Colloidal stability of Al2O3 nanoparticles as affected by coating of structurally different humic acids

The colloidal stability of three structurally different humic acid (HA)-coated Al(2)O(3) nanoparticles (HAs-Al(2)O(3) NPs) was studied in the presence of Ca(2+). HAs were obtained after sequential extractions of Amherst Peat Soil. Highly polar HA1-coated Al(2)O(3) NPs exhibited strong aggregation in the presence of Ca(2+). HA3 and HA7-coated NPs showed weaker aggregation due to their increased aliphaticity and low polarity. HA7-Al(2)O(3) NPs displayed the weakest aggregation behavior even at relatively high Ca(2+) concentration. The inverse stability ratio (alpha = 1/W) was the lowest for HA7-Al(2)O(3) NPs, reflecting that strong steric stabilization enhanced colloidal stability. Atomic force microscopy (AFM) of pure Al(2)O(3) NPs on Ca(2+)-saturated mica clearly demonstrated significant aggregation following classical Derjaguin-Landau-Verwey-Overbeek (DLVO) model for hard spheres. On the contrary, weakly polar HA fraction produced approximately 10 nm thick corona of adsorbed layer around each Al(2)O(3) NP, thus stabilizing coated NP suspension through steric effect. Under alkaline conditions and at low ionic strength, adsorbed HA chains swelled, increasing their osmotic potential, which in turn resulted in stabilization of the colloids. Inherent structural variations of natural organic matter (NOM) played a significant part in colloidal stability of the coated NPs. Thus, development of sterically stabilized NPs may have potential application for water remediation in marine and high salinity conditions.