Photoassisted Fenton degradation of nonbiodegradable azo dye (Orange II) in Fe-free solutions mediated by cation transfer membranes

Photoassisted degradation of nonbiodegradable Orange II is shown to be catalyzed by Nafion cation-transfer membranes exchanged with Fe ions in the presence of H2O2. The Nafion membranes in the oxidative media used degraded Orange II with similar kinetics as found in the homogeneous Fe3+/H2O2 photoassisted catalysis, avoiding the drawbacks of the homogeneous treatment. The treatment of this model textile dye is shown to proceed via a Fenton-like process without sludge production because of the selective H2O2 decomposition on the Fe ions exchanged on the membrane. The effect of the concentration of H2O2, solution pH, azo dye concentration, and light intensity (visible light) on the degradation of Orange is reported in detail. The activity of the membranes during the Orange II decomposition was tested for 1500 h and was observed to remain fairly stable within this period. The Fe/Nafion membranes consisted mainly of Fe2O3 (78%) before reaction and Fe2O3 (14%) after light irradiation during Orange II oxidation, as found by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The size of the Fe particles in the Nafion was investigated by transmission electron microscopy (TEM) and turned out to be 37 +/- 4 Angstrom. These Q-sized Fe particles on the Nafion absorbed directly the light energy, avoiding the losses due to absorption by the contaminants as it is the case in homogeneous photoassisted Fenton processes. A simplified reaction mechanism for Orange II decomposition is suggested that is consistent with the experimental findings for solutions up to pH 4.8. The Fe redox reactions in the membranes under light were studied via XPS and spectrophotometric techniques. The effect of pretreatment of the azo dye making possible subsequent biological degradation was tested by BOD5. A drastic increase of the BOD5 values for the pretreated solutions was found with respect to the zero BOD5 value observed for nonpretreated Orange II.