Ebullition-Enhanced Release of Microplastics from Aquatic Sediment to Overlying Water: An Overlooked Transport Pathway

Abstract Microplastics (MPs) in aquatic systems are typically attributed to wastewater, runoff and atmospheric inputs, whereas the remobilization of sediment-associated MPs by benthic gas ebullition remains poorly understood. Here, we show that air-purged ebullition mobilized four common MPs, polyethylene (PE), polyethylene terephthalate (PET), polyvinyl chloride (PVC), and polylactic acid (PLA), from sediments into the overlying water, with release ratios of 8.9%–12.1% over 9 d. Suspended MP concentrations in the overlying water increased rapidly during the first 48 h, followed by slower release and eventual plateauing, with the decelerating release behavior empirically described by a power-law model. Theoretical calculations suggest that aggregation with suspended sediment can reduce the flotation velocities of MPs from 9.38–9.99 cm·s–1 to 8.64–9.95 cm·s–1. Combined experimental observations and theoretical analyses support a coupled flotation mechanism involving MPs attachment to rising bubbles, bubble-driven flow that counteracts gravitational settling, and transient pressure gradients. These processes could convert sediments from a long-term sink into a secondary source of MPs during ebullition. Overall, this study identifies benthic gas ebullition as an underappreciated pathway for MP remobilization and provides a mechanistic framework for understanding the fate and transport of sediment-associated MPs in aquatic environments.