Microplastic mineralization rate in Fenton reactions depends on polymer type

Despite microplastics (MPs) being highly inert pollutants, Fenton-type reactions-using hydrogen peroxide (H2O2) and iron(II) ions (Fe2+)-may effectively initiate chain cleavage and induce mineralization. However, mineralization rates and mechanisms for different MP types at varying Fenton reagent concentrations remain unclear. This study examined the mineralization of four MPs- low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polyester (PES)-by measuring CO2 release across varying H2O2 concentrations. Mineralization rates depended on both polymer type and H2O2 concentration. PES showed the highest degree of mineralization, followed by PP, while LDPE and PS exhibited the lowest rates. Increased H2O2 concentrations enhanced CO2 release and accelerated reaction saturation, especially for PES and PP, suggesting efficient mineralization due to elevated hydroxyl radical (OH˙) production. In contrast, PS and LDPE showed no significant increase in mineralization above certain H2O2 levels. Despite visible Fe-oxide precipitates, scanning electron microscopy did not provide evidence of surface changes associated with mineralization. Contrary to expectations, aromatic PS was less susceptible to Fenton mineralization than aliphatic PP, possibly due to structural factors. The study also emphasizes the importance of radical recombination and scavenging at high OH˙ concentrations, which can lower mineralization efficiency. Non-integer reaction orders suggest a multi-step mineralization process influenced by both polymer structure and radical dynamics. These findings underscore the high environmental persistence of MPs, as natural mineralization, such as by fungi utilizing Fenton-like mechanisms, occurs at even slower rates than those observed under controlled lab conditions.