The ever-increasing use of engineered nanomaterials will lead to heightened levels of these materials in the environment. The present review aims to provide a comprehensive overview of current knowledge regarding nanoparticle transport and aggregation in aquatic environments. Nanoparticle aggregation and deposition behavior will dictate particle transport potential and thus the environmental fate and potential ecotoxicological impacts of these materials. In this review, colloidal forces governing nanoparticle deposition and aggregation are outlined. Essential equations used to assess particle-particle and particle-surface interactions, along with Hamaker constants for specific nanoparticles and the attributes exclusive to nanoscale particle interactions, are described. Theoretical and experimental approaches for evaluating nanoparticle aggregation and deposition are presented, and the major findings of laboratory studies examining these processes are also summarized. Finally, we describe some of the challenges encountered when attempting to quantify the transport of nanoparticles in aquatic environments.

Recent reports underline the potential environmental and public health risks linked to the "nano" revolution, yet little is known regarding the environmental fate and impacts of most nanomaterials following release in natural soils and groundwaters. Quantum dots (QDs) are one example of engineered nanomaterials that have been demonstrated to exhibit cytotoxic effects; hence the fate of this material in aqueous environments is of particular interest. In this study, a quartz crystal microbalance (QCM) was used to examine the interaction of a commercially available carboxyl terminated CdTe QD with a model sand (i.e., silica) surface. The deposition kinetics of the QD onto clean silica coated QCM crystals were measured over a wide range of solution conditions, in the presence of either monovalent (K + ) or divalent cations (Ca 2+ ). QD deposition rates onto silica were significantly greater in the presence of calcium versus potassium. Solution pH also influenced QD deposition behavior, with increased deposition observed at a lower pH value. The rate of QD release from the silica surface was also monitored using QCM measurements and found to be comparable to the rate of particle deposition when the monovalent salt was used. In contrast, the rate of QD release was considerably lower than the rate of deposition when particles were deposited in the presence of Ca 2+ . Physicochemical characterization of the QD suspended in varying electrolytes provided insights into the role of solution chemistry on particle size and electrophoretic mobility (surface charge). Measurements of QD size using dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to interpret the QD deposition behavior in different solution chemistries. Lower particle deposition rates observed at high ionic strengths were attributed to aggregation of the QDs resulting in decreased convective-diffusive transport to the silica surface.

A quartz crystal microbalance with dissipation (QCM-D) monitoring can be an alternative tool to characterize nanoparticle size by virtue of its acoustic principle to sense adsorbed mass. In this study, sizes obtained by QCM-D for polymer-coated metallic nanoparticles and polydisperse polystyrene latex particle suspensions were compared with those obtained by transmission electron microscopy (TEM) and dynamic light scattering (DLS). We describe the obtained "QCM-D mass", which is weighted over surface area, by a general particle height distribution equation that can be used to determine the average particle diameter of a distribution of particles deposited on the QCM-D surface. Because the particle height distribution equation can be used for any particle geometry and surface packing geometry, it is described how the QCM-D can also be used to study the orientation of deposited nonspherical particles. Herein, the mean nanoparticle sizes obtained by QCM-D were generally in closer agreement with the primary particle size determined by TEM than with the sizes obtained by DLS, suggesting that primarily smaller particles within the particle population deposited on the sensor surface. Overall, the results from this study demonstrate that QCM-D could serve as an alternative and/or complementary means to characterize the size of nanoparticles deposited on a surface from suspensions of varying complexity.

A clear understanding of the factors controlling the deposition behavior of engineered nanoparticles (ENPs), such as quantum dots (QDs), is necessary for predicting their transport and fate in natural subsurface environments and in water filtration processes. A quartz crystal microbalance with dissipation monitoring (QCM-D) was used to study the effect of particle surface coatings and water chemistry on the deposition of commercial QDs onto Al2O3. Two carboxylated QDs (CdSe and CdTe) with different surface coatings were compared with two model nanoparticles: sulfate-functionalized (sPL) and carboxyl-modified (cPL) polystyrene latex. Deposition rates were assessed over a range of ionic strengths (IS) in simple electrolyte (KCl) and in electrolyte supplemented with two organic molecules found in natural waters; namely, humic acid and rhamnolipid. The Al2O3 collector used here is selected to be representative of oxide patches found on the surface of aquifer or filter grains. Deposition studies showed that ENP deposition rates on bare Al2O3 generally decreased with increasing salt concentration, with the exception of the polyacrylic-acid (PAA) coated CdTe QD which exhibited unique deposition behavior due to changes in the conformation of the PAA coating. QD deposition rates on bare Al2O3 were approximately 1 order of magnitude lower than those of the polystyrene latex nanoparticles, likely as a result of steric stabilization imparted by the QD surface coatings. Adsorption of humic acid or rhamnolipid on the Al2O3 surface resulted in charge reversal of the collector and subsequent reduction in the deposition rates of all ENPs. Moreover, the ratio of the two QCM-D output parameters, frequency and dissipation, revealed key structural information of the ENP-collector interface; namely, on bare Al2O3, the latex particles were rigidly attached as compared to the more loosely attached QDs. This study emphasizes the importance of considering the nature of ENP coatings as well as organic molecule adsorption onto particle and collector surfaces to avoid underestimating ENP mobility in natural and engineered aquatic environments.