Comparative assessment of cadmium(II) carrier-effects derived by polyethylene microplastics with heterogeneous sources

Microplastics (MPs), as wastes derived from various plastic commodities, have distinct heterogeneity in their sources. The overlook of MP sources may decrease the prediction reliability of their carrier-effects and the execution efficiency of their refined control. Hence, this study focuses the influence of source heterogeneity on the Cd(II) carrier-effect of MPs, aiming to improve the insights on Cd(II)-pollution pattern reshaped by MPs. This study employed three polyethylene (PE) MPs: bottle PE-MPs (BPE), impregnating-powder PE-MPs (IPE), and packaging/bag PE-MPs (PPE), to conduct indoor batch-type adsorption experiments and highlight the controlled variable of source heterogeneity. This study further considered effects of environmental factors (e.g., pH and the presence of fulvic acid (FA)), and made instrument characterization and theoretical simulations (density functional theory, DFT; molecular dynamics, MD) to explain relevant mechanisms. PPE resulted in the highest degree of Cd(Ⅱ) adsorption; BPE resulted in the most flexible desorption capability; and IPE resulted in the greatest adsorption capacity per unit specific surface area, which suggested significant differences among Cd(Ⅱ) carrier-effects of the three PE-MPs. The excellent adsorption capacity of PPE was likely attributed to its abundant pores, larger specific surface area, and additional -OH surface modification. Whereas the three PE-MPs showed consistency in the adsorption model fitting, where the superior fits of the Freundlich over Langmuir models, and the pseudo-second-order over pseudo-first-order models were confirmed for all PE-MPs. Mechanically, after adsorption, the FTIR spectra showed limited new peaks or peak shifts, which implies that the adsorptions of three PE-MPs were dominated by physical interactions. Additionally, this study found that both the increased pH and FA presence promoted Cd(Ⅱ)-adsorption across all three PE-MPs, which mainly owing to the strengthening of electrostatic interactions and the formation of molecular bridging, respectively, according to DFT, MD, and Zeta potentials. This study is expected to provide enlightenment on deciphering the intricate environmental behaviors of MPs with source heterogeneity and advancing source-targeted strategies of MP management.