Optimization of Surface Treatment Using Sodium Hypochlorite

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Sustainability Engineering and Green Chemistry

Optimization of surface treatment using sodium hypochlorite facilitates co-separation of ABS and PC from WEEE plastics by flotation Jianchao Wang, Hui Wang, and Dongbei Yue Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b06432 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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Environmental Science & Technology

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Optimization of surface treatment using sodium hypochlorite facilitates co-separation of

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ABS and PC from WEEE plastics by flotation

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Jianchao Wang †, ‡, §, Hui Wang §,*, Dongbei Yue †, ‡,*

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† School of Environment, Tsinghua University, Beijing 100084, PR China;

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‡ Key Laboratory of Solid Waste Management and Environment Safety (Tsinghua University), Ministry of

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Education, Tsinghua University, Beijing 100084, PR China;

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§School of Chemistry and Chemical Engineering, Central South University, Changsha, 410083 Hunan, China;

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*Co-corresponding author, Hui Wang (e-mail: [email protected], tel./fax: +8673188879616) and Dongbei

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Yue ([email protected], tel./fax: +86 1062773693);

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ORCID ID: Jianchao Wang: 0000-0002-6576-2044; Hui Wang: 0000-0003-1015-7653; Dongbei Yue: 0000-00015983-1157; Word counts: 4349; Tables: 1; Figures: 4.

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1 ACS Paragon Plus Environment


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Abstract

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Waste acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) as dominant

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components in waste electrical and electronic equipment (WEEE) plastics show

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significant potential for recycling, which is severely restricted by efficient separation

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method. We proposed a novel surface treatment method using sodium hypochlorite for

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facilitating co-separation of ABS and PC from WEEE plastics by flotation for recycling.

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Optimization of surface treatment process was performed with response surface

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methodology using Box-Behnken design. A quadratic model was generated for

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predicting the floating rate of ABS and PC, and it was also used to optimize the co-

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separation performance. The optimum conditions were determined and included

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concentration of 0.05 M, temperature of 69.5 °C, contact time of 56.5 min, and stirring

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rate of 200 rpm. Under optimum conditions, the co-separation of ABS and PC was

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effectively achieved; the recovery and the purity of ABS and PC reached 97.4% and

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100.0%, respectively. The formation of oxygen-bearing groups and morphological

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changes were confirmed as major mechanism to induce the surface hydrophilization of

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ABS and PC. Consequently, this method is feasible for selective co-separation of ABS

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and PC from WEEE plastics, and it provides technological insights in the sustainable

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deposal of WEEE plastics.

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Key words: optimization, surface treatment, WEEE plastics, separation, ABS, PC


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Abstract graphic

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Introduction

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The consumption of electrical and electronic equipment is one of the fast-growing

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sectors due to rapid upgrading of products, and consequentially the corresponding

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generation of waste electrical and electronic equipment (WEEE) is expected to

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persistently increase.1-3 It was estimated that the global generation of WEEE in 2017

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would reach 72 million tons with a growth rate of 3-5%.1,

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approximately 30% of WEEE, including acrylonitrile butadiene styrene (ABS),

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polycarbonate (PC), polymethyl methacrylate (PMMA), polystyrene (PS), polyvinyl

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chloride (PVC), polypropylene (PP), polyethylene (PE), etc.4-7 ABS and PC are widely

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used in electronic and electrical equipment, automobile industry, and construction

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materials. Furthermore, the blended ABS/PC is also widely utilized to improve

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physicochemical, thermal, and mechanical properties of single polymer, and thus it also

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has a widespread application in printed circuit boards of computers, mobile phones, and

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television, etc. Accordingly, waste ABS, PC, and blended ABS/PC are dominant

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plastics in WEEE plastics, accounting for approximately 50% of WEEE plastics.5, 8, 9

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Particularly in cathode ray tube (CRT) monitors and copying equipments, the

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proportion of ABS, PC, and blended ABS/PC can reach up to 89% and 71%,

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respectively.10

4, 5

Plastics take up to

Generally, about 90% of WEEE plastics is disposed by incineration and landfill,

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11, 12

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and only 10% is treated by recycling.

Recycling is considered as a promising

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approach to simultaneously address environmental concerns and resource issues.4, 9, 11,

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13-19

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polybrominated dibenzodioxins and dibenzofurans which are generated in the

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incineration of brominated flame retardants (BFRs), a typical additive in ABS.3, 9, 10

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Toxic leachates and soil resources crisis associated with landfill can be effectively

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mitigated by recycling.5, 20 It has been demonstrated that waste ABS and PC can be

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together sent to mechanical recycling, and the composition range between 10%-20%

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PC is most desired to obtain remarkable properties of recycled products.19,

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Intriguingly, 36.75% of ABS and 4.99% of PC can be found in WEEE plastics and are

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perfectly suited to recycling.5 Thus, the recycling of waste ABS and PC of WEEE

For example, it can be expected to relieve the detrimental emissions including


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plastics through blending shows significant potential for recycling. However, the

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recycling of waste ABS and PC is challenging because they are generally mixed with

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other plastics. The target plastic can be contaminated by others due to chemical

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incompatibility, discoloration, and degradation.16, 19

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Flotation is a prospective method for the separation of plastics because three

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significant advantages: (1) it is more applicative for waste plastics with similar densities;

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(2) it is more effective and cheaper; (3) it has simpler operation procedures.19, 22-28

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Intrinsically, the mechanism of flotation is the selective attachment of air bubbles on

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hydrophilic surface, resulting in distinct floating/submerging behaviors in flotation and

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thereby enabling the separation of plastics.29 Owning to inherently hydrophobic surface

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of plastics, selective surface hydrophilization of target-plastic is more crucial prior to

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flotation separation. Tremendous surface hydrophilization methods have been reported

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for promoting plastic flotation, including nano-Fe/Ca/CaO ozonization,30 mild-heat

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treatment,31 nanometallic Ca/CaO treatment,1 ZnO/microwave treatment,27 ZnO

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coating,26 powder activated carbon coating,25 and Fenton oxidation.

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these methods only focus on single separation of ABS or PVC from other plastics, and

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study on co-separation of ABS and PC cannot be found in the pertinent literature.

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Furthermore, single parameter experiment design was generally utilized and rational

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experimental design for optimization is overlooked. Single parameter experiment

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monitors the effect of one parameter at a time on an experimental response, and only

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one parameter is changed at a constant level of other parameters.34, 35 This methodology

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not only fails to depict the complete effect of parameters but also increase the run

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numbers of experiment. Thus, optimum conditions obtained from single parameter

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experiment is unreliable because it ignores the effect of interactions between

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parameters.35

32, 33

However,

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We emphasize that the co-separation of ABS and PC from WEEE plastics is of

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significance for recycling, and optimization with full factorial designs is necessary to

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obtain factual optimum conditions. Therefore, this study aims to optimize surface

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treatment using sodium hypochlorite (NaClO) by response surface methodology (RSM)

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combined with Box-Behnken designs for facilitating co-separation of ABS and PC 5 ACS Paragon Plus Environment


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from WEEE plastics by flotation. The first objective of this work was to optimize

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surface treatment process using Box-behnken design, generate a model for predicting

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floating rate of ABS and PC, determine the optimum conditions, and verify the co-

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separation performance of ABS and PC. The second objective was to ascertain the

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changes in surface properties of plastics induced by surface treatment with the aid of

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electron dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS),

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Fourier transform infrared spectroscopy (FT-IT), and Water Contact angles (WCA).

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Materials and methods

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Materials

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Five plastics included ABS, PC, PMMA, PS, and PVC with densities of 1.109,

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1.140, 1.190, 1.051 and 1.410 g/cm3, respectively. The colors of plastics were different,

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and this made it easy to evaluate separation performance by manual sorting.

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Summarized preparation information of plastic samples has been provided in the

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previous reporting.36 The particle size used in this study was 3.2-4.0 mm because it

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comes within the most suited particle size for plastic flotation 33, 36. Molecular structures

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and practical pictures of these plastics are shown in Fig. S1. NaClO with analytical

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purity obtained from Jiangshun Chemical Technology Co., Ltd. (Guangdong province,

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China). Terpineol as frother with analytical purity was used for maintaining air bubbles

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generated by airflow and sand core in flotation process, and purchased from Wancong

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Chemical Engineering Co., Ltd. (Guangdong province, China). Tap water as flotation

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medium with temperature of 25±2 ℃ was used for flotation tests.

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Surface treatment and flotation tests

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Plastic mixtures of 10 g was treated with NaClO (100 mL) in a thermostat-

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controlled water bath (DF101S, corporation of Yuhua apparatus, Gongyi, Henan

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province, China). After treatment, plastics were filtrated from NaClO solution, washed

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with tap water for three times, and sent to perform flotation tests. Flotation tests were

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performed using an integrative device consisting a self-designed flotation column and

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an air pump (Guangdong Hailea Group Co., Ltd.). The height and inside diameter of

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flotation column are 580 mm and 60 mm. The adjustable air-pump was used to generate

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airflow which can pass a sand core installed at the bottom of flotation column to 6 ACS Paragon Plus Environment

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produce homogeneous-size air bubbles. Flotation tests were carried out under

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conditions of flotation time of 4.0 min, frother concentration of 24.0 mg/L, and airflow

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rate of 7.2 mL/min. After flotation tests, floated components and submerged

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components of each plastic were sorted manually according to colors, washed with tap

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water, dried at room temperature, and weighed.

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Significant variables in surface treatment on the floatability of plastics were

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predetermined by single parameter experiment. The conditions and results of single

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parameter experiment are shown in Table S1 and Fig. S2, respectively. The floating rate

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was the indicator of floatability and can be calculated by Eqs. (1).

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∅=𝑤

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where ∅ is the floating rate of plastic 𝑖, %; 𝑤𝑖𝑓 and 𝑤𝑖𝑠 are the weight of floated and

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submerged plastic 𝑖, respectively, g.

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Box-Behnken design and statistical analysis

𝑤𝑖𝑓 𝑖𝑓 +𝑤𝑖𝑠

× 100%,

(1)

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The co-separation performance of ABS and PC from WEEE plastics is determined

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by the expected difference between the floating rate of target plastics (ABS and PC)

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and other plastics. Therefore, optimization design was carried out to ascertain the

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interactions of significant variables, optimize surface treatment process, and generate a

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model for predicting the floating rate of ABS and PC. RSM combined with Box-

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Behnken design using Design Expert@ software, Version 8.6 (Stat-Ease, Minneapolis,

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USA) was utilized to design optimization experiment 37. A design of 3 variables with

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3-levels (see Table S2) was used to optimize surface treatment process. The total

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number of experimental runs (N) is determined by Eq. (2).

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N = 2K (K-1) + C0,

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where K is the number of variables and C0 is the number of central points.

(2)

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The total number of runs (N) were 17 including 12 factorial points and 5 central

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points. The experimental design matrix for variables and experimental results are

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shown in Table 1. The floating rate of ABS and PC was selected as response Y. A

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quadratic model equation was used to predict response Y as a function of variables

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which is expressed by Eq. (3). 7 ACS Paragon Plus Environment


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𝑘 Y = 𝛽0 + ∑𝑘𝑖=1 𝛽𝑖 𝑋𝑖 + ∑𝑘𝑖=1 𝛽𝑖𝑖 (𝑋𝑖 )2 + ∑𝑘−1 𝑖=1 ∑𝑗=2 𝛽𝑖𝑗 𝑋𝑖 𝑋𝑗 ,

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where Y is predicted response value, β0 is constant term, βi is liner effect term, βii is

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squared effect term, βij is interactive effect term, and Xi and Xj are independent variables.

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Analysis of variance (ANOVA), a typical collection of statistical models, was used to

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estimate the statistical significance of independent variables, interactions, coefficients

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and residues. Table 1 Box-Behnken design matrix for variables and experimental results

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(3)

Run

A/concentration (M/L)

B/temperature (℃)

C/contact time (min)

Response Y/ the floating rate of ABS and PC (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

0.02 0.04 0.02 0.05 0.04 0.02 0.05 0.05 0.04 0.02 0.04 0.05 0.04 0.04 0.04 0.04 0.04

50.00 70.00 50.00 70.00 50.00 70.00 50.00 50.00 50.00 30.00 30.00 30.00 50.00 50.00 70.00 30.00 50.00

30.00 30.00 60.00 45.00 45.00 45.00 30.00 60.00 45.00 45.00 30.00 45.00 45.00 45.00 60.00 60.00 45.00

73.75 25.25 64.11 0.00 63.25 14.25 50.75 32.75 45.75 100.00 92.25 88.25 67.75 48.25 3.75 81.06 48.05

Confirmatory experiment

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Plastic mixtures of 10 g was subjected to surface treatment under optimum

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conditions including concentration of 0.05 M, temperature of 69.5 °C, and contact time

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of 56.5 min, and stirring rate of 200 rpm. Flotation tests were performed under

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conditions of flotation time of 4.0 min, frother concentration of 24.0 mg/L, and airflow

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rate of 7.2 mL/min. ABS and PC as submerged product, and other plastics as floated

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product were collected. The recovery and the purity of ABS and PC were selected to

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evaluate the co-separation performance, and calculated by Eqs. (4) and (5).

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𝛿=𝑤

𝑤𝑠

𝑠 +𝑤𝑓

× 100%,

(4) 8 ACS Paragon Plus Environment

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𝑤𝑠

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𝜗=𝑤

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where 𝛿 and 𝜗 are the recovery and purity of ABS and PC. 𝑤𝑠 , 𝑤𝑓 and 𝑤𝑠∗ are the

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weight of submerged ABS and PC, floated ABS and PS, and submerged other plastics,

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respectively, g.

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Surface characterization

∗ 𝑠 +𝑤𝑠

× 100%,

(5)

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The changes in surface properties of plastics before and after treatment under

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optimum conditions of NaClO concentration of 0.05 M, temperature of 69.5 °C, and

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contact time of 56.5 min, and stirring rate 200 rpm were investigated using SEM-EDS,

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XPS, FT-IT, and WCA analyses. SEM-EDS (EVO18, Carl Zeiss of Germany) was used

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to ascertain morphological changes and elemental composition. XPS (K-Alpha 1063,

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Thermo Fisher Scientific) was also utilized to confirm elemental compositions. FT-IR

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(Nicolet Avatar 360, Nicolet Magua Corporation of USA) was utilised to examine the

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changes in functional groups. WCA was measured with a JJC-I contact angle measuring

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instrument (Changchun Optional Instrument Factory, China); every value reported is a

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mean value of five measurements.

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Results and discussion

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Significant parameters

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Surface treatment can selectively suppress the floating rate of ABS and PC, which

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may be expected to co-separate ABS and PC from WEEE plastics (see Fig. S2). Within

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the tested interval, concentration, temperature, and contact time are the significant

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variables that markedly affect the floating rate of ABS and PC, and thus these

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parameters serve as the significant variables. Nonetheless, conventional single

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parameter experiment design (manipulation of one variable at a time) cannot

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comprehensively depict the effects of variables and cannot obtain factually optimum

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conditions due to its neglect in the interactions of variables.35, 37 Furthermore, this

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experimental design is time-consuming and needs a large number of tests, thereby

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leading to a remarkable increase in the consumption of reagents and material. Therefore,

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a rational and comprehensive optimization design using RSM, a statistic technique, is

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necessary to systematically explain and optimize the surface treatment process for 9 ACS Paragon Plus Environment


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complete co-separation of ABS and PC from WEEE plastics by flotation.

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Optimization experiment

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The fitting model and statistical analysis

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A quadratic model was generated by the Design Expert@ software, Version 8.6

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(Stat-Ease, Minneapolis, USA) based on experimental results. The model for the coded

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unit can be shown as Eqs. (6). Positive and negative coefficients in model represent a

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synergistic and an antagonistic effect on the response, respectively. It can be observed

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that the consistent 7.36 is independent of any variables and interactions between

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variables. Furthermore, the linear terms including A (concentration), B (temperature),

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and C (contact time), the interaction terms containing AB and BC, and the second-order

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term B2 all have negative effects on the response Y (the floating rate of ABS and PC).

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Therefore, the floating rate of ABS and PC decreases with the increasing of these terms.

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Y = 7.36 − 1.02A − 3.41B − 0.71C − 0.79AB − 0.62BC − 1.27𝐵2

(6)

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ANOVA results for the model and different terms was obtained (see Table S3).

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The model ‘F-value’ is 37.97, implying the terms of this model are significant to the

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response. There is only a 0.01% chance that the model ‘F-Value’ could occur due to

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noise. Any variables or their interactions with ‘Prob > F’ < 0.05 are significant. A, B,

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C, and B2 with lower ‘Prob > F’ than 0.05 are the significant model terms for the

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response. Particularly, amongst these significant terms, B is the most significant term

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that affects the response with ‘F-value’ of 182.36 and ‘Prob>F’ < 0.0001. ‘R-Squared’

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of 0.9580 manifests that the model has a good correlation between the experimental

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values and predicted values for the response. High ‘Adj R-squared’ displays the

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significance of the model and guarantees approving adjustment of the experimental

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values to this model. The ‘Pred R-Squared’ and the ‘Adj R-squared’ are close with

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values of 0.8032 and 0.9327 respectively and their difference is less than 0.20, which

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demonstrates a reasonable consistency. The C. V. of 10.55% is low, which denotes good

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accuracy and reliability of the experiment. As well, the Adeq Precision of 19.317 is

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much larger than desirable value of 4, indicating credible fitness of the model.

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Consequently, this model is reliable and can be used in surface treatment process to

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predict values for the floating rate of ABS and PC. 10 ACS Paragon Plus Environment

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The interaction effect of variables

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Fig. 1 displays the interaction effect of concentration, temperature, and contact

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time on the floating rate of ABS and PC with (a) contour plot and (b) three-dimensional

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response surface. The significance of interactions between variables is indicated with

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the shape of contour plot; circular and elliptical contour plot reveal negligible and

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apparent significance, respectively.23, 38 It can be observed that the interactions between

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the variables are not significant. Additionally, the near-linear slopes of the response

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surface demonstrate that the interactions between the variables are not significant,

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which can be also confirmed by the relatively high ‘Prob>F’ values (AB and BC are

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0.0507 and 0.1121, respectively) obtained in the ANOVA (see Table S3).

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241 242 243

Fig. 1 Contour plot and three-dimensional response surface of variables.

The determination of optimum conditions and confirmatory experiment

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A relatively low floating rate of ABS and PC is desired after surface treatment,

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thereby ensuring the high-efficiency co-separation of ABS and PC from WEEE plastics.

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The minimum floating rate of ABS and PC was determined by numerical optimization

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of the overall desirability function using Design-Expert Software. The variables 11 ACS Paragon Plus Environment


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including concentration, temperature, and contact time were set as in range without a

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target. The floating rate of ABS and PC was set as minimization with a target of 0.0%.

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The minimized the floating rate of ABS and PC is 1.54E-007% with the overall

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desirability of 100.0%; the predicted conditions are concentration of 0.05 M,

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temperature of 69.79 °C, and contact time of 56.36 min. For conveniently and

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accurately controlling variables in confirmatory experiment, the predicted conditions

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were amended, and concentration of 0.05 M, temperature of 69.5 °C, and contact time

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of 56.5 min combined with stirring rate of 200 rpm are considered as optimum

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conditions. Triplicate confirmatory experiments were performed under optimum

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conditions. The predicted and experimental values for variables, the floating rate of

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ABS and PC, and the recovery and purity of ABS and PC were summarized (see Table

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S4). The experimental value of the floating rate of ABS and PC is 2.58±0.58%,

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indicative of a good reproducibility of this method. The recovery and the purity of ABS

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and PC are 97.4% and 100.0%, implying an efficient co-separation of ABS and PC.

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Overall, the mean experimental value is in close agreement with the predicted value

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obtained from the model. The above results demonstrate that the RSM using Box-

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Behnken design is a reliable method for the optimization of surface treatment

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concerning co-separation of ABS and PC from WEEE plastics. The environmentally

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friendly NaClO and simple flotation procedure are advantages of this method.

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Furthermore, this method firstly proposed co-separation of ABS and PC from WEEE

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plastics for recycling, and provides technical insight into the sustainable disposal of

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WEEE plastics.

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The Mechanism of surface treatment

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Surface morphology analysis

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Fig. 2 displays comparison of surface morphology with different magnifications

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between plastic samples before and after surface treatment. Demonstrable changes in

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surface morphology are observed on ABS and PC surface after surface treatment, while

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other plastics (involving PMMA, PS, and PVC) have no morphological changes on

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surface. Numerous damascenes or cloudlike structures are formed on ABS surface after

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surface treatment, which is also confirmed by the image with a larger magnification. 12 ACS Paragon Plus Environment

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Similarly, corrugated structures are also observed on PC surface after surface treatment.

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These results is similar to the reporting by N.T. Thanh Truc et al. 26, 27 that ZnO coating

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combined with microwave treatment selectively induced the formation of numerous

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large wrinkles on ABS surface in two-component plastics. Additionally, it was also

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observed in our previous studies 33, 36 that surface modification with potassium ferrate

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and Fenton reagent can promote the formation of obvious protrusions on PC surface in

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two-component and three-component plastic mixtures, respectively. In terms of other

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plastics, both samples before and after surface treatment present a relatively smooth

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surface. Thus, surface treatment using sodium hypochlorite promotes the formation of

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numerous damascenes on ABS and PC surface, thereby resulting in a rough surface.

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289

290

291



292 293

Fig. 2 Comparison of SME micrographs of plastic samples before and after surface treatment

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Element composition analysis

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XPS analysis

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Elemental ratios of almost of all plastics (see Table S5) are affected after surface

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treatment at varying degrees by surface treatment. Particularly for ABS and PC, the

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O/C ratio has an obvious enhancement by 11.65% and 4.61% respectively, and the N/C

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ratio of ABS declines by 0.79%. Differently, the O/C ratios of PMMA, PS, and PVC

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all display a relatively slight increasing by 3.07%, 1.60%, and 0.72%, respectively.

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Furthermore, the Cl/C ratio of chlorine-containing PVC descends slightly by 2.58%.

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Hence, it can be concluded surface treatment enhances the O/C ratio of ABS and PC

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clearly compared with other plastics. In order to validate the XPS results, SEM-EDS

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analysis of ABS and PC was performed (see Fig. S3). The SEM-EDS results

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demonstrate that the O/C ratio on ABS and PC surface increases by 13.26% and 9.37%

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respectively, and this along with XPS results provides valid evidences that the O/C ratio

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of ABS and PC is enhanced by surface treatment. S.R. Mallampati 1 reported that the

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increasing in the O/C ratio is the results of the formation of oxygen-bearing groups.

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Consequently, this enhancement in the O/C ratio may be attributed to the formation of

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oxygen-bearing groups. To validate the hypothesis of formation of oxygen-bearing

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groups, the C 1s peak in XPS spectra of plastic samples was further analyzed with the

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high resolution spectra of C 1s peak. This method, employed to elucidate the C 1s peak,

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is available for studying the oxygen-bearing groups in carbon structure of polymers.39

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Fig. 3 presents the high-resolution C 1s peaks of plastics before and after surface

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treatment. For the C 1s peak, both ABS and PC possess significant changes after surface

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treatment, while the C 1s peak of other plastics including PMMA, PS and PVC has little

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variations. For ABS before surface treatment, the C 1s peak is well fitted with five 14 ACS Paragon Plus Environment

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peaks, aliphatic C-H/C-C at 285.00 eV, aromatic C-H/C-C at 284.50 eV, C≡N at 286.50

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eV, and π → π* of the aromatic ring structure at 291.00 eV. Interestingly, surface

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treatment facilitate the formation of C-O peak at 286.00 eV and O=C-O peak at 289.00

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eV. Moreover, the disappearing of C≡N peak may be ascribed to the combined effect of

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the overlap with C-O peak and the hydrolysis of C≡N group.26 In terms of PC, the C 1s

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peak of both samples before and after surface treatment can be well fitted with five

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peaks including aliphatic C-H/C-C at 285.00 eV, aromatic C-H/C-C at 284.50 eV, C-O

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at 286.00 eV, O=C-O at 289.00 eV, and π→π* of the aromatic ring structure at 291.00

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eV. Obviously, the intensity of C-O peak and O=C-O peak is enhanced. As for as

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PMMA, its C 1s peak includes three peaks, aliphatic C-H/C-C at 285.00 eV, C-O at

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286.00 eV, and O=C-O at 289.00 eV, and surface treatment facilitate the slight

329

increasing in the intensity of O=C-O. In the case of PS, three peaks including aliphatic

330

C-H/C-C at 285.00 eV, aromatic C-H/C-C at 284.50 eV, and π→π* of the aromatic ring

331

structure at 291.00 eV can be found in the C 1s peak before surface treatment. After

332

surface treatment, the C-O at 286.00 eV is introduced on PS surface. In regard to PVC,

333

the C 1s peak of samples before and after surface treatment contains aliphatic C-H/C-

334

C at 285.00 eV and C-Cl peak at 286.20 eV, and surface treatment induce no changes

335

on its C 1s peak. Consequently, it can be concluded that the results of peak-fitting of

336

the C 1s peak confirm the formation of hydroxyl and carbonyl groups, and this accounts

337

for the increasing in the O/C ratio on ABS and PC surface.

338



339

340

341

342 343

Fig. 3 Comparison of high-resolution C 1s peak of plastic sample before and after surface

344

treatment

345

Functional groups analysis 16 ACS Paragon Plus Environment

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346

Fig. 4 presents comparison of FT-IR spectra of plastic samples before and after

347

surface treatment. The functional groups of ABS, PC, and PMMA show significant

348

variations, and that of PS and PVC possesses no changes, which is well in agreement

349

with XPS and SEM-EDS results. For ABS after surface treatment, its FT-IR spectra

350

shows several remarkable differences: (1) the absorption band at 3200-3700 cm-1 is

351

obviously enhanced, which may be ascribed to the C-O and N-H; (2) the C≡N

352

absorption band at 2236 cm-1 evidently declines; (3) the O=C-O absorption band at

353

1800 cm-1 and 1740 cm-1 appears; (4) the N-H absorption band at 1540 cm-1 emerges.

354

These results in the FT-IR spectra of ABS well agree with XPS results that are the

355

formation of C-O and O=C-O peak, and the declining of C≡N peak along with the

356

emerging of N-H, which is ascribed to the oxidation and hydrolysis effect induced by

357

sodium hypochlorite.26 In terms of PC, the absorption bands of C-O at 3200-3700 cm-1

358

and O=C-O at 1680-1840 cm-1 clearly increase after surface treatment, which provides

359

further evidences for the oxidation effect of sodium hypochlorite obtained from XPS

360

results in which the intensity of C-O and O=C-O peak is remarkably enhanced.24 As for

361

as PMMA, the enhancement of O=C-O absorption band at 1740 cm-1 is well in

362

agreement with XPS results and confirms the oxidation effect of sodium hypochlorite.

363

Differently, the corresponding spectra of PS and PVC are not dramatically changed,

364

indicating that the oxidation effect of sodium hypochlorite is not capable to change the

365

functional groups on plastics. Particularly, the formation of hydroxyl group on PS

366

indicated by the C 1s fitted peak is not found in the FT-IR spectrum of PS after surface

367

treatment, which indicates that the weak signal of hydroxyl due to its low content in

368

molecular structure.

369 17 ACS Paragon Plus Environment


370

371 372 373

Fig. 4 Comparison of FT-IR spectra of plastic samples before and after surface treatment

Surface hydrophilicity analysis

374

Based on SEM, XPS, SEM-EDS, and FT-IR results, it can be concluded that

375

surface treatment selectively induce morphological changes and the formation of

376

oxygen-bearing groups on ABS and PC surface. Compared smooth surface, the rough

377

surface can reduce capturing ability for air bubbles, thereby enhancing the surface

378

hydrophilicity of plastics.26 Moreover, hydroxyl and carbonyl groups formed on plastic

379

surface are polar and have an affinity for water, and thus these groups have been

380

considered hydrophilic.1, 26, 31 These changes in surface morphology and functional

381

groups can increase the surface hydrophilicity of ABS and PC and suppress the

382

attachment of air bubbles on their surface, which was also reported by N.T.T. Truc 31

383

and S.R. Mallampati.1 Therefore, the submerging of ABS and PC in flotation after

384

surface treatment using sodium hypochlorite can be attributed to the increasing of

385

surface hydrophilicity, and this was also elucidated by hydrophilicity analysis using

386

WCA measurements (see Table S6). It can be observed that the WCA of ABS and PC

387

descends by 19.91°and 13.67°respectively, and that of PMMA, PS, and PVC decreases 18 ACS Paragon Plus Environment

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388

slightly by 4.79°, 2.84°, and 2.45°respectively. The declining in WCA indicates that

389

surface treatment induce the enhancement in surface hydrophilicity of five type of

390

plastics at different degrees. Particularly for ABS and PC, their surface hydrophilicity

391

is enhanced significantly due to the morphological changes and the formation of

392

oxygen-bearing groups. Consequently, surface treatment using sodium hypochlorite

393

facilitates the surface hydrophilization of ABS and PC by inducing the morphological

394

changes and the formation of hydrophilic oxygen-bearing groups, which can enhance

395

their surface hydrophilicity and suppress their floatability in flotation.

396

Acknowledgment

397

This research was supported by National Natural Science Foundation of China

398

(21878343).

399

Supporting Information

400

The molecular structures and pictures of plastics (Fig. S1); results of single parameter

401

experiment for surface treatment (Fig. S2); SEM-DES spectra for the ABS and PC

402

surface before and after surface treatment (Fig. S3); conditions of single parameter

403

experiment in surface treatment (Table S1); level and code of variables for Box-

404

Behnken design (Table S2); ANOVA for the model and different terms (Table S3);

405

optimum and confirmative values of the floating rate, recovery, purity of ABS and PC

406

(Table S4); Elemental ratio on the surface of plastics before and after surface treatment

407

(Table S5); water contact angle of plastic samples before and after surface treatment

408

(Table S6).

409

References

410

1. Mallampati, S. R.; Heo, J. H.; Park, M. H., Hybrid selective surface

411

hydrophilization and froth flotation separation of hazardous chlorinated plastics from

412

E-waste with novel nanoscale metallic calcium composite. Journal of Hazardous

413

Materials 2016, 306, 13-23; DOI 10.1016/j.jhazmat.2015.11.054.

414

2. Kan, Y.; Yue, Q.; Liu, S.; Gao, B., Effects of Cu and CuO on the preparation of

415

activated carbon from waste circuit boards by H3PO4 activation. Chemical Engineering

416

Journal 2017, 331,93-101; DOI 10.1016/j.cej.2017.08.113.

417

3. WäGer, P. A.; Mathias, S.; Esther, M.; Rolf, G., RoHS regulated substances in 19 ACS Paragon Plus Environment


418

mixed plastics from waste electrical and electronic equipment. Environmental Science

419

& Technology 2011, 46 (2), 628-635; DOI 10.1021/es202518n.

420

4. Ma, C.; Yu, J.; Wang, B.; Song, Z.; Xiang, J.; Hu, S.; Su, S.; Sun, L., Chemical

421

recycling of brominated flame retarded plastics from e-waste for clean fuels production:

422

A review. Renewable & Sustainable Energy Reviews 2016, 61, 433-450; DOI

423

10.1016/j.rser.2016.04.020.

424

5. Dimitrakakis, E.; Janz, A.; Bilitewski, B.; Gidarakos, E., Small WEEE:

425

determining recyclables and hazardous substances in plastics. Journal of Hazardous

426

Materials 2009, 161 (2), 913-919; DOI 10.1016/j.jhazmat.2008.04.054.

427

6. Kunwar, B.; Cheng, H. N.; Chandrashekaran, S. R.; Sharma, B. K., Plastics to fuel:

428

a review. Renewable & Sustainable Energy Reviews 2016, 54, 421-428; DOI

429

10.1016/j.rser.2015.10.015.

430

7. Cui, J.; Forssberg, E., Mechanical recycling of waste electric and electronic

431

equipment: a review. Journal of Hazardous Materials 2003, 99 (3), 243-263; DOI

432

10.1016/S0304-3894(03)00061-X.

433

8. Schlummer, M.; Gruber, L.; Mäurer, A.; Wolz, G.; Eldik, R. V., Characterisation of

434

polymer fractions from waste electrical and electronic equipment (WEEE) and

435

implications for waste management. Chemosphere 2007, 67 (9), 1866-1876; DOI

436

10.1016/j.chemosphere.2006.05.077.

437

9. Takaretu, I.; Stephan, H.; Rothenbacher, K. P., Comparison of the recyclability of

438

flame-retarded plastics. Environmental Science & Technology 2003, 37, 652-656; DOI

439

10.1021/es025771c.

440

10. Wang, R.; Xu, Z., Recycling of non-metallic fractions from waste electrical and

441

electronic equipment (WEEE): a review. Waste Management 2014, 34, 1455-1469; DOI

442

10.1016/j.wasman.2014.03.004.

443

11. Chunfei, W.; Nahil, M. A.; Norbert, M.; Jun, H.; Williams, P. T., Processing real-

444

world waste plastics by pyrolysis-reforming for hydrogen and high-value carbon

445

nanotubes. Environmental Science & Technology 2015, 48, (1), 819-826; DOI 819-826;

446

10.1021/es402488b.

447

12. Wang, Y.; Zhang, F. S., Degradation of brominated flame retardant in computer 20 ACS Paragon Plus Environment

Page 20 of 24

Page 21 of 24


448

housing plastic by supercritical fluids. Journal of Hazardous Materials 2012, 205-206

449

(3), 156-163; DOI 10.1016/j.jhazmat.2011.12.055.

450

13. Wong, S. L.; Ngadi, N.; Abdullah, T. A. T.; Inuwa, I. M., Current state and future

451

prospects of plastic waste as source of fuel: A review. Renewable & Sustainable Energy

452

Reviews 2015, 50 (C), 1167-1180; DOI 10.1016/j.rser.2015.04.063.

453

14. Cucchiella, F.; D'Adamo, I.; Koh, S. C. L.; Rosa, P., Recycling of WEEEs: An

454

economic assessment of present and future e-waste streams. Renewable & Sustainable

455

Energy Reviews 2015, 51, 263-272; DOI 10.1016/j.rser.2015.06.010.

456

15. Johansson, J.; Luttropp, C., Material hygiene: improving recycling of WEEE

457

demonstrated on dishwashers. Journal of Cleaner Production 2009, 17 (1), 26-35; DOI

458

10.1016/j.jclepro.2008.02.010.

459

16. Chintala, V.; Godkhe, P.; Phadtare, S.; Tadpatrikar, M.; Pandey, J. K.; Kumar, S., A

460

comparative assessment of single cylinder diesel engine characteristics with plasto-oils

461

derived from municipal mixed plastic waste. Energy Conversion And Management

462

2018, 166, 579-589; DOI 10.1016/j.enconman.2018.04.068.

463

17. Zhao, P.; Xie, J.; Gu, F.; Sharmin, N.; Hall, P.; Fu, J. Z., Separation of mixed waste

464

plastics via magnetic levitation. Waste Management 2018, 76, 46-54; DOI

465

10.1016/j.wasman.2018.02.051.

466

18. Zhang, L.; Xu, Z., Separating and Recycling Plastic, Glass and Gallium from Waste

467

Solar Cell Modules by Nitrogen Pyrolysis and Vacuum Decomposition. Environmental

468

Science & Technology 2016, 50 (17), 9242-9250; DOI 10.1021/acs.est.6b01253.

469

19. Vilas Ganpat, P., Upcycling: converting waste plastics into paramagnetic,

470

conducting, solid, pure carbon microspheres. Environmental Science & Technology

471

2010, 44, (12), 4753-4759; DOI 10.1021/es100243u.

472

20. Saquing, J. M.; Saquing, C. D.; Knappe, D. R. U.; Barlaz, M. A., Impact of plastics

473

on fate and transport of organic contaminants in landfills. Environmental Science &

474

Technology 2010, 44, (16), 6396-6402; DOI 10.1021/es101251p.

475

21. Balarta, R.; García, D.; Salvador, M. D., Recycling of ABS and PC from electrical

476

and electronic waste. Effect of miscibility and previous degradation on final

477

performance of industrial blends. European Polymer Journal 2005, 41 (9), 2150-2160. 21 ACS Paragon Plus Environment


Page 22 of 24

478

DOI 10.1016/j.eurpolymj.2005.04.001.

479

22. Wang, C. Q.; Wang, H.; Fu, J. G.; Liu, Y. N., Flotation separation of waste plastics

480

for

481

10.1016/j.wasman.2015.03.027.

482

23. Wang, C.; Wang, H.; Liu, Y.; Huang, L., Optimization of surface treatment for

483

flotation separation of polyvinyl chloride and polyethylene terephthalate waste plastics

484

using response surface methodology. Journal of Cleaner Production 2016, 139, 866-

485

872; DOI 10.1016/j.jclepro.2016.08.111.

486

24. Wang, J.; Hui, W.; Wang, C.; Zhang, L.; Tao, W.; Long, Z., A novel process for

487

separation of hazardous poly(vinyl chloride) from mixed plastic wastes by froth

488

flotation. Waste Management 2017, 69, 59-65; DOI 10.1016/j.wasman.2017.07.049.

489

25. Truc, N. T. T.; Lee, C. H.; Lee, B. K.; Mallampati, S. R., Development of

490

hydrophobicity and selective separation of hazardous chlorinated plastics by mild heat

491

treatment after PAC coating and froth flotation. Journal of Hazardous Materials 2017,

492

321, 193-202; DOI 10.1016/j.jhazmat.2016.09.014.

493

26. Thanh Truc, N. T.; Lee, B. K., Selective separation of ABS/PC containing BFRs

494

from ABSs mixture of WEEE by developing hydrophilicity with ZnO coating under

495

microwave treatment. Journal of Hazardous Materials 2017, 329, 84-91; DOI

496

10.1016/j.jhazmat.2017.01.027.

497

27. Thanh Truc, N. T.; Lee, B. K., Combining ZnO/microwave treatment for changing

498

wettability of WEEE styrene plastics (ABS and HIPS) and their selective separation by

499

froth

500

10.1016/j.apsusc.2017.04.075.

501

28. Reddy, M. S.; Kurose, K.; Okuda, T.; Nishijima, W.; Okada, M., Separation of

502

polyvinyl chloride (PVC) from automobile shredder residue (ASR) by froth flotation

503

with ozonation. Journal of Hazardous Materials 2007, 147 (3), 1051-1055; DOI

504

10.1016/j.jhazmat.2007.01.137.

505

29. Fraunholcz, N., Separation of waste plastics by froth flotation-a review, part I.

506

Minerals Engineering 2004, 17 (2), 261-268; DOI 10.1016/j.mineng.2003.10.028.

507

30. Mallampati, S. R.; Lee, B. H.; Mitoma, Y.; Simion, C., Selective sequential

recycling-A

flotation.

review.

Waste

Applied

Surface

Management

Science

2015,

2017,


41,

420,

28-38;

746-752;

DOI

DOI

Page 23 of 24


508

separation of ABS/HIPS and PVC from automobile and electronic waste shredder

509

residue by hybrid nano-Fe/Ca/CaO assisted ozonisation process. Waste Management

510

2017, 60, 428-438; DOI 10.1016/j.wasman.2017.01.003.

511

31. Truc, N. T. T.; Lee, B. K., Sustainable and Selective Separation of PVC and ABS

512

from a WEEE Plastic Mixture using Microwave and/or Mild-heat Treatment with Froth

513

Flotation. Environmental Science & Technology 2016, 50 (19), 10580–10587; DOI

514

10.1021/acs.est.6b02280.

515

32. Wang, J. C.; Wang, H.; Huang, L. L.; Wang, C. Q., Surface treatment with Fenton

516

for separation of acrylonitrile-butadiene-styrene and polyvinylchloride waste plastics

517

by flotation. Waste Management 2017, 67, 20-26; DOI 10.1016/j.wasman.2017.05.009.

518

33. Wang, J. C.; Wang, H., Fenton treatment for flotation separation of polyvinyl

519

chloride from plastic mixtures. Separation & Purification Technology 2017, 187, 415-

520

425; DOI 10.1016/j.seppur.2017.06.076.

521

34. Bezerra, M. A.; Santelli, R. E.; Oliveira, E. P.; Villar, L. S.; Escaleira, L. A.,

522

Response surface methodology (RSM) as a tool for optimization in analytical chemistry.

523

Talanta 2008, 76 (5), 965-977; DOI 10.1016/j.talanta.2008.05.019.

524

35. Ray, S.; Lalman, J. A.; Biswas, N., Using the Box-Benkhen technique to

525

statistically model phenol photocatalytic degradation by titanium dioxide nanoparticles.

526

Chemical Engineering Journal 2009, 150 (1), 15-24; DOI 10.1016/j.cej.2008.11.039.

527

36. Wang, H.; Wang, J.; Zou, Q.; Liu, W.; Wang, C.; Huang, W., Surface treatment

528

using potassium ferrate for separation of polycarbonate and polystyrene waste plastics

529

by

530

10.1016/j.apsusc.2018.04.091.

531

37. Ray, S.; Lalman, J. A., Using the Box-Benkhen design (BBD) to minimize the

532

diameter of electrospun titanium dioxide nanofibers. Chemical Engineering Journal

533

2011, 169 (1), 116-125; DOI 10.1016/j.cej.2011.02.061.

534

38. Muralidhar, R. V.; Chirumamila, R. R., A response surface approach for the

535

comparison of lipase production by Canida cylindracea using two different carbon

536

sources. Biochemical Engineering Journal 2001, 9 (1), 17-23; DOI 10.1016/S1369-

537

703X(01)00117-6.

froth

flotation.

Applied

Surface

Science

2018,


448,

219-229;

DOI


538

39. Wang, R.; Shen, Y.; Zhang, C.; Yan, P.; Shao, T., Comparison between helium and

539

argon plasma jets on improving the hydrophilic property of PMMA surface. Applied

540

Surface Science 2016, 367, 401-406; DOI 10.1016/j.apsusc.2016.01.199.

541


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Optimization of Surface Treatment Using Sodium Hypochlorite

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