Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H₂O₂) and hydroxyl radical (^•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical ^•OH production was primarily attributed to conduction band reduction reactions, with the reduction of O₂ by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.