Photolysis of disposable face masks facilitates abiotic manganese oxide formation

During the COVID-19 pandemic, billions of face masks were discarded into aquatic environments, releasing micro/nanoplastics. This release threatens aquatic ecosystems, influences pollutant transport, and generates reactive oxygen species (ROS). These ROS can affect redox-active metal ions, such as manganese (Mn), in water. Mn oxide solids are commonly found in nature and serve as both electron donors and acceptors in various biogeochemical reactions of trace elements, metal ions, and organics in the environment. However, it remains unclear how disposable face masks, primarily made of polypropylene (PP), impact Mn oxidation and Mn oxides formation in natural surface waters under sunlight. This study, for the first time, reports the photolysis of PP mask layers and their impacts on the kinetics of Mn²⁺ (aq) oxidation to Mn oxide nanoparticles. We found that mask layers enhanced Mn²⁺(aq) photo-oxidation kinetics as their surface material packing density increased. Furthermore, the local concentrations of oxidized Mn²⁺ near the mask surfaces were two orders of magnitude greater than the bulk solution, facilitating heterogeneous Mn oxide formation near mask surfaces. Photoaging of masks further expedited Mn²⁺ oxidation. Superoxide radicals (O₂^•−) generated by mask photolysis were the main responsible ROS for boosting Mn oxidation. These findings highlight the influences of mask photolysis on Mn redox chemistry. Mn oxides formed on mask materials can alter the fate and transport of pollutants such as heavy metals and organic compounds, impacting surface water quality.