Stability and aggregation of nanoscale titanium dioxide particle (nTiO2): Effect of cation valence, humic acid, and clay colloids

Fate and transport of engineered nanoscale titanium dioxide (nTiO2) have received much attention during the past decade. The aggregation and stability of nTiO2 in water with complicated components, however, have not been fully examined. The objective of this paper is to determine the individual and synergistic effect of cation valence, humic acid, and clay colloids on nTiO2 stability and aggregation, and elucidate the related mechanisms. We conducted systematic laboratory experiments to determine nTiO2 stability and aggregation in NaCl and MgCl2 solutions, both in the absence and presence of humic acid and illite colloids. Results showed that Mg2+, in comparison to Na+, could make the zeta potential of nTiO2 more positive, and shift the point of zero charge of nTiO2 (pHpzc,TiO2) towards higher pH. We also found that nTiO2 are destabilized by illite colloids at pH < pHpzc,TiO2 through formation of illite-nTiO2 hetero-aggregates, but are not interfered by illite colloids at higher pH. HA was found to make nTiO2 stable via electrostatic and steric effects, both in the absence and presence of illite colloids. Calculated interaction energy based on DLVO theory revealed that instability of the nTiO2 suspensions is mainly caused by primary minima, and that secondary minima normally do not destabilize the suspension, even though they are found to promote aggregation.