The role of humic acid in stabilizing fullerene (C60) suspensions

Natural organic matter (NOM) has a profound effect on the colloidal stability of discharged C-60 nanoparticles in the water environment, which influences the environmental behaviors and risks of C-60 and therefore merits more specific studies. This study investigates the effects of humic acid (HA), as a model NOM, on the aqueous stabilization of C-60 powder and the colloidal stability of a previously suspended C-60 suspension (aqu/nC(60)) with variations of pH values and ionic strengths. Our results reveal that HA could disperse C-60 powder in water to some degree, but was unable to stably suspend them. The aqu/nC(60) could remain stable at pH > 4 but was destabilized at lower pH values. However, the colloidal stability of aqu/nC(60) in the presence of HA was insensitive to pH 3-11, owing to the adsorption of HA onto nC(60) and the increased electrosteric repulsions among nC(60) aggregates. The colloidal stability of aqu/nC(60), with and without HA, decreased as we increased the valence and concentration of the added cations. HA was found to mitigate the destabilization effect of Na+ on the colloidal stability of aqu/nC(60) by increasing the critical coagulation concentration (CCC) of Na+, while HA lowered the CCCs of Ca2+ and La3+ probably by the bridging effect of nC(60) with HA aggregates formed through the intermolecular bridging of the HA macromolecules via cation complexation at high concentrations of cations with high valences.