Quantification and global emission estimates of antifouling paint-derived microplastics released during ship hull...

In-water cleaning (IWC) is conducted underwater to remove biofouling from ship hulls, thereby reducing hydrodynamic drag and improving ship's fuel efficiency, but it can unintentionally release ship coating-derived materials into the marine environment. This study investigated microplastic (MP) pollution derived from ship coatings released during IWC, with the aim of quantifying MP emissions to the marine environment. Polymer type, abundance, mass concentration, and size distribution of MPs in IWC wastewaters were determined before and after wastewater treatment within the particle size range of 20 µm to 5 mm. Acrylate-based antifouling (AF) paint particles dominated the MP fraction, accounting for approximately 99% of total particles by both number and mass. Following wastewater treatment, number-based AF-MP concentrations decreased from 503 to 26 particles mL^-1 (96.6% removal), while mass-based concentrations decreased from 24.7 to 1.2 mg L^-1 (96.2% removal). AF-MPs smaller than 100 µm represented 73.8% of total particle numbers and 28.8% of total mass prior to treatment. Global AF-MP emissions associated with IWC were preliminarily estimated by combining the measured AF-MP emission factor (0.65 g m^-²), the global ship wetted surface area (3.26 × 10⁸ m²), and an average hull-cleaning frequency (0.368 yr^-1). Annual AF-MP emissions were estimated at 78.3 t yr^-1 without wastewater treatment, decreasing to approximately 3.0 t yr^-1 under the assumption that all generated wastewaters were processed through wastewater treatment systems. These results demonstrate that IWC represents a significant yet potentially controllable source of paint-derived microplastics and underscore the importance of effective wastewater capture and treatment in mitigating marine microplastic pollution.