Cleaning of contaminated soils using ionic liquids and supercritical carbon dioxide

Abstract

The aim of this research was to utilize the ability of ionic liquids (IL) to dissolve contaminants at near ambient conditions from soil along with the ability of scCO2 to recover the contaminants from the IL extract, in order to develop a process for the cleaning of contaminated soils. Naphthalene was used as the model component to represent a group of soil contaminants, and 1-n-butyl 3-methylimidazolium hexafluorophosphate ([bmim][PF6]) was used as the IL. The model component was extracted from soil by IL and then it was recovered from the IL extract by scCO2 to recycle the IL. The feasibility of using scCO2 to recover the naphthalene dissolved in the IL was investigated by the supercritical extraction experiments conducted at 25, 35, 40°C and 80, 100, 120, 140 bar to observe the effects of temperature and pressure on the naphthalene recovery. The supercritical extraction experiments were performed for two, four and six hours to study the influence of contact time. Finally, ILs cleaned by scCO2 under different conditions were reused in the extraction of other soils samples to observe the efficiency of IL recycling. Results of soil extraction experiments demonstrated that soil contaminated with naphthalene was cleaned using [bmim][PF6], and the naphthalene remaining in the soil was below the maximum allowable concentration limit given. Results of scCO2 extraction experiments suggest that extraction pressure, temperature and time positively affect the naphthalene recovery from IL. At 140 bar and 40°C and four hours of extraction period, naphthalene recovery from IL by scCO2 reached a value of 83.8 per cent, whereas at the low end of the pressure and temperature range studied, i.e. at 80 bar, 25°C and four hours of extraction period, the recovery was 12.0 per cent. This study is expected to be the first in the literature which investigates soil-model compound-IL-scCO2 system. On the basis of this research a process flowsheet was suggested using IL extraction of contaminated soils and continuous scCO2 extraction of the contaminants from IL extract.