CLEANUP OF COMPLEX ORGANIC MIXTURES USING SUPERCRITICAL FLUIDS AND SELECTIVE ADSORBENTS

The selectivity of various adsorbents toward the fractionation of complex organic mixtures consisting of compounds having similar solubility parameters and containing polychlorinated dibenzo-p-dioxins (PCDDs) as target analytes was investigated using supercritical fluids. Tenax, Florisil, alumina, carbon, and chemically modified silica (C18 and CN) were used as adsorbents. On the basis of the results, a simple fractionation scheme was proposed. A 15-min extraction with supercritical carbon dioxide (CO2) at 3000 psi removed over 75% of the chlorinated benzenes and PCBs from Florisil. Full recoveries of the PCDDs were subsequently obtained by extracting with ritrous oxide (N2O) for 90 min at 6000 psi. The fractionation process was used to clean-up municipal incinerator fly ash extracts before quantitation of PCDDs. Differences in the basic properties between CO2 and N2O are used to explain the differences in extraction recoveries from Florisil. The affinity of Florisil toward PCDDs is associated with the presence of magnesium ions fused into the silica lattice. Heavier metals in the lattice, which are present in many common environmental matrices such as fly ash, also result in chemisorption of PCDDs, but the adsorption process is very slow because of high energy barrier of adsorption. Good fractionation between PCDDs and PCBs can be also achieved by using activated carbon or Porous Graphitic Carbon; however, poor recoverles of target analytes (accuracy) and poor precision makes this method unsuitable for practical applications. The results provide a better understanding about the processes which control the extraction rates of analytes from environmental samples.