Sustainable and Selective Separation of PVC and ABS from a WEEE

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Sustainable and Selective Separation of PVC and ABS from a WEEE Plastic Mixture using Microwave and/or Mild-heat Treatment with Froth Flotation Nguyen Thi Thanh Truc, and Byeong-Kyu Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02280 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 7, 2016

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Environmental Science & Technology

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Sustainable and Selective Separation of PVC and ABS from a WEEE Plastic Mixture using

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Microwave and/or Mild-heat Treatment with Froth Flotation

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Nguyen Thi Thanh Truc, Byeong-Kyu Lee*

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Department of Civil and Environmental Engineering, University of Ulsan, Daehakro 93, Namgu,

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Ulsan 680-749, Republic of Korea

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Corresponding Author: *Byeong-Kyu Lee, Professor, Tel: 82-52-259-2864, Fax: 82-52-259-2629,

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E-mail: [email protected]

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Abstract

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This study reports simple, selective and sustainable separation of chlorinated plastic (polyvinyl

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chloride, PVC) and acrylonitrile butadiene styrene (ABS) containing brominated flame retardants

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(BFRs) from mixed waste electrical and electronic equipment (WEEE) plastics using microwave

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and/or mild-heat treatment. Microwave treatment after plastic coating with powdered activated carbon

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(PAC) selectively increased the hydrophilicity of the PVC surface, which facilitated PVC separation

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(100% recovery and purity) from the WEEE plastic mixture under the optimum flotation conditions.

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A further mild-heat treatment for 100 sec facilitated selective separation with the highest recovery and

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purity (100%) of PAC-coated ABS containing BFRs from the remaining plastic mixture due to

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selective formation of a twisted structure with a lower density than water and the untreated ABS.

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Mild-heat treatment only of PAC-coated WEEE plastic mixture resulted in successful recovery of

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(100%) the ABS and PVC. However, the recovered PVC had slightly reduced purity (96.8%) as

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compared to that obtained using the combined heat and microwave treatments. The combination of

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both treatments with flotation facilitated selective and sustainable separation of PVC and ABS from

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WEEE plastics to improve their recycling quality.

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Key words: PVC; ABS; microwave; mild-heat treatment; surface modification; recycling.

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1. Introduction

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Tremendous use of electrical and electronic devices in modern life and for industrial purposes has

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produced a large amount of waste electrical and electronic equipment (WEEE) plastics. It has also

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resulted in serious environment problems and global issues associated with their disposal1-3. The

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estimated WEEE production rate in South Korea was ~28.3 million units of products in 20104.

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Plastic materials have been reported to account for ~30–50% (w/w) of the content of WEEE

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including acrylonitrile–butadiene–styrene (ABS), polyvinyl chloride (PVC), polycarbonate (PC),

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high-impact polystyrene (HIPS), polypropylene (PP), polystyrene (PS), styrene-acrylonitrile (SAN),

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polyesters, polyurethane (PU), polyamide (PA), PC/ABS blends, et cetera 5-8. Halogenated or halogen-

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containing plastics account for the majority of WEEE and their polymer backbone structures contain

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halogen atoms or they inherently have halogenated flame retardants (HFRs). The halogen atoms are

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improve the performance or reduce fire damage by acting as flame retardants. Around 41% of EEE

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plastics include halogenated structures such as PVC, or HFRs such as ABS to improve their fire

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resistance7,

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tetrabromobisphenol- A (TBBPA), decabromodiphenyl oxide (DDO),1,2,5,6,9,10-hexabromocyclo-

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decane (HBCD) and decabromodiphenyl ethane (DDE) are widely used7.

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. The common HFRs in WEEE plastics are BFRs with about 75 types in which

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PVC products contain ~36.3%12 chlorine suchs as computer cables and wires have several

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additives as chlorinated flame retardants1. ABS is commonly used in computers, television sets and

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various electrical appliances and contains ~10 wt% brominated flame retardants

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boards can have BRF contents of >30% (w/w)13. Halogenated plastics are considered to be toxic and

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exert a severe negative environmental impact 1, 10. In end-of-life vehicles (ELVs), PVC and ABS can

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form toxic halogenic dioxin or furan, HCl, and HBr through incineration and landfilling activities.

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. Printed circuit

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Many studies have focused on the recycling of plastic waste, in particular, WEEE plastics.

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However, most have involved pyrolysis, thermal decomposition or degradation of individual plastics

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for preparation of nanomaterial composites or fuel7, 12, 14. Since each plastic has different properties

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and characteristics, it is not easy to get optimized conditions for better recycling processes. Recycling

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of mixed plastics waste incurs a high capital cost, including operational and environmental fees due to

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release of several hazardous components such as halogenic elements and HFRs. In particular,

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brominated compounds containing plastics such as ABS greatly affect recycled product quality and

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reuse of plastics7. Therefore, the value of recycled products depends mainly on identification and the 2 ACS Paragon Plus Environment


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quality of separation of the individual plastics. Various studies of selective separation of hazardous

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plastics containing halogenic elements have been performed. However, these methods have limited

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applications due to the considerable separation costs.

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ABS is a copolymer composed of styrene (65–76%), butadiene and acrylonitrile (24– 35%)15.

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Because of the structural instability of C=C in butadiene, ABS is vulnerable to oxidative aging by

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ozone, free radicals and hydro peroxides produced during handling or processing14, 16. To minimize

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oxidative degradation of ABS, interactive heating processes using microwave or mild-heat treatment

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are considered to make selective change of ABS from plastic mixture without altering their main

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properties.

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Since PVC has a higher dielectric-loss coefficient than other plastics17,

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, interactive energy

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application could facilitate its selective separation. A selective surface reaction on PVC can

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increase/decrease the amount of hydrophilic functional groups—such as ether, hydroxyl and carboxyl

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moieties—as compared with other plastics. The study reports the method to get different density and

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wettability of ABS and PVC, respectively from the remaining WEEE plastic mixture by after

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applying an interactive microwave/ mild-heat treatment after PAC coating. Then, the aim of this study

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was to achieve selective separation of PVC and ABS from a WEEE plastic mixture by optimizing

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froth flotation conditions.

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2. Materials and methods

2.1. Materials and chemicals

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WEEE plastics were obtained from Ulsan Resources Recycling Co., Ltd, Korea after removal from

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parts of other electrical and electronic equipment. After collection, the WEEE was manually separated

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with removal of metals, rubber, glasses and non-plastic materials. The WEEE plastics were classified

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depending upon their IR spectrum obtained using a FTIR spectrometer. The composition of WEEE

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plastics was analyzed using a scale with a deviation of 0.01 g.

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Normally, the obtained plastic samples have numerous sizes and shapes. However, in this study,

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the plastic samples were cut to 10  10 mm using a stainless saw and nipper. Prior to microwave or

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mild-heat treatment, plastic samples were washed with tap water and then with distilled water in an 3 ACS Paragon Plus Environment

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ultrasonic generator for 10 min to yield a clean plastic surface. Also, the cleaned plastics were coated

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with powdered activated carbon (PAC) with d50 of 15 μm (Activated Charcoal Norit® , Sigma-Aldrich

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Co.). 2-Methyl-4-pentanol (MIBC) was used as a cavitation-promoting agent to assist froth flotation.

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These chemicals purchased from Daejung Chemicals and Metals Co., Ltd., Korea were used without

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pretreatment.

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2.2. Surface modification of plastics by microwave/ heat treatment

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Plastic samples were manually shaken with a commercial PAC (2 g/100 pieces of each plastic) for

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15 min. Then, PAC was spread evenly over the plastic surface. After lightly sifting using a sieve with

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a mesh size of 4.75 mm, the PAC-coated plastic samples were placed in a microwave oven (Dongbu

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Daewoo Electronics Corp., KR-G20EW) for microwave treatment or in a thermal oven (Model KT-

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1800H, Kitchen-Art Co., LTD., Korea) for mild-heat treatment. The rated microwave oven was

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operated at a frequency of 2,450 MHz with 1,120 W input. Thermal heat treatment (1.4 KWh) was

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used to maintain 150C during the duration of treatment. The treated plastic samples were cooled to

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25 ± 1C at room temperature and subjected to surface characterization tests (See Fig. S1, Supporting

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Information).

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This study compared the physicochemical properties of the selected four types of plastic subjected

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to four surface modification methods. In this, we define PAC@MW is as the combination of a PAC

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coating with microwave treatment for 30 sec, while PAC@MH is a PAC coating with mild-heat

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treatment for 100 sec. Plastic samples without PAC coating were subjected to microwave treatment

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for 90 sec and mild-heat treatment for 80 sec, which were termed MW and MH, respectively. The

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times stated above the treatments were the optimum condition for each surface modification method.

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2.3. Surface characterization tests

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To measure the contact angles (CA) of the plastic surfaces, a contact angle meter (FEMTOFAB

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Co., Ltd.) was employed using a drop of distilled water (2 l). The average of at least five

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measurements of different surfaces was considered the final CA of the plastic. To observe the changes

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in surface morphology and roughness caused by microwave/mild heat, atomic force microscopy

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(AFM, DI 3100 Dimension AFM, Veeco) was employed. AFM data were analyzed using the 4 ACS Paragon Plus Environment


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Gwyddion 2.41 software to extract the surface roughness (calculated on a 2 μm). The average of 3

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measurements at 3 random positions on a plastic surface was calculated as the reported root mean

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square (RMS) value. To identify the elemental states of carbons, an X-ray photoelectron spectroscopy

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(XPS) spectrometer (K-Alpha, Thermo Scientific, USA) was employed. XPSPEAK41 ver. 4.1 was

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used to fit XPS peaks and determine the relative peak areas. Each surface characterization test was

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performed before and after the treatments.

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2.4.Measurement of plastic density

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Based on the American Standard Test Methods for Density and Specific Gravity of Plastics by

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Displacement, ASTM D792-08, with a pycnometer was employed to measure the density of the

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plastics in water with the following equation: 𝜌𝑠 =

𝑀(𝑝+𝑠) − 𝑀𝑝 × 𝜌𝑤 (𝑀(𝑝+𝑤) − 𝑀𝑝 ) − (𝑀(𝑝+𝑠+𝑤) − 𝑀(𝑝+𝑠) )

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where M (g) is the mass at room temperature (23C) with pycnometer (p), water (w), plastic samples

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(s). ρw is the density of the distilled water, 0.9975 g/cm3 at 23C. The reported result was the average

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of three measurements.

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2.5.Froth flotation experiment

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Plastic samples (10 samples of each plastic) after microwave/mild-heat treatment were placed in a

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flotation cell (150 mm height and 70 mm inner diameter),s which was filled with tap water (0.40 L).

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The froth flotation experiment was clearly shown in Fig. S1, Supporting Information. To enhance

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flotation efficiency, MIBC (1 g/mL) was added to the flotation cell without a significant volume

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change. Bubbles were produced by dry airflow through a ceramic diffuser plate located at the bottom

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of the flotation cell. The dry airflow was supplied by an air-pump (MP-Σ300, Sibata, Japan) with an

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alterable flow rate. The mixture of plastic samples was stirred using an auto overhead stirrer (WiseStir,

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Daihan scientific Co., Ltd.) at various speeds. After each experiment, the recovery and purity of each

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plastic was calculated by the amount of sample that settled and was removed from the flotation cell.

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Each experiment was performed in triplicate and the average value reported.

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3. Results and discussion


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3.1. Composition of WEEE plastics

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After the separation and removal of non-plastic materials, the remaining plastics, WEEE plastics,

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were analyzed for typical compositions. The WEEE comprises >10 types of engineering plastic,

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including PP, PE, polyethylene terephthalate (PET), ABS, HIPS, PS, polymethyl methacrylate

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(PMMA), PC, PVC, polyoxymethylene (POM), PA and others, as identified by FT-IR peak analysis

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(See in Fig. S2, Supporting Information). ABS had the largest weight fraction in the WEEE plastic

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mixture, followed by PP + PE, PS, PVC, PC, etc. in order. The halogenated or halogen-containing

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plastics (PVC or ABS) comprised ~51% (w/w) of the WEEE plastics. In other studies, halogen-

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containing plastics, in particular ABS, accounted for a large proportion of WEEE plastics (See in

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Table S1, Supporting Information). Martinho et al. (2012) reported that ABS waste can comprise up

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to 44% and 69% of CPUs and CRT monitors, respectively, and CRT televisions and large cooling

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appliances also have high ABS content. Yang et al. (2013) and Maris et al. (2015) reported that the

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proportion of ABS in WEEE plastic was 29–30%

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total WEEE. Other thermoplastics—such as PET, HIPS, PC, PMMA and POM—were also present in

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large quantities7, 18, 20, 21. In this study, however, their proportions were 2.18, 2.74, 6.52, 2.46 and 1%,

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respectively. The floating plastics in water, including PE and PP, comprised about 18% of the total

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WEEE plastics and were easily separated by gravity separation in water due to their low densities 21, 22.

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Among the plastic types in the WEEE, four major heavy plastic components—ABS, PS, PVC and

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PC—which exhibited larger weight fractions and high densities were selected for surface modification

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by microwave/mild-heat treatment and separation using the froth flotation technique.

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3.2. Selective separation of ABS

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. However, PVC waste comprises ~3% of the

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The principal mechanism of separation of ABS from the WEEE plastic mixture was selective twist

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formation of ABS using microwave or mild-heat treatment. The twisted ABS has a lower apparent

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density and thus, can be easily separated by gravity separation. Under the heat interactions, the shape

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of plastics changes depending on their expansion or contraction. Microwave/mild-heat treatment of

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the plastics for certain treatment time had a marked effect on the floating rates of the treated ABS

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Fig. 1a shows the floating rate of PAC@MH treated ABS treated increased for 60, 80, and 100 %

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for 80, 90, and 100 sec, respectively. The ABSs treated by MW, MH or PAC@MW remained settled

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after 120 sec treatment.

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Tensile elasticity or Young’s modulus is described as the linear elastic ability of materials to return

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to their initial shape. The elasticity of ABS with BFRs is usually lower than that of other plastics23.

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The C=C in butadiene of ABS can be easily broken because the α-hydrogen in its structure is easily

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altered. Therefore, upon heating in the presence of oxygen, ABS is easily oxidized which can help on

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the development of free radicals and hydroperoxides resulting in modification of ABS chemical

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structure

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treatment. The molecular rearrangement of ABS monomer components induced a change in the

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molecular weight and mechanical properties of ABS14, 16. Therefore, under mild-heat treatment (150

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C), the PAC-coated ABS is easily twisted and thus, its density was lower than that of ABS and other

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plastics with and without heat treatment and with heat treatment but not a PAC coating.

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. PAC as a heat/oxygen absorbent led to molecular rearrangement under mild-heat

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Fig. 1b shows the difference in density of tested plastics before and after the treatments. The

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density of water at room temperature is 0.9975 g/cm3, which is higher than that of the PAC-coated

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ABS after mild heat treatment (0.850 g/cm3). Therefore, the treated ABS can float on water. As shown

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in Fig. 1b, the ABS treated by PAC@MH for 80 sec started to float. In particular, when PAC-coated

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ABS was treated with mild-heat for 100 sec, 100% floated on water. Thus, PAC@MH ABS can be

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selectively separated from the plastic mixture.

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The intense heat generated by microwaves mobilizes molecules in plastics even in the presence of

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a PAC coating. Therefore, plastics were not twisted and their apparent density was not changed before

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and after microwave treatment. Thus, the MW-treated plastics did not float on water.

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Fig. 2a shows an AFM image with summarizing scans 2 × 2 m 3D of ABS without any treatment

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and that treated with PAC@MC and PAC@MH. As shown in Fig. 2a, nano-texture measurement

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facilitated evaluation of nanoscale roughness. The nanoscale texture of the ABS surfaces was altered

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by microwave and mild-heat treatments. The PAC@MW treated ABS was filled with interlaced

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grooves compared with that before the treatment. However, the ABS treated by PAC@MH had large 7 ACS Paragon Plus Environment

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grooves. The treated ABS surfaces were twisted, resulting in the presence of spaces, which trapped a

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large amount of air, leading to lower apparent density. During froth flotation testing, therefore,

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removing the air trapped inside the twist structure is difficult. The resulting twisted ABS has a lower

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density and so is easily separated from the plastic mixture by froth flotation.

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3.3. Surface characteristics of plastics after pre-treatments

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3.3.1.

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Contact angles

Fig. 3 shows the contact angle (CA) of a water droplet on the surface of PC, PVC, ABS and PS plastics before and after MW, PAC@MW, MH and PAC@MH treatments to determine wettability.

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Before the treatments, the CA of the heavy plastics ranged from 78.8 to 87.7, indicating

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hydrophobicity. After microwave treatment of plastics without PAC coating for 90 sec, the CA value

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decreased, while that of treated PVC, ABS and PS increased. The highest CA after microwave

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treatment was in MW-treated PVC, 88.7. However, 30 sec microwave treatment of PAC-coated PVC

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resulted in a significantly reduced CA (69.9), which differs markedly from that of the PAC-coated

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treated PC, ABS and PS. The CA values of the PAC@MW-treated PC, ABS and PS increased

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following the combination of microwave treatment and PAC coating. Mild-heat treatment for 80 sec

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greatly reduced the CA of the PS to 70.8o but increased the CA of PVC and ABS to 90.5 and 82.1,

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respectively. After mild-heat treatment for 100 sec, the CA of the PVC with a PAC coating was

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significantly increased to 98.3 and could reach 101.1, whereas the CAs of the PAC-coated PC, ABS

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and PS decreased from 87.7 to 76.0, from 78.8 to 75.4, and from 80.5 to 67.2, respectively.

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In addition, the PVC (disparity) in CAs between PVC and PC, ABS and PS shows the CAs

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different changes between PVC and the remaining plastics, which indicate selective PVC separation

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in the froth flotation (see in Table S2, Supporting Information). After the PAC@MW, the PVC

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between PVC and PC, ABS and PS (19.6, 11.1 and 20.3, respectively) were significantly higher than

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those of disparities after the MW (4, 7.3 and 1.3, respectively). Similarly, the PVC after the

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PAC@MH (22.3, 22.9 and 31.1) were also extremely higher than the PVC after the MH (4.9, 8.4

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and 19.7) between PVC and PC, ABS and PS, respectively. Therefore, the PAC@MW and



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PAC@MH was also more effective than the MW and MH in the treatment of plastic for selective

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separation of PVC in froth flotation

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Thus, the CA results indicate that microwave and mild-heat treatments of PVC after PAC coating

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altered the wettability of PVC compared with other plastics. The PAC@MW-treated PVC became

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more hydrophilic and the PAC@MH-treated PVC more hydrophobic. These results indicate that PVC

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can be selectively separated from the WEEE plastic mixture by microwave and mild-heat treatments.

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3.3.2.

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Rearrangement of elements on plastic surfaces after pretreatments

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XPS was used to investigate the changes in the elemental states of carbons on the plastic surfaces.

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Then, XPSPEAK41 ver. 4.1 was used to fit XPS peaks and analyze the relative peak areas between

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hydrophilic and hydrophobic groups on the plastic surfaces before and after treatments. Hydrophilic

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groups in ABS include C–O (ether carbon), O–(C=O)–O (carbonyl carbon) in PC, C–Cl (chlorine

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carbon) in PVC and C–N (nitriles carbon), while C–C, C=C, and C–H (neutral carbon) are considered

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hydrophobic groups. Under the interacting environmental conditions, the plastic surfaces are

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rearranged to respond to the change in the environment24-26. Table 1 shows the changes in relative

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peak area fractions of hydrophilic moieties after the treatments. Following microwave treatment, the

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relative peak area fractions of hydrophilic moieties of PVC were virtually unchanged. However,

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microwave treatment after PAC coating of PVC surfaces greatly increased the relative fraction of

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hydrophilic groups from 22.9 to 28.5%. Moreover, mild-heat treatment of PVC with/without a PAC

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coating reduced the relative fractions of hydrophilic moieties, particularly on PAC-coated PVC, by

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22.9 to 16.9%. Treatment of PC did not significantly affect the relative composition of hydrophilic

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moieties on its surface, with the exception of a 2.2% increase following mild-heat treatment without a

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PAC coating. The MW/MH treatment of ABS also resulted in a negligible change in the relative peak

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areas of hydrophilic moieties, with the exception of a significant change (6.8% increase) upon the

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PAC@MH-treated ABS.

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The changes in the relative peak areas of hydrophilic moieties are consistent with the CA results.

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After the treatments, the CAs increased when the proportion of hydrophilic moieties decreased, and

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vice versa. Thus, PAC coated PVC surface had a greater fraction of hydrophilic moieties due to the 9 ACS Paragon Plus Environment

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microwave treatment. However, mild-heat treatment resulted in an increased fraction of hydrophobic

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moieties.

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Table 1. Relative peak area fractions (%) of hydrophilic moieties as determined by XPS analysis

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before and after treatments. Plastics

Before

MW

PAC@MW

MH

PAC@MH

PC

13.1

13.7

12.1

15.2

12.1

PVC

22.9

22.0

28.5

20.3

16.9

ABS

9.9

9.1

8.9

8.7

16.7

PS

–

–

–

–

–

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3.3.3.

Changes in surface morphology

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AFM results showed the morphological changes in the treated PVC surfaces (Fig. 2b). After the

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PAC@MH, the ABS surfaces were significantly pitted with large scales (Fig. 2a), whereas the treated

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PVC surfaces showed smaller and thinner protrusions (Fig. 2b), which are slightly rougher than those

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of the untreated PVC and treated ABS.

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However, the RMS value of the PAC@MH-treated PVC surface was markedly higher than that of

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the PAC@MW-treated PVC surface. After PAC coating and microwave treatment for 30 sec, the root

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mean square surface roughness (RMS) values of PC, PVC, ABS and PS increased significantly by

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7.78, 3.18, 2.96 and 3.66 nm, respectively, compared with those of samples before treatment. The

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PAC@MW-treated PVC had the lowest RMS value among the four tested plastics. The increase in

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nanoscale roughness increases the CA and hydrophobicity of the treated plastic surfaces27-30. This

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result indicates that the PAC@MW-treated PVC has a more hydrophilic surface than the other three

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plastics. After PAC@MH for 100 sec, the RMS value of PC was virtually unchanged before and after

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treatment, from 3.02 to 3.23 nm. However, the treated PVC, ABS and PS surfaces exhibited RMS

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values significantly increased by 14.47, 22.24 and 23.76 nm, respectively (See Table S3, Supporting

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Information). XPS and CA analyses revealed that the PVC surface was more hydrophobic than the

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other plastics after the PAC@MH. This observation is the opposite to the result of microwave

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Based on this surface structure information, the interaction area between the water drop and the

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PAC@MH-treated PVC surface would be decreased as compared with the untreated PVC, which has

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lower roughness. This explained the marked increase in the CA value of the PAC@MH-treated PVC.

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Also, the increase in the fraction of hydrophobic moieties resulted in reduced wettability of the

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PAC@MH-treated PVC surface.

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Surface roughness can affect the surface wettability with 2 concepts which the water drop contact

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with full of rough surface and the drop contact with protrusion of the surface only 31-33. The change in

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nanoscale morphology on the plastic surface after the microwave/mild-heat treatment, in particular

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PVC, affected the surface wettability. Thus, using froth flotation, the bubbles may be unable to reach

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the spaces between the protrusions on the surface of the PAC@MH-treated PVC due to its higher

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RMS value. However, the bubbles can interact with the surface of the PAC@MW-treated PVC due to

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its lower RMS value and increased fraction of hydrophilic moieties.

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3.4. Froth flotation separation of treated PVC

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3.4.1.

Mechanism of PVC separation from WEEE plastics

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A schematic illustration of treated PVC separation by froth flotation is shown in Fig. 4. PVC and

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other plastics had considerable flotability before the treatments due to adherence of bubbles adhere to

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the plastic surfaces. Fig. 4a–4c and 4d–4f show photographs of water drops on the PVC surface and a

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schematic illustration of its surface topography, respectively. In the froth flotation, because of the

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increased contact of air bubbles with the plastic surfaces, all plastics could float (Fig. 4g).

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Following PAC@MW-treated PVC, the increased proportion of hydrophilic moieties on the

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treated PVC surface blocked air bubble attachment. In addition, the greater penetration of nanoscale

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water drops into the spacing between the protrusions on the PVC surface increased the distance

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between the air bubble and PVC surface, as reported by Wenzel32. Also, bubbles will rapidly stretch

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on the surface because of the high external and internal pressure due to mixing effective. Then, the air

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bubbles attached to the PVC surface are released, resulting in greater contact between water and the

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PVC surface, which increases wettability. Therefore, the measured CAs of the PVC with PAC coating

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and microwave treatment decreased (Fig. 4a and 4b). Therefore, the PAC@MW-treated PVC setteled 11 ACS Paragon Plus Environment

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on the bottom of the flotation cell while, the other plastics are floated on the water with bubbles

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attached (Fig. 4h). This facilitated selective separation of PVC from WEEE plastics.

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In contrast, PAC@MH-treated PVC created slender protrusions and dramatically increased the

293

surface roughness. Therefore, bubbles could not penetrate the spaces among protrusions so that the

294

flotation medium (water) was present between the bubbles and the PVC surface. Similar to the study

295

by Cassie–Baxter31 , water can probably pass through interstices between the bubble and the surface

296

(Fig. 4f). Water passage enhances the movement (release) of the bubbles from the PVC surface. In

297

addition, the interaction between the air bubbles on the hydrophobized PVC surface and the bubble

298

attachment behavior are reduced by the hydrodynamic effects of mixing. The optimized flotation

299

condition (mixing speed of 150 rpm, airflow rate 0.5 L/m, flotation medium temperature 45–50C,

300

MIBC concetration 0.8–0.9 mg/L and flotation time 2 min) created hydrodynamic pressure that

301

pushed the bubbles out of the surface. Bubbles with a lower surface tension are fragile and easily

302

dissolved in floataion medium. Therefore, the bubbles left on the surface of the treated PVC greatly

303

decrease, resulting in settling of the PVC at the bottom of the flotation cell, whereas other plastics

304

remained floating (Fig. 4i). Since the PAC@MH-treated ABS was separated from the WEEE plastic

305

by gravity separation (section 3.2), in froth flotation, the PVC was also successfully separated from

306

the remaining WEEE plastic mixture subjected to mild-heat treatment after PAC coating.

307

3.4.2.

Effect of mixing speed

308

Fig. 5a shows that the mixing speed of the froth flotation significantly affects the floatability of

309

PVC after microwave/mild-heat treatments of PAC-coated WEEE plastics. An appropriate mixing

310

speed improves recovery of settled PVC in froth flotation. Selective separation of treated PVC at low

311

mixing speeds (0, 50 or 100 rpm) results in recovered PVC of high purity (100%), but its recovery

312

rate was low. At a mixing speed of 150 rpm, microwave and mild-heat treated PVC after PAC coating

313

showed 100 and 90% recovery rates and 76.9 and 75% purity, respectively. The PVC purity was

314

decreased at higher mixing speeds due to increased contamination by PS and ABS for microwave

315

treatment, and increased contamination by PS for mild-heat treatment. Thus, a mixing speed of 150

316

rpm is the optimum for froth flotation for selective PVC separation. 12 ACS Paragon Plus Environment


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3.4.3.

Page 14 of 25

Effect of temperature of flotation medium

318

As shown as Fig. 5b, at 15–20C, the recovery rates of the PAC-coated PVC subjected to

319

microwave and mild-heat treatments were 40 and 6.7%, respectively. However, the purity of

320

PAC@MW-treated PVC was 100% at this temperature. As the temperature increased, the purity of

321

the recovered PVC (after microwave treatment) was decreased due to contamination by PS and PC.

322

At room temperature (25–30C), the recovered PVC showed 90% recovery and purity. Recovery after

323

PAC coating and microwave treatment decreased with increasing flotation medium temperature. In

324

contrast, the recovery of the PAC@MH treated PVC increased with increasing flotation medium

325

temperature, and reached 100% at 35–40C and 45–50C. However, the purity of the recovered PVC

326

was 88.2% at 45–50C, which was slightly reduced due to co-settling of PS. Therefore, 25–30C and

327

45–50C are the optimum temperatures of the froth flotation medium for PAC-coated plastics

328

subjected to microwave and mild-heat treatments, respectively.

329

3.4.4.

Effect of floating agent concentration

330

In the absence of MIBC, all plastics exhibited a low flotation ability. The recovery and purity of

331

the treated PVC with PAC coating increased with increasing frother concentration. At a frother

332

concentration of >0.6–0.7 mg/L of MIBC, a 100% recovery rate was obtained of the treated PVC with

333

PAC coating and microwave treatment. However, the treated PVC did not show a significant higher

334

PVC recovery rate. At 0.8 and 0.9 mg/L MIBC, the PAC@MW/PAC@MH treated PVC exhibited the

335

maximum recovery (100%) and purity (96.8%). Therefore, the optimum frother concentration was

336

0.8–0.9 mg/L (Fig. 5c).

337

3.5. Sequence separation of PVC and ABS from WEEE plastics

338

In previous studies, surface treatments for selective wetting for separation of ABS or PVC from

339

plastic mixtures were investigated. However, most previous methods were limited to separation of one

340

type of plastic (ABS or PVC) from a plastic mixture, which can result in reduced recovery and purity.

341

In the present study, microwave/mild-heat treatments of the PAC-coated plastics can facilitate

342

selective separation of ABS and PVC (100% recovery and purity) from WEEE under the optimum

343

conditions for froth flotation (See Table S4, Supporting Information). Also, the combination of 13 ACS Paragon Plus Environment

Page 15 of 25


344

microwave and heat treatment enabled complete separation of PVC and ABS via froth flotation and

345

the highest purity.

346

Fig. 6 shows a flow diagram of the separation of PVC and ABS from a WEEE plastic mixture.

347

Selective separation involves three main steps: PAC coating, microwave treatment-froth flotation and

348

mild-heat treatment. The WEEE plastics are coated with a thin layer of PAC after resizing to less than

349

10  10 mm. The PAC-coated plastics are first treated in a microwave for 30 sec; this increases

350

wetting of PVC significantly, while that of the other plastics is unaffected. Under the optimum

351

conditions for froth flotation, the PVC is easily separated from the mixture due to the selectively

352

increased hydrophilicity. The remaining plastics are screened and dried at room temperature. Then,

353

PAC is again coated on the plastic surface to replace that lost during froth flotation. ABS separation is

354

enhanced by the subsequent mild-heat treatment. Treatment at 150C for 100 sec reduces the density

355

of ABS due to formation of twisted structure, which facilitates recovery of ABS from the mixture by

356

gravity separation.

357

This combination enables complete separation of PVC and ABS from WEEE plastics with

358

maximum recovery and purity. In addition, the combined process reduces the energy required due to

359

the lack of heat treatment for PVC separation. The short duration of the combined treatments (30 sec

360

for microwave and 100 sec for mild-heat treatment) and simple froth flotation procedure are

361

advantages of the sequential separation method. Moreover, this method also has economics

362

advantages in terms of reducing operational and environmental costs. For example, use of ozone for

363

surface modification can release/generate remaining ozone issues or problems such as odor pollution,

364

in the absence of effective control efforts. Plasma and flame treatment can physically modify the

365

plastic surface by reaction or exposure to the generated corona discharge. However, these methods

366

require costly instrument installation. The use of chemical reagents not only requires a relatively long

367

time for surface modification but also generates contaminated wastewater.

368

Acknowledgments

369 370

This research was supported by the National Research Foundation of Korea (NRF) through the Basic Science Research Program funded by the Ministry of Education (2014R1A1A2055487). 14 ACS Paragon Plus Environment


371

Page 16 of 25

Supporting Information

372

We attached some descriptive information for the manuscript describing the experimental setup

373

and composition of WEEE plastic samples which used in this study. Besides, the CAs results was in

374

Table S2 and the AFM results - RMS - shown in Tables S3, supported plastic morphology changes

375

before and after the treatments. Finally, Table S4 showed some PVC and ABS separation results in

376

previous studies in compared with the present study.

377

References

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Fig. 1. Floating behavior of ABS (a) and change of plastic densities (b) before and after microwave treatment (MW), PAC coating with microwave treatment (PAC@MW), mild-heat treatment (MH) and PAC coating with mild-heat treatment (PAC@MH).



Fig. 2. Changes in ABS (a) and PVC (b) surfaces before and after PAC coating with microwave treatment (PAC@MW) and PAC coating with mild-heat treatment (PAC@MH).


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Fig. 3. Water contact angles of plastic before and after microwave treatment (MW), PAC coating with microwave treatment (PAC@MW), mild-heat treatment (MH), PAC coating with mild-heat treatment (PAC@MH).



Fig. 4 Contact angles of PVC surfaces before and after treatments (a, b, c), a schematic illustration of air bubble adhesion to PVC surfaces (d, e, f), and the settling behavior of the treated PVC after treatment in froth flotation (g).


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Fig. 5. Recovery and purity of treated PVC as function of mixing speed during froth flotation (a); water temperature (b); floating agent concentration (c) (airflow rate 0.5 L/m and flotation time 2 min).



Fig. 6. Flowchart of the simplified froth flotation process for selective separation of PVC and ABS from WEEE plastics.


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TOC/Abstract Art


Sustainable and Selective Separation of PVC and ABS from a WEEE

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