Effect of natural organic matter on the aggregation kinetics of CeO2 nanoparticles in KCl and CaCl2 solutions: measurements and modeling

To characterize the environmental transport and quantify the risk of nanoparticles (NPs), it is important to fundamentally understand the aggregation of NPs and to describe this process quantitatively. This study investigates the aggregation kinetics of CeO(2) NPs in the presence of KCl, CaCl(2) and humic acid (HA) using time-resolved dynamic light scattering. In KCl solutions, regardless of their concentration, HA drastically reduces the aggregation kinetics of CeO(2) NPs. However, the effect of HA was more complicated in CaCl(2) solutions. At low CaCl(2) concentrations, HA inhibited NP aggregation, whereas at high CaCl(2) concentrations, HA promoted aggregation. The critical coagulation concentration (CCC) in KCl in the absence of HA is approximately 36.5mM. In presence of both 1 ppm and 10 ppm HA in KCl solutions, extremely low aggregation kinetics were observed even at very high KCl concentrations (500 mM), implying KCl-CCCs in presence of HA were larger than 500 mM. The CCCs under conditions of no HA, 1 ppm HA and 10 ppm HA in CaCl(2) solutions are approximately 9.5, 8.0 and 12.0mM, respectively. These observations were analyzed in the framework of extended Derjaguin-Landau-Verwey-Overbeek (EDLVO) theory. Moreover, a kinetic model was used to predict the aggregation kinetics of CeO(2) NPs. The model predictions are in close agreement with experimental observations. To the best of our knowledge, this work is the first to model quantitatively the aggregation of NPs in the presence of natural organic matter.