5:15 PM - 6:45 PM
[MIS09-P10] Behaviors of Small Microplastics in Hiroshima Bay Based on their Characteristics of Size and Polymer Types
Keywords:Small microplastics, Sinking velocity, Bottom sediments, Residual current
The size and types of polymers significantly influence the behaviors of microplastics (MP) in the marine environment. Haave et al. (2019) investigated the bottom sediments of Norway and found that the MP concentrations increased as the size decreased. They also observed a higher diversity of polymer types in particles smaller than 500 µm compared to those larger than 500 µm. Sagawa et al. (2018) examined the MP concentrations and types of polymers in the bottom sediments of Hiroshima Bay. Their results indicated that MP with a lower density than seawater, particularly foamed polystyrene (FPS) used in oyster aquaculture facilities, dominated. However, MP smaller than 300 µm was not discussed, especially the FPS pollution and the variations in polymer types. This study aims to understand the characteristics of MP based on size differences and investigate the abundance and types of polymers of small MP in the bottom sediments of Hiroshima Bay.
Sampling of the bottom sediments was conducted using a Smith-McIntyre grab sampler (4.2 L) at three stations in Hiroshima Bay. Approximately 50 g of wet sediment was taken from the collected sediments. Extraction was performed using a method that involved removing organic matter with a Fenton reagent, density separation with a sodium iodide solution, suction filtration using a 109 µm mesh, and identifying types of polymers identification using ATR-FTIR and Micro-FTIR. A portion of the solution that passed through the 109 µm mesh was filtered using a 15 µm mesh. Tests were conducted using plastic pellets of known quantities to investigate the impact of solution volume and the amount of sample taken on MP recovery. The results confirmed no significant difference in recovery counts across all verification cases, indicating that the solution extraction did not affect the MP extraction counts in this study.
The collected MP in Hiroshima Bay ranged from 20 µm to 999 µm, with an average concentration of 17.91 million particles/m². Of these, over 99% were MP smaller than 100 µm, and the density increased as the size decreased.
Polystyrene (PS) and PE were found in all size fractions below 1000 µm. Among particles larger than 350 µm, PS constituted 56%, confirming FPS contamination as observed by Sagawa et al. (2018). In contrast, the proportion of polystyrene decreased to 17% in particles smaller than 350 µm, revealing the emergence of polymers with a higher density than seawater, such as epoxy (22%, specific density 1.1). Additionally, all discovered epoxy were below 150 µm, demonstrating an increase in polymer diversity as the size decreased.
Although plastics such as PE and PS are commonly found in various marine environments, epoxy was rarely detected (Wang et al., 2021; Eo et al., 2022). Jaini et al. (2023) reported that epoxy is used in ship paints. Considering that Hiroshima Bay has the highest ownership and disposal rates of pleasure boats nationwide, with active ship usage, this study suggests that epoxy contamination originates from ships in Hiroshima Bay.
However, epoxy larger than 150 µm was not found. The study estimated the impact of density and size on the dispersion of epoxy in the ocean by calculating sedimentation rates and horizontal transport distances. The results indicated that larger epoxy particles had higher sedimentation rates, settling quickly to the seabed. Therefore, it is suggested that these larger particles may accumulate on the seabed in fishing ports or marinas. On the other hand, smaller particles with slower sedimentation rates may selectively transport more easily into the bay. As a result, this study did not find epoxy particles larger than 150 µm; it only identified MP smaller than 150 µm.
In the future, sampling of the bottom sediments in fishing ports and marinas should be conducted to investigate the concentration and size distribution of epoxy larger than 150 µm. Furthermore, it is necessary to investigate the sinking of large polymers, such as nylon, with higher density near river mouths.
Sampling of the bottom sediments was conducted using a Smith-McIntyre grab sampler (4.2 L) at three stations in Hiroshima Bay. Approximately 50 g of wet sediment was taken from the collected sediments. Extraction was performed using a method that involved removing organic matter with a Fenton reagent, density separation with a sodium iodide solution, suction filtration using a 109 µm mesh, and identifying types of polymers identification using ATR-FTIR and Micro-FTIR. A portion of the solution that passed through the 109 µm mesh was filtered using a 15 µm mesh. Tests were conducted using plastic pellets of known quantities to investigate the impact of solution volume and the amount of sample taken on MP recovery. The results confirmed no significant difference in recovery counts across all verification cases, indicating that the solution extraction did not affect the MP extraction counts in this study.
The collected MP in Hiroshima Bay ranged from 20 µm to 999 µm, with an average concentration of 17.91 million particles/m². Of these, over 99% were MP smaller than 100 µm, and the density increased as the size decreased.
Polystyrene (PS) and PE were found in all size fractions below 1000 µm. Among particles larger than 350 µm, PS constituted 56%, confirming FPS contamination as observed by Sagawa et al. (2018). In contrast, the proportion of polystyrene decreased to 17% in particles smaller than 350 µm, revealing the emergence of polymers with a higher density than seawater, such as epoxy (22%, specific density 1.1). Additionally, all discovered epoxy were below 150 µm, demonstrating an increase in polymer diversity as the size decreased.
Although plastics such as PE and PS are commonly found in various marine environments, epoxy was rarely detected (Wang et al., 2021; Eo et al., 2022). Jaini et al. (2023) reported that epoxy is used in ship paints. Considering that Hiroshima Bay has the highest ownership and disposal rates of pleasure boats nationwide, with active ship usage, this study suggests that epoxy contamination originates from ships in Hiroshima Bay.
However, epoxy larger than 150 µm was not found. The study estimated the impact of density and size on the dispersion of epoxy in the ocean by calculating sedimentation rates and horizontal transport distances. The results indicated that larger epoxy particles had higher sedimentation rates, settling quickly to the seabed. Therefore, it is suggested that these larger particles may accumulate on the seabed in fishing ports or marinas. On the other hand, smaller particles with slower sedimentation rates may selectively transport more easily into the bay. As a result, this study did not find epoxy particles larger than 150 µm; it only identified MP smaller than 150 µm.
In the future, sampling of the bottom sediments in fishing ports and marinas should be conducted to investigate the concentration and size distribution of epoxy larger than 150 µm. Furthermore, it is necessary to investigate the sinking of large polymers, such as nylon, with higher density near river mouths.