Concomitant releases of various engineered nanoparticles (NPs) into the environment have resulted in concerns regarding their combined toxicity to aquatic organisms. It is however, still elusive to distinguish the contribution to toxicity of components in NP mixtures. In the present study, we quantitatively evaluated the relative contribution of NPs in their particulate form (NP) and of dissolved ions released from NPs (NP) to the combined toxicity of binary mixtures of ZnO NPs and graphene oxide nanoplatelets (GO NPs) to three aquatic organisms of different trophic levels, including an alga species (Scenedesmus obliquus), a cladoceran species (Daphnia magna), and a freshwater fish larva (Danio rerio). Our results revealed that the effects of ZnO NPs and GO NPs were additive to S. obliquus and D. magna but antagonistic to D. rerio. The relative contribution to toxicity (RCT) of the mixture components to S. obliquus decreased in the order of RCT > RCT > RCT, while the RCT of the mixture components to D. magna and D. rerio decreased in the order of RCT > RCT > RCT. This finding also implies that the suspended particles rather than the dissolved Zn-ions dictated the combined toxicity of binary mixtures of ZnO NPs and GO NPs to the aquatic organisms of different trophic level. The alleviation of the contribution to toxicity of the ionic form of ZnO NPs was caused by the adsorption of the dissolved ions on GO NPs. Furthermore, the ZnO NP and GO NP displayed a different contribution to the observed mixture toxicity, dependent on the trophic level of the aquatic organisms tested. The difference of the contributions between the two particulate forms was mainly associated with differences in the intracellular accumulation of reactive oxygen species. Our findings highlight the important role of particles in the ecological impact of multi-nanomaterial systems.

No abstract

Metal-based nanoparticles (NPs) are the most widely used engineered nanomaterials. The individual toxicities of metal-based NPs have been plentifully studied. However, the mixture toxicity of multiple NP systems (n ≥ 3) remains much less understood. Herein, the toxicity of titanium dioxide (TiO2) nanoparticles (NPs), silicon dioxide (SiO2) NPs and zirconium dioxide (ZrO2) NPs to unicellular freshwater algae Scenedesmus obliquus was investigated individually and in binary and ternary combination. Results show that the ternary combination systems of TiO2, SiO2 and ZrO2 NPs at a mixture concentration of 1 mg/L significantly enhanced mitochondrial membrane potential and intracellular reactive oxygen species level in the algae. Moreover, the ternary NP systems remarkably increased the activity of the antioxidant defense enzymes superoxide dismutase and catalase, together with an increase in lipid peroxidation products and small molecule metabolites. Furthermore, the observation of superficial structures of S. obliquus revealed obvious oxidative damage induced by the ternary mixtures. Taken together, the ternary NP systems exerted more severe oxidative stress in the algae than the individual and the binary NP systems. Thus, our findings highlight the importance of the assessment of the synergistic toxicity of multi-nanomaterial systems.

Graphene-family nanomaterials (GFNs) exhibit universal applications and consequently will inevitably enter aquatic systems. However, both the fate and behavior of GFNs in aquatic environments have not been completely explored at field relevant conditions. Herein, we have systematically investigated the aqueous aggregation and stability of graphene nanoplatelets (GNPs), graphene oxide (GO), and reduced graphene oxide (RGO) under varied solution chemistry parameters (pH, divalent cations, and dissolved organic carbon (DOC)) during 21 days of incubation in simulated natural environmental conditions. Results indicate that pH values from 6 to 9 had a notable impact on the aqueous behaviors of the three GFNs. Divalent cations (Ca and Mg) at the concentrations of 2.5 and 10 mM remarkably increased the extent of aggregation of the three GFNs and resulted in severe sedimentation, independently of surface chemical functionalization. The presence of only DOC ranging from 0.5 to 2 mg C/L significantly elevated the dispersion stability of GNPs and RGO in a dose-dependent manner, whereas no effects were observed on GO. Furthermore, DOC at the studied concentrations and surface functionality were insufficient to counterbalance the impact of the divalent cations. Direct visual and in situ observations further supported the conclusions on the effects of divalent cations or/and DOC. These findings further underline that the environmental behaviors of GFNs are controlled by the complex interplay between water chemistry parameters and GFN surface properties.

This study investigated the impact of dissolved organic matters (DOM) on the ecological toxicity of aluminum oxide nanoparticles (AlONPs) at a relatively low exposure concentration (1 mg L). The unicellular green alga Scenedesmus obliquus was exposed to AlONP suspensions in the presence of DOM (fulvic acid) at various concentrations (1, 10, and 40 mg L). The results show that the presence of DOM elevated the growth inhibition toxicity of AlONPs towards S. obliquus in a dose-dependent manner. Moreover, the combination of DOM at 40 mg L and AlONPs resulted in a synergistic effect. The relative contribution of Al-ions released from AlONPs to toxicity was lower than 5%, indicating that the presence of the particles instead of the dissolved ions in the suspensions was the major toxicity sources, regardless of the presence of DOM. Furthermore, DOM at 10 and 40 mg L and AlONPs synergistically induced the upregulation of intercellular reactive oxygen species levels and superoxide dismutase activities. Analysis of the plasma malondialdehyde concentrations and the observation of superficial structures of S. obliquus indicated that the mixtures of DOM and AlONPs showed no significant effect on membrane lipid peroxidation damage. In addition, the presence of both DOM and AlONPs contributed to an enhancement in both the mitochondrial membrane potential and the cell membrane permeability (CMP) in S. obliquus. In particular, AlONPs in the presence of 10 and 40 mg L DOM caused a greater increase in CMP compared to AlONPs and DOM alone treatments. In conclusion, these findings suggest that DOM at high concentrations and AlONPs synergistically interrupted cell membrane functions and triggered subsequent growth inhibition toxicity.