Photogenerated Fe(II) Trapped as Humboldtine (FeC2O4·2H2O): Abiotic Secondary Mineral Formation During Citrate-Promoted Photodissolution of Hematite Under Aerobic Conditions

Light-driven reductive dissolution of iron oxides by organic ligands controls Fe and C cycling in sunlit environments, yet the fate of photogenerated Fe(II) under oxygenated conditions remains unresolved. It may persist in solution, be re-oxidized, or be captured into secondary minerals. Here we show that photogenerated Fe(II) can be kinetically trapped as humboldtine (FeC2O4·2H2O), a ferrous oxalate secondary mineral, during citrate-promoted photodissolution of hematite in aerobic environments. In batch experiments with citric acid (pH 5.0) under N2-purged (nominally anoxic) or air-equilibrated conditions (light or dark), ligand-to-metal charge transfer from surface-bound citrate to lattice Fe(III) released 4.17 mg/g dissolved Fe(II) together with carbon-centered radicals (EPR spin trapping) under N2, whereas air suppressed Fe(II) release to 0.87 mg/g and intensified superoxide, shifting the system from reduction toward oxidation. SAED identified humboldtine on the recovered solids only under light + air, while XRD documented the persistence of hematite with a small lattice shift. We propose that Fe(II) is trapped by rapid precipitation with photogenerated oxalate, outcompeting O2-mediated re-oxidation, a kinetic pathway previously overlooked in Fe-C cycling. These findings imply that in organic-rich, sunlight-exposed systems such as soils, sediments, and surface waters, light can divert dissolved Fe(II) into stable ferrous-oxalate mineral pools, thereby modulating Fe bioavailability, contaminant fate, and carbon cycling.