Microplastics as adsorbent for Pb²⁺ and Cd²⁺: A comparative study of polypropylene, polyvinyl chloride, high-density polyethylene, and low-density polyethylene

Microplastics (MPs) in aquatic environments adsorb heavy metals, thereby posing potential environmental risks. However, further research is needed to elucidate the adsorption behavior of different types of MPs for various heavy metals. The aim of this study was to characterize four types of MPs: polypropylene (PP), polyvinyl chloride (PVC), high-density polyethylene (HDPE), and low-density polyethylene (LDPE). Moreover, their Pb²⁺ and Cd²⁺ adsorption properties were determined to investigate the differences in their capacity to function as heavy metal adsorbents. MPs were characterized via scanning electron microscopy (SEM) using energy dispersive X-ray spectrometer (EDS), Brunauer-Emmett-Teller (BET) analysis, and Fourier transform infrared spectroscopy (FTIR). Adsorption experiment data were analyzed using the Langmuir and Freundlich isotherm models to evaluate the adsorption capacity of the MPs. Based on the results of the adsorption isotherm models and 2D-COS FTIR, the presence of oxygen-containing functional groups, including hydroxyl, carbonyl, and carboxyl groups influences the adsorption process of Pb²⁺ and Cd²⁺ onto PP and PVC, with the maximum adsorption capacities (Q_m) being 0.759 mg/g and 0.495 mg/g, respectively. Combination of the adsorption isotherm data and characteristics of MPs revealed that the following order of adsorption efficiencies of MPs for each heavy metal: PP > LDPE > PVC > HDPE for Pb²⁺ and PP > PVC > LDPE > HDPE for Cd²⁺. The results of this study suggest that MPs, particularly PP and PVC, may serve as vectors for heavy metal transport in aquatic environments, highlighting the need for further research to assess their environmental impact.