Effects of polystyrene fragments on the transport of Pb2+ in saturated porous media: The role of microplastics characteristics and flow velocity

Microplastics (MPs) could interact with heavy metals via multiple mechanisms in subsurface environment. Understanding the effect of MPs on the fate of heavy metals is essential for the prediction of their ecological impacts. In this study, laboratory columns were conducted to investigate the effects of pristine/aged secondary polystyrene fragments of different sizes and dosages on the transport of Pb in saturated porous media. MPs generally promoted Pb mobility, and the promotion degree was greater with increasing MP size and dosage. The enhancement of Pb mobility by naturally aged MPs was slightly weaker than that by pristine MPs. The sensitivity of Pb mobility to MPs was correlated with the flow velocity, the promotion impact was more significant at larger flow velocity. The enhanced mobility of Pb was mainly attributed to the decrease Pbsorption capacity of the media resulting from dilution effect, as well as alterations in pore structure and porosity caused by the high heterogeneity of MP fragments. Findings of this study indicated that the fate and transport of heavy metals are strongly influenced by the co-contamination with MPs. Evaluation of their cotransport in the subsurface is essential for accurately predicting their environmental risks.