The Salinity Difference and Clay Mineral Types Affect the Distribution of Microplastics in the Seabed: New Evidence from the Western North Yellow Sea

Abstract

Salinity and clay mineral types have been shown to influence the migration and settlement efficiency of microplastics (MPs) under restrictive experimental conditions. However, current research is limited to deep trenches or laboratory conditions, and studies in the semi-enclosed sea area of the continental shelf are still lacking. We investigated the effects of bottom seawater salinity and clay mineral types on MPs distribution in surface sediments using the western part of the North Yellow Sea as an example, where current conditions are complex and salinity changes rapidly over short distances. Under detection conditions with a minimum detection limit of 10 μm, the abundance range of MPs in the investigated sea area reached 24–1134 items/(g dry weight). The distribution of MPs was in good agreement with the isohaline of the bottom seawater, and MPs tended to converge in the high salinity area. However, there is an exceptional case in which the temperature and salinity difference caused by the cold water mass can create a frontal flow that blocks the transport of terrigenous materials to the middle of the cold water mass. This phenomenon causes MPs to settle at the edge of the cold water mass. A significant positive correlation exists between montmorillonite with expansive properties and fragment MPs and MPs with particle size > 100 μm, which have a larger surface area (p < 0.05). The negative charges on the surface of MPs and clay minerals are neutralized, promoting the heterogeneous aggregation between clay minerals and MPs and accelerating the sedimentation process of MPs in the ocean. This is another important reason for the accumulation of MPs in the high-salinity region. This study provides a basis for pollution prevention and control of MPs in the shallow sea, supplying new insights into the effects of bottom seawater salinity and clay mineral type on the distribution of MPs.

1. Introduction

With the increasing consumption of plastic products, plastic pollution has become one of the environmental problems that we can hardly ignore. It is estimated that between 800,000 and 23 million tons of plastic enter the world’s oceans every year [1]. When plastic enters the ocean, it is broken down into tiny particles by UV radiation, wind, and waves [2,3]. MPs are typically defined as plastics that are smaller than 5 mm in size [4,5]. In addition to the in situ degradation of large plastic items, MPs enter the ocean via river input, atmospheric transport, and loss of marine economic activities [6,7,8,9,10].

MPs in the ocean can enter biogeochemical cycles and eventually have a range of harmful impacts on marine ecosystems. For instance, their presence can significantly alter microbial community structure and the nitrogen cycle, thereby affecting the primary marine environment [11]. MPs are highly adsorbent to many toxic pollutants, and when marine organisms ingest them, the rate of accumulation of these pollutants in marine ecosystems accelerates. This process has adverse effects on human health through the food web [12,13,14]. These effects tend to be more pronounced in seafloor surface sediments, as most marine MPs sink to the seafloor [15,16], where they are ingested by microorganisms, and harmful substances begin the process of accumulation in the food chain. As seabed sediments are a sink for MPs, it is essential to understand and study the distribution of MPs in marine surface sediments.

Previous studies have investigated the impact of variations in seawater salinity on the distribution of MPs. Kane et al. found that thermohaline-driven currents create extensive seafloor sediment accretion, which controls the distribution of MPs and creates seafloor MP hotspots [17]. Bergmann et al. found large amounts of MPs in deep Arctic sediments and suggested that these MPs are transported from low latitudes to the Arctic via the thermohaline circulation [18]. However, these studies have focused only on deep-sea regions, and there are few studies investigating the effects of differences in bottom seawater salinity on the distribution of MPs in shallow-sea regions, especially in semi-enclosed continental shelf areas where seafloor flow paths show complex variations. Notably, clay mineral types have also been demonstrated to facilitate the co-aggregation of MPs in concentrated salt environments [19]. The changes in ionic strength can alter the surface charge of MPs and clay minerals, thereby affecting their interaction energy. Relevant experiments have shown that in the context of clay minerals and MPs coexisting in a system, the deposition rate coefficient and the maximum deposition exhibit a linear decrease with the logarithm of the energy potential barrier of DLVO [20]. The high-salt environment reduces the energy potential barrier between particles, which promotes particle aggregation and deposition. Nevertheless, the current study is limited to laboratory conditions with stable conditions, and the natural sea area with complex hydrodynamics has not yet been effectively explored.

Given the limitations of the studies mentioned above, the Western North Yellow Sea, a semi-enclosed continental shelf area adjacent to the Bohai Strait, was selected for this study. This region serves as a nexus for maritime economic activities and a vital material exchange area between the Bohai Sea and the Yellow Sea. MPs from the Shandong Peninsula, Liaodong Peninsula, and Bohai Coast are prone to gather here. However, the complex underwater flow environment makes it difficult to determine the path of MP migration. The aim of this survey is twofold: (1) Utilizing the western North Yellow Sea as a case study, investigate the influence of thermohaline currents in the semi-enclosed sea area on the distribution of MPs in marine surface sediments. (2) Investigating the effects of different clay mineral types on the distribution of MPs with different grain sizes and morphologies in surface sediments under high salinity environments at the seafloor and further analyzing the genesis.

2. Materials and Methods

2.1. Sampling Site and Sample Collection

Surface sediment samples (0–5 cm depth) were collected from 20 stations in the Western North Yellow Sea (37°46′–38°04′ N, 121°09′–121°48′ E) during June 2023 using a box corer. Subsamples were immediately divided into two aliquots, wrapped in pre-cleaned aluminum foil, and stored at −20 °C for subsequent microplastic (MP) and clay mineral analyses.

2.2. Analysis of Microplastics

2.2.1. Sample Pretreatment

Frozen sediments were lyophilized (SCIENTZ-30ND, Ningbo, China) for 8 days, homogenized with an agate mortar, and sieved through a 35-mesh stainless steel sieve (5 mm aperture). Aliquots (50 g) were transferred to 5 L borosilicate beakers for density separation.

2.2.2. Density Flotation

Firstly, saturated NaCl solution (1.2 g/cm³) and saturated NaI solution (1.8 g/cm³) were prepared in advance for density flotation, and the preparation method was described in Text S1 in detail. The sediment-NaCl solution mixtures (1:40 w/v) were aerated (0.05 L/s) under peristaltic pumping (1.0 L/min) until a 2 L supernatant volume. After magnetic stirring (500 rpm, 10 min) and 8 h of sedimentation, supernatants were vacuum-filtered through 15 μm stainless steel sieves to achieve primary flotation. After that, retained particles were resuspended in NaI solution (1.5 L), stirred (10 min), and settled for 6 h. Suspensions were filtered onto 5 μm cellulose nitrate membranes with triple NaI rinses to achieve secondary flotation.

2.2.3. Organic Digestion and Detection

Membranes were digested with 30% H₂O₂ (40 mL, 12 h), rinsed with Milli-Q water, and refiltered through 0.22 μm PTFE membranes. Sequential ultrasonic extractions (40 kHz, 35 °C) in 99% ethanol (2 × 30 min) preceded solvent evaporation to 1 mL (90 °C water bath). Clarified extracts were stored in amber HPLC vials at 4 °C. Ethanol-suspended MPs (10 μL) were deposited on ZnSe windows. After solvent evaporation, particles were analyzed using an Agilent 8700 LDIR (Agilent Technologies, Santa Clara, CA, USA) with spectral library matching (400–4000 cm⁻¹, 8 cm⁻¹ resolution). Turbid samples underwent repeat flotation prior to analysis.

2.3. Analysis of Clay Mineral

Sediment subsamples (50 g) were dispersed in 1 L Milli-Q water. Clay fractions (<2 μm) were isolated via Stokes’ settling and dried at 50 °C. Oriented mounts were prepared by saturation with magnesium-glycerol/ethanol and stored in Ca(NO₃)₂-humidity chambers for 24 h.

The sample was detected by an X-ray diffractometer (Rigaku Ultima IV, Rigaku, Japan) operated at 40 kV and 40 mA. The data were collected at a scan rate of 10° min⁻¹ within the range of scattering angle 2θ of 10° to 80°. Comparing the scanning map with relevant data to determine the name of the clay mineral family. The relative content of clay minerals in sediments was calculated by the Biscaye semi-quantitative analysis method [21].

2.4. Statistical Analysis

SPSS software (SPSS Inc., Chicago, IL, USA, version 25.0) was used for statistical analysis, and the analyzed item is the correlation of experimental data. Pearson’s correlation coefficient was chosen for the type of correlation analysis. When the p value is lower than 0.05, it is considered that the difference between the data is significant.

3. Results and Discussion

3.1. Abundance Distribution of Microplastics

MPs were discovered in significant quantities in the surface sediments located in the Western North Yellow Sea. Figure 1 illustrates the abundance of MPs. All 20 sediment samples tested positive for MPs with an average concentration of 340.13 items/(g dry weight) (items/gDW). The variability in MP abundance between samples indicates that MPs are not uniformly distributed in seafloor sediments. Concentrations of MPs at different sites ranged from 24 to 1134 items/gDW (Table S1). The distribution of MPs is more concentrated in near-shore areas and seas with water depths greater than 40 m. Three sampling sites (A4, B5, and C5) with water depths greater than 40 m all contained MPs over 500 items/gDW. B5 had the highest concentration of MPs in the surveyed area, amounting to 1134 items/gDW. Previous studies suggest that the average abundance of MPs in the marine environment decreases progressively with distance from shore and water depth [16,22]. However, the results of this survey are not consistent with the findings of previous studies, as they indicate that there is a high abundance of MPs on the northern and southern sides of the surveyed sea area, but a low abundance in the middle. This result is due to the fact that the investigation area is located in the Western North Yellow Sea, near the Bohai Strait, which is a channel for material exchange between the Bohai Sea and the Yellow Sea. The complex monsoon and ocean current conditions in this area affect the temperature and salinity of the seawater, making the material transport path more complex. Especially, the low salt current from the middle of the Bohai Strait to the southeast invading the North Yellow Sea affected the hydrodynamic conditions and salinity distribution of the sea area [23], causing MPs to converge toward both sides of the low salt current, forming an unusual MP distribution pattern.

The Bohai Sea is a typical semi-enclosed sea area that receives a lot of rivers, including the Yellow River, Haihe River, Liao River, and Luanhe River. These rivers carry a significant amount of MPs from North China through surface runoff. During winter, seawater accumulates on the north coast of the Shandong Peninsula, forming a strong coastal current that flows from west to east along the north coast of the Shandong Peninsula. The coastal current passes through the Bohai Strait and transports materials to the North Yellow Sea [24]. The weakening of the summer monsoon makes the coastal current of the Shandong Peninsula disappear, resulting in a decrease in seawater sediment transport capacity. As a result, a pattern of seawater transport to surface sediments occurs in winter and sedimentation in summer on the north coast of the Shandong Peninsula. Research has shown that the coastal current of the Shandong Peninsula is usually located in the coastal area with a shallow contour of 20–30 m [25]. In the surveyed sea area, stations shallower than the 30 m isobath (e.g., B1, C1, C2, D2, and D3) had a high average MP concentration of 549.60 items/gDW, which was significantly higher than the overall average of 340.13 items/gDW. This suggests that the coastal current of the Shandong Peninsula has an impact on the distribution of MPs in the nearshore side of the Western North Yellow Sea. The input of MPs from the Bohai Sea is an important source of MPs in surface sediments here.

3.2. Effect of Salinity in Bottom Seawater on the Distribution of Microplastics

Numerous studies have shown that cold water masses exist in the North Yellow Sea during summer [26,27]. Strong winter winds cause vertical mixing of seawater, leading to the formation of low-temperature, high-brine clumps in the North Yellow Sea during winter. In summer, the surface water temperature rises and the salinity decreases. The weaker summer winds make the upper layer of high-temperature, low-salt water more stable, resulting in a stable mass of cold water at the bottom. This cold water mass is situated in a trough 50 m deeper than the isobath, centered around 122.5° E and 38.5° N [23]. The mass has a weak internal circulation and limited material exchange with the external water body. The salinity difference formed at the edge of the cold water mass creates a front that obstructs the transport of terrigenous materials to the middle of the North Yellow Sea. This phenomenon may be an important reason for the sedimentation of MPs at the edge of the cold water mass. Yao et al. measured the summer bottom salinity of the Western North Yellow Sea and formed an isosalinity map, as shown in Figure 2 [23]. It can be seen that the abundance of MPs was in good agreement with the isosalinity line. A low salt current from the middle of the Bohai Strait to the southeast, invading the North Yellow Sea, greatly influences the distribution of MPs in the area, causing them to converge toward the high salinity area. The density of MPs is less than 200 items/gDW in most sea areas with a salinity lower than 31.2, while in the sea area with salinity higher than this range, the concentration of MPs showed an increasing trend, forming a pattern of high distribution on both sides of the survey sea area and low distribution of MPs in the middle survey sea area. It can be inferred that A4, B5, and C5, located at a depth of more than 40 m and close to the edge of the North Yellow Sea cold water mass, are affected by the frontal currents formed by the temperature and salt difference between the cold water mass and the outer sea. The frontal currents mainly intercept the MPs transported from the Liaodong Peninsula and the Bohai Strait to the interior of the cold water mass, which causes high concentrations of MPs at the edge of the cold water mass.

Plastics are known for their chemical structure stability, which makes them difficult to decompose. They can exist in the natural environment for centuries, and breaking down into smaller particles is often the case when exposed to external forces. In the Western North Yellow Sea, the MP concentration decreased with the particle size decrease (Figure 3A). The highest concentration was found in MPs with a particle size of 10–30 μm, which accounted for an average of 68.79%. Followed by MPs with a particle size of 30–50 μm, accounting for an average of 25.47%, and the proportion of MPs with a particle size of 50–100 μm and >100 μm was smaller, accounting for an average of 4.96% and 0.78%, respectively. The distribution of MPs with different particle sizes in surface sediments of the investigated sea area is shown in Figure 3B–E (specific data are shown in Table S1). Previous research shows that near the North Yellow Sea’s cold water mass, the water body mainly transports terrigenous materials with a particle size < 64 μm in summer [28]. This can lead to the accumulation of small particle size MPs in the Western North Yellow Sea, which is significantly affected by the cold water mass. The results of this survey indicate that MPs with smaller particle sizes are more susceptible to ocean currents formed by cold water masses (Figure 3B). It can be proved by the content of MPs with a particle size of 10–30 μm near the southwest edge of the cold water masses increases, accounting for an average of 75.64% (the area circled in Figure 3B), and becoming the most important constituent part of MPs in the seabed sediments here. According to the analysis in 3.1, the middle of the survey area has low salinity, and MPs with a particle size greater than 50 μm are more distributed here (Figure 3D,E). This phenomenon demonstrates that high-salt ocean currents are more likely to form a collection of MPs with small particle sizes (<50 μm), whereas MPs with larger particles are more likely to gather in low-salt areas.

In recent years, advancements in detection technology have enabled scientists to detect much smaller MP particles, revealing a huge number of tiny plastic particles that were previously unknown. So there is a large increase in the amount of MPs reported in the recent study compared to previous studies. Studies have shown that the relationship between the concentration of MPs and their size follows a power exponential equation [29]. If particles smaller than 30 μm are ignored, and only those larger than 30 μm are considered, the average abundance of MPs in the Yellow Sea will decrease by 45.8% [30]. Therefore, it is necessary to take into account the particle size range of MP detection when comparing the abundance of MPs in different regions. Under the premise that the minimum detected particle size is consistent with 20–30 um, the abundance of MPs in the offshore area of the South Yellow Sea is 54.81 ± 19.36 items/g [31], which is lower than the survey results of the Western North Yellow Sea. This indicates that the Western North Yellow Sea has received MPs from the Bohai Sea and the northern cities of the Shandong Peninsula, resulting in a high level of MP enrichment here.

MPs found in the Western North Yellow Sea are primarily in pellet and fragment forms, with an average proportion of 54.32% and 45.02%, respectively. On the other hand, fiber MPs are the least distributed in this area, with an average proportion of only 0.66% (Figure 4A). Figure S1 shows the laser infrared imaging of distinct forms of MPs. The pellet and fragment MPs, which constitute the majority of the proportion, are dispersed in a zonal pattern parallel to the coastline. This is due to different forms of MPs are influenced by the coastal current of the Shandong Peninsula, the low salt current from the middle of the Bohai Strait to the southeast, and the marine front current formed by the cold water mass, so that MPs have a zonal screening in the southwest to northeast direction (Figure 4B–D).

The materials of MPs found in the surface sediments of the Western North Yellow Sea were tested (Figure 5). A total of 24 types of MPs made from different materials were detected. Among them, 6 types represented more than 5% of the total detected MPs, namely Polyvinyl alcohol (26.53%), Polyamide (17.16%), Polyvinyl chloride (12.44%), Chlorinated polyethylene (8.35%), Polyethylene (6.02%), and Polyurethane (5.40%). These six types accounted for 75.90% of the total detected MPs and were the most prevalent in the surveyed sea area (specific data are shown in Table S2). Polyvinyl alcohol, which was the most frequently detected, is the most biodegradable vinyl polymer. The alcohol groups of Polyvinyl alcohol are easily oxidized to keto groups by enzymes, and keto groups are further hydrolyzed, resulting in molecular chain breakage. This process completes the degradation of Polyvinyl alcohol [32]. The remaining 5 types of MPs are not easily biodegradable and hydrolyzed due to their stable molecular structure [33,34,35]. As a result, they can exist stably in the environment for a long time. The average particle size of these 6 types of MPs ranges from 27.36 to 33.93 μm, and their small size characteristics make them more bioavailable. Persistent organic pollutants such as polychlorinated biphenyls, pesticides, and polycyclic aromatic hydrocarbons are often associated with marine plastic debris. These hydrophobic molecules have a greater affinity for plastics than for sediment or water [36]. Therefore, more toxic substances attached to MPs are ingested by benthic organisms and accumulate through the food chain, ultimately posing a threat to human health. The MP types with larger particle sizes include Ethylene vinyl acetate (mean grain size: 62.21 μm), Polytetrafluoroethylene (mean grain size: 54.04 μm), HDPE (mean grain size: 52.09 μm), and Polystyrene (mean grain size: 47.43 μm). Among them, Ethylene vinyl acetate and Polytetrafluoroethylene are plastic materials with good impact toughness and environmental stress resistance, which allows them to maintain large particle sizes under complex stress conditions in the seabed [37,38].

3.3. Effect of Clay Minerals on the Distribution of Microplastics

The above research shows that the distribution of MPs in the Western North Yellow Sea is uneven. The main clay mineral types in the surface sediments were detected to further understand the factors that may affect the distribution of MPs here, including montmorillonite, illite, kaolinite, and chlorite. A correlation analysis was conducted, taking into account the morphology and particle size distribution of MPs. The results are illustrated in Figure 6. It was observed that montmorillonite had a significant positive correlation with fragment MPs and MPs greater than 100 μm (p < 0.05). Additionally, illite was also found to be significantly correlated with MPs that have a particle size ranging from 10 to 30 μm (p < 0.05).

The density of most plastics is lower than that of seawater, yet MPs are still present in substantial amounts in seafloor sediments. This phenomenon is closely related to the presence of clay minerals in the ocean. In high-salinity water, such as estuaries and oceans, the negative charges on the surface of MPs and clay minerals can be neutralized by rich cations, which weakens the electrostatic repulsion between them. Then the van der Waals force generated by the comprehensive action of weak electric intermolecular forces will facilitate the heterogeneous aggregation between MPs and clays, helping them overcome the buoyancy and settle on the sea floor (Figure S2) [20,39,40]. In addition to the influence of ocean currents caused by temperature and salt differences mentioned in chapter 3.1, the salinity-controlled aggregation between MPs and clay minerals is another important reason for the accumulation of MPs in high-salinity areas.

According to the results of correlation analysis, there is a significant positive correlation between montmorillonite and fragmented MPs (p < 0.05), which can be attributed to the following three main reasons: Firstly, fragment MPs have a larger specific surface area that provides more space for the adhesion of clay minerals. This feature is conducive to the formation of heterogeneous aggregates. Second, montmorillonite’s presence affects MPs’ migration in the surface sediments, which play a role as the loose porous medium. Research by Li et al. has shown that bentonite, a non-metallic material with montmorillonite as its main component, readily forms heterogeneous aggregates with MPs. In an environment of high ionic strength, the migration of bentonite and MPs in the porous medium decreases, and the settlement increases. This results in the co-transport or co-deposition of the two [41]. Lastly, due to the small interlayer connection, montmorillonite is easy to hydrate and dilate in water [42]. With an increase in montmorillonite content, the viscosity of the colloid formed by hydration increases [43], making MPs with large specific surface areas less likely to re-float into seawater. This increases the abundance of fragment MPs in the high distribution area of montmorillonite, causing the distribution of montmorillonite and fragment MPs to gradually become consistent.

Illite is the most abundant clay mineral in the surface sediments of the Western North Yellow Sea. It has a significant positive correlation with the distribution of MPs that have a size ranging from 10 to 30 μm (p < 0.05). Unlike montmorillonite, which is easy to hydrate and dilate, illite has a larger interlayer connection and is not easy to dilate. This makes its clay viscosity poorer compared with montmorillonite [44]. Due to the poor clay viscosity and the shear stress caused by complex ocean currents at the bottom of the Western North Yellow Sea, the formation of large-particle aggregates dominated by illite is inhibited. The MPs with a small particle size of 10–30 μm are mostly pellets, and their low specific surface area makes it unfavorable to aggregate with clay minerals to form heterogeneous aggregates. The disadvantaged position of illite and 10–30 μm MPs in the formation of heterogeneous aggregates may be the primary reason for the significant positive correlation between the distribution of the two.

4. Conclusions and Prospection

Numerous MPs have been discovered in surface sediments of the Western North Yellow Sea. Under detection conditions with a minimum detection limit of 10 μm, the abundance range of MPs in the investigated sea area was 24–1134 items/gDW, presenting a trend of decreasing from shore to the sea and then increasing. The shallower sea area, less than 30 m deep, is influenced by the coastal current from the Shandong Peninsula, which deposits MPs from the Bohai coast. The salinity line of the bottom seawater is in good agreement with the abundance distribution of MPs, indicating that salinity plays a significant role in the distribution of MPs in the marine environment. MPs tend to converge in the high-salinity area, but there are exceptions where the temperature and salinity difference caused by cold water masses will form frontal currents. These currents block the transport of land-based materials into the center of the cold water masses and cause MPs to settle at the edge of the cold water masses. Ocean currents screen different forms of MPs, often in strips parallel to the coast. In high salinity environments, a significant positive correlation exists between montmorillonite with expansive properties and fragment MPs and MPs with particle size > 100 μm, which have a larger surface area (p < 0.05). The negative charges on the surfaces of microplastics and clays can be neutralized by abundant cations in high salinity waters, promoting their heteroaggregation. We hypothesize that this process enhances the co-transport of clay minerals and microplastics in marine environments, which may be another important reason for the enrichment of MPs in high salinity areas. This study clarifies the distribution characteristics of MPs in the Western North Yellow Sea and reveals the effects of bottom seawater salinity and clay mineral type on the distribution of MPs, which provides a reference for the study of MP transport characteristics in semi-enclosed waters of the continental shelf.

Due to the unique geological terrain conditions, the channel outlet in the semi-closed sea area forms a distinctive ocean current characteristic. This causes a significant salinity difference in the bottom seawater over a short distance, which affects the movement of MPs in the marine surface sediments. In the Western North Yellow Sea, near the Bohai Strait, MPs in surface sediments tend to gather in the high salinity area. However, further investigation is needed in the open sea where salinity changes slowly. When evaluating and controlling MP pollution in semi-enclosed sea areas, the focus areas can be determined based on the distribution of salinity in the bottom seawater. This will help avoid cumbersome large-scale sampling investigations. Given that the regionally dominant clay mineral types in most of the world’s current marine areas have basically been identified, exploring the influence of marine clay mineral types on the sedimentation characteristics of MPs of different sizes and morphologies in surface sediments makes it easier to determine the characteristics of MPs that are susceptible to enrichment in the target area, thus improving the targeting of pollution prevention, control or treatment efforts for MPs in marine areas.