AbstractPlastic litter in the urban environment has been identified as a source of microplastics and stormwater a pathway for its transportation to freshwater and marine environments. However, few studies exist on the potential for litter to contribute to microplastics in a land-based system. This laboratory-based study involves simulation of the weathering of four polymers [low-density polyethylene (PE-LD), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET)] in a land-based environment using accelerated photodegradation with three exposure times. Microplastics generated were quantified with Fourier transform infrared spectroscopy and identified using a spectra reference library. The results showed differences in release patterns and number of particles produced. For LD-PE, no clear pattern of UV-degradation was demonstrated, because the number of particles released from exposed and unexposed (control) samples was in the same order of magnitude. PS and PET showed similar patterns, where the number of particles released increased with exposure duration. The numbers of particles detected were, on average, 1, 8, and 31 particles/cm2 for PS and 3, 3, and 16 particles/cm2 for PET for exposures of seven, 28, and 56 days, respectively. PP produced the largest number of particles after 28 days exposure (ca. 58 particles/cm2) which then decreased after 56 days (ca. 21 particles/cm2). It was hypothesized that the number of particles increased with exposure time and that the generated particles then further fragmented into pieces of undetectable particle size (<10 μm).IntroductionLittering occurs in urban environments with plastic being one of the most common forms of debris. The large amounts of small plastic particles released annually into the aquatic and marine environment is a growing environmental problem. These particles originate from various sources, both on land and in oceans (Jambeck et al. 2015). For example, Jambeck et al. (2015) estimated that between 4.8 and 12.7 million tonnes of plastic debris reached the ocean in 2010. Microplastics are usually defined as plastic particles ranging in size from 5 mm to 1 μm (Arthur et al. 2009). Fragmentation of plastics into microplastics in nature can be caused by photo, biological, thermal, and mechanical degradation (Yousif and Haddad 2013). Reduced elasticity due to oxidation caused by absorption of ultraviolet (UV) radiation makes plastic brittle and easily breakable (Song et al. 2017; Yousif and Haddad 2013). Plastic debris, both small and large, can become fragile, then fragment into microplastic.Stormwater is recognized as a pathway for transporting pollutants, including microplastics, from sources on land in urban areas into the receiving water bodies (Magnusson et al. 2016; Shruti et al. 2021). Littering has been identified as a source of secondary microplastics in the environment, along with construction work, road and tire wear, artificial turf, and atmospheric fallout (Dris et al. 2016; Järlskog et al. 2020; Liu et al. 2019; Magnusson et al. 2016). However, it is not possible to quantify littering as a source of microplastics with available monitoring data (Magnusson et al. 2016).In this laboratory study, four common plastic debris were exposed to UV radiation. These are products commonly found in litter in the urban environment, from products made of polymers that are among those with the highest production volumes (present and past) (PlasticsEurope 2019). In studies of microplastics in stormwater and stormwater-related sediments, PP was found to be the most abundant polymer. Other types of polymers, such as PS and PE, have also been found but in smaller amounts in comparison to PP. As shown in a study by Liu et al. (2019) of microplastics (size range 10–2,000 μm) in sediments from seven urban and highway stormwater retention ponds, PP was the most common polymer found, both in terms of particle number and mass. Most of the identified microplastic particles were small (10–250 μm) (Liu et al. 2019). PP was also among the most commonly found polymers in a study of microplastics (20–100 μm) in highway runoff (Lange et al. 2021). The overall aim was to increase the knowledge of one of the processes generating secondary microplastics in a land-based urban environment. This was achieved by observing plastic debris degradation behaviors and patterns due to UV exposure over time, and by quantifying the release of microplastic particles.MethodsFour common plastic debris in the form of a plastic bag, a chocolate bar wrapper, a plastic coffee cup lid, and a plastic bottle, made of PE-LD, PP, PS, and PET, respectively, were tested. All tested materials were white except for the PET bottle, which was transparent blue.The sample materials, from new plastic products to ensure no previous wear, were cut into pieces (4×4 cm2) to allow comparison between the different litters and were put into glass beakers which were placed under UV light at 20°C. Four single UV lamps from Q-LAB (type: UVA 340 nm 40 W T12 lamp) were placed 15 cm above the plastic materials giving a total irradiation of 66.4Wh/m2. The UVA lamps generated wavelengths between 295 and 365 nm (emission peak: 340 nm) and simulated outdoor weathering due to sunlight (Q-LAB 2019). The open beakers with the UV light source placed above allowed the samples to be exposed directly to the light without it having to go through glass before reaching the sample material.Three different exposure times were used (7, 28, and 56 days) to identify the effects of the duration of exposure on the degree of fragmentation, and to find whether it differed for the different polymers. The samples were exposed 24 h a day. The exposure times of 7, 28, and 56 days were selected to correspond to approximately a quarter of a year, 1, and 2 years, respectively, outdoors in Sweden (UVA radiation in Luleå: 41863 W/m2×yr) or approximately one eighth of a year, half, and 1 year, respectively, near the equator. As it was assumed that the UV lamps produced less radiation towards their edges, the sample containers were placed on trays, which were rotated 180° weekly to ensure an even distribution of radiation. The experiments for all polymer types and exposure times were duplicated.After exposure, a volume of 30 mL of filtered deionized water was added to samples of each polymer (duplicates) and a control sample (nonexposed sample kept in glass beakers covered with aluminum foil in a dark space) and blanks (empty beaker placed along with the samples under the UV lamp). The beakers containing samples, blanks, and controls were placed in an ultrasonic bath (Elmasonic S 50 R) for 10 min per sample (5 min per sample side) to release loose particles. The liquid samples were transferred into glass bottles for further analysis of the microplastics. The glass beakers and sample materials were rinsed with 10 mL deionized water each, making a total volume of 50 mL per sample.To avoid contamination, all equipment including the plastic sample materials was thoroughly rinsed in deionized water three times prior to the experiment. Throughout the study, the deionized water used was filtered through a Whatman 1.2 μm glass microfiber filter to avoid contamination from the purification equipment and its tubing.The samples (50 ml per sample), including blanks and controls, were filtered through 10 μm metal filters (1 cm2 surface area), and the filters were analyzed using micro–Fourier Transform Infrared Spectroscopy (μFTIR; PerkinElmer, Spotlight 400 with automatic μATR) at the ALS Scandinavia ABs laboratory. μFTIR is a common analytical technique for MP analysis in environmental samples (e.g., Liu et al. 2019; Lange et al. 2021). Briefly, 1/4th of the whole filter area was scanned in point mode for counting the number of particles on the filter, and image mode for determination of the area of the particles on the filter. The particles were identified and quantified using a library of reference spectra containing more than 40,000 different polymers. Thereby, only PS particles were counted for the PS coffee cup lid, PE-LD for the plastic bag, and so on. For a selection of samples, the mass of microplastics was calculated, based on the particle surface area and the density of the respective polymers, using SiMPle® software. Briefly, an ellipsoid shape was assumed for the particles, and the thickness was assumed to be 67% of each particle’s minor dimension, as described in, e.g., Liu et al. (2019). This calculation was carried out for PP and PS samples with exposure times of 28 and 56 days. The number of particles per area (particles/cm2) was calculated for each sample based on the number of particles detected and the area of sample material.Results and DiscussionConcentrations of microplastics in the UV-exposed samples showed different degradation behaviors over time for the polymers included in this study. Degradation was apparent for three of the studied polymers, PP, PS, and PET, due to UV radiation [Figs. 1(a–d)]. For the fourth polymer, PE-LD, no clear degradation due to UV exposure was observed.The low number of particles released from exposed PE-LD samples and unexposed (control) samples have the same order of magnitude [Fig. 1(a)]. In theory, the bonds in the PE-LD plastic are also susceptible to degradation due to UV exposure; however, the process takes longer for PE in comparison with, for example, PP and PS (Gewert et al. 2015). An explanation for the results in this study would, therefore, be that the degradation due to UV exposure is slow, and that additional exposure time is needed to observe degradation. It is worth noting that PE-LD microplastics found in urban waters are broken down by a combination of weathering processes. Furthermore, the durability of PE plastics also largely depends on which additives, for example UV stabilizers, are included in the plastic product (Gewert et al. 2015).PS and PET show similar degradation patterns, where particles released increase with the exposure duration. The numbers of particles detected were, on average, 1, 8, and 31 particles/cm2 for PS and 3, 3, and 16 particles/cm2 for PET for exposure times of 7, 28, and 56 days, respectively. The PP produced the largest number of particles after 28 days exposure (ca. 58 particles/cm2) which decreased after 56 days (ca. 21 particles/cm2). It is suggested this may be because the number of particles released from the PP sample increases with exposure time, and the particles become too small to detect, smaller than 10 μm (limited by both filter pore size and the μFTIR technique).The blanks indicated a low level of contamination from the experiment and analysis process. The low concentrations of microplastics in the nonexposed control samples are an indication of a low presence of microplastics in the sample materials prior to the experiment, and that no degradation due to other types of impact apart from the UV radiation took place in the exposed samples. A few particles of polymers other than the four included in the present study were observed, both in exposed samples, blanks, and controls, and were considered to be contamination.These results show degradation of three of the studied polymers due to UV radiation, which is in line with previous studies (Song et al. 2017; Lambert and Wagner 2016). A study by Lambert and Wagner (2016) showed an increase in nanoplastics (in the size range 30–2,000 nm) released from a UV-exposed plastic lid for a take-away coffee cup made of PS over time. However, this study was also carried out in water. A laboratory experiment by Song et al. (2017), set up to simulate weathering on beaches (mechanical abrasion and photodegradation), was carried out in water on three types of polymer pellets: PE-LD, PP, and EPS (expanded polystyrene). It showed an increase in degradation with longer UV exposure times, with a larger increase for PP than PE. The results of our study showed that PP degrades to the greatest extent due to UV exposure. This is in line with results of studies using analysis with μFTIR to quantify and identify microplastics particles in urban stormwater (Liu et al. 2019; Lange et al. 2021).As a complement to the particle counts for each plastic, the masses of microplastics in exposed PP and PS samples were determined (Table 1). The selection of PP and PS was based on their elevated concentrations in comparison with PET and LDPE (Fig. 1). They showed an increase in the mass of microplastics released and an increase in average particle weight with the exposure duration. This also supports the hypotheses that the particles released from PP samples after 28 days of exposure (the exposure time resulting in the largest number of particles) were smaller than the particles released after 56 days of exposure, and that particles detected in samples after 28 days of exposure continued to degrade into smaller particles than 10 μm.Table 1. Number of particles and mass of particles released per cm2 and per item of PP and PS samples exposed for 28 and 56 days. Mean values of duplicate measurements are shownTable 1. Number of particles and mass of particles released per cm2 and per item of PP and PS samples exposed for 28 and 56 days. Mean values of duplicate measurements are shownCategoryUnitExposure time (days)PS coffee cup lid (total area: 70 cm2)PP chocolate bar wrapper (total area: 600 cm2)Concentrationparticles/cm2 (particles/item)287.6 (530)58 (17,000)5631 (2,200)21 (13,000)Massμg/cm2 (μg/item)283.8 (270)5.1 (3,100)562,900 (203,000)3,700 (2200,000)Average particle weight(μg/particle)280.50.15693176In addition, the results for exposed PS samples showed a presence of PP particles, but the reason for this has not been determined. The hypothesis is that the PS sample, a plastic lid for a take-away coffee cup, also contained PP; however, this has not been confirmed.ConclusionsIn conclusion, this study showed that there was clear degradation of PS, PP, and PET from a coffee cup lid, a chocolate bar wrapper, and a plastic bottle, respectively, due to UV exposure in comparison with unexposed (control) samples. For these materials, degradation patterns showed an increased release of microplastics with longer UV exposure times. PP was most sensitive to UV exposure followed by PS and PET. Due to outdoor UV exposure in the urban environment, litter made of this type of polymer can contribute to microplastics in stormwater and the spread of microplastics to water bodies and oceans. For the plastic bag made of PE-LD, degradation due to UV exposure was not observed over the exposure times used because the numbers of particles released from exposed and unexposed (control) samples were in the same order of magnitude. Future studies are needed to investigate the degradation of plastic litter and its contribution to microplastics in stormwater. For further studies of microplastics, a lower size limit of detection should be used, and, as a complement, the total mass of polymers should be analyzed, and also a combination of degradation mechanisms. Investigations of the same polymers from different sources will also be of interest, because the degradation is not only dependent on the polymer but also on additions of additives, pigments, and UV stabilizers.Data Availability StatementAll data that support the findings of this study are available from the corresponding author upon reasonable request.AcknowledgmentsWe gratefully acknowledge the financial support provided by the VINNOVA (Swedish Governmental Agency for Innovation Systems) DRIZZLE—Centre for Stormwater Management (Grant No. 2016-05176) and The Swedish Environmental Protection Agency project μrban plastics (Ref. No. 208-0182-18). The authors also thank Peter Rosander for assisting with the experimental setup.References Arthur, C., J. Baker, and H. Bamford. 2009. Proceedings of the international research workshop on the occurrence, effects and fate of microplastic marine debris. NOAA Technical Memorandum NOS-OR&R-30. Silver Spring, MD: NOAA Marine Debris Division. 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