The aggregation and deposition kinetics of fullerene C60 nanoparticles have been investigated over a wide range of monovalent and divalent electrolyte concentrations by employing time-resolved dynamic light scattering (DLS) and quartz crystal microbalance (QCM), respectively. Aggregation kinetics of the fullerene nanoparticles exhibited reaction-limited (slow) and diffusion-limited (fast) regimes in the presence of both electrolytes, having critical coagulation concentrations (CCC) of 120 and 4.8 mM for the monovalent (NaCl) and divalent (CaCl2) salts, respectively. The measured stability ratios of the aggregating fullerene nanoparticles were in very good agreement with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, with a derived Hamaker constant of 6.7 x 10-21 J for the fullerene nanoparticles in aqueous medium. For the deposition kinetics studies, the rate of fullerene nanoparticle deposition increased with increasing electrolyte concentrations, as was indicated in the aggregation kinetics results. However, at electrolyte concentrations approaching or exceeding the CCC, the rate of deposition dropped sharply due to significant concurrent aggregation of the fullerene nanoparticles. The deposition of the fullerene nanoparticles was further shown to be mostly irreversible, with immediate detachment of the nanoparticles observed only when exposed to a solution of high pH.

The aggregation kinetics of silver nanoparticles (AgNPs) that were coated with two commonly used capping agents—citrate and polyvinylpyrrolidone (PVP)—were investigated. Time-resolved dynamic light scattering (DLS) was employed to measure the aggregation kinetics of the AgNPs over a range of monovalent and divalent electrolyte concentrations. The aggregation behavior of citrate-coated AgNPs in NaCl was in excellent agreement with the predictions based on Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, and the Hamaker constant of citrate-coated AgNPs in aqueous solutions was derived to be 3.7 × 10-20 J. Divalent electrolytes were more efficient in destabilizing the citrate-coated AgNPs, as indicated by the considerably lower critical coagulation concentrations (2.1 mM CaCl2 and 2.7 mM MgCl2 vs. 47.6 mM NaCl). The PVP-coated AgNPs were significantly more stable than citrate-coated AgNPs in both NaCl and CaCl2, which is likely due to steric repulsion imparted by the large, non-charged polymers. The addition of humic acid resulted in the adsorption of the macromolecules on both citrate- and PVP-coated AgNPs. The adsorption of humic acid induced additional electrosteric repulsion that elevated the stability of both nanoparticles in suspensions containing NaCl or low concentrations of CaCl2. Conversely, enhanced aggregation occurred for both nanoparticles at high CaCl2 concentrations due to interparticle bridging by humic acid clusters.

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The early stage aggregation kinetics of bare and alginate-coated hematite nanoparticles are acquired through time-resolved dynamic light scattering (DLS). Varying concentrations of monovalent (NaCl) and divalent (MgCl2 and CaCl2) electrolytes are employed to induce aggregation. In the presence of NaCl and MgCl2, the alginate-coated hematite nanoparticles undergo aggregation through electrostatic destabilization as described by the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This is ascertained through examination of the favorable and unfavorable regimes of the stability curves depicting the attachment efficiency as a function of salt concentration. Additional evidence may be found in the aggregation kinetics of alginate-coated particles, which, under favorable aggregation conditions, are reasonably close to that of bare hematite nanoparticles. However, in the presence of CaCl2, the aggregate growth rate of alginate-coated hematite nanoparticles is much higher than that which conventional diffusive aggregation predicts. Dispersed hematite primary particles and lower-order aggregates enmeshed within extended alginate gel networks were observed under transmission electron microscope (TEM). The proposed mechanism for enhanced aggregation suggests an apparent increase in the collision radii of alginate-coated hematite nanoparticles through alginate gel network formation from the particle surface. Additionally, cross-linking between unadsorbed (suspended) alginate macromolecules may form bridges between hematite-alginate gel clusters. It is further established that the presence of background electrolyte NaCl in solution is detrimental to the calcium-induced enhanced aggregation.

Carbon nanotubes (CNTs) are currently incorporated into various consumer products, and numerous new applications and products containing CNTs are expected in the future. The potential for negative effects caused by CNT release into the environment is a prominent concern and numerous research projects have investigated possible environmental release pathways, fate, and toxicity. However, this expanding body of literature has not yet been systematically reviewed. Our objective is to critically review this literature to identify emerging trends as well as persistent knowledge gaps on these topics. Specifically, we examine the release of CNTs from polymeric products, removal in wastewater treatment systems, transport through surface and subsurface media, aggregation behaviors, interactions with soil and sediment particles, potential transformations and degradation, and their potential ecotoxicity in soil, sediment, and aquatic ecosystems. One major limitation in the current literature is quantifying CNT masses in relevant media (polymers, tissues, soils, and sediments). Important new directions include developing mechanistic models for CNT release from composites and understanding CNT transport in more complex and environmentally realistic systems such as heteroaggregation with natural colloids and transport of nanoparticles in a range of soils.

The deposition kinetics of fullerene (C60) nanoparticles onto bare silica surfaces and surfaces precoated with humic acid and alginate are investigated over a range of monovalent (NaCI) and divalent (CaCl2) salt concentrations using a quartz crystal microbalance. Because simultaneous aggregation of the fullerene nanoparticles occurs, especially at higher electrolyte concentrations, we normalize the observed deposition rates by the corresponding favorable (transport-limited) deposition rates to obtain the attachment efficiencies, alpha. The deposition kinetics of fullerene nanoparticles onto bare silica surfaces are shown to be controlled by electrostatic interactions and van der Waals attraction, consistent with the classical particle deposition behavior where both favorable and unfavorable deposition regimes are observed. The presence of dissolved humic acid and alginate in solution leads to significantly slower deposition kinetics due to steric repulsion. Precoating the silica surfaces with humic acid and alginate exerts similar steric stabilization in the presence of NaCl. In the presence of CaCl2, the deposition kinetics of fullerene nanoparticles onto both humic acid- and alginate-coated surfaces are relatively high, even at relatively low (0.3 mM) calcium concentration. This behavior is attributed to the macromolecules undergoing complex formation with calcium ions, which reduces the charge and steric influences of the adsorbed macromolecular layers.

The aggregation and deposition kinetics of two multiwalled carbon nanotubes (MWNTs) with different degrees of surface oxidation are investigated using time-resolved dynamic light scattering (DLS) and quartz crystal microbalance with dissipation monitoring (QCM-D), respectively. Carboxyl groups are determined to be the predominant oxygen-containing surface functional groups for both MWNTs through X-ray photoelectron spectroscopy (XPS). The aggregation and deposition behavior of both MWNTs is in qualitative agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) of the highly oxidized MWNTs (HO-MWNTs) is significantly higher than the lowly oxidized MWNTs (LO-MWNTs) in the presence of NaCl (210 and 53 mM, respectively) since HO-MWNTs have a higher surface charge density. In contrast, the aggregation inverse stability profiles of HO-MWNTs and LO-MWNTs are identical and yield comparable CCCs (0.9 and 1.0 mM, respectively) in the presence of CaCl(2). Similar to the results obtained from the aggregation study, HO-MWNTs are considerably more stable to deposition on silica surfaces compared to LO-MWNTs in the presence of NaCl. However, both MWNTs have the same propensity to undergo deposition in the presence of CaCl(2). The remarkable similarity in the aggregation and deposition kinetics of HO-MWNTs and LO-MWNTs in CaCl(2) may be due to Ca(2+) cations having a higher affinity to form complexes with adjacent carboxyl groups on HO-MWNTs than with isolated carboxyl groups on LO-MWNTs.

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