Aggregation behavior of zinc oxide nanoparticles and their biotoxicity to Daphnia magna: Influence of humic acid and sodium alginate.

The widespread applications of zinc oxide nanoparticles (ZnO NPs) have raised increasing concerns due to their adverse environmental effects. The ubiquitous natural organic matter in natural aqueous environments can interact with ZnO NPs, thereby affecting their aggregation, sedimentation and biotoxicity. Here, we systematically investigated the effects of humic acid (HA) and sodium alginate (SA) on the aggregation behavior of ZnO NPs and their biotoxicity to Daphnia magna. High concentrations (9.0 mg/L) of HA and SA accelerated the aggregation of ZnO NPs with maximum aggregation rates (ΔD/Δt) of 22.1 and 19.2 nm/min, respectively. Both HA and SA led to 31.2% and 30.1% decrease of ZnO NPs concentration compared with the control experiment. The results calculated by Derjaguin-Landau-Verwey-Overbeek theoretical formula were consistent with these of aggregation and sedimentation of ZnO NPs. Furthermore, excitation-emission-matrix fluorescence spectroscopy verified that the carboxylic groups of HA and SA have high complexation capacity with ZnO NPs. Daphnia magna was used to evaluate the biotoxicity of ZnO NPs, and the toxicity of ZnO NPs to Daphnia magna was alleviated as the HA concentration increased from 0 to 1.2 mg/L. Toxicity mitigation experiments confirmed that photocatalytic generation of reactive oxygen species was more toxic to Daphnia magna than dissolved Zn in acute and chronic toxicity tests. Moreover, the attacks of active oxygen free radical damaged the antioxidant system of Daphnia magna. The information obtained will help us to improve the understanding of the impacts of ZnO NPs on freshwater ecosystems.