Research interests:
In-situ microscopy of colloidal contaminants in water
We present the first real-time observations of the diffusion of individual asbestos fibers in water. We first scaled up a technique for fluorescent tagging and imaging of chrysotile-asbestos fibers, and prepared samples with a distribution of fiber lengths ranging from 1 to 20 um.
Experiments were then conducted by placing a 20, 100 or 150-ppm solution of these fibers in a liquid cell mounted on a spinning-disk confocal microscope. Using automated elliptical-particle detection methods, we determined the translation and rotation, and 2D trajectories, of thousands of diffusing chrysotile fibers. We find that fiber diffusion is size-dependent, and in reasonable agreement with theoretical predictions for the Brownian motion of rods. This agreement is remarkable given that experiments involved non-idealized particles at environmentally-relevant concentrations in a confined cell, in which particle-particle and particle-wall interactions might be expected to cause deviations from theory. Experiments also confirmed that highly-elongated chrysotile fibers exhibit anisotropic diffusion at short timescales, a predicted effect that may have consequences for aggregate formation and transport of asbestos in confined spaces. The examined fibers vary greatly in their lengths and were prepared from natural Chrysotile. Our findings thus indicate that the diffusion rates of a wide range of natural colloidal particles can be predicted from theory – so long as the particle aspect ratio is properly accounted for. This is an important first step for understanding aggregate formation and transport of non-spherical contaminant particles, in the environment and in vivo.
Experiments were then conducted by placing a 20, 100 or 150-ppm solution of these fibers in a liquid cell mounted on a spinning-disk confocal microscope. Using automated elliptical-particle detection methods, we determined the translation and rotation, and 2D trajectories, of thousands of diffusing chrysotile fibers. We find that fiber diffusion is size-dependent, and in reasonable agreement with theoretical predictions for the Brownian motion of rods. This agreement is remarkable given that experiments involved non-idealized particles at environmentally-relevant concentrations in a confined cell, in which particle-particle and particle-wall interactions might be expected to cause deviations from theory. Experiments also confirmed that highly-elongated chrysotile fibers exhibit anisotropic diffusion at short timescales, a predicted effect that may have consequences for aggregate formation and transport of asbestos in confined spaces. The examined fibers vary greatly in their lengths and were prepared from natural Chrysotile. Our findings thus indicate that the diffusion rates of a wide range of natural colloidal particles can be predicted from theory – so long as the particle aspect ratio is properly accounted for. This is an important first step for understanding aggregate formation and transport of non-spherical contaminant particles, in the environment and in vivo.
Single-stem efficiency theory of colloid filtration in dense emergent vegetation in overland flow
Experimental Analysis of Colloid Capture by a Cylindrical Collector in Laminar Overland Flow
Although colloid-facilitated contaminant transport in water flow is a well-known contamination process, little research has been conducted to investigate the transport of colloidal particles through emergent vegetation in overland flow. In this study, a series of laboratory experiments were conducted to measure the single-collector contact efficiency (η0) of colloid capture by a simulated plant stem in laminar lateral flow. Florescent microspheres of various sizes were used as experimental colloids. The colloid suspensions were applied to a glass cylinder installed in a small size flow chamber at different flow rates. Two cylinder sizes were tested in the experiment and silicone grease was applied to the cylinder surface to make it favorable for colloid deposition. Our results showed that increases in flow rate and collector size reduced the value of η0 and a minimum value of η0 might exist for a colloid size. The experimental data were compared to theoretical predictions of different single-collector contact efficiency models. The results indicated that existing single-collector contact efficiency models underestimated the η0 of colloid capture by the cylinders in laminar overland flow. A regression equation of η0 as a function of collector Reynolds number (Rec) and Peclet number (NPe) was developed and fitted the experimental data very well. This regression equation can be used to help construct and refine mathematical models of colloid transport and filtration in laminar overland flow on vegetated surfaces.
Although colloid-facilitated contaminant transport in water flow is a well-known contamination process, little research has been conducted to investigate the transport of colloidal particles through emergent vegetation in overland flow. In this study, a series of laboratory experiments were conducted to measure the single-collector contact efficiency (η0) of colloid capture by a simulated plant stem in laminar lateral flow. Florescent microspheres of various sizes were used as experimental colloids. The colloid suspensions were applied to a glass cylinder installed in a small size flow chamber at different flow rates. Two cylinder sizes were tested in the experiment and silicone grease was applied to the cylinder surface to make it favorable for colloid deposition. Our results showed that increases in flow rate and collector size reduced the value of η0 and a minimum value of η0 might exist for a colloid size. The experimental data were compared to theoretical predictions of different single-collector contact efficiency models. The results indicated that existing single-collector contact efficiency models underestimated the η0 of colloid capture by the cylinders in laminar overland flow. A regression equation of η0 as a function of collector Reynolds number (Rec) and Peclet number (NPe) was developed and fitted the experimental data very well. This regression equation can be used to help construct and refine mathematical models of colloid transport and filtration in laminar overland flow on vegetated surfaces.
Single collector attachment efficiency of colloid capture by a cylindrical collector in laminar overland flow
Little research has been conducted to investigate fate and transport of colloids in shallow overland flow through dense vegetation under unfavorable chemical conditions. In this work, the single collector attachment efficiency (α) of colloid capture by a simulated plant stem (i.e. cylindrical collector) in laminar overland flow was measured directly in laboratory flow chamber experiments. Fuorescent microspheres of two sizes were used as experimental colloids. The colloid suspensions flowed towards a glass cylindrical rod installed in a small size flow channel at different laminar flow rates. Different solution ionic strengths (IS) were used in the experiments to simulate unfavorable attachment conditions. Our results showed that α increased with IS and decreased with flow velocity. Existing theoretical and empirical models of colloid attachment efficiency for porous media were used to simulate the experimental measurements of α and found to fall short in matching the experimental data. A new dimensionless (regression) equation was proposed that predicts the α of colloid capture by a cylindrical collector in laminar overland flow with reasonable accuracy. In addition, the equation was also effective in predicting the attachment efficiency of colloid deposition in porous media.
Little research has been conducted to investigate fate and transport of colloids in shallow overland flow through dense vegetation under unfavorable chemical conditions. In this work, the single collector attachment efficiency (α) of colloid capture by a simulated plant stem (i.e. cylindrical collector) in laminar overland flow was measured directly in laboratory flow chamber experiments. Fuorescent microspheres of two sizes were used as experimental colloids. The colloid suspensions flowed towards a glass cylindrical rod installed in a small size flow channel at different laminar flow rates. Different solution ionic strengths (IS) were used in the experiments to simulate unfavorable attachment conditions. Our results showed that α increased with IS and decreased with flow velocity. Existing theoretical and empirical models of colloid attachment efficiency for porous media were used to simulate the experimental measurements of α and found to fall short in matching the experimental data. A new dimensionless (regression) equation was proposed that predicts the α of colloid capture by a cylindrical collector in laminar overland flow with reasonable accuracy. In addition, the equation was also effective in predicting the attachment efficiency of colloid deposition in porous media.
Extended single stem efficiency theory of colloid filtration in surface dense vegetation
Understanding colloid and colloid-facilitated contaminant transport in overland flow through dense vegetation is essential to protect water quality for the environment. In previous studies, a single-stem efficiency theory for rigid and clean stem systems has been developed to predict colloid filtration by plant stems of vegetation in laminar overland flow. Hence, in order to improve the accuracy of the single-stem efficiency theory to real dense vegetation system, a new dimensionless number was incorporated into that accounts for the effect of plant surface properties on the filtration of colloids by stems. Laboratory dense vegetation flow chamber experiments and model simulations were used to determine the kinetic deposition (filtration) rate of colloids in the vegetation system under various conditions. The results showed that, in addition to flow hydrodynamics and solution chemistry, steric repulsion afforded by biopolymer brush player on the plants stem surface could also play a significant role in controlling colloid deposition on vegetation in overland flow. For the first time, an extended single-stem efficiency theory which considers the steric repulsion effect and describes the experimental data with good accuracy is developed. The extended theory can be used to not only help construct and refine mathematical models of colloid transport in real vegetation systems in overland flow but also inform the development of theories of colloid deposition on various polymer brush surfaces in natural, engineered, and biomedical systems.
Understanding colloid and colloid-facilitated contaminant transport in overland flow through dense vegetation is essential to protect water quality for the environment. In previous studies, a single-stem efficiency theory for rigid and clean stem systems has been developed to predict colloid filtration by plant stems of vegetation in laminar overland flow. Hence, in order to improve the accuracy of the single-stem efficiency theory to real dense vegetation system, a new dimensionless number was incorporated into that accounts for the effect of plant surface properties on the filtration of colloids by stems. Laboratory dense vegetation flow chamber experiments and model simulations were used to determine the kinetic deposition (filtration) rate of colloids in the vegetation system under various conditions. The results showed that, in addition to flow hydrodynamics and solution chemistry, steric repulsion afforded by biopolymer brush player on the plants stem surface could also play a significant role in controlling colloid deposition on vegetation in overland flow. For the first time, an extended single-stem efficiency theory which considers the steric repulsion effect and describes the experimental data with good accuracy is developed. The extended theory can be used to not only help construct and refine mathematical models of colloid transport in real vegetation systems in overland flow but also inform the development of theories of colloid deposition on various polymer brush surfaces in natural, engineered, and biomedical systems.
Deposition and aggregation of carbon nanotubes (CNTs) and graphene oxide in aquatic system
DLVO Interactions of Carbon Nanotubes with Isotropic Planar Surfaces
Knowledge of interaction between carbon nanotubes (CNTs) and planar surfaces is essential to optimize CNT applications as well as to reduce their environmental impacts. In this work, the surface element integration (SEI) technique was coupled with the DLVO theory to determine the orientation-dependent interaction energy between a single-walled carbon nanotube (SWNT) and an infinite isotropic planar surface. For the first time, an analytical formula was developed to accurately describe the interaction between not only pristine but also surface charged CNTs and planar surfaces with arbitrary rotation angles. Compared to other methods, the new analytical formulas were either more convenient or more accurate to describe the interaction between CNTs and planar surface especially with respect to arbitrary angles. The results revealed complex dependences of both force and torque between SWNTs and planar surfaces on the separation distances and rotation angles. With minor modifications, the analytical formulas derived for SWNTs can also be applied to multi-walled carbon nanotubes (MWNTs). The new analytical expressions presented in this work can be used as a robust tool to describe the DLVO interaction between CNTs and planar surfaces under various conditions and thus to assist the design and application of CNT-based products.
Knowledge of interaction between carbon nanotubes (CNTs) and planar surfaces is essential to optimize CNT applications as well as to reduce their environmental impacts. In this work, the surface element integration (SEI) technique was coupled with the DLVO theory to determine the orientation-dependent interaction energy between a single-walled carbon nanotube (SWNT) and an infinite isotropic planar surface. For the first time, an analytical formula was developed to accurately describe the interaction between not only pristine but also surface charged CNTs and planar surfaces with arbitrary rotation angles. Compared to other methods, the new analytical formulas were either more convenient or more accurate to describe the interaction between CNTs and planar surface especially with respect to arbitrary angles. The results revealed complex dependences of both force and torque between SWNTs and planar surfaces on the separation distances and rotation angles. With minor modifications, the analytical formulas derived for SWNTs can also be applied to multi-walled carbon nanotubes (MWNTs). The new analytical expressions presented in this work can be used as a robust tool to describe the DLVO interaction between CNTs and planar surfaces under various conditions and thus to assist the design and application of CNT-based products.
Aggregation Kinetics of Aqueous Suspensions of Graphene Oxide: Measurement, Mechanism and Modeling
While significant improvements have been made in the applications of graphene oxide (GO), its environmental fate and behaviors are still unclear. In this study, the aggregation kinetics of GO sheet were investigated using time-resolved dynamic light scattering under different solution chemistry conditions (e.g., cation valence and pH). Both cation valence and pH had significant effect on the GO aggregation kinetics. The calculated critical coagulation concentrations were in good agreement with the empirical extension of Schulze-Hardy rule. Ca2+ and Mg2+ are more efficient than Na+ in GO sheet aggregation due to the chemical cross-linking between GO sheets through specific chemical interactions between the functional groups of the GO sheets and the divalent metal ions. Deprotonation of carboxylic groups was found to play key role in increasing GO sheet stability and surface charge development when pH increase. Edge to edge and face to face were the dominant interaction modes of GO aggregation in the presence of divalent metal ions and H+, respectively. Based on the combination of Maxwell-Boltzman distribution and DLVO theory, attachment efficiency (α) equation was modified and found to be effective in prediction of the ionic strength and pH effect on the GO sheet aggregation kinetics. Overall, the modified model provided an alternative theoretical approach for predicting attachment efficiency in GO sheet aggregation kinetics studies.
While significant improvements have been made in the applications of graphene oxide (GO), its environmental fate and behaviors are still unclear. In this study, the aggregation kinetics of GO sheet were investigated using time-resolved dynamic light scattering under different solution chemistry conditions (e.g., cation valence and pH). Both cation valence and pH had significant effect on the GO aggregation kinetics. The calculated critical coagulation concentrations were in good agreement with the empirical extension of Schulze-Hardy rule. Ca2+ and Mg2+ are more efficient than Na+ in GO sheet aggregation due to the chemical cross-linking between GO sheets through specific chemical interactions between the functional groups of the GO sheets and the divalent metal ions. Deprotonation of carboxylic groups was found to play key role in increasing GO sheet stability and surface charge development when pH increase. Edge to edge and face to face were the dominant interaction modes of GO aggregation in the presence of divalent metal ions and H+, respectively. Based on the combination of Maxwell-Boltzman distribution and DLVO theory, attachment efficiency (α) equation was modified and found to be effective in prediction of the ionic strength and pH effect on the GO sheet aggregation kinetics. Overall, the modified model provided an alternative theoretical approach for predicting attachment efficiency in GO sheet aggregation kinetics studies.
Analytical and experimental analysis of solute transport in heterogeneous porous media.Knowledge of solute transport in heterogeneous porous media is crucial to monitor contaminant fate and transport in soil and groundwater systems. In this study, we present new findings from experimental and mathematical analysis to improve current understanding of solute transport in structured heterogeneous porous media. Three saturated columns packed with different sand combinations were used to examine the breakthrough behavior of bromide, a conservative tracer. Experimental results showed that bromide had different breakthrough responses in the three types of sand combinations, indicating that heterogeneity in hydraulic conductivity has a significant effect on the solute transport in structured heterogeneous porous media. Simulations from analytical solutions of a two-domain solute transport model matched experimental breakthrough data well for all the experimental conditions tested. Experimental and model results show that under saturated flow conditions, advection dominates solute transport in both fast-flow and slow-flow domains. The sand with larger hydraulic conductivity provided a preferential flow path for solute transport (fast-flow domain) that dominates the mass transfer in the heterogeneous porous media. Importantly, the transport in the slow-flow domain and mass exchange between the domains also contribute to the flow and solute transport processes and thus must be considered when investigating contaminant transport in heterogeneous porous media.