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Results from this study provide an atomic-based insight into a promising thermal recycling route of e-waste. Page 2 of 33 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 IntroductionCo-pyrolysis of metal oxides with halogen-bearing materials attracts technological interests on two compelling grounds: (i) recycling of bromine and chlorine-containing objects and (ii)pyro-metallurgical extraction of metals from their oxides. Thermal treatment of the ever increasing electronic and electric waste (e-waste) constitutes a real-world case where the recycling of bromine and extraction of metals overlap. 1 The non-metallic fraction in e-waste bears a significant load of halogenated hydrocarbons, mainly in the form of brominated flame retardants (BFRs). [1][2][3][4][5] On the other hand, exposing the metallic constituents in e-waste to oxygen at elevated temperature transforms them readily into metal oxides. 3 The interest in studying the co-pyrolysis of BFRs with metal oxides stems from their ability to act as bromine fixation agents, 6 the process that ultimately leads to reductive debromination of BFRs. Of particular industrial as well as health importance are ferric oxides that make up most of the ferric fraction in electric arc furnace dust (EAFD). It is estimated that, 4.3 -5.7 million tonnes of EAFD arise annually worldwide during crude steel production. 7 Thermal degradation of BFRs in the presence of metal oxides achieves a dual-benefit, reducing the overall toxicity of the decomposition products of BFRs and forming metal bromides that could be easily leached out.The potential for the conversion of BFRs into hazardous brominated compounds (most notably the notorious polybrominated dibenzo-p-dioxins and furans, PBDD/Fs) often overshadows the environmental and economic benefits of thermal recycling of e-waste. 8, 9The co-existence of aromatic brominated precursors with metal oxides typically act as a perfect recipe for the catalytic synthesis of PBDD/Fs through prominent intermediate steps. 10While HCl represents an inactive chlorinating agent for generation of PCDD/Fs, 11 conversionPage 3 of 33 ACS Paragon Plus EnvironmentThe Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 We estimate reaction rate constants based on the conventional transition state theory (TST).In the TST calculations, vibrational frequencies yield activation enthalpies and entropies at the temperature of interest (300 -1000 K). A-factors for barrierless reactions are estimated based on the difference in entropies between reactants and products. We report electronic charges for the α-Fe 2 O 3 cl...

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Hydrogen halides (HCl/HBr) represent major halogen fragments from the thermal decomposition of halogen laden materials, most notably PVC and brominated flame retardants (BFRs). Co-pyrolysis of halogen-containing solid waste with metal oxides is currently deployed as a mainstream strategy to treat halogen content as well as to recycle the valuable metallic fraction embedded in electric arc furnace dust (EAFD) and e-waste. However, designing an industrial-scale recycling facility necessitates accurate knowledge on mechanistic and thermo-kinetic parameters dictating the interaction between metal oxides and hydrogen halides. In this contribution, we investigate chemical interplay between HCl/HBr and zincite surfaces as a representative model for structures of zinc oxides in EAFD by using different sets of functionals, unit cell size and energy cut-off. In the first elementary step, dissociative adsorption of the HCl/HBr molecules affords oxyhalide structures (Cl/Br-Zn, H-O) via modest activation barriers. Conversion of the oxyhalide structure into zinc halides occurs through two subsequent steps, further dissociative adsorption of HCl/Br over the same surface Zn atom as well as the release of a HO molecule. Evaporation (or desorption of zinc halide molecules) signifies a bottleneck for the overall halogenation of ZnO. Our simplified kinetic model on the HCl + ZnO system concurs very well with experimentally reported TGA weight loss profiles on two grounds: accumulation of oxyhalides until ∼700 K and desorption of ZnCl at higher temperatures. The thermo-kinetic and mechanistic aspects reported herein could be useful in the pursuit of a design of a large-scale catalytic upgrading unit that operates to extract valuable zinc loads from EAFD.

The principal objective in the treatment of e-waste is to capture the bromine released from the brominated flame retardants (BFRs) added to the polymeric constituents of printed circuits boards (PCBs) and to produce pure bromine-free hydrocarbons. Metal oxides such as calcium hydroxide (Ca­(OH)2) have been shown to exhibit high debromination capacity when added to BFRs in e-waste and capturing the released HBr. Tetrabromobisphenol A (TBBA) is the most commonly utilized model compound as a representative for BFRs. Our coauthors had previously studied the pyrolytic and oxidative decomposition of the TBBA:Ca­(OH)2 mixture at four different heating rates, 5, 10, 15, and 20 °C/min, using a thermogravimetric (TGA) analyzer and reported the mass loss data between room temperature and 800 °C. However, in the current work, we applied different machine learning (ML) and chemometric techniques involving regression models to predict the TGA data at different heating rates. The motivation of this work was to reproduce the TGA data with high accuracy in order to eliminate the physical need of the instrument itself, so that this could save significant experimental time involving sample preparation and subsequently minimizing human errors. The novelty of our work lies in the application of ML techniques to predict the TGA data from e-waste pyrolysis since this has not been conducted previously. The significance of our work lies in the fact that e-waste is ever increasing, and predicting the mass loss curves faster will enable better compositional analysis of the e-waste samples in the industry. Three ML models were employed in our work, namely Linear, random forest (RF), and support vector regression (SVR), out of which the RF method exhibited the highest coefficient of determination (R 2) of 0.999 and least error of prediction as estimated by the root mean squared error (RMSEP) at all 4 heating rates for both pyrolysis and oxidation conditions. An 80:20 split was used for calibration and validation data sets. Furthermore, for showing versatility and robustness of the best-predicting RF model, it was also trained using all the data points in the lower heating rates of 5 and 10 °C/min and predicted on all the data points for the higher heating rates of 15 and 20 °C/min to again obtain a high R 2 of 0.999. The excellent performance of the RF model showed that ML techniques can be used to eliminate the physical use of TGA equipment, thus saving experimental time and potential human errors, and can further be applied in other real-time e-waste recycling scenarios.

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Polyvinyl Chloride (PVC) plastics constitutes a large fraction of buildings, packaging and electronic devices, whereas, the annual emission electric arc furnace dust (EAFD) from steel manufacturing operations has recently peaked at nearly 6 Mt. Co-pyrolysis of PVC with EAFD currently represents a focal abatement technology for both categories of pollutants. However, despite of several experimental investigations; the mechanisms underlying interaction between EAFD and PVC remain largely speculative. Herein, we examine theoretically reactions of major products from thermal degradation of PVC with nanoclusters of iron (III) oxide, α-Fe 2 O 3 (hematite) as a representative model for the various metal oxides in EAFD. The facile nature for the H-Cl bond fission over hematite is in line with experimental findings, pointing out to formation of iron chlorides from pyrolysis of Fe 2 O 3-PVC mixtures. Interaction of selected chlorinated C 1-C 3 cuts with the hematite structure preferentially proceeds via a dissociative adsorption pathway. Results from this study shall be instrumental to understand, on a precise molecular basis, fixation of halogens on transitional metal oxides; a viabjjle thermal recycling approach for polymeric materials laden with halogenated constituents. Highlights • Reactions of HCl and chlorinated VOCs with iron (III) oxide have been analysed. • Successive dissociation of HCl on F-O bonds converts Fe 2 O 3 into iron chloride. • Decomposition of chlorinated VOCs mainly occurs by dissociative addition. • Results herein demonstrate the chlorine fixation ability of iron oxides.

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