Formation Of Polychlorinated Dibenzofurans Research Articles

Environmental transformation and hazards of decachlorobiphenyl on suspended particles under sunlight irradiation

Decachlorobiphenyl (PCB-209) can be widely detected in suspended particles and sediments due to its large hydrophobicity, and some of its transformation products may potentially threaten organisms through the food chain. Here we investigate the photochemical transformation of PCB-209 on suspended particles from the Yellow River. It was found that the suspended particles had an obvious shielding effect to largely inhibit the photodegradation of PCB-209. Meanwhile, the presence of inorganic ions (e.g. Mg2+ and NO3-) and organic matters (e.g. humic acid, HA) in the Yellow River water inhibited the reaction. The main transformation products of PCB-209 were lower-chlorinated and hydroxylated polychlorinated biphenyls (OH-PCBs), and small amounts of pentachlorophenol (PCP) and polychlorinated dibenzofurans (PCDFs) were also observed. The mechanisms of PCP formation by double •OH attacking carbon bridge and PCDFs formation by elimination reaction of ionic state OH-PCBs were proposed using theoretical calculations, which provided some new insights into the inter-transformations between persistent organic pollutants. In combination with VEGA and EPI Suite software, some intermediates such as PCDFs were more toxic to organisms than PCB-209. This study deepens the understanding of the transformation behavior of PCB-209 on suspended particles under sunlight.

Inhibition of element sulfur and calcium oxide on the formation of PCDD/Fs during co-combustion experiment of municipal solid waste

In order to investigate the effect of element sulfur (S) and calcium oxide (CaO) on the formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) during municipal solid waste (MSW) combustion process, MSW was incinerated with S or CaO in a laboratory-scale incinerator and heated in the flow of N2-O2 gas mixture at 800°C. It can be concluded that 25% of oxygen concentration is the best condition for the following inhibitors experiments through a series of oxygen variation experiments. With adding S and CaO in MSW incineration, seventeen kinds of 2, 3, 7, 8-substituted PCDD/Fs congeners were analysed with high-resolution chromatography and mass spectrometry method. The results show that S and CaO obviously suppress PCDD/Fs formation. Comparing inhibition effect of S with that of CaO, the inhibitory effect of S on HpCDD/Fs formation was most remarkable, of around 88.1%; while CaO could inhibit the formation of HxCDD/Fs more evidently than sulfur and the inhibition was 85.1%. PCDFs were the main components of dioxins produced from MSW incineration in the experiments, and S and CaO were unable to change the dominating generation route of PCDD/Fs. S and CaO could mainly consume chlorine sources or weaken the chlorination in the PCDD/Fs formation process to restrain the PCDFs formation, but the inhibition mechanisms were different. In addition, some dioxins or precursors might be decomposed by S and CaO.

Suppression of PCDD/Fs during thermal desorption of PCBs-contaminated soil.

Thermal treatment of polychlorinated biphenyls (PCBs) contaminated soil was shown in earlier work to generate new PCBs, as well as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). In this study, this thermal desorption was conducted with addition of three distinct inhibitors, including ammonium sulphate, urea and calcium oxide, to inhibit the formation of PCDDs and PCDFs when remediating PCBs-contaminated soil. Experiments were conducted for 40min at 400°C after adding 1wt.% of inhibitor. Both the total PCDD/Fs and international toxic equivalent quantity (I-TEQ) reduced when inhibitors were introduced. Of the three compounds tested, CaO shows the highest inhibition efficiency, 92.2% for total PCDD/Fs and 95.6% for I-TEQ. The amount of CaO added also influences the suppression efficiency of PCDD/Fs. These results suggest that promoting desorption and destruction of precursors is probably the mechanism of suppression.

Theoretical Mechanistic and Kinetic Studies on Homogeneous Gas-Phase Formation of Polychlorinated Naphthalene from 2-Chlorophenol as Forerunner.

Polychlorinated naphthalenes (PCNs) are dioxins-like compounds and are formed along with polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in thermal and combustion procedures. Chlorophenols (CPs) are the most important forerunners of PCNs. A comprehensive comprehension of PCN formation procedure from CPs is a precondition for reducing the discharge of PCNs. Experiments on the formation of PCNs from CPs have been hindered by PCN toxicity and short of precise detection methods for active intermediate radicals. In this work, PCN formation mechanism in gas-phase condition from 2-chlorophenol (2-CP) as forerunner was studied by quantum chemistry calculations. Numbers of energetically advantaged formation routes were proposed. The rate constants of key elementary steps were calculated over 600–1200 K using canonical variational transition-state theory (CVT) with small curvature tunneling contribution (SCT) method. This study illustrates formation of PCNs with one chlorine atom loss from 2-CP is preferred over that without chlorine atom loss. In comparison with formation of PCDFs from 2-CP, PCN products are less chlorinated and have lower formation potential.

Thermal desorption of PCBs from contaminated soil with copper dichloride.

Copper dichloride is an important catalyst both in the dechlorination of chlorinated aromatic compounds and the formation of PCDD/Fs. The effect of copper dichloride on polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs) was studied in treated soil and off gas after thermal desorption of PCB-contaminated soil at 300, 400, 500, 600 °C. The presence of copper dichloride clearly enhances thermal desorption by promoting PCBs removal, destruction, and dechlorination. After thermal treatment at 600 °C for 1 h, the removal efficiency and destruction efficiency for PCBs reached 98.1 and 93.9%, respectively. Compared with the positive influence on PCBs, copper dichloride catalyzed large amount of PCDFs formation at 300 °C, with the concentration ratio of 2.35. The effect of CuCl2 on PCDFs formation weakened with the rising temperature since PCDFs destruction became dominant under higher temperature. Different from PCDFs, PCDDs concentration in treated soil and off gas decreased continuously with the increasing temperature.

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.

Behavior of PCDDs/PCDFs in remediation of PCBs-contaminated sediments by thermal desorption

Thermal desorption is an effective method for removing volatile and semivolatile organic matter in contaminated solid remediation. Formation of polychlorinated dibenzo- p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) in many polychlorinated biphenyls (PCBs) destruction processes has been reported, but the removal pathways are poorly understood. We therefore investigated the behaviors of PCBs and PCDDs/PCDFs in thermal desorption of PCBs-contaminated sediment and predicted the reaction pathways. Four thermal desorption experiments using PCB-contaminated sediments containing different doses of PCBs were carried out. In all the experiments, decomposition of 48–70% of PCBs was achieved, resulting in formation of PCDFs. Despite the PCBs decomposition levels, toxic equivalencies (TEQs) in the treated and volatilized samples were 2.8–6.3 times and 8.0–10.5 times as high as the TEQs in the initial samples, respectively, indicating increased toxicity after treatment. Further analysis revealed that PCDFs with higher numbers of chlorine atoms are likely to remain in the sediments than those of volatilized PCDFs; this is supported by the positive correlation between the vapor pressures of PCDFs and the ratios of volatilized PCDFs.

Effects of sulfur on PCDD/F formation under stable and transient combustion conditions during MSW incineration

SO 2 levels in the flue gas from a laboratory-scale fluidized bed reactor combusting artificial municipal solid waste (MSW) were varied (resulting in four different SO 2:HCl ratios 0, 0.2, 0.7 and 2.7 (by mass)) to study the effects of sulfur on the formation of polychlorinated dibenzo- p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzothiophenes (PCDTs). Sampling was performed simultaneously at three fixed points in the post-combustion zone with temperatures of 400, 300 and 200 °C, under normal combustion conditions and both during and after transient combustion conditions. The findings indicate that sulfur has a greater inhibitory effect on PCDF formation than on PCDD formation and that the PCDD/PCDF ratio in the flue gas depends on both the SO 2:HCl ratio in the flue gas and memory effects arising from transient combustion conditions. The results also indicate that the relative importance of different pathways shifts in the post-combustion zone; condensation products increasing with reductions in temperature and increases in residence time. However, these changes appear to depend on the SO 2:HCl ratio in the flue gas and combustion conditions. Sulfur seems to inhibit the chlorination of PCDFs. A tendency for increased SO 2 levels in the flue gas to increase levels of PCDTs was also detected, but the increases were much less significant than the reductions in PCDF levels.

Prevention of PCDD/F Formation and Minimization of Their Emission at the Stack of a Secondary Aluminum Casting Plant

Results of an extensive 5 year study on a full-scale plant with the specific aim to investigate polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzo furan (PCDF) formation and release in a secondary aluminum casting plant are reported. PCDD and PCDF concentrations were evaluated for all the gaseous and solid streams (no liquid stream was present) as well as for the flue gas upstream and downstream of every single unit of the flue gas cleaning system. The study highlights PCDD and PCDF formation particularly in the narrow 320-360 degrees C range. To prevent formation reactions and/or minimize PCDD and PCDF concentration at the stack, effects of the fabric filter substitution, a quenching chamber and a postcombustor installation together with working conditions are investigated. The flue gas cleaning system results in PCDD and PCDF emission at stack of 0.1-0.2 ng I-TEQ/N m3 and in a mass flow of 250-550 nmol/h. The total PCDD and PCDF release into the environment is 0.06 g I-TEQ/yr and the corresponding emission factor, 0.35 microg I-TEQ/ton. It is shown that the global effects of the technological innovation on the reaction mechanisms are the prevention of PCDD/F formation by de novo synthesis and the minimization of their emission.

Formation of chlorinated phenols, dibenzo- p-dioxins, dibenzofurans, benzenes, benzoquinnones and perchloroethylenes from phenols in oxidative and copper (II) chloride-catalyzed thermal process

Formation of polychlorinated dibenzo- p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and chlorinated phenols on CuCl 2 from unsubstituted phenol and three monochlorophenols was studied in a flow reactor over a temperature range of 100–425 °C. Heated nitrogen gas streams containing 8.0% oxygen were used as carrier gas. The 0.00024 mol of unsubstituted phenol and 0.00039 mol of each monochlorophenol were passed through a 1 g and 1 cm SiO 2 particle containing 0.5% (Cu by mass) CuCl 2. Chlorination preferentially occurred on ortho-(2, 6) and para-(4) positions. Chlorination increased up to 200 °C, and thereafter decreased as temperature increased. Chlorination of phenols plays an important role in the formation of the more chlorinated PCDD/Fs. Chlorinated benzenes are formed possibly from both chlorination of benzene and chlorodehydroxylation of phenols. Chlorinated phenols with ortho chlorine formed PCDD products, and major PCDD products were produced via loss of one chlorine. For PCDF formation, at least one unchlorinated ortho carbon was required.

Experimental and modeling study of de novo formation of PCDD/PCDF on MSW fly ash

The effect of sulfur dioxide (SO 2) on the formation of polychlorinated dibenzo- p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) has been studied in an entrained-flow reactor (EFR) under simulated waste combustion conditions. A chlorination model based on conditional probability was employed to evaluate the homologue patterns of PCDDs and PCDFs. Results revealed that the presence of SO 2 did not alter the formation pathway although SO 2 suppressed PCDD/F formation. The prediction model of PCDF showed good agreement with the experimental data ( R=0.95), whereas the prediction for PCDDs did not correlate well with the experimental data. This may be explained because potential chlorination pathways play a significant role in PCDF formation, whereas PCDDs are mainly formed through condensation reactions. Furthermore, the result indicated that the steric hindrance during formation has more effects on PCDD than on PCDF due to the symmetric molecular structures of PCDDs.

Effect of water on catalyzed de novo formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans

The effect of water vapor on catalyzed de novo formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) was investigated through experiments conducted on a fixed-bed apparatus. The results indicated that water vapor could promote the formation of PCDD and PCDF. The dominant pathway was activating fly ash in this work, while water also reacted with chlorine to change the equilibrium of Deacon reaction, which influenced the final yield of PCDD/PCDF. Also a suppression effect of water on CuCl2 was found according to the values of the catalysis indicator. It is possible that water reduced the catalysis of CuCl2 or prevented its direct chlorination. But the overall effect of water on the formation of PCDD and PCDF was promotion rather than inhibition.

Is phenol condensation one of the major pathways in the formation of polychlorinated dibenzofurans in municipal waste incinerators?: Model prediction vs. field observation

The role of phenol condensation pathways in the formation of polychlorinated dibenzofurans (PCDF) in a municipal waste incinerator was assessed by comparing predicted PCDF homologue and isomer patterns with those obtained from the incinerator. A two-phenol condensation model, dependent only on the distribution of phenols, was used to predict the distributions of PCDF congeners in the incinerator. Complete distributions of phenols and PCDF congeners were obtained from the incinerator. To quantify the degree of agreement between obtained isomer distributions and those predicted by the model, R-squared values from linear correlations were calculated for the dichlorinated- through hexachlorinated-isomers. They ranged from 0.001 to 0.1. Agreement between obtained and predicted PCDF isomers was very poor for all homologues, suggesting that phenol condensation pathways are unlikely to be the primary route in the formation of PCDF in the incinerator. However, dibenzofuran (DF) is likely to be produced from a condensation of two phenols. This paper shows the use of PCDF homologue and isomer patterns calculated by the two-phenol condensation model for testing PCDF formation mechanism attribution in a municipal waste incinerator.

CuCl 2-catalyzed PCDD/F formation and congener patterns from phenols

Homologue and isomer patterns of polychlorinated dibenzo- p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) in CuCl 2-catalyzed formation were studied in an isothermal flow reactor using a distribution of 20 phenols as measured in municipal waste incinerator (MWI) exhaust gases. A mixture of 20 phenols was synthesized and used as reactants for this study because phenols are known to be key precursors in the formation of PCDD/F. Experiments were conducted at 400 °C. The 92% of nitrogen (N 2) and 8% of oxygen (O 2) were used as a carrier gas. PCDD/F homologue and isomer patterns with dibenzo- p-dioxin (DD) and dibenzofuran (DF) were obtained from a mixture of 20 phenols. DF + PCDF formation was favored over DD + PCDD formation. The major homologue groups formed were non-chlorinated DD and DF, and PCDD/F homologue fraction decreased with the degree of chlorination. PCDD/F homologue and isomer distributions were almost constant. Phenol and lower chlorinated phenols present in high amount played an important role in PCDD/F congener distributions. The results presented here can be used as characteristics or fingerprints for homologue and isomer patterns of PCDD/F formation attribution in CuCl 2-catalyzed reaction from phenols.

Formation of polychlorinated naphthalenes from chlorophenols

To test the hypothesis that polychlorinated naphthalenes (PCNs) can be formed from chlorinated phenols via cyclopentadienyl radicals in post-combustion gas containing 8% oxygen, the slow combustion of three chlorophenols (CPs) at 600 °C was studied in a laminar flow reactor. Contrary to the a priori hypothesis, different distributions of PCN isomers were produced from each CP. To explain these distributions and the observed correlation between polychlorinated dibenzofuran (PCDF) and PCN yields, a mechanism is proposed that builds on published mechanisms of PCDF formation from chlorinated phenols and naphthalene formation from dihydrofulvalene. This mechanism involves phenoxy radical coupling at unsubstituted ortho-carbon sites followed by CO elimination to produce dichloro-9,10-dihydrofulvalene intermediates. Naphthalene products are formed by loss of H and/or Cl atoms and rearrangement. Of the three chlorophenols, 3-CP produced the highest PCN and PCDF yields, as well as the greatest number of PCN and PCDF product; 2-CP produced the lowest PCN and PCDF yields. Phenol, formed by hydrodechlorination of CP, led to the formation of naphthalene and monochloronaphthalene products. This work provides a better understanding of isomer-specific PCN formation in post-combustion gas and may be developed and used as a tool for mechanism attribution in combustion systems.

An Elementary Reaction Kinetic Model of the Gas-Phase Formation of Polychlorinated Dibenzofurans from Chlorinated Phenols

A 30-reaction core kinetic model has been developed that accounts for the major features in the homogeneous formation of polychlorinated dibenzofurans (PCDF) from the high-temperature oxidation of 2,4,6-trichlorophenol (TCP). The core, purely gas-phase scheme provides agreement within a factor of 1.5−2 between experimental and calculated yields of PCDD/Fs using the REACT kinetic program. It is demonstrated that the formation of PCDF is due to the conversion of TCP to 2,4-dichlorophenol (DCP), which subsequently forms a keto resonance structure of the 2,4-dichlorophenoxyl radical (DCP•). The conversion of TCP to DCP provides an ortho hydrogen that is necessary for the formation of PCDF. Ab initio calculations were utilized to calculate the activation energies of the key reactions involved in the gas-phase formation of PCDF.

PCB destruction in subcritical and supercritical water--evaluation of PCDF formation and initial steps of degradation mechanisms.

The reduction of polychlorinated biphenyl (PCB) emissions to the environment are contemporary issues of global efforts, and possible destruction technologies have to be selected and evaluated for PCB remediation. In this study PCB destruction in subcritical and supercritical water were assessed under oxidative conditions and nonoxidative, alkaline conditions. In both cases PCBs could be destroyed by more than 99%. The formation of polychlorinated dibenzofurans (PCDFs) during PCB destruction was evaluated. Under both oxidative and nonoxidative treatments of sub- and supercritical conditions, the formation of PCDFs was observed. The PCDFs formed in the early stages of PCB destruction resulted in up to 47-fold increase in terms of toxic equivalency (TEQ) compared to the initial PCB mixture. However, the PCDFs were destroyed together with the PCBs under more severe conditions i.e., at higher temperature or prolonged residence time. The mechanism of PCDF formation and the initial step of PCB degradation was evaluated. Our laboratory-scale investigation indicates that PCB destruction under supercritical water conditions is feasible, but because of the PCDF formation potential, in particular the high ratio of toxic 2,3,7,8-substituted congeners, conditions have to be carefully selected.

Effects of copper chloride on formation of polychlorinated dibenzofurans in model waste incineration in a laboratory-scale fluidized-bed reactor

Combustion experiments in a laboratory-scale fluidized-bed reactor have been performed to clarify the effects of copper chloride as a catalyst on polychlorinated dibenzofurans (PCDFs) formation in municipal waste incineration. We used model wastes with and without copper chloride (CuCl 2·2H 2O) as a catalyst, both of which contained polyvinyl chloride (PVC) as a chlorine source. Combustion temperature was set to 900°C, and the amount of air supplied was twice as much as the theoretical amount. The experimental setup had been carefully planned to ensure avoidance of the influences of previous experiments. Results of these present experiments revealed that copper chloride in the waste increased the amount of PCDFs formed and made the homologue profile shifted towards the highly chlorinated species. Copper chloride contributes to PCDFs formation by promoting chlorination via catalytic reactions, whereby the reaction could be important in that organic matters are chlorinated directly by chlorinated compounds related to Deacon reaction such as copper chloride. It was elucidated that characteristic isomer distribution patterns appeared in case the waste contained copper chloride. It is probable in our experiment with copper chloride that PCDFs are mainly formed via catalytic reactions of copper compounds and carbon.

Low temperature decomposition of PCB by TiO2-based V2O5/WO3 catalyst: evaluation of the relevance of PCDF formation and insights into the first step of oxidative destruction of chlorinated aromatics

The reduction of polychlorinated biphenyls (PCB) emissions to the environment are contemporary issues of global efforts. Within this goal, the destruction of PCB is a challenge also for applied catalysis, in particular with respect to the destruction of PCB waste and the off-gas cleaning of incineration facilities.In this study, PCB were destroyed on a V2O5/WO3 supported titanium catalyst at low temperature in the range of 150–300°C. At a space velocity of 5000h−1 more than 98% could be removed. Below 250°C, the higher chlorinated PCB remained partly unchanged on the catalyst for several minutes. In contrast, the oxidation process lasted up to hours at a temperature of 150°C.At around 200°C and below a significant part of the PCB were oxidised to the more toxic polychlorinated dibenzofurans (PCDF). The PCDF remained mainly adsorbed on the catalyst. At 250°C, no significant amount of PCDF were detected and at 300°C no byproducts were found.The oxidation of selected individual PCB isomers and the isomer specific analysis of PCDF formed give an insight into the initial step of degradation of chlorinated aromatics on V2O5/WO3-TiO2 catalysts. The results indicated that hydrogen abstraction is the preferred step and an initial abstraction of chlorine is only a minor pathway. This suggests an electrophilic oxygen transfer during the destruction of PCB.During the destruction experiments, a slight chlorination of the non-degraded PCB occurred. On the other hand, no measurable dechlorination/hydrogenation was observed.

Formation of PCDF, PCDD, PCB, and PCN in de novo synthesis from PAH: Mechanistic aspects and correlation to fluidized bed incinerators

The difference of polychlorinated dibenzofurans (PCDF) isomer patterns between stoker type incinerators and some fluidized bed incinerators (FBI) is a key to understand the formation mechanisms in both types of incinerators. The total yield and the isomer patterns of PCDF, polychlorinated biphenyls (PCB), polychlorinated naphthalenes (PCN), and polychlorinated benzenes (PCBz) formed via de novo synthesis from polycyclic aromatic hydrocarbons (PAH) indicate that chlorinated aromatics in the FBI are formed as a result of PAH breakdown. The detailed analysis of the isomer patterns of PCDF, PCB and PCN gives a first insight into the transformation mechanism of the PAHs and the sequence of degradation, chlorination and oxygen insertion. The major chlorination takes part at the position of the C–C cleavage during degradation of the PAHs. Further chlorination of the hydrogen position of the former PAH takes part preferably in ortho-position to this chlorination or is directed by incorporated oxygen. A perylene structure in soot is proposed as basis for the observed PCDF pattern in the FBI. Polychlorinated dibenzo- p-dioxins (PCDD) and polychlorinated phenols (PxCP) were formed in lower concentrations from the de novo experiments indicating an additional formation pathway for these compounds in the FBI.