Co-transport of degradable microplastics with Cd(Ⅱ) in saturated porous media: Synergistic effects of strong adsorption affinity and high mobility

With the utilization of degradable plastics in the agricultural film and packaging industries, degradable microplastics (MPs) with strong mobility distributed in the underground environment may serve as carriers for heavy metals. It is essential to explore the interaction of (aged) degradable MPs with Cd(Ⅱ). The adsorption and co-transport behavior of different types of (aged) MPs (polylactic acid (PLA), polyvinyl chloride (PVC)) with Cd(Ⅱ) were investigated through batch adsorption experiments and column experiments under different conditions, respectively. The adsorption results showed that the adsorptive capacity of (aged) PLA with O-functional groups, polarity, and more negative charges was stronger than that of PVC and aged PVC, which was attributed to the binding of (aged) PLA to Cd(Ⅱ) through complexation and electrostatic attraction. The co-transport results indicated that the promotion of Cd(Ⅱ) transport by MPs followed the order of aged PLA > PLA > aged PVC > PVC. This facilitation was more pronounced under conditions of stronger transport of MPs and favorable attachment of Cd(Ⅱ) to MPs. Overall, the combination of strong adsorption affinity and high mobility helped (aged) PLA act as effective carriers for Cd(Ⅱ). The DLVO theory well explains the transport behavior of Cd(Ⅱ)-MPs. These findings provide new insights into the co-transport of degradable MPs and heavy metals in the subsurface environment.