Comparative analysis of kinetics and mechanisms for Pb(II) sorption onto three kinds of microplastics

Microplastics (MPs), a kind of novel contaminant, have potential to concentrate and transport heavy metals in the aquatic environment. This feature may affect the distribution and bioavailability of heavy metals. In order to determine the sorption behaviors of heavy metals onto the MPs, the sorption kinetics and mechanisms were investigated between the MPs (polyvinylchloride PVC, polyethylene PE, polystyrene PS) and Pb(II). The results suggested that the Pb(II) sorption onto the MPs were pH- and ionic strength-dependent. The sorption processes were best fitted by the pseudo-second-order model, and the rate-limiting steps were the intraparticle diffusion and final equilibrium process. The maximum sorption capacities of PVC, PE and PS were 483.1 μg/g, 416.7 μg/g and 128.5 μg/g under the condition of 0.01 M NaCl, pH 6.0, T = 298 K. The sorption rate constants were in the following order: PVC<PE<PS. According to the Fourier transformed infrared and X-ray photoelectron spectroscopy, no new bonds were formed between the MPs and Pb(II). Physisorption was the main driven force for Pb(II) sorption. In summary, the investigation reveals the Pb(II) sorption kinetics and mechanisms onto the MPs, which will improve the understanding of the interactions between the MPs and heavy metals.