Combined Effects of UV Exposure Duration and Mechanical Abrasion

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Combined Effects of UV Exposure Duration and Mechanical Abrasion on Microplastic Fragmentation by Polymer Type Young Kyoung Song, Sang Hee HONG, Mi Jang, Gi Myung Han, Seung Won Jung, and Won Joon Shim Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06155 • Publication Date (Web): 02 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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Environmental Science & Technology

Combined Effects of UV Exposure Duration and Mechanical Abrasion on Microplastic Fragmentation by Polymer Type

Young Kyoung Song1,2, Sang Hee Hong1,2, Mi Jang1,2, Gi Myung Han1, Seung Won Jung1,2, Won Joon Shim1,2* 1

Oil and POPs Research Laboratory, Korea Institute of Ocean Science and Technology, Geoje 53201, Republic of Korea 2 Department of Marine Environmental Sciences, Korea University of Science and Technology, Daejeon 34113, Republic of Korea

*To whom correspondence should be addressed

Manuscript for “Environmental Science and Technology”

Mailing Address: Won Joon Shim, Ph. D Oil and POPs Research Laboratory Korea Institute of Ocean Science and Technology Tel: +82-55-639-8671 Fax: +82-639-8689 E-mail: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: It is important to understand the fragmentation processes and mechanisms of

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plastic litter to predict microplastic production in the marine environment. In this study,

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accelerated weathering experiments were performed in the laboratory, with ultraviolet (UV)

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exposure for up to 12 months followed by mechanical abrasion (MA) with sand for 2 months.

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Fragmentation of low-density polyethylene (PE), polypropylene (PP), and expanded

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polystyrene (EPS) was evaluated under conditions that simulated a beach environment. PE

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and PP were minimally fragmented by MA without photooxidation by UV (8.7 ± 2.5 and 10.7

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± 0.7 particles/pellet, respectively). The rate of fragmentation by UV exposure duration

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increased more for PP than PE. A 12-month UV exposure and 2-month MA of PP and PE

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produced 6,084 ± 1,061 and 20 ± 8.3 particles/pellet, respectively. EPS pellets were

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susceptible to MA alone (4,220 ± 33 particles/pellet), while the combination of 6 months of

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UV exposure followed by 2 months of MA produced 12,152 ± 3,276 particles/pellet. The

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number of fragmented polymer particles produced by UV exposure and mechanical abrasion

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increased with decreasing size in all polymer types. The size-normalized abundance of the

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fragmented PE, PP, and EPS particles according to particle size after UV exposure and MA

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was predictable. Up to 76.5% of the initial EPS volume was unaccounted for in the final

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volume of pellet produced particle fragments, indicating that a large proportion of the

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particles had fragmented into undetectable sub-micron particles.

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Keywords: fragmentation, UV exposure, mechanical abrasion, microplastic

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■ INTRODUCTION

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Plastics that are lightweight, durable, and cheap are suitable for a very wide range of

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products used in everyday life. However, 4.8–12.7 million tons of plastic waste was estimated

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to have entered oceans from land-based sources in 192 coastal countries in 2010.1 This

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accounted for ~1.6–4.2% of the global plastic production, based on data from 2014.2 It has

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been predicted that there could be more plastic than fish in the ocean, by weight, by 2050

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unless significant actions are taken.3 Among marine plastic debris, microplastics smaller than

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5 mm are not only ubiquitously distributed in coastal, oceanic, and polar waters4-6 but are also

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found in the gastrointestinal tracts of fish7-9 and deposit- and filter-feeding invertebrates such

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as polychaetes,10 crustaceans,11 and bivalves.12,13 Ingested microplastics can be transferred

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along the planktonic food web.14 Moreover, microplastics and/or associated chemicals have 1


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been shown to cause adverse biological effects on organisms in laboratory experiments.13,15-20

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Abandoned plastics are likely to be continuously fragmented in the environment, and

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the proportion of fragments to total marine debris will continue to rise.21,22 Fragmented

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secondary microplastic particles typically account for the majority of microplastics in field

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surveys,23,24 except in some freshwater environments, such as rivers25 and lakes.26 Therefore,

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it is important to understand plastic litter fragmentation processes and mechanisms in the

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marine environment to predict the fate of microplastics in the environment. To date, polymer

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weathering (or degradation) has been studied extensively to develop products with longer

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service lives for outdoor applications in terrestrial environments.27-29 However, the process of

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plastic degradation under marine conditions has only been explained and classified

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theoretically.21 The photodegradation rate of exposed floating debris has been compared

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between marine and freshwater environments, and the physicochemical characterization of

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samples has been performed via mathematical analyses to understand microplastic

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fragmentation.30,31 However, data regarding the fragmentation of plastics at the micro-scale is

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limited. A few studies have reported weathering of plastics on beaches, although

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fragmentation via surface weathering has only been indirectly demonstrated with

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photographs.32,33 The formation nano- and micro-sized fragments was investigated under

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aqueous conditions with laboratory-accelerated photodegradation.34-36 A recent study

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confirmed the fragmentation of three polymer strips exposed to a salt marsh habitat for eight

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months,37 but the loss of fragmented plastics in the open exposure system may have resulted

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in an underestimation of the number of fragmented particles produced. Without data on the

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process of fragmentation of plastics in marine environments collected following rigorous

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methods, it is difficult to estimate the size, number, and production rate of secondary

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microplastic particles formed via weathering.

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Polymer weathering is commonly caused by photo-oxidation, photothermal oxidation,

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mechanical abrasion (MA), hydrolysis, and biodegradation.38 Most synthetic polymers with a

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chromophoric group undergo a photochemical reaction to absorb ultraviolet (UV) radiation.39

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Photochemical reactions caused by UV radiation absorption induce oxidation, which makes

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plastics brittle and easy to break up due to their decreasing elasticity.40 Small and large plastic

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litter floating on sea surfaces or washed ashore can become fragmented into

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microplastics.23,41 Once plastics are discarded into harsh environments, their surfaces become

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weak due to UV radiation exposure and oxidation, which likely generates microplastics via 2



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friction by wind, waves, and sand.21 Common polymers exposed to the marine environment

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are adversely affected by solar radiation (primarily UV-B), which initiates photo-oxidative

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degradation.42 Photo-oxidative and photothermal oxidative degradation may have greater

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effects on plastics on beaches than on the sea surface and floor due to the higher availability

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of UV radiation and oxygen in combination with a higher temperature.21,30,33 In addition, MA

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of plastics by sand is likely greater on beaches due to wind and wave action, and tidal

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currents. Beaches are considered to be the most favorable environment for plastic weathering

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and fragmentation, although this hypothesis has not been tested.

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We evaluated the fragmentation of plastics, with an accelerated weathering experiment

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simulating beach environments in the laboratory. Three polymers, low-density polyethylene

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(PE), polypropylene (PP), and expanded polystyrene (EPS), were exposed to UV for up to 12

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months in a UV chamber, followed by MA with sand for 2 months in the laboratory. We

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determined the degree of fragmentation by MA alone, the effects of UV exposure duration on

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the degree of fragmentation, the difference in fragmentation among polymers, and the

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abundance and size distribution of fragmented microplastic particles produced by weathering.

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■ MATERIALS AND METHODS

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Experimental setting. The experiments were designed to simulate a beach environment.

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The two major components of weathering included in this study were UV irradiance and MA.

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It was difficult to simulate and control both UV irradiance and MA simultaneously. Therefore,

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we first exposed the model plastics to UV, and then examined MA. Experiments were

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performed in four sets with different UV exposure times (0, 2, 6, and 12 months) to identify

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the effects of the duration of exposure on the degree of fragmentation. The MA experiment

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was conducted for all of the experimental sets, and period was fixed for 2 months. Three

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polymers (PE, PP, and EPS) were selected to examine differences in weathering

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characteristics according to polymer type. PE and PP were reported as the two major polymer

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types in a marine debris monitoring study,5 and EPS was identified as being exceptionally

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abundant on Korean beaches.43,44 PE and PP pre-production resin pellets were purchased from

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a wholesaler in Busan, Korea, in the form of white granules with a volume of 26±0.8 and

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19±0.9 mm3, respectively (see Fig. S1, Supporting Information). They contained a UV

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stabilizer and antioxidant, which were identified by Fourier transform infrared (FTIR)

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spectroscopy (Fig. S1) and confirmed by spectrum matching.45 EPS spherules were detached 3


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from a 62 L EPS float used in aquaculture. The detached EPS was spherical with a mean

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volume of 22±2.2 mm3, and no additives were detected by FTIR.

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UV exposure. For the UV exposure experiment, virgin pellets of PE and PP and spherules

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detached from an EPS float (hereafter described as pellets) were placed separately in soda-

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lime glass Petri dishes with covers. Triplicate samples of each polymer were exposed to UV

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light under a metal halide lamp (UV-A: 11.01 W/m2, UV-B: 0.12 W/m2, UV-C: 0.04 W/m2)

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for 60, 180, or 360 d. The temperature of the UV chamber was maintained at 43–45°C for the

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duration of the UV exposure. Natural UV was measured for comparison with the UV

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chamber 10 times a day in 1-h intervals during the day at the Geoje Island campus of the

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Korea Institute of Ocean Science and Technology on the southern coast of Korea for one year

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from March 2015 to February 2016 (UV-A: 1.95–9.80; 5.37 ± 2.87 W/m2 [min-max; mean ±

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standard deviation], UV-B: 0.09–0.64; 0.33 ± 0.20 W/m2, UV-C: 0.02–0.08; 0.05 ± 0.02

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W/m2). The UV-A intensity in the UV chamber was slightly higher than in natural sunlight,

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while UV-B and -C fell within the natural range. Control samples were placed in soda-lime

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glass Petri dishes and were wrapped in aluminum foil before exposure in the UV chamber

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under the same conditions as the exposed samples.

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Mechanical abrasion. To simulate a real beach environment, sand was collected from

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Heungnam Beach on Geoje Island, Korea. The sand was pretreated before the experiment to

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remove organic matter and plastic. First, it was sieved within a size range of 63–1,000 µm to

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remove mud particles (< 63 µm) and particles > 1 mm. Low-density particles, including

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plastics, were removed by density separation with a saturated NaCl solution, after which the

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salt was washed out with distilled water. The washed sand was combusted at 450°C in a

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furnace for 4 h to eliminate organic material. Ten of each fresh and UV-exposed PE, PP, and

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EPS pellets were placed in separate amber bottles (60 mL) with the pretreated sand (50 g).

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The three bottles for each group were placed and rotated on a roller mixer for 2 months at 37

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rpm. Triplicate control samples were prepared containing only pretreated sand and were

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rotated for 2 months under the same conditions as the samples.

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Surface weathering observations. Physical damage and structural changes in the plastic

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surface were examined by scanning electron microscopy (SEM: JEOL JSM-7600F) with 3–5

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kV electron accelerating voltage. All plastic samples were coated with platinum before the

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analysis to prevent surface charging. The oxidation status of the plastic surface was measured

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in the subsamples of UV-exposed pellets by Fourier transform infrared spectroscopy (FTIR) 4



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for 12 months in one-month intervals. The carbonyl index (CI), which can be used to

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represent the degree of weathering and surface oxidation,46,47 was used to characterize the

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degree of photo-oxidation of PE, PP, and EPS. The CI is defined as the ratio of absorption

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intensity at carbonyl groups around 1,870–1,650 cm-1 to an internal constant band. Relative

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absorbance intensities of the carbonyl bond at 1,712 cm-1 (maximum carbonyl peak for PP)

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and a band at 1,375 cm-1 attributed to –CH3 bending were used in the CI analysis of PE and

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PP.30,48 For EPS, the CI was calculated by a comparison of the FTIR absorption band at 1,720

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cm-1 with the reference band at 1,450 cm-1.49 The FTIR measurements were performed using

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a Nicolet 6700 spectrometer (Thermo Fisher Scientific) equipped with attenuated total

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reflectance (ATR), consisting of a diamond crystal at an incident angle of 42°. The spectra

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were recorded as the average of 32 scans in the spectral range of 650–4,000 cm-1.

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Quantification of microplastic fragments. Fragmented PE, PP, and EPS particles were

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extracted from the sand by density separation using HPLC-grade water (Daejung, Korea)

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through a black membrane polycarbonate filter paper (47 mm Ø, 0.2 µm pore size). HPLC-

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grade water was used to avoid contamination. In addition, water rather than saturated NaCl

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solution was used for better separation of interfering dense inorganic particles from less

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dense PE, PP, and EPS fragments. The recovery rate (n=3) of water separation was 99.4% for

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the most dense PE (100–500 µm in size) among the three polymers tested (Table S1). The

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filter papers were dried at room temperature and stored in glass Petri dishes. The plastic

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fragments on the filter papers were identified and quantified using fluorescence microscopy

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(excitation: 450–490 nm, emission: 515–565 nm, 50× and 200×) after Nile Red (NR)

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staining.50 NR solution in hexane (5 mg/L, total volume: 200 µL) was used to dye the

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fragmented particles on the filter paper, and the filter paper was washed with 200 µL of

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hexane. The particles were categorized into twelve size classes based on their maximal length

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(< 50, 50–100, 100–200, 200–300, 300–400, 400–500, 500–600, 600–700, 700–800, 800–

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900, 900–1,000, and > 1,000 µm). While all NR-stained fragmented particles were counted

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with 50× magnification, all experimental sets for EPS and 12-month UV exposure group for

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PP were counted in an area 0.7–5% that of the filter paper under 200× magnification due to

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the large numbers of fragmented particles < 100 µm in size. At least 500 microplastics were

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counted in 0.7% of the filter paper, and observations (n=3) of 0.7, 5, 10, and 20% of the filter

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paper showed no significant difference (ANOVA followed by Duncan’s multiple-range test)

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in the total particle counts. 5


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Although the sand was pretreated before MA experiments to remove NR-stainable

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plastics and organic matter, the control samples (n = 3) contained 65 ± 20 particles/bottle

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(mean ± standard error [SE]). The particles in the control samples stained with Nile Red were

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less than 300 µm in size and irregularly shaped. The number of fragmented plastic particles in

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the experimental groups differed significantly (t-test; p < 0.05) from the control except for the

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2- and 6-month UV exposure groups for PE (PE UV2 and UV6). The PE UV2 and UV6

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groups were excluded from the data analysis. The mean number of NR-stained particles of

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the control (65 particles/bottle) was subtracted from the total fragmented particles in the

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bottles of each experimental group, which were then converted into particles/pellet. The

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results of fragmented plastic particles were expressed as the mean number of particles/pellet

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from triplicate samples (mean ± SE).

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Statistical analysis. The difference in the number of fragmented particles and change in

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plastic volume among the three polymer types by UV exposure duration were compared and

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tested with an analyses of variance (ANOVA) and Duncan’s multiple-range test.

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■ RESULTS

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Surface weathering by UV exposure. The surfaces of UV-exposed pre-production PE

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and PP resin pellets and pellets detached from an EPS float were examined with SEM. The

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surface of PE pellets exposed to UV for 2 months did not show any changes compared to the

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control (Fig. S2a); however, cracks were apparent after 6 months. After 12 months, hairline

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cracks began to appear in larger cracks. In the case of PP, surface cracking was apparent after

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2 months of UV exposure, and the PP pellet was crushed by the pressure of the FTIR-ATR

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probe tip during the spectral observation (Fig. S2b). After 6 months, the PP cracks were

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thicker and linked together, appearing as larger cracks. After 12 months, fine cracks began to

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appear along with the larger cracks. The surface of the EPS pellets yellowed, became fragile

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and brittle, and showed cracks after 2 months of UV exposure (Fig. S2c). In addition,

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powder-like white fine particles were produced on the surface of EPS after 2 months. After 6

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months, the surface turned yellow and the cracks became thick. More white fine particles

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were generated, and the EPS pellets were easily crushed. After 12 months, the parent EPS

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pellets had shrunk and large fragmented pieces remained, while the cracks in the surface

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became thicker. In addition, micrometer-sized fine particles were detected inside the cracks.

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While the outer surface was severely damaged, a non-damaged fresh inner surface was 6



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observed inside the cracks. Aggregations of small particles < 1 µm were confirmed on the

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inner surface of cracks under high magnification. The surface weathering of UV-exposed

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samples was tracked by chemical changes using FTIR at one-month intervals for one year

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(Fig. S3). The carbonyl content of PE showed a linear increase with UV exposure duration

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(Fig. S3a). However, the CI of PP rapidly increased, reached a plateau after 2 months, and

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began to increase again after 6 months (Fig. S3b). In EPS, the CI began to increase after 2

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months of exposure until it peaked at about five months, and then gradually declined (Fig.

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S3c).

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Fragmentation by UV exposure and mechanical abrasion.

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Comparison of fragmentation by UV exposure duration. MA alone or accompanied with

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UV exposure produced micro-sized plastic fragments (Fig. 1). The degree of fragmentation

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according to UV exposure duration varied among the polymer types (Fig. 2). The UV

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exposure duration of 0 (UV0) and 12 months (UV12) followed by 2 months of MA produced

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8.7 ± 2.5 (mean ± SE) and 20 ± 8.3 particles/pellet, respectively (Fig. 2a). Significantly more

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(p < 0.05) PE particles were produced after UV exposure for 12 months compared with those

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in the 0-month exposure group. The number of PP fragments was 10.7 ± 0.7 particles/pellet

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for UV0, 69 ± 6.1 particles/pellet for UV2, 166 ± 39 particles/pellet for UV6, and 6,084 ±

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1,061 particles/pellet for UV12 (Fig. 2b). Significantly more (p < 0.05) PP particles were

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produced after UV exposure for 12 months compared with those in the UV0, UV2, and UV6

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groups. Regarding EPS production, 4,220 ± 33, 7,612 ± 833, 12,152 ± 3,276, and 10,501 ±

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1,718 particles/pellet were produced in the UV0 UV2, UV6, and UV12 groups, respectively

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(Fig. 2c). MA without UV exposure produced a considerable amount of EPS fragments.

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Significantly more (p < 0.05) EPS particles were produced after 6 months of UV exposure

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compared with those in the UV0 and UV2 groups. However, there was no significant

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difference (p > 0.05) between the UV6 and UV12 groups.

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Comparison of fragmentation by polymer type. Significantly more (p < 0.05) EPS

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particles were produced by MA alone (UV0) and all of the UV exposure groups compared to

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the respective exposures for PE and PP particles (Fig. 3). The number of PE and PP particles

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produced by MA alone did not differ significantly. However, the number of PP particles

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increased much more steeply than PE during the 12 months of UV exposure, resulting in

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significantly more (p < 0.05) fragmented PP particles than PE particles (Fig. 3d).

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Size and volume distribution. The number of fragmented polymer particles produced by

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UV exposure and MA increased with decreasing particle size (Fig. 4). This trend was

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observed for all polymer types and UV exposure durations. The particle-size normalized

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abundance of fragmented plastic particles could be modeled by assuming a steady state.51 The

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relationship between the size-normalized particle abundance (the abundance of fragmented

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particles was divided by each size-class interval) and particle size was evaluated, except for

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particles > 1 mm for PP and EPS. The size-normalized abundance of the fragmented PE, PP,

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and EPS particles according to particle size after UV exposure for 12 months and MA for 2

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months showed a significant (p < 0.05) relationship in the regression analysis (r2: PE =

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0.9498, PP = 0.9823, and EPS = 0.9710) with exponential equations (Fig. 4). Particles < 100

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µm accounted for 86.0 and 88.6% of the total fragmented PE in the UV0 and UV12 exposure

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groups, respectively (Fig. S4), and 95.0 (UV0), 88.7 (UV2), 73.7 (UV6), and 99.8% (UV12)

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for PP (Fig. S5). For EPS, over 99.8% of fragmented particles were < 100 µm in all UV

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exposure groups (Fig. S6).

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The volume of the remaining parent pellets was calculated from the measured mean

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diameters, and the volume of produced particles was calculated on the assumption that all

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particles were spherical with a diameter at the mid-point of each size category (e.g., 150 µm

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for 100–200 µm). The total volume of the remaining pellets plus the fragmented particles was

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converted into the proportion of the original volume of the corresponding parent pellets for

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each UV exposure group (Fig. 5). While the volume of parent PE pellets remained almost

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intact (> 90%) in all exposure groups, the missing portion of PE volume after weathering

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increased from –1.3 and 9.1% according to the UV exposure duration (Fig. 5a). In addition,

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98.3 and 95.6% of PP parent pellets remained after exposure to UV for 0 and 2 months,

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respectively, with corresponding unaccounted for proportions of 1.1 and 3.1%. After 6 and 12

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months, the parent PP pellet volume decreased to 80.5 and 80.6%, respectively, while

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fragmented particles accounted for 11.6 and 20.6%, respectively (Fig. 5b). After exposure to

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UV for 0 and 2 months, parent EPS pellets accounted for 77.7 and 35% of the original

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volume, while 16.7 and 24.1% of the pellets were fragmented, respectively. After 6 and 12

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months, no visible parent pellets were apparent, but the volume of fragmented particles

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accounted for only 42.1 and 23.5% of the original parent pellet volume, respectively. The

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unaccounted for proportions of EPS pellets gradually increased over UV exposure duration,

8



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accounting for 5.60, 40.9, 57.9, to 76.5% in the UV0, UV2, UV6, and UV12 groups,

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respectively (Fig. 5C).

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The volume density of PE particles increased with increasing particle size in all UV

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exposure groups (Fig. S7a). The volume density of PP particles showed a peak at 400–700

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µm in the group exposed only to MA (Fig. S7b), while the volume density peak continuously

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decreased over the UV exposure duration to 100–300 µm in UV2 and UV6 and 50–100 µm

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in UV12. After MA of EPS, the peak volume density of fragments was in the 50–100 µm size

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class, except for particles > 1 mm. The peak gradually shifted to the 0–50 µm size class with

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increasing UV exposure duration from 0 to 6 months (Fig. S7c), while the volume density of

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the parent pellets and fragment particles > 1 mm rapidly decreased during the same period.

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■ DISCUSSION

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Fragmentation by mechanical abrasion. MA with sand produced 8.7 ± 2.5

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particles/pellet of PE and 10.7 ± 0.7 particles/pellet of PP, while producing 4,220 ± 33

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particles/pellet of EPS. These results indicate that PE and PP are unlikely to be fragmented by

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MA without photo-oxidation by UV; however, EPS pellets are susceptible to abrasion with

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sand. These results can be explained by the lower mechanical strength of EPS than PE and PP.

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Polystyrene (1.04–1.1 g/cm3) is generally denser than PE (0.92–0.97 g/cm3) and PP (0.9–0.91

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g/cm3),5 but its density steeply decreases to 0.010–0.035 g/cm3 after expansion with a

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blowing agent and steam.52 In addition, the tensile strength of EPS (3–14 kPa) (Australian

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Urethane & Styrene EPS Technical Data) is one or two order(s) of magnitude lower than that

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of low-density PE (203 kPa) and PP (783 kPa).53 Although EPS, as a foamed plastic, has an

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excellent capacity to absorb shock and insulate heat, it is susceptible to frictional forces and

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can be torn apart readily by sand particles.

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Fragmentation according to UV exposure duration. In this study, fragmentation of

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polymers increased with increasing UV exposure time in combination with MA, even though

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the rate of fragmentation differed by polymer type. The bond dissociation energy of C–C

282

bonds (375 kJ/mol) and C–H bonds (420 kJ/mol) is equivalent to UV radiation of 320 and

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290 nm, respectively.54 UV light has enough energy to produce initial free radicals as the

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primary photochemical products, and causes C–C and C–H bonds to dissociate from the

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polymer backbone.28 This can subsequently lead to one or more chemical changes, resulting

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in embrittlement at the surface of polymers, visible as cracks and fractures. UV oxidation 9


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produces embrittlement at the surface of polymers to a depth of more than 100 µm caused by

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cross-linking and chain reactions.28 This brittle surface layer easily formed cracks and became

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more obvious and thicker by increasing the UV exposure time of three polymers, as shown by

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SEM (Fig. S2). Interestingly, the brittle surface and cracks from photo-oxidation (or

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weathering) did not directly result in the ‘fragmentation’ of polymers. Although many large

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and small cracks were observed on the surfaces of PE and PP pellets, no fragments were

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detected with UV exposure alone by SEM, and their formation required subsequent MA. This

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implies that additional physical force is required to enhance the breaking of embrittled

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plastics.

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The increase in the fragmentation rate of PP (UV12/UV0 ratio = 804) according to UV

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exposure duration was higher than for PE (3.7) and EPS (2.6). In fact, field samples of pellets

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or fragments of PP had a higher degree of chemical weathering, or were less resistant to

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weathering, than PE samples, based on SEM and FTIR.55 Every other carbon atom in the PP

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backbone is a tertiary carbon that is more susceptible to abiotic attack than the secondary

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carbons of PE.56 In addition, the maximum sensitivity, as determined by the bond dissociation

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energies, are in the order of: PE (96 kcal/mol at 300 nm) > PS (90 kcal/mol at 318 nm) > PP

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(77 kcal/mol at 370 nm). This indicates that less energy is required to dissociate chemical

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bonds in PP than PE and EPS. EPS fragments were observed on the pellet surface after 2

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months of UV exposure in this study, indicating that UV exposure alone, without MA, was

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sufficient to break EPS into micro-sized fragments. Although EPS has unsaturated double

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bonds that are susceptible to photo-initiated degradation,56 the foamed structure of EPS may

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influence both photo-oxidation and subsequent fragmentation. The depth of UV penetration,

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diffusion of radicals, and availability of oxygen is critical in the propagation and rate of

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photo-oxidation of polymers.57 The mesoporous structure of EPS could assist photo-oxidation

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by facilitating the penetration of UV radiation and diffusion of radicals, while increasing

312

oxygen availability. Expansion of polystyrene changes the bulk plastic into a fused small

313

balloon-like structure with a thin polystyrene envelope. The progression of cracks and fusion

314

with other cracks in the thin layer more readily fragmented than the bulk plastic (Fig. S2c). In

315

addition, the gradual decrease in the CI of the surface of EPS pellets after 6 months of UV

316

exposure (Fig. S3c) supported the assumption that fragmentation of the surface layer exposed

317

the underlying relatively less oxidized surface. According to polymer type, the size of

318

fragmented particles by chemical and mechanical weathering differed and the rate of 10



319

fragmentation was in the order of: EPS > PP >> PE. The physicochemical properties of

320

polymers determined the degree and rate of weathering and fragmentation in combination

321

with environmental factors. The fragmentation of PE and PP could be impeded by additives,

322

such as UV stabilizers and antioxidants, compared with EPS. Therefore, not only the amount

323

of each polymer used in production and the polymer composition of marine macroplastic

324

debris, but also the weathering and fragmentation characteristics of polymers, are critical

325

determining factors for predicting microplastic abundance in the environment.

326

Size distribution of fragmented microplastics. The number of fragmented particles

327

produced by UV exposure and MA in all three tested polymers increased with the decreasing

328

size of fragmented microplastics. Similar size distribution patterns have been found in coastal

329

waters and sediments,24,58 although other studies have observed that the abundance increased

330

down to a size of a few millimeters, and then decreased.51,59 Fragmented microplastics < 100

331

µm accounted for over 73% of the total microplastic fragments produced, and for over 97%

332

of fragments smaller than 300 µm in this study. Oyster feeding modifications and

333

reproduction were affected by exposure to polystyrene microplastics (2 and 6 µm).60 In this

334

study, the unaccounted proportion of EPS by volume after 12 months of UV exposure and 2

335

months of MA was 76.5% of the parent pellet volume. These results imply that 76.5% of EPS

336

by volume was fragmented into particles that could not be recovered or detected using the

337

current analytical method. Even if the recovery rate of PE (100–500 µm) particles was 99.4%

338

(Table S1), it is possible that the fragmented particles less than 100 µm were lost during the

339

density separation. The low rising velocity of small plastic particles may affect their low

340

recovery. Even at the highest magnification (×1,000) of the fluorescence microscope, the

341

smallest discernible NR-stained microplastics were several micrometers wide. It is, however,

342

likely that a large proportion of EPS was fragmented into sub-micron particles. The

343

continuous increase in the number of EPS particles with decreasing particle size to the lowest

344

size class (1–50 µm) supports the existence of sub-micron particles, which may have been

345

more numerous than the micro-sized particles. Moreover, the smaller plastic particle fraction

346

increased with exposure time in the laboratory weathering experiment,34-36,61 and nanoplastics

347

formed from five different polymers, including PE, PP, and PS, with only UV exposure in

348

aqueous condition.35 Nano-sized polystyrene plastics have been frequently used for toxicity

349

tests of microplastics, and have demonstrated toxic effects at high exposure levels.62

350

Considering the general cut-off size (> 300 µm) of microplastics sampled with a Neuston net 11


Page 12 of 26

Page 13 of 26


351

or sieving, a large proportion of microplastics produced by fragmentation could be excluded

352

from sampling. Therefore, their abundance in the environment may be underestimated.

353

Estimation of fragmentation in the field. In this study, the fragmentation process of

354

plastics on a beach was simulated by MA after UV exposure, because we expected that

355

photo-oxidation by UV is required before MA to enhance fragmentation. This was proven by

356

the significant enhancement of fragmentation with UV exposure and subsequent MA

357

compared with MA only for all three polymers (Fig. 2). In addition, two processes were

358

sequentially adopted to measure UV exposure duration exactly and to evaluate its role in the

359

fragmentation mechanism. However, UV exposure and MA of plastics may occur

360

simultaneously in real beach environments. Furthermore, the UV exposure and MA are not

361

independent variables. UV exposure after MA and simultaneous UV exposure and MA may

362

produce different results. In UV exposure after MA, attached or embedded sand particles in

363

the plastic surface as a result of MA may reduce the UV-exposed surface area, and the rough

364

plastic surface formed by MA may scatter or enhance the penetration of UV light.

365

Simultaneous UV exposure and MA may significantly reduce the UV exposure time as a

366

result of the rolling and burial of plastics or a shading effect caused by sand particles.

367

Natural organic matters that interfere with Nile Red staining were removed in this study.

368

Although the organic matter content of beach sand is generally low,63 the interaction of

369

organic matter with plastics probably influences the weathering process. The absorption or

370

attachment of organic matter on the plastic surface may reduce the penetration of UV light to

371

the plastic surface or the friction force of sand particles.

372

The UV intensity in the experimental chamber used in this study was comparable to or

373

rather lower (UV-B and -C) than that of natural sunlight in Geoje, Korea, suggesting that the

374

level of photo-oxidation by UV radiation in the chamber represents that of sunny days.

375

Temperature, which influences photothermal oxidation, was maintained in the chamber (43–

376

45°C) by cooling to fall within the environmentally relevant range of beach sand during the

377

summer in Geoje, Korea (rooftop exposure from 9 AM to 6 PM in August; range

378

21.5−49.5°C; average±SE 42.1±5.76°C). The plastics were weathered in the UV chamber for

379

24 h a day to shorten the exposure time. The total irradiance time of sunlight in Geoje from

380

March 2015 to February 2016 was 2,087 h (86.9 d).64 The UV exposure time of 2, 6, and 12

381

months in the chamber converted into the equivalent time in Geoje was 8.4 (0.7 yr), 25.2 (2.1

382

yr), and 50.4 (4.2 yr) months, respectively. One year of exposure in the UV chamber 12



383

corresponded to 4.2 yr outdoors in Geoje. However, the time of plastics exposure to UV in a

384

real environment could be decreased by burying the plastics in sand, attached organic matter,

385

and the amounts of additives. Many plastic products and beached plastic debris contain

386

additives, including antioxidants and UV stabilizers to retard UV effects.65 Therefore, it

387

requires more than 4.2 yr in a real beach environment to produce the same degree of

388

fragmentation found in this study. On beaches, photo-oxidized plastics may experience

389

various mechanical forces. Wind, tidal currents, and waves can cause friction with air, water,

390

and sand on the beach. In addition, human activities on popular recreational beaches may

391

generate mechanical forces. However, it is difficult to estimate the average intensity and

392

duration of mechanical forces experienced by plastic debris in the environment to convert the

393

2-month period of MA in this study to an equivalent time in the environment. Further studies

394

are required to consider a variety of environmental factors, and the properties of polymers, to

395

estimate the fragmentation of plastics in real marine environments and model the

396

fragmentation processes.

397 398

■ ACKNOWLEDGMENTS

399

This study was supported by the Ministry of Oceans and Fisheries, Korea under a research

400

project titled “Environmental Risk Assessment of Microplastics in the Marine Environment”.

401 402

■ ASSOCIATED CONTENT

403

Supporting Information

404

Material and methods: Quantification of microplastic fragments

405

Table addressing the (1) recovery test. Figures addressing: (1) test material; (2) the

406

surface of UV-exposed pre-production resin pellets; (3) the carbonyl index according to UV

407

exposure duration; (4) the size distribution of fragmented PE particles; (5) the size

408

distribution of fragmented PP particles; (6) the size distribution of fragmented EPS particles;

409

and (7) the volume density distribution of fragmented particles.

410

13


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Page 20 of 26

Page 21 of 26


(a)

(b)

(c)

UV0

a) UV0

UV2

UV0

b) UV2

UV12

UV6

UV12

UV6 C)

UV12 d)

Figure 1. Photographs of fragmented (a) polyethylene (PE), (b) polypropylene (PP), and (c) expanded polystyrene (EPS) particles after UV exposure (0–12 months) and subsequent mechanical abrasion (MA) with sand (2 months). Plastic particles were stained with Nile Red and photographed under a fluorescence microscope. Upper left: 2-months MA only; upper right: 2-months UV + 2-months MA; lower left: 6-months UV + 2-months MA; lower right: 12-months UV + 2-months MA. The stained PE particles are shown for only two experiment sets, after UV exposure (0 and 12 months) and subsequent mechanical abrasion (MA) with sand (2 months).

20



30

(a)

10000

PE(UV0) UV0+MA PE(UV12) UV2+MA UV6+MA UV12+MA

b

25 20 15 a

10

5000

Particles/pellet

Fragments Particles/pellet (particles/pellet)

35

(b) PP(UV0) PP(UV2) PP(UV6) PP(UV12)

b

Page 22 of 26

20000

15000

(c) EPS(UV0) EPS(UV2) EPS(UV6) EPS(UV12)

c

250

bc a

200

10000

ab

150 100

5

50

0

0

a

5000

a

a 0

Figure 2. Number of (a) polyethylene (PE), (b) polypropylene (PP), and (c) expanded polystyrene (EPS) plastic particles produced by UV exposure for 0 (UV0), 2 (UV2), 6 (UV6), and 12 months (UV12) followed by 2 months of mechanical abrasion (MA). Data were subjected to an ANOVA followed by Duncan’s multiple-range test. Groups with different letters are statistically different (p < 0.05).

21


Page 23 of 26


104

(a)

104

(b) b

Particles/pellet Fragments (particles/pellet)

b 103

103

102

102

101

a

a

PE

PP

Particles/pellet Fragments (particles/pellet)

a

101

100

100

105

a

PP

EPS

(c)

105

EPS

(d)

b 104

104

103

103

b

c

a 102

102

101

1

a 10 PP

PE

EPS

PP

EPS

Figure 3. Comparison of the number of fragmented particles among three polymer types according to UV exposure duration for (a) 0, (b) 2, (c) 6, and (d) 12 months followed by 2 months of mechanical abrasion with sand. Data were subjected to an ANOVA followed by Duncan’s multiple-range test. Groups with different letters are statistically different (p < 0.05).

`

22



(a)

100

PE (UV0) UV0+MA PE (UV2) UV2+MA PE (UV6) UV6+MA PE (UV12) UV12+MA

101

10-2 100

10-3 10-1

10-4 10-5 101

105 Fragments (particles/pellet) Particles/pellet

y=5604.1x-2.742 r2=0.9498 p

Combined Effects of UV Exposure Duration and Mechanical Abrasion

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