Suspended state heteroaggregation kinetics of kaolinite and fullerene (nC60) in the presence of tannic acid: Effect of π-π interactions

The colloidal heteroassociation between natural mineral colloids and engineered nanoparticles (ENPs) can reduce the bioavailability and toxicity of the ENPs. However, the efficacy of this heteroassociation-based entrapment of ENPs depends on the intrinsic material properties of the particles and the physicochemical parameters of the aquatic environment. Natural organic matter (NOM)-induced surface modifications of clay colloids, functionalization of ENPs, and efficiency of counterions as effective coagulants profoundly affect the effectiveness of heteroaggregation-based attenuation of anthropogenic colloids. In this study, tannic acid (TA), a surrogate of NOM, prevented the edge-to-face self-association of sodium-saturated kaolinite (Na-kaolinite) at acidic pH, as evaluated from the transverse proton spin–spin relaxation data (T₂). Likewise, fullerene water suspension (FWS) adhesion to Na-kaolinite prevented the self-association of Na-kaolinite and enhanced the colloidal stability. At pH 4 and diffusion-limited aggregation regime salt concentrations, the Na-kaolinite and FWS heteroaggregation rates were lower than the Na-kaolinite homoaggregation rates, and eventually reached a plateau. The higher colloidal stability of the Na-kaolinite and FWS binary mixture than that of Na-kaolinite, regardless of stronger charge screening by Ca²⁺, reflects steric stabilization. However, at pH 7, the increased electrostatic barrier reduces the feasibility of colloidal heteroassociation between Na-kaolinite and FWS; thus, higher salt concentrations are required to initiate aggregation. Weak adsorption of TA on Na-kaolinite at pH 7 facilitated stronger π-π interactions with FWS. All suspensions exhibited faster aggregate growth at pH 7 than pH 4, possibly due to the stronger cation response at pH 7. In situ atomic force microscopy imaging and line profile plots of Na-kaolinite, TA, and FWS mixture in CaCl₂ further corroborated the difference in the heteroaggregation rates observed at the two different pH values. Thus, TA-induced surface functionalization of FWS and the consequent increased electrostatic barrier to heteroassociation with Na-kaolinite may facilitate the environmental mobility of FWS in aquatic media.