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Title: Determining parameters and mechanisms of colloid retention and release in porous media

Author
item Bradford, Scott
item TORKZABAN, SAEED - Commonwealth Scientific And Industrial Research Organisation (CSIRO)

Submitted to: Langmuir
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2015
Publication Date: 10/20/2015
Citation: Bradford, S.A., Torkzaban, S. 2015. Determining parameters and mechanisms of colloid retention and release in porous media. Langmuir. doi: 10.1021/acs.langmuir.5b03080.

Interpretive Summary: An understanding and ability to predict the transport of colloids in soils and aquifer material is needed for many environmental and industrial applications, including assessing the risks of pathogenic microorganisms. A computer program was developed to determine fundamental parameters (sticking and release efficiencies, and the maximum concentration of retained colloids) and mechanisms controlling colloid retention and release in porous media. The roles of nanoscale chemical heterogeneity, nanoscale roughness, microscopic roughness, solution ionic strength, pore water velocity, grain size, and colloid size on retention and release were systematically investigated with the model to provide insight on controlling mechanisms and sites for retention and release. This information will be of interest to scientists and engineers concerned with predicting the risks of colloid transport in the environment.

Technical Abstract: A framework is presented to determine fundamental parameters and mechanisms controlling colloid (including microbes and nanoparticles) retention and release on hypothetical porous medium surfaces that exhibit distributions of nanoscale chemical heterogeneity, nano- to microscale roughness, and spatial variations in water velocity. Primary and/or secondary minimum interactions in the zone of electrostatic influence were determined over the heterogeneous solid surface. The Maxwellian kinetic energy model was subsequently employed to determine the probability of immobilization and diffusive release of colloids from each of these locations. In addition, a balance of applied hydrodynamic and resisting adhesive torques was conducted to determine locations of immobilization and hydrodynamic release in the presence of spatially variable water flow and microscopic roughness. Locations for retention had to satisfy both energy and torque balance conditions for immobilization, whereas release could occur either due to diffusion or hydrodynamics. Summation of energy and torque balance results over the porous medium surface provided estimates for colloid retention and release parameters that are critical to predicting environmental fate, including: the sticking and release efficiencies, and the maximum concentration of retained colloids on the solid phase. The roles of nanoscale heterogeneity, microscopic roughness, solution ionic strength, pore water velocity, and colloid radius on colloid retention and release were subsequently investigated to provide insight on controlling mechanisms and sites for retention. Nanoscale roughness and chemical heterogeneity produced many primary minimum interactions even when mean conditions were unfavorable that controlled long-term immobilization. Microscopic roughness played a dominant role in initial colloid immobilization under low ionic strength and high hydrodynamic conditions, especially for larger colloids.