Enhanced extraction of PAHs hindered in fly ashes with supercritical water

Many polycyclic aromatic hydrocarbons (PAHs) which are toxic and suspected carcinogens are frequently found in incineration fly ashes. However, there is a lack of quality data on the levels of PAHs in fly ashes primarily due to the fact that the conventional Soxhlet extraction fails to yield significant recovery of any of the PAHs from fly ashes. With the supercritical water (SCW) at 673 K and 240 bar, PAHs hindered in fly ashes containing a wide range (0.1–21%) of unburned carbon can be extracted, which gives a more detailed finger-print PAH pattern. The extraction efficiencies of PA, BbF, BaP, and BghiP from the incineration fly ash with SCW are greater than those with Soxhlet extraction by 15, 25, 56, and 13 times, respectively. The relatively high extraction efficiency of PAHs from the fly ashes with SCW may be mainly associated with the solvent property (i.e., dielectric constant). In addition, as the unburned-carbon contents in fly ashes increase, more PAHs (NaP excluded) can be extracted.