Study on the Adsorption Behavior of Cadmium by the MPs and Its Environmental Factors

Abstract

This work examined the adsorption mechanism of microplastics made of polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) on Cd2+ and its effects on the environment. The findings demonstrated that microplastics’ crystallinity and unique functional groups also affected the adsorption of Cd2+, besides being a critical factor regulating the particle size of the MPs. The three MPs’ Cd2+ adsorption kinetics conformed to the quasi-secondary kinetic model, confirming that the adsorption was nonlinear. Besides, the three MPs’ Cd2+ isothermal adsorption process can be adequately described by the Langmuir and Freundlich models, demonstrating that monolayer and multilayer adsorption coexist and are heterogeneous adsorption processes. As indicated by the findings of the energy spectrum study, PVC (0.78%), PP (0.25%), and PE (0.18%) exhibited the maximum concentrations of Cd2+ adsorbed in MPs. As revealed by the findings of the effect of environmental conditions on the adsorption of Cd2+ by the MPs, the three MPs exhibited the maximum adsorption capacity at 35ºC and a pH of 6.0. The equilibrium adsorption of Cd2+ by PP, PE, and PVC was reduced by 3.3, 8, and 13 times, respectively, compared with no salt at a salt concentration of 1 mg/L salt solution.