A Simple Method to Quantify PC and PET Microplastics in the

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A Simple Method to Quantify PC and PET Microplastics in the Environmental Samples by LC-MS/MS Lei Wang, Junjie Zhang, Shaogang Hou, and Hongwen Sun Environ. Sci. Technol. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.estlett.7b00454 • Publication Date (Web): 02 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Environmental Science & Technology Letters

1

A Simple Method to Quantify PC and PET Microplastics in the

2

Environmental Samples by LC-MS/MS

3 4

Lei Wang*, Junjie Zhang, Shaogang Hou, and Hongwen Sun

5 6

Ministry of Education Key Laboratory of Pollution Processes and Environmental

7

Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control,

8

College of Environmental Science and Engineering, Nankai University, Tianjin

9

300350, China

10

*

11

Nankai University

12

#38 Tongyan Road

13

Tianjin, China 300071

14

Phone: +86-22-2350-4262

15

Fax: +86-22-2350-8807

16

E-mail:[email protected]

17

For submission to: Environmental Science & Technology Letters

Corresponding author: L. Wang

1

ACS Paragon Plus Environment


18

Abstract

19

Occurrence of microplastics (MPs) in the environments have been frequently reported.

20

However, studies on the quantification methods for MPs are still needed. Plastics are

21

polymers of different degrees of polymerization. In this study, alkali assisted thermal

22

hydrolysis was applied to depolymerize two plastics containing ester groups,

23

polycarbonate (PC) and polyethylene terephthalate (PET), in pentanol or butanol

24

system. By determining the concentrations of the depolymerized building block

25

compounds, i.e. bisphenol A (BPA) and para-phthalic acid (PTA), the amounts of PC

26

and PET MPs in the environmental samples were quantified. Recoveries of 87.2-97.1%

27

were obtained for the PC and PET plastics particles spiked in the landfill sludge. The

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method was successfully applied to determine the occurrence of PC and PET MPs in

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the samples of sludge, marine sediments, indoor dust, digestive residues in mussel and

30

clam, as well as in sea salt and rock salt. High concentrations of 246 and 430 mg/kg

31

were determined for PC and PET type MP in an indoor dust, respectively. In addition,

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63.7 mg/kg of PC and 127 mg/kg of PET were detected in the digestive residues of a

33

clam.

2


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INTRODUCTION

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Microplastics (MPs) are members of the newest emerging contaminants in the

36

environments.1 Global plastics production was estimated to be 300 million metric tons

37

approximately,2 while around 10% of the produced plastics will enter the

38

environments and fragment into MPs eventually.3 Considering the low degradability

39

of most plastics, high levels of MPs might accumulate in the natural environments.4

40

Analysis methods for MPs mainly focus on their morphological and physical

41

characterization using different microscopy and spectroscopy instruments,5 while the

42

chemical characterization of MPs were identified by infrared and roman spectroscopy.

43

5,6

44

MPs from the samples, and might lead to great systematic errors.5 Chemical

45

identification of MPs is seldom performed with mass spectrometry. For example, a

46

pyrolysis-gas chromatography-mass spectrometry (Py-GCMS) method was used to

47

identify the separated MPs from marine sediments.7 Very recently, Py-GCMS was

48

applied to directly analyze the spiked MPs in fish.8 However, it failed to quantify MPs

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in the original fish samples, because of the small indicator signals for quantification.8

However, MPs are quantified mainly by counting, which need pre-separation of

50

Polycarbonate (PC) and polyethylene terephthalate (PET) are important plastics

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with estimated global annual production of 4.4 and 53.3 million tonnes,

52

respectively.9,10 Although the occurrence of PC and PET MPs in the environments has

53

been reported, 11-17 the detailed concentrations of these MPs in the environmental

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samples are still unknown. To quantify the PC and PET MPs in the environments, we

55

developed a direct quantification method by depolymerizing these plastics and 3



56

determining the emerging building block compounds.

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MATERIALS AND METHODS

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Chemicals and environmental samples.

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Standard of bisphenol A (BPA, ≥99%), para-phthalic acid (PTA, 99%) and

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bisphenol A diglycidyl ether (BADGE, ≥95%), as well as 1-pentanol (>99%) and

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1-butanol (99%) solvent were used. The PC and PET particles (0.075-0.15 mm) were

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provided by Yineng Plastic Co. (Dongguan, China).

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Landfill sludge was collected from a municipal sludge landfill in Tianjin, China.

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Two marine sediments were collected from Bohai Bay, with the site location

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information shown in Figure S1 in the Supporting Information. An indoor dust sample

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was collected from a student dorm by sweeping of floors. The digestive residues from

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two mussels and two clams purchased from a local market were also collected for the

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MPs quantification. Commercial crude sea salt and rock salt were randomly

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purchased from a supermarket. Environmental samples were stored at -20°C after

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lyophilization.

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Depolymerization of PC and PET MPs.

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Alkali assisted heating depolymerization method has been studied for plastic

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recycling.18 In this study, depolymerization of PC and PET was performed according

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to alkaline hydrolysis reactions (Figure 1a&b). In brief, PET or PC particles together

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with KOH was added into 1-pentanol solution. The system was heated at 135 °C for

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different time. Then the depolymerization products in pentanol solution were

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extracted by water. The water extracts were diluted 1×105 times, and 1.0 mL of the 4


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diluted extract was taken out and spiked with and 13C12-BPA and D4-PTA (10 ng for

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each) before the liquid chromatograph tandem mass spectrometer (LC-MS/MS)

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analysis of the newly formed BPA and PTA. Detailed depolymerization procedure was

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shown in Table S1. The depolymerization experiments were performed in triplicates.

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The control experiments indicated BPA and PTA were stable during the

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depolymerization process, while the procedure blank showed no detectable

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background of BPA and PTA.

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Depolymerization was also performed in the solvent of 1-butanol (Figure 1).

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Then the butanol solvent was removed by rotary evaporation, and the solutes were

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re-dissolved in water/acetonitrile (1:1 v/v) for LC-MS/MS analysis.

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Spiking and recovery of PC and PET MPs in the sludge.

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The recoveries of PC and PET MPs spiked in the landfill sludge were determined

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at three spiking levels, i.e. 10, 100, and 1000 mg/kg, with the detailed spiking

91

procedure shown in Table S1. After sufficient mixing, 1.0 g of the sludge containing

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1000 mg/kg of spiking MPs were weighted and transferred to the 1-pentanol

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depolymerization system as described above. At the optimized reaction time, the free

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form BPA and PTA were extracted twice by 30+20 mL of water. The water extract

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was diluted 100 times, and 1.0 mL of the diluted extract spiked with 10 ng of

96

13

97

shown in Table S1, before the LC-MS/MS analysis.

C12-BPA and 10 ng of D4-PTA were purified by solid phase extraction (SPE) as

98

For determining the concentrations of BPA and PTA in the depolymerization

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systems of samples with spiking MPs of 10 and 100 mg/kg, water extracts were 5



100

diluted 100 times and 10 times, respectively, before the SPE treatment. At each MPs

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spiking levels, six parallels of spiked sludge samples were analyzed to identify the

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error of determination. Six sludge samples without MPs spiked were analyzed as

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blank controls. Therefore, contribution of the spiked MPs to the newly formed BPA

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and PTA can be calculated. Accordingly, spiked PC and PET MPs amounts can be

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calculated, with the equations introduced below.

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Method application.

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To determine PC and PET MPs in the sludge sample, BPA and PTA in the

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original sludge were firstly detected in a control experiment. Sludge was treated in the

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similar procedure, only with the heating process cancelled. Therefore, the newly

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formed BPA and PTA from the depolymerization of PC and PET MPs can be

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calculated by subtracting the amount of BPA and PTA in the original sludge from the

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total BPA and PTA compounds in the depolymerized systems, and then the

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corresponding occurrence of PC and PET MPs can be quantified. Determination of

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PC and PET MPs in marine sediments, indoor dust and digestive residues from the

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biological samples was also carried out with the same procedure. It should be noted

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that, although BPA is also commonly used as the monomer of epoxy resin, 19 BADGE,

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the monomer of epoxy resin, is proved to be stable in the depolymerization system,

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while no emerging BPA was detected. Besides, in addition to PET, the newly formed

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PTA during depolymerization may also come from other terephthalate type plastics,

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such as polybutylene terephthalate (PBT) and polytrimethylene terephthalate plastics

121

(PTT). However, considering the much lower production of PBT and PTT and their 6


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similar structure and application with PET plastics,20 PET calculated in this study

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could represent the occurrence of all these three plastics.

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Two hundred grams of the sea salt or rock salt were weighted and dissolved in

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hot water, respectively. Then the insoluble matters were filtered by 0.22 µm nylon

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membrane filters, and dried to a constant weight at 48 oC for 24h. Then the insoluble

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matters were depolymerized in 1-butanol for 30 min. After the addition of 10 ng of

128

13

129

and the target compounds were re-dissolved in water/acetonitrile (1:1 v/v) and filtered

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by nylon membrane filter before analysis.

C12-BPA and 10 ng of D4-PTA, butanol solvent was removed by rotary evaporation,

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For each sample of sludge, marine sediments, dust, digestive residues and salts,

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two parallel samples were analyzed and the average values were calculated. It should

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be note that, the newly formed BPA and PTA during the depolymerization was

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assumed to be derived entirely from the depolymerization of PC and PET in the

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samples. This will be further discussed in detail below.

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Instrument analysis.

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Detailed information of chromatographic separation and tandem MS system to

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quantify BPA and PTA was shown in Table S2 and S3. The MRM chromatograms of

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BPA and PTA in the depolymerized sludge were shown in Figure S2, and their

140

concentrations were corrected by the corresponding internal standards, i.e. 13C12-BPA

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and D4-PTA, respectively.

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RESULTS AND DISCUSSION

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Depolymerization of PC and PET MPs in pentanol and butanol 7



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Both of PC and PET MPs can be readily depolymerized via hydrolysis within 30

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min in 1-pentanol or 1-butanol solution with potassium hydroxide. Visual

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disappearance of PC and PET particles in both of the 1-pentanol and 1-butanol

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solutions can be observed. Accordingly, formation of BPA and PTA can be determined.

148

For example, at 30 min, 0.42 g of BPA was detected in the 1-pentanol system initially

149

containing 0.5 g of PC MPs (Figure 1a). Based on the structural formula of PC

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polymer,

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approximately 89.5% (f(BPA-2H), w/w) in PC plastic, indicating a theoretical mass of

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0.45 g of emerging BPA might form from 0.5 g of depolymerized PC plastic.

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Therefore, >93% of depolymerization efficiency of PC plastic within 30 min can be

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verified.

i.e.

[-O-C6H4-C(CH3)2-C6H4-O-C(O)-]n,

(BPA-2H)

accounts

for

155

Similarly, 0.42 g of PTA formed at 30 min in the hydrolysis system containing

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1-pentanol and 0.5 g of PET plastic (Figure 1b). According to the PET polymer

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structure formula, i.e. [-C(O)-C6H4-C(O)-O-CH2-CH2-]n, (PTA-H2O) accounts for

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approximately 77.4% (f(PTA-H2O), w/w) in PET plastic. Therefore, 0.43 g of emerging

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PTA might form from 0.5 g of depolymerized PET. This demonstrated almost all of

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PTA (98%) depolymerized from PET plastic. Similar depolymerization efficiencies

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were also obtained in the solution of 1-butanol. Therefore, by determining the amount

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of hydrolysis products, i.e. BPA and PTA, quantification of PC and PET plastics can

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be conducted according to equation 1&2, where f(BPA-2H) and f(PTA-H2O) are mass

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percentage of (BPA-2H) and (PTA-H2O) in PC and PET plastics respectively, and

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MW represents molecular weight. 8


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Unheated control indicated that, heating is necessary for the depolymerization.

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At room temperature (25 oC), the plastic particles were insoluble in pentanol and

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butanol, with the mass changes of PC or PET particles of < 2%.

169

=

170

#$ =

171

( ) ! ( )× "( ) (& ( % % )× "(&

')

(1)

')

! &

(2)

Quality Assurance / Quality Control.

172

As mentioned above, a mass ratio of newly formed BPA (∆BPA=()*+,-. −

173

()*+,-. ) and the depolymerized PC MPs of 90% can be theoretically assumed,

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and the method recovery of PC MPs could be calculated as

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Similarly, recovery of PET MPs spiked in the sludge samples could be calculated by

176

the amount of newly formed PTA (∆PTA= $)*+,-. − $)*+,-. ) as

177

∆ % /9:% 56

∆ /23% 56

× 100%.

× 100%. Ideal recoveries of PC and PET MPs spiked in the sludge samples

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can be obtained for MPs at three levels in 1-pentanol. In sludge samples with 1000

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mg/kg of PC or PET MPs spiked, 843 mg/kg of BPA or 850 mg/kg of PTA can be

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determined in the depolymerized samples. In comparison, 14.1 mg/kg of BPA or 12.7

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mg/kg of PTA were detected in the background samples without spiked MPs. This

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indicates the concentrations of newly formed BPA and PTA of 829 and 837 mg/kg

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(Figure S3a), accounting for the recoveries of 92.0±1.1% and 97.1±2.1% for the

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spiked PC and PET MPs in the sludge (n=6) (Table S4). Using the same method, the

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newly formed BPA by depolymerization of the spiked PC MPs at 100 and 10 mg/kg in

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the sludge were determined to be 72.3 and 8.73 mg/kg, while the newly formed PTA

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were 78.9 and 7.56 mg/kg, respectively (Figure S3a). Therefore, the ideal recoveries 9



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of 87.2~92.0 % (for PC) and 87.9~97.1 % (for PET) can be obtained for the different

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spiking levels MPs in the sludge (Table S4). Depolymerized BPA and PTA were also

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detected in the 1-butanol depolymerization systems, but with greater deviation values,

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especially for the spiked MPs at 100 and 10 mg/kg (Figure S3b). Detection recoveries

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of PC and PET particles were lower if the 1-butanol depolymerization and rotary

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evaporation concentration process were applied (Table S4). This was due to the more

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significant matrix interference effects of this method, since no purification treatment

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was included in the butanol depolymerization-rotary evaporation process. In

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comparison, both of the water extraction and SPE used in the1-pentanol

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depolymerization method could purify the samples efficiently.

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Instrumental calibration was verified by the injection of a 10-point calibration

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standard ranging in concentrations from 0.1 to 20 µg/L for BPA and 0.5 to 200 µg/L

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for PTA, with the regression coefficients (R) of the calibration curves ≥0.99.

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Accordingly, the linear ranges of this method for PC and PET MPs in sludge was

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accounted for 5.56~1110 and 29.1~11600 µg/kg, respectively. The limits of

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quantification for PC and PET MPs were calculated to be 8.32 and 53.0 µg/kg, based

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on the instrumental limits of quantification (S/N 10) and the recoveries of BPA and

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PTA.

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Determination of PC and PET MPs in environmental samples

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To determine PC and PET MPs in the environmental samples, the concentrations

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of original free form BPA and PTA should be deducted. In the unheated treatment,

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5.03 mg/kg of BPA and 2.41 mg/kg of PTA were determined in sludge sample. After 10


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these background monomers were deducted, PC MPs in the sludge was calculated to

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be 10.1 mg/kg, while PET was 12.0 mg/kg (Table 1), according to eq. 1&2,

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respectively. Although MPs in the marine sediments and Lagoon sediments were

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quantified by counting,

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reported until now. In this study, in the marine sediment A, 1.60 mg/kg of PC MPs

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and 10.3 mg/kg of PET MPs were detected (Table 1). Very high concentrations of PC

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(246 mg/kg) and PET MPs (430 mg/kg) were detected in the indoor dust sampled in

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Tianjin, indicating high exposure risk of these MPs in human through dust.

12,16,21

no detailed mass concentration of MPs has been

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Bivalves are commonly applied as indictors of environmental contaminants.22 PC

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MPs of 2.33 and 4.41 mg/kg were detected in the digestive residues from mussel A

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and B, while the concentrations for PET MPs were calculated to be 75.4 and 17.5

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mg/kg, respectively (Table 1). In addition, much higher PC and PET MPs were

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detected in the digestive residues from a clam, with concentrations of 63.7 and 127

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mg/kg, respectively.

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MPs can be detected in sea salts.14 In the two commercial salts, i.e. a sea salt and

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a rock salt, the solid insoluble matters were weighted to be 3280 and 490 mg/kg,

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respectively. These solid matters were depolymerized in 1-butanol. In the insoluble

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matters from sea salts, the PC and PET MPs were calculated to be 26.6 and 31.0

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mg/kg in the insoluble matters. Therefore, the concentrations of PC and PET MPs in

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the sea salts were 0.088 and 0.102 mg/kg, respectively (Table 1). By comparison,

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much less concentrations of MPs, 1×10-4 mg/kg for PC and 2×10-3 mg/kg for PET,

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were detected in the rock salts. 11



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In current method, separation of MPs from the samples is unnecessary, which

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increases the analyzing efficiency and avoids the MPs loss or error-picking caused by

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separation. Attention should also be paid to the specificity of the building block

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compound of MPs. For instance, for PC MPs containing other key building block

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compounds, e.g. bisphenol S (BPS), detection of only BPA will lead to an

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underestimation.

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depolymerization process was assumed to be derived entirely from the hydrolysis of

239

PC and PET MPs in the samples, respectively. Although BPA was also added into

240

some plastic materials as an antioxidant, it may affect little to the PC quantification.

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Firstly, contribution of BPA additive is much lower than that of the building block

242

BPA in PC. For example, BPA released from a PVC plastic was six orders of

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magnitude lower than that from the depolymerized PC plastic (Table S5). Secondly,

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most released BPA from the PVC particles can migrate in the non-depolymerization

245

treatment (Table S5), which can be deducted as “ ()*+,-. ” according to

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equation (1). Similarly, PTA is also added in PVC plastic as a plasticizer in the form

247

of lipids. However, emerging PTA released from PVC plastics during the

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depolymerization process was four to five orders of magnitude lower than that from

249

PET (Table S5), indicating limited contribution of PVC to detected PTA after the

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depolymerization process. Besides, it should be noted that the particle size

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distribution of MPs in the samples cannot be analyzed by this method. In addition, if

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polymers containing the same building block compound present and can be

253

depolymerized at the same condition, it will be difficult to distinguish the source of

Besides,

the

newly

formed

BPA and

12


PTA during

the

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the depolymerization products.

255

ASSOCIATED CONTENT

256

Supporting Information

257

Additional information as noted in the text. This material is available free of charge

258

via the internet at http://pubs.acs.org.

259

Detailed procedure of depolymerization, MPs spiking, and SPE (Table S1); HPLC

260

gradient elution programs for BPA and PTA (Table S2); MS parameters for the

261

analysis of BPA and PTA (Table S3); Recoveries of the spiked MPs in sludge samples

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(Table S4); Amounts of BPA and PTA (mg) released from 0.5 g of PVC a, PC or PET

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plastics before and after depolymerization (Table S5); Sampling location of marine

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sediment samples (Figure S1); Chromatograms of BPA and PTA in the extracts from

265

depolymerized sludge without/with PET and PC MPs spiked (Figure S2); The

266

detected concentrations of BPA and PTA depolymerized from spiked MPs at different

267

spiking levels in 1-pentanol or 1-butanol system. (Figure S3).

268

AUTHOR INFORMATION

269

*Corresponding Author

270

Phone:

271

[email protected]

272

ORCID

273

Lei Wang: 0000-0002-8193-9954

274

Junjie Zhang: 0000-0002-4383-1096

275

Shaogang Hou: 0000-0001-6192-9428

+86-22-2350-4262.

Fax:

+86-22-2350-8807.

13


E-mail:


Page 14 of 21

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Hongwen Sun: 0000-0003-1948-5047

277

Notes

278

The authors declare that there are no conflicts of interest.

279

ACKNOWLEDGEMENTS

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This work was supported by National Science Foundation for Natural Science of

281

China

282

(2017CB41722304), and 111 Program, Ministry of Education, China (T2017002).

283

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(19) Groshart, C.; Okkeman, P.; Pijnenburg, A. Chemical Study on Bisphenol A; RIKZ: Rijkswaterstaat, 2001. (20) Plastic Europe, 2016. Plastics – the Facts 2016: An analysis of European plasticsproduction, demand and waste data. 2016. (21) Liebezeit, G.; Dubaish, F. Microplastics in beaches of the East Frisian islands 16


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213-217.

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(22) Wang, L.; Sun, H.; Yang, L.; He, C.; Wu, W.; Sun, S. Liquid

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chromatography/mass spectrometry analysis of perfluoroalkyl carboxylic acids

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and perfluorooctanesulfonate in bivalve shells: Extraction method optimization.

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17



FIGURE LEGEND Figure 1. The depolymerization efficiencies of PC (a) and PET (b) plastics at different time in 1-pentanol and 1-butanol. The diagram represents average value of the triplicates, while the error bar represents SD.

18


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Figure 1

(a)

(b)

19



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Table 1. BPA, PTA and the calculated PC and PET MPs in the environmental samples (mg/kg) a BPAnon-depolym BPAdepolym PC MP Sludge

PTAnon-depolym PTAdepolym PET MP

5.03

14.1

10.1

2.41

12.7

12.0

Sediment A

0.0214

1.47

1.60

0.482

9.31

10.3

Sediment B

0.208

0.898

0.765

0.318

2.84

2.93

Indoor dust

0.770

222

246

217

587

430

Mussel digestive residues Ab

1.20

3.30

2.33

4.17

69.0

75.4

Mussel digestive residues Bb

0.234

4.20

4.41

4.31

19.4

17.5

Clam digestive residues Ab

0.740

58.1

63.7

7.04

116

127

Clam digestive residues Bb

0.659

4.91

4.72

14.8

19.8

5.81

SIMsc in sea salt

A Simple Method to Quantify PC and PET Microplastics in the

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