Aggregation and ecotoxicity of CeO₂ nanoparticles in synthetic and natural waters with variable pH, organic matter concentration and ionic strength.

The influence of pH (6.0-9.0), natural organic matter (NOM) (0-10 mg C/L) and ionic strength (IS) (1.7-40 mM) on 14 nm CeO₂ NP aggregation and ecotoxicity towards the alga Pseudokirchneriella subcapitata was assessed following a central composite design. Mean NP aggregate sizes ranged between 200 and 10000 nm. Increasing pH and IS enhanced aggregation, while increasing NOM decreased mean aggregate sizes. The 48 h-E(r)C20s ranged between 4.7 and 395.8 mg CeO₂/L. An equation for predicting the 48 h-E(r)C20 (48 h-E(r)C20 = -1626.4 × (pH) + 109.45 × (pH)² + 116.49 × ([NOM]) - 14.317 × (pH) × ([NOM]) + 6007.2) was developed. In a validation study with natural waters the predicted 48 h-E(r)C20 was a factor 1.08-2.57 lower compared to the experimental values.