Preparing and characterizing environmentally aged microplastics

Microplastics (MPs) are persistent and widely distributed pollutants that have become an emerging environmental problem. When present in the environment, they are increasingly being incorporated into the carbon cycle and thus participate in the Earth's biochemical cycle alongside other pollutants. However, upon entering the environment (as primary or secondary MPs), various environmental factors collectively influence the physical and chemical characteristics of MPs, leading to changes in surface morphology, particle size and microstructure. These alterations can change their physical behavior and the effect that they have on living organisms. The aging processes of MPs are complex and influenced by multiple factors relating to both the size and composition of the MPs themselves and the environmental conditions. We developed a protocol for studying MP aging across different environmental settings (soil, water, air and organisms) over timescales from several weeks to months, applicable to common types of MPs. This protocol integrates modular aging experiments with an array of analytical techniques (for example, scanning electron microscopy, Fourier transform IR spectroscopy and pyrolysis-gas chromatography-mass spectrometry) to characterize MP aging according to micromorphology, chemical characteristics, dissolution characteristics and mass fraction. We have established a quantitative method for evaluating MP aging along with graded property assessment criteria for measuring the degree of aging-the composite aging index (CAI). This protocol takes approximately 4 days to 6 months, depending on the specific environmental aging conditions selected, the sample matrix and the suite of analytical techniques used. By implementing this protocol, we can gain mechanistic insights into MP aging processes, ultimately contributing to sustainable plastics development through strategic design that guides their fate and behavior.