Homologue and isomer patterns of polychlorinated dibenzo-p-dioxins and dibenzofurans from phenol precursors: comparison with municipal waste incinerator data

The role of phenol precursors in polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) formation in municipal waste incinerators is assessed on the basis of homologue and isomer patterns. Homologue and isomer patterns of PCDD and PCDF congeners formed from phenols both in the gas phase and via particle-mediated reactions were studied in an isothermal flow reactor. A mixture of unsubsitituted phenol and 19 chlorinated phenols in relative concentrations found in a municipal waste incinerator (MWI) stack gas was used for this study. PCDD and PCDF homologue and isomer patterns obtained from the phenol experiments were compared with those observed in MWI data. From the phenol experiments, gas-phase formation at 600-700 degrees C favors PCDF formation whereas particle-mediated formation at 400 degrees C favors PCDD formation. Unsubstituted phenol, which was present in high concentration, played a significant role in the formation of PCDD/F congeners under both sets of experimental conditions. PCDD/F distributions in MWI flue gas and fly ash samples were differentfrom those observed in the phenol experiments, suggesting that direct phenol condensation was not the primary route of PCDD/F formation at the incinerators. Gas-phase phenol condensation is a source of dibenzofuran, with subsequent particle-mediated chlorination resulting in PCDF formation. In the case of PCDD formation, phenol condensation may be responsible for the formation of certain highly chlorinated congeners. In this paper we demonstrate the use of homologue and isomer patterns for PCDD/F formation mechanism attribution in municipal waste incinerators.     

Potential role of chlorination pathways in PCDD/F formation in a municipal waste incinerator

The role of chlorination reactions in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in a municipal waste incinerator was assessed by comparing predicted chlorination isomer patterns with incinerator flue gas measurements. Complete distributions of PCDD and PCDF congeners were obtained from a stoker-type municipal waste incinerator operated under 13 test conditions. Samples were collected from the flue gas prior to the gas cleaning system. While total PCDD/F yields varied by a factor of 5 to 6, the distributions of congeners were similar. A conditional probability model, dependent only on the observed distribution of monochlorinated isomers, was developed to predictthe distributions of polychlorinated isomers formed by chlorination of dibenzo-p-dioxin (DD) and dibenzofuran (DF). Agreement between predicted and measured PCDF isomer distributions was high for all homologues, supporting the hypothesis that DF chlorination can play an important role in the formation of PCDF byproducts. The PCDD isomer distributions, on the other hand, did not agree well with model predictions, suggesting that DD chlorination was not a dominant PCDD formation mechanism at this incinerator. This work demonstrates the use of PCDD/F isomer patterns for testing formation mechanism hypotheses, and the findings are consistent with those from other municipal waste combustion studies.     

Distribution of mono to octa-chlorinated PCDD/Fs in fly ash from a municipal solid-waste incinerator

We have estimated the concentration and distribution of the mono to octa-chlorinated congeners of polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) in fly ashes at various sampling points in a large-scale municipal solid waste incinerator at Ume?, Sweden, as they cooled from 700 to 170 degrees C. Differences between the ashes were observed, the PCDD homologue profile was found to vary with temperature. The total amount of PCDD and PCDF increased as the temperature decreased in the postcombustion zone. The increase was due to both adsorption to the fly ash and formation of PCDD and PCDF. Mono-to trichlorinated PCDD predominated at high temperatures, whereas hepta- and octachlorinated PCDD predominated at temperatures below 400 degrees C. PCDF predominated over PCDD in the whole temperature range. However,the changes in homologue profile for PCDFwere minor. The isomer distribution within the homologue groups was not changed asthetemperature decreased in the postcombustion zone.     

De novo synthesis of polychlorinated dibenzo-p-dioxins and dibenzofurans on fly ash from a sintering process

Fly ash, collected in the electrostatic precipitator of a sinter plant in Belgium, has been examined and characterized in terms of its behavior with respect to thermal polychlorodibenzo-p-dioxins (PCDD) and polychlorodibenzofurans (PCDF) formation. Thermal experiments of the fly ash were conducted in a flow of air. The temperature was varied from 250 to 450 degrees C, and the reaction time varied from 30 min to 6 h. For comparison, the oxidative degradation of carbon in the fly ash was studied by differential scanning calorimetry (DSC) in the temperature range from 50 to 500 degrees C. Besides the known maximum of formation of PCDD/Fs around 325 degrees C generally found on experiments with incinerator fly ash, a second maximum of formation around 400 degrees C is observed on the sinter fly ash used in this study. DSC measurements on the fly ash show that the oxidative degradation of carbon appears at these two different temperatures confirming that the de novo synthesis on this kind of fly ash take place at two different optimum temperatures. About the reaction time, already after 30 min, an important quantity of PCDD/Fs is formed; the fast increase in PCDD/Fs amount is followed by a slower formation rate between 2 and 4 h. At longer reaction time, the formation slows down, and decomposition reactions become important. Analysis of homologue distribution indicates that the profile of PCDD/Fs is independent of the reaction time but that an increase of the temperature leads to a rise of lower chlorinated species. In all experiments, PCDF are formed preferentially (total PCDF/PCDD ratios larger than 5). The PCDF/PCDD ratio is clearly independent of the reaction time. Concerning the temperature, the apparently better stability of PCDF at high temperature (PCDF/PCDD ratio higher at high temperature) results in the fact of different PCDF/PCDD ratios for the different family and modifications of homologue distribution with the temperature. The isomer distribution shows little reaction time or temperature dependency, which is an argument in favor of a thermodynamic control of the isomer distribution during de novo formation of PCDD/Fs. Differences within the isomer distribution patterns of PCDD/Fs obtained from the laboratory de novo synthesis experiments and the original fly ash, reflecting the formation under the industrial process, suggest a different mechanism of formation in the sinter plant for the PCDD and PCDF. The de novo synthesis is sufficient to explain the PCDF formation in the real process, but synthesis from precursors must play a role for the PCDD formation.     

Formation of polychlorinated dibenzo-p-dioxins and dibenzofurans from a mixture of chlorophenols over fly ash: influence of water vapor

Recent efforts have been made to establish readily measurable surrogate compounds, such as chlorophenols, for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), that would enable plant operations to limit formation. Despite the extensive studies conducted on PCDDs/Fs formation from chlorophenols, very few studies have been carried out in real combustion conditions with a realistic concentration of precursors and the presence of water. In the present study, low (10(-9) M), stable concentrations of chlorinated phenols that are representative of concentrations of such compounds in municipal waste incinerator (MWI) raw flue gas were used in experiments investigating the formation of PCDDs/Fs over fly ash. Different mixtures of the chlorophenols (CPs) studied (2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, and 2,3,4,6-tetrachlorophenol) were passed through a bed of oxidized fly ash (carbon-free) and glass beads with and without the presence of water. The chlorophenol reactants used in this study were found to favor PCDD over PCDF formation. The presence of water was observed to considerably reduce the yields of all PCDD/F formed (< 0.3% phenol conversion). The PCDD homologue and isomer distributions were not affected by the presence of water, unlike the PCDF compounds. The major PCDD homologue groups formed were tetra- and penta-, both with or without water in the gas stream. The major PCDF homologue groups were mostly the lower chlorinated ones in the experiments performed in the presence or absence of water. These results contribute to the understanding of PCDD/Fs formation in realistic combustion conditions, including very low concentrations of precursors and the presence of water in the flue gas.     

Near-real-time combustion monitoring for PCDD/PCDF indicators by GC-REMPI-TOFMS

The boiler exit flue gas of a municipal waste combustor was sampled to evaluate an online monitoring system for chlorobenzene congeners as indicators of polychlorinated dibenzodioxin and dibenzofuran (PCDD/PCDF) concentrations. Continuous measurements of chlorobenzene congeners using gas chromatography coupled to a resonance-enhanced multiphoton ionization - time-of-flight mass spectrometry (GC-REMPI-TOFMS) system were compared over 5-min periods with conventional sampling methods for PCDD/PCDF. Three pairs of values were taken every hour over a period of three days to characterize the combustor's response to transient operating conditions (shutdowns and startups). Isolation of specific chlorobenzene congeners from other same-mass compounds was accomplished by using a GC column separator ahead of the REMPI-TOFMS. The 50-fold variation of PCDD/PCDF concentration was paralleled by similar changes in monitored compounds of 1,4-dichlorobenzene, 1,2,4-trichlorobenzene, 1,2,3-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene. A correlation of R = 0.85 and 0.89 was established between 40 pairs of simultaneous 5-min GC-REMPI-TOFMS measurements of 1,2,4-trichlorobenzene and 5 min conventional sampling and analysis for the TEQ and Total measures of PCDD/PCDF, respectively. The GC-REMPI-TOFMS system can be used to provide frequent measures of correlative PCDD/PCDF concentration thereby allowing for an understanding of measures to minimize PCDD/PCDF formation and develop operational feedback to limit emissions.     

Polychlorinated naphthalenes, -biphenyls, -dibenzo-p-dioxins, and -dibenzofurans in double-crested cormorants and herring gulls from Michigan waters of the Great Lakes

Concentrations of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), naphthalenes (PCNs), and biphenyls (PCBs) were measured in eggs of double-crested cormorants and herring gulls collected from Michigan waters of the Great Lakes. Concentrations of PCNs in eggs of double-crested cormorants and herring gulls were in the ranges of 380-2400 and 83-1300 pg/g, wet wt, respectively. Concentrations of 2,3,7,8-substituted PCDDs and PCDFs were 10-200 times less than those of PCNs in eggs whereas those of total PCBs (380-7900 ng/g, wet wt) were 3-4 orders of magnitude greater. While the profile of PCB isomers and congeners between double-crested cormorants and herring gulls was similar, the PCN isomer profile differed markedly between these two species. PCN congeners 66/67 (1,2,3,4,6,7/1,2,3,5,6,7) accounted for greater than 90% of the total PCN concentrations in herring gulls, whereas their contribution to total PCN concentrations in double-crested cormorants ranged from 18 to 40% (mean, 31%). The ratios of concentrations of PCDDs to PCDFs were greater in herring gulls than in double-crested cormorants collected from the same locations, suggesting the ability of the former to metabolize PCDF congeners relatively rapidly. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) equivalents (TEQs) contributed by PCNs in double-crested cormorant and herring gull eggs were 2-3% of the sum TEQs of PCBs, PCDDs, PCDFs, and PCNs. PCB congener 126 (3,3',4,4',5-PeCB) accounted for 57-72% of the total TEQs in double-crested cormorant and herring gull eggs.     

Different catalytic effects by copper and chromium on the formation and degradation of chlorinated aromatic compounds in fly ash

Fly ash from municipal solid waste incineration may catalytically enhance the formation and degradation of chlorinated aromatic compounds. The activities of three Deacon catalysts in this process were investigated in a statistically designed experiment. Chlorides of copper, chromium, and nickel were added to fly ash samples and the resulting samples heated at 300 degrees C for 2 h in an air atmosphere. The addition of copper increases the formation of all chlorinated aromatic compounds except the low chlorinated congeners of polychlorinated dibenzo-p-dioxins and dibenzofurans. The addition of chromium decreased the formation of most chlorinated aromatic compounds except the highest chlorinated species, where it was without effect. The addition of nickel did not show any significant effect. The outcome of the experiment can be interpreted as two competing processes: the chlorination of aromatic rings and the oxidation of carbon-carbon and carbon-oxygen bonds. The delicate balance between chlorination and oxidation could probably be further exploited to minimize both the emissions and the net production of chlorinated aromatic compounds from combustion.     

Characterization of trophic transfer for polychlorinated dibenzo-p-dioxins, dibenzofurans, non- and mono-ortho polychlorinated biphenyls in the marine food web of Bohai Bay, North China

Many investigations have highlighted the bioaccumulation of dioxins in animals, but little is known about the trophodynamics of dioxins in the food web. In this study, the trophic transfer of nine dibenzo-p-dioxin (PCDD) congeners, eleven dibenzofuran (PCDF) congeners, and twelve non-, mono-ortho polychlorinated biphenyl (non- and mono-ortho PCBs) congeners in a marine food web were determined. The concentrations of PCDDs, PCDFs, non- and mono-ortho PCBs were analyzed in phytoplankton/ seston, zooplankton, three invertebrate species, six fish species, and one seabirds species collected from Bohai Bay, representing approximately 4 trophic levels based on stable nitrogen isotope values. Positive relationships were found between trophic levels and lipid equivalent concentrations of non- and mono-ortho PCBs except for PCB-77, PCB-81, PCB-126, PCB-156, and PCB-167, indicating bioaccumulation of these compounds in this food web. But lipid equivalent concentrations of low chlorinated 2,3,7,8-substituted-PCDD/Fs did not exhibit statistically significant trends with trophic levels. And lipid equivalent concentrations of high chlorinated 2,3,7,8-substituted-PCDD/Fs and three non-2,3,7,8-substituted-PCDD/Fs declined significantly with increasing trophic levels providing that these isomers undergo trophic dilution. The similarity in log Kow values for non-, mono-ortho PCBs, non-2,3,7,8-substituted-PCDD/Fs, and some 2,3,7,8-substituted-PCDD/Fs suggests that the difference of trophic transfer is mainly due to their different metabolic transformation rates.     

Transformation processes,pathways, and possible sources of distinctive polychlorinated dibenzo-p-dioxin signatures in sink environments

In recent years, studies on environmental samples with unusual dibenzo-p-dioxin (PCDD) congener profiles were reported from a range of countries. These profiles, characterized by a dominance of octachlorinated dibenzodioxin (OCDD) and relatively low in dibenzofuran (PCDF) concentrations, could not be attributed to known sources or formation processes. In the present study, the processes that result in these unusual profiles were assessed using the concentrations and isomer signatures of PCDDs from dated estuarine sediment cores in Queensland, Australia. Increases in relative concentrations of lower chlorinated PCDDs and a relative decrease of OCDD were correlated with time of sediment deposition. Preferred lateral, anaerobic dechlorination of OCDD represents a likely pathway for these changes. In Queensland sediments, these transformations result in a distinct dominance of isomers fully chlorinated in the 1,4,6,9-positions (1,4-patterns), and similar 1,4-patterns were observed in sediments from elsewhere. Consequently, these environmental samples may not reflect the signatures of the original source, and a reevaluation of source inputs was undertaken. Natural formation of PCDDs, which has previously been suggested, is discussed; however, based on the present results and literature comparisons, we propose an alternative scenario. This scenario hypothesizes that an anthropogenic PCDD precursor input (e.g. pentachlorophenol) results in the contamination. These results and hypothesis imply further investigations are warranted into possible anthropogenic sources in areas where natural PCDD formation has been suggested.     

PCDD/F TEQ indicators and their mechanistic implications

Stack gas samples from two incinerator facilities with different operating conditions were investigated to understand how indicators of toxic equivalency (TEQ) from among the 210 polychlorinated dibenzo-p-dioxin/furan (PCDD/F) isomers varied. This effort was motivated by the need to find more easily monitored indicator compound(s) of TEQ and to reconcile the varying indicator compounds reported in the literature. The measured isomer patterns were compared with those expected from known formation mechanisms to identify the dominant mechanism(s) and explain why certain compounds are relevant TEQ indicators. Despite differences in the facility types and operating conditions, a common pattern was found for the highly chlorinated (4Cl and higher) PCDDs/Fs. A combination of chlorination/dechlorination reactions as the dominant formation mechanism for PCDF was consistent with the observed isomer patterns, whereas condensation reactions of phenolic precursors appeared to be responsible for PCDD formation. PCDF isomers, ratherthan the PCDD isomers, were more closely related to the TEQ measure, likely because the chlorination mechanism favors 2,3,7,8-Cl-substitution more than the phenol condensation mechanism. Unlike highly chlorinated PCDD/F isomer patterns, less chlorinated PCDD/F patterns (especially, mono- and di-CDF) were sensitive to operating conditions and facility type. Competing formation mechanisms were inferred from the variation of observed isomer distribution patterns; this sensitivity resulted in relatively low correlations of these isomers with PCDD/F TEQ values. This suggests that any use of the low-chlorinated compounds as TEQ indicators for online monitoring processes are likely best suited for plant-specific, rather than universal, applications. In addition to many of the highly chlorinated (penta-CDF, hexa-, and heptaCDD/F) isomers being identified as strong TEQ indicators, 1 of 12 (8%), 5 of 17 (29%), and 5 of 28 (18%) of the separable tri-CDD, tri-CDF, and tetra-CDF isomers, respectively, were identified as strong (R2 > 0.7) TEQ indicators in both incinerators.     

Formation of PCDD/Fs from the copper oxide-mediated pyrolysis and oxidation of 1,2-dichlorobenzene

Formation of polychorinated dibenzo-p-dioxins (PCDDs) has been demonstrated to occur via surface-mediated reactions of chlorinated phenols. However, polychlorinated dibenzofurans (PCDFs) are observed in much lower yields in laboratory studies than in full-scale combustors where PCDFs are in higher concentrations than PCDDs. This has led to the suggestion that at least PCDFs are formed from elemental carbon in the de novo process. However, the potential for PCDF formation from reactions of chlorinated benzenes has been largely overlooked. In this study, we investigated the potential contribution of chlorinated benzenes to formation of PCDD/Fs using 1,2-dichlorobenzene as a surrogate for reactions of other chlorinated benzenes and CuO/silica (3 wt % Cu) as a surrogate for fly ash. Results were similar for oxidative and pyrolytic conditions with a slight increase in more chlorinated products under oxidative conditions. Reaction products included chlorobenzene, polychlorinated benzenes, phenol, 2-monochlorophenol (2-MCP), dichlorophenols, and trichlorophenols with yields ranging from 0.01 to 2% for the phenols and from 0.01 to 10% for chlorinated benzenes. 4,6-Dichlorodibenzo furan (4,6-DCDF) and dibenzofuran (DF) were observed in maximum yields of 0.2% and 0.5%, respectively, under pyrolytic conditions and 0.1% and 0.3%, respectively, under oxidative conditions. In previous studies of the pyrolysis of 2-MCP under identical conditions, 4,6-DCDF and dibenzo-p-dioxin (DD) were observed with maximum yields of ～0.2% and ～0.1%, respectively, along with trace quantities of 1-monochlorodibenzo-p-dioxin (1-MCDD). Under oxidative conditions, 1-MCDD, DD, and 4,6-DCDF were observed with maximum yields of 0.3%, 0.07% and 0.1%, respectively. When combined with the fact that measured concentrations of chlorinated benzenes are 10-100× that of chlorinated phenols in full-scale combustion systems, the data suggest surface-mediated reactions of chlorinated benzenes can be a significant source of PCDD/F emissions.     

Polychlorinated naphthalenes in the Global Atmospheric Passive sampling (GAPS) study

Air concentrations of polychlorinated naphthalenes (PCNs) were measured as part of the Global Atmospheric Passive Sampling (GAPS) study to assess their spatial distribution on a worldwide basis for the first sampling period between December 2004 and March 2005. Results from more than 40 sites on seven continents show that PCNs are widespread, and highest levels are detected in urban/industrial locations consistent with other air sampling studies. The geometric mean air concentration of sigmaPCN is 1.6 pg/m3, ranging from below detection limit to 32 pg/m3. With technical PCN mixtures largely no longer produced, combustion inputs may be contributing increasingly to contemporary PCN air burden globally. Enrichment of combustion-related congeners, e.g., PCN-52/60, -50, -51,-54, and -66/67, is observed in the congeneric compositions of air at nearly all sites compared to relatively minor contribution of these congeners in technical PCN formulations. Further evidence of current combustion sources influencing global PCN levels is a higher relative abundance of combustion-related congeners quantified by sigmaPCNcombustion/sigmaPCN. The relative contribution by combustion sources and emissions from technical PCN mixtures is expected to vary among sites since it depends on the combustion sources and the technical mixture used in a particular country or region.     

Formation of chlorinated aromatics by reactions of Cl*, Cl2, and HCl with benzene in the cool-down zone of a combustor

Conversion of benzene to chlorobenzenes and monochlorophenols by reaction with chlorine radicals (Cl*) in the cool-down zone of a plug-flow combustor has been studied, and a mechanistic analysis of the initial steps of the oxy-chlorination process is proposed. Superequilibrium concentrations of Cl* are formed during combustion of chlorocarbon species and persist at significant concentration levels even after a substantial reduction in the flue gas temperature (T = 500-700 degrees C). At these temperatures, Cl* attack on benzene present in trace concentrations (initial benzene concentration of 300 ppmv or 1080 ppmv were used for the experiments) in the post-flame gas is shown to result in stable chlorinated products (chlorobenzenes and chlorophenols) and loss of benzene. These results suggest that Cl* attack on trace level aromatics and possibly other organic species may be the initial step in the formation of a broad class of chlorinated and oxy-chlorinated pollutants in the post combustion zone.     

Chlorinated aromatics from combustion: influence of chlorine,combustion conditions,and catalytic activity

Research on the formation of chlorinated aromatics in combustion processes has mainly taken place in the laboratory. Previous attempts to correlate observation data from commercial plants have been inconclusive. This study reports on the outcome of an industrial experiment in a full-scale afterburner. The influence of chlorine input, combustion temperature, and catalytic activity was investigated in a factorial design with two blocks. Polychlorinated benzenes, dibenzo-p-dioxins, and dibenzofurans were formed both at combustion temperatures and below 400 degrees C. The results show that all three experimental factors have statistically significant impact on the formation and release of these toxic byproducts. The quantitative dependence between chlorine input and the occurrence of chlorinated aromatics is of particular interest due to previous controversy. The purpose with this study was to ensure that the installation of a boiler for energy recovery would not cause elevated emissions of chlorinated aromatics. The experiment demonstrated that this risk is probably low, since the presence of catalytic material or an increase in chlorine input is required for this to happen. A general conclusion was that industrial experimentation employing the principles of statistical design could improve the validity in recommendations regarding commercial plant operation.     

Origin of carbon in polychlorinated dioxins and furans formed during sooting combustion

The importance of solid- and gas-phase carbon precursors for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) during sooting combustion was investigated in an entrained flow reactor (EFR). Experiments were performed at various methane (CH4) flame equivalence ratios with or without gas-phase chlorine (Cl2) and fly ash, to provide a realistic environment for carbon reactions and PCDD/DF formation. Selected experiments were conducted with labeled 13CH4 and 37Cl2 to investigate the relative roles of different carbon and chlorine species for the formation of PCDD/DF. The presence of soot and ash were the two major factors controlling the PCDD/DF yields. The 16 PCDD/DF homologues as well as other analyzed chlorinated aromatics were formed by reaction pathways that varied with degree of chlorination. The mono- and dichlorinated homologues were formed by gas-phase, catalytic, or noncatalytic flame product reactions, occurring during soot formation in the near flame zone and/or at lower reaction temperatures (<650 degrees C) in the postcombustion zone. Meanwhile, the higher (tri- to octa-) chlorinated homologues were mainly formed in the postcombustion zone (<650 degrees C) by fly ash-catalyzed de novo synthesis of the soot. Of these, the PCDD/DFs were formed from high carbon number (>C12) fragments in the solid soot structure, while the PCDDs, at least in part, were also formed by reaction of two C6 fragments. The tri- to hexachlorinated DD/DF homologues were formed via a relatively fast de novo synthesis occurring during the first minutes of reactions on the continuously formed soot particles, whereas de novo synthesis on an aged soot matrix was the major pathway for the hepta- and octachlorinated congeners.     

Key parameters for de novo formation of polychlorinated dibenzo-p-dioxins and dibenzofurans

De novo formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) was investigated in an Entrained Flow Reactor (EFR) to simulate combustion conditions. The parameters investigated were carbon content and nature in fly ash; type of gas-phase environment (oxidative versus reducing conditions) influence of combustion gases such as water, carbon monoxide, and carbon dioxide; amount of gas-phase chlorine; reaction temperature (250-600 degrees C); and reaction time (minutes vs hours). The comprehensive data set was further evaluated with principal component analysis (PCA) to statistically determine the role and importance of each parameter for de novo formation of PCDDs and PCDFs. Results revealed that an initial fast de novo formation occurs within the first minutes with a formation rate in the orders of hundreds of pmol per minutes; however, the reactivity of the ash was found to decline with time. An average formation rate as low as 3 pmol/min was measured after 6 h. The slower de novo formation of PCDDs and PCDFs was found to be through different reaction mechanisms and, thus, controlled by different parameters. The amount of Cl2 in the gas phase was observed to be an important parameter for PCDFs formation; meanwhile the levels of O2 were not found to be a PCDF rate controlling parameter. The formation rate of PCDDs was significantly lower than the PCDFs, and two mechanisms appear to be controlling the formation, one depending on the amount of O2 and one on the amount of Cl2 present in the gas phase. Overall the most significant parameter for the rate of formation for both PCDDs and PCDFs was revealed to be the reaction temperature. A maximum rate of formation was observed between 300-400 degrees C for the PCDDs and 400-500 degrees C for the PCDFs.     

A quantum mechanical study on the formation of PCDD/Fs from 2-chlorophenol as precursor

The most direct route to the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in combustion and thermal processes is the gas-phase reaction of chemical precursors such as chlorinated phenols. Detailed insight into the mechanism and kinetics properties is a prerequisite for understanding the formation of PCDD/Fs. In this paper, we carried out molecular orbital theory calculations for the homogeneous gas-phase formation of PCDD/Fs from 2-chlorophenol (2-CP). The profiles of the potential energy surface were constructed, and the possible formation pathways are discussed. The single-point energy calculation was carried out at the MPWB1K/ 6-311+G(3f,2p) level. Several energetically favorable formation pathways were revealed for the first time. The rate constants of crucial elementary steps were deduced over a wide temperature range of 600 approximately 1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT). The rate-temperature formulas were fitted. The ratio of PCDD to PCDF formed shows strong dependency on the reaction temperature and chlorophenoxy radicals (CPRs) concentration.     

An isomer prediction model for PCNs, PCDD/Fs, and PCBs from municipal waste incinerators

Isomer patterns of polychlorinated naphthalenes (PCNs), polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated biphenyls (PCBs), and polychlorinated dibenzofurans (PCDFs) from municipal waste incinerators (MWIs) were predicted by a model based on symmetry numbers and preferential chlorination positions. Fly ash isomer patterns from five stoker and seven fluidized bed incinerators were compared to validate the prediction model. The isomer patterns of the highly chlorinated PCN homologues from stoker type incinerators were successfully predicted. The relative equilibrium concentrations of tetrachloronaphthalenes (TeCNs), calculated by an ab initio method, cannot explain the field isomer patterns. Formation pathways involving chlorophenol precursor condensation reactions should be examined to see whether these isomer patterns provide a better fit to the field PCDD data. The PCB isomer patterns were fit reasonably well, but this finding could merely be an artifact of the limited data and the large number of isomers. The prediction equations of PCDFs, revised from prior work to include a symmetry number for each isomer, represented the field data patterns for the higher chlorinated isomers very well. Successful prediction of isomer patterns for partial homologue ranges suggests that these patterns are determined by a mechanism governed by Cl-position-specific preferences.     

Formation and control of toxic polychlorinated compounds during incineration of wastes containing polychlorinated naphthalenes

To estimate the potential impact on municipal solid waste (MSW) incinerator toxic equivalent (TEQ) emissions of treating wastes containing polychlorinated naphthalenes (PCNs), pilot-scale thermal treatment experiments were conducted. MSW (run 1) and MSW fortified with synthetic rubber belts containing PCNs (runs 2 and 3) were incinerated. Flue-gas and ash samples were analyzed for polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), coplanar polychlorinated biphenyls (co-PCBs), and PCNs. Final exhaust-gas WHO-TEQ emissions were all less than 0.1 ng/Nm3. Flue-gas TEQs were mainly from PCDFs (58-74%). When 2,3,7,8-tetrachlorodibenzo-p-dioxin relative potency factors (REPs) of specific PCN congeners from previous reports were used as estimated toxic equivalency factors to compute estimated PCN TEQs and total TEQs along with PCDDs, PCDFs, and co-PCBs, the contributions of PCNs to the total TEQs were small in ash samples and up to 28% in final exhaust gas. The TEQs in primary combustion flue gases increased through the formation of dioxins and PCNs and then decreased via secondary combustion, fabric filtration, and activated carbon adsorption. From this pilot-scale study, the incremental impact of incinerating PCN-containing wastes on annual TEQ emissions in Japan is estimated as 0.27 g of total TEQ.