Influence of Structurally Different Humic Acids On Colloidal Behavior of Aluminum Oxide Nanoparticles.

Production and use of engineered nanomaterials have increased significantly in the last decade. One major group of frequently used engineered nanomaterials is metal oxide nanoparticles (NP). Metal oxide nanoparticles can cause oxidative stress and inflammation in living organisms. Interactions of these nanomaterials with environmental colloids may influence their availability and potential toxicity to aquatic species. Natural organic matter (NOM) is ubiquitous in the environment and varies significantly in their physico-chemical behavior. Therefore, the colloidal behavior of structurally different humic acids (HA)-coated Al₂O₃ NP was examined in the presence of divalent cation (Ca²⁺) by dynamic light scattering (DLS), atomic force microscopy (AFM), and zeta potential measurements. Fast aggregation kinetics of three structurally different HAs-coated Al₂O₃ NPs was investigated in the presence of Ca²⁺. This aggregation study was carried out in both acidic and alkaline solution conditions. Critical coagulation concentration (CCC) of Ca²⁺ was determined for each HA-coated Al₂O₃ NP system. The CCC increased with decreasing polarity of the adsorbed HA on the nanoparticle surface. The CCC values also increased in alkaline condition compared to acidic condition. Low polar, high molecular weight HA fractions bound to the nanoparticle surface strongly enhanced colloidal stability even at higher ionic strength. Steric stabilization mechanism is likely to be operative in these systems. This study highlights the prominent role of NOM on the mobility and fate of engineered metal oxide nanoparticles if exposed to the natural aquatic environment. Therefore, ecological exposure and risk assessment of NPs should be evaluated with consideration of their interactions with NOM.