Efficient removal of triphenylmethane dyes from aqueous medium by in situ electrogenerated Fenton’s reagent at carbon-felt cathode

Fenton’s reagent (Fe2+ + H2O2) has been electrogenerated in situ in an undivided electrolytic cell from the effective reduction of Fe3+ and O2 at carbon-felt cathode for the treatment of aqueous solutions of four triphenylmethane dyes (TPMs), namely malachite green (MG), crystal violet (CV), methyl green (MeG) and fast green FCF (FCF), at pH 3.0 and room temperature. MG has been used as a model among them to study the influence of some experimental parameters on the decay kinetics, COD removal and current efficiency. The results in such electro-Fenton system are explained in terms of the many parasitic reactions involving OH. Higher efficiency values are obtained with rising organic content and decreasing applied current. The first stage of the mineralization process, involving aromatic by-products, leads to fast decoloration as well as quick initial COD removal that fit well to a pseudo-first-order kinetics. At prolonged electrolysis time, the mineralization rate and efficiency decrease due to the formation of hardly oxidizable compounds and the enhancement of wasting reactions. Solutions of all four TPMs are quickly degraded following a pseudo-first-order decay kinetics. The absolute rate constant (kTPM) for their reaction with OH increases in the order MeG < FCF < CV < MG. Their degradation rate decreases when they are mixed due to competitive oxidation by OH. Finally, a mixture containing all four dyes with initial COD ca. 1000 mg l−1 is totally depolluted with efficiency near 100% at the beginning of the treatment. A general scheme for the mineralization of TPMs is proposed.