Fulvic acids concentration and pH influence on the stability of hematite nanoparticles in aquatic systems

In aquatic systems, fulvic acids (FAs) are expected to play key roles on the stability and aggregation behavior of manufactured nanoparticles (NPs). The exact conditions under which aggregation or dispersion occurs will depend on the nanoparticle surface charge properties, FAs concentration as well as solution conditions, such as pH and ionic strength. The systematic calculation of stability (aggregation versus disaggregation) diagrams is therefore a key aspect in the prediction of the environmental fate and behavior of manufactured nanoparticles in aquatic systems. In this study, the responses to changes in pH and FAs concentrations on the resulting surface charge of purified iron oxide nanoparticles (53 nm nominal diameter) is investigated. By adjusting the pH, different nanoparticle surface charge electrostatic regions are found, corresponding to positively, neutral, and negatively charged nanoparticle solutions. For each situation, the adsorption of negatively charged FAs at variable concentrations is considered by analyzing surface charge modifications and calculating experimental kinetics aggregation rates. Results show that, under the conditions used, and range of FAs environmental relevant conditions, the nanoparticle aggregation process is promoted only when the nanoparticle positive surface charge (solution pH less than the charge neutralization point) is compensated by the adsorption of FAs. In all the other cases, FAs adsorption and increase of FAs concentration are expected to promote not only the NPs stabilization but also the disaggregation of NPs aggregates. In addition, our study suggest that very low concentrations of FAs 0.1 mg/l are sufficient to rapidly stabilize iron hydroxide NPs solutions at concentration less than 5 mg/l