Bacteria transport in a porous medium: Retention of bacillus and pseudomonas on silica surfaces

Pseudomonas fluorescens and Bacillus subtilis transport was studied in water flowing through laboratory columns packed with 0.5-mm silica beads. The rate of bacterial attachment to the silica was shown to reach steady state after about one pore volume of bacteria-containing solution had passed through the column. Column-outlet concentrations were 6-80% of inlet concentrations. There was less P. fluorescens retardation at pH 5.5 versus 7.0; since the pH_ieps (isoelectric point) for both bacteria are at lower pHs, this difference indicates that the change in electrostatic repulsion over this pH range was relatively unimportant in determining bacteria retention in the porous medium. Transport of both species was highly retarded, suggesting that neither could be quickly transported over long distances in sandy soils. Results were adequately described using a steady-state transport model with bacterial attachment to soil as a pseudo-first-order process. Sticking efficiency (α), calculated using the steady-state colloid-filtration model, was near one. The continued appearance of bacteria in the column effluent for several pore volumes after switching to a bacteria-free inlet solution shows attachment to be reversible, though slowly reversible.