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On the basis of laboratory experiments with model mixtures (active carbon+CuBr2 at different loads), this work studies the formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) by de novo synthesis. For the different samples, the temperature of the maximum carbon oxidation rate was determined by thermogravimetric analysis, and a kinetic model was proposed for the degradation of the materials in an oxidizing atmosphere (synthetic air). The effect of the addition of different amounts of CuBr2 was studied, finding that its presence accelerates the degradation of the carbonaceous structure in the presence of oxygen. The thermal degradation of the samples in air is satisfactorily described by a first-order single-reaction model. In addition, combustion runs of one of the mixtures (consisting of activated carbon+50 wt % CuBr2, pyrolyzed at 700 °C) were performed in a quartz horizontal laboratory furnace. The analysis of the emissions and the solid residue proved the formation of brominated dioxins and furans at 300, 400, and 500 °C, with a maximum yield at 300 °C (91.7 ng/g of total PBDD/Fs) and a higher bromination degree with increasing temperature.

This study presents a detailed mechanistic and kinetic investigation that explains the experimentally observed high yields of formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) from the polybrominated diphenyl ethers (PBDEs), commonly deployed in brominated flame retardants (BFRs). Theoretical calculations involved the accurate meta hybrid functional of M05-2X. The previously suggested pathways of debromination and generation of bromophenols/bromophenoxys/bromobenzenes were found to be unimportant corridors for the formation of PBDD/Fs. A loss of an ortho Br or H atom from PBDEs, followed by a ring-closure reaction, is the most accessible pathway for the production of PBDFs via modest reaction barriers. The initially formed peroxy-type adduct (RO₂) is found to evolve in a complex, nevertheless very exoergic, mechanism to produce PBDDs. Results indicate that, degree and pattern of bromination, in the vicinity of the ether oxygen bridge, has a minor influence on governing mechanisms and that even fully brominated isomers of BFRs are capable of forming PBDD/Fs. We thoroughly discuss bimolecular reactions of PBDEs with Br and H, as well as the Br-displacement reaction by triplet oxygen. The rate of the Br-displacement reaction significantly exceeds that of the unimolecular inititiation reactions due to loss of ortho Br or H. Results presented herein address conclusively the intriguing question of how PBDEs form PBDD/Fs, a matter that has been in the center of much debate among environmental chemists.

The photochemical conversion of selected hydroxylated polybrominated diphenyl ethers (OH-PBDEs) to dioxins and other products was investigated. OH-PBDEs, which are both transformation products of polybrominated diphenyl ethers and naturally occurring compounds, undergo direct photolysis to yield a number of products that may have a higher toxicity than their parent. The compounds investigated were 6-OH-PBDE 99, 6'-OH-PBDE 100, and 6'-OH-PBDE 118. Of special interest was 6'-OH-PBDE 118, a potential transformation product of PBDE 153 that is capable of photochemically generating 2,3,7,8-tetrabromodibenzo-p-dioxin, the most toxic brominated dioxin congener. Photolysis experiments were conducted at two different pH values to assess the photochemical behavior of both the phenol and phenolate form of the compounds. The percent conversion to dioxin and other photoproducts was determined and the natural product, 6-OH-PBDE 99, was found to have the highest conversion to dioxin (7%). The reaction quantum yields ranged from 0.027 to 0.16 across all photolysis conditions. In addition, it is shown that all three compounds are capable of photochemically generating other compounds of concern, including brominated phenols and a dibenzofuran.

This study investigated the homogeneous gas-phase formation of polybrominated dibenzo-p-dioxin/dibenzofurans (PBDD/Fs) from 2-BP, 2,4-DBP, and 2,4,6-TBP as precursors. First, density functional theory (DFT) calculations were carried out for the formation mechanism. The geometries and frequencies of the stationary points were calculated at the MPWB1K/6-31+G(d,p) level, and the energetic parameters were further refined by the MPWB1K/6-311+G(3df,2p) method. Then, the formation mechanism of PBDD/Fs was compared and contrasted with the PCDD/F formation mechanism from 2-CP, 2,4-DCP, and 2,4,6-TCP as precursors. Finally, the rate constants of the crucial elementary reactions were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) correction over a wide temperature range of 600-1200 K. Present results indicate that only BPs with bromine at the ortho position are capable of forming PBDDs. The study, together with works already published from our group, clearly shows an increased propensity for the dioxin formations from BPs over the analogous CPs. Multibromine substitutions suppress the PBDD/F formations.

Density functional theory (DFT) calculations have been carried out to explore the potential energy surface (PES) associated with the gas-phase reaction between 2,4,5-trichlorophenol and CuO. A gas-phase model was constructed to account, from a theoretical perspective, for the most important reaction steps reported experimentally for the interaction between chlorinated phenol and a CuO surface. This involves the facile production of the chlorophenoxy radical through hydroxyl H abstraction, formation of HOCu-2,4,5-trichlorophenolate complex, and reduction of Cu-(II) into Cu(I) through chlorophenoxy desorption from the chlorophenolate complex. The overall process: 2,4,5-trichlorophenol + CuO --> 2,4,5-trichlorophenoxy radical + CuOH is significantly exothermic and facile (unlike the strongly endothermic process of 2,4,5-trichlorophenol --> 2,4,5-trichlorophenoxy radical + H) suggesting that in the gas phase, at least, CuO would be an efficient catalyst for production of polychlorinated phenoxy radicals which are known precursors of dioxins. Hence, the present study should be an important preliminary to a detailed investigation of the efficacy of CuO surfaces toward catalysis of dioxin formation. Lastly, we estimate the reaction energies for the reaction 2,4,5-trichlorophenol + MO --> 2,4,5-trichlorophenoxy radical + MOH for the first-row transition metal monoxides. This reaction only becomes exothermic for elements which have at least a half-filled d shell. Although the results of the present thermodynamic analysis match the observed catalytic effect toward dioxin formation, kinetic considerations are expected to play a major role as well.

The homogeneous, gas-phase pyrolytic thermal degradation of a 50:50 mixture of 2-bromophenol and 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a total concentration of 88 ppm, reaction time of 2.0 s, and temperatures from 300 to 1000 degrees C. Observed products included (in decreasing yield) naphthalene, dibenzo-p-dioxin (DD), phenol, dibenzofuran (DF), bromobenzene, chloronaphthalene, 4-bromo-6-chlorodibenzofuran (4-B,6-CDF), bromonaphthalene, benzene, 4,6-dichlorodibenzofuran (4,6-DCDF), chlorobenzene, 4-monobromodibenzofuran (4-MBDF), 4-monochlorodibenzofuran (4-MCDF), 1-mono-bromodibenzo-p-dioxin (1-MBDD), 2-chloro,4-bromophenol, 2,4-dibromophenol, and 2-bromo-4-chlorophenol. Unlike the case for the pyrolysis of pure 2-chlorophenol, 4,6-DCDF was observed, but the analogous 4,6-DBDF remained undetected similar to the individual results with 2-MBP. This indicates that the presence of bromine increases the concentration of chlorine atoms available for the formation of 4,6-DCDF. Due to bromine atoms acting as better leaving groups than chlorine atoms, the yield of DD was increased over that observed for the pyrolysis of 2-chlorophenol. The addition of bromine to a chlorinated hydrocarbon system results in an increase in the total yield of PCDD/Fs as well as PBDD/Fs and mixed PBCDD/Fs due to the ease of bromine elimination reactions as well as an increase of the chlorine atom concentration.

The homogeneous, gas-phase oxidative thermal degradation of 2-bromophenol was studied in a 1 cm i.d., fused silica flow reactor at a concentration of 88 ppm, reaction time of 2.0 s, over a temperature range from 300 to 1000 degrees C. Observed products in order of yield were dibenzo-p-dioxin (DD) > 4,6-dibromodibenzofuran (4,6-DBDF) > 4-monobromodibenzofuran (4-MCDF), dibenzofuran (DF), 1-monobromodibenzo-p-dioxin (1-MBDD), naphthalene, bromonaphthalene, 2,4-dibromophenol, 2,6-dibromophenol, phenol, bromobenzene, and benzene. This result is in contrast to the oxidation of 2-chlorophenol, where the major product is 4,6-dichlorodibenzofuran (4,6-DCDF). 4,6-DBDF was observed in high yields in contrastto our previous results for the pyrolysis of 2-bromophenol, where 4,6-DBDF was not detected. The increase in 4,6-DBDF yields is attributed to hydroxyl radical being the major chain carrier under oxidative conditions, which favors hydrogen-abstraction reactions that lead to formation of 4,6-DBDF. However, DD is still the highest yield product under oxidative conditions because of the relative ease of displacement of Br in the ring-closure reaction.

The total amounts of dioxins found in exhaust gases from combustion of polyethylene (PE), polystyrene (PS), and poly(ethylene terephthalate) (PET) with approximately 3% (w/ w) NaCl were 6.07, 17.7, and 28.9 ng/g, respectively. Plastics containing benzene rings produced more dioxins than plastic containing no benzene ring. The amounts of dioxin formed in the exhaust gases from the combustion of newspapers impregnated with CaCl2, KCl, and NaCl were 18.6, 28.6, and 49.0 ng/g, respectively. Dioxin formation was associated with the bond energy between metal atom and chlorine atom. When newspapers impregnated with four different amounts of NaCl were combusted, the highest NaCl content newspaper (chlorine content 4.08%, w/w; lignin content 19.8%, w/w) produced the greatest amount of dioxins (174 ng/g). Pulp with NaCl (chlorine content 4.25%, w/w; lignin content 0.69%, w/w) produced more dioxins (6.71 ng/g) than pulp alone (0.799 ng/g) did upon combustion. The lignin content in a combustion sample correlated with the amount of dioxin formation. The results exhibited that combustion conditions with low CO concentration (<2 ppm) produced much less dioxins than conditions with high CO concentration (159 ppm).