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
Environmental Science & Technology
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
2 ACS Paragon Plus Environment
Page 2 of 30
Page 3 of 30
42
Environmental Science & Technology
TOC
43 44
3 ACS Paragon Plus Environment
Environmental Science & Technology
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
4 ACS Paragon Plus Environment
Page 5 of 30
Environmental Science & Technology
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
5 ACS Paragon Plus Environment
Environmental Science & Technology
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
6 ACS Paragon Plus Environment
Page 6 of 30
Page 7 of 30
Environmental Science & Technology
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
7 ACS Paragon Plus Environment
Environmental Science & Technology
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
8 ACS Paragon Plus Environment
Page 8 of 30
Page 9 of 30
Environmental Science & Technology
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
Environmental Science & Technology
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)
10 ACS Paragon Plus Environment
Page 11 of 30
204
Environmental Science & Technology
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