Use of a carbon felt–iron oxide air-diffusion cathode for the mineralization of Malachite Green dye by heterogeneous electro-Fenton and UVA photoelectro-Fenton processes

Abstract

A novel carbon felt cathode coated with iron oxides has been prepared by Fe3+ electrodeposition. The deposited iron content was analyzed by X-ray fluorescence, the composition of iron oxides formed by X-ray diffraction and the morphology of deposits by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Freshly prepared electrodes were used as air-diffusion cathodes in a stirred tank reactor with a boron-doped diamond (BDD) anode to degrade solutions of Malachite Green dye at pH3.0 by heterogeneous electro-Fenton (EF) and photoelectro-Fenton (PEF) processes. Oxidizing agents were hydroxyl radicals formed at the BDD surface from water oxidation and at the cathode surface or in the bulk from Fenton's reaction between deposited or leached Fe2+ and electrogenerated H2O2 in both treatments, along with the photo-oxidation by UVA light in heterogeneous PEF. The decolorization and mineralization of dye solutions were faster for the latter process due to the photolysis of Fe(III) complexes with initial oxalate and generated carboxylic acids. TOC decay always obeyed a pseudo-first-order kinetics. The increase in dye concentration decelerated the decolorization efficiency and mineralization rate, but enhanced the mineralization current efficiency. A test of 10 consecutive cycles of heterogeneous EF treatment of 50mgL−1 of Malachite Green at 21.7mAcm−2 showed a gradual loss of iron oxides from the cathode with decreasing dye mineralization rate. An improvement of the stability of the novel cathode is then necessary for its reuse with a better keeping of the oxidation power of heterogeneous EF and PEF treatments of organic pollutants in waters.