PHOTODEGRADATION OF EDTA IN THE PRESENCE OF LEPIDOCROCITE

Based on laboratory experiments combined with kinetic modeling, we propose a conceptual model for the photodegradation of initially uncomplexed EDTA in the presence of gamma-FeOOH (lepidocrocite), as follows: Free EDTA becomes adsorbed at the surface of gamma-FeOOH and is initially photooxidized as a surface species. Thereby, gamma-FeOOH is reductively dissolved. Our results suggest that photooxidation of adsorbed EDTA, coupled to reductive dissolution of gamma-FeOOH, occurs through photolysis of the Fe(111)EDTA surface complex. The photochemically formed Fe(ll) then catalyzes the thermal dissolution of the solid phase in the presence of EDTA. This process results in production of dissolved Fe(111)EDTA, which is subsequently photolyzed. Hence, in these heterogeneous systems, initially uncomplexed EDTA is photooxidized via two pathways: (i) photooxidation at the surface of gamma-FeOOH and (ii) photolysis of dissolved Fe(111)EDTA that is formed in the Fe(ll)-catalyzed dissolution of gamma-FeOOH. Which pathway predominates depends on the relative rates of Fe(ll) oxidation and of Fe(ll)-catalyzed formation of dissolved Fe(111)EDTA. At pH 3, photooxidation of EDTA occurred predominantly through photolysis of dissolved Fe(111)EDTA, whereas at pH 7, photooxidation of adsorbed EDTA was more important in our aerated heterogeneous systems, because of the faster Fe(II) oxidation at pH 7, compared to pH 3. Our results indicate that not only dissolved Fe(111)EDTA but also Fe(111)EDTA surface complexes are efficiently photolyzed.