Photolysis of Citrate on the Surface of Lepidocrocite: An in situ Attenuated Total Reflection Infrared Spectroscopy Study

The photodecomposition of citrate adsorbed to gamma-FeOOH (lepidocrocite) was investigated by batch photodissolution experiments and by in situ attenuated total reflection infrared spectroscopy (ATR-FTIR). Batch photodissolution experiments in suspensions of 125 mg/L gamma-FeOOH and 100 mu M C-14 radio-labeled citrate revealed that the alpha-hydroxycarboxylic acid functional group of citrate was selectively photooxidized at pH 4 and pH 6. ATR-FTIR spectra recorded during the irradiation of gamma-FeOOH-layers with adsorbed citrate showed that the primary photoproduct of citrate was acetonedicarboxylic acid. In the presence of excess citrate, the adsorbed photoproduct was exchanged in a ligand-exchange reaction indicating that citrate forms stronger surface complexes than acetonedicarboxylic acid. The primary photooxidation reaction was resolved from the subsequent ligand-exchange reaction by the application of a relatively high photon flux (5-10 W/cm(2), 300-500 nm). Despite consecutive ligand-exchange reactions, the photoconversion of adsorbed citrate to acetonedicarboxylic acid was almost complete at pH 4 within 22 min. At pH 6, only a small photodecomposition was observed. This result was interpreted in terms of (i) different fractions of inner- and outer-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 revealed that adsorbed acetonedicarboxylic acid was further decomposed to acetoacetate at pH 4 but not at pH 6. This study shows that the photooxidation of adsorbed citrate leads to the same products as the photodecomposition of dissolved ferric-citrate complexes. Moreover, it highlights the potential of ATR-FTIR spectroscopy for investigating photoreactions at iron oxide surfaces at the molecular level.