The environmental transformations of silver nanoparticles (AgNPs) are correlated with their behaviors and ecological risks. Humic acid (HA) is ubiquitous in natural water environments. However, the influence of HA on the environmental behavior, bioavailability, and toxicity of AgNPs is little known. This study investigated the processes of AgNP bioaccumulation in and toxicity to Lemna minor in an aquatic environment. AgNP effects were assessed in the presence of HA, which has been shown to interact with AgNP and modify its behavior and toxicity to organisms. Results showed that AgNP dissolution declined with the addition of HA. Carboxylate groups and C–O and C–O–C bonds were found to be involved in the HA and AgNP interaction, while XPS results suggested that HA caused Ag⁺ to be reduced to Ag⁽⁰⁾. The addition of HA inhibited the absorption and accumulation of Ag in Lemna minor roots and leaves. When exposed to AgNP treatments alone, the plant morphology and ultrastructure were damaged by root atrophy and a reduction in vessel number and cross-sectional areas (CSAs) was observed. However, the addition of HA mitigated AgNP toxicity, resulting in no significant difference in the chlorophyll biomass and concentration between the AgNP–HA and unexposed treatment groups. The addition of HA was found to inhibit the generation of ROS and the depolarization of mitochondrial membrane potential (ΔΨm) caused by AgNPs in root cells. Overall, these results establish the mechanism of the effect of HA on AgNP behavior and highlight the influence of HA on the uptake and toxicity of AgNPs to Lemna minor.

中文翻译：

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腐殖酸调节银纳米颗粒对Lemna minor的环境行为和植物毒性

银纳米颗粒（AgNPs）的环境转化与其行为和生态风险相关。腐殖酸（HA）在自然水环境中无处不在。但是，HA对AgNPs的环境行为，生物利用度和毒性的影响鲜为人知。这项研究调查了在水生环境中AgNP在Lemna minor中的生物蓄积过程及其毒性。在HA存在下评估了AgNP的作用，已证明HA与AgNP相互作用并改变其行为和对生物的毒性。结果表明，添加HA降低了AgNP的溶解度。发现羧酸酯基团以及C–O和C–OC–C键参与HA和AgNP的相互作用，而XPS结果表明HA导致了Ag ⁺还原为Ag ^（0）。HA的添加抑制了Lemna minor中Ag的吸收和积累根和叶。当单独接受AgNP处理时，根部萎缩会破坏植物的形态和超微结构，并观察到血管数量和横截面积（CSA）的减少。但是，添加HA减轻了AgNP的毒性，导致AgNP–HA与未暴露的治疗组之间的叶绿素生物量和浓度没有显着差异。发现添加HA可抑制根细胞中AgNP引起的ROS的产生和线粒体膜电位（ΔΨm）的去极化。总的来说，这些结果建立了HA对AgNP行为的作用机理，并突出了HA对AgNPs对小Lemna的吸收和毒性的影响。