Mixture toxicity of metal oxide nanoparticles and silver ions on Daphnia magna

Because of the ubiquitous production and use of silver nanoparticles (AgNPs), silver ions (Ag + ) released from AgNPs can not only singly pose significant toxicity to aquatic ecosystems but can also mix with other coexisting metal oxide nanoparticles (MONPs), such as ZnO NPs and TiO 2 NPs to provoke combined toxicity. However, information regarding the combined impact of MONPs on aquatic organisms is limited. In this study, the impact of exposure to mixtures composed of Ag + and two different MONPs (i.e., ZnO NPs and TiO 2 NPs) on Daphnia magna was examined. The toxicity of the mixtures containing Ag + concentrations exceeding 0.5–1.5 μg/L and two different MONPs (Ag + -two different MONP mixture) was higher than that of the intrinsic toxicity of each component, indicating a synergistic effect. However, the concentrations of the two different MONPs did not have a strong relationship with the occurrence of the synergistic or antagonistic effect between components in the mixtures. Moreover, the combined risk of the Ag + -two different MONP mixture estimated based on a whole-value risk for the mixture (VaR:192–198) was up to eight times higher than that estimated using a component-based value risk (VaR:736–1623) considering the predicted environmental concentration of Ag + (20 μg/L). These results imply that the component-based approach could not determine the synergistic effect between the components in the Ag + -two different MONP mixtures. Additionally, when mixed with two different MONPs, Ag + as a major toxicant induced synergistic effects among the components in the mixture. Therefore, to evaluate the interactive effects and for environmental risk assessment of mixtures, a whole-mixture approach is recommended rather than a component-based approach. Graphical abstract