Contrasting effects of physical and chemical aging of microplastics on the transport of lead and copper in sandy soil

Microplastic (MP) contamination in the terrestrial environment has received increasing concern in recent years. Due to the richness of surface chemistry in MPs after aging, the complexity of aged MPs with heavy metals often happens and their cotransport in soil could pose a high ecological risk. This study investigated the transport of Pb and Cu in saturated sandy soil coexisting with polystyrene MPs aged by physical coating and chemical oxidation. Iron oxide and extracellular polymeric substances coatings as physical aging processes enabled MPs surface covered with Fe and carboxyl and amide groups, respectively, increasing the MPs surface charge and particle size. Consequently, the inhibited transport of aged MPs and their enhanced sorption capacity resulted in greater accumulation of Pb by11.6 %-31.5 % and Cu by 23.2 %-30.9 % in aged-MPs soil compared to that in the pristine-MPs soil. Chemical aging by ultraviolet irradiation and persulfate oxidation caused surface roughness of MPs with particle size reduction. Correspondently, the MPs transport was enhanced, and co-transport of Pb and Cu in soil increased by up to 27.2 % and 36.2 %, respectively, in comparison with that in the pristine MPs soil. Overall, the transport of Pb and Cu in soil was contrast to physical and chemical aging of plastics in the terrestrial environment which may cause the possibility of the combined pollution of heavy metals with plastics in surface soil and subsurface down to the groundwater, respectively.