Biosurfactant-enhanced remediation of phenanthrene contaminated soil using a two-stage leaching and treatment process

Abstract

Bioremediation of phenanthrene contaminated soil by rhamnolipid biosurfactants was studied. Preparation of biosurfactants, biosurfactant-enhanced soil washing, and the mineralization of contaminant in washing solution were the main attempts of this work. In the preparation of biosurfactants, a glycolipid type of biosurfactants was obtained from a strain which had been isolated from soil. The cell was identified as Pseudomonas aeruginosa from taxonomic characteristics and was designated as YPJ80. Thin layer chromatography and deoxyhexose detection tests were done to verify the type of biosurfactant. Steps for the recovery was investigated by comparing the yield of glycolipid from ultrafiltration, adsorption, and solvent extraction. Further spectroscopic analysis such as IR, $^1H$-NMR, $^{13}C-NMR$ and FAB-MS was done and the structure of the biosurfactant was verified to be rhamnolipid type Ⅰ and Ⅲ. Critical micelle concentration of the surfactant was observed to be 50 ppm and the minimum surface tension was 30.1 mN/m. As an emulsifier, rhamnolipids showed superior activity to emulsan in the emulsification of crude Arabian light oil. In soil washing, effect of rhamnolipid addition was investigated in terms of enhancement of solubilizing power of the washing solution and the increment in the rate of phenanthrene transfer into washing solution. Significant enhancement upto 50 folds was observed in the phenanthrene solubility over that of control by the addition of 2 g/L rhamnolipids. Further enhancement was observed in the presence of cations such as $Na^+$ and $Mg^{++}$. Upto 10 times higher molar solubility ratio was obtained with 1.0 mM $MgCl_2$ when compared with that of cation free rhamnolipid. The coefficient ($κ_L$) of the phenanthrene dissolution rate was increased from 1.8 to 107.2 by the addition of 2 g/L rhamnolipids in the solution. From the uncertainty analysis of the batch equilibrium of the soil washing process, surfactant concentration was verified...