Adsorption mechanisms of metal ions (Pb, Cd, Cu) onto polyamide 6 microplastics: New insight into environmental risks in comparison with natural media in different water matrices

Microplastics (MPs) are ubiquitous in the environment and have a great potential to coexist with metal ions. This work investigated the adsorption–desorption behaviors of three divalent metal ions (Cu, Cd, Pb) on polyamide 6 (PA6) MPs and natural media (riverside soil and marine sediment) and compare their environmental risks. The result showed that the adsorption amounts of metal ions onto PA6 MPs followed the order of Cd(II) (1.210 μmol/g) < Cu(II) (2.299 μmol/g) < Pb(II) (3.756 μmol/g). The adsorption of metal ions on PA6 was significantly affected by water chemistry, including solution pH, salinity, and the concentration of citric acid (CA) and humic acid (HA). Natural water matrices (Yangtze River water and Seawater) enhanced the uptake of three metal ions by PA6 MPs. The analysis of Fourier transform-infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) highlighted the importance of amide groups in controlling the adsorption of metal ions onto PA6 MPs. Site energy distribution indicated that the adsorption of metal ions mainly involved chemisorption. The retention factor (R_f) calculated by the adsorption–desorption behavior of three heavy metal ions indicated that the risk of metal ions posed by PA6 MPs was higher than that of natural media. Among all water matrices, the metal ion/PA6 MPs combinations posed the highest environmental risk in the simulated gastric fluid. This work highlights the behavior of PA6 MPs as metal ions vectors and provides strong evidence the assessment of environmental risk posed by MPs/metal ions combinations, opening new insight into understanding the environmental impacts of MPs.