PHOTODEGRADATION OF EDTA IN THE PRESENCE OF LEPIDOCROCITE

Based on laboratory experiments combined with kinetic modeling, we pro pose a conceptual model for the photodegradation of initially uncomple xed EDTA in the presence of gamma-FeOOH (lepidocrocite), as follows: F ree EDTA becomes adsorbed at the surface of gamma-FeOOH and is initial ly photooxidized as a surface species. Thereby, gamma-FeOOH is reducti vely dissolved. Our results suggest that photooxidation of adsorbed ED TA, coupled to reductive dissolution of gamma-FeOOH, occurs through ph otolysis of the Fe(111)EDTA surface complex. The photochemically forme d 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 heterog eneous systems, initially uncomplexed EDTA is photooxidized via two pa thways: (i) photooxidation at the surface of gamma-FeOOH and (ii) phot olysis 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 pre dominantly through photolysis of dissolved Fe(111)EDTA, whereas at pH 7, photooxidation of adsorbed EDTA was more important in our aerated h eterogeneous 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 .