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[MIS02-03] Effect of sand properties on foamed polystyrene fragmentation
Keywords:Microplastic Fragmentation, Size Distribution, Shape Analysis, Development of Protocol
Foamed polystyrene (FPS) is characterized by low abrasion resistance and a tendency to fragment easily into small pieces, particularly on beaches. Although fragmentation experiments have been conducted, no research has quantitatively analyzed both the quantity and shape of the fragments produced. This study developed a protocol to elucidate the fragmentation modes of FPS microplastics through size distribution and shape analysis of FPS fragments and parent spheres. Furthermore, experiments were conducted using three types of sand with varying grain sizes and morphologies to evaluate the effects of the sand properties on the FPS fragmentation mode.
The basic and comparative cases of fragmentation experiments were performed by mixing sand, water, and virgin FPS spheres in a pot mill. The basic cases utilized the beach sand collected from the foreshore of Itsukushima in Hiroshima Bay in 2023. The milling time was varied for seven basic cases: 6 h, 12 h, 1 d, 2 d, 3 d, 5 d, and 10 d. To elucidate the effect of sand properties on the fragmentation mode, two types of sand were utilized in this study: beach sand obtained from Itsukushima in 2024, and river sand. Sand properties were categorized according to grain size distribution, circularity, and angle of repose, exhibiting the following characteristics. The median grain size was comparable for the beach sand collected in 2023 and river sand (approximately 1.4 mm) and exceeded that of the beach sand collected in 2024 (approximately 0.85 mm). River sand has the highest circularity. In descending order, angles of repose were observed for beach sand collected in 2023, beach sand collected in 2024, and river sand. Comparative cases were conducted by milling each sand, water, and 100 virgin FPS spheres for 10 days.
Based on the basic cases, temporal tracking identified two distinct fragmentation modes: one dominated by fragments in the 5–100 µm range (size class 1) and the other by fragments in the 200–1000 µm range (size class 2). Size class 1 was observed in all comparative cases, whereas size class 2 was present only in the case of beach sand collected in 2023. The fragmentation mode varied with different sand properties. Sands with a larger angle of repose are likely to agglomerate and descend in the pot mill, and a larger median grain size will promote the fragmentation of FPS spheres.
The observation that the fragmentation mode varied with sand properties suggests that the size and quantity of the generated fragments are likely to differ depending on the beach and location (foreshore or backshore) where microplastics wash ashore. Subsequent experiments should be conducted with diverse sand properties, polymer types and degradation states of plastics, and pot mill rotational speeds.
The basic and comparative cases of fragmentation experiments were performed by mixing sand, water, and virgin FPS spheres in a pot mill. The basic cases utilized the beach sand collected from the foreshore of Itsukushima in Hiroshima Bay in 2023. The milling time was varied for seven basic cases: 6 h, 12 h, 1 d, 2 d, 3 d, 5 d, and 10 d. To elucidate the effect of sand properties on the fragmentation mode, two types of sand were utilized in this study: beach sand obtained from Itsukushima in 2024, and river sand. Sand properties were categorized according to grain size distribution, circularity, and angle of repose, exhibiting the following characteristics. The median grain size was comparable for the beach sand collected in 2023 and river sand (approximately 1.4 mm) and exceeded that of the beach sand collected in 2024 (approximately 0.85 mm). River sand has the highest circularity. In descending order, angles of repose were observed for beach sand collected in 2023, beach sand collected in 2024, and river sand. Comparative cases were conducted by milling each sand, water, and 100 virgin FPS spheres for 10 days.
Based on the basic cases, temporal tracking identified two distinct fragmentation modes: one dominated by fragments in the 5–100 µm range (size class 1) and the other by fragments in the 200–1000 µm range (size class 2). Size class 1 was observed in all comparative cases, whereas size class 2 was present only in the case of beach sand collected in 2023. The fragmentation mode varied with different sand properties. Sands with a larger angle of repose are likely to agglomerate and descend in the pot mill, and a larger median grain size will promote the fragmentation of FPS spheres.
The observation that the fragmentation mode varied with sand properties suggests that the size and quantity of the generated fragments are likely to differ depending on the beach and location (foreshore or backshore) where microplastics wash ashore. Subsequent experiments should be conducted with diverse sand properties, polymer types and degradation states of plastics, and pot mill rotational speeds.