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-diox ins (dioxins) from by ash samples, collected at different municipal wa ste incinerators, was investigated using supercritical CO2 and compare d to the classical Soxhlet extraction. Results were correlated to fly ash composition, which is strongly related to the fume purification sy stem used in the incinerators. Fly ash collected at the bottom of the electrostatic precipitator is composed of dust coming from the combust ion unit, but also of lime and eventually of activated charcoal inject ed in the fumes for acids and pollutants removal. When only lime is us ed for the fume purification, SFE of dioxins from fly ash leads to bet ter results than Soxhlet extraction. The use of a binary cosolvent (tr ifluoroacetic acid in toluene) greatly increases the percentage recove ry. When activated charcoal is used in conjunction with lime for the f ume purification, SFE under classical extraction conditions is not pow erful enough to extract dioxins, which are strongly adsorbed to the re sidual activated charcoal. (C) 1998 Elsevier Science B.V. All rights r eserved.