Exploring the Potential of Hematite as a Debromination Agent for 2,4,6-Tribromophenol

Debromination of novel brominated flame retardants using Zn-based additives: A viable thermochemical approach in the mitigation of toxic effects during e-waste recycling

Brominated flame retardants (BFRs) are bromine-bearing additives added to the polymeric fraction in various applications to impede fire ignition. The Stockholm Convention and various other legislations abolished legacy BFRs usage and hence, the so-called novel BFRs (NBFRs) were introduced into the market. Recent studies spotlighted their existence in household dust, aquifers and aquatic/aerial species. Co-pyrolysis of BFRs with metal oxides has emerged as a potent chemical recycling approach that produces a bromine-free stream of hydrocarbon. Herein, we investigate the debromination of two prominent two NBFRs; namely tetrabromobisphenol A 2,3-dibromopropyl ether (TD) and tetrabromobisphenol A diallyl ether (TAE) through their co-pyrolysis with zinc oxide (ZnO) and franklinite (ZnFe2O4). Most of the zinc content in electrical arc furnace dust (EAFD) exists in the form of these two metal oxides. Conversion of these metal oxides into their respective bromides could also assist in the selective extraction of the valuable zinc content in EAFD. The debromination potential of both oxides was unveiled via a multitude of characterization studies to analyze products (char, gas and condensates). The thermogravimetric analysis suggested a pyrolytic run up to 500 °C and the TAE treatment with ZnO produced only a trivial amount of brominated compounds (relative area, 0.83%). Phenol was the sole common compound in condensable products; potentially formed by the β-scission debromination reaction from the parental molecular skeleton. Inorganic compounds and methane were the major constituents in the gaseous products. The pyrochar analyses confirmed the presence of metal bromides retained in the residue, averting the bromine release into the atmosphere. The ion chromatography analysis portrayed <8% of HBr gas release into the atmosphere upon pyrolysis with ZnO. The ZnO dominance herein envisaged further probes into other spinel ferrites in combating brominated polymers.

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.

Combating toxic emissions from thermal recycling of polymeric fractions laden with novel brominated flame retardants (NBFRs) in e-waste: an in-situ approach using Ca(OH)2

Legacy brominated flame retardants (BFRs) in printed circuit boards are gradually being replaced by novel BFRs (NBFRs). Safe disposal and recycling of polymeric constituents in the polymeric fractions of e-waste necessitate the removal of their toxic and corrosive bromine content. This is currently acquired through thermal recycling operations involving the pyrolysis of BFRs-containing materials with metal oxides. Nonetheless, the debromination capacity toward NBFRs is yet to be established. Thus, this study aims to address these two crucial gaps in the current knowledge pertaining to the plausible formation of brominated toxicants from the thermal decomposition of NBFRs and their thermal recycling potential. Herein, we investigate the pyrolysis of a mixture of 2,4,6-tribromophenol (TBP), allyl 2,4,6-tribromophenyl ether (ATE) and Tetrabromobisphenol A-bis (2,3-dibromo propyl ether) (TBBPA-DBPE) in the presence of acrylonitrile butadiene styrene (ABS) polymers at various loads. To demonstrate a viable debromination route, pyrolysis of NBFRs-ABS mixture with Ca(OH)2 was also investigated. The latter is a potent debromination agent for legacy BFRs. Upon pyrolysis with Ca(OH)2, the bromine content in the collected oil was reduced up to 80.49% between 25-500 °C. Products of the co-pyrolysis process generally feature non-brominated aromatic and aliphatic compounds; a finding that indicates an effective thermal recycling approach. As evident by IC measurements, no HBr emission could be detected when Ca(OH)2 is added to the mixture. As XRD patterns show, Ca(OH)2 is partially converted into CaBr2. DFT calculations provide pathways for the observed surface debromination characterized by surface-assisted fission of aromatic C-Br bonds and the formation of CaBr sites. Outcomes reported herein are instrumental to designing and operating a thermal recycling facility of polymeric materials contaminated with high loads of bromine, i.e., most notably during scenarios encountered in the thermal recycling of e-waste.

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.

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.