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

Abstract

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 \5 mg/l.

Formation of complexes between hematite nanoparticles and a non-conventional galactomannan gum. Toward a better understanding on interaction processes

“…Z-average diameter variations were found different starting from different pH due to the formation of large aggregates when the PZC region was reached. As shown by Palomino and Stoll (2013), the zeta potential was not found to exhibit hysteresis by increasing or decreasing the pH values. This indicates that the hematite charging process is not dependent on the aggregate formation.…”

“…The NPs were stored as a 0.2 g/L suspension in a cold and dark environment. After image analysis of more than 50 SEM pictures by Palomino and Stoll (2013), the mean SEM diameter of the individual nanoparticles was found equal to 53 ± 5 nm confirming that the nanoparticle sizes were reasonably monodispersed. The Z-average hydrodynamic diameter was found equal to 94 ± 3 nm at pH = 3, indicating the presence of dimmers and trimers in solution.…”

“…The pH value at which the charges are neutralized (point of zero charge or PZC) was found equal to 6.3 ± 0.1. Several authors observed that the hematite PZC was strongly dependent on the size or crystalline phase of the synthesized NPs, and thus Fe 2 O 3 PZC were found in the range of 5.5-9.5 (Dickson et al, 2012;He et al, 2008;Nidhin et al, 2012;Palomino and Stoll, 2013). These differences between PZC values were also attributed to synthesis and storage conditions, presence of impurities, and slow aging of the dispersions (i.e.…”

“…The interaction of hematite NPs with natural organic matter (NOM) has been studied before to investigate the environmental impact of hematite nanoparticles when entering natural aquatic systems (Baalousha, 2009;Nidhin et al, 2012;Palomino and Stoll, 2013). The adsorption of negatively charged humic substances on the positively charged surface of hematite NPs was found to strongly modify their surface charge by inducing charge inversion and thus enhanced their stability through electrostatic repulsion and steric effects.…”

Dissipative particle dynamic simulation and experimental assessment of the impacts of humic substances on aqueous aggregation and dispersion of engineered nanoparticles

“…This means that at this concentration the extent of disaggregation of the NP colloids in the presence of the concentration of humic substances is the highest. Palomino and Stoll [35] also found that very low concentrations of fulvic acids (0.1-5 mg/L) showed a similar effect on iron hydroxide NP solutions. Figure 4 provides a quantitative extent of disaggregation of the 12 NP colloids in the pure water and the algae medium when the humic substance is present at the concentration.…”