Aggregation and disaggregation of Al2O3 nanoparticles: influence of solution pH, humic acid, and electrolyte cations.

Extensive use of Al₂O₃ nanoparticles in consumer and industrial products has led to concerns about their potential environmental impacts in the recent years. In most studies concerning Al₂O₃ aggregation and disaggregation, more was to consider the single factor that influences their environmental behaviors. Understanding the combined abiotic factors that influence the fate, transport, and stability of nanoparticles in a complex aquatic system has become extremely important. Here, we reported and analyzed the major abiotic factors such as typical solution pH, electrolyte cations in different valences (Na⁺ and Ca²⁺), and the presence of humic acid (HA) that influence the stability, aggregation, and disaggregation behaviors of Al₂O₃ nanoparticles in a complex aquatic system. Dynamic light scattering technique combined with fluorescence spectroscopic analysis was used to explore the aggregation mechanisms. Experimental results indicated that Al₂O₃ nanoparticle stability was mainly controlled by the steric hindrance, van der Walls, and electrostatic interactions between HA and Al₂O₃ nanoparticles. Aggregation kinetics and attachment efficiency studies induced by the addition of Na⁺ and Ca²⁺ cations confirmed that divalent electrolytes could reduce the large energy barrier between the charged colloidal particles more efficiently, and induce a more aggressive aggregation of the particles. Additionally, the bridging effect of HA with Ca²⁺ was also an important mechanism for the aggregation enhancement, which had been confirmed by the fluorescence excitation-emission matrix (EEM) spectra analysis. These findings are useful in understanding the environmental challenges of inorganic colloidal particles in natural environments. [ABSTRACT FROM AUTHOR]