Title

Modeling the mechanics of colloid detachment in environmental systems

Date of Completion

January 1999

Keywords

Engineering, Chemical|Environmental Sciences|Engineering, Environmental

Degree

Ph.D.

Abstract

Colloids may become detached from surfaces in environmental systems if the conditions are favorable. These mobile colloids may subsequently facilitate transport of hydrophobic contaminants. In this work, a mathematical construct is presented to quantitatively predict the thermodynamics and hydrodynamics inducing colloid detachment from surfaces. This technique was utilized to accurately quantify polystyrene latex colloid detachment from a model porous media, and natural colloids from soil during batch mixing. Subsequently, the effect of generated colloids on the apparent aqueous concentration of hydrophobic compounds was predicted. ^ Detachment of polystyrene latex colloids from a packed column of glass beads was exceptionally well-predicted with a physicochemically-based model. The polystyrene colloids were attached and subsequently detached by varying the solution chemistry and fluid shear rate. Extended-DLVO theory was used to determine repulsive interaction energies in porous media, which were experimentally found to follow a gamma distribution indicative of heterogeneous surface sites. Hydrodynamic conditions favorable for detachment were ascertained through a force and moment balance approach. Upon detachment, the applied rolling moment from hydrodynamic shear overcomes the resistance to rolling, which was characterized with an energy hysteresis loss factor. ^ To apply these concepts to natural media, batch testing was used to provide a uniform hydrodynamic shear field on heterogeneous soil grains. Natural colloids were generated during agitation in batch leach testing, and force and moment balances were utilized to explain the detachment of these colloids. The desorption of hydrophobic contaminants from these mobilized colloids was mathematically coupled with desorption from the soil grains in the batch leach test to predict increases in apparent aqueous concentration with mixing time. ^ This technique will have utility in the many engineering applications where fluid/particle separation is expected in dispersive energy fields, such as: filtration, ion-exchange, activated carbon adsorption, and catalytic reactors. This work also has serious implications for the impact of colloids on the apparent aqueous concentration of hydrophobic compounds. ^