Biosurfactant-enhanced removal of residual hydrocarbon from soil

An anionic monorhamnolipid biosurfactant produced by Pseudomonas aeruginosa was investigated for its potential to remove residual hexadecane from sand columns. In a series of column experiments, residual hexadecane saturation was established by pumping ¹⁴C-hexadecane into water-saturated sand columns and then flushing with water at a velocity of 25 cm h^-1. Monorhamnolipid solutions of varying concentration were then applied to the columns at a velocity of 15 cm h^-1 to remove the residual hexadecane. Of the rhamnolipid concentrations tested, which ranged from 40 to 1500 mg l^-1, the optimal concentration for residual removal was 500 mg l^-1, approximately ten times the critical micelle concentration (cmc). Approximately 84% of the residual was removed from the column packed with 20/30 mesh sand, and 22% was removed from the 40/50 mesh column. The primary mechanism for residual removal was mobilization (displacement and dispersion), whereas solubilization was found to be insignificant. The performance of monorhamnolipid was compared with that of two synthetic surfactant solutions on a mass basis (500 mg l^-1) for the 40/50 mesh sand. Sodium dodecyl sulfate (0.2 x cmc), and polyoxyethylene (20) sorbitan monooleate (38 x cmc), removed 0% and 6.1% of the residual saturation, respectively.