Revealing the hidden environmental risks: A two-step sequential extraction of mobile and immobile (mineral-bound) microplastics in coral reef sediments.

Despite increasing attention on microplastics (MPs) in marine environments, their entrapment within biogenic reef sediments remains understudied. This study applies a dual-fraction pretreatment method to quantify mobile MPs (MMPs) and immobile MPs (IMPs) in carbonate-rich sediments. Sediment samples were sequentially treated with hydrogen peroxide to digest organic matter and isolate MMPs, followed by hydrochloric acid to dissolve inorganic carbonates and release IMPs. Seasonal monitoring of coral reef sediments in Penghu Archipelago, Taiwan, revealed MMP abundances ranging from 23 ± 16 to 153 ± 46 MP/kg, while IMPs ranged from 20 ± 9 to 73 ± 20 MP/kg. MMPs exhibited higher diversity (MPDII = 0.55), predominantly fibrous in shape, <1 mm in size, and a broader spectrum of synthetic polymers (e.g., polypropylene, polyester). IMPs showed lower diversity (MPDII = 0.42) and selective retention of polymers such as rayon and cellulose (together ∼48 %). Conventional indices (CF, PLI, PRI, PERI) identified MMPs as higher-risks, but underestimated IMP risks. IMPs may persist as a chronically stable yet potentially mobilizable pool in carbonate-rich sediments through mineral embedding and environmental disturbances. These findings highlight the need for bioavailability-aware refinements to ecological risk assessments. This study provides the first seasonal monitoring of dual-fraction partitioning of MPs in reef sediments, offering new insights into their long-term fate and behaviour.