Organotin Release from Polyvinyl Chloride Microplastics and

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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

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Environmental Science & Technology

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Organotin Release from Polyvinyl Chloride (PVC) Microplastics

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and Concurrent Photodegradation in Water: Impacts from Salinity,

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Dissolved Organic Matter and Light Exposure

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

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a Key

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

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Environmental Science and Engineering, Tongji University, Shanghai, China

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b Shanghai

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

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c State

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

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Engineering, Tongji University, Shanghai, China.

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d

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Environmental Research Institute, Lyndhurst, New Jersey, USA

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e John

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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

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Corresponding author:

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

1 ACS Paragon Plus Environment


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Abstract

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Photochemical weathering leads to degradation of microplastics and releases chemical

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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formed and reacted with OTCs from PVC microplastics.

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

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TOC

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1. Introduction

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

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

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

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

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

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released from weathered plastics

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

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

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mammals) and hydrophobic pollutants

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

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

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from microplastics has not been well understood.

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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.

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endocrine disrupting effects

Besides as biocides and fungicides, OTCs have been used as

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

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

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

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

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

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

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plasticizer) from PVC-based products

15, 16.

12-14.

Thus,

However, there is a paucity of information on the


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

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environmental fate and potential impacts of microplastics.

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

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

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

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

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

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

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photodegradation rates up to five-fold

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

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

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

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

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chemicals 21.

18, 19.

For instance, Parker et al.

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17.

have reported the

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

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

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

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

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

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

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

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

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



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

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

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hazardous effects in natural water environment.

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2. Materials and Methods

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2.1. Materials

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

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

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

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

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bromide (Br-)

17, 22.

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

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

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

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

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respectively.

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2.2. Characterization of PVC Microplastics

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

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Surface morphology of PVC microplastics was examined by a Scanning Electron

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

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

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

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

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

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


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

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

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

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

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being dried in a dark desiccator.

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

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

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

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

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

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concentration is below 10-3 M at room temperature

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

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

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

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

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

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Thermo Scientific).

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2.3. Release experiment design

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2.3.1. Release kinetics of organotins from PVC microplastics

23, 24.

About 1 mL of liquid samples was

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

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

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

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



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

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

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

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

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

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

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investigate how many amounts of OTCs attached on PVC surfaces.

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

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

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

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

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

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

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total tin as mentioned above.

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2.4. Analytical detection

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2.4.1. Determination of the total surface OTCs on PVC microplastics

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

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

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

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

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(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

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

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


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

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

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(GC-MS).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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system stability and no contamination during sample preparation.

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

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

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

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

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


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

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

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

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microplastics The concentration change (

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

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

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

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

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

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

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V

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A(t )  Ao exp(k3t )

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

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

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

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

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

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

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

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

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dC SAok3  exp(k3t )  k2C dt V

dC  k1 A(t )  k2CV dt

(Eq. 1)

(Eq. 2)

(Eq. 3)


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

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dC SAok3  exp(k3t ) dt V

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

(Eq. 4)

Physical meaning

Small size

t V C

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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.

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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

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

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

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

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

Organotin Release from Polyvinyl Chloride Microplastics and

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