Effect Of Low-concentration Rhamnolipid On The Transport Of Pseudomonas Aeruginosa ATCC 9027 In Natural Porous Media

The effect of low-concentrations of monorhamnolipid biosurfactant on transport of Pseudomonas aeruginosa ATCC 9027 in natural porous media (silica sand and a sandy soil) was studied with miscible-displacement experiments using artificial groundwater as the background solution. Transport of two types of cells was investigated, glucose-and hexadecane-grown cells with lower and higher cell surface hydrophobicity (CSH), respectively. Effect of hexadecane presence as residual non-aqueous phase liquids (NAPL) on the transport was also examined. A clean-bed colloid deposition model was used to calculate deposition rate coefficients (k) for quantitative assessment. Significant cell retention was observed in the sand (80% and 83% for glucose- and hexadecane-grown cells, respectively). Addition of low-concentration rhamnolipid enhanced cell transport, with 40 mg/L of rhamnolipid reducing retention to 51% and 60% for glucose- and hexadecane-grown cells, respectively, k for both glucose- and hexadecane-grown cells correlates linearly with rhamnolipid-dependent CSH represented as bacterial-adhesion-to-hydrocarbon rate of cells. Retention of cells by the soil was nearly complete (>99%).40 mg/L of rhamnolipid attenuated the retention and caused 5% of cell recovery. The presence of NAPL in the sand enhanced retention of cells, and hexadecane-grown cells (higher CSH) exhibited greater retention than glucose-grown cells. Transport of cells in the presence of NAPL was enhanced by rhamnolipid at all concentrations tested, and the relative enhancement was greater than in the absence of NAPL. This study shows the importance of hydrophobic interaction on bacterial transport in natural porous media and the potential of using low-concentration rhamnolipid for facilitating the transport in subsurface for bioaugmentation efforts.