New insights into the decrease in Cd2+ bioavailability in sediments by microplastics: Role of geochemical properties

Microplastics (MPs) are considered to influence the bioavailability of heavy metals through direct adsorption, but this neglects the distribution of heavy metal chemical speciation indirectly influenced by MPs by altering geochemical properties. The present study aims to explore the link between the fractionation distribution of cadmium (Cd2+) and changes in geochemical properties in sediments induced by polyethylene terephthalate microplastics (PET-MPs). The PET-MPs reduced the acid-soluble fraction of Cd2+ in sediments and increased its organically bound fraction. In addition, the concentration of bioavailable Cd2+ in the sediment decreased by 4.09–25.96 % with increasing PET-MPs doses and aging. Whereas the thermal aging of PET-MPs led to a decrease in the molar ratio of oxygen-containing functional groups and the BET surface area, which is not favorable for improving the adsorption capacity of PET-MPs. On the other hand, the correlation analysis demonstrated the key role of PET-MPs in increasing the content of sediment organic matter and its humification level, which indirectly led to a decrease in Cd2+ bioavailability. The microbial analysis demonstrated that PET-MPs increase the relative abundances of Chloroflexi, hexokinase, and 6-phosphofructose kinase in sediments, thereby increasing the humification level of sediment organic matter. The present study provides a new perspective for understanding the environmental risks of MPs-altered heavy metals.