Abstract
Emerging nanomaterials are being manufactured with varying particle sizes, morphologies, and crystal structures in the pursuit of achieving outstanding functional properties. These variations in these key material properties of nanoparticles may affect their environmental fate and transport. To date, few studies have investigated this important aspect of nanoparticles' environmental behavior. In this study, the aggregation kinetics of ten different TiO2 nanoparticles (5 anatase and 5 rutile each with varying size) was systematically evaluated. Our results show that, as particle size increases, the surface charge of both anatase and rutile TiO2 nanoparticles shifts toward a more negative value, and, accordingly, the point of zero charge shifts toward a lower value. The colloidal stability of anatase sphere samples agreed well with DLVO theoretical predictions, where an increase in particle size led to a higher energy barrier and therefore greater critical coagulation concentration. In contrast, the critical coagulation concentration of rutile rod samples correlated positively with the specific surface area, i.e., samples with higher specific surface area exhibited higher stability. Finally, due to the large innate negative surface charge of all the TiO2 samples at the pH value (pH = 8) tested, the addition of natural organic matter was observed to have minimal effect on TiO2 aggregation kinetics, except for the smallest rutile rods that showed decreased stability in the presence of natural organic matter.
Highlights
Given the accelerating production of existing and emerging engineered nanoparticles (ENPs), the accidental spill and usephase or end-of-product-life release of nanoparticles into the environment may be inevitable [1,2,3]
We report here the distinctly different aggregation behaviors of a set of TiO2 nanoparticles with varying size, crystal structure, and morphology
The isoelectric points of both anatase spheres and rutile rods shift towards a lower pH value as the particle size increases
Summary
Given the accelerating production of existing and emerging engineered nanoparticles (ENPs), the accidental spill and usephase or end-of-product-life release of nanoparticles into the environment may be inevitable [1,2,3]. The effect of solution chemistry on the aggregation of mostly spherical ENPs has been extensively studied [13,14,15,16,17,18,19] and is relatively well understood. PH alters the colloidal stability of the ENPs system by modulating the protonation/deprotonation equilibrium and further altering the electrostatic repulsion [13,17]. The presence of natural organic matter, depending on the concentration, can either stabilize nanoparticles by providing additional electrostatic repulsion and/or steric hindrance, or bridge multiple particles and enhance aggregation [16,19]. Our recent study revealed that natural clay minerals can coagulate either positively or negatively charged nanoparticles due to their edge-face charge heterogeneity [22]
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