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A Detailed Mechanism of The Surface-Mediated Formation of PCDD/F from the Oxidation of 2-Chlorophenol on CuO/ Silica Surface
Abstract
The formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) via a Cu(II)O-mediated reaction of 2-chlorophenol (2-MCP) has been studied in a packed bed reactor over a temperature range of 200500 oC. Under oxidative conditions, the principle PCDD/F products were 1-monochlorodibenzo-p-dioxin (MCDD) > 4,6-dichlorodibenzofuran (DCDF) > dibenzo-p-dioxin (DD). EPR studies indicated the presence of a carbon-centered phenoxyl radical on the surface, which is attributed to chemisorption of 2-MCP at a copper oxide site followed by electron transfer to Cu(II) to form Cu(I) and a phenoxyl radical. The presence of a surface bound phenoxyl radical and the formation of MCDD, DCDF, and DD, which were also observed as the principle products of the gas-phase oxidation of 2-MCP, strongly suggest a surface-mediated mechanism involving many of the same radical and molecular species involved in the gas-phase formation of PCDD/F from 2-MCP. Reaction orders of 0.51.0 were observed for MCDD and DD formation, i
... The most notable of the POPs from EPFRs are PCDD/Fs. Scheme 1A and 1B depict the general pathway for the formation of PCDD/Fs from EPFRs [74]. Accumulation of PCDFs is more likely to dominate as they are formed from two surface-species compared to PCDDs that require another gas-phase species [75]. ...
... Condensation of phenoxyl radicals will form cyclopentadienyl radicals [76], which upon recombination will produce naphthalene [76]. Whereas recombination of chloro/phenoxyl EPFRs will form diphenyl ethers [48,74]. Scheme 2 depicts the formation of 2,3'-dichloro-2'-hydroxydiphenyl ether [73]. ...
... Black bar represents the surface. Adapted and simplified from the mechanism proposed by Lomnicki and Dellinger [74]. Scheme 2 Formation of 2,3'-dichloro-2'-hydroxydiphenyl ether from two chlorophenoxyl radicals [73]. ...
Environmentally persistent free radicals (EPFRs) are a new class organic pollutant sharing some of the attributes of persistent organic pollutants (POPs). This opinion/short review aims to describe the properties of EPFRs that merit their recognition as an additional and potentially significant source of POPs. EPFRs are ubiquitous in diverse environments because of multiple factors: (1) organic precursors from anthropogenic, biogenic, and other natural emission sources are abundant; multiple mechanisms in PM and soils form (2) EPFRs; and (3) EPFRs are stable and persist for a long time, thereby, accumulate in the environment and potentially transported long range. The hazards of EPFRs arise from their ability to induce oxidative stress and the formation of hazardous byproducts. EPFRs are ultimately deactivated by reactive processes, yielding molecular recombination byproducts that are structurally similar to those classified as POPs. It is plausible that EPFRs may form POPs in vivo in organisms; therefore, they are potential additional sources of exogenous POPs. Understanding the formation of EPFRs and extensive investigation of the pollutants generated from their recombination will add to the growing body of knowledge on their environmental and health hazards.
... The former refers to the reactions of surface-bound species with gaseous species, while the latter involves the coupling of two adsorbed surface species. Varieties of experimental research have proved that the L-H mechanism is responsible for the formation of PCDTs and the E-R mechanism is mainly related to the formation of PCTAs [17,[22][23][24][25]. Nganai et al. [25] have elucidated differences between the L-H and E-R mechanisms, which contribute to different rate orders during the oxidation of 2-chlorophenol on the CuO/Silica surface. ...
... Varieties of experimental research have proved that the L-H mechanism is responsible for the formation of PCDTs and the E-R mechanism is mainly related to the formation of PCTAs [17,[22][23][24][25]. Nganai et al. [25] have elucidated differences between the L-H and E-R mechanisms, which contribute to different rate orders during the oxidation of 2-chlorophenol on the CuO/Silica surface. Typically, L-H reactions exhibit lower reaction orders compared to similar reactions in the E-R mechanism [25]. However, there are still a lot of uncertainties about the detailed heterogeneous mechanism of PCTA/DTs. ...
Silica (SiO2), accounting for the main component of fly ash, plays a vital role in the heterogeneous formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) in high-temperature industrial processes. Silica clusters, as the basic units of silica, provide reasonable models to understand the general trends of complex surface reactions. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. By employing density functional theory, this study examined the formation of 2-chlorothiophenolate from 2-CTP adsorbed on the dehydrated silica cluster ((SiO2)3) and the hydroxylated silica cluster ((SiO2)3O2H4). Additionally, this study investigated the formation of pre-PCTA/DTs, the crucial intermediates involved in PCTA/DT formation, from the coupling of two adsorbed 2-chlorothiophenolates via the Langmuir–Hinshelwood (L–H) mechanism and the coupling of adsorbed 2-chlorothiophenolate with gas-phase 2-CTP via the Eley–Rideal (E–R) mechanism on silica clusters. Moreover, the rate constants for the main elementary steps were calculated over the temperature range of 600–1200 K. Our study demonstrates that the 2-CTP is more likely to adsorb on the termination of the dehydrated silica cluster, which exhibits more effective catalysis in the formation of 2-chlorothiophenolate compared with the hydroxylated silica cluster. Moreover, the E–R mechanism mainly contributes to the formation of pre-PCTAs, whereas the L–H mechanism is prone to the formation of pre-PCDTs on dehydrated and hydroxylated silica clusters. Silica can act as a relatively mild catalyst in facilitating the heterogeneous formation of pre-PCTA/DTs from 2-CTP. This research provides new insights into the surface-mediated generation of PCTA/DTs, further providing theoretical foundations to reduce dioxin emission and establish dioxin control strategies.
... 11−13 It was hypothesized that some types of tobacco radicals are consistent with surface-associated, carbon-centered radicals, in which an unpaired electron is vicinal to an oxygen-containing functional group or a partially delocalized electron is associated with the bulk of a phenoxyltype polymeric matrix. 11 Dellinger and co-workers, in parallel with the tobacco research, widely developed and realized the formation of persistent free radicals (PFRs) as surface-bound radicals in the formation of dioxins, 14,15 the formation of PFRs on metals/ metal oxide surfaces 15 (lately known as EPFRs), and the role of PFRs in the toxicity of airborne fine particulate matter, PM, 1, 16 as a new class of pollutants. 17 The toxicological consequences of EPFRs are widely shown in numerous publications from LSU since 2000 1, 16 and the LSU Superfund Research Program (SRP) since 2009. ...
... 11−13 It was hypothesized that some types of tobacco radicals are consistent with surface-associated, carbon-centered radicals, in which an unpaired electron is vicinal to an oxygen-containing functional group or a partially delocalized electron is associated with the bulk of a phenoxyltype polymeric matrix. 11 Dellinger and co-workers, in parallel with the tobacco research, widely developed and realized the formation of persistent free radicals (PFRs) as surface-bound radicals in the formation of dioxins, 14,15 the formation of PFRs on metals/ metal oxide surfaces 15 (lately known as EPFRs), and the role of PFRs in the toxicity of airborne fine particulate matter, PM, 1, 16 as a new class of pollutants. 17 The toxicological consequences of EPFRs are widely shown in numerous publications from LSU since 2000 1, 16 and the LSU Superfund Research Program (SRP) since 2009. ...
To assess contribution of the radicals formed from biomass burning, our recent findings toward the formation of resonantly stabilized persistent radicals from hydrolytic lignin pyrolysis in a metal-free environment are presented in detail. Such radicals have particularly been identified during fast pyrolysis of lignin dispersed into the gas phase in a flow reactor. The trapped radicals were analyzed by X-band electron paramagnetic resonance (EPR) and high-frequency (HF) EPR spectroscopy. To conceptualize available data, the metal-free biogenic bulky stable radicals with extended conjugated backbones are suggested to categorize as a new type of metal-free environmentally persistent free radicals (EPFRs) (bio-EPFRs). They can be originated not only from lignin/biomass pyrolysis but also during various thermal processes in combustion reactors and media, including tobacco smoke, anthropogenic sources and wildfires (forest/bushfires), and so on. The persistency of bio-EPFRs from lignin gas-phase pyrolysis was outlined with the evaluated lifetime of two groups of radicals being 33 and 143 h, respectively. The experimental results from pyrolysis of coniferyl alcohol as a model compound of lignin in the same fast flow reactor, along with our detailed potential energy surface analyses using high-level DFT and ab initio methods toward decomposition of a few other model compounds reported earlier, provide a mechanistic view on the formation of C- and O-centered radicals during lignin gas-phase pyrolysis. The preliminary measurements using HF-EPR spectroscopy also support the existence of O-centered radicals in the radical mixtures from pyrolysis of lignin possessing a high g value (2.0048).
... EPFRs can be produced from primary emissions such as vehicle emissions, solid fuel combustions in industry and residential sectors (Fang et al., 2014;Kiruri et al., 2013;Tian et al., 2009;Wang et al., 2020;Zhao et al., 2022) and secondarily formed through reactions between transition oxide metals (e.g., CuO, NiO and ZnO) and organic precursors containing benzene rings (e.g., Polycyclic Aromatic Hydrocarbons) (B. Dellinger et al., 2001;Lomnicki & Dellinger, 2003;Dellinger et al., 2007;Jia et al., 2017;Yang et al. 2017). Epidemiological studies suggested that exposure to EPFRs can be associated with adverse health outcomes (Pan et al., 2019). ...
Environmental persistent free radicals (EPFRs) are receiving growing concerns owing to their potentially adverse impacts on human health. Road dust is one important source of air pollution in most cities and may pose significant health risks. Characteristics of EPFRs in urban road dusts and its formation mechanism(s) are still rarely studied. Here, we evaluated occurrence and size distributions of EPFRs in road dusts from different functional areas of an urban city, and assessed relationship between EPFRs and some transition metals. Strong electron paramagnetic resonance signals of 6.01 × 10¹⁶ − 1.3 × 10¹⁹ spins/g with the mean g value of 2.0029 ± 0.0019 were observed, indicating that EPFRs consisted of a mixture of C-centered radicals, and C-centered radicals with an adjacent oxygen atom in the urban road dust. Much more EPFRs enriched in finer dust particles. EPFRs significantly correlated with the total Fe, but not water-soluble Fe, suggesting different impacts of water-soluble and insoluble metals in the formation of EFPRs. Health risk assessment results indicated high risk potentials via the ingestion and dermal exposure to EPFRs in road dusts. Future studies are calling to look into formation mechanisms of EPFRs in urban road dusts and to quantitatively evaluate its potential risks on human health.
... The makeup of the precursors that can create dioxins in e-waste is the underlying cause of the variable PCDD/F ratio (Nganai et al. 2014). Furthermore, the presence of metal oxides (particularly Fe and Cu) causes created PCDDs to persist on the surface, whereas newly formed PCDFs enter the gas phase immediately and settle in the surrounding soil (Lomnicki and Dellinger 2003). As a result, PCDFs are produced in high concentrations during e-waste recycling and chemical operations. ...
Purpose
Informal electronic waste (e-waste) recycling has been identified as a significant source of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs), which pose a health and environmental danger. The impact of PCDD/Fs in the e-waste dismantling sites depends on their concentration in environmental matrices. This study aimed to investigate the vertical distribution characteristic of PCDD/F content collected from an abandoned e-waste dismantling site in Guiyu, China, as well as the nearby paddy field and stream.
Material and methods
One year after the e-waste dismantle site was abandoned, soil, sediment, and mud cores were taken from the site, the neighboring stream, and the paddy field. The source and distribution characteristics of PCDD/Fs in samples were revealed using a variety of statistical analytic methods.
Results and discussion
The concentrations of PCDD/Fs steadily decreased with increasing soil depth at all sampling sites. The highest concentration of PCDD/Fs was discovered in the sub-layer (10–20 cm) of the e-waste disposal site, at 263,332.72 ng/kg. PCDD/Fs concentrations in soil, sediment, and mud samples were 314.21–16,037.39, 0.01–0.11, and 6.93–97.44 ng I-TEQ/kg, respectively, according to the International Toxicity Equivalency Quantity (I-TEQ). The health risk assessment revealed the harsh working environment for the workers. The association between the three sampling sites was determined using principal component analysis (PCA) and hierarchical cluster analysis (HCA). The main source of PCDD/Fs was e-waste recycling, and one of the explanations for the similarity of PCDD/Fs distribution between the non-plough layer (0–20 cm) of paddy field and sediments was thought to be water management.
Conclusion
Decades of informal e-waste recycling have led to PCDD/Fs reaching deep into the soil. The order of pollution levels was as follows: e-waste dismantling site > paddy field > stream. The utilization of the e-waste dismantling site and its neighboring lands necessitates meticulous planning and oversight.
... Precursor rearrangement formed most PCDDs (mostly CP route in MSW incineration), together with a very small fraction of de novo synthesis and dibenzodioxin (DD) chlorination (Lomnicki and Dellinger 2003;Nganai et al. 2014). Accordingly, the ratios of PCDD and PCDF were calculated to verify the formation pathway in the incinerators (Luijk et al. 1994;Ryu et al. 2006). ...
Few studies focused on the emission of polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/F) from different kinds of waste incinerators. This study was conducted in a full-scale MSW incineration plant to investigate the influence of different incinerator types on PCDD/F. Experimental results indicated that the 2,3,7,8-PCDD/F concentration in the inlet gas of the air pollution control system (APCS) in the studied fluidized bed was higher (2.03 ng I-TEQ/Nm³) than that of the grate (0.77 ng I-TEQ/Nm³). But gas in the outlet of APCS from both incinerators had an approximate concentration, lower than the Chinese emission limit of 0.1 ng I-TEQ/Nm³. Similar distribution patterns were observed for 2,3,7,8-PCDD/Fs, as well as 136 PCDD/F congeners. Specifically, OCDD and 1,2,3,4,6,7,8-HpCDD were major isomer constituents for 2,3,7,8-PCDD/F isomers. In terms of formation pathways, a similar formation mechanism was observed based on fingerprint characteristics of 136 PCDD/F congeners. De novo synthesis was the dominating formation pathway for both incinerators. Meanwhile, DD/DF chlorination was another contributor to PCDD/F formation, which in the fluidized bed was higher. In addition, little correlation (0.009 < R² < 0.533) between conventional pollutants (HCl, CO, PM) and PCDD/Fs was found, suggesting little high-temperature synthesis observed and verifying the dominance of de novo synthesis.
Graphical abstract
... The lower promotion effect of metal oxides on PCDD/F formation could be ascribed to the lack of chlorine source and the lower activity level of oxygen atoms than that of chlorine atoms (Chin et al., 2011). In three metal oxides, CuO showed higher catalytic ability due to the promotion effect on chlorination of aliphatic intermediates and ring formations (Lomnicki and Dellinger, 2003;Qian et al., 2005). Besides, the added CuO would enhance the PCDD/F formation by the synergistic effect of the Fe2O3 in SS and/or coal (Liu et al., 2019;Potter et al., 2018). ...
... It was proved in previous studies that the condensation of chlorophenols (CP-routes) was one of the typical patterns to form PCDD/Fs (Lomnicki and Dellinger 2003;Zhan et al. 2019;Wang et al. 2020). Congeners of PCDD/Fs generated by CP-routes are considered as 1, 3,7,9-and 1,3,6,8-TCDD;1,2,4,7,9 + 1,2,4,6,8-, 1,2,3,6,8-and 1,2,3,7,9-PeCDD;and 1,2,3,4,6,8-HxCDD (Luijk et al. 1994;Addink et al. 1995). ...
This study emphasized on the removal performance of polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/Fs) and mercury by different activated carbon injection (ACI) rates from a full-scale (700 t/d) MSW incinerator. The result exhibited that the emission standard of PCDD/Fs and mercury could be met when the ACI rate reached 50 mg/Nm³ and 30 mg/Nm³, respectively. Lower chlorinated PCDD/Fs and PCDFs showed higher removal efficiencies compared with highly chlorinated PCDD/Fs and PCDDs, which could be attributed to the larger competitiveness of highly volatile congeners in AC adsorption than the lower volatile ones. AC turned out to have different adsorption selectivity for CP-routes PCDD/Fs congeners, among which 1379-TCDD was preferred to be absorbed while others exhibited little or poor selectivity for AC adsorption. The removal efficiency of PCDD/Fs was positively correlated with ACI rate at 99% confidence interval with a linear relationship (R² = 0.98). Also, the outlet concentration of mercury decreased with the increase of ACI rate in a nearly linear function (R² = 0.96). These results will be meaningful for the rational use of AC for pollutants control.
... 有研究指出, 在PVC燃烧过程中可以生成Cl-PAHs 等多种UP-POPs [54,56] . Wang等 [54] 在管式炉中模拟了 [21,63,72] . 有研究发现, 当氧含 量<10%时, PCDD的生成量与氧含量呈正相关关系 [66] . ...
... In recent years, a new type of PM-associated pollutant has been identified and studied-environmentally persistent free radicals (EPFRs) (Balakrishna et al. 2011;Yang et al. 2017;Guo et al. 2020;Qian et al. 2020), with potentially significant health implications. EPFRs are long-lived surface-stabilized radicals (Dellinger et al. 2007;Lomnicki et al. 2008;Gehling and Dellinger 2013) formed in the cool zone of combustion processes through the interaction of transition metal oxide with substituted aromatic compounds on the surface of particles (Lomnicki and Dellinger 2003;Cormier et al. 2006;Lomnicki et al. 2008). New studies have emerged, focusing on EPFR formation under other conditions, including at the presence of metals other than transition metals (Wu et al. 2020), through heterogeneous oxidation mechanisms (Borrowman et al. 2016), or on the microplastics under the light irradiation ). ...
Ambient air particulate matter (PM) and PM-associated environmentally persistent free radicals (EPFRs) have been documented to contribute to pollution-related health effects. Studies of ambient air PM potentially bear artifacts stemming from the collection methods. We have investigated the applicability of PM phytosampling (PHS) as a supplementary tool to a classic PM sampler in respect of achieving better PM chemical composition assessment (primarily organic fraction). Phytosampling is a static PM collection method relying on the particle entrapment by the plant’s leaf through electrostatic forces and surface trichomes. We have investigated the differences in the EPFR and polycyclic aromatic hydrocarbon (PAH) speciation and concentration on ambient air PM for PHS and high-volume PM sampler (HVS). The advantages of PHS are easy particle recovery from the matrix, collection under natural environmental conditions, and the ability to apply a dense collection network to accurately represent spatial pollutant distribution. The experimental results show that the PHS can provide valuable speciation information, sometimes different from that observed for HVS. For PM collected by PHS, we detected the larger contribution of oxygen-centered EPFRs, different decay behavior, and more consistent PAH distribution between different PM sizes compared to the PM from HVS. These results indicate that the isolation of samples from the ambient during HVS sampling and exposure to high-volume airflow may alter the chemical composition of the samples, while the PHS method could provide details on the original speciation and concentration and be more representative of the PM surface. However, PHS cannot evaluate an absolute air concentration of PM, so it serves as an excellent supplementary tool to work in conjunction with the standard PM collection method.
... Among various existing solid waste management strategies, incineration is considered a preferential volume-reduction technique (Duan et al., 2008;Mari et al., 2009), however, extremely toxic polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/Fs) are unintentionally formed during waste incineration. Two main pathways have been presented for their formation during waste incineration (Altarawneh et al., 2009;McKay, 2002;Stanmore, 2004): (1) homogeneous reactions in the gas phase at high temperature (Altarawneh et al., 2009;Khachatryan et al., 2003;Stanmore, 2004), (2) heterogeneous reactions from precursor such as chlorophenols and chlorobenzenes (Lomnicki and Dellinger, 2002;Lomnicki and Dellinger, 2003) or via de novo synthesis from macromolecular carbon and chlorine (McKay, 2002;Altarawneh et al., 2019). ...
Inhibition mechanisms of sulfur-, nitrogen- and phosphorus- based inhibitors on the de novo synthesis of polychlorinated dibenzo-p-dioxins, and dibenzofurans (PCDD/F) were studied by exploring speciation evolution of carbon, chorine and copper in fly ash under laboratory-scale experiments. Significant inhibition of PCDD/Fs by thiourea (TUA) and ammonium dihydrogen phosphate (ADP) was observed as 97.2% and 98.2%, respectively, except for potassium dihydrogen phosphate (PDP). ADP and PDP exhibited better inhibition on PCDFs than on PCDDs, whereas TUA exhibited the opposite effect. After adding inhibitors, the proportion of C-O/C=O/O-C=O bonds at the surface of fly ash increased, and stronger oxidation of carbon occurred, together with the conversion from Cu²⁺ to Cu⁺ and the inhibition of organic chlorine formation. Kinetic model results indicated that TUA might either suppress the carbon gasification or promote the decomposition of PCDD/Fs, resulting in a remarkable inhibition of PCDD/Fs formation. Simulated chemical reaction equilibrium further comfirmed that catalytic metal could be deactivated into CuS and Cu2S by sulfur, and into Cu2P2O7 by phosphorus. Moreover, NH3, decomposed from TUA and ADP, was able to convert Cl2 into HCl, albeit with a weaker chlorination ability. This study of inhibition mechanisms is useful for the exploration and utilization of efficient inhibitors in full-scale incinerators.
... These decreases could be explained by several phenomena. For instance, EPFRs may form new compound, e.g., through Eley-Rideal or Langmuir-Hinshelwood surface reactions (Lomnicki and Dellinger 2003). Alternatively, EPFRs may be thermally decomposed or stabilized by radical-radical recombination, leading to the formation of new, secondary, molecular contaminants (Maskos and Dellinger 2008). ...
Environmentally persistent free radicals (EPFRs) are emerging contaminants of increasing concern due to their toxicity for life and ecosystems, yet their formation, behavior and fate are poorly known. In particular, there is actually no knowledge on the formation of those radicals during the thermal treatment of soils containing polycyclic aromatic hydrocarbons. Such knowledge is important because thermal treatment is a remediation method used to decontaminate soils by removing and degrading PAHs. Here, we studied the formation of radicals in three types of cultivated soils, bauxite soil, fluvo-aquic soil and chernozem soil, artificially contaminated by benzo[a]pyrene, during thermal treatment from 100 to 200 °C for 1 h, using electron paramagnetic resonance. Results show spins densities of radicals up to of 2.079 × 1017 spins/g for bauxite soil, 1.481 × 1017 spins/g for fluvo-aquic soil and 8.592 × 1016 spins/g for chernozem soil at 175 °C. The formed radicals exhibited multiple decays during their observable time and the shortest 1/e lifetimes of radicals up to 757.58 h. These findings are strengthened by EPFR-induction of reactive oxygen species (ROS), O2·− and ·OH, which increased in concentrations from 100 to 200 °C. Overall, our results demonstrates for the first time that thermal treatment of PAHs-contaminated soils induces the formation of EPFRs and suggests that thermal treatment might not be a fully clean remediation method for soils as thermal treatment creates new contaminants.
Copper oxides are vital catalysts in facilitating the formation of polychlorinated thianthrenes/dibenzothiophenes (PCTA/DTs) through heterogeneous reactions in high-temperature industrial processes. Chlorothiophenols (CTPs) are the most crucial precursors for PCTA/DT formation. The initial step in this process is the metal-catalyzed production of chlorothiophenoxy radicals (CTPRs) from CTPs via dissociation reactions. This work combines density functional theory (DFT) calculations with ab initio molecular dynamics (AIMD) simulations to explore the formation mechanism of the adsorbed 2-CTPR from 2-CTP, with the assistance of CuO(111). Our study demonstrates that flat adsorption configurations of 2-CTP on the CuO(111) surface are more stable than vertical configurations. The CuO(111) surface acts as a strong catalyst, facilitating the dissociation of 2-CTP into the adsorbed 2-CTPR. Surface oxygen vacancies enhance the adsorption of 2-CTP on the CuO(111) surface, while moderately suppressing the dissociation of 2-CTP. More importantly, water molecules and surface hydroxyl groups actively promote the dissociation of 2-CTP. Specifically, water directly participates in the reaction through "water bridge", enabling a barrier-free process. This research provides molecular-level insights into the heterogeneous generation of dioxin with the catalysis of metal oxides in fly ash from static and dynamic aspects, providing novel approaches for reducing dioxin emission and establishing dioxin control strategies.
Phosphorus-containing compounds are considered as the potential alternatives of traditional inhibitors for suppressing the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), but the suppression characteristics are scarcely studied. In this study, ammonium dihydrogen phosphate (ADP) was selected as the inhibitor to inhibit the PCDD/F formation via de novo synthesis at 350 °C. The influence of oxygen content and addition method on PCDD/F inhibition was systematically investigated by means of statistical analysis and morphological characterization. The results showed that oxygen enhanced the formation of PCDD/Fs from 1470 ng g−1 (9.78 ng I-TEQ g−1) to 2110 ng g−1 (14.8 ng I-TEQ g−1). ADP significantly inhibited the PCDD/F formation, with inhibition efficiencies ranging from 82.0% to 97.7%. Herein, a higher oxygen content and the premixed way intensified the suppression effect. Dibenzo-p-dioxin (DD)/dibenzofuran (DF) chlorination was proven to be effectively suppressed while chlorophenol (CP) route was not obviously influenced. With the addition of ADP, Cl source was significantly reduced and the formation of organic Cl was effectively inhibited. Also, it decreased the proportion of C–O/C=N and C=O, revealing the efficient inhibition of carbon oxidation. Meanwhile, the formation of copper phosphate and copper pyrophosphate was observed in XPS (X-ray photoelectron spectroscopy) spectra, indicating that the catalytic metal Cu was chelated and passivated by ADP. The premixed way had a better effect on reducing Cl resources, inhibiting oxidation and chelating metals, due to the direct contact with inhibitor. However, the separation method could only depend on the decomposed gases, resulting in a lower inhibition efficiency.
Particulate matter (PM) containing environmentally persistent free radicals (EPFR) is formed by the incomplete combustion of organic wastes, resulting in the chemisorption of pollutants to the surface of PM containing redox-active transition metals. In prior studies in mice, EPFR inhalation impaired endothelium-dependent vasodilation. These findings were associated with aryl hydrocarbon receptor (AhR) activation in the alveolar type-II (AT-II) cells that form the air-blood interface in the lung. We thus hypothesized that AhR activation in AT-II cells promotes the systemic release of mediators that promote endothelium dysfunction peripheral to the lung. To test our hypothesis, we knocked down AhR in AT-II cells of male and female mice and exposed them to 280 µg/m3 EPFR lo (2.7e + 16 radicals/g) or EPFR (5.5e + 17 radicals/g) compared with filtered air for 4 h/day for 1 day or 5 days. AT-II-AhR activation-induced EPFR-mediated endothelial dysfunction, reducing endothelium-dependent vasorelaxation by 59%, and eNOS expression by 50%. It also increased endothelin-1 mRNA levels in the lungs and peptide levels in the plasma in a paracrine fashion, along with soluble vascular cell adhesion molecule-1 and iNOS mRNA expression, possibly via NF-kB activation. Finally, AhR-dependent increases in antioxidant response signaling, coupled to increased levels of 3-nitrotyrosine in the lungs of EPFR-exposed littermate control but not AT-II AhR KO mice suggested that ATII-specific AhR activation promotes oxidative and nitrative stress. Thus, AhR activation at the air-blood interface mediates endothelial dysfunction observed peripheral to the lung, potentially via release of systemic mediators.
Incineration technology has been widely adopted to safely dispose of hazardous waste (HW). While the incineration process causes the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Due to its extreme toxicity, many scholars have been committed to determining the PCDD/F formation process and reducing emissions in incinerators. Previous studies ignored the impact of incineration and fluctuation of feeding materials on PCDD/F formation in hazardous waste incinerators (HWIs). In this study, differences in PCDD/F formation between HWIs and municipal solid waste incinerators (MSWIs) were pointed out. The incineration section in HWIs should be carefully considered. Laboratory experiments, conventional analysis and thermogravimetry experiments were conducted. An obvious disparity of PCDD/F formation between 12 kinds of HWs was found. Distillation residue was found with remarkably higher PCDD/F concentrations (11.57 ng/g). Except for the Cl content, aromatic rings and C-O bond organics were also found with high correlation coefficients with PCDD/F concentrations (>0.92). And PCDD/Fs were formed through a chlorination process and structure formation process. All of these are helpful to further understand the PCDD/F formation process during HW incineration, optimize the operation conditions in HWIs and reduce the emission pressure of PCDD/Fs in the future.
Particulate matter (PM) containing environmentally persistent free radicals (EPFRs) results from the incomplete combustion of organic wastes which chemisorb to transition metals. This process generates a particle-pollutant complex that continuously redox cycles to produce reactive oxygen species. EPFRs are well characterized, but their cardiopulmonary effects remain unknown. This publication provides a detailed approach to evaluating these effects and demonstrates the impact that EPFRs have on the lungs and vasculature. Combustion-derived EPFRs were generated (EPFR lo: 2.1e-16 radical/g, EPFR hi: 5.5e-17 radical/g), characterized, and verified as representative of those found in urban areas. Dry particle aerosolization and whole-body inhalation were established for rodent exposures. To verify that these particles and exposures recapitulate findings relevant to known PM-induced cardiopulmonary effects, male C57BL6 mice were exposed to filtered air, ∼280 μg/m3 EPFR lo or EPFR hi for 4 h/d for 5 consecutive days. Compared to filtered air, pulmonary resistance was increased in mice exposed to EPFR hi. Mice exposed to EPFR hi also exhibited increased plasma endothelin-1 (44.6 vs 30.6 pg/mL) and reduced nitric oxide (137 nM vs 236 nM), suggesting vascular dysfunction. Assessment of vascular response demonstrated an impairment in endothelium-dependent vasorelaxation, with maximum relaxation decreased from 80% to 62% in filtered air vs EPFR hi exposed mice. Gene expression analysis highlighted fold changes in aryl hydrocarbon receptor (AhR) and antioxidant response genes including increases in lung Cyp1a1 (8.7 fold), Cyp1b1 (9 fold), Aldh3a1 (1.7 fold) and Nqo1 (2.4 fold) and Gclc (1.3 fold), and in aortic Cyp1a1 (5.3 fold) in mice exposed to EPFR hi vs filtered air. We then determined that lung AT2 cells were the predominate locus for AhR activation. Together, these data suggest the lung and vasculature as particular targets for the health impacts of EPFRs and demonstrate the importance of additional studies investigating the cardiopulmonary effects of EPFRs.
EPFRs (Environmentally Persistent Free Radicals) are a class of pollutants that have been identified as potential environmental contaminants due to their persistence and ability to generate reactive oxygen species (ROS) that can cause oxidative stress in living organisms. However, no study has comprehensively summarized the production conditions, influencing factors and toxic mechanisms of EPFRs, impeding exposure toxicity assessments and risk prevention strategies. To bridge the gap between theoretical research and practical application, a thorough literature review to summarize the formation, environmental effects, and biotoxicity of EPFRs are conducted. A total of 470 relevant papers were screened in Web of Science Core collection databases. The transfer of electrons between interfaces and the cleavage of covalent bonds of persistent organic pollutants is crucial to the generation of EPFRs, which is induced by external sources of energy, including thermal energy, light energy, transition metal ions, and others. In the thermal system, the stable covalent bond of organic matter can be destroyed by heat energy at low temperature to form EPFRs, while the formed EPFRs can be destroyed at high temperature. Light can also accelerate the production of free radicals and promote the degradation of organic matter. The persistence and stability of EPFRs are synergistically influenced by individual environmental factors such as environmental humidity, oxygen content, organic matter content, and environmental pH. Studying the formation mechanism of EPFRs and their biotoxicity is essential for fully understanding the hazards posed by these emerging environmental contaminants.
Understanding the mechanisms through which persistent organic pollutants (POPs) form during combustion processes is critical for controlling emissions of POPs, but the mechanisms through which most POPs form are poorly understood. Polyhalogenated dibenzo-p-dioxins and dibenzofurans (PXDD/Fs) are typical toxic POPs, and the formation mechanisms of PXDD/Fs are better understood than the mechanisms through which other POPs form. In this study, a framework for identifying detailed PXDD/Fs formation mechanisms was developed and reviewed. The latest laboratory studies in which organic free radical intermediates of PXDD/Fs have been detected in situ and isotope labeling methods have been used to trace transformation pathways were reviewed. These studies provided direct evidence for PXDD/Fs formation pathways. Quantum chemical calculations were performed to determine the rationality of proposed PXDD/Fs formation pathways involving different elementary reactions. Many field studies have been performed, and the PXDD/Fs congener patterns found were compared with PXDD/Fs congener patterns obtained in laboratory simulation studies and theoretical studies to mutually verify the dominant PXDD/Fs formation mechanisms. The integrated method involving laboratory simulation studies, theoretical calculations, and field studies described and reviewed here can be used to clarify the mechanisms involved in PXDD/Fs formation. This review brings together information about PXDD/Fs formation mechanisms and provides a methodological framework for investigating PXDD/Fs and other POPs formation mechanisms during combustion processes, which will help in the development of strategies for controlling POPs emissions.
At present, the researches on photocatalysis were mainly focused on the design, improvement and development of catalysts, and less attention was paid to the existing characteristics of environmentally persistent free radicals (EPFRs) during the process of photocatalytic oxidation. In this study, A flower-like Z-type heterojunction ZnO/ZnIn2S4 (ZnO/ZIS) and typical antibiotic ceftriaxone sodium (CS) were taken as study objects, concentrating on the generation characteristics of EPFRs during the degradation of CS by ZnO/ZIS, and clarifying the degradation mechanism of CS in which EPFRs participated. The results showed that the degradation efficiency of 10 mg/L CS by 0.40 g/L ZnO/ZIS reached 85.3% in 150 min under the irradiation of 500 W xenon lamp. It was clear that ·O2- and h+ play major roles in CS degradation by ZnO/ZIS under visible light, and ·OH plays an auxiliary role. Furthermore, the formation mechanism of EPFRs during photocatalytic degradation processes of CS by ZnO/ZIS were first investigated thoroughly via experimental analysis and density functional theory (DFT) calculations. The concentration level of EPFRs centered on oxygen atoms is 1011 spin/mm3, which were generated in the process of degradation of CS by ZnO/ZIS under visible light. The production of EPFRs chiefly includes two procedures: chemical adsorption and transfer of electrons. The adsorption energy of precursor P8 on ZnIn2S4 side is -1.91 eV, the electrons transferred from precursor P8 and P11 to ZnO/ZnIn2S4 heterojunction. Surprisingly, EPFRs have little negative effects on the degradation process of CS by ZnO/ZIS. The study was not only a key field in the development of photocatalysis technology, but also a new way to study the removal mechanism of antibiotics.
Organic free radical intermediates are pivotal to our understanding of toxic chemicals formation from chlorophenols that widely exist in thermal processes. However, in most cases, multiple free radical intermediates exist and produce complex spectra that are hard to deconvolute. Identification of free radical intermediates is the current difficulty for detailed formation mechanisms of toxic products from chlorophenols. In this study, a universal bottom-up method was developed to identify the organic free radical intermediates. Candidate organic free radicals were firstly speculated according to the critical parameters obtained from experimental electron paramagnetic resonance (EPR) spectra and the calculated bond dissociation energies of precursors. Their theoretical spectra were then used retrospectively to justify the accordance with the experimental EPR spectra. Identification of the organic free radicals provides straightforward evidence for the formation pathways of pollutants from chlorophenol. Internal factors influencing formation of radical intermediates and the toxic products were also studied, including the ortho effect of the precursor, spin densities of the organic free radical intermediates, and steric hindrance effects of the molecular intermediates. In combination of the experimental results and theoretical calculations, detailed formation mechanisms of toxic pollutants intermediating by organic free radicals from thermal oxidation of chlorophenol were strongly evidenced.
Metal compounds play important roles in the formation of organic pollutants during thermal-related processes. However, the metal-catalyzed predominant organic pollutants have not previously been characterized nor have any detailed catalytic mechanisms been clarified. Here, we preciously distinguished the multiple organic free radical intermediates on metal catalyst surfaces during the organic pollutant formation through laboratory and theoretical studies. Differences between the organic free radical intermediate species, concentrations, and formation mechanisms under the catalysis of different metal compounds were investigated. The results were verified mutually with the differed characteristics of organic pollutant products. CuO predominantly catalyzed the formation of highly chlorinated phenoxy radical intermediates and dioxins. High proportions of semiquinone radicals and oxygen-containing derivatives were found on ZnO surfaces. Differently, methyl-substituted phenoxy radicals and long-chain products formed on Al2O3 surfaces. The results will be instructive for the target emission control of priority organic pollutants during thermal-related processes rich in different metal compounds.
The control of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from the flue gas in hazardous waste incinerators (HWIs) is an intractable problem. To figure out the formation mechanism of PCDD/Fs and reduce the emission, a field study was carried out in a full-scale HWI. Ca(OH)2 & (NH4)H2PO4 or CH4N2S & (NH4)H2PO4 were injected into the quench tower, and the detailed inhibition effect on PCDD/Fs formation by the inhibitors coupled with quench tower was studied. Gas and ash samples were collected to analyze PCDD/Fs. XPS, EDS characterization and Principal component analysis were adopted to further analyze the de novo and precursors synthesis. The PCDD/Fs emissions reduced from 0.135 ng I-TEQ/Nm³ to 0.062 or 0.025 ng I-TEQ/Nm³ after the injection of Ca(OH)2 & (NH4)H2PO4 or CH4N2S & (NH4)H2PO4, respectively. The quench tower was found mainly hindering de novo synthesis by reducing reaction time. CP-route was the dominant formation pathway of PCDD/Fs in quench tower ash. Ca(OH)2 & (NH4)H2PO4 effectively inhibit precursors synthesis and reduce proportions of organic chlorine from 4.11% to 2.86%. CH4N2S & (NH4)H2PO4 show good control effects on both de novo and precursors synthesis by reducing chlorine content and inhibiting metal-catalysts. Sulfur-containing inhibitors can cooperate well with the quench tower to inhibit PCDD/Fs formation and will be effective to reduce dioxins formation in high chlorine flue gas. The results pave the way for further industrial application of inhibition to reduce PCDD/Fs emissions in the HWIs flue gas.
Incineration has gradually become the most effective way to deal with MSW due to its obvious volume reduction and weight reduction effects. However, since heavy metals and organic pollutants carried by municipal solid waste incinerator fly ash (MSWI FA) pose a serious threat to the ecological environment and human health, they need to be handled carefully. In this study, the current status of MSWI FA disposal was first reviewed, and the harmless and resourceful disposal technologies of heavy metals and organic pollutants in MSWI FA are summarized as well. A summary of the advantages and disadvantages of each technology, including sintering, melting/vitrification, hydrothermal treatment, mechanochemistry, solidification/stabilization of MSWI FA, is compared. Finally, the research work that needs to be strengthened in the future (such as codisposal of multiple wastes, long-term stability research of disposal products, etc.) was proposed. Through comprehensive analysis, some reasonable and feasible suggestions were provided for the effective and safe disposal of MSWI FA in the future.
In thermal treatment of Municipal Solid Waste (MSW), oxygen-induced low-temperature pre-dechlorination coupled with hydrogen-assisted Cl capture in subsequent pyrolysis was proposed as an economical and competitive way for the inhibition of chloroaromatics. Dechlorination and pyrolysis processes were experimentally investigated, and formation of chloroaromatics in both processes was quantitatively compared, using PVC as the model. Due to the low bond energy of CCl, most of Cl released at low temperature, ahead of common volatiles, providing a feasible temperature window for the pre-dechlorination of feedstock. Chlorobenzene and benzyl chloride were the major chloroaromatics in pyrolysis product, formed at 300 °C and higher temperature. Therefore, dechlorination should be proceeded below 300 °C to prevent the generation of chloroaromatics from itself. However, under inert atmosphere, dechlorination efficiency was heavily dependent on temperature, being poor at 250 °C and 275 °C. O2 had a clear inductive effect on low-temperature dechlorination, lowering the peak temperature of HCl release to 234 °C. After a 30 min oxygen-induced dechlorination at 250 °C, 97.525% of Cl in feedstock was removed, and chloroaromatics in the subsequent pyrolysis was reduced by 90.2%. For a further inhibition of chloroaromatics, 2% H2 was introduced for in-situ Cl capture in the pyrolysis of pre-dechlorinated feedstock, as a result, no chloroaromatic was detected.
Polycyclic aromatic hydrocarbons (PAHs) formation from the pyrolysis of waste tires is inevitable because of the complexity of tire formulations and the addition of certain chemicals. In this study, the formation behavior and distribution of PAHs in three-phases were investigated from waste tires under pyrolysis conditions. The influencing factors including the temperature, heating rate, holding time, particle size, catalyzer, and atmosphere, were systematically evaluated. The results showed that PAHs were mainly concentrated in pyrolysis oil (94.59–99.03%), followed by the gas phase (0.96–5.34%), and their content was very low in the solid phase (0.01–0.99%). A higher temperature and slower heating rate lead to partial PAHs decomposition, thus reducing their emissions. The overall formation of PAHs can be inhibited when pyrolyzing coarse-grained tire powder. Furthermore, the PAHs formation mechanisms in waste tires were determined through reaction molecular dynamics, electron paramagnetic resonance, and intermediate products. Tires were mainly decomposed into benzene series, *C2H3, and *CH3; therefore, it was determined that PAHs were formed by the joint action of the hydrogen abstraction, and vinyl radical addition and methyl addition cyclization mechanisms. Among them, low and middle-ring PAHs were formed more easily, particularly naphthalene. The generation of PAHs can be inhibited by reducing the concentration of hydrocarbons and monocyclic benzene series. Regarding the distribution law and generation pathways of PAHs, our results provide guidance for reducing PAHs formation and emissions.
Copper compounds are considered the strongest promotors of unintentional persistent organic pollutants (POPs) formation, while calcium compounds are widely used to inhibit this process. Understanding the mechanisms underlying these processes is important for synergetic control of multiple POPs. Herein, the promoting ability of CuCl2 on the synergetic formation of POPs from anthracene and the underlying mechanisms were clarified. During thermochemical reaction with CuCl2, Cu²⁺ was first reduced to Cu⁺, and finally oxidized to CuO. Furthermore, CuCl2 promoted the formation of environmentally persistent free radicals (EPFRs) during thermochemical reaction. The formed EPFRs were identified as anthrone-type and anthraquinone-type radicals using electron paramagnetic resonance spectroscopy and gas chromatography/quadrupole time-of-flight mass spectrometry. Synergetic inhibition of POPs and EPFRs was achieved using CaO. The inhibiting mechanism of CaO was attributed to the absorption of chlorine originating from CuCl2 through the formation of CaClOH and the inhibition of free radical intermediates for POPs formation.
Dielectric barrier discharge coupled with 10 wt% Co/γ-Al2O3 catalyst was developed to degrade chlorobenzene in this study. The effects of experimental parameters including applied voltage, flow rate, initial chlorobenzene concentration, and their interactions on the chlorobenzene degradation performance were investigated by the response surface methodology integrated with a central composite design. Results indicated that applied voltage was the most significant parameter affecting the mineralization rate and the concentration of ozone generated, while energy yield was mainly determined by initial chlorobenzene concentration. As a key precursor of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorophenols were found during the identification of the intermediates produced during chlorobenzene degradation through GC-MS. Furthermore, HRGC-HRMS was used to detect the remaining byproducts on the catalyst surface after 3 and 10 h discharge time, and three types of PCDD/Fs (2,3,7,8-TCDF, 1,2,3,4,6,7,8-HCDF and OCDD) were detected after 10 h of discharge. The degradation mechanism of chlorobenzene was analyzed based on these detected intermediates, and the possible formation mechanisms of the three PCDD/Fs were proposed for the first time in plasma catalytic degradation of chlorobenzene.
Previous literature data showed that pristine aromatic allotropes of carbon such as graphene and carbon nanotubes (CNT) don’t show good interaction with carbon-based pollutants. On the other hand, defects or metal/non-metal doping in these inexpensive available catalysts could increase their strength for the adsorption and dissociation of different pollutants. In this work, by using periodic DFT calculations, adsorption of Tetrachlorodibenzofuran (TCDF) on pristine, co-adsorbed, defected, and M-doped carbon nanotubes (M=Al, Fe, Mn) is studied, and possible pathways for its first step dissociation is investigated. Data show that all three models of dissociations of TCDF need very high barrier energies on the surface of pristine CNT. Its minimum barrier energy is 3.31 eV which belongs to C−Cl bond cleavage. On the other hand, co-adsorbed, defected, and M-doped CNTs need less barrier energies. Minimum barrier energy belongs to C−H bond dissociation on oxygen co-adsorbed CNT with 0.92 eV which is almost equal to the temperature of 355 K. Co-adsorbed CNT catalyst can also activate C−Cl chemical bond by 0.93 eV. Some catalysts such as Al-doped CNT is selective for C−O bond cleavage, but some other catalysts, such as co-adsorbed CNT for C−H and C−Cl, have similar barrier energy for two models of dissociations. With considering kinetics point of view (by barrier energies) and thermodynamics point of view (by reaction energy), data show that co-adsorbed, defected, and Al-doped CNTs are good catalysts for the first step dissociation of TCDF.
Airborne particulate matter (PM) is associated with an increased risk for cardiovascular diseases. When incomplete combustion of waste and fuels occurs, organics chemisorb to transition metals to generate PM-containing environmentally persistent free radicals (EPFRs). EPFRs have been detected in PM found in diesel and gasoline exhaust, woodsmoke, and urban air. Prior in vivo studies demonstrated that EPFRs reduce cardiac function secondary to increased pulmonary arterial pressure. In vitro studies showed that EPFRs increase ROS and cytokines in pulmonary epithelial cells. We thus hypothesized that EPFR inhalation would promote lung inflammation and oxidative stress, leading to inflammation, endothelial injury, and a decline in vascular function. Mice were exposed to EPFRs for 4h or for 4h/d for 10d, and lung and vascular function were assessed. After 4h, plasma nitric oxide (NO) was reduced while endothelin-1 (ET-1) was increased, but lung function was not altered. After 10d, plasma NO and ET-1 levels were again altered and lung tidal volume was reduced. These studies suggested the vasculature may be an early target of injury. To test this hypothesis, a 3d time point was selected. Though the mice exhibited no marked inflammation, we did note reduced endothelial function concurrent with a reduction in lung tidal volume and an elevation in Annexin V protein levels in the lung. Although vascular dysfunction was not dependent upon inflammation, it may be associated with an injury at the air-blood interface. Gene expression analysis suggested roles for oxidative stress and aryl hydrocarbon receptor signaling.
Environmentally persistent free radicals (EPFRs) are emerging organic pollutants found widely in environmental media, which can induce reactive oxygen species generation and DNA damage. EPFRs have lifetimes of hours, days, or even months. EPFRs form through chemical bonds of organic compounds (e.g., phenols and hydroquinones) rupture or through electron transfer between organic compounds and metal ions during thermal-related processes. Metal compounds play critical roles in EPFR formation and stabilization. Different metal compounds and concentrations catalyze the formation of EPFRs and give different EPFR lifetimes. Information on EPFR catalysis by different metal compounds is important for understanding EPFR formation mechanisms. The influences of metal compounds at different species, concentrations and in different crystal forms on EPFRs are systematically summarized here. Information on the roles metal compounds play in EPFR formation and stabilization is important to understanding EPFR characteristics during thermal processes involving different metals, and will therefore allow effective strategies to be developed to control EPFRs and certain persistent organic pollutants intermediated by EPFRs.
Environmentally persistent free radicals are pollutants recently detected in most environmental matrices such as fly ash, aerosols, soils and sediments. Their generation and transformation is poorly known, notably in the atmopshere. Here we modeled the effect of dioxygen O2, hydroxyl radical •OH, and nitrate radical NO3 on Cu(II)O surface-bound phenoxyl radical, using quantum chemical calculations and kinetics analysis. Results show that additional stabilization of the surface-bound phenoxyl radical is provided by the metal-oxide surface, implying that self-decomposition is not likely to occur. The addition reactions of hydroxyl and nitrate radicals with surface-mediated radicals are both thermodynamically and kinetically favorable, whereas the role of O2 appears negligible. The tropospheric lifetime of the Cu(II)O-based surface-bound phenoxyl radical is only few seconds to about one hour, in agreement with experimental observations from the literature.
Urea ((NH2)2CO) is widely applied to the reduction of NOX in modern full-scale solid waste incineration systems, but there is a lack of knowledge about how urea affects the formation and emission of Cl-aromatics. In this study, we investigated the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and polychlorinated polychlorinated naphthalenes (PCNs) via electrophilic chlorination and precursor pathway mediated by model fly ashes containing Cu and Fe species with or without urea addition. The results indicated that the addition of urea promoted the direct chlorination of parent aromatics over Cu (Ⅱ) chlorides and the coupling reaction of chlorophenols over Fe species, while suppressed the catalytic chlorination of parent aromatics over Fe (Ⅲ) chlorides and the coupling reaction of chlorophenols over Cu species. The diverse effects should be mainly attributed to the formation of complex salts containing NH3 and NH4⁺. The formation of complex salts of Fe chlorides and NH4Cl could hinder the oxidization of Fe chlorides, and thus maintain the high activity of Fe species for catalyzing the coupling reaction of chlorophenols. The formation of complex salts of Cu (Ⅱ) chloride and NH3 could prevent the chemical sorption of phenoxyl groups, and thus suppress the coupling reaction of chlorophenols. NH3 released from the thermal decomposition of urea could not only react with Cl2 to suppress the catalytic chlorination of aromatics, but also neutralize HCl to accelerate the direct chlorination of aromatics. In general, urea should act as inhibitor for suppressing the formation of Cl-aromatics in solid waste incineration systems.
Boiler ash formed at different temperature ranges in a typical mechanical grate incinerator is collected and systemically studied, with the aim of providing a reference for ash disposal and revealing the formation routes and distribution of polychlorinated ρ-dibenzodioxins and dibenzofurans (PCDD/Fs). Key physical and chemical properties are carefully analyzed, including chemical component, ash fusion temperatures (AFTs), crystalline phases, chemical species, and PCDD/Fs. Several fouling and slagging indices are introduced and their relationships with AFT are revealed. The fouling index (Fu) and a slagging index (Rb/a×Na) are well fitted with ash flow temperatures, with correlation coefficient (R2) of 0.82 and 0.82, respectively; these could be better potential indices for disposal applications of municipal solid waste incineration fly ash. C=C/C–C/C–H (69.25-80.93%) and inorganic chlorine (94.23-98.68%) are the dominant carbon and chlorine species, respectively. The increasing AFT is mainly attributed to the changing components, the increasing proportions of crystalline CaSO4, NaCl and KCl and the decreasing crystallinity and content of SiO2. Twice as much PCDD/Fs is generated by the low-temperature heterogeneous reaction (6.71-19.22 ng/g) than by the high-temperature homogeneous reaction (0.59-6.71 ng/g). The proportions of highly chlorinated homologues increase and gradually become the main component. Principal component analysis reveals that PCDD/Fs is positively correlated with Cl, Cu, Pb, Sn, Sb, Zn and C=C/C–C/C–H but negatively correlated with less volatile elements, e.g., Ni, Mn, Al, Ti, Si, and Cr. These results can benefit further research on boiler ash disposal and PCDD/F formation routes in the post-combustion area of incinerators.
Municipal solid waste incineration (MSWI) fly ash, containing heavy metals and dioxins that can migrate and accumulate in biosphere, poses severe threats to human health and the environment. This review summarizes the sources and characteristics of heavy metals and persistent organic pollutants in MSWI fly ash, and comprehensively compares and discusses various treatment methods of MSWI fly ash, such as thermal treatment, pyrolysis process, hydrothermal treatment, solidification/stabilization (S/S) method and leaching process. Besides, the mechanism of dioxins degradation, and the principle of heavy metals solidification and recovery are analyzed in detail. Moreover, the pre-removal methods of chlorides are compared due to its adverse effect on the treatment processes of MSWI fly ash. However, to date, most of the disposal processes only have good effect on dioxins or heavy metals, except for pre-washing thermal treatments (sintering, melting and vitrification etc.) which need high energy consumption and complex equipment. Finally, low-temperature catalytic pyrolysis with electrolytic manganese residue (EMR) and S/S combined process of MSWI fly ash, and collaborative treatment of MSWI fly ash with melting furnace to degrade dioxins and enrich heavy metals are proposed. Those can provide some reasonable development directions for complete harmless treatment of MSWI fly ash based on the comprehensive analysis of all hazardous components.
To clarify the dominant formation mechanism of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and reduce PCDD/F emissions in full-scale hazardous waste incinerators (HWIs), three tests were designed by adding different PCDD/F precursors in phenol-containing raw material. Flue gas from three stages of the incineration facility, as well as bottom ash and fly ash were collected to investigate formation pathways, emission characteristics and mass balance of PCDD/Fs. The results showed that in tests A and C, the PCDD/F emission levels were 0.02 and 0.83 ng I-TEQ Nm⁻³, with adding naphthalene and p-dichlorobenzene, respectively. Test B, the control group, only incinerated raw materials, resulting in 0.72 ng I-TEQ Nm⁻³ PCDD/F emissions. PCDD/F formation mechanism analysis suggested that high-temperature radical-molecule reaction was the dominate pathway in test A, while for test B, memory effect in the air pollution control devices (APCDs) led to high PCDD/F emissions. With the addition of p-dichlorobenzene in test C, PCDD/F levels at the quenching tower outlet were one order of magnitude higher than those observed at the inlet, indicating that the quenching tower failed to suppress the formation of PCDD/Fs. The PCDD/PCDF ratios indicate that with the abundance of PCDD/F precursors, surface-mediated precursor reaction is the dominant formation mechanism in low-temperature stages. These finding raise the following strategies for industry to control PCDD/F emissions: (1) strict regulation of the organochlorine content in feed material; (2) frequent and thorough cleaning the APCDs; (3) optimizing the injection rate of activated carbon.
This study is carried out in two full-scale (300 t/d) municipal solid waste incinerators (MSWI), focusing on the inhibition effect on polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/F) formation by the Sulfur-, Phosphorus-, and Nitrogen-containing inhibitors. The inhibition efficiencies of total PCDD/F range from 45.77 % to 58.55 %, meanwhile, from 50.1 % to 57.6 % for toxic PCDD/F. X-ray photoelectron spectroscopy results conduct the inhibition effect on the three key factors of PCDD/F formation: catalytic metal, carbon source and chlorine source. Inhibitors can increase the proportion of inorganic chlorine form at the ash surface. The changes of sulfur and phosphorus forms support the inhibition mechanisms of PCDD/F. De novo synthesis is the stable inhibition pathway in this study, meanwhile, the chlorophenols-route and dibenzodioxin and dibenzofuran chlorination also work in some tests. The results are the basics for further industrial application of PCDD/F inhibitors and benefit in controlling the PCDD/F emission from MSWI.
The formation of dibenzofuran (DF), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and environmentally persistent free radicals (EPFRs) from 1,2,3-trichlorobenzene (1,2,3-TrCBz) over metal oxide / silica surface were investigated using a tubular furnace. PCDD/Fs increased exponentially from 250 to 550 °C over copper oxide / silica surface and PCDD/Fs had the maximum growth from 400 to 450 °C. The ratio of PCDD / PCDF was much less than 1, especially when the temperature raised from 450 to 550 °C. Pentachlorianated dibenzo-p-furan (PeCDF) dominated among the homologues, which contributed 45–61% to the total PCDD/Fs. Two peaks of the yield of DF occurred at 400 °C and 500 °C respectively. Furthermore, the oxygen contents have different effects for PCDD and PCDF formation, and low oxygen could promote PCDD production, especially for tetrachlorinated dibenzo-p-dioxin (TCDD). More PCDF were formed on the oxygen rich condition, indicating that the oxygen promoted the chlorination of DF. Iron oxides are better than copper oxides to catalyze the formation of PCDD/Fs from 1,2,3-TrCBzs at 350 °C, especially for PCDF. The major EPFRs on the catalysts were formed with g values in the range of 2.0040 to 2.0049, which were phenoxy radicals and semiquinone occurred with higher g value of 2.0075 when the temperature increased to 550 °C, and more EPFRs were produced with the temperature increasing. The addition of iron oxides reduced the spins concentrations of oxygen-centered radicals but increase the spins concentrations of signals with lower g values. The different possible formation pathways of PCDD and PCDF from 1,2,3-TrCBz over metal oxide surface were also proposed.
Waste incineration is a preferred method in China to dispose the municipal solid waste, but controlling the production of highly toxic polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans effectively during incineration is both challenging and imperative. In this study, the suppression of PCDD/Fs by various phosphorus-containing compounds was explored, and the mechanisms responsible for the inhibition were studied in detail. The experiments took place in a lab-scale vertical tubular reactor at 350 °C under a simulated flue gas (12 vol% O2 in N2 flow), and both the off-gases and residues were collected for PCDD/Fs analysis. The scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the reaction residues. The experimental results revealed that NH4H2PO4 and (NH4)2·HPO4 showed the highest inhibitory effect (57.2% and 57.3%, respectively) on the PCDD/Fs formation, followed by CaHPO4 with inhibition efficiency of 39.1%. In contrast, KH2PO4 and K2HPO4 barely inhibited the generation of the PCDD/Fs. The inhibitory effect of NH4H2PO4 and (NH4)2·HPO4 was similar to that of nitrogen-based inhibitors. At the same time, it was proven that the inhibitory activity of CaHPO4 might be due to the reaction of it with Cu²⁺ forming stable compounds.
Since the discovery of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the process of municipal solid waste incineration (MSWI), a large number of researches have been conducted to reveal their formation mechanisms and emission characteristics. As one of national priority control pollutants, chlorinated organics are inclined to transfer into PCDD/Fs in the heterogeneously catalyzed process, which has been considered to be one of great challenges in environmental catalysis. However, so far direct evidences to support such a conversion process are insufficient, and the reaction mechanisms are lack of exploration. This study investigated the catalytic elimination of chlorobenzene (CBz) over a range of industrially applied active species including Pt, Ru, V, Ce and Mn oxides, and explored their reaction byproducts, chlorine adsorption/desorption behaviors and PCDD/F formations. We found that all of these species could generate the PCDD/Fs, amongst which, Mn species were the most active for PCDD/F formation. Approximately 140 ng I-TEQ g⁻¹ PCDD/Fs were detected on the Mn-CNT surface after ageing at 250 °C for 30 h. Even using the dichloromethane (DCM) as a precursor, significant PCDD/Fs were still detected. The Ru and V species were shown to generate much less polychlorinated byproducts and PCDD/Fs, owning to their sufficiently high abilities in Cl desorption, which were through the semi-Deacon and Brønsted H reactions, respectively.
The direct chlorination of dibenzo-p-dioxin (DD) and dibenzofuran (DF) is an important source of dioxins in combustion flue gas. The chlorination reaction mainly occurs via electrophilic substitution induced by Cu and Fe chlorides, which must cohabit on particulate matters in mixed state. To explore the mechanism for DD/DF chlorination in real combustions flue gas, 8 kinds of CuO/Fe2O3/CuCl2/FeCl3 composites impregnated onto silica powder were prepared to simulate the coexisting state of Cu and Fe species in combustion flue gas. Mixed Cu and Fe oxides and chlorides induced a significant synergistic effect on electrophilic chlorination of DD/DF. The efficiencies of DD/DF chlorination over composites containing both Cu and Fe species were 1–2 orders of magnitude higher than those over composites containing only Fe species at 250 °C. CuCl2 species were highly active sites for electrophilic chlorination. FeCl3 acted as an excellent promoter to accelerate DD/DF chlorination over CuCl2 species. The elevated proportion of Cu and Fe oxides was also favorable for electrophilic chlorination. Compared with DF, DD was more prone to be chlorinated. Chlorine substitution primarily occurred at 2, 3, 7 and 8 positions of DD and DF. Furthermore, the possible mechanism for synergistic effect on electrophilic chlorination of DD/DF was speculated.
A series of magnesia-supported palladium catalysts (Pd loading in the range 0.5È7.0 wt.%) has been prepared by impregnation from aqueous solutions of and and characterised by PdCl 2 , Pd(NH 3) 4 Cl 2 Pd(CH 3 COO) 2 X-ray diraction (XRD), CO chemisorption and high resolution transmission electron microscopy (HRTEM). The gas-phase hydrogenation of phenol was employed as a model reaction to probe the dependence of catalytic activity/selectivity on changes in Pd particle size and surface acidÈbase properties. The catalyst prepared from the acetate precursor exhibited the greatest Pd dispersion when compared with the chloride and amine precursors. The surface mobility of the metal chloride resulted in larger Pd particles from the Cl-containing precursors while the presence of residual Cl in the activated catalysts lowered the hydrogenation rate and was responsible for a decline in activity with time-on-stream. The eects of varying such process variables as temperature, hydrogen/phenol mol ratio and inlet molar phenol feed rate are presented and discussed while the question of structure sensitivity is addressed. The reaction exhibited a negative dependence on phenol partial pressure up to 503 K but a positive dependence was evident at higher temperatures. The order of the reaction with respect to hydrogen remained positive and was close to unity at 563 K ; an apparent activation energy of 63 kJ mol~1 was recorded. The eect of doping the support with calcium and Ñuoride has shown that modiÐcations to the acidÈbase properties of magnesia can be used to control catalytic activity/selectivity.
Almost any combination of C, H, O, and Cl can yield some polychlorinated dioxons/furans under suitable conditions of time and temperature. The quantities formed in incinerators, ng m -3 or ng g -1, need to be explained in terms of time/temperature regimes relevant to those units. Current evidence suggests that surface catalyzed reactions at relatively low temperatures (250-400°) may play an important role. This paper reviews relevant evidence from incinerators, laboratory combustors and surface-catalyzed experiments using single precursors. A global kinetic model is proposed and assumptions and parameters are discussed.
The objective of this work is to acquire a better understanding of the gas-phase thermolysis of volatile chlorophenoxyaluminum compounds and to clarify the thermolysis on the fly ash chemisorbed chlorophenols. The purpose of this work was to elucidate the mechanism of the thermolysis of 3-chlorophenol chemisorbed on the surface of amorphous aluminum oxide or hydrated aluminum oxide. This is an example of the fly ash mediated surface catalysis reaction where 3,3′-dichlorodiphenylether and three derivates of dichloro-hydroxy-diphenyls are formed as the main thermolysic products. The proposed formation mechanism of 3,3′-dichlorodiphenylether and dichloro-hydroxy-diphenyls isomers is based on the dimerisation of the resonance stabilised 3-chlorophenoxy radical with the 3-chlorophenyl radical, which is formed by the expulsion of CO2 in the reaction of CO with the 3-chlorophenoxy radical. In this work some electronic and structural properties of aluminum-3-chlorophenolate intermediates are discussed. The results support and offer explanation for the previously suggested mass spectrometric fragmentation pathways and the mechanism of thermolysis. The influence of A1 atom as catalyst is explicitly provided for in the computational model.
The formation of chlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) downstream from the combustion zone of a heterogeneous combustor occurred within seconds in the temperature range of 430-380°C, but not at substantially lower temperatures. A spouted bed reactor was used with sand as the heterogeneous medium and a fuel mixture of 1,2-dichlorobenzene/heptane. Total PCDD/F-concentrations were in the range of 100 ng m−3; chlorobenzenes other than the starting compound and chlorophenols were formed. The formation of PCDD/F may be mediated by high surface area particles originating from the bed due to attrition. Homogeneous gas phase mechanisms cannot account readily for the levels observed.
Based on laboratory experiments with fly ash from municipal waste incinerators and model mixtures, the main reactions for the denovosynthesis of PCDD/PCDF were identified. According to the results, particles of carbonaceous material from incomplete combustion were converted to a series of chlorinated aromatic compounds including PCDD/PCDF, In the process, two basic reactions were involved: a) the transfer of inorganic chloride to the macromolecular carbonaceous structure with formation of carbon-chlorine bonds, and b) the oxidative degradation of the structure. In both processes, transition metal ions, especially Cu(II), were involved as reaction partners or catalysts. Important parameters for the reaction are: the composition of the fly ash, the temperature range from 300 to 350 degrees C, and the presence of oxygen in the gas phase.
2,3,7,8-Tetrachlorodibenzodioxin toxicity equivalents (I-TE) were closely correlated with the concentrations of other chloroaromatic compounds like pentachlorophenol, pentachlorobenzene, and hexachlorobenzene in the flue gas of three waste incinerators. To predict I-TE from these indicator parameters, regression equations were established for different sampling points of three hazardous waste incinerators. The sampling points were located in the boiler, directly upstream and downstream of the electrostatic precipitator, and downstream of the wet scrubber. Typically, the slopes of the regression lines were similar for the different incinerators at a given sampling point, but the intercepts were different. These findings indicate the possibility of transferring part of the regression results (the values of the slope) from one incinerator to another. The intercept of the regression line, which can be different, can then be determined with only a few simultaneous measurements of I-TE and the indicator compound.
The TiO2-mediated photocatalytic degradation of 4-chlorocatechol is studied as a branch of the degradation of 4-chlorophenol. In addition to some basic kinetic studies, the identities of many of the cyclic and acyclic intermediates, verified in most cases with authentic samples, are reported. From 4-chlorocatechol, the major product is hydroxylation to form 5-chloro-1,2,4-benzenetriol. A small amount of 4-chloropyrogallol is also produced. Substitution to give 1,2,4-benzenetriol is observed as is oxidative cleavage of the C1−C2 bond to give the diacid. The major products of all of the triols are those of oxidative cleavages, occurring mainly between ortho hydroxy-substituted carbons to give diacids but also between one hydroxy and one unsubstituted carbon to give acid-aldehydes. Many smaller intermediates in the degradations are identified, and pathways are proposed for the larger compounds.
The reaction of phenol on Al(111) was investigated between 100 and 800 K with temperature programmed desorption and Auger electron spectroscopy. Molecular phenol desorbed at 200 K from the multilayer. The first monolayer of phenol decomposed into hydrogen, benzene, and adsorbed oxygen and carbon as the surface was heated. The sequence of bond scission was determined using deuterium labeling. Three H2 desorption peaks were observed. The first H2 desorption peak was from the dissociation of the hydroxyl group and was observed around 380 K. The second H2 desorption peak, which started around 450 K and reached a maximum at 510 K, resulted from the scission of the CH bonds in the 2 or 6 positions on the ring. The third H2 desorption peak was observed at 680 K and was from scission of the CH bonds on the remaining hydrocarbon fragments on the surface. Benzene desorption occurred between 450 and 700 K with maxima at 490 and 630 K resulting from hydrogen addition to the phenyl group from OH and CxHy moieties, respectively. HD exchange with the hydrogen on the phenyl ring also was observed. However, there was no evidence for hydrogenation (saturation) of any of the double bonds of the ring. After heating to 800 K. some O and C remained on the surface.
In the present paper a detailed investigation of a well known photoreaction, i.e. phenol photodegradation in the presence of TiO2, has been carried out. The Fourier transform infrared (FT-IR) technique has been used to characterize the catalyst surface and to follow the photoprocess by “in situ” measurements in a gas—solid regime, simulating the aqueous liquid—solid regime in which this photodegradation reaction has been usually carried out. The influence on the photoprocess of the surface hydroxylation of the catalyst has been investigated and the presence of some reaction intermediates has been revealed by FT-IR. The experimental results have demonstrated that the chosen gas—solid system in the presence of water vapour is an acceptable simulation of the aqueous liquid—solid regime.
Since polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) were quantified in the emissions from the Amsterdam municipal solid waste incinerator (MSWI) in 1977, more than 20 years of intensive research gave many answers on how PCDD and PCDF can form in thermal processes. As a result, it can be concluded that PCDD/PCDF can be formed in all combustion processes where organic carbon, oxygen, and chlorine are present. Although there are still open questions, findings can be summarized as follows: PCDD/PCDF can be formed in the gas phase as well as in the heterogeneous phase. Geometry of the combustion chamber, time, temperature, feeding rate, input (chlorine), and so forth may have an influence on the formation of PCDD/PCDF. There may be different processes dominating to form PCDD or PCDF. The amount of PCDD and PCDF formed differs between types of thermal processes. Within the same process categories, e.g., MSWIs, hazardous waste incinerators, steel mills, etc., the PCDD/PCDF pattern is very similar. Formation of PCDD/PCDF can be prevented by addition of "inhibitors" such as sulfur- or nitrogen-containing agents. Results from field investigations, i.e., waste wood combustion and a new technology for thermal treatment of municipal solid waste, support these basic findings.
Liquid-phase oxidation using a solid catalyst provides a potential method for removal of dissolved toxic organic pollutants from waste waters. Low-temperature heterogeneous oxidation of phenol on a cobalt oxide system modified with Fe has been studied. The conversion of phenol into nontoxic compounds occurs in air-equilibrated aqueous media. The major hydroxylated aromatic intermediates have been identified by UV–VIS spectroscopy and HPLC: catechol, hydroquinone and hydroxyhydroquinone. An oxidation mechanism has been proposed based on the results of the kinetic investigations and on the data obtained by IR and UV spectroscopic studies of the sample. The results obtained show that the Co–Fe-oxide system is a promising catalyst for practical application.
If one understands and controls free radical chemistry, one could then design much more efficient synthetic fuels. This book attempts to ascertain the importance of this thesis by examining the available evidence from electron spin resonance (ESR) studies of coal-free radicals. Proceeding from elementary concepts to the more specialized topics, it is based primarily on the work done by the authors in their systematic investigation of the utility of ESR in the study of coal-free radicals and their relevance to coal hydroliquefaction. Although dealing with coal and coal conversion, the book is also applicable to the conversion of other fossil fuels.
A review on fly ash, an important but problematic waste material is presented. The article deals with several aspects of the characterisation, extraction of metal values leaching and associated environmental pollution problems due to fly ash. Characterisation includes classification, physicochemical and mineralogical analysis, presence of organic carbon and the significance of cenospheres. Utilisation aspects are confined to extraction of metal values and discuss the elemental distribution in fly ash while also covering several aspects of leaching including the leaching of specific metals. Effect of fly ash on surroundings, including ground water pollution and associated problems during disposal, are considered while dealing with the environmental aspects. Some remedial measures are also suggested.© 2001 Society of Chemical Industry
Decontamination of hazardous wastes containing dioxins (dibenzo-p-dioxin and its chlorinated analogues) remains a serious problem and new approaches for the degradation or alteration of dioxins are needed. We describe here dibenzo-p-dioxin radical cation formation and polymerization on a simple clay mineral (Cu(II)-smectite) in mild reaction conditions. Existence of the radical species was conclusively demonstrated using electron spin resonance (ESR), ultraviolet-visible, and infrared spectroscopy. The radical cation polymerized to form dimers and trimers as revealed by mass spectroscopy. Radical cations of 1- and 2-chlorodibenzo-p-dioxin were also formed on Cu(II)-smectite. The radical cations are formed by electron transfer from the dioxins to Cu(II). The formation of dioxin radicals may provide the basis for new detoxification methods in which the reactive radical species is coupled to other molecules or degraded by subsequent reactions.
Complete mineralization of 4-chlorophenol in water can be achieved by photocatalytic degradation of oxygenated solutions containing suspended TiO2. The chemical pathways of this degradation are complex, and in this paper, that which begins with hydroquinone is examined. Hydroxylation to form 1,2,4-benzenetriol is the first step, though a very small amount of cleavage of the C1−C2 bond is observed. The first major group of acyclic compounds derives from oxidative cleavage of either the C1−C2 or C3−C4 bond of 1,2,4-benzenetriol. It is argued that this results from single electron oxidation and capture by superoxide. Many smaller compounds have also been identified, and routes to various ones of them are proposed. Nearly all of the compounds are verified by use of authentic samples.
Emissions of organic chlorinated compounds from municipal waste incineration, in particular polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), have been a cause of public concern for many years because of the high toxicity of these compounds. PCDD/F formation in incineration processes is being studied widely using lab-scale apparatus, but pilot-plant investigations are quite rare. The correlation between TEQ-related PCDD/Fs and chlorophenols (ClPhs) was studied here using a pilot-scale plant. The results suggest that almost all the ClPh isomers correlate strongly with PCDD/Fs in the gas phase, but only certain isomers, in particular 2,3,4,6- and 2,3,4,5,6-ClPh, are of importance in the particle phase. The relationship of TEQ-related PCDD/Fs to ClPhs is so close that even predictive partial least-squares (PLS) modeling is feasible. In view of our results, some aspects of the mechanism of PCDD/F formation are discussed. From a practical point of view, the results suggest that ClPhs may be a good surrogate of TEQ-related PCDD/Fs in different incineration processes.
Research aimed at understanding polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) formation in combustion and incineration processes constitutes an important component in developing strategies for controlling their emission. Incinerator ashes from six different process sources have been examined and characterized in terms of their behavior with respect to PCDD/F formation in laboratory experiments. The effects of varying the carbon content of one ash has been investigated by replacing the native carbon content with activated charcoals and pyrocarbons from paper and PVC. Analysis of homologue totals and full isomer profiles indicate that PCDDs tend to form in preferential isomer groups, while PCDF isomers are distributed more broadly. PCDD formation is consistent with condensation of chlorophenols, together with stepwise chlorination and dechlorination. Experiments with a pentachlorophenol precursor show that fly ashes are more reactive dechlorinators than model systems, probably due to the presence of alkali elements. PCDF formation probably occurs via condensation of nonchlorinated phenol followed by chlorination of the dibenzofuran skeleton; this gives schemes of formation for both PCDDs and PCDFs that start with phenol. Demonstration of the formation of phenols in ashes and simulates is not currently satisfactory, but aromatization of residual aliphatic oils is believed to be the most probable source. A relationship was found between the amounts of such oils present on the original carbons/ashes and the quantities of PCDD and PCDF formed.
Chlorinated dioxins and dibenzofurans (PCDD/F) were measured in 107 soil samples globally, and depositional fluxes were determined. Deposition to land surfaces was estimated by dividing the earth into five depositional zones based on climatic and geographical factors. Mean depositional fluxes to these zones ranged from 18 to 610 ng m-2 yr-1. Low fluxes were observed in most zones not impacted by industrialization. Total global deposition from the atmosphere to land was estimated to be 12 500 ± 1300 kg/yr. Based on limited data, deposition to the oceans was estimated to be about 610 ± 1500 kg/yr, yielding a total global deposition of 13 100 ± 2000 kg/yr from the atmosphere. Emissions of PCDD/F to the global atmosphere were estimated by determining emission factors and production rates for the major PCDD/F sources. The major sources considered in this study were municipal waste incineration, biomass combustion, steel and copper mill emissions, cement kiln emissions, medical waste incineration, and emissions from automobiles Total annual emissions were estimated to be 3000 ± 600 kg. Global deposition (see above) is roughly four times greater than annual emissions. This suggests that sources of PCDD/F are not well-characterized. More data are needed on emission factors (particularly from developing countries) and on introduction rates of PCDD/F to the global atmosphere.
Acetylene was reacted with HCl/air under heterogeneous combustion conditions between 300 and 600 °C. Model catalyst mixtures of SiO2/metal oxides (Al2O3, Fe2O3, TiO2, and CuO) were compared with municipal waste incinerator (MWI) fly ash. Chlorinated benzenes (ClxBz), chlorinated phenols (ClxPh), and PCDD/F were detected in both gas-phase products as well as catalyst-adsorbed products. ClxBz production increased exponentially to 600 °C (HCB: 4 × 104 ng/g of C2H2), while ClxPh production (2.5 × 104 ng/g of C2H2) was maximized at 500 °C (ca. 104−105 ng/g of C2H2). PCDF displayed a dramatic peak at 500 °C (TCDF: 2 × 106 pg/g of C2H2), in contrast to the generally accepted de novo and aromatic precursor formation temperature of 300 °C. Comparison of gas-phase ClxBz congener distribution patterns revealed that the CuO-catalyzed reaction closely matched the ClxBz pattern produced in the reactions with MWI fly ash, providing further evidence of the importance of Cu in the fly ash matrix. Comparison of fly ash-catalyzed gas-phase ClxBz with MWI flue gas and 300 °C de novo synthesized ClxBz demonstrated the relevance of mid-temperature (600 °C) heterogeneous combustion reactions of C2 aliphatics in real combustion systems.
Tetrachlorophenol (T4CP) reactions to PCDD products in the presence of MSWI fly ash were measured as a function of gas-phase precursor concentration, reaction time, and reaction temperature. Tetrachlorophenol concentrations ranged from 150 to 700 ng/mL in 10% O2, and temperatures ranged from 250 to 400 °C, which resulted in PCDD formation rates from about 0 to 35 μg of PCDD (g-fly ash)-1 min-1. Reaction conditions were found such that the reactor approached differential behavior, which led to the breakthrough of the T4CP and the desorption of adsorbed PCDD products into the gas-phase. At constant temperature, the surface coverage of chlorophenols on fly ash was found to be constant for reaction times between 2 and 120 min, suggesting that the system had quickly reached adsorption/desorption equilibrium. Yields of PCDD were observed to increase with increasing precursor concentration but either peaked, leveled off, or increased as the temperature was raised. Studies of T4CP reactions on different fly ashes showed a correlation between total conversion of T4CPto unknown productsand total yield of the trace reaction forming PCDD.
4-Chloroanisole was found to react with Cu(II)-smectite forming a blue clay-organic complex. The presence of radical cation intermediates in the complex was confirmed by electron spin resonance and infrared spectroscopy. The radical cation intermediates were formed via an initial one electron oxidation of 4-chloroanisole by Cu(II)-smectite. Coupling of the radical cation of 4-chloroanisole (I) with a neutral 4-chloroanisole molecule gave a biphenyl radical cation (VI). Single electron transfer from the dimerized radical cation (VI) to Cu(II)-smectite resulted in the formation of a nonradical dication biphenyl intermediate (V). Reaction of the blue clay-organic complex with methanol resulted in the formation of the final dechlorinated dimeric product, viz., 4,4'-dimethoxybiphenyl. Chloride ion was recovered form the methanol extract. It is suggested that Cu(II)-smectite may be a useful catalyst in the oxidative polymerization and dechlorination of chlorinated aromatic toxicants. The products of these reactions should be significantly less toxic than the parent compounds. 15 references, 4 figures, 2 tables.
The aryl coupling reaction of bromobenzene on alumina-supported copper catalysts has been studied as model for dioxin formation. The reaction was monitored in situ by transmittance FTIR spectroscopy. Time-dependent changes in the spectra were recorded during addition of bromobenzene to the carrier gas stream. Both coupling of phenyl intermediates to yield biphenyls and formation of phenol and phenolate were observed. Novel approaches to inhibit the coupling reaction or aryl halides by addition of ethanolamine to the catalyst surface were studied. The resulting inhibition was attributed to site blocking and irreversible deactivation of the copper surface due to imine and nitride formation.
The copper(II)-catalyzed polymerization of 2,4,6-trichlorophenol and 2,4,6-trichloro-3-methylphenol were studied. Their polymer structures were confirmed by elemental analysis, infrared (IR), and nuclear magnetic resonance (NMR). The 2,4,6-trichlorophenoxo–copper(II) complex and 2,4,6-trichloro-3-methylphenoxo–copper(II) complex intermediates were isolated. The elemental analysis and electron spin resonance (ESR) measurement of the phenoxo–copper(II) complexes were discussed in relation to their structures. Decomposition of the phenoxo–copper(II) complex in refluxing benzene yielded poly(dichlorophenylene oxide). ESR measurements on the phenoxo–copper(II) complex in the solid state at 120°C indicated that the phenoxy radical was generated during the period of decomposition and the intensity of the ESR spectra based on the copper(II) ion decreased with the measurement time. A single-electron transfer reaction mechanism was proposed for the phenoxo–copper(II) complex intermediate. © 1996 John Wiley & Sons, Inc.
Kinetic and thermodynamic parameters for ferrate(VI) oxidation of phenol have been measured in isotopic solvents, H 2 O and D 2 O, using ambient and high-pressure stopped-flow UV-visible spectroscopy. An increase (fast stage) and then a decrease (slow stage) in absorbance at 400 nm are observed when potassium ferrate (K 2 FeO 4) and aqueous phenol solutions are mixed rapidly. This suggests that small amounts of unstable intermediate 4,4′-biphenoquinone are produced during this redox process. An electron paramagnetic resonance signal for the reaction mixture of ferrate and phenol trapped by spin-trap R-(4-pyridyl-1-oxide)-N-tert-butylnitrone indicates a radical reaction pathway. Gas chromatographic/mass spectrometric measurements show p-benzoquinone is a major organic product, and the red ferric thiocyanate complex formed from addition of potassium thiocyanate to the spent reaction solution indicates that Fe(VI) is reduced to Fe(III). Activation enthalpy, entropy, and volume changes have been determined. There is a primary isotope effect for the formation of the intermediate (fast stage), k fast (H 2 O)/k fast (D 2 O)) 2.4 (0.6. Because the phenol hydroxylic hydrogen is deuterated in D 2 O, this isotope effect suggests that a hydrogen bond is formed in the transition state.
The reactions of (halogenated) phenoxy radicals with (ortho) chlorinated or brominated phenols may play: a role in "dioxin" formation mechanisms. Therefore, we have recently investigated the model reaction (la) by using the slow combustion of phenol/chlorobenzene mixtures in a well-stirred reactor. This study has now been extended to include bromobenzene. Phi O + Phi X --> Phi O Phi + X, (a) X = Cl, (b) X = Br 2 Phi O --> DF The rate coefficient for reaction (la) was derived from the determination of the direct reaction product diphenylether (Phi O Phi) in comparison to dibenzofuran (DF) which is formed in the competing self-reaction (2) and found to obey (728 < T < 984 K) k(1a) = 4.8 * 10(8) exp(-24.5 kcal/mol/RT) L/mol s. This method with Phi Br (reaction Ib) instead of Phi Cl is ambiguous since the HBr produced from the slow combustion of Phi Br affects the overall rate coefficient for Df: formation from phenoxy radicals, Nevertheless, we have carried out Phi OH/Phi Br experiments under conditions similar to those for Phi OH/Phi Cl. The measured DI: and Phi O Phi, yields suggested that reactions (Ib) and (la) have nearly equal rates. To obtain a better insight we have investigated the co-combustion of Phi Br/Phi Cl mixtures with Phi OH, i.e., the reaction Phi O + (Phi Br was measured in competition to Phi O + Phi Cl (805 < T < 953 K). Since the direct reaction product is in both cases Phi O Phi one of these two reactions has to be labelled and this was achieved by using deuterated bromobenzene, d(5)-Phi Br. The respective diphenylethers were measured by GC/MS. Some H/D scrambling at higher temperatures was accounted for. The rate coefficient for reaction (Ib) was found to obey k(1b) = 1.4 * 10(8) exp(-21.4 kcal/mol/RT) L/mol s. This rate coefficient is much too small for the high yields of polybrominated dibenzodioxins as observed by Sidhu et al. (1995) in flow reactor experiments with 2,4,6-tribromophenol. This means that a radical-radical reaction of(ortho) brominated phenoxy radicals is the most likely source of halogenated dioxins, in analogy to the related case of slow combustion of 2,4,6-trichlorophenol.
The adsorption of 2-chlorophenol, 2,3- and 2,4-dichlorophenols and 2,4,6-trichlorophenol in liquid and gas phase on iron, titanium and aluminum oxides seem to proceed in a similar way. Higher adsorption of chlorophenols either from gas phase or from aqueous solution was observed on α-Fe2O3 than on α-FeOOH. The low adsorption of chlorophenols from aqueous solution on oxide surfaces suggests that hydrophobic chlorophenols cannot effectively compete with water for the absorption on hydrophilic oxide surface sites. The adsorption of chlorophenols on iron, titanium and aluminum oxides was followed by the adsorption isotherm, HPLC and diffuse reflectance FT-IR (DRIFT) spectroscopy. The adsorption of the chlorophenols on the oxides under study is related to the amount of interfacial water content on the iron oxide. The alumina–chlorophenolate surface complex was found to be weak when compared with either the iron or titanium analogs as seen by the CO stretching vibrations, leading to a lower adsorption on alumina than on iron and titanium oxides.
The pyrolysis of chlorinated phenates at a temperature of about 280°C results in the formation of definite chlorinated dibenzodioxin (PCDD) congeners [1–3].It is shown that in gas phase reactions chlorophenols react in the presence of oxygen above 340°C not only to PCDD but also to chlorinated dibenzofurans (PCDF). The mechanism of this reaction of chlorophenols to PCDD and PCDF was elucidated. In a first step phenoxyradicals are formed which are capable of forming PCDDs and PCDFs. This is confirmed by the oxygen dependency of the reaction. In an argon atmosphere no dimerization of chlorophenols could be observed at 420°C.By the identification of intermediates and by analyzing the PCDF isomers formed from individual chlorophenols the reaction pathway is elucidated. As intermediates in the formation of PCDFs polychlorinated dihydroxybiphenyls (DOHB) were identified. These are most likely formed by the dimerization of two phenoxy radicals at the hydrogen substituted carbons in ortho-positions under simultaneous movement of the hydrogen atoms to the phenolic oxygen atoms.PCDDs are formed in the gas phase via ortho-phenoxyphenols (POP) analogous to the pyrolysis of phenates, but due to the radical mechanism in the first condensation step to POPS not only a chlorine atom is capable for substitution but also the hydrogen atoms.The formation of the DOHBs and their condensation to PCDFs and hydroxylated PCDFs as well as the ratio of PCDD to PCDF formed show a strong dependency on the reaction temperature, the substitution pattern of the chlorophenols and the oxygen concentration.
By using selectively substituted iodobenzenes, we have been able to study the kinetics of the Ullmann phenyl coupling reaction on the Cu(111) surface. The substituted iodobenzenes used in these experiments were (iodobenzene (C6H5I), 3-iodotoluene (m-CH3C6H4I), 1-fluoro-2-iodobenzene (o-FC6H4I), 1-fluoro-3-iodobenzene (m-FC6H4I) and 1-fluoro-4-iodobenzene (p-FC6H4I)). Variable heating rate temperature programmed reaction (TPR) experiments were used to determine the energy barriers and preexponential factors in the rate constant for phenyl coupling. These studies have allowed us to probe the nature of charge separation in the transition state. Fluorination of iodobenzene in the meta or para position decreased the activation barrier, indicating that the transition state is electron rich with respect to the initial state. Reaction rate constants were calculated from the kinetic parameters and linear free energy relationships (LFER) or Hammett plots were constructed using tabulated values of the substituent constants. The reaction constant, ϱ, was determined to be 6.0 ± 3.3 from the Hammett plot. The sign of the reaction constant indicates the electron rich character of the transition state relative to the initial state.
The experimental observations on dioxin formation from various combustion sources, from detailed incinerator measurements and from laboratory simulation studies including de novo synthesis and precursor formation pathways are discussed in this paper. The de novo synthesis seems to be the dominant mechanism of dioxin formation in actual combustion systems. As de novo synthesis experiments indicate that carbon morphology of certain degenerated graphitic structure is essential for dioxin formation, the morphologies of the particulate emissions from actual combustion sources are examined and it appears that soot particles formed in gas phase combustion reactions consisting of degenerated graphitic structures are the plausible source for de novo synthesis of dioxins. With this understanding dioxin formation in combustion systems is described as a two-stage process: (1) the formation of the graphitic structure of soot particles in the combustion zone; and (2) the conversion of the graphitic structure of soot particles to aromatic compounds including PCDD/Fs in the postcombustion zone. New explanations are given for some seemingly unrelated experimental observations including the high dioxin emissions from municipal waste incineration, the low dioxin emissions from coal combustion, the similar dioxin “fingerprint” from all combustion sources and the discrepancy between the dioxin formation rates observed in laboratory experiments and incinerator measurements.
The homogeneous, gas-phase formation of polychlorinated dibenzo-p-dioxins (PCDD) and polybrominated dibenzo-p-dioxins (PBDD) has been observed from the high-temperature thermal decomposition of 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4,6-tribromophenol (2,4,6-TBP), respectively. Experiments were conducted in a 1.0-cm-i.d. flow reactor over a temperature range of 300°–800°C with reactant concentrations of ∼ 3.0 × 10−7 mol/L in a reaction atmosphere of dry air. The 1,3,6,8- and 1,3,7,9-tetra chlorinated isomers were the dominant PCDDs observed from the thermal oxidation of 2,4,6-TCP with maximum yields of 0.05% each. The corresponding tetrabrominated isomers were observed from the thermal oxidation of 2,4,6-TBP; however, the maximum yields were approximately 500 times higher. The observed PCDD/PBDD yields and the temperature of their formation can be readily accounted for using a modified form of the original gas-phase formation model of Shaub and Tsang, if the activation energy for the formation of diphenyl ether by displacement of from halophenol by phenoxy is decreased from 26 to 19.5 and 8.8 kcal/mol, for the chlorinated and brominated systems, respectively. This suggests that gas-phase formation reactions make a significant contribution to observed dioxin and furan yields in full-scale incinerator.
The temperature dependence of the gas-phase, rate-limited formation of dichlorodibenzo-p-dioxin (DCDD) and dichlorodibenzofuran (DCDF) isomers from 2,6-dichlorophenol and 3-chlorophenol, respectively, has been studied experimentally in an isothermal flow reactor over the range 300–900°C under pyrolytic, oxidative and catalytic conditions and computationally using semi-empirical molecular orbital methods. At high temperatures, distributions of sets of DCDD/F condensation products are consistent with the calculated thermodynamic distributions, indicating that the relative rates of formation are governed by differences in symmetry and steric hindrance present in the isomer product structures. At low temperatures, however, this is not the case. In the case of 1,6- and 1,9-DCDD formed from 2,6-dichlorophenol via Smiles rearrangement, the 1,6 isomer is favored at low temperatures more than thermodynamically predicted. This result appears to be consistent with kinetic effects of either the expansion of the five-membered ring Smiles intermediate or a lower activation energy six-membered ring intermediate pathway that produces only the 1,6 isomer. For formation of 1,7-, 3,7- and 1,9-DCDF from 3-chlorophenol, the 1,7 isomer fraction increases at low temperatures whereas thermodynamics predicts a decrease. This result can be attributed to steric effects in alternative “sandwich-type” approach geometries of phenoxy radicals to form the o,o′-dihydroxybiphenyl (DOHB) intermediate via its keto-tautomers. Higher level molecular theory (ab initio) is needed to provide a more quantitative description of these kinetics.
Acetylene is readily converted to perchlorinated gas-phase intermediates including hexachlorobenzene, hexachlorobutadiene, and tetrachloroethylene and heavier perchlorinated species via heterogeneous gas-solid reactions with HCl and cupric oxide on borosilicate under postcombustion conditions. Experiments were conducted using an integrated gas-solid flow-reactor and analytical system at temperatures ranging from 150 to 500°C for gas-phase residence times of 2.0 s and total reaction times of 60 min. Chlorine addition and chlorine net substitution mechanisms mediated by the conversion of Cu(II)Cl2 to Cu(I)Cl are proposed to account for the observed or inferred C2 reaction products including tetrachloroethylene, trichloroethylene, and dichloroacetylene. The formation of condensation products including tetrachlorovinylacetylene, hexachlorobutadiene, and hexachlorobenzene are proposed to be catalyzed by copper chloride species and involve the following steps: (1) chemisorption of a chlorinated ethylene or acetylene by HCl elimination or 1,2-Cu−Cl addition, respectively: (2) physisorption of additional chlorimated ethylenes or acetylenes followed by cis-insertions: and (3) carbon-to-copper chlorine transfer followed by desorption of the molecular growth product. The mechanism accounts for product isomer distributions and branching desorption of the higher molecular weight products, and regeneration of the copper chloride catalyst.
This study represents an effort to fit a Langmuir–Hinshelwood-type mechanism which involves a proposed hydrogen transfer step from a carbonaceous deposit with observed rate data, spectroscopic data, and reported literature results on a supported Pd/γ-Al2O3 catalyst prepared by the sol–gel method. Our data was taken in a differential reactor as the temperature was varied from 100 to 225°C in six increments. A change in the apparent reactant orders and activation energies support a multiple mechanism rate expression. Our data correlates best with an expression composed of two mechanisms: a competitive, low-temperature mechanism (mechanism I) with an activation energy of 29 kJ/mol, and a non-competitive, high-temperature mechanism (mechanism II) with an activation energy of 168 kJ/mol. Both mechanisms were fit using an enthalpy of adsorption of −124 kJ/mol for acetylene. The enthalpy of the hydrogen transfer was found to be 133 kJ/mol, indicating an endothermic process. Mechanism II is predicted to emerge and dominate at high temperatures.
The operation of Municipal Waste Incinerators (MWI’s) results in the emission of organochlorine compounds including trace amounts of hazardous polychlorinated dibenzodioxins and dibenzofurans. Although the presence of PCDDs and PCDFs in stack gases and on fly ash is well established, little is known about the mechanisms and kinetics of formation and the phase(s) — e.g. pyrolysis, burning, or fly-ash catalysis — in which these compounds and/or their precursors are formed. Proper insight into these chemical features may learn how to improve a MWI installation so as to reduce, or eliminate, these emissions.
The sensitized photocatalytic degradation of mono-, di- and trichlorophenols on iron oxides aqueous suspensions of α-Fe2O3 and α-FeOOH is reported in detail. The degradation of these compounds followed pseudo-first-order kinetics when α-Fe2O3 was used as photocatalyst. α-FeOOH was found to be inactive for chlorophenols degradation with the exception of 2,4-dichlorophenol (2,4-DCP) where a modest effect was observed. The formation of a surface complex by the chlorophenols with the iron oxide and the solubility of the particular chlorophenol in aqueous solution were observed to be the controlling parameters during the photodegradation. The results obtained with the most active catalyst α-Fe2O3 are compared with TiO2. Total mineralization of chlorophenols was observed on TiO2 while on α-Fe2O3 only partial mineralization was observed. In either case, the intermediates produced in solution during the photodegradation were found to be significantly more biodegradable than the initial compound. For mono-, di- and trichlorophenols the overall photocatalytic degradation was observed to increase in the order: 2,4,6-trichlorophenol (2,4,6-TCP)<2,3-dichlorophenol (2,,4-DCP. The former sequence shows that the recalcitrant 2,4-DCP degrades more rapidly than other chlorophenols tested during this study. The photodegradation of chlorophenols on α-Fe2O3 and TiO2 proceeds mechanistically through para-hydroxylation of the initial compound as suggested by the intermediates found by high-pressure liquid chromatography HPLC during the course of the degradation.
Processes which may contribute to the formation of polychlorinated dibenzo-p-dioxins (PCDDs) in incinerators are examined. A model mechanism has been constructed to investigate the possibility of homogeneous gas phase formation of PCDDs from chlorophenols in an incinerator environment. Numerical calculations have been made. The results lead to the conclusion that the probability of gas-phase formation of PCDDs is likely to be very low at high temperatures, ~>1200 K, if mixing between fuel and air is efficient. In addition, production of PCDDs from chlorophenols is found to depend upon the square of the chlorophenol concentration. Probable sources of nonidealities in practical incinerators are examined. Effects of use of auxiliary hydrocarbon fuel and excess air are examined. The potential role of non-gas-phase effects is considered. A discussion of some of the significant problems which complicate a further understanding of PCDD formation processes in incinerators has been presented in a manner that highlights future research needs.
Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans are toxic compounds formed during natural processes and human activities. The basic questions about PCDD/F formation such as (1) what is the influence of process parameters on the formation process, (2) what reaction mechanisms are involved in formation, and (3) what kinetics describes PCDD/F formation are discussed, and recommendations are given.
Chlorinated phenols and anisoles formed radical cations on copper(II)-smectite in mild reaction conditions. Electron spin resonance was used to demonstrate the formation of radical cations for pentachlorophenol, 4-chlorophenol, and 3-chloroanisole. Electron transfer from the aromatic species to Cu(II) resulted in the formation of the aromatic radical cation and Cu(I). The electron-withdrawing properties of Cl substituents did not prohibit oxidation of the aromatic species. The pentachlorophenol and 3-chloroanisole radicals formed dimers as revealed by mass spectrometry. The formation of dimers when 3-chloroanisole is reacted with Cu(II)-smectite was supported by changes in the UV spectrum. Pentachlorophenol was also dechlorinated forming 2,3,5,6-tetrachlorophenol. The formation of radical cations on Cu(II)-smectite may provide the basis for a new detoxication technology in which recalcitrant halogenated aromatic molecules are converted to less toxic products through a variety of potential reactions (e.g., polymerization or dechlorination) involving the reactive radical species.
The concentrations of hexa- and pentachlorobenzene in the flue gases of waste incinerators are known to be strongly correlated with the emission of polychlorinated dibenzo-p-dioxins/furans PCDD/F, measured in international toxicity equivalents, I-TEQ. In this work it was investigated whether polychlorinated benzenes (PCBz) with a lower degree of chlorination and mono- and dichlorinated dibenzo-p-dioxins/furans can be used for estimation of the dioxin emission (I-TEQ) in the flue gases of hazardous waste incinerators (HWI). The HWI process consists of a primary oxidative pyrolysis step in a rotary kiln followed by a post-combustion of the pyrolysis gases. The investigation indicated, that lower chlorinated PCDD/F homologues are very weakly correlated with the dioxin emission (I-TEQ) and thus are probably not useful as indicator parameters. In contrast to this finding, mono- to trichlorobenzenes are present in relatively high amounts in flue and stack gases and are correlated with the PCDD/F I-TEQ value.
Some attempts to balance atmospheric emissions of dioxins with deposition have suggested "missing sources." But the evidence disappears in the uncertainties in deposition data.
The presence of polychlorinated p-dibenzodioxins and polychlorinated dibenzofurans (hereafter PCDD/F) in municipal waste incinerator effluents has sparked a variety of laboratory investigations in the last few years in the hope of learning more about the formation and destruction mechanisms of these compounds. In the course of the investigations a variety of approaches have been employed. It is the purpose of this paper to review these designs and to comment on them in general as well as in specifics. Since under the conditions of interest the quantities of PCDD/Fs are miniscule compared with other products and reactants and because there are nearly 200 congeners, quantitative information is difficult and expensive to obtain. A second aspect concerns the reaction rates involved. No intrinsic kinetic studies have been conducted and no intrinsic values for PCDD/F rates of formation exist. Such information will be necessary, however. As will be shown below, there seems to exist a basic conflict between reaction rates as may be inferred from some laboratory experiments and rates seemingly required by the time/temperature histories of gases and particles in incinerators. The paper is divided into four major parts: homogeneous formation and destruction, heterogeneous formation and destruction, comments on discrepancies, and suggested designs.
Simplified thermal formation experiments have been conducted using dioxin-free fly ash as a catalyst with many kinds of combustible samples such as newspaper, kerosene, paraffin, PE (polyethylene), PP (polypropylene) and PVC. Chlorine sources were PVC, NaCl and HCl. The combustion of samples containing chlorine in the absence of dioxin-free fly ash produced dioxins at a low level although HCl was present in the gas stream. On the other hand, the combustion of samples without chlorine with dioxin-free fly ash increased dioxins formation to a level around 10 times higher than that upon heating dioxin-free fly ash alone. This result is considered to be due to the presence of metal chloride in the fly ash and hydrocarbons in the gas stream. The combustion of samples containing either an organic or inorganic chlorine source or using a HCl stream with dioxin-free fly ash increased dioxin level dramatically.
We performed experiments on two different matrices with 2,4,6-trichlorophenol as precursor to Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD)/F. A municipal solid waste incinerators (MSWI) and a model fly ash were spiked in two different ways. The experiments demonstrated a three times higher formation potential of the trichlorophenol to PCDD on MSWI fly ash compared with the model fly ash used. For both fly ashes the PCDD yield was higher when gaseous trichlorophenol was fed continuously compared to mixing the fly ashes prior to the experiments with the total amount of the precursor. Despite dilution of the fly ashes tenfold with an inactive matrix the conversion of the chlorophenol was very high.
A kinetic model is developed for PCDD formation from chlorophenol catalysed by incinerator fly ash. The key step in the model is a Langmuir-Hinshelwood type elementary step for the coupling of two adsorbed chlorophenol species to PCDD. Kinetic expression is derived which can relate PCDD formation rates with process variables including temperature, precursor concentration, fly ash loading and number of active sites in fly ash. Calculated PCDD formation rates based on this kinetic model are in good agreement with laboratory measurements reported in the literature. When the model is applied to industrial incinerator conditions, at maximum a PCDD yield of 10(-3) microg/N m3 is calculated.
The oxidation of selected chlorophenols (2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol) was studied in aqueous solutions using UV/H2O2 and O3 methods. The formation of oxidation intermediates was measured to elucidate their importance in the treatment of chlorophenols. Results indicated that chlorophenols can be treated efficiently by the methods studied, but the dechlorination of the compounds was insufficient. Analysis of intermediates in the acetylated extraction fractions showed that hydroxylation of chlorophenols and formation of dimeric products were involved in the oxidation of chlorophenols in both treatment processes. The majority of the intermediates detected were transient, and thus were not detectable after an extended treatment time. The presence of a complicated mixture of intermediates suggests the need for toxicity testing to confirm the detoxifying effect of the chemical oxidation of chlorophenols.
The transformation of 3-chlorophenol (3CP) photoinduced by iron(II) in aqueous solution has been investigated under monochromatic irradiation (lambda(exc) = 365 nm) representative of atmospheric solar emission. Hydroxyl radicals are formed via an intramolecular photoredox process in iron(III) excited hydroxy-complexes. Fe(OH)2+ is the most active complex in terms of HO* formation and according to our experiments and calculations, it appears that Fe(OH)2+ is the only iron(III) species involved in 3CP oxidation process. Hydroxyl radicals react very rapidly with 3CP, which is eliminated from the solution. The primary intermediates do not accumulate in the medium but rapidly degraded to non-absorbing compounds by a subsequent action of hydroxyl radicals.
Thermal treatment of 2,4,6-trichlorophenol on a magnesium silicate-based model fly ash in the temperature range between 250 degrees C and 400 degrees C leads predominantly to carbon monoxide and carbon dioxide. The fraction of 2,4,6-trichlorophenol which is oxidized to CO and CO2 increases from 3% at 250 degrees C to 75% at 400 degrees C. Further products are polychlorinated benzenes, dibenzo-p-dioxins, dibenzofurans and phenols. The homologue and isomer patterns of the chlorobenzenes suggest chlorination in the ipso-position of the trichlorophenol. The formation of PCDD from 2,4,6-trichlorophenol and 2,3,4,6-tetrachlorophenol on municipal solid waste incinerator fly ashes and model fly ash were compared and the reaction order calculated.
In the de-novo synthesis and formation of PCDD/PCDF, the transfer of inorganic chlorine to the carbonaceous material of fly ash plays an important role. Here, copper acts as a catalyst in the chlorination reaction. In experiments in the range of 250-350 degrees C under helium, we determined the stoichiometry of the chlorination reaction with model systems. Therefore, it was necessary to develop a method to quantify the copper(II) and copper(I) ions. In a combination of solid electron paramagnetic (spin) resonance spectroscopy (EPR) for Cu(I), and X-ray fluorescence spectroscopy (XRFA) analysis for Cu (total), we found a way for the quantification of copper(I) and (II). With these experiments, we can show that the chlorination reaction is relatively fast and comes to a stop under helium, after the copper(II) is reduced. The ratio between the organic chlorine formed and copper(II) reduced is, at the end of the reaction, 0.5, which is in agreement with the following reaction: 2CuCl2 + R-H-->2CuCl + R-Cl + HCl.