Manganese oxide-mediated inhibition of microplastic photodegradation: insight into the photoreduction mechanism

Microplastics (MPs) are widely distributed in aquatic environments and undergo photodegradation. Although clays and iron (hydr)oxides are known to promote MP photodegradation, the role of manganese oxides (MnO_x), one of the most abundant transition metal oxides, remains poorly understood. Here, we examined the impact of MnO_x on the photodegradation of two prevalent MPs, polystyrene (PS) and polyvinyl chloride (PVC), under irradiation from a 500 W mercury lamp. Results indicated that MnO_x significantly inhibited the photodegradation of both PS and PVC, with their MP-derived dissolved organic matter (MP-DOM) release rates reduced by 58.5% and 57.7%, respectively. MnO_x acted as a reactive oxygen species (ROS) scavenger through photoreductive dissolution, consuming superoxide radicals (O₂^•-) and reducing hydroxyl radicals (•OH) formation, thereby limiting ROS-dependent degradation pathways. Additionally, the inhibition of PS degradation progressively intensified with irradiation, whereas the inhibitory effect on PVC gradually weakened as MnO_x was consumed. This difference occurred because soluble Mn(III) released during MnO_x photoreduction was stabilized by PS-DOM, forming Mn(III)-DOM complexes with strong UV absorption. These persistent complexes imposed an additional light-shielding effect, further inhibiting PS photodegradation. In contrast, such complexation was absent in PVC systems. Importantly, the inhibitory effects of MnO_x on MP photodegradation were also evident in natural waters from estuarine, riverine, and lake environments. Overall, MnO_x inhibited MP photodegradation through both ROS scavenging and complexation-mediated light-shielding. These findings challenge the conventional view that minerals promote MP photodegradation and highlight the critical role of MnO_x in modulating MP photodegradation.