key: cord-0957181-05kwhr6q
authors: Wang, Zheng; An, Chunjiang; Chen, Xiujuan; Lee, Kenneth; Zhang, Baiyu; Feng, Qi
title: Disposable masks release microplastics to the aqueous environment with exacerbation by natural weathering
date: 2021-09-05
journal: J Hazard Mater
DOI: 10.1016/j.jhazmat.2021.126036
sha: 7310cd84a998716ef21111e533eef94ee073acad
doc_id: 957181
cord_uid: 05kwhr6q

The COVID-19 pandemic has driven explosive growth in the use of masks has resulted in many issues related to the disposal and management of waste masks. As improperly disposed masks enter the ocean, the risk to the marine ecological system is further aggravated, especially in the shoreline environment. The objective of this study is to explore the changing characteristics and environmental behaviors of disposable masks when exposed to the shoreline environment. The transformation of chain structure and chemical composition of masks as well as the decreased mechanical strength of masks after UV weathering were observed. The melt-blown cloth in the middle layer of masks was found to be particularly sensitive to UV irradiation. A single weathered mask can release more than 1.5 million microplastics to the aqueous environment. The physical abrasion caused by sand further exacerbated the release of microplastic particles from masks, with more than 16 million particles released from just one weathered mask in the presence of sand. The study results indicate that shorelines are not only the main receptor of discarded masks from oceans and lands, but also play host to further transformation of masks to plastic particles.

With the spread of the coronavirus (SARS-CoV-2), as of March 2021, a total of more than 115 million cases have been diagnosed around the world (Johns Hopkins University Coronavirus Resources Cenrer, 2021). Based on emerging evidence early in the pandemic, the World Health Organization (WHO) recommended wearing surgical masks or other face coverings to prevent airborne transmission of the virus (World Health Organization, 2020), leading to a dramatic increase in the demand for disposable face masks (Gereffi, 2020) . Due to the shortage of disposable mask stock and the increase in prices, the public was encouraged to use alternative face coverings in the early stage of the pandemic. However, with increasing production capacity, the supply and use of disposable masks have gradually increased to unprecedented levels. For example, the daily production of disposable masks in China increased from 20 million in January, 2020, to 200 million in March, 2020 (Organisation for Economic Co-operation and Development, 2020) . It is estimated that more than 129 billion face masks are consumed globally every month .

However, the explosive growth in the use of masks has introduced numerous issues related to the disposal and management of waste masks (Patricio Silva et al., 2021 . A large amount of mask waste is discarded without any treatment, as a result of inappropriate waste management and the lack of environmental awareness (Cordova et al., 2021 , Haque et al., 2021 . Various types of masks have been widely observed on shorelines, rivers, lakes and in the aqueous environment (Aragaw, 2020 , Canning-Clode et al., 2020 , Fadare and Okoffo, 2020 . The masks themselves pose a direct threat to marine life (Guzzetti et al., 2018) . Moreover, the plastic fragments or microplastics derived from masks can further cause many other environmental problems. Microplastics (less than 5 mm) even nano-size plastic particles (less than 100 nm) are widely present in marine (Andrady, 2011) , sediments (Lin et al. 2020) , soil (Corradini et al. 2019 , Yin et al. 2021 , freshwater (Li et al. 2018, Wang and Hao, 2020) , the atmosphere , and other environmental matrices (Geilfus et al. 2019 , Kanhai et al. 2019 . Recent studies have revealed that microplastic can also be released from teabags (Hernandez et al. 2019) , cigarette butts (Belzagui et al. 2020) , food containers (Du et al. 2020 , disposable paper cups (Ranjan et al. 2020) , personal care products (Hernandez et al. 2017b) , and other household and industrial products (Dibke et al. 2021 , Hernandez et al. 2017a . Not surprisingly, evidences of microplastics in living organisms is now being discovered. Microplastics can accumulate and transfer within plankton, fish, and birds through the food chain (de Sa et al. 2018) . What is more shocking is that microplastics have been found in the human colon (Ibrahim et al. 2021) , and even in the human placenta (Ragusa et al. 2021) . Therefore, it is necessary to investigate the potential release of microplastics from disposable face masks to determine the pollution risk and improve waste mask management.

The commonly used disposable masks are made from polymeric materials, especially polypropylene. A standard disposable mask typically consists of 3 layers: the outer hydrophobic spun bond layer, a middle melt-blown layer, and an inner non-woven layer. The plastic can be weathered when it is subjected to UV irradiation, mechanical abrasion, oxidation, and biodegradation, leading to changes in physical and chemical properties. On the one hand, the surface of the virgin polymer will become rougher after weathering, making the plastic more friable and prone to form small fragments, even microplastics and nano-plastics (Cai et al. 2018 , Song et al. 2017 ). On the other hand, the adsorption ability of weathered plastic to organic matter or other pollutants will increase after disposal and introduced to the environment (Liu et al. 2020a (Liu et al. , 2020b . As large numbers of improperly disposed masks enter the ocean, the risk disposed plastics pose to the marine ecological system is exacerbated, particularly in shoreline environments . The coast, it should be noted, is a complex environment that comprises seawater, groundwater, soil, atmosphere, and other environmental factors (Bi et al. 2021 . Discarded disposable masks on beaches will be further weathered when exposed to natural factors such as sunlight (including UV irradiation), sand abrasion, and sea waves (De-la-Torre and Aragaw, 2021 , Kotek et al. 2004 , Song et al. 2017 . With the increasing presence of waste masks on shorelines, it is critical that the environmental effect of disposable masks on the shoreline environment be evaluated in a timely manner. In this context, the main objective of the study described in this paper was to explore the changing characteristics and environmental behaviors of disposable masks when exposed to the shoreline environment. Disposable masks were subjected to weathering, followed by the characterization through various methods. The phenomenon of microplastics being released from disposable masks to the aqueous environment, compounded by natural weathering, was then explored.

Ecoparksg disposable masks were purchased from Fisher Scientific (Canada). The elastic earloop and nose bridge were first removed, and the rest of the mask was then cut into strips (1.5 cm × 14 cm). Some virgin mask strips were placed in glass Petri dishes uncovered, then exposed to UV light (254 nm, 20 mW/cm 2 ) for 1-48 h in a UV chamber (CX-2000 Crosslinker, Analytik Jena, USA) . Control samples were placed in glass Petri dishes and wrapped in aluminum foil before exposure in the UV chamber under the same conditions as the exposed samples. The mask strips (outer layer, middle layer, and inner layer) with different irradiation times and 50 mL of deionized (DI) water were added into the Erlenmeyer flask in the absence and presence of sand (20 g). The flasks were placed in a New Brunswick Innova 42R incubator shaker (Eppendorf, USA) with 300 rpm and 25 ℃ for 24 h. The calcinated quartz standard sand obtained from MilliporeSigma (Canada), was washed with DI water, then dried in an oven at 60 ℃ for 24 h to remove any impurities. The same procedure was applied to observe the particles released from each of the mask layers. Triplicate slices of mask layer were added to the Erlenmeyer flask. Control experiments without masks were conducted following the same procedure to explore the potential effect of sand (Fig. 1 ).

The virgin and UV weathered mask strips were analyzed using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR, Tensor 27, Bruker, USA). Spectra were acquired in wavenumber ranges of 4000-400 cm -1 with 32 scans. The background spectrum was obtained for each sample. Four measurements were taken for each sample on the surface. The FTIR spectra were compared with a set of reference data in order to identify the peaks. The mechanical properties of the mask strips before and after UV irradiation were measured using the Model 42 Material Test System (MTS Criterion, USA). Each measurement was repeated three times, and the average values were calculated. An Innova atomic force microscope (AFM, Bruker, USA) with RTESPA-150 silicon probes was used to analyze single fiber of the mask surface in tapping mode. The probe had a spring constant of 6 N/m and a resonance frequency of 150 kHz. The AFM images obtained were analyzed using Nanoscope Analysis v1.8 (Bruker, USA). Physical damage and structural changes on the mask surface were examined using an S-3400N scanning electron microscope (SEM, Hitachi, Japan) with 5 kV electron accelerating voltage. The mask strip was cut into small squares (0.5 cm × 0.5 cm) and placed on the carbon tape that affixed on the surface of the sample holder. The sample holder was then coated with a 2 nm layer of gold for SEM imaging. The mask samples were viewed under different magnifications ranging from 100 to 5000. In addition, the hydrophobicity of the weathered mask surface was evaluated by the water contact angle, which was measured by a contact angle goniometer (VCA Optima, AST Products, USA).

The particle size distribution (PSD) in the water sample was analyzed using a laser in-situ scattering and transmissometry analyzer (LISST-200X, Sequoia, USA). The detection range of the analyzer was 1-500 µm, and the concentration of particulate matter (μL/L) was obtained by calculating the equivalent volume of the irregular spheres. Samples of 20 mL were added to the small chamber of the LISST, and live data collecting mode was used to scan the samples. Each sample was scanned for 1 min, and a total of 60 data points were collected (Serra et al. 2020 , Yang et al. 2021 . Control samples containing only sand were also tested. The DI water in the control samples was applied as the background for each test. To view the shape of particles released in the leachates, 20 μL of water was carefully drop onto the sample holder and dried for SEM observation. In order to avoid the "coffee ring effect" during drying of the leachate, the sample holder was washed with ethanol to make the surface hydrophilic prior to adding the water samples (Hernandez et al. 2019) . After drying, the sample holders were coated with a 2 nm layer of gold for imaging.

ATR-FTIR analysis was performed in order to analyze structural changes of the mask strips after weathering. Fig. 2 presents the FTIR spectra of three layers of both virgin and weathered mask strips. The three layers of the mask displayed indistinguishable spectra, with four large peaks in the wavenumber ranging from 3000 to 2800 cm -1 and two large peaks in the range from 1540 to 1300 cm -1 . The peaks at 2918 and 2835 cm -1 were attributable to asymmetric and symmetric stretching vibrations of CH 2 groups, while the peaks at 2950 and 2870 cm -1 were due to the asymmetric and symmetric stretching vibrations of CH 3 groups (Morent et al. 2008 , Ullah et al. 2020 . This results also indicates that the CH 3 asymmetric vibrations or CH 2 scissor vibrations led to the peak at 1456 cm -1 , while the peak at 1375 cm -1 was the result of CH 3 symmetric deformation. In addition, the small peaks between 1200 and 700 cm -1 were identified as the characteristic bands of the polypropylene crystal structure, including C-C asymmetric and symmetric stretching, C-H wagging vibrations, CH 3 asymmetric rocking, and CH 2 asymmetric rocking vibrations. The formation of C˭C and C˭O structures contributed to the peaks in the 1750-1600 cm -1 range (Morent et al. 2008 , Ullah et al. 2020 .

The FTIR spectra of the virgin masks indicated that all three mask layers were made from polypropylene. The obvious peaks caused by the vibration of CH 3 and CH 2 , meanwhile, indicated that these functional groups were the main components of the mask. Comparing the spectra of masks under different durations of UV weathering, it was found that the intensity of all peaks gradually decreased as the UV weathering duration increased, with some small peaks appeared between 1750 and 1600 cm -1 simultaneously. These minor changes imply that some C-H bonds were broken and double bond structures were formed (Aslanzadeh and Haghighat Kish, 2010, Aslanzadeh and Kish, 2005) . Moreover, it is worth noting that the resistance to UV weathering varied depending on the layer. The inner layer exhibited strong resistance, followed by the outer layer, while the middle layer was the most vulnerable to UV weathering. With respect to the middle layer, after 18 h of weathering, a tiny peak appeared in the range of 1750-1600 cm -1 , and after 36 h of weathering, a more pronounced peak appeared at 1710 cm -1 . These peaks can be attributed to the formation of C˭C and C˭O structures. After 36 h of weathering, a peak at 1710 cm -1 was found in the spectrum of the outer layer, while, for the inner layer, a more obvious peak resulting from the formation of double bond structures did not appear until 48 h of weathering. The weathering of masks is due to photo-oxidation, resulting from free-radical chain reactions (Cai et al. 2018 ). In the present case, the UV radiation provided sufficient energy for the C-C and C-H bonds to be broken, generating alkoxy radicals and peroxyl radicals. This, in turn, led to chain scission or crosslinking. Meanwhile, some double bond groups, such as carbonyl groups were also formed.

To explore the surface characteristics of masks, AFM and SEM were applied to observe the micro changes in the mask fibers before and after UV weathering. It can be seen from SEM images (Fig. 3 ) that the majority of fibers were intact and had a smooth surface before weathering. Only a small part of the surface was accidented or attached with tiny particles. It also can be seen that the diameter of individual fibers in the outer and inner layers was about 30 µm, while the fibers in the middle layer were roughly 5 µm in diameter. After 18 h of weathering, the outer and inner layers showed a similar change in fiber surface, i.e., obvious deformation or even damage on the surface. This was caused by irradiation at the intersections of fibers, which were easy to bend. At the same time, some small particles and fiber fragments formed where the damage occurred. The middle layer showed signs of more severe weathering. Not only did the surface of the fiber became abrasive, but some fractures of fiber occurred. When weathered for 36 h, most of the fibers in all three layers became fractured, forming fiber fragments of varying lengths. Meanwhile, as the weathering duration increased, small particles attached to the mask fibers began to appear. The surface roughness of mask fiber could also be observed using AFM. Comparing the AFM images of the virgin fibers with those of the fibers subjected to weathering for 24 h, it can be seen that the surface of the virgin fiber was relatively smoother. After UV weathering, some protrusions and depressions, resulting in increased roughness, could be observed. The roughness of the mask fibers before and after UV weathering is summarized in Table 1 . As can be seen in the table, regardless of the arithmetic roughness average of the surface (Ra) or the root mean square of the surface roughness (Rq), the virgin roughness of the outer and inner layers was similar. After weathering for 24 h, the increase in roughness of the inner layer was significantly greater than that of the outer layer. Although the fiber of the middle layer was too thin to be analyzed using AFM, the roughness of the middle can be described by SEM imaging (Fig. 3 and Fig. S1 ). It can be seen that the fiber surface of virgin middle layer was smooth, and there were few particles attached. After 18 h of weathering, cracks, flakes, grooves or pits appeared on the surface. Compared with the outer and inner layers, the fibers of the middle layer were rougher, and some of the fibers were completely broken. When subjected to weathering for 36 h, all the fibers of middle layer were broken into small fragments. Fig. 4 shows the changes in the maximum load force of different mask layers with UV weathering. As can be seen, the general tendency for all layers was that, as the weathering duration increased, the maximum load force decreased. The initial maximum load force for the outer and inner layers was in the range of 7-8 N. Within 15 h of UV weathering, the maximum load force changed little. However, the maximum load force was markedly reduced when subjected to weathering for more than 15 h. The force of the outer and inner layers decreased by 87.2% and 59.5%, respectively, compared with initial load after 36 h of weathering, while, if weathered for more than 36 h, the outer and inner layers became very brittle and difficult to test on the instrument. The outer and inner layers exhibited a similar change when subjected to weathering, while changes observed in the middle layer differed. The initial maximum load force of the middle layer was about 3.5 N, lower than of the outer and the inner layers. When weathered for more than 9 h, the maximum load force decreased considerably, and it was lower than the instrument detection limit when weathered more than 15 h. This result matched well the FTIR spectra, in which the middle layer was the layer most sensitive to UV weathering.

The hydrophilicity of masks can also change during weathering. The contact angles of different layers affected by weathering are shown in Fig. S2 . For the virgin mask, the water droplet cannot be absorbed on the surface regardless of which layer was tested. After subjected to weathering for 24 h, the water droplets adhered to the outer and inner layers, with contact angles of 123.7 • and 127.8 • , respectively. Meanwhile, the contact angle decreased to 100 • and 92 • for the outer and inner layers, respectively, when subjected to weathering for 48 h. On the contrary, water droplet still cannot be absorbed on the middle layer after weathering. Nevertheless, some small particles or fiber fragments were adsorbed on the surface of the water droplets when the water droplets left the surface of the middle layer. This phenomenon was more evident for the middle layer after weathered for 48 h.

With different weathering durations, the surface and mechanical properties of the mask exhibited various aging characteristics. In general, the outer and inner layers of the mask showed a comparable weathering trend under UV weathering, while weathering in the middle layer was different. For the virgin mask, the outer and inner layers had consistent properties in terms of chemical inertness, hydrophobic performance, and mechanical performance. If the UV weathering duration was less than 12 h, little changes was observed in the mechanical performance and chemical structure of these two layers. When weathered for 12 h, however, and even more so weathered for 12-18 h, the physical properties of the outer and inner layers changed significantly. Due to damage to or tearing of the mask fibers, the maximum load force decreased sharply. The chemical composition of the mask, however, changed little according to the results of FTIR. Although the intensity of the peak representing the C-H bond decreased, no obvious new peaks appeared. It can be inferred that the main chain was broken and the molecular weight was reduced, resulting in decreased mechanical strength (Gewert et al. 2018 , Shi et al. 2021 . When weathered for more than 36 h, most of the fibers became fractured, resulting in a very low maximum load force that was in fact too low to be measured by the instrument. Meanwhile, based on the FTIR spectra, some C-H bonds changed to C˭C or C˭O bonds. Moreover, the decrease in contact angle also indicates that some hydrophilic double bond groups, such as carbonyl groups, were formed on the surface of the mask.

As those observations indicate, the middle layer of the mask was found to be more sensitive to UV weathering compared with the outer and inner layers. This is attributable to the fact that the middle layer of masks is composed of melt-blown fabric, which has a smaller fiber diameter than non-woven fabrics. In this regard, some studies have found that the filtration performance of the middle layer is significantly reduced after chemical treatment (Ullah et al. 2020) . With respect to the virgin mask, except for the difference in maximum load force, the middle layer had the same characteristics as the other two layers. However, after weathering for just 12 h, the maximum load force decreased to an undetectable level, while the chain-like chemical structure changed, causing the fibers to become brittle. Fig. S2 also proves that most fibers of the middle layer broken into small fragments, causing the layer to lose its basic mechanical features.

The particle release from masks under different conditions was further investigated. Fig. 5(a)-(c) shows the morphology of particles in leachates of masks after different durations of weathering. It was found that the number of particles in the leachate of the virgin mask was low and the particle size was small, with the exception of a small number of particles having a particle size in the tens of microns, most of these being distributed at the nanometer range. In reference to the SEM image of the mask (Fig. 3) , it can be seen that most of the virgin mask fibers had good mechanical features and a smooth surface, with only a small fraction of the fibers having small particles or impurities. This suggests that few particles from the masks entered the water phase as a result of subjection to shaking conditions. After 18 h of UV weathering, however, the number of particles released to the water increased, including a significant increase in the number of particles of micro size being released. Interestingly, due to the accumulation of nano-size particles during water evaporation, a film formed on the surface of the sample holder. The middle layer of the masks not only released a large quantity of small particles, but also produced larger fiber fragments. Moreover, the outer and inner layers were important sources of microplastics released under shaking conditions, corresponding with a decrease in mechanical features after 18 h of weathering. When weathered for 36 h, the mask fibers could easily enter the water since most of them have been broken down, while irregular fiber fragments were observed in the water phase, readily agglomerating to form flocs. In addition, most of the fibers were observed to be tens to hundreds of microns in length, even visible to the naked eye. In addition to large fiber fragments, there were individual nano-size particles were difficult to observe, while a more distinct film was observed on the surface of the sample holder, this film having been formed by millions of small particles. Fig. 5(d) shows the distribution of particles released to the water from masks after different weathering durations. Regarding to virgin mask leachates, two main particle size distribution ranges were observed, which were 10-20 µm and 50-250 µm. The total concentration of microplastics in the virgin mask was 3.49 μL/L. After 6 h of UV weathering, the concentration of microplastics in the solution was only 0.61 μL/L, and more than 90% of particles were less than 100 µm in diameter. When weathered for 12 h, the total concentration of microplastics in the solution was 1.52 μL/L, and about 80% of the particles had a size greater than 30 µm. It should be noting that the release of microplastics significantly decrease after UV weathering for 6 h and 12 h. Previous studies validated that the polymers (e.g., polyethylene, polypropylene, and polystyrene) and some monomer or functional groups can be crosslinked under short term UV irradiation, leading to the maintenance or even enhancement of mechanical properties (Bendjaouahdou and Bensaad, 2018 , Liao et al. 2019 , Shyichuk et al. 2001 . Chemical additives, such as stabilizers, may accelerate or slow down the photolysis process, and further affect the crosslinking or scission caused by UV irradiation Shyichuk, 2007, White et al. 2006) . Although the molecular weight decreases under UV irradiation, the scission and crosslinking may exist simultaneously (Cui et al. 2013, Guan and Xu, 2001) . According to the results of the tension test and FTIR, we hypothesize that short-time UV weathering caused self-crosslinking in the main chain, thereby increasing the density and maintaining the mechanical strength of the mask, inhibiting the release of microparticles. For masks weathered for 18 h, the size of the microplastics released increased overall, and only 2% of the particle size was less than 10 µm. Compared to the concentrations after 6 and 12 h of weathering, the concentration of particles released after 18 h of weathering increased dramatically to 4.35 μL/L. This phenomenon demonstrated that, once again, the most marked changes in mask characteristics occurred in the weathering duration range of 12-18 h. As the weathering duration extended to 24, 30 and 36 h, the concentration of microplastics released from the masks showed an overall increasing trend, with respective concentrations of 2.59, 3.82 and 7.10 μL/L observed. In addition, the respective concentrations of microplastics with a diameter greater than 10 µm after these weathering durations were 95.0%, 91.1% and 98.9%. Comparing and analyzing the size distribution of the microplastics released after different weathering durations, it was found that, for short UV weathering durations, most of the particles released fell within the size distribution of less than 200 µm. As the weathering duration increased, the size of particles increased correspondingly, a result aligned with the SEM images of the mask leachates.

The characteristics of the microplastics released from different mask layers were also explored, with the results shown in Fig. 6(a)-(c) . For the virgin mask, the concentrations of microplastics released from the outer, middle, and inner layers were 4.47, 1.52 and 0.80 μL/L, respectively.

The microplastics released from the outer layer was mainly distributed in two size ranges, which were 15-50 µm and 200-500 µm, accounting for 24.2% and 44.3%, respectively, of the total concentration, while the Fig. 6 . Size distributions for particles released from three mask layers under different weathering conditions. microplastics released from the middle and inner layers mostly ranged between 10 and 200 µm in diameter, contributing 76.2% and 83.6% to the total concentration, respectively. After 24 h of UV weathering, the concentrations of microplastics released from the outer, middle, and inner layers were 6.19, 27.81 and 4.27 μL/L, representing increases by 0.39, 17.30 and 4.33 times, respectively, compared with the virgin masks leachates. The concentration of microplastics released from the middle layer was much higher than those from the outer and inner layers. This was due damage to the mechanical structure of the middle layer, resulting in a mass of small particles and fiber fragments being formed that could readily enter the water phase.

Most of the particles released were less than 200 µm in size for all three mask layers, with this trend being particularly pronounced for the middle layer, where this size distribution accounted for 91.2% of the total concentration. When weathered for 36 h, the respective concentrations of microplastics released from the outer, middle, and inner mask layers increased to 70.73, 60.36 and 11.44 μL/L. Since the inner layer was the most resistant to weathering among the three layers, the concentration of particles released from the inner layer was significantly less than that from the other two layers. The particle size distribution of the microplastics released from the different layers, meanwhile, exhibited different modes. Regarding the leachates of the outer and inner layers, the particle size distributions presented a bimodal pattern, while the middle layer showed a unimodal pattern. Specifically, the particle from the outer layer was mainly distributed in the range of 20-100 µm and 100-500 µm, accounting for 51.3% and 38.1%, respectively, of the total concentration. The size of released particles from the inner layer was mainly distributed in 30-100 µm and 100-500 µm, contributing 56.8% and 35.9%, respectively. For the middle layer, the particle size of the microplastics released was mainly distributed between 50 and 200 µm, accounting for 56.9% of the total concentration.

To assess the behavior of the masks in the shoreline environment, the changes in the properties of particles released from the mask after physical abrasion were evaluated. The analysis of the particle size of the sand showed that the presence of sand had no effect on the measurement of plastic particles released from masks (Supplementary Material). The Fig. 5 (e) shows the distribution of particles released from the masks to the water in the presence of sand. For the virgin mask, a total of 3.13 μL/ L particles were released, with the particle size mainly distributed within the ranges 15-30 µm and 100-250 µm, accounting for 33.9% and 44.4% of total concentrations, respectively. When the masks were subjected to weathering for 6 h, 6.08 μL/L of microplastics were released, and their particle size was mainly distributed in two ranges, 5-50 µm and 100-250 µm, accounting for 66.7% and 17.9%, respectively, of the total concentration. For masks weathered for 18 h, the microplastics released sharply increased to 23.78 μL/L. Similarly, most of the particles after this weathering duration were less than 50 µm in diameter, with this size distribution accounting for 72.8% of total concentration. When the masks were weathered for 30 and 36 h, the microplastics released were 21.9 and 20.8 μL/L, respectively. In addition, the particles showed a bimodal mode particle size distribution, with size distributed between 5-50 µm and 50-250 µm. However, it is worth noting that there was an increasing trend when the particle size was higher than 400 µm. Compared with the case where there was no sand, the presence of sand in the aqueous environment promoted the release of microplastics from masks. In addition to the virgin mask, the concentration of particles released increased 4.5 times on average for the weathered masks in the presence of sand. Moreover, the particles released from the mask after 18 h of weathering increased sharply compared to the mask subjected to weathering for 12 h. It was found that, after 24 h of shaking, masks that have been weathered for more than 30 h broken down into small fragments, with this phenomenon being particularly pronounced when sand was present in flasks. It should be noted in this regard that particles with a size greater than 500 µm or less than 1 µm were not calculated by the size analyzer, leading to the underestimation of concentration.

The effect of sand on particle release from different mask layers was also investigated, with the results shown in Fig. 6 (d)-(f). For the virgin mask, the concentrations of particles released from the outer, middle, and inner mask layers were 14.6, 6.2 and 7.0 μL/L, respectively, when shaken in water for 24 h in the presence of sand. The particle size distribution of the microplastics released was observed to be unimodal for all layers under these conditions, with most of the particles falling within the size range of 10-50 µm. The proportions in the outer, middle, and inner layers were found to be 84.9%, 84.1% and 54.6%, respectively. The concentrations of microplastics released from the three mask layers weathered for 12 h were 10.2, 10.7 and 11.4 μL/L, respectively.

For the outer and middle layers, most of the particles released were less than 50 µm in diameter, with this size distribution accounting for 76.9% and 71.5% of the total concentration for the outer and middle layers, respectively. For the inner layer, however, the particles were distributed in two ranges, 5-50 µm and 300-500 µm, which contributed to 43.2% and 44.7%, respectively, of total concentrations. With regard to the mask weathered for 24 h, the respective concentrations of microplastics released from the outer, middle and inner layers were 5.41, 20.52 and 17.49 μL/L. Different particle size distributions were observed among the different mask layers. Most of the particles released from the outer layer were small in diameter, (i.e., less than 50 µm), with this size distribution accounting for 76.0% of the total concentration. The particles released from the middle layer were mainly distributed between 10 and 200 µm, with this distribution accounting for 87.4% of the total concentration. For the inner layer, the particle size of the microplastics released was mainly distributed in two ranges, 5-50 µm and 100-500 µm, accounting for 48.0% and 47.4%, respectively, of the total concentrations. When the masks weathered for 36 h were shaken in the water with sand, the respective concentrations of particles released from the outer, middle, and inner layers were found to be 132.52, 210.23 and 186.94 μL/L. The particles released from the outer and inner layers had the same size distribution, mainly distributed in the two ranges of 20-100 µm and 200-500 µm. However, the particles released from the middle layer was more evenly distributed, with most of the particles distributed between 10 and 200 µm, this range accounting for 68.65% of the total concentration. When UV weathering exceeded 30 h, the particle size distribution of the microplastics released from the mask was similar regardless of whether or not sand was present.

Masks have been proven to be effective in preventing the spread of COVID-19 (Eikenberry et al. 2020 , Okuku et al. 2021 , and it can be expected that they will be a normal part of everyday life both during and after the pandemic (Rab et al. 2020, Xu and Ren, 2021) . However, this preventive measure puts tremendous pressure on plastic waste management (Ammendolia et al. 2020 , Patricio Silva et al. 2020 . Shorelines already suffer from the large quantities of plastic debris, and the discarded masks pose a new challenge for the shoreline environment around the world. The environmental conditions on shoreline are complex and discarded masks on shorelines may undergo a series of changes (Li et al. 2020b ). The present study has shown that the physicochemical features of disposable masks can change dramatically under UV weathering. After being exposed to weathering conditions, the transformation of chain structure and chemical composition, and decreased mechanical strength were observed. These changes caused the masks to be broken into small fragments, triggering the release a large quantity of microplastics to the aqueous environment. It should be noted that masks may contain some chemical additives, and some of them can also be released to the aqueous phase in the form of particles. Around 483,888 plastic particles can be released from one virgin disposable mask, and 1,566,560 particles from the weathered mask. Although shortwave UV in particular was applied in this study, the findings serve to demonstrate that photo-oxidation plays a crucial role in the release of microplastics from discarded masks. The middle layer of the mask is found to be the most sensitive part to UV irradiation and is the easiest to break, releasing a large amount of microplastics to water. Moreover, in a shoreline environment (i.e., in the presence of both water and sand), the physical abrasion caused by sand can further exacerbate the release of particles from masks. In this regard, we found that as many as 16,001, 943 particles can be released from one weathered mask in the presence of sand. The cumulative amount of microplastic particles produced during the 24 h breathing simulation of the disposable mask was ranged from 4678 to 24,643, and even the cumulative amount after 720 h was less than 2 million (Li et al. 2020a ). Our findings indicate that the quantity of microplastics released from the mask to the water is significantly larger than that released through inhalation during normal mask use. Shorelines are not only the main receptor of discarded masks from oceans and lands, but also play host to further transformation of masks to plastic particles under weathering conditions. Given these considerations, appropriate strategies need to be developed to better assess and mitigate the risks posed by discarded masks in order to protect coastal ecosystem. In addition, the release of chemical compounds from masks under different conditions can also be studied in future research.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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This research was supported by the Multi-partner Research Initiative of Fisheries and Oceans Canada, Environment and Climate Change Canada, and the Natural Sciences and Engineering Research Council of Canada. The authors are also grateful to the editor and the anonymous reviewers for their insightful comments and suggestions.

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jhazmat.2021.126036.