From Mineral State to Electron Flux: Photoredox Chemistry at Iron-Bearing Mineral–Water Interfaces

Light-driven chemistry at iron-bearing mineral-water interfaces couples interfacial electronic structure to Fe redox cycling, reactive oxygen species formation, contaminant transformation, and mineral evolution. Yet photocurrent, Fe(II), ROS, contaminant loss, and dissolution are often treated as interchangeable measures of mineral photoactivity even though they probe different steps in interfacial electron flow. This Review organizes evidence from natural and model systems around a state-dependent electron-flow framework in which Mineral State0 comprises phase, crystallinity, Fe coordination and valence, surface Fe–OH configuration, formation history, and persistent organic or mineral associations. Reducing equivalents enter through intrinsic mineral excitation, ligand-to-metal charge transfer, or heterointerfacial electron transfer. Fe(III)-to-Fe(II) reduction can transiently store reducing power in aqueous, surface-associated, or solid-associated pools, whereas other pathways bypass resolvable Fe(II) storage. Downstream electrons partition among O2-derived oxidants, external acceptors, Fe cycling, and biological or biogeochemical demand, while irradiation simultaneously drives dissolution, atom exchange, recrystallization, and surface restructuring to generate State1. The central implication is that the hydrated interfacial state, rather than bulk phase identity alone, is the appropriate unit for comparing mineral photoredox behavior. A predictive description of mineral photoredox chemistry therefore requires linking measurable interfacial state to electron generation, redox storage or bypass, downstream utilization, and light-driven restructuring rather than assigning a single photoactivity to a mineral phase.