Enhanced Adsorption of Cadmium onto Microplastics Derived from Polyethylene Mulching Film by Weathering

The interaction between microplastics (MPs) and heavy metals would significantly determine their environmental impacts. The aim of this study was to investigate the influences of weathering on the affinity of MPs derived from polyethylene mulching film towards Cd(II). Weathered (w-PEMP) and pristine MPs (p-PEMP) were prepared from mulching film sampled from a garlic planting field and the pristine one, respectively. MPs were characterized by spectrum, surface charge and hydrophobicity analysis to determine the variations in physicochemical properties during the weathering process. Batch adsorption and desorption experiments were conducted to compare the affinity of MPs towards Cd(II) before and after the weathering process. Results showed that the Pseudo-second-order dynamics model satisfactorily described the adsorption of Cd(II) onto w-PEMP, indicating the different binding sites on the surface and the main role of chemical adsorption in the interaction. According to Langmuir model, maximum adsorption capacity of w-PEMP was more than 4 times higher than that of p-PEMP, corresponding to the coarse microscopic surface and the exogenous elements and more oxygen containing groups introduced in weathering process. A higher K_f value of w-PEMP in Freundlich model suggested that the weathered microplastics surfaces had a higher adsorption affinity for the Cd(II) than p-PEMP. The weathering process also significantly increased desorption capacity of the MPs for Cd(II), and mitigated the influence of pH on the adsorption of Cd(II) onto the MPs. The weathering process markedly altered the surface morphology and physicochemical properties of MPs, and significantly enhanced the adsorption and desorption capacity of Cd(II).