Formation and removal of hydrocarbon residual in porous media: Effects of attached bacteria and biosurfactants

Column studies were used to investigate the fate of a representative nonaqueous-phase liquid (NAPL), hexadecane, with specific regard to (1) the effect of attached bacteria on the formation of residual saturation and (2) the role of biodegradation and biosurfactants on the removal of residual NAPL. Residual saturation of hexadecane was determined using sterile sand (40/50 mesh) columns and was found to be 19.0 ± 4.8% of the pore volume. Columns loaded with bacterial biomass (Pseudomonas aeruginosa ATCC 15442, 10⁹ cells g^-1) showed no difference in residual hexadecane formation as compared to sterile sand columns. In further column studies examining the effect of ATCC 15442 and biosurfactants on the removal of hexadecane residual, results showed that biodegradation alone removed approximately 50% of the [¹⁴C]hexadecane, in the form of ¹⁴CO₂ and undefined cellular metabolites, during elution with at least 200 pore volumes of mineral salts medium. The columns were then eluted with 1 mM rhamnolipid biosurfactant, which increased total removal to 65%. Rhamnolipid addition resulted in (1) the mobilization of hexadecane free product and (2) a transitory 3-12-fold increase in the rate of hexadecane mineralization. In a separate study, the column was eluted from the beginning with a low (0.1 mM) concentration of rhamnolipid. This lower concentration of biosurfactant enhanced the removal of hexadecane by mobilization, but had no effect on the rate of biodegradation of residual hexadecane. Analysis of residual radioactivity within two columns revealed only 2% remaining as intact hexadecane. These results suggest that a combination of biodegradation and rhamnolipid treatment could be used to maximize the removal of residual NAPL from porous media.