A resource-utilization way of the waste printed circuit boards to prepare silicon carbide nanoparticles and their photocatalytic application

Journal of Hazardous Materials 373 (2019) 640–648

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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

A resource-utilization way of the waste printed circuit boards to prepare silicon carbide nanoparticles and their photocatalytic application

T

Juan Yanga,b, Jinxi Fenga,b, Waiqing Lia,b, Xixi Chena,b, Xiaosheng Liua,b, Jujun Ruana,b, ⁎ ⁎ Rongliang Qiua,b, Ya Xionga,b, , Shuanghong Tiana,b, a b

School of Environmental Science and Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, PR China Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Printed circuit boards (PCBs) Resourcelization Silicon carbide (SiC) p-type photocatalyst

A resource-utilization strategy of the waste PCBs was developed: preparation of high value-added silicon carbide (SiC) nanoparticles using the waste PCBs as both silica and carbon precursors. The preparation process contained three optimized steps: acid wash pretreatment with 3 mol L−1 nitric acid at 60 °C for 96 h, low-temperature pyrolysis at 500 °C to allow the epoxy resin to decompose into carbon, and high-temperature pyrolysis at 1600 °C (in situ carbothermal reduction) to gain pure SiC nanoparticles. The pseudo first-order reaction rate constant (k) of the p-n heterojunction of SiC/TiO2 towards the photocatalytic degradation of methylene blue was 0.0219 min−1, 3.42 and 3.98 times that of TiO2 and no acid washed-SiC/TiO2, respectively.

1. Introduction Waste Electrical and Electronic Equipment (WEEE) is an emerging global issue that its growing rate remains at an exponentially pace. Printed circuit boards (PCBs) are the major components in almost every WEEE. The waste PCBs not only impose damage to the environment and human health but also are rich in reusable resources, therefore,

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researchers, industry manufacturers and government are showing more and more interests in resourcing the waste PCBs [1–3]. The waste PCBs are generally consisted of two main components, namely, the metallic fractions (MFs, 30 wt.%) and the nonmetallic fractions (NMFs, 70 wt.%). In light of the high market value of the MFs, various techniques, such as hydrometallurgical processes, chemical leaching, bio-leaching, organic solvent-leaching, pyrometallurgical

Corresponding authors at: School of Environmental Science and Engineering, Sun Yat-Sen (Zhongshan) University, Guangzhou, 510275, PR China. E-mail addresses: [email protected] (Y. Xiong), [email protected] (S. Tian).

https://doi.org/10.1016/j.jhazmat.2019.03.115 Received 13 August 2018; Received in revised form 25 March 2019; Accepted 26 March 2019 Available online 28 March 2019 0304-3894/ © 2019 Published by Elsevier B.V.

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waste PCBs via a three-step process, in which much attention will be paid to explore the effects of acid wash pre-treatment of PCBs, (2) construction of a p-n heterojunction photocatalyst between the prepared SiC and the classic TiO2; (3) activity characterization of the nanoSiC as a p-type semiconductor photocatalyst. The aim of these investigations was to develop a resource way of waste printed circuit boards to prepare high value-added SiC nanoparticles with p-type semiconductor photocatalytic activity.

treatment, pulverization, plasma treatment, have been developed for recovering expensive metals such as Cu, Au, and so on [2,4–8]. In contrast, the NMFs, consisting of epoxy resin, glass fiber and inorganic additives, are generally treated by landfill or incineration [9–11]. Due to the current constraints for landfill disposal and incineration, serious environmental pollution, as well as the economic incentives, more sustainable physical and chemical methods have been developed for recycling NMFs. The epoxy resin as one of the main components in NMFs is thermosetting and crosslinked, hence it is impossible to separate the epoxy resin with the glass fiber (another main NMF) by melting or dissolving the epoxy resin. As a typical physical recycling method, NMFs powder as mechanic filler for reinforced polymer, decorating agent, adhesives, insulators and similar construction materials are widely investigated [12–16]. Chemical recycling methods including pyrolysis, gasification, and activation, are also studied. Hadi et al. prepared NMFs adsorbents by activating the mixture of NMFs and KOH at 250 °C [12]. Du et al. prepared porous carbons from NMFs by carbonization and activation with KOH at 800 °C [17]. Sun et al. recovered the glass fibers from the NMFs by breaking down the plastic fraction at 500 °C in a fluidized bed [18]. Rajagopal et al. recycled the glass fibers by pyrolyzing the NMFs at 1000 °C and then mixed the glass fibers, external silica and char to produce glass fiber-nano silicon carbide composite [19]. Gao synthesized oil-based resin using pyrolysis oil produced by pyrolysis of PCBs [20]. However, the production of high value-added and pure SiC using the PCBs as both carbon and silicon resources have not been reported in the literatures. SiC materials have been widely used for various applications including photocatalytic processes for water purification. Since the first synthesis of SiC nanorods through reacting carbon nanoclusters with SiO at 1700 °C by Zhou et al. in 1994 [21], various methods in production of SiC materials have been developed, such as carbothermal reduction, solvothermal and hydrothermal synthesis, electrochemical and chemical etching, chemical vapor deposition, pyrolysis of different organic precursors, inorganic molten salt (MS) synthesis [22], nanocasting, sol–gel, vapor–liquid–solid, laser ablation [23], magnesiothermic reduction (MR) [24]. Particularly, the carbothermal reduction of silica with carbonaceous rich species is one of the most promising methods. Various pairs of precursors have been exploited, including SiO powder and grapheme foam [25], carbon nanotubes as template and the tetraethyl orthosilicate (TEOS) sol as the siliceous precursor [26], exfoliated graphite containing silica particles [27], Gangue and carbon black [28] etc. However, synthesis of nano-SiC particles at low cost is still of great interest and remains a challenge. It is noticed that the PCB base plates were consisted of carbon, silicon, and some inorganic additives, within which carbon makes up about 20 wt.% and Si element about 50 mol% in the inorganic elements [12]. The PCBs base plate was a perfect mixture of carbonaceous and siliceous material, thus there would be no need for precursors mixing step. The reactants already in intimate contact would favor a solid-state reaction. Therefore, the waste PCBs must be a suitable candidate to produce silicon carbide (SiC). In this paper, the waste PCBs were used as both silica and carbon precursors to produce high value-added SiC nanoparticles. Considering that SiC is a well-known p-type semiconductor and the photocatalytic degradation is an important advanced oxidation process in environmental remediation [29–31], the p-type semiconductor activity of the prepared SiC was investigated by constructing a p-n heterojunction photocatalyst with a typical n-type TiO2 semiconductor. Hence, three aspects were mainly focused on: (1) preparation of nano-SiC from the

2. Experimental 2.1. Materials The PCB scraps were collected from waste site in Guangzhou China. Nitrate acid (HNO3, 65 wt.%), chloride acid (HCl, 37 wt.%), sulfuric acid (H2SO4, 98 wt.%), commercial SiC (99.5 wt.%), Al2O3, CaO, sodium sulfate (Na2SO4, 99%), isopropyl titanate, and methylene blue etc. were purchased from Fuchen Chemical Co., Ltd. (Tianjin, China). Ethanol and glacial acetic acid were supplied by Guangzhou Chemical Reagent Factory. If no specified, all the chemicals are analytic reagents. 2.2. Preparation of SiC from the waste PCBs The waste PCBs were firstly pre-washed with chloride, sulfuric or nitrate acid, respectively, resulting PCB-Cl, PCB-S and PCB-N. Then, in a tube electric furnace under inert atmosphere, 5 g of the acid washed PCBs were placed in a ceramic ark and heated to 500 °C, (PCB-500) followed by a further heating to 1500, 1600 and 1700 °C at a heating rate of 10 °C min−1, then kept for 2 h. The resulting powders were washed with HNO3 again to remove any inorganic residues, then washed with deionized water until clean. The dry solid was ground to 200mesh and stored in desiccator. The final products were denoted as SiC1500, SiC-1600 and SiC-1700, respectively. The preparation procedure is illustrated in Fig. 1. SiC resulting from various acid washing were denoted as SiC-Cl, SiC-S and SiC-N. In addition, a SiC sample obtained without any acid pre-wash was denoted as SiC-blank. 2.3. Preparation of SiC/TiO2 0.125 g SiC dispersed in 30 mL ethanol was sonicated for 15 min. Then the mixture was heated to 70 °C, in which isopropyl titanate was slowly added drop-wise. After the drop wise finished, 55.0 wt.% mixture of water and glacial acetic acid (weight ratio = 4:1) was promptly added. After stirring for 5 h, the evaporated mixture was ground and calcined at 450 °C for 3 h. The obtained SiC/TiO2 powder was stored in desiccator for further use. For comparison, pure TiO2 was prepared by the same method of preparing SiC/TiO2 except that no SiC was added to the reaction mixture. 2.4. Characterizations X-ray diffraction (XRD) analysis was performed at a D-max2200 VPC (RIGAKU, Japan), using Cu Kα radiation (40 kV, 26 mA, λ =0.15406 nm) in the angular 20–80° with the scan speed of 0.01° s−1. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer thermal analyzer TG209F1 libra and the sample was heated from 25 to 900 °C at a heating rate of 10 °C min−1, under the purified nitrogen flow rate of 40 mL min−1. Morphologies of the products were examined by

Fig. 1. Procedure of producing SiC from the waste PCBs. 641

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(e.g. 1500 °C) was required to produce SiC. Apart from the peaks belonging to carbon and quartz SiO2 for both PCB and PCB-500, there are many other peaks, such as 25.9°, 28.7°, 31.5°, 32.8°, 43.0°, attributed to the oxides impurity containing Al, Ca, Mg, B. These ions in the second high-temperature pyrolytic process can form inorganic impurities-vitrified coating, which is difficult to be removed from the SiC product and therefore deteriorate the activity of SiC. Hence, it is necessary to wash away the inorganic impurities to provide a relatively pure in situ carbothermal reduction environment, as a result, ensuring the purity of the final SiC. To wash away the inorganic impurities, acid wash pretreatment of the waste PCBs was performed. Fig. 4 showed XRD spectra of the SiC obtained under different acid wash conditions. The acid types, acid concentrations, washing temperature and washing time were optimized. Three acids, including HNO3, HCl and H2SO4 were used, and their resulting SiC samples were named as SiC-N, SiC-Cl and SiC-S (see Fig. 4a). It was interesting to find out that only SiC-N possessed relatively pure SiC diffraction peaks, while SiC-Cl and SiC-S did not. Furthermore, we altered HNO3 washing conditions with different temperatures (40 °C, 60 °C, 80 °C) and concentrations (2 mol L−1, 3 mol L−1 and 4 mol L−1), and their corresponding samples were named as SiCN40, SiC-N60, SiC-N80, SiC-2 M, SiC-3 M and SiC-4 M, respectively. As shown in Fig. 4b and c, two groups of samples possessed somewhat similar SiC peaks with satisfied degree of crystallinity. Some weak impurity peaks appear in the XRD spectra of SiC-N40 and SiC-2 M while high purity was obtained by the other four samples. We could conclude that temperature above 60 °C and concentration above 3 mol L−1 pretreatment are suitable for producing pure SiC product. Finally, different acid pre-treatment time of 72, 96 and 120 h was tried. Samples obtained from longer acid washing times possessed sharper and stronger SiC diffraction peaks (see Fig. 4d). This is because the longer pretreatment time might enhance the breakdown of the inner-structure of the thermosetting epoxy resin in waste PCBs [35], favoring the carbon production during pyrolysis and therefore facilitating the conversion of C and Si resources into a better crystallized SiC. The PCBs samples before and after acid washing treatment with 3 mol L−1 nitric acid at 60 °C for 96 h (named as virgin PCBs and AW-PCBs, respectively) were also analyzed. From the SEM images (Fig. S1), the morphology did not change too much except that more small particles appeared in AWPCBs. The EDAX analysis (Fig. S2) shows that only C, O, Si were detected on the surface of AW-PCBs, indicating that acid washing effectively removed the other inorganic elements. Furthermore, the ash and organic contents in virgin PCBs and AW-PCBs were listed in Table S1. In AW-PCBs, the ash content is 0.1543 g, which should be SiO2 according to the EDAX analysis of AW-PCBs. Moreover, the ash content (0.1543 g) is in accordance with the silica content in the virgin PCBs, demonstrating that acid washing did not affect the content of SiO2. In contrast, the organic content is 0.1491 g in AW-PCBs, which is 97.5% of the total organic content (0.1530 g) in the virgin PCBs. It reveals that the acid washing process might breakdown the inner structure of the epoxy resin (Fig. S1) but did not dissolve it. Then the ions dissolved in the acid washing solution were decided by ICP measurement. As shown in Table S2, the filtrate contained many ions of Al, Ca, Ba, Cu, Mg, Fe, B, P and so on, but mainly Al3+ and Ca2+. These results indicate that nitric acid washing is effective for removing the inorganic ions but have no obvious effect on the organic contents. It was reported that the reaction of C and SiO was feasible at 1600 °C while the partial pressure of CO in the system is below 15% [36]. Therefore, our pre-pyrolyzed PCBs powder was further heated to 1500, 1600 and 1700 °C, respectively. Fig. 5a in the centre showed the overall XRD spectra of the three SiC samples, where triangle symbols indicated the feature peaks of Quartz-SiO2 (JCPDS#65-0466), club symbols indicated the peaks of carbon (JCPDS #26-1076), while diamond symbols indicated those of the face-cubic centered 3C-SiC (JCPDS Card #00291129). Fig. 5b–d, and e showed the amplified details at theta range of 20–32 °, 33–38 °, 59–61 ° and 70–75 °, aiming to see the details of the

Scanning Electron Microscopy (SEM) on a FEI/OXFORD/HKL Quanta 400 F microscope equipped with an EDX accessory. UV–vis absorption spectra were collected using a UV-2501PC spectrophotometer fitted with an integrating sphere, the scan range was 200–800 nm. The photoelectrochemical properties were investigated in a conventional threeelectrode cell using a CHI660 analyzer. The as-prepared film, a Pt nod and an Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively. The Mott–Schottky curves were measured at a certain DC potential range with AC amplitude of 5 mV and a frequency of 1000 Hz. Inductively coupled plasma optical emission spectrometry (ICP-OES, optima 5300DV, Perkin-Elmer) was used to measure the ion concentrations in the acid washing solution. 2.5. Photocatalytic tests The catalytic tests were performed at 25 °C using methylene blue (MB) solution of 10 mg L−1 and the catalyst dose of 500 mg L−1 under the irradiation of a Hg lamp (12 W). The dye concentration was monitored with a UV–vis spectrophotometer (UV-5100 Yuanxi Ltd. Shanghai, china) at 664 nm, MB maximum absorbance wavelength. 3. Results and discussion 3.1. Preparation of nano-SiC from the waste PCBs Fig. 2 studied the decomposition behavior of the waste PCBs powders by the TG analysis. The weight loss started at 300 °C. The first major weight loss of 25% occurred between 300–450 °C, corresponding to the rapid decomposition of epoxy resin into small molecular gases as well as the formation of pyrolytic carbon. The TG curve became flat after 500 °C and the maximum decomposition weight reached 30% at 900 °C. These were in good agreement with the values reported for typical epoxy resins [32]. In addition, the epoxy resin applied in electronic manufacturing industry are required to meet the minimum thermal stability (250–280 °C) and their maximum decomposition temperature is around 405 °C [33,34]. It was determined that 500 °C was the optimal first step pyrolysis temperature of the waste PCBs, at which almost all the organic hydrocarbons were evaporated or decomposed, leaving the pyrolytic carbon behind. This was evidenced that black powder was obtained at this temperature (Fig. 1). Fig. 3a and b compared the XRD spectra of the original PCB and PCB-500 that had been pyrolyzed at 500 °C. Comparing with the PCBs, the pyrolyzed PCB showed stronger diffraction peak at around 26.6°, confirming the early occurrence of pyrolytic carbon. The co-existence of peaks at both 20.9° and 26.6° could be assigned to the feature peaks of the SiO2(101) and SiO2(011) (JCPDS#65-0466), respectively, indicating the existence of silicate oxides as the siliceous resource. However, at this relatively low temperature, no SiC diffraction peak was observed, meaning no reaction between the epoxy resin and silicate oxides took place yet. A second pyrolytic process at higher temperature

Fig. 2. Thermo-Gravimetric (TG) curve of the PCBs powders. 642

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Fig. 3. XRD spectra of the (a) original PCBs; (b) PCBs that pyrolyzed at 500 °C.

Fig. 4. XRD spectra of the synthesized SiC via various acid pre-treatments: (a) HNO3, H2SO4 and HCl were used as washing agents; (b) HNO3 washing temperature was adjusted as 40, 60 and 80 °C; (c) HNO3 concentration was adjusted as 2 M, 3 M and 4 M; (d) HNO3 washing time was adjusted to 72, 96 and 120 h.

for SiC-1600, it showed strong diffraction peaks at around 35.6°, 41.4°, 60.0° and 71.7° (Fig. 5a), which are the featured peaks for face-cubic centered 3C-SiC (JCPDS Card #29-1129), confirming the successful conversion from waste PCBs to SiC. Moreover, it did not show the peaks for either Carbon or SiO2 (Fig. 5b), which means the SiC-1600 was relatively pure. In Fig. 5c, the dominated SiC(111) peak intensity of SiC1600 was almost 4 times that of SiC-1700 and 10 times that of SiC1500, and a very similar peak intensity order was observed for peak SiC(311) at 71.7° (Fig. 5e). All these observations indicated a well-developed, fine crystallized, highly purified SiC was obtained at 1600 °C. Lastly, in Fig. 5b, SiC-1700 showed an obvious peak at around 26 °,

different featured diffraction peaks of C (002) @ 26°, SiC(111) @ 35.6°, SiC (220) @ 60° and SiC(311) @ 71.7°, respectively. Firstly, for sample SiC-1500, the characteristic peak of SiC(111) at around 35.6° was observed, indicating when the temperature was raised to 1500 °C, SiC started to form. However, SiC-1500 still possessed both clear characteristic peaks of Quartz-SiO2 at both 20° and 26°, signifying the presence of silica oxides. SiC-1500 showed a much weaker SiC(111) peak intensity than those of the other two samples (Fig. 5c), and it did not show as obvious SiC(220)& (311) peak as its counterparts (Fig. 5d and e). We concluded that 1500 °C was not the optimal pyrolysis temperature to produce SiC in this waste PCBs recycling strategy. Secondly,

Fig. 5. (a–e) XRD spectra of the SiC-1500,1600 & 1700; (f) FTIR spectra of SiC-1600. 643

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Table 1 The peak positions, crystal lattice parameters and average particle size of the SiC-1600 & SiC-1700 and their comparisons to the standard 3C-SiC (#29-1129). Samples

2θ(111)/°

2θ(200)/°

2θ(220)/°

2θ(311)/°

d111/nm

d200/nm

d220/nm

d311/nm

Average size/nm

SiC-1600 SiC-1700 SiC(#29-1129)

35.77 35.71 35.60

41.54 41.46 41.38

60.15 60.05 60.00

71.92 71.91 71.78

0.2508 0.2513 0.2520

0.2172 0.2176 0.2188

0.1537 0.1540 0.1541

0.1312 0.1312 0.1314

63.46 56.69 /

fitted in the nano-material category. Based on all the results and analysis above, we could conclude that the waste PCBs can be successful resourcelized into a nano-SiC material by a proper nitrate acid wash pre-treatment and a subsequently two step pyrolysis at 500 and 1600 °C.

without an accompany peak at 20°, indicating the existence of carbon (JCPDS #26-1076). This could be because when the pyrolysis temperature increased to 1700 °C, SiC might start to decompose, broke down the well crystallized SiC structure and re-generated carbon. Even SiC-1700 possessed the entire feature peaks for 3C-SiC across the whole range, it always showed weaker feature peak intensity than those of SiC-1600. To sum up, herein 1600 °C was the optimal pyrolysis temperature to produce SiC. Fig. 5f showed the FTIR spectra of sample SiC1600. The presence of strong peaks at 819 and 1621 cm−1 could be attributed to the Si-C and SieOH vibrations, confirming the successful production of SiC from the waste PCBs. Based on the XRD results, the detailed peak positions, crystal lattice parameters and average particle size of the SiC-1600 and SiC-1700 and the 3C-SiC (JCPDS Card #0029-1129) were extracted and listed in Table 1. The SiC-1500 sample was not considered in this study because of its poor SiC quality. The average particle size was estimated from the XRD spectra by the Scherrer equation (Eq. (1)) [37,38], while the crystalline lattice spacing of each diffraction peaks can be obtained by the Bragg’s equation (Eq. (2)) [39]. D=(Kλ)/(βcosθ)

3.2. Construction of a p-n heterojunction photocatalyst SiC is a famous p-type semiconductor with low oxidation potential. It is necessary to combine SiC with another semiconductor, especially ntype semiconductor to construct a p-n heterojunction photocatalyst applied in the photocatalytic degradation processes. The optical properties of SiC and TiO2 was measured by UV–vis DRS. As shown in Fig. 7a, SiC can absorb the light in a wide range from UV to visible region. The band structures of the photocatalysts can be estimated by combined optical and electrochemical analyses. Specifically, the band gap energy (Eg) of SiC-1600 is estimated from the plots of (Ahν) 2 versus (hν) while that of TiO2 from the plots of (Ahν) 1/2 versus (hν) based on the optical absorption data in Fig. 7a, where A is the absorbance, ν is the frequency of light, and h is Planck's constant [40,41]. The Eg of SiC1600 and TiO2 was estimated to be 2.61 eV and 3.03 eV, respectively (Fig. 7b and c). Fig. 7d and e exhibited the Mott-schottky plots for SiC-1600 and TiO2 respectively. Mott-schottky measurement is an excellent tool for determining the conduction type, conduction band/valance band (CB/ VB) position and carrier concentration of a photocatalyst. In Fig. 7d, SiC-1600 exhibited a linear relationship over the voltage range with a negative slope, indicating their p-type conductive behavior, with hole (h+) conduction. Oppositely, the prepared TiO2 was a classic n-type semiconductor (Fig. 7e). The values of flat band potential (Efb) could be extracted from the x-intercept of the linear region of the curve 1/C2 vs.V, which is +1.26 and -0.37 eV vs. Ag/AgCl electrode for SiC-1600 and TiO2, respectively. The Efb value vs. Ag/AgCl can be converted to that vs. reversible hydrogen electrode (RHE) according to the Nernst equation (Eq. (3)),

(1)

Where D is the diameter of the grain or the layer, K is the shape factor (0.89), λ is the X-ray wavelength of Cu Kα (0.154 nm), β is the experimental full-width half maximum of the respective diffraction peak, and θ is the Bragg angle. 2dsinθ=nλ

(2)

Where d is the inter-planar spacing, λ is X-ray wavelength of Cu Kα (0.154 nm), θ is Bragg angle and n is 1. The peak positions and lattice structure of SiC-1600 and SiC-1700 matched well with the standard JCPDS #29-1129, which confirmed their high purity and well crystallinity. SiC-1600 showed a larger average particle size of 63.46 nm than that of SiC-1700 (56.69 nm), implying that manipulating the pyrolysis temperature could effectively control the size of SiC. Such correlation inspired a future research direction that manipulating the reaction conditions to achieve SiC with a higher purity and smaller size (approaching the Bohr radius), hence manipulating its optical, light adsorption and photocatalysis efficiency. Fig. 6 showed the morphologies of the obtained SiC by SEM. Many small particles and some aggregated coarse fragments were observed, particles size was ranging from a few tens to hundreds of nanometers. By analyzing the size of particles in the SEM images with the software “image J”, the average size of the particles in Fig. 6a–c is 81.8, 78.4, 72.1 nm, respectively. It could be confirmed that the synthesized SiC

ERHE = EAg/AgCl + 0.059pH + E0Ag/AgCl

(3)

Where ERHE is the converted potential vs. reversible hydrogen electrode (RHE), E0Ag/AgCl = 0.1976 at 25 °C, and EAg/AgCl is the experimentally measured potential against Ag/AgCl reference [42,43]. Then the Efb values of SiC-1600 and TiO2 were calculated to be +1.84 and +0.21 eV (vs. RHE), respectively. It was well known that for a p-type semiconductor, its valence band energy (EVB) is treated equal to its Efb while for a n-type semiconductor, its conductor band energy (ECB) is usually

Fig. 6. SEM images of (a) SiC-1500; (b) SiC-1600; and (c) SiC-1700. 644

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Fig. 7. (a) UV–Vis Diffuse Spectra of SiC-1600 and TiO2; (b) Plots of (Ahν)2 vs. (hν) for estimating the Eg of SiC-1600; (c) Plots of (Ahν)1/2 vs. (hν) for estimating the Eg of TiO2; (d) & (e) Mott-Schottky plots of SiC-1600 and TiO2 in 0.1 M Na2SO4 solution (pH 6.5); (f) Schematic diagram of the synergetic band of the SiC-TiO2 p-n heterojunction.

MB. Therefore, enhanced degradation rates of MB were expected for the p-n heterojunction of SiC-TiO2.

approximately 0.1 eV more negative than its Efb [44]. Hence, the EVB of obtained p-type SiC was determined as +1.84 eV (vs. RHE) and the ECB of TiO2 was +0.11 eV (vs. RHE). According to Eq. (4), the ECB of p-type SiC was calculated to be -0.77 eV (vs. RHE) and the EVB of TiO2 was +3.14 eV (vs. RHE). ECB = EVB-Eg

3.3. Activity characterization of the nano-SiC as a p-type photocatalyst The photocatalytic activities of the obtain SiC was investigated by the MB photodegradation behaviors. In Fig. 8a and b, the effect of acid wash pre-treatment of the PCBs on photocatalytic activity of the final SiC products was explored. Fig. 8a compared the MB photodegradation behaviors of no catalyst, SiC-Cl, SiC-N and SiC-S with SiC-blank; while Fig. 8b compared that of SiC-N40, SiC-N6, SiC-N80 with SiC-blank. Here, all SiC samples were the p-n heterojunction of SiC and TiO2, which were prepared according to the method described in the experimental part. In Fig. 8a, the degradation efficiency of MB is only 24.2% by the photolysis without any catalysts. The degradation efficiencies of all three acid-treated samples showed evidently enhancement over SiC-blank. SiC-N displayed the highest degradation efficiency of 82.8%, while SiC-Cl and SiC-S achieved 68.0 and 64.3%. SiC-blank only showed a removal of 36.6%, indicating acid pre-treatment was essential to make sure pure SiC production from the waste PCBs thus allowing the final superior photocatalytic activity. Similarly, Fig. 8b compared the MB degradation rates of SiC-N40, SiC-N60 and SiC-N80

(4)

Where ECB, EVB and Eg are the conductor band energy, the valence band energy and the band gap energy of the semiconductor. The resulting p-n heterojunction between SiC and TiO2 could be constructed as in Fig. 7f. Under light irradiation, electrons on the VB of the SiC were excited and transferred to the VB band of TiO2, the holes transfer in the opposite direction, which made the separation of electron-hole pairs more effective and reduced the recombination of electron-hole pairs [45]. The CB band of SiC was more negative than the formation of the ·O2– radicals at -0.33 eV, therefore the enriched electrons on the CB of the SiC could have been used for reducing O2 to ·O2– radicals, then the ·O2– radicals degraded the MB. At the same time, ntype TiO2 possessed a positive enough potential (+3.14 eV), therefore the holes on the VB of TiO2 were able to oxidize H2O (+2.27 eV) and OH− (+2.38 eV) to ·OH radicals or directly oxidized the pollutants. Both the ·OH radicals and the holes on the VB of TiO2 could degrade 645

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Fig. 8. MB degradation performance: (a) & (b) SiC-Cl, SiC-N, SiC-S & SiC-N40, N60, N80, compared with no catalysts and SiC-blank; (c) & (d) various p-n heterojunction SiC-TiO2 photocatalysts & their pseudo first-order kinetics fittings.

concentration at time t (mg L−1), t was time(min), k was rate constant, t1/2 was half-life (min), which was the time required to degrade half of the MB. The adsorption performance of various catalysts towards MB was shown in Fig. S3. It is observed that the MB removal by the adsorption did not exceed 8.0% except that on Comm. SiC/TiO2 reached 13.5%. In Fig. 8c, the SiC-1600/TiO2 (ST) showed the best photocatalytic activity among the 6 samples, signifying the SiC synthesized from the recycled PCBs could be a promising candidate as the future p-type photocatalyst for dye removal. The UV–vis spectral changes of MB in the SiC-1600/ TiO2 (ST) mediated photocatalytic degradation process are shown in Fig. S4. The main peak intensity of MB located at about 664 nm progressively decreased along with the reaction time. The ST and commercial SiC/TiO2 (CT) p-n heterojunction showed the highest and comparable degradation rate of 82.8% and 77.8%, both was double of that of the pure TiO2 (41.0%). In contrast, the pure SiC1600 only removed 7.4% of MB (data not shown). Considering that the BET specific surface area (SBET) of TiO2, SiC1600, and SiC-1600/TiO2 (ST) are 2.3, 11.9, 12.8 m2 g−1, respectively, the high photocatalytic activity of ST can be attributed to the p-n heterojunction, not the SBET. The pseudo first-order reaction rate constant (k) of ST for MB was 0.0219 min−1, which was the highest among all the samples. This was 25.1% higher

with SiC-blank to see the effect of pre-treatment temperatures. SiC-N60 andSiC-N80 showed parallel degradation rate of 82.8 and 85.2%, respectively, which were more than double of that of SiC-balnk (36.6%). SiC-N40 exhibited a removal rate of 67.7%, which also presented better photocatalytic activity than the SiC-blank. It was concluded that hot nitrate acid pre-treatment was beneficial for the SiC photocatalytic performance, however when the temperature was higher than 60 °C no thriving improvement could be achieved. To reveal the origin of the high catalytic activity of SiC1600/TiO2, the MB photodegradation behaviors of SiC/TiO2 based catalysts were shown in Fig. 8c and the corresponding pseudo first-order kinetics fittings in Fig. 8d. The pseudo first-order kinetics equation were listed below as Eqs. (5) & (6). The calculated degradation rate, rate constant, correlation coefficients and half life were summarized in Table 2, where catalysts abbreviations were also given [46–48].

C In ⎛ 0 ⎞ = kt ⎝C⎠ t1/2 =

(5)

In2 k

(6) −1

Where C0 was the initial MB concentration(mg L

), C was the MB

Table 2 MB photodegradation rate, rate constants (k), half life (t1/2) the with various catalysts. Catalysts

Deg. rate (%) in 80 min

k(/min−1)

t1/2(min)

R2

SiC-1600/TiO2 (ST) Commer. SiC/TiO2 (CT) TiO2 (T) SiC-blank/TiO2 (SBT) Commer.SiC + impurities/TiO2 (CIT)* Calcined Commer.SiC + impurities/TiO2 (CCIT)**

82.8 77.8 41.0 36.6 32.8 22.9

0.0219 0.0175 0.0064 0.0055 0.0051 0.0032

32 40 108 127 136 217

0.9944 0.9550 0.9887 0.9844 0.9907 0.9930

* The CIT sample were prepared by mechanically mixing 16.8 wt.% of CaO, 11.2 wt.% of Al2O3, 52.0 wt.%. *In CIT sample, 16.8 wt.% CaO and 11.2 wt.% Al2O3 was added since the main impurities in PCBs are Al3+ and Ca2+ during the preparation of SiC/TiO2. **In CCIT sample, the mixture of 16.8 wt.% CaO, 11.2 wt.% Al2O3, and 52.0 wt.% commercial SiC was calcined at 1600 °C for 1 h under the protection of N2 before forming p-n heterojunction with TiO2. 646

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than that of the second highest of CT of 0.0175 min-1, which was possibly due to the unique nano-morphology of the synthesized nano-SiC. Moreover, the k of ST was 3.98 times of that of SBT because SiC-blank did not experience the acid wash pretreatment therefore contained residual inorganic impurities. These results proved the importance of acid wash pretreatment of the waste PCBs. As shown in Table S2, the main inorganic additive ions in the virgin PCBs are Al3+ (2478 mg L-1) and Ca2+ (4983 mg L-1). To directly understand the effects of the Al and Ca species on the overall photocatalytic activity, two extra samples, namely the commer.SiC + impurities/TiO2 (CIT) and the calcined Commer.SiC + impurities/TiO2 (CCIT) were prepared. In these samples, additional Al2O3 and CaO were added into the commercial SiC sample when constructing the p-n heterojunction photocatalysts. Their preparation methods were undernoted in Table 2. As shown in Fig. 8c and Table 2, sample CIT showed a MB removal rate of 32.8% while sample CCIT showed a rate of 22.9%, both of which were significantly lower than that of CT of 77.8%. Also, the k value of the CIT and CCIT were 0.0051 and 0.0032 min−1 that were much lower than that of CT of 0.0175 min-1, indicating the additional impurity oxides, such as CaO and Al2O3, could largely reduce the photodegradation performance. The catalytic activity of CCIT was even lower than that of CIT. It was deduced that the CaO and Al2O3 impurities were vitrified at 1600 °C and coated on the SiC, thus inactivated the SiC. We could see that both sample CIT and CCIT exhibited weaker photocatalytic performance than the sample CT, indicating the introduction of impurities oxide deteriorated the photocatalytic activity, consequently confirmed the proper acid wash pre-treatment was essential to avoid the formation of vitrified coating on the final SiC and ensure a superior photocatalytic activity. The half-life (t1/2) showed coherent trends with the previous photocatalytic activity parameters. ST illustrated the first-rank photocatalytic activity with the lowest half-life of only 32 min, while the CT, T, SBT, CIT, and CCIT had the values of 40, 108, 127, 136, 217 min, respectively. Many photocatalysts were used to degrade MB and several review papers have mentioned them [49–52]. Moreover, the degradation efficiency of MB depended on the intensity of UV-lamps, the solution volume, the MB concentration and the catalyst dosage, and so on except the catalyst type. To evaluate the catalytic activity of the obtained SiC1600/TiO2 in this paper more comprehensively, the performance of some other typical and novel photocatalysts reported in recent 3 years were listed in Table S3. It can be seen that the performance of SiC1600/TiO2 also ranked high level comparing to the other photocatalysts. Then the effect of initial solution pH on the MB degradation was studied (Fig. S5). At the initial solution pH of 3.0, the degradation rate of MB reached 61.7%. It has been observed that the degradation rate increases with the increase of the initial solution pH from 3.0 to 6.9 and reached the highest value of 82.8% at pH 6.9. Further increasing pH to 9.0 slightly decreased the removal of MB. That is because the negative charges carried by the catalysts decreased with the increase of the solution pH, which could weaken the repulsive force between the catalyst surface and the MB cations and enhanced the degradation of MB [30]. However, the Fermi energy level shifted positively (59 mV) when the solution pH increased one unit. This implies that the oxidation stability of the holes is weaker under higher pH. Hence, the degradation rate of MB slightly decreased at pH 9.0. The results show that the SiC-1600/ TiO2 (ST) heterojunction is suitable for working around neutral pH. Effect of ionic strength on the degradation rate of MB at irradiation time of 80 min was investigated in a concentration range from 0 to 0.5 mol L−1 of sodium chloride. As shown in Fig. S6, the presence of inorganic salts had a negative effect on the photocatalytic activity of SiC1600/TiO2 in the degradation of MB. Higher salt concentration results in a more deactivation effect. Two deactivation reasons by sodium chloride concentration were proposed, quenching free radicals by Clions and blockage of catalyst active sites by adsorption of anions to form a surrounding layer [30,53]. The other typical pollutants including anionic acid orange 7 (AO7)

dye, 4-chlorophenol (4-CP) and trimethoprim (TMP), were also employed as model contaminants. As shown in Fig. S7, within the reaction time of 80 min, the removal efficiencies of MB, AO7, 4-CP, TMP reached 82.8, 89.5, 76.5, and 66.2%, respectively. The result indicates that SiC/ TiO2 is also active towards the degradation of the other pollutants. 4. Conclusions Pure silicon carbide (SiC) nanoparticles using the waste PCBs as both silica and carbon precursors were successfully prepared via a three-step process: acid wash pretreatment, low-temperature pyrolysis, and high-temperature carbon-thermal reduction. UV–vis and MottSchottky analysis revealed that the nano-SiC could be used as a p-type semiconductor to construct a p-n heterojunction photocatalyst with the n-type semiconductor TiO2. The synergetic effect ensured excellent MB degradation efficiencies of the resulting SiC/TiO2 composites. The photocatalytic mechanism was also illustrated. Acknowledgments This research is supported by the Nature Science Foundations of China (21677180, 21777196), the Science and Technology Key Projects of Guangdong Province (2014B020216004 and 2015B020237005). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.03.115. References [1] J. Wu, J. Li, Z. Xu, Electrostatic separation for recovering metals and nonmetals from waste Printed Circuit Board: problems and improvements, Environ. Sci. Technol. 42 (2008) 5272–5276. [2] R.H. Estrada-Ruiz, R. Flores-Campos, H.A. Gámez-Altamirano, E.J. VelardeSánchez, Separation of the metallic and non-metallic fraction from printed circuit boards employing green technology, J. Hazard. Mater. 311 (2016) 91–99. [3] J. Wang, Z. Xu, Disposing and recycling waste Printed Circuit Boards: disconnecting, resource recovery, and pollution control, Environ. Sci. Technol. 49 (2015) 721–733. [4] Y. Zhou, K. Qiu, A new technology for recycling materials from waste printed circuit boards, J. Hazard. Mater. 175 (2010) 823–828. [5] Y. Tang, P.-H. Lee, K. Shih, Copper sludge from Printed Circuit Board production/ recycling for ceramic materials: a quantitative analysis of copper transformation and immobilization, Environ. Sci. Technol. 47 (2013) 8609–8615. [6] A. Işıldar, J. van de Vossenberg, E.R. Rene, E.D. van Hullebusch, P.N.L. Lens, Twostep bioleaching of copper and gold from discarded printed circuit boards (PCB), Waste Manage. 57 (2016) 149–157. [7] P. Hadi, J. Barford, G. McKay, Toxic heavy metal capture using a novel electronic waste-based material—mechanism, modeling and comparison, Environ. Sci. Technol. 47 (2013) 8248–8255. [8] J. Li, Z. Xu, Environmental friendly automatic line for recovering metal from waste Printed Circuit Boards, Environ. Sci. Technol. 44 (2010) 1418–1423. [9] K. Huang, J. Guo, Z.M. Xu, Recycling of waste printed circuit boards: a review of current technologies and treatment status in China, J. Hazard. Mater. 164 (2009) 399–408. [10] G.H. Song, X.F. Zhu, W.Y. Yuan, C.L. Zhang, W. Meng, Recycling and disposal technology for nonmentallic materials from waste printed circuitboards (WPCBs) in China, Procedia Environ. Sci. 31 (2016) 935–940. [11] P. Hadi, M. Xu, C.K.S. Lin, C.W. Hui, G. McKay, Waste printed circuit board recycling techniques and product utilization, J. Hazard. Mater. 283 (2015) 234–243. [12] P. Hadi, P. Gao, J.P. Barford, G. McKay, Novel application of the nonmetallic fraction of the recycled printed circuit boards as a toxic heavy metal adsorbent, J. Hazard. Mater. 252-253 (2013) 166–170. [13] P. Hadi, C. Ning, W. Ouyang, M. Xu, C.S.K. Lin, G. McKay, Toward environmentallybenign utilization of nonmetallic fraction of waste printed circuit boards as modifier and precursor, Waste Manage. 35 (2015) 236–246. [14] J. Guo, J. Guo, Z. Xu, Recycling of non-metallic fractions from waste printed circuit boards: a review, J. Hazard. Mater. 168 (2009) 567–590. [15] J. Guo, B. Cao, J. Guo, Z. Xu, A plate produced by nonmetallic materials of pulverized waste Printed Circuit Boards, Environ. Sci. Technol. 42 (2008) 5267–5271. [16] J. Guo, J. Guo, S. Wang, Z. Xu, Asphalt modified with nonmetals separated from pulverized waste Printed Circuit Boards, Environ. Sci. Technol. 43 (2009) 503–508. [17] X. Du, L. Wang, W. Zhao, Y. Wang, T. Qi, C.M. Li, Preparation of hierarchical porous carbon from waste printed circuitboards for high performance electric double-layer capacitors, J. Power Sources 323 (2016) 166–173.

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