Biofilm alters tetracycline and copper adsorption behaviors onto polyethylene microplastics

Journal Pre-proofs Biofilm alters tetracycline and copper adsorption behaviors onto polyethylene microplastics Yuan Wang, Xuejiang Wang, Yuan Li, Jing Li, Fei Wang, Siqing Xia, Jianfu Zhao PII: DOI: Reference:

S1385-8947(19)33223-1 https://doi.org/10.1016/j.cej.2019.123808 CEJ 123808

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

5 October 2019 9 December 2019 11 December 2019

Please cite this article as: Y. Wang, X. Wang, Y. Li, J. Li, F. Wang, S. Xia, J. Zhao, Biofilm alters tetracycline and copper adsorption behaviors onto polyethylene microplastics, Chemical Engineering Journal (2019), doi: https:// doi.org/10.1016/j.cej.2019.123808

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1

Biofilm alters tetracycline and copper adsorption behaviors onto

2

polyethylene microplastics

3 4

Yuan Wanga,b, Xuejiang Wanga,b,*, Yuan Lia,b, Jing Lia,b, Fei Wanga,b, Siqing Xiaa,b, Jianfu Zhaoa,b

5

a

6

Resource Reuse, Tongji University, Shanghai 200092, PR China

7

b Shanghai

8

*Corresponding author:

9

Tel.: +86 021 65984268;

College of Environmental Science and Engineering, State Key Laboratory of Pollution Control and

Institute of Pollution Control and Ecological Security, Shanghai 200092, PR China

10

E-mail address: [email protected] (X. Wang)

11

Abstract

12

In this study, the adsorption properties of Cu(II) and tetracycline (TC) onto virgin

13

and biofilm-developed polyethylene (PE) microplastics were investigated in batch

14

sorption experiments. PE microplastics were placed at sewage outlets (Shanghai, China)

15

for 20 days to develop biofilm on their surface. The adsorption and desorption

16

isotherms of Cu(II) and TC were well fitted by the Freundlich model, and revealed that

17

biofilm could enhance the adsorption and stabilization of Cu(II) and TC on

18

microplastics. The linearity test of the film diffusion model in kinetic experiments

19

suggested that the adsorption on virgin and biofilm-developed microplastics was

20

dominated by intra-particle diffusion and film diffusion, respectively. Compared with

21

the virgin microplastics, the adsorption of Cu(II) and TC on biofilm-developed

22

microplastics was additionally affected by pH-dependent complexation interactions in

23

the biofilm and competition interactions. Cu(II) pre-adsorbed on the biofilm could be

24

released into solution because of competition effects of TC. Fourier transform infrared 1

25

spectroscopy (FTIR) and density functional theory (DFT) further confirmed that the

26

enhanced adsorption of TC on the biofilm could be attributed to the complexation of

27

TC, Cu(II) and components in biofilm. This study illustrated that biofilms could

28

enhance the role of microplastics in the Cu(II) and TC migration by changing their

29

adsorption properties on microplastics.

30

Keywords

31

Microplastics; Biofilms; Copper; Tetracycline; Adsorption

32

1. Introduction

33

In the last 60 years, extensive production and applications of plastic materials have

34

increased the release of plastic residues into the environments[1, 2]. Recently,

35

microplastics with the diameter of <5 mm have attached much attention because of their

36

toxic effects on water environments[3]. It was reported that more than 35000 tonnes of

37

microplastics were floating in the oceans globally[4]. Moreover, they were also

38

frequently detected in freshwater lakes [5], rivers [6] and drinking water [7]. Attributed

39

to the peculiar properties of microplastics, e.g. high surface area and mobility capacity

40

[8], microplastics are able to accumulate a great number of pollutants and serve as a

41

carrier for long migrations in aquatic ecosystems [9-13].

42

At present, many studies have been conducted on the adsorption behaviors of

43

organics and heavy metals onto microplastics [14-17]. The results showed that the

44

adsorption capacities of microplastics for pollutants varied with the properties of

45

microplastics, adsorbates and environmental factors [18-20]. For instance, Guo et al.[11]

46

and Li et al.[15] found that the rubbery abundance, crystallinity degree and polarity of 2

47

polymer particles affected the adsorption of organic pollutants by microplastics. Hüffer

48

and Hofmann [9] investigated the adsorption of aliphatic and aromatic organic

49

compounds onto polyamide (PA), polyethylene (PE), polyvinylchloride (PVC) and

50

polystyrene(PS) microplastics. The results showed that the adsorption processes were

51

related with the hydrophobicity of the organic compounds. Wang et al. [19] explored

52

the adsorption of two perfluorochemicals onto PS microplastics, which suggested that

53

functional groups in adsorbates affected the affinity of adsorbates for microplastics. As

54

for the effects of environmental factors, Turner and Holmes [10] reported that

55

increasing pH of river water could enhance the adsorption of Ag, Ni and Zn on

56

microplastics. Xu et al. [21] also revealed that the adsorption of TC on PE microplastics

57

peaked at pH 6.0 because TC species varied with the change of solution pH, and they

58

also found that fulvic acid could compete with microplastics for TC in solution.

59

Velzeboer et al. [22] found that salinity could increase the adsorption of hydrophobic

60

organic contaminants (HOCs) on PS and PE microplastics.

61

In fact, microplastic particles are susceptible to various abiotic and biological aging

62

processes once they enter the environment, which also had a certain effect on their

63

adsorption capacity. Zhang et al. [18] investigated the effects of weathering on the

64

adsorption capacities of PS microplastics for oxytetracycline, and they found that

65

weathering could alter the surface properties of microplastics, which increased their

66

adsorption capacity for oxytetracycline. At the same time, Johansena et al. [17] found

67

that the development of biofilm on microplastics accumulated more adionuclides than

68

the virgin microplastics. However, as compared with the effects of abiotic aging process, 3

69

attentions focused on the effects of biological aging process on the adsorption capacity

70

of microplastics are limited to few researches.

71

Recently, the combined pollution of antibiotics and heavy metals has become an

72

emerging issue around the world [23-26]. As one of the most widely used antibiotics in

73

human therapy and livestock production [27], a great quantity of tetracycline (TC) is

74

discharged into environment each year [28]. The increasing abundance of TC in

75

environment could result in acute and chronic toxicity to humans[29] as well as the

76

propagation of antibiotic-resistant genes in the microflora[30]. As a heavy metal, Cu(II)

77

is also widely used in multiple industries, e.g. farming, electroplating industry and

78

dyestuff industry[31]. It could be highly toxic to wildlife when beyond certain limits

79

[32]. More seriously, attributed to the high ionic potential of Cu(II) and electron-donor

80

groups in TC, they tend to form complexes in natural conditions [33, 34]. Lian et al.

81

[35] reported Cu-TC complex species were more toxic than their individual toxicity,

82

and Cu(II) and TC co-adsorbed on the microplastics could be more pernicious than their

83

individual during the migration of microplastics in the wild.

84

The objective of this study was to investigate the adsorption properties of Cu(II) and

85

tetracycline (TC) onto PE microplastics following biofilm development in sewage. The

86

adsorption behaviors were investigated by using kinetics, isotherms, desorption, pH

87

effects and ordinal addition experiments. Moreover, the components in biofilm were

88

identified by GC-MS, and their binding sites for Cu(II) as well as ion bridge effects of

89

Cu(II) at the molecular level were further revealed by quantum chemical methods.

90

2. Materials and methods 4

91

2.1. Materials and chemicals

92

PE microplastics were obtained from Dongguan Jingyun Plastic Market (China) with

93

60-150μm size classes. PE microplastics were placed at sewage outlets in Shanghai to

94

develop biofilm on their surface. The detailed process of biofilm development was

95

shown

96

chromatography grade and the other reagents were of analytical grade purchased from

97

Aladdin (USA).

98

2.2. Experimental procedures

in

supporting

materials.

Acetonitrile

was

high-performance

liquid

99

Batch adsorption experiments were managed to investigate Cu(II) and TC

100

adsorption by microplastics. Generally, 0.1 g adsorbents were added into 40 mL glass

101

tubes with 20mL solution containing 0.01 M NaNO3, 0.003 M NaN3 and different

102

concentrations of adsorbates at 25℃. Solution pH was continuously measured and kept

103

at around 5.8. Except sorption kinetics experiments, the solution was obtained after

104

shaking in dark for 4 days. The obtained samples were centrifuged and filtered through

105

0.22 μm filters, and then the adsorbate concentrations in each sample were detected. All

106

the adsorption experiments were repeated three times to get the mean value and had the

107

blank control group.

108

In sorption kinetics experiments, three series of solution containing 0.25mg/L

109

Cu(II); 5mg/L TC; 0.25mg/L Cu(II) and 5mg/L TC were added with virgin PE

110

microplastics and were marked as PE+0 mg/L TC, PE+0 mg/L Cu, PE+5 mg/L TC or

111

PE+0.25 mg/L Cu, respectively. Cu(II) and TC concentrations in solution were detected

112

successively ranging from 0 to 96 h. When using biofilm-developed microplastics as

113

adsorbents, the adsorbate concentrations were 1mg/L Cu(II); 5 mg/L TC; 1 mg/L Cu(II) 5

114

and 5 mg/L TC; 0.25 mg/L Cu(II) and 5 mg/L TC, which were marked as Bio-PE+0

115

mg/L TC, Bio-PE+0 mg/L Cu, Bio-PE+5 mg/L TC and Bio-PE+0.25 mg/L Cu,

116

respectively.

117

To investigate TC sorption isotherms, two series of solution were prepared. In one

118

series of solution, 0-15 mg/L TC was added into solution to be adsorbed by virgin

119

microplastics or biofilm-developed microplastics, remarked as PE+0 mg/L Cu, Bio-

120

PE+0 mg/L Cu, respectively. In the other series of solution, 0-15 mg/L TC and 0.5

121

mg/L Cu(II) were added into solution to be adsorbed by virgin microplastics or biofilm-

122

developed microplastics, remarked as PE+0.5 mg/L Cu, Bio-PE+0.5 mg/L Cu,

123

respectively. Cu(II) sorption isotherms were investigated in a similar way as described

124

above. Cu(II) was added into solution with a range of concentrations (0-5 mg/L) with

125

0 or 5 mg/L TC, and then virgin microplastics and biofilm-developed microplastics

126

were used as adsorbents. Finally, four experimental groups were obtained, remarked as

127

PE+0 mg/L TC, Bio-PE+0 mg/L TC, PE+5 mg/L TC and Bio-PE+5 mg/L TC,

128

respectively.

129

Desorption experiments were carried out directly at the points with highest

130

concentrations in adsorption isotherms experiments. After centrifugation, 10mL

131

supernatant in the glass tube was replaced by deionized water, and the mixture was

132

shaken in dark for 4 days. 10 mL supernatant that was taken out from the solution was

133

used to detect adsorbate concentrations. This process was repeated four times for each

134

sample.

135

In the pH effect experiments, the solution containing 3mg/L TC with and without 6

136

1mg/L Cu(II) was prepared to investigate pH effects on TC adsorption by virgin and

137

biofilm-developed microplastics in the sole and binary system. The solution containing

138

3mg/L Cu(II) with and without 5mg/L TC was prepared to investigate pH effects on

139

Cu(II) adsorption by virgin and biofilm-developed microplastics in the sole and binary

140

system. And the pH was controlled at the range from 2 to 10.

141

In the ordinal addition experiments, to investigate the adsorption by virgin

142

microplastics three sets of glass tubes were added with 10 mg/L TC, 1 mg/L Cu(II)

143

simultaneously or separately. The first set of glass tubes were added with 10 mg/L TC,

144

1 mg/L Cu(II) separately and marked as TC-Cu. The second set of glass tubes were

145

added with 1 mg/L Cu(II), 10 mg/L TC separately and marked as Cu-TC. The third set

146

of glass tubes were added with 10 mg/L TC, 1 mg/L Cu(II) simultaneously and marked

147

as TC&Cu. During investigating the adsorption by biofilm-developed microplastics, to

148

ensure the effects of biofilm on TC and Cu(II) more obvious, sewage sludge, obtained

149

from the same place as biofilm development, was used to represent biofilm. To

150

investigate TC adsorption, three sets of glass tubes were added with 200 mg/L TC, 10-

151

20 mg/L Cu(II) simultaneously or separately. And to investigate Cu(II) adsorption,

152

three sets of glass tubes were added with 10 mg/L Cu(II), 50-200 mg/L TC

153

simultaneously or separately.

154

2.3 Characterization and analytical method

155

X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), N2-BET

156

methods, zeta potentials, microscope images and scanning electron microscope-energy

157

dispersive X-ray spectroscopy (SEM-EDS) were conducted to characterize adsorbents 7

158

and adsorption products. TC and Cu(II) concentration was detected by high-

159

performance liquid chromatography, Inductively Coupled Plasmas Atomic Emissive

160

Spectrometry (ICP-AES), respectively. Gas chromatograph-mass spectrometer (GC-

161

MS) was used to qualitatively analyze the component of biofilm on PE microplastics.

162

And Cu(II) in the biofilm was also detected by ICP-AES. The detailed information was

163

shown in the supplementary material.

164

2.4 Data analysis

165 166

Cu(II) and TC species in solution was simulated by MINEQL[36], and corresponding constants were given in the supplementary material.

167

The experimental data of adsorption kinetic experiments were fitted by the intra-

168

particle diffusion model. The film diffusion model was used to further identify the

169

speed-limiting step during adsorption. These models were listed as follows.

170

Intra-particle diffusion model[37]: 𝑞𝑡 = 𝐾𝑖𝑑 × 𝑡1/2 +𝐶

171 172

(1)

Film diffusion model[38]:

(

173

𝑞𝑡

)

𝐵𝑡 = ― ln 1 ― 𝑞𝑒 ―0.4977

(2)

174

Where 𝑞𝑒(ug g-1) is equilibrium adsorption amount of Cu(II) or TC; 𝑞𝑡(μg g-1) is

175

adsorption amount at time t; 𝐾𝑖𝑑 (μg g-1 t1/2) is the constant related with the diffusion

176

rate; 𝐶 (μg g-1) is the constant related with effects of boundary layer; 𝐵𝑡 is the Boyd

177

constant.

178 179

The models used to describe adsorption and desorption isotherms of Cu(II) and TC were listed as follows[15]: 8

180

The Freundlich model: 𝑞𝑒 = 𝐾𝑓𝐶1/𝑛 𝑒

181 182

The Langmuir model: 𝑞𝑚𝑎𝑥𝐾𝐿𝐶𝑒

183 184

(3)

𝑞𝑒 = 1 + 𝐾𝐿𝐶𝑒

(4)

The Liner model:

185

(5)

𝑞𝑒 = 𝐾𝑑𝐶𝑒

186

Where 𝐶𝑒(mg L-1) was the concentration of Cu(II) and TC at equilibrium; 𝐾𝑓((μg

187

g-1)(mg L-1)-n) and n were the constants of the Freundlich model; qmax(μg g-1) was the

188

maximum adsorption capacity in the Langmuir model; KL(L mg-1) was the constants of

189

the Langmuir model; 𝐾𝑑(mL g-1) was partition coefficient in the Liner model.

190 191

Thermodynamic irreversibility index (TII) was used to estimate the reversibility of Cu(II) and TC adsorption, and it was calculated as follows[39]: 𝑛𝑓

192 193

TII = 1 ― (𝑛𝑓𝑑)

(6)

Where nf and nfd were the constants of the Freundlich model in adsorption and

194

desorption, respectively.

195

2.5 Quantum chemical methods

196

GaussView 5.0 and Gaussian 09 were used to optimize the molecule structure,

197

minimize the energy of the molecule and calculate charge distribution as well as

198

superficial electrostatic potential utilizing Density Functional Theory (DFT) B3LYP

199

functional.

200

3. Results and discussion

201

3.1 Characterization of PE microplastics with and without biofilm 9

202

Fig.1a showed the microscope images of PE microplastics. The shapes of PE

203

microplastics were irregular. The surface of the microplastics were furtherly displayed

204

in SEM micrographs and shown in Fig.1c and Fig.1d. A great number of pores were

205

found to exist in virgin PE microplastics and the surface of virgin PE microplastics was

206

rugged (Fig.1c). However, compared with virgin PE microplastics, the pore width

207

became smaller and some tiny components were stuck on the surface of biofilm-

208

developed microplastics (Fig.1d). This phenomenon accorded with the results of N2

209

adsorption/desorption isotherms in Fig.1b and Table S1. As shown in pore size

210

distribution, after biofilm development large pores in virgin microplastics

211

(Diameter>10nm) were transformed into smaller pores (Diameter< 3nm) (Fig.1b),

212

resulting smaller pore volume and average pore size (Table S1). And the larger specific

213

surface area in biofilm-developed microplastics (Table S1) could be contributed by

214

components on the surface of biofilm. XRD spectrum of PE microplastics was depicted

215

in Fig.1e. XRD spectrum of virgin PE microplastics was identified with Wang et al.’s

216

results[19]. Compared with virgin microplastics, the peak of 2θ at 31.614° in biofilm-

217

developed microplastics could result from the components on the surface of biofilm-

218

developed microplastics. ------Fig. 1------

219 220

3.2 Adsorption kinetics

221

Adsorption of Cu(II) and TC by virgin PE microplastics or biofilm-developed PE

222

microplastics was a complicated process, which was controlled through the following

223

consecutive steps: bulk transportation, film diffusion, pore transportation and 10

224

adsorption[40]. Intra-particle diffusion model hypothesized that the adsorption process

225

was proceeded by adsorbate diffusion into the internal particles[18], and it was utilized

226

to investigate the speed-limiting step during adsorption in this study. Boyd plots of Bt

227

vs t were also used to ulteriorly identify whether film or particle diffusion was the

228

speed-limiting step[38]. The Plots of 𝑞𝑡 of Cu(II) and TC vs 𝑡1/2 were shown in

229

Fig.2a and Fig.2b, and plots of Bt vs t were displayed in Fig.2c and Fig.2d. The values

230

of corresponding Kid and C were listed in Table 1.

231

As displayed in Fig.2a and Fig.2b,the plots were divided into two or three linear

232

segments, which suggested different adsorption stages of Cu(II) and TC during

233

adsorption processes. The initial linear segments of Cu(II) and TC represented film

234

diffusion and the pursuant linear segments were related with intraparticle diffusion[41].

235

As shown in Table 1, relatively high r2 values of linear segments fitted from the data

236

of virgin PE microplastics substantiated that intra-particle diffusion played greater role

237

in the adsorption by virgin PE microplastics than biofilm-developed microplastics.

238

Besides, compared with virgin microplastics, the larger intercept of linear segments

239

from the adsorption data of biofilm-developed microplastics also elucidated the surface

240

adsorption, rather than intraparticle diffusion, could dominate the pollutant adsorption

241

by biofilm-developed microplastics [38, 41].

242

To further identify the speed-limiting step during adsorption, Linearity test of the

243

film diffusion model was utilized. If the plots were linear and passed through origin,

244

the speed-limiting step was intraparticle diffusion, otherwise it was film diffusion [18,

245

38]. As shown in Fig.2c and Fig.2d, plots from virgin PE microplastics appeared linear 11

246

and close to origin, which illustrated that intraparticle diffusion could be involved

247

during the adsorption by virgin PE microplastics. However, plots from biofilm-

248

developed microplastics were not linear or across origin, suggesting adsorption by

249

biofilm-developed microplastics could be governed by film diffusion.

250

------Fig. 2------

251

------Table 1------

252 253

3.3 Adsorption isotherms

254

Adsorption isotherms of TC and Cu(II) by PE microplastics with and without biofilm

255

were shown in Fig.3a and Fig.3b. The adsorption amount of TC and Cu(II) increased

256

with their equilibrium concentrations. As shown in Fig.3a and Table 2, the amount of

257

TC adsorbed followed the order of Bio-PE+0.5 mg/L Cu (qmax=950.1 μg/g) > PE+0.5

258

mg/L Cu (qmax=911.2 μg/g) > Bio-PE+0 mg/L Cu (qmax=762.2 μg/g) > PE+0 mg/L Cu

259

(qmax=237.5 μg/g), which suggested that the existence of Cu(II) and the development

260

of biofilm on PE microplastics enhanced TC adsorption. The synergistic adsorption of

261

TC and Cu(II) could be due to the formation of ion bridge [33]. In addition, the positive

262

effects of biofilm on the metal adsorption were also reported by Johansena et al.[17].

263

However, in Fig.3b the presence of TC was not conductive to Cu(II) adsorption onto

264

biofilm-developed microplastics. Moreover, despite of the higher Cu(II) adsorption

265

amount by biofilm-developed microplastics than virgin microplastics at high Cu(II)

266

concentration, at low Cu(II) concentration Cu(II) was even released from biofilm-

267

developed microplastics. These phenomena implied that there were Cu(II) pre-adsorbed 12

268

in biofilm, which was also confirmed by detecting Cu(II) in the biofilms. It was found

269

that 256.1 μg Cu(II) in biofilms were detected in 1 g biofilm-developed microplastics.

270

Besides, TC could compete for Cu(II) to release Cu(II) from biofilm. Similar

271

competition effects of TC were also reported by Parolo et al.[42].

272

These assumption could be additionally verified by investigating Kd of TC and Cu(II)

273

in Fig.3c and Fig.3d. The higher Kd values suggested the higher affinity of adsorbents

274

for adsorbates. In Fig.3c, Kd of TC adsorption by biofilm-developed microplastics was

275

more invariable than that by virgin microplastics, which indicated that the enhanced TC

276

adsorption by biofilm-developed microplastics was mainly contributed by Cu(II) in

277

biofilm rather than in solution. Besides, decreased Kd of Cu(II) with the increase of

278

initial TC concentration demonstrated the increasing competition between TC and

279

biofilm for Cu(II).

280

The parameters obtained from the Freundlich and Langmuir model were

281

summarized in Table 2. Both the Freundlich model and the Langmuir model well fitted

282

adsorption isotherms, but the Freundlich model was more suitable to describe

283

adsorption isotherms, as revealed by its higher r2 values. According to the deviation of

284

n values from 1, both Cu(II) and TC adsorption was nonlinear, which suggested that

285

the adsorption sites could heterogeneously distribute on microplastics’ surface[43], and

286

the adsorption could not only related with hydrophobic interactions but also other

287

mechanisms such as the electrostatic interactions [21] and pore-filling mechanisms[18].

288

In addition, compared with virgin microplastics higher Kf values of biofilm-developed

289

microplastics substantiated the high affinity of biofilm-developed microplastics for 13

290

adsorbates.

291

------Fig. 3------

292

------Table 2------

293

3.4 Cu(II) and TC desorption from PE microplastics

294

Desorption isotherms of TC and Cu(II) from microplastics with and without biofilm

295

were shown in Fig.4. Thermodynamic irreversibility index (TII) was used to estimate

296

the reversibility of Cu(II) and TC adsorption. TII=0 indicated that the adsorption

297

process was fully reversible, and TII=1 indicated that the adsorption process was fully

298

irreversible. Considering the release of Cu(II) from biofilm-developed microplastics at

299

low Cu(II) equilibrium concentration, the point with lowest Cu(II) or TC concentration

300

in isotherms was regarded as the origin. The detailed parameters of adsorption and

301

desorption isotherms were shown in Table S2, and the corresponding TII value was

302

also exhibited in Fig.4.

303

As shown in Fig.4, the adsorption of Cu(II) and TC in the binary system was more

304

irreversible than that in the sole system, which suggested that Cu-TC complex species

305

were more difficult to be desorbed from microplastics, and the affinity of microplastics

306

for Cu-TC complex was more stronger than their individual. Moreover, the TII value

307

of biofilm-developed microplastics was obviously higher than virgin microplastics,

308

which suggested that Cu(II) and TC adsorbed on biofilm-developed microplastics were

309

more stabilized than that adsorbed on the virgin microplastics. It could be due to the

310

strong interactions between the components in biofilm and adsorbates[44]. Lissalde et

311

al. [45] also reported that biofilm coated on the surface of polymer could impede mass 14

312

from transferring out of the polymer. In addition, as exhibited in Fig.4, although Cu(II)

313

and TC were more stable on biofilm-developed microplastics, the amount of Cu(II) and

314

TC desorbed from biofilm-developed microplastics was not less than that from virgin

315

microplastics because of the high adsorption capacities of biofilm-developed

316

microplastics for Cu(II) and TC. ------Fig. 4------

317 318

3.5 Effects of pH

319

The pH effects on the zeta potential of microplastics and the adsorption of Cu(II)

320

and TC by PE microplastics with and without biofilm as well as corresponding

321

distributions of TC and Cu species were shown in Fig.5. In the sole and binary system,

322

TC adsorption by virgin microplastics peaked at pH 5.8 (Fig.5a, Fig.5b), which could

323

be explained by electrostatic interactions. The effects of pH on the zeta potential of

324

virgin microplastics and the charge of TC species are consistent with each other. With

325

the solution pH increasing from 3 to 9, the majority of TC changed from a cation

326

(H3TC+) to an anion (HTC-) or a zwitterion (CuTC) (Fig.5a and Fig.5b), and the zeta

327

potential of virgin microplastics changed from 11.3 to -29.2 (Fig.5e). Thus, the

328

electrostatic repulsion restricted TC adsorption by virgin PE microplastics at acidic

329

and alkaline conditions. However, TC adsorption by biofilm-developed microplastics

330

in the sole and binary system did not exhibit the similar tendency, which could be

331

attributed to the complexation effects of biofilm. Zhang et al. [18] found that after

332

weathering more carboxyl as well as ester carbonyl existed on the surface of

333

polystyrene microplastics to increase oxytetracycline adsorption by H-bonding 15

334

attraction. Thus, we speculated that the interactions between TC, Cu(II) in solution

335

and biofilm as well as other components in biofilm could explain why TC adsorption

336

by biofilm-developed microplastics did not decrease at high pH. In addition,

337

competition effects of H+ could lead to low TC adsorption amount[34]. Therefore, the

338

interactions between Cu(II), TC and the components in biofilm could dominate TC

339

adsorption by biofilm-developed microplastics.

340

In Fig.5c, Cu(II) adsorption by virgin and biofilm-developed microplastics in the

341

sole system increased with increasing equilibrium pH attributed to the formation of

342

hydroxides phase of copper[34]. Besides, at acidic conditions low Cu(II) adsorption

343

amount on microplastics could be due to the electrostatic repulsion and H+

344

competition effects. In the binary system, Cu(II) adsorption by virgin microplastics

345

peaked at around pH 5.8 because at 5
346

Cu(II) species and virgin microplastics were higher than other pH range, in which TC

347

existed as a kation (CuHTC+) and virgin microplastics were electronegative (Fig.5e).

348

In the Cu(II) adsorption by biofilm-developed microplastics, Cu(II) was released from

349

biofilm at pH<5 which was attributed to the competition effects of H+ and less

350

generation of copper complex. At pH>5 Cu(II) adsorption was high, which could

351

result from less competition effects of H+ and TC (Fig.5b) as well as more copper

352

complex formation (Fig.5d).

353 354 355

------Fig. 5-----3.6 Synergetic and competitive adsorption by PE microplastics To further illustrated synergetic and competitive adsorption of Cu(II) and TC by 16

356

PE microplastics and how competition and complexation interactions affected Cu(II)

357

and TC adsorption by microplastics, the adsorbates were sequentially adsorbed by

358

microplastics. Adsorption isotherms of TC and Cu(II) onto virgin PE microplastics

359

with different addition orders were shown in Fig.6. Adsorption isotherms were fitted

360

by the Langmuir model and the corresponding parameters were exhibited in Table

361

S3. The adsorption amount of Cu(II) and TC on virgin PE microplastics was changed

362

with different addition orders. This phenomenon suggested that Cu(II) and TC

363

adsorption by virgin microplastics simultaneously involved with the complexation

364

and competition interactions. To further illustrate the effects of the complexation and

365

competition during Cu(II) and TC adsorption, these effects were estimated by using

366

𝑞𝑚𝑎𝑥 in the Langmuir model and described as follows: [34, 46]. 𝑞1

367

i = 𝑞0

(7)

368

𝐸𝑐𝑜𝑚 = 𝑖𝑎&𝑏,𝑏 ―1

(8)

369

𝐸𝑝𝑟𝑒 = 𝑖𝑎 ― 𝑏,𝑏 ― 𝑖𝑎&𝑏,𝑏

(9)

370

𝐸𝑟𝑒𝑝 = 𝑖𝑏 ― 𝑎,𝑏 ― 𝑖𝑎&𝑏,𝑏

(10)

371

Where i was the effect ratio; 𝑞1 was the adsorption amount of Cu(II) or TC with

372

different addition orders in the binary system; 𝑞0was was the adsorption amount of

373

Cu(II) or TC in the sole system; 𝒂 and 𝒃 represented two kinds of adsorbates (Cu(II)

374

and TC); 𝐸𝑐𝑜𝑚 represented comprehensive effects of 𝒂 on 𝒃 adsorption; 𝐸𝑝𝑟𝑒

375

represented the effects of 𝒂 pre-adsorption on 𝒃 adsorption; 𝐸𝑟𝑒𝑝 represented the

376

effects of 𝒃 replacement by 𝒂; 𝑖𝑎&𝑏,𝑏 represented effect ratio of 𝒃 with

377

simultaneous Cu(II) and TC addition; 𝑖𝑎 ― 𝑏,𝑏 represented effect ratio of 𝒃 with 17

378

sequential addition of 𝒂 and 𝒃; 𝑖𝑏 ― 𝑎,𝑏 represented effect ratio of 𝒃 with sequential

379

addition of 𝒃 and 𝒂.

380

The negative pre-adsorption effects (𝐸𝑝𝑟𝑒 = ―1.36) of Cu(II) on TC adsorption

381

and the void replacement effects of TC (𝐸𝑟𝑒𝑝 = 0.00) on Cu(II) adsorption elucidated

382

that little Cu(II) adsorbed on the virgin microplastics could be replaced by TC.

383

Moreover, replacement effects of Cu(II) (𝐸𝑟𝑒𝑝 = 2.49) and pre-adsorption effects of

384

TC (𝐸𝑝𝑟𝑒 = 0.26) were positive, which suggested that the subsequent addition of Cu(II)

385

enabled more adsorption sites to be adsorbable for TC by complexation. In addition,

386

the large difference between comprehensive effects and the other effects also

387

suggested that electrostatic adsorption by complexation of Cu(II) and TC in solution

388

could play a major role during Cu(II) and TC adsorption by virgin microplastics.

389

To obtain a clearer understanding of Cu(II) and TC adsorption mechanism on

390

microplastics shrouded with biofilm, adsorbates were successively adsorbed by

391

sewage sludge, and the results were shown in Fig.6. In Fig.6c and Fig.6e, the TC

392

effect ratio with ordinal addition of Cu(II) and TC was the highest within three

393

treatments, and the pre-adsorption effects of Cu(II) was positive, which illustrated that

394

the pre-adsorbed Cu(II) could contribute to the adsorption of TC. In Fig.bd and Fig.6f,

395

Cu(II) adsorption amount was the lowest with successive addition of Cu(II) and TC,

396

and it was mainly restricted by subsequent addition of TC according to the lowest

397

value of TC replacement effects within three effects, which suggested that TC

398

subsequently added into solution might compete with biofilm for Cu(II). Besides, the

399

value of pre-adsorption effects of TC was also negative because of the complexation 18

400

of TC and Cu(II) in the biofilm. And then, TC adsorbed by biofilm-developed

401

microplastics would show little ability to complex with Cu(II) in solution. Moreover,

402

the pre-adsorbed TC could also compete with Cu(II) in solution for adsorption sites

403

leading less Cu(II) adsorption[47]. In addition, with the increase of TC concentration,

404

the absolute value of pre-adsorption effects and replacement effects decreased and the

405

absolute value of comprehensive effects increased, which suggested that at high TC

406

concentration Cu(II) and TC complexation in solution might hinder Cu(II) adsorption.

407

Thus, at low TC concentration the adsorption of Cu(II) on biofilm-developed

408

microplastics was mainly decreased by replacement effects of TC, while at high TC

409

concentration Cu(II) adsorption was decreased by complexation interactions. ------Fig. 6------

410 411

3.7 Adsorption mechanism

412

The SEM-EDS elemental mapping of phosphorus and copper on the surface of

413

microplastics with and without biofilm was shown in Fig.7. Phosphorus, an essential

414

element for microbial growth[48], could reflect the abundance of microorganism on

415

microplastics to some extent. Compared with virgin microplastics, P and Cu(II)

416

heterogeneously distributed on biofilm-developed microplastics and located in the

417

similar regions, which accorded with the cumulation of contaminants in deposits

418

biologically facilitated[17], and manifested that Cu(II) adsorption by biofilm-

419

developed microplastics was closely related with biofilm.

420

In Fig.7F, FT-IR was conducted to investigate the interactions between Cu(II), TC

421

and microplastics. The obvious peaks at 723 cm-1, 1466cm-1, 2850cm-1 and 2919cm-1 19

422

were contributed by carbon chain of PE microplastics and attributed to rocking

423

deformation, bending deformation, CH2 symmetric stretching and CH2 symmetric

424

stretching, respectively[49]. Peaks at range from 1400cm-1 to 1700cm-1 could

425

characterize lots of functional groups. Compared with virgin microplastics, more

426

peaks were shown in biofilm-developed microplastics, which could be attributed to

427

the functional groups such as benzene ring (1618cm-1) [50], C=N (1681cm-1) [51] and

428

aliphatic ketone (1627cm-1) [52, 53] contained in biofilm. These functional groups

429

could be attributed to the extracellular polymeric substance (EPS) and HOCs adsorbed

430

by biofilm-developed microplastics [44, 54, 55]. The peak at 1514cm-1 was attributed

431

to amid II in TC on ring D[56, 57]. The peak at 1586cm-1 was attributed to the C=O

432

stretching in TC on ring B[58], and the peak at 1652cm-1 was attributed to the

433

stretching of carboxide in the amide in TC on ring D as well as the benzene ring[59-

434

61]. The product of TC adsorption by microplastics with and without biofilm

435

exhibited these peaks, which suggested that TC was adsorbed by microplastics.

436

However, with the addition of Cu(II) the disappearance of the peak at 1586cm-1

437

indicated that Cu(II) could be complexed with the carboxide on ring B. In Fig.7E, a

438

red shift generated in the UV-vis spectra of TC solution after the addition of Cu(II)

439

also confirmed the complexation of Cu(II) and TC[33]. Moreover, compared with

440

biofilm-developed microplastics, there are more peaks appeared in the product of

441

Cu(II) adsorption by biofilm-developed microplastics, which elucidated that Cu(II)

442

could not only be complexed with TC, but also interact with the components in biofilm.

443

------Fig. 7-----20

444

GC-MS was used to identify the potential low-boiling components in biofilm.

445

These components were shown in Table 3. Similar compounds in biofilm were widely

446

reported[44, 62]. 1,2-Benzenedicarboxylic acid, 1,2-dimethyl ester (DMP) and 2,4-

447

Dimethyl-7,8-benzoquinoline as well as 1,2-Benzenedicarboxylic acid, 1-(2-

448

ethylhexyl) ester were the major components detected. Electron-donating groups,

449

such as amino, oxhydryl and carbonyl, were contained in these components, which

450

were known to be able to be complexed with Cu(II)[63-66]. To further investigate the

451

complexation reaction between these components and Cu(II) as well as the ion bridge

452

effects of Cu(II), the charge distribution of atoms and surface electrostatic potential

453

with the lowest energy level of these components and complexes were calculated by

454

Gaussian 09 program. The charge distribution and surface electrostatic potential of

455

these components were shown in Fig.8, and the detailed information of the charge

456

distribution of each atom in these components was listed in Table S4. In the DMP

457

molecule the charge of No.12 O atom and No.19 O atom was -0.498, -0.485

458

respectively, and the electron density around No.12 O atom and No.19 O atom was

459

higher than other atoms (Fig.8a), which suggested that No.12 O atom and No.19 O

460

atom could be the binding sites with Cu(II). Likewise, No.21 N atom was the most

461

negative charged atom in 2,4-Dimethyl-7,8-benzoquinoline with the highest electron

462

density, and the charge of No.41 O atom in 1,2-Benzenedicarboxylic acid, 1-(2-

463

ethylhexyl) ester was -0.551. Thus, No.21 N atom and No.41 O atom (Fig.8b and

464

Fig.8c)

465

Benzenedicarboxylic acid, 1-(2-ethylhexyl) ester could be the possible binding sites

respectively

in

2,4-Dimethyl-7,8-benzoquinoline

and

1,2-

21

466

with Cu(II). Furthermore, in our previous studies Cu(II) could be complexed with TC

467

via O atom at ring B and ring C[34]. Thus, Cu(II) could be the ion bridge between

468

these components and TC by complexation. The potential complexing way of Cu(II),

469

TC and 1,2-Benzenedicarboxylic acid, 1,2-dimethyl ester with most energetically

470

stable configuration was shown in Fig.8d. To minimize the electrostatic repulsion, the

471

benzene ring in TC should be parallel to Cu atom, and the benzene ring in DMP and

472

Cu atom should lie in one plane [67]. In addition, TC could be adsorbed by biofilm

473

through π-π interactions[68].

474

------Table 3------

475

------Fig. 8------

476

Thus, according to the above results, the adsorption mechanisms of Cu(II) and TC

477

by microplastics with and without biofilm were shown in Fig.9 and summarized as

478

follows. Contributed to the development of biofilm on microplastics, physical and

479

chemical properties of microplastics were changed (Fig.1 and Fig.7), and more Cu(II)

480

and TC were adsorbed by biofilm-developed microplastics (Fig.3) through

481

complexation with the components in biofilm (Fig.5 and Fig.8). Cu(II) and TC were

482

adsorbed by virgin PE microplastics through intra-particle diffusion, while they were

483

adsorbed by biofilm-developed microplastics through film diffusion (Fig.2).

484

Synergetic Cu(II) and TC adsorption by virgin microplastics (Fig.3) resulted from

485

complexation of Cu(II) and TC as well as ion bridge effects (Fig.6) by electrostatic

486

interactions (Fig.5). Compared with virgin microplastics, more TC was adsorbed by

487

biofilm-developed microplastics attributed to Cu(II) pre-adsorbed by biofilm. 22

488

However, TC decreased Cu(II) adsorption on biofilm-developed microplastics (Fig.3)

489

because of the competition effects (Fig.6). In addition, adsorbates were more difficult

490

to be desorbed from biofilm-developed microplastics than virgin microplastics, but

491

the amount of adsorbates desorbed from biofilm-developed microplastics was not less

492

than that from virgin microplastics because of the different adsorption capacities of

493

adsorbents (Fig.4). ------Fig. 9------

494 495

4. Conclusions

496

TC and Cu(II) adsorption behaviors by virgin and biofilm-developed polyethylene

497

(PE) microplastics were investigated in this study. The physical and chemical surface

498

properties of PE microplastics were changed with the development of biofilm, which

499

resulted in different adsorption properties of the microplastics for Cu(II) and TC. With

500

the biofilm development on microplastics, the diffusion mechanism transformed from

501

intra-particle diffusion into film diffusion. Besides, biofilm enhanced the adsorption

502

and stabilization of Cu(II) and TC on microplastics, because compared with the

503

electrostatic affinity of virgin microplastics for adsorbates, the complexation

504

interactions between Cu(II), TC and the components in biofilm were more stronger.

505

Moreover, different from the synergetic adsorption of Cu(II) and TC on virgin

506

microplastics, Cu(II) could be released from biofilm because of competition effects of

507

TC. This study illustrated that biofilm could enhance the role of microplastics in the

508

Cu(II) and TC migration by changing their adsorption properties on microplastics.

509

Acknowledgements 23

510 511

We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21976136, 51678421, 41571301, 21777120).

512

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Astrophysics 416 (2004) 983-990.

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[53] J. Cervantes-Uc, J. Cauich-Rodríguez, H. Vázquez-Torres, A. Licea-Claveríe,

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TGA/FTIR study on thermal degradation of polymethacrylates containing

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carboxylic groups, Polym. Degradation Stab. 91 (2006) 3312-3321.

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exopolymers provide a mechanism for bioaccumulation of contaminants, MicEc

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[55] J.V. Headley, J. Gandrass, J. Kuballa, K.M. Peru, Y. Gong, Rates of sorption and

674

partitioning of contaminants in river biofilm, Environ. Sci. Technol. 32 (1998)

675

3968-3973.

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[56] S. Chahardahmasoumi, M.N. Sarvi, S.A.H. Jalali, Modified montmorillonite

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nanosheets as a nanocarrier with smart pH-responsive control on the antimicrobial

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activity of tetracycline upon release, Applied Clay Science 178 (2019) 105135.

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[57] P. Kulshrestha, R.F. Giese, D.S. Aga, Investigating the molecular interactions of

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oxytetracycline in clay and organic matter: insights on factors affecting its

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mobility in soil, Environ. Sci. Technol. 38 (2004) 4097-4105.

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[58] P.-H. Chang, J.-S. Jean, W.-T. Jiang, Z. Li, Mechanism of tetracycline sorption on rectorite, Colloids Surf. Physicochem. Eng. Aspects 339 (2009) 94-99.

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investigation on the complexation of tetracycline with dissolved organic matter 31

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derived from algae and macrophyte, Chemosphere 187 (2017) 421-429.

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dissolved organic matter: insights from multi-spectroscopic techniques,

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Chemosphere 155 (2016) 225-233.

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the evolution, redox, and metal binding properties of dissolved organic matter

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from municipal solid wastes using two-dimensional correlation spectroscopy,

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Chemosphere 117 (2014) 701-707.

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endocrine-disrupting chemicals on natural sediments: role of biofilm covered on

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surface, Environ. Sci. Pollut. Res. 22 (2015) 1380-1388.

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[63] X. Li, M. Bera, G.T. Musie, D.R. Powell, Copper (II) promoted imidazolidine ring

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formation and complexation: A unique reaction course, Inorg. Chim. Acta 361

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hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES buffer), J. Inorg.

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Biochem. 99 (2005) 1653-1660.

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W.-H. Chen, Synthesis, characterization and potent DNA-cleaving activity of

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copper (II)-complexed berberine carboxylate, Bioorg. Med. Chem. Lett. 22 (2012)

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(II) selective sensor based on dimethyl 4, 4′(o-phenylene) bis (3-thioallophanate)

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714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 33

730 731

Fig. 1:

732 733

Fig.1. Microscope images of PE microplastics (a); N2 adsorption isotherms and

734

corresponding BJH pore size distribution of PE microplastics with and without biofilm

735

(b); SEM of PE microplastics (c) and biofilm-developed PE microplastics (d); XRD (e)

736

of PE microplastics with and without biofilm. 34

737 738

Fig. 2:

739 740

Fig.2. Intraparticle diffusion plots for TC (a) and Cu(II) (b) adsorption in the sole and

741

binary systems as well as plots of Bt vs time for TC (c) and Cu(II) (d) adsorption by PE

742

microplastics with and without biofilm.

743 744 745 746 747 748 35

749 750 751

Table 1:

752

Table 1. Parameters of intraparticle diffusion for adsorption of Cu(II) and TC by

753

PE microplastics with and without biofilm. Cu(II) adsorption Parameters

PE

TC adsorption

Biofilm+PE

PE

Biofilm+PE

0

5

0

5

0 mg/L

0.25

0 mg/L

0.25

mg/L

mg/L

mg/L

mg/L

Cu(II)

mg/L

Cu(II)

mg/L

TC

TC

TC

TC

Kid,1 (μg g-1 t1/2)

6.96

5.04

9.07

5.85

29.35

50.68

40.82

149.53

C1 (μg g-1)

-2.90

-2.71

4.41

0.23

-13.61

-9.01

-17.37

-71.37

R2

0.98

0.99

0.96

0.97

0.99

0.99

0.99

0.97

Kid,2 (μg g-1 t1/2)

0.64

0.94

0.50

0.45

18.11

19.27

58.28

52.94

C2 (μg g-1)

25.13

25.99

23.02

17.92

94.58

198.39

-13.10

206.31

R2

0.80

0.99

0.77

0.71

0.97

0.95

0.99

0.98

Kid,3 (μg g-1 t1/2)

10.04

-4.73

C3 (μg g-1)

247.20

517.72

R2

0.41

0.98

Cu(II)

Cu(II)

754 755 756 36

757 758 759 760

Fig. 3:

761 762

Fig.3. Adsorption isotherms of TC (a) and Cu(II) (b) onto virgin and biofilm-developed

763

PE microplastics as well as Kd of TC (c) and Cu(II) (d) with different initial

764

concentration of Cu(II) and TC.

765 766 767 768 37

769 770 771 772 773

Table 2:

774

Table 2. Parameters obtained from adsorption isotherms for adsorption of TC and Cu(II)

775

on PE microplastics with and without biofilm. Sorb

Initial

Freundlich

ent

concentration

n

(mg L-1)

Kf((μg 1)(mg

Langmuir g- r2 L-1)-

qmax(μ

KL(L

g/g)

mg-1)

r2

n)

Bio-

TC=0

PE PE

TC=0

Cu=0

Cu=0

1.57

92.4

0.986

762.2

0.115

0.977

Cu=0.5

1.58

173.2

0.982

950.1

0.230

0.943

Cu=0

1.49

27.1

0.990

237.5

0.126

0.989

Cu=0.5

1.54

138.2

0.987

911.2

0.156

0.985

TC=0

3.90

19.5

0.973

30.8

1.97

0.968

TC=5

0.987

15.6

0.969

1205

0.0135

0.962

776 777 778 779 780 38

781 782 783 784 785 786

Fig. 4:

787 788

Fig.4. Adsorption and desorption isotherms of TC and Cu(II) onto PE microplastics

789

with and without biofilm as well as corresponding TII.

790 791 792 793 794 795 796 797 798 39

799 800

Fig. 5:

801 802

Fig.5. Effect of pH on adsorption of TC (a, b) and Cu(II) (c, d) and the corresponding

803

distribution of TC and Cu species in the sole (a, c) and binary (b, d) systems as well as

804

Zeta potential with 0.01M NaNO3(e) of PE microplastics with and without biofilm.

805 40

806

Fig. 6:

807 808

Fig.6. Adsorption isotherms of TC (a) and Cu(II) (b) onto virgin PE microplastics and

809

the effect ratio of TC (c) and Cu(II) (d) adsorption by biofilm-developed microplastics

810

as well as different effects during TC (e) and Cu(II) (f) adsorption by biofilm-developed

811

microplastics. TC alone represents only TC was added into solution; TC&Cu represents

812

Cu(II) and TC were synchronously added into solution; Cu-TC represents Cu(II) was 41

813

added into the solution in advance, and after equilibrium TC was added into the solution;

814

TC-Cu represents TC was added into the solution in advance.

815 816

Fig. 7:

817 818

Fig.7 SEM-EDS elemental mapping of phosphorus on the surface of virgin

819

microplastics (A), biofilm-developed microplastics (B) and copper on the surface of the 42

820

product of Cu(II) adsorption by virgin microplastics (C), biofilm-developed

821

microplastics (D); UV-vis spectra of the solution of TC with and without Cu(II) (E);

822

FT-IR (F) of virgin microplastics (a), biofilm-developed microplastics (b), product of

823

TC adsorption by virgin microplastics (c), product of TC adsorption by biofilm-

824

developed microplastics (d), product of TC and Cu(II) adsorption by virgin

825

microplastics (e), product of Cu(II) adsorption by biofilm-developed microplastics (f).

826 827

Table 3:

828

Table 3 GC-MS analysis of biofilm. The components present at greater than 2.5% were

829

indicated by bold font. No.

SIa

RTb

Compound

Composition

Identification

(%)c

methodd

1

62

2.741

6-Chloro-5-nitrouracil

0.8

EI-MS

2

67

3.371

1,7-Diamino-4-azaheptane

0.34

EI-MS

3

71

5.451

Cyanoacetamide

0.34

EI-MS

4

77

7.173

4-Guanidinobutanoic acid

0.31

EI-MS

5

65

7.572

1,3-Isobenzofurandione

0.99

EI-MS

6

95

8.434

1,2-Benzenedicarboxylic

52.43

EI-MS

acid, 1,2-dimethyl ester 7

72

8.784

Methyl 2-propionylbenzoate

1.77

EI-MS

8

68

9.785

2,4-Dimethylamphetamine

0.43

EI-MS

9

79

9.911

2-Methylpiperazine

0.73

EI-MS 43

10

60

10.163

Hydrazinecarboximidamide,

0.82

EI-MS

1.22

EI-MS

0.70

EI-MS

32.39

EI-MS

0.66

EI-MS

2-[(2-nitrophenyl) methylene]11

63

10.758

1-[.alpha.-(1Adamantyl)benzylidene]thi osemicarbazide

12

78

10.828

2-(1-Oxopropyl)benzoic acid

13

66

12.404

2,4-Dimethyl-7,8benzoquinoline

14

72

13.832

3,3-Diethoxy-1,1,1,5,5,5hexamethyltrisiloxane

15

43

13.986

2-Phenyl-5-methylindole

0.30

EI-MS

16

61

14.035

2,4-Dimethyl-7,8-

0.49

EI-MS

0.36

EI-MS

4.50

EI-MS

benzoquinoline 17

60

14.084

1,1,1,3,3,5,5,7,7,7Decamethyltetrasiloxane

18

72

14.196

1,2-Benzenedicarboxylic acid, 1-(2-ethylhexyl) ester

830

a

SI: Library search purity value.

831

b

RT: Retention time.

832

c

Percentage based on FID peak area normalization. 44

833 834

d

Identification based on computer matching of electron ionization mass spectra

using Wiley and NIST libraries for the GC-MS system.

835 836

Fig.8:

837 838

Fig.8. The charge distribution and surface electrostatic potential of 1,2-

839

Benzenedicarboxylic acid, 1,2-dimethyl ester (a), 2,4-Dimethyl-7,8-benzoquinoline(b)

840

and 1,2-Benzenedicarboxylic acid, 1-(2-ethylhexyl) ester (c) as well as the potential

841

complexing way of Cu(II), TC and 1,2-Benzenedicarboxylic acid, 1,2-dimethyl ester (d)

842

with most energetically stable configuration using GaussView 5.0.

843 844 45

845

Fig.9:

846

847 848

Fig.9. The adsorption mechanisms of Cu(II) and TC by microplastics with and without

849

biofilm.

850 851 46

852

47

853

854 48

855

856 49

857

50

858

51

859

52

860 861

Highlights

862



Co-adsorption of Cu(II) and TC on PE microplastics with biofilm was first studied

863



Biofilm enhanced the adsorption and stabilization of Cu(II) and TC on microplastics

864 865 866



Competition and complexation interactions were involved in the adsorption process 53

867



Microplastics with biofilm act as carriers of heavy metals and antibiotics in water

868 869 870

Declaration of interests

871 872 873

☒ 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.

874 875 876

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

877 878 879 880 881

54