Adsorption behaviors and mechanisms of humic acid on virgin and aging microplastics

Because of the wide distribution and strong adsorption ability, microplastics (MPs) in aquatic environment tend to adsorb natural organic matter (NOM). This study aimed to investigate behaviors and underlying mechanisms of adsorption of humic acid (HA) on virgin and aging polyamide 66 (mPA66) and polypropylene (mPP). The kinetic data of HA adsorption on virgin and aging MPs well fitted the pseudo-second-order kinetics model (R2 > 0.95), while the adsorption isotherms better followed the Freundlich models (R2 > 0.92) than Langmuir models. The adsorption process of HA on the virgin and aging MPs was spontaneous and exothermic according to thermodynamics analysis. The effect of ionic strength on adsorption indicated that Ca2+ promoted more adsorption of HA on MPs than Na+ due to the formation of more cation bridge between Ca2+ and HA. With the pH increase, the adsorption capacity of HA first decreased and then increased. Desorption experiment showed that desorption efficiencies of HA adsorbed on virgin MPs were all relatively higher than those on corresponding aging MPs. Particularly, higher desorption efficiencies occurred in ultrapure water rather than in surface water, and HA desorption was apparently pH-dependent but slightly effect by ionic strength. Generally, HA had poor adsorption toward on virgin and aging MPs, but the following desorption efficiencies were relatively high, indicating that the adsorption of HA on virgin and aging mPA66 and mPP MPs tended to be reversible. This result was attributed to physical adsorption process, hydrophobic interactions, polarity and n-π electron donor-acceptor (EDA) interactions. The HA adsorption was mainly dominated by surface area of MPs, instead of crystallinity and functional groups based on XRD patterns, FTIR and XPS.