Heterogeneous Fenton reactions: oxidation of adsorbing and non-adsorbing probe molecules via H2O2-promoted reduction of iron oxides

Hydroxyl radicals (HO^•), produced through reactions between H₂O₂ and iron oxides, drive biogeochemical transformations, mediate organism toxicity, and facilitate advanced oxidation processes. The effectiveness of these processes depends on the spatial proximity between HO^• generation and target substrates. Consequently, the oxidation mechanisms should be governed by interfacial interactions among H₂O₂, substrates, and iron oxide surfaces. Substrate oxidation by iron oxide/H₂O₂ systems were studied using two probes simultaneously: terephthalate (TPA), forming outer-sphere surface complexes, and coumarin, exhibiting no surface interactions. The reactions were followed as a function of pH, time and H₂O₂ concentration. Complementary experiments were performed with oxalate inner-sphere surface complexes. Both ferrihydrite and goethite were studied, representing differences in reduction potential. Solution analyses were combined with in-situ infrared spectroscopy probing the interfacial reactions. Both probes were oxidized by ferrihydrite/H₂O₂. Between pH 5.5-6.5, substantial amounts of TPA outer-sphere surface complexes were oxidized, while coumarin was mainly oxidized at pH ≤ 4.5, coinciding with a decrease in TPA oxidation. At all investigated pH values, H₂O₂ reduced ferrihydrite, and the partitioning of Fe(II) controlled the location of HO^• generation. At low pH, Fe(II) diffused into solution triggering homogeneous Fenton reactions, while adsorption and re-oxidation at higher pH confined radical generation to the near-surface region. Oxalate inner-sphere complexes resisted oxidation. Oxidation by the goethite/H₂O₂ system was low compared to ferrihydrite, consistent with the lower reduction potential of goethite. This work demonstrates that H₂O₂-promoted reduction of iron oxides is a key reaction leading to HO^• oxidation of organic outer-sphere surface complexes.