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| - Photolysis of Citrate on the Surface of Lepidocrocite: An in situ Attenuated TotalReflection Infrared Spectroscopy Study
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| - The photodecomposition of citrate adsorbed to γ-FeOOH (lepidocrocite) was investigated by batchphotodissolution experiments and by in situ attenuated total reflection infrared spectroscopy (ATR-FTIR).Batch photodissolution experiments in suspensions of 125 mg/L γ-FeOOH and 100 μM 14C radio-labeledcitrate revealed that the α-hydroxycarboxylic acid functional group of citrate was selectively photooxidizedat pH 4 and pH 6. ATR-FTIR spectra recorded during the irradiation of γ-FeOOH-layers with adsorbedcitrate showed that the primary photoproduct of citrate was acetonedicarboxylic acid. In the presence ofexcess citrate, the adsorbed photoproduct was exchanged in a ligand-exchange reaction indicating that citrateforms stronger surface complexes than acetonedicarboxylic acid. The primary photooxidation reaction wasresolved from the subsequent ligand-exchange reaction by the application of a relatively high photon flux(5−10 W/cm2, 300−500 nm). Despite consecutive ligand-exchange reactions, the photoconversion of adsorbedcitrate to acetonedicarboxylic acid was almost complete at pH 4 within 22 min. At pH 6, only a smallphotodecomposition was observed. This result was interpreted in terms of (i) different fractions of inner- andouter-sphere citrate surface complexes at pH 4 and pH 6 and (ii) different photoreactivity of different inner-sphere complexes. Furthermore, both batch photodissolution experiments and ATR-FTIR spectroscopy revealedthat adsorbed acetonedicarboxylic acid was further decomposed to acetoacetate at pH 4 but not at pH 6. Thisstudy shows that the photooxidation of adsorbed citrate leads to the same products as the photodecompositionof dissolved ferric−citrate complexes. Moreover, it highlights the potential of ATR-FTIR spectroscopy forinvestigating photoreactions at iron oxide surfaces at the molecular level.
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