Aging behavior of microplastics accelerated by mechanical fragmentation: alteration of intrinsic and extrinsic properties

Microplastics (MPs) inevitably undergo multiple aging processes during their life cycle in the environment. However, the information regarding the mechanical fragmentation behavior of MPs remained unclear, including the changes in the intrinsic properties of aged MPs, the measurement of aging degree, the underlying mechanism, and the interaction with heavy metals. Here, MPs (PS, PP, PET) were aged by crushing (-CR) and ball-milling (-BM) to simulate mild and severe mechanical fragmentation, respectively. Our results indicated that mechanical fragmentation significantly affected the morphology of MPs. The aging degree of MP-BM was deeper compared to MP-CR owing to smaller particle size, larger specific surface area, poorer heat resistance, better hydrophilicity, and richer oxygen-containing functional groups. The carbonyl index (CI) and O/C ratio were used to measure the aging degree of the two mechanical aging treatments. Besides, the mechanism was proposed and the discrepancy between the two treatments was elaborated from three aspects including the excitation energy source, reaction interface, and reaction dynamics. Furthermore, the extrinsic properties of MPs altered with the increase of aging degree; specifically, the adsorption capacities of heavy metals were enhanced. Meanwhile, it was unveiled that the CI value and O/C ratio played a vital role in estimating the adsorption ability of heavy metals. The findings not only reveal the mechanical fragmentation behavior of MPs but also provide new insights into the assessment of the potential risks of the aged MPs via chemical indexes.

Automated summary

Microplastics (MPs) undergo aging processes in the environment, but the mechanical fragmentation behavior of MPs and their interaction with heavy metals remained unclear. This study simulated mild and severe mechanical fragmentation of MPs (PS, PP, PET) and found that it significantly affected their morphology. The aging degree of MPs was influenced by particle size, surface area, heat resistance, hydrophilicity, and oxygen-containing functional groups. The carbonyl index (CI) and O/C ratio were used to measure aging degree, and the mechanism and discrepancy between the two treatments were proposed. The adsorption capacities of heavy metals increased with the aging degree, and the CI value and O/C ratio played a vital role in estimating the adsorption ability of heavy metals.