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 (MnOx), one of the most abundant transition metal oxides, remains poorly understood. Here, we examined the impact of MnOx on the photodegradation of two prevalent MPs, polystyrene (PS) and polyvinyl chloride (PVC), under irradiation from a 500 W mercury lamp. Results indicated that MnOx 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. MnOx 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 MnOx was consumed. This difference occurred because soluble Mn(III) released during MnOx 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 MnOx on MP photodegradation were also evident in natural waters from estuarine, riverine, and lake environments. Overall, MnOx 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 MnOx in modulating MP photodegradation.