Simultaneous Desorption of Polycyclic Aromatic Hydrocarbons and Heavy Metals from Contaminated Soils by Rhamnolipid Biosurfactants

Hydrophobic organic compounds (HOCs) and heavy metals (HMs) are toxins that usually coexist in natural environments. Due to the differences in their properties, it remains challenging to simultaneously remove HMs and HOCs. In this study, the removal of phenanthrene (Phe) and lead (Pb) from co-contaminated soils by single rhamnolipid (RL) and mixed RL-sophorolipid (SL) biosurfactants were evaluated via soil column experiments. Biosurfactant micelle sizes were determined by dynamic light scattering, and the mechanisms of micelle solubilization were studied. The effects of biosurfactant concentrations, pH, washing agent salinity and the ageing time of polluted soils on Phe and Pb desorption efficiencies were also assessed. The substantial removal of Phe and Pb using mixed RL-SL systems, when molar fractions of RLs were 0.7, was attributed to large mixed micelle formation and lower sorption losses of these systems. The optimal pH value was 6.0, while Phe desorption was favoured at high RLs and low ionic strengths. However, the RLs concentration and ionic strength had no obvious influence on Pb removal. In addition, both Phe and Pb desorption decreased with increased ageing of the polluted soils. Combined RL-SL biosurfactants can be effective for simultaneously removing HOCs and HMs from polluted soils.