Objective The interaction between microplastics and heavy metals and its effect on the availability of heavy metals in soil were studied, so as to provide some references for further developing the effects of microplastics-heavy metal combined pollution on crops and the prevention and control of soil ecological pollution.

Method The hard-to-degrade microplastics polyethylene (PE) and degradable microplastics polylactic acid (PLA) with a particle size of 150 μm were selected for soil simulation and adsorption experiments, and different concentrations of heavy metals (Cd, Pb) were set to pollute the soil, and low concentrations of contaminated soil were Cd 1 mg kg⁻¹ and Pb 46 mg kg⁻¹. Cd 15 mg kg⁻¹ and Pb 65 mg kg⁻¹ in high concentration polluted soil. By using scanning electron microscope (SEM), Fourier infrared spectroscopy (FTIR) and inductively coupled plasma mass spectrometry (ICP-MS), the effects of refractory and degradable microplastics on the availability of Cd and Pb in soil were studied on the basis of analyzing the adsorption mechanism of heavy metals in microplastics.

Result The adsorption capacity and characteristics of heavy metals in microplastics are related to the species of microplastics and the concentration of environmental pollution. According to the species of microplastics, the equilibrium adsorption capacities of refractory microplastics PE for Cd and Pb are 0.480 mg g⁻¹ and 1.202 mg g⁻¹, respectively, while the equilibrium adsorption capacities of degradable microplastics PLA for Cd and Pb are 0.157 mg g⁻¹ and 0.315 mg g⁻¹, respectively. The adsorption of heavy metals Cd and Pb by PE was mainly single-layer physical adsorption, accompanied by chemical adsorption and surface adsorption, while PLA adsorbed heavy metals Cd and Pb. From the point of view of environmental pollution concentration, the addition of microplastics to the soil with high concentration of heavy metals increased the available contents of Cd and Pb. Adding microplastics to PE surface under high concentration heavy metal pollution significantly increased the Cd content. Due to the aging of microplastics in soil for a long time, the surface became rough, the specific surface area and porosity increased, and the oxygen-containing functional groups increased. Under the stress of high concentration heavy metal pollution, the mass transfer of Cd accelerated, and more Cd was adsorbed and utilized.

Conclusion Compared with PLA, PE shows stronger adsorption capacity and capacity for Cd and Pb, and the addition of microplastics generally improves the availability of heavy metals in soil, and PLA has higher activation of available forms of heavy metals than PE, which may be related to the changes of surface morphology during degradation.