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 H 2 O 2 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 H 2 O 2 , substrates, and iron oxide surfaces. Substrate oxidation by iron oxide/H 2 O 2 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 H 2 O 2 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/H 2 O 2 . 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, H 2 O 2 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/H 2 O 2 system was low compared to ferrihydrite, consistent with the lower reduction potential of goethite. This work demonstrates that H 2 O 2 -promoted reduction of iron oxides is a key reaction leading to HO • oxidation of organic outer-sphere surface complexes. • Dual molecular probes resolve HO • oxidation sites in heterogeneous Fenton systems. • HO • is generated via H 2 O 2 -promoted reduction of iron oxides. • pH-dependent fate of Fe(II) determines the site of HO • generation. • Organic outer-sphere complexes are vulnerable to HO • generated on iron oxides. • Interactions between iron oxides and substrates control oxidation efficiency.