Polyethylene and polylactic acid microplastics affect the migration of Cr(VI) and Cr(III) in acidic clay soil via...

The coexistence of microplastics (MPs) and heavy metals in agricultural soils leads to complex interactions during their adsorption and migration processes. Understanding their fate under realistic soil conditions is crucial for assessing the associated environmental risks. Therefore, virgin and aged polyethylene (PE) and polylactic acid (PLA) MPs were used to investigate their influence on the adsorption and migration of chromium (Cr) in unsaturated steady-state soils via batch and column experiments. Batch experiments revealed that soil adsorbed less Cr(VI) than Cr(III) (0.44 vs. 1.45 mg/g). The addition of MPs inhibited Cr adsorption (specifically, 0.40 and 1.23 mg/g for Cr(VI) and Cr(III), respectively, at 7% MP addition), with adsorption capacity decreasing as MP dosage increased. Column experiments showed Cr(VI) reached adsorption equilibrium faster than Cr(III) (25 vs. 41 pore volumes); its migration was primarily controlled by soil hydraulic conditions, and MPs exerted an inhibitory effect. In contrast, Cr(III) migration was predominantly governed by adsorption, and MP addition promoted its migration. Regarding the MPs themselves, PE exhibits greater migration capacity, which decreased with aging, whereas PLA showed the opposite trend. Interestingly, the presence of Cr(III) reduces the migration of MPs compared to Cr(VI). These findings clarify the effects and mechanisms of MPs on Cr migration under realistic soil conditions, and enhance the understanding of migration dynamics and environmental fate of coexisting pollutants.