Modeling colloid and microorganism transport and release with transients in solution ionic strength

The transport and fate of colloids, microorganisms, and nanoparticles in subsurface environments is strongly influenced by transients in solution ionic strength (IS). A sophisticated dual-permeability transport model was modified and a theory was developed to mechanistically account for the transport, retention, and release of colloids with transients in IS. In particular, colloid release in the model was directly related to the balance of applied hydrodynamic and resisting adhesive torques that determined the fraction of the solid surface area that contributed to colloid immobilization (S-f). The colloid sticking efficiency (alpha) and S-f were explicit functions of IS that determined the rates of colloid interaction with the solid, immobilization on the solid, colloid release from the solid and back into the bulk aqueous phase, and the maximum amount of colloid retention. The developed model was used to analyze experimental transport and release data with transients in IS for 1.1 and 0.11 mu m latex microspheres, E. coli D21g, and coliphage phi X174. Comparison of experimental values of Sf (IS) with predictions based on mean interaction energies indicated that predictions needed to account for the influence of physical and/or chemical heterogeneity on colloid immobilization. This was especially true for smaller colloids because they were more sensitive to microscopic heterogeneities that produced mainly irreversible interaction in a primary minimum and greater hysteresis in S-f (IS) with IS. Significant deviations between experimental and predicted values of alpha(IS) were observed for larger colloids when hydrodynamic forces were not accounted for in the predictions. A sensitivity analysis indicated that colloid release with IS transients was not diffusion controlled, but rather occurred rapidly and with low levels of dispersion. The calibrated model provided a satisfactory description of the observed release behavior for a range of colloid types and sizes and a general theoretical foundation to develop predictions for the influence of solution chemistry on the transport, retention, and release of colloids.