Effect of different seawater environmental factors on zinc ion adsorption behavior by PE microplastics

Polyethylene (PE) microplastic is a new type of marine pollutant, which is easy to adsorb Zn²⁺ in seawater and can cause complex ecotoxicity to marine ecological health. However, environmental factors in different seawater have different effects on the adsorption behavior of Zn²⁺ by PE microplastic. The effects of concentration, dosage, salinity and pH on the adsorption behavior of Zn²⁺ were also investigated, and the adsorption kinetics and thermodynamics were discussed. The results showed that the optimal saturation time for Zn²⁺ adsorption on the three kinds of PE microplastics with different mesh numbers was 4 h, and the increasing adsorption capacities of different mesh numbers were in the order of 30 mesh < 100 mesh < 500 mesh. The maximum equilibrium adsorptions were 1.428 mg/g, 1.454 mg/g and 1.498 mg/g, respectively, and salinity played an inhibitory adsorption effect. The adsorption capacity increased as pH increases, and the optimal dosage was 0.1 g. Freundlich model was used to fit the best adsorption isotherm experiment; the fitting correlation coefficient ( R²) of the three kinds of PE microplastics with different mesh numbers were all > 0.982 ( p < 0.01), which indicated the existence of a multilayer chemisorption. The quasi second order adsorption kinetic model was used to fit the best kinetic experiment. The fitting correlation coefficient of the three kinds of PE microplastics with different mesh number was statistically significant ( R² > 0.991; p < 0.01), indicating that the adsorption process was mainly chemisorption. In adsorption thermodynamics experiment, the ∆G < 0 and ∆H > 0 indicated that the adsorption process of PE microplastics for Zn ²⁺ was spontaneous and endothermic.