Organotin Release from Polyvinyl Chloride (PVC) Microplastics and

Subscriber access provided by Nottingham Trent University

Environmental Processes

Organotin Release from Polyvinyl Chloride (PVC) Microplastics and Concurrent Photodegradation in Water: Impacts from Salinity, Dissolved Organic Matter and Light Exposure Chunzhao Chen, Ling Chen, Ying Yao, Francisco Artigas, Qinghui Huang, and Wen Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03428 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

Environmental Science & Technology

1

Organotin Release from Polyvinyl Chloride (PVC) Microplastics

2

and Concurrent Photodegradation in Water: Impacts from Salinity,

3

Dissolved Organic Matter and Light Exposure

4 5

Chunzhao Chen,a,e Ling Chen,b,c Ying Yao,d Francisco Artigas,d Qinghui Huang,a,c Wen Zhang e*

6 7

a Key

Laboratory of Yangtze River Water Environment of the Ministry of Education, College of

8

Environmental Science and Engineering, Tongji University, Shanghai, China

9

b Shanghai

Institute of Pollution Control and Ecological Security, Shanghai, China.

10

c State

Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and

11

Engineering, Tongji University, Shanghai, China.

12

d

13

Environmental Research Institute, Lyndhurst, New Jersey, USA

14

e John

15

Technology, Newark, New Jersey, USA.

Rutgers University Newark, Department of Earth and Environmental Science, Meadowlands

A. Reif, Jr. Department of Civil and Environmental Engineering, New Jersey Institute of

16 17

Corresponding author:

18

Wen Zhang. Phone: +1- (973) 596-5520; Fax: (973) 596-5790; Email: [email protected]

1 ACS Paragon Plus Environment


19

Abstract

20

Photochemical weathering leads to degradation of microplastics and releases chemical

21

additives, polymeric fragments and/or byproducts. This study evaluated the release kinetics of

22

organotin compounds (OTCs) from three different sized (10-300 µm) polyvinyl chloride (PVC)

23

microplastics under UV and visible light irradiation. Four OTCs, dimethyltin (DMT),

24

monomethyltin (MMT), dibutyltin (DBT) and monobutyltin (MBT), were found to release from

25

PVC particles after 24-h leaching in darkness ranging from 2 to 20 µg·g-PVC-1. Under

26

UV/visible light irradiation, only DMT and DBT were detectable, whereas MMT and MBT were

27

not detected due to rapid photodegradation. The total tin concentrations (including organic and

28

inorganic tins) in the aqueous phase monotonically increased under light exposure. By contrast,

29

they reached plateaus after 24 h in darkness, confirming the photodegradation of OTCs. A

30

release kinetics model was established and correctly interpreted the microplastics size effect on

31

OTC release process. Finally, the impacts of salinity and dissolved organic matter (DOM) were

32

investigated. The release and photodegradation of OTCs were both inhibited at high salinity

33

conditions, probably due to the enhanced re-adsorption of OTCs on PVC microplastics and the

34

formation of halogen radicals that were less reactive towards neutral OTCs. The presence of

35

DOM, however, increased OTCs release probably because excited state triplet DOM (3DOM*)

36

formed and reacted with OTCs from PVC microplastics.

37 38

Keywords: Organotins; PVC microplastics; Photodegradation; Reactive radicals; Salinity; DOM;

39 40 41


Page 2 of 30

Page 3 of 30

42


TOC

43 44



46

Page 4 of 30

1. Introduction

47

Weathering or aging of microplastics leads to the release of a variety of chemicals, additives,

48

polymeric fragments and ultrafine particles (e.g., nanoplastics). For instance, phthalate is a

49

typical plasticizer used to soften polyvinyl chloride (PVC)-based materials. Tricolosan is

50

commonly applied as antimicrobial agent in plastic toys and trash bags 1. Besides chemical

51

additives, monomers such as propylene oxide, bisphenol A, styrene and vinyl chloride can be

52

released from weathered plastics

53

carcinogens, therefore causing potential harmful effects on organisms 3. Microplastics in natural

54

waters are likely to undergo complex interactions with aquatic species (e.g., bacteria, algae,

55

mammals) and hydrophobic pollutants

56

adsorption and desorption behavior of organic pollutants and heavy metals on microplastics

57

Nevertheless, the environmental fate of released chemical additives or degradation byproducts

58

from microplastics has not been well understood.

59

2, 3.

These plastic leachates may be endocrine disruptors and

4, 5.

Previous studies have mainly focused on the 6-8.

Organotin compounds (OTCs) are one of the man-made organometallic derivatives with 9, 10.

60

endocrine disrupting effects

Besides as biocides and fungicides, OTCs have been used as

61

light and thermal stabilizers in PVC plastics for more than 40 years, accounting for 3.5% of the

62

total tin consumption worldwide 11. Meanwhile, PVC polymers have broad applications in pipes,

63

hoses, toys, medical devices and automotive parts and account for about 73% of the annual

64

global production of plasticizers (e.g., phthalate, benzophenones and also OTCs)

65

waste PVC or its debris may enter the environment and contribute to the release of embedded

66

chemicals. Some studies have reported the release behavior of diethylhexyl phthalate (a type of

67

plasticizer) from PVC-based products

15, 16.

12-14.

Thus,

However, there is a paucity of information on the


Page 5 of 30


68

release process of OTCs from PVC materials, which is important for understanding the

69

environmental fate and potential impacts of microplastics.

70

Photochemical weathering of microplastics in natural waters such as estuaries and oceans

71

may experience impacts from transiting salinity or dissolved organic matter (DOM). The

72

seawater-specific halide ions have been considered as hydroxyl radicals (•OH) scavengers (i.e.,

73

•OH + Br- → Br•+OH-) and can also drive the decomposition of organic contaminants

74

Compared to freshwater conditions, reactive halogen radicals (RHS) in marine waters increase

75

the radical-mediated oxidation rates by 1-2 orders of magnitude and increase overall

76

photodegradation rates up to five-fold

77

enhanced degradation of organic contaminants (e.g., dimethyl sulfide and dienes) by halogen

78

radicals in seawater. On the other hand, photochemical degradation of organic pollutants and

79

micro-polymers not only depends on the direct absorbance of light irradiance, but also the levels

80

of DOM since some reactive intermediates of DOM may directly or indirectly react with organic

81

chemicals 21.

18, 19.

For instance, Parker et al.

20

17.

have reported the

82

This study investigated OTC release kinetics from PVC microplastics in simulated seawater

83

under UV or visible light irradiation. Three different sizes of PVC microplastics were prepared

84

and rigorously characterized. We compared the release kinetics of OTCs from these PVC

85

microplastics and attributed the photochemical weathering and the OTC release to the photo-

86

induced information of hydroxyl radicals that were experimentally measured. A new kinetics

87

model was established to analyze the microplastic size effect on the release kinetics. The

88

modeling analysis is also aimed to provide a quantitative assessment of the contributions from

89

surface desorption and photodegradation in the release kinetics of OTCs. Furthermore, the

90

influences of photo-induced reactive intermediates (i.e., Cl•, Br•, ClBr•- and 3DOM*) on OTCs



91

release and photodegradation were also investigated under UV irradiation. These results are

92

expected to provide new insight into the weathering process of microplastics and their potential

93

hazardous effects in natural water environment.

94

2. Materials and Methods

95

2.1. Materials

96

Three different sized PVC microplastics were prepared by grinding the PVC thin sheet and

97

sieving (Fig. S1) in the Supporting Information (SI). The artificial seawater (20 ‰ of salinity)

98

used in OTC release experiments contained 312 mM NaCl and 0.312 mM NaBr since the

99

chloride (Cl-) concentrations in natural marine waters are about thousand times higher than

100

bromide (Br-)

17, 22.

101

NaBr) were prepared to investigate the halogen radical influences on OTC release and

102

photodegradation. Humic acid (Sigma-Aldrich, CAS No.1415-93-6) was used as a DOM

103

surrogate. A UV lamp (365 nm; UVP, UVL-21, Analytik Jena, Upland, CA) and a LED lamp

104

(400 nm, BLCC-04, Prizamix) were used to provide light irradiation at 2.0 W·m-2 and 1.5 W·m-2,

105

respectively.

106

2.2. Characterization of PVC Microplastics

Reaction solutions with different halide ion concentrations (i.e., NaCl and

107

Surface morphology of PVC microplastics was examined by a Scanning Electron

108

Microscopy-Energy Dispersive Spectrometer (SEM-EDS; JSM-5610LV, JEOL, Japan), which

109

also determined the element abundance on PVC microplastic surfaces. The diameter distribution

110

was analyzed from the SEM images using the ImageJ software. The specific surface areas were

111

determined by the Brunauer-Emmett-Teller (BET) method using a Micromeritics analyzer

112

(Autochem Ⅱ 2920). The zeta potential of microplastics was measured by a dynamic light

113

scattering (DLS) with a Malvern Zetasizer (Nano ZS, Malvern Instruments, UK). The surface


Page 6 of 30

Page 7 of 30


114

functional groups were examined by attenuated total reflection-Fourier Transformation Infrared

115

Spectroscopy (ATR-FTIR, Cary 660, Agilent Technologies, the USA) with non-destructive

116

determination of sample surfaces. Three replicate spectrums of each sample were collected at 64

117

scans over the range of 4000-400 cm-1. Analysis for the treated samples was performed after

118

being dried in a dark desiccator.

119

Hydroxyl radicals (•OH) in UV-irradiated PVC suspension were detected using terephthalic

120

acid (TA) as the probe molecules, which forms a highly fluorescent product, 2-

121

hydroxyterephthalic acid (2-HTA). Briefly, 0.15 g of PVC microplastics was mixed with 150 mL

122

of the 0.5 mM TA solution (previously dissolved in 2 mM NaOH) and irradiated by UV365 to

123

induce the hydroxylation reaction of TA by •OH radicals, which occurs when the TA

124

concentration is below 10-3 M at room temperature

125

withdrawn every 30 min for 1.5 h and filtered through a 0.45-µm membrane filter before the

126

analysis on a fluorescence spectrophotometer (F-4500, HITACHI, Japan). 2-HTA was identified

127

at the emission wavelength of approximately 425 nm under an excitation wavelength of 315 nm.

128

The UV-visible light absorption spectrum of PVC microplastics in artificial seawater was

129

recorded in the range of 300-500 nm using a UV-Visible spectrophotometer (Evolution 201,

130

Thermo Scientific).

131

2.3. Release experiment design

132

2.3.1. Release kinetics of organotins from PVC microplastics

23, 24.

About 1 mL of liquid samples was

133

To examine the release of OTCs from PVC microplastics, 2 g of PVC microplastics and 500

134

mL artificial seawater were added into a glass beaker and stirred under UV (365 nm) or visible

135

light (400 nm) radiation (Fig. S2). Dark controls were exposed to the same conditions except

136

with no light irradiation. At different time intervals (0.5, 1, 2, 5, 8, 16, 24, 32, 40 and 56 h), 7.5



137

mL of water solution were withdrawn and filtered through a 0.22-μm glass fiber membrane

138

(Thermo Fisher Scientific, the USA). 5 mL of filtrate were used to measure the OTC

139

concentrations, and the remaining 2.5 mL were kept in refrigerator (4 ℃) for the total tin analysis.

140

The liquid-to-solid ratio in reaction solutions changed more than 10% during the sampling

141

procedure, which was considered in the subsequent data analysis. Prior to the release

142

experiments, OTC determination was also carried out on pristine PVC microplastics to

143

investigate how many amounts of OTCs attached on PVC surfaces.

144

2.3.2. Effects of halide ions and DOM on organotin release from PVC microplastics

145

Considering that OTC amounts leached from large PVC microplastics were relatively low,

146

only small and medium sized PVC microplastics were used in this experimental assessment.

147

Briefly, 0.1 g of PVC microplastics were added in 25 mL DI water with various amounts of Cl-

148

and Br- ions as well as with/without 10 mg·L-1 of humic acid. Then, this solution was exposed to

149

the UV365 irradiation for 24 h, followed by measuring the concentrations of dissolved OTC and

150

total tin as mentioned above.

151

2.4. Analytical detection

152

2.4.1. Determination of the total surface OTCs on PVC microplastics

153

To determine the total OTC contents on PVC microplastics (g-OTC·g-PVC-1), 50 mg of

154

microplastics were mixed in 10 mL of methanol solutions containing 10% acetic acid and 0.03%

155

tropolone, followed by a 30 min ultrasonic extraction and then 5 min centrifugation (1000×g). 5

156

mL of supernatant was collected and spiked into 40 mL of a sodium acetate/acetic acid buffer

157

(pH 4.5, 1 mol L-1) with addition of 1 g of NaCl, 4 mL of hexane, 0.1 mL of TPrT (as internal

158

standard) and 600 μL of NaBEt4 (w/w=1%). The mixture was horizontally shaken for 1 h. Then,

159

2 mL of organic phase was passed through a purification column filled with Florisil (500 mg/6


Page 8 of 30

Page 9 of 30


160

mL, CNW) and eluted with 5 mL of hexane and diethyl ether (v/v=9:1). After concentrated to

161

0.5 mL via nitrogen gas, OTCs were determined by gas chromatography-mass spectrometer

162

(GC-MS).

163

OTC determination and quantification were performed on an Agilent 5973-6890 GC-MS

164

(USA) using a HP-5 capillary column. The injection was made in the split-less mode at 280 °C,

165

and ion source was kept at 230 °C. The oven temperature started at 35 °C during the first 2 min,

166

followed by a 10 °C·min-1 increase to 200 °C and held for 5 min. Mass spectrometer was

167

operated in selected ion monitoring (SIM) mode with at least three qualified ions. High purified

168

helium gas (99.99%) was used as GC carrier gas at a flow of 1 mL·min-1.

169

2.4.2. Organotin determination in the aqueous phase of the water suspension of PVC

170

5 mL of filtered water sample were mixed with 0.2 mL of an internal standard (TPrT, 1

171

mg·L-1) in a 60 mL glass tube, followed by addition of 20 mL of a sodium acetate/acetic acid

172

buffer (1 mol·L-1, pH 4.5), 2 mL of n-hexane and 600 μL of NaBEt4 solution (1%, w/w) as the

173

derivatization agent. Then, they were horizontally shaken for 1 h for complete derivatization.

174

After standing for 30 min, 1 mL of hexane phase was taken and subjected to the GC-MS analysis

175

as mentioned above. For each batch (12 samples), 5 mL of blank seawater sample and 5 mL of

176

OTC standards (50 μg·L-1) were also determined as quality check (QC) to ensure analytical

177

system stability and no contamination during sample preparation.

178

2.4.3. Total tin determination in the aqueous phase of the water suspension of PVC

179

2.5 mL of filtered water samples were microwave digested using 2 mL HNO3 and 0.5 mL

180

HCl and then diluted to 10 mL by DI water. The total tin concentrations were determined by

181

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS, Agilent 7700, the USA). One of the

182

isotope tin, 118Sn, was selected for quantification based on its smaller analytical interference and 9 ACS Paragon Plus Environment


Page 10 of 30

183

larger sensitivity. The analysis sequence was as one reagent blank (0.2% of HNO3) and several

184

samples by turns, and at the end tin standard was analyzed.

185

2.5. Kinetics modeling of coupled release and photodegradation of organotins from PVC

186

microplastics The concentration change (

187

dC ) of released OTCs in the bulk solution (µg∙L-1∙h-1) is related dt

188

to the release rate of OTCs from PVC microplastics and their subsequent photodegradation rate.

189

As shown in Eq. 1-2, we hypothesized that (1) OTC release rate is proportional to its surface

190

coverage on PVC microplastics; (2) OTC photodegradation follows a first order kinetics with its

191

solution concentration and (3) the reduction rate of OTC-covered PVC surfaces exponentially

192

decreases with time. All the parameters used for this model are listed in Table 1.

193

V

194

A(t )  Ao exp(k3t )

195

where V is the volume of reaction solution (L); C is the remaining concentration of OTCs (µg·L-1)

196

in the aqueous mixed with PVC microplastics at release time, t; k1 is the specific release rate

197

constant of OTCs (µg·m-2·h-1); A(t) is the available surface area (m2) that still release OTCs at

198

time, t; k2 is the first-order photodegradation rate constant of OTCs (h-1). A0 is the initial surface

199

area of PVC microparticles which could be calculated from the BET results. k3 is a decrease rate

200

constant for A(t) (h-1). k1 and k3 are correlated via the surface coating or coverage density (S) (i.e.,

201

k1  S  k3 ), where S indicates the quantity of OTCs per unit surface areas of PVC microplastics

202

(µg∙m-2). Thus, rearranging Eq. 1 and Eq. 2 leads to:

203

dC SAok3  exp(k3t )  k2C dt V

dC  k1 A(t )  k2CV dt

(Eq. 1)

(Eq. 2)

(Eq. 3)


Page 11 of 30

204


In dark conditions, OTC photodegradation is ignored and the Eq. 3 can be simplified to:

205

dC SAok3  exp(k3t ) dt V

206

Table 1. Parameters used for modeling the OTC release kinetics from PVC microplastics. Parameter

(Eq. 4)

Physical meaning

Small size

t V C

207

Release time (h) Solution volume (L) Concentrations of OTCs in PVC-mixed water (µg·L-1) Initial surface area of PVC microplastics (m2) in the A0 suspension, calculated from specific surface area and the corresponding spiked mass of PVC microplastics (m2) Coating density (µg·m-2), calculated from the released OTC S levels in pristine PVC microplastics and A0 k2 First-order kinetics degradation rate constant (h-1) Rate constant describing how fast the OTC-covered surface c area is decreased (h-1) as desorption or release occurs k1 Specific release rate constant (µg·m-2·h-1) a Coating density of DMT on three sized PVC microplastic surfaces.

208

2.6. Statistical Analysis

Value Medium size Large size 0.5-56 0.5 Experimental data

10.72±0.28

9.44±0.13

6.47±0.20

4.99±0.20a

3.63±0.05a

0.72±0.02a

To be determined via data fitting To be determined via data fitting k1  S  k3

209

Each experimental condition was carried out in triplicates at 25 ± 3 °C of room temperature.

210

The presented results are the mean values ± standard deviation (SD) from three independent

211

experiments. Statistical analysis was performed by the t-test using SPSS Statistics 19.0 software

212

(SPSS Inc., USA). The significance was set at p

Organotin Release from Polyvinyl Chloride (PVC) Microplastics and

Recommend Documents