Comparisons of Soxhlet extraction, pressurized liquid extraction, supercritical fluid extraction and subcritical water extraction for environmental solids: recovery, selectivity and effects on sample matrix

Extractions of a polycyclic aromatic hydrocarbon (PAH)-contaminated soil fr om a former manufactured gas plant site were performed with a Soxhlet appar atus (18 h), by pressurized liquid extraction (PLE) (50 min at 100 degrees C), supercritical fluid extraction (SFE) (1 h at 150 degrees C with pure CO 2), and subcritical water (1 h at 250 degrees C, or 30 min at 300 degrees C ). Although minor differences in recoveries for some PAHs resulted from the different methods, quantitative agreement between all of the methods was g enerally good. However, the extract quality differed greatly. The organic s olvent extracts (Soxhlet and PLE) were much darker, while the extracts from subcritical water (collected in toluene) were orange, and the extracts fro m SFE (collected in CH2Cl2) were light yellow. The organic solvent extracts also yielded more artifact peaks in the gas chromatography (GC)-mass spect rometry and GC-flame ionization detection chromatograms, especially compare d to supercritical CO2. Based on elemental analysis (carbon and nitrogen) o f the soil residues after each extraction, subcritical water, PLE, and Soxh let extraction bad poor selectivity for PAHs versus bulk soil organic matte r (similar to 1/4 to 1/3 of the bulk soil organic matter was extracted alon g with the PAHs), while SFE with pure CO, removed only 8% of the bulk organ ic matrix. Selectivities for different compound classes also vary with extr action method. Extraction of urban air particulate matter with organic solv ents yields very high concentrations of n- and branched alkanes (similar to C-18 to C-30) from diesel exhaust as well as lower levels of PAHs, and no selectivity between the bulk alkanes and PAHs is obtained during organic so lvent extraction. Some moderate selectivity with supercritical CO2 can be a chieved by first extracting the bulk alkanes at mild conditions, followed b y stronger conditions to extract the remaining PAHs, i.e., the least polar organics are the easiest organics to extract with pure CO2. In direct contr ast, subcritical water prefers the more polar analytes, i.e., PAHs were eff iciently extracted from urban air particulates at 250 degrees C, with littl e or no extraction of the alkanes. Finally, recent work has demonstrated th at many pollutant molecules become "sequestered" as they age for decades in the environment (i.e., more tightly bound to soil particles and less avail able to organisms or transport). Therefore, it may be more important for an extraction method to only recover pollutant molecules that are environment ally-relevant, rather than the conventional attempts to extract all polluta nt molecules regardless of how tightly bound they are to the soil or sedime nt matrix. Initial work comparing SFE extraction behavior using mild to str ong conditions with bioremediation behavior of PAHs shows great promise to develop extraction methodology to measure environmentally-relevant concentr ations of pollutants in addition to their total concentrations. (C) 2000 El sevier Science B.V. All rights reserved.