Extrapolated long-term stability of titanium dioxide nanoparticles and multi-walled carbon nanotubes in artificial freshwater

Abstract

Long-term stability of two engineered nanomaterials (ENMs), i.e., the inorganic n-TiO₂ and the organic Multi-Walled Carbon Nanotubes (MWCNTs), dispersed in artificial freshwater (5–100 mg l⁻¹), was investigated from short-term settling velocity, particle size distribution, and surface charge. Hydrodynamic diameter and ζ-pot, calculated by means of dynamic and electrophoretic light scattering, respectively, qualitatively indicated a general ENMs dispersion instability over 1 h time. Sedimentation results, obtained by centrifugal separation analysis using the LUMiSizer over approx. 30 min analysis time, allowed to estimate the quantitative long-term (over 30 days) stability of ENMs. Settling data fitted satisfactorily with a first-order kinetic equation (R ² in the range of 0.918–0.989). The settling rate constant k values extrapolated at gravity spanned one order of magnitude, i.e., from 7.21 × 10⁻⁵ to 4.12 × 10⁻⁴ s⁻¹, and with the increasing of initial ENMs concentration. Sedimentation velocities were in good agreement with short- to long-term literature data (7.8 × 10⁻²–1.7 × 10⁻1 m day⁻¹ vs. 5 × 10⁻⁴–3 × 10⁻¹ m day⁻¹ for n-TiO₂ and 5.9 × 10⁻²–3.4 × 10⁻¹ m day⁻¹ vs. 2 × 10⁻¹–1.2 m day⁻¹ for MWCNTs). n-TiO₂ showed a higher long-term stability with respect to MWCNTs (average: 1 × 10⁻¹ ± 3.4 × 10⁻² m day⁻¹ instead of 1.7 × 10⁻¹ ± 1.1 × 10⁻¹ m day⁻¹, respectively).