Separation of hazardous polyvinyl chloride from waste plastics by flotation assisted with surface modification of ammonium persulfate: Process and mechanism

Journal of Hazardous Materials 389 (2020) 121918

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Separation of hazardous polyvinyl chloride from waste plastics by flotation assisted with surface modification of ammonium persulfate: Process and mechanism

T

Yingshuang Zhanga, Hongru Jianga, Hui Wanga,*, Chongqing Wangb,* a b

College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, China School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, 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

Editor: R Teresa

Plastic separation becomes an effective method to improve the plastic recycling by concentrating a single component from complex plastic mixtures. Based on advanced oxidation process, surface modification assisted by ammonium persulfate ((NH4)2S2O8) was applied to selectively wet plastic surface, achieving the separation of hazardous polyvinyl chloride (PVC) from acrylonitrile butadiene styrene (ABS), polystyrene (PS), and polycarbonate (PC) in forth flotation. The mechanisms were investigated through contact angle, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), as well as scanning electron microscope (SEM). The floatability of PS, PC, and ABS reduces owing to the introduction of carbonyl (O = CeO), hydroxyl (−OH), and amide (O = C–NH2) on plastic surfaces, which is the result of the oxidation by sulfate radical (SO∙− 4 ) and the hydrolysis of nitrile group (C^N) and butadiene (C]C). Then, available reaction equations of ABS, PS, and PC were established to supplement the mechanisms of surface modification. The optimal conditions for flotation separation of PVC are (NH4)2S2O8 concentration 0.2 M, temperature 70 °C, pretreatment time 30 min, pH 10, flotation time 4 min, terpineol dosage 20 mg/L, and particle size 3–4 mm. The recovery and purity of PVC reach 100 % and 99.7 ± 0.2 % respectively, favoring the reuse of separated waste plastic.

Keywords: Plastic flotation Hazardous plastic Surface modification Advanced oxidation Ammonium persulfate

⁎

Corresponding authors. E-mail addresses: [email protected] (H. Wang), [email protected] (C. Wang).

https://doi.org/10.1016/j.jhazmat.2019.121918 Received 27 August 2019; Received in revised form 14 December 2019; Accepted 16 December 2019 Available online 19 December 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 389 (2020) 121918

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1. Introduction

Fang et al., 2012; Mahdi Ahmed et al., 2012). Persulfate can be activated by heat, laser flash, and zero value metal to produce enough SO4·− for the oxidation of contamination (Huie and Clifton, 1990; Criquet and Leitner, 2009). It is evident that surface modification by (NH4)2S2O8 is low toxic and environmentally friendly (Alzate et al., 2016). In addition, according to published literature, (NH4)2S2O8 has not been applied in surface modification for plastic flotation. Therefore, it is a meaningful attempt to introduce (NH4)2S2O8 into the surface modification for efficient plastic flotation. Herein, we put forward a novel surface modification to alter plastic hydrophobicity, thereby achieving the flotation separation of hazardous PVC from other waste plastics. (NH4)2S2O8 assisted by heating activation, as a typical AOP, was carried out to change the wettability of ABS, PS, and PC. Besides, mechanisms of surface modification were explored by the contact angle, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM). Finally, we clarified the surface reactions of hydrophilic plastics in the pretreatment system of (NH4)2S2O8, possibly promoting the application of surface modification by AOP in plastic flotation.

The total production of plastics has been 6300 Mt till now, in which 4900 Mt of plastics are discarded in the environment (Geyer et al., 2017). With the increasing attention on the consumption of plastics and the circular economy, plastic recycling has become an essential scheme to deal with the environmental pollution and energy depletion caused by mismanagement of waste plastics. However, different types of plastics are collected together as mixtures, making it hard to recycle. For example, the product from chemical recycling of plastic mixtures is too complex to use directly due to quite various pyrolysis conditions of different plastics (Lopez et al., 2017). Besides, the recycled plastic product is unqualified on account of the chemical and physical incompatibility of impurities and target plastics (Wang et al., 2016a). Therefore, the separation of each plastic from mixtures is beneficial for the recycling of waste plastic, which can be illustrated by life cycle assessment as well (Al-Salem et al., 2009). Three categories of waste plastic mixtures mostly researched in plastic separation include automobile shredder residue (ASR) (Reddy et al., 2007), municipal solid waste (MSW) (Wang et al., 2016b), and waste electrical and electronic equipment (WEEE) (Mallampati et al., 2015). Wherein, PVC always mixes with ABS, PS, and PC, because PVC is widely used in packaging, electronic equipment, construction, and automobile. However, even a small amount of undesired PVC can deteriorate the quality of recycled plastics and damage the oil of plastic pyrolysis. Besides, PVC, a chlorine-rich plastic, is considered as a hazardous material, because soil and water are exposed to heavy metals and persistent organic contaminants that migrate from discarded PVC (Yu et al., 2016; Mallampati et al., 2018). Furthermore, chloride in PVC can contribute to carcinogenic dioxins after incineration and decomposition. Therefore, it is particularly significant to separate PVC from other plastics. Many methods have been proposed to separate plastic mixtures including spectroscopic selection (Beigbeder et al., 2013; Mauruschat et al., 2016), electrostatic selection (Singh et al., 2017; Tilmatine et al., 2009), density separation (Moroni et al., 2017; Gent et al., 2018), and froth flotation separation (Wang et al., 2015a, a; Wang et al., 2013b). In comparison to other methods, flotation, based on selective adsorption of air bubbles on the surface of different plastics, has the advantage of separating plastic mixtures with similar chemical and physical characteristics (Mallampati et al., 2015). Therefore, froth flotation is worthy of being further researched for its application in plastic separation. Due to the natural hydrophobicity of plastics, it is necessary to modify the plastic surface before plastic flotation (Guney et al., 2013; Wang et al., 2014a). The selectivity of flotation is realized by different hydrophobicity of plastic surfaces through surface modification. In the past few years, many modification methods were applied to plastic flotation including depressant reagents (Wang et al., 2013a, b; Guney et al., 2013), ozone treatment (Reddy et al., 2008), chemical oxidation (Wang et al., 2015b, a; Wang et al., 2017b), mild heat treatment (Thanh Truc et al., 2017), steam treatment (Hu et al., 2010), boiling treatment (Wang et al., 2014b), microwave treatment (Thanh Truc and Lee, 2017a), ZnO coating (Thanh Truc and Lee, 2017b), and dehalogenation (Mallampati et al., 2017; Nah et al., 2008). Then, hydrophilic groups such as carboxyl (O = CeO), carbonyl (C]O), hydroxyl (−OH), or amide (O = C–NH2), are introduced on the plastic surface to selectively change plastic wettability when the reactions of oxidation, hydrolysis, ammonolysis, and adsorption occur. Thus far, the advanced oxidation processes (AOPs) are emerging methods of surface modification for plastic flotation, which is deserved intensive research, and available reactions on plastic surface based on AOPs are still unclear. Ammonium persulfate ((NH4)2S2O8) is widely applied as a powerful oxidant in AOPs, owing to the generation of sulfate radical anion (SO4·−). For example, some organic pollutants in wastewater, such as methyl orange, polychlorinated biphenyls, and sulfamethoxazole, can be removed effectively by the oxidation of persulfate (Li et al., 2014;

2. Materials and methods 2.1. Materials and reagents Waste plastic mixtures (ABS, PS, PC, and PVC) were collected from the waste materials markets in Miluo (Hunan, China). Then, plastic materials were broken into particles by the plastic crushing machine (SCP-180-2, Yinbao machinery company, Cixi, Zhejiang, China). Further, plastic particles were divided into 0.8–2.0 mm, 2.0–3.0 mm, 3.0–4.0 mm, and 4.0–5.0 mm by screening to research the effect of size on plastic flotation, because it was verified that the optimum separation of plastics can always be obtained in the size range of 0.8–5.0 mm (Wang et al., 2018). These waste plastics were washed with tap water for 24 h by continuous stirring using a DJ1C-enhanced electric agitator (Shanghai Lichen Instrument Technology Co., Ltd) and dried at room temperature (25 °C). It was convenient to sort different plastics by hand and conduct statistical analysis after flotation experiments, because the plastic samples had different colors (Fig. S1). (NH4)2S2O8 and terpineol were applied as surface modification reagent and frother, respectively. The pH value of the pretreatment solution was adjusted by 0.1–1 M HCl and NaOH. These chemical reagents were analytical purity, and the flotation medium used in all experiments was deionized water. 2.2. Surface modification Waste plastics (PVC, PC, PS, and ABS) of 10 g with equal mass ratio were added to the 100 mL (NH4)2S2O8 solution in the process of surface modification, to research the influence of (NH4)2S2O8 pretreatment on plastic floatability. The solid-to-liquid ratio was about 10 % determined by a large number of previous experiments (Wang et al., 2017b). Surface modification was carried out in the thermostatic water bath device (Dongfeng 101 s, Yuhua instrument company, Gongyi, Henan, China) with 300 r/min. In surface modification, (NH4)2S2O8 dosage (0–0.4 M), pH (1–13), pretreatment temperature (10–80 °C), and pretreatment time (0–50 min) were studied under flotation conditions of flotation time 4 min, terpineol dosage 20 mg/L, and particle size fraction 2–4 mm. The NaOH and HCl solutions were transferred by 1 mL syringe. The treated plastic particles were filtered out from the (NH4)2S2O8 solution, washed with deionized water, and transferred into a flotation column for the flotation experiments (Fig. 1). 2.3. Conditions experiments and flotation separation The pneumatic flotation column with a porous sand core in the bottom was designed to carry out all of the flotation experiments. This 2

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Fig. 1. Flow chart of pretreatment and flotation.

flotation column with a 580 mm diameter and 60 mm height was made of a transparent polymethyl methacrylate, which favors the observation of flotation separation. An adjustable micro air compressor (SaierS-88B Saier Libaco, Ltd, Beijing, China) connected with the column could generate micro air bubbles when gas flowed through the porous sand core. Flotation time (0–5 min) and terpineol dosage (0–60 mg/L) were investigated in flotation experiments. The factors matrix for experiments of surface modification and flotation separation were designed in Table S1. Besides, the experiments of surface modification and flotation separation were conducted with three replications. Subsequently, floating and submerged plastics were collected and rinsed separately after the flotation experiment. Then flotation percentage, purity, and recovery were calculated after drying at room temperature (25 °C). The flotation percentage, purity, and recovery of PVC and other plastics were calculated by Eqs. 1–10.

Flotation percentage:FloPVC =

Floother plastics =

PVCf × 100% PVCs + PVCf

Other plastics f × 100% Other plasticss + Other plasticsf

(4)

× 100%

Other plasticss × 100% Other plasticsf + Other plasticss

(8)

(9)

(10)

Where PVCf and Other plastics f represent the mass of PVC and other plastics in floating components. PVCs and Other plasticss are the mass of PVC and other plastics in submerged components. Fig. 1 describes the diagram of equipment connection in flotation separation.

Contact angle, Fourier transform infrared spectrum (FT-IR), Scanning electron microscope (SEM), and X-ray photoelectron spectrum (XPS) were used to characterize the waste plastic after surface modification by (NH4)2S2O8. All samples for surface characterization were modified under conditions of (NH4)2S2O8 concentration 0.2 M, pH 10, temperature 70 °C, and pretreatment time 30 min, and then dried at 70 °C for 12 h in a vacuum drying oven (Jinghong DZF-6050, Tianheng Instrument Co., Ltd, Changsha, China). The samples for contact angle and SEM measurements were micro plastic boards in a dimension of 3 × 3 mm and 1 mm thick. However, the samples for FT-IR and XPS measurements were ground into microparticles with a size of less than 0.074 mm. Thereinto, the hydrophilicity of different plastics could be described by the contact angle that was measured by the statistic contact angle measuring instrument (JJC-I, Changchun, China). Surface morphology was researched by scanning electron microscope (MIRA3, TESCAN, Czech Republic). XPS and FT-IR analysis were carried out in order to detect the functional groups and chemical composition on plastic surfaces by Fourier transform infrared spectrometer (Nicolet Avatar 360, Nicolet Magua Corporation, USA) and X-ray photoelectron spectrometer (K-Alpha1063, Thermo Fisher Scientific, USA). In addition, FT-IR spectra were recorded in the scanning range of 4000–400 cm−1 by thin KBr film with plastic powders of 3 % mass ratio, and the XPS spectra of powdery plastics were acquired by monochromatic Al

(5)

Other plastics f × 100% Other plasticsf + Other plasticss

Purity of plastics in sunken components:SPurPVC =

S RecOther plastics =

(2)

Other plastics f × 100% Other plasticsf + PVCf

FRecother plastics =

Recovery of plastics in sunken components PVCs :SRecPVC = × 100% PVC f + PVCs

(1)

(3)

Recovery of plastics in floating components PVCf :FRecPVC = × 100% PVC f + PVCs

Other plasticss × 100% Other plasticss + PVCs

2.4. Analysis of surface characterization

Purity of plastics in floating components PVCf :FPurPVC = × 100% Other plasticsf + PVCf

FPurother plastics =

SPurOther plastics =

(6)

PVCs Other plasticss + PVCs (7) 3

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Fig. 2. Effect of (a) (NH4)2S2O8 concentration, (b) temperature, (c) pH and (d) pretreatment time on the floatability of plastic mixtures.

Kα X-rays of 1486.6 eV with the binding energy of 1200–0 eV.

PS, PC, and ABS (Wang et al., 2017a).

3. Results and discussion

3.2.2. Surface morphology analysis The surface roughness of ABS, PC, and PS before and after (NH4)2S2O8 treatment is given in Fig. 3. As shown in Fig. 3a, 3c, and 3e, some scratches, pits, and protuberances appear on the surface of waste plastics. It is concluded that waste plastics obtain a high roughness after the intended use. Considering the high floatability of plastics before (NH4)2S2O8 treatment, original surface roughness cannot provide hydrophilic surfaces for waste plastics. As shown in Fig. 3b, d, and f, the surface roughness of ABS, PC, and PS are enhanced slightly after (NH4)2S2O8 treatment. After surface modification, the surfaces of ABS and PS have more granular protuberances compared to original surfaces, and some flaky structures present on the PC surface, manifesting that (NH4)2S2O8 treatment slightly etches plastic surfaces. SEM results show that oxidation may increase surface roughness and further improve hydrophilicity.

3.1. Pretreatment experiments As shown in Fig. 2, the flotation percentage of unmodified plastics is about 100 % due to the natural hydrophobicity of plastic. After pretreatment by (NH4)2S2O8, the flotation percentage of PS, PC, and ABS decrease from 100 % to around 3 %, 1 %, and 0 % respectively with increasing (NH4)2S2O8 concentration, treatment temperature, pH value, and treatment time, while the flotation percentage of PVC remains 100 %. The optimal pretreatment conditions of separating PVC from mixtures are as follows: (NH4)2S2O8 concentration 0.2 M, pH value 10, temperature 70 °C, and treatment time 30 min. As shown in Fig. 2d, the sequence of flotation percentage is PS > PC > ABS in the same pretreatment time. The different floatability of PC, PS, and ABS might be on account of the different chemical structures and functional groups on the surface of plastics.

3.2.3. Functional groups analysis FT-IR was conducted to verify the existence of hydrophilic groups on surfaces of PS, ABS, and PC after the (NH4)2S2O8 pretreatment. The FT-IR spectra of PVC are displayed in Fig. 4a. Both treated and untreated PVC have absorbing bands such as C–Cl stretching at 688 cm−1, CeH twisting at 960 cm−1, CeH rocking at 1265 cm−1, CeH deformation at 1390 cm−1, and CeH stretching at 2924 cm−1 (Wang et al., 2016a, b). There are no obvious differences between the infrared spectra of untreated and treated samples, suggesting no new polar hydrophilic group exists on the surfaces of treated PVC. As shown in Fig. 4b, there are some characteristic bands in infrared spectrum of untreated PC: 1247 cm−1 (CeO stretching vibration), 1767 cm-1 (C]O stretching vibration), 677 cm−1, 1504 cm−1,

3.2. Mechanism of ammonium persulfate pretreatment 3.2.1. Wettability analysis As shown in Table S2, the mean value of contact angles of PS, PC, ABS, and PVC decrease by 12.20°, 13.06°, 17.30°, and 0.13°, respectively. These results are consistent with the flotation percentage and the floatability order of PVC, PC, PS, and ABS. (NH4)2S2O8 pretreatment enhance the hydrophilicity of PS, PC, and ABS with a limited effect on PVC, and thus decreases the floatability of PC, PS, and ABS. According to other oxidation methods, it is expectable that (NH4)2S2O8 pretreatment may introduce some hydrophilic polar groups on the surfaces of 4

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Fig. 3. SEM of ABS, PC, and PS before and after (NH4)2S2O8 treatment.

3081 cm-1 (aromatic ring vibration), 2863 cm−1, and 2963 cm−1 (CeH vibration in CH3) (Wang et al., 2018). After (NH4)2S2O8 pretreatment, new peaks appear at 1250 cm−1 and 3500 cm−1 in the FT-IR spectrum of PC. Both new bands may be caused by the CeO stretching vibration and OeH vibration. In Fig. 4c, it is obvious that molecular vibration adsorption peaks in PS are at 732 cm−1, 756 cm−1 (CeH out-of-plane deformation in the aromatic ring), 1490 cm−1, 1600 cm−1 (C]C stretching in the aromatic ring), 3027 cm−1 (CeH stretching in the aromatic ring), and 2848 cm−1 (−CH2– vibration). Some new peaks emerge in the infrared spectrum of PS after (NH4)2S2O8 pretreatment: 1260 cm−1 (CeO stretching vibration) and 3500 cm−1 (OeH stretching) (Raval et al., 2018). Much more variation exists between the infrared spectra of unmodified and modified ABS (Fig. 4d). The infrared spectrum of unmodified ABS shows characteristic adsorption bands of C^N stretching at 2320 cm−1, vibration in benzene ring at 710 cm−1, 765 cm−1, 1450 cm−1, 1490 cm−1, and 3033 cm−1, and C]C stretching of butadiene at 1635 cm−1 (Wang et al., 2016b). After modification by

(NH4)2S2O8, adsorbing bands of C^N and C]C decrease, and three other bands in the infrared spectrum of modified ABS are located at 1630 cm−1 (CeO), 1050 cm−1 (C]O), and 3500 cm−1 (OeH), implying the existence of O = CeO, −OH, or O = C–NH2 on the surface of modified ABS (Raval et al., 2018; Ibrahim et al., 2005). It is expectable that O = CeO, −OH, or O = C–NH2 generates on the surface of ABS, PS, and PC to raise their hydrophilicity and then decrease their floatability. 3.2.4. Chemical bonds analysis It is necessary to compare the XPS spectra of the treated plastics and untreated counterparts to further ascertain the chemical bonds on plastic surfaces. The relative ratio of different chemical bonds on the plastic surface is listed in Table S3, and the C 1s XPS spectra of PS, PC, and ABS are presented in Fig. 5. It can be seen in Fig. 5a and 4b that the unmodified and modified PC are in accordance with the following bands: aliphatic CeH/C–C (285.0 eV), aromatic CeH (284.5 eV), CeOH (286.2 eV), O = CeO (288 eV), and π→π* (291.0 eV). After (NH4)2S2O8 5

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Fig. 4. FT-IR spectra of untreated and treated (a) PVC, (b) PC, (c) PS, and (d) ABS.

3.2.5. Mechanism of oxidation by S2O82− The chemical structural formulas of PVC, PS, PC, and ABS are listed in Table S4 to facilitate an explanation of surface modification. The functional groups in plastics that may participate in the chemical reaction of surface modification are represented by blue groups. For ABS, the increased O = C–NH2 or O = CeO can be attributed to the hydrolyzation of C^N under the conditions of alkaline solution and heating treatment (Eq. 11) (Thallaj et al., 2008). The hydrolyzation of C^N on ABS surface is consistent with the decrease of C^N and appearance of O = C–NH2 and O = CeO in FT-IR and XPS of ABS (Figs. 4d and 5 f). Both O = C–NH2 and O = CeO are hydrophilic groups that increase the wettability of ABS. Besides, SO∙− 4 can attack the C]C in butadiene due to the existing of active α-hydrogen (Eq. 12) (Thanh Truc and Lee, 2017a). This result coincides with the disappearance of C]C and the increased −OH on the surface of ABS (Figs. 3d and 4 f). The mechanisms of PC and PS oxidation by (NH4)2S2O8 are listed as Eqs. 13–18: persulfate anion (S2 O82 −) can produce SO∙− 4 by heating (Eq. 13) (Huie and Clifton, 1990). The hydroxyl radical (OH∙−) can be generated by the reaction between SO∙− 4 and water (Eq. 14) (Lee et al., ∙− 2010). What follows these two reactions is the attack of SO∙− 4 and OH on organic compounds (RH) on the plastic surface. The above two reactions can generate organic radicals (R%) due to electron transfer or deprotonation (Eqs. 15–16) (Wang et al., 2017a; Luo et al., 2017). Subsequently, hydroxyl addition can introduce −OH on the surface of plastics (Eq. 17). Besides, phenyl bonded with donating groups, such as hydrocarbon alkyl (R–) and ester (OeC = O), can facilitate −OH introduction, resulting from a single-electron transfer (Luo et al., 2017). There are appropriate phenyl groups in PC and PS for their oxidation and hydroxyl addition (Eq. 18), which is consistent with the FT-IR and XPS of modified PC and PS (Figs. 3 and 4). All these reactions can bring hydrophilic groups (O = CeO, CeOH, or O = C–NH2) on the surface of ABS, PS, and PC, and make them hydrophilic. However, PVC can avoid oxidation and remain hydrophobic after pretreatment due to the lack of reactive sites.

pretreatment, the relative ratio of CeOH increases by 6 % with a slight reduction of aliphatic CeH/C–C, indicating the introduction of more hydroxyl groups in the backbone chain on PC surface (Table S3). As shown in Figs. 5c and 4 d, the C 1s spectrum of the unmodified PS can fit three peaks including aromatic C–H/C–C (284.5 eV), aromatic C–H/C–C (285.0 eV), and π→π* (291.0 eV) (Wang et al., 2016b). However, the C 1s spectrum of modified PS can be matched by four peaks at 286.2 eV (C−OH), 291.0 eV (π→π*), 285.0 eV (aliphatic C–H/ C–C), and 284.5 eV (aromatic C–H/C–C). The formation of C−OH can be observed after (NH4)2S2O8 pretreatment, which is consistent with the appearance of −OH in FT-IR of modified PS (Fig. 4c). Based on Table S3, the introduction of −OH is verified by the increased relative ratio of C−OH and decreasing relative ratio of aliphatic C–H/C–C. The XPS analysis is convictive for the existence of 16 % more −OH on the backbone chain of PS. The C 1s XPS spectra of modified and unmodified ABS are revealed in Figs. 5e and 4 f. The peaks of aromatic C–H/C–C (284.5 eV), aliphatic C–H/C–C (285.0 eV), aliphatic C=C (285.5 eV), C≡N (286.5 eV), and the benzene ring structure of π→π* (291.0 eV) can imitate the C 1s spectrum of unmodified ABS (Wang et al., 2017a). While the C 1s spectrum of modified ABS fits with the benzene ring structure of π→π* at 291.0 eV, O=C–O at 289 eV, O=C–NH2 at 287.2 eV, C−OH at 286.2 eV, aliphatic C–H/C–C at 285.0 eV, and aromatic C–H/C–C at 284.5 eV. It is reported that the C≡N might be substituted with O = C–NH2 and O = C–O, which is testified by a similar variation in the relative ratio of O = C–NH2, O = C–O, and C≡N (Noguchi et al., 2015). As shown in Table S3, aliphatic C = C in butadiene decrease by 4 % after (NH4)2S2O8 modification. In fact, aliphatic C = C can be substituted with −OH (Thanh Truc and Lee, 2017a). The redundant −OH of 10 % on the ABS surface might come from the oxidation of the backbone chain. These results are consistent with the infrared spectra of ABS (Fig. 4d).

6

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Fig. 5. The C 1s XPS peaks of (a) untreated PC, (b) treated PC, (c) untreated PS, (d) treated PS (e) untreated ABS, and (f) treated ABS. Δ

-NH+ 4

R-C≡N+ H2 O+ OH− → R-CONH2 → R-COO−

3.3. Flotation separation of PVC from mixtures

(11)

3.3.1. Frother dosage The microbubbles used in flotation should be sufficient, stable, and homogeneous. Terpineol, as a kind of surfactant, can lower surface tension to produce appropriate microbubbles rather than affect the wettability of plastics. Therefore, terpineol dosage must be suitable for plastic separation (Wang et al., 2016a, b). PS, PC, ABS, and PVC (with a mass ratio of 1:1:1:1) were treated by (NH4)2S2O8 before flotation separation. As shown in Fig. 6a, the flotation percentage of PVC and other plastics remain about 100 % and 0 %, respectively when the dosage of terpineol ranges from 20 mg/L to 60 mg/L. Therefore, the optimum concentration of terpineol is 20 mg/L.

(12) Δ

S2 O82- → SO∙4- + SO∙4-

(13)

SO∙4- + H2 O→HSO−4 + HO∙

(14)

RH+ HO∙→H2 O+ R∙

(15)

RH+ SO∙4-→R∙+ HSO−4

(16)

2R∙+ S2 O82- + 2H2 O →2ROH+2HSO−4

(17)

3.3.2. Flotation time The effect of flotation time on plastic flotation separation is presented in Fig. 6b. The flotation percentage of PVC sharply increases to 100 % within 5 min. However, the flotation percentages of PC, PS, and ABS remain 0 % with the increasing flotation time, which indicates the floatability of PC, PS, and ABS is unaffected by flotation time. The

(18)

7

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Fig. 6. Effect of (a) flotation time and (b) frother concentration on the floatability of plastic.

flotation percentage of PVC can be 100 % at flotation time of 4 min. Therefore, the optimal flotation time for PVC separation is 4 min.

Table 2 The effect of mass ratio on the purity and recovery of PVC and other plastics.

3.3.3. Particle size It is inevitable to produce a wide particle size fraction after plastic crushing. Therefore, different particle sizes were mixed with a mass ratio of 1:1:1:1 to research the effect of particle particles on plastic flotation after (NH4)2S2O8 pretreatment. The recovery and purity were calculated in Table 1, where F represents the floated plastic, and S is the sunken product. The flotation percentage of PVC maintains 100 % with the increase of particle size. What dominates plastic flotation is the density of plastic-bubble aggregation when the size of plastic particles is under 4 mm, and the density of plastic-bubble aggregation can be calculated by empirical formula (Shen et al., 2001). In comparison to large particle-bubble aggregation, the density of small particle-bubble aggregation deviates much more from the actual density of the plastic particle. Therefore, some hydrophilic plastics will float with PVC plastic when particle size is less than 3 mm. Unlike the small particles (1.0–3.0 mm), sunken plastic particles may be entrained by amounts of floating components in flotation separation when particle size is greater than 4.0 mm, which attributes to the decrease of purity of PVC. Therefore, the optimal size fraction is 3–4 mm for flotation separation of PVC from plastic mixtures.

Purity (%)

Feed F S Feed F S Feed F S Feed F S

100 43.5 56.5 100 30.8 69.3 100 25.4 74.6 100 25.9 74.1

PVC 25 57.4 0 25 81.3 0 25 98.3 0 25 96.7 0

(0.8–2.0)

(2.0–3.0)

(3.0–4.0)

(4.0–5.0)

± 1.5 ± 1.5 ± 0.85 ± 0.85 ± 0.3 ± 0.3 ± 0.9 ± 0.9

± 2.2

± 2.2

± 0.8

± 3.3

PVC – 100 0 NA 100 0 – 100 0 – 100 0

Feed (PVC 75 %) F S Feed (PVC 50 %) F S Feed (PVC 25 %) F S

100 75.2 24.8 100 50.2 49.8 100 25.5 74.5

PVC 75 99.7 ± 0.2 0 50 99.5 ± 0.3 0 25 97.9 ± 2.1 0

± 0.2 ± 0.2 ± 0.2 ± 0.2 ± 0.5 ± 0.5

Recovery (%) Others 25 0.3 ± 0.2 100 50 0.5 ± 0.3 100 75 2.1 ± 2.1 100

PVC NA 100 0 NA 100 0 NA 100 0

Others NA 0.7 ± 0.5 99.3 ± 0.5 NA 0.5 ± 0.4 99.5 ± 0.4 NA 0.7 ± 0.7 99.3 ± 0.7

(NH4)2S2O8 assisted by heating activation broadens the ways to modify plastic surfaces, achieving the flotation separation of PVC from other plastics. According to optimal conditions from this study, we can summarize a process flowsheet of separating PVC from other plastics (Fig. S2). As shown in Table S5, various methods for surface modification of waste plastics have been reported in recent years. Compared with previous methods, this technology can separate PVC from more types of plastics with simple equipment. It can be concluded that surface oxidation and decomposition dominate the methods for the surface hydrophilization of plastics. An initial cost estimation was performed to evaluate the feasibility of this technology for separation of PVC from plastic mixtures (Table S6). Compared with other modification technologies, this process can achieve significant separation at a relatively low cost. Long pretreatment time and high temperature may be pervasive problems in surface modification for plastic flotation. At last, we put forward to the preliminary reaction equations of ABS, PC, and PS under heating-activated (NH4)2S2O8 pretreatment for the fundamental research, and efficient catalyst may be a potential scheme to obtain mild pretreatment conditions. Additionally, surface modification can alter the wettability of near surfaces and maintain a stable bulk performances of recycled plastics. Therefore, surface modification does not obstruct the reuse of waste plastics.

Recovery (%) Others 75 42.6 ± 2.2 100 75 18.7 ± 2.2 100 75 1.7 ± 0.8 100 75 3.3 ± 3.3 100

Purity (%)

4. Assessment and perspectives

Table 1 The effect of size fraction on the purity and recovery of PVC and other plastics. Yields (%)

Yields (%)

with different mass ratios.

3.3.4. Mass ratio The mass ratio of PC, PS, ABS, and PVC may be different in certain plastic mixtures. The flotation experiments of plastic mixtures with different mass ratios were conducted after pretreatment. The results are exhibited in Table 2. The purity of PVC can reach 99.7 ± 0.2 %, 99.5 ± 0.3 %, and 97.9 ± 2.1 % when the mass ratio of PVC is 75 %, 50 %, and 25 %. PVC can be separated by flotation from the mixtures of PS, PC, and ABS under the appropriate pretreatment conditions with different mass ratios. Therefore, the flotation separation with (NH4)2S2O8 pretreatment is feasible to separate PVC from other plastics

Products (mm)

Products (mm)

Others – 24.7 ± 2.1 75.3 ± 2.1 NA 7.7 ± 1.1 92.3 ± 1.1 – 0.6 ± 0.4 99.4 ± 0.4 – 0.2 ± 0.2 99.8 ± 0.2

5. Conclusion This research puts forward a novel surface modification based on (NH4)2S2O8 pretreatment to selectively enhance the hydrophilicity of PC, PS, and ABS, which assists the flotation separation of PVC from other waste plastics efficiently. The mechanism of (NH4)2S2O8 pretreatment was investigated by the contact angle, FT-IR, XPS, and SEM. It can be verified that the surface of PS, PC, and ABS introduces hydrophilic groups (CeOH, O = CeO, or O = C–NH2) by the oxidation by 8

Journal of Hazardous Materials 389 (2020) 121918

Y. Zhang, et al.

sulfate radical anion (SO∙− 4 ) oxidation and the hydrolysis of nitrile group (C^N) and butadiene (C]C). The proper reaction equations were given in this research for the universal application of this technology. The optimal conditions of separating PVC from other waste plastics by flotation are (NH4)2S2O8 concentration 0.2 M, pretreatment time 30 min, temperature 70 °C, pH 10, terpineol dosage 20 mg/L, and flotation time 4 min. The optimal size fraction for flotation separation is 3.0–4.0 mm, and the flotation separation is stable with different mass ratios of PVC. At a relatively low cost, the purity and recovery of PVC are up to 99.7 ± 0.2 % and 100 %, and other waste plastics were collected as mixtures.

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CRediT authorship contribution statement Yingshuang Zhang: Writing - original draft. Hongru Jiang: Validation, Writing - review & editing, Formal analysis. Hui Wang: Resources, Supervision, Funding acquisition. Chongqing Wang: Investigation, Data curation, Funding acquisition. Declaration of Competing Interest The authors declare that there are no conflicts of interest. Acknowledgements This work is supported by the National Natural Science Foundation of China (21878343 and 51804276) and the China Postdoctoral Science Foundation (2018M630838). 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.2019.121918. References Al-Salem, S.M., Lettieri, P., Baeyens, J., 2009. Recycling and recovery routes of plastic solid waste (PSW): a review. Waste Manage. 29, 2625–2643. Alzate, A., López, M.E., Serna, C., 2016. Recovery of gold from waste electrical and electronic equipment (WEEE) using ammonium persulfate. Waste Manage. 57, 113–120. Beigbeder, J., Perrin, D., Mascaro, J., Lopez-Cuesta, J., 2013. Study of the physico-chemical properties of recycled polymers from waste electrical and electronic equipment (WEEE) sorted by high resolution near infrared devices. Resour. Conserv. Recy. 78, 105–114. Criquet, J., Leitner, N.K., 2009. Degradation of acetic acid with sulfate radical generated by persulfate ions photolysis. Chemosphere 77, 194–200. Fang, G., Dionysiou, D.D., Wang, Y., Al-Abed, S.R., Zhou, D., 2012. Sulfate radical-based degradation of polychlorinated biphenyls: effects of chloride ion and reaction kinetics. J. Hazard. Mater. 227, 394–401. Gent, M., Sierra, H.M., Menéndez, M., de Cos Juez, F.J., 2018. Evaluation of ground calcite/water heavy media cyclone suspensions for production of residual plastic concentrates. Waste Manage. 71, 42–51. Geyer, R., Jambeck, J.R., Law, K.L., 2017. Production, use, and fate of all plastics ever made. Sci. Adv. 3, e1700782. https://doi.org/10.1126/sciadv.1700782. Guney, A., Poyraz, M.I., Kangal, O., Burat, F., 2013. Investigation of thermal treatment on selective separation of post consumer plastics prior to froth flotation. Waste Manage. 33, 1795–1799. Hu, B., Fraunholz, N., Rem, P., 2010. Wetting technologies for high-accuracy sink-float separations in water- based media. Open Waste Manage. J. 3, 71–80. Huie, R.E., Clifton, C.L., 1990. Temperature dependence of the rate constants for reactions of the sulfate radical, S04−, with anions. J. Phys. Chem. C. 94, 8561–8567. Ibrahim, M., Nada, A., Kamal, D.E., 2005. Density functional theory and FT-IR spectroscopic study of carboxyl group. Indian J. Pure Appl. Phys. 43, 911–917. Lee, Y., Lo, S., Chiueh, P., Liou, Y., Chen, M., 2010. Microwave-hydrothermal decomposition of perfluorooctanoic acid in water by iron-activated persulfate oxidation. Water Res. 44, 886–892. Li, H., Guo, J., Yang, L., Lan, Y., 2014. Degradation of methyl orange by sodium persulfate activated with zero-valent zinc. Sep. Purif. Technol. 132, 168–173. Lopez, G., Artetxe, M., Amutio, M., Bilbao, J., Olazar, M., 2017. Thermochemical routes for the valorization of waste polyolefinic plastics to produce fuels and chemicals. A review. Renew. Sust. Energy Rev. 73, 346–368. Luo, S., Wei, Z., Yang, Z., Ye, T., Xiao, R., 2017. Mechanistic insight into reactivity of sulfate radical with aromatic contaminants through single-electron transfer pathway.

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