Green flotation of polyethylene terephthalate and polyvinyl chloride assisted by surface modification of selective CaCO3 coating

Journal Pre-proof Green flotation of polyethylene terephthalate and polyvinyl chloride assisted by surface modification of selective CaCO3 coating Yingshuang Zhang, Hongru Jiang, Kangyu Wang, Hui Wang, Chongqing Wang PII:

S0959-6526(19)33311-6

DOI:

https://doi.org/10.1016/j.jclepro.2019.118441

Reference:

JCLP 118441

To appear in:

Journal of Cleaner Production

Received Date: 24 April 2019 Revised Date:

19 August 2019

Accepted Date: 15 September 2019

Please cite this article as: Zhang Y, Jiang H, Wang K, Wang H, Wang C, Green flotation of polyethylene terephthalate and polyvinyl chloride assisted by surface modification of selective CaCO3 coating, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.118441. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

1

Green Flotation of Polyethylene Terephthalate and Polyvinyl Chloride assisted

2

by surface modification of selective CaCO3 Coating

3

Yingshuang Zhang 1, Hongru Jiang 1, Kangyu Wang1, Hui Wang 1,* ,

4

Chongqing Wang 2,*

5

1

6

410083 Hunan, P.R. China

7

2

8

450001, P.R. China.

9

*

10

School of Chemistry and Chemical Engineering, Central South University, Changsha,

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou

Corresponding author: Tel: +86 13973138633; E-mail: [email protected]

5760 Words (including references)

1

Green Flotation of Polyethylene Terephthalate and Polyvinyl Chloride assisted by surface modification of selective CaCO3 Coating Yingshuang Zhang 1, Hongru Jiang 1, Kangyu Wang1, Hui Wang 1,* , Chongqing Wang 2,* 1

School of Chemistry and Chemical Engineering, Central South University, Changsha,

410083 Hunan, P.R. China 2

School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou

450001, P.R. China. *

Corresponding author: Tel: +86 13973138633; E-mail: [email protected]

1

1

Abstract: Plastic flotation was a promising separation method for efficient recycling

2

of waste plastics. Aiming to avoid the destruction of original surface and secondary

3

pollution, a novel flotation process based on calcium carbonate (CaCO3) coating was

4

proposed for separation of polyvinyl chloride (PVC) and polyethylene terephthalate

5

(PET). The mechanism of CaCO3 coating was researched via scanning electron

6

microscope (SEM), X–ray photoelectron spectroscopy (XPS), X–ray diffraction

7

(XRD), zeta potential, Fourier transformed infrared spectroscopy (FT–IR), and

8

solution chemistry analysis. High polarity of C–Cl on PVC surface resulted in

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selective adherence of Ca2+ on PVC surface, and Ca2+ was a bridge between plastic

10

surface and CaCO3 particles. The separation of PVC and PET was optimized by

11

response surface methodology (RSM) combining Box–Behnken design (BBD).

12

Optimal pretreatment conditions for flotation separation of PVC and PET were 0.11 g

13

CaCO3, temperature 50.6 °C, treatment time 20 min, and pH 10.1. The purity and

14

recovery of PVC could be 100% and 99%, respectively.

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Keywords: Surface coating; green separation; calcium carbonate; flotation; waste

16

plastic

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1.

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A rising production of plastics is caused by increasing demand and market

19

consumption, and plastics irreversibly replace traditional materials due to their

20

excellent properties (Eygen et al., 2016). The production of artificial polymers and

21

fibers was only 2 million metric tons in 1950, while growing to 245 million metric

22

tons in 2006. In 2015, the production of plastics reached 380 million metric tons with

Introduction

2

23

about 8.2% compound annual growth rate (Yu et al., 2016). Environmental crisis also

24

appears along with the development of plastics industry. Post–consumer plastics

25

stagnate in ecosystem due to irreconcilable conflict between high production and slow

26

degradation of plastics. Even in Europe, only about 30% of waste plastics were

27

subjected to recycling in 2014 (Rahimi and García, 2017). A substantial proportion of

28

waste plastics would persist in environment and threaten ecosystem and human health.

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For example, the accumulation of endogenous additives and adsorbed hazardous

30

materials from micro plastics may be toxic for respiratory, immune, and reproductive

31

system (Li et al., 2018; Wright and Kelly, 2017).

32

Several common plastics dominate the global plastic demand: polyethylene (PE, 33%),

33

polypropylene (PP, 21%), polyvinyl chloride (PVC, 17%), polystyrene and

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expandable polystyrene (PS, 8%), and polyethylene terephthalate (PET, 7%)

35

(Hahladakis et al., 2018). Above plastics usually exist in packaging of consumer,

36

apparel, food, beverage, health care, and cosmetics, accounting for a high proportion

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of 40% of waste packaging plastics in waste plastics stream (Groh et al., 2019). In a

38

research article about the lifecycle of waste plastics, it is notable that log–normal

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distribution for the life time of packaging plastics has a mean at less than one year

40

(Geyer et al., 2017). Therefore, it is significant to improve the recycling of packaging

41

plastics for alleviating problem of environmental pollution and energy burden (Wang

42

et al., 2019).

43

Recycling methods of packaging plastics involve energy recycling, chemical

44

recycling, and physical recycling. The direct combustion of waste plastics requires 3

45

complex equipment to achieve low discharge of harmful volatile components

46

(Glushkov et al., 2019). In addition, relatively simple and predictable feedstock is

47

desired in chemical recycling for pure gas and oil in order to avoid unnecessary cost

48

of separation and purification (Canopoli et al., 2018). Physical recycling depends on

49

similar physical and chemical properties between recycled materials and virgin

50

plastics (Hahladakis and Iacovidou, 2018). Therefore, separation of different plastics

51

into pure component becomes the bottleneck of plastic recycling. PP and PE, as light

52

constituents in packaging plastics whose density are less than water, can be separated

53

from PVC (1.10–1.45 g cm –3) and PET (1.38–1.40 g cm –3) by density separation

54

(Bakker et al., 2009). However, the separation of PVC and PET is difficult due to

55

their similar density. Therefore, an efficient separation method is required to separate

56

PVC from PET for plastic recycling.

57

Numerous processes were developed to separate different plastics, including spectrum

58

technology (Costa et al., 2017; Kassouf et al., 2014), hardness separation

59

(Krolikowski and Piszczek, 2017), density separation (Fu et al., 2017), and flotation

60

separation (Thanh Truc and Lee, 2017). Wherein, flotation separation is attractive for

61

its high selectiveness and low cost. Considering the natural hydrophobicity of plastics,

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surface modification is usually required before flotation to improve wettability of

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target plastics, such as Nano-Fe/Ca/CaO catalytic ozonation (Mallampati et al., 2017),

64

Fenton pretreatment (Wang and Wang, 2017), and depressant pretreatment (Güney et

65

al., 2015). For flotation of PVC and PET, Wang et al. reported that surface

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modification by potassium permanganate made PVC hydrophilic while PET remained 4

67

hydrophobicity (Wang et al., 2015; Wang et al., 2016). Weak ester link in the PET

68

backbone could be decomposed to carboxyl group in 10 wt% sodium hydroxide

69

solution under high temperature (Wang et al., 2015). Desired separation of PET and

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PVC were also obtained through ozonation treatment (Reddy et al., 2007), depressants

71

coating (Abbasi et al., 2010), and flame treatment (Pascoe and O Connell, 2003).

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However, current pretreatment methods are limited to defective surface modification,

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damaging the original surface of plastics and producing secondary contamination. It is

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essential to develop a novel modification process assisting flotation separation of

75

PVC and PET to eliminate the surface erosion and environmental pollution during

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pretreatment. Interestingly, calcium carbonate (CaCO3) is applied as filler in plastics

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improves the characteristics of scratch resistance (Charde et al., 2018), flame

78

resistance (Polli et al., 2006), and hardness (Croitoru et al., 2018) of polymers,

79

stimulating an idea of CaCO3 coating assisting plastic flotation. Besides, rational

80

experimental design, as practical scheme to depict the interactions between

81

parameters, is usually ignored in flotation separation of plastics. Therefore, response

82

surface methodology (RSM) is desired to optimize experimental parameters, explain

83

interactions of parameters, and reveal the effect of experimental parameters on

84

response value (Wang et al., 2018).

85

This work researched the flotation separation of PVC and PET assisted with coating

86

by CaCO3. Response surface methodology (RSM) combined with Box–Behnken

87

design (BBD) facilitated optimization of flotation separation of PVC and PET. The

88

mechanism of CaCO3 coating was revealed by scanning electron microscope (SEM), 5

89

X–ray photoelectron spectroscopy (XPS), X–ray diffraction (XRD) Fourier transform

90

infrared spectroscopy (FT–IR), and zeta potential. The flotation phenomena were

91

explained by solution chemistry of flotation. This research indicated that CaCO3

92

coating could depress PVC selectively and accomplish flotation separation of PVC

93

and PET without surface damage and secondary contamination.

94

2. Materials and methods

95

2.1 Preparation of plastic samples

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Waste PVC and PET were collected from Tongli Recycling Ltd., Miluo, Hunan, China.

97

Firstly, waste PVC and PET were crushed by cutting miller (SCP-180-2, Cixi Yinbo

98

Plastics Machinery Co., Ltd., Zhejiang, China) into 2–4 mm that was verified as a

99

proper size for plastic flotation (Wang et al., 2016; Wang et al., 2017). Then, all

100

plastic particles for surface modification were rinsed for 24 h in stirring machine

101

(JJ-4A-B Six electric agitators, Changzhou, China) and dried at room temperature

102

(25±5 °C). Plastic samples for flotation were mixtures of PVC and PET with mass

103

ratio of 1:1. Partial plastic particles were further ground into plastic powder under 300

104

meshes by an herbal medicine pulverizer (Taisite instrument Ltd., Tianjin, China) for

105

characterization. Waste PVC was dark and PET was light, which was convenient to

106

sort them manually after flotation separation. Chemical structure formula of PVC and

107

PET were listed in Table S1: C–Cl bonds and benzene ring existed on PVC and PET

108

surface, respectively.

109

2.2 Surface coating and flotation separation

110

Single parameter experiments (SPE) were conducted to select the significant factors 6

111

in surface coating on plastic flotation, narrow parameters for responsive surface

112

analysis (RSM), and determine the response value. 10 g obtained waste plastic

113

particles were added in CaCO3 solution to be treated several minutes under specific

114

temperature. Wherein, CaCO3 was used as agent for surface coating. The pH value of

115

modification solution was adjusted by 0.1–1 M HCl and NaOH, and terpineol was

116

used as frother in all flotation experiments. CaCO3, HCl, NaOH, and terpineol were of

117

analytical purity and purchased from sinopharm chemical reagent Ltd. in Shanghai,

118

China. In surface coating procedure, we researched the effect of CaCO3 dosage (0.02–

119

0.18 g), temperature (20–80°C), treatment time (5–30 min), and pH (6–13.5) on

120

plastic flotation. Table S2 in supplementary information listed the parameters

121

researched in SPE. After surface coating, modified plastic samples were added to

122

transparent flotation column (height×inner diameter, 580×60 mm) with tap water

123

and frother for plastic flotation. The flotation column with a sand pore in the bottom

124

connected air pump that provided stable airflow (SaierS-88B Saier Libaco, LTD.,

125

Beijing, China). Amounts of bubbles formed with the help of frother to finish the

126

plastic flotation when air flows through sand pore. Flotation separation was conducted

127

under conditions of flotation time 4 min, frother concentration 24.0 mg L-1 and

128

airflow rate 7.2 mL min−1. Flotation percentage (%), purity (%) and recovery (%) of

129

PVC were calculated by Eqs. 1–3. The same equations can be used to calculate the

130

flotation percentage, purity and recovery of PET (as floating product).

131

Per

=

× 100%

(1)

132

Pur

=

× 100%

(2) 7

133

Rec

=

× 100%

134

2.3 Characterization

135

X–ray diffraction (XRD) patterns of plastics/CaCO3 composites was conducted by X–

136

ray diffractometer (Siemens Electronic Instruments D500, Germany) at ambient

137

temperature with Cu Kα radiation (30 kV). The scanning rate was 5° in the range of

138

5–80°. The chemical bonds on plastic surface were monitored by X–ray photoelectron

139

spectroscopy (XPS) with Al X–ray source at 15 kV (K-Alpha 1063, Thermo Fisher

140

Scientific, USA). The functional groups on plastic surface was characterized by

141

Fourier transform infrared spectrometer with 3% mass ratio of plastic powders in KBr

142

films (Nicolet 6700 FT-IR, USA). Surface morphology of waste plastics was observed

143

with the help of scanning electron microscope at an acceleration voltage of 15 kV

144

(JSM-6360LV, Japan). Zeta potential of waste plastics was conducted by zeta

145

potential analyzer (ZetaPALS, Bruker, America). In each Zeta potential measurement,

146

plastic powders must be dispersed well with the aid of significant shaking. The

147

concentration of CaCl2 solution used in zeta potential measurement was 0.13g L−1 that

148

is determined by the concentration of Ca2+ in saturated CaCO3 solution. The plastic

149

samples for SEM measurements were 2–4 mm plastic particles, while that for XRD,

150

FT–IR, XPS, and Zeta potential measurements were plastic powder under 300 meshes.

151

All samples for characterization were treated under conditions of CaCO3 0.11 g,

152

temperature 50.6 °C, treatment time 20 min and pH 10.1. Solution chemistry of

153

CaCO3 was calculated based on the dissolution equilibrium of Ca2+ in virtue of visual

154

MINTEQ 3.1.

(3)

8

155

2.4 Design of response surface analysis

156

Optimization of flotation experiments was required because flotation was a process

157

influenced by multi-factors. The performance of flotation separation of PVC and PET

158

could be determined by floating percentage of PET and PVC. Because the flotation

159

percentage of PET was generally stable, flotation percentage of PVC was chosen as

160

response value (Y) to ascertain the interaction among significant parameters, optimize

161

process of surface coating, and generate a predicted model for optimal separation

162

(Mat Rosid et al., 2018). Box–Behnken design (BBD) was used to conduct response

163

surface analysis with the help of Design Expert® software, Version 10 (Stat-Ease,

164

Minneapolis, 176 MN) on account of its efficient and economical nature. Four

165

variables (dosage of CaCO3, temperature, treatment time and pH) with three levels

166

(low, middle and high) were utilized in optimization on the basis of SPE analysis

167

(Table S3). The value of experimental runs was 29, including 24 factorial points and 5

168

repetitive central points. Table 1 clarifies design matrix and experimental conditions

169

about response surface method. As shown in Eq. 4, flotation percentage of PVC, as a

170

response value (Y), was predicted by quadratic model.

171

Y=

172

Where

173

interactive term,

174

experiment. The positive or negative coefficient of every term means synergistic or

175

antagonistic effect on the response value.

176

2.5 Confirmation experiments

+∑

+∑

( ) +∑

is constant term, and

!

"

∑!

!

is linear term,

(4)

!

is quadratic term,

!

is

are variables. $ is the number of parameters in

9

177

The quadratic model was established to predict the optimal conditions where

178

minimum response value can be obtained (minimum flotation percentage of PVC).

179

Then, confirmation experiments were conducted under pretreatment conditions of

180

0.11 g CaCO3, temperature 50.6 °C, treatment time 20 min and pH 10.1. The flotation

181

experiments were conducted under conditions of flotation time 1.5 min, frother

182

concentration 24 mg L−1 and airflow rate 7.2 mL min−1. Above experiment procedures

183

of SPE and RSM were summarized in Fig. S1.

184

3. Results and discussion

185

3.1 Single parameter experiments

186

As shown in Fig. 1, the flotation percentage of PVC decreases along with increasing

187

CaCO3 dosage, temperature, and treatment time, then plateaus at about 0% when

188

CaCO3 dosage was 0.1 g, temperature 50 °C, and treatment time 15 min. The

189

wettability of PVC is changed significantly after CaCO3 treatment, possibly resulting

190

from CaCO3 coating on surface. However, the flotation percentage of PET always

191

remains 100% during flotation separation. It is obvious that the hydrophobicity of

192

PET is not affected by surface coating, and bubbles can adhere to PET surface easily.

193

Therefore, PET remains its natural floatability after CaCO3 coating. Intriguingly, the

194

pH value of 8–11 is appropriate for sunken PVC and floating PET. Considering the

195

varying molar ratio of ions in overstaturated CaCO3 solution at different pH values,

196

the floatability of PVC and PET may be explained by existing status of CaCO3 in

197

solution. As shown in Table S4, the high purity and recovery of PVC and PET

198

manifest that the surface modification method based on CaCO3 coating favors an 10

199

excellent flotation separation of PVC and PET.

200

According to SPE results, CaCO3 dosage, temperature, treatment time, and pH value

201

are significant parameters for flotation percentage of PVC. The optimal conditions for

202

flotation of PVC and PET can de narrowed to CaCO3 dosage of 0.08–0.12 g,

203

treatment time of 10–20 min, treatment temperature of 40–60 °C, and pH value 8–12.

204

3.2 SEM analysis

205

As shown in Fig. 2, inherent scratches and pits exist on surface of uncoated PVC and

206

PET, which might be caused by original using (Fig. 2a and 2c). Compared with virgin

207

surface of waste plastics, surface roughness does not change a lot after CaCO3 coating,

208

which can be certified by the stable morphology of plastic substrate before and after

209

pretreatment. We can speculate that CaCO3 coating has no destruction on plastic

210

surfaces. After pretreatment by CaCO3, PVC and PET surfaces support some

211

prismatic particles, that are highly similar to the crystal structure of CaCO3 (Fig. 2b

212

and 2d). Intriguingly, more particles exist on the PVC surface than that on the PET

213

surface. It seems that the above particles are inclined to adhere on the surface of PVC

214

rather than the PET surface, possibly caused by specific chemical bonds on PVC

215

surfaces. According to literature, CaCO3 particles on PVC surface can not only offer

216

hydrophilic points to increase the surface energy of PVC (Wang et al., 2013) but also

217

isolate bubbles from original surface of plastics (Thanh Truc and Lee, 2016).

218

Therefore, it is reasonable to speculate that prismatic particles are CaCO3, and CaCO3

219

coating leads to the hydrophilicity of PVC. However, the amount of CaCO3 particles

220

on PET surface is too little to change the wettability of waste PET, possibly ascribing 11

221

to the weak adsorption of CaCO3 on the PET surface.

222

3.3 XRD analysis

223

XRD is a useful measurement to clarify that the coating particles on plastic surfaces

224

are CaCO3. As shown in Fig. 3, XRD patterns are applied to analyze the waste plastics

225

after CaCO3 coating in the angle range of 5–80º. The broad peaks in the range of 2θ

226

=10–40º are attributed to the amorphous PVC and PET. CaCO3 significantly appears

227

on PVC surface, verified by the characteristic peaks at 2θ = 29.405º, 47.489º, 39.401º,

228

35.965º (JCPSD file No. 05-0586). On the contrary, the XRD pattern of PET remains

229

steady after CaCO3 coating, manifesting limited amount of CaCO3 on PET surface.

230

According to the XRD analysis, the prismatic particles on PVC surface are testified as

231

CaCO3.

232

3.4 XPS analysis

233

XPS was conducted in order to clarify the surface reaction on waste plastics. The XPS

234

spectra of PVC are shown in Fig. 4. The C 1s spectrum of PVC before CaCO3 coating

235

is composed of aliphatic C–C/C–H (284.5 eV), C–Cl (286.2 eV) and C=O (288.6 eV)

236

with relative ratio of 76.18%, 22.40% and 1.42% respectively, which is consistent

237

with the works reported by Wang (Wang et al., 2019). Carbonyl on PVC surface

238

might come from plasticizers. The C 1s spectrum of PVC after CaCO3 coating is

239

fitted with C=O (288.6 eV), C–Cl (286.2 eV), aliphatic C–C/C–H (284.5 eV) and C–

240

O in carbonate (288.9 eV) with relative ratio of 75.4%, 22.13%, 1.14% and 1.32%,

241

respectively (Baer and Moulder, 1993; Wang et al., 2007). The existence of C–O in

242

carbonate suggests the adherence of CaCO3 on PVC surface. But relative ratio of 12

243

several chemical bonds on PVC surface changes less, revealing that no side reactions

244

occur along with CaCO3 coating on the surface of PVC.

245

3.5 Zeta potential analysis

246

Zeta potential analysis is an efficient method to reveal the interaction between various

247

components in solution. As shown in Fig. 5, Zeta potentials of raw PVC and PET are

248

very similar in the pH range of 2–11. Zeta potentials of PVC, PET, and CaCO3 are

249

negative at pH is 7–11, illustrating low possibility of adherence between CaCO3 and

250

artificial polymers. When waste plastic samples are immersed in Ca2+ solution,

251

surface potentials of PVC and PET soar to some extent. It is obvious that higher

252

surface potential appears on PVC compared with PET, which may be the result of

253

selective adsorption of Ca2+ on PVC surface. Further, Ca2+ may bridge PVC surfaces

254

and CaCO3 particles, causing the hydrophilic surface of PVC.

255

3.6 Solution chemistry analysis

256

As shown in Fig. 6, numerous Ca2+, CaCl+, CaOH+, CaHCO3+, and Ca(OH)2 exist in

257

solution with pH less than 7 or higher than 12. Ca2+ and CaCO3 are extremely

258

abundant in solution when pH in the range of 8–11. According to SPE results, PVC

259

and PET are floating products when pH is less than 7 or higher than 12, while waste

260

PVC can sink well after CaCO3 coating with pH range of 8–11. It is expectable that

261

the sinking of PVC is resulted from the synergistic effect of Ca2+ and CaCO3.

262

Combined with Zeta potential analysis in Fig. 5, solution chemistry analysis reveals

263

that Ca2+ works as a bridge between polymer and CaCO3 that have same surface

264

potential, promoting the adsorption of CaCO3 on plastic surfaces. According to 13

265

pertinent literatures, due to the strong polarity of C–Cl on PVC surface, PVC can

266

attract the Ca2+ in calcium carbonate solution (Brennecke et al., 2016). However,

267

interaction between benzene and cations (π–cation interaction) might be too weak for

268

PET to adsorb much CaCO3 and change wettability (Mahadevi and Sastry, 2013).

269

4. Optimization of pretreatment

270

4.1 Predicted model

271

The flotation percentage of PVC is strongly dependent on the pretreatment conditions

272

of CaCO3 dosage, temperature, treatment time, and pH value. A modified quadratic

273

model is suggested to be suitable for predicting response value (Wang et al., 2019).

274

According to the analysis of experiments designed by Design Expert 10, a quadratic

275

model with transformation of square root for coded factors is generated as Eq. 5. The

276

equation in terms of coded factors can be used to make predictions about the response

277

for given levels of factors that include linear terms (A, B, C, and D), quadratic terms

278

(A2, B2, C2, and D2), interaction terms (AC, AD, and AD), and response value (Y).

279

According to the coefficients in model, linear terms of A (dosage of CaCO3), B

280

(temperature), and C (treatment time), and interaction terms of AC and AD have

281

negative effect on flotation percentage of PVC. However, linear term of D (pH),

282

interaction term of CD, and all quadratic terms (A2, B2, C2, and D2) are positive for

283

the response value. Then, the quality of obtained model is verified by analysis of

284

variance (ANOVA).

285

Sqrt (Y) = −0.23A−3.70B−0.74C+1.20D−3.29AC−1.85AD+1.08CD+2.48A2+1.60B2

286

+2.09C2 +3.81D2

(5) 14

287

The result of ANOVA for predicted model is showed in Table 2. The reliance and

288

statistical significance of model for prediction of optimal pretreatment conditions are

289

analyzed by F-value, P-value, and determination coefficient (R2). Similarly, the

290

significance of the terms in model is demonstrated by P-value. Besides, predicted R2

291

(%

292

the accuracy of prediction by regression model. The high F-value of 18.84 along with

293

the P-value less than 0.0000001 suggests only 0.00001% probability of generating

294

such a large F-value due to noise, implying the model is extremely significant to

295

response value. The model variables seem to be significant when P-value is less than

296

0.05 (Sabiha et al., 2017). It is obvious that model terms of B, D, AC, AD, A2, B2, C2,

297

and D2 are significant, while the terms of A, C, and CD are not significant. Besides,

298

the high determination coefficient (R2=92.42) manifests a fairly good fit between

299

predictable values and experimental values. Considering difference is less than 0.2,

300

the %

301

consistency between predictable values and adjustable values. The adequate precision

302

of 12.78 is higher than 4, demonstrating that the range of predicted response relative

303

to associated error is acceptable, and the predicted model can navigate the design

304

space. Based the above analysis, this response surface model is constructive to predict

305

the response value (flotation percentage of PVC).

306

4.2 Interaction analysis

307

The interaction among CaCO3 dosage, treatment time, and pH are depicted by the

308

shape of contour plots, three-dimensional response surface plots, and the P-value of

&'( ),

adjusted R2 (%)(! ), and the value of adequate precision are applied to evaluate

&'(

of 0.7412 is substantially consistent with %)(! of 0.8751, implying a high

15

309

interaction terms. Based on the research of (Wang et al., 2016) and (Wang et al.,

310

2019), a regular shape of response surface and contour plots manifests an insignificant

311

interaction effect, while elliptical contour plots suggest significant interaction between

312

parameters. According to the elliptical contour plots and irregular response surface

313

plots in Fig. S2 and Fig. 7, the interaction effects between parameters of AD (CaCO3

314

dosage and pH) and AC (CaCO3 dosage and treatment time) are significant, which is

315

further confirmed by low P-values of AD and AC terms in Table 2 (0.01 and 0.0001,

316

respectively). However, there is no obvious interaction effect observed between

317

parameters of CD (treatment time and pH value) with high P-value of 0.13.

318

As shown in Fig. 7, the optimal modification conditions for flotation of PVC and PET

319

locate inside the given ranges of critical parameters, verified by evident trough in

320

response surface plots. When the pH is constant of 10, flotation percentage of PVC

321

decreases with increasing CaCO3 dosage and treatment time. However, flotation

322

percentage of PVC increases with increasing pH at a constant treatment time of 15

323

min. The minimum flotation percentage of PVC occurs near the range of central

324

values (CaCO3 dosage of 0.1−0.12 g, treatment time of 15−20 min, temperature of

325

50−60°C, and pH of 10−12).

326

4.3 Confirmation experiments

327

In this process, a low flotation percentage of PVC is desired for separation of PVC

328

and PET. Based on the function of numerical optimization in Design Expert, the

329

minimum flotation percentage of PVC is confirmed via overall desirability function.

330

During numerical optimization, the parameters of CaCO3 dosage, temperature, 16

331

treatment time, and pH are set as in their range, and the flotation percentage of PVC is

332

set to be minimum value. Finally, the predicted flotation percentage of PVC is 0.982%,

333

which can be obtained under optimal conditions of 0.11 g CaCO3, temperature

334

50.6 °C, treatment time 20 min, and pH 10.1. Confirmation experiments were

335

performed three times, obtaining the average experimental values of flotation

336

percentage, purity, and recovery of 1%, 100%, and 99%, respectively. The

337

experimental value is significantly consistent with predicted value, demonstrating that

338

the RSM along with BBD is an efficient method for optimizing flotation separation of

339

PVC and PET.

340

5. Protection of original surface

341

Challenges with flotation separation of waste plastics are secondary pollution and

342

preservation of original surface of waste plastics. Current surface modification

343

generally amends chemical composition and element state on the surface of waste

344

plastics. It is difficult to evaluate the impact on original surface of waste plastics from

345

surface modification. Strict environmental laws and regulations limit the application

346

of many chemical reagents in the separation of waste plastics. However, the green

347

separation of PET and PVC based on CaCO3 coating can avoid the above defects of

348

surface modification. FT−IR was applied to further monitor the chemical composition

349

and functional groups on plastic surface. As shown in Fig. 8, infrared spectra of waste

350

samples after CaCO3 coating are similar to that of raw samples, suggesting that this

351

technology can protect original surface of plastic well. The above conclusions are

352

consistent with results of SEM (Fig. 2), XRD (Fig. 3) and XPS (Fig. 4). Therefore, 17

353

green flotation based on CaCO3 coating is a novel technology for plastic separation

354

without any secondary pollution and damage on plastic surface. The comparison

355

among types of surface modification before flotation is listed in Table 3.

356

6. Conclusion

357

Plastic flotation was a promising separation method for efficient recycling of waste

358

polyvinyl chloride (PVC) and polyethylene terephthalate (PET). Due to the natural

359

hydrophobicity of plastic, surface modification is necessary for plastic flotation. In

360

this work, a novel surface modification based on CaCO3 coating was proposed to

361

assist flotation separation of PVC and PET. The conditions in pretreatment were

362

optimized by response surface methods combining Box–Behnken design. A reliable

363

fitting model was established to obtain the optimal pretreatment conditions, and the

364

significant interaction effects exist between parameters of CaCO3 dosage and pH,

365

CaCO3 dosage and treatment time. Optimal pretreatment conditions for flotation

366

separation of PVC and PET are CaCO3 dosage 0.11 g, temperature 50.6 °C, treatment

367

time 20 min, and pH 10.1. The purity and recovery of PVC can be 100% and 99%,

368

respectively. The mechanism of surface coating was revealed through SEM, XPS,

369

XRD, zeta potential, and solution chemistry analysis. High polarity of C–Cl on PVC

370

surface results in selective adherence of Ca2+ on PVC surface, and Ca2+ works as a

371

bridge between PVC surface and CaCO3 particles. Flotation separation of PVC and

372

PET assisted with surface coating by CaCO3 avoids defects of other modification

373

methods: destruction of original plastic surface and secondary pollution of

374

pretreatment. 18

375

Acknowledgements

376

This work is supported by the National Natural Science Foundation of China

377

(21878343 and 51804276) and the China Postdoctoral Science Foundation

378

(2018M630838).

379

Supplementary Information

19

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25

Figure captions: Fig. 1. Effect of dosage of CaCO3 (a), temperature (b), treatment time (c) and pH (d) on flotation percentage of waste plastics Fig. 2. SEM of PVC and PET before and after CaCO3 coating Fig. 3. XRD patterns of PVC and PET before and after CaCO3 coating Fig. 4. XPS spectra of PVC before and after CaCO3 coating Fig. 5. Zeta potential of PVC and PET Fig. 6. Solution chemistry of CaCO3 in separation of PVC and PET Fig. 7. Response surface of interaction among parameters Fig. 8. FT−IR of PVC and PET before and after CaCO3 coating

26

Table captions： Table 1. Design matrix and experimental conditions of response surface method Table 2. Analysis of variance for predicted model Table 3. Comparison of surface modification in flotation of PVC and PET

27

Table 1. Box-Behnken design of variables and response value Run A: Dosage of CaCO3 (g) 1 0.12 2 0.1 3 0.1 4 0.1 5 0.08 6 0.1 7 0.1 8 0.1 9 0.1 10 0.1 11 0.08 12 0.1 13 0.1 14 0.1 15 0.1 16 0.12 17 0.1 18 0.08 19 0.08 20 0.12 21 0.08 22 0.12 23 0.1 24 0.1 25 0.12 26 0.08 27 0.12 28 0.1 29 0.1

B: Temperature (ºC) 50 50 40 40 60 60 60 50 50 50 50 50 40 40 60 50 50 50 50 60 50 50 50 50 40 40 50 60 50

C: Time (min) 20 20 20 15 20 20 15 20 20 25 25 20 25 20 20 20 20 20 15 20 20 15 15 25 20 20 25 25 15

D: pH

8 10 8 10 10 12 10 10 10 12 10 10 10 12 8 12 10 8 10 10 12 10 12 8 10 10 10 10 8

Response value: Floating percentage of PVC (%) 40.5 0 57.5 28 0 17.5 0 0 0 47.5 52 0 57 81 0 21 0 11 4.5 0 80 65 77.5 5 90 93.5 0 0 72

Table 2. Analysis of variance for predictable model Source Model A-Dosage of CaCO3 B-Temperature C-Time D-pH AC AD CD A2 B2 C2 D2 Residual Lack of Fit Pure Error Cor. Total

Sum of squares 374.98 0.64 164.26 6.56 17.33 43.24 13.72 4.70 39.86 16.70 28.35 94.05 30.77 30.77 0.00 405.74

Degree of freedom 11.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 17.00 13.00 4.00 28.00

Mean square

F-value

34.09 0.64 164.26 6.56 17.33 43.24 13.72 4.70 39.86 16.70 28.35 94.05 1.81 2.37 0.00

18.84 0.35 90.77 3.63 9.58 23.90 7.58 2.60 22.02 9.23 15.67 51.97

P-value (Prob>F) <0.0000001 0.56 <0.0000001 0.07 0.01 <0.0001 0.01 0.13 <0.0001 0.01 <0.001 <0.00001

Table 3. Comparison of surface modification in flotation of PVC and PET Agent

Conditioning time (min) 10

Recovery of PVC (%) 96.7

Mechanism

Authors

Ozone

Temperature (ºC) Room temperature

Surface oxidation

Flame

140

0.14 s

>90%

Surface oxidation

KMnO4

66.5

38

98%

Surface oxidation

Tannic acid

Room temperature

30 s

99%

NaOH

70

20

>98%

CaCO3

50

15

100%

Adherence of depressant Hydrolysis of ester links Surface coating

(Reddy et al., 2007) (Pascoe and O Connell, 2003) (Wang et al., 2016) (Abbasi et al., 2010) (Wang et al., 2015) This research

Highlights: Green flotation of polyvinyl chloride and polyethylene terephthalate is achieved. CaCO3 coating avoids surface destruction and secondary pollution in pretreatment. High polarity of C-Cl bond triggers selective adherence of Ca2+. Ca2+ is a bridge between plastic surface and CaCO3 particles. Conditions are optimized by response surface methodology.