1 Introduction

The application of plastics has led to various improvements in everyday life [1, 2]. However, plastics have become a global concern due to a wide range of negative impacts on the environment. Plastic accumulation in the marine and coastal environment has increased since the 1970s, with only about 10% of plastic waste being recycled [3,4,5]. According to reports, an estimated amount of 4.8 to 12.7 million MT of plastic waste had entered the ocean in 2010 [6]. The main ingredients of plastics are polymers such as polypropylene, polyethylene, polystyrene, polyvinyl chloride, polyamide, and polyethylene terephthalate. The breakdown of larger plastics ultimately procedure tiny particles known as microplastics (MPs), which are, according to most definitions, tiny plastic particles with a diameter of less than 5 mm [7,8,9]. The size ranges of plastic debris can be identified as mesoplastics (5–25 mm), large microplastics (1–5 mm), small microplastics (20 μm–1 mm), and nanoplastics (1–1000 nm) [10, 11]. In addition, microplastics can be divided into primary (i.e. enter directly into the environment as nurdles, pellets, and microbeads) and secondary sources [12,13,14]. Primary MPs are manufactured at microscopic sizes as raw materials in plastic products for various industrial and domestic applications [9, 15]. Secondary MPs are derived from large plastic debris/macroplastics (e.g. plastic bags, bottles, and packing) under degradation of physical, photo-oxidative, chemical, thermal, and biological processes [13, 16,17,18].

Taking account of plastics in marine environments, major sinks for MPs can be identified as coastal belts, sea, and ocean [19, 20]. Major sources of MPs in marine and coastal environments include beach littering, atmospheric transport, rivers, fishing activities, and aquaculture [8, 17, 18]. The widespread distribution of MPs across larger distances, both on land and within marine systems is aided primarily by the smaller size (< 5 mm), low density, shape diversity, and lightweight [17, 21]. However, the type and concentration of MPs along the Sri Lankan coastlines are poorly constrained. The recent MV X-Press Pearl accident has triggered MP research, concerning potential harm to human and marine life. The MV X-Press Pearl accident is the largest and most catastrophic plastic-based marine pollution in the Indian Ocean. For example, this accident released about 1750 tons of plastics (i.e. belonging to the main category of primary MPs) into the Indian Ocean. About 1610 tons of plastic debris have been removed after the disaster along the directly affected coastline of Sri Lanka [22, 23]. Accordingly, there is a huge threat to spread and accumulate the remained primary MPs over a large area. These primary MPs will also remain in nature for a long time, and degrade into the secondary MPs.

Although several studies related to the disaster were done focusing on the separation of MPs from saline water and beach sediments [2, 23, 24], characterization of how the ship fire changed the physical and chemical properties of the nurdles [23,24,25], nurdle pollution and the effectiveness of beach cleaning [25], and analysis of potentially toxic elements (PTEs) [23], the MV X-Press Pearl accident has left several unresolved questions for future research, including (i) what is the weathering and degradation rate of these contaminated primary MP pellets?, and hence (ii) what is the accumulation/enhancement of secondary MPs after a given time? Such qualitative estimations are thus limited without baseline data (i.e. accumulated secondary MPs before the MV X-Press Pearl accident) in the affected areas of the disaster. Therefore, the current study aims to understand the degradation process of plastics with the influence of maritime disasters, with the aim of providing baseline data for future studies. Although significant and irreversible impacts occurred during the MV X-Press Pearl accident, the findings of this research will assist coastal zone management and future risk assessment.

2 Materials and methods

2.1 Sampling strategy and study area

The MV X-Press Pearl accident took place during the southwest monsoon season, spanning from May to September, during which, the prevailing summer monsoon current is dominant in the Central Indian Ocean, flowing from west to east. Hence, the west coast of Sri Lanka was significantly contaminated by different colour plastic pellets (Fig. 1b), after the shipwreck. Accordingly, beach sediment samples were collected systematically covering the west coast, a distance of 17 km from Colombo (Uswetakeiyawa: L1) to Negambo (Basiyawatta: L40). Sampling was done at 40 locations, 6 to 8 days after the MV X-Press Pearl accident, with special permission due to movement restrictions and curfew laws followed by the Covid-19 global pandemic. Approximately 2 kg of samples were taken separately from the mean sea level and the berm at each location (Fig. 1a), and finally 80 systematic samples of 2 kg each, in total. A random sampling was conducted in collecting beach sediments from 20 locations around the country as a baseline study. These samples were collected from the berm that contains sediments deposited by wave action beyond the mean sea level. Therefore, sampling was conducted by focusing on the minimal impact of the maritime disaster on locations around the country compared to the significantly affected areas observed along the west coast of Sri Lanka. Both systematic and random sampling were done using a simple steel hand shovel. Plastic usage was minimized to avoid contamination and the addition of external plastics to samples.

Fig. 1
figure 1

a Sri Lanka map shows the location of the X-Press Pearl shipwreck and sampling sites (where SMC = summer monsoon current; EICC: east India coastal current), b polluted beaches 6 to 8 days after the explosion, and c observed primary MP contaminants

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Collected samples were observed considering the presence of nurdles, plastic wires, melted/burnt plastics, organic debris, and other occurrences. The amount, size, colour, and shape of nurdles, wires and burnt and deformed plastic particles were noted (Additional file 1). Since plastic particles smaller than 5 mm in size were considered, all visually identified macro and contaminated plastic nurdles were removed manually before further analytical steps [26]. In the pre-treatment and separation steps, different types of primary MP contaminants were also removed from the filtrates (Fig. 1c).

2.2 Pre-treatment and separation of MPs

All the samples were oven-dried (Jeio Tech ON-02G) at 40 °C for one hour. Dried samples were sieved for 15 min using a sieve set (UTEST ISO 3310-1, BS: 410-1, EN 933) of 4.75, 2.00, 1.00, 0.50, and 0.25 mm. Retained samples in each sieve were taken, and 50 g from each sample was separated using the coning and quartering method. These 50 g samples were oven-dried again at 40 °C to remove moisture. Visible primary MPs were separated before the removal of organic matter (Fig. 1c).

The removal of organic matter was done following the Wet Peroxide Oxidation (WPO) process under the National Oceanic and Atmospheric Administration (NOAA) protocol [27]. In this procedure, 7.5 g of FeSO4·7H2O was dissolved in 500 ml of water and mixed with 3 ml of concentrated sulphuric acid to prepare Fe(II) solution in 0.05 M concentration. A 20 ml of this solution was added to beach sand samples along with 10 ml of 50% H2O2. Samples were kept for 5 min to allow the reaction between organic matter and added solutions. After that, the sample container was covered with a watch glass and was introduced to the laboratory magnetic stirrer with a hot plate at 75 °C for about 20 min. A small amount of distilled water was added from time to time to reduce the aggressiveness of the reaction. The procedure was continued until there was no evidence of the presence of organic materials. For example, the bubbling was over when organic matter was dissolved completely. Later, an approximate amount of 6 g of table salt (NaCl) per 20 ml of sample was added to the container to increase the density of the aqueous solution. Heating was continued at 75 °C until the dissolution of salt. Subsequently, the solution was transferred to a separation funnel, and solid remains (i.e. composed of degraded organic matter and inorganic portion/sand) in the wet peroxide oxidation beaker were also transferred to the separation funnel. These funnels were then covered with the lid and allowed to settle overnight.

Settled solids were drained and discarded. However, floating solids and liquids were filtered, as MPs floated on a saturated sodium chloride solution. Samples were filtered using 70 mm millipore 1.2 μm glass microfiber filters (Whatman GF/C™ CAT No. 1822-070). The separation funnels were rinsed several times with distilled water to transfer all solids to the filter papers. These filter papers were dried inside a desiccator for 24 h.

The quality assurance and quality control protocols were followed during the entire processing and analytical steps. For example, the sampling containers were cleaned three times with ultrapure water and alcohol, and then covered by aluminium foil. In addition, none of the plastic containers were used during the analysis. Blanks were also prepared using the same procedure as the other samples. Accordingly, this study ensures a negligible amount of contamination during the entire experimental procedure.

2.3 Identification of secondary MPs

2.3.1 Observations

The retained and dried MPs in the filtrates (n = 100, i.e., 80 systematic samples from the affected region and 20 random samples around the country as the baseline study) were observed with the naked eye. The number of visible MPs (known as large MPs with sizes larger than approximately 0.5 mm) was counted (items per 1 mm2), and other observations were noted, before moving to microscopic analysis. After that, the retained and dried MPs in the filtrates (n = 100) were also observed using a Euromex iScope® microscope under 40×, 100×, and 200× magnifications. The number of total MPs was counted using a grid under 40× magnification, considering two of 1 mm2, and obtained the average value (known as total secondary MPs).

2.3.2 Fourier transform infra-red (FTIR) analysis

Seven representative MP samples were used for FTIR-ATR (Attenuated Total Reflection) analysis. These samples are representatives of all types of MPs found within samples and were analysed using a Bruker Alpha spectrophotometer over the range of 500–4000 cm−1 with a resolution of 4 cm−1. The transmittance spectra were compared to determine the origin and compositions of secondary MPs.

3 Results and discussion

3.1 Abundance of MPs

The spatial variation of large and total secondary MPs (sum of MPs at mean sea level and berm) on the west coast are shown in Fig. 2. All sediment samples (n = 80) collected from the affected coastal region from Colombo (L1: Uswetakeiyawa) to Negambo (L40: Basiyawatta) contain large secondary MPs (Fig. 2). The average value of large secondary MPs at mean sea level and the berm are 33 ± 56 items per 1 mm2 and 61 ± 154 items per 1 mm2, respectively (Fig. 2). Similarly, the average value of total secondary MPs at mean sea level and the berm are 61 ± 66 items per 1 mm2 and 106 ± 165 items per 1 mm2, respectively (Fig. 2). The lower values at the mean sea level suggest that a significant amount of secondary MPs are transported by oceanic currents. For example, meteorological and oceanographic conditions promote primary MP distribution with representative oceanic circulations in the Central Indian Ocean [22, 24, 25, 28]. The MV X-Press Pearl erupted on 20th May 2021 and burned for nearly 2 weeks until the entire vessel sank on 17th June 2021 [2, 22]. Accordingly, this accident occurred during the southwest monsoon (from May to September). In this period, the summer monsoon current is predominant in the Central Indian Ocean, from west to east directions [29, 30]. Therefore, the powerful longshore drift can carry MPs along with the summer monsoon current (SMC), and then combine with the east India coastal current (EICC) (Fig. 1a). Consequently, the negative impacts of MPs are not limited to the west coast of Sri Lanka but also spread over larger areas in the Indian Ocean. Similarly, previous studies indicate the transboundary impacts of the MV X-Press Pearl accident due to the spreading of pellets across Indian Ocean coastlines from Indonesia and Malaysia to Somalia [22].

Fig. 2
figure 2

Variation in the sum of large and total secondary MPs at mean sea level and berm, along the sampling sites

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Figure 3 shows the total secondary MPs corresponding to 20 samples collected from the berms at other coastal areas in Sri Lanka. The average value of total secondary MPs at the berm is 53 ± 66 items per 1 mm2 (Fig. 3). Accordingly, the average value of total secondary MPs at the berm is about two times higher in the affected west coast (106 ± 165 items per 1 mm2 in Fig. 2b) compared to other coastal regions in Sri Lanka (53 ± 66 items per 1 mm2 in Fig. 3). However, large secondary MPs are absent on all filter papers of those 20 samples (B1 to B20 in Fig. 1). Therefore, the MV X-Press Pearl accident made a significant impact on generating secondary MPs on the west coast of Sri Lanka. In contrast, the number of primary MPs in 1 kg of sand sample is defined as the plastic pellets pollution index (PPI), and it is a direct indicator for primary MPs contamination after the X-Press Pearl accident [26]. In this study, primary MPs were removed (i.e. plastic nurdles and remnants of burnt plastics) during pre-treatment and separation of MPs. Consequently, the distribution of secondary MPs can provide new evidence on the weathering and degradation of plastic nurdles after the MV X-Press Pearl accident.

Fig. 3
figure 3

Variation in the number of total secondary MPs along the coastal region of Sri Lanka

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The numbers of large secondary MPs are high in Epamulla (L19, n = 588) and Sarakkuwa (L27, n = 1232) beaches (Fig. 2). Similarly, the higher plastic pellet pollution index (PPI) values are also recorded at the same locations (L19, PPI = 1940–3364 and L27, PPI = 2158–3466) [26]. These values were obtained by considering the number of plastic pellets found within 1 kg of samples, by drying each sample to remove moisture first and followed by sieving. Later the ratio between the numbers of pellets (n) in 1 kg of sample had been considered in determining the PPI values [26]. In addition, the large (r2 = 0.85) and total (r2 = 0.75) secondary MPs show a good correlation with the plastic pellets pollution index (Fig. 4). Consequently, these observations suggest that the accumulation of secondary MPs is significantly influenced by the MV X-Press Pearl accident on the west coast of Sri Lanka.

Fig. 4
figure 4

Relationship between large and total secondary MPs versus plastic pellets pollution index (PPI). The PPI values were obtained from the literature [26]

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Different processes including physical/mechanical degradation, photodegradation, chemical degradation, and biodegradation break plastics into MPs fragments (Fig. 5). This phenomenon is known as the weathering and degradation of plastics [2, 31]. Environmental factors [e.g. ultraviolet (UV) radiation, wave action] and material characteristics (e.g. polymer type) also controlled the rate of weathering and degradation of plastics [21, 32]. However, weathering and degradation of plastics into MPs is a very slow process. In this study, sand samples were collected 6 to 8 days after the MV X-Press Pearl accident. Therefore, the comparatively higher value of secondary MPs identified from the samples collected at Epamulla and Sarakkuwa, and the good correlations in Fig. 4 suggest an extremely high plastic degradation rate along the west coast of Sri Lanka. According to the cargo manifest, the ship was carrying 25 metric tons of nitric acid [22]. In addition, the ship was on fire for 13 days around 17 km northwest of the Colombo Port (Fig. 1a). Consequently, chemical and thermal degradations are mainly influenced to derive secondary MPs during the MV X-Press Pearl accident. For example, nitric acid can break and oxidize polymer chains of plastic into MPs. In thermal degradation, depolymerization and statistical fragmentation are the main mechanisms for breaking plastics into MPs [33]. Furthermore, in situ, laboratory experiments concluded (i) the change of white colour plastic nurdles into different colours (e.g. yellow and black) with the reaction of nitric acid and heat, and (ii) alter the polymer structures of plastic nurdles above 60 °C [26].

Fig. 5
figure 5

Microscopic images of secondary MPs retained in the filter papers (magnification: ×40)

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3.2 Fourier transform infra-red (FTIR) observations

The FTIR spectra of selected different types of representative secondary MPs, obtained from samples (n = 80) collected on the affected west coast, are shown in Fig. 6. In addition, the spectra with respective IR transmittance peak assignments are shown in Table 1. The FTIR spectra of MPs contain some common peaks. The peak ranging from 3570 to 3200 cm−1 (broad) is assigned to the hydroxyl group. The peak at 2935–2915 cm−1 is the methylene (–CH2) asymmetric stretch, whereas the peak at 2865–2845 cm−1 is the methylene symmetric stretch. The peak at 1485–1445 cm−1 corresponds to the methylene bending deformation. In addition, peaks at 750–720 cm−1 represent the methylene rocking deformations [36, 37]. According to the previous study, MPs were released directly from the wreck. Several containers are composed mainly of plastic pellets of low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) [22]. Therefore, several peaks can be used to differentiate these polymer types, according to different degrees of branching sizes [35]. For example, LDPE contains a stronger peak at 1377 cm−1 compared to 1366 cm−1, whereas LLDPE contains a stronger peak at 1366 cm−1. The peak at 1377 cm−1 is absent for HDPE [23, 35]. Consequently, few samples belonged to HDPE such as plastic wire samples (Fig. 6g).

Fig. 6
figure 6

FTIR spectra of selected MPs

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Table 1 Peak assignments for IR transmittance spectra of selected secondary MPs
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The CH stretching peaks between 3000 and 2850 cm−1 indicate the presence of methyl and methylene. Peaks within the range of 2935–2915 cm−1 suggest a methylene (–CH2) asymmetric stretch, and peaks within the range of 2865–2845 cm−1 suggest a methylene symmetric stretch. Polyethylene consists of only methylene, and two CH peaks between 3000 and 2850 cm−1 are indicators for methylene [38, 40]. Therefore, the analysed samples except for the plastic wire can be identified as polyethylene. The LDPE and LLDPE contain a single –CH2 rocking vibration peak around 720 cm−1, whereas HDPE contains two –CH2 rocking peaks at 730 and 720 cm−1 [36,37,38, 40]. These facts confirmed that the LDPE and LLDPE were dominant in secondary MPs (Fig. 6). In addition, LLDPE contains a small methyl umbrella mode peak at 1378 cm−1 [38, 40]. Consequently, samples of perfect sphere-shaped black (burned) particles and burnt and deformed MPs are derived from LLDPE (Fig. 6a–f).

The newly identified transmittance bands corresponding to –C=O, –C–O–C, and –S=O are indicators for the alteration/deformation of the chemical composition of MPs in the coastal/marine environment [23, 37]. The peak position around 1030 cm−1 suggests the presence of –S=O bonds (Fig. 6b), and it indicates the polymerisation reactions with sulphur-associated compounds [41]. However, the impact of added concentrated sulphuric acid during the WPO method can be neglected due to dilution effects [27].

The FTIR confirmed that the majority of MPs including sphere-shaped black (burned) particles and burnt and deformed microplastics belonged to LDPE or LLDPE (Fig. 6). It can be followed by the release of plastic pellet bags from a single container during the MV X-Press Pearl disaster [22, 23]. Therefore, it suggests that a significant amount of primary plastic nurdles has already degraded to secondary MPs, and accumulated up to a considerable extent. Polypropylene (PP) and polyethene (PE) are low-density polymers. Therefore, MPs composed of the LDPE or LLDPE can float on water and spread rapidly over a wide area by ocean currents [42].

The long-term accumulation of MPs causes direct and non-direct environmental and health hazards such as asthma, cancer, cardiac diseases, and dyspnea [16, 43,44,45,46,47]. Land-based plastic wastes also transport and accumulate on beaches and into the oceans. In addition, the leaching of pollutants such as monomers and toxic additives from MPs can lead to carcinogenesis and endocrine disruption [42, 43, 46, 48,49,50]. However, the removal of MPs becomes still more difficult and expensive due to their fine particle sizes.

4 Conclusion

Marine and coastal environments are identified as the most suitable sites for plastic degradation. This study confirmed that the MV X-Press Pearl accident released secondary MPs extremely high rate (perhaps 1000 times fast) than the normal degradation rate. The variance in average values of secondary MPs at mean sea level (large MPs = 33 ± 56 items per 1 mm2 and total MPs = 61 ± 66 items per 1 mm2) and the berm (large = 61 ± 154 items per 1 mm2 and total MPs = 106 ± 165 items per 1 mm2) suggested significant dispersal of large quantities of MPs to other areas in the Indian Ocean with oceanic currents. The number of total MPs in other areas (average 53 ± 66 items per 1 mm2) can indicate the baseline value of secondary MPs before the MV X-Press Pearl accident. The good correlation between the higher plastic pellet pollution index (PPI) and secondary MPs (large and total) suggests the high rate of plastic degradation within a short period of 6 to 8 days after the disaster. These findings further suggest that nitric acid and heat/fire are major drivers for plastic degradation during the disaster. The secondary MPs are mainly derived from low-density polyethylene (LDPE) and linear low-density polyethylene (LLDPE), which were confirmed through Fourier Transform Infrared (FTIR) observations. Since low-density secondary MPs can easily be widespread in the Indian Ocean, there is a high probability to occur global impacts on the ecosystem in marine environments. In recommendation, future research must be conducted to quantify the MP release into marine and coastal environments, mitigate their adverse impacts, and establish a proactive coastal management system for the country.