On the use of biosurfactants for the removal of heavy metals from oil-contaminated soil

The feasibility of using biodegradable biosurfactants to remove heavy metals from an oil-contaminated soil was evaluated by batch washes with surfactin, a rhamnolipid and a sophorolipid. The soil contained 890 mg/kg of zinc and 420 mg/kg of copper with a 12.6% oil and grease content. Highest levels of zinc removal were obtained using 12% rhamnolipid (19.5% of the zinc) and 4% sophorolipid/0.7% HCl (15.8% of the zinc). Highest copper removal rates were achieved (greater than 25%) with 12% rhamnolipid or with 2% rhamnolipid/1% NaOH or 0.25% surfactin/1% NaOH. A series of five batch washes removed 70% of the copper with 0.1% surfactin/1% NaOH while 4% sophorolipid/0.7% HCl was able to remove 100% of the zinc. Sequential extraction procedures showed that the carbonate and oxide fractions accounted for over 90% of the zinc present in the soil and the organic fraction in the soil constituted over 70% of the copper. Sequential extraction of the soil after washing with the surfaction or rhamnolipid (both with 1% NaOH) indicated that these surfactants could remove the organically-bound copper and that the sophorolipid with acid could remove the carbonate and oxide -bound zinc. In conclusion, the results clearly indicated the feasibility of removing the metals with the anionic biosurfactants tested even though the exchangeable metal fractions were very low.