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

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 zeta-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 x 10(-5) to 4.12 x 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 x 10(-2) -1.7 x 10(-1) m day(-1) vs. 5 x 10(-4) -3 x 10(-1) m day(-1) for n-TiO2 and 5.9 x 10(-2) -3.4 x 10(-1) m day(-1) vs. 2 x 10(-1) -1.2 m day(-1) for MWCNTs). n-TiO2 showed a higher long-term stability with respect to MWCNTs (average: 1 x 10(-1) +/- 3.4 x 10(-2) m day(-1) instead of 1.7 x 10(-1) +/- 1.1 x 10(-1) m day(-1), respectively).