Formation of polybrominated dibenzofurans (PBDFs) and polybrominated diphenyl ethers (PBDEs) from oxidation of brominated flame retardants (BFRs)

A mechanistic and kinetic investigation on the oxidative thermal decomposition of decabromodiphenyl ether

The thermal processes of materials containing decabromodiphenyl ether (BDE-209) normally result in the exposure of BDE-209 to high-temperature environments, generating a series of hazardous compounds. However, the evolution mechanisms of BDE-209 during oxidative thermal processes remain unclear. Thus, this paper presents a detailed investigation on the oxidative thermal decomposition mechanism of BDE-209 by utilizing density functional theory methods at the M06/cc-pVDZ theoretical level. The results show that the barrierless fission of the ether linkage dominates the initial degradation of BDE-209 at all temperatures, with branching ratio over 80%. The decomposition of BDE-209 in oxidative thermal processes is mainly along BDE-209 → pentabromophenyl and pentabromophenoxy radicals → pentabromocyclopentadienyl radicals → brominated aliphatic products. Additionally, the study results on the formation mechanisms of several hazardous pollutants indicate that the ortho-phenyl-type radicals created by ortho-C-Br bond fission (branching ratio reached 15.1% at 1600 K) can easily be converted into octabrominated dibenzo-p-dioxin and furan, which require overcoming the energy barriers of 99.0 and 48.2 kJ/mol, respectively. The O/ortho-C coupling of two pentabromophenoxy radicals also acts as a non-negligible pathway for the formation of octabrominated dibenzo-p-dioxin. The synthesis of octabromonaphthalene involves the self-condensation of pentabromocyclopentadienyl radicals, followed by an intricately intramolecular evolution. Results presented in this study can enhance our understanding of the transformation mechanism of BDE-209 in thermal processes, and offer an insight into controlling the emissions of hazardous pollutants.

Theoretical study on pyrolysis mechanism of decabromodiphenyl ether (BDE-209) using DFT method

Decabromodiphenyl ether (BDE-209), as a brominated flame retardant (BFR), is widely applied to various consumer products due to its superior performance and affordable pricing to improve the flame resistance of materials. To better comprehend the pyrolysis behavior of BDE-209 and the evolution process of main pyrolysis products, the thermal degradation mechanism of BDE-209 was studied using density functional theory (DFT) method at the theoretical level of M06/cc-pVDZ, and thermodynamic parameters were calculated in this paper. Unimolecular degradation was dominated by cleavage of the ether linkage, which results in a high yield of hexabromobenzene, and fission of the ortho-position C-Br bond is the main competitive reaction channel. In the system of BDE-209 + H, the pyrolysis reaction is majorly characterized by debromination, leading to the formation of considerable HBr and low-brominated diphenyl ethers. Additionally, the hydrogen-derived splitting of the ether bond acts as a mainly competitive channel, which is the source of polybromophenols and polybromobenzenes. The formation of polybrominated dibenzofuran (PBDF) derives from the cyclization reaction of ortho-phenyl-type radicals, which are readily generated through the ortho-position Br atom abstraction by H radical. The formation of polybrominated dibenzo-p-dioxin (PBDD) involves the ortho-C-O coupling reaction of polybromophenoxy radicals, debromination reaction, and cyclization reaction. And the total yield of PBDD/Fs was significantly increased when H was involved. Results presented in this work will provide the helpful information for the treatment and reuse of BDE-209-containing waste plastics through using pyrolysis technology.

Unintentional formation of mixed chloro-bromo diphenyl ethers (PBCDEs), dibenzo-p-dioxins and dibenzofurans (PBCDD/Fs) from pyrolysis of polybrominated diphenyl ethers (PBDEs)

This study presents the comprehensive investigation for formation pathways of chloro-bromo-mixed products from the pyrolysis of polybrominated diphenyl ethers (PBDEs). In the study, a total of 23 PBDEs with bromination levels from mono-to deca-were selected. Each PBDE standard was sealed in the glass vial and then heated under 450 °C in the muffle furnace to simulate the pyrolysis process. The results demonstrated that PBDEs in the glass vials can unintentionally transform into chloro-bromo diphenyl ethers (PBCDEs) and dibenzo-p-dioxin and dibenzofurans (PBCDD/Fs) during the pyrolysis process. Atmosphere pressure gas chromatography (APGC) coupled with high-resolution mass spectrometry (HRMS) was used to identify these pyrolysis products, which demonstrated that all investigated nPBDEs (n represents the number of bromine substituents) can unintentionally transform into Cl₁-(n-1)BDEs, Cl₂-(n-2)BDEs, Cl₁-(n-1)BDFs, and Cl₁-(n-3)BDDs, while some nPBDEs can transform into Cl₁-(n-2)PBDD/Fs during pyrolysis. Experimental phenomena assisted with density functional theory (DFT) calculations reveal that Cl atom can substitute at C-Br rather than C-H, and Cl₁-(n-1)BDEs can be easily generated by Cl atom attacking at C-Br sites with low energy barriers (3.66-11.9 kcal/mol). In addition, nPBDEs with lower bromination levels are more favorable to generate Cl₁-(n-1)BDEs than those with higher bromination levels. Further DFT calculations suggest that PBDEs are preferentially first transformed into Cl₁-(n-1)BDEs, then subsequentially transform into PBCDD/Fs. We believe the results of this study can greatly improve our understanding of the transformation mechanism from PBDEs to cholo-bromo-mixed products in thermal treatment processes and provide new insight into controlling the emission of toxic cholo-bromo-mixed products.

Oxidative and pyrolytic decomposition of an evaporated stream of 2,4,6-tribromophenol over hematite: A prevailing scenario during thermal recycling of e-waste

Brominated flame retardants (BFRs) constitute a major load in the polymeric fraction of e-waste. Degradation of BFRs-laden plastics over transition metal oxides is currently deployed as a mainstream strategy in the disposal and treatment of the non-metallic segment of e-waste. However, interaction of pyrolysis's products of BFRs with transition metal oxides is well-known to facilitate the formation of notorious pollutants. Despite recent progress to comprehend the germane chemistry of this interaction, several important pertinent aspects remain to be addressed. To fill in this gap, an integrated experimental and simulation account of the pyrolytic and oxidative decomposition of a gaseous stream of 2,4,6-tribromophenol (TBP) over hematite (Fe2O3) has been reported herein. TBP is utilized as a model compounds of BFRs as their most common formulations include brominated phenolic rings. Overall, hematite entails a rather low cracking capacity under pyrolytic conditions. Analysis of condensate products indicates that oxidative degradation of a gaseous stream of TBP results mainly in the formation of brominated alkanes such as bromoethane and bromo-pentane. Likewise, Ion chromatography (IC) measurements disclosed a noticeable reduction in the concentrations of escaped HBr. Transformation of iron oxides into iron bromides (possibly in the form of FeBr2) during pyrolysis and combustion operations is evident through XRD measurements. Density functional theory (DFT) calculations map out important reactions pathways that operate in the initial degradation of the TBP molecule. From a broader perspective, outlined results shall be instrumental to precisely assess the effectiveness of using iron oxides in thermal catalytic recycling of e-waste and the likely emission of brominated toxicants.

Variation in the formation characteristics of PBDD/F, brominated PAH, and PBDE congeners along the secondary copper smelting processes

Simultaneous determination of 58 congeners of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs), brominated polycyclic aromatic hydrocarbons (Br-PAHs), and polybrominated diphenyl ethers (PBDEs) from multiple stages of industrial-scale secondary copper smelting plants was conducted with the aim of understanding their variations and control. In addition to the historical manufacture of PBDEs as brominated flame retardants, this study confirmed that PBDEs can be unintentionally produced and released from the secondary copper industry. The average mass emission factors of PBDD/Fs, PBDEs, and Br-PAHs from different sources were 10.0, 5.21 × 10³, and 7.24 × 10³ μg t^-1, respectively. Therefore, the emission of brominated persistent organic pollutants (POPs) in the secondary copper industry should be of concern. The concentration of brominated POPs increased from the gas cooling stage to stack outlet due to the possible "memory effect" and the regenerated POPs were mainly low-brominated homologs. A comparison of brominated POPs with corresponding chlorinated analogs in the same process indicated that the formation pathway of Br-PAHs was consistent with that of chlorinated PAHs. However, unlike chlorinated dioxins and furans, PBDD/Fs can also be formed from PBDEs as precursors, leading to obvious increases in highly brominated furans. Therefore, inhibiting the unintentional formation of PBDEs is important for controlling brominated POPs emissions.

Simulation degradation of bromophenolic compounds in chlorine-based advanced oxidation processes: Mechanism, microscopic and apparent kinetics, and toxicity assessment

Chlorine-based advanced oxidation processes (AOPs) have been extensively studied to remove contaminants through generating HO^• and reactive chlorine species, including ClO^• and Cl^•. In this work, 2,4,6-tribromoanisole (246TBA) and 2,4,6-tribromophenol (246TBP) were selected as model to investigate the reaction mechanisms and micro-kinetics of brominated contaminants with HO^•, ClO^• and Cl^• in chlorine-based AOPs. Also, the apparent degradation kinetics of two compounds were simulated at pH 3.0-9.5 under UV/H₂O₂, UV/chlorine and UV/NH₂Cl. Calculated results showed that neutral 246TBA and 246TBP exhibited similar reactivity to HO^• and ClO^•, which was different from anionic 2,4,6-tribromophenolate (246TBPT): radical adduct formation (RAF) and H atom abstraction (HAA) were predominant mechanisms for the HO^• and ClO^• initiated reactions of 246TBA and 246TBP, while RAF and single electron transfer (SET) for 246TBPT; the reaction rate constants of 246TBA and 246TBP with HO^• and ClO^• were lower than 10⁷ M^-1 s^-1, and such rate constants dramatically increased to 10¹⁰ M^-1 s^-1 once 246TBP was deprotonated to 246TBPT. The apparent degradation kinetics of 246TBA at pH 3.0-9.5 was simulated in the order of UV/NH₂Cl > UV/chlorine > UV/H₂O₂, and UV/chlorine and UV/NH₂Cl were more effective for the removal of 246TBP and 246TBPT than UV/H₂O₂. UV and/or Cl^• dominated 246 TBA degradation under three AOPs. The main radicals mediating 246TBP and 246TBPT degradation are respectively HO^• under UV/H₂O₂, ClO^• under UV/chlorine, and HO^• and Cl^• under UV/NH₂Cl. The transformation products of 246TBA, 246TBP and 246TBPT, especially methoxylated and hydroxylated polybrominated diphenyl ethers (MeO-PBDEs and HO-PBDEs), were still toxic pollutants.

Temperature-dependent profiles of dioxin-like toxicants from combustion of brominated flame retardants

Literature provides detailed mechanisms underpinning the formation of a wide array of bromine (Br)-containing molecules with a prime focus on dioxin-like compounds. However, from a more applied point of view, the practical deployment of attained thermo-kinetic parameters remains inadequate in the absence of a robust kinetic model that connects bromine transformation at the molecular level with pertinent experimental observations. Herein and to fill in this gap, this study constructs a chemical kinetic model to account for the "homogenous gas phase" emission of Br-aromatic pollutants from the oxidative thermal decomposition of a monobromobenzene molecule (MBZ). The latter serves as a model compound for brominated flame retardants (BFRs) present in e-waste. The model consists of sub-mechanisms (that include reaction rate constants and thermochemical T-dependent functions) for HBr oxidation, combustion mechanism of C₁-C₆ species, bromine transformation, and synthesis of Br dioxin-like compounds. Reaction rate parameters were obtained for a large array of reactions that constitute the core of the model. For instance, the obtained activation energies for the initial pathways in the formation of brominated biphenyls reside in the range of ~15-45 kJ/mol. Considering oxidation of 5000 ppm MBZ in a plug flow reactor, the model reasonably predicts the temperature-dependent profiles (between 500 and 1200 °C at atmospheric pressure) of a few PBDD/Fs (i.e., polybrominated dibenzo-p-dioxins) isomers in reference to limited corresponding experimental measurements. Most Br dioxin-like compounds appear in the narrow temperature window of 600-1000 °C and achieve their highest abundance at molar yields in the range of 1.0-15 mmol/mol MBZ. A high load (100-120 mmol/mol MBZ) of brominated environmentally persistent free radicals (Br-EPFR) emerges and shifts from bromophenoxy radicals to bromocyclopentadienyl radicals around 700 °C. Oxidation of a 2-bromophenol molecule results in the formation of higher yields of Br-toxicants when compared with that of MBZ. The assembled model provides an informed hazards assessment into the potential emission inventories of Br-compounds in the gas phase at conditions encountered in real scenarios, such as open burning and primitive treatment of e-waste. Via an atomic-base understanding of the complex bromine chemistry and speciation, the model allows the underlying operational conditions that reduce the emission of Br-notorious pollutants to be surveyed and fine-tuned.

Removal of Bromine from the non-metallic fraction in printed circuit board via its Co-pyrolysis with alumina

The effectiveness of a recycling approach of the printed circuit board (PCBs), and, thus, the quality of polymeric constituents, primarily rests on the capacity to eliminate the bromine content (mainly as HBr). HBr is emitted in appreciable quantities during thermal decomposition of PCB-contained brominated flame retardants (BFRs). The highly corrosive, yet relatively reactive HBr, renders recovery of bromine-free hydrocarbons streams from brominated polymers in PCBs very challenging. Via combined experimental and theoretical frameworks, this study explores the potential of deploying alumina (Al₂O₃) as a debromination agent of Br-containing hydrocarbon fractions in PCBs. A consensus from a wide array of characterization techniques utilized herein (ICP-OES, IC, XRD, FTIR, SEM-EDX, and TGA) clearly demonstrates the transformation of alumina upon its co-pyrolysis with the non-metallic fractions of PCBs, into aluminum bromides and oxy-bromides. ICP-OES measurements disclose the presence of high concentration of Cu in the non-metallic fraction of PCB, along with minor levels of selected valuable metals. Likewise, elemental ionic analysis by IC demonstrates an elevated concentration of bromine in washed alumina-PCBs pyrolysates, especially at 500 °C. The Coats-Redfern model facilitates the derivation of thermo-kinetic parameters underpinning the thermal degradation of alumina-PCB mixtures. Density functional theory calculations (DFT) establish an accessible reaction pathway for the HBr uptake by the alumina surface, thus elucidating chemical reactions governing the observed alumina debromination activity. Findings from this study illustrate the capacity of alumina as a HBr fixation agent during the thermal treatment of e-waste.

Low-temperature oxidation of monobromobenzene: Bromine transformation and yields of phenolic species

Brominated benzenes and phenols constitute direct precursors in the formation of bromine-bearing pollutants; most notably PBDD/Fs and other dioxin-type compounds. Elucidating accurate mechanisms and constructing robust kinetic models for the oxidative transformation of bromobenzenes and bromophenols into notorious Br-toxicants entail a comprehensive understanding of their initial oxidation steps. However, pertinent mechanistic studies, based on quantum chemical calculations, have only focused on secondary condensation reactions into PBDD/Fs and PBDEs. Literature provide kinetic parameters for these significant reactions, nonetheless, without attempting to compile the acquired Arrhenius coefficients into kinetic models. To fill in this gap, this study sets out to illustrate primary chemical phenomena underpinning the low-temperature combustion of a monobromobenzene molecule (MBZ) based on a detail chemical kinetic model. The main aim is to map out temperature-dependent profiles for major intermediates and products. The constructed kinetic model encompasses several sub-mechanisms (i.e, HBr and benzene oxidation, bromination of phenoxy radicals, and initial reaction of oxygen molecules with MBZ). In light of germane experimental observations, the formulated kinetic model herein offers an insight into bromine speciation, conversion profile of MBZ, and formation of higher brominated congeners of benzene and phenol. For instance, the model satisfactorily accounts for the yields of dibromophenols from oxidation of a 2-bromophenol (2-MBP) molecule, in reference to analogous experimental measurements. From an environmental perspective, the model reflects the accumulation of appreciable loads of 2-bromophenoxy radicals at intermediate temperatures (i.e., a bromine-containing environmental persistent free radical, EPFR) from combustion of MBZ and 2-MBP molecules. Acquired mechanistic/kinetic parameters shall be useful in comprehending the complex bromine transformation chemistry in real scenarios, most notably those prevailing in thermal recycling of brominated flame retardants (BFRs).

Insights into the metabolic mechanism of PBDEs catalyzed by cytochrome P450 enzyme 3A4: A QM/MM study

Elucidating the metabolic mechanism and the derivatives of polybrominated diphenyl ethers (PBDEs) is significant to risk assessment. This study delineated the metabolic mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) catalyzed by P450 enzymes using a combination of molecular dynamic (MD), quantum mechanics/molecular mechanics (QM/MM) and density functional theory (DFT). The calculation results reveal that the electrophilic addition is the main pathway for the biotransformation of BDE-47 catalyzed by P450 enzymes. 6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-47) is a more kinetically preferable product than 5-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (5-OH-BDE-47). Electrophilic addition reaction can lead to the formation of polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs). The ecotoxicity assessment indicates that the final products of BDE-47 are still toxic to aquatic organisms, but the solubility increase of the hydroxylated products can accelerate their excretion from the body. We expect that the established metabolic mechanism and the derivatives will be used to predict the biotransformation of other PBDE congeners catalyzed by P450 enzymes in human livers.