Adsorption of three bivalent metals by four chemical distinct microplastics

Microplastics (MPs), the appearance of which has gained considerable interest, can act as vectors to transport other pollutants such as metals into organisms. In this study, the sorption isotherms of three model heavy metals (i.e., Cu²⁺, Cd²⁺, and Pb²⁺) on four virgin plastic particles including chlorinated polyethylene (CPE), PVC, and two polyethylene plastic particles (i.e., LPE and HPE). HPE and LPE were investigated. The results showed that MPs can load high amounts of Pb²⁺, Cu²⁺ and Cd²⁺. The sorption affinity of the three metals to the model MPs followed the sequence of CPE > PVC > HPE > LPE. The adsorption process was affected by the chemical structure and electronegativity of the sorbents, and seemed irrelevant to the crystallinity of MPs. For the three metals, Pb²⁺ exhibited significantly stronger sorption than did Cu²⁺ and Cd²⁺ due to the strong electrostatic interaction. Moreover, pH can significantly affect the sorption of metals on MPs, but ionic strength exerted a relatively slight effect on this process. In brief, the electrostatic interaction played an important role in the sorption of Pb²⁺ to model MPs. For Cd²⁺ and Cu²⁺, sorption was determined by electrostatic interaction together with surface complexation onto the plastic surface. This study indicates that depending on the surface physicochemical properties of MPs the adsorption behavior can vary significantly. Therefore, the adsorption process of metals on MPs should be readily affected by other environmental mediums in the environment. The study provides additional insight into the behavior of MPs as a vector of metals.