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.     

Role of chlorine in combustion field in formation of polychlorinated dibenzo-p-dioxins and dibenzofurans during waste incineration

Combustion experiments performed in the presence of hydrogen chloride (HCl) in a laboratory-scale fluidized-bed reactor were carried out to elucidate the role of chlorine in the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs; together: PCDD/Fs) in various sections of a municipal waste incinerator. We first demonstrated that the homologue profile and the pattern of the congener proportions of PCDD/Fs for a model waste containing poly(vinyl chloride) (PVC) combusted in the absence of HCl were similar to those for a PVC-free waste combusted in the presence of HCl. This showed no difference between PVC in the waste and injected HCl in the role as a chlorine source in PCDD/F formation during incineration. Next, to investigate PCDD/F formation in each section of the incinerator, we carried out combustion experiments with the PVC-free waste, injecting HCl at different locations of the incinerator. The amounts of PCDDs and PCDFs formed were significantly reduced when HCI was not supplied to the main combustion section. The presence of HCI in the main combustion section was essential for the formation of PCDD/Fs, even in the downstream sections. This finding indicates that compounds that were able to form PCDD/Fs in the downstream sections were mainly formed in the main combustion section in the presence of HCl.     

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.     

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.     

Effect of moisture, charge size, and chlorine concentration on PCDD/F emissions from simulated open burning of forest biomass

Loblolly pine (Pinus taeda) was combusted at different charge sizes, fuel moisture, and chlorine content to determine the effect on emissions of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/Fs) as well as copollutants CO, PM, and total hydrocarbons. The experiments were performed in an enclosed chamber under conditions simulating open, prescribed burns of forest biomass. Burn progress was monitored through on line measurement of combustion gases and temperature while PCDD/F concentrations were determined by ambient sampling methods. PCDD/F toxic equivalency (TEQ) and total (tetra- to octa-CDD/F) emission factors were independent of charge size (1-10 kg) and moisture content (7-50%). However, the lower chlorinated, mono- to tri-CDD/F compounds were increased by poor combustion conditions: combustion efficiency lower than 0.919 was generally found when the moisture content was higher than 30%. The increase of fuel matrix chlorine from 0.04% to 0.8% using a brine bath resulted in about a 100-fold increase of PCDD/F to about 90 ng TEQ/kg of carbon burned, C(b). These emission factors were linearly dependent on Cl concentration in the biomass. PCDD 2,3,7,8-Cl-substituted congeners and homologue patterns were also influenced by the addition of chlorine resulting in emissions with a higher abundance of the most toxic congeners (TeCDD and PeCDD). When both chlorine and moisture content were increased in the fuel, a simultaneous effect of the two parameters was observed. The increased TEQ values expected from higher Cl concentrations were mitigated by the presence of water, giving MCE = 0.868, promoting formation of mono- to tri-PCDD/F, and lowering the TEQ value. Open burn simulations were used to study PCDD/F formation in different combustion conditions providing a mathematical correlation between PCDD/F emissions and chlorine and moisture content in the fuel.     

Effect of nitrogen-containing compounds on polychlorinated dibenzo-p-dioxin/dibenzofuran formation through de novo synthesis

An experimental study was conducted to clarify the suppression effect of nitrogen-containing compounds, that is, ammonia and urea, on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) through the de novo synthesis reaction. In the experiment, graphite and copper chloride contained in a mixture were used as sources of carbon and chlorine, respectively. The granulated sample mixture was charged as a packed-bed in the glass tube and heated at 300 degrees C in the flow of Ar-O2 gas mixture. In some cases, urea was added as aqueous solution to the sample, while ammonia was added to the gas flowed through the sample bed. The amount of PCDD/Fs formed decreases significantly by the addition of both ammonia and urea. Particularly, the addition of urea reduces the amount of PCDD/Fs discharged in the outlet gas by approximately 90%. The oxidation rate of carbon in the early stage of the experiment, that is, the heating period, is promoted bythe addition of nitrogen-containing compounds. However, soon afterthe temperature reaches 300 degrees C, the formation rate becomes lower than that of the case without the addition of nitrogen-containing compounds. On the other hand, organic compounds containing amino (-NH2) or cyanide (-CN) groups and those containing nitrogen within the carbon ring frame were detected in the outlet gas in the case of urea addition. Typically observed aromatic compounds are chlorobenzonitriles, chlorobenzeneamines, and chloropyridines. This suggests a possibility that hydrogen and/or chlorine combined with PCDD/Fs are also substituted by such nitrogen-containing groups, and this decreases the formation rate of their frame of carbon rings. This phenomenon was also consistent with the fact that a significant reduction was observed in the amount of PCDD/Fs released to the outlet gas when urea was added.     

In situ formed soot deposit as a carbon source for polychlorinated dibenzo-p-dioxins and dibenzofurans

The aim of this study was to investigate the role of in situ formed soot deposits generated during a combustion process for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs). In situ formed soot deposits were generated in an entrained flow reactor by using a sooting methane (CH4) flame (sooting phase), with or without chlorine doped into the flame, and fly ash added into the gas phase. The presence of fly ash in the soot deposit was found to be critical, as a catalyst for formation and/or a chlorinating agent. The presence of chlorinated aromatic structures in the soot matrix was not enough to promote de novo formation of PCDDs/Fs without the presence of fly ash. PCDFs were formed via direct release of the molecule backbone structure from the soot. PCDDs were formed via a similar mechanism as well as an equally important formation pathway of condensation reactions of C6 compounds. The formation rate of the soot/ash depositwas still at half its original activity 34 h after the deposits were formed, suggesting a persistent de novo formation occurring for a long time after the sooting incidences (memory effect).     

Amines compounds as inhibitors of PCDD/Fs de novo formation on sintering process fly ash

The polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are highly toxic compounds produced by some natural processes and different human activities. Waste incineration as well as steel and iron industries, in particular, the sintering process, are among the principal sources of these pollutants. In this paper, two inhibitors, triethanolamine (TEA) and monoethanolamine (MEA) are tested relating to their abilityto prevent the de novo formation of PCDD/Fs on sinter plant fly ash. The amounts of both PCDDs and PCDFs, formed by thermal treatment of the fly ash, decrease when inhibitors are added. The best results, up to 90% reduction of the PCDD/ Fs formation, are obtained when MEA is mixed with the fly ash at the highest concentration tested (2 wt %). The addition of inhibitors modifies the PCDFs/PCDDs ratios and, under some experimental conditions, the PCDD/Fs homologue distributions, suggesting that more than one pathway for the de novo formation of PCDD/Fs exist. On the other hand, no modification in the PCDD/Fs isomer distributions is observed as a result of the addition of inhibitors, in accordance with the possible thermodynamic control of these distributions. The temperature tested, 325 and 400 degrees C, does not affect the inhibition activity; however, longer reaction times (4 h instead of 2 h) give better percentages of PCDD/Fs reduction. The results suggest that the two inhibitors and especially MEA can reduce the PCDD/Fs formation on sinter plant fly ash under various conditions of temperature and reaction time, making them suitable for use in the real process. Tests performed in parallel at a real sinter plant are in good agreement with the laboratory experiments and confirm that the use of inhibitors is an appropriate technique for the prevention of PCDD/Fs emissions from sintering processes.     

Influence of metallic chlorides on the formation of PCDD/Fs during low-temperature oxidation of carbon

Experimental study was conducted to clarify the formation behavior of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from carbonaceous materials through a de novo synthesis route. Samples were prepared by changing mixing method and composite state of metallic chloride in graphite powder in order to simulate the texture of "unburned carbonaceous particles", i.e., soot, formed in thermal processes. Reagents of KCl, CaCl2. 2H2O, FeCl3 x 6H2O, and CuCl2 x 2H2O were used as chlorine sources and were added to graphite powder with different methods. The composite state of metallic chloride was varied by preliminary treatments: hand-mixing, mixed-grinding using a high-intensity ball mill, and preheating at different temperatures between 500 and 1100 degrees C. In the de novo experiment, reaction temperature and oxygen concentration of flowed gas were set at 300 degrees C and 2.5 mol %, respectively. During the experiment, formation rates of CO and CO2 and the formed amounts of organic chlorine and PCDD/Fs were measured. The results show a reasonable relation between the amount of formed organic chlorine and oxidation rate of carbon, and the order of the activity of metallic chlorines was obtained as KCl < CaCl2 < FeCl3 < CuCl2. Furthermore, it was found that the effect of the composite state of metallic chloride on the formation of PCDD/Fs significantly depends on the kind of metal. The results will give useful information to examine the formation mechanism of PCDD/Fs from unburned carbon particles in thermal processes.     

Experimental study on the effect of SO2 on PCDD/F emissions: determination of the importance of gas-phase versus solid-phase reactions in PCDD/F formation

Cofiring coal in municipal solid waste incinerators (MSWls) has previously been reported to reduce polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) emissions due to increasing the flue gas SO2 concentration. The present study was focused on understanding the primary mechanism responsible for the suppressant effect of SO2 on total PCDD/F formation and toxic equivalent (TEQ) emissions. The addition of SO2, simulating the effect of coal addition on the flue gas composition, resulted in significant reductions in the TEQ emissions due to reactions involving SO2 in the postcombustion zone. However, emissions of total PCDDs/Fs, unlike the TEQ value, were dependent upon the Cl2 and SO2 injection temperatures due to increases in non-TEQ correlated isomers. The conversion of metal chlorides in the fly ash to sulfates, thus reducing the sites responsible for chlorination/oxidation reactions, was concluded to be the main suppressant mechanism; proposed reactions for copper and iron are presented. This mechanism was found to be independent of combustion conditions and could have prolonged effects on PCDD/F emissions from deposits formed with high flue gas S/Cl ratios.     

Assessing forest fire as a potential PCDD/F source in Queensland, Australia

Forest fires are suggested as a potential and significant source of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs), even though no studies to date provide sufficient evidence to confirm forest fires as a source of PCDD/Fs. Recent investigations in Queensland, Australia have identified a widespread contamination of PCDDs (in particular OCDD) in soils and sediments in the coastal region from an unknown source of PCDD/Fs. Queensland is predominately rural; it has few known anthropogenic sources of PCDD/Fs, whereas forest fires are a frequent occurrence. This study was conducted to assess forest fires as a potential source of the unknown PCDD/F contamination in Queensland. A combustion experiment was designed to assess the overall mass of PCDD/Fs before and after a simulated forest fire. The results from this study did not identify an increase in sigmaPCDD/Fs or OCDD after the combustion process. However, specific non-2,3,7,8 substituted lower chlorinated PCDD/Fs were elevated after the combustion process, suggesting formation from a precursor. The results from this study indicate that forest fires are unlikely to be the source of the unknown PCDD contamination in Queensland, rather they are a key mechanism for the redistribution of PCDD/Fs from existing sources and precursors.     

Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) in fires of arsenic-free treated wood: role of organic preservatives

This article demonstrates that biocidal organochlorines such as tebuconazole and permethrin, employed in formulations of wood preservatives, produce significant quantities of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran (PCDD/F) when subjected to thermal decomposition under oxidative conditions. Both tebuconazole and permethrin form PCDD/F during gas-phase oxidation, but much greater yields occurred in the presence of surrogate ash corresponding to wood treated with copper-based fungicides. The significant yields have implications for the increased toxicity of PCDD/F emissions during fires of wood impregnated by combination of organic and copper-based preservatives. The oxidative pyrolysis of tebuconazole and permethrin over simulated wood ash generated amounts of PCDD/F exceeding those of untreated wood by 3 orders of magnitude. We obtained yields of 1000 ng WHO97-TEQ/g tebuconazole and 5500 ng WHO97-TEQ/g permethrin when reacting the organochlorines in an oxidizing atmosphere over surrogate wood ash. Gas-phase oxidation also produce measurable quantities of PCDD/F, corresponding to 1 ng WH097-TEQ/g tebuconazole and 36 ng WHO97-TEQ/g permethrin. In the case of tebuconazole, the present measurements correlate well with those obtained from oxidative pyrolysis of CBA-treated wood in the cone calorimeter. It appears that permethrin and tebuconazole provide phenyl and diphenyl precursors to formation of PCDD/F and both constitute a source of chlorine upon fragmentation.     

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.     

Discrimination of aerial deposition sources of polychlorinated dibenzo-p-dioxin and polychlorinated dibenzofuran downwind from a pulp mill near Ketchikan, Alaska

Drinking water is supplied by individual roof-catchment systems for homes and businesses near a dissolving sulfite pulp mill (now closed) located just north of Ketchikan in southeast Alaska. This study was conducted to determine if polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/Fs) found in the sediments of the roof-catchment cisterns resulted from historical deposition of stack emissions from the pulp mill's multi-fuel power boilers. Fly ash from the power boilers had maximum total PCDD/F concentrations of 3.08 x 10(5)-3.10 x 10(6) ng/kg, which resulted from combustion of bleach plant wastewater sludge and saltwater-soaked wood waste. Cistern sediments had maximum total PCDD/F concentrations of 7.71 x 10(4) ng/kg. Potential sources of PCDDs/Fs in the cistern sediments were considered to be automobile exhaust, heating oil combustion, and private trash burning as well as pulp mill boiler emissions. Discriminant analysis was used to analyze differences between profiles of tetra through octa homologue classes of PCDDs/ Fs (defined as proportional contributions to total concentration) from different source terms. Homologue profiles of potential sources from Ketchikan included in this analysis were fly ash collected from the mill's power boilers and soils collected from background areas (areas with similar PCDD/F sources as the residences [e.g., auto exhaust and burn barrels] near the mill but beyond the zone of aerial deposition of emissions from the mill). Profiles for emissions from automobile exhaust, fertilizers, oil heating, residential trash burning, and residential wood heating were also included in the source "training" data set (for the discriminant analysis) using data from published literature. The classification rules developed from the discriminant analysis were applied to the following test media sampled at Ketchikan: roof-catchment cistern sediments and soils collected from areas in the vicinity of the mill's power boilers (i.e., nearby residential or commercial [developed] areas, on the mill property, and nearby forestlands). The homologue profiles of cistern sediment and nearby developed area soil samples were similar to background soils, whereas the profiles for the forestland soil samples (influenced by emissions from the mill but not other anthropogenic sources) closely matched the fly ash pattern. The homologue profiles of the emission sources from published data were more similar to one another than either background soils or fly ash. Soil samples from the mill property were classified as members of all source groups. On the basis of these analyses, the composition of PCDDs/Fs detected in the cistern sediments is typical of Ketchikan background conditions and not reflective of mill emissions.     

Secondary effects of catalytic diesel particulate filters: copper-induced formation of PCDD/Fs

Potential risks of a secondary formation of polychlorinated dibenzodioxins/furans (PCDD/Fs) were assessed for two cordierite-based, wall-through diesel particulate filters (DPFs) for which soot combustion was either catalyzed with an iron- or a copper-based fuel additive. A heavy duty diesel engine was used as test platform, applying the eight-stage ISO 8178/4 C1 cycle. DPF applications neither affected the engine performance, nor did they increase NO, NO2, CO, and CO2 emissions. The latter is a metric for fuel consumption. THC emissions decreased by about 40% when deploying DPFs. PCDD/F emissions, with a focus on tetra- to octachlorinated congeners, were compared under standard and worst case conditions (enhanced chlorine uptake). The iron-catalyzed DPF neither increased PCDD/F emissions, nor did it change the congener pattern, even when traces of chlorine became available. In case of copper, PCDD/F emissions increased by up to 3 orders of magnitude from 22 to 200 to 12 700 pg I-TEQ/L with fuels of < 2, 14, and 110 microg/g chlorine, respectively. Mainly lower chlorinated DD/Fs were formed. Based on these substantial effects on PCDD/F emissions, the copper-catalyzed DPF system was not approved for workplace applications, whereas the iron system fulfilled all the specifications of the Swiss procedures for DPF approval (VERT).     

Open dumping site in Asian developing countries: a potential source of polychlorinated dibenz-p-dioxins and polychlorinated dibenzofurans

Open landfill dumping areas for municipal wastes in Asian developing countries have recently received particular attention with regard to environmental pollution problems. Because of the uncontrolled burning of solid wastes, elevated contamination by various toxic chemicals including dioxins and related compounds in these dumping sites has been anticipated. In this study, concentrations of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and coplanar polychlorinated biphenyls (PCBs) were determined in soils from dumping sites in the Philippines, Cambodia, India, and Vietnam. Residue concentrations of PCDD/Fs and coplanar PCBs in dumping site soils were apparently greater than those in soils collected in agricultural or urban areas far from dumping sites, suggesting that dumping sites are potential sources of PCDD/Fs and related compounds. Observed PCDD/F concentrations in soils from dumping sites in the Philippines and Cambodia were comparable or higher than those reported for dioxin-contaminated locations in the world (e.g., near the municipal waste incinerators and open landfill dumping sites). Homologue profiles of PCDD/Fs in dumping site soils from the Philippines and, to a lesser extent, from Cambodia and India reflected patterns of samples representing typical emissions, while profiles of agricultural or urban soils were similar to those of typical environmental sinks. This result suggests recent formation of PCDD/Fs in dumping site areas and that open dumping sites are a potential source of dioxins in Asian developing countries. Uncontrolled combustions of solid wastes by waste pickers, generation of methane gas, and low-temperature burning can be major factors for the formation of dioxins in dumping sites. Elevated fluxes of PCDD/Fs to soils in dumping sites were encountered in the Philippines, Cambodia, India, and Vietnam-Hanoi, and these levels were higher than those reported for other countries. Considerable loading rates of PCDD/Fs in the dumping sites of these countries were observed, ranging from 20 to 3900 mg/yr (0.12-35 mg TEQ/yr). PCDD/F concentrations in some soil samples from the Philippines, Cambodia, India, and Vietnam-Hanoi exceeded environmental guideline values, suggesting potential health effects on humans and wildlife living near these dumping sites. The estimated intakes of dioxins via soil ingestion and dermal exposure for children were higher than those for adults, suggesting greater risk of dioxin exposure for children in dumping sites. To our knowledge, this is the first comprehensive study on PCDD/Fs contamination in open dumping sites of Asian developing countries. On the basis of the result of this study, we have addressed a new environmental issue that open dumping sites are potential sources of PCDD/Fs and related compounds, and dioxin contamination in dumping sites may become a key environmental problem in developing countries.     

Investigation of PCDD/F emissions from mobile source diesel engines: impact of copper zeolite SCR catalysts and exhaust aftertreatment configurations

This study investigated the impact of copper zeolite selective catalytic reduction (SCR) catalysts and exhaust aftertreatment configurations on the emissions of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) from mobile source diesel engines. Emissions of PCDD/Fs, reported as the weighted sum of 17 congeners called the toxic equivalency quotient (TEQ), were measured using a modified EPA Method 0023A in the absence and presence of exhaust aftertreatment. Engine-out emissions were measured as a reference, while aftertreatment configurations included various combinations of diesel oxidation catalyst (DOC), diesel particulate filter (DPF), Cu-zeolite SCR, Fe-zeolite SCR, ammonia oxidation catalyst (AMOX), and aqueous urea dosing. In addition, different chlorine concentrations were evaluated. Results showed that all aftertreatment configurations reduced PCDD/F emissions in comparison to the engine-out reference, consistent with reduction mechanisms such as thermal decomposition or combined trapping and hydrogenolysis reported in the literature. Similarly low PCDD/F emissions from the DOC-DPF and the DOC-DPF-SCR configurations indicated that PCDD/F reduction primarily occurred in the DOC-DPF with no noticeable contribution from either the Cu- or Fe-zeolite SCR systems. Furthermore, experiments performed with high chlorine concentration provided no evidence that chlorine content has an impact on the catalytic synthesis of PCDD/Fs for the chlorine levels investigated in this study.     

Assessment of chlorinated dibenzo-p-dioxin and dibenzofuran trends in sediment and crab hepatopancreas from pulp mill and harbor sites using multivariate- and index-based approaches

Mill process changes and source controls instituted in the late 1980s at pulp and paper mills along the British Columbia (BC) coast produced dramatic reductions in both the concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and the TEQ (2,3,7,8-TCDD toxic equivalents). Nevertheless, questions remained as to whether the sediments were acting as contaminant sources or sinks, whether crab uptake pathways have stayed the same since the mills ceased producing PCDD/Fs, and whether improvement was faster at some sites than others. To examine the processes governing the changes in PCDD/F composition, PCDD/F sources and compositional trends for the coastal BC sites were interpreted using sediment and crab PCA (principal components analysis) models and two indices based on the three main groups of PCDD/Fs that covary in all PCA models. One index measured the relative inputs of chlorine bleaching PCDFs and PCP (pentachlorophenol)-related PCDDs at each mill and harbor site whereas the other measured the relative inputs of PCP wood preservatives and pulp mill effluents. Results indicated that PCDFs produced during chlorine bleaching accumulate from sediments to crabs more readily than the PCDDs but that the chlorine bleaching TCDFs have generally decreased in importance at all mill sites. In the years just before 1995, little change was observed in either the proportion of toxic 2,3,7,8-chlorinated congeners or the PCDD/F concentrations, and the improvements seemed to have largely ceased by 1995. The lack of change in PCA composition for the harbors provided a further, disturbing indication that ecosystem recovery may have stalled. Results also implied that the main route of PCDD/Fs into crabs shifted from uptake via a pelagic food web incorporating effluent-borne PCDD/Fs associated with suspended particulate matter to uptake via a benthic food web.     

Formation of polychlorinated dibenzo-p-dioxins/dibenzofurans from soot of benzene and o-dichlorobenzene combustion

Soots were prepared from flame combustion of benzene and o-dichlorobenzene (ODCB), creating one soot without carbon-chlorine bonds (benzene soot) and one with such bonds (ODCB soot). ODCB soot was tested for PCDD/F formation between 277 and 600 degrees C without additional chlorine, but levels were very low. Copper and Cu2O were added as potential catalysts for ODCB soot oxidation, but levels of PCDD/F observed were even lower than without these additives. Both benzene soot and ODCB soot produced PCDD/F after adding CuCl2 to the reaction mixtures, suggesting that a (volatile) metal chloride was needed in order for PCDD/F formation to take place. Under the various conditions of [Cu2+], time, and temperature tested, ODCB soot was always more reactive than benzene soot in forming PCDD/F. It seemed plausible that, despite the fact that CuCl2 was very effective in creating C-Cl bonds in benzene soot, the C-Cl bonds created in ODCB soot during preparation were of a reactivity so as to make this soot especially prone to PCDD/F formation. High temperature (gas phase) chlorination of soots by HCI or other chlorinating agents, followed by deposition of these soots and condensed metal chlorides on the ducts and walls of the postcombustion zone, could create an effective mechanism for de novo formation of PCDD/F.     

Mechanism and direct kinetic study of the polychlorinated dibenzo-p-dioxin and dibenzofuran formations from the radical/radical cross-condensation of 2,4-dichlorophenoxy with 2-chlorophenoxy and 2,4,6-trichlorophenoxy

A direct density functional theory (DFT) kinetic calculation is carried out for the homogeneous gas-phase formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from the cross-condensation of 2,4-dichlorophenoxy radical (2,4-DCPR) with 2-chlorophenoxy radical (2-CPR) and 2,4,6-trichlorophenoxy radical (2,4,6-TCPR). The possible formation mechanism is investigated and compared with the PCDD/F formation mechanism from the self-condensation of 2,4-DCPR, 2-CPR, and 2,4,6-TCPR. The rate constants and their temperature dependence of the crucial elementary reactions are computed by the canonical variational transition-state theory with the small curvature tunneling contribution over the temperature range of 600-1200 K. This study shows that the multichlorine substitutions suppress the PCDD/F formations. Because of a lack of experimental kinetic data, the present theoretical results are expected to be useful and reasonable to estimate the kinetic properties, such as the pre-exponential factors, the activation energies, and the rate constants, of the elementary reactions involved in the formation of PCDD/Fs.