Article pubs.acs.org/est
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*,†,‡ †
Oil and POPs Research Group, Korea Institute of Ocean Science and Technology, Geoje 53201, Republic of Korea Department of Marine Environmental Sciences, Korea University of Science and Technology, Daejeon 34113, Republic of Korea § Library of Marine Samples, Korea University of Science and Technology, Geoje 53201, Republic of Korea Downloaded via UNIV OF CAMBRIDGE on July 9, 2018 at 11:31:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: It is important to understand the fragmentation processes and mechanisms of plastic litter to predict microplastic production in the marine environment. In this study, accelerated weathering experiments were performed in the laboratory, with ultraviolet (UV) exposure for up to 12 months followed by mechanical abrasion (MA) with sand for 2 months. Fragmentation of low-density polyethylene (PE), polypropylene (PP), and expanded polystyrene (EPS) was evaluated under conditions that simulated a beach environment. PE and PP were minimally fragmented by MA without photooxidation by UV (8.7 ± 2.5 and 10.7 ± 0.7 particles/pellet, respectively). The rate of fragmentation by UV exposure duration increased more for PP than PE. A 12-month UV exposure and 2-month MA of PP and PE produced 6084 ± 1061 and 20 ± 8.3 particles/pellet, respectively. EPS pellets were susceptible to MA alone (4220 ± 33 particles/pellet), while the combination of 6 months of UV exposure followed by 2 months of MA produced 12,152 ± 3276 particles/pellet. The number of fragmented polymer particles produced by UV exposure and mechanical abrasion increased with decreasing size in all polymer types. The size-normalized abundance of the fragmented PE, PP, and EPS particles according to particle size after UV exposure and MA was predictable. Up to 76.5% of the initial EPS volume was unaccounted for in the final volume of pellet produced particle fragments, indicating that a large proportion of the particles had fragmented into undetectable submicron particles.
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microplastics in field surveys,23,24 except in some freshwater environments, such as rivers25 and lakes.26 Therefore, it is important to understand plastic litter fragmentation processes and mechanisms in the marine environment to predict the fate of microplastics in the environment. To date, polymer weathering (or degradation) has been studied extensively to develop products with longer service lives for outdoor applications in terrestrial environments.27−29 However, the process of plastic degradation under marine conditions has only been explained and classified theoretically.21 The photodegradation rate of exposed floating debris has been compared between marine and freshwater environments, and the physicochemical characterization of samples has been performed via mathematical analyses to understand microplastic fragmentation.30,31 However, data regarding the fragmentation of plastics at the microscale is limited. A few studies have reported weathering of plastics on beaches, although
INTRODUCTION Plastics that are lightweight, durable, and cheap are suitable for a very wide range of products used in everyday life. However, 4.8−12.7 million tons of plastic waste was estimated to have entered oceans from land-based sources in 192 coastal countries in 2010.1 This accounted for 1.6−4.2% of the global plastic production, based on data from 2014.2 It has been predicted that there could be more plastic than fish in the ocean, by weight, by 2050 unless significant actions are taken.3 Among marine plastic debris, microplastics smaller than 5 mm are not only ubiquitously distributed in coastal, oceanic, and polar waters4−6 but are also found in the gastrointestinal tracts of fish7−9 and deposit- and filter-feeding invertebrates such as polychaetes,10 crustaceans,11 and bivalves.12,13 Ingested microplastics can be transferred along the planktonic food web.14 Moreover, microplastics and/or associated chemicals have been shown to cause adverse biological effects on organisms in laboratory experiments.13,15−20 Abandoned plastics are likely to be continuously fragmented in the environment, and the proportion of fragments to total marine debris will continue to rise.21,22 Fragmented secondary microplastic particles typically account for the majority of © 2017 American Chemical Society
Received: Revised: Accepted: Published: 4368
December 7, 2016 March 2, 2017 March 2, 2017 March 2, 2017 DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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Environmental Science & Technology
major polymer types in a marine debris monitoring study,5 and EPS was identified as being exceptionally abundant on Korean beaches.43,44 PE and PP preproduction resin pellets were purchased from a wholesaler in Busan, Korea, in the form of white granules with a volume of 26 ± 0.8 and 19 ± 0.9 mm3, respectively (Figure S1, Supporting Information). They contained a UV stabilizer and antioxidant, which were identified by Fourier transform infrared (FTIR) spectroscopy (Figure S1) and confirmed by spectrum matching.45 EPS spherules were detached from a 62 L EPS float used in aquaculture. The detached EPS was spherical with a mean volume of 22 ± 2.2 mm3, and no additives were detected by FTIR. UV Exposure. For the UV exposure experiment, virgin pellets of PE and PP and spherules detached from an EPS float (hereafter described as pellets) were placed separately in sodalime glass Petri dishes with covers. Triplicate samples of each polymer were exposed to UV light under a metal halide lamp (UV-A: 11.01 W/m2; UV-B: 0.12 W/m2; UV−C: 0.04 W/m2) for 60, 180, or 360 d. The temperature of the UV chamber was maintained at 43−45 °C for the duration of the UV exposure. Natural UV was measured for comparison with the UV chamber 10 times a day in 1-h intervals during the day at the Geoje Island campus of the Korea Institute of Ocean Science and Technology on the southern coast of Korea for one year from March 2015 to February 2016 (UV-A: 1.95−9.80; 5.37 ± 2.87 W/m2 [min−max; mean ± standard deviation], UV-B: 0.09−0.64; 0.33 ± 0.20 W/m2; UV-C: 0.02−0.08; 0.05 ± 0.02 W/m2). The UV-A intensity in the UV chamber was slightly higher than in natural sunlight, while UV-B and UV-C fell within the natural range. Control samples were placed in sodalime glass Petri dishes and were wrapped in aluminum foil before exposure in the UV chamber under the same conditions as the exposed samples. Mechanical Abrasion. To simulate a real beach environment, sand was collected from Heungnam Beach on Geoje Island, Korea. The sand was pretreated before the experiment to remove organic matter and plastic. First, it was sieved within a size range of 63−1000 μm to remove mud particles (1 mm. Low-density particles, including plastics, were removed by density separation with a saturated NaCl solution, after which the salt was washed out with distilled water. The washed sand was combusted at 450 °C in a furnace for 4 h to eliminate organic material. Ten of each fresh and UVexposed PE, PP, and EPS pellets were placed in separate amber bottles (60 mL) with the pretreated sand (50 g). The three bottles for each group were placed and rotated on a roller mixer for 2 months at 37 rpm. Triplicate control samples were prepared containing only pretreated sand and were rotated for 2 months under the same conditions as the samples. Surface Weathering Observations. Physical damage and structural changes in the plastic surface were examined by scanning electron microscopy (SEM: JEOL JSM-7600F) with 3−5 kV electron accelerating voltage. All plastic samples were coated with platinum before the analysis to prevent surface charging. The oxidation status of the plastic surface was measured in the subsamples of UV-exposed pellets by FTIR for 12 months in 1-month intervals. The carbonyl index (CI), which can be used to represent the degree of weathering and surface oxidation,46,47 was used to characterize the degree of photooxidation of PE, PP, and EPS. The CI is defined as the ratio of absorption intensity at carbonyl groups around 1870− 1650 cm−1 to an internal constant band. Relative absorbance intensities of the carbonyl bond at 1712 cm−1 (maximum
fragmentation via surface weathering has only been indirectly demonstrated with photographs.32,33 The formation of nanoand microsized fragments was investigated under aqueous conditions with laboratory-accelerated photodegradation.34−36 A recent study confirmed the fragmentation of three polymer strips exposed to a saltmarsh habitat for eight months,37 but the loss of fragmented plastics in the open exposure system may have resulted in an underestimation of the number of fragmented particles produced. Without data on the process of fragmentation of plastics in marine environments collected following rigorous methods, it is difficult to estimate the size, number, and production rate of secondary microplastic particles formed via weathering. Polymer weathering is commonly caused by photooxidation, photothermal oxidation, mechanical abrasion (MA), hydrolysis, and biodegradation.38 Most synthetic polymers with a chromophoric group undergo a photochemical reaction to absorb ultraviolet (UV) radiation.39 Photochemical reactions caused by UV radiation absorption induce oxidation, which makes plastics brittle and easy to break up due to their decreasing elasticity.40 Small and large plastic litter floating on sea surfaces or washed ashore can become fragmented into microplastics.23,41 Once plastics are discarded into harsh environments, their surfaces become weak due to UV radiation exposure and oxidation, which likely generates microplastics via friction by wind, waves, and sand.21 Common polymers exposed to the marine environment are adversely affected by solar radiation (primarily UV-B), which initiates photooxidative degradation.42 Photooxidative and photothermal oxidative degradation may have greater effects on plastics on beaches than on the sea surface and floor due to the higher availability of UV radiation and oxygen in combination with a higher temperature.21,30,33 In addition, MA of plastics by sand is likely greater on beaches due to wind and wave action and tidal currents. Beaches are considered to be the most favorable environment for plastic weathering and fragmentation, although this hypothesis has not been tested. We evaluated the fragmentation of plastics, with an accelerated weathering experiment simulating beach environments in the laboratory. Three polymers, low-density polyethylene (PE), polypropylene (PP), and expanded polystyrene (EPS), were exposed to UV for up to 12 months in a UV chamber, followed by MA with sand for 2 months in the laboratory. We determined the degree of fragmentation by MA alone, the effects of UV exposure duration on the degree of fragmentation, the difference in fragmentation among polymers, and the abundance and size distribution of fragmented microplastic particles produced by weathering.
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MATERIALS AND METHODS Experimental Setting. The experiments were designed to simulate a beach environment. The two major components of weathering included in this study were UV irradiance and MA. It was difficult to simulate and control both UV irradiance and MA simultaneously. Therefore, we first exposed the model plastics to UV, and then examined MA. Experiments were performed in four sets with different UV exposure times (0, 2, 6, and 12 months) to identify the effects of the duration of exposure on the degree of fragmentation. The MA experiment was conducted for all of the experimental sets, and period was fixed for 2 months. Three polymers (PE, PP, and EPS) were selected to examine differences in weathering characteristics according to polymer type. PE and PP were reported as the two 4369
DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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Environmental Science & Technology
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-month MA; lower left: 6-month UV + 2-month MA; lower right: 12-month 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).
carbonyl peak for PP) and a band at 1375 cm−1 attributed to −CH3 bending were used in the CI analysis of PE and PP.30,48 For EPS, the CI was calculated by a comparison of the FTIR absorption band at 1720 cm−1 with the reference band at 1450 cm−1.49 The FTIR measurements were performed using a Nicolet 6700 spectrometer (Thermo Fisher Scientific) equipped with an attenuated total reflectance (ATR) probe, consisting of a diamond crystal at an incident angle of 42°. The spectra were recorded as the average of 32 scans in the spectral range of 650−4000 cm−1. Quantification of Microplastic Fragments. Fragmented PE, PP, and EPS particles were extracted from the sand by density separation using HPLC-grade water (Daejung, Korea) through a black membrane polycarbonate filter paper (47 mm Ø, 0.2 μm pore size). HPLC-grade water was used to avoid contamination. In addition, water rather than saturated NaCl solution was used for better separation of interfering dense inorganic particles from less dense PE, PP, and EPS fragments. The recovery rate (n = 3) of water separation was 99.4% for the most dense PE (100−500 μm in size) among the three polymers tested (Table S1). The filter papers were dried at room temperature and stored in glass Petri dishes. The plastic fragments on the filter papers were identified and quantified using fluorescence microscopy (excitation: 450−490 nm; emission: 515−565 nm; 50× and 200× ) after Nile Red (NR) staining.50 NR solution in hexane (5 mg/L, total volume: 200 μL) was used to dye the fragmented particles on the filter paper, and the filter paper was washed with 200 μL of hexane. The particles were categorized into 12 size classes based on their maximal length (1000 μm). While all NR-stained fragmented particles were counted with 50× magnification, all experimental sets for EPS and 12-month UV exposure group for PP were counted in an area 0.7−5% that of the filter paper under 200× magnification due to the large numbers of fragmented particles 0.05) in the total particle counts.
Although the sand was pretreated before MA experiments to remove NR-stainable plastics and organic matter, the control samples (n = 3) contained 65 ± 20 particles/bottle (mean ± standard error [SE]). The particles in the control samples stained with Nile Red were less than 300 μm in size and irregularly shaped. The number of fragmented plastic particles in the experimental groups differed significantly (t-test; p < 0.05) from the control except for the 2- and 6-month UV exposure groups for PE (PE UV2 and UV6). The PE UV2 and UV6 groups were excluded from the data analysis. The mean number of NR-stained particles of the control (65 particles/ bottle) was subtracted from the total fragmented particles in the bottles of each experimental group, which were then converted into particles/pellet. The results of fragmented plastic particles were expressed as the mean number of particles/pellet from triplicate samples (mean ± SE). Statistical Analysis. The difference in the number of fragmented particles and change in plastic volume among the three polymer types by UV exposure duration were compared and tested with an analyses of variance (ANOVA) and Duncan’s multiple-range test.
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RESULTS Surface Weathering by UV Exposure. The surfaces of UV-exposed preproduction PE and PP resin pellets and pellets detached from an EPS float were examined with SEM. The surface of PE pellets exposed to UV for 2 months did not show any changes compared to the control (Figure S2a); however, cracks were apparent after 6 months. After 12 months, hairline cracks began to appear in larger cracks. In the case of PP, surface cracking was apparent after 2 months of UV exposure, and the PP pellet was crushed by the pressure of the FTIRATR probe tip during the spectral observation (Figure S2b). After 6 months, the PP cracks were thicker and linked together, appearing as larger cracks. After 12 months, fine cracks began to appear along with the larger cracks. The surface of the EPS pellets yellowed, became fragile and brittle, and showed cracks after 2 months of UV exposure (Figure S2c). In addition, powder-like white fine particles were produced on the surface of EPS after 2 months. After 6 months, the whole surface turned yellow and the cracks became thick. More white fine particles were generated, and the EPS pellets were easily 4370
DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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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).
crushed. After 12 months, the parent EPS pellets had shrunk and large fragmented pieces remained, while the cracks in the surface became thicker. In addition, micrometer-sized fine particles were detected inside the cracks. While the outer surface was severely damaged, a nondamaged fresh inner surface was observed inside the cracks. Aggregations of small particles 0.05) between the UV6 and UV12 groups. Comparison of Fragmentation by Polymer Type. Significantly more (p < 0.05) EPS particles were produced by MA alone (UV0) and all of the UV exposure groups compared to the respective exposures for PE and PP particles (Figure 3). The number of PE and PP particles produced by MA alone did not differ significantly. However, the number of PP particles
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).
increased much more steeply than PE during the 12 months of UV exposure, resulting in significantly more (p < 0.05) fragmented PP particles than PE particles (Figure 3d). Size and Volume Distribution. The number of fragmented polymer particles produced by UV exposure and MA increased with decreasing particle size (Figure 4). This trend was observed for all polymer types and UV exposure durations. The particle-size normalized abundance of fragmented plastic particles could be modeled by assuming a steady state.51 The relationship between the size-normalized particle abundance (the abundance of fragmented particles was divided by each size-class interval) and particle size was evaluated, except for particles >1 mm for PP and EPS. The size-normalized abundance of the fragmented PE, PP, and EPS particles according to particle size after UV exposure for 12 months and MA for 2 months showed a significant (p < 0.05) relationship in the regression analysis (r2: PE = 0.9498, PP = 0.9823, and 4371
DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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Environmental Science & Technology
Figure 4. Size distribution of fragmented (a) polyethylene (PE), (b) polypropylene (PP), and (c) expanded polystyrene (EPS) particles after UV exposure for 0 (UV0), 2 (UV2), 6 (UV6), and 12 months (UV12) followed by 2 months of mechanical abrasion (MA) with sand. The measured size distributions of the normalized abundance by width (μm) of the size class of fragmented (d) PE, (e) PP, and (f) EPS particles after UV exposure for 12 months (UV12) followed by 2 months of mechanical abrasion (MA) with sand, on a logarithmic scale.
EPS = 0.9710) with exponential equations (Figure 4). Particles 1 mm. The peak gradually shifted to the 0−50 μm size class 4372
DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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strength of EPS than PE and PP. Polystyrene (1.04−1.1 g/cm3) is generally denser than PE (0.92−0.97 g/cm3) and PP (0.9− 0.91 g/cm3),5 but its density steeply decreases to 0.010−0.035 g/cm3 after expansion with a blowing agent and steam.52 In addition, the tensile strength of EPS (3−14 kPa) (Australian Urethane & Styrene EPS Technical Data) is one or two order(s) of magnitude lower than that of low-density PE (203 kPa) and PP (783 kPa).53 Although EPS, as a foamed plastic, has an excellent capacity to absorb shock and insulate heat, it is susceptible to frictional forces and can be torn apart readily by sand particles. Fragmentation According to UV Exposure Duration. In this study, fragmentation of polymers increased with increasing UV exposure time in combination with MA, even though the rate of fragmentation differed by polymer type. The bond dissociation energy of C−C bonds (375 kJ/mol) and C− H bonds (420 kJ/mol) is equivalent to UV radiation of 320 and 290 nm, respectively.54 UV light has enough energy to produce initial free radicals as the primary photochemical products and causes C−C and C−H bonds to dissociate from the polymer backbone.28 This can subsequently lead to one or more chemical changes, resulting in embrittlement at the surface of polymers, visible as cracks and fractures. UV oxidation produces embrittlement at the surface of polymers to a depth of more than 100 μm caused by cross-linking and chain reactions.28 This brittle surface layer easily formed cracks and became more obvious and thicker by increasing the UV exposure time of three polymers, as shown by SEM (Figure S2). Interestingly, the brittle surface and cracks from photooxidation (or weathering) did not directly result in the “fragmentation” of polymers. Although many large and small cracks were observed on the surfaces of PE and PP pellets, no fragments were detected with UV exposure alone by SEM, and their formation required subsequent MA. This implies that additional physical force is required to enhance the breaking of embrittled plastics. The increase in the fragmentation rate of PP (UV12/UV0 ratio = 804) according to UV exposure duration was higher than for PE (3.7) and EPS (2.6). In fact, field samples of pellets or fragments of PP had a higher degree of chemical weathering, or were less resistant to weathering, than PE samples, based on SEM and FTIR.55 Every other carbon atom in the PP backbone is a tertiary carbon that is more susceptible to abiotic attack than the secondary carbons of PE.56 In addition, the maximum sensitivity, as determined by the bond dissociation energies, are in the order of PE (96 kcal/mol at 300 nm) > PS (90 kcal/mol at 318 nm) > PP (77 kcal/mol at 370 nm). This indicates that less energy is required to dissociate chemical bonds in PP than PE and EPS. EPS fragments were observed on the pellet surface after 2 months of UV exposure in this study, indicating that UV exposure alone, without MA, was sufficient to break EPS into microsized fragments. Although EPS has unsaturated double bonds that are susceptible to photoinitiated degradation,56 the foamed structure of EPS may influence both photooxidation and subsequent fragmentation. The depth of UV penetration, diffusion of radicals, and availability of oxygen is critical in the propagation and rate of photooxidation of polymers.57 The mesoporous structure of EPS could assist photooxidation by facilitating the penetration of UV radiation and diffusion of radicals, while increasing oxygen availability. Expansion of polystyrene changes the bulk plastic into a fused small balloonlike structure with a thin polystyrene envelope. The progression of cracks and fusion with other cracks in the thin layer more readily fragmented than the bulk plastic (Figure S2c). In
Figure 5. Change in the total volume of plastics calculated from the sum of the remaining parent pellets (solid bar) and fragmented particles (open bar) according to UV exposure duration for (a) lowdensity polyethylene (LDPE), (b) polypropylene (PP), and (c) expanded polystyrene (EPS).
with increasing UV exposure duration from 0 to 6 months (Figure S7c), while the volume density of the parent pellets and fragment particles >1 mm rapidly decreased during the same period.
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DISCUSSION Fragmentation by Mechanical Abrasion. MA with sand produced 8.7 ± 2.5 particles/pellet of PE and 10.7 ± 0.7 particles/pellet of PP, while producing 4220 ± 33 particles/ pellet of EPS. These results indicate that PE and PP are unlikely to be fragmented by MA without photooxidation by UV; however, EPS pellets are susceptible to abrasion with sand. These results can be explained by the lower mechanical 4373
DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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compared with MA only for all three polymers (Figure 2). In addition, two processes were sequentially adopted to measure UV exposure duration exactly and to evaluate its role in the fragmentation mechanism. However, UV exposure and MA of plastics may occur simultaneously in real beach environments. Furthermore, the UV exposure and MA are not independent variables. UV exposure after MA and simultaneous UV exposure and MA may produce different results. In UV exposure after MA, attached or embedded sand particles in the plastic surface as a result of MA may reduce the UV-exposed surface area, and the rough plastic surface formed by MA may scatter or enhance the penetration of UV light. Simultaneous UV exposure and MA may significantly reduce the UV exposure time as a result of the rolling and burial of plastics or a shading effect caused by sand particles. Natural organic matters that interfere with Nile Red staining were removed in this study. Although the organic matter content of beach sand is generally low,63 the interaction of organic matter with plastics probably influences the weathering process. The absorption or attachment of organic matter on the plastic surface may reduce the penetration of UV light to the plastic surface or the friction force of sand particles. The UV intensity in the experimental chamber used in this study was comparable to or rather lower (UV-B and UV-C) than that of natural sunlight in Geoje, Korea, suggesting that the level of photooxidation by UV radiation in the chamber represents that of sunny days. Temperature, which influences photothermal oxidation, was maintained in the chamber (43− 45 °C) by cooling to fall within the environmentally relevant range of beach sand during the summer in Geoje, Korea (rooftop exposure from 9 AM to 6 PM in August; range 21.5− 49.5 °C; average ± SE 42.1 ± 5.76 °C). The plastics were weathered in the UV chamber for 24 h a day to shorten the experimental time. The total irradiance time of sunlight in Geoje from March 2015 to February 2016 was 2087 h (86.9 d).64 The UV exposure time of 2, 6, and 12 months in the chamber converted into the equivalent time in Geoje was 8.4 (0.7 yr), 25.2 (2.1 yr), and 50.4 (4.2 yr) months, respectively. One year of exposure in the UV chamber corresponded to 4.2 yr outdoors in Geoje. However, the time of plastics exposure to UV in a real environment could be decreased by burying the plastics in sand, attached organic matter, and the amounts of additives. Many plastic products and beached plastic debris contain additives, including antioxidants and UV stabilizers to retard UV effects.65 Therefore, it requires more than 4.2 yr in a real beach environment to produce the same degree of fragmentation found in this study. On beaches, photooxidized plastics may experience various mechanical forces. Wind, tidal currents, and waves can cause friction with air, water, and sand on the beach. In addition, human activities on popular recreational beaches may generate mechanical forces. However, it is difficult to estimate the average intensity and duration of mechanical forces experienced by plastic debris in the environment to convert the 2-month period of MA in this study to an equivalent time in the environment. Further studies are required to consider a variety of environmental factors, and the properties of polymers, to estimate the fragmentation of plastics in real marine environments and model the fragmentation processes.
addition, the gradual decrease in the CI of the surface of EPS pellets after 6 months of UV exposure (Figure S3c) supported the assumption that fragmentation of the surface layer exposed the underlying relatively less oxidized surface. According to polymer type, the size of fragmented particles by chemical and mechanical weathering differed and the rate of fragmentation was in the order of EPS > PP ≫ PE. The physicochemical properties of polymers determined the degree and rate of weathering and fragmentation in combination with environmental factors. The fragmentation of PE and PP could be impeded by additives, such as UV stabilizers and antioxidants, compared with EPS. Therefore, not only the amount of each polymer used in production and the polymer composition of marine macroplastic debris but also the weathering and fragmentation characteristics of polymers are critical determining factors for predicting microplastic abundance in the environment. Size Distribution of Fragmented Microplastics. The number of fragmented particles produced by UV exposure and MA in all three tested polymers increased with the decreasing size of fragmented microplastics. Similar size distribution patterns have been found in coastal waters and sediments,24,58 although other studies have observed that the abundance increased down to a size of a few millimeters and then decreased.51,59 Fragmented microplastics 300 μm) of microplastics sampled with a Neuston net or sieving, a large proportion of microplastics produced by fragmentation could be excluded from sampling. Therefore, their abundance in the environment may be underestimated. Estimation of Fragmentation in the Field. In this study, the fragmentation process of plastics on a beach was simulated by MA after UV exposure because we expected that photooxidation by UV is required before MA to enhance fragmentation. This was proven by the significant enhancement of fragmentation with UV exposure and subsequent MA 4374
DOI: 10.1021/acs.est.6b06155 Environ. Sci. Technol. 2017, 51, 4368−4376
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Environmental Science & Technology
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b06155. Material and methods: Quantification of microplastic fragments. Table S1 addresses the recovery test. Figure S1−S7 address (S1) test material, (S2) surface of UVexposed preproduction resin pellets, (S3) carbonyl index according to UV exposure duration, (S4) size distribution of fragmented PE particles, (S5) size distribution of fragmented PP particles, (S6) size distribution of fragmented EPS particles, and (S7) volume density distribution of fragmented particles. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel: +82-55-639-8671. Fax: +82-639-8689. E-mail: wjshim@ kiost.ac.kr. ORCID
Won Joon Shim: 0000-0002-9591-8564 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the Ministry of Oceans and Fisheries, Korea, under a research project titled “Environmental Risk Assessment of Microplastics in the Marine Environment”.
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