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

Abstract

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 CeO2 NPs in the presence of KCl, CaCl2 and humic acid (HA) using time-resolved dynamic light scattering. In KCl solutions, regardless of their concentration, HA drastically reduces the aggregation kinetics of CeO2 NPs. However, the effect of HA was more complicated in CaCl2 solutions. At low CaCl2 concentrations, HA inhibited NP aggregation, whereas at high CaCl2 concentrations, HA promoted aggregation. The critical coagulation concentration (CCC) in KCl in the absence of HA is approximately 36.5mM. In presence of both 1ppm and 10ppm HA in KCl solutions, extremely low aggregation kinetics were observed even at very high KCl concentrations (500mM), implying KCl–CCCs in presence of HA were larger than 500mM. The CCCs under conditions of no HA, 1ppm HA and 10ppm HA in CaCl2 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 CeO2 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.