Characterization of the micro-interfacial interactions of heterogeneous particulate matter (fine-grained sediment and microplastics) with copper ions

Particulate matter, such as sediment and microplastics (MPs), in aqueous environments has comparable micro-interfacial characteristics. However, the micro-interfacial characteristics of heterogeneous particulate matter are complex and are not fully understood. In this study, micro-interfacial kinetics and the interactions governing copper (Cu) ion adsorption to PP–sediment were investigated using micro-interfacial characterization experiments and kinetic experiments for Cu ion absorption to sediment, polypropylene (PP), and PP–sediment particles. The results show that (1) a pseudo-second-order model can be used to accurately describe the micro-interfacial kinetics of Cu ion adsorption to PP–sediment, indicating that chemical adsorption dominates the process. For Cu, the adsorption capacity, q_e, of PP–sediment is approximately 0.4808 mg/g, between that of sediment and PP. (2) Film diffusion is the rate-controlling process in the early adsorption stage. Intraparticle diffusion gradually becomes the controlling process as the film diffusion constant (k_fd) decreases and the film diffusion model becomes less applicable, but intraparticle diffusion is weaker in the PP–sediment system than in the sediment-only system. (3) The applicability of the intraparticle diffusion model for Cu ion adsorption to PP–sediment in the later stage of adsorption is between that of the sediment and PP systems. Furthermore, the heterogeneous aggregation of sediment and PP affects the micro-interfacial interactions of both types of particulate matter. The current study contributes to a better understanding of the micro-interfacial interactions of heterogeneous particulate matter (such as fine-grained sediment and MPs) with heavy metal ions in aqueous environments.