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 Å. 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.

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Accelerated bleaching, photobleaching and mineralization of the non-biodegradable azo-dye, Orange II, was observed with oxone in solutions with Co 2+ -ions. The bleaching rate of Orange II in the dark was found to follow a first-order kinetics with respect to [Co 2+ ] with a rate constant of 20 M −1 s −1 . Fitting of the Orange II photobleaching experimental points in the presence of the Co 2+ /oxone reagent was carried out and followed the trend known for reactions presenting a chain radical branched mechanism. The photobleaching trace could be fitted by a single mathematical expression with an error <5% with respect to the experimental data. The bleaching trace observed for the Orange II solution in the dark followed zero-order decay kinetics. In a typical run, Orange II (0.20 mM or TOC 30 mg C/l) in the presence of oxone and Co 2+ was bleached under visible light within ∼15 s. The Co 2+ -ion concentrations necessary to catalyze the Orange II mineralization by oxone was observed to be ∼100 times lower than the oxone concentration. A 100% TOC decrease, under visible light irradiation, was attained for an Orange II (0.2 mM) solution in the presence of Co 2+ -ions (0.06 mM) and oxone (20 mM) within times ∼70 min for solutions purged with oxygen. Under visible light irradiation, Orange II mineralization in the presence of O 2 involves the photo-dissociation of reaction intermediates leading to organic peroxides in the second step of the mineralization process.

Iron alginate gel beads were prepared starting from sodium alginate solutions. Fe complexed with carboxylate is active during the Fenton-enhanced decoloration/degradation of Orange II via the encapsulated Fe catalyst. By energy-dispersive X-ray microanalysis (EDX) it was shown that a significant amount of Fe was on the catalyst surface and only a smaller fraction inside the cross-linked Fe alginate. The Fe alginate beads were ∼2 mm in diameter containing highly dispersed Fe species with sizes of about 0.5 nm. The Fe alginate mediated decoloration of Orange II takes place in less than an hour in the presence of H 2 O 2 under visible light irradiation, at pH values between 5 and 8. Repetitive decoloration of Orange II solutions was observed by addition of the azo dye at the beginning of each cycle.

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