Aggregation behavior of aqu/nC produced via extended mixing: Influence of sunlight and agitation intensity.

Aggregation of C, as an important process governing its mobility and toxicity, has been quantitatively investigated. However, effects of sunlight and agitation intensity on the aggregation behavior of aqu/nC produced via extended mixing, have not been clarified. Therefore, in the present study, the aggregation behavior of aqu/nC produced at 500 and 800 rpm in the absence and presence of sunlight was investigated. Aggregation with increasing concentrations could be accelerated, while changes of Z and zeta potential were not obvious. Critical coagulation concentrations (CCCs) of aqu/nC obtained at 800 rpm in the absence/presence of sunlight and that at 500 rpm under sunlight were 330, 205 and 170 mM NaCl, and 10.0, 2.6 and 3.1 mM CaCl, respectively. These CCCs indicated that the aqu/nC prepared by the extended mixing were more stable than those produced by other methods. Salt-induced aggregation occurred more easily for aqu/nC formed under sunlight than that formed in the dark. Extra surface oxidation induced by high agitation intensity remarkably increased the stability of aqu/nC in NaCl solutions. In contrast, in CaCl solutions, aqu/nC formed at high agitation intensity had similar stability or even inadequate stability to that obtained at low agitation intensity due to the charge neutralization and cross-link bridging.