Elements in the Crystals Determined the Distribution of Bromine in

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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Elements in the Crystals Determine the Distribution of Bromine in Nonmetallic Particles of Crushed Waste Printed Circuit Boards Jujun Ruan,* Jiaxin Huang, Zhihui Yuan, and Rongliang Qiu Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, School of Environmental Science and Engineering, Sun Yat-Sen University, 135 Xingang Xi Road, Guangzhou 510275, People’s Republic of China

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S Supporting Information *

ABSTRACT: The recovery of waste printed circuit boards (WPCBs) has been a hot topic in the area of environmental science. WPCBs contain heavy metals (Cu, Zn, Pb, etc.) and nonmetals (fiberglass, plastics, and additives). Many technologies have been developed to recover the metals. However, little attention has been given to the disposal of nonmetals. Nonmetallic fractions originating from crushed WPCBs have become one of the largest hazardous waste streams during the recycling of e-wastes in China. Bromine is a major hazardous element in the plastic additives. In this study, we discovered an interesting phenomenon that bromine preferred to concentrate on the surface of fiberglass particles. The causes of this phenomenon were explained from the views of crystal structure of the nonmetallic particles and electron orbitals of the elements. We found that glass fibers have the ability to absorb bromine, because of chemical and physical adsorption. Na and Ca in the crystal structure of fiberglass captured bromine on the surface of fiberglass. Bromine in the nonmetals of crushed WPCBs can be removed by separating the fiberglass particles. This discovery contributes to the development of green technology for removing bromine from crushed WPCBs. Meanwhile, the toxicity of bromine should be a concern in regard to the reuse of fiberglass. KEYWORDS: WPCBs, Nonmetallic particles, Bromine distribution, Crystal structure, Fiberglass

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INTRODUCTION E-waste recovery has become a severe global issue, because of its rapid and huge increase.1,2 SInce it contains a huge amount of renewable resource, waste printed circuit boards (WPCBs) have attracted more and more attention. The main compositions of WPCBs are copper foil, bromine resin, fiberglass, and solder (see Figure 1b).3 Physical, chemical, and biological methods have been developed to recover metals from WPCBs.4,5 WPCBs are always crushed into mixed particles. The metallic fractions and nonmetallic fractions then are separated by mechanical separation.6 Metallic fractions have attracted much attention, because of the huge economic benefits. However, little attention has been paid to nonmetallic fractions. In fact, nonmetallic fractions account for 60−70 wt % of WPCBs.7 They are mainly comprised of resin, fiberglass, and the residual metals. Toxic organic substances in nonmetallic fractions limit their reuse. Brominated flame retardants (BFRs) are toxic organic substances, which will cause great harm to the environment and human health.8,9 Because of higher toxicity and lower economic value, few technologies are developed to recover nonmetallic fractions of WPCBs. In China, nonmetallic fractions are often dumped directly on the ground or openly incinerated.10 Many BFRs are released into the soil, groundwater, and atmosphere.11,12 In Guiyu, the concentration of PBDEs detected in residential areas is more than 2000 ng/g in 2014.8 Some studies focus on © XXXX American Chemical Society

recovering nonmetallic fractions by pyrolysis, but polybrominated dibenzo-p-dioxins, dibenzofurans (PBDD/Fs), and other toxic pollutants are still formed during pyrolysis.13,14 Therefore, green and effective recovery technologies are required for recovery nonmetallic fractions of WPCBs. To develop a suitable recovery technology of nonmetallic fractions, the toxic element must be removed first. Understanding the distribution of toxic element in nonmetallic fractions has great significance. This paper provided the information about the distribution of bromine in the nonmetallic particles of crushed WPCBs. Meanwhile, the reason for this distribution was discussed. This paper contributes to develop green technology of disposing nonmetallic fraction of crushed WPCBs.

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MATERIALS AND METHODS

The nonmetallic particles were collected from the crushed WPCBs (see Figure 1a). The WPCBs were mainly collected from waste televisions and some from waste computers, which were provided by an electronic waste recycling site in Guangzhou (Kang Xiang Metal Recycling Co., Ltd.). WPCBs were first crushed into mixed particles 1−2 mm in size. They then were separated into metallic particles and nonmetallic fractions by magnetic separation and corona electrostatic separation.15 The separation rate of metallic and nonmetallic particles Received: August 7, 2018 Revised: October 13, 2018 Published: October 16, 2018 A

DOI: 10.1021/acssuschemeng.8b03884 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. (a) SEM image of fine nonmetallic particles, (b) microstructure of printed circuit board, and (c) SEM images of particle (1), (2), and (3).

Table 1. Elemental Distribution of the Three Fine Nonmetallic Particles Composition (wt %) component

C

O

Si

Pb

Al

Br

Ca

others

① solder particles ② resin powders ③ fiberglass powders

0.00 32.80 37.89

17.44 29.06 27.20

17.57 9.03 13.03

55.30 10.02 0.00

0.26 3.49 2.84

0.00 0.00 7.81

0.00 1.96 7.79

9.69 15.6 14.07

can reach ∼98.8%.16 Nonmetallic fractions were mainly comprised of resin, fiberglass, and a small amount of unseparated metals. The nonmetallic particles were sieved using 360 mesh screen to obtain fine nonmetallic particles. The surface morphology and elemental distribution analysis of nonmetallic particles were performed using a scanning electron microscopy (SEM) microscope (Zeiss, Model EVO MA-10) that was equipped with an energy-dispersive spectrometry (EDS) system (OXFORD, Model INCA ENERGY 250). Microstructural characterization at high magnification was performed using SEM-EDS (FEI/OXFORD/HKL, Model Quanta 400F). The surface chemical composition of product composed of nonmetallic particles was measured by X-ray photoelectron spectroscopy (XPS) (Model ESCALAB 250, Thermo-VG Scientific, USA). All the apparatuses were provided by the Instrument Analysis and Research Center at Sun Yat-Sen University.

separating. After being sieved, particles with diameters of 95 wt %.17,18 Generally, the BFRs would concentrate in resin particles. Therefore, the purpose of separating fiberglass was to remove fiberglass from the pollutants of BFRs. The fiberglass might be reused with less concern, with regard to environmental risks. However, is it credible to believe that the fiberglass powders are free of BFRs? A Discovery about the Concentration of Bromine on Fiberglass Particles. To confirm this case, we collected nonmetallic particles from WPCBs after crushing and B


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Figure 2. SEM pictures and EDS mapping of fiberglass and resin in fine nonmetallic particles: (1) top, left: SEM pictures; (2) bottom, left: integrated mapping of elemental Al, Ca, Br, Si, Fe, Cl and S; and (3) Right: individual mapping of Br, Na, and Ca.

Figure 3. Survey scan XPS spectrum of (a) metallic particles and (b) nonmetallic particles of crushed WPCBs; (c) Br 3d XPS spectrum of nonmetallic particles and (d) Cu 2p XPS spectrum of metallic particles and nonmetallic particles.

To give more convincing evidence, an EDS mapping of Br and other element of nonmetallic particles was performed. The result is shown in Figure 2, as well as Table S1 in the Supporting Information. The distributions of elemental Na, Ca, and Br on the surface of fiberglass and resin indicated that the elemental Ca and Na obviously originate from fiberglass,

and Br was mostly distributed on the resin surface. However, there was still some dark color in the reign of fiberglass of Br mapping, indicating some bromine transferred from resin to fiberglass. Furthermore, to investigate whether BFR transferred to the surface of metallic particles, XPS analysis was conducted on C


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relatively weak bond. This crystal structure of fiberglass provided the chance for Br atoms to combine with Ca and Na atoms. The resin particle was comprised of epoxy resin and the additive of BFRs. The main elements contained in resin particle were carbon, oxygen, hydrogen, and bromine. Why did elemental bromine present a high concentration on the surface of fiberglass particle? Generally, BFRs were added into the resin to decrease the combustibility of PCBs. Bromine should be concentrated on the resin particle, not on the surface of the fiberglass particle. We found that the crystal structure and the elements contained in fiberglass caused this interesting phenomenon. It is well-known that the crushing process generates high temperature in the cavity of crusher. This temperature might reach ∼500−600 °C if there is no condensing system on the crusher. Even though the condensing system is fixed, the temperature will still reach ∼150−200 °C. The high temperature will destroy the structure of BFRs molecule in resin. Most of BFRs has a lower volatilization and decomposition temperature than the resin and will release easier from resin particles, because of their poor thermal stability. Meanwhile, the mechanism of crushing involves the use of mechanical force to destroy the molecular bond and liberate the chemical elements. Therefore, because of the weaker bond energy of the C−Br bond, the C−Br bond in the resin will break first. Abundant free-radical bromine and volatile BFRs molecular will exist in the cavity of the crusher. Thus, mechanical crushing is not a perfectly safe technology to liberate the materials of e-waste. After crushing, when the temperature returned to room temperature and there is no mechanical force to destroy the molecular bond. Bromine free radical will combine with the other elements to form a relative stable substance. From the view of the orbital distribution of electron of bromine element, we found that the outermost electron of elemental bromine was in the 4p orbital and there was only one electron in the third orbital of 4p. According to the principle of minimum energy, the single outermost electron prefers to pair with another single electron in the outmost electron orbital of the other element. In consideration of the principle of minimum energy, the single electron in the third orbital of 4p of bromine prefers to pair with the single electron in the orbital of s of the other element. Thus, among the elements contained in the fiberglass, resin, and solder, bromine prefers to combine with hydrogen, sodium, calcium, and copper. Thus, in the nonmetallic particles of crushed WPCBs, there was no bromine attached on a solder particle. In addition, compared to the elemental hydrogen and sodium, the 4s orbital energy of elemental copper is higher than that of the 1s orbital of hydrogen, the 3s orbital of sodium, and the 4s orbital of calcium. Therefore, bromine preferred to combine with hydrogen, calcium, and sodium than copper. However, the H atoms in resin polymers were combined with C tightly, so the Br free radical was easier to combine with calcium and sodium atom in glass fiber. Thus CaBr2 and NaBr can be formed by strong chemical adsorption. Meanwhile, when the temperature returned to room temperature, the volatile gaseous BFRs would condense again. According to the principle of minimum energy, BFRs and free radical bromine in the cavity prefers to attach onto the solid surface to reduce surface energy. Thus physical adsorption occurred due to the attraction of intermolecular force. Metal surface may have weaker ability to capture BFRs, because of its smaller specific surface area and lesser chance of contact than the nonmetal fraction. BFRs are mainly absorbed on the surface of resin and

metallic particles separated from WPCBs. The results are presented in Figure 3a. The elemental contents of C and O were high, because of the adsorption of CO2 and O2 in air bythe particle surface. Cu and Mg were the main metals of metallic particles separated from crushed WPCBs; Br was not detected. Thus, the recovered metallic particles from crushed WPCBs have an absence of Br and can be directly used for the metal refining process. XPS analysis was also performed on nonmetallic particles separated from crushed WPCBs. The results were presented in Figure 3b. Bromine and lead were detected and their contents were ∼0.47 at. % and 0.1 at. %. It indicated that solder and BFRs were contained in the nonmetallic particles. This was in agreement with the results of EDS analysis. According to these results, as an accumulation place of toxic substances, nonmetallic particles of WPCBs should be concerned as hazardous waste. The results of XPS analysis of metallic particles and nonmetallic particles of crushed WPCBs reconfirmed that the liberated elemental bromine had a tendency to concentrate on the surface of nonmetallic particles. From the Br 3d XPS spectrum (Figure 3c), we can find that the Br 3d binding energy of the brominated compound in nonmetallic particles was 68.20 and 70.77 eV. The former value (68.20 eV) was similar to the Br 3d5/2 binding energy of KBr (68.7 eV), which can represent inorganic bromine in nonmetallic particles.19 The other value, 70.77 eV, was similar to the Br 3d5/2 binding energy of methyl bromide (70.75 eV). This peak can represent the organic bromine compounds (BFRs) in nonmetallic particles. It means that, after crushing, part of the organic bromine in BFR changed to inorganic bromine. Figure 3d shows the Cu 2p XPS spectrum of metallic particles and nonmetallic particles. The intensity of the Cu 2p peak of metallic particles was significantly higher than that of nonmetallic particles. For metallic particles, the Cu 2p binding energy of copper compound was 932.8 and 934.6 eV. The former value (932.8 eV) was close to the Cu 2p1/2 binding energy of Cu (932.7 eV), and the latter value (934.6 eV) was close to the Cu 2p3/2 binding energy of CuO (933.8 eV). For nonmetallic particles, the Cu 2p binding energy of the copper compound in nonmetallic particles was 933.7 and 935.1 eV, which was close to Cu 2p3/2 binding energy of CuO (933.8 eV) and CuSiO2(OH)2 (935.2 eV). It indicates that part of copper particles might be oxidized to Cu(II) oxide during crushing, and more Cu(II) oxide and few copper in nonmetallic particles existed. Reason for the Concentration of Bromine on Fiberglass. To explain this reason, we should gain the elements and crystal structures of the particles of crushed WPCBs. According to the components of the WPCBs, copper particle, solder particle, resin particle, and fiberglass particle were contained in the crushed WPCBs. A certain portion of the copper particle might be changed to copper oxide in the air. Based on the results of EDS and XPS, the main element of the solder particle was lead. It is known that fiberglass is comprised of a large amount of SiO2 and Al2O3 and a small amount of additive (CaO, Na2O). Therefore, sodium, silicon, aluminum, calcium, and oxygen are the main elements of fiberglass. SiO2 is the skeleton of glass fiber while CaO and Na2O exist in the form of Ca, Na, and O atoms among the net structure of SiO2 skeleton. The crystal structure of fiberglass was given in Figure S1 in the Supporting Information. It can be seen that abundant Ca and Na atoms filled in the net structure of SiO2, and most of the Ca and Na atoms bonded with O atoms in the net structure with a D



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glass fibers. According to this principle, there is no bromine element detected by XPS in the metallic particles of crushed PCBs. Meanwhile, it is said that the metallic particles can be recycled by neglecting the toxicity of bromine. The bromine flame retardants are enclosed in the structure of epoxy resin. Thus, in the incision of the resin particle in crushing, Br atoms and volatile BFRs will release from the resin particle. In the uncrushed portion of the resin particle, Br atoms are still enclosed by epoxy resin molecule structure. Therefore, there is less Br atoms on the surface of resin particles. Based on the above analysis, we conclude that abundant bromine is released during the crushing process of WPCBs. Besides some reported brominated organic matter, the most probable destination of the released elemental bromine is to combine with calcium and sodium to form CaBr2 and NaBr. The formation of CaBr2 and NaBr is the combination of the released Br atoms with the Ca and Na atoms that originate from the surface of fiberglass particles. Therefore, Br atoms are attached to and concentrated on the surface of fiberglass particles. Meanwhile, physical adsorption of BFRs also occurred on the surface of fiberglass particles. In conclusion, the glass fibers have the ability to absorb bromine, because of chemical and physical adsorption. The Contribution of This Discovery. Fine nonmetallic particles of WPCBs contain a considerable portion of toxic substance (lead and BFRs) and have the potential to release hazardous substances, which will cause serious environmental pollution. Numerous of nonmetallic particles of WPCBs have been dumped in China. Their disposal is urgent. However, there is little suitable technology to dispose of these huge amounts of nonmetallic particles. We found that abundant hazardous element of bromine were concentrated on the surface of fiberglass. In other words, if fiberglass is separated from the nonmetallic particles of WPCBs, most of the hazardous materials will be removed from the nonmetallic fractions. The residual resin particles can be reused as lowquality plastic particles. The fiberglass particles also can be reused in the removal of elemental bromine. In addition, the separation technologies, which are based on the principles of electrical conductivity and density, may be suitable for separating fiberglass particles from the nonmetallic particles of WPCBs. No matter which method will be employed, the bromine that is attached to fiberglass particles must be carefully treated.

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ACKNOWLEDGMENTS This work was supported by the 111 Project (No. B18060), the Science and Technology Programs of Guangdong Province (Nos. 2015B020237005 and 2016A020221014), the Pearl River Star of Science and Technology (No. 201710010032), the Fundamental Research Funds for the Central Universities (No. 17lgzd22).

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REFERENCES

(1) Ruan, J.; Xu, Z. Constructing environment-friendly return road of metals from e-waste: Combination of physical separation technologies. Renewable Sustainable Energy Rev. 2016, 54, 745−760. (2) Zeng, X.; Mathews, J.; Li, J. urban mining of e-waste is becoming more cost-effective than virgin mining. Environ. Sci. Technol. 2018, 52, 4835−4841. (3) Wang, R.; Xu, Z. Recycling of non-metallic fractions from waste electrical and electronic equipment (WEEE): a review. Waste Manage. 2014, 34, 1455−1469. (4) Ruan, J.; Zhu, X.; Qian, Y.; Hu, J. A new strain for recovering precious metals from waste printed circuit boards. Waste Manage. 2014, 34, 901−907. (5) Zhan, L.; Li, J.; Xie, B.; Xu, Z. Recycling arsenic from gallium arsenide scraps through sulfurizing thermal treatment. ACS Sustainable Chem. Eng. 2017, 5, 3179−3185. (6) Li, J.; Xu, Z. Environmental friendly automatic line for recovering metal from waste printed circuit boards. Environ. Sci. Technol. 2010, 44, 1418−1423. (7) Guo, J.; Jiang, Y.; Hu, X.; Xu, Z. Volatile organic compounds and metal leaching from composite products made from fiberglass-resin portion of printed circuit board waste. Environ. Sci. Technol. 2012, 46, 1028−1034. (8) Zhang, S.; Xu, X.; Wu, Y.; Ge, J.; Li, W.; Huo, X. Polybrominated diphenyl ethers in residential and agricultural soils from an electronic waste polluted region in South China: Distribution, compositional profile, and sources. Chemosphere 2014, 102, 55−60. (9) Ebert, J.; Bahadir, M. Formation of PBDD/F from flameretarded plastic materials under thermal stress. Environ. Int. 2003, 29, 711−716. (10) Lu, S.; Li, Y.; Zhang, T.; Cai, D.; Ruan, J.; Huang, M.; Wang, L.; Zhang, J.; Qiu, R. Effect of e-waste recycling on urinary metabolites of organophosphate flame retardants and plasticizers and their association with oxidative stress. Environ. Sci. Technol. 2017, 51, 2427−2437. (11) Yu, D.; Duan, H.; Song, Q.; Liu, Y.; Li, Y.; Li, J.; Shen, W.; Luo, J.; Wang, J. Characterization of brominated flame retardants from ewaste components in China. Waste Manage. 2017, 68, 498−507. (12) Sun, B.; Hu, Y.; Cheng, H.; Tao, S. Kinetics of brominated flame retardant (BFR) releases from granules of waste plastics. Environ. Sci. Technol. 2016, 50, 13419−13427. (13) Guo, J.; Zhang, R.; Xu, Z. PBDEs emission from waste printed wiring boards during thermal process. Environ. Sci. Technol. 2015, 49, 2716−2723. (14) Gao, W.; Song, J.; Cao, H.; Lin, X.; Zhang, X.; Zheng, X.; Zhang, Y.; Sun, Z. Selective recovery of valuable metals from spent lithium-ion batteries−Process development and kinetics evaluation. J. Cleaner Prod. 2018, 178, 833−845. (15) Veit, H. M.; Diehl, T. R.; Salami, A. P.; Rodrigues, J. S.; Bernardes, A. M.; Tenório, J. A. S. Utilization of magnetic and electrostatic separation in the recycling of printed circuit boards scrap. Waste Manage. 2005, 25, 67−74. (16) Wu, J.; Li, J.; Xu, Z. Electrostatic separation for recovering metals and nonmetals from waste printed circuit board: problems and improvements. Environ. Sci. Technol. 2008, 42, 5272−5276. (17) Li, J.; Gao, B.; Xu, Z. New technology for separating resin powder and fiberglass powder from fiberglass-resin powder of waste printed circuit boards. Environ. Sci. Technol. 2014, 48, 5171−5178. (18) Li, J.; Gao, K.; Xu, Z. Charge-decay electrostatic separation for removing Polyvinyl chloride. J. Cleaner Prod. 2017, 157, 148−154.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b03884.

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Letter

Crystal structure of fiberglass in the crushed WPCBs (Figure S1) and EDS element analysis of the mapping area of Figure 2 (Table S1) (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: + 86 20 84113620. Fax: +86 20 84113620. E-mail: [email protected]. ORCID

Jujun Ruan: 0000-0001-8194-2988 Notes

The authors declare no competing financial interest. E


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ACS Sustainable Chemistry & Engineering (19) National Institute of Standards and Technology (NIST). NIST X-ray Photoelectron Spectroscopy Database. Available via the Internet at: https://srdata.nist.gov/xps/Default.aspx.

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Elements in the Crystals Determined the Distribution of Bromine in

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