Model s9i

Macroplastics (>5 mm) were excluded from the study. Microplastics were counted and measured after flotation using a stereomicroscope (Model S9i, Leica AG, Wetzlar, Germany). Microplastics (2e5 mm, not including fibers) were identified using Fourier transform infrared spectrometry (Nicolet iS10, Thermo Fisher, Waltham, MA) fitted with an ATR accessory (ATR-FTIR). The spectral range was 650e4000 cm À1 at 32 scans. Representative particles (<2 mm) and fibers were identified by m-FTIR (Nicolet iN10, Thermo Fisher, Waltham, MA). The spectral range was set at 650-4000 cm À1 and samples were scanned 16 times. A scanning electron microscope (SEM) (s-4800, Hitachi, Tokyo, Japan) was used to observe the surface morphology of the microplastics, and the elemental composition of selected surface micro-domains was analyzed by combination with an energy dispersive spectrometer (EX-350, Horiba, Kyoto, Japan).
The microplastics were classified into four categories based on their characteristics and morphology, namely fibers (elongated strings), fragments (hard angular pieces), films (soft transparent flakes), and granules (irregular stereo particles) (Fig. S1). Here, the category polypropylene comprised poly(ethylene)propylene copolymers, polyethylene consisted of low-density and high-density polyethylene, and polyester (PES) encompassed polyester fibers and polyethylene terephthalate (PET).

Soil and sludge samples were filtered after digestion for further analysis and the microplastics were observed under a stereomicroscope (Model S9i, Leica, Wetzlar, Germany). The particle size range of microplastics detected in our study is 0.021-4.996 mm. Representative soil microplastics were selected for identification of composition using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) (Nicolet iS5, Thermo Fisher, Waltham, MA) and micro-Fourier transform infrared spectroscopy (μ-FTIR) (Spotlight 400, Per-kinElmer, Waltham, MA). Each sample was scanned at a rate of 16 scans in the spectral range 750-4000 cm -1 . Polyester (PES) consisted of polyester fibers and polyethylene terephthalate (PET). Polyethylenepolypropylene copolymer was included in polypropylene (polypropylene accounted for the majority). The microplastics in the sludges were separated from non-plastics by stereomicroscopic observation. The composition of the microplastics was not determined. A scanning electron microscope (SEM) (Model s-4800, Hitachi, Tokyo, Japan) was used to observe the surface morphology of the microplastics extracted from the soil samples.