Effects of chemical pretreatments on microplastic extraction in sewage sludge and their physicochemical characteristics

Journal Pre-proof Effects of chemical pretreatments on microplastic extraction in sewage sludge and their physicochemical characteristics Xiaowei Li, Lubei Chen, Yanyan Ji, Man Li, Bin Dong, Guangren Qian, John Zhou, Xiaohu Dai PII:

S0043-1354(19)31153-4

DOI:

https://doi.org/10.1016/j.watres.2019.115379

Reference:

WR 115379

To appear in:

Water Research

Received Date: 29 August 2019 Revised Date:

3 December 2019

Accepted Date: 6 December 2019

Please cite this article as: Li, X., Chen, L., Ji, Y., Li, M., Dong, B., Qian, G., Zhou, J., Dai, X., Effects of chemical pretreatments on microplastic extraction in sewage sludge and their physicochemical characteristics, Water Research (2020), doi: https://doi.org/10.1016/j.watres.2019.115379. 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.

Sludge pretreatment

Improve MP extraction efficiency in sludge

EPS Cell

EPS

MP extraction efficiency changes with SCOD content

Microplastics

Heavy metals

Exert an influence on MP property

POPs

Pathogens

1

Effects of chemical pretreatments on microplastic

2

extraction in sewage sludge and their physicochemical

3

characteristics

4 5

Xiaowei Li 1, Lubei Chen 1, Yanyan Ji 1, Man Li 1, Bin Dong 2*, Guangren Qian 1, John Zhou 3

, Xiaohu Dai 2*

1

School of Environmental and Chemical Engineering, Organic Compound Pollution Control

6 7 8 9 10

Engineering, Ministry of Education, Institute for the Conservation of Cultural Heritage, Shanghai University, Shanghai 200444, People R. China
 2

11

State Key Laboratory of Pollution Control and Resources Reuse, National Engineering

12

Research Center for Urban Pollution Control, College of Environmental Science and

13

Engineering, Tongji University, Shanghai 200092, People R. China 3

14 15

School of Civil and Environmental Engineering, University of Technology Sydney, 15

Broadway, Sydney, NSW 2007, Australia

16 17

*Corresponding Author

18

Phone: 86-021-66137746; Email addresses: [email protected] (Dong B.);

19

[email protected] (Dai X.)

20

1

21

Abstract

22

Sewage sludge is a primary pathway for microplastics (MPs) entering into terrestrial

23

ecosystems. However, a standardized method to analyze MP in sludge is lacking due to its

24

high organic matter. This study investigated the extraction efficiency of six MPs in five solid

25

matrices, i.e. sewage sludge, cattle manure, soil, sediment and silicon dioxide. Results show

26

lower extraction efficiency of 87.2% for MPs in sludge compared with that in other matrices,

27

especially polyethylene terephthalate (PET) (only 27.8%). The possible reason was that the

28

presence of extracellular polymeric substances within the sludge hinders the MPs to float.

29

Therefore, five protocols, i.e. hydrogen peroxide (H2O2), Fenton, nitric acid (HNO3),

30

hydrochloric acid (HCl) and sodium hydroxide (NaOH) were used to pretreat the sludge and

31

optimize the MP extraction. The sludge pretreated by H2O2, Fenton and 1 M of acids had

32

higher MP extraction efficiency than the raw sludge due to higher extraction of the PET. The

33

MP extraction efficiency in the sludge first increased, and subsequently decreased with the

34

soluble chemical oxygen demand (SCOD) content, implying that moderate dissolution of

35

sludge organic matter is beneficial to the MP extraction. Quantitative analysis of the changes

36

in the MP physicochemical characteristics after the pretreatments indicated that polyamide

37

(PA) and PET are not resistant to acid and alkali treatment, respectively. Principal component

38

analysis shows that the effect of pretreatments on the MPs follows a decreasing sequence:

39

alkali> high concentration of acids >low concentration of acids > H2O2 and Fenton.

40

Additionally, the susceptibility of the MPs to the pretreatments follows a decreasing sequence:

41

PET, PA and polymethyl methacrylate (PMMA) > polystyrene (PS) > polyethylene (PE) and

42

polypropylene (PP). The findings supply novel insights into the effect of chemical

43

pretreatments on MP extraction in sewage sludge.

44

Keywords: Microplastics; Wastewater treatment plants; Sewage sludge; Microplastic

45

extraction; Chemical pretreatment; Physicochemical property 2

46 47

1. Introduction

48

Wastewater treatment plants (WWTPs) are a significant point source of microplastics

49

(MPs) in the environment in recent years (Mason et al. 2016, Murphy et al. 2016, Sun et al.

50

2019). However, only a small amount of MPs in wastewater is directly discharged into the

51

aquatic environment, and more than 98% of the MPs are trapped in sewage sludge resulting

52

in high concentrations of MPs in the sludge (Carr et al. 2016). Mahon et al. (2017) found that

53

the MP contents in the sludge from the WWTPs in the Netherlands reach 4196 – 15385

54

particles kg-1 dry sludge. Lassen (2015) reported a MPs concentration of 1000 – 24,000

55

particles kg-1 dry sludge in Germany. Li et al. (2018) found an average MP contents of 22.7 ±

56

12.1 × 103 particles kg-1 dry sludge in China. The total amount of MPs inputted into the soil

57

via land and agricultural use of sludge in Europe, North America and China can reach 63,000

58

– 430,000 tons, 44,000 – 300,000 tons and 1.56 × 1014 particles per year, respectively

59

(Nizzetto et al. 2016). Once MPs are introduced into the soil and accumulated to a certain

60

extent, they will affect soil properties, soil functions and biodiversity, and cause potential

61

damage to terrestrial ecosystems (He et al. 2018). Therefore, MP pollution in sludge and its

62

potential environmental risks need more attention. Until now, our knowledge about the MPs

63

in sewage sludge is limited, probably due to the lack of standardized methods for the MP

64

analysis, which are a prerequisite to effectively characterize the environmental behavior of

65

the MPs.

66

Currently, the extraction of MPs in sludge mainly refers to the method used in the

67

sediment system, i.e. the separation of light-target components from heavy-impurity

68

components using density separation (Mahon et al. 2017). Nevertheless, the extraction

69

efficiency of MPs from sludge is not often satisfactory at present, especially for fibrous MPs

70

(Lares et al. 2019). Sludge contains high concentrations of organic particles, bacterial

3

71

micelles and filamentous bacteria produced in the sewage treatment process, all of which are

72

encapsulated in a network structure composed of extracellular polymeric substances (EPS)

73

(Bala Subramanian et al. 2010). EPS are an important component of activated sludge flocs,

74

which are widely distributed in the inner and outer layers of flocs, and have an important

75

influence on sludge flocculation, sedimentation and dewatering performance (Bala

76

Subramanian et al. 2010). Thus, MPs may be also embedded in the sludge flocs by the EPS,

77

thus affecting the MPs’ floating velocity. In addition, flocculants are widely used in WWTPs

78

to improve dewatering and sedimentation performance of sludge (Dai et al. 2014). Hence, it

79

is hypothesized that the low extraction efficiency of MPs in sludge may be related to the EPS

80

and flocculants, and thus leading to the complication of extraction methods for sludge-based

81

MPs.

82

At present, sludge pretreatment processes are widely used to promote the breakdown of

83

EPS and microbial cell walls of sludge cells, to improve sludge energy conversion, and for

84

dewatering (Zhen et al. 2017). Therefore, the sludge pretreatment may be able to optimize

85

MP extraction by removing the negative impact of EPS. Chemical pretreatment methods are

86

widely applied because of their high processing speed and treatment efficiency (Carrere et al.

87

2010), which can be divided into two major categories: acid-alkali treatment and oxidation

88

treatment. The pH is adjusted through adding acid or alkali reagents to the excess sludge. A

89

lower pH can destroy the floc structure of the microorganisms, while a higher pH can

90

effectively dissolve and destroy the cell wall and cell membrane (Torres and Llorens 2008).

91

Meanwhile, to facilitate MP quantification and identification, hydrogen peroxide (H2O2),

92

Fenton, alkaline and acid pretreatments have been used to remove organic matter and purify

93

the MPs. (Hurley et al. 2018, Lares et al. 2019, Sun et al. 2019). Although Hurley et al. (2018)

94

suggested that the ordering of the analytical procedure (organic matter removal followed by

95

density separation, and vice versa) has no significant effect on the recovery of the different

4

96

MPs, the information about the effect of chemical pretreatments on the MP extraction in

97

sewage sludge is limited.

98

Additionally, MP physicochemical property may be affected during the process of

99

sludge pretreatment. Nuelle et al. (2014) noted that H2O2 has an effect on the polymer, and

100

causes a decrease in surface area of the polymer. Polyamide (PA) particles were destroyed

101

after treated by H2O2 at 70°C (Hurley et al. 2018). In contrast, Tagg et al. (2015)

102

demonstrated that Fourier Transform Infrared Spectroscopy (FTIR) spectra of the MPs

103

remain unchanged, and their surface properties are not affected after seven days of exposure

104

of the samples to H2O2 (30%). Most MPs have almost no change in size during the oxidation

105

process (Nuelle et al. 2014). The above results are based on the qualitative analysis using

106

scanning electron microscopy (SEM) and/or FTIR, which may not properly reflect the effect

107

of pretreatment on the MP surface properties due to their high subjectivity and low sensitivity.

108

Zhang et al. (2018) found enhanced adsorption for oxytetracycline in the beached foams

109

compared with that in the virgin foams which may not be visually reflected on the SEM

110

image. Therefore, quantitative indicators like adsorption potentials of MPs were used to

111

assess the impact of pretreatments in this study, in order to fully understand their effect.

112

The objectives of this study are to: 1) verify extraction efficiency of MPs in the sludge

113

and explore the reasons for limiting the MP extraction; 2) improve the MP extraction in

114

sludge through chemical pretreatments such as acid-alkali and oxidation treatments; 3) and

115

evaluate the effect of the pretreatments on the MP physicochemical properties using various

116

techniques such as FTIR, SEM, mass-size loss and adsorption potentials to Cd.

117

2. Materials and Methods

118

2.1 Materials and reagents

119

Five solid matrices, i.e., sewage sludge, cattle manure, soil, sediment and silicon dioxide

120

(SiO2), were used for MP extraction. Other four matrices were selected to favorably analyze

5

121

possible reasons for limiting MP extraction in the sludge. Sewage sludge was collected from

122

a WWTP in Shanghai, China; cattle manure samples were obtained from a cattle farm in

123

Zaozhuang, Shandong Province, China; soil samples were collected from a farm in Qingpu

124

District, Shanghai, China; sediment samples were gained from Hongguang River along

125

Shanghai University in Shanghai, China; and SiO2 samples were taken from Jingyou Sand

126

Co., Ltd. in Anhui Province, China. Physicochemical properties of the five samples including

127

total solids (TS) content, volatile solids (VS)/TS, pH, electrical conductivity, and EPS

128

content were analyzed. The detailed methods and results of the analyses are outlined in the

129

Supporting Information (SI) and Table S1 of the SI.

130

Polyethylene (PE), polypropylene (PP), polystyrene (PS), polyamide (PA), polymethyl

131

methacrylate (PMMA) and polyethylene terephthalate (PET) ranging from 270 – 550 µm

132

were obtained through cutting the corresponding plastic products and sieving. The sample

133

colors were distinct to facilitate their measurement; they are blue, yellow, white, red, black

134

and green, respectively. The density of MPs is provided in Table S2 of the SI. All metal

135

standard solutions and standard bovine serum albumin were purchased from Aladdin

136

Industrial Corp., Shanghai, China. Other reagents are analytically pure and were obtained

137

from Sinopharm Group Corp, Shanghai, China.

138

2.2 MP Extraction in five matrices

139

Thirty particles of each kind of plastic and a total of 180 polymer particles were added

140

to 30 g of each solid matrix for each replicate. The MP shapes were granular except for PET

141

which was fibrous. The MP particles were thoroughly mixed into the solid matrices for 24 h.

142

Then, the MPs were extracted using a two-step method (Hurley et al. 2018). In the first step,

143

30 g of a solid matrix was added to an Erlenmeyer flask with 300 mL deionized water in

144

which sodium chloride had been added to saturation (1.2 g cm-3) (Li et al. 2018). After

145

stirring for 15 min, the mixture was allowed to settle for 2 h. Then, the liquid supernatant was

6

146

filtered in a vacuum-filtration unit using a stainless-steel sieve with a pore size of 37 µm. In

147

the second step, zinc chloride solution (ZnCl2, 1.6 – 1.7 g cm-3) was used to extract higher

148

density MPs, and the same settling and vacuum procedures were conducted as described

149

above. After the extraction, the polymer particles were separately counted. The tests were

150

carried out in triplicate.

151

In all the steps of sample processing, the openings were wrapped with an aluminum foil

152

to avoid contamination with other polymer particles. Synthetic clothing was avoided, and the

153

work surface was cleaned with alcohol before use. When analyzing the filter paper, a blank

154

piece of filter paper was placed under open laboratory conditions to assess the possibility of

155

ambient air pollution.

156

2.3 Floating velocity of MPs in the presence of EPS and flocculant

157

Fibrous and granular PETs, as representative MPs, were used to test the effect of EPS

158

and flocculant on the MP floating velocity during the extraction process. An EPS solution

159

was prepared using bovine serum albumin and sodium alginate, in which the protein content

160

was 80 mg L-1 and the polysaccharide content was 100 mg L-1 (Priyananda and Chen 2006).

161

Polyacrylamide (PAM) was chosen as a representative flocculant (0.1 g L-1 PAM). The

162

floating velocities of the two PETs were measured in the following three solutions: (1) a

163

ZnCl2 solution, (2) a ZnCl2+EPS solution and (3) a ZnCl2+EPS+PAM solution. The content

164

of ZnCl2 in the solutions was same as described above. The tests were carried out in a settling

165

column with a diameter of 60 mm and a height of 800 mm. The column was filled with the

166

simulated solution to a height of 700 mm. The device diagram is shown in Figure S1 of the SI

167

(Kowalski et al. 2016). One polymer particle was used for each test. The time elapsed during

168

the particle floating from the bottom of the column to the top was recorded. Floating velocity

169

of the particle was gained through the height of the solution over the time. Each test was

170

conducted five to eight times.

7

171

2.4 Pretreatment experiment

172

Five main protocols, i.e. H2O2 (30%), Fenton, nitric acid (HNO3, 1 M and 5 M),

173

hydrochloric acid (HCl, 1 M and 5 M), sodium hydroxide (NaOH, 1 M, 5 M and 10 M), were

174

selected according to the review of current study (Hurley et al. 2018, Sun et al. 2019). The

175

selection of the protocols are detailed in the SI. They were marked as 1, 2, 3a, 3b, 4a, 4b, 5a,

176

5b and 5c, respectively.

177

To evaluate the effect of the pretreatments on the extraction efficiency of the MPs and

178

the dissolution of organic matter in sewage sludge, 30 particles of each MP were added to 30

179

g of sewage sludge (wet weight) and thoroughly mixed. Then, 30 mL of each pretreatment

180

reagent was added to the mixture and reacted at 60 °C for 24 h (Sujathan et al. 2017). The

181

pretreated samples of H2O2 and Fenton were placed in a cold bath in the early stage of the

182

reaction to control the temperature at 60 °C. After the reaction, the MP extraction in the

183

pretreated sludge was proceeded as described in Section 2.2. Meanwhile, the pretreated

184

sludge was centrifuged at 4000 rpm for 20 min, and the supernatant was used for soluble

185

chemical oxygen demand (SCOD) analysis. The control was conducted the same procedure

186

with the deionized water. In addition, to investigate the effect of the pretreatments on the MP

187

properties, 1 g of each MP was added to 50 mL of each reagent. After pretreatment at 60 °C

188

for 24 h, the reaction solution was filtered using vacuum filtration, and the plastic particles

189

were washed with deionized water for 15 minutes and then placed in a clean petri dish. The

190

particles were applied for the following analysis after air-dried at room temperature.

191

2.5 MP physicochemical analysis

192

SEM analysis was conducted using Hitachi SU-1500 SEM (Hitachi High Technologies

193

Corp., Japan), to detect any changes in the surface structures of the MPs before and after

194

pretreatment. FTIR spectra were obtained through a Nico 380 MX FTIR spectrometer using

195

an attenuated total reflectance module (Thermo Fisher Scientific Inc., USA). The carbonyl

8

196

index (CI) was used to represent the degree of weathering and surface oxidation of MPs,

197

which was defined as the ratio of the carbonyl absorption intensity around 1870 –1650 cm-1

198

to an internal constant band (Liu et al. 2019, Song et al. 2017). The details are provided in

199

Table S3 of the SI.

200

The MP mass was measured using the gravimetric method (Hurley et al. 2018), while

201

the size was determined using a Stemi 508 stereomicroscope (Carl Zeiss Jena, Germany) that

202

was equipped with a high-definition electronic microscope and scaleplate. The MP

203

adsorption potential to Cd iron was evaluated according to our previous study (Li et al. 2019).

204

The adsorption was carried out in a centrifuge tube containing 0.1 g of MP particles and 10

205

mL of 10 mg L-1 metal solution. The blank group was carried out in the same metal solution

206

using the same procedure, but without MP particles. The metal concentration in the filtrate

207

was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES),

208

and the metal content adsorbed onto the MPs was calculated through the difference in Cd

209

concentration of the filtrate between the test and blank groups. Each test was run in triplicate.

210

The MP Zeta potentials were estimated using a zeta potential analyzer (Zetasizer Nano ZS90,

211

Malvern Instruments Ltd., England).

212

2.6 Statistical analysis

213

All the data in this study were the mean values (± standard deviation, SD) of replicas.

214

Statistical analysis was accomplished using the SPSS 19.0 program. Significant differences

215

among the tests were analyzed through one-way analysis of variance (ANOVA) followed by

216

Duncan’s multiple range test. Nine kinds of pretreatment and six types of MPs were

217

categorized using a principal component analysis (PCA), based on the changes in the mass,

218

size, CI, adsorption to Cd, and the zeta potentials of the MPs after the pretreatments. The data

219

of these changes are estimated as equation (1).

9

− =

× 100%

(1)

220

in which, P is the change of the MP after the pretreatment, VA is the value of the pretreated

221

MP, and VC is the value of the corresponding virgin MP.

222

3. Results and Discussion

223

3.1 Extraction efficiency of MPs in sludge and the effect of EPS

224

As shown in Figure 1A, extraction efficiency of the MPs in the sludge (87.2%) was

225

significantly lower than that in other four matrices, implying that sludge has a low MP

226

extraction efficiency, corresponding to the previous study (Hurley et al. 2018). Hurley et al.

227

(2018) reported that the average extraction efficiency of PE particles and PET fibers in the

228

sludge (85.57%) is lower than that in soil (about 90%). The MP extraction efficiency in the

229

sediment can reach more than 95% (Yu et al. 2016, Zhao et al. 2018). The physicochemical

230

characteristics of the five matrices were further investigated to identify possible mechanism

231

responsible for the low MP extraction in the sludge. The results were shown in Table S1 of

232

the SI. The sludge had the highest EPS contents in all of the matrices, implying that the EPS

233

may be an important factor limiting the MP extraction in the sludge. Manara and Zabaniotou

234

(2012) reported that sludge is a viscous substance cross-linked by EPS and has a high affinity

235

for most polymer surfaces. Jorand et al. (1998) reported that the protein in EPS contributes to

236

the sludge’s hydrophobicity, which is mainly made up of hydrophobic amino acids, thus

237

causing the entrapment of hydrophobic MPs in the sludge floc. Polysaccharides in EPS with a

238

high molecular weight (>100 KDa) have a long carbon skeleton structure with active side

239

chains, leading to high flocculation of the EPS (Yin et al. 2015). They trap the MPs in the

240

sludge floc, causing the floating MP particles to sink (Cole et al. 2016, Long et al. 2015), and

241

thus hampering the MP extraction in sludge.

242

The extraction efficiencies of the six MPs in the five matrices are shown in Figure 1B.

243

PET had the lowest extraction efficiency of all the MPs. The shape of the PET samples used 10

244

in this study was fibrous (different from with the other plastics, which had a granular shape),

245

indicating that the MP shape has an effect on the MP extraction efficiency. Lares et al. (2019)

246

suggested that PET fibers are more flexible than other fibers, enabling them to pass through

247

sieves more easily. The dimensions and shapes of MP fibers are not symmetrical, and thus

248

they are difficult to collect from environment samples by sieving. Compared with the other

249

matrices, sludge had the lowest extraction efficiency of PETs (only 27.8%) that significantly

250

contributed to low total extraction efficiency of the MPs in the sludge. The possible reason

251

was that the microfibers are more easily entangled with sludge organic matter to form

252

compact and rigid structures, compared with the MP particles (Bayo et al. 2016).

253

Schmiedgruber et al. (2019) found that the vast majory of MP fibers (approximatly 99%) are

254

found to associate with the sludge floc.

255

A simulation experiment was conducted to complement and confirm the effect of EPS

256

on the extraction efficiency of granular and fibrous MPs in the sludge. Compared with the

257

ZnCl2 solution, the floating velocity of granular and fibrous PET in the ZnCl2+EPS solution

258

decreased from 1.34 cm s-1 to 1.28 cm s-1 and 0.75 cm s-1 to 0.57 cm s-1, respectively (Figure

259

2). ANOVA shows that the difference is insignificant for granular PET (P>0.05), but

260

significant for fibrous PET (P<0.05), indicating that the EPS have a significantly negative

261

impact on the fibrous-MP extraction. Compared with the ZnCl2+EPS solution, the

262

ZnCl2+EPS+PAM solution has a lower floating velocity for the two PETs, but the differences

263

are insignificant (P>0.05), implying that the PAM has a negligible effect on the MP floating

264

velocity in the presence of EPS. In addition, the granular PET has a significantly higher

265

floating velocity than the fibrous PET in all the three solution. The results indicate that the

266

granular MPs are more easily extracted compared with the fibrous MPs, and the EPS have a

267

significantly negative impact on the extraction of fibrous MPs.

11

268

3.2 Effect of pretreatment on MP extraction and organic matter dissolution

269

As shown in Figure 3A, total extraction efficiencies of the MPs in the sludge pretreated

270

by H2O2, Fenton, 1 M HNO3 and 1 M HCl were higher than those in raw sludge, but the

271

extraction efficiencies by 5 M HNO3, 5 M HCl, 1 M NaOH and 5 M NaOH were lower. The

272

results indicate that pretreatments of oxidation and low-concentration acids can improve the

273

MP extraction in the sludge by 2.9% – 7.6%, while other pretreatments cannot, compared

274

with raw sludge. Further analysis found that the pretreatments caused little changes in the

275

extraction efficiencies of PE, PP, PS and PMMA except for 5 M NaOH (Figure 3B).

276

Compared with raw sludge, the extraction efficiency of PA was reduced in the sludge

277

pretreated by 5 M HNO3 and HCl, because nylon is not resistant to strong acids (Avio et al.

278

2015, Dehaut et al. 2016). The extraction efficiency of PET fibers increased by 40% – 144%

279

after pretreatments using H2O2, Fenton, 1 M of HNO3 and HCl, but decreased using 5 M of

280

HNO3 and HCl, and all alkali, corresponding to the results from the total MP extraction

281

efficiency (Figure 3A). The results indicate that the pretreatments have an important

282

influence on the extraction of PET fibers in sludge, and thus on the total MP extraction

283

efficiency.

284

The effect of the pretreatments on the release of the organic matter in the sludge was

285

further investigated, since the above results show that the EPS and sludge floc have

286

significantly negative impacts on the MP extraction. In this study, SCOD were used to reflect

287

disintegration degree of sludge organic matter (Xiao et al. 2015). Compared with raw sludge

288

(RS), all of the pretreated sludge had higher SCOD contents (Figure 4A), implying that the

289

pretreatments promote the dissolution of sludge organic matter. The extraction efficiencies of

290

the total MPs and PET fibers initially increased, and then decreased as the SCOD content

291

(Figure 4B and 4C). Similar phenomena are also found between sludge dissolution degree

292

and its dewaterability (Yu et al. 2009, Yuan et al. 2011). At a low SCOD content by H2O2

12

293

and Fenton, the floc structure may be disrupted because the EPS are released into the aqueous

294

phase (Zhang et al. 2012), but the microbial cells were not destroyed due to the relatively

295

minimal change (Yu et al. 2009, Yuan et al. 2011). This caused the release of the MPs from

296

the sludge floc, and improved the MP extraction. However, under a higher-concentration

297

acid/alkali, the microbial cell walls are prone to swelling and rupture, and then intracellular

298

protein and carbohydrates are released to the aqueous phase (Liu et al. 2009), causing high

299

SCOD content. Excessive chemical action may cause the released substances to form

300

functional groups such as hydroxyl and carbonyl groups, leading to reflocculation of the

301

cracked sludge (Zhang et al. 2007) and the MPs. Additionally, small molecules from the

302

dissolution of the sludge organic matter may adhere to the MP surface (Kowalski et al. 2016),

303

hindering their natural floating. Therefore, proper disintegration of the sludge organic matter

304

is vital for improving the MP extraction, especially for PET fibers. The findings need to be

305

further verified using the same chemical reagents under different concentrations as different

306

chemical pretreatments may have different mechanisms of organic matter disruption and

307

other influences on the MP extraction. For example, in the alkaline pretreatment, ZnCl2 used

308

in the two-step extraction will forms a precipitate due to the reaction of Zn2+ and hydroxyl

309

ions, which may be not beneficial for MP flotation.

310

3.3

Effect of pretreatment on MP properties

311

SEM analysis showed that the MP surface morphologies are slightly rough and broken

312

after alkaline pretreatments (Figures S2-S7 of the SI), especially for PETs, implying that the

313

alkaline pretreatments have significant impacts on the MP surface properties. Compared with

314

other pretreatments, the H2O2 and Fenton pretreatments caused a hardly change in the SEM

315

graphs of the MPs, indicating that the oxidative treatments have little effect on the surface

316

morphology of the MPs (Avio et al. 2015, Sujathan et al. 2017). Meanwhile, the FTIR spectra

317

of the pretreated MPs showed no significant difference in the surface functional groups from

13

318

the virgin samples (Figures S8-S10 of the SI), in addition to PA groups by 5 M of acid

319

solution without available spectra. Although SEM and FTIR are widely used to evaluate

320

effect of the pretreatments on the MP properties, using them alone may not allow us to fully

321

understand the pretreatments’ influence due to their limited qualitative information.

322

Therefore, quantitative indicators such as the mass, size, CI, adsorption potential of Cd and

323

zeta potentials were used to systematically evaluate the MP changes after the pretreatments in

324

this study.

325

Table 1 shows the changes in mass and size of the six MPs after the chemical

326

pretreatments. It is suggested that the mass or size loss rates of more than 5% are valid

327

changes and those of less than 5% are caused by the experimental errors. Eight data points

328

with valid changes were found in the MP mass analysis, while 20 data points in the size

329

analysis. The results indicate that changes in the MP size are more sensitive to the effect of

330

the pretreatments, compared with the mass. The mass and size of PA were reduced by 100%

331

after the pretreatment by 5 M of HNO3 and HCl. Similar results were also found in other

332

studies (Avio et al. 2015, Dehaut et al. 2016), as the high-concentration acid solution caused

333

the hydrolysis of peptide bonds in PA molecules. The mass and size of PET decreased by

334

30.2% – 53.5% and 9.4% –16.7% after the alkali pretreatments, respectively. The possible

335

reason was that the saponification reaction occurs in ester linkages of PET with the alkali

336

during alkaline pretreatment, thus causing a significant decrease in the PET mass (Hurley et

337

al. 2018).

338

CI is often used to quantitatively describe surface alteration properties of the MPs during

339

aging process (Hüffer et al. 2018, Liu et al. 2019, Müller et al. 2018). As shown in Table S4

340

of the SI, virgin PET had higher CI values than virgin PMMA and PA, followed by virgin PS,

341

PP and PE, showing that the CI values are significantly dependent on the MP types. In

342

general, the CI values of the pretreated MPs increased with varying degree, compared with

14

343

the virgin MPs, implying that the chemical pretreatments cause surface aging of the MPs, in

344

accordance with the previous results from UV aging (Hüffer et al. 2018, Liu et al. 2019,

345

Müller et al. 2018). Compared with other pretreatments, the alkaline pretreatments led to

346

higher aging for PP, PS, PA and PET, and the oxidation pretreatments (H2O2 and Fenton)

347

caused less aging for all the MPs, corresponding to the results of the mass and size. Liu et al.

348

(2019) also found that the alteration degree of Fenton for PE and PS is lower than that of

349

heat-activated K2S2O8 system according to the CI values. The changing CI degree of the six

350

pretreated MPs followed a decreasing order: PET > PMMA and PA > PS > PP > PE,

351

implying that the chemical composition and inherent structures of polymers affect their

352

alteration

353

higher-molecular-weight monomers seem more liable to aging under the chemical

354

pretreatment due to easier penetration of oxygen (Lv et al. 2017). Researchers also reported

355

that the CI values (surface weathering) of PE, PP and PS have the different changes as the

356

UV-exposure duration due to their different chemical structures (Liu et al. 2019, Song et al.

357

2017).

properties.

The

polymers

with

higher

oxygen-containing

groups

and

358

In this study, Cd was used as representative metal to evaluate the effect of chemical

359

pretreatment on MP adsorption potentials (Li et al. 2019), and the results are shown in Figure

360

5. Compared with virgin MPs, the Cd adsorption on the six corresponding MPs pretreated by

361

the alkaline solution significantly increased. Particularly, the Cd adsorption of PET pretreated

362

by 10 M NaOH reached nearly 148 times that of the virgin one. The results imply that the

363

alkaline pretreatment has a significant impact on the vector effect of MPs to pollutants. The

364

possible reason was that the alkaline pretreatment brings more hydroxyl functional groups on

365

the MP surface, thus promoting the adsorption of MPs to cations, to achieve balanced surface

366

charges (Turner and Holmes 2015). As shown in Figure S11 of the SI, the zeta potentials of

367

the MPs significantly decreased after alkaline pretreatments, confirming an increase in the

15

368

negative charge on the MP surfaces. Unlike the alkaline pretreatment, the acid and oxidation

369

pretreatments changed the MP adsorption potentials with a low degree, except for PMMA

370

and PA by acid pretreatment, implying that they exert little impact on the vector effects of the

371

MPs. The PA adsorption potential decreased significantly after the acid pretreatment,

372

possibly attributed to the positive hydrogen ions being attracted to the negatively charged

373

surface of the MPs (Melo et al. 2013), thus competing with Cd for the MP adsorption sites.

374

Compared with the virgin PA, zeta potential of the PA pretreated by the acid solution

375

increased, complementing and confirming the results of the adsorption potentials (Figure S11

376

of the SI). Additionally, the pretreated PMMA and PET had higher adsorption potentials of

377

Cd than the corresponding virgin MPs, respectively, possibly ascribed to the significant

378

increase in the CI values after the pretreatment (Table S4 of the SI). Yang et al. (2019) found

379

a positive correlation between the Cu adsorption capacities and the number of carbonyl

380

groups of the MPs.

381

PCA was conducted according to the five quantitative indicators of the MPs, in order to

382

comprehensively understand the effect of the pretreatments. As shown in Figure 6A, the

383

pretreatments were classified into four main categories: Category P1 (1 M, 5 M and 10 M

384

NaOH), Category P2 (5 M HNO3 and HCl), Category P3 (1 M HNO3 and HCl) and Category

385

P4 (H2O2 and Fenton). The above results imply that the effect of the pretreatments on the

386

MPs follows a decreasing sequence: Category P1 > Category P2 > Category P3 > Category

387

P4. In general, all of the alkaline pretreatments produced a strong effect on most of the MP

388

physicochemical characteristics and types especially for adsorption potentials, and PA and

389

PET were more susceptible to alkali treatment than other MPs. Unlike alkaline pretreatments,

390

high-concentration acid pretreatments had a higher effect on MPs than low-concentration

391

ones, especially for PA. As shown in Figure 6B, the six MPs are divided into three main

392

categories, Category M1 (PET, PA and PMMA), Category M2 (PS) and Category M3 (PE

16

393

and PP). The above results indicate that the resistance of the MPs to the chemical

394

pretreatments tends to increase as following: Category M1 < Category M2 < Category M3.

395

The MPs in Category M1 share the characteristics with polar carbonyl group and heteroatoms

396

in the main chain, which are different from PS, PE and PP solely built of carbon atoms, and

397

thus are potentially susceptible to hydrolytic cleavage of, e.g., ester or amide bonds (Gewert

398

et al. 2015). According to the glass transition temperatures, PS belongs to glassy plastics,

399

while PE and PP pertain to rubbery plastics (Alimi et al. 2018, Teuten et al. 2009).

400

Researchers found that glassy polymers possess dense structures and closed internal

401

nanoscale pores (Teuten et al. 2009). The presence of nanoscale pores may be conducive for

402

chemical materials to attack on the PS carbon backbone during the pretreatment. Kelkar et al.

403

(2019) reported that PS is less resistant to chlorination than PP and PE, as the presence of

404

plastic additives promotes the direct attack by chlorine. The results indicate that different

405

chemical pretreatments exert different influences on the different MPs. Overall, the

406

application of H2O2 and Fenton pretreatments will induce the lowest effect on the

407

physiochemical properties of PP and PE.

408

3.4

Limitations of this study

409

This study investigates various chemical pretreatments that have widely been used, but

410

have lacked the evaluations of physical and biological methods. A gentler pretreatment

411

should be sought out in the future to optimize the extraction efficiency and reduce the effects

412

on the extracted MPs. Enzyme pretreatment are able to eliminate more than 97% of

413

biomaterial from seawater and biota during the purification process of MPs (Cole et al. 2014,

414

Löder et al. 2017). However, it is unknown whether enzyme pretreatment can effectively

415

improve the MP extraction in the sludge due to the complex composition of sludge organic

416

matter, although protease and polysaccharase have been used in sludge pretreatment to

417

accelerate the hydrolysis process (Luo et al. 2019). Meanwhile, some physical pretreatments

17

418

such as ultrasonic waves constitute mature sludge decomposition technologies that contribute

419

to the decomposition of sludge flocs and the release of intercellular substances (Zhen et al.

420

2017). Therefore, it is necessary to further evaluate the effects of physical and biological

421

pretreatments, and their combination on MP extraction and physicochemical characteristics.

422

In addition, the MPs used in this study were virgin commercial plastics, and not derived from

423

the practical sewage sludge. Previous research reported that the surface characteristics of

424

virgin and sludge-based MPs are different (Li et al. 2019), and thus the results of this study

425

need to be further verified for the sludge-based MPs.

426 427

4. Conclusions

428

Compared with the other solid matrices, sewage sludge showed the lowest MP

429

extraction efficiency, especially for PET. The possible reason was that the EPS in the sludge

430

floc produce a negative effect on the extraction of MPs, by hindering their floating. Part of

431

chemical pretreatments such as H2O2 and Fenton can improve the MP extraction in sludge,

432

through the disintegration of the floc structure and the release of organic matter at proper

433

degree. High-concentration acid and alkaline pretreatments cause excessive dissolution of the

434

sludge organic matter, which decreases the MP extraction efficiency. The pretreatments affect

435

the MP property such as size, surface morphology and adsorption potentials, especially for

436

alkaline pretreatments. Compared with the PE, PP and PS solely built of carbon atoms, the

437

PET and PA with heteroatoms in their main chains are more susceptible to chemical

438

pretreatments. Therefore, an underestimate for MP abundance or an alteration of their real

439

physicochemical properties in sludge and other matrices may happen if improper

440

pretreatments are used in the MP extraction. Further investigation needs to be conducted in

441

order to discover more effective and gentler pretreatments for MP extraction in sludge.

442

18

443

Acknowledgements

444

The work was financially supported by the National Key R&D Program of China

445

(2018YFC1903201), Shanghai Committee of Science and Technology (19DZ1204702),

446

National Natural Scientific Foundation of China (51408423, 51578397 and 51538008),

447

Program of Shanghai Technology Research Leader Grant (17XD1420500), Key Program for

448

International S&T Cooperation Projects of China (2016YFE0123500), and Key Projects of

449

National Water Pollution Control and Management of China (2017ZX07403002).

450 451 452 453

Declaration of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

454 455 456 457

Appendix A. Supplementary data Additional tables and figures are as mentioned in the main text. This supporting information is available free of charge via the Internet.

458 459

References

460

Alimi, O.S., Farner Budarz, J., Hernandez, L.M. and Tufenkji, N. (2018) Microplastics and

461

Nanoplastics in Aquatic Environments: Aggregation, Deposition, and Enhanced

462

Contaminant Transport. Environ. Sci. Technol. 52, 1704-1724.

463

Avio, C.G., Gorbi, S. and Regoli, F. (2015) Experimental development of a new protocol

464

for extraction and characterization of microplastics in fish tissues: First observations in

465

commercial species from Adriatic Sea. Mar. Environ. Res. 111, 18-26.

466

Bala Subramanian, S., Yan, S., Tyagi, R.D. and Surampalli, R.Y. (2010) Extracellular

467

polymeric substances (EPS) producing bacterial strains of municipal wastewater sludge:

468

isolation, molecular identification, EPS characterization and performance for sludge

469

settling and dewatering. Water Res. 44(7), 2253-2266. 19

470

Bayo, J., Olmos, S., López-Castellanos, J. and Alcolea, A. (2016) Microplastics and

471

microfibers in the sludge of a municipal wastewater treatment plant. International

472

Journal of Sustainable Development and Planning 11(5), 812-821.

473

Carr, S.A., Liu, J. and Tesoro, A.G. (2016) Transport and fate of microplastic particles in

474

wastewater treatment plants. Water Res. 91, 174-182.

475

Carrere, H., Dumas, C., Battimelli, A., Batstone, D.J., Delgenes, J.P., Steyer, J.P. and Ferrer, I.

476

(2010) Pretreatment methods to improve sludge anaerobic degradability: a review. J.

477

Hazard. Mater. 183(1-3), 1-15.

478

Cole, M., Lindeque, P.K., Fileman, E., Clark, J., Lewis, C., Halsband, C. and Galloway, T.S.

479

(2016) Microplastics Alter the Properties and Sinking Rates of Zooplankton Faecal

480

Pellets. Environ. Sci. Technol. 50(6), 3239-3246.

481

Cole, M., Webb, H., Lindeque, P.K., Fileman, E.S., Halsband, C. and Galloway, T.S. (2014)

482

Isolation of microplastics in biota-rich seawater samples and marine organisms. Sci

483

Rep-Uk 4.

484

Dai, X.H., Luo, F., Yi, J., He, Q.B. and Dong, B. (2014) Biodegradation of polyacrylamide by

485

anaerobic digestion under mesophilic condition and its performance in actual

486

dewatered sludge system. Bioresource. Technol. 153, 55-61.

487

Dehaut, A., Cassone, A.L., Frere, L., Hermabessiere, L., Himber, C., Rinnert, E., Riviere, G.,

488

Lambert, C., Soudant, P., Huvet, A., Duflos, G. and Paul-Pont, I. (2016) Microplastics in

489

seafood: Benchmark protocol for their extraction and characterization. Environ. Pollut.

490

215, 223-233.

491

Gewert, B., Plassmann, M.M. and MacLeod, M. (2015) Pathways for degradation of

492

plastic polymers floating in the marine environment. Environ. Sci. Proc. Imp. 17(9),

493

1513-1521.

494

He, D., Luo, Y., Lu, S., Liu, M., Song, Y. and Lei, L. (2018) Microplastics in soils: Analytical

495

methods, pollution characteristics and ecological risks. TrAC Trends Anal. Chem. 109,

496

163-172.

497

Hüffer, T., Weniger, A.-K. and Hofmann, T. (2018) Sorption of organic compounds by aged

498

polystyrene microplastic particles. Environ. Pollut. 236, 218-225.

499

Hurley, R.R., Lusher, A.L., Olsen, M. and Nizzetto, L. (2018) Validation of a Method for

500

Extracting Microplastics from Complex, Organic-Rich, Environmental Matrices. Environ.

501

Sci. Technol. 52(13), 7409-7417.

502

Jorand, F., Boue-Bigne, F., Block, J. and Urbain, V. (1998) Hydrophobic/hydrophilic 20

503

properties of activated sludge exopolymeric substances. Water Sci. Technol. 37(4-5),

504

307-315.

505

Kowalski, N., Reichardt, A.M. and Waniek, J.J. (2016) Sinking rates of microplastics and

506

potential implications of their alteration by physical, biological, and chemical factors.

507

Mar. Pollut. Bull. 109(1), 310-319.

508

Lares, M., Ncibi, M.C., Sillanpää, M. and Sillanpää, M. (2019) Intercomparison study on

509

commonly used methods to determine microplastics in wastewater and sludge samples.

510

Environ. Sci. Pollut. Res. 26(12), 12109-12122.

511

Lassen, C., Hansen, S.F., Magnusson, K., Norén, F., Hartmann, N.I.B., Jensen, P.R., Nielsen,

512

T.G., Brinch, A. (2015) Microplastics-Occurrence, effects and sources of releases to the

513

environment in Denmark. Agency., D.E.P. (ed).

514

Li, X., Chen, L., Mei, Q., Dong, B., Dai, X., Ding, G. and Zeng, E.Y. (2018) Microplastics in

515

sewage sludge from the wastewater treatment plants in China. Water Res. 142, 75-85.

516

Li, X., Mei, Q., Chen, L., Zhang, H., Dong, B., Dai, X., He, C. and Zhou, J. (2019)

517

Enhancement in adsorption potential of microplastics in sewage sludge for metal

518

pollutants after the wastewater treatment process. Water Res. 157, 228-237.

519

Liu, P., Qian, L., Wang, H., Zhan, X., Lu, K., Gu, C. and Gao, S. (2019) New Insights into the

520

Aging Behavior of Microplastics Accelerated by Advanced Oxidation Processes. Environ.

521

Sci. Technol. 53(7), 3579-3588.

522

Liu, X.L., Liu, H., Du, G.C. and Chen, J. (2009) Improved bioconversion of volatile fatty

523

acids from waste activated sludge by pretreatment. Water Environ. Res. 81(1), 13-20.

524

Lö der, M.G., Imhof, H.K., Ladehoff, M., Lö schel, L.A., Lorenz, C., Mintenig, S., Piehl, S.,

525

Primpke, S., Schrank, I. and Laforsch, C. (2017) Enzymatic purification of microplastics

526

in environmental samples. Environ. Sci. Technol. 51(24), 14283-14292.

527

Long, M., Moriceau, B., Gallinari, M., Lambert, C., Huvet, A., Raffray, J. and Soudant, P.

528

(2015) Interactions between microplastics and phytoplankton aggregates: Impact on

529

their respective fates. Mar. Chem. 175, 39-46.

530

Luo, K., Pang, Y., Yang, Q., Wang, D.B., Li, X., Lei, M. and Huang, Q. (2019) A critical review

531

of volatile fatty acids produced from waste activated sludge: enhanced strategies and its

532

applications. Environ. Sci. Pollut. Res. 26(14), 13984-13998.

533

Lv, Y., Huang, Y., Kong, M., Yang, Q. and Li, G. (2017) Multivariate correlation analysis of

534

outdoor weathering behavior of polypropylene under diverse climate scenarios. Polym.

535

Test. 64, 65-76. 21

536

Mahon, A.M., O'Connell, B., Healy, M.G., O'Connor, I., Officer, R., Nash, R. and Morrison, L.

537

(2017) Microplastics in Sewage Sludge: Effects of Treatment. Environ. Sci. Technol.

538

51(2), 810-818.

539

Manara, P. and Zabaniotou, A. (2012) Towards sewage sludge based biofuels via

540

thermochemical conversion - A review. Renew. Sust. Energ. Rev. 16(5), 2566-2582.

541

Mason, S.A., Garneau, D., Sutton, R., Chu, Y., Ehmann, K., Barnes, J., Fink, P., Papazissimos,

542

D. and Rogers, D.L. (2016) Microplastic pollution is widely detected in US municipal

543

wastewater treatment plant effluent. Environ. Pollut. 218, 1045-1054.

544

Melo, D.Q., Neto, V.O.S., Oliveira, J.T., Barros, A.L., Gomes, E.C.C., Raulino, G.S.C.,

545

Longuinotti, E. and Nascimento, R.F. (2013) Adsorption Equilibria of Cu2+, Zn2+, and

546

Cd2+ on EDTA-Functionalized Silica Spheres. J. Chem. Eng. Data 58(3), 798-806.

547

Müller, A., Becker, R., Dorgerloh, U., Simon, F.-G. and Braun, U. (2018) The effect of

548

polymer aging on the uptake of fuel aromatics and ethers by microplastics. Environ.

549

Pollut. 240, 639-646.

550

Murphy, F., Ewins, C., Carbonnier, F. and Quinn, B. (2016) Wastewater Treatment Works

551

(WwTW) as a Source of Microplastics in the Aquatic Environment. Environ. Sci. Technol.

552

50(11), 5800-5808.

553

Nizzetto, L., Futter, M. and Langaas, S. (2016) Are Agricultural Soils Dumps for

554

Microplastics of Urban Origin? Environ. Sci. Technol. 50(20), 10777-10779.

555

Nuelle, M.T., Dekiff, J.H., Remy, D. and Fries, E. (2014) A new analytical approach for

556

monitoring microplastics in marine sediments. Environ. Pollut. 184, 161-169.

557

Priyananda, P. and Chen, V. (2006) Flux decline during ultrafiltration of protein-fatty

558

acid mixtures. J. Membr. Sci. 273(1-2), 58-67.

559

Schmiedgruber, M., Hufenus, R. and Mitrano, D.M. (2019) Mechanistic understanding of

560

microplastic fiber fate and sampling strategies: Synthesis and utility of metal doped

561

polyester fibers. Water Res. 155, 423-430.

562

Song, Y.K., Hong, S.H., Jang, M., Han, G.M., Jung, S.W. and Shim, W.J. (2017) Combined

563

effects of UV exposure duration and mechanical abrasion on microplastic fragmentation

564

by polymer type. Environ. Sci. Technol. 51(8), 4368-4376.

565

Sujathan, S., Kniggendorf, A.K., Kumar, A., Roth, B., Rosenwinkel, K.H. and Nogueira, R.

566

(2017) Heat and Bleach: A Cost-Efficient Method for Extracting Microplastics from

567

Return Activated Sludge. Arch. Environ. Contam. Toxicol. 73(4), 641-648.

568

Sun, J., Dai, X., Wang, Q., van Loosdrecht, M.C.M. and Ni, B.-J. (2019) Microplastics in 22

569

wastewater treatment plants: Detection, occurrence and removal. Water Res. 152,

570

21-37.

571

Tagg, A.S., Sapp, M., Harrison, J.P. and Ojeda, J.J. (2015) Identification and Quantification

572

of Microplastics in Wastewater Using Focal Plane Array-Based Reflectance Micro-FT-IR

573

Imaging. Anal. Chem. 87(12), 6032-6040.

574

Teuten, E.L., Saquing, J.M., Knappe, D.R.U. and Barlaz, M.A. (2009) Transport and release

575

of chemicals from plastics to the environment and to wildlife. Philos. Trans. R. Soc. B.

576

364(1526), 2027-2045.

577

Torres, M.L. and Llorens, M.D.E. (2008) Effect of alkaline pretreatment on anaerobic

578

digestion of solid wastes. Waste Manage. 28(11), 2229-2234.

579

Turner, A. and Holmes, L.A. (2015) Adsorption of trace metals by microplastic pellets in

580

fresh water. Environ. Chem. 12(5), 600-610.

581

Xiao, B.Y., Liu, C., Liu, J.X. and Guo, X.S. (2015) Evaluation of the microbial cell structure

582

damages in alkaline pretreatment of waste activated sludge. Bioresource. Technol. 196,

583

109-115.

584

Yang, J., Cang, L., Sun, Q., Dong, G., Ata-Ul-Karim, S.T. and Zhou, D. (2019) Effects of soil

585

environmental factors and UV aging on Cu 2+ adsorption on microplastics. Environ. Sci.

586

Pollut. Res., 1-10.

587

Yin, C.Q., Meng, F.G. and Chen, G.H. (2015) Spectroscopic characterization of

588

extracellular

589

ammonia-oxidizing bacteria. Water Res. 68, 740-749.

590

Yu, Q., Lei, H., Yu, G., Feng, X., Li, Z. and Wu, Z. (2009) Influence of microwave irradiation

591

on sludge dewaterability. Chem. Eng. J. 155(1-2), 88-93.

592

Yu, X., Peng, J., Wang, J., Wang, K. and Bao, S. (2016) Occurrence of microplastics in the

593

beach sand of the Chinese inner sea: the Bohai Sea. Environ. Pollut. 214, 722-730.

594

Yuan, H.-p., Yan, X.-f., Yang, C.-f. and Zhu, N.-w. (2011) Enhancement of waste activated

595

sludge dewaterability by electro-chemical pretreatment. J. Hazard. Mater. 187(1-3),

596

82-88.

597

Zhang, H., Wang, J., Zhou, B., Zhou, Y., Dai, Z., Zhou, Q., Chriestie, P. and Luo, Y. (2018)

598

Enhanced adsorption of oxytetracycline to weathered microplastic polystyrene: Kinetics,

599

isotherms and influencing factors. Environ. Pollut. 243(Pt B), 1550-1557.

600

Zhang, L.L., Feng, X.X., Zhu, N.W. and Chen, J.M. (2007) Role of extracellular protein in the

601

formation and stability of aerobic granules. Enzyme Microb. Tech. 41(5), 551-557.

polymeric

substances

from

23

a

mixed

culture

dominated

by

602

Zhang, X., Lei, H., Chen, K., Liu, Z., Wu, H. and Liang, H. (2012) Effect of potassium ferrate

603

(K2FeO4) on sludge dewaterability under different pH conditions. Chem. Eng. J. 210,

604

467-474.

605

Zhao, J., Ran, W., Teng, J., Liu, Y., Liu, H., Yin, X., Cao, R. and Wang, Q. (2018) Microplastic

606

pollution in sediments from the Bohai Sea and the Yellow Sea, China. Sci. Total Environ.

607

640-641, 637-645.

608

Zhen, G.Y., Lu, X.Q., Kato, H., Zhao, Y.C. and Li, Y.Y. (2017) Overview of pretreatment

609

strategies for enhancing sewage sludge disintegration and subsequent anaerobic

610

digestion: Current advances, full-scale application and future perspectives. Renew. Sust.

611

Energ. Rev. 69, 559-577.

24

612

Table 1. Changes in the mass and size of the six MPs after different chemical pretreatments (mean value ± SD, n=3). The data marked in orange are

613

more than 5%. MPs

30% H2O2

Fenton

1 M HNO3

5 M HNO3

1 M HCl

5 M HCl

1 M HaOH

5 M HaOH

10 M HaOH

PE

-1.1 ± 4.7

2.0 ± 1.0

0.3 ± 1.9

2.4 ± 3.7

-1.3 ± 2.2

2.1 ± 3.2

-2.2 ± 1.6

0.3 ± 1.1

-3.1 ± 2.6

PP

-0.4 ± 0.7

0.8 ± 0.2

0.4 ± 0.8

1.8 ± 0.7

0.6 ± 1.4

1.3 ± 2.6

-2.4 ± 3.3

0.4 ± 1.8

-2.4 ± 1.8

PS

-0.5 ± 0.9

0.3 ± 0.1

2.1 ± 0.6

0.2 ± 1.1

-1.8 ± 1.4

-1.0 ± 1.5

-0.9 ± 1.0

0.2 ± 1.1

-2.1 ± 1.8

PA

-0.4 ± 2.0

-0.4 ± 1.1

1.7 ± 2.3

-100

1.1 ± 2.5

-100

-7.1 ± 4.0

-2.4 ± 1.1

-6.5 ± 4.0

PMMA

-1.6 ± 1.5

3.8 ± 2.9

-0.7 ± 1.3

-0.04 ± 2.0

-0.1 ± 1.2

2.1 ± 2.4

-6.1 ± 4.2

-0.01 ± 1.8

-2.5 ± 5.2

PET

-2.7 ± 2.8

0.6 ± 0.7

0.1 ± 2.1

1.9 ± 1.7

0.6 ± 1.4

3.0 ± 2.6

-30.2 ± 6.2

-38.5 ± 5.8

-53.5 ± 7.2

PE

-2.8 ± 1.1

1.8 ± 4.1

4.1 ± 0.3

5.6 ± 0.6

-2.9 ± 6.3

-1.6 ± 1.0

-3.7 ± 0.3

-5.5 ± 4.1

-2.5 ± 1.6

PP

-2.6 ± 3.6

2.6 ± 1.5

0.7 ± 0.2

-3.5 ± 2.0

-6.0 ± 2.4

-6.2 ± 3.6

-3.3 ±1.9

-2.3 ± 1.6

-4.6 ± 2.1

PS

1.4 ± 0.6

0.9 ± 3.1

-3.2 ± 2.5

-6.0 ± 0.8

-3.3 ± 2.2

-6.9 ± 5.1

-2.3 ± 4.1

-1.3 ± 2.5

-5.4 ± 4.5

PA

-5.2 ± 7.1

2.1 ± 6.3

-7.6 ± 1.7

-100

-4.8 ± 8.1

-100

-11.8 ± 2.1

-8.9 ± 3.5

-9.2 ± 3.1

PMMA

-2.7 ± 1.6

-2.9 ± 1.6

2.6 ± 4.2

-1.2 ±1.6

-2.3 ± 1.2

-6.8 ± 2.2

-7.7 ± 5.1

-3.2 ± 1.7

-4.4 ± 0.5

PET

-4.6 ± 5.0

-3.4 ± 2.8

-5.6 ± 3.0

-2.4 ± 1.2

1.1 ± 0.9

-4.1 ± 0.2

-9.4 ± 5.8

-13.4 ± 3.9

-16.7 ± 3.5

Mass loss (%)

Size loss (%)

614

25

615

Figure A a

Microplastics recovery efficiency (%)

100

90

a a

a

Soil

Sediment

b

80

70

60

50

Sludge Cattle manure

SiO2

616

B

First step Second step

MP extraction efficiency (%)

100

75

50

25

0 PE PP PS PA MA PET PM

Sludge

PE PP PS PA MA PET PM

PE PP PS PA MA PET PM

PE PP PS PA MA PET PM

PE PP PS PA MA PET PM

Soil

Sediment

SiO2

Cattle manure

617 618

Figure 1. Extraction efficiency of MPs in five solid matrices (mean value ± SD, n=3). (A)

619

Total extraction efficiency; (B) respective extraction efficiency of six MPs in the

620

first-step and second-step extraction. Different letters along the vertical bars

621

indicate the significant difference between treatments (P < 0.05).

26

1.5

a

granular PET

a

fibrous PET

-1

Floating velocity (cm s )

a

1.0

b c

c

0.5

0.0

PS PS Cl 2 M M nCl 2 +E Zn +E PA PA Z Cl 2 Cl 2 S+ S+ n P P Z Zn E + +E Cl 2 Cl 2 Zn Zn

622 623

Figure 2. Floating velocity of granular and fibrous PET in the ZnCl2, ZnCl2+EPS and

624

ZnCl2+EPS+PAM solutions (n=5-8). Different letters along the vertical bars

625

indicate the significant difference between treatments (P < 0.05).

27

A 100 a

a

a

ab

MP extraction efficiency (%)

b

bc cd de

80

ef f

60

40 RS

1

2

3a

3b 4a Pretreatment

4b

5a

5b

5c

626

MP extraction efficiency (%)

B First step Second step

100

75

50

25

30% H 2 O 2

Fenton

1 M HNO 3

5 M HNO 3 1 M HCl

5 M HCl

PE PP PS P MPA M A PE T PE PP PS PMP A M A PE T

PE PP PS P MP A M A PE T PE PP PS P MPA M A PE T PE PP PS P MPA M A PE T PE PP PS P MPA M A PE T PE PP PS P MPA M A PE T PE PP PS PM PA M A PE T PE PP PS P MPA M A PE T

0 1 M NaOH 5 M NaOH 10 M NaOH

627 628

Figure 3. Changes in extraction efficiency of MPs before and after pretreatments (mean value

629

± SD, n=3). (A) Total extraction efficiency; (B) respective extraction efficiency of

630

six MPs in the first-step and second-step extraction. Different letters along the

631

vertical bars indicate the significant difference between treatments (P < 0.05). RS, 1,

632

2, 3a, 3b, 4a, 4b, 5a, 5b and 5c represent raw sludge, and the pretreatments of 30%

633

H2O2, Fenton, 1 M HNO3, 5 M HNO3, 1 M HCl, 5 M HCl, 1 M NaOH, 5 M NaOH

634

and 10 M NaOH, respectively. 28

-1

SCOD content (mg g dry weight)

70

A

60 50 40 30 20 10 0 Control

1

2

3a

3b

4a

4b

5a

5b

5c

Pretreatment

635 Total extraction efficiency of the MPs (%)

95

B Total extraction efficiency Gauss fitting curve

90

85

80

75

70 10

20

30

40

50

60

70

-1

SCOD content (mg g dry weight)

636 80

Extraction efficiency of the PET fibre (%)

C PET extraction efficiency Gauss fitting curve 60

40

20

0

10

20

30

40

-1

50

60

70

SCOD content (mg g dry weight)

637 638

Figure 4. Changes in SCOD contents in the sludge before and after the pretreatments (A,

639

mean value ± SD, n=3), and the Gauss fitting curves of SCOD content and

640

extraction efficiency of total MPs (B) and PET (C), respectively. Control, 1, 2, 3a,

641

3b, 4a, 4b, 5a, 5b and 5c represent the pretreatments of deionized water, 30% H2O2,

642

Fenton, 1 M HNO3, 5 M HNO3, 1 M HCl, 5 M HCl, 1 M NaOH, 5 M NaOH and

643

10 M NaOH, respectively.

29

1200

1200

PP Adsorption of Cd on PP (mg kg )

900

-1

-1

Adsorption of Cd on PE (mg kg )

PE

600

300

0

900

600

300

0 1

Ps Virgin M

2

3a

3b

4a

4b

5a

5b

5c

Ps Virgin M

Pretreatment

1200

3a

3b

4a

4b

5a

5b

5c

4b

5a

5b

5c

4b

5a

5b

5c

Pretreatment

PA Adsorption of Cd on PA (mg kg )

900

-1

-1

2

1200

PS Adsorption of Cd on PS (mg kg )

1

600

300

0

900

600

300

0 1

Ps Virgin M

2

3a

3b

4a

4b

5a

5b

5c

Ps Virgin M

Pretreatment

1

2

3a

3b

4a

Pretreatment

4000

1200

PET

PMMA -1

Adsorption of Cd on PET (mg kg )

-1

Adsorption of Cd on PMMA (mg kg )

3500 900

600

300

0 Ps Virgin M

1

2

3a

3b

4a

Pretreatment

4b

5a

5b

5c

3000 1200 900 600 300 0

Ps Virgin M

1

2

3a

3b

4a

Pretreatment

644 645

Figure 5. Adsorbed amounts of Cd on virgin and pretreated MPs (mean value ± SD, n=3). 1, 2,

646

3a, 3b, 4a, 4b, 5a, 5b and 5c represent the pretreatments of 30% H2O2, Fenton, 1 M

647

HNO3, 5 M HNO3, 1 M HCl, 5 M HCl, 1 M NaOH, 5 M NaOH and 10 M NaOH,

648

respectively.

30

0.8

0.8

PC 3 (11.8%)

5M HCl 5M NaOH

0.2

10 M NaOH

0.0 30% H2O2

-0.2

1 M HNO3 1 M HCl

0.0 0.4 0.8

PE

0.2 PP

0.0 -0.2

0.0

-0.4

) 0.4 3% . 25 0.6 ( 2 0.8 PC

-0.4

649 650

PA

PMMA

-0.4

0.2

-0.8 (55 .4% )

PET

0.4

Fenton

-0.4

PC 1

PS

B

0.6

1M NaOH

0.4

PC 2 (27.6%)

0.6

1.0

5M HNO3

A

0.0

-0.8 -0.4 PC 1 (3 0.0 4.7% 0.4 )

1.0

0.4 0.8

PC

) .8% 19 ( 3

0.8

Figure 6. Principal component analysis (PCA) of nine pretreatments (A) and six MP types (B)

651

according to the changes in the mass, size, carbonyl index (CI), adsorption of Cd

652

and zeta potentials of the MPs before and after the pretreatments

653

31

Highlights Extraction efficiency of MPs in sludge (87.2%) was lower than other solid matrices. The entanglement of sludge flocs formed by EPS led to low extraction of MPs. Proper pretreatments can destroy the sludge flocs and improve the MP extraction. Pretreatments affect the MP property, especially for alkaline pretreatments. The MPs with heteroatoms in main chain are more susceptible to the pretreatments.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: