Effects of Ozonation and Anaerobic Digestion on the Physicochemical Properties of Low-Density Polyethylene, Polypropylene, and Polyamide 66 Microplastics

Microplastics (MPs) are a growing environmental concern due to their ubiquitous presence, especially in wastewater treatment plants (WWTPs), where they are transferred and accumulated in sludge and can be reintroduced into the environment through sludge reuse. The persistence of MPs highlights the need for effective and tailored treatment strategies to enhance their removal or management. This study investigates the effects and impacts of ozonation as a pretreatment method for sludge, followed by anaerobic digestion (AD), on low-density polyethylene (LDPE), polypropylene (PP), and polyamide 66 (PA(66)) MPs. Different ozone doses, ranging from 5 to 50 gO3/gMPs, were tested in both deionized water and synthetic sludge. The study evaluated MPs degradation through mass variation measurements, Fourier Transform Infrared spectroscopy (FTIR), Differential Scanning Calorimetry (DSC), and Carbonyl Index (CI) analysis. Results showed that ozonation induced chemical modifications in MPs, increasing CI values and leading to the formation of oxygen-containing functional groups, particularly carbonyls. FTIR analysis confirmed the development of new absorption peaks at 1716 cm−1 and 1710 cm−1 for LDPE and PP, respectively, while PA(66) exhibited a shift in its carbonyl peak from 1739 cm−1 to 1754 cm−1. DSC analysis revealed a reduction in crystallinity for all tested polymers, suggesting increased structural disorder. However, no significant MPs mass reduction was observed, and AD did not further enhance MPs degradation. These findings highlight ozonation as a promising strategy for modifying MPs surface chemistry and potentially increasing their environmental degradability.