Photo-redox reactions of dicarboxylates and α-hydroxydicarboxylates at the surface of Fe(III)(hydr)oxides followed with in situ ATR-FTIR spectroscopy

• Photo-degradation of dicarboxylates on iron(hydr)oxides was followed by FTIR.
• Slow degradation of malonate and succinate indicates low yield of OH-radicals.
• Fast degradation of oxalate, tartronate and malate indicates MLCT and h+ pathways.
• Fast degradation by MLCT or valence band holes supports inner-sphere coordination.
• Iron(hydr)oxides are selective photocatalysts and produce OH only in low yields.

Colloidal mineral-phases play an important role in the adsorption, transport and transformation of organic and inorganic compounds in the atmosphere and in aqueous environments. Artificial UV-light and sunlight can induce electron transfer reactions between metal ions of the solid phases and adsorbed compounds, leading to their transformation and degradation. To investigate different possible photo-induced oxidation pathways of dicarboxylates adsorbed on iron(III)(hydr)oxide surfaces, we followed UV-A induced photoreactions of oxalate, malonate, succinate and their corresponding α-hydroxy analogues tartronate and malate with in situ ATR-FTIR spectroscopy in immersed particle layers of lepidocrocite, goethite, maghemite and hematite at pH 4. UV-A light (365 ± 5 nm) lead to fast degradation of oxalate, tartronate and malate, while malonate and succinate were photo-degraded at much slower rates. Efficient generation of OH-radicals can be excluded, as this would lead to fast and indiscriminate degradation of all tested compounds. Rapid photo-degradation of adsorbed oxalate and the α-hydroxydicarboxylates must be induced by direct ligand-to-metal charge transfer (LMCT) or by selectively oxidizing valence band holes, both processes requiring inner-sphere coordination with direct ligand-to-metal bonds to enable efficient electron-transfer. The slow photo-degradation of malonate and succinate can be explained by low-yield production of OH-radicals at the surface of the iron(III)(hydr)oxides.

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