J. Aziz

Bio: J. Aziz is an academic researcher from Rice University. The author has contributed to research in topics: Collision frequency & Fractal dimension. The author has an hindex of 1, co-authored 1 publications receiving 72 citations.

Flocs Restructuring during Aggregation: Experimental Evidence and Numerical Simulation

Abstract: Aggregate structure, i.e., fractal dimension, has a coupled geometric and hydrodynamic impact on the aggregate collision frequencies. In this work, we evaluated different collision frequency kernel calculation schemes to simulate experimentally determined latex particle aggregation kinetics. Light scattering was used to simultaneously measure aggregate size and fractal dimension throughout the latex aggregation experiments. The intermediate collision scheme proposed by Veerepanneni and Wiesner ( J. Colloid Interface Sci. 177, 45–57 (1996)) is in agreement with experimental aggregation kinetics if considered along with experimentally determined fractal dimension evolution. Aggregate restructuring was found to be an important mechanism in explaining aggregation kinetics.

Development of Environmental Fate Models for Engineered Nanoparticles—A Case Study of TiO2 Nanoparticles in the Rhine River

Abstract: For a proactive risk assessment of engineered nanoparticles (ENPs) it is imperative to derive predicted environmental concentration (PEC) values for ENPs in different environmental compartments; PECs can then be compared to effect thresholds. From the basis of established multimedia environmental fate models for organic pollutants, we develop a new concept of environmental fate modeling for ENPs with process descriptions based on the specific properties of ENPs. Our new fate modeling framework is highly flexible and can be adjusted to different ENPs and various environmental settings. As a first case study, the fate and transport of TiO2 NPs in the Rhine River is investigated. Predicted TiO2 NP concentrations lie in the ng/L range in the water compartment and mg/kg in the sediment, which represents the main reservoir for the nanoparticles. We also find that a significant downstream transport of ENPs is possible. A fundamental process, the heteroaggregation between TiO2 NPs and suspended particulate matter...

Heteroaggregation of Titanium Dioxide Nanoparticles with Model Natural Colloids under Environmentally Relevant Conditions

Abstract: The heteroaggregation of engineered nanoparticles (ENPs) with natural colloids (NCs), which are ubiquitous in natural surface waters, is a crucial process affecting the environmental transport and fate of ENPs. Attachment efficiencies for heteroaggregation, αhetero, are required as input parameters in environmental fate models to predict ENP concentrations and contribute to ENP risk assessment. Here, we present a novel method for determining αhetero values by using a combination of laser diffraction measurements and aggregation modeling based on the Smoluchowski equation. Titanium dioxide nanoparticles (TiO2 NPs, 15 nm) were used to demonstrate this new approach together with larger silicon dioxide particles (SiO2, 0.5 ?m) representing NCs. Heteroaggregation experiments were performed at different environmentally relevant solution conditions. At pH 5 the TiO2 NPs and the SiO2 particles are of opposite charge, resulting in αhetero values close to 1. At pH 8, where all particles are negatively charged, αhetero was strongly affected by the solution conditions, with αhetero ranging from <0.001 at low ionic strength to 1 at conditions with high NaCl or CaCl2 concentrations. The presence of humic acid stabilized the system against heteroaggregation.

Heteroaggregation of titanium dioxide nanoparticles with natural clay colloids.

Abstract: To better understand and predict the fate of engineered nanoparticles in the water column, we assessed the heteroaggregation of TiO2 nanoparticles with a smectite clay as analogues for natural colloids. Heteroaggregation was evaluated as a function of water salinity (10(-3) and 10(-1) M NaCl), pH (5 and 8), and selected nanoparticle concentration (0-4 mg/L). Time-resolved laser diffraction was used, coupled to an aggregation model, to identify the key mechanisms and variables that drive the heteroaggregation of the nanoparticles with colloids. Our data show that, at a relevant concentration, nanoparticle behavior is mainly driven by heteroaggregation with colloids, while homoaggregation remains negligible. The affinity of TiO2 nanoparticles for clay is driven by electrostatic interactions. Opposite surface charges and/or high ionic strength favored the formation of primary heteroaggregates via the attachment of nanoparticles to the clay. The initial shape and dispersion state of the clay as well as the nanoparticle/clay concentration ratio also affected the nature of the heteroaggregation mechanism. With dispersed clay platelets (10(-3) M NaCl), secondary heteroaggregation driven by bridging nanoparticles occurred at a nanoparticle/clay number ratio of greater than 0.5. In 10(-1) M NaCl, the clay was preaggregated into larger and more spherical units. This favored secondary heteroaggregation at lower nanoparticle concentration that correlated to the nanoparticle/clay surface area ratio. In this latter case, a nanoparticle to clay sticking efficiency could be determined.