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Lab-scale thermal analysis of electronic waste plastics
Accepted Manuscript Title: Lab-scale thermal analysis of electronic waste plastics Author: Wu-Jun Liu Ke Tian Hong Jiang Han-Qing Yu PII: DOI: Reference:
S0304-3894(16)30171-6 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.02.044 HAZMAT 17477
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
23-11-2015 20-1-2016 20-2-2016
Please cite this article as: Wu-Jun Liu, Ke Tian, Hong Jiang, Han-Qing Yu, Labscale thermal analysis of electronic waste plastics, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.02.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Lab-scale thermal analysis of Electronic Waste Plastics
Wu-Jun Liu, Ke Tian, Hong Jiang*, Han-Qing Yu
CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
Corresponding author: Dr. Hong Jiang
E-mail: [email protected]
1
GRAPHICAL ABSTRACT Br
Br
Br
Br Br
PBDD/Fs
Br Br
Br
Br
Br
Pyrolysis
Br
Br
HO
OH Br
PBDD/Fs
Br
Highlights
We provided the experimental evidence that WEEE can be recovered by pyrolysis method We explored the thermochemical behaviors of WEEE using online TG-FTIR-MS technology The intramolecular oxygen atoms play a pivotal role in the formation of PBDD/Fs
Abstract In this work, we experimentally revealed the thermochemical decomposition pathway of Decabromodiphenyl ethane (DBDPE) and tetrabromobisphenol A (TBBPA) containing electronic waste plastics using an online thermogravimetric–fourier transform infrared–mass spectroscopy (TG-FTIR-MS) system, a high resolution gas chromatography/high resolution mass (HRGC-MS) spectroscopy, and a fixed-bed reactor. We found the distribution and species of produced bromides can be easily controlled by adjusting pyrolytic temperature, which is particularly crucial to their recycle. From the analysis of the liquid and solid phase obtained from the fixed-bed reactor, we proposed that the •Br radicals formed during the pyrolysis process may be captured by organic species derived from the depolymerization of plastics to form 2
brominated compounds or by the inorganic species in the plastics, and that these species remained in the char residue after pyrolysis. Our work for the first time demonstrates intramolecular oxygen atoms play a pivotal role in the formation of PBDD/Fs that pyrolysis of oxygen-free BFRs is PBDD/Fs-free, whereas pyrolysis of oxygen-containing BFRs is PBDD/Fs-reduced.
Keywords: Electronic waste plastics; TG–FTIR–GC/MS; PBDD/Fs; BFRs; Pyrolysis
1. Introduction Polybrominated dibenzo-p-dioxins and furans (PBDD/Fs) have similar toxic properties to their chlorinated relatives, and may cause cancer, multifocal hemorrhages, yolk sac edema, pericardial edema, craniofacial malformations, and reduced growth.[1-4] Furthermore, because of their persistence, bioaccumulation, and long distance transport in environment, PBDD/Fs would pose great risk to global ecosystem even long after the prohibition of the usage of their precursors.[5] The incineration of waste electrical and electronic equipment (WEEE) containing brominated flame retardants (BFRs) is one of the main contributions to the formation of PBDD/Fs.[6] WEEE is one of the fastest growing waste streams and 80% of electronic plastics contained the BFRs, such as the tetrabromobisphenol A (TBBPA) and ploybromodiphenyl ethers (PBDEs).[7] For instance, the global production of WEEE increases by about 40 million tons every year, according to a report from the United Nations Environment Program in 2009,[8] two times faster than the increase in 3
other types of municipal waste.[9] Decabromodiphenyl ethane (DBDPE) is first introduced by the Albemarle Corporation (Richmond, USA) in the early 1990s as an alternative to the commercial decabromodiphenyl ether, which accounts for 83% of the brominated flame retardants used in the WEEE. It was estimated that the production of commercial DBDPE in China was 12,000 tons in 2006 and to increase with a rate of 85% every year.[10] Regardless of putting in landfills, or subjecting to combustion or machine processing, PBDD/Fs can be inevitably formed from the decomposition of BFRs.
[11]
Therefore, environmentally benign recycling BFRs
contained WEEE would drastically decrease the emission and accumulation of global PBDD/Fs. Pyrolysis, the thermal decomposition of organic materials in the absence of oxygen at a mediate temperature,[12, 13] is considered as an environmentally friendly and widely used method for producing renewable energy from biomass.[12-17] Pyrolysis technology have many advantages: (i) the energy consumption of the pyrolysis process is very low (only ~10% of the energy content of the WEEE is consumed in the pyrolysis process);[18] (ii) in the pyrolysis process, the polymer structures of the plastics can be thermally decomposed and converted into gas and oil that can be used as fuels and chemical feedstocks.[14,
19-22]
Recent studies have
demonstrated that pyrolysis is feasible for recycling WEEE plastics, and useful information about the yields of oil, gas, and char under different pyrolytical conditions is available.[15, 22-28] Theoretically, since the pyrolysis of BFRs is performed in the absence of oxygen, 4
the formation of PBDD/Fs can be greatly suppressed. For instance, based on the theoretical quantum chemical calculations, Altarawneh et al.[29,
30]
proposed the
possible thermal decomposition ways of several oxygen contained BFRs (e.g., 1,2-Bis(2,4,6-tribromophenoxy)ethane and polybrominated biphenyl ethers) and the mechanisms for the formation of PBDD/Fs. It has been proposed that the reactive radical species present in the pyrolytic environment are the main contributions to the formation of PBDD/Fs. The degree and pattern of bromination in the vicinity of the ether oxygen bridge, have a minor influence on governing mechanisms, and that even the fully brominated isomers of oxygen contained BFRs are capable of forming PBDD/Fs. However, quantum chemical calculations are usually conducted in the ideal conditions, which cannot fully reflect the mechanism of PBDD/Fs formation in the practical conditions. Therefore, an experimental elucidation to fully illustrate the mechanism based on the elaborate combination of advanced analytic methods is necessary.
Unfortunately,
since
the
fast
and
complicated
thermochemical
decomposition reactions happen in pyrolysis, the intermediates are multitudinous and constantly changed, and the pathways of BFRs decomposition are difficult to track. Whether or not, or to what extent, formation of PBDD/Fs in the pyrolysis of BFRs-containing WEEE is still unknown. Herein, we experimentally investigated the decomposition pathways of oxygen-containing and oxygen-free BFRs (TBBPA and DBDPE) and the formation mechanism of PBDD/Fs during pyrolysis of WEEE by means of an online thermogravimetric–fourier
transform
infrared–mass 5
spectroscopy
system
(TG–FTIR–MS),
high
resolution
gas
chromatography/high
resolution
mass
spectroscopy (HRGC/HRMS), and a fixed-bed reactor. Based on the understanding of thermochemical decomposition mechanism of BFRs contained WEEE, we propose a PBDD/Fs-free and PBDD/Fs-reduced recycling approach for oxygen-free and oxygen-containing BFRs, respectively. Our results will be useful to recover most of the WEEE in an environmentally friendly way and curb the increasing trend of PBDD/Fs accumulation in global ecosystem.
2. Experimental Section 2.1 Materials The electronic waste plastics, with main component of high-impact polystyrene (HIPS), were collected from a local WEEE recycling plant. Prior to use, the plastic was crushed using a high-speed rotary cutting mill and sieved to collect the particles smaller than 0.12 mm for further use. The BFRs incorporated into the HIPS are DBDPE and TBBPA, new nondiphenyl-ether based BFRs alternative to conventional PBDEs.[31] The main elemental composition (C, H, and O) of the plastics was analyzed through an elemental analyzer (VARIO EL III, Elementar Inc., Germany) and the Br content was determined using inductively coupled plasma-sector field-mass spectrometry (ICP-MS, Plasma Quad 3, Thermo-VG Elemental., UK).[32] The general characteristics of the plastics and the BFRs (DBDPE and TBBPA) are in Tables S1 and S2 and in Fig. S1 of the Supporting Information (SI).
6
2.2 TG–FTIR–MS Analysis of the Pyrolysis Products In TG–FTIR–MS technology, TG measurement can show the decomposition behavior and kinetics of the plastics, while the FTIR and MS data can provide detailed information about the concentration and structures of the evolved degradation products, which can help us understand the mechanism by which BFRs degrade. The pyrolysis of the BFRs contained plastics was performed in a TG analyzer. Approximately 5 mg of the plastics was heated from room temperature to 800 °C in the TG analyzer at heating rates of 200 K/min under 100 mL/min of helium flow. The evolved compounds during the pyrolysis process were analyzed using coupled FTIR and MS. The coupling systems (TL-9000) between the TG–FTIR–MS were also heated to prevent condensation of evolved compounds. The FTIR scanning range was from 4000 to 400 cm−1, and was conducted every 2.5 seconds. The MS scanning was operated at 70 eV. The specific charge values were set as 0<(m/z)<200 which were considered to be the representatives of evolved compounds identified in the pyrolysis of plastics containing BFRs. To comprehensively analyze the evolved compounds with an m/z higher than 200, an online gas chromatography (GC)–MS method was applied. Briefly, the evolved compounds at certain temperatures (~350 and ~450 °C, the peak values of the DTG results) were swept by high-purity helium through a heated transporting tube to a downstream online GC–MS. The MS was run at 70 eV and the m/z scanning range was from 0 to 500 amu. All mass spectra were compared to the NIST mass spectrum library to identify the possible structure of the evolved compounds. 7
2.3 Post-Pyrolysis Analysis After pyrolysis, the obtained pyrolysis char was imaged by scanning electron microscope (SEM, Sirion 200, FEI electron optics company, USA) and the results are presented in Fig. S2. The X-ray photoelectron spectroscopy (XPS) spectra of the pyrolysis char were obtained through an X-ray photoelectron spectrometer (ESCALAB250, Thermo-VG Scientific Inc., UK) with monochromatized Al Kα radiation (1486.92 eV). The detailed process for the determination of PBDD/Fs formed in the pyrolysis of BFRs contained electronic plastics: First, the pyrolysis vapor formed in the pyrolysis process was completely absorbed by the organic solvent n-hexane. The PBBDD/Fs congeners in the absorption liquid were identified and quantified using isotopic dilution high–resolution gas chromatography combined with high–resolution mass spectrometry (HRGC/HRMS).[33] In Brief, the stack samples were spiked with a
13
C12–labeled PBDD/F internal standard mixture (EDF–5408,
Cambridge Isotope Laboratories, USA) and extracted with 250 mL of toluene for 24 h. The spiked extract was then concentrated with a rotary evaporator and subjected to a series of clean–up process, including a multilayer silica gel column and a basic alumina column. Once the clean–up steps completed, an active carbon–impregnated silica column was employed to separate PBDD/Fs from other organic pollutants and the PBDD/Fs contained fraction was concentrated to about 20 µL through a rotary evaporator under a mild N2 stream. Finally, another
13
C12–labeled PBDD/F standard
(EDF–5409, Cambridge Isotope Laboratories) was added to the sample as a reference 8
to calculate the recoveries of the internal standards. The PBDD/Fs congeners were quantified by a trace Ultra gas chromatograph coupled to a DFS mass spectrometer (Thermo, USA) with an electron impact ion source. The high–resolution MS was operated in selected ion monitoring mode with a resolution of about 10 000. A DB–5 MS capillary column (15 m×0.25 mm×0.1 μm, Agilent, USA) was used for separating the PBDD/F congeners. The electron emission energy was set to 45 eV, and the source temperature was 280 °C.
3. Results and Discussion 3.1 Pyrolysis process analysis TG–FTIR–MS offers a simultaneous and online method of measuring multiple evolved degradation products in complex mixtures,[34,
35]
making it the optimal
technology for investigating the fate of the BFRs during the pyrolysis process. The pyrolysis of the WEEE containing BFRs at different heating rates was first analyzed by TG and differential thermogravimetry (DTG). Figure 1a shows the TG and DTG profiles of DBDPE containing plastics at heating rate of 200 K/min, from which we can see that the pyrolysis process can be subdivided into two main stages. The first stage which took placed at 280–380 °C with a weight loss of ~23% (Fig. 1a) is attributed to the decomposition of the DBDPE, while the second stage that happened at 380–550 °C with a big weight loss of ~71% is corresponding to the degradation of the main composition of the plastics (HIPS) into volatile products. Two peaks is observed in the DTG curve with a heating rate of 200 °C/min: The first peak that 9
occurred at 374 oC may correspond to the volatilization and decomposition of DBDPE, while the second one which happened at 486 °C can be attributed to the decomposition of main composition (HIPS) of the electronic plastics. For comparison, Fig. S3 provides the TG and DTG curves of plastics without BFRs, which shows only one pyrolysis stage happening at 380–550 °C, and one similar peak value in the DTG curve, in agreement with the aforementioned analysis. Figure 1b presents the TG and DTG profiles of TBBPA contained plastics at heating rate of 200 K/min. Differently, it shows there is only one stage in the pyrolysis process, and only one peak was found in the DTG curve at 497 oC, with a total weight loss of 97.3%, suggesting that the TBBPA has a similar decomposition behavior with the plastics, but very different from the DBDPE (see Fig. S4). 3.2 Analysis of evolved compounds using TG–FTIR The small molecular compounds were identified using TG–FTIR. We used FTIR to monitor three evolved gas compounds (HBr, CH3Br, and CH4). The emission profiles of HBr, CH3Br, and CH4, each of which has only one characteristic infrared (IR) absorbance, at 2450, 954, and 3028 cm–1, respectively,[36-38] are presented in Figs. 2a, b, and c. These three compounds are mainly formed at a temperature range from 300–650 °C, which corresponds to the main weight loss stage of the TG curves. Compared to the TBBPA contained plastics which has only one peak in the FTIR absorbance curve, there are two peaks in the curve of the DBDPE contained electronic plastics, with much higher absorbance values, indicated that the releases of HBr, CH3Br, and CH4 from the DBDPE containing electronic plastics are much higher than 10
those from the TBBPA contained electronic plastics. The FTIR absorbance curves of these three compounds are similar with their DTG curves, with the peak temperatures in the FTIR absorbance curves are about 15–35 °C higher than those in the DTG curves. One possible explanation for this phenomenon is that these three compounds are not formed directly in the decomposition of the plastics, but in the further reaction of the initial degradation species. Figures 2d and e shows the 3D FTIR spectra for the total pyrolysis emission at 200 K/min. The temperature dependence of FTIR absorbance (Abs) agrees well with that of DTG curve, and the Abs of the pyrolysis emission reaches its maximum value at 490 °C, which is just the temperature of the maximum weight loss rate in the pyrolysis process (see Figs. 1a and b). The band positions for the function groups identified are listed in Table S3. The presence of the brominated hydrocarbons was confirmed by the band position at 931, 954, and 1054 cm-1 (attributed to methylene, methylic, and aromatic C–Br stretching, respectively).[39, 40] The band positions at 2800–3100 cm–1 were attributed to the C–H and C–C (C=C) stretching, respectively, while the band positions at 700–1200 cm–1 were attributed to the C–H and C–C (C=C) bending.[41] The band positions at higher than 3300 cm–1 and in the range of 1200–1600 cm–1 were attributed to the stretching of O-H and C–O (C=O), respectively, which were found in the 3D spectra of the TBBPA contained plastics, but not in those of DBDPE contained plastics, suggesting that only hydrocarbons and brominated hydrocarbons were formed during the pyrolysis of DBDPE contained plastics. O–H and C–O stretching, the typical IR absorbance of the PBDD/Fs, are 11
observed in the 3D FTIR spectra of pyrolysis of TBBPA contained plastics, suggesting PBDD/Fs may form during their pyrolysis. 3.3 Analysis of evolved compounds using TG–MS Although TG–FTIR provided a lot of useful information about the evolution of compounds during the pyrolysis of plastics containing BFRs, the TG–FTIR technique has drawbacks. For example, the TG–FTIR technique cannot be used to identify the exact molecular structures of evolved compounds. Moreover, the strong IR signal of CO2 may obscure the IR signals of other compounds. To comprehensively analyze evolved compounds, we performed further investigation using coupled TG–MS. Six representative compounds—three most abundant aromatic hydrocarbons in the pyrolysis process (see the GC-MS spectra of the evolved compounds during the pyrolysis process, Fig. S5) and three typical brominated hydrocarbons—were monitored as the temperature increased (Figs. 3a-f). The release rates of the three aromatic hydrocarbons (Figs. 3a–c) during the pyrolysis of DBDPE contained plastics were slightly higher than those of the TBBPA contained plastics, which supports the FTIR results. The release profiles of the aromatic hydrocarbons during the pyrolysis of DBDPE contained plastics had a peak at 510 °C, while the peak of the release profiles for the TBBPA contained plastics is in 580 °C. Among all the aromatic compounds, the benzene and styrene—the directly depolymerized products of the HIPS plastics[42]—showed the highest abundance, with MS intensities of about 17×106 and 31×106, respectively. It is well-established that benzene and styrene, as well as other aromatic hydrocarbons 12
like toluene, are important feedstock for the production of many fine chemicals (such as pharmaceuticals, pesticides, and dyes),[43-45] which demonstrates that pyrolysis is a feasible method of recycling plastics. Brominated hydrocarbons showed very different release trends. As shown in Figs. 3d and 3e, the release trends of bromoethane and bromobenzene during the pyrolysis of DBDPE and TBBPA contained plastics were similar, which have the highest release rates at about 600 °C, suggesting that these two compounds may form in a similar way as aromatic hydrocarbons. In contrast, (bromomethyl)benzene shows the peak release at 600 °C during the pyrolysis of DBDPE contained plastics, while this peak appears at about 700 °C in the pyrolysis of TBBPA contained plastics, suggesting that the formation of (bromomethyl)benzene is in different ways in the pyrolysis of DBDPE and TBBPA contained plastics. Another phenomenon should be mentioned is that the MS abundances of brominated hydrocarbons in the pyrolysis of DBDPE contained plastics are much higher than those in the TBBPA contained plastics, which may be attributed to the higher Br contents in the DBDPE. The heating rate is a important factor affecting the pyrolysis behavior of the BFRs containing electronic plastics, take the DBDPE contained plastics as an example, the influence of heating rate on the compounds evolved during pyrolysis process was investigated. Figs. S6-S8 show the pyrolysis process and compounds evolved during the pyrolysis of DBDPE contained plastics at different heating rates (10, 50, and 200 K/min) and the detailed discussion on these results are presented therein. 13
3.4 Quantificationally Determination of PBDD/Fs Althrough TG–FTIR–MS provides a fast and online analysis of pyrolytic products of BFRs contained WEEE, it is not perfect to detect the ultra low concentrations of PBDD/Fs. To quantitatively determine the PBDD/Fs during pyrolysis of BFRs contained WEEE, the volatile from a pyrolytic reactor was absorbed by n-hexane, and quantified by HRGC/HRMS. Figure 4a shows the emission of 10 typical PBDD/Fs congeners (the chemical structures of these ten congeners are shown in Fig. S9) during the pyrolysis of BFRs contained plastics. It can be seen that during the pyrolysis of DBDPE contained plastics, only two PBDD congeners, 123789-HxBDD and 1234678-HpBDD, were found in the pyrolysis products with extremely low concentrations of 0.8 and 2.3 pg g–1, respectively, suggesting that the formation of dioxin compounds can be well avoided in the pyrolysis of DBDPE contained electronic waste plastics. For the PBDFs, four congeners, 2378-TeBDF, 2468-TeBDF, 12378-PeBDF, and 23478-PeBDF, were found in the pyrolysis products, with low concentrations of 0.02, 0.06, 0.26, and 0.16 ng g–1. The total PBDD/Fs concentrations in the pyrolysis of DBDPE contained electronic waste plastics only 0.50 ng g–1, and for a comparison, the total PBDD/Fs emissions from the incineration of some typical WEEEs are more than 179 ng g–1,[46] suggesting that the external oxygen plays a crucial role in the formation of PBDD/Fs during the thermal decomposition of the BFRs contained plastics. Apart from the external oxygen, the intramolecular oxygen atoms in the BFRs also play an important role in the formation of PBDD/Fs. To confirm this deduction, 14
the emission of typical PBDD/Fs congeners was also determined during the pyrolysis of another typical BFR, TBBPA, which contains 5.9 wt.% of oxygen. As shown in Fig. 4b, all the 10 PBDD/Fs congeners can be found in the pyrolysis products of TBBPA plastics and the total PBDD/Fs concentration in the pyrolysis products is 14 ng g–1, much higher than those in the pyrolysis products of DBDPE contained electronic waste plastics (0.50 ng g–1), suggesting that the inherent oxygen in the TBBPA may significantly increase the emission of PBDD/Fs in the pyrolysis process. However, compare to the PBDD/Fs in the incineration process, this value is still very low, indicating that the pyrolysis process can greatly suppress the formation of PBDD/Fs, which make the pyrolysis be an effective and environmentally friendly approach for the recycle of BFRs contained electronic waste plastics. 3.5 Formation pathways of PBDD/Fs Based on the results and analysis above, the degradation pathway of the BFRs during the pyrolysis process was illustrated (Fig. 5). For the DBDPE, debromination is the primary reaction in the pyrolysis process, producing polybrominated biphenyl compounds with <10 Br atoms (S1) and highly reactive •Br radicals.[47] S1 compounds can be further degraded to form several brominated monoaromatic compounds and more •Br radicals with the further increase of the temperature. As evidenced by the GC–MS spectra, there are two polybrominated biphenyl compounds (compound S1) at 374 °C (the first peak value of the DTG curve, as shown in Fig. S4), while at 486 °C (the second peak value of the DTG curve, as shown in Fig. S4), all 15
the brominated compounds found in the GC–MS spectrum are brominated monoaromatic compounds (compound S2). For the HIPS plastics, during the pyrolysis process, many radicals (e.g., •H, •CH3, •CH2CH3) and monoaromatic compounds (e.g., benzene, toluene, styrene, and ethylbenzene) can be formed by depolymerizing the polystyrene chain. These radicals and compounds can capture the •Br radicals to form many other brominated compounds (e.g., HBr, CH3Br, CH2CH3Br, and brominated monoaromatic compounds). For the formation of PBDD/Fs, as discussed before, the PBDD/Fs can only form in the pyrolysis of TBBPA contained plastics, therefore, we speculate that the intramolecular oxygen atoms in the TBBPA may be the main contribution to the formation of PBDD/Fs. As shown in Fig. 4b, under the pyrolysis conditions, the TBBPA may go through two different ways: First, a debromination process may happen to the TBBPA in the pyrolysis, which is similar to the DBDPE, producing a lot of •Br radicals. Second, unlike the DBDPE, the TBBPA have two phenol hydroxyl groups, which has high reaction activity in the pyrolysis conditions, and can form bromophenoxy radicals. The bromophenoxy radicals, with high reactivity, can react with each other to form the precursor of the PBDD/Fs. This precursor, further reacting with the •Br radicals formed in the debromination process, finally produces the PBDD/Fs with different Br contents. During pyrolysis, Br is mainly distributed in both pyrolysis oil and char. Because the presence of Br in pyrolysis oil makes the oil environmentally unfriendly, the distribution of Br in pyrolysis oil must be effectively decreased.[48] The heating rate 16
greatly influences the pyrolysis behavior of the BFRs, so the Br content in the oil phase can be controlled by adjusting the heating rate during the pyrolysis process. Furthermore, as shown in the XPS spectra, many •Br radicals are captured by inorganic species in the char and retained in the final solid residues (Fig. S10). Two peaks at 71.1 and 72.0 eV were found in the Br 3d spectrum of the plastics before pyrolysis, which can be identified as the Br atoms covalently bonded to sp2 and sp3 carbon atoms, respectively.[49] After pyrolysis at different heating rates, the two peaks shifted significantly to low binding energies (~69 eV and ~68 eV, respectively), which can be attributed to the Br atoms in the form of mineral bromide (Br–) compounds,[50] suggesting inorganic species (e.g., Na, Ca, and Mg, Table S4) can capture •Br radicals in the char residue. Other, inorganic species (e.g., FeOOH or CaCO3 [51, 52]) have also been reported to capture •Br radicals and thus decrease the Br content in the oil.
4. Conclusion In summary, we demonstrate a PBDD/Fs-free pyrolysis approach for recovering ever-increasing WEEEs. The ●Br radicals formed in the pyrolysis process can be captured by organic species derived from the depolymerization of plastics to form brominated compounds or by the inorganic species in the plastics. These bromide species remained in the char residue after pyrolysis and can be easily reclaimed by leaching or extraction. Oxygen atoms were demonstrated to play a pivotal role in the formation of PBDD/Fs. For oxygen-free DPDEB, a PBDD/Fs-free pyrolysis of 17
DBDPE-containing WEEE is affirmative, whereas for the oxygen-containing TBBPA, the PBDD/Fs-free recovery is also prospective by capturing the oxygen-containing radicals in pyrolysis.
Acknowledgements The authors gratefully acknowledge financial support from the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (No.2012ZX07103-001), National 863 Program (2012AA063608-01), National Key Technology R&D Program of the Ministry of Science and Technology (2012BAJ08B00), and China Postdoctoral Science Foundation (2015M580553).
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[42] P. Kannan, J.J. Biernacki, D.P. Visco Jr, W. Lambert, Kinetics of thermal decomposition of expandable polystyrene in different gaseous environments, J. Anal. Appl. Pyrolysis 84 (2009) 139-144. [43] A.M. Birch, S. Groombridge, R. Law, A.G. Leach, C.D. Mee, C. Schramm, rationally designing safer anilines: the challenging case of 4-aminobiphenyls, J. Med. Chem. 55 (2012) 3923-3933. [44] S. Vishnoi, V. Agrawal, V.K. Kasana, Synthesis and structure−activity relationships of substituted cinnamic acids and amide analogues: a new class of herbicides, J. Agri. Food Chem. 57 (2009) 3261-3265. [45] M.R. Yazdanbakhsh, A. Mohammadi, E. Mohajerani, H. Nemati, N.H. Nataj, A. Moheghi,
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25
100
-0.6
60 40
-0.4
20
-0.2
0
0.0 200
400
600
Temperature
(oC)
b -2.0
80 Weight (%)
80
DTG (% min-1)
Weight ( %)
-0.8
-1.5
60 -1.0 40 -0.5
20
0.0
0
800
DTG (% min-1)
-1.0 a
100
0
200 400 600 o Temperature ( C)
800
Fig. 1. (a) TG-DTG curves of the DBDPE contained electronic plastics; (b) TG-DTG curves of the TBBPA contained electronic plastics.
26
0.30 b
HBr
0.25
CH3Br
0.20 Abs
Abs
0.056 a 0.048 0.040 0.032 0.024 0.016 0.008 0.000 0
DPDBE TBBPA
0.15
DPDBE TBBPA
0.10 0.05 0.00
200 400 600 Temperature (C)
0
800
200 400 600 Temperature (C)
800
0.20 c
Abs
0.15
CH4
0.10
DPBDE TBBPA
0.05 0.00 0
200 400 600 Temperature (C)
800
Fig. 2. (a–c) The FTIR response of some evolved compounds during the pyrolysis process at 200 K/min; (d-e) 3D FTIR spectra of the evolved compounds during the pyrolysis process: (d) TBBPA contained plastics, (e) the DBDPE contained plastics. 27
5000000 Intensity (a.u.)
Intensity (a.u.)
18000000 a 15000000 12000000 9000000 6000000 TBBPA DBDPE
3000000
3000000 toluene
2000000
0 0
35000000
200 400 600 Temperature (C)
0
800
c
Intensity (a.u.)
Intensity (a.u.)
25000000 20000000 15000000 10000000
TBBPA DBDPE
5000000 0 0
200 400 600 Temperature (C)
40000 CH3CH2Br 30000 20000 TBBPA DBDPE
10000
0
200 400 600 Temperature (oC)
30000
Br
Intensity (a.u.)
Intensity (a.u.)
800
0
800
60000 e
40000 30000 TBBPA DBDPE
10000
200 400 600 Temperature (C)
50000 d
30000000
20000
TBBPA DPDBE
1000000
0
50000
b
4000000
f Br
25000 20000 15000 10000
TBBPA DBDPE
5000
0
800
0 0
200 400 600 Temperature (C)
0
800
200 400 600 Temperature (C)
800
Fig. 3. The MS response of the evolved compounds during the pyrolysis process.
28
Fig. 4. Emission of the PBDD/Fs during the pyrolysis of BFRs contained plastics.
29
Br(y) Br
Br
HO
b
HO
OH
Br
OH (x)Br
Br
(n)Br
Br
n<4
O
PBDD/Fs
TBBPA HO
O Br
Br
O
Br
Br
Br
Br
OH O Br
O
OH Br Br
Br
Br
OH Br O Br
OH Br
HBr
Fig. 5. Mechanism schematic for the degradation and transformation of DBDPE and TBBPA in the pyrolysis process. 30
S0304-3894(16)30171-6 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.02.044 HAZMAT 17477
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
23-11-2015 20-1-2016 20-2-2016
Please cite this article as: Wu-Jun Liu, Ke Tian, Hong Jiang, Han-Qing Yu, Labscale thermal analysis of electronic waste plastics, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.02.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Lab-scale thermal analysis of Electronic Waste Plastics
Wu-Jun Liu, Ke Tian, Hong Jiang*, Han-Qing Yu
CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China
Corresponding author: Dr. Hong Jiang
E-mail: [email protected]
1
GRAPHICAL ABSTRACT Br
Br
Br
Br Br
PBDD/Fs
Br Br
Br
Br
Br
Pyrolysis
Br
Br
HO
OH Br
PBDD/Fs
Br
Highlights
We provided the experimental evidence that WEEE can be recovered by pyrolysis method We explored the thermochemical behaviors of WEEE using online TG-FTIR-MS technology The intramolecular oxygen atoms play a pivotal role in the formation of PBDD/Fs
Abstract In this work, we experimentally revealed the thermochemical decomposition pathway of Decabromodiphenyl ethane (DBDPE) and tetrabromobisphenol A (TBBPA) containing electronic waste plastics using an online thermogravimetric–fourier transform infrared–mass spectroscopy (TG-FTIR-MS) system, a high resolution gas chromatography/high resolution mass (HRGC-MS) spectroscopy, and a fixed-bed reactor. We found the distribution and species of produced bromides can be easily controlled by adjusting pyrolytic temperature, which is particularly crucial to their recycle. From the analysis of the liquid and solid phase obtained from the fixed-bed reactor, we proposed that the •Br radicals formed during the pyrolysis process may be captured by organic species derived from the depolymerization of plastics to form 2
brominated compounds or by the inorganic species in the plastics, and that these species remained in the char residue after pyrolysis. Our work for the first time demonstrates intramolecular oxygen atoms play a pivotal role in the formation of PBDD/Fs that pyrolysis of oxygen-free BFRs is PBDD/Fs-free, whereas pyrolysis of oxygen-containing BFRs is PBDD/Fs-reduced.
Keywords: Electronic waste plastics; TG–FTIR–GC/MS; PBDD/Fs; BFRs; Pyrolysis
1. Introduction Polybrominated dibenzo-p-dioxins and furans (PBDD/Fs) have similar toxic properties to their chlorinated relatives, and may cause cancer, multifocal hemorrhages, yolk sac edema, pericardial edema, craniofacial malformations, and reduced growth.[1-4] Furthermore, because of their persistence, bioaccumulation, and long distance transport in environment, PBDD/Fs would pose great risk to global ecosystem even long after the prohibition of the usage of their precursors.[5] The incineration of waste electrical and electronic equipment (WEEE) containing brominated flame retardants (BFRs) is one of the main contributions to the formation of PBDD/Fs.[6] WEEE is one of the fastest growing waste streams and 80% of electronic plastics contained the BFRs, such as the tetrabromobisphenol A (TBBPA) and ploybromodiphenyl ethers (PBDEs).[7] For instance, the global production of WEEE increases by about 40 million tons every year, according to a report from the United Nations Environment Program in 2009,[8] two times faster than the increase in 3
other types of municipal waste.[9] Decabromodiphenyl ethane (DBDPE) is first introduced by the Albemarle Corporation (Richmond, USA) in the early 1990s as an alternative to the commercial decabromodiphenyl ether, which accounts for 83% of the brominated flame retardants used in the WEEE. It was estimated that the production of commercial DBDPE in China was 12,000 tons in 2006 and to increase with a rate of 85% every year.[10] Regardless of putting in landfills, or subjecting to combustion or machine processing, PBDD/Fs can be inevitably formed from the decomposition of BFRs.
[11]
Therefore, environmentally benign recycling BFRs
contained WEEE would drastically decrease the emission and accumulation of global PBDD/Fs. Pyrolysis, the thermal decomposition of organic materials in the absence of oxygen at a mediate temperature,[12, 13] is considered as an environmentally friendly and widely used method for producing renewable energy from biomass.[12-17] Pyrolysis technology have many advantages: (i) the energy consumption of the pyrolysis process is very low (only ~10% of the energy content of the WEEE is consumed in the pyrolysis process);[18] (ii) in the pyrolysis process, the polymer structures of the plastics can be thermally decomposed and converted into gas and oil that can be used as fuels and chemical feedstocks.[14,
19-22]
Recent studies have
demonstrated that pyrolysis is feasible for recycling WEEE plastics, and useful information about the yields of oil, gas, and char under different pyrolytical conditions is available.[15, 22-28] Theoretically, since the pyrolysis of BFRs is performed in the absence of oxygen, 4
the formation of PBDD/Fs can be greatly suppressed. For instance, based on the theoretical quantum chemical calculations, Altarawneh et al.[29,
30]
proposed the
possible thermal decomposition ways of several oxygen contained BFRs (e.g., 1,2-Bis(2,4,6-tribromophenoxy)ethane and polybrominated biphenyl ethers) and the mechanisms for the formation of PBDD/Fs. It has been proposed that the reactive radical species present in the pyrolytic environment are the main contributions to the formation of PBDD/Fs. The degree and pattern of bromination in the vicinity of the ether oxygen bridge, have a minor influence on governing mechanisms, and that even the fully brominated isomers of oxygen contained BFRs are capable of forming PBDD/Fs. However, quantum chemical calculations are usually conducted in the ideal conditions, which cannot fully reflect the mechanism of PBDD/Fs formation in the practical conditions. Therefore, an experimental elucidation to fully illustrate the mechanism based on the elaborate combination of advanced analytic methods is necessary.
Unfortunately,
since
the
fast
and
complicated
thermochemical
decomposition reactions happen in pyrolysis, the intermediates are multitudinous and constantly changed, and the pathways of BFRs decomposition are difficult to track. Whether or not, or to what extent, formation of PBDD/Fs in the pyrolysis of BFRs-containing WEEE is still unknown. Herein, we experimentally investigated the decomposition pathways of oxygen-containing and oxygen-free BFRs (TBBPA and DBDPE) and the formation mechanism of PBDD/Fs during pyrolysis of WEEE by means of an online thermogravimetric–fourier
transform
infrared–mass 5
spectroscopy
system
(TG–FTIR–MS),
high
resolution
gas
chromatography/high
resolution
mass
spectroscopy (HRGC/HRMS), and a fixed-bed reactor. Based on the understanding of thermochemical decomposition mechanism of BFRs contained WEEE, we propose a PBDD/Fs-free and PBDD/Fs-reduced recycling approach for oxygen-free and oxygen-containing BFRs, respectively. Our results will be useful to recover most of the WEEE in an environmentally friendly way and curb the increasing trend of PBDD/Fs accumulation in global ecosystem.
2. Experimental Section 2.1 Materials The electronic waste plastics, with main component of high-impact polystyrene (HIPS), were collected from a local WEEE recycling plant. Prior to use, the plastic was crushed using a high-speed rotary cutting mill and sieved to collect the particles smaller than 0.12 mm for further use. The BFRs incorporated into the HIPS are DBDPE and TBBPA, new nondiphenyl-ether based BFRs alternative to conventional PBDEs.[31] The main elemental composition (C, H, and O) of the plastics was analyzed through an elemental analyzer (VARIO EL III, Elementar Inc., Germany) and the Br content was determined using inductively coupled plasma-sector field-mass spectrometry (ICP-MS, Plasma Quad 3, Thermo-VG Elemental., UK).[32] The general characteristics of the plastics and the BFRs (DBDPE and TBBPA) are in Tables S1 and S2 and in Fig. S1 of the Supporting Information (SI).
6
2.2 TG–FTIR–MS Analysis of the Pyrolysis Products In TG–FTIR–MS technology, TG measurement can show the decomposition behavior and kinetics of the plastics, while the FTIR and MS data can provide detailed information about the concentration and structures of the evolved degradation products, which can help us understand the mechanism by which BFRs degrade. The pyrolysis of the BFRs contained plastics was performed in a TG analyzer. Approximately 5 mg of the plastics was heated from room temperature to 800 °C in the TG analyzer at heating rates of 200 K/min under 100 mL/min of helium flow. The evolved compounds during the pyrolysis process were analyzed using coupled FTIR and MS. The coupling systems (TL-9000) between the TG–FTIR–MS were also heated to prevent condensation of evolved compounds. The FTIR scanning range was from 4000 to 400 cm−1, and was conducted every 2.5 seconds. The MS scanning was operated at 70 eV. The specific charge values were set as 0<(m/z)<200 which were considered to be the representatives of evolved compounds identified in the pyrolysis of plastics containing BFRs. To comprehensively analyze the evolved compounds with an m/z higher than 200, an online gas chromatography (GC)–MS method was applied. Briefly, the evolved compounds at certain temperatures (~350 and ~450 °C, the peak values of the DTG results) were swept by high-purity helium through a heated transporting tube to a downstream online GC–MS. The MS was run at 70 eV and the m/z scanning range was from 0 to 500 amu. All mass spectra were compared to the NIST mass spectrum library to identify the possible structure of the evolved compounds. 7
2.3 Post-Pyrolysis Analysis After pyrolysis, the obtained pyrolysis char was imaged by scanning electron microscope (SEM, Sirion 200, FEI electron optics company, USA) and the results are presented in Fig. S2. The X-ray photoelectron spectroscopy (XPS) spectra of the pyrolysis char were obtained through an X-ray photoelectron spectrometer (ESCALAB250, Thermo-VG Scientific Inc., UK) with monochromatized Al Kα radiation (1486.92 eV). The detailed process for the determination of PBDD/Fs formed in the pyrolysis of BFRs contained electronic plastics: First, the pyrolysis vapor formed in the pyrolysis process was completely absorbed by the organic solvent n-hexane. The PBBDD/Fs congeners in the absorption liquid were identified and quantified using isotopic dilution high–resolution gas chromatography combined with high–resolution mass spectrometry (HRGC/HRMS).[33] In Brief, the stack samples were spiked with a
13
C12–labeled PBDD/F internal standard mixture (EDF–5408,
Cambridge Isotope Laboratories, USA) and extracted with 250 mL of toluene for 24 h. The spiked extract was then concentrated with a rotary evaporator and subjected to a series of clean–up process, including a multilayer silica gel column and a basic alumina column. Once the clean–up steps completed, an active carbon–impregnated silica column was employed to separate PBDD/Fs from other organic pollutants and the PBDD/Fs contained fraction was concentrated to about 20 µL through a rotary evaporator under a mild N2 stream. Finally, another
13
C12–labeled PBDD/F standard
(EDF–5409, Cambridge Isotope Laboratories) was added to the sample as a reference 8
to calculate the recoveries of the internal standards. The PBDD/Fs congeners were quantified by a trace Ultra gas chromatograph coupled to a DFS mass spectrometer (Thermo, USA) with an electron impact ion source. The high–resolution MS was operated in selected ion monitoring mode with a resolution of about 10 000. A DB–5 MS capillary column (15 m×0.25 mm×0.1 μm, Agilent, USA) was used for separating the PBDD/F congeners. The electron emission energy was set to 45 eV, and the source temperature was 280 °C.
3. Results and Discussion 3.1 Pyrolysis process analysis TG–FTIR–MS offers a simultaneous and online method of measuring multiple evolved degradation products in complex mixtures,[34,
35]
making it the optimal
technology for investigating the fate of the BFRs during the pyrolysis process. The pyrolysis of the WEEE containing BFRs at different heating rates was first analyzed by TG and differential thermogravimetry (DTG). Figure 1a shows the TG and DTG profiles of DBDPE containing plastics at heating rate of 200 K/min, from which we can see that the pyrolysis process can be subdivided into two main stages. The first stage which took placed at 280–380 °C with a weight loss of ~23% (Fig. 1a) is attributed to the decomposition of the DBDPE, while the second stage that happened at 380–550 °C with a big weight loss of ~71% is corresponding to the degradation of the main composition of the plastics (HIPS) into volatile products. Two peaks is observed in the DTG curve with a heating rate of 200 °C/min: The first peak that 9
occurred at 374 oC may correspond to the volatilization and decomposition of DBDPE, while the second one which happened at 486 °C can be attributed to the decomposition of main composition (HIPS) of the electronic plastics. For comparison, Fig. S3 provides the TG and DTG curves of plastics without BFRs, which shows only one pyrolysis stage happening at 380–550 °C, and one similar peak value in the DTG curve, in agreement with the aforementioned analysis. Figure 1b presents the TG and DTG profiles of TBBPA contained plastics at heating rate of 200 K/min. Differently, it shows there is only one stage in the pyrolysis process, and only one peak was found in the DTG curve at 497 oC, with a total weight loss of 97.3%, suggesting that the TBBPA has a similar decomposition behavior with the plastics, but very different from the DBDPE (see Fig. S4). 3.2 Analysis of evolved compounds using TG–FTIR The small molecular compounds were identified using TG–FTIR. We used FTIR to monitor three evolved gas compounds (HBr, CH3Br, and CH4). The emission profiles of HBr, CH3Br, and CH4, each of which has only one characteristic infrared (IR) absorbance, at 2450, 954, and 3028 cm–1, respectively,[36-38] are presented in Figs. 2a, b, and c. These three compounds are mainly formed at a temperature range from 300–650 °C, which corresponds to the main weight loss stage of the TG curves. Compared to the TBBPA contained plastics which has only one peak in the FTIR absorbance curve, there are two peaks in the curve of the DBDPE contained electronic plastics, with much higher absorbance values, indicated that the releases of HBr, CH3Br, and CH4 from the DBDPE containing electronic plastics are much higher than 10
those from the TBBPA contained electronic plastics. The FTIR absorbance curves of these three compounds are similar with their DTG curves, with the peak temperatures in the FTIR absorbance curves are about 15–35 °C higher than those in the DTG curves. One possible explanation for this phenomenon is that these three compounds are not formed directly in the decomposition of the plastics, but in the further reaction of the initial degradation species. Figures 2d and e shows the 3D FTIR spectra for the total pyrolysis emission at 200 K/min. The temperature dependence of FTIR absorbance (Abs) agrees well with that of DTG curve, and the Abs of the pyrolysis emission reaches its maximum value at 490 °C, which is just the temperature of the maximum weight loss rate in the pyrolysis process (see Figs. 1a and b). The band positions for the function groups identified are listed in Table S3. The presence of the brominated hydrocarbons was confirmed by the band position at 931, 954, and 1054 cm-1 (attributed to methylene, methylic, and aromatic C–Br stretching, respectively).[39, 40] The band positions at 2800–3100 cm–1 were attributed to the C–H and C–C (C=C) stretching, respectively, while the band positions at 700–1200 cm–1 were attributed to the C–H and C–C (C=C) bending.[41] The band positions at higher than 3300 cm–1 and in the range of 1200–1600 cm–1 were attributed to the stretching of O-H and C–O (C=O), respectively, which were found in the 3D spectra of the TBBPA contained plastics, but not in those of DBDPE contained plastics, suggesting that only hydrocarbons and brominated hydrocarbons were formed during the pyrolysis of DBDPE contained plastics. O–H and C–O stretching, the typical IR absorbance of the PBDD/Fs, are 11
observed in the 3D FTIR spectra of pyrolysis of TBBPA contained plastics, suggesting PBDD/Fs may form during their pyrolysis. 3.3 Analysis of evolved compounds using TG–MS Although TG–FTIR provided a lot of useful information about the evolution of compounds during the pyrolysis of plastics containing BFRs, the TG–FTIR technique has drawbacks. For example, the TG–FTIR technique cannot be used to identify the exact molecular structures of evolved compounds. Moreover, the strong IR signal of CO2 may obscure the IR signals of other compounds. To comprehensively analyze evolved compounds, we performed further investigation using coupled TG–MS. Six representative compounds—three most abundant aromatic hydrocarbons in the pyrolysis process (see the GC-MS spectra of the evolved compounds during the pyrolysis process, Fig. S5) and three typical brominated hydrocarbons—were monitored as the temperature increased (Figs. 3a-f). The release rates of the three aromatic hydrocarbons (Figs. 3a–c) during the pyrolysis of DBDPE contained plastics were slightly higher than those of the TBBPA contained plastics, which supports the FTIR results. The release profiles of the aromatic hydrocarbons during the pyrolysis of DBDPE contained plastics had a peak at 510 °C, while the peak of the release profiles for the TBBPA contained plastics is in 580 °C. Among all the aromatic compounds, the benzene and styrene—the directly depolymerized products of the HIPS plastics[42]—showed the highest abundance, with MS intensities of about 17×106 and 31×106, respectively. It is well-established that benzene and styrene, as well as other aromatic hydrocarbons 12
like toluene, are important feedstock for the production of many fine chemicals (such as pharmaceuticals, pesticides, and dyes),[43-45] which demonstrates that pyrolysis is a feasible method of recycling plastics. Brominated hydrocarbons showed very different release trends. As shown in Figs. 3d and 3e, the release trends of bromoethane and bromobenzene during the pyrolysis of DBDPE and TBBPA contained plastics were similar, which have the highest release rates at about 600 °C, suggesting that these two compounds may form in a similar way as aromatic hydrocarbons. In contrast, (bromomethyl)benzene shows the peak release at 600 °C during the pyrolysis of DBDPE contained plastics, while this peak appears at about 700 °C in the pyrolysis of TBBPA contained plastics, suggesting that the formation of (bromomethyl)benzene is in different ways in the pyrolysis of DBDPE and TBBPA contained plastics. Another phenomenon should be mentioned is that the MS abundances of brominated hydrocarbons in the pyrolysis of DBDPE contained plastics are much higher than those in the TBBPA contained plastics, which may be attributed to the higher Br contents in the DBDPE. The heating rate is a important factor affecting the pyrolysis behavior of the BFRs containing electronic plastics, take the DBDPE contained plastics as an example, the influence of heating rate on the compounds evolved during pyrolysis process was investigated. Figs. S6-S8 show the pyrolysis process and compounds evolved during the pyrolysis of DBDPE contained plastics at different heating rates (10, 50, and 200 K/min) and the detailed discussion on these results are presented therein. 13
3.4 Quantificationally Determination of PBDD/Fs Althrough TG–FTIR–MS provides a fast and online analysis of pyrolytic products of BFRs contained WEEE, it is not perfect to detect the ultra low concentrations of PBDD/Fs. To quantitatively determine the PBDD/Fs during pyrolysis of BFRs contained WEEE, the volatile from a pyrolytic reactor was absorbed by n-hexane, and quantified by HRGC/HRMS. Figure 4a shows the emission of 10 typical PBDD/Fs congeners (the chemical structures of these ten congeners are shown in Fig. S9) during the pyrolysis of BFRs contained plastics. It can be seen that during the pyrolysis of DBDPE contained plastics, only two PBDD congeners, 123789-HxBDD and 1234678-HpBDD, were found in the pyrolysis products with extremely low concentrations of 0.8 and 2.3 pg g–1, respectively, suggesting that the formation of dioxin compounds can be well avoided in the pyrolysis of DBDPE contained electronic waste plastics. For the PBDFs, four congeners, 2378-TeBDF, 2468-TeBDF, 12378-PeBDF, and 23478-PeBDF, were found in the pyrolysis products, with low concentrations of 0.02, 0.06, 0.26, and 0.16 ng g–1. The total PBDD/Fs concentrations in the pyrolysis of DBDPE contained electronic waste plastics only 0.50 ng g–1, and for a comparison, the total PBDD/Fs emissions from the incineration of some typical WEEEs are more than 179 ng g–1,[46] suggesting that the external oxygen plays a crucial role in the formation of PBDD/Fs during the thermal decomposition of the BFRs contained plastics. Apart from the external oxygen, the intramolecular oxygen atoms in the BFRs also play an important role in the formation of PBDD/Fs. To confirm this deduction, 14
the emission of typical PBDD/Fs congeners was also determined during the pyrolysis of another typical BFR, TBBPA, which contains 5.9 wt.% of oxygen. As shown in Fig. 4b, all the 10 PBDD/Fs congeners can be found in the pyrolysis products of TBBPA plastics and the total PBDD/Fs concentration in the pyrolysis products is 14 ng g–1, much higher than those in the pyrolysis products of DBDPE contained electronic waste plastics (0.50 ng g–1), suggesting that the inherent oxygen in the TBBPA may significantly increase the emission of PBDD/Fs in the pyrolysis process. However, compare to the PBDD/Fs in the incineration process, this value is still very low, indicating that the pyrolysis process can greatly suppress the formation of PBDD/Fs, which make the pyrolysis be an effective and environmentally friendly approach for the recycle of BFRs contained electronic waste plastics. 3.5 Formation pathways of PBDD/Fs Based on the results and analysis above, the degradation pathway of the BFRs during the pyrolysis process was illustrated (Fig. 5). For the DBDPE, debromination is the primary reaction in the pyrolysis process, producing polybrominated biphenyl compounds with <10 Br atoms (S1) and highly reactive •Br radicals.[47] S1 compounds can be further degraded to form several brominated monoaromatic compounds and more •Br radicals with the further increase of the temperature. As evidenced by the GC–MS spectra, there are two polybrominated biphenyl compounds (compound S1) at 374 °C (the first peak value of the DTG curve, as shown in Fig. S4), while at 486 °C (the second peak value of the DTG curve, as shown in Fig. S4), all 15
the brominated compounds found in the GC–MS spectrum are brominated monoaromatic compounds (compound S2). For the HIPS plastics, during the pyrolysis process, many radicals (e.g., •H, •CH3, •CH2CH3) and monoaromatic compounds (e.g., benzene, toluene, styrene, and ethylbenzene) can be formed by depolymerizing the polystyrene chain. These radicals and compounds can capture the •Br radicals to form many other brominated compounds (e.g., HBr, CH3Br, CH2CH3Br, and brominated monoaromatic compounds). For the formation of PBDD/Fs, as discussed before, the PBDD/Fs can only form in the pyrolysis of TBBPA contained plastics, therefore, we speculate that the intramolecular oxygen atoms in the TBBPA may be the main contribution to the formation of PBDD/Fs. As shown in Fig. 4b, under the pyrolysis conditions, the TBBPA may go through two different ways: First, a debromination process may happen to the TBBPA in the pyrolysis, which is similar to the DBDPE, producing a lot of •Br radicals. Second, unlike the DBDPE, the TBBPA have two phenol hydroxyl groups, which has high reaction activity in the pyrolysis conditions, and can form bromophenoxy radicals. The bromophenoxy radicals, with high reactivity, can react with each other to form the precursor of the PBDD/Fs. This precursor, further reacting with the •Br radicals formed in the debromination process, finally produces the PBDD/Fs with different Br contents. During pyrolysis, Br is mainly distributed in both pyrolysis oil and char. Because the presence of Br in pyrolysis oil makes the oil environmentally unfriendly, the distribution of Br in pyrolysis oil must be effectively decreased.[48] The heating rate 16
greatly influences the pyrolysis behavior of the BFRs, so the Br content in the oil phase can be controlled by adjusting the heating rate during the pyrolysis process. Furthermore, as shown in the XPS spectra, many •Br radicals are captured by inorganic species in the char and retained in the final solid residues (Fig. S10). Two peaks at 71.1 and 72.0 eV were found in the Br 3d spectrum of the plastics before pyrolysis, which can be identified as the Br atoms covalently bonded to sp2 and sp3 carbon atoms, respectively.[49] After pyrolysis at different heating rates, the two peaks shifted significantly to low binding energies (~69 eV and ~68 eV, respectively), which can be attributed to the Br atoms in the form of mineral bromide (Br–) compounds,[50] suggesting inorganic species (e.g., Na, Ca, and Mg, Table S4) can capture •Br radicals in the char residue. Other, inorganic species (e.g., FeOOH or CaCO3 [51, 52]) have also been reported to capture •Br radicals and thus decrease the Br content in the oil.
4. Conclusion In summary, we demonstrate a PBDD/Fs-free pyrolysis approach for recovering ever-increasing WEEEs. The ●Br radicals formed in the pyrolysis process can be captured by organic species derived from the depolymerization of plastics to form brominated compounds or by the inorganic species in the plastics. These bromide species remained in the char residue after pyrolysis and can be easily reclaimed by leaching or extraction. Oxygen atoms were demonstrated to play a pivotal role in the formation of PBDD/Fs. For oxygen-free DPDEB, a PBDD/Fs-free pyrolysis of 17
DBDPE-containing WEEE is affirmative, whereas for the oxygen-containing TBBPA, the PBDD/Fs-free recovery is also prospective by capturing the oxygen-containing radicals in pyrolysis.
Acknowledgements The authors gratefully acknowledge financial support from the Key Special Program on the S&T for the Pollution Control and Treatment of Water Bodies (No.2012ZX07103-001), National 863 Program (2012AA063608-01), National Key Technology R&D Program of the Ministry of Science and Technology (2012BAJ08B00), and China Postdoctoral Science Foundation (2015M580553).
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25
100
-0.6
60 40
-0.4
20
-0.2
0
0.0 200
400
600
Temperature
(oC)
b -2.0
80 Weight (%)
80
DTG (% min-1)
Weight ( %)
-0.8
-1.5
60 -1.0 40 -0.5
20
0.0
0
800
DTG (% min-1)
-1.0 a
100
0
200 400 600 o Temperature ( C)
800
Fig. 1. (a) TG-DTG curves of the DBDPE contained electronic plastics; (b) TG-DTG curves of the TBBPA contained electronic plastics.
26
0.30 b
HBr
0.25
CH3Br
0.20 Abs
Abs
0.056 a 0.048 0.040 0.032 0.024 0.016 0.008 0.000 0
DPDBE TBBPA
0.15
DPDBE TBBPA
0.10 0.05 0.00
200 400 600 Temperature (C)
0
800
200 400 600 Temperature (C)
800
0.20 c
Abs
0.15
CH4
0.10
DPBDE TBBPA
0.05 0.00 0
200 400 600 Temperature (C)
800
Fig. 2. (a–c) The FTIR response of some evolved compounds during the pyrolysis process at 200 K/min; (d-e) 3D FTIR spectra of the evolved compounds during the pyrolysis process: (d) TBBPA contained plastics, (e) the DBDPE contained plastics. 27
5000000 Intensity (a.u.)
Intensity (a.u.)
18000000 a 15000000 12000000 9000000 6000000 TBBPA DBDPE
3000000
3000000 toluene
2000000
0 0
35000000
200 400 600 Temperature (C)
0
800
c
Intensity (a.u.)
Intensity (a.u.)
25000000 20000000 15000000 10000000
TBBPA DBDPE
5000000 0 0
200 400 600 Temperature (C)
40000 CH3CH2Br 30000 20000 TBBPA DBDPE
10000
0
200 400 600 Temperature (oC)
30000
Br
Intensity (a.u.)
Intensity (a.u.)
800
0
800
60000 e
40000 30000 TBBPA DBDPE
10000
200 400 600 Temperature (C)
50000 d
30000000
20000
TBBPA DPDBE
1000000
0
50000
b
4000000
f Br
25000 20000 15000 10000
TBBPA DBDPE
5000
0
800
0 0
200 400 600 Temperature (C)
0
800
200 400 600 Temperature (C)
800
Fig. 3. The MS response of the evolved compounds during the pyrolysis process.
28
Fig. 4. Emission of the PBDD/Fs during the pyrolysis of BFRs contained plastics.
29
Br(y) Br
Br
HO
b
HO
OH
Br
OH (x)Br
Br
(n)Br
Br
n<4
O
PBDD/Fs
TBBPA HO
O Br
Br
O
Br
Br
Br
Br
OH O Br
O
OH Br Br
Br
Br
OH Br O Br
OH Br
HBr
Fig. 5. Mechanism schematic for the degradation and transformation of DBDPE and TBBPA in the pyrolysis process. 30