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

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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.
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Document Type: Research Article

Publication date: March 1, 2013

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  • Journal of Colloid Science and Biotechnology is an international multidisciplinary peer- reviewed journal covering all aspects of colloid science including colloids preparation, characterization, structure-property relationships, spectroscopy, chemical and physical properties and applications of colloids in biotechnology, medicine and pharmaceuticals. This journal deals with interdisciplinary research areas of chemistry, physics, biology, materials science, polymer science, nanotechnology covering all topics related to colloids such as latexes, emulsions, suspensions, micellar systems, gels, composites, hybrids, surfactants, aerosols, foams, minerals, soft matter, microfluidics, encapsulation processes of active molecules, biomolecules, surfacial and interfacial processes, catalysis, electrochemistry, theoretical aspects, computer simulations and colloids in biotechnology and medicine.
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