Escherichia coli transport and release experiments were conducted to investigate the pore-water velocity (v) dependency of the sticking efficiency (α), the fraction of the solid surface area that contributed to retention (Sf), the percentage of injected cells that were irreversibly retained (Mirr), and cell release under different (10-300mM) ionic strength (IS) conditions. Values of α, Sf, and Mirr increased with increasing IS and decreasing v, but the dependency on v was greatest at intermediate IS (30 and 50mM). Following the retention phase, successive increases in v up to 100 or 150mday-1 and flow interruption of 24h produced negligible amounts of cell release. However, excavation of the sand from the columns in excess electrolyte solution resulted in the release of >80% of the retained bacteria. These observations were explained by: (i) extended interaction energy calculations on a heterogeneous sand collector; (ii) an increase in adhesive strength with the residence time; and (iii) torque balance consideration on rough surfaces. In particular, α, Sf, and Mirr increased with IS due to lower energy barriers and stronger primary minima. The values of α, Sf, and Mirr also increased with decreasing v because the adhesive strength increased with the residence time (e.g., an increased probability to diffuse over the energy barrier) and lower hydrodynamic forces diminished cell removal. The controlling influence of lever arms at microscopic roughness locations and grain-grain contacts were used to explain negligible cell removal with large increases in v and large amounts of cell recovery following sand excavation. Results reveal the underlying causes (interaction energy, torque balance, and residence time) of the velocity dependency of E. coli retention and release parameters (ksw, α, and Sf) that are not accounted for in colloid filtration theory.
Keywords: Bacteria; Flow velocity; Residence time; Retention; Sticking efficiency; Surface roughness.
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