Washing Enhancement By Rhamnolipid Combined With Biochar Adsorption To Remediate PHE-Contaminated Soil

As one of polycyclic aromatic hydrocarbons(PAHs), phenanthrene(PHE) is a carcinogenic organic pollutant, and it is widespread in the natural environment. Due to its hydrophobicity and refractory, PHE in soil environment will cause persistent pollution to the soil thus pose a threat to health of human beings. Surfactant-enhanced soil washing is an efficient technique to remediate this kind of contaminated soil. However, soil washing solution containing pollutants still needs further treatment in order to be discharged or recycled. In recent years, biochar with high carbon content and stable property has often been used as adsorbent or soil conditioner in the study of groundwater and soil remediation. This paper studies soil washing enhancement by surfactant combined with biochar adsorption to repair PHE-contaminated soil, provides reference for practical soil remediation.In the research of washing PHE from contaminated soil by biosurfactant rhamnolipid, it's indicated that rhamnolipid could solubilize PHE effectively, and rhamnolipid with different concentrations caused different influences on adsorption and desorption behavior of PHE in soil-water system. Adsorptive rhamnolipid on soil similar to soil organic matter would enhance distribution of PHE in the phase. So adsorption of PHE would be promoted when rhamnolipid was rare in the system. While the content of rhamnolipid in liquid phase was above critical micelle concentration, rhamnolipid micelles in the liquid phase came into being and solubilized PHE effectively. Rhamnolipid with a concentration above critical washing concentration could enhance desorption of PHE from soil into the liquid phase so as to achieve the purpose of PHE removal from contaminated soil.In the study of biochar adsorption of PHE from soil washing solution, it's found that with the increase of pyrolysis temperature(250~650?), carbonization degree of biochar, and its hydrophobicity, aromaticity as well as structural properties such as specific surface area and pore volume were all promoted, no matter it's derived from rice hull or cottonseed hull. At a same pyrolysis temperature, ash content of rice hull-derived biochar was higher, and the parameters of its properties such as aromaticity and specific surface area were smaller than that of cottonseed hull-derived biochar, so its adsorption capacity was inferior to cottonseed hull-derived biochar. As the specific surface area of biochar pyrolysed at high temperature(?550?) was greater, the amount of adsorptive rhamnolipid on biochar increased significantly. Consequently, the adsorptive rhamnolipid could enhance distribution of PHE in the organic phase. Therefore, the linear degree of adsorption and adsorptive content of PHE increased on biochars which were pyrolysed at high temperature. Due to the combined effect of adsorptive rhamnolipid, micropore structure, aromaticity and hydrophobicity, cottonseed hull-derived biochar pyrolysed at 650? has the strongest adsorbability for PHE in the soil washing solution and also the highest selectivity for PHE among the tested biochars.