Sunlight-Driven Production of Hydroxyl Radical from Manganese Minerals

Manganese (Mn) minerals are abundant across Earth's surface and have recently been recognized as sunlight-active materials. Here, we report that Mn minerals exhibit unexpectedly high photochemical reactivity for the generation of reactive oxygen species (ROS) including hydroxyl radicals (^•OH) and hydrogen peroxide (H₂O₂). Through systematic evaluation of five representative Mn minerals (pyrolusite, psilomelane, rhodochrosite, hausmannite, and vernadite), we revealed ^•OH production rates reaching 16.2-168.6 μM over 120 min sunlight irradiation. Both valence band (VB) water oxidation and conduction band (CB) oxygen reduction contributed to ROS productions, with relative contributions varying among minerals. Remarkably, ROS yields from Mn minerals exceed those from Fe minerals by more than an order of magnitude; for instance, vernadite exhibited a steady-state ^•OH concentration of 4.9 × 10^-16 M, compared to 9.5 × 10^-18 M for hematite. Higher ROS production correlated with greater charge separation efficiency, as indicated by high photocurrent densities (e.g., 2.5 × 10^-6 A cm^-2 for vernadite) and substantial photoinduced surface potential shifts (e.g., +150 mV for vernadite). The ^•OH generated by Mn minerals oxidized aromatic-rich natural organic matter (NOM) into more labile forms and reshapes the microbial community composition. These results indicate that Mn minerals are an important and previously underappreciated source of ROS in sunlit environments, with implications on oxidative processes at mineral-water interfaces and their role in global biogeochemical cycles.