Toxicity of GO to Freshwater Algae in the Presence of Al2O3 Particles with Different Morphologies: Importance of Heteroaggregation

The roles of Al2O3 particles with different morphologies in altering graphene oxide (GO) toxicity to Chlorella pyrenoidosa were investigated. Algal growth inhibition by GO with coexisting Al2O3 particles was much lower than the sum of inhibitions from the individual materials for all the three Al2O3, showing the toxicity mitigation by Al2O3. The lowest GO toxicity was observed at the concentrations of 300, 150, and 100 mg/L for Al2O3 nanoparticles (NPs, 8-10 nm), bulk particles (BPs, 100-300 nm), and fibers (diameter: 10 nm; length: 400 nm), respectively. GO-Al2O3 heteroaggregation was responsible for the observed toxicity reduction. GO-induced algal membrane damage was suppressed by the three types of Al2O3 due to GO-Al2O3 heteroaggregation, and the reduction in intracellular reactive oxygen species generation and physical contact were confirmed as two main mechanisms. Moreover, the exposure sequence of GO and Al2O3 could highly influence the toxicity, and the simultaneous exposure of individual GO and Al2O3 showed the lowest toxicity due to minimum direct contact with algal cells. Humic acid further decreased GO-Al2O3 toxicity due to enhanced steric hindrance through surface coating of GO-Al2O3 heteroaggregates. This work provides new insights into the role of natural mineral particles in altering the environmental risk of GO.