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

Abstract

Natural organic matter (NOM) has a profound effect on the colloidal stability of discharged C60 nanoparticles in the water environment, which influences the environmental behaviors and risks of C60 and therefore merits more specific studies. This study investigates the effects of humic acid (HA), as a model NOM, on the aqueous stabilization of C60 powder and the colloidal stability of a previously suspended C60 suspension (aqu/nC60) with variations of pH values and ionic strengths. Our results reveal that HA could disperse C60 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/nC60 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 nC60 aggregates. The colloidal stability of aqu/nC60, 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/nC60 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. © 2014 Zhejiang University and Springer-Verlag Berlin Heidelberg.