Multi-metal adsorption behavior of PP and PVC microplastics: polymer-dependent interactions and mechanisms

Microplastics (MPs) can act as carriers for co-occurring contaminants in aquatic environments, potentially influencing the mobility, partitioning, and environmental fate of trace metals. In this study, the adsorption behavior of Mn, Ni, Co, and Ba onto pristine MP particles, specifically polypropylene (PP) and polyvinyl chloride (PVC), was investigated in a multi-metal system. MP-metal interactions were characterized using complementary surface, spectroscopic, microscopic, and elemental analyses (BET, FTIR-ATR, SEM-EDX, ICP-OES). The results indicate generally higher adsorption for Co, Ni, and Mn compared to Ba for both MPs, with equilibrium adsorption capacities ranging from 0.011 to 0.032 mg/g for PVC and 0.016 to 0.041 mg/g for PP after 48 h. Kinetic analyses indicated that adsorption onto PP was best described by the pseudo-first-order model, whereas PVC followed the Elovich model, suggesting differences in surface heterogeneity and interaction behavior. Equilibrium data were well fitted the Langmuir model for both polymers, reflecting apparent monolayer-like behavior in the multi-metal system rather than true monolayer adsorption on heterogeneous MP surfaces. Surface analyses revealed polymer-dependent metal distributions and changes in characteristic functional group regions, which were more pronounced for PP. These findings provide insight into the factors governing metal adsorption onto MPs under multi-metal conditions and highlight their potential role as supplementary carriers influencing metal distribution in aquatic systems.