Research Initiation Award: Novel Approaches to Probing the Mechanism of Brownian Colloid Release from Packed Bed Surfaces

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ABSTRACT Joseph Ryan CTS-9410301 The transport of low-solubility contaminants like radionuclides, metals, and hydrophobic organic compounds is directly linked to the transport of colloids. The diffusion-driven release of Brownian colloids from porous media surfaces is influenced by the chemistry of the colloid and porous media surfaces, the solution chemistry (pH, ionic strength), the flow velocity, and the colloid and grain sizes. In the past, the kinetics of colloid release were modeled as a first-order exchange between attached and detached colloids in a manner analogous to Arrhenius kinetics. Under certain conditions, this approach has failed, owing to peculiarities in the calculation of the intersurface potential energy. Preliminary experimental results have indicated that colloid release is actually a two-step mechanism consisting of (1) detachment from the surface controlled by surface and solution chemistry and (2) diffusion across the boundary layer controlled by the effect of flow velocity diffusion coefficient. The experiments proposed here are designed to proof the two-step mechanism by attempting to isolate and quantify each step. The detachment step will be isolated by conducting experiments in non-moving fluid. The diffusion step will be isolated by removing the energy barrier. The results of the experiments will be analyzed to develop a quantitative kinetic model of colloid release. The project addresses environmental concerns in groundwaters.