Stability of co-existing ZnO and TiO2 nanomaterials in natural water: Aggregation and sedimentation mechanisms

The use of diverse engineered nanomaterials (ENMs) potentially leads to the coexistence of multiple ENMs in the natural environment. The fate such as colloidal stability, transport, and transformation of individual ENMs are dedicated to the coexistence of other types of ENMs in the environment. Here, we for the first time investigated the sedimentation behaviors of two most widely used ENMs (i.e. ZnO and TiO2 nanomaterials, nZnO and nTiO2) copresented in the natural water of China. Sedimentation rates (Vs), homo-aggregation (khom, crit) and hetero-aggregation (khet, crit) rate of nZnO and nTiO2 were calculated based on Von Smoluchowski-Stokes equation and the sedimentation mechanisms were systematically analyzed. The results showed that the coexistence of like negative charge nZnO and nTiO2 effectively enhanced the stability of either ENM by competing hetero-aggregation with natural colloids (NCs) and reducing to form homo-aggregates by the hindrance effect on particle collision. In the natural water, homo-aggregation, hetero-aggregation between ENMs and NCs, as well as the hetero-aggregation between nZnO and nTiO2 were the main aggregation and sedimentation mechanisms. The coexistence of nZnO and nTiO2 made Vs of nZnO decreased by 30.7-49.1% and Vs of nTiO2 decreased by a factor of 42.4%. Value of khet.crit between nZnO and NCs was 0.084-0.132 L mg−1 day−1, was 0.038 L mg−1 day−1 between nTiO2 and NCs, and was 0.011-0.014 L mg−1 day−1 between nZnO and nTiO2.