Supercritical fluid extraction of polychlorinated dibenzo-p-dioxins from fly ash: the importance of fly ash origin and composition on extraction efficiency

Supercritical fluid extraction (SFE) of polychlorinated dibenzo-p-dioxins (dioxins) from by ash samples, collected at different municipal waste incinerators, was investigated using supercritical CO2 and compared to the classical Soxhlet extraction. Results were correlated to fly ash composition, which is strongly related to the fume purification system used in the incinerators. Fly ash collected at the bottom of the electrostatic precipitator is composed of dust coming from the combustion unit, but also of lime and eventually of activated charcoal injected in the fumes for acids and pollutants removal. When only lime is used for the fume purification, SFE of dioxins from fly ash leads to better results than Soxhlet extraction. The use of a binary cosolvent (trifluoroacetic acid in toluene) greatly increases the percentage recovery. When activated charcoal is used in conjunction with lime for the fume purification, SFE under classical extraction conditions is not powerful enough to extract dioxins, which are strongly adsorbed to the residual activated charcoal. (C) 1998 Elsevier Science B.V. All rights reserved.