title: Photo-redox reactions of dicarboxylates and α-hydroxydicarboxylates at the surface of Fe(III)(hydr)oxides followed with in situ ATR-FTIR spectroscopy.
abstract: 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.
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland. Electronic address: paul.borer@eawag.ch.
authors.forename: Paul
authors.initials: P
authors.lastname: Borer
authors.affiliation: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland. Electronic address: stephan.hug@eawag.ch.
authors.forename: Stephan J
authors.initials: SJ
authors.lastname: Hug
mesh_terms: D000327:Adsorption; D005290:Ferric Compounds / Q000737:chemistry; D058085:Iron Compounds / Q000737:chemistry; D007700:Kinetics; D008293:Malates / Q000737:chemistry*; D008314:Malonates / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry; D019815:Oxalic Acid / Q000737:chemistry*; D010084:Oxidation-Reduction; D010782:Photolysis; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D019802:Succinic Acid / Q000737:chemistry*; D013646:Tartronates / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D008293:Malates; D008314:Malonates; D008903:Minerals; D013646:Tartronates; C094886:goethite; C000499:ferric oxide; C032973:tartronic acid; C030298:malic acid; D019815:Oxalic Acid; C030290:malonic acid; D019802:Succinic Acid
keywords: ATR-FTIR; Carboxylic acids; Iron(III)(hydr)oxides; Photo-oxidation; Photo-reactivity; α-Hydroxycarboxylic acids