The Impacts of Microplastics on Sorption and Desorption Specifications of Iron in Soil

Global awareness of microplastics in soils, especially agricultural soils, is growing. Nevertheless, the effects of microplastics (MPs) on the sorption and removal of pollutants in farming soils need to be investigated. This study analyzed the role of high-density polyethylene (HDPE) microplastics on iron (Fe) adsorption and desorption in soil. Batch experiments were performed in a sandy loamy field soil under different situations. Both sorption and desorption of iron in soils in the presence or absence of microplastics attained equilibrium within 120 min. The largest Fe adsorption amount was 997.03 mg/kg in the control treatment (no MPs). The Fe adsorption amount in the HDPE treatment was 932.05 mg/kg. The sorption kinetics of Fe confirmed the quasi-second-order model, and the sorption isotherm was more accurately adapted to the Langmuir model. In general, the adding of microplastics reduced iron sorption but enhanced excretion, and the results changed with microplastic dosage, size, and pH. The reduction of iron adsorption caused by microplastics and the increase of iron excretion were more noticeable at higher microplastic dosage and bigger particle size but changed with pH. Energy-dispersive X-ray spectroscopy (EDS) analysis affirmed the sorption of iron on the surface of microplastics. Both types of microplastic samples before and after Fe sorption indicated similar XRD schemes, showing that the microplastics had high crystallinity, and no new crystalline phase was created. As a result, the introduction of microplastics into the soil may increase the motility of iron by reducing the adsorption capacity of the soil, thus causing more risks to the agricultural ecosystem.