- Email: [email protected]
H2O2
Journal of Environmental Chemical Engineering 7 (2019) 103354
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Selective flotation separation of ABS/PC from ESR plastic wastes mixtures assisted by ultrasonic catalyst/H2O2 Trinh Duy Nguyena,b, T.M. Al Tahtamounic, Pham Thi Huongd, Phan Quang Thange,f,
T
⁎
a
NTT Hi-Tech Institute, Nguyen Tat Thanh University, 300A Nguyen Tat Thanh Street, Dist. 4, Ho Chi Minh City, Vietnam Center of Excellence for Green Energy and Environmental Nanomaterials, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam c Materials Science and Technology Program, College of Arts and Sciences, Qatar University, Doha, 2713, Qatar d Center for Advanced chemistry, Institute of research and development, Duy Tan University, 550000, DaNang, Vietnam e Division of Computational Mathematics and Engineering, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam f Faculty of Environment & Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Vietnam b
A R T I C LE I N FO
A B S T R A C T
Keywords: Waste electrical Plastic waste Recycles Ultrasonication Hydroxyl radical
The present study investigated the potential recycles of acrylonitile butadiene styrene terpolymers (ABS) from their blends with polymethylmehtacrylate (PMMA), polyCarbonate (PC) and high impact polyStyrene (HIPS) using flotation separation assisted by ultrasonic catalyst/ H2O2. The effect of various factors such as H2O2 dose, duty cycle and contact time of the ultrasonication on the recovery rate and purity of ABS/PC were conducted. The results showed that H2O2 dose significantly influenced on the recovery rate of ABS/PC and the optimized H2O2 dose was found at 2%. The recovery rate and the purity of submerged ABS/PC reached 99.5 and 98.6%, respectively, at 300 s of contact time. The duty cycle and the contact time of the ultrasonification also exhibited highly effective for the recycle of ABS/PC from the plastic waste mixtures. The radical scavengers and the mechanism of flotation separation of ABS/PC by ultrasonic catalyst/ H2O2 were proposed. Additionally, the economic potential and environmental impacts were discussed. These findings are crucial for flotation separation of ABS/PC from the plastic waste mixtures assisted by ultrasonic catalyst/ H2O2 with high recovery rate and purity of ABS/PC in order to produce further value added products as well as reduce the environmental impact of plastic waste.
1. Introduction Waste Electrical and Electronic Equipment (WEEE) is one of the fastest growing waste streams with approximately 44, 7 million metric tons that were generated worldwide in 2016. WEEE is a heterogeneous mixture of various electrical equipments like computers, televisions, washing machines and other household appliances. WEEE components also contain different valuable materials such as metals (70%), plastics (20%) and refractories (10%). Consequently, most of the plastics isolated or segregated from the WEEE has been disposed by landfill and incineration, and only 10% is treated by recycling, thus causing environmental pollution. Therefore, WEEE is receiving worldwide attention due to the increasing environmental issues, management and disposal [1–4]. Until now, WEEE recycling is challenged by a large mix of different materials, often containing hazardous substances such as heavy metals and some types of brominated flame retardants that are today restricted in developed countries. In contrast, WEEE recycling encompasses many opportunities to reduce environmental impacts, as ⁎
well as generating economic value [5–9]. The recycle of WEEE can be expected to relieve the detrimental emissions including polybrominated dibenzodioxins and dibenzofurans which are generated in the incineration of brominated flame retardants. It has been demonstrated that the components of plastic in WEEE considered top quality grades of acrylonitile butadiene styrene terpolymers (ABS) and their blends with polymethylmehtacrylate (PMMA) or polyCarbonate (PC), as well as high impact polyStyrene (HIPS). Intriguingly, 36.75% of ABS and 4.99% of PC can be found in WEeEE plastics and are perfectly suited to recycling. Also, the blends of ABS/PC have considerable market importance because their excellent characteristics have led to many applications [10]. The ABS and PC of WEeEE plastic through blending show significant potential for recycling. However, the recycling of waste ABS and PC is challenging because they are generally mixed with other plastics. A number of promising technologies were investigated for recycles of WEEE such as hydrocyclone, sink-float and electrostatic separation. Plastics flotation is an alternative method, which shows advantages such as cost-effective and higher separation efficiency,
Corresponding author. E-mail addresses: [email protected] (T.D. Nguyen), [email protected] (T.M. Al Tahtamouni), [email protected] (P.Q. Thang).
https://doi.org/10.1016/j.jece.2019.103354 Received 23 April 2019; Received in revised form 9 August 2019; Accepted 9 August 2019 Available online 11 August 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103354
T.D. Nguyen, et al.
Fig. 1. The effect of H2O2 dose on the separation of the plastic wastes.
especially for plastics with similar density. The mechanism of flotation is the selective attachment of air bubbles on hydrophilic surface, resulting in distinct floating/submerging behaviors in flotation and thereby enabling the separation of plastics. However, the plastics have low energy surfaces, they are naturally hydrophobic. Flotation separation of plastics requires selective hydrophilicity of certain kind of plastics and surface treatment is an effective way to change the hydrophilicity of plastics [23–26]. The methods of surface treatment with nano-Fe/Ca/CaO ozonization, mild-heat treatment, nanometallic Ca/ CaO treatment, ZnO/microwave treatment, ZnO coating, powder activated carbon coating, and Fenton oxidation have been investigated. However, these surface treatment methods have been done with only single separation of ABS or PVC from the plastic waste mixed [11]. Advanced oxidation processes (AOPs) for remediation of contaminated soils, groundwater, and sediments have received considerable attention in recent years. These processes are based mostly on activation of hydrogen peroxide (H2O2), titanium dioxide (TiO2), persulfate (PDS) and peroxymonosulfate (PMS) to generate oxidizing species such as sulfate radical (SO4•-) (2.5–3.1 V) and hydroxyl radical (•OH) (2.4–3.0 V), which are highly reactive oxidants toward organic compounds. Previous researches reported that AOPs technology is promising method for the treatment of plastic surface [12,13]. Hydrogen peroxide is the most widely used for surface modification of plastic due to its low-cost, simple operation, low energy requirement and environmentally friendly. As reported from previous the literature, the hydroxyl radical can selectively increase the hydrophilicity of the plastics surface (ABS, PS and PC), leading to increase the separation rate during flotation process [12–15]. Nevertheless, the ultrasound has been applied effectively as an emerging advanced oxidation process (AOP). The main advantage is that the ultrasound process does not require added chemicals, oxidants or catalysts, and does not generate additional waste streams as compared with other processes [11–16]. Hydrogen peroxide is one of the most effective additives used to enhance sonochemical degradation of organic pollutants. During ultrasound irradiation, H2O2 can dissociate into hydroxyl radicals, though these have a very short lifetime and tend to combine and form H2O2. As hydrogen peroxide present at high concentration can act as a radical scavenger. The combine of ultrasonic treatment of plastic in a liquid medium during peroxide bleaching is associated with the occurrence of a number of physical and chemical phenomena, such as cavitation, redox reactions, polymerization and depolymerization [16–20]. Therefore, the combine of ultrasonic and H2O2 for the surface treatment of plastic waste is an idea method to increase the hydrophilic functional groups on the surface of plastic in order to improve the separation capacity and recycles ability of ABS/ PC. This study aim to investigate the potential application of flotation separation of ABS/PC from WEEE mixed assisted by ultrasonic/H2O2. The H2O2 dose, duty cycle and contact time of ultrasonication was optimized for the ABS/PC separation process. To identify the major oxidation species involved during the surface treatment of plastic waste, the experiment for radical scavenger was conducted. The
mechanism and environmental impact were also discussed in detail. 2. Materials and methods 2.1. Materials The WEEE samples were collected from Ho Chi minh City, Vietnam without contain any uniform shapes or sizes of the styrene plastic pieces obtained from the shredded residue. The collected samples were segregated washed, dried and cut into similar size with an average diameter of 0.4-0.45 cm, prior to specimen preparation. After classify of obtained plastic wastes, the samples were denoted as PLW1, PLW2, PLW3 and PLW4, corresponding to the ABS, PS/HIPS, ABS/PMMA and ABS/PC, respectively. 2.2. Methods The method use to separation of PLW1, PLW2, PLW3 and PLW4 was prepared according to previously reported [19–24]. The details of synthetization method is shown in the Supporting Information. 3. Results and discussion 3.1. Effect of H2O2 dose The effect of H2O2 concentration on the recovery and purity of plastics were investigated under the conditions of duty cycle of 50%, irradiation time of 5 min, and air supply time of 2 min. The PLW4 was separated on the reactor bottom while the remaining plastics remained floating on top of the medium. The recovery and purity rates of ESR plastics were evaluated on the floating rate of PLW1, PLW2 and PLW3 and the submerging rate of PLW4 as a function of H2O2 dose, as shown in Fig. 1a. It can be observed that, without adding of H2O2, the PLW1, PLW2 and PLW3 completely floated on top of the floating medium due to the inherent hydrophobic surface of styrene plastics helps the adherence of air bubbles. However, the H2O2 dose significantly affected on the recovery rate of PLW1, PLW2, PLW3 and PLW4. As shown in Fig.1a, the recovery rate of the submerged PLW4 increased from 0 to 85.6% with an increase in H2O2 content from 0 to 2%, then slightly effected with further increasing in H2O2 content (3–5%). The H2O2 dose had no effect on the recovery rate by floating PLW3 and moderately affected the recovery rate of PLW2. The PLW2 recovery rate by floating decreased to 81.3% with increasing H2O2 content to 3%, but then increased to 89.8 and 92.5% with further increasing H2O2 concentration up to 4 and 5%, respectively. The recovery of PLW1 by floating was remarkably decreased from 96.9 to 91.3 and 51.2% with the increase of the H2O2 amount from 3 to 4 and 5%, respectively. Fig.1 b showed the influent of H2O2 content on the purity of these plastics after flotation separation assisted by ultrasonic catalyst H2O2. It can be seen that, the purity of submerged PLW4 reached 89.9% at 2% of H2O2 dose, but then slowly decreased 2
Journal of Environmental Chemical Engineering 7 (2019) 103354
T.D. Nguyen, et al.
Fig. 2. The effect of contact time in ultrasonification on the separation of plastic wastes.
increasing in the duty cycle from 0 to 70%, respectively. However, the duty cycle had slightly effect on the purity of PLW1, PLW2 and PLW3 (less than 20% was change in the purity when duty cycles increase from 0 to 90%). The PLW4 showed higher in the purity than those of other samples due to the different wettability of PLW4 surface. The duty cycles of 70% was optimized for further experiments.
with further increase in H2O2 dose. At an appropriate amount of H2O2, It will interact with chemical species on the plastic surface. Therefore, some hydrophobic species in the interfacial plastic can be broken down by hydroxyl radicals. The oxidation of radicals then generated carbonyl (C]O) and carboxyl (COOH) groups. The C]C bonds in the butadiene and styrene of ABS were easily broken via alteration of the α-hydrogen in its structure, leading to an increase in hydrophilic groups of PLW4 fraction. However, excessive loading of H2O2 can induce the destruction of plastic surface and thereby decrease hydrophilic group, which might be the reason for the negative effect on recovery rate [24–27]. The H2O2 dose showed slightly effect on the purity of PLW1, PLW2 and PLW3, less than 10% of purity increased with an increasing in H2O2 dose from 0 to 5%. Based on the results, the optimum H2O2 dose of 2% was chosen for further experiments.
3.3. Radical scavengers and proposed mechanism In order to propose the mechanism of the PLW4 separation from plastics waste mixture, tert-butanol (t-BuOH, 20 mM) was added to scavenge the •OH radicals [32–38]. Fig. 4 showed the recovery rate of PLW1, PLW2, PLW3 and PLW4. It can be seen that, the recovery rate of PLW4 was 0 and 78.6% by adding 0 and 2% of H2O2 into the system without scavenger, respectively. However, the recovery rate of PLW4 was almost completely decreased at 10.2% by adding t- BuOH indicated that *OH radical is the prominent species for PLW4 separation form the mixtures, while other radicals species such as *O-, *OOH- which released from ultrasonic process displayed as minor effect. The flotation separation of PLW4 from the mixtures assisted by ultrasonic catalyst/ H2O2 is based on the detected radicals and the reaction pathway reported from literature [32–37]. Firstly, the H2O2 generated to hydroxyl radicals as showed in reaction (1):
3.2. Effect of contact time and the duty cycle in ultrasonication Fig. 2(a) displayed the recovery rate of PLW4 increased significantly with increasing contact time, whereas the recovery rate of floated PLW1 and PLW2 were maintained in an approximate range of 98.2–99.7%. The floating recovery rate of PLW3 slightly fluctuated at 60 s contact time with a decrease to 74.9%. The recovery of PLW4 rapidly increased and reached 86.8% after 180 s of contact time, and further up to 90.1 and 99.5% after 240 and 300 s, respectively. From Fig. 2 (b), it clearly supported to the enhancement in the purity of PLW4 after ultrasonication time of 180, 240 and 300 s at 96.2, 92.5 and 98.6%, respectively. Therefore, the optimum contact time of ultrasonication for separation of PLW4 from the mixtures plastics wastes was 300 s. In addition, compared to others methods which used for separation of ABS/PS, such as surface modification by chemicals [25–29, 37] or solvent-based [38], the flotation separation assisted by ultrasonic catalyst/ H2O2 showed less impact for environmental, shorter separation time, simple and high recovery rate. Therefore, it would be a promising method for industrial application of plastic waste treatment. A duty cycle of ultrasonication defies as the length of each pulse in which ultrasonic energy is active in discontinuous mode. For example, a duty cycle of 50% means ultrasonication to be on for 0.5 s, then off for 0.5 s and repeated. From Fig. 3 (a) it can be seen that, all of plastic samples (excepted for PLW4) were floated without assisted of ultrasonication and the recovery rate of PLW1, PLW2 and PLW3 were almost 98.8% while it was 0% for PLW4. However, the recovery rate of PLW4 was rapidly increased with an increasing in the duty cycle from 10 to 90%. The recovery rate of PLW4 was found at 99.8% at the duty cycle of 70% and a little decreased with further increasing in the duty cycle up to 90%. Besides, the duty cycle showed slightly influent on the recovery of PLW1, PLW2 and PLW3, the recovery rate was almost 100% at 70% of duty cycle. The effect of duty cycle on the floating purities of these samples were identified and the results are shown in Fig. 3(b). The purity of PLW4 significantly increased from 52.4 to 97.8% with an
H2O2 →2*OH
(1)
At the same time, by applied the ultrasound, it induces the sonolysis of water molecules and thermal dissociation of oxygen molecules, to produce different kinds of reactive species such as *OH, *O- and *OOHas previous studies [32–38], following the reaction (2) – (6): H2O + Ultrasonic → *OH + H*
(2)
O2 + Ultrasonic → 2O*
(3)
*OH + O* → *OOH
(4)
*O + H2O → 2*OH
(5)
*H + *O2→ *OOH
(6)
Therefore, the hydroxyl radicals from H2O2 and the radicals generated from ultrasonification process above will react with plastic surface and replacement of hydrophilic moieties on the PLW4 surface, and the separation of the submerged PLW4 and other floated plastics in the mixtures including PLW1, PLW2 and PLW3. The result indicated that, the modification of the plastic surface not only increased the hydrophilic moieties, but also improved the wettability of the PLW4 surface, leading to high separation efficiency between PLW4 and others in the mixtures. The hydroxyl radicals generated from H2O2 and during cavitation were respond for the oxidative plastic surface in the system.
3
Journal of Environmental Chemical Engineering 7 (2019) 103354
T.D. Nguyen, et al.
Fig. 3. The Effect of duty cycles on the separation of plastic wastes.
separation of PLW4 was proposed and the oxidized of PLW4 surface by hydroxyl radicals, leading to replace of hydrophilic moieties on the PLW4 surface. In addition the economic potential and the environmental impacts were discussed for further confirmed potential application of this study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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.jece.2019.103354. References Fig. 4. Radical scavengers.
[1] C.P. Baldé, V. Forti, V. Gray, R. Kuehr, P. Stegmann, The Global E-waste Monitor– 2017, United Nations University (UNU), International Telecommunication Union (ITU) & International Solid Waste Association (ISWA), Bonn/Geneva/Vienna, 2017. [2] P.A. Wäger, R. Hischier, Life cycle assessment of post-consumer plastics production from waste electrical and electronic equipment (WEEE) treatment residues in a Central European plastics recycling plant, Sci. Total Environ. 529 (2015) 158–167. [3] A. Buekens, J. Yang, Recycling of WEEE plastics: a review, J. Mater. Cycles Waste Manag. 16 (2014) 415–434. [4] F. Cucchiella, I. D’Adamo, S.C. Lenny Koh, P. Rosa, Recycling of WEEEs: an economic assessment of present and future e-waste streams, Renewable Sustainable Energy Rev. 51 (2015) 263–272. [5] Pyrolysis kinetics of tetrabromobisphenol a (TBBPA) and –Electric arc furnace dust mixtures, Thermochem Acta 660 (2018) 61–69. [6] Leaching of valuable metals from electric arc furnace dust—tetrabromobisphenol A pyrolysis residues, J. Anal. Appl. Pyrol. 125 (2017) 50–60. [7] Treatments of electric arc furnace dust and halogenated plastic wastes: a review, J. Environ. Chem. Eng. 7 (2019) 102856. [8] Thermal analysis on the pyrolysis of tetrabromobisphynol a (TBBPA) and electric arc furnace dust mixtures, Metall. Mater Trans. B (2017), https://doi.org/10.1007/ s11663-017-1121-7) In Press. [9] Bromine fixing ability of electric arc furnace dust during thermal degradation of tetrabromobisphenol: experimental and thermodynamic analysis study, J. Anal. Appl. Pyrol. 134 (2018) 503–509. [10] European Environment Agency, Typical charge (gate fee and landfill tax) for legal landfilling of non-hazardous municipal waste in EU Member States and regions, Eur. Environ. Agency (2019) [WWW Document]. URL https://www.eea.europa.eu/ data610 and-maps/figures/typical-charge-gate-fee-and. [11] J. Cui, Forssberg, Mechanical recycling of waste electric and electronic equipment: a review, J. Hazard. Mater. 99 (2003) 243–263. [12] H. Sun, X. Peng, S. Zhang, S. Liu, Y. Xiong, S. Tian, J. Fang, Activation of peroxymonosulfate by nitrogen-functionalized sludge carbon for efficient degradation of organic pollutants in water, Bioresour. Technol. 241 (2017) 244–251. [13] G. Fang, C. Liu, J. Gao, D.D. Dionysiou, D. Zhou, Manipulation of persistent free radicals in biochar to activate persulfate for contaminant degradation, Environ. Sci. Technol. 49 (2015) 5645–5653. [14] Y. Zhang, A.N. Mohamad, W. Ch, T.W. Paul, Pyrolysis–catalysis of waste plastic using a nickel–stainless-steel mesh catalyst for high-value carbon products, Environ. Technol. 38 (2017) 2889–2897. [15] F. Ghanbaric, M. Ahmadi, Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants: review, Chem. Eng. J. 310 (2017) 41–62. [16] A.-S. S M, A. Antelava, A. Constantinou, G. Manos, A. Dutta, A review on thermal
3.4. Economic potential and environmental impact The WEEE components are considered top quality grades of acrylonitile butadiene styrene terpolymers and their blends with polymethylmehtacrylate or polycarbonate, as well as high impact polyStyrene and its blend with polyphenyleneether [1–3]. These special engineering blends have market prices up to 4 times higher compared to standard packaging polymers such as polyethylene, polypropylene and polyethylene terephthalate [4]. Besides, the WEEE collection rates are increasing year by year around the world, thus recovery of WEEE has potential to add-value of plastics waste as well as reduce the effect for environmental. The prices of plastics recycles can be significantly lower than the prices of the virgin plastics, largely depending on their quality, supply volumes and reliability. The outcome of flotation separation of ABS/PC from the plastic waste mixtures assisted by ultrasonic catalyst/H2O2 indicated the high potential of this method to recover value added plastic wastes along with reduce the environmental impacts [1,2,10,11]. 4. Conclusions In this study, the separation of ABS/PC plastic from the plastics waste mixtures using flotation assisted by ultrasonic catalyst/H2O2 was investigated. The influent of major factors such as H2O2 dose, duty cycle and contact time on the separation process were identified. The results showed that PLW4 mostly recovery under optimum condition of 2% H2O2, 70% duty cycle and 300 s of contact time. The ultrasonic/ H2O2 provided the hydroxyl radicals as the prominent species responded for PLW4 separation form the plastic waste mixtures. The mechanism of the combined ultrasonication catalyst/ H2O2 for 4
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[27] S. Chen, Y. Zhang, C. Guo, Y. Zhong, K. Wang, H. Wang, Separation of polyvinyl chloride from waste plastic mixtures by froth flotation after surface modification with sodium persulfate, J. Clean. Prod. 218 (2019) 167–172. [28] Y. Zhang, S. Chen, H. Wang, M. Luo, Separation of polyvinylchloride and acrylonitrile-butadiene-styrene combining advanced oxidation by S2O82−/Fe 2+ system and flotation, Waste Manage. 91 (2019) 80–88. [29] X. Ma, Y.Q. Cheng, Y.J. Ge, H.D. Wu, Q.S. Li, N. Gao, J. Deng, Ultrasound-enhanced nanosized zero-valent copper activation of hydrogen peroxide for degradation of norfloxacin, Ultrason. Sonochem. 40 (2018) 763–772. [32] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/Visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics, J. Phys. Chem. A 115 (2011) 13211–13241. [33] P. Bansal, A. Verma, Synergistic effect of dual process (photocatalysis and photoFenton) for the degradation of Cephalexin using TiO2 immobilized novel clay beads with waste fly ash/foundry sand, J. Photochem. Photobiol. A: Chem. 342 (2017) 131–142. [34] M. Lim, Y. Son, J. Khim, The effects of hydrogen peroxide on the sonochemical degradation of phenol and bisphenol A, Ultrason. Sonochem. 21 (2014) 1976–1981. [35] R.A. Torres-Palma, E.A. Serna-Galvis, Sonolysis, Adv. Oxid. Process, Wastewater Treat. Emerg. Green Chem. Technol. 217 (2018) 177–213. [36] G. Fang, J. Gao, D.D. Dionysiou, Y. Wang, D. Zhou, Key role of persistent free radicals in hydrogen peroxide activation by biochar: implications to organic contaminant degradation, Environ. Sci. Technol. 48 (2014) 1902–1910. [37] G. Ch, Q. Zou, J. Wang, H. Wang, C. Sh, Y. Zhong, Application of surface modification using sodium hypochlorite for helping flotation separation of acrylonitrilebutadiene-styrene and polystyrene plastics of WEEE, Waste Manag. 82 (2018) 167–176. [38] Y.B. Zhao, X.-D. Lv, H.G. Ni, Solvent-based separation and recycling of waste plastics: areview, Chemosphere 209 (2018) 707–720.
and catalytic pyrolysis of plastic solid waste (PSW), J. Environ. Manag. 197 (2017) 177–198. S.Y. Shibashova, A.V. Shibashov, Effect of low frequency ultrasound waves on the hydrogen peroxide degradation process, Energies. 50 (2016) 165–168. J. Fu, H. Zhang, A. Zhang, G. Jiang, E-waste recycling in China: a challenging field Environ, Sci. Technol. 52 (2018) 6727–6728. T. Takoungsakdakun, S. Pongstabodee, Separation of mixed post-consumer PET–POM–PVC plastic waste using selective flotation, Sep. Purif. Technol. 54 (2007) 248–252. E. Dimitrakakis, A. Janz, B. Bilitewski, E. Gidarakos, Small WEEE: determining recyclables and hazardous substances in plastics, J. Hazard. Mater. 161 (2009) 913–919 2009. M.S. Reddy, K. Kurose, T. Okuda, W. Nishijima, M. Okada, Separation of polyvinyl chloride (PVC) from automobile shredder residue (ASR) by froth flotation with ozonation, J. Hazard. Mater. 147 (2007) 1051–1055. J.C. Wang, H. Wang, J. Fu, Y. Liu, Flotation separation of waste plastics for recycling- a review, Waste Manage 41 (2015) 28–38. J. Wang, H. Wang, D. Yue, Optimization of surface treatment using sodium hydrochlorite facilitates coseparation of ABS and PC from WEEE plastic by flotation, Environ. Sci. Technol. 53 (2019) 4086-2094. J.C. Wang, H. Wang, Fenton treatment for flotation separation of polyvinyl chloride from plastic mixtures, Sep. Purif. Technol. 187 (2017) 415–425. C. Wang, H. Wang, Y. Cao, Waste printed circuit boards as novel potential engineered catalyst for catalytic degradation of orange II, J. Clean. Prod. 221 (2019) 234–241. C. Gui, Q. Zuo, J. Wang, H. Wang, S. Chen, Y. Zhong, Application of surface modification using sodiumhydrochlorite for helping flotation separeation of acrylonitrile-butadiene-stryrene and polystryrene plastics of WEEE, Waste Manage. 82 (2018) 167–176.
5
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Selective flotation separation of ABS/PC from ESR plastic wastes mixtures assisted by ultrasonic catalyst/H2O2 Trinh Duy Nguyena,b, T.M. Al Tahtamounic, Pham Thi Huongd, Phan Quang Thange,f,
T
⁎
a
NTT Hi-Tech Institute, Nguyen Tat Thanh University, 300A Nguyen Tat Thanh Street, Dist. 4, Ho Chi Minh City, Vietnam Center of Excellence for Green Energy and Environmental Nanomaterials, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam c Materials Science and Technology Program, College of Arts and Sciences, Qatar University, Doha, 2713, Qatar d Center for Advanced chemistry, Institute of research and development, Duy Tan University, 550000, DaNang, Vietnam e Division of Computational Mathematics and Engineering, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam f Faculty of Environment & Labour Safety, Ton Duc Thang University, Ho Chi Minh City, Vietnam b
A R T I C LE I N FO
A B S T R A C T
Keywords: Waste electrical Plastic waste Recycles Ultrasonication Hydroxyl radical
The present study investigated the potential recycles of acrylonitile butadiene styrene terpolymers (ABS) from their blends with polymethylmehtacrylate (PMMA), polyCarbonate (PC) and high impact polyStyrene (HIPS) using flotation separation assisted by ultrasonic catalyst/ H2O2. The effect of various factors such as H2O2 dose, duty cycle and contact time of the ultrasonication on the recovery rate and purity of ABS/PC were conducted. The results showed that H2O2 dose significantly influenced on the recovery rate of ABS/PC and the optimized H2O2 dose was found at 2%. The recovery rate and the purity of submerged ABS/PC reached 99.5 and 98.6%, respectively, at 300 s of contact time. The duty cycle and the contact time of the ultrasonification also exhibited highly effective for the recycle of ABS/PC from the plastic waste mixtures. The radical scavengers and the mechanism of flotation separation of ABS/PC by ultrasonic catalyst/ H2O2 were proposed. Additionally, the economic potential and environmental impacts were discussed. These findings are crucial for flotation separation of ABS/PC from the plastic waste mixtures assisted by ultrasonic catalyst/ H2O2 with high recovery rate and purity of ABS/PC in order to produce further value added products as well as reduce the environmental impact of plastic waste.
1. Introduction Waste Electrical and Electronic Equipment (WEEE) is one of the fastest growing waste streams with approximately 44, 7 million metric tons that were generated worldwide in 2016. WEEE is a heterogeneous mixture of various electrical equipments like computers, televisions, washing machines and other household appliances. WEEE components also contain different valuable materials such as metals (70%), plastics (20%) and refractories (10%). Consequently, most of the plastics isolated or segregated from the WEEE has been disposed by landfill and incineration, and only 10% is treated by recycling, thus causing environmental pollution. Therefore, WEEE is receiving worldwide attention due to the increasing environmental issues, management and disposal [1–4]. Until now, WEEE recycling is challenged by a large mix of different materials, often containing hazardous substances such as heavy metals and some types of brominated flame retardants that are today restricted in developed countries. In contrast, WEEE recycling encompasses many opportunities to reduce environmental impacts, as ⁎
well as generating economic value [5–9]. The recycle of WEEE can be expected to relieve the detrimental emissions including polybrominated dibenzodioxins and dibenzofurans which are generated in the incineration of brominated flame retardants. It has been demonstrated that the components of plastic in WEEE considered top quality grades of acrylonitile butadiene styrene terpolymers (ABS) and their blends with polymethylmehtacrylate (PMMA) or polyCarbonate (PC), as well as high impact polyStyrene (HIPS). Intriguingly, 36.75% of ABS and 4.99% of PC can be found in WEeEE plastics and are perfectly suited to recycling. Also, the blends of ABS/PC have considerable market importance because their excellent characteristics have led to many applications [10]. The ABS and PC of WEeEE plastic through blending show significant potential for recycling. However, the recycling of waste ABS and PC is challenging because they are generally mixed with other plastics. A number of promising technologies were investigated for recycles of WEEE such as hydrocyclone, sink-float and electrostatic separation. Plastics flotation is an alternative method, which shows advantages such as cost-effective and higher separation efficiency,
Corresponding author. E-mail addresses: [email protected] (T.D. Nguyen), [email protected] (T.M. Al Tahtamouni), [email protected] (P.Q. Thang).
https://doi.org/10.1016/j.jece.2019.103354 Received 23 April 2019; Received in revised form 9 August 2019; Accepted 9 August 2019 Available online 11 August 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 103354
T.D. Nguyen, et al.
Fig. 1. The effect of H2O2 dose on the separation of the plastic wastes.
especially for plastics with similar density. The mechanism of flotation is the selective attachment of air bubbles on hydrophilic surface, resulting in distinct floating/submerging behaviors in flotation and thereby enabling the separation of plastics. However, the plastics have low energy surfaces, they are naturally hydrophobic. Flotation separation of plastics requires selective hydrophilicity of certain kind of plastics and surface treatment is an effective way to change the hydrophilicity of plastics [23–26]. The methods of surface treatment with nano-Fe/Ca/CaO ozonization, mild-heat treatment, nanometallic Ca/ CaO treatment, ZnO/microwave treatment, ZnO coating, powder activated carbon coating, and Fenton oxidation have been investigated. However, these surface treatment methods have been done with only single separation of ABS or PVC from the plastic waste mixed [11]. Advanced oxidation processes (AOPs) for remediation of contaminated soils, groundwater, and sediments have received considerable attention in recent years. These processes are based mostly on activation of hydrogen peroxide (H2O2), titanium dioxide (TiO2), persulfate (PDS) and peroxymonosulfate (PMS) to generate oxidizing species such as sulfate radical (SO4•-) (2.5–3.1 V) and hydroxyl radical (•OH) (2.4–3.0 V), which are highly reactive oxidants toward organic compounds. Previous researches reported that AOPs technology is promising method for the treatment of plastic surface [12,13]. Hydrogen peroxide is the most widely used for surface modification of plastic due to its low-cost, simple operation, low energy requirement and environmentally friendly. As reported from previous the literature, the hydroxyl radical can selectively increase the hydrophilicity of the plastics surface (ABS, PS and PC), leading to increase the separation rate during flotation process [12–15]. Nevertheless, the ultrasound has been applied effectively as an emerging advanced oxidation process (AOP). The main advantage is that the ultrasound process does not require added chemicals, oxidants or catalysts, and does not generate additional waste streams as compared with other processes [11–16]. Hydrogen peroxide is one of the most effective additives used to enhance sonochemical degradation of organic pollutants. During ultrasound irradiation, H2O2 can dissociate into hydroxyl radicals, though these have a very short lifetime and tend to combine and form H2O2. As hydrogen peroxide present at high concentration can act as a radical scavenger. The combine of ultrasonic treatment of plastic in a liquid medium during peroxide bleaching is associated with the occurrence of a number of physical and chemical phenomena, such as cavitation, redox reactions, polymerization and depolymerization [16–20]. Therefore, the combine of ultrasonic and H2O2 for the surface treatment of plastic waste is an idea method to increase the hydrophilic functional groups on the surface of plastic in order to improve the separation capacity and recycles ability of ABS/ PC. This study aim to investigate the potential application of flotation separation of ABS/PC from WEEE mixed assisted by ultrasonic/H2O2. The H2O2 dose, duty cycle and contact time of ultrasonication was optimized for the ABS/PC separation process. To identify the major oxidation species involved during the surface treatment of plastic waste, the experiment for radical scavenger was conducted. The
mechanism and environmental impact were also discussed in detail. 2. Materials and methods 2.1. Materials The WEEE samples were collected from Ho Chi minh City, Vietnam without contain any uniform shapes or sizes of the styrene plastic pieces obtained from the shredded residue. The collected samples were segregated washed, dried and cut into similar size with an average diameter of 0.4-0.45 cm, prior to specimen preparation. After classify of obtained plastic wastes, the samples were denoted as PLW1, PLW2, PLW3 and PLW4, corresponding to the ABS, PS/HIPS, ABS/PMMA and ABS/PC, respectively. 2.2. Methods The method use to separation of PLW1, PLW2, PLW3 and PLW4 was prepared according to previously reported [19–24]. The details of synthetization method is shown in the Supporting Information. 3. Results and discussion 3.1. Effect of H2O2 dose The effect of H2O2 concentration on the recovery and purity of plastics were investigated under the conditions of duty cycle of 50%, irradiation time of 5 min, and air supply time of 2 min. The PLW4 was separated on the reactor bottom while the remaining plastics remained floating on top of the medium. The recovery and purity rates of ESR plastics were evaluated on the floating rate of PLW1, PLW2 and PLW3 and the submerging rate of PLW4 as a function of H2O2 dose, as shown in Fig. 1a. It can be observed that, without adding of H2O2, the PLW1, PLW2 and PLW3 completely floated on top of the floating medium due to the inherent hydrophobic surface of styrene plastics helps the adherence of air bubbles. However, the H2O2 dose significantly affected on the recovery rate of PLW1, PLW2, PLW3 and PLW4. As shown in Fig.1a, the recovery rate of the submerged PLW4 increased from 0 to 85.6% with an increase in H2O2 content from 0 to 2%, then slightly effected with further increasing in H2O2 content (3–5%). The H2O2 dose had no effect on the recovery rate by floating PLW3 and moderately affected the recovery rate of PLW2. The PLW2 recovery rate by floating decreased to 81.3% with increasing H2O2 content to 3%, but then increased to 89.8 and 92.5% with further increasing H2O2 concentration up to 4 and 5%, respectively. The recovery of PLW1 by floating was remarkably decreased from 96.9 to 91.3 and 51.2% with the increase of the H2O2 amount from 3 to 4 and 5%, respectively. Fig.1 b showed the influent of H2O2 content on the purity of these plastics after flotation separation assisted by ultrasonic catalyst H2O2. It can be seen that, the purity of submerged PLW4 reached 89.9% at 2% of H2O2 dose, but then slowly decreased 2
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Fig. 2. The effect of contact time in ultrasonification on the separation of plastic wastes.
increasing in the duty cycle from 0 to 70%, respectively. However, the duty cycle had slightly effect on the purity of PLW1, PLW2 and PLW3 (less than 20% was change in the purity when duty cycles increase from 0 to 90%). The PLW4 showed higher in the purity than those of other samples due to the different wettability of PLW4 surface. The duty cycles of 70% was optimized for further experiments.
with further increase in H2O2 dose. At an appropriate amount of H2O2, It will interact with chemical species on the plastic surface. Therefore, some hydrophobic species in the interfacial plastic can be broken down by hydroxyl radicals. The oxidation of radicals then generated carbonyl (C]O) and carboxyl (COOH) groups. The C]C bonds in the butadiene and styrene of ABS were easily broken via alteration of the α-hydrogen in its structure, leading to an increase in hydrophilic groups of PLW4 fraction. However, excessive loading of H2O2 can induce the destruction of plastic surface and thereby decrease hydrophilic group, which might be the reason for the negative effect on recovery rate [24–27]. The H2O2 dose showed slightly effect on the purity of PLW1, PLW2 and PLW3, less than 10% of purity increased with an increasing in H2O2 dose from 0 to 5%. Based on the results, the optimum H2O2 dose of 2% was chosen for further experiments.
3.3. Radical scavengers and proposed mechanism In order to propose the mechanism of the PLW4 separation from plastics waste mixture, tert-butanol (t-BuOH, 20 mM) was added to scavenge the •OH radicals [32–38]. Fig. 4 showed the recovery rate of PLW1, PLW2, PLW3 and PLW4. It can be seen that, the recovery rate of PLW4 was 0 and 78.6% by adding 0 and 2% of H2O2 into the system without scavenger, respectively. However, the recovery rate of PLW4 was almost completely decreased at 10.2% by adding t- BuOH indicated that *OH radical is the prominent species for PLW4 separation form the mixtures, while other radicals species such as *O-, *OOH- which released from ultrasonic process displayed as minor effect. The flotation separation of PLW4 from the mixtures assisted by ultrasonic catalyst/ H2O2 is based on the detected radicals and the reaction pathway reported from literature [32–37]. Firstly, the H2O2 generated to hydroxyl radicals as showed in reaction (1):
3.2. Effect of contact time and the duty cycle in ultrasonication Fig. 2(a) displayed the recovery rate of PLW4 increased significantly with increasing contact time, whereas the recovery rate of floated PLW1 and PLW2 were maintained in an approximate range of 98.2–99.7%. The floating recovery rate of PLW3 slightly fluctuated at 60 s contact time with a decrease to 74.9%. The recovery of PLW4 rapidly increased and reached 86.8% after 180 s of contact time, and further up to 90.1 and 99.5% after 240 and 300 s, respectively. From Fig. 2 (b), it clearly supported to the enhancement in the purity of PLW4 after ultrasonication time of 180, 240 and 300 s at 96.2, 92.5 and 98.6%, respectively. Therefore, the optimum contact time of ultrasonication for separation of PLW4 from the mixtures plastics wastes was 300 s. In addition, compared to others methods which used for separation of ABS/PS, such as surface modification by chemicals [25–29, 37] or solvent-based [38], the flotation separation assisted by ultrasonic catalyst/ H2O2 showed less impact for environmental, shorter separation time, simple and high recovery rate. Therefore, it would be a promising method for industrial application of plastic waste treatment. A duty cycle of ultrasonication defies as the length of each pulse in which ultrasonic energy is active in discontinuous mode. For example, a duty cycle of 50% means ultrasonication to be on for 0.5 s, then off for 0.5 s and repeated. From Fig. 3 (a) it can be seen that, all of plastic samples (excepted for PLW4) were floated without assisted of ultrasonication and the recovery rate of PLW1, PLW2 and PLW3 were almost 98.8% while it was 0% for PLW4. However, the recovery rate of PLW4 was rapidly increased with an increasing in the duty cycle from 10 to 90%. The recovery rate of PLW4 was found at 99.8% at the duty cycle of 70% and a little decreased with further increasing in the duty cycle up to 90%. Besides, the duty cycle showed slightly influent on the recovery of PLW1, PLW2 and PLW3, the recovery rate was almost 100% at 70% of duty cycle. The effect of duty cycle on the floating purities of these samples were identified and the results are shown in Fig. 3(b). The purity of PLW4 significantly increased from 52.4 to 97.8% with an
H2O2 →2*OH
(1)
At the same time, by applied the ultrasound, it induces the sonolysis of water molecules and thermal dissociation of oxygen molecules, to produce different kinds of reactive species such as *OH, *O- and *OOHas previous studies [32–38], following the reaction (2) – (6): H2O + Ultrasonic → *OH + H*
(2)
O2 + Ultrasonic → 2O*
(3)
*OH + O* → *OOH
(4)
*O + H2O → 2*OH
(5)
*H + *O2→ *OOH
(6)
Therefore, the hydroxyl radicals from H2O2 and the radicals generated from ultrasonification process above will react with plastic surface and replacement of hydrophilic moieties on the PLW4 surface, and the separation of the submerged PLW4 and other floated plastics in the mixtures including PLW1, PLW2 and PLW3. The result indicated that, the modification of the plastic surface not only increased the hydrophilic moieties, but also improved the wettability of the PLW4 surface, leading to high separation efficiency between PLW4 and others in the mixtures. The hydroxyl radicals generated from H2O2 and during cavitation were respond for the oxidative plastic surface in the system.
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Fig. 3. The Effect of duty cycles on the separation of plastic wastes.
separation of PLW4 was proposed and the oxidized of PLW4 surface by hydroxyl radicals, leading to replace of hydrophilic moieties on the PLW4 surface. In addition the economic potential and the environmental impacts were discussed for further confirmed potential application of this study. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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.jece.2019.103354. References Fig. 4. Radical scavengers.
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3.4. Economic potential and environmental impact The WEEE components are considered top quality grades of acrylonitile butadiene styrene terpolymers and their blends with polymethylmehtacrylate or polycarbonate, as well as high impact polyStyrene and its blend with polyphenyleneether [1–3]. These special engineering blends have market prices up to 4 times higher compared to standard packaging polymers such as polyethylene, polypropylene and polyethylene terephthalate [4]. Besides, the WEEE collection rates are increasing year by year around the world, thus recovery of WEEE has potential to add-value of plastics waste as well as reduce the effect for environmental. The prices of plastics recycles can be significantly lower than the prices of the virgin plastics, largely depending on their quality, supply volumes and reliability. The outcome of flotation separation of ABS/PC from the plastic waste mixtures assisted by ultrasonic catalyst/H2O2 indicated the high potential of this method to recover value added plastic wastes along with reduce the environmental impacts [1,2,10,11]. 4. Conclusions In this study, the separation of ABS/PC plastic from the plastics waste mixtures using flotation assisted by ultrasonic catalyst/H2O2 was investigated. The influent of major factors such as H2O2 dose, duty cycle and contact time on the separation process were identified. The results showed that PLW4 mostly recovery under optimum condition of 2% H2O2, 70% duty cycle and 300 s of contact time. The ultrasonic/ H2O2 provided the hydroxyl radicals as the prominent species responded for PLW4 separation form the plastic waste mixtures. The mechanism of the combined ultrasonication catalyst/ H2O2 for 4
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