Discoloration of Azo-dyes at biocompatible pH-values through an Fe-histidine complex immobilized on nafion via Fenton-like processes

The Fe/histidine complex immobilized on a Nafion membrane was employed in the bleaching and photobleaching of the azo-dye Orange II. This photocatalyst shows to be effective in the pH range 3-10. The Fe/histidine/Nafion membrane is able to bleach/photobleach Orange II in the presence of H2O2 in a repetitive fashion. The catalyst can be recovered after each cycle and reused for further runs. This Fe/histidine/ Nafion membrane was also observed to be surprisingly stable and active toward the oxidation of the azo-dye in the presence of H2O2. The percentages of the homogeneous and heterogeneous processes leading to Orange II disappearance on the Fe/histidine/Nafion surface have been quantified as a function of the pH. The results obtained indicate that at pH values > 5.5 heterogeneous catalysis accounts for more than 96% of the Orange II disappearance. The nature of the Fe species (Fe-O-Fe) on the histidine/Nafion surface has been identified and evaluated by Fourier transform infrared spectroscopy (FTIR). The iron content on the Fe/histidine/Nafion membrane was investigated by elemental analysis (EA). No difference was found in the iron content (2.02% w/w) after seven repetitive Orange II discoloration cycles, indicating that leaching of the iron from the Fe/ histidine complex does not occur. High-resolution transmission electron microscopy (TEM) revealed iron clusters with a size of similar to6 nm. X-ray photoelectron spectroscopy (XPS) of the Fe complex on the Nafion surface indicated a periodic change in the Fe oxidation state within the photobleaching time of Orange II. The Fe concentration remained fairly stable in the catalyst inner layers as observed by Ar sputtering. This shows that the Fe/histidine complex successfully penetrated some,surface layers of the Nafion membrane. Based on the experimental data, a discoloration mechanism of Orange II on Fe/histidine/Nafion membranes is suggested.