Toxicity of iron-based nanoparticles to green algae: Effects of particle size, crystal phase, oxidation state and environmental aging

With the increasing environmental application and discharge of iron-based nanoparticles (NPs), a comprehensive understanding of their fate and ecotoxicological effect in the aquatic environment is very urgent. In this study, toxicities of 4 zero-valent iron NPs (nZVI) of different sizes, 2 Fe₂O₃ NPs of different crystal phases, and 1 type of Fe₃O₄ NPs to a green alga (Chlorella pyrenoidosa) were investigated, with a focus on the effects of particle size, crystal phase, oxidation state, and environmental aging. Results show that the algal growth inhibition of nZVI increased significantly with decreasing particle size; with similar particle sizes (20–30 nm), the algal growth inhibition decreased with oxidation of the NPs with an order of nZVI > Fe₃O₄ NPs > Fe₂O₃ NPs, and α-Fe₂O₃ NPs presented significantly higher toxicity than γ-Fe₂O₃ NPs. The NP-induced oxidative stress was the main toxic mechanism, which could explain the difference in algal toxicity of the NPs. The NP-cell heteroagglomeration and physical interactions also contributed to the nanotoxicity, whereas the effect of NP dissolution was negligible. The aging in distilled water and 3 surface water samples for 3 months increased surface oxidation of the iron-based NPs especially nZVI, which decreased the toxicity to algae. These findings will be helpful for the understanding of the fate and toxicity of iron-based NPs in the aquatic environment. - Highlights: • The algal toxicity of nZVI decreased with increasing particle size. • α-Fe₂O₃ NPs exhibited higher algal toxicity than γ-Fe₂O₃ NPs. • The iron-based NPs with higher oxidation presented lower algal toxicity. • The aging in the water samples increased surface oxidation of nZVI. • The aging in the water samples mitigated algal toxicity of iron-based NPs. - The algal toxicity of nZVI decreased with increasing particle size, surface oxidation, and environmental aging.