Supercritical Fluid Extraction of Polycyclic Aromatic Hydrocarbon Mixtures from Contaminated Soils

Removal of polycyclic aromatic hydrocarbons from soil: a comparison between bioremoval and supercritical fluids extraction

Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic substances which are resistant to environmental degradation due to their highly hydrophobic nature. Soils contaminated with PAHs pose potential risks to human and ecological health, therefore concern over their adverse effects have resulted in extensive studies on their removal from contaminated soils. The main purpose of this study was to compare experimental results of PAHs removal, from a natural certified soil polluted with PAHs, by biological methods (using bioaugmentation and biostimulation in a solid-state culture) with those from supercritical fluid extraction (SFE), using supercritical ethane as solvent. The comparison of results between the two methods showed that maximal removal of naphthalene, acenaphthene, fluorene, and chrysene was performed using bioremediation; however, for the rest of the PAHs considered (fluoranthene, pyrene, and benz(a)anthracene) SFE resulted more efficient. Although bioremediation achieved higher removal ratios for certain hydrocarbons and takes advantage of the increased rate of natural biological processes, it takes longer time (i.e. 36 d vs. half an hour) than SFE and it is best for 2-3 PAHs rings.

Supercritical CO(2) extraction of PAHs on spiked soil Co-solvent effect and solvent regeneration by ozonization

The supercritical CO(2) extraction of four PAHs (acenaphthene, phenanthrene, anthracene and fluoranthene) from an artificially contaminated soil has been investigated. The effect of temperature (40-60 degrees C), pressure (300-500 bar) and extraction time (90-150 min) has been assessed by conducting a Box-Behnken experimental design. The results suggest the existence of perturbation variables other than the aforementioned controlled variables leading to a significant dispersion of extraction recoveries. With the exception of anthracene, an optimum in temperature (50 degrees C) is envisaged when extracting the PAHs. Analogously, with the exception of anthracene (positive effect), pressure does not have a significant influence. The recovery yield increases as extraction time is increased to a value of 120 min. No further improvement is experienced thereafter. If a co-solvent is used (H(2)O(2) aqueous solution) a beneficial effect can be noticed. Hydrogen peroxide concentration did exert no significant influence in the process. Methanol used to collect the extracted PAHs could be regenerated by gaseous ozone and reused in several consecutive runs.

Decontamination of hazardous substances from solid matrices and liquids using supercritical fluids extraction: a review

Supercritical fluid has been adopted as an extraction media to remove various kinds of substances from distinct types of solid matrices since three decades ago. Compared to conventional extraction mode, supercritical fluid extraction technology is preferred because of the flexibility in adjusting its dissolving power and inherent elimination of organic solvent which means reducing time and money needed for subsequent purification. Utilization of this method as an environmental remedial technology, however, has become a trend only after its accomplishment in analytical chemistry was acknowledged. This review tries to summarize in a comprehensive manner the multitude aspects involved in hazardous compounds removal from miscellaneous class of environmental matrices. The industrial adsorbent regeneration using supercritical fluid technology is also discussed. Although, this technology has been successfully realized for environmental remediation in laboratory and on pilot-plant scale, its commercialization attempts still lack significant technology improvement in order to reach the economic feasibility.

Effect of supercritical fluid extraction parameters and clay properties on the efficiency of phenanthrene extraction

Clay soils have specific properties that cause difficulty in the assessment and remediation of contaminated sites. Furthermore, polyaromatic hydrocarbons, when present in soil, are difficult to extract due to their nonpolar, high molecular weight characterization. In this study, the supercritical fluid (carbon dioxide) extraction (SFE) technique, with and without methanol modifier, was used for removal of PAHs (phenanthrene) from kaolinite, illite, and montmorillonite soils. The impact of SFE parameters (fluid pressure, fluid temperature, and time), and of clay properties (such as clay minerals content, initial moisture content, soil porosity or equivalent pores size, clay surface area, cation-exchange capacity, and clay-swelling index) on the removal efficiency of PAHs from clayey soils were investigated. The results of this investigation were used to develop a semi-empirical correlation between the recovery (i.e. the extraction efficiency) at any time and above mentioned parameters and properties.

Supercritical fluid extraction of polycyclic aromatic hydrocarbons from marine sediments and soil samples

Supercritical fluid extraction (SFE) was used to extract polycyclic aromatic hydrocarbons (PAH) from a certified sample of marine sediment. This sample contains a great number of organic pollutants that are present in low concentrations. The extractions were carried out at 50 and 80 degrees C, at a pressure varying from 230 to 600 bar and using CO2 in the supercritical phase and the effect of three organic modifiers (methanol, n-hexane and toluene), added at 5%/vol, at the same temperature and pressure conditions, were then considered. PAHs were characterized by GC-MS and the recover yield was estimated for 6 PAHs that were representative of those present in the sample, according to their molecular weight and to the number of condensed rings. The analytical conditions giving the best recovery efficiency were used on an unpolluted soil sample spiked with 11 PAHs of environmental importance at a concentration similar to that certified for the sediment sample. An increase in the yield of recovered PAHs, using methanol as co-solvent, was observed while higher temperatures caused a negative effect on the quantity of recovered pollutants. The recovery yield for PAHs from the spiked soil sample was measured and found to be greater than 90%. Better recoveries were obtained for those compounds with higher molecular weight.

Kinetics of the extraction of pyrene using carbon dioxide in dense phase

Bath extraction experiments of pyrene from two spiked soils with different textural composition (sandy, sandy-clay) were performed with near-critical and supercritical CO(2) at different pressures (7.5, 15, 20, 25 MPa) and temperatures (40, 50, 60 degrees C) in order to obtain mass transfer rate data. An empirical equation whose parameters depend on type of soil but are independent of pressure and temperature is proposed. The comparison between individual mass transfer coefficients shows that the extraction is controlled by the resistance within the particle. Effective diffusivities between 0.4 and 6.0 x 10(-10) m(2)/s are deduced.

Application of supercritical and subcritical fluids for the extraction of hazardous materials from soil

Subcritical and supercritical extractions are novel, non destructive techniques which can be applied for the removal of hazardous compounds from contaminated soil without any changes of the soil composition and structure. The aim of the presented review paper is to give information on up-to day results of this method commonly applied by several institutions worldwide. Interest in the application of SC CO2 has been more expressed in the last two decades, which may be related to its favorable characteristics (non-toxic, non-flammable, increase diffusion into small pores, low viscosity under SC conditions, low price and others). However, interest in wet oxidation (WO) and especially in SCWO (the application of water under supercritical conditions with air) has also increased in the last few years. Interest in H2O as a SC fluid, as well as in extraction with water under subcritical conditions may also be related to specific characteristics and the enhanced rate of extraction. Moreover, the solubility of some specific compounds present in soil can be easily changed by adjusting the pressure and temperature of extraction. The high price of the units designed to operate safely at a pressure and temperature much higher than the a critical one of the applied fluids is the main reason why, at present, there is no more broader application of such techniques for the removal hazardous materials from contaminated soil. In the present paper, among many literature citations and their overall review, some specific details related to the development of specific analytical methods under SC conditions are also considered.