Effects of humic substances on the aqueous stability of cerium dioxide nanoparticles and their toxicity to aquatic organisms.

The impacts of humic substances (HS) on the aquatic stability and toxicity of nano‑cerium dioxide (nCeO) to three organisms with different exposure characteristics were investigated. Addition of HS to suspensions of nCeO lowered the surface zeta potential of the particles, reduced their hydrodynamic size, and increased the energy barrier as indicated by the total potential energy profile. This resulted in a more stable suspension compared to suspensions without HS added. Moreover, a higher concentration of HS further stabilized nCeO in the suspension. Acute toxicity of the suspensions to the unicellular green alga Raphidocelis subcapitata and to the crustacean Chydorus sphaericus was lower as compared to exposure without HS added. The acute toxicity of nCeO suspensions to the zebrafish (Danio rerio) eleutheroembryo was on the other hand significantly enhanced (additive and synergistic) upon increasing HS concentration. Our findings emphasize that HS is important to stabilize the nano-suspensions and that its impact on nCeO toxicity differs across different aquatic organisms. Emphasizing the exposure characteristics of each of the organisms selected from the trophic levels can explain how particle stability impacts particle toxicity.