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A modelingframework is presented to determine fundamental parametersand controlling mechanisms of colloid (microbes, clays, and nanoparticles)retention and release on surfaces of porous media that exhibit widedistributions of nanoscale chemical heterogeneity, nano- to microscaleroughness, and pore water velocity. Primary and/or secondary minimuminteractions in the zone of electrostatic influence were determinedover the heterogeneous solid surface. The Maxwellian kinetic energymodel was subsequently employed to determine the probability of immobilizationand diffusive release of colloids from each of these minima. In addition,a balance of applied hydrodynamic and resisting adhesive torques wasconducted to determine locations of immobilization and hydrodynamicrelease in the presence of spatially variable water flow and microscopicroughness. Locations for retention had to satisfy both energy andtorque balance conditions for immobilization, whereas release couldoccur either due to diffusion or hydrodynamics. Summation of energyand torque balance results over the elementary surface area of theporous medium provided estimates for colloid retention and releaseparameters that are critical to predicting environmental fate, includingthe sticking and release efficiencies and the maximum concentrationof retained colloids on the solid phase. Nanoscale roughness and chemicalheterogeneity produced localized primary minimum interactions thatcontrolled long-term retention, even when mean chemical conditionswere unfavorable. Microscopic roughness played a dominant role incolloid retention under low ionic strength and high hydrodynamic conditions,especially for larger colloids. [ABSTRACT FROM AUTHOR]

Transport of colloidal particles in porous media is governed by the rate at which the colloids strike and stick to collector surfaces. Classic filtration theory has considered the influence of system hydrodynamics on determining the rate at which colloids strike collector surfaces, but has neglected the influence of hydrodynamic forces in the calculation of the collision efficiency. Computational simulations based on the sphere-in-cell model were conducted that considered the influence of hydrodynamic and Derjaguin−Landau−Verwey−Overbeek (DLVO) forces on colloid attachment to collectors of various shape and size. Our analysis indicated that hydrodynamic and DLVO forces and collector shape and size significantly influenced the colloid collision efficiency. Colloid attachment was only possible on regions of the collector where the torque from hydrodynamic shear acting on colloids adjacent to collector surfaces was less than the adhesive (DLVO) torque that resists detachment. The fraction of the collector surface area on which attachment was possible increased with solution ionic strength, collector size, and decreasing flow velocity. Simulations demonstrated that quantitative evaluation of colloid transport through porous media will require nontraditional approaches that account for hydrodynamic and DLVO forces as well as collector shape and size. [ABSTRACT FROM AUTHOR]