Microplastic Aging: Mechanisms, Quantification, and Applications from Laboratory Studies to Environmental Samples

Microplastics (MPs) released into the environment rarely remain in the same state as the pristine materials used in many laboratory studies. After exposure to chemical, physical, and biological aging, they can undergo a range of physical and chemical transformations that alter their material properties. These transformations are important because they influence how MPs persist, fragment, interact with surrounding materials, and bring analytical challenges in characterizing them in matrices. A clearer understanding of MP aging is needed to interpret their environmental behaviour, evaluate their potential biological relevance, and improve methods for measuring aged particles in realistic matrices. The measurement challenge is particularly important in environmental samples such as biosolids and soil, where MPs are often present at low concentrations, altered by aging-induced property changes, and are embedded within complex matrices. To address these challenges, a mass-based quantification method for MPs was developed and validated using a tiered analytical approach centered on modulated differential scanning calorimetry (MDSC). Calibration models for polyethylene (PE), polypropylene (PP), polyamide 6 (PA6), and polyethylene terephthalate (PET) exhibited strong linearity, high sensitivity, low limit of quantification (LOQ), and high recovery in biosolids matrices. This tiered workflow, incorporating MDSC, Raman spectroscopy, and thermogravimetric analysis (TGA), was applied to real wastewater biosolid samples, enabling mass-based quantification of multiple polymer types and demonstrating that the method can reliably measure MP burdens in complex environmental samples. Recognizing that aging-induced transformations represent a major source of uncertainty in microplastic quantification in biosolids, an investigation of characterization of microplastic aging under controlled conditions was therefore conducted. The thermal-oxidative aging of polystyrene (PS) resulted in surface oxidation, spectral changes consistent with ring-opening reactions, chain scission, and particle fragmentation, showing that laboratory aging can drive both chemical alteration and physical breakdown of PS particles. Building upon the findings, the work was expanded to ultraviolet C (UVC)-induced aging of PE, PP, and PET, which showed that PP was more responsive to UVC exposure than PE and PET. Thus, additional focus was placed on PP materials from different product sources and particle sizes. The characterization results demonstrate that smaller particles generally exhibited greater thermal and chemical changes, and polymer formulation modulates the depth of surface oxidation and crack propagation. Transparent PP particles exhibited more extensive surface cracking than pigmented microplastics, indicating the role of additives in mitigating UV-induced degradation. These differences suggest that material source can influence the apparent aging response of MPs, which should be considered when comparing laboratory-aged particles with environmental or consumer-derived plastics. Rather than relying on a single aging descriptor, these variables were integrated using a principal component analysis (PCA)-based aging score to integrate structural, thermal, chemical, and spectroscopic descriptors to capture the evolution of microplastics from three aged plastics. The Shapley additive explanations (SHAP) approach was then used to examine which measured properties contributed most strongly to the aging score. By combining these descriptors, the aging score captured coordinated changes in surface oxidation, crystallinity, melting behaviour, and particle size that would be difficult to interpret using a single indicator alone. It shows the feasibility of comparing aging extent across the polymer types and laboratories. To evaluate the biological relevance of these aging-induced changes, aged PS, PE, PP, and PET produced under similar laboratory conditions were tested using simplified in vitro assays. Cell viability, reactive oxygen species (ROS) generation, and membrane integrity were used as endpoints. Aging-dependent differences were observed in cell viability and ROS responses, with PS showing the clearest differences between pristine and aged particles across the tested cell models. ROS responses for PE, PP, and PET were more cell-type dependent, with aging-related differences observed only in A549 cells. However, no differences in lactate dehydrogenase (LDH) release were observed for PS, suggesting that the measured responses were not associated with measurable membrane damage. Overall, this work shows that MP aging is a coupled physical and chemical process that affects both environmental interpretation and analytical measurement. It develops mechanistic, quantitative, and analytical foundations for the study of microplastic aging and delivers a validated mass-based approach for measuring microplastic abundance in real environmental matrices.