Bacterial transport in laboratory columns and filters: Influence of ionic strength and pH on collision efficiency

The influence of ionic strength and pH on the transport of Pseudomonas fluorescens P17 in porous media was investigated using continuous-flow laboratory columns and a rapid screening technique in which radiolabeled cells were applied to large-pore, glass-fiber filters. Colloid-filtration theory was used to interpret P17 transport results in the two systems. Bacterial retention was directly related to the ionic strength of the carrying solution. Decreasing the ionic strength from 10^-1 to 10^-5 M caused the bacterial collision efficiency, α, to decrease nearly 90% (from 0.18 to 0.026 in screening experiments and from 0.12 to 0.015 in column experiments). This change in α is qualitatively consistent with double-layer theory, but suggests that very large changes in ionic strength are needed to influence transport. Bacterial transport was unaffected by changes in pH in the range of 5.5 < pH < 7.0 in both systems. This suggests that changes in surface properties of the bacteria and the silica collectors over the pH range that is commonly encountered in soil and ground water are not sufficient to influence cell attachment. The screening technique provided a rapid alternative to column experiments. Even though estimated α's were different in the two systems, the responses to changes in ionic strength and pH were similar.