Facilitated transport of a PAH mixture by a rhamnolipid biosurfactant in porous silica matrices

The facilitated transport of a mixture of three polycyclic aromatic hydrocarbons (PAH) (naphthalene, fluorene, and phenanthrene) by a rhamnolipid biosurfactant was determined with silica, octadecyl-coated silica, and humic acid-coated silica columns. The retardation factors for the contaminants in the absence of rhamnolipid ranged from 1.8 (naphthalene with silica) to 708 (phenanthrene with octadecyl-coated silica). The retardation factors for phenanthrene were up to eightfold lower in the presence of 500 mg/l rhamnolipid compared to the situation where no rhamnolipid was present. The facilitated transport of the contaminants in the presence of rhamnolipid could be predicted by accounting for sorption, solubilization and admicellar sorption. In the octadecyl-coated matrix, transport of fluorene and phenanthrene was enhanced even in the presence of sub-micellar solution of rhamnolipid (20 mg/l), showing that adsorbed surfactant reduced the affinity of these compounds to this stationary phase. Linear free-energy relations (LFERs) indicated that the matrix–water, micelle–water, and admicelle–water partitioning constants for PAH were correlated with their 1-octanol–water partitioning constants. These correlations were used to simulate the facilitated transport of contaminants of varying hydrophobicity by a surfactant. It was concluded that surfactants have the greatest transport facilitating effect on the more hydrophobic components. The effectiveness of surfactants for enhancing removal of the less hydrophobic compounds (log Kow<3.4) may considerably be reduced due to admicellar sorption.