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 nTiO(2)) copresented in the natural water of China. Sedimentation rates (V-s), homo-aggregation (k(hom), (crit)) and hetero-aggregation (k(het), (crit)) rate of nZnO and nTiO(2) 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 nTiO(2) 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 nTiO(2) were the main aggregation and sedimentation mechanisms. The coexistence of nZnO and nTiO(2) made V-s of nZnO decreased by 30.7-49.1% and V-s of nTiO(2) decreased by a factor of 42.4%. Value of k(het.crit) between nZnO and NCs was 0.084-0.132 L mg(-1) day(-1), was 0.038 L mg(-1) day(-1) between nTiO(2) and NCs, and was 0.011-0.014 L mg(-1) day(-1) between nZnO and nTiO(2). (C) 2017 Elsevier Ltd. All rights reserved.