Abstract: Taking methyl orange (MO) as target pollutant and microplastics as carrier, we compared the adsorptive capacities of four typical plastic materials including common plastic polypropylene (PP), common engineering plastic nylon (PA), polyformaldehyde (POM) and special engineering plastic polytetrafluoroethylene (PTFE) in adsorption kinetics and isothermal adsorption experiments. We characterized PA before and after adsorption by scanning electron microscopy (SEM), specific surface area testing (BET), micro-Raman spectroscopy (Raman), and Zeta potential and attenuated total reflection Fourier transform infrared spectrum (ATR-FTIR). We investigated the effects of plastics materials and PA particle size on MO adsorption to explore likely adsorption mechanisms. The adsorptive capacities of microplastics for MO were obviously different for different plastic materials, among which PA had the largest adsorptive capacity of 7.39 mg/g. The adsorption process for PA included initial fast and subsequent slow reactions following two-stage second order kinetics. The results of isothermal adsorption experiments showed heterogeneous chemical adsorption of MO on PA. For different kinds of microplastics, the adsorptive capacity differences were mainly due to electrostatic effect and formation of intermolecular hydrogen bond. The particle size of microplastics was negatively correlated with the adsorptive capacity for MO (R2=0.995, P<0.05), which was closely related to the effective adsorption sites provided by microplastics. This study provided a basis for the scientific evaluation of the complicated behavior of microplastics and their capabilities, as carriers, of removing pollutants from the environment.

Key words: microplastics, nylon, methyl orange, adsorption