Microplastic pollution in the salt marshes of the Maheshkhali Canal, Bangladesh

Abundance of deputies

In Table 1, the abundance values ​​of MP (mean value ± standard deviation) are presented by category of shape, size, color and type of polymer for each sampling site. PM was found in all salt samples analyzed, including pellets, fibers, fragments, films and lines (Fig. 3). Total MP abundance values ​​per site ranged from 74.7 to 136.7 particles kg-1 in the following order of increasing abundance: S3

Table 1 Abundance of microplastics (Particles kg-1) (mean value ± SD) by shape, color, size range and polymer categories in the unconditioned coarse salt samples from stations S1 to S8 (n = 3).
figure 3

Photographs of different forms of PM found in salt samples: (a) red fragment; (b) blue fragment; (vs) lozenge; (D) line.

Figure 4

Box plot of the abundance of PM (particles kg-1) for sampling sites S1 to S8.

PM in seawater is expected to be the main source of marine salt contamination19,20,21,22. Based on these results, PM contamination in the salt marshes of the Maheshkhali Canal coast can be classified into three areas of increasing pollution: a lower zone (S1 to S3), an intermediate zone (S4 and S5), and an upper zone (S6 to S8). One possible explanation for these values ​​could be the different arrangement of the study sites in the channel and the differences in hydrology and current (speed of water movement). According to Misra et al.16, the seawater current flows with a higher intensity from the south of the Bay of Bengal to the north. Therefore, any PM in seawater flowing from the Bay of Bengal into the MC (direction S1 to S8) is likely to accumulate in the inner part of the channel. However, the higher concentration of PM in S7 compared to S8 suggests that other environmental variables or anthropogenic sources could affect the presence of PM in MC, and more variables should be evaluated in future studies to arrive at better conclusions. . It should also be mentioned that the plastic film used in salt marshes for desiccation has a high potential as a source of PM. Other possible sources of PM could be plastic pollution from fishing, urbanization and tourist activities in the surrounding area. In addition, land runoff and atmospheric fallout could also be potential contributing pathways.23, especially given that Bangladesh is subject to the influence of the monsoon, with high rainfall values ​​and salt production developing after the monsoon season9. Future research, including samples of seawater and atmospheric PM, should be considered to confirm these hypotheses.

Although there have been no previous studies on the abundance of PM in salt samples, further studies have been carried out in the coastal area of ​​the Bay of Bengal adjacent to Bangladesh. A study by Rahman et al.24 in the beach sediments of Cox’s Bazar beach recorded relatively low PM values ​​of 8.1 ± 2.9 particles kg-1 while, in a study of intertidal sediments from the same region by Hossain et al.25, higher MP values ​​of 368.68 ± 10.65 particles kg-1 have been reported. Both studies attributed their spatial variation in PM to tidal current, wave energy, beach orientation, river flows, and human activity. Finally, another study in beach sediments found concentrations of PM up to 1100 kg particles-1, attributing these high values ​​to increasing urbanization and tourism26.

The results obtained in this study were compared with other salt studies around the world (Table 2). However, it should be mentioned that most of the studies were developed with samples of refined commercial sea salt and not from field salt marshes as in this case. In addition, the different analytical methods used for the determination of PM make it difficult to compare the results. The PM concentrations found in MC salts are similar to those reported by studies in Brazil, Mexico, South Korea and Indonesia12, 27 (Table 2). On the other hand, studies of salt samples from the Atlantic and Indian Oceans12, the Pacific Ocean (China and Thailand)19, and the Mediterranean Sea (Croatia, Italy)27, 28 had higher PM concentrations (Table 2). These studies detected the presence of PM smaller in size than those recorded here, supporting the observation of the highest values. One would expect that the fragmentation of PM particles during salt processing for commercial salts could also contribute to the increase in the number of particles found in salt samples.

Table 2 Comparison of current and global values ​​of microplastic abundance, size range, and polymer types in commercial sea salt samples.

Shape, size and color of the MP

The categories of fragments and MP films were the most abundant form types (Table 1, Figure 5), coinciding with the results reported for sea salt samples worldwide.12. The order of distribution based on the MP form was: fragments (48%)> films (22%)> fibers (15%)> pellets and lines (both 9%). Higher amounts of fragments and films were also reported for Indian salt samples.20. Studies analyzing MP in Cox’s Bazar sediments have recorded fibers and fragments dominating the composition of the form15, 23, 24, 25, 26. Since this area is very touristy and urbanized, plastic fibers from clothing and fabrics could be a major source.

Figure 5

Abundance of microplastics (particles kg-1) by shape category recorded at sampling sites S1 to S8.

The abundance of MP in salt samples by color and size range is shown in Figs. 6 and 7 and Table 1. Colors identified include white, blue, green, black, pink, transparent and colorless. The distribution was: white (37%)> black (17%)> blue (15%)> green and transparent (10% each)> pink (6%)> colorless (5%). In terms of size, most particles were in the 500 to 1000 µm category, with the exception of S3 (1000 to 5000 µm) (Table 1). The distribution of PM particles according to size range was: 500 to 1000 µm (40%)> 1000 to 5000 µm (34%)> 250 to 500 µm (26%). For Atlantic and Pacific Ocean salts, originating in Brazil, the United Kingdom and the United States, Kim et al.12 reported higher abundance of PM in the size range 100–1000 µm while sizes between 100 and 5000 µm were reported for samples of Black Sea salt. Seth and Shriwastay20 found that 80% of the fibers found in Indian sea salt samples were less than 2000 m in length. It is suggested that the differences in PM size between the various studies depend on the degree of alteration for a given environment.30, different climatic conditions such as wind, rain, temperature, salinity and waves influencing the composition of the size range. Additionally, for runoff, rivers, and atmospheric fallout transport, smaller PM size ranges can be expected to be associated with a longer range from the original plastic sources.31.32.33. Nonetheless, more detailed information on the characteristics of MP polymers / colors in size ranges is needed to reach more solid conclusions on potential long / short range sources.

Figure 6

Abundance of microplastics (particles kg-1) by color in the sea salt samples from stations S1 to S8.

Figure 7

Abundance of microplastics (particles kg-1) by size range in sea salt samples from stations S1 to S8.

MP polymer composition

Four types of polymers, namely polypropylene (PP), polystyrene (PS), polyethylene (PE) and polyethylene terephthalate (PET), have been identified with FT-MIR-NIR (Supplementary Figure S1). These results are consistent with those reported for salt samples in other studies around the world (Table 1). These types of polymers are widely used in everyday products, packaging, single-use plastics, and clothing, contributing to plastic pollution around the world.21. PET presented the highest contribution at all sampling sites, at ~ 48%, while PS was found to be the lowest, at ~ 15% (Fig. 8, Table 1). Iñiguez et al.34 also reported a predominance of PET (83.3%) in Spanish table salt samples. The predominance of PET could be explained by its high density (1.30 g cm-3), making the particles prone to sedimentation during the salt crystallization process19. PE (0.94 g cm-3), PP (0.90 g cm-3) and PS (1.05 cm-3) had densities lower than or similar to seawater (~ 1.02 g cm-3), which makes them more prone to flotation and possible wind loss during drying.

Figure 8
figure 8

Abundance of microplastics (particles kg-1) by polymer composition in the sea salt samples from stations S1 to S8.

Risk Assessment

During degradation, PM tends to emit monomers and different types of additives, which can potentially harm ecological systems and health.18, 35. The results of the polymer risk indices are presented in figure 9. According to the polymer risk classification, all the salt samples showed low risks, being similar to the whole study area. To date, none of the published studies have applied chemometric models to assess PM pollution in salts, which poses difficulties when comparing our results. Information on the dangers of PM linked to ingestion for human health is still very unclear. Apart from exposure, the fate and transit of PM ingested in the human body, including intestinal digestion and bile flow, has not been determined in previous research and has remained largely unclear.36. Some studies have conducted impact evaluations based on in vitro models37.38. However, it is not conclusive that the exposure concentrations used in such studies indicate the PM consumed and collected in humans. Previous studies have shown that toxicity, oxidative stress and inflammation may result from exposure to PM, including effects of immune disruption and neurotoxicity, among others.39. Therefore, immediate effort is required to assess the health consequences of these PMs when they reach the human body.

Figure 9

Polymeric risk indices for the types of PM in the salts of stations S1 to S8.