Brominated flame retardants extraction from waste electrical and electronic equipment-derived ABS using supercritical carbon dioxide
Introduction
Fast technological innovations in the field of electrical and electronic equipment (EEE), their continuous production and their shorter lifespan led to an increasing waste electrical and electronic equipment (WEEE) stream. In 2019, 53.6 million tons of WEEE were generated in the world (Cardamone et al., 2021) and only 20% was properly collected and recycled (Baldé et al., 2017). These WEEE are a significant source of pollution as they contain heavy metals and substances recognized as persistent organic pollutants (POPs) because of their toxicity, transport properties, persistence in the environment and bioaccumulation in humans and wildlife (Stockholm Convention, 2008). To limit the pollution and the consumption of non-renewable resources, the European Directive on WEEE decrees valorization and recycling objectives (European Parliament and Council, Directive N°2012/19/EU, 2012).
Plastic materials represent 18 %wt of the total mass of WEEE (Fangeat et al., 2019). Considering the higher and optimistic targets set by the European Commission, the recovery of polymer materials through re-use and recycling becomes very important. Most polymer materials used in WEEE contain flame retardant additives to comply with the safety flammability standards. Brominated flame retardants (BFR) were the most extensively used and some of them are nowadays restricted. The Stockholm convention and the European Directive on hazardous substances (RoHS) aim to phase out or limit the use of some brominated flame retardant molecules recognized as POP, such as polybrominated biphenyls (PBB) and polybromodiphenylethers (PBDE) in marketed and recycled products (2019/1021/EU, 2019, UNEP, 2017a).
Moreover, commonly used disposal or recycling techniques of plastics such as incineration or mechanical recycling have the disadvantage of releasing toxic substances (e.g. polybrominated dibenzofurans and dibenzodioxins) and corrosive gases if the plastic materials contain BFR (UNEP, 2017b). Consequently, physico-chemical treatment, incineration, and the use as fuel or other means to generate energy are the only allowed operations for the end-of-life of BFR containing plastics. Nevertheless, if the restricted BFR contents are below the concentration limits specified in the directive 2019/1021/EU (e.g. the sum of tetrabromodiphenylether (C12H6Br4O), pentabromodiphenylether (C12H5Br5O), hexabromodiphenylether (C12H4Br6O), heptabromodiphenylether (C12H3Br7O) and decabromodiphenylether concentrations is <1000 mg/kg), the plastic waste is considered as non-hazardous and can, therefore, be recycled mechanically (European Parliament and Council, Directive N°2019/1021/EU, 2019).
Physico-chemical methods such as extraction of hazardous additives from polymer materials before recycling could be a solution to allow BFR containing plastics to be recycled. Using supercritical fluids to separate BFR from polymer matrices has been reported to be a promising green technique as the solubility characteristics of these fluids are similar to those of organic solvents and their diffusion properties are comparable to those of gases (Altwaiq and mnim, Wolf, M., van Eldik, R., 2003, Ügdüler et al., 2020). Indeed, the extraction efficiency depends both on the extractant solubility in the supercritical fluid and on its diffusion across the polymer matrix. Sc-CO2 can also interact with polymers by plasticizing and swelling them which favors the diffusion and thus the extraction. The solvation of molecules is linked to sc-CO2 density which varies with pressure and temperature. CO2 is defined as a non-polar solvent but is compatible with slightly polar molecules thanks to its molecular quadrupole. Another advantage of Sc-CO2 extraction compared to organic solvent extraction is the ability to recover directly a solvent-free extract without a post-treatment. Co-solvents can be added to increase the solvation by changing the fluid polarity and/or to stimulate the diffusion by increasing the polymer swelling (Altwaiq and mnim, Wolf, M., van Eldik, R., 2003, Lou et al., 1995, Ügdüler et al., 2020, Vandenburg et al., 1997). Apart from these parameters, particle size, flow rate or velocity of sc-CO2 in the extractor could also influence the BFR extraction yield (Gamse et al., 2000, Knez et al., 2019).
Bunte et al. (1996) studied the extraction of tetrabromophtalic anhydride (TBBA), tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCDD) from an acrylonitrile–butadienestyrene (ABS) matrix (1.8–2.2 mm particle size) using scCO2. They found out that the optimal parameters of the extraction were 450 bars and 100 °C during 45 min extraction time but did not specify the extraction rate obtained. Then, Gamse et al., (2000) confirmed that the solubilities of pure HBCDD and TBBPA in sCO2 were enhanced when high temperature and pressure (100 °C and 500 bars) were applied. Xia et al. (2021) measured the solubilities of TBBPA, decabromodiphenylether (decaBDE) and HBCDD in sc-CO2, their extraction kinetics and the effect of polymeric matrix such as ABS, polystyrene (PS) and high impact polystyrene (HIPS) on the extraction efficiency.
Since the sc-CO2 extraction of BFR was less efficient at low pressure (100–120 bars), Altwaiq et al. (2003) studied mixtures of sc-CO2 and organic solvents to facilitate solubilization and improve extraction effectiveness at low pressure. The authors demonstrated that the co-solvent played a key role in enhancing the BFR extraction efficiency. For instance, using toluene as a co-solvent to extract 1,2-bis (tribromophenoxy)-ethane (TBPE) from a WEEE ABS matrix increased the bromine extraction efficiency from 42.5% to 76.4%. Moreover, the addition of a mixture of toluene and 1-propanol (1:1) to WEEE-derived HIPS sample containing decaBDE enhanced the extraction rate from 26.3% to 90.8% (Altwaiq et al., 2003).
The objective of the present study is to demonstrate the efficiency of sc-CO2 as a green technique to extract BFR molecules from a real WEEE-derived plastic batch prior to mechanical recycling. This study concentrates on ABS because of its high proportion in WEEE (around 30%)(Maris et al., 2015). Different parameters such as temperature, granulometry, pressure and the addition of a co-solvent were studied in order to understand the impact of each parameter on the extraction efficiency. For the first time in this application, high pressure (up to 700 bars) was applied. Compared to other published studies, our work investigates at the same time 22 flame retardants including PBDE, PBB, HBCDD and TBBPA in a WEEE-derived ABS and focuses on reducing the POP concentration to satisfy the regulation. The influence of the sample particle size and the addition of a co-solvent on the extraction yield were also discussed. While in a previous study, solvents such as toluene, methanol and THF were tested (Altwaiq et al., 2003), in this study, non-toxic co-solvents such as ethanol and diethyl ether were chosen for safety and environmental reasons. This study goes further in the determination of the impact of the sc-CO2 process on the polymer material with a view to assess its recyclability. For each parameter set, the polymer material was characterized to determine both the bromine extraction efficiency and the effect of the extraction process on the polymer matrix.
Section snippets
Materials
Ground polymer samples (1–2 cm) used for this study are derived from WEEE. They are mainly constituted of brominated ABS. Liquid nitrogen provided by Air Liquide (France) was used to prevent the sample from self-heating during grinding.
Toluene used for the extraction of BFR was a chromatographic grade of 100 purity obtained from VWR (Radnor, USA) filtered to 0.2 µm and conditioned under nitrogen atmosphere before use.
BFR reference compounds used for gas chromatography coupled with mass
Characterization of the brominated WEEE samples before sc-CO2 treatment
The composition of the studied batch was first estimated by analyzing 100 samples of 1–2 cm size, collected from the big bag center (sample N°2 in Fig. 1), using FTIR-ATR. 89 samples showed a typical ABS spectrum with specific absorption peaks around 1450–1600 cm−1 (ν(C = C)), 2240 cm−1 (ν(C≡N)) and 3025–3085 cm−1 (aromatic ν(C-H)). As expected, the batch is mainly constituted of ABS. Spectrum similar to that of ABS but without the 2240 cm−1 (ν(C≡N)) band was recorded suggesting the presence of
Conclusion
The sc-CO2 extraction of BFR was performed on WEEE-derived ABS with the objective to satisfy the 2019/1021/EU regulation by removing BFR molecules, recognized as POP, prior to mechanical recycling. The effect of temperature, pressure, granulometry and co-solvent was determined in this study. The maximum extracted bromine was 43.5 ± 0.9% when the extraction was performed at 40 °C, 500 bars during 6 h on <500 µm particle size samples using ethanol as a co-solvent. However, the tested conditions
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors thank Klaudia Bulanda (IFS) very much for her work on the sc-CO2 experiments, Alexandre Benard and Marion Mercent (Le Mans Université) for the SEC analysis, Fabien Boucher (Le Mans Université) for XRF analysis and SGS Multilab Rouen for the GC-MS analysis.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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