Electro-Fenton degradation of synthetic dyes.

The electrochemical removal of a synthetic solution containing 120mgL(-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 Fe(2+) 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 Fe(2+) ions and the electrogenerated H(2)O(2). After 4h 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 1.0mM was the optimal concentration of ferrous ions and the single step or the stepwise addition of Fe(2+) 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.