Properties of residual titanium dioxide nanoparticles after extended periods of mixing and settling in synthetic and natural waters

Abstract

Titanium dioxide nanoparticle (TiO2 NP) discharged into water bodies can affect ecosystems and human health adversely. We studied the properties of residual TiO2 NPs with and without gentle mixing (to simulate a natural environment more closely) and after settling for 12-h periods. Surface complexation, dynamic particle size changes, and TiO2 NP destabilization in synthetic and lake waters were investigated. The accumulation of inert ions (Na+ and Cl-) in the diffuse layer which was not discussed in other studies was supposed to be the main reason that aggregation occurred slowly and continuously. PO43- stabilized and destabilized TiO2 NPs at 10 mM and 100 mM, respectively. Destabilization occurred because high ionic strength overwhelmed increased negative charges of TiO2 NPs by complexation with PO43-. TiO2 NP destabilization was achieved in approximately 12 h in synthetic and lake waters, and is attributed to the slow diffusion of ions into aggregates. Despite the presence of moderately high concentrations of natural organic matter, which tends to stabilize TiO2 NPs, the addition of 20 mM PO43- destabilized the TiO2 NPs in lake water. Smaller aggregate sizes formed compared with those before destabilization, which indicates that stable residual TiO2 NPs could exist in aquatic environments after extended periods.