ABSTRACT
Octachlorodibenzo-p-dioxin (OCDD)-added, pretreated fly ash was heated under vacuum at sample temperatures ranging from T(s)=450 to 650 K. The fly ash and liquid nitrogen-cooled trap samples were analyzed for DD/DF through OCDD/DF. The total amounts of DD through OCDD decrease with increasing T(s), which indicates that dechlorination/hydrogenation (DCH) reactions are not the only reaction channels. Reduction of toxic equivalent (TEQ) for PCDDs by more than 99% was achieved in the fly ash by the vacuum heat treatment at T(s)=650 K for 4 h. The total amount of PCDDs and DD detected in the liquid nitrogen-cooled trap relative to that of added OCDD was about 17%, i.e., PCDDs and DD which were adsorbed to the fly ash surfaces can evaporate into the gaseous phase. The difference between the evaporation behavior of PCDDs in the present and the previous studies is discussed in the light of their states of existence.
Subject(s)
Air Pollutants/analysis , Carbon/chemistry , Hot Temperature , Incineration , Polychlorinated Dibenzodioxins/analogs & derivatives , Coal Ash , Gas Chromatography-Mass Spectrometry , Hydrogenation , Incineration/standards , Models, Chemical , Particulate Matter , Polychlorinated Dibenzodioxins/analysis , VacuumABSTRACT
Temperature dependence of PCDD/DF and dioxin-like polychlorinated biphenyl (dl-PCB) concentrations in fly ash from a municipal waste incinerator (MWI) heated under vacuum has been investigated as a function of sample temperature ranging from T(s)=425 to 800 K to find out if PCDDs/DFs in fly ash evaporate and are trapped in a liquid nitrogen-cooled trap. The results show that more than 99.98% of PCDDs/DFs in TEQ is removed from fly ash by vacuum heat treatment at T(s)>650 K for 4 h. Almost no PCDDs/DFs were detected in the liquid nitrogen-cooled trap. Homologue distributions indicate that dechlorination/hydrogenation (DCH) reactions proceed in fly ash at T(s)>450 K. Arrhenius rate parameters for the DCH reactions have been determined for each homologue assuming that only DCH reactions occur. The fly ash heated under vacuum at 650 or 800 K was reheated at 573 K (300 degrees C) in a stream of dry or humid air to see how much PCDDs/DFs and dl-PCBs are regenerated. We have found that (1) PCDDs/DFs are regenerated in both 650 K and 800 K treated fly ash, whereas dl-PCBs are regenerated in 650 K treated fly ash, (2) formation of PCDFs predominates over that of PCDDs or dl-PCBs, and (3) less chlorinated homologues are abundant for PCDDs/DFs and dl-PCBs.
