Bacterial adhesion and transport in porous media: role of the secondary energy minimum

The adhesion of a well-characterized Escherichia coli bacterial strain to quartz sediment grains in the presence of repulsive electrostatic interactions is systematically examined. An increase in the ionic strength of the pore fluid results in an increase in bacterial attachment, despite DLVO calculations indicating a sizable electrostatic energy barrier to deposition. Bacterial deposition is likely occurring in the secondary energy minimum, which DLVO calculations indicate increases in depth with ionic strength. A decrease in the ionic strength of the pore fluid--thereby eliminating the secondary energy minimum--resulted in release of the majority of previously deposited bacteria, suggesting that these cells were deposited reversibly in the secondary minimum. Additionally, bacterial attachment to a quartz surface in a radial stagnation point flow system was absent at ionic strengths less than 0.01 M and resulted in attachment efficiencies over an order of magnitude lower than in the packed-bed column experiments at higher ionic strengths. Because of the hydrodynamics in the radial stagnation point flow system, this observation supports our conclusion that the majority of bacterial deposition in the packed bed occurs in a secondary energy minimum.