- Email: [email protected]
A novel management strategy for removal and degradation of polybrominated diphenyl ethers (PBDEs) in waste printed circuit boards
Waste Management 100 (2019) 191–198
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
A novel management strategy for removal and degradation of polybrominated diphenyl ethers (PBDEs) in waste printed circuit boards Fu-Rong Xiu a,b,⇑, Xuan Yu a, Yingying Qi a,b, Yifan Li a, Yongwei Lu a, Yixiao Wang a, Jiahuan He a, Ke Zhou a, Zhiqi Song a, Xiang Gao a,b a b
College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi’an 710054, China
a r t i c l e
i n f o
Article history: Received 6 June 2019 Revised 30 August 2019 Accepted 15 September 2019
Keywords: WEEE Waste printed circuit boards PBDEs Degradation Subcritical methanol
a b s t r a c t Waste printed circuit boards (PCBs) contain a high level of brominated flame retardants (BFRs), among which polybrominated biphenyl ethers (PBDEs) are the most widely used additive BFRs. PBDEs are considered to be a type of persistent organic pollutants (POPs). The efficient removal/degradation of PBDEs in waste PCBs is an urgent problem in electronic waste treatment, but the degradation of PBDEs is a great challenge due to their extreme stability and persistence in nature. In this study, a novel management strategy was developed for removal and degradation of PBDEs in waste PCBs by using a simple subcritical methanol (SubCM) process. The results showed that reaction temperature, residence time, solid-to-liquid ratio, and additive NaOH are key factors influencing the removal of PBDEs from waste PCBs. Under optiP mal conditions (200 °C, 60 min, 1:20 g/mL), the removal efficiency of 8PBDEs from waste PCBs could reach 91.3% and 98.8% for the proposed process of SubCM and SubCM + NaOH, respectively. When the temperature is below 200 °C, highly brominated PBDEs congeners in waste PCBs were degraded into 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) and 2,4,40 -Tribromodiphenyl ether (BDE28) after SubCM treatment. 4-Bromophenyl ether (BDE4) and diphenyl ether were generated by the further debromination of BDE71 and BDE28 with the increase of treatment temperature. The debromination temperature of PBDEs congeners in SubCM could be markedly lowered by adding 4 g/L of NaOH. The complete debromination of PBDEs congeners in waste PCBs could be achieved at 300 °C and 250 °C for the developed process of SubCM and SubCM + NaOH, respectively. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The generation of waste electrical and electronic equipment (WEEE) is increasing drastically in the last decade due to the rapid development of information technology and product upgrade (Hadi et al., 2015). As the most important unit of electrical and electronic equipment, the global production amount of waste printed circuit boards (PCBs) is staggering (Xiu et al., 2019). Waste PCBs accounts for over 4% of total WEEE (Lu and Xu, 2016). It is estimated that more than 500,000 tonnes of waste PCBs are needed to dispose each year in China (Zhang et al., 2012). Waste PCBs includes both metallic and non-metallic components. In addition to Cu and precious metals with recycling value, the metallic component also contains a large number of other heavy metals such as Pb, Sn, Zn, and Ni (Zhang et al., 2017). Non-metallic component mainly contains ⇑ Corresponding author at: College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China. E-mail address: [email protected] (F.-R. Xiu). https://doi.org/10.1016/j.wasman.2019.09.022 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
resin, brominated flame retardants (BFRs), and glass fiber (Shen, 2018; Qi et al., 2019). BFRs are combined with resin materials by addition or reaction in PCBs. Typical BFRs are polybrominated diphenyl ethers (PBDEs), polybrominated biphenyls (PBBs), hexabromocyclododecane (HBCD), and tetrabromobisphenol-A (TBBPA). As a kind of additive flame retardants widely used in the manufacture of PCBs, PBDEs are considered to be a type of persistent organic pollutants (POPs) that can bioaccumulate and amplify (Yang et al., 2016; Cowell et al., 2018; Alharbi et al., 2018). The traditional disposal method of electronic wastes containing PBDEs, such as incineration and landfill, can easily produce highly toxic polybrominated dibenzo-dioxins and benzo-furans (PBDD/Fs) or lead to secondary pollution of soil and groundwater by PBDEs (Duan et al., 2012; Zhu et al., 2018). Therefore, it is of great significance to develop safe and effective debromination and detoxification technology to solve the increasingly serious pollution of PBDEs contained in waste PCBs. Degradation of PBDEs is a great challenge due to their extreme stability and persistence in nature (Yang et al., 2016). Many
192
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
processes were proposed for debromination and degradation of PBDEs such as microbial degradation (He et al., 2006; Robrock et al., 2008), electrocatalysis (Liu et al., 2014), photocatalysis (Miller et al., 2017; Gupta et al., 2011; Rajendran et al., 2016), and zerovalent iron reduction (Zhuang et al., 2010; Wang et al., 2018). However, all of the reported methods are only suitable for the treatment of low level of PBDEs in environment, and are difficult to be applied in the actual engineering treatment for the removal and degradation of PBDEs in waste PCBs, which are heterogeneous mix of BFRs, polymer, metals and glass fiber. In recent years, the application of supercritical fluids for detoxification and degradation of toxic organic pollutants has become a very promising and fast-.0growing branch of green chemistry (Zou et al., 2013; Xiu et al., 2018). Supercritical water (SCW) is the most intensively used supercritical fluid for decomposition of toxic wastes (Cao et al., 2017; Qi et al., 2018). However, the condition of high temperature/pressure of SCW greatly limits its practical application (Xiu et al., 2018). Therefore, supercritical methanol (SCM) has attracted increasing attention and has been widely used in green chemical processes (Qi et al., 2019; Erdocia et al., 2016). The critical temperature and pressure of methanol (240 °C, 8.1 MPa) are low when compared to water (374 °C, 22 MPa). The operating conditions of SCM are much milder in comparison with water, weakening the corrosion of the reaction equipment. In addition, the boiling point of methanol is only 64.7 °C, which is beneficial to the separation and recycling of methanol after treatment. In the critical condition, methanol possesses unique physicochemical properties. The ion product of methanol increases greatly with the increase of pressure, and the methanol possesses acid-base catalytic activity (Kusdiana and Saka, 2004). Supercritical methanol molecule also has strong nucleophilic reactivity due to the greater electronegativity of the oxygen in methanol (Narayan et al., 2017). In addition, previous studies have also found that SCM has strong reducibility (Xiu et al., 2017; Sun et al., 2006). These unique properties of SCM encouraged us to test its effectiveness for the decomposition of epoxy resin (the main organic components contained in waste PCBs) in our previous study (Xiu et al., 2010). It was found that the epoxy resin of waste PCBs could be efficiently decomposed in SCM (>300 °C) and the decomposition products mainly include phenol and its derivatives. However, the reaction behavior of PBDEs contained in waste PCBs during the process of critical methanol is still unknown. In this study, we attempt to develop an effective process for the removal and degradation of PBDEs in waste PCBs by using subcritical methanol (SubCM, T 240 °C, P 8.1 MPa) process. In addition, the introduction of alkaline species in SubCM may play a positive role in promoting the debromination of PBDEs by trapping the acidic debromination products, and the debromination temperature of PBDEs in SubCM may be lowered. There is no study investigating the removal and degradation of PBDEs contained in waste PCBs, and the influence of SubCM treatment on the behavior of PBDEs is unclear. In critical conditions, the methanol molecule possesses unique physicochemical
properties such as swelling enhancement, acid-base catalytic activity, and strong nucleophilic reactivity. In the proposed process, SubCM is used as either a reaction medium or a reactant for removal and degradation of PBDEs contained in waste PCBs. Therefore, the purposes of this study are (1) to evaluate the removal, debromination and degradation behavior of PBDEs in waste PCBs in SubCM medium and (2) to examine the boost function of alkaline species such as NaOH for the removal and degradation of PBDEs in waste PCBs. On this basis, a novel and highefficiency strategy for removal and degradation of PBDEs in waste PCBs by using SubCM and SubCM + NaOH process was successfully developed. In the proposed process, PBDEs contained in waste PCBs could be efficient removal and further be completely transformed into nontoxic substances. 2. Materials and methods 2.1. Materials and chemicals Waste PCBs samples were obtained from Shaanxi Xintiandi Solid Waste Material Management Company (China). The samples were dismantled from abandoned instruments and apparatus. The type of the boards is mainly the paper-based copper clad board. The electronic components on waste PCBs were removed manually before the subsequent experiments. Then the samples were cut into small pieces (2 cm 1 cm) by hammer and scissor. The obtained small pieces were sent to comminute by using a pulverizer. The particle size was smaller than 0.5 mm after pulverization. Chemical reagents were obtained from Sinopharm Chemical Reagent Company. 2.2. SubCM experiments of waste PCBs The treatment process of waste PCBs by SubCM is shown in Fig. 1. The SubCM experiments were carried out in a supercritical fluid reactor with a volume of 100 mL. Firstly, 2 g of waste PCBs sample and methanol with a certain volume were added to the reactor, and the solid-liquid ratio (S/L) was controlled at 1:10, 1:15, 1:20, 1:25, and 1:30 g/mL. Then the reactor was heated by turning on the power. The temperature in the reactor increased rapidly to the set value. SubCM experiments were performed with reaction temperature of 100, 150, 200, 250 and 300 °C. The residence time was controlled at 30, 45, 60, 75, and 90 min. Table 1 shows the reaction conditions of each experiment. After the reaction, the power supply of the device was shut down, and the reactor was cooled to room temperature by using an ice-water bath. Then the cooled product was taken out from the reactor, and a membrane filter (pore size = 1.0 mm) was used to separate the mixture products of solid residue and liquid. A rotary evaporator (RE-2000B, YARONG, China) was used to recover the reaction medium methanol from the separated liquid. After the
Fig. 1. Schematic drawing of the SubCM treatments of waste PCBs.
193
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198 Table 1 Conditions of the SubCM and SubCM + NaOH treatments (CNaOH = 4 g/L). Experiment
Temperature (oC)
Pressure (MPa)
Time (min)
Solid-to-liquid ratio (g/mL)
SubCM 1 SubCM 2 SubCM 3 SubCM 4 SubCM 5 SubCM 6 SubCM 7 SubCM 8 SubCM 9 SubCM 10 SubCM 11 SubCM 12 SubCM 13 SubCM 14 SubCM 15 SubCM + NaOH1 SubCM + NaOH2 SubCM + NaOH3 SubCM + NaOH4
100 150 200 250 300 250 250 250 250 250 250 250 250 250 250 100 150 200 250
0.5 1.6 3.5 6.1 9.3 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 0.5 1.6 3.5 6.1
60 60 60 60 60 30 45 60 75 90 60 60 70 80 90 60 60 60 60
1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:10 1:15 1:20 1:25 1:30 1:20 1:20 1:20 1:20
removal of methanol, the obtained oil was dissolved in 10-ml toluene and diluted 5000 times. Finally, the chemical composition of the oil was analyzed by Gas chromatography-mass spectrometry (GC-MS) equipment (Agilent 7890A). NIST database was used for the components measurement by GC-MS. According to the analysis result of GC-MS, the component of the obtained oil mainly included low brominated PBDEs and Br-free decomposition products such as diphenyl ether and anisole. 2.3. Extraction and analysis of PBDEs in original and SubCM-treated PCBs samples The extraction and the analysis of PBDEs in original waste PCBs samples and SubCM-treated PCBs samples were performed according to a previous report (Guo et al., 2015). In brief, waste PCBs sample was extracted by using microwave-assisted extraction with 20 mL of acetone/hexane solvent mixture (1:1 v/v). 13C12-labeled polychlorinated biphenyl (PCB) congener was added as recovery standards before extraction. Then the sample was concentrated and transferred into hexane. The further purification was conducted by a multilayer silica gel column. Finally, dichloromethane/hexane mixture (1:1 v/v) was used to elute the sample. The obtained extracts were analyzed by GC-MS equipment (Agilent 7890A) with a DB-5HT capillary column using negative chemical ionization. Eight PBDEs congeners were detected in original waste PCBs samples: 2,4,40 -Tribromodiphenyl ether (BDE28), 2,20 ,4,40 -Tet rabromodiphenyl ether (BDE47), 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71), 2,20 ,4,40 ,5-Pentabromodiphenyl ether (BDE99), 2,20 ,4,40 ,6-Pentabromodiphenyl ether (BDE100), 2,20 ,4,40 ,5,50 -Hex abromodiphenyl ether (BDE153), 2,20 ,4,40 ,5,60 -Hexabromodiphenyl ether (BDE154), and Decabromodiphenyl ether (BDE209). The average recoveries of surrogate standard were 93.6–104.2%. 3. Results and discussion 3.1. The removal efficiency of PBDEs from waste PCBs by SubCM and SubCM + NaOH treatments Eight PBDEs congeners (BDE28, BDE47, BDE71, BDE99, BDE100, BDE153, BDE154, and BDE209) were detected in original waste PCBs sample. The composition and content of PBDEs congeners in the original waste PCBs sample are shown in Table 2. The content P of 8PBDEs in the original waste PCBs sample was 3376.12 mg/kg. It can be seen from Table 2 that tetrabromodiphenyl ether,
Table 2 The composition and content of PBDEs congeners in the original waste PCBs sample. PBDEs
Congener
Content (mg/kg)
Monobromo-tetrabromodiphenyl ether (Mono-TetraBDE)
BDE28
92.33
BDE47 BDE71
318.50 557.25
Pentabromodiphenyl ether (PentaBDE)
BDE99 BDE100
728.45 126.48
Hexabromodiphenyl ether (HexaBDE)
BDE153 BDE154
190.67 119.06
Decabromodiphenyl ether (DecaBDE) P 8PBDEs
BDE 209
1243.38 3376.12
pentabromodiphenyl ether, and decabromodiphenyl ether are the most important PBDEs congeners in the original waste PCBs sample. The content of BDE71, BDE99, and BDE209 accounts for 16.5%, 21.6%, and 36.8% of the total PBDEs in waste PCBs, respectively. Fig. 2 presents the effect of reaction conditions on the removal efficiency (RE) of PBDEs from PCBs by the process of SubCM and SubCM + NaOH. The reaction conditions studied are temperature, time, and solid-to-liquid ratio. The RE of PBDEs from waste PCBs was calculated as follows:
REð%Þ ¼ ðM1 M 2 Þ 100=M1 P where M1 is the initial content of 8PBDEs in the original waste P PCBs sample, M2 the content of 8PBDEs in SubCM-treated PCBs sample. It can be found from Fig. 2A that the increase of reaction temperature can significantly increase the RE of PBDEs from waste PCBs. Hydrogen bonds can be easily formed between methanol molecules at low temperature. This is similar to water molecules. The energy of 5–7 kcal/mol is needed to break a hydrogen bond, whose bond energy is much weaker than chemical bond bonds (25–100 kcal/mol). Hydrogen bonds become unstable with increasing temperature, and about 70% of the hydrogen bonds in methanol molecules will be broken near its critical point. When the temperature rises to 300 °C (10 MPa), only about 10% of intermolecular hydrogen bonds exist in critical methanol (Asahi and Nakamura, 1998). Therefore, more methanol molecules exist as a single molecule due to the significant decrease of hydrogen bonds
194
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
Fig. 2. Effect of temperature (1:20 g/mL, 60 min), reaction time (1:20 g/mL, 200 °C), and solid-to-liquid ratio (200 °C, 60 min) on the removal efficiency of PBDEs from waste PCBs by SubCM and SubCM + NaOH.
between methanol molecules with the increase of temperature, which makes it easier for more methanol molecules to contact with the reactants (PBDEs in waste PCBs) and promote the removal and decomposition reactions. In addition, ion product is a very important parameter for the chemical reaction. The ion product of supercritical fluid will increase greatly with the increase of pressure. Hence, critical methanol, like critical water, has certain acidbase catalytic properties (Kusdiana and Saka, 2004). The pressures inside the reactor at different temperatures are shown in Table 1. The pressure of the reaction system increased markedly with the increase of temperature. It sharply increased from 0.5 to 9.3 MPa when the reaction temperature increased from 100 to 300 °C. The higher pressure is very beneficial to the enhancement of the acid-base catalytic activity of critical methanol, and facilitates the decomposition and removal of PBDEs contained in waste PCBs. The RE of PBDEs from waste PCBs also dramatically increased with the increase of reaction time and the decrease of solid-to-liquid ratio (Fig. 2B and C). The optimized treatment conditions for PBDEs removal were 200 °C, 60 min, and 1:20 g/mL. Under the optimized P conditions of SubCM, 91.3% of the 8PBDEs could be removed from waste PCBs. Generally, the bromine in brominated flame retardants of waste PCBs exists as an acid species after debromination such as HBr. Therefore, the introduction of basic species into the reaction system may have a good capture effect on acidic species, and promote the debromination, decomposition and removal of PBDEs from waste PCBs. On the other hand, the participation of alkaline species in SubCM reaction is likely to further enhance the acid-base catalytic activity of critical methanol, and is also beneficial to the removal process of PBDEs. In this study, NaOH of 4 g/L was added
to the reaction system for the RE comparison of PBDEs between SubCM and SubCM + NaOH under the same reaction conditions. The results are shown in Fig. 2. It indicated that the introduction of NaOH could significantly improve the RE of PBDEs from waste PCBs. When the reaction conditions were controlled at 200 °C, 60 min, and 1:20 g/mL, the RE of PBDEs from waste PCBs by SubCM + NaOH treatment was approximately 100%. 3.2. Debromination and degradation pathway of PBDEs in waste PCBs by SubCM The chemical composition information of the debromination and degradation products of PBDEs after SubCM or SubCM + NaOH treatment at different temperatures can be obtained by GC-MS analysis of the oil. Fig. 3A shows the GC-MS results of the oil obtained from waste PCBs after SubCM treatment at 100 °C, and the relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after SubCM treatment are shown in Table 3. From the analysis result of GC-MS, the component of the oil mainly included low brominated PBDEs and Br-free decomposition products. Under the low-temperature condition of 100, 150, and 200 °C, two PBDEs congeners were generated in oil phase products: 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) and 2,4,40 Tribromodiphenyl ether (BDE28). The two PBDEs congeners generated at 100, 150, and 200 °C may be derived from the initial BDE71 and BDE28 in waste PCBs, which dissolved into the critical methanol medium during the treatment process. However, it can be found from Table 2 that the content of BDE71 and BDE28 accounts for only P 19.2% of the total 8PBDEs in the original waste PCBs. It also can be
195
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
Fig. 3. GC-MS analysis results of the oil obtained from waste PCBs after SubCM treatment at (A) 100 °C and (B) 300 °C.
Table 3 The relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after the SubCM treatment in the absence of NaOH. Compound
0
2,’3,4 ,6-Tetrabromodiphenyl ether (BDE71) 2,4,40 -Tribromodiphenyl ether (BDE28) 4-Bromophenyl ether (BDE4) Diphenyl ether Anisole 1-Methoxy-4-methyl-benzene 1-Methoxy-4-(1-methylethyl)-benzene Toluene o-Xylene
Relative peak area (%) 100 °C
150 °C
200 °C
250 °C
300 °C
30.71 20.04 0 0 0 0 0 0 0
27.16 35.34 0 0 0 0 0 0 0
23.07 38.47 0 0 0 0 0 0 0
0 0 19.37 16.65 13.43 12.51 0 1.43 1.51
0 0 0 6.08 20.19 12.31 6.17 9.42 9.96
P seen in Fig. 2 that more than 68.5, 83.6, and 91.3% of the total 8PBDEs are removed from the waste PCBs and transferred to the oil phase products after the reaction at 100, 150, and 200 °C, respectively. Therefore, it can be inferred that another important source of BDE71 and BDE28 formed in oil phase products after SubCM treatments is the debromination of PBDEs congeners containing more than four Br when the reaction temperature is below 200 °C. From the molecular structure, 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) can be degraded either from BDE154 or BDE209. However, the source of BDE28 was more complex in comparison with BDE71. It can be produced by the debromination of any of the following PBDEs congeners in original waste PCBs: BDE47, BDE99, BDE100, BDE153, BDE154, and BDE209. In addition to PBDEs congeners, it also can be found from Fig. 3A that the produced oil phase product contains another flame retardant frequently-used in electronic waste: phosphorous flame retardants. Three organophosphorus flame retardants such as triphenyl phosphate, 2-methylphenyl diphenyl phosphoric acid ester, and bis(4-methylphenyl) phenyl phosphoric ester could be identified in the oil obtained from waste PCBs after the SubCM process at 100 °C (Fig. 3A). It indicated that phosphorous flame retardants also could be efficiently removed from waste PCBs during the SubCM process at lower temperature. When the reaction temperature increased to 250 °C, an important change in the chemical composition of oil phase products is that 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) and 2,4,40 Tribromodiphenyl ether (BDE28) disappeared, and only one Brcontaining chemical compound (4-Bromophenyl ether, BDE4) was detected. BDE4 was derived from the further debromination of BDE28 and BDE71. This indicated that the para-bromine in the molecule of PBDEs congeners is much more stable in comparison with other positions in the treatment process of SubCM. This
debromination characteristic is also observed in the previous report about the debromination pathways of PBDEs by the reductive debromination of microbial degradation (Zhao et al., 2018). At reaction temperature of 250 °C, diphenyl ether, the complete debromination product of PBDEs congeners, also was found in the oil phase product, indicating that the debromination performance of SubCM was more complete when compared to other reported methods of PBDEs debromination such as photocatalytic degradation (Miller et al., 2017) and thermocatalytic degradation (Yang et al., 2016). In the photocatalytic or thermocatalytic degradation, the bromination end product of PBDEs is usually the monobromodiphenyl ether (Yang et al., 2016; Miller et al., 2017). After the SubCM reaction at 300 °C, no Br-containing compound can be detected in the oil product. Fig. 3B shows the GC-MS analysis results of the oil phase product obtained from waste PCBs after the SubCM treatment at 300 °C. The relative peak areas of the debromination products of PBDEs in oil phase are presented in Table 3. In addition to diphenyl ether (the final bromination product of PBDEs at temperatures of 250 and 300 °C), several derivatives of benzene and anisole were produced in the oil phase product. The derivatives of benzene and anisole include anisole, 1-methoxy-4-methyl-benzene, 1-methoxy-4-(1-methylethyl)-ben zene, toluene, and o-xylene. The critical temperature and pressure of methanol are 240 °C and 8.1 MPa. From Table 1, it can be found that the reaction conditions of 250 and 300 °C approach or even exceed the critical point of methanol. It has been found that methanol molecule can be used as either reaction medium or reactant in the high-temperature super-critical state (Narayan et al., 2017; Alenezi et al., 2010). Supercritical methanol molecule can take part in a variety of reactions such as esterification, alkylation, and oxyalkylation. The ether bond of diphenyl ether, the final debromination product of PBDEs, has weaker bond energy (around
196
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
263 kJ/mol). The ether bond could be broken under the reaction condition of supercritical methanol, then the methylation reaction occurred and anisole was generated. The chemical compounds of 1-methoxy-4-methyl-benzene and 1-methoxy-4-(1-methylethyl)benzene were produced by the further alkylation of anisole. In addition, the toluene and o-xylene also could be generated by further decomposition and methylation of anisole. The debromination and degradation pathway of PBDEs in waste PCBs by SubCM treatment is described in Fig. 4. 3.3. Debromination and degradation pathway of PBDEs in waste PCBs by SubCM + NaOH To understand the influence of NaOH on the debromination and degradation of PBDEs in waste PCBs by SubCM process, 4 g/L of NaOH was added to the SubCM reaction system, and the GC-MS analysis of the oil after SubCM + NaOH treatment was conducted. The relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after the SubCM treatment in the presence of NaOH are shown in Table 4. At the treatment temperature of 100 °C and 150 °C, the main debromination and degradation products of PBDEs were 2,’3,40 ,6Tetrabromodiphenyl ether (BDE71), 2,4,40 -tribromodiphenyl ether (BDE28), 4,40 -dibromophenyl ether (BDE15), 4-bromophenyl ether (BDE4), 1,3,5-tribromo-2,4,6-trimethoxybenzene, and diphenyl ether. Low brominated PBDEs congeners, such as mono-BDE and di-BDE, were found in the products, which was completely different from the reaction system of SubCM in the absence of NaOH. In SubCM system, original PBDEs congeners in waste PCBs could only be degraded to Tetrabromo-BDE and Tribromo-BDE when temperature was lower than 200 °C (Table 3 and Fig. 3). In addition, the
Br
Br O
Br Br
BrBr O
Br Br Br
Br O
Br
BrBr O
Br Br
Br
BDE-28
BDE-47
BDE-71
BrBr O
Br Br
BDE-99
BDE-100
Br O
Br
Br Br O
Br
Br
Br
Br Br
completely debrominated product, diphenyl ether, was detected after SubCM + NaOH treatment at 100 °C (3.06% of relative peak areas) and 150 °C (4.96% of relative peak areas), indicating that the debromination ability of SubCM for PBDEs in waste PCBs could be efficiently enhanced by introducing alkaline NaOH. When the reaction temperature of SubCM + NaOH increased to 200 °C, the products mainly include 1,3,5-tribromo-2,4,6-trime thoxybenzene, diphenyl ether, and anisole. Anisole and diphenyl ether became the primary products with the temperature increasing to 250 °C, and no Br-containing compound could be detected. In all of the products, the relative peak areas of anisole and diphenyl ether reached 46.06% and 14.69%, respectively. The results showed that the PBDEs congeners in the initial sample of the waste PCBs could be completely debrominated and decomposed into diphenyl ether and anisole. Therefore, after NaOH was introduced into the reaction system of SubCM, the complete debromination could be realized at low temperature of 250 °C, and high level of anisole (46.06%) could be obtained and recovered at the same time. The complete debromination temperature of SubCM + NaOH for PBDEs could be reduced by 50 °C in comparison with SubCM (300 °C, Table 3). The facilitation function of NaOH for the debromination of PBDEs in SubCM can result from two reasons. On the one hand, the acid-base catalytic activity of critical methanol can be further enhanced by the introduction of alkaline NaOH, and the debromination of PBDEs can be enhanced efficiently. On the other hand, the capture function of alkaline NaOH for the acidic products such as HBr formed after the debromination of PBDEs, can accelerate the escape of the generated HBr from waste PCBs matrix. Then the reaction moved to the direction which was favorable to the bromination reaction of PBDEs. Thus, the debromination efficiency of PBDEs was significantly improved.
Br Br Br
BDE-153
Br O
Br Br O Br
Br Br Br Br
BDE-154
Br
Br
BDE-28
BDE-71
Br
O
O
Br
BDE-4
Br
BDE-15
O
CH3 OH
O
CH 3
CH3
O CH3
Fig. 4. Debromination and degradation pathway of PBDEs in waste PCBs by SubCM and SubCM + NaOH.
Br
Br Br O
Br
Br Br
Br
Br
BDE-209
197
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198 Table 4 The relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after the SubCM treatment in the presence of NaOH (CNaOH = 4 g/L). Compound
Relative peak area (%) 100 °C
150 °C
200 °C
250 °C
2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) 2,4,40 -Tribromodiphenyl ether (BDE28) 4,40 -Dibromophenyl ether (BDE15) 4-Bromophenyl ether (BDE4) 1,3,5-Tribromo-2,4,6-trimethoxybenzene Diphenyl ether Anisole 1-Methoxy-4-methyl-benzene Toluene 1,3,5-Trimethyl-benzene o-Xylene
48.88 17.85 12.92 1.65 9.66 3.06 0 0 0 0 0
47.01 32.04 0 11.66 3.75 4.96 0 0 0 0 0
0 0 0 0 9.63 42.34 33.25 0 0 0 0
0 0 0 0 0 14.69 46.06 13.18 5.75 7.29 5.36
One interesting result is that a new chemical compound is formed after the SubCM + NaOH treatment in comparison with SubCM treatment (Table 3). It can be seen from Table 4 that the newly generated compound was 1,3,5-Tribromo-2,4,6-trimethoxy benzene. In terms of the molecular structure, 1,3,5-Tribromo-2,4, 6-trimethoxybenzene could not be produced from the debromination of any of the eight PBDEs congeners in the initial sample of waste PCBs. We inferred that it was formed by complicated multi-step reactions of debromination, ether bond cracking, methylation, and methoxylation. Fig. 1S presents the schematic diagram of the possible formation pathway of 1,3,5-Tribromo-2,4, 6-trimethoxybenzene during SubCM + NaOH process. Firstly, 2,4,6-tribromodiphenyl ether was generated from the debromination of BDE-154 or BDE-209. Then the Ph-O ether bond in 2,4,6-tribromodiphenyl ether was broken under the reaction conditions of critical methanol, and the (Br3)-Ph-O- group was formed. Simultaneously the generated (Br3)-Ph-O- group further reacted with critical methanol molecule to produce 2,4,6-tribromoanisole by the methylation reaction. Finally, the further methylation reaction on the meta-position of 2,4,6-tribromoanisole resulted in the generation of 1,3,5-tribromo-2,4,6-trimethoxybenzene. Several studies (Wang et al., 2007a, 2007b, 2007c) found that the reactivity of supercritical methanol molecule could be markedly enhanced by adding catalysts such as metallic oxide, organic amine, and alkali. Among these catalysts, alkali was found to have the highest reactivity. The alkaline catalyst such as NaOH not only had the excellent catalytic activity of transesterification for the preparation of biodiesel from rapeseed oil, but also did not cause saponification (Wang et al., 2007c). Therefore, the introduction of NaOH in the SubCM system could significantly promote the reactivity of methylation and methoxylation between PBDEs congeners and methanol molecules. This can explain the proposed formation pathway of 1,3,5-tribromo2,4,6-trimethoxybenzene from PBDEs in waste PCBs during SubCM + NaOH treatment. 4. Conclusions In this study, the SubCM process was tested for the first time to remove and degrade PBDEs contained in waste PCBs. In comparison with the reported works on the removal of low level of PBDEs in environmental media, high-content PBDEs in electronic waste could be efficiently removed and decomposed by the proposed SubCM process. In SubCM process, the increase of the reaction temperature/time and the decrease of the solid-to-liquid ratio could improve the removal efficiency of PBDEs from waste PCBs. The increase of reaction temperature could significantly enhance the debromination ability of SubCM. PBDEs congeners in waste PCBs could be degraded into monobromodiphenyl ether (BDE4) by SubCM at 250 °C, and the complete debromination of PBDEs could be realized at 300 °C. The increase of temperature could lead
to methylation and methoxylation reactions between debrominated products and critical methanol molecules. The introduction of alkaline additive NaOH in SubCM process could significantly enhance the removal and degradation of PBDEs in waste PCBs. The complete debromination of PBDEs by SubCM + NaOH process could be achieved at lower temperature (250 °C) in comparison with SubCM process due to the enhancement effect of NaOH on the acid-base catalytic activity of critical methanol molecule. According to the degradation products after treatments, no significant difference could be found for debromination and degradation pathway of PBDEs between SubCM and SubCM + NaOH process. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (No. 21605018), the Natural Science Basic Research Project of Shaanxi Province of China (No. 2018JM5149), the Foundation Research Project of Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation (No. MTy2019-05), and the Natural Science Foundation of Fujian Province of China (No. 2017 J01472). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.09.022. References Alharbi, O.M.L., Basheer, A.A., Khattab, R.A., Ali, I., 2018. Health and environmental effects of persistent organic pollutants. J. Mol. Liq. 263, 442–453. Alenezi, R., Leeke, G.A., Winterbottom, J.M., Santos, R.C.D., Khan, A.R., 2010. Esterification kinetics of free fatty acids with supercritical methanol for biodiesel production. Energ. Convers. Manage. 51, 1055–1059. Asahi, N., Nakamura, Y., 1998. Chemical shift study of liquid and supercritical methanol. Chem. Phys. Lett. 290, 63–67. Cao, C.Q., Xu, L., He, Y., Guo, L., Jin, H., Huo, Z., 2017. High-efficiency gasification of wheat straw blackliquor in supercritical water at high temperatures for hydrogenproduction. Energ. Fuel. 31, 3970–3978. Cowell, W.J., Sjodin, A., Jones, R., Wang, Y., Wang, S., Herbstman, J.B., 2018. Determinants of prenatal exposure to polybrominated diphenyl ethers (PBDEs) among urban, minority infants born between 1998 and 2006. Environ. Pollut. 233, 774–781. Duan, H.B., Li, J., Liu, Y., Yamazakib, N., Jiang, W., 2012. Characterizing the emission of chlorinated/brominated dibenzo-p-dioxins and furans from low-temperature thermal processing of waste printed circuit board. Environ. Pollut. 161, 185– 191. Erdocia, X., Prado, R., Fernández-Rodríguez, J., Labidi, J., 2016. Depolymerization of different organosolv lignins in supercritical methanol, ethanol, and acetone to produce phenolic monomers. ACS Sustain. Chem. Eng. 4, 1373–1380. Guo, J., Zhang, R., Xu, Z., 2015. PBDEs emission from waste printed wiring boards during thermal process. Environ. Sci. Technol. 49, 2716–2723. Gupta, V.K., Jain, R., Nayak, A., Agarwal, S., Shrivastava, M., 2011. Removal of the hazardous dye-tartrazine by photodegradation on titanium dioxide surface. Mat. Sci. Eng. C 31, 1062–1067.
198
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
Hadi, P., Xu, M., Lin, C.S.K., Hui, C.W., McKay, G., 2015. Waste printed circuit board recycling techniques and product utilization. J. Hazard. Mater. 283, 234–243. He, J.Z., Robrock, K.R., Alvarez-Cohen, L., 2006. Microbial reductive debromi-nation of polybrominated diphenyl ethers (PBDEs). Environ. Sci. Techno. 40, 4429– 4434. Kusdiana, D., Saka, S., 2004. Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresource Technol. 91, 289–295. Liu, Y.M., Chen, S., Quan, X., Fan, X., Zhao, H., Zhao, Q., Yu, H., 2014. Nitrogen-doped nanodiamond rod array electrode with superior performance for electroreductive debromination of polybrominated diphenyl ethers. Appl. Catal B: Environ. 154–155, 206–212. Lu, Y., Xu, Z.M., 2016. Precious metals recovery from waste printed circuit boards: A review for current status and perspective. Resour. Conserv. Recy. 113, 28–39. Miller, E.B., Zahran, E.M., Knecht, M.R., Bachas, L.G., 2017. Metal oxide semiconductor nanomaterial for reductive debromination: Visible light degradation of polybrominated diphenyl ethers by Cu2O@Pd nanostructures. Appl. Catal. B: Environ. 213, 147–154. Narayan, R.C., Lamba, N., Javed, A., Madras, G., 2017. Kinetics of esterificat-ion of 10undecenoic and ricinoleic acids with near-critical and supercritical methanol. Sustain. Energ. Fuels 1, 1425–1436. Qi, Y., He, J., Xiu, F., Nie, W., Chen, M., 2018. Partial oxidation treatment of waste polyvinyl chloride in critical water: preparation of benzaldehyde/acetophenone and dichlorination. J. Clean. Prod. 196, 331–339. Qi, Y., He, J., Xiu, F., Yu, X., Gao, X., Li, Y., Lu, Y., Song, Z., 2019. A convenient chemiluminescence detection for bisphenol A in E-waste dismantling site based on surface charge change of cationic gold nanoparticles. Microchem. J. 147, 789–796. Qi, Y., He, J., Xiu, F.-R., Yu, X., Li, Y., Lu, Y., Gao, X., Song, Z., Li, B., 2019. A facile chemiluminescence sensing for ultrasensitive detection of heparin using charge effect of positively-charged AuNPs. Spectrochim. Acta A 216, 310–318. Rajendran, S., Khan, M.M., Gracia, F., Qin, J., Gupta, V.K., Arumainathan, S., 2016. Ce3+-ion-induced visible-light photocatalytic degradation and electrochemical activity of ZnO/CeO2 nanocomposite. Sci. Rep. 6, 31641. Robrock, K.R., Korytár, P., Alvarez-Cohen, L., 2008. Pathways for the anaerobic microbial debromination of polybrominated diphenyl ethers. Environ. Sci. Technol. 42, 2845–2852. Shen, Y., 2018. Effect of chemical pretreatment on pyrolysis of non-metallic fraction recycled from waste printed circuit boards. Waste Manage. 76, 537–543. Sun, Z.Y., Fu, L., Liu, Z., Han, B., Liu, Y., Du, J., 2006. Synthesis of nobl-e metal/carbon nanotube composites in supercritical methanol. J. Nanosci. Nanotechno. 6, 691– 697. Wang, L.Y., He, H., Xie, Z., Yang, J., Zhu, S., 2007a. Transesterification of the crude oil of rapeseed with NaOH in supercritical and subcritical methanol. Fuel Process. Technol. 88, 477–481.
Wang, L.Y., Tang, Z., Xu, W., Yang, J., 2007b. Catalytic transesterification of crude rapeseed oil by liquid organic amine and co-catalyst in supercritical methanol. Catal. Commun. 8, 1511–1515. Wang, L.Y., Yang, J.C., 2007. Transesterification of soybean oil with nano-MgO or not in supercritical and subcritical methanol. Fuel. 86, 328–333. Wang, R., Tang, T., Lu, G., Huang, K., Yin, H., Lin, Z., Wu, F., Dang, Z., 2018. Rapid debromination of polybrominated diphenyl ethers (PBDEs) by zero valent metal and bimetals: mechanisms and pathways assisted by density function theory calculation. Environ. Pollut. 240, 745–753. Xiu, F.R., Li, Y., Qi, Y., Yu, X., He, J., Lu, Y., Gao, X., Deng, Y., Song, Z., 2019. A novel treatment of waste printed circuit boards by low-temperature near-critical aqueous ammonia: Debromination and preparation of nitrogen-containing fine chemicals. Waste Manage. 84, 355–363. Xiu, F.R., Qi, Y., Wang, S., Nie, W., Weng, H., Chen, M., 2018. Application of critical water-alcohol composite medium to treat waste printed circuit boards: oil phase products characteristic and debromination. J. Hazard. Mater. 344, 333– 342. Xiu, F.R., Weng, H., Qi, Y., Yu, G., Zhang, Z., Zhang, F.-S., Chen, M., 2017. A novel recovery method of copper from waste printed circuit boards by supercritical methanol process: preparation of ultrafine copper materials. Waste Manage. 60, 643–651. Xiu, F.R., Zhang, F.S., 2010. Materials recovery from waste printed circuit boards by supercritical methanol. J. Hazard. Mater. 178, 628–634. Yang, F., Li, Q., Su, G., Huang, X., Li, B., Zhao, Y., Miao, X., Zheng, M., 2016. Thermal degradation of 2,2’,4,4’-tetrabromodiphenyl ether (BDE-47) over synthesized Fe-Al composite oxide. Chemosphere. 150, 445–452. Zhang, S., Li, Y., Wang, R., Xu, Z., Wang, B., Chen, S., Chen, M., 2017. Superfine copper powders recycled from concentrated metal scraps of waste printed circuit boards by slurry electrolysis. J. Clean. Prod. 152, 1–6. Zhang, Y.H., Liu, S., Xie, H., Zeng, X., Li, J., 2012. Current status on leaching precious metals from waste printed circuit boards. Procedia Environ. Sci. 16, 560–568. Zhao, S.Y., Rogers, M.J., Ding, C., He, J., 2018. Reductive debromination of polybrominated diphenyl ethers-microbes, processes and dehalogenases. Front. Microbiol. 9, 1292-. Zhuang, Y., Ahn, S., Luthy, R.G., 2010. Debromination of polybrominated diphenyl ethers by nanoscale zerovalent iron: pathways, kinetics, and reactivity. Environ. Sci. Technol. 44, 8236–8242. Zhu, X., Beiyuan, J., Lau, A.Y.T., Chen, S.S., Tsang, D., Graham, N.J.D., Lin, D., Sun, J., Pan, Y., Yang, X., Li, X., 2018. Sorption, mobility, and bioavailability of PBDEs in the agricultural soils: Roles of co-existing metals, dissolved organic matter, and fertilizers. Sci. Total Environ. 619–620, 1153–1162. Zou, D., Chi, Y., Fu, C., Dong, J., Wang, F., Ni, M., 2013. Co-destruction of organic pollutants in municipal solid waste leachate and dioxins in fly ash under supercritical water using H2O2 as oxidant. J. Hazard. Mater. 248–249, 177–184.
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
A novel management strategy for removal and degradation of polybrominated diphenyl ethers (PBDEs) in waste printed circuit boards Fu-Rong Xiu a,b,⇑, Xuan Yu a, Yingying Qi a,b, Yifan Li a, Yongwei Lu a, Yixiao Wang a, Jiahuan He a, Ke Zhou a, Zhiqi Song a, Xiang Gao a,b a b
College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi’an 710054, China
a r t i c l e
i n f o
Article history: Received 6 June 2019 Revised 30 August 2019 Accepted 15 September 2019
Keywords: WEEE Waste printed circuit boards PBDEs Degradation Subcritical methanol
a b s t r a c t Waste printed circuit boards (PCBs) contain a high level of brominated flame retardants (BFRs), among which polybrominated biphenyl ethers (PBDEs) are the most widely used additive BFRs. PBDEs are considered to be a type of persistent organic pollutants (POPs). The efficient removal/degradation of PBDEs in waste PCBs is an urgent problem in electronic waste treatment, but the degradation of PBDEs is a great challenge due to their extreme stability and persistence in nature. In this study, a novel management strategy was developed for removal and degradation of PBDEs in waste PCBs by using a simple subcritical methanol (SubCM) process. The results showed that reaction temperature, residence time, solid-to-liquid ratio, and additive NaOH are key factors influencing the removal of PBDEs from waste PCBs. Under optiP mal conditions (200 °C, 60 min, 1:20 g/mL), the removal efficiency of 8PBDEs from waste PCBs could reach 91.3% and 98.8% for the proposed process of SubCM and SubCM + NaOH, respectively. When the temperature is below 200 °C, highly brominated PBDEs congeners in waste PCBs were degraded into 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) and 2,4,40 -Tribromodiphenyl ether (BDE28) after SubCM treatment. 4-Bromophenyl ether (BDE4) and diphenyl ether were generated by the further debromination of BDE71 and BDE28 with the increase of treatment temperature. The debromination temperature of PBDEs congeners in SubCM could be markedly lowered by adding 4 g/L of NaOH. The complete debromination of PBDEs congeners in waste PCBs could be achieved at 300 °C and 250 °C for the developed process of SubCM and SubCM + NaOH, respectively. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction The generation of waste electrical and electronic equipment (WEEE) is increasing drastically in the last decade due to the rapid development of information technology and product upgrade (Hadi et al., 2015). As the most important unit of electrical and electronic equipment, the global production amount of waste printed circuit boards (PCBs) is staggering (Xiu et al., 2019). Waste PCBs accounts for over 4% of total WEEE (Lu and Xu, 2016). It is estimated that more than 500,000 tonnes of waste PCBs are needed to dispose each year in China (Zhang et al., 2012). Waste PCBs includes both metallic and non-metallic components. In addition to Cu and precious metals with recycling value, the metallic component also contains a large number of other heavy metals such as Pb, Sn, Zn, and Ni (Zhang et al., 2017). Non-metallic component mainly contains ⇑ Corresponding author at: College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China. E-mail address: [email protected] (F.-R. Xiu). https://doi.org/10.1016/j.wasman.2019.09.022 0956-053X/Ó 2019 Elsevier Ltd. All rights reserved.
resin, brominated flame retardants (BFRs), and glass fiber (Shen, 2018; Qi et al., 2019). BFRs are combined with resin materials by addition or reaction in PCBs. Typical BFRs are polybrominated diphenyl ethers (PBDEs), polybrominated biphenyls (PBBs), hexabromocyclododecane (HBCD), and tetrabromobisphenol-A (TBBPA). As a kind of additive flame retardants widely used in the manufacture of PCBs, PBDEs are considered to be a type of persistent organic pollutants (POPs) that can bioaccumulate and amplify (Yang et al., 2016; Cowell et al., 2018; Alharbi et al., 2018). The traditional disposal method of electronic wastes containing PBDEs, such as incineration and landfill, can easily produce highly toxic polybrominated dibenzo-dioxins and benzo-furans (PBDD/Fs) or lead to secondary pollution of soil and groundwater by PBDEs (Duan et al., 2012; Zhu et al., 2018). Therefore, it is of great significance to develop safe and effective debromination and detoxification technology to solve the increasingly serious pollution of PBDEs contained in waste PCBs. Degradation of PBDEs is a great challenge due to their extreme stability and persistence in nature (Yang et al., 2016). Many
192
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
processes were proposed for debromination and degradation of PBDEs such as microbial degradation (He et al., 2006; Robrock et al., 2008), electrocatalysis (Liu et al., 2014), photocatalysis (Miller et al., 2017; Gupta et al., 2011; Rajendran et al., 2016), and zerovalent iron reduction (Zhuang et al., 2010; Wang et al., 2018). However, all of the reported methods are only suitable for the treatment of low level of PBDEs in environment, and are difficult to be applied in the actual engineering treatment for the removal and degradation of PBDEs in waste PCBs, which are heterogeneous mix of BFRs, polymer, metals and glass fiber. In recent years, the application of supercritical fluids for detoxification and degradation of toxic organic pollutants has become a very promising and fast-.0growing branch of green chemistry (Zou et al., 2013; Xiu et al., 2018). Supercritical water (SCW) is the most intensively used supercritical fluid for decomposition of toxic wastes (Cao et al., 2017; Qi et al., 2018). However, the condition of high temperature/pressure of SCW greatly limits its practical application (Xiu et al., 2018). Therefore, supercritical methanol (SCM) has attracted increasing attention and has been widely used in green chemical processes (Qi et al., 2019; Erdocia et al., 2016). The critical temperature and pressure of methanol (240 °C, 8.1 MPa) are low when compared to water (374 °C, 22 MPa). The operating conditions of SCM are much milder in comparison with water, weakening the corrosion of the reaction equipment. In addition, the boiling point of methanol is only 64.7 °C, which is beneficial to the separation and recycling of methanol after treatment. In the critical condition, methanol possesses unique physicochemical properties. The ion product of methanol increases greatly with the increase of pressure, and the methanol possesses acid-base catalytic activity (Kusdiana and Saka, 2004). Supercritical methanol molecule also has strong nucleophilic reactivity due to the greater electronegativity of the oxygen in methanol (Narayan et al., 2017). In addition, previous studies have also found that SCM has strong reducibility (Xiu et al., 2017; Sun et al., 2006). These unique properties of SCM encouraged us to test its effectiveness for the decomposition of epoxy resin (the main organic components contained in waste PCBs) in our previous study (Xiu et al., 2010). It was found that the epoxy resin of waste PCBs could be efficiently decomposed in SCM (>300 °C) and the decomposition products mainly include phenol and its derivatives. However, the reaction behavior of PBDEs contained in waste PCBs during the process of critical methanol is still unknown. In this study, we attempt to develop an effective process for the removal and degradation of PBDEs in waste PCBs by using subcritical methanol (SubCM, T 240 °C, P 8.1 MPa) process. In addition, the introduction of alkaline species in SubCM may play a positive role in promoting the debromination of PBDEs by trapping the acidic debromination products, and the debromination temperature of PBDEs in SubCM may be lowered. There is no study investigating the removal and degradation of PBDEs contained in waste PCBs, and the influence of SubCM treatment on the behavior of PBDEs is unclear. In critical conditions, the methanol molecule possesses unique physicochemical
properties such as swelling enhancement, acid-base catalytic activity, and strong nucleophilic reactivity. In the proposed process, SubCM is used as either a reaction medium or a reactant for removal and degradation of PBDEs contained in waste PCBs. Therefore, the purposes of this study are (1) to evaluate the removal, debromination and degradation behavior of PBDEs in waste PCBs in SubCM medium and (2) to examine the boost function of alkaline species such as NaOH for the removal and degradation of PBDEs in waste PCBs. On this basis, a novel and highefficiency strategy for removal and degradation of PBDEs in waste PCBs by using SubCM and SubCM + NaOH process was successfully developed. In the proposed process, PBDEs contained in waste PCBs could be efficient removal and further be completely transformed into nontoxic substances. 2. Materials and methods 2.1. Materials and chemicals Waste PCBs samples were obtained from Shaanxi Xintiandi Solid Waste Material Management Company (China). The samples were dismantled from abandoned instruments and apparatus. The type of the boards is mainly the paper-based copper clad board. The electronic components on waste PCBs were removed manually before the subsequent experiments. Then the samples were cut into small pieces (2 cm 1 cm) by hammer and scissor. The obtained small pieces were sent to comminute by using a pulverizer. The particle size was smaller than 0.5 mm after pulverization. Chemical reagents were obtained from Sinopharm Chemical Reagent Company. 2.2. SubCM experiments of waste PCBs The treatment process of waste PCBs by SubCM is shown in Fig. 1. The SubCM experiments were carried out in a supercritical fluid reactor with a volume of 100 mL. Firstly, 2 g of waste PCBs sample and methanol with a certain volume were added to the reactor, and the solid-liquid ratio (S/L) was controlled at 1:10, 1:15, 1:20, 1:25, and 1:30 g/mL. Then the reactor was heated by turning on the power. The temperature in the reactor increased rapidly to the set value. SubCM experiments were performed with reaction temperature of 100, 150, 200, 250 and 300 °C. The residence time was controlled at 30, 45, 60, 75, and 90 min. Table 1 shows the reaction conditions of each experiment. After the reaction, the power supply of the device was shut down, and the reactor was cooled to room temperature by using an ice-water bath. Then the cooled product was taken out from the reactor, and a membrane filter (pore size = 1.0 mm) was used to separate the mixture products of solid residue and liquid. A rotary evaporator (RE-2000B, YARONG, China) was used to recover the reaction medium methanol from the separated liquid. After the
Fig. 1. Schematic drawing of the SubCM treatments of waste PCBs.
193
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198 Table 1 Conditions of the SubCM and SubCM + NaOH treatments (CNaOH = 4 g/L). Experiment
Temperature (oC)
Pressure (MPa)
Time (min)
Solid-to-liquid ratio (g/mL)
SubCM 1 SubCM 2 SubCM 3 SubCM 4 SubCM 5 SubCM 6 SubCM 7 SubCM 8 SubCM 9 SubCM 10 SubCM 11 SubCM 12 SubCM 13 SubCM 14 SubCM 15 SubCM + NaOH1 SubCM + NaOH2 SubCM + NaOH3 SubCM + NaOH4
100 150 200 250 300 250 250 250 250 250 250 250 250 250 250 100 150 200 250
0.5 1.6 3.5 6.1 9.3 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 0.5 1.6 3.5 6.1
60 60 60 60 60 30 45 60 75 90 60 60 70 80 90 60 60 60 60
1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:20 1:10 1:15 1:20 1:25 1:30 1:20 1:20 1:20 1:20
removal of methanol, the obtained oil was dissolved in 10-ml toluene and diluted 5000 times. Finally, the chemical composition of the oil was analyzed by Gas chromatography-mass spectrometry (GC-MS) equipment (Agilent 7890A). NIST database was used for the components measurement by GC-MS. According to the analysis result of GC-MS, the component of the obtained oil mainly included low brominated PBDEs and Br-free decomposition products such as diphenyl ether and anisole. 2.3. Extraction and analysis of PBDEs in original and SubCM-treated PCBs samples The extraction and the analysis of PBDEs in original waste PCBs samples and SubCM-treated PCBs samples were performed according to a previous report (Guo et al., 2015). In brief, waste PCBs sample was extracted by using microwave-assisted extraction with 20 mL of acetone/hexane solvent mixture (1:1 v/v). 13C12-labeled polychlorinated biphenyl (PCB) congener was added as recovery standards before extraction. Then the sample was concentrated and transferred into hexane. The further purification was conducted by a multilayer silica gel column. Finally, dichloromethane/hexane mixture (1:1 v/v) was used to elute the sample. The obtained extracts were analyzed by GC-MS equipment (Agilent 7890A) with a DB-5HT capillary column using negative chemical ionization. Eight PBDEs congeners were detected in original waste PCBs samples: 2,4,40 -Tribromodiphenyl ether (BDE28), 2,20 ,4,40 -Tet rabromodiphenyl ether (BDE47), 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71), 2,20 ,4,40 ,5-Pentabromodiphenyl ether (BDE99), 2,20 ,4,40 ,6-Pentabromodiphenyl ether (BDE100), 2,20 ,4,40 ,5,50 -Hex abromodiphenyl ether (BDE153), 2,20 ,4,40 ,5,60 -Hexabromodiphenyl ether (BDE154), and Decabromodiphenyl ether (BDE209). The average recoveries of surrogate standard were 93.6–104.2%. 3. Results and discussion 3.1. The removal efficiency of PBDEs from waste PCBs by SubCM and SubCM + NaOH treatments Eight PBDEs congeners (BDE28, BDE47, BDE71, BDE99, BDE100, BDE153, BDE154, and BDE209) were detected in original waste PCBs sample. The composition and content of PBDEs congeners in the original waste PCBs sample are shown in Table 2. The content P of 8PBDEs in the original waste PCBs sample was 3376.12 mg/kg. It can be seen from Table 2 that tetrabromodiphenyl ether,
Table 2 The composition and content of PBDEs congeners in the original waste PCBs sample. PBDEs
Congener
Content (mg/kg)
Monobromo-tetrabromodiphenyl ether (Mono-TetraBDE)
BDE28
92.33
BDE47 BDE71
318.50 557.25
Pentabromodiphenyl ether (PentaBDE)
BDE99 BDE100
728.45 126.48
Hexabromodiphenyl ether (HexaBDE)
BDE153 BDE154
190.67 119.06
Decabromodiphenyl ether (DecaBDE) P 8PBDEs
BDE 209
1243.38 3376.12
pentabromodiphenyl ether, and decabromodiphenyl ether are the most important PBDEs congeners in the original waste PCBs sample. The content of BDE71, BDE99, and BDE209 accounts for 16.5%, 21.6%, and 36.8% of the total PBDEs in waste PCBs, respectively. Fig. 2 presents the effect of reaction conditions on the removal efficiency (RE) of PBDEs from PCBs by the process of SubCM and SubCM + NaOH. The reaction conditions studied are temperature, time, and solid-to-liquid ratio. The RE of PBDEs from waste PCBs was calculated as follows:
REð%Þ ¼ ðM1 M 2 Þ 100=M1 P where M1 is the initial content of 8PBDEs in the original waste P PCBs sample, M2 the content of 8PBDEs in SubCM-treated PCBs sample. It can be found from Fig. 2A that the increase of reaction temperature can significantly increase the RE of PBDEs from waste PCBs. Hydrogen bonds can be easily formed between methanol molecules at low temperature. This is similar to water molecules. The energy of 5–7 kcal/mol is needed to break a hydrogen bond, whose bond energy is much weaker than chemical bond bonds (25–100 kcal/mol). Hydrogen bonds become unstable with increasing temperature, and about 70% of the hydrogen bonds in methanol molecules will be broken near its critical point. When the temperature rises to 300 °C (10 MPa), only about 10% of intermolecular hydrogen bonds exist in critical methanol (Asahi and Nakamura, 1998). Therefore, more methanol molecules exist as a single molecule due to the significant decrease of hydrogen bonds
194
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
Fig. 2. Effect of temperature (1:20 g/mL, 60 min), reaction time (1:20 g/mL, 200 °C), and solid-to-liquid ratio (200 °C, 60 min) on the removal efficiency of PBDEs from waste PCBs by SubCM and SubCM + NaOH.
between methanol molecules with the increase of temperature, which makes it easier for more methanol molecules to contact with the reactants (PBDEs in waste PCBs) and promote the removal and decomposition reactions. In addition, ion product is a very important parameter for the chemical reaction. The ion product of supercritical fluid will increase greatly with the increase of pressure. Hence, critical methanol, like critical water, has certain acidbase catalytic properties (Kusdiana and Saka, 2004). The pressures inside the reactor at different temperatures are shown in Table 1. The pressure of the reaction system increased markedly with the increase of temperature. It sharply increased from 0.5 to 9.3 MPa when the reaction temperature increased from 100 to 300 °C. The higher pressure is very beneficial to the enhancement of the acid-base catalytic activity of critical methanol, and facilitates the decomposition and removal of PBDEs contained in waste PCBs. The RE of PBDEs from waste PCBs also dramatically increased with the increase of reaction time and the decrease of solid-to-liquid ratio (Fig. 2B and C). The optimized treatment conditions for PBDEs removal were 200 °C, 60 min, and 1:20 g/mL. Under the optimized P conditions of SubCM, 91.3% of the 8PBDEs could be removed from waste PCBs. Generally, the bromine in brominated flame retardants of waste PCBs exists as an acid species after debromination such as HBr. Therefore, the introduction of basic species into the reaction system may have a good capture effect on acidic species, and promote the debromination, decomposition and removal of PBDEs from waste PCBs. On the other hand, the participation of alkaline species in SubCM reaction is likely to further enhance the acid-base catalytic activity of critical methanol, and is also beneficial to the removal process of PBDEs. In this study, NaOH of 4 g/L was added
to the reaction system for the RE comparison of PBDEs between SubCM and SubCM + NaOH under the same reaction conditions. The results are shown in Fig. 2. It indicated that the introduction of NaOH could significantly improve the RE of PBDEs from waste PCBs. When the reaction conditions were controlled at 200 °C, 60 min, and 1:20 g/mL, the RE of PBDEs from waste PCBs by SubCM + NaOH treatment was approximately 100%. 3.2. Debromination and degradation pathway of PBDEs in waste PCBs by SubCM The chemical composition information of the debromination and degradation products of PBDEs after SubCM or SubCM + NaOH treatment at different temperatures can be obtained by GC-MS analysis of the oil. Fig. 3A shows the GC-MS results of the oil obtained from waste PCBs after SubCM treatment at 100 °C, and the relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after SubCM treatment are shown in Table 3. From the analysis result of GC-MS, the component of the oil mainly included low brominated PBDEs and Br-free decomposition products. Under the low-temperature condition of 100, 150, and 200 °C, two PBDEs congeners were generated in oil phase products: 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) and 2,4,40 Tribromodiphenyl ether (BDE28). The two PBDEs congeners generated at 100, 150, and 200 °C may be derived from the initial BDE71 and BDE28 in waste PCBs, which dissolved into the critical methanol medium during the treatment process. However, it can be found from Table 2 that the content of BDE71 and BDE28 accounts for only P 19.2% of the total 8PBDEs in the original waste PCBs. It also can be
195
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
Fig. 3. GC-MS analysis results of the oil obtained from waste PCBs after SubCM treatment at (A) 100 °C and (B) 300 °C.
Table 3 The relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after the SubCM treatment in the absence of NaOH. Compound
0
2,’3,4 ,6-Tetrabromodiphenyl ether (BDE71) 2,4,40 -Tribromodiphenyl ether (BDE28) 4-Bromophenyl ether (BDE4) Diphenyl ether Anisole 1-Methoxy-4-methyl-benzene 1-Methoxy-4-(1-methylethyl)-benzene Toluene o-Xylene
Relative peak area (%) 100 °C
150 °C
200 °C
250 °C
300 °C
30.71 20.04 0 0 0 0 0 0 0
27.16 35.34 0 0 0 0 0 0 0
23.07 38.47 0 0 0 0 0 0 0
0 0 19.37 16.65 13.43 12.51 0 1.43 1.51
0 0 0 6.08 20.19 12.31 6.17 9.42 9.96
P seen in Fig. 2 that more than 68.5, 83.6, and 91.3% of the total 8PBDEs are removed from the waste PCBs and transferred to the oil phase products after the reaction at 100, 150, and 200 °C, respectively. Therefore, it can be inferred that another important source of BDE71 and BDE28 formed in oil phase products after SubCM treatments is the debromination of PBDEs congeners containing more than four Br when the reaction temperature is below 200 °C. From the molecular structure, 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) can be degraded either from BDE154 or BDE209. However, the source of BDE28 was more complex in comparison with BDE71. It can be produced by the debromination of any of the following PBDEs congeners in original waste PCBs: BDE47, BDE99, BDE100, BDE153, BDE154, and BDE209. In addition to PBDEs congeners, it also can be found from Fig. 3A that the produced oil phase product contains another flame retardant frequently-used in electronic waste: phosphorous flame retardants. Three organophosphorus flame retardants such as triphenyl phosphate, 2-methylphenyl diphenyl phosphoric acid ester, and bis(4-methylphenyl) phenyl phosphoric ester could be identified in the oil obtained from waste PCBs after the SubCM process at 100 °C (Fig. 3A). It indicated that phosphorous flame retardants also could be efficiently removed from waste PCBs during the SubCM process at lower temperature. When the reaction temperature increased to 250 °C, an important change in the chemical composition of oil phase products is that 2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) and 2,4,40 Tribromodiphenyl ether (BDE28) disappeared, and only one Brcontaining chemical compound (4-Bromophenyl ether, BDE4) was detected. BDE4 was derived from the further debromination of BDE28 and BDE71. This indicated that the para-bromine in the molecule of PBDEs congeners is much more stable in comparison with other positions in the treatment process of SubCM. This
debromination characteristic is also observed in the previous report about the debromination pathways of PBDEs by the reductive debromination of microbial degradation (Zhao et al., 2018). At reaction temperature of 250 °C, diphenyl ether, the complete debromination product of PBDEs congeners, also was found in the oil phase product, indicating that the debromination performance of SubCM was more complete when compared to other reported methods of PBDEs debromination such as photocatalytic degradation (Miller et al., 2017) and thermocatalytic degradation (Yang et al., 2016). In the photocatalytic or thermocatalytic degradation, the bromination end product of PBDEs is usually the monobromodiphenyl ether (Yang et al., 2016; Miller et al., 2017). After the SubCM reaction at 300 °C, no Br-containing compound can be detected in the oil product. Fig. 3B shows the GC-MS analysis results of the oil phase product obtained from waste PCBs after the SubCM treatment at 300 °C. The relative peak areas of the debromination products of PBDEs in oil phase are presented in Table 3. In addition to diphenyl ether (the final bromination product of PBDEs at temperatures of 250 and 300 °C), several derivatives of benzene and anisole were produced in the oil phase product. The derivatives of benzene and anisole include anisole, 1-methoxy-4-methyl-benzene, 1-methoxy-4-(1-methylethyl)-ben zene, toluene, and o-xylene. The critical temperature and pressure of methanol are 240 °C and 8.1 MPa. From Table 1, it can be found that the reaction conditions of 250 and 300 °C approach or even exceed the critical point of methanol. It has been found that methanol molecule can be used as either reaction medium or reactant in the high-temperature super-critical state (Narayan et al., 2017; Alenezi et al., 2010). Supercritical methanol molecule can take part in a variety of reactions such as esterification, alkylation, and oxyalkylation. The ether bond of diphenyl ether, the final debromination product of PBDEs, has weaker bond energy (around
196
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
263 kJ/mol). The ether bond could be broken under the reaction condition of supercritical methanol, then the methylation reaction occurred and anisole was generated. The chemical compounds of 1-methoxy-4-methyl-benzene and 1-methoxy-4-(1-methylethyl)benzene were produced by the further alkylation of anisole. In addition, the toluene and o-xylene also could be generated by further decomposition and methylation of anisole. The debromination and degradation pathway of PBDEs in waste PCBs by SubCM treatment is described in Fig. 4. 3.3. Debromination and degradation pathway of PBDEs in waste PCBs by SubCM + NaOH To understand the influence of NaOH on the debromination and degradation of PBDEs in waste PCBs by SubCM process, 4 g/L of NaOH was added to the SubCM reaction system, and the GC-MS analysis of the oil after SubCM + NaOH treatment was conducted. The relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after the SubCM treatment in the presence of NaOH are shown in Table 4. At the treatment temperature of 100 °C and 150 °C, the main debromination and degradation products of PBDEs were 2,’3,40 ,6Tetrabromodiphenyl ether (BDE71), 2,4,40 -tribromodiphenyl ether (BDE28), 4,40 -dibromophenyl ether (BDE15), 4-bromophenyl ether (BDE4), 1,3,5-tribromo-2,4,6-trimethoxybenzene, and diphenyl ether. Low brominated PBDEs congeners, such as mono-BDE and di-BDE, were found in the products, which was completely different from the reaction system of SubCM in the absence of NaOH. In SubCM system, original PBDEs congeners in waste PCBs could only be degraded to Tetrabromo-BDE and Tribromo-BDE when temperature was lower than 200 °C (Table 3 and Fig. 3). In addition, the
Br
Br O
Br Br
BrBr O
Br Br Br
Br O
Br
BrBr O
Br Br
Br
BDE-28
BDE-47
BDE-71
BrBr O
Br Br
BDE-99
BDE-100
Br O
Br
Br Br O
Br
Br
Br
Br Br
completely debrominated product, diphenyl ether, was detected after SubCM + NaOH treatment at 100 °C (3.06% of relative peak areas) and 150 °C (4.96% of relative peak areas), indicating that the debromination ability of SubCM for PBDEs in waste PCBs could be efficiently enhanced by introducing alkaline NaOH. When the reaction temperature of SubCM + NaOH increased to 200 °C, the products mainly include 1,3,5-tribromo-2,4,6-trime thoxybenzene, diphenyl ether, and anisole. Anisole and diphenyl ether became the primary products with the temperature increasing to 250 °C, and no Br-containing compound could be detected. In all of the products, the relative peak areas of anisole and diphenyl ether reached 46.06% and 14.69%, respectively. The results showed that the PBDEs congeners in the initial sample of the waste PCBs could be completely debrominated and decomposed into diphenyl ether and anisole. Therefore, after NaOH was introduced into the reaction system of SubCM, the complete debromination could be realized at low temperature of 250 °C, and high level of anisole (46.06%) could be obtained and recovered at the same time. The complete debromination temperature of SubCM + NaOH for PBDEs could be reduced by 50 °C in comparison with SubCM (300 °C, Table 3). The facilitation function of NaOH for the debromination of PBDEs in SubCM can result from two reasons. On the one hand, the acid-base catalytic activity of critical methanol can be further enhanced by the introduction of alkaline NaOH, and the debromination of PBDEs can be enhanced efficiently. On the other hand, the capture function of alkaline NaOH for the acidic products such as HBr formed after the debromination of PBDEs, can accelerate the escape of the generated HBr from waste PCBs matrix. Then the reaction moved to the direction which was favorable to the bromination reaction of PBDEs. Thus, the debromination efficiency of PBDEs was significantly improved.
Br Br Br
BDE-153
Br O
Br Br O Br
Br Br Br Br
BDE-154
Br
Br
BDE-28
BDE-71
Br
O
O
Br
BDE-4
Br
BDE-15
O
CH3 OH
O
CH 3
CH3
O CH3
Fig. 4. Debromination and degradation pathway of PBDEs in waste PCBs by SubCM and SubCM + NaOH.
Br
Br Br O
Br
Br Br
Br
Br
BDE-209
197
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198 Table 4 The relative peak areas of the debromination and degradation products of PBDEs in waste PCBs after the SubCM treatment in the presence of NaOH (CNaOH = 4 g/L). Compound
Relative peak area (%) 100 °C
150 °C
200 °C
250 °C
2,’3,40 ,6-Tetrabromodiphenyl ether (BDE71) 2,4,40 -Tribromodiphenyl ether (BDE28) 4,40 -Dibromophenyl ether (BDE15) 4-Bromophenyl ether (BDE4) 1,3,5-Tribromo-2,4,6-trimethoxybenzene Diphenyl ether Anisole 1-Methoxy-4-methyl-benzene Toluene 1,3,5-Trimethyl-benzene o-Xylene
48.88 17.85 12.92 1.65 9.66 3.06 0 0 0 0 0
47.01 32.04 0 11.66 3.75 4.96 0 0 0 0 0
0 0 0 0 9.63 42.34 33.25 0 0 0 0
0 0 0 0 0 14.69 46.06 13.18 5.75 7.29 5.36
One interesting result is that a new chemical compound is formed after the SubCM + NaOH treatment in comparison with SubCM treatment (Table 3). It can be seen from Table 4 that the newly generated compound was 1,3,5-Tribromo-2,4,6-trimethoxy benzene. In terms of the molecular structure, 1,3,5-Tribromo-2,4, 6-trimethoxybenzene could not be produced from the debromination of any of the eight PBDEs congeners in the initial sample of waste PCBs. We inferred that it was formed by complicated multi-step reactions of debromination, ether bond cracking, methylation, and methoxylation. Fig. 1S presents the schematic diagram of the possible formation pathway of 1,3,5-Tribromo-2,4, 6-trimethoxybenzene during SubCM + NaOH process. Firstly, 2,4,6-tribromodiphenyl ether was generated from the debromination of BDE-154 or BDE-209. Then the Ph-O ether bond in 2,4,6-tribromodiphenyl ether was broken under the reaction conditions of critical methanol, and the (Br3)-Ph-O- group was formed. Simultaneously the generated (Br3)-Ph-O- group further reacted with critical methanol molecule to produce 2,4,6-tribromoanisole by the methylation reaction. Finally, the further methylation reaction on the meta-position of 2,4,6-tribromoanisole resulted in the generation of 1,3,5-tribromo-2,4,6-trimethoxybenzene. Several studies (Wang et al., 2007a, 2007b, 2007c) found that the reactivity of supercritical methanol molecule could be markedly enhanced by adding catalysts such as metallic oxide, organic amine, and alkali. Among these catalysts, alkali was found to have the highest reactivity. The alkaline catalyst such as NaOH not only had the excellent catalytic activity of transesterification for the preparation of biodiesel from rapeseed oil, but also did not cause saponification (Wang et al., 2007c). Therefore, the introduction of NaOH in the SubCM system could significantly promote the reactivity of methylation and methoxylation between PBDEs congeners and methanol molecules. This can explain the proposed formation pathway of 1,3,5-tribromo2,4,6-trimethoxybenzene from PBDEs in waste PCBs during SubCM + NaOH treatment. 4. Conclusions In this study, the SubCM process was tested for the first time to remove and degrade PBDEs contained in waste PCBs. In comparison with the reported works on the removal of low level of PBDEs in environmental media, high-content PBDEs in electronic waste could be efficiently removed and decomposed by the proposed SubCM process. In SubCM process, the increase of the reaction temperature/time and the decrease of the solid-to-liquid ratio could improve the removal efficiency of PBDEs from waste PCBs. The increase of reaction temperature could significantly enhance the debromination ability of SubCM. PBDEs congeners in waste PCBs could be degraded into monobromodiphenyl ether (BDE4) by SubCM at 250 °C, and the complete debromination of PBDEs could be realized at 300 °C. The increase of temperature could lead
to methylation and methoxylation reactions between debrominated products and critical methanol molecules. The introduction of alkaline additive NaOH in SubCM process could significantly enhance the removal and degradation of PBDEs in waste PCBs. The complete debromination of PBDEs by SubCM + NaOH process could be achieved at lower temperature (250 °C) in comparison with SubCM process due to the enhancement effect of NaOH on the acid-base catalytic activity of critical methanol molecule. According to the degradation products after treatments, no significant difference could be found for debromination and degradation pathway of PBDEs between SubCM and SubCM + NaOH process. Acknowledgements This work was supported financially by the National Natural Science Foundation of China (No. 21605018), the Natural Science Basic Research Project of Shaanxi Province of China (No. 2018JM5149), the Foundation Research Project of Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation (No. MTy2019-05), and the Natural Science Foundation of Fujian Province of China (No. 2017 J01472). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.09.022. References Alharbi, O.M.L., Basheer, A.A., Khattab, R.A., Ali, I., 2018. Health and environmental effects of persistent organic pollutants. J. Mol. Liq. 263, 442–453. Alenezi, R., Leeke, G.A., Winterbottom, J.M., Santos, R.C.D., Khan, A.R., 2010. Esterification kinetics of free fatty acids with supercritical methanol for biodiesel production. Energ. Convers. Manage. 51, 1055–1059. Asahi, N., Nakamura, Y., 1998. Chemical shift study of liquid and supercritical methanol. Chem. Phys. Lett. 290, 63–67. Cao, C.Q., Xu, L., He, Y., Guo, L., Jin, H., Huo, Z., 2017. High-efficiency gasification of wheat straw blackliquor in supercritical water at high temperatures for hydrogenproduction. Energ. Fuel. 31, 3970–3978. Cowell, W.J., Sjodin, A., Jones, R., Wang, Y., Wang, S., Herbstman, J.B., 2018. Determinants of prenatal exposure to polybrominated diphenyl ethers (PBDEs) among urban, minority infants born between 1998 and 2006. Environ. Pollut. 233, 774–781. Duan, H.B., Li, J., Liu, Y., Yamazakib, N., Jiang, W., 2012. Characterizing the emission of chlorinated/brominated dibenzo-p-dioxins and furans from low-temperature thermal processing of waste printed circuit board. Environ. Pollut. 161, 185– 191. Erdocia, X., Prado, R., Fernández-Rodríguez, J., Labidi, J., 2016. Depolymerization of different organosolv lignins in supercritical methanol, ethanol, and acetone to produce phenolic monomers. ACS Sustain. Chem. Eng. 4, 1373–1380. Guo, J., Zhang, R., Xu, Z., 2015. PBDEs emission from waste printed wiring boards during thermal process. Environ. Sci. Technol. 49, 2716–2723. Gupta, V.K., Jain, R., Nayak, A., Agarwal, S., Shrivastava, M., 2011. Removal of the hazardous dye-tartrazine by photodegradation on titanium dioxide surface. Mat. Sci. Eng. C 31, 1062–1067.
198
F.-R. Xiu et al. / Waste Management 100 (2019) 191–198
Hadi, P., Xu, M., Lin, C.S.K., Hui, C.W., McKay, G., 2015. Waste printed circuit board recycling techniques and product utilization. J. Hazard. Mater. 283, 234–243. He, J.Z., Robrock, K.R., Alvarez-Cohen, L., 2006. Microbial reductive debromi-nation of polybrominated diphenyl ethers (PBDEs). Environ. Sci. Techno. 40, 4429– 4434. Kusdiana, D., Saka, S., 2004. Effects of water on biodiesel fuel production by supercritical methanol treatment. Bioresource Technol. 91, 289–295. Liu, Y.M., Chen, S., Quan, X., Fan, X., Zhao, H., Zhao, Q., Yu, H., 2014. Nitrogen-doped nanodiamond rod array electrode with superior performance for electroreductive debromination of polybrominated diphenyl ethers. Appl. Catal B: Environ. 154–155, 206–212. Lu, Y., Xu, Z.M., 2016. Precious metals recovery from waste printed circuit boards: A review for current status and perspective. Resour. Conserv. Recy. 113, 28–39. Miller, E.B., Zahran, E.M., Knecht, M.R., Bachas, L.G., 2017. Metal oxide semiconductor nanomaterial for reductive debromination: Visible light degradation of polybrominated diphenyl ethers by Cu2O@Pd nanostructures. Appl. Catal. B: Environ. 213, 147–154. Narayan, R.C., Lamba, N., Javed, A., Madras, G., 2017. Kinetics of esterificat-ion of 10undecenoic and ricinoleic acids with near-critical and supercritical methanol. Sustain. Energ. Fuels 1, 1425–1436. Qi, Y., He, J., Xiu, F., Nie, W., Chen, M., 2018. Partial oxidation treatment of waste polyvinyl chloride in critical water: preparation of benzaldehyde/acetophenone and dichlorination. J. Clean. Prod. 196, 331–339. Qi, Y., He, J., Xiu, F., Yu, X., Gao, X., Li, Y., Lu, Y., Song, Z., 2019. A convenient chemiluminescence detection for bisphenol A in E-waste dismantling site based on surface charge change of cationic gold nanoparticles. Microchem. J. 147, 789–796. Qi, Y., He, J., Xiu, F.-R., Yu, X., Li, Y., Lu, Y., Gao, X., Song, Z., Li, B., 2019. A facile chemiluminescence sensing for ultrasensitive detection of heparin using charge effect of positively-charged AuNPs. Spectrochim. Acta A 216, 310–318. Rajendran, S., Khan, M.M., Gracia, F., Qin, J., Gupta, V.K., Arumainathan, S., 2016. Ce3+-ion-induced visible-light photocatalytic degradation and electrochemical activity of ZnO/CeO2 nanocomposite. Sci. Rep. 6, 31641. Robrock, K.R., Korytár, P., Alvarez-Cohen, L., 2008. Pathways for the anaerobic microbial debromination of polybrominated diphenyl ethers. Environ. Sci. Technol. 42, 2845–2852. Shen, Y., 2018. Effect of chemical pretreatment on pyrolysis of non-metallic fraction recycled from waste printed circuit boards. Waste Manage. 76, 537–543. Sun, Z.Y., Fu, L., Liu, Z., Han, B., Liu, Y., Du, J., 2006. Synthesis of nobl-e metal/carbon nanotube composites in supercritical methanol. J. Nanosci. Nanotechno. 6, 691– 697. Wang, L.Y., He, H., Xie, Z., Yang, J., Zhu, S., 2007a. Transesterification of the crude oil of rapeseed with NaOH in supercritical and subcritical methanol. Fuel Process. Technol. 88, 477–481.
Wang, L.Y., Tang, Z., Xu, W., Yang, J., 2007b. Catalytic transesterification of crude rapeseed oil by liquid organic amine and co-catalyst in supercritical methanol. Catal. Commun. 8, 1511–1515. Wang, L.Y., Yang, J.C., 2007. Transesterification of soybean oil with nano-MgO or not in supercritical and subcritical methanol. Fuel. 86, 328–333. Wang, R., Tang, T., Lu, G., Huang, K., Yin, H., Lin, Z., Wu, F., Dang, Z., 2018. Rapid debromination of polybrominated diphenyl ethers (PBDEs) by zero valent metal and bimetals: mechanisms and pathways assisted by density function theory calculation. Environ. Pollut. 240, 745–753. Xiu, F.R., Li, Y., Qi, Y., Yu, X., He, J., Lu, Y., Gao, X., Deng, Y., Song, Z., 2019. A novel treatment of waste printed circuit boards by low-temperature near-critical aqueous ammonia: Debromination and preparation of nitrogen-containing fine chemicals. Waste Manage. 84, 355–363. Xiu, F.R., Qi, Y., Wang, S., Nie, W., Weng, H., Chen, M., 2018. Application of critical water-alcohol composite medium to treat waste printed circuit boards: oil phase products characteristic and debromination. J. Hazard. Mater. 344, 333– 342. Xiu, F.R., Weng, H., Qi, Y., Yu, G., Zhang, Z., Zhang, F.-S., Chen, M., 2017. A novel recovery method of copper from waste printed circuit boards by supercritical methanol process: preparation of ultrafine copper materials. Waste Manage. 60, 643–651. Xiu, F.R., Zhang, F.S., 2010. Materials recovery from waste printed circuit boards by supercritical methanol. J. Hazard. Mater. 178, 628–634. Yang, F., Li, Q., Su, G., Huang, X., Li, B., Zhao, Y., Miao, X., Zheng, M., 2016. Thermal degradation of 2,2’,4,4’-tetrabromodiphenyl ether (BDE-47) over synthesized Fe-Al composite oxide. Chemosphere. 150, 445–452. Zhang, S., Li, Y., Wang, R., Xu, Z., Wang, B., Chen, S., Chen, M., 2017. Superfine copper powders recycled from concentrated metal scraps of waste printed circuit boards by slurry electrolysis. J. Clean. Prod. 152, 1–6. Zhang, Y.H., Liu, S., Xie, H., Zeng, X., Li, J., 2012. Current status on leaching precious metals from waste printed circuit boards. Procedia Environ. Sci. 16, 560–568. Zhao, S.Y., Rogers, M.J., Ding, C., He, J., 2018. Reductive debromination of polybrominated diphenyl ethers-microbes, processes and dehalogenases. Front. Microbiol. 9, 1292-. Zhuang, Y., Ahn, S., Luthy, R.G., 2010. Debromination of polybrominated diphenyl ethers by nanoscale zerovalent iron: pathways, kinetics, and reactivity. Environ. Sci. Technol. 44, 8236–8242. Zhu, X., Beiyuan, J., Lau, A.Y.T., Chen, S.S., Tsang, D., Graham, N.J.D., Lin, D., Sun, J., Pan, Y., Yang, X., Li, X., 2018. Sorption, mobility, and bioavailability of PBDEs in the agricultural soils: Roles of co-existing metals, dissolved organic matter, and fertilizers. Sci. Total Environ. 619–620, 1153–1162. Zou, D., Chi, Y., Fu, C., Dong, J., Wang, F., Ni, M., 2013. Co-destruction of organic pollutants in municipal solid waste leachate and dioxins in fly ash under supercritical water using H2O2 as oxidant. J. Hazard. Mater. 248–249, 177–184.