Quantification of bacterial mass recovery as a function of pore-water velocity and ionic strength

Transport of bacteria in aquifer systems plays an important role in bioaugmentation, which relies upon successful bacterial delivery to a target area. In the present study, we conducted a set of laboratory column experiments under various conditions of pore-water velocity (υ_ω) and ionic strength (IS) of culture medium for Pseudomonas aeruginosa, known to be a benzene-degrading bacteria, in order to investigate their relationship to mass recovery in saturated quartz sands. The column experiments revealed that both peak concentrations and mass recoveries of bacteria were lower than those of a conservative tracer KCl when deionized water was used as leaching water for all ranges of pore-water velocity (0.18-6.23 cm/min). Thus, the parameter responsible for transport of P. aeruginosa was only the deposition coefficient. Bacterial cells could not be attached to the mineral surfaces by predominance of electrostatic charge or repulsive forces over hydrophobicity or attractive forces due to the very low ionic strength (≈0 mM) of deionized water. The loss of bacterial mass was attributed to the deposition in the crevice formed on the quartz surfaces, as evidenced by SEM images. For a given pore-water velocity, the ionic strength markedly influenced bacterial deposition, showing decreased peak concentrations and mass recoveries with increasing ionic strength of column leaching water. An optimum range of υ_ω and IS for achieving bacterial mass recovery higher than 70% in the studied quartz sand was found such that: (i) at low IS (≈0 mM), a pore-water velocity higher than 0.30 cm/min, and (ii) at pore-water velocity of 0.52 cm/min, an IS lower than 290 mM, were required, respectively.