Mechanistic insights into dissolved black carbon-mediated mutual transformation of silver nanoparticles and silver ions under alternating dark/light conditions in aquatic environments

The role of dissolved black carbon (DBC), an emerging organic component, in mediating the redox-driven interconversion between silver nanoparticles (AgNPs) and silver ions (Ag + ) under environmentally realistic dark/light alternations remains poorly understood. Through five alternating cycles of dark/light exposure, this study systematically investigated DBC-mediated AgNPs oxidation and Ag + reduction, revealing structure-dependent mechanisms influenced by coexisting anions (NO 3 − /HCO 3 − ). Radical probe experiments identified photochemically produced reactive intermediates (PPRIs) as key drivers: (i) Small-molecular DBC (BAM-DBC) containing tyrosine-like components generated persistent singlet oxygen ( 1 O 2 ) under light, enabling continuous Ag + release across cycles. (ii) Nitrogen/sulfur-rich DBC (WH-DBC) facilitated Ag + reduction through superoxide (·O 2 − ) photogeneration and Ag + complexation in darkness, causing oscillatory Ag + dynamics. (iii) Photoreactive anions modulated transformation pathways between AgNPs and Ag + by altering PPRI production. These findings provide critical insights into the environmental fate of AgNPs under alternating dark/light cycles, emphasizing the need to account for dynamic light regimes in risk assessments.