KINETIC-STUDY OF SUPERCRITICAL-FLUID EXTRACTION OF ORGANIC CONTAMINANTS FROM HETEROGENEOUS ENVIRONMENTAL-SAMPLES WITH CARBON-DIOXIDE AND ELEVATED-TEMPERATURES

Supercritical fluid extraction (SFE) rates of spiked polychlorinated d ibenzo-p-dioxins (PCDDs) from Florisil, spiked [C-13]PCDDs and native PCDDs from fly ash, and spiked [H-2]polycyclic aromatic hydrocarbons ( PAHs) and native PAHs from marine sediment and railroad bed soil were examined at 40, 120, and 200 degrees C, while constant fluid density ( d = 0.67 g/mL) and now rate were maintained, Over 30 min (150 void vol umes of CO2) was required to quantitatively remove both spiked 2,3,7,8 -tetrachlorodibenzo-p-dioxin and 1,2,3,7,8-pentachlorodibenzo-p-dioxin from Florisil at 40 degrees C, while SFE at 200 degrees C significant ly improved the elution rate so complete removal was achieved in simil ar to 10 min, Elution rates of spiked PCDDs from Florisil were slower with a 5-mL vessel (12 cm long) than a 0.5-mL vessel (6 cm long), Incr easing the temperature from 40 to 120 and 200 degrees C enhanced the S FE rates of spiked [C-13]PCDDs and native PCDDs from fly ash, as well as [H-2]PAHs and native PAHs from marine sediment and railroad bed soi l, In all cases, native analytes were extracted more slowly than spike d analytes, suggesting that additional processes affect SFE rates of n ative analytes, A kinetic model was used that could help distinguish b etween these processes and included terms for matrix-fluid mass transp ort, as well as partitioning and bulk mass transport in the supercriti cal fluid. Using a three-rate constant desorption model to describe ma ss transport, good correlations (r(2) > 0.9 in most cases) were obtain ed with experimental data for native analytes, and desorption rate con stants suggest that analyte-matrix interactions are strong, The result s of this study show that increasing the extraction temperature is a s imple and effective method to increase SFE rates while still exploitin g the advantages of supercritical CO2, and can be used regardless of w hether slow SFE rates are due to poor partitioning into the fluid or l imited by slow desorption due to strong analyte-matrix interactions.