Exploring the interaction between microplastics and heavy metals: unveiling the impact of microplastics on lead sorption and desorption in soil

The global attention on microplastics in soils, particularly in agricultural soils, has been growing. However, there is a need to investigate the impact of microplastics on the ability of agricultural soils to add and remove pollutants. To address this, a series of batch experiments were conducted to examine how high-density polyethylene microplastics affect the adsorption and desorption of lead in a field soil under different conditions. The equilibrium for both lead adsorption and desorption, with and without microplastics, was reached within 120 min. The adsorption kinetics of lead followed the quasi-second-order model, and the Langmuir model provided a more accurate fit for the adsorption isotherm compared to the Freundlich model. Generally, the addition of microplastics decreased lead adsorption but increased its excretion. The effects varied depending on the dosage of microplastics, particle size, and solution pH. Higher microplastic dosage and larger particle size resulted in a more pronounced reduction in lead adsorption and an increase in lead excretion, although these effects also depended on the solution pH. Analysis using energy-dispersive X-ray spectroscopy confirmed the adsorption lead on the surface of microplastics. Both types of microplastic samples, before and after lead adsorption, exhibited similar X-ray diffraction patterns, indicating that the microplastics maintained their high crystallinity and did not undergo any new crystalline phase formation. Consequently, the introduction of microplastics into the soil may enhance the mobility of lead by reducing the soil's adsorption capacity, thereby posing greater risks to the agricultural ecosystem.