Feasibility of Tea Saponin-Enhanced Soil Washing in a Soybean Oil-Water Solvent System to Extract PAHs/Cd/Ni Efficiently from a Coking Plant Site

Mixed contaminated brownfield sites have brought serious risks to human health and environmental safety. With the purpose of removing polycyclic aromatic hydrocarbons (PAHs) and heavy metals from a coking plant site, an innovative technology for ex-situ washing was developed in the present work. The combination of 15.0 mL L−1 soybean oil and 7.5 g L−1 tea saponin proved an effective method to extract co-pollutants from soil. After two consecutive washing cycles, the efficiency rates of removal for 3-, 4-, 5(+6)-ring, and total PAHs, Cd, and Ni were approximately 98.2%, 96.4%, 92.3%, 96.3%, 94.1%, and 89.4%, respectively. Meanwhile, as evaluated by Tenax extraction method and metal stability indices, the residual PAHs and heavy metals after consecutive washing mainly existed in the form with extremely low bioaccessibility in the soil. Thus, in the soil after two washing cycles, there appeared limited environmental transfer risk of co-pollutants. Moreover, a subsequent precipitation method with alkaline solution and PAH-degrading strain Sphingobium sp. PHE9 inoculation effectively removed 84.6%-100% of Cd, 82.5%-91.7% of Ni, and 92.6%-98.4% of PAHs from the first and second washing solvents. The recovered solvents also exhibited a high recycling effectiveness. Therefore, the combined cleanup strategy proposed in this study proved environmentally friendly, which also played a major role in risk assessment and management in mixed polluted sites.