Surfactant-induced adsorption of Pb(II) on the cracked structure of microplastics.

Surfactant molecules can change the hydrophobic nature of microplastic surfaces, thereby affecting the adsorption of heavy metals in the environment onto the microplastics. It is essential to explore the role of crack structure of microplastics in the adsorption of heavy metals, especially in the presence of surfactants. In this study, polyethylene (PE) and polypropylene (PP) were evaluated for Pb(II) adsorption and desorption mechanism in the presence of two surfactants: cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS). The experimental results were analyzed using kinetics and the isothermal model fitting and spectrogram (FTIR, XPS). This study showed that the application of surfactants could greatly enhance the Pb(II) adsorption capacities of PE and PP by promoting Pb(II) into the fissures. The Pb(II), S, and N contents did not significantly decrease at different depths in the presence of surfactants, and the Pb(II) content without surfactants decreased with an increasing depth. The adsorption behavior was consistent with the Bangham channel diffusion model and the DR model, suggesting that the adsorption process was related to the pore structure of the microplastics. Furthermore, the release of Pb(II) from desorption using high concentration of surfactant solution was less than that of low concentration, it was difficult to release heavy metals primarily because of the crack structure of the microplastics, especially when more surfactant molecules entered the pores. This work contributes to a better understanding of the adsorption mechanism of heavy metals on microplastics in the presence of surfactants, which will better control the ecological risks of microplastics.