Occurrence, ecological risk of microplastics in campus athletic fields runoff and their adsorption behavior towards heavy metals.

The occurrence of microplastics in athletic fields and their risk of ecological pollution have attracted widespread attention. The abundance, particle size, morphology, color and type of microplastics as well as their ecological risk are conducted in five types of athletic fields runoff on a campus in Beijing. The concentration of microplastics in the stormwater runoff of the five athletic fields ranges 2433 ± 493 to 5067 ± 839 particles/L, composed of fibers, granules and fragments. Fibers microplastics (41-64%) are the most abundant in stormwater runoff samples from most athletic fields, followed by granules (26-45%), and fragments (8-18%). ATR-FTIR and micro-FTIR identify the types of microplastics in runoff from athletic fields as EPDM, SBR, PE, PP, PO, rayon, and nylon. The degree of microplastic pollution is ranked level II-III pollution, which posing potential health and ecological risks. The adsorption behavior is tested for three types of microplastic particles including ethylene propylene diene monomer (EPDM), styrene-butadiene rubber (SBR) and aged-SBR particles derives from athletic fields surface materials towards runoff typical heavy metals Pb and Zn. The adsorption isotherms are more in line with the Langmuir model, indicating a chemical monolayer adsorption. The maximum adsorption capacity towards Pb and Zn follow the order of EPDM (2.67 mg/g) > aged-SBR (1.50 mg/g) > SBR (0.13 mg/g), and EPDM (2.61 mg/g) > aged-SBR (1.50 mg/g) > SBR (0.56 mg/g), respectively. Aged microplastics are subjected to processes such as UV aging and weathering, the surface layer is more likely to acquire charges and adsorb metals to maintain charge balance. EPDM, SBR and aged-SBR particles all contain Ca, Zn, and Mg, which can undergo displacement reactions with Pb and Zn. FTIR results indicate that the adsorption of heavy metals may alter the surface chemical properties of microplastics, rendering them more polar. XPS results reveal that the changes in surface functional groups of EPDM are more pronounced before and after adsorption compared to SBR and aged-SBR, indicating that chemical adsorption plays a dominant role in this process. Microplastics in runoff from athletic fields is an important source of microplastic release, and the occurrence of microplastics needs to attract further attention. The adsorption of microplastics and pollutants in athletic field runoff could exacerbate their combined pollution, thus their ecological risks cannot be ignored.