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

Quantum chemical pathways for the formation of 2,3,7,8-tetrachloro dibenzo-p-dioxin (TCDD) from 2,4,5-trichlorophenol: a mechanistic and thermo-kinetic study

CONTEXT

Dioxins, specifically 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD), are highly toxic dioxins known for their severe health impacts and persistent environmental pollutants. This study focuses on understanding the formation pathways of TCDD from its precursor molecule 2,4,5-trichlorophenol (2,4,5-TCP). In our exploration of reaction pathways from 2,4,5-trichlorophenol (TCP), we delve into three reaction mechanisms: free-radical, direct condensation, and anionic. Our findings highlight the significance of the radical mechanism, particularly propagated by H radicals, with a notable increase in dioxin formation around 900 K. These results are consistent with experimental observations indicating an increase in the conversion of trichlorophenol from 600 to 900 K in the non-catalytic gas phase reaction. Thermodynamic parameters (∆H, ∆S, and ∆G), reaction barriers, and rate constants (k) were calculated across a temperature range of 300-1200 K to support the findings and provide insights into the optimal temperature range for controlling dioxins during the incineration process.

METHOD

In this study, quantum chemical calculations were conducted using density functional theory (DFT) with the B3LYP functional and the 6-311 +  + G(d,p) basis set in Gaussian 16 software. Stationary points, including transition states (TS), were confirmed with frequency calculations. Intrinsic reaction coordinate (IRC) calculations ensured minimum energy paths between TS and products, visualized in GaussView 6.0 Program. Single-point energy calculations utilized a more precise basis set, 6-311 +  + G(3df,2p), for enhanced energy accuracy, incorporating zero-point vibrational energy (ZPE) and other energy corrections. These calculations were repeated over a temperature range of 298.15-1200 K at 1 atm pressure. Finally, rate constant (k) expressions associated with TCDD formation were determined using transition state theory (TST).

Predicting the Decomposition Mechanism of the Serine α-Amino Acid in the Gas Phase and Condensed Media

Comprehending the nitrogen combustion chemistry during the thermal treatment of biomass demands acquiring a detailed mechanism for reaction pathways that dictate the degradation of amino acids. Serine (Ser) is an important α-amino acid that invariably exists in various categories of biomass, most notably algae. Based on density functional theory (DFT) coupled with kinetic modeling, this study presents a mechanistic overview of reactions that govern the fragmentation of the Ser compound in the gas phase as well as in the crystalline form. Thermokinetic parameters are computed for a large set of reactions and involved species. The initial decomposition of Ser is solely controlled by a dehydration channel that leads to the formation of a 2-aminoacrylic acid molecule. Decarboxylation and deamination routes are likely to be of negligible importance. The falloff window of the dehydration channel extends until the atmospheric pressure. Bimolecular reactions between two Ser compounds simulate the widely discussed cross-linking reactions that prevail in the condensed medium. It is demonstrated that the formation of the key experimentally observed products (NH3, CO2, and CO) may originate from direct bond fissions in the melted phase of Ser prior to evaporation. A constructed kinetic model (with 24 reactions) accounts for the primary steps in the degradation of the Ser molecule in the gas phase. These steps include dehydration, decarboxylation, deamination, and others. The kinetic model presents an onset decomposition temperature of 700 K with the complete conversion attained at ∼1090 K. Likewise, the model portrays the temperature-dependent increasing yields of CO2 and NH3. The results presented in this work offer a detailed analysis of the intricate chemical processes involved in nitrogen transformations, specifically in relation to amino acids. Amino acids play a crucial role as the primary nitrogen carriers in biomass, such as microalgae and protein-rich biomass.

Update of the risk assessment of polybrominated diphenyl ethers (PBDEs) in food

The European Commission asked EFSA to update its 2011 risk assessment on polybrominated diphenyl ethers (PBDEs) in food, focusing on 10 congeners: BDE-28, -47, -49, -99, -100, -138, -153, -154, -183 and ‑209. The CONTAM Panel concluded that the neurodevelopmental effects on behaviour and reproductive/developmental effects are the critical effects in rodent studies. For four congeners (BDE-47, -99, -153, -209) the Panel derived Reference Points, i.e. benchmark doses and corresponding lower 95% confidence limits (BMDLs), for endpoint-specific benchmark responses. Since repeated exposure to PBDEs results in accumulation of these chemicals in the body, the Panel estimated the body burden at the BMDL in rodents, and the chronic intake that would lead to the same body burden in humans. For the remaining six congeners no studies were available to identify Reference Points. The Panel concluded that there is scientific basis for inclusion of all 10 congeners in a common assessment group and performed a combined risk assessment. The Panel concluded that the combined margin of exposure (MOET) approach was the most appropriate risk metric and applied a tiered approach to the risk characterisation. Over 84,000 analytical results for the 10 congeners in food were used to estimate the exposure across dietary surveys and age groups of the European population. The most important contributors to the chronic dietary Lower Bound exposure to PBDEs were meat and meat products and fish and seafood. Taking into account the uncertainties affecting the assessment, the Panel concluded that it is likely that current dietary exposure to PBDEs in the European population raises a health concern.

Prediction of Thermogravimetric Data in the Thermal Recycling of e-waste Using Machine Learning Techniques: A Data-driven Approach

The release of bromine-free hydrocarbons and gases is a major challenge faced in the thermal recycling of e-waste due to the corrosive effects of produced HBr. Metal oxides such as Fe2O3 (hematite) are excellent debrominating agents, and they are copyrolyzed along with tetrabromophenol (TBP), a lesser used brominated flame retardant that is a constituent of printed circuit boards in electronic equipment. The pyrolytic (N2) and oxidative (O2) decomposition of TBP with Fe2O3 has been previously investigated with thermogravimetric analysis (TGA) at four different heating rates of 5, 10, 15, and 20 °C/min, and the mass loss data between room temperature and 800 °C were reported. The objective of our paper is to study the effectiveness of machine learning (ML) techniques to reproduce these TGA data so that the use of the instrument can be eliminated to enhance the potential of online monitoring of copyrolysis in e-waste treatment. This will reduce experimental and human errors as well as improve process time significantly. TGA data are both nonlinear and multidimensional, and hence, nonlinear regression techniques such as random forest (RF) and gradient boosting regression (GBR) showed the highest prediction accuracies of 0.999 and lowest prediction errors among all the ML models employed in this work. The large data sets allowed us to explore three different scenarios of model training and validation, where the number of training samples were varied from 10,000 to 40,000 for both TBP and TBP + hematite samples under N2 (pyrolysis) and O2 (combustion) environments. The novelty of our study is that ML techniques have not been employed for the copyrolysis of these compounds, while the significance is the excellent potential of enhanced online monitoring of e-waste treatment and extension to other characterization techniques such as spectroscopy and chromatography. Lastly, e-waste recycling could greatly benefit from ML applications since it has the potential to reduce total and operational costs and improve overall process time and efficiency, thereby encouraging more treatment plants to adopt these techniques, resulting in reducing the increasing environmental footprint of e-waste.

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.

Recycling Hazardous and Valuable Electrolyte in Spent Lithium-Ion Batteries: Urgency, Progress, Challenge, and Viable Approach

Recycling spent lithium-ion batteries (LIBs) is becoming a hot global issue due to the huge amount of scrap, hazardous, and valuable materials associated with end-of-life LIBs. The electrolyte, accounting for 10-15 wt % of spent LIBs, is the most hazardous substance involved in recycling spent LIBs. Meanwhile, the valuable components, especially Li-based salts, make recycling economically beneficial. However, studies of electrolyte recycling still account for only a small fraction of the number of spent LIB recycling papers. On the other hand, many more studies about electrolyte recycling have been published in Chinese but are not well-known worldwide due to the limitations of language. To build a bridge between Chinese and Western academic achievements on electrolyte treatments, this Review first illustrates the urgency and importance of electrolyte recycling and analyzes the reason for its neglect. Then, we introduce the principles and processes of the electrolyte collection methods including mechanical processing, distillation and freezing, solvent extraction, and supercritical carbon dioxide. We also discuss electrolyte separation and regeneration with an emphasis on methods for recovering lithium salts. We discuss the advantages, disadvantages, and challenges of recycling processes. Moreover, we propose five viable approaches for industrialized applications to efficiently recycle electrolytes that combine different processing steps, ranging from mechanical processing with heat distillation to mechanochemistry and in situ catalysis, and to discharging and supercritical carbon dioxide extraction. We conclude with a discussion of future directions for electrolyte recycling. This Review will contribute to electrolyte recycling more efficiently, environmentally friendly, and economically.

Recent advances in flame retardant and mechanical properties of polylactic acid: A review

The large-scale application of ecofriendly polymeric materials has become a key focus of scientific research with the trend toward sustainable development. Mechanical properties and fire safety are two critical considerations of biopolymers for large-scale applications. Polylactic acid (PLA) is a flammable, melt-drop carrying, and strong but brittle polymer. Hence, it is essential to achieve both flame retardancy and mechanical enhancement to improve safety and broaden its application. This study reviews the recent research on the flame retardant functionalization and mechanical reinforcement of PLA. It classifies PLA according to the type of the flame retardant strategy employed, such as surface-modified fibers, modified nano/micro fillers, small-molecule and macromolecular flame retardants, flame retardants with fibers or polymers, and chain extension or crosslinking with other flame retardants. The functionalization strategies and main parameters of the modified PLA systems are summarized and analyzed. This study summarizes the latest advances in the fields of flame retardancy and mechanical reinforcement of PLA.

Degradation of tetrabromobisphenol A (TBBA) with calcium hydroxide: a thermo-kinetic analysis

Thermal treatment of bromine-contaminated polymers (i.e., as in e-waste) with metal oxides is currently deployed as a mainstream strategy in recycling and resources recovery from these objects. The underlying aim is to capture the bromine content and to produce pure bromine-free hydrocarbons. Bromine originates from the added brominated flame retardants (BFRs) to the polymeric fractions in printed circuits boards, where tetrabromobisphenol A (TBBA) is the most utilized BFR. Among notable deployed metal oxides is calcium hydroxide, i.e., Ca(OH)₂ that often displays high debromination capacity. Comprehending thermo-kinetic parameters that account for the BFRs:Ca(OH)₂ interaction is instrumental to optimize the operation at an industrial scale. Herein, we report comprehensive kinetics and thermodynamics studies into the pyrolytic and oxidative decomposition of a TBBA:Ca(OH)₂ mixture at four different heating rates, 5, 10, 15, and 20 °C min^-1, carried out using a thermogravimetric analyser. Fourier Transform Infrared Spectroscopy (FTIR) and a carbon, hydrogen, nitrogen, and sulphur (CHNS) elemental analyser established the vibrations of the molecules and carbon content of the sample. From the thermogravimetric analyser (TGA) data, the kinetic and thermodynamic parameters were evaluated using iso-conversional methods (KAS, FWO, and Starink), which were further validated by the Coats-Redfern method. The computed activation energies for the pyrolytic decomposition of pure TBBA and its mixture with Ca(OH)₂ reside in the narrow ranges of 111.7-112.1 kJ mol^-1 and 62.8-63.4 kJ mol^-1, respectively (considering the various models). Obtained negative ΔS values suggest the formation of stable products. The synergic effects of the blend exhibited positive values in the low-temperature ranges (200-300 °C) due to the emission of HBr from TBBA and the solid-liquid bromination process occurring between TBBA and Ca(OH)₂. From a practical point of view, data provided herein are useful in efforts that aim to fine-tune operational conditions encountered in real recycling scenarios, i.e., in co-pyrolysis of e-waste with Ca(OH)₂ in rotary kilns.

On the formation chemistry of brominated polycyclic aromatic hydrocarbons (BrPAHs)

Brominated polycyclic aromatic hydrocarbons (BrPAHs) have been consistently detected in various environmental matrices, and measured at alarming rates in stack emissions. However, formation mechanisms and bromination patterns of BrPAHs remain unclear. This contribution constructs detailed mechanistic pathways for the synthesis of selected BrPAHs (namely bromine-bearing naphthalene, acenaphthylene, anthracene, and phenanthrene). Mapped-out pathways follow the Bittner-Howard's route in the hydrogen abstraction acetylene addition (HACA) mechanism, in which a second C₂HBr molecule is added to the first one. Constructed kinetic model portrays temperature-dependent profiles of major and minor species. Direct loss of an H atom from the acetylenic fragment appears to be more important at elevated temperatures, when compared with further addition of C₂HBr cuts or ring-cyclization reactions. The occurrence of closed-shell Diels-Alder pathway should be inhibited owing to sizable enthalpic barriers. Fukui Indices for electrophilic substitutions (f^-1) establish bromination' s pattern of selected BrPAHs. The diradical character of BrPAHs coupled with electron-deficient C(Br) sites, render BrPAHs as potent precursors for the formation of environmentally persistent free radicals (EPFRs). Findings reported herein shall be useful in comprehending the formation chemistry of BrPAHs, a less-investigated category of toxicants in thermal systems.

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.