Stability of TiO₂ Nanoparticles in Presence of Fulvic Acids. Importance of pH

Manufactured nanoparticles are now present in many daily use commercial products such as cosmetics, paints, food packaging, etc. One of the main consequences regarding the environment is their uncontrolled release and diffusion in the aquatic systems. To explore their environmental behavior in term of stability, coagulation and dissociation, it is important to perform investigation at the laboratory scale as a prerequisite for understanding their environmental behaviors. In this study, the stability of titanium dioxide (TiO₂) nanoparticles as a function of pH and fulvic acid concentration (one of the major organic component found in aquatic systems) is systematically examined by measuring the size and zeta potential variations of TiO₂ nanoparticles, fulvic acids mixtures and aggregates they can form. Experiments are also conducted at three different pH values corresponding to the three possible TiO₂ surface charge states: positive, neutral, and negative. The TiO₂ size and zeta potential evolution is then examined as a function of the concentration of negatively charged fulvic acids (FAs) by increasing successively the fulvic acids concentrations. Our results point out that in absence of fulvic acids, TiO₂ aggregation is achieved for pH values between 5 and 8 i.e., when the nanoparticle surface charge is close to the point of zero charge. At low and high pH values, nanoparticle surface charges result in strong electrostatic repulsions hence preventing aggregation. In presence of fulvic acids fast aggregation is achieved at low pH and with fulvic acids concentration comprised between 4 and 8 mg/L. In such conditions the nanoparticle surface charge neutralization is achieved via the adsorption of the negative fulvic acids on the positively charged nanoparticles. By increasing further the fulvic acids concentration, surface charge inversion is obtained then resulting in the restabilization of the nanoparticle solution via aggregate fragmentation (disaggregation). At high pH, the negatively charged fulvic acids are not found to adsorb significantly on the negatively charged nanoparticles. These results clearly pointed out the role of the pH, electrostatic interactions and FAs concentrations on the stability of NPs such as TiO₂. It is shown that the environmental FAs concentrations are important enough to promote the dispersion of 50 mg/L TiO₂ nanoparticle solutions including aggregates.