Microplastics pollution in salt pans from the Maheshkhali Channel, Bangladesh

MPs abundance

In Table 1 MP abundance (mean value ± standard deviation) values are presented by shape, size range, color and polymer type categories for each sampling site. MP were found in all analyzed salt samples including pellets, fibers, fragments, films and lines (Fig. 3). MP total abundance values per site ranged from 74.7 to 136.7 particles kg−1 in the following order of increasing abundance: S3 < S1 < S2 < S4 < S5 < S6 < S8 < S7 (Fig. 4, Table 1). The measured values show significant difference among sites (Kruskal Wallis, p = 0.013). The Dunn’s test results reveal significant differences, with S1, S2 and S3 differing from S7 (p = 0.006; p = 0.007 and p = 0.005, respectively) and S8 (p = 0.023; p = 0.028 and p = 0.019, respectively), S3 also being significantly different from S6 (p = 0.05).

Table 1 Microplastic abundance (Particles kg−1) (Mean value ± SD) by shape, color, size range and, polymer categories in unpacked coarse salt samples from stations S1 to S8 (n = 3).
Figure 3

Photographs of different MP shapes found in salt samples: (a) red fragment; (b) blue fragment; (c) pellet; (d) line.

Figure 4

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

It is expected that MP in seawater was the primary source of contamination of the sea salts19,20,21,22. According to these results, MP contamination in the salt pans from the Maheshkhali Channel coast can be classified into three zones of increasing pollution: a lower zone (S1 to S3), an intermediate zone (S4 and S5), and a higher zone (S6 to S8). A possible explanation for these values could be the different disposition of the study sites in the channel and the hydrology and current (rate of water movement) differences. According to Misra et al.16, seawater current flows with higher intensity from the south of the Bay of Bengal to the north. Therefore, any MP in seawater flowing from the Bay of Bengal inside MC (S1 to S8 direction) will probably accumulate in the inner part of the channel. However, the higher MP concentration in S7 with respect to S8 suggests that other environmental variables or anthropogenic sources could affect MP presence in MC, and more variables should be assessed in future studies to arrive at better conclusions. It should also be mentioned that the plastic film used in salt pans for desiccation is of high potential as a source of MP. Other possible MP sources could be plastic pollution from fishing, urbanization, and tourism activities in the surrounding area. Furthermore, runoff from the land and atmospheric fallout could also be potential contributory pathways23, not least given that Bangladesh is subject to the influence of the monsoon, with high rain values and salt production developing after the monsoon season9. Future research, including seawater and atmospheric MP samples, should be considered to confirm these assumptions.

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

The results obtained in this study have been compared with other salt studies worldwide (Table 2). However, it should be mentioned that most studies were developed with refined commercial sea salt samples and not from field salt pans as in this case. In addition, the different analytical methods used for MP determination difficult results comparison. The MP concentrations found in the salts from MC 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 presented higher MP concentrations (Table 2). These studies detected the presence of MP of smaller size than those registered here, supporting the observation of the higher values. It could be expected that the fragmentation of MP particles during salt processing for commercial salts could also be contributing to the increasing number of particles found in salt samples.

Table 2 Comparison of present and worldwide values for microplastics abundance, size range, and polymer types in commercial sea salt samples.

MP shape, size, and color

The fragment and film MP categories were the most abundant shape types (Table 1, Fig. 5), coinciding with the results reported for sea salts samples worldwide12. The order of distribution based on MP shape was: fragments (48%) > films (22%) > fibers (15%) > pellets and lines (both 9%). Higher quantities of fragments and films were also reported for Indian salts samples20. The studies analyzing MP in Cox’s Bazar sediments registered fibers and fragments dominating shape composition15, 23,24,25,26. Given that this area is highly touristic and urbanized, plastic fibers from clothes and fabrics could be a major source.

Figure 5

Microplastics abundance (particles kg−1) by shape category registered at the sampling sites S1 to S8.

The abundance of MP in salt samples by color and size range is presented in Figs. 6 and 7 and Table 1. The 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 category 500–1000 µm, except for S3 (1000–5000 µm) (Table 1). The distribution of MP particles based on size range was: 500–1000 µm (40%) > 1000–5000 µm (34%) > 250–500 µm (26%). For salts from the Atlantic and the Pacific Ocean, originating from Brazil, the United Kingdom, and the USA, Kim et al.12 reported a higher abundance of MP in size range 100–1000 µm while sizes in the range 100–5000 µm were reported for salt samples from the Black Sea. Seth and Shriwastay20 found that 80% of fibers found in salt samples from the Indian Sea were smaller than 2000 μm in length. MP size range differences among the various studies are suggested to depend on the degree of weathering for a given environment30, different climatic conditions such as wind, rain, temperature, salinity, and waves influencing size range composition. Also, for runoff, rivers, and atmospheric fallout transportation, smaller MP size ranges can be expected to be associated with a longer range from the initial plastic sources31,32,33. Nevertheless, more detailed information about MP polymer/color features within the size ranges are needed to achieve stronger conclusions about potential long/short-range sources.

Figure 6

Microplastics abundance (particles kg−1) by color in sea salt samples from stations S1 to S8.

Figure 7

Microplastics abundance (particles kg−1) by size range in sea salt samples from stations S1 to S8.

MP polymer composition

Four types of polymer, namely polypropylene (PP), polystyrene (PS), polyethylene (PE), and polyethylene terephthalate (PET), were identified with FT-MIR-NIR (Supplementary Figure S1). These results are in accordance with those reported for salt samples in other studies worldwide (Table 1). These polymer types are widely used in daily life products, packaging, single-use plastics, and clothes, contributing to plastic pollution worldwide21. PET presented the highest contribution at all sampling sites, at ~ 48%, whereas PS was found to be least, at ~ 15% (Fig. 8, Table 1). Iñiguez et al.34 also reported PET predominance (83.3%) in Spanish table salt samples. PET predominance could be explained by its high density (1.30 g cm−3), making 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) presented lower or similar densities to seawater (~ 1.02 g cm−3), making these more prone to flotation and possible loss due to wind during desiccation.

Figure 8

Microplastics abundances (particles kg−1) by polymer composition in sea salt samples from stations S1 to S8.

Risks assessment

During degradation, MP tends to emit monomers and different types of additives, these having the potential to cause harm to ecological systems and health18, 35. Results for the polymeric risks indices are presented in Fig. 9. According to polymer risk classification, all salts samples showed low risks, being similar to the entire study area. To date, none of the published studies have applied chemometric models in evaluating MP pollution in salts, posing difficulties when comparing our results. Information on the hazards of MP from ingestion to human health is still highly unclear. Other than exposure, the destiny and transit of ingested MP in the human body, including intestinal digestion and biliary discharge, have not been determined in previous research and remained largely unclear36. Some studies conducted impact assessments based on in vitro models37,38. However, whether the exposure concentrations used in such studies indicate the MP consumed and collected in humans is inconclusive. Previous studies found that toxicity, oxidative stress, and inflammation could result from MP exposure, including immune disruption and neurotoxicity effects, among others39. Therefore, an immediate effort is required to assess the health consequences of these MP when they reach the human body.

Figure 9

Polymeric risk indices for MP types in salts from stations S1 to S8.

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