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.

水体中银纳米粒子 (AgNPs) 和离子的命运和毒性在很大程度上取决于天然有机物 (NOM) 介导的氧化还原循环。然而，NOM 介导的昼夜循环氧化还原循环过程仍然难以捉摸。在这项研究中，在受控的明暗条件下研究了腐植酸 (HA) 引起的AgNPs 和 Ag ⁺离子之间的相互转化。结果表明，HA在模拟阳光下诱导 Ag ⁺还原为 AgNPs，并在黑暗中氧化 AgNPs 以释放 Ag ⁺。动力学数据表明，AgNPs 形成和溶解的速率随着 HA 浓度的增加而增加。随着 pH 值的增加，Ag ⁺加速，但抑制了 AgNPs 的氧化溶解。在昼夜循环中，AgNPs 和 Ag ⁺浓度表现出相似的波形变化曲线。AgNPs 和 Ag ⁺离子的峰值分别出现在晚上 7 点和早上 7 点。AgNPs/Ag ⁺对大肠杆菌的毒性主要由溶解的Ag ⁺浓度决定，但也受颗粒特异性毒性的影响。HA 的双重作用意味着应该重新考虑之前关于NOM 将Ag ⁺光还原为 AgNPs 的报道，并且 HA 在黑暗中的氧化性强烈影响 AgNPs 在水中的转化和毒性。