Supercritical CO₂ Extraction Of Polycyclic Aromatic Hydrocarbons From Soil

Supercritical carbon dioxide (SC CO₂) has specific characteristics in supercritical state and many other particular merits of cheap price, nontoxicity, being apyrous, and easy reuse. In recent years SC CO₂ is very attractive and has been applied in many fields including extraction, environment remediation, chemical engineering, material science, etc. It shows a promising application in the future. Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants and hazardous to human health. Soil is an important medium for the accumulation and transportation of PAHs. PAHs can also transport into the water and atmosphere by the geochemical cycle. It is very critical to detect and remove these PAHs in soils. The object of this thesis is to study the extraction of PAHs from soils using supercritical CO₂ fluid. From a lot of the first hand data, the thermodynamic theory of supercritical fluid mixtures will be supplemented. In addition, it will provide necessary parameters of supercritical fluid extraction (SFE) for soil remediation.Single factor experiments and orthogonal experiments were both designed for extraction of PAHs from soils by supercritical CO₂. The factors of system pressure, temperature, co-solvents and soil moisture were evaluated and optimized.The results showed in the following. (1) The extractions of most PAHs had a certain relationship with changed pressure and temperature. However, such relationship depended on different kinds of PAHs. The extraction of naphthalene increased with pressure increase, while some other PAHs of complicated molecular structure such as fluoranthene, benzo(b)fluoranthene changed little. At both 10MPa and 15 MPa, the extraction of naphthalene at 60℃was higher than that at 40℃,while the extraction of fluoranthene at 60℃was higher than that at 40℃at 10MPa, and lower than that at 40℃at 15MPa . (2) The co-solvent methanol showed better enhancement for the extraction of naphthalene, benzo(b)fluoranthene than hexane and toluene. Hexane had once and for all effect on eight kind species of PAHs. (3) At 40℃, 15MPa, the extraction efficiency of dibenzo(a,h)anthracene's was 83.98% with methanol as a co-solvent. By contrast, it was hard to get such high efficiency using other general methods in a short time. The extraction efficiency of anthracene, naphthalene, benzo(b)fluoranthene was 65.80%, 49.76%, 49.02%, respectively. (4) The descending impact of the four factors for the extraction efficiency of naphthalene was pressure, soil moisture, co-solvent quantity, temperature. However, for fluoranthene extraction, this sort of descending impact was temperature, pressure, moisture, co-solvent quantity.