Heteroaggregation of engineered nanoparticles and kaolin clays in aqueous environments

Abstract

a b s t r a c tThe increasing and wide use of nanoparticles (NPs), including TiO 2 and Ag NPs, have raised concerns due to their potential toxicity and environmental impacts. Kaolin is a very common mineral in aquatic systems, and there is a very high probability that nanoparticles (NPs) will interact with these clay minerals. We studied the effect of kaolin particles on the aggregation of NPs under different conditions, including the role of pH, ionic strength (IS), and humic acid (HA). We show that kaolin reduces the energy barrier and the Critical Coagulation Concentration (CCC) at pH 4. At pH 8, even though the energy barrier of the system without kaolin increases, kaolin promotes NP aggregation via heteroaggregation. When IS is equal to or greater than the CCC, on the one hand HA promotes aggregation of TiO 2 NPs, but on the other hand HA decreases the rate of Ag NP aggregation because the existence of a surface coating may limit the adsorption of HA on these Ag NPs. In addition, the presence of HA increases the energy barrier and the CCC of the binary system (kaolin þ NPs). Thus, the complex interactions of clay, NPs, IS, pH, and HA concentration determine the colloidal stability of the NPs. We find that kaolin is a potential coagulant for removal of NPs that behave like Ag and TiO 2 .

Heteroaggregation of nanoparticles with biocolloids and geocolloids

“…Even though the measured surface charge is similar at pH 4 and pH 8, montmorillonite is much less stable at pH 4 (critical coagulation concentration (CCC) around 35 mM) than it is at pH 8 (CCC around 100 mM). In a recent study [97], we investigated the effect of kaolin particles on the aggregation of TiO 2 and Ag NPs under different conditions. We found that kaolin reduces the energy barrier and the CCC at pH 4, but it promotes NPs aggregation via heteroaggregation at pH 8.…”

Heteroaggregation of CeO2 and TiO2 engineered nanoparticles in the aqueous phase: Application of turbiscan stability index and fluorescence excitation-emission matrix (EEM) spectra

“…To detect the dominant effect of CeO 2 and TiO 2 ENPs on heteroaggregation, the experiments were conducted by mixing CeO 2 and TiO 2 ENPs stock solution with different mass ratios (0.1/9.9, 1/9, 5/5, 9/1, 9.9/0.1), for a total concentration of 10 mg/L (Our preliminary study showed [26,52,58] that 10 mg/L ENPs could more clearly indicate the aggregation behavior even if it may be unable to represent low environment concentration of ENPs like ng/L or μg/L). 5 mmol/L NaCl was used to provide IS.…”

“…Most heteroaggregation studies have used dynamic light scattering (DLS) and attachment efficiency for understanding these processes [14,26,27]. Dusak et al suggested that the mass ratios of multiwall carbon nanotubes (MCNTs) and hematite nanoparticles (Hem ENPs) (15 g/L) have influence on the heteroaggregation rates as measured by DLS and their zeta potential [28].…”

“…Ultraviolet-visible spectrophotometry is usually used to analyze homoaggregation of samples with characteristic peaks, such as nano-Ag, nano-ZnO, nano-Cu and nanoCeO 2 [40][41][42][43][44]. TSI is influenced by particle size and volume concentration of the system, and is also appropriate for measuring extreme unstable system compared with DLS, which can only reflect the changes of the particle size [45][46][47][48].…”

“…Particularly, NOM can adsorb onto the surface of TiO 2 and CeO 2 ENPs, form a coating layer, result in steric hinderance and electrostatic repulsion and have effect on the heteroaggregation of ENPs [8,26,51]. Luo et al [52] showed that the properties of humic acid (HA) and fulvic acid (FA) influence the adsorption and aggregation of TiO 2 ENPs (10 mg/L), indicating that NOM can adsorb onto the surface of TiO 2 ENPs.…”