Electro-Fenton degradation of synthetic dyes

The electrochemical removal of a synthetic solution containing 120 mg L−1 of alizarin red has been studied by electro-Fenton process using a gas-diffusion cathode to produce in situ hydrogen peroxide by oxygen reduction. The effect of operating conditions such as Fe2+ concentration, applied current, solution pH and temperature on the efficacy of the process was investigated. It is shown that alizarin red and its products may be effectively degraded by the OH radicals produced by the reaction between the Fe2+ ions and the electrogenerated H2O2.After 4 h of electrolysis COD removal was only 45% when no ferrous ions were added to the solutions, while the presence of ferrous ion greatly improved COD removal up to more than 90%. In particular 1.0 mM was the optimal concentration of ferrous ions and the single step or the stepwise addition of Fe2+ ions enables the same COD removal. The oxidation rate increased with increasing of current density and temperature and with decreasing of pH. The UV–vis analysis indicated that the discoloration of the solution occurs simultaneous with the destruction of aromatic rings and alizarin red is oxidised firstly to colourless intermediates (mainly phthalic acid, small carbonyl species) and then to carbon dioxide.