizing nature. The compiled results reveal the importance of an understanding of not only the inherent environmental behaviors of individual substances but also their interactions with concurrent substances in the environment. We demonstrated that the transformation of silver under sunlight by carbonaceous materials with different characteristics could alter the properties and potential risks of metallic species in aquatic environments.