On the capability of biosurfactants for the removal of heavy metals from soil and sediments

Batch soil washing experiments were used to evaluate the feasibility of using biosurfactants for the removal of heavy metals from a contaminated soil and sediments. Surfactin from Bacillus subtilis, rhamnolipids from Pseudomonas aeruginosa and sophorolipid from Torulopsis bombicola were evaluated using a soil contaminated with hydrocarbon and metals (890 mg/kg zinc, 420 mg/kg copper, 12.6% oil and grease) and metal contaminated sediments (110 mg/kg copper, 3300 mg/kg zinc).Although water alone removed insignificant levels of metals, results showed that the biosurfactants could remove 5% of the zinc (with 12% rhamnolipid) and 19.5% of the zinc (with 4% sophorolipid with 0.7% HCl). Copper could also be removed and was most efficiently extracted (greater than 25%) with 12% rhamnolipid or with 2% rhamnolipid with 1% NaOH. 1% NaOH alone removed only 5% of the copper and 2% zinc. After a series of five batch washes, 90% of the copper could be removed by 0.1% surfactin with 1% NaOH while 4% sophorolipid with 0.7% HCl was able to remove 100% of the zinc. From the sediment, a single washing with 0.5% rhamnolipid removed 65% of the copper and 18% of the zinc whereas 4% sophorolipid removed 25% of the copper and 60% of the zinc.Sequential extraction procedures were used on the soil and sediments. For both matrices, the carbonate and the oxide fractions accounted for over 90% of the zinc present in the soil. The organic fraction constituted over 70% of the copper in the soil and sediments. Sequential extraction of the soil and sediments after washing with the various surfactants indicated that the biosurfactants, rhamnolipid or surfactin with NaOH, could remove the organically-bound copper and that the sophorolipid with acid could remove the carbonate and oxide bound zinc and cadmium.Concerning the mechanism for metal removal by the surfactants, the techniques of octanol-water partitioning, ultrafiltration and zeta potential measurements indicated that the surfactants removed the metals first by sorption at the soil interphase, followed by desorption of the metal through interfacial tension lowering and fluid forces and then solubilization of the metal within the micelle