Inter-transformation between silver nanoparticles and Ag⁺ induced by humic acid under light or dark conditions

The fate and toxicity of silver nanoparticles (AgNPs) and ions in water bodies is largely determined by the natural organic matter (NOM)-mediated redox cycling. However, the process of NOM-mediated redox cycling in the day/night cycles remains elusive. In this study, the inter-transformation between AgNPs and Ag⁺ ion caused by humic acid (HA) was investigated under controlled light and dark conditions. It was shown that HA induced the reduction of Ag⁺ into AgNPs in simulated sunlight, and also oxidize AgNPs to release Ag⁺ in darkness. Kinetics data demonstrated that the rates of AgNPs formation and dissolution increased along with the increment of HA concentrations. Along with the pH increase, the reduction of Ag⁺ accelerated, but the oxidative dissolution of AgNPs was inhibited. In day-night cycles, the AgNPs and Ag⁺ concentrations exhibited similar wave-shaped change curves. The peaks of AgNPs and Ag⁺ ion appeared at 7 p.m. and 7 a.m., respectively. The toxicity of AgNPs/Ag⁺ to Escherichia coli was determined primarily by the concentration of dissolved Ag⁺, but also affected by the particle-specific toxicity. The dual role of HA implied that previous reports about the photo-reduction of Ag⁺ to AgNPs by NOM should be reconsidered, and the oxidizability of HA in darkness strongly affect the transformation and toxicity of AgNPs in water.