Ozonation facilitates the aging and mineralization of polyethylene microplastics from water: Behavior, mechanisms, and pathways

Microplastics (MPs) are ubiquitous in the environment, of which 94 % undergo the aging process. Accelerated aging induced by advanced oxidation processes (AOPs) is significant in explaining the formation pathway of secondary MPs and enables possible mineralization. In this study, ozonation coupled with hydrogen peroxide (O3/H2O2), a type of AOPs, was applied for the aging of MPs (polyethylene, PE). Physiochemical properties of aged PE MPs were analyzed through scanning electron microscope, Fourier-transform infrared spectroscopy-attenuated total reflection, and X-ray photoelectron spectroscopy. The mechanism regarding the contribution of reactive oxygen species (•OH) was determined using chemical probe (p-chlorobenzoic acid) and quencher (tert-butanol). Possible transformation pathways were modeled via two-dimensional correlation spectroscopy. Mineralization of MPs, associated with aging was also studied, with the percentage of PE degradation determined by mass loss. Our results confirmed that ozonation promoted fragmentation of PE, with 20 mM H2O2 facilitating the production of •OH. The growth of oxygen-containing functional groups on the surface of PE was consistent with the alteration of the oxygen-to‑carbon atom ratio, revealing the formation of CO, CO, and C-O-C. The enhanced adsorption property of aged PE for triclosan was due to the increased specific surface area and negative charges on the surface. Moreover, the percentage of PE degradation was higher at lower concentrations, and the mass loss reached 32.56 % at a PE concentration of 0.05 g/L after 8-h ozonation. These results contribute to revealing the long-term aging behavior of MPs and providing significant guidance for employing AOPs to achieve efficient removal.