Unexpected Spontaneous Generation of Superoxide Radicals: Water-Solid Collision as an Atypical Source

Spontaneous generation of reactive oxygen species (ROS) driven by water-solid contact electrification was a new source of redox chemistry. However, superoxide radicals (O₂ ^•-), a kind of ROS that was of great significance in both natural and engineering environments, remain overlooked and poorly understood at the water-solid interface. Here, we found O₂ ^•- could be generated spontaneously when water collided with the solid interface even under dark conditions, and its production was quantified by CL. ¹⁸O isotope labelling experiments and solid-liquid contact electrification detection confirmed that the generation of O₂ ^•- was from the reduction of dissolved oxygen by electrons at the water-solid interface and that the kinetic energy of the water energized the reaction. Given the ubiquitous nature of solid-liquid collisions, such as rain colliding with mineral surfaces during rainfall, we further systematically investigated the O₂ ^•- generation and their decay kinetics at three distinct rain-mineral interfaces. Results showed that O₂ ^•- generation ranged from 0.015 to 1.1 nM, comparable to the O₂ ^•- production by photochemical pathways. Our findings established the water-solid collision interface as a previously unrecognized global source of O₂ ^•- and highlight a universal yet overlooked physicochemical mechanism underpinning multiphase redox chemistry.