A unified framework for simulating aging process of typical microplastics

Natural aging is a driving factor for changes in the toxicity of microplastics (MPs) and absorbed pollutants. However, there is a lack of quantitative techniques to describe the aging degree, which is a bottleneck in the assessment of changes in ecological risks with MPs aging. Here, aging experiments and characterization have been conducted on two types of MPs (polyvinyl chloride (PVC) and polylactic acid (PLA)) with different aging times. Using multifractal detrended fluctuation analysis and Frenkel-Halsey-Hill model, the morphology and pore structure were quantitatively described by fractal parameters. The fractal results showed that aged MPs surface becomes rougher, enhancing the spatial heterogeneity in morphology with aging time. Changes in pore fractal dimensions suggested that mesopores developed more significantly with aging for both PVC and PLA. Meanwhile, the increment of micropores of PLA is more prominent than that of PVC. Based on the principal component analysis, a comprehensive aging index F_z was developed to quantify MPs aging degree. The linear increase of F_z with aging time means that MPs aging was a steady process. Based on the relations between F_z and aging time, the aging kinetics models for the two types of MPs were further constructed. It was discovered that the aging rate constant of PLA was slightly higher than that of PVC, which indicates the stronger degradability of PLA. This work has significant implications for identifying the aging degree of MPs and further evaluating the combined ecological risk of aged MPs in the natural environment.