Influence of polyethylene-microplastic on environmental behaviors of metals in soil

Microplastics (MPs) in terrestrial ecosystems have attracted increasing attention all over the world. The adsorption-desorption behavior and bioavailability of metals in soil would affect its toxicity to organisms. However, the influences of MPs on adsorption-desorption behavior between metals and soil as well as bioavailability of metals in soils are scarcely investigated. Herein, different percentage (0, 0.1%, 1%, 10%) of polyethylene-microplastic (PE-MP) were thoroughly mixed into the soil to investigate the impacts of PE-MP on adsorption-desorption and bioavailability of metals (Zn²⁺, Pb²⁺) in the soil. A series of characterization were carried out to determine the change of PE-MP before and after adsorption to investigate the mechanisms. When MP100 (average size: 129 μm) content in soil increased to 10%, the adsorption capacities of soil with Pb²⁺ and Zn²⁺ were 3.73 and 4.56 mg/g, respectively, which were significantly (p < 0.05) lower than that of pure soil. When MP300 (average size: 293 μm) content in soil increased to 10%, the extraction fraction of Zn²⁺ and Pb²⁺ from soil by diethylenetriaminepentaacetic acid reached 12.35% and 23.96%, respectively, which were significantly (p < 0.05) higher than that of pure soil, indicating high concentration (10%) of MPs in soil would decrease the adsorption capability of soil to metals and increase the mobility of metals in terrestrial environment. However, when MPs content in soil was 0.1%, the extraction fraction of Zn²⁺ and Pb²⁺ showed no significant difference with that of pure soil, indicating that actual MPs in soil is unlikely to bring significant influence on metal bioavailability.