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Updated: Sep 21, 2025
Sampling, Sorting, and Characterizing Microplastics in Aquatic Environments with High Suspended Sediment Loads and Large Floating Debris
Published on: July 28, 2018
Integrated effects of polymer type, size and shape on the sinking dynamics of biofouled microplastics
Siguang Liu1, Yifeng Huang2, Dehua Luo3
1Key Laboratory of Watershed Sciences and Health of Zhejiang Province, School of Public Health and Management, Wenzhou Medical University, Wenzhou 325035, China; Fujian Institute of Oceanography, Xiamen 361013, China.
Abstract:
Sinking of microplastics (MPs) after biofouling is considered an important mechanisms responsible for the downward transport/sedimentation of MPs in the ocean and freshwaters. Previous studies demonstrated MP sinking caused by an increase in the composite density of MPs after biofouling, while MPs with smaller size or shapes with higher surface area to volume ratios (SA:V), such as films, are speculated to sink faster. In this study, we designed an in situ microcosm to simulate the ambient environmental conditions experienced by floating MPs to elucidate the biofouling and sinking of polyethylene (PE), polypropylene (PP), and expanded-polystyrene (EPS) MPs of various sizes and shapes. Our results showed smaller PE and PP MP granules sank faster than large ones. Even EPS granules of 100 μm diameter, having a much lower density (0.02 mg/mm3) than water, started to sink after 2 weeks of biofouling. Moreover, PE film and fiber MPs with higher SA:V did not sink faster than PE MP granules of the same mass, implying that mechanisms other than SA:V, such as fouling contact area and drag coefficient, play a role in the regulation of biofouling and sinking of MPs.
Insights
Biofouling causes microplastics (MPs) to sink. Smaller polyethylene (PE) and polypropylene (PP) MP granules sank faster than larger ones, and even low-density expanded polystyrene (EPS) MPs sank after biofouling.
Area of Science:
- Environmental Science
- Marine Biology
- Polymer Science
Background:
- Biofouling increases microplastic (MP) density, facilitating their sedimentation in aquatic environments.
- Smaller MPs or those with high surface area to volume ratios (SA:V) like films are hypothesized to sink faster.
- Understanding MP sinking mechanisms is crucial for assessing their environmental fate.
Purpose of the Study:
- To investigate the biofouling and sinking dynamics of various microplastics (PE, PP, EPS) under simulated environmental conditions.
- To determine the influence of MP size, shape, and material on their sedimentation rates.
- To elucidate the key factors governing microplastic biofouling and sinking.
Main Methods:
- An in situ microcosm was utilized to simulate natural aquatic conditions for microplastic exposure.
- Polyethylene (PE), polypropylene (PP), and expanded polystyrene (EPS) microplastics of diverse sizes and shapes were studied.
- Biofouling accumulation and subsequent sinking of microplastics were monitored over time.
Main Results:
- Smaller PE and PP microplastic granules exhibited faster sinking rates compared to larger granules.
- Expanded polystyrene (EPS) microplastics, even those with very low density, began to sink after two weeks of biofouling.
- Microplastic films and fibers with higher SA:V did not necessarily sink faster than granules of equivalent mass, suggesting other factors are involved.
Conclusions:
- Microplastic size is a significant factor influencing sinking rates, with smaller particles sedimenting faster.
- Biofouling is a critical process enabling the sinking of even buoyant microplastics like EPS.
- Factors beyond SA:V, including fouling contact area and drag coefficient, play important roles in microplastic biofouling and sedimentation.
