Influence of Ionic Strength, pH, and Cation Valence on Aggregation Kinetics of Titanium Dioxide Nanoparticles

Abstract

The extensive use of titanium dioxide nanoparticles (nano-TiO2) in many consumer products has raised concerns about possible risks to the environment The magnitude of the threat may depend on whether nano-TiO2 remains dispersed in the environment, or forms much larger-sized aggregates or clusters. Currently, limited information is available on the issue. In this context, the purpose of the present article is to report initial measurements of the morphology and rate of formation of nano-TiO2 aggregates in aqueous suspensions as a function of ionic strength and of the nature of the electrolyte in a moderately acid to circumneutral pH range typical of soil and surface water conditions. Dynamic light scattering results show that 4-5 nm titanium dioxide particles readily form stable aggregates with an average diameter of 50-60 nm at pH approximately 4.5 in a NaCl suspension adjusted to an ionic strength of 0.0045 M. Holding the pH constant but increasing the ionic strength to 0.0165 M, leads to the formation of micron-sized aggregates within 15 min. At all other pH values tested (5.8-8.2), micron-sized aggregates form in less than 5 min (minimum detection time), even at low ionic strength (0.0084-0.0099 M with NaCl). In contrast, micron-sized aggregates form within 5 min in an aqueous suspension of CaCl2 at an ionic strength of 0.0128 M and pH of 4.8, which is significantly faster than observed for NaCI suspensions with similar ionic strength and pH. This result indicates that divalent cations may enhance aggregation of nano-TiO2 in soils and surface waters. Optical micrographs show branching aggregates of sizes ranging from the 1 microm optical limit of the microscope to tens of micrometers in diameter.

Environmental and biological influences on the stability of silver nanoparticles

“…These findings indicate that Ag NPs will tend to aggregate in high ionic strength and valence solutions, and lose their nanoscale properties. This observation is consistent with an earlier study on nano-TiO 2 [22].…”

Metal‐based nanoparticles in soil: Fate, behavior, and effects on soil invertebrates

“…Nanoparticles sorbed less strongly to soils of low ionic strength and high dissolved organic matter content, suggesting that these factors may affect the bioavailability of metal-based NPs in soils, assuming that bioavailability is related to the particle fraction suspended in the pore water and not the fraction associated with the soil matrix. Other studies showed that aggregation of TiO 2 NPs occurred at an ionic strength higher than 4.5 mM [62] and that increasing ionic strength from 1 to 100 mM caused a 50-fold increase in TiO 2 NP diameter [36].…”

“…In these studies, the particles are typically found to be agglomerated/aggregated, for example, TiO 2 [53,62,65,66], ZnO [43,53,[66][67][68], Ag [23,69], SiO 2 [70], and CeO 2 [53]. Manzo et al [71] analyzed an ZnO NP-contaminated soil by the Brunauer-Emmett-Teller method and found that the particles were not aggregated.…”

“…Indeed, NPs will dissolve partly and form aggregates and agglomerates. Kinetics experiments have shown TiO 2 NPs to be very stable after forming aggregates in water and soil solution [62]. For such long-term stable NPs, bioaccumulation may occur, because NPs may be readily taken up if present in the soil pore water [64].…”

Stability of nTiO 2 particles and their attachment to sand: Effects of humic acid at different pH

“…nTiO2 normally exist as aggregates in water instead of individual nanoscale particles (Chen et al, 2012;French et al, 2009;Lin et al, 2010;Lee et al, 2015). Aggregation of nTiO2 are influenced by factors such as pH, ionic strength, surfactants and natural organic matters (NOM) such as humic acid (HA) (Hsu and Chang, 2000;Tkachenko et al, 2006, Dunphy Guzman et al, 2006Fang et al, 2009;French et al, 2009, Godinez andDarnault, 2011).…”