Soil remediation via an ionic liquid and supercritical CO2

The potential of ionic liquids (ILs) to dissolve soil contaminants at ambient conditions and the ability of supercritical carbon dioxide (scCO2) to recover these contaminants from IL extracts are utilized serially to clean contaminated soils. Naphthalene is used as the model component to represent a group of soil contaminants, i.e. PAHs, and 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) is used as the IL. Naphthalene is extracted from soil by the IL, and then recovered from the IL extract by scCO2 extraction. The feasibility of using scCO2 to recover the naphthalene dissolved in the IL is investigated at 25, 35, 40 °C and 80, 100, 120, 140 bar conditions and for 2, 4 and 6 h extraction times. ILs cleaned by scCO2 under different conditions are re-used in soil-sample extractions to observe the efficiency of IL recycling. The results show that naphthalene-contaminated soil is cleaned using [bmim][PF6], and the amount of naphthalene remaining in the soil is below the allowable contamination limit. The extraction pressure, temperature and extraction time positively affect naphthalene recovery from IL. At 140 bar, 40 °C and 4 h extraction conditions, the naphthalene recovery from the IL by scCO2 reaches a value of 83.8%, whereas at 80 bar, 25 °C and 4 h extraction conditions, the recovery is 12.0%. This study is novel as it investigates a soil/model-contaminant/IL/dense-CO2 system. On the basis of the findings a process flowsheet for the IL extraction of contaminated soils and continuous scCO2 extraction of the contaminants from IL extracts is also suggested.