AGGREGATION AND COLLOIDAL STABILITY OF COMMERCIALLY AVAILABLE AL2O3 NANOPARTICLES IN AQUATIC ENVIRONMENTS

Nanoparticles (NPs), particles with dimensions in the range of 1 – 100 nm, are one of the major classes of nanomaterials (NMs). Among the currently available manufactured NPs, aluminum oxide (Al2O3) NPs are 2nd ranked in the total market production, accounting for approximately 20% of the NMs manufactured and produced. Due to the extensive, existing and new uses of Al2O3 NPs in consumer products, their entrance in aquatic environment is inevitable. In this study, we investigated the colloidal behavior of gamma-Al2O3 NPs with three different sizes (5, 10, and 20-30 nm) as a function of solution pH and ionic strength (NaCl and CaCl2), and in the presence of humic acid (HA) and clay minerals. Montmorillonite, a natural, swelling-smectite clay, was used as a model clay. For HA and montmorillonite, we chose to study three pHs (5, 7.5 ~ 8, and 10) to further examine the pH-dependence on NPs aggregation and stability in their presence. Dynamic light scattering and transmission electron microscopy were employed for the NPs characterization and measurements. Surface charge of all the NPs decreased with increasing pH and reached the point of zero charge (PZC) at pH 7.5-8. NPs tend to aggregate around PZC due to surface charge neutralization. Significant NPs aggregation was also observed at higher ionic strength conditions (1 M NaCl, 100 mM and 1 M CaCl2). However, they were stable in the pHs far from PZC or in ionic strength ranges typically found in natural waters. Further, the presence of HA, even at the lowest concentration (1 mg/L), had a significant impact on the NPs stability around PZC, yet in higher pH, very little interaction between HA and the NPs were found due to electrostatic repulsion. A significant interaction of montmorillonite with NPs (0.05 w/w) was also observed in all three pHs; in low pHs the negatively-charged montmorillonite base planes play an important role in surface interactions with the NPs, whereas at PZC and in higher pHs the edge sites do. We, therefore, conclude that commercially available Al2O3 NPs are stable in water conditions typically found in nature. With the presence of clay minerals and HA, their stability can be further enhanced even around PZC. Thus, the heavy use of manufactured Al2O3 NPs may raise concerns over the potentially adverse impacts on the aquatic environment.