| The formation of chlorinated dibenzo-p-dioxins (CDDs) and chlorinated dibenzofurans (CDFs) on copper chloride (CuCl2) from chlorinated phenol (CP), dibenzo-p-dioxin (DD), and dibenzofuran (DF) precursors in the temperature range of 100–500°C has been investigated in the laboratory. Experimental results are shown on chlorination and phenol condensation reaction pathways.; Chlorination patterns of phenol and chlorinated phenols were favored on ortho and para carbons, resulting in 2,6- or 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol. These are found in the most abundant amount in typical municipal waste incinerators. Unlike gas-phase CDD/F formation from phenols at higher temperatures, CuCl2-mediated formation favored CDD products. Also, more highly chlorinated CDD/F products were formed on particle surfaces than in the gas phase. 2,4,6-Trichlorophenol and pentachlorophenol were most reactive in producing CDDs. A maximum yield of CDD/F formation occurred at about 400°C. CDFs are formed without the loss of chlorine from chlorinated phenols, and CDDs are formed through the loss of one or two ortho chlorines. Total CDD/F yield was strongly dependent on exhaust gas conditions such as temperature and gas velocity, but CDD/F isomer distributions were almost constant. CDD/F isomer patterns identified in this work may provide a fingerprint for the formation of CDD/F from precursors.; Chlorination of DD and DF was favored at the four lateral positions (2,3,7,8) and resulted in the production of the most toxic compounds. DD/DF chlorination was highly efficient at temperatures between 200 and 300°C. The degree of chlorination was strongly dependent on gas velocity (gas-particle contact time). Based on stastistical factors and partitioning between mono- and di-chlorinated products, a simple model was developed to predict CDD/F isomer distribution from DD and DF chlorination.; To investigate the source of CDD/F byproducts, CDD/F isomer distributions obtained from eight municipal waste incinerators were compared with CDD/F isomer distributions obtained from this study. The results suggest that CDD and CDF isomers are formed through different mechanisms. CDD products appear to be formed from chlorinated phenol precursors. CDF products appear to be derived through a combination of the chlorination of DF, likely formed in the gas phase from phenol, and the dechlorination of OCDF (octachlorodibenzofuran), which is possibly formed from particulate carbon.; This work provides the information needed to develop measurement tools and control strategies to minimize the formation of these toxic compounds. |
