Adsorption of Fulvic Acid on Virgin and Aging Microplastics

Abstract

Due to the wide distribution and strong adsorption ability of microplastics (MPs) for organic matter in aquatic environments, the interaction between MPs and natural organic matter (NOM) cannot be ignored. In this study, virgin and aging polyamide 66 (PA66) and polypropylene (PP) MPs were used to adsorb fulvic acid (FA) in order to understand the effect of MPs on NOM. The results indicated that the kinetics experimental data of FA adsorption on virgin and aging MPs well fitted the pseudo-second-order model (R2>0.94), and the adsorption equilibrium was reached at 48 h. Compared to that of PP, the adsorption capacity of FA on PA66 were relatively higher, and the aging process improved the adsorption ability of MPs for FA. Freundlich models were well fitting with the adsorption isotherms experimental data compared to Langmuir models, indicating that the adsorption of FA on the virgin and aging MPs was a multi-layer heterogeneous physical process. The thermodynamics analysis revealed that the adsorption was spontaneous and endothermic. With the increase in pH, the adsorption capacity of FA first decreased and then increased. The desorption experiment indicated that the FA desorbed from the tested MPs in ultrapure water obtained higher desorption rates than that in surface water, and the desorption rates of aging MPs were less than that of the corresponding virgin ones. The aging process had a great influence on the structure of MPs, which resulted in a distinct increase in surface area and roughness of MPs, but slightly affected functional groups. Specific surface area and polarity of MPs were the main influencing factors for the adsorption process, and the main mechanism of FA adsorption on the tested MPs was hydrophobic and π-π interactions.